Kim, Sung-Jin; Wang, Fang; Burns, Mark A; Kurabayashi, Katsuo
2009-06-01
Micromixing is a crucial step for biochemical reactions in microfluidic networks. A critical challenge is that the system containing micromixers needs numerous pumps, chambers, and channels not only for the micromixing but also for the biochemical reactions and detections. Thus, a simple and compatible design of the micromixer element for the system is essential. Here, we propose a simple, yet effective, scheme that enables micromixing and a biochemical reaction in a single microfluidic chamber without using any pumps. We accomplish this process by using natural convection in conjunction with alternating heating of two heaters for efficient micromixing, and by regulating capillarity for sample transport. As a model application, we demonstrate micromixing and subsequent polymerase chain reaction (PCR) for an influenza viral DNA fragment. This process is achieved in a platform of a microfluidic cartridge and a microfabricated heating-instrument with a fast thermal response. Our results will significantly simplify micromixing and a subsequent biochemical reaction that involves reagent heating in microfluidic networks.
Kim, Sung-Jin; Wang, Fang; Burns, Mark A.; Kurabayashi, Katsuo
2009-01-01
Micromixing is a crucial step for biochemical reactions in microfluidic networks. A critical challenge is that the system containing micromixers needs numerous pumps, chambers, and channels not only for the micromixing but also for the biochemical reactions and detections. Thus, a simple and compatible design of the micromixer element for the system is essential. Here, we propose a simple, yet effective, scheme that enables micromixing and a biochemical reaction in a single microfluidic chamber without using any pumps. We accomplish this process by using natural convection in conjunction with alternating heating of two heaters for efficient micromixing, and by regulating capillarity for sample transport. As a model application, we demonstrate micromixing and subsequent polymerase chain reaction (PCR) for an influenza viral DNA fragment. This process is achieved in a platform of a microfluidic cartridge and a microfabricated heating-instrument with a fast thermal response. Our results will significantly simplify micromixing and a subsequent biochemical reaction that involves reagent heating in microfluidic networks. PMID:19419189
Energy Technology Data Exchange (ETDEWEB)
Babcock, D.F.; Bernath, L.; Menegus, R.L.; Ring, H.F.
1956-05-01
A previous report described the conceptual design of a plutonium producing reactor that may be characterized as follows: Power output (2000 MW); cooling - (natural convection of light water through the reactor, up through a draft tube to an evaporative cooling pond, then back to the reactor, and fuel (400 to 500 tons of uranium enriched to 1.2% U-235). Because this reactor would be cooled by the natural convection of light water, it is believed that the construction costs would be significantly less than for a Savannah or Hanford type reactor. Such expensive items as water treatment and water pumping facilities would be eliminated entirely. The inventory of 500 tons of slightly enriched uranium, however, is an unattractive feature. It represents not only a large dollar investment but also makes the reactor less attractive for construction during periods of national emergency because of the almost certain scarcity of even slightly enriched uranium at that time. The Atomic Energy Commission asked that the design be reviewed with the objective of reducing the inventory of uranium, The results of this review are given in this report.
Natural convection from circular cylinders
Boetcher, Sandra K S
2014-01-01
This book presents a concise, yet thorough, reference for all heat transfer coefficient correlations and data for all types of cylinders: vertical, horizontal, and inclined. This book covers all natural convection heat transfer laws for vertical and inclined cylinders and is an excellent resource for engineers working in the area of heat transfer engineering.
Topology optimisation of natural convection problems
DEFF Research Database (Denmark)
Alexandersen, Joe; Aage, Niels; Andreasen, Casper Schousboe
2014-01-01
This paper demonstrates the application of the density-based topology optimisation approach for the design of heat sinks and micropumps based on natural convection effects. The problems are modelled under the assumptions of steady-state laminar flow using the incompressible Navier-Stokes equations...... coupled to the convection-diffusion equation through the Boussinesq approximation. In order to facilitate topology optimisation, the Brinkman approach is taken to penalise velocities inside the solid domain and the effective thermal conductivity is interpolated in order to accommodate differences...... for designing heat sink geometries cooled by natural convection and micropumps powered by natural convection. Copyright © 2013 John Wiley & Sons, Ltd....
A Simple Classroom Demonstration of Natural Convection
Wheeler, Dean R.
2005-01-01
This article explains a simple way to demonstrate natural convection, such as from a lit candle, in the classroom using an overhead projector. The demonstration is based on the principle of schlieren imaging, commonly used to visualize variations in density for gas flows.
Solar Hot Water Heating by Natural Convection.
Noble, Richard D.
1983-01-01
Presents an undergraduate laboratory experiment in which a solar collector is used to heat water for domestic use. The working fluid is moved by natural convection so no pumps are required. Experimental apparatus is simple in design and operation so that data can be collected quickly and easily. (Author/JN)
LAMINAR NATURAL CONVECTION IN TRIANGULAR ENCLOSURES
Kent, Emin Fuad
2016-01-01
Natural convection in non-rectangular enclosures is numerically analyzed in this study. Streamlines and isotherms are presented for different triangular enclosures with different boundary conditions and Rayleigh numbers. Mean Nusselt numbers on hot walls are also calculated in order to make comparisons between different cases.
Natural convection flow between moving boundaries | Chepkwony ...
African Journals Online (AJOL)
The laminar steady natural convection flow of viscous, incompressible fluid between two moving vertical plates is considered. It is assumed that the plates are moving in opposite direction with equal velocity. The two-point boundary value problem governing the flow is characterized by a non-dimensional parameter K. It is ...
Thermal Efficiency of Natural Convection Solar Dryer
Seetapong, N.; Chulok, S.; Khoonphunnarai, P.
2017-09-01
The purpose of this research is to study the thermal efficiency of natural convection. The working principle of natural convection solar dryer is, once the air in the solar dryer is heated by solar energy, the air relative humidity will drop and floating up through the drying. This air will take moisture out of the product and flow out to the ambient air. It was found from the experiment that, in the duration of 8.00 am - 4.00 pm on a clear sky day, an all-day average ambient air and inside the chamber temperature were 38.34°C and 63.19°C respectively. At the solar radiation intensity of 759.53W/m2, mass flow rate of air was 0.023 kg/s and the thermal efficiency of the solar dryer was 2.59%.
On natural solutal convection in magnetic fluids
Ivanov, A. S.; Pshenichnikov, A. F.
2015-09-01
An experiment was carried out to investigate natural solutal convection in a magnetic fluid caused by non-homogeneous initial distribution of colloidal particles in a vertical Hele-Shaw cell. For experiment, we used a dilute magnetic fluid of the "magnetite-kerosene-oleic acid" type. The initial distribution of particles was formed by magnetophoresis of the drop-like aggregates and their sedimentation on the surface of the diamagnetic disk located in the center of the cell. Application of the magnetic field on the system led to the onset of the Rayleigh-Taylor instability and formation of descending convective jets. The velocity of the flow at the front of descending jets was measured for different values of cell thickness (up to 0.18 mm) and strength of the magnetic field generating the drop-like aggregates (up to 21 kA/m). The solutal Rayleigh numbers varied in the range Ra = 50-105. It was shown that the intensity of the convective flow characterized by the Reynolds number Re, increases with the Rayleigh number according to the power law: Re = 1.16 × 10-5Ra0.86.
Natural convection through enclosed disconnected solid blocks
Energy Technology Data Exchange (ETDEWEB)
Lao, Fernando Cesar De; Junqueira, Silvio L.M.; Franco, Admilson T. [Universidade Tecnologica Federal do Parana (UTFPR), Curitiba, PR (Brazil)]. E-mails: fernandodelai@gmail.com; silvio@utfpr.edu.br; admilson@utfpr.edu.br; Lage, Jose L. [Southern Methodist University (SMU), Dallas, TX (United States)]. E-mail: JLL@smu.edu
2008-07-01
In this study, the natural convection inside a fluid filled, enclosure containing several solid obstructions and being heated from the side is modeled and numerically simulated. The solid obstructions are equally spaced, conducting, and disconnected square blocks. The mathematical model is based on the balance equations of mass, momentum and energy, which are then solved numerically via the finite-volume method with the SIMPLEST algorithm and the HYBRID scheme. The effects of varying the solid-fluid thermal conductivity ratio (K), the fluid volume-fraction or porosity ({phi}), the number of solid blocks (N) and the heating strength (represented by the Rayleigh number, Ra) of the enclosure on the Nusselt number based on the surface-averaged heat transfer coefficient along the heated wall of the enclosure are studied. The results indicate a competing effect caused by the proximity of the solid blocks to the heated and cooled walls, vis-a-vis hindering the boundary layer growth, hence reducing the heat transfer effectiveness, and at the same time enhancing the heat transfer when the blocks' thermal conductivity is larger than that of the fluid. An analytical estimate of the minimum number of blocks beyond which the convection hindrance becomes predominant is presented and validated by the numerical results. (author)
Further on integrator circuit analogy for natural convection
Energy Technology Data Exchange (ETDEWEB)
Khane, Vaibhav [Nuclear Engineering, Missouri University of Science and Technology, 225 Fulton Hall, 300W. 13th St., Rolla, MO-65409 (United States); Usman, Shoaib, E-mail: usmans@mst.ed [Nuclear Engineering, Missouri University of Science and Technology, 225 Fulton Hall, 300W. 13th St., Rolla, MO-65409 (United States)
2010-03-15
This research is an extension of the previous work on the development of an integrator (RC) circuit analogy for natural convection. This analogy has been proven experimentally as well as by numerical simulations. Additional Rayleigh-Benard convection numerical simulations were performed to investigate DELTAT (temperature difference between source and sink) dependence of the thermal resistance of a natural convection system. Our results suggest that analogous to voltage dependent resistor (VDR) in electrical engineering, DELTAT dependent thermal resistance is observed in natural convection system. This DELTAT dependent thermal resistance leads to a variable time constant. Moreover, this research also suggests that for a natural convection system, in addition to the thermal capacitance a kinetic energy capacitance also exists. The relative contribution of kinetic energy capacitance depends on Rayleigh number. These results provide significant step forward towards development of a new inexpensive modeling and transient analysis tool for a natural convection system.
Vertical natural convection: application of the unifying theory of thermal convection
Ng, C.S.; Ooi, A.; Lohse, Detlef; Chung, D.
2015-01-01
Results from direct numerical simulations of vertical natural convection at Rayleigh numbers 1.0×10 5 –1.0×10 9 and Prandtl number 0.709 support a generalised applicability of the Grossmann–Lohse (GL) theory, which was originally developed for horizontal natural (Rayleigh–Bénard) convection. In
Natural convection in an open cavity
Sernas, V.; Kyriakides, I.
Two-dimensional natural convection in an open air cavity of aspect ratio 1.0 at a Grashof number of 10 to the 7th was investigated experimentally. The heat transfer rates along the inner hot vertical wall and ceiling were measured with a Wollaston prism schlieren interferometer. Velocity profiles near the hot inner vertical wall and the ceiling were measured with a Laser Doppler Anemometer. Streamline patterns within the open cavity were visualized by injecting cigarette smoke into the cavity. The measured local heat flux distribution curve along the inner hot vertical wall, except in the region close to the upper corner, agreed well with the distribution curve predicted for an isothermal vertical flat plate at the same Grashof number. The vertical velocity profiles along the inner hot vertical wall of the open cavity agreed well with the profiles predicted for an infinite medium.
Natural convection in a fluid layer periodically heated from above.
Hossain, M Z; Floryan, J M
2014-08-01
Natural convection in a horizontal layer subject to periodic heating from above has been studied. It is shown that the primary convection leads to the cooling of the bulk of the fluid below the mean temperature of the upper wall. The secondary convection may lead either to longitudinal rolls, transverse rolls, or oblique rolls. The global flow properties (e.g., the average Nusselt number for the primary convection and the critical conditions for the secondary convection) are identical to those of the layer heated from below. However, the flow and temperature patterns exhibit phase shifts in the horizontal directions.
Natural Convection in Enclosed Porous or Fluid Media
Saatdjian, Esteban; Lesage, François; Mota, José Paulo B.
2014-01-01
In Saatdjian, E., Lesage, F., and Mota, J.P.B, "Transport Phenomena Projects: A Method to Learn and to Innovate, Natural Convection Between Porous, Horizontal Cylinders," "Chemical Engineering Education," 47(1), 59-64, (2013), the numerical solution of natural convection between two porous, concentric, impermeable cylinders was…
Natural convection in superposed fluid-porous layers
Bagchi, Aniruddha
2013-01-01
Natural Convection in Composite Fluid-Porous Domains provides a timely overview of the current state of understanding on the phenomenon of convection in composite fluid-porous layers. Natural convection in horizontal fluid-porous layers has received renewed attention because of engineering problems such as post-accident cooling of nuclear reactors, contaminant transport in groundwater, and convection in fibrous insulation systems. Because applications of the problem span many scientific domains, the book serves as a valuable resource for a wide audience.
Natural convective heat transfer from short inclined cylinders
Oosthuizen, Patrick H
2014-01-01
Natural Convective Heat Transfer from Short Inclined Cylinders examines a heat transfer situation of significant, practical importance not adequately dealt with in existing textbooks or in any widely available review papers. Specifically, the book introduces the reader to recent studies of natural convection from short cylinders mounted on a flat insulated base where there is an “exposed” upper surface. The authors considers the effects of the cylinder cross-sectional shape, the cylinder inclination angle, and the length-to-cross sectional size of the cylinder. Both numerical and experimental studies are discussed and correlation equations based on the results of these studies are reviewed. This book is ideal for professionals involved with thermal management and related systems, researchers, and graduate students in the field of natural convective heat transfer, instructors in graduate level courses in convective heat transfer.
Natural convection in sheep's wool and paper insulation
DEFF Research Database (Denmark)
Kristiansen, Finn Harken; Rode, Carsten
1999-01-01
The natural convection of two types of alternative insulation material has been measured in the convection apparatus of Department of Buildings and Energy. Measurements have been made on a type of sheep's wool (Herawool) with support fibres from the firm of Heraklith and a type of paper insulation...... alternative insulation materials have been compared to previous measurements on a traditional insulation material (Rockwool).Calculations of the convection conditions in the two materials have been made by means of a computer program CHConP. The measurements have been compared with these calculations....
Modeling approaches to natural convection in porous media
Su, Yan
2015-01-01
This book provides an overview of the field of flow and heat transfer in porous medium and focuses on presentation of a generalized approach to predict drag and convective heat transfer within porous medium of arbitrary microscopic geometry, including reticulated foams and packed beds. Practical numerical methods to solve natural convection problems in porous media will be presented with illustrative applications for filtrations, thermal storage and solar receivers.
Natural convection heat transfer within horizontal spent nuclear fuel assemblies
Energy Technology Data Exchange (ETDEWEB)
Canaan, R.E.
1995-12-01
Natural convection heat transfer is experimentally investigated in an enclosed horizontal rod bundle, which characterizes a spent nuclear fuel assembly during dry storage and/or transport conditions. The basic test section consists of a square array of sixty-four stainless steel tubular heaters enclosed within a water-cooled rectangular copper heat exchanger. The heaters are supplied with a uniform power generation per unit length while the surrounding enclosure is maintained at a uniform temperature. The test section resides within a vacuum/pressure chamber in order to subject the assembly to a range of pressure statepoints and various backfill gases. The objective of this experimental study is to obtain convection correlations which can be used in order to easily incorporate convective effects into analytical models of horizontal spent fuel systems, and also to investigate the physical nature of natural convection in enclosed horizontal rod bundles in general. The resulting data consist of: (1) measured temperatures within the assembly as a function of power, pressure, and backfill gas; (2) the relative radiative contribution for the range of observed temperatures; (3) correlations of convective Nusselt number and Rayleigh number for the rod bundle as a whole; and (4) correlations of convective Nusselt number as a function of Rayleigh number for individual rods within the array.
Natural thermal convection in fractured porous media
Adler, P. M.; Mezon, C.; Mourzenko, V.; Thovert, J. F.; Antoine, R.; Finizola, A.
2015-12-01
In the crust, fractures/faults can provide preferential pathways for fluid flow or act as barriers preventing the flow across these structures. In hydrothermal systems (usually found in fractured rock masses), these discontinuities may play a critical role at various scales, controlling fluid flows and heat transfer. The thermal convection is numerically computed in 3D fluid satured fractured porous media. Fractures are inserted as discrete objects, randomly distributed over a damaged volume, which is a fraction of the total volume. The fluid is assumed to satisfy Darcy's law in the fractures and in the porous medium with exchanges between them. All simulations were made for Rayleigh numbers (Ra) fracture aperture (or fracture transmissivity), fracture density and fracture length is studied. Moreover, these models are compared to porous media with the same macroscopic permeability. Preliminary results show that the non-uniqueness associated with initial conditions which makes possible either 2D or 3D convection in porous media (Schubert & Straus 1979) is no longer true for fractured porous media (at least for 50fracture density and fracture aperture on the Nusselt number (Nu) is highly Ra dependent. The effect of the damaged zone on Nu is roughly proportional to its size. All these models also allows us to determine for which range of fracture density the fractured porous medium is in good agreement with an unfractured porous medium of the same bulk permeability.
Study of Natural Convection Passive Cooling System for Nuclear Reactors
Abdillah, Habibi; Saputra, Geby; Novitrian; Permana, Sidik
2017-07-01
Fukushima nuclear reactor accident occurred due to the reactor cooling pumps and followed by all emergencies cooling systems could not work. Therefore, the system which has a passive safety system that rely on natural laws such as natural convection passive cooling system. In natural convection, the cooling material can flow due to the different density of the material due to the temperature difference. To analyze such investigation, a simple apparatus was set up and explains the study of natural convection in a vertical closed-loop system. It was set up that, in the closed loop, there is a heater at the bottom which is representing heat source system from the reactor core and cooler at the top which is showing the cooling system performance in room temperature to make a temperature difference for convection process. The study aims to find some loop configurations and some natural convection performances that can produce an optimum flow of cooling process. The study was done and focused on experimental approach and simulation. The obtained results are showing and analyzing in temperature profile data and the speed of coolant flow at some point on the closed-loop system.
Natural convection in rectangular enclosures with one thermally ...
African Journals Online (AJOL)
Natural convective fluid flow and heat transfer in rectangular enclosures bounded by three adiabatic walls and one thermally active and differentially heated vertical side were predicted by using the finite difference method. The effects of different temperature functions, aspect ratio and Rayleigh numbers on the natural ...
Solution of heat removal from nuclear reactors by natural convection
Directory of Open Access Journals (Sweden)
Zitek Pavel
2014-03-01
Full Text Available This paper summarizes the basis for the solution of heat removal by natural convection from both conventional nuclear reactors and reactors with fuel flowing coolant (such as reactors with molten fluoride salts MSR.The possibility of intensification of heat removal through gas lift is focused on. It might be used in an MSR (Molten Salt Reactor for cleaning the salt mixture of degassed fission products and therefore eliminating problems with iodine pitting. Heat removal by natural convection and its intensification increases significantly the safety of nuclear reactors. Simultaneously the heat removal also solves problems with lifetime of pumps in the primary circuit of high-temperature reactors.
Energy Technology Data Exchange (ETDEWEB)
Ghalambaz, M.; Noghrehabadi, A.; Ghanbarzadeh, A., E-mail: m.ghalambaz@gmail.com, E-mail: ghanbarzadeh.a@scu.ac.ir [Department of Mechanical Engineering, Shahid Chamran University of Ahvaz, Ahvaz (Iran, Islamic Republic of)
2014-04-15
In this paper, the natural convective flow of nanofluids over a convectively heated vertical plate in a saturated Darcy porous medium is studied numerically. The governing equations are transformed into a set of ordinary differential equations by using appropriate similarity variables, and they are numerically solved using the fourth-order Runge-Kutta method associated with the Gauss-Newton method. The effects of parametric variation of the Brownian motion parameter (Nb), thermophoresis parameter (Nt) and the convective heating parameter (Nc) on the boundary layer profiles are investigated. Furthermore, the variation of the reduced Nusselt number and reduced Sherwood number, as important parameters of heat and mass transfer, as a function of the Brownian motion, thermophoresis and convective heating parameters is discussed in detail. The results show that the thickness of the concentration profiles is much lower than the temperature and velocity profiles. For low values of the convective heating parameter (Nc), as the Brownian motion parameter increases, the non-dimensional wall temperature increases. However, for high values of Nc, the effect of the Brownian motion parameter on the non-dimensional wall temperature is not significant. As the Brownian motion parameter increases, the reduced Sherwood number increases and the reduced Nusselt number decreases. (author)
Numerical investigation of natural convection heat transfer of nano ...
African Journals Online (AJOL)
An enhanced cell-centered finite-volume procedure has been presented for solving the natural convection of the laminar Al O /Water nanofluid flow in a Γ shaped micro-channel in the slip flow region, including the effects of velocity slip and temperature jump at the wall, which are the main characteristics of flow in the slip ...
Application of the Random Vortex Method to Natural Convection ...
African Journals Online (AJOL)
Natural convection flows in channels have been studied using numerical tools such as finite difference and finite element techniques. These techniques are much demanding in computer skills and memory. Random Vortex Element method which has been used successfully in fluid flow was adopted in this work in view of its ...
Natural convection mud type solar dryers for rural farmers | Eke ...
African Journals Online (AJOL)
A prototype direct mod'e natural convection mud type solar dryer was designed, fabricated and tested. Mud was selected as a major dryer material based on its thermal properties, availability and workability by the rural dwellers who .are the end users. Consequent to the laboratory test which dried maize from initial moisture ...
Integral method for analyzing natural convection of non-newtonian ...
African Journals Online (AJOL)
An analytical study of natural convection boundary-layer flow along a vertical plate embedded in an anisotropic porous medium saturated by a non-Newtonian fluid has been conducted. The principal axis of permeability ani-sotropy was oriented in oblique direction to the gravity vector. A power-law variation of wall ...
Directory of Open Access Journals (Sweden)
MD. FAISAL KADER
2012-10-01
Full Text Available In the present paper, the effect of solar radiation on automobiles has been studied by both experimentally and numerically. The numerical solution is done by an operation friendly and fast CFD code – SC/Tetra with a full scale model of a SM3 car and turbulence is modeled by the standard k-ε equation. Numerical analysis of the three-dimensional model predicts a detailed description of fluid flow and temperature distribution in the passenger compartment during both the natural convection due to the incoming solar radiation and mixed convection due to the flow from defrost nozzle and radiation. It can be seen that solar radiation is an important parameter to raise the compartment temperature above the ambient temperature during summer. During natural convection, the rate of heat transfer is fast at the initial period. In the mixed convection analyses, it is found that the temperature drops down to a comfortable range almost linearly at the initial stage. Experimental investigations are performed to determine the temperature contour on the windshield and the local temperature at a particular point for further validation of the numerical results.
Natural convection in polygonal enclosures with inner circular cylinder
Directory of Open Access Journals (Sweden)
Habibis Saleh
2015-12-01
Full Text Available This study investigates the natural convection induced by a temperature difference between cold outer polygonal enclosure and hot inner circular cylinder. The governing equations are solved numerically using built-in finite element method of COMSOL. The governing parameters considered are the number of polygonal sides, aspect ratio, radiation parameter, and Rayleigh number. We found that the number of contra-rotative cells depended on polygonal shapes. The convection heat transfer becomes constant at L / D > 0 . 77 and the polygonal shapes are no longer sensitive to the Nusselt number profile.
EFFECT OF CONVECTIVE BOUNDARY CONDITIONS AT BOTTOM WALL ON NATURAL CONVECTIONS IN A SQUARE CAVITY
Directory of Open Access Journals (Sweden)
ASWATHA
2013-04-01
Full Text Available Simulations were carried out for natural convection in a square cavity using finite volume based computational procedure with biased quadratic elements to investigate the influence of convective boundary conditions at bottom wall. Parametric study has been carried out for a wide range of Rayleigh number (Ra (103 ≤ Ra ≤ 108, Prandtl number (Pr (0.7 ≤ Pr ≤ 17 and heat transfer coefficient (h (0.1 ≤ h ≤ 104 W/m2 K. It is observed from the present study that the heat transfer is primarily due to conduction for Rayleigh number up to 104. Convection dominant heat transfer is observed at higher Ra values. The intensity of circulation increases with increase in Ra number. The average heat transfer rate at the bottom wall is found to be invariant for all values of heat transfer coefficient for Ra up to 104. The power law correlations between average Nusselt number and Rayleigh numbers are presented for convection dominated regimes.
Nonlinear Multiplicative Schwarz Preconditioning in Natural Convection Cavity Flow
Liu, Lulu
2017-03-17
A natural convection cavity flow problem is solved using nonlinear multiplicative Schwarz preconditioners, as a Gauss-Seidel-like variant of additive Schwarz preconditioned inexact Newton (ASPIN). The nonlinear preconditioning extends the domain of convergence of Newton’s method to high Rayleigh numbers. Convergence performance varies widely with respect to different groupings of the fields of this multicomponent problem, and with respect to different orderings of the groupings.
Modeling the natural convective flow of micropolar nanofluids
Bourantas, Georgios
2014-01-01
A micropolar model for nanofluidic suspensions is proposed in order to investigate theoretically the natural convection of nanofluids. The microrotation of the nanoparticles seems to play a significant role into flow regime and in that manner it possibly can interpret the controversial experimental data and theoretical numerical results over the natural convection of nanofluids. Natural convection of a nanofluid in a square cavity is studied and computations are performed for Rayleigh number values up to 106, for a range of solid volume fractions (0 ≤ φ ≤ 0.2) and, different types of nanoparticles (Cu, Ag, Al2O3 and TiO 2). The theoretical results show that the microrotation of the nanoparticles in suspension in general decreases overall heat transfer from the heated wall and should not therefore be neglected when computing heat and fluid flow of micropolar fluids, as nanofluids. The validity of the proposed model is depicted by comparing the numerical results obtained with available experimental and theoretical data. © 2013 Elsevier Ltd. All rights reserved.
Predicting flow reversals in chaotic natural convection using data assimilation
Directory of Open Access Journals (Sweden)
Kameron Decker Harris
2012-04-01
Full Text Available A simplified model of natural convection, similar to the Lorenz system, is compared to computational fluid dynamics simulations of a thermosyphon in order to test data assimilation (DA methods and better understand the dynamics of convection. The thermosyphon is represented by a long time flow simulation, which serves as a reference ‘truth’. Forecasts are then made using the Lorenz-like model and synchronised to noisy and limited observations of the truth using DA. The resulting analysis is observed to infer dynamics absent from the model when using short assimilation windows. Furthermore, chaotic flow reversal occurrence and residency times in each rotational state are forecast using analysis data. Flow reversals have been successfully forecast in the related Lorenz system, as part of a perfect model experiment, but never in the presence of significant model error or unobserved variables. Finally, we provide new details concerning the fluid dynamical processes present in the thermosyphon during these flow reversals.
Natural convection heat transfer along vertical rectangular ducts
Energy Technology Data Exchange (ETDEWEB)
Ali, M. [King Saud University, Mechanical Engineering Department, Riyadh (Saudi Arabia)
2009-12-15
Experimental investigations have been reported on steady state natural convection from the outer surface of vertical rectangular and square ducts in air. Seven ducts have been used; three of them have a rectangular cross section and the rest have square cross section. The ducts are heated using internal constant heat flux heating elements. The temperatures along the vertical surface and the peripheral directions of the duct wall are measured. Axial (perimeter averaged) heat transfer coefficients along the side of each duct are obtained for laminar and transition to turbulent regimes of natural convection heat transfer. Axial (perimeter averaged) Nusselt numbers are evaluated and correlated using the modified Rayleigh numbers for laminar and transition regime using the vertical axial distance as a characteristic length. Critical values of the modified Rayleigh numbers are obtained for transition to turbulent. Furthermore, total overall averaged Nusselt numbers are correlated with the modified Rayleigh numbers and the area ratio for the laminar regimes. The local axial (perimeter averaged) heat transfer coefficients are observed to decrease in the laminar region and increase in the transition region. Laminar regimes are obtained at the lower half of the ducts and its chance to appear decreases as the heat flux increases. (orig.)
A new method to optimize natural convection heat sinks
Lampio, K.; Karvinen, R.
2017-08-01
The performance of a heat sink cooled by natural convection is strongly affected by its geometry, because buoyancy creates flow. Our model utilizes analytical results of forced flow and convection, and only conduction in a solid, i.e., the base plate and fins, is solved numerically. Sufficient accuracy for calculating maximum temperatures in practical applications is proved by comparing the results of our model with some simple analytical and computational fluid dynamics (CFD) solutions. An essential advantage of our model is that it cuts down on calculation CPU time by many orders of magnitude compared with CFD. The shorter calculation time makes our model well suited for multi-objective optimization, which is the best choice for improving heat sink geometry, because many geometrical parameters with opposite effects influence the thermal behavior. In multi-objective optimization, optimal locations of components and optimal dimensions of the fin array can be found by simultaneously minimizing the heat sink maximum temperature, size, and mass. This paper presents the principles of the particle swarm optimization (PSO) algorithm and applies it as a basis for optimizing existing heat sinks.
National Research Council Canada - National Science Library
Luo, Kang; Yi, Hong-Liang; Tan, He-Ping
2014-01-01
Transitions and bifurcations of transient natural convection in a horizontal annulus with radiatively participating medium are numerically investigated using the coupled lattice Boltzmann and direct...
An inverse natural convection problem of estimating the strength of a heat source
Energy Technology Data Exchange (ETDEWEB)
Park, H.M.; Chung, O.Y. [Sogang University, Seoul (D.P.R. of Korea). Dpt. of Chemical Engineering
1999-12-01
The inverse problem of determining the time-varying strength of a heat source, which causes natural convection in a two-dimensional cavity, is considered. The Boussinesq equation is used to model the natural convection induced by the heat source. The inverse natural convection problem is posed as a minimization problem of the least-square criterion, which is solved by a conjugate gradient method employing the adjoint equation to determine the descent direction. The present method solves the inverse natural convection problem accurately without any simplification of the governing Boussinesq equation. (author)
Adjoint optimization of natural convection problems: differentially heated cavity
Saglietti, Clio; Schlatter, Philipp; Monokrousos, Antonios; Henningson, Dan S.
2017-12-01
Optimization of natural convection-driven flows may provide significant improvements to the performance of cooling devices, but a theoretical investigation of such flows has been rarely done. The present paper illustrates an efficient gradient-based optimization method for analyzing such systems. We consider numerically the natural convection-driven flow in a differentially heated cavity with three Prandtl numbers (Pr=0.15{-}7) at super-critical conditions. All results and implementations were done with the spectral element code Nek5000. The flow is analyzed using linear direct and adjoint computations about a nonlinear base flow, extracting in particular optimal initial conditions using power iteration and the solution of the full adjoint direct eigenproblem. The cost function for both temperature and velocity is based on the kinetic energy and the concept of entransy, which yields a quadratic functional. Results are presented as a function of Prandtl number, time horizons and weights between kinetic energy and entransy. In particular, it is shown that the maximum transient growth is achieved at time horizons on the order of 5 time units for all cases, whereas for larger time horizons the adjoint mode is recovered as optimal initial condition. For smaller time horizons, the influence of the weights leads either to a concentric temperature distribution or to an initial condition pattern that opposes the mean shear and grows according to the Orr mechanism. For specific cases, it could also been shown that the computation of optimal initial conditions leads to a degenerate problem, with a potential loss of symmetry. In these situations, it turns out that any initial condition lying in a specific span of the eigenfunctions will yield exactly the same transient amplification. As a consequence, the power iteration converges very slowly and fails to extract all possible optimal initial conditions. According to the authors' knowledge, this behavior is illustrated here for
Natural convection of ferrofluids in partially heated square enclosures
Energy Technology Data Exchange (ETDEWEB)
Selimefendigil, Fatih, E-mail: fatih.selimefendigil@cbu.edu.tr [Department of Mechanical Engineering, Celal Bayar University, 45140 Manisa (Turkey); Öztop, Hakan F., E-mail: hfoztop1@gmail.com [Department of Mechanical Engineering, Technology Faculty, Fırat University, 23119 Elazığ (Turkey); Al-Salem, Khaled, E-mail: kalsalem@ksu.edu.sa [Department of Mechanical Engineering, College of Engineering, King Saud University, Riyadh (Saudi Arabia)
2014-12-15
In this study, natural convection of ferrofluid in a partially heated square cavity is numerically investigated. The heater is located to the left vertical wall and the right vertical wall is kept at constant temperature lower than that of the heater. Other walls of the square enclosure are assumed to be adiabatic. Finite element method is utilized to solve the governing equations. The influence of the Rayleigh number (10{sup 4}≤Ra≤5×10{sup 5}), heater location (0.25H≤y{sub h}≤0.75H), strength of the magnetic dipole (0≤γ≤2), horizontal and vertical location of the magnetic dipole (−2H≤a≤−0.5H, 0.2H≤b≤0.8H) on the fluid flow and heat transfer characteristics are investigated. It is observed that different velocity components within the square cavity are sensitive to the magnetic dipole source strength and its position. The length and size of the recirculation zones adjacent to the heater can be controlled with magnetic dipole strength. Averaged heat transfer increases with decreasing values of horizontal position of the magnetic dipole source. Averaged heat transfer value increases from middle towards both ends of the vertical wall when the vertical location of the dipole source is varied. When the heater location is changed, a symmetrical behavior in the averaged heat transfer plot is observed and the minimum value of the averaged heat transfer is attained when the heater is located at the mid of vertical wall. - Highlights: • Free convection of ferrofluids in a partially heated cavity is numerically studied. • Velocities are sensitive to the magnetic dipole source strength and its position. • Averaged Nu increases with decreasing x-location values of the magnetic dipole. • Averaged Nu increases from middle towards ends when y-location of dipole changes.
Directory of Open Access Journals (Sweden)
Minea Alina Adriana
2015-01-01
Full Text Available The aim of this paper is to present a theoretical analysis of a few convection problems. The investigations were started from the geometry of a classic muffle manufactured furnace. During this analytical study, different methodologies have been carefully chosen in order to compare and evaluate the effects of applying different analytical methods of the convection heat transfer processes. In conclusion, even if there are available a lot of analytical methods, natural convection in enclosed enclosures can be studied correctly only with numerical analysis. Also, in this article is presented a case study on natural convection application in a closed heated enclosure.
NUMERICAL ANALYSIS OF NATURAL CONVECTION IN A PRISMATIC ENCLOSURE
Directory of Open Access Journals (Sweden)
Walid AICH
2011-01-01
Full Text Available Natural convection heat transfer and fluid flow have been examined numerically using the control-volume finite-element method in an isosceles prismatic cavity, submitted to a uniform heat flux from below when inclined sides are maintained isothermal and vertical walls are assumed to be perfect thermal insulators, without symmetry assumptions for the flow structure. The aim of the study is to examine a pitchfork bifurcation occurrence. Governing parameters on heat transfer and flow fields are the Rayleigh number and the aspect ratio of the enclosure. It has been found that the heated wall is not isothermal and the flow structure is sensitive to the aspect ratio. It is also found that heat transfer increases with increasing of Rayleigh number and decreases with increasing aspect ratio. The effects of aspect ratio become significant especially for higher values of Rayleigh number. Eventually the obtained results show that a pitchfork bifurcation occurs at a critical Rayleigh number, above which the symmetric solutions becomes unstable and asymmetric solutions are instead obtained.
Natural convection heat transfer analysis of ATR fuel elements
Energy Technology Data Exchange (ETDEWEB)
Langerman, M.A.
1992-05-01
Natural convection air cooling of the Advanced Test Reactor (ATR) fuel assemblies is analyzed to determine the level of decay heat that can be removed without exceeding the melting temperature of the fuel. The study was conducted to assist in the level 2 PRA analysis of a hypothetical ATR water canal draining accident. The heat transfer process is characterized by a very low Rayleigh number (Ra {approx} 10{sup {minus}5}) and a high temperature ratio. Since neither data nor analytical models were available for Ra < 0.1, an analytical approach is presented based upon the integral boundary layer equations. All assumptions and simplifications are presented and assessed and two models are developed from similar foundations. In one model, the well-known Boussinesq approximations are employed, the results from which are used to assess the modeling philosophy through comparison to existing data and published analytical results. In the other model, the Boussinesq approximations are not used, thus making the model more general and applicable to the ATR analysis.
Lattice Boltzmann model for melting with natural convection
Energy Technology Data Exchange (ETDEWEB)
Huber, Christian [Department of Earth and Planetary Science, University of California - Berkeley, 307 McCone Hall 4767, Berkeley, CA 94720-4767 (United States)], E-mail: chuber@seismo.berkeley.edu; Parmigiani, Andrea [Computer Science Department, University of Geneva, 24, Rue du General Dufour, 1211 Geneva 4 (Switzerland)], E-mail: andrea.parmigiani@terre.unige.ch; Chopard, Bastien [Computer Science Department, University of Geneva, 24, Rue du General Dufour, 1211 Geneva 4 (Switzerland)], E-mail: Bastien.Chopard@cui.unige.ch; Manga, Michael [Department of Earth and Planetary Science, University of California - Berkeley, 177 McCone Hall 4767, Berkeley, CA 94720-4767 (United States)], E-mail: manga@seismo.berkeley.edu; Bachmann, Olivier [Department of Earth and Space Science, University of Washington, Johnson Hall 070, Seattle WA 98195-1310 (United States)], E-mail: bachmano@u.washington.edu
2008-10-15
We develop a lattice Boltzmann method to couple thermal convection and pure-substance melting. The transition from conduction-dominated heat transfer to fully-developed convection is analyzed and scaling laws and previous numerical results are reproduced by our numerical method. We also investigate the limit in which thermal inertia (high Stefan number) cannot be neglected. We use our results to extend the scaling relations obtained at low Stefan number and establish the correlation between the melting front propagation and the Stefan number for fully-developed convection. We conclude by showing that the model presented here is particularly well-suited to study convection melting in geometrically complex media with many applications in geosciences.
Investigation of Solar Drying of Plantain and Cassava Slices under Natural Convection
Directory of Open Access Journals (Sweden)
2016-11-01
Full Text Available Solar drying of cassava and plantain under natural convection were investigated in this study. Slices of cassava and plantain weighing 451.2 and 369.8 g respectively were dried in a natural convection solar dryer while equal masses were dried in the open sun. Moisture removal of 71 and 68.2% were obtained for the cassava and plantain slices in the solar dryer while 55.7 and 55.6% were obtained in the open sun. The study further revealed that the slices dried in the solar drier were neater with no coloration. Natural convection solar drier is thus recommended for use in the rural areas.
MHD Natural Convection with Convective Surface Boundary Condition over a Flat Plate
Directory of Open Access Journals (Sweden)
Mohammad M. Rashidi
2014-01-01
Full Text Available We apply the one parameter continuous group method to investigate similarity solutions of magnetohydrodynamic (MHD heat and mass transfer flow of a steady viscous incompressible fluid over a flat plate. By using the one parameter group method, similarity transformations and corresponding similarity representations are presented. A convective boundary condition is applied instead of the usual boundary conditions of constant surface temperature or constant heat flux. In addition it is assumed that viscosity, thermal conductivity, and concentration diffusivity vary linearly. Our study indicates that a similarity solution is possible if the convective heat transfer related to the hot fluid on the lower surface of the plate is directly proportional to (x--1/2 where x- is the distance from the leading edge of the solid surface. Numerical solutions of the ordinary differential equations are obtained by the Keller Box method for different values of the controlling parameters associated with the problem.
National Research Council Canada - National Science Library
Ahmed Kadhim Hussein; Kolsi Lioua; Ramesh Chand; S. Sivasankaran; Rasoul Nikbakhti; Dong Li; Borjini Mohamed Naceur; Ben Aïssia Habib
2016-01-01
Numerical computation of unsteady laminar three-dimensional natural convection and entropy generation in an inclined cubical trapezoidal air-filled cavity is performed for the first time in this work...
Energy Technology Data Exchange (ETDEWEB)
Yamamoto, Satoru; Niiyama, Daisuke [Tohoku Univ., Sendai (Japan). Dept. of Aeronautics and Space Engineering; Byeong Rog Shin [Tohoku Univ., Sendai (Japan). Inst. of Fluid Science
2004-12-01
Natural convection around a horizontal circular pipe coupled with heat conduction in the solid structure is numerically investigated using a preconditioning method for solving incompressible and compressible Navier-Stokes equations. In this method, fundamental equations are completely reduced to an equation of heat conduction when the flow field is static (zero velocity). Therefore, not only compressible flows but also very slow flows such as natural convection in a flow field and heat conduction in a static field can be simultaneously calculated using the same computational algorithm. In this study, we first calculated the compressible flow around a NACA0012 airfoil with conduction in the airfoil and then simulated natural convections around a horizontal circular pipe with a different heat conductivity. Finally, we numerically investigated the effect of heat conductivity of the pipe on natural convection. (author)
Wu, Jilian; Shen, Jie; Feng, Xinlong
2017-11-01
We construct in this paper two Gauge-Uzawa schemes, one in conserved form and the other in convective form, for solving natural convection problems with variable density, and prove that the first-order versions of both schemes are unconditionally stable. We also show that a full discretized version of the conserved scheme with finite elements is also unconditionally stable. These schemes lead to a sequence of decoupled elliptic equations to solve at each step, hence, they are very efficient and easy to implement. We present several numerical tests to validate the analysis and demonstrate the effectiveness of these schemes for simulating natural convection problems with large density differences.
Role of natural convection in the dissolution of sessile droplets
Dietrich, Erik; Visser, Claas Willem; Hofhuis, Kevin; Kooij, E Stefan; Zandvliet, Harold J W; Lohse, Detlef
2016-01-01
The dissolution process of small (initial (equivalent) radius $R_0 Ra_t$, where $Ra_t = 12$ is the transition Ra-number as extracted from the data. For $Ra < Ra_t$ and smaller, convective transport is progressively overtaken by diffusion and the above scaling relations break down.
Spectrally-consistent regularization modeling of turbulent natural convection flows
Trias, F. Xavier; Verstappen, Roel; Gorobets, Andrey; Oliva, Assensi
2012-01-01
The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Reynolds numbers because of the almost numberless small scales produced by the non-linear convective
analysis and computer simulation of a natural convective solar
African Journals Online (AJOL)
A model has been developed to predict the outlet air temperature and air flow rate from a solar collector based on the theory of thermal buoyancy. A high capacitance solar collector directly coupled to an animal building absorbs solar radiation, which heats up air and forces entry into the building by convection. In order to ...
Nature, theory and modelling of geophysical convective planetary boundary layers
Zilitinkevich, Sergej
2015-04-01
Geophysical convective planetary boundary layers (CPBLs) are still poorly reproduced in oceanographic, hydrological and meteorological models. Besides the mean flow and usual shear-generated turbulence, CPBLs involve two types of motion disregarded in conventional theories: 'anarchy turbulence' comprised of the buoyancy-driven plumes, merging to form larger plumes instead of breaking down, as postulated in conventional theory (Zilitinkevich, 1973), large-scale organised structures fed by the potential energy of unstable stratification through inverse energy transfer in convective turbulence (and performing non-local transports irrespective of mean gradients of transporting properties). C-PBLs are strongly mixed and go on growing as long as the boundary layer remains unstable. Penetration of the mixed layer into the weakly turbulent, stably stratified free flow causes turbulent transports through the CPBL outer boundary. The proposed theory, taking into account the above listed features of CPBL, is based on the following recent developments: prognostic CPBL-depth equation in combination with diagnostic algorithm for turbulence fluxes at the CPBL inner and outer boundaries (Zilitinkevich, 1991, 2012, 2013; Zilitinkevich et al., 2006, 2012), deterministic model of self-organised convective structures combined with statistical turbulence-closure model of turbulence in the CPBL core (Zilitinkevich, 2013). It is demonstrated that the overall vertical transports are performed mostly by turbulence in the surface layer and entrainment layer (at the CPBL inner and outer boundaries) and mostly by organised structures in the CPBL core (Hellsten and Zilitinkevich, 2013). Principal difference between structural and turbulent mixing plays an important role in a number of practical problems: transport and dispersion of admixtures, microphysics of fogs and clouds, etc. The surface-layer turbulence in atmospheric and marine CPBLs is strongly enhanced by the velocity shears in
Directory of Open Access Journals (Sweden)
Mohammad Jafari
2015-09-01
Full Text Available The effects of Single Walled Carbon Nanotube and Copper nanoparticles on natural convection heat transfer in an open cavity are investigated numerically. The problem is studied for different volume fractions of nanoparticles (0–1% and aspect ratio of the cavity (1–4 when Rayleigh number varies from 103 to 105. The volume fraction of added nanoparticles to Water is lower than 1% to make a dilute suspension. Although, results show that adding nanoparticles to the base fluid enhances the heat transfer, make a comparison between SWCNT and Cu-nanoparticles shows that the SWCNT-nanoparticle has better performance to enhance the convection rate. It is found that the aspect ratio of the cavity plays an important role on natural convection. An increase of this parameter leads to heat transfer reduction in the target problem. It is concluded that the Carbon Nanotubes can be applied as a passive way to enhance heat transfer in convection problems.
Study of natural convection in a freezer at different modes of operation of the refrigeration unit
Directory of Open Access Journals (Sweden)
Krasnoshlykov Aleksander S.
2015-01-01
Full Text Available The paper discusses a study of natural convective in the volume of air and water in the freezer, under various operating conditions of the refrigeration unit. It was obtained changing the temperature of the water and air depending on the time of installation. It was revealed, that convective flows in a study area produce influence on the temperature field being frozen liquid. The values obtained by mathematical modelling are in good agreement with experimental data.
Energy Technology Data Exchange (ETDEWEB)
Arevalo J, P
1998-12-31
At this thesis it is studied the heat transfer by natural convection in an horizontal cavity, it is involved a boiling`s part that is described the regimes and correlations differences for boiling`s curve. It is designed a horizontal cavity for realize the experimental part and it`s mention from equipment or instrumentation to succeed in a experimentation that permits to realize the analysis of heat transfer, handling as water fluid at atmospheric pressure and where it`s present process from natural convection involving part boiling`s subcooled. The system consists of heater zone submerged in a horizontal cavity with water. Once part finished experimental with information to obtained it`s proceeded to obtain a correlation, realized starting from analysis dimensionless such as: Jakob, Bond and Grasoft (Boiling) besides of knows in natural convection: Prandtl and Nusselt. The mathematical model explains the behavior for natural convection continued part boiling`s subcooled. It is realize analysis graphics too where it`s show comparing with Globe Dropkin and Catton equations by natural convection with bottom heating. (Author)
Huang, Zhu
2015-03-01
The periodic unsteady natural convection flow and heat transfer in a square enclosure containing a concentric circular cylinder is numerically studied. The temperature of the inner circular cylinder fluctuates periodically with time at higher averaged value while the temperature of the enclosure keeps lower constant, and the natural convection is driven by the temperature difference. The two-dimensional natural convection is simulated with high accuracy temporal spectral method and local radial basis functions method. The Rayleigh number is studied in the range 103 ≤ Ra ≤ 106, the temperature pulsating period ranges from 0.01 to 100 and the temperature pulsating amplitudes are a = 0.5, 1.0 and 1.5. Numerical results reveal that the fluid flow and heat transfer is strongly dependent on the pulsating temperature of inner cylinder. Comparing with the steady state natural convection, the heat transfer is enhanced generally for the time-periodic unsteady natural convection, and the local maximum heat transfer rate is observed for Ra = 105 and 106. Moreover, the phenomenon of backward heat transfer is discussed quantitatively. Also, the influence of pulsating temperature on the unsteady fluid flow and heat transfer are discussed and analyzed.
Numerical simulation of natural convection in wedge-shaped domain with isothermal free surface
Yaroslavtseva, N. A.; Ivanov, N. G.
2017-11-01
The contribution deals with 2D laminar unsteady natural convection in a wedge-shaped reservoir model induced by the isothermal surface heating of a water basin being colder than surrounding atmosphere. The problem formulation considered corresponds to large-scale convection development during a cloudy day when the solar radiation impact is negligible. Numerical simulation was performed using an in-house Navier-Stokes code SINF. The focus of the paper is on the accurate resolution of the initial period of the convective circulation pattern development. The dependence of the predicted convective structures on the computational domain size as well as on the boundary condition at the free surface is analysed. The influence of geometry on the buoyancy-induced flow formation is discussed.
Phase-field-lattice Boltzmann studies for dendritic growth with natural convection
Takaki, Tomohiro; Rojas, Roberto; Sakane, Shinji; Ohno, Munekazu; Shibuta, Yasushi; Shimokawabe, Takashi; Aoki, Takayuki
2017-09-01
Simulating dendritic growth with natural convection is challenging because of the size of the computational domain required when compared to the dendrite scale. In this study, a phase-field-lattice Boltzmann model was used to simulate dendritic growth in the presence of natural convection due to a difference in solute concentration. To facilitate and accelerate the large-scale simulation, a parallel computing code with multiple graphics processing units was developed. The effects of the computational domain size as well as those of gravity on the dendritic morphologies were examined by performing two-dimensional free dendritic growth simulations with natural convection. The effects of the gravity direction on the dendrite spacing and morphology were also investigated by simulating unidirectional solidification from multiple seeds.
Directory of Open Access Journals (Sweden)
Shestakov Igor A.
2015-01-01
Full Text Available The article shows the results of mathematical modeling of convective heat transfer in the low-temperature storage of liquefied natural gas. Regime of natural convection in an enclosure with different intensity of the heat flux at the external borders are investigated. Was examined two-dimensional nonstationary problem within the model of Navier-Stokes in dimensionless variables “vorticity - stream function - temperature”. Distributions of hydrodynamic parameters and temperatures that characterize the basic regularities of the processes are obtained. Circulating flows are determined and carried out the analysis of vortices formation mechanism and the temperature distribution in solution at conditions of natural convection when the Grashof number (Gr = 106. A significant influence of heat transfer rate on solutions boundary on flow structure and temperature field in LNG storage tanks.
Inverse natural convection problem of estimating wall heat flux using a moving sensor
Energy Technology Data Exchange (ETDEWEB)
Park, H.M.; Chung, O.Y.
1999-11-01
Inverse heat transfer problems have many applications in various branch of science and engineering. Here, the inverse problem of determining heat flux at the bottom wall of a two-dimensional cavity from temperature measurement in the domain is considered. The Boussinesq equation is used to model the natural convection induced by the wall heat flux. The inverse natural convection problem is posed as a minimization problem of the performance function, which is the sum of square residuals between calculated and observed temperature, by means of a conjugate gradient method. Instead of employing several fixed sensors, a single sensor is used which is moving at a given frequency over the bottom wall. The present method solves the inverse natural convection problem accurately without a priori information about the unknown function to be estimated.
Open Channel Natural Convection Heat Transfer on a Vertical Finned Plate
Energy Technology Data Exchange (ETDEWEB)
Park, Joo Hyun; Heo, Jeong Hwan; Chung, Bum Jin [Kyung Hee Univ., Yongin (Korea, Republic of)
2013-10-15
The natural convection heat transfer of vertical plate fin was investigated experimentally. Heat transfer systems were replaced by mass-transfer systems, based on the analogy concept. The experimental results lie within the predictions of the existing heat transfer correlations of plate-fin for the natural convections. An overlapped thermal boundary layers caused increasing heat transfer, and an overlapped momentum boundary layers caused decreasing heat transfer. As the fin height increases, heat transfer was enhanced due to increased inflow from the open side of the fin spacing. When fin spacing and fin height are large, heat transfer was unaffected by the fin spacing and fin height. Passive cooling by natural convection becomes more and more important for the nuclear systems as the station black out really happened at the Fukushima NPPs. In the RCCS (Reactor Cavity Cooling System) of a VHTR (Very High Temperature Reactor), natural convection cooling through duct system is adopted. In response to the stack failure event, extra cooling capacity adopting the fin array has to be investigated. The finned plate increases the surface area and the heat transfer increases. However, the plate of fin arrays may increase the pressure drop and the heat transfer decreases. Therefore, in order to enhance the passive cooling with fin arrays, the parameters for the fin arrays should be optimized. According to Welling and Wooldridge, a natural convection on vertical plate fin is function of Gr, Pr, L, t, S, and H. The present work investigated the natural convection heat transfer of a vertical finned plate with varying the fin height and the fin spacing. In order achieve high Rayleigh numbers, an electroplating system was employed and the mass transfer rates were measured using a copper sulfate electroplating system based on the analogy concept.
Transient performance and temperature field of a natural convection air dehumidifier loop
Fazilati, Mohammad Ali; Sedaghat, Ahmad; Alemrajabi, Ali-Akbar
2017-07-01
In this paper, transient performance of the previously introduced natural convection heat and mass transfer loop is investigated for an air dehumidifier system. The performance of the loop is studied in different conditions of heat source/heat sink temperature and different startup desiccant concentrations. Unlike conventional loops, it is observed that natural convection of the fluid originates from the heat sink towards the heat source. The proper operation of the cycle is highly dependent on the heat sink/heat source temperatures. To reduce the time constant of the system, a proper desiccant concentration should be adopted for charge of the loop.
Numerical Studies on Natural Convection Heat Losses from Open Cubical Cavities
Directory of Open Access Journals (Sweden)
M. Prakash
2013-01-01
Full Text Available The natural convection heat losses occurring from cubical open cavities are analysed in this paper. Open cubical cavities of sides 0.1 m, 0.2 m, 0.25 m, 0.5 m, and 1 m with constant temperature back wall boundary conditions and opening ratio of 1 are studied. The Fluent CFD software is used to analyse the three-dimensional (3D cavity models. The studies are carried out for cavities with back wall temperatures between 35°C and 100°C. The effect of cavity inclination on the convective loss is analysed for angles of 0° (cavity facing sideways, 30°, 45°, 60°, and 90° (cavity facing vertically downwards. The Rayleigh numbers involved in this study range between 4.5 × 105 and 1.5 × 109. The natural convection loss is found to increase with an increase in back wall temperature. The natural convection loss is observed to decrease with an increase in cavity inclination; the highest convective loss being at 0° and the lowest at 90° inclination. This is observed for all cavities analysed here. Nusselt number correlations involving the effect of Rayleigh number and the cavity inclination angle have been developed from the current studies. These correlations can be used for engineering applications such as electronic cooling, low- and medium-temperature solar thermal systems, passive architecture, and also refrigeration systems.
Natural Convection Cooling of the Advanced Stirling Radioisotope Generator Engineering Unit
Lewandowski, Edward J.; Hill, Dennis
2011-01-01
After fueling and prior to launch, the Advanced Stirling Radioisotope Generator (ASRG) will be stored for a period of time then moved to the launch pad for integration with the space probe and mounting on the launch vehicle. During this time, which could be as long as 3 years, the ASRG will operate continuously with heat rejected from the housing and fins. Typically, the generator will be cooled by forced convection using fans. During some of the ground operations, maintaining forced convection may add significant complexity, so allowing natural convection may simplify operations. A test was conducted on the ASRG Engineering Unit (EU) to quantify temperatures and operating parameters with natural convection only and determine if the EU could be safely operated in such an environment. The results show that with natural convection cooling the ASRG EU Stirling convertor pressure vessel temperatures and other parameters had significant margins while the EU was operated for several days in this configuration. Additionally, an update is provided on ASRG EU testing at NASA Glenn Research Center, where the ASRG EU has operated for over 16,000 hr and underwent extensive testing.
Numerical Study of Natural Convection in Vertical Enclosures Utilizing Nanofluid
Directory of Open Access Journals (Sweden)
M. Alipanah
2014-01-01
Full Text Available Enhancement of buoyancy-driven convection heat transfer within vertical cavities containing nanofluids subjected to different side wall temperatures and various aspect ratios is investigated. The computations are based on an iterative, finitevolume numerical procedure (SIMPLE that incorporates the Boussinesq approximation to simulate the buoyancy term. With the base fluid being water, three different nanoparticles (Cu, TiO2, and Al2O3 are considered as the nanofluids. This study has been carried out for the pertinent parameters in the following ranges: the Rayleigh number, Raf = 105–107 and the volumetric fraction of nanoparticle between 0 and 5 percent. The results are presented for different length-to-height ratios varying from 0.1 to 1.0. The comparisons show that the mean Nusselt numbers and velocity magnitudes increase with volume fraction for the whole range of the Rayleigh numbers. The predictions show a noticeable heat transfer enhancement compared to pure fluid. It is also found that the heat transfer enhancement utilizing nanofluid is more pronounced at low aspect ratios than high aspect ratios. Moreover, the results depict that the addition of nanoparticles to the pure fluid has more effects at lower Rayleigh numbers.
Scaled model studies of decay heat removal by natural convection for sodium cooled reactors
Energy Technology Data Exchange (ETDEWEB)
Hoffmann, H. (Institut fuer Angewandte Thermo- und Fluiddynamik (IATF), Kernforschungszentrum Karlsruhe (Germany)); Weinberg, D. (Institut fuer Angewandte Thermo- und Fluiddynamik (IATF), Kernforschungszentrum Karlsruhe (Germany)); Marten, K. (Institut fuer Angewandte Thermo- und Fluiddynamik (IATF), Kernforschungszentrum Karlsruhe (Germany)); Schnetgoeke, G. (Institut fuer Angewandte Thermo- und Fluiddynamik (IATF), Kernforschungszentrum Karlsruhe (Germany))
1993-06-01
Thermohydraulic experiments were performed with water in order to simulate decay heat removal by natural convection in a pool-type sodium cooled reactor. Two water test rigs of different scales were used, namely, RAMONA (1:20) and NEPTUN (1:5). RAMONA was taken to study the transition from nominal operation by forced convection to decay heat removal operation by natural convection. Steady-state similarity tests were carried out in both facilities. All tests provide a basis for verification of computer programs. Calculations performed with the three-dimensional code FLUTAN proved that the thermohydraulic processes are quantitatively mastered, even for the very complex geometry of the NEPTUN test rig. (orig.)
The Fractional Step Method Applied to Simulations of Natural Convective Flows
Westra, Douglas G.; Heinrich, Juan C.; Saxon, Jeff (Technical Monitor)
2002-01-01
This paper describes research done to apply the Fractional Step Method to finite-element simulations of natural convective flows in pure liquids, permeable media, and in a directionally solidified metal alloy casting. The Fractional Step Method has been applied commonly to high Reynold's number flow simulations, but is less common for low Reynold's number flows, such as natural convection in liquids and in permeable media. The Fractional Step Method offers increased speed and reduced memory requirements by allowing non-coupled solution of the pressure and the velocity components. The Fractional Step Method has particular benefits for predicting flows in a directionally solidified alloy, since other methods presently employed are not very efficient. Previously, the most suitable method for predicting flows in a directionally solidified binary alloy was the penalty method. The penalty method requires direct matrix solvers, due to the penalty term. The Fractional Step Method allows iterative solution of the finite element stiffness matrices, thereby allowing more efficient solution of the matrices. The Fractional Step Method also lends itself to parallel processing, since the velocity component stiffness matrices can be built and solved independently of each other. The finite-element simulations of a directionally solidified casting are used to predict macrosegregation in directionally solidified castings. In particular, the finite-element simulations predict the existence of 'channels' within the processing mushy zone and subsequently 'freckles' within the fully processed solid, which are known to result from macrosegregation, or what is often referred to as thermo-solutal convection. These freckles cause material property non-uniformities in directionally solidified castings; therefore many of these castings are scrapped. The phenomenon of natural convection in an alloy under-going directional solidification, or thermo-solutal convection, will be explained. The
Combined effect of natural convection and non-gray gas radiation ...
Indian Academy of Sciences (India)
The present paper reports numerical results of combined effects of non-gray gas radiation and natural convection between two vertical plates with partial heating at walls. The plates are symmetrical and made of two equal zones alternately isotherm and insulated. The idea is to predict that thermal radiation will attenuate the ...
Visualization of Natural Convection Heat Transfer on a Single Sphere using the Electroplating System
Energy Technology Data Exchange (ETDEWEB)
Lee, Dong Young; Chung, Bum Jin [Kyunghee University, Yongin (Korea, Republic of)
2016-05-15
The natural convective flows on outer sphere rise along surface. At top of sphere, the flows are lifted-up plume shape. For laminar flows, the local heat transfer shows maximum at the bottom of sphere and a monotonic decreases as flows approached to the top. The laminar natural convection heat transfer on a single sphere has been studied experimentally and numerically by several researchers. However, relatively less study has been performed for turbulent flows as it requires large facilities to achieve high Rayleigh numbers. The flows, which occur transition, is hard to experiment because of unstable. This study tried measurement of heat transfer and visualization external natural convection on a single sphere. The basic idea is that the plating patterns of copper on the sphere in mass transfer system will reveal the amount of heat transfer according to angular distance from the bottom. This study simulated natural convection on a single sphere and performed a mass transfer experiment using heat and mass transfer analogy concept. For visualization experiment, streak form plating pattern was observed. In this case, it seems that turbulence sets on the top of sphere and increases local heat transfer.
Numerical two-dimensional natural convection in an air filled square ...
African Journals Online (AJOL)
Numerical two-dimensional natural convection in an air filled square enclosure, tilted (25° and 65°) in relation to the horizontal plane, heated from two opposite sides for rayleigh numbers ranging between 10 3 and 2.10 6.
Large scale three-dimensional topology optimisation of heat sinks cooled by natural convection
DEFF Research Database (Denmark)
Alexandersen, Joe; Sigmund, Ole; Aage, Niels
2016-01-01
This work presents the application of density-based topology optimisation to the design of three-dimensional heat sinks cooled by natural convection. The governing equations are the steady-state incompressible Navier-Stokes equations coupled to the thermal convection-diffusion equation through...... topologies verify prior conclusions regarding fin length/thickness ratios and Biot numbers, but also indicate that carefully tailored and complex geometries may improve cooling behaviour considerably compared to simple heat fin geometries. (C) 2016 Elsevier Ltd. All rights reserved....
Carbon-nanotube nanofluid thermophysical properties and heat transfer by natural convection
Li, Y.; Suzuki, S.; Inagaki, T.; Yamauchi, N.
2014-11-01
We measured the thermophysical properties of suspensions of carbon nanotubes in water as a type of nanofluid, and experimentally investigated their heat transfer characteristics in a horizontal, closed rectangular vessel. Using a previously constructed system for high- reliability measurement, we quantitatively determined their thermophysical properties and the temperature dependence of these properties. We also investigated the as yet unexplained mechanism of heat transport in carbon-nanotube nanofluids and their flow properties from a thermal perspective. The results indicated that these nanofluids are non-Newtonian fluids, whose high viscosity impedes convection and leads to a low heat transfer coefficient under natural convection, despite their high thermal conductivity.
MHD natural convection in open inclined square cavity with a heated circular cylinder
Hosain, Sheikh Anwar; Alim, M. A.; Saha, Satrajit Kumar
2017-06-01
MHD natural convection in open cavity becomes very important in many scientific and engineering problems, because of it's application in the design of electronic devices, solar thermal receivers, uncovered flat plate solar collectors having rows of vertical strips, geothermal reservoirs, etc. Several experiments and numerical investigations have been presented for describing the phenomenon of natural convection in open cavity for two decades. MHD natural convection and fluid flow in a two-dimensional open inclined square cavity with a heated circular cylinder was considered. The opposite wall to the opening side of the cavity was first kept to constant heat flux q, at the same time the surrounding fluid interacting with the aperture was maintained to an ambient temperature T∞. The top and bottom wall was kept to low and high temperature respectively. The fluid with different Prandtl numbers. The properties of the fluid are assumed to be constant. As a result a buoyancy force is created inside the cavity due to temperature difference and natural convection is formed inside the cavity. The Computational Fluid Dynamics (CFD) code are used to discretize the solution domain and represent the numerical result to graphical form.. Triangular meshes are used to obtain the solution of the problem. The streamlines and isotherms are produced, heat transfer parameter Nu are obtained. The results are presented in graphical as well as tabular form. The results show that heat flux decreases for increasing inclination of the cavity and the heat flux is a increasing function of Prandtl number Pr and decreasing function of Hartmann number Ha. It is observed that fluid moves counterclockwise around the cylinder in the cavity. Various recirculations are formed around the cylinder. The almost all isotherm lines are concentrated at the right lower corner of the cavity. The object of this work is to develop a Mathematical model regarding the effect of MHD natural convection flow around
Danaila, Ionut; Moglan, Raluca; Hecht, Frédéric; Le Masson, Stéphane
2014-10-01
We present a new numerical system using finite elements with mesh adaptivity for the simulation of solid-liquid phase change systems. In the liquid phase, the natural convection flow is simulated by solving the incompressible Navier-Stokes equations with Boussinesq approximation. A variable viscosity model allows the velocity to progressively vanish in the solid phase, through an intermediate mushy region. The phase change is modeled by introducing an implicit enthalpy source term in the heat equation. The final system of equations describing the liquid-solid system by a single domain approach is solved using a Newton iterative algorithm. The space discretization is based on a P2-P1 Taylor-Hood finite elements and mesh adaptivity by metric control is used to accurately track the solid-liquid interface or the density inversion interface for water flows. The numerical method is validated against classical benchmarks that progressively add strong non-linearities in the system of equations: natural convection of air, natural convection of water, melting of a phase-change material and water freezing. Very good agreement with experimental data is obtained for each test case, proving the capability of the method to deal with both melting and solidification problems with convection. The presented numerical method is easy to implement using FreeFem++ software using a syntax close to the mathematical formulation.
Campbell, A N
2015-07-14
When any exothermic reaction proceeds in an unstirred vessel, natural convection may develop. This flow can significantly alter the heat transfer from the reacting fluid to the environment and hence alter the balance between heat generation and heat loss, which determines whether or not the system will explode. Previous studies of the effects of natural convection on thermal explosion have considered reactors where the temperature of the wall of the reactor is held constant. This implies that there is infinitely fast heat transfer between the wall of the vessel and the surrounding environment. In reality, there will be heat transfer resistances associated with conduction through the wall of the reactor and from the wall to the environment. The existence of these additional heat transfer resistances may alter the rate of heat transfer from the hot region of the reactor to the environment and hence the stability of the reaction. This work presents an initial numerical study of thermal explosion in a spherical reactor under the influence of natural convection and external heat transfer, which neglects the effects of consumption of reactant. Simulations were performed to examine the changing behaviour of the system as the intensity of convection and the importance of external heat transfer were varied. It was shown that the temporal development of the maximum temperature in the reactor was qualitatively similar as the Rayleigh and Biot numbers were varied. Importantly, the maximum temperature in a stable system was shown to vary with Biot number. This has important consequences for the definitions used for thermal explosion in systems with significant reactant consumption. Additionally, regions of parameter space where explosions occurred were identified. It was shown that reducing the Biot number increases the likelihood of explosion and reduces the stabilising effect of natural convection. Finally, the results of the simulations were shown to compare favourably with
Energy Technology Data Exchange (ETDEWEB)
Alloui, I., E-mail: imene.alloui@gmail.co [Universite Kasdi Merbah, BP 156 Rouissat, Ouargla 30130 (Algeria); Benmoussa, H. [Universite Hadj Lakhdar, 1 rue Chahid Boukhlouf, Batna 05000 (Algeria); Vasseur, P. [Ecole Polytechnique, C.P. 6079, Succ ' Center Ville' , Montreal, Quebec, H3C 3A7 (Canada)
2010-04-15
This paper reports an analytical and numerical study of the combined Soret and thermosolutal effects on natural convection in a shallow rectangular cavity filled with a binary mixture. Neumann boundary conditions for temperature and concentration are applied to the horizontal walls of the enclosure, while the two vertical ones are assumed impermeable and insulated. The governing parameters for the problem are the thermal Rayleigh number, Ra{sub T}, the Lewis number Le, the buoyancy ratio phi, the solute flux imposed on the horizontal boundaries j, the Prandtl number Pr, the aspect ratio of the cavity A, and the real number a (a = 0 for double diffusive convection and a = 1 for the coexistence of double diffusion convection and Soret effect). For convection in an infinite layer (A >> 1), analytical solutions for the stream function, temperature and concentration fields are obtained using a parallel flow approximation in the core region of the cavity and an integral form of the energy and constituent equations. The critical Rayleigh numbers for the onset of supercritical and subcritical convection are predicted explicitly by the present model. A linear stability analysis of the parallel flow model is conducted and the critical Rayleigh number for the onset of Hopf's bifurcation is predicted numerically. Also, results are obtained for finite amplitude convection for which the flow and heat and solute transfers are presented in terms of the governing parameters of the problem. Numerical solutions of the full governing equations are obtained for a wide range of the governing parameters. A good agreement is observed between the analytical model and the numerical simulations.
Integral transform solution of natural convection in a square cavity with volumetric heat generation
Directory of Open Access Journals (Sweden)
C. An
2013-12-01
Full Text Available The generalized integral transform technique (GITT is employed to obtain a hybrid numerical-analytical solution of natural convection in a cavity with volumetric heat generation. The hybrid nature of this approach allows for the establishment of benchmark results in the solution of non-linear partial differential equation systems, including the coupled set of heat and fluid flow equations that govern the steady natural convection problem under consideration. Through performing the GITT, the resulting transformed ODE system is then numerically solved by making use of the subroutine DBVPFD from the IMSL Library. Therefore, numerical results under user prescribed accuracy are obtained for different values of Rayleigh numbers, and the convergence behavior of the proposed eigenfunction expansions is illustrated. Critical comparisons against solutions produced by ANSYS CFX 12.0 are then conducted, which demonstrate excellent agreement. Several sets of reference results for natural convection with volumetric heat generation in a bi-dimensional square cavity are also provided for future verification of numerical results obtained by other researchers.
Energy Technology Data Exchange (ETDEWEB)
Zhang, Youjie [Institute on Nuclear Energy Technology, Tsinghua Univ., Beijing (China); Takeda, Tetsuaki; Inaba, Yoshitomo [Japan Atomic Energy Research Inst., Oarai, Ibaraki (Japan). Oarai Research Establishment
2000-11-01
To investigate a heat transfer characteristic in a vertical cavity between the pressure vessel and the cooling panel of a high-temperature engineering test reactor (HTTR), we carried out an experiment of natural convection coupled with thermal radiation in a vertical enclosure of a double coaxial cylinder. Rayleigh number based on the width of the double coaxial cylinder was set to be 5.6x10{sup 5} < Ra{sub d} < 1.04x10{sup 8}. A heat transfer coefficient of natural convection coupled with thermal radiation was obtained as function of Rayleigh number, aspect ratio of the enclosure, and the temperature of the hot and cold surface. We also carried out the numerical analysis using a heat transfer and fluid flow analytical code, which is named FLUENT/UNS. The numerical results of the temperature distribution in the apparatus showed good agreement with the experimental ones. (J.P.N.)
Study on the natural convection heat transfer characteristics in the air duct
Energy Technology Data Exchange (ETDEWEB)
Kim, Y. K.; Lee, Y. B.; Choi, S. K.; Hwang, J. S.; Nam, H. Y. [Korea Atomic Energy Research Institute, Taejon (Korea, Republic of)
1997-12-31
Temperature distribution measurements in the mockup apparatus of reactor vessel were performed to determine the effective thermal conductivity of porous media with different geometry and to obtain the experimental data for the heat transfer processes by natural convection occurring in the air duct. The temperature distributions at four separated sections with different arrangements of porous media have different slopes according to the geometrical configuration. From the measured temperature distribution, effective thermal conductivity have been derived using the least square fitting method. The test at air duct was performed to the high heat removal at 3.4kW/m{sup 2} by the natural convection from the outer wall to the air. And also the temperature distributions in the sir duct agree well with the 1/7th power-law turbulent temperature distribution. The obtained heat transfer data have been compared with the Shin`s and Sieger`s correlations. 10 refs., 6 figs. (Author)
Double-diffusive natural convection in an enclosure filled with nanofluid using ISPH method
Directory of Open Access Journals (Sweden)
Abdelraheem M. Aly
2016-12-01
Full Text Available The double-diffusive natural convection in an enclosure filled with nanofluid is studied using ISPH method. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. In addition the thermal energy equations include regular diffusion and cross-diffusion terms. In ISPH algorithm, a semi implicit velocity correction procedure is utilized and the pressure is implicitly evaluated by solving pressure Poisson equation. The results are presented with flow configurations, isotherms, concentration and nanoparticle volume fraction contours and average Nusselt and Sherwood numbers for different cases. The results from this investigation are well validated and have favorable comparisons with previously published results. It is found that, among all cases, a good natural convection can be obtained by considering the double diffusive case. An increase in Soret number accompanied by a decrease in Dufour number results in an increase in average Nusselt number and a decrease in average Sherwood number.
Moment-based boundary conditions for lattice Boltzmann simulations of natural convection in cavities
Allen, Rebecca
2016-06-29
We study a multiple relaxation time lattice Boltzmann model for natural convection with moment-based boundary conditions. The unknown primary variables of the algorithm at a boundary are found by imposing conditions directly upon hydrodynamic moments, which are then translated into conditions for the discrete velocity distribution functions. The method is formulated so that it is consistent with the second order implementation of the discrete velocity Boltzmann equations for fluid flow and temperature. Natural convection in square cavities is studied for Rayleigh numbers ranging from 103 to 108. An excellent agreement with benchmark data is observed and the flow fields are shown to converge with second order accuracy. Copyright © 2016 Inderscience Enterprises Ltd.
van der Sman, R G M
2006-08-01
In this paper we present lattice Boltzmann (LB) schemes for convection diffusion coupled to fluid flow on two-dimensional rectangular lattices. Via inverse Chapman-Enskog analysis of LB schemes including source terms, we show that for consistency with physics it is required that the moments of the equilibrium distributions equal those of the Maxwell-Boltzmann distribution. These constraints can be satisfied for the rectangular D2Q9 lattice for only fluid flow in the weakly compressible regime. The analysis of source terms shows that fluxes are really defined on the boundaries of the Wigner-Seitz cells, and not on the lattice sites where the densities are defined-which is quite similar to the staggered grid finite-volume schemes. Our theoretical findings are confirmed by numerical solutions of benchmark problems for convection diffusion and natural convection. The lattice Boltzmann scheme shows remarkably good performance for convection diffusion, showing little to non-numerical diffusion or numerical dispersion, even at high grid Peclet numbers.
Thermal performance of a porus radial fin with natural convection and radiative heat losses
Directory of Open Access Journals (Sweden)
Darvishi M.T.
2015-01-01
Full Text Available An analytic (series solution is developed to describe the thermal performance of a porous radial fin with natural convection in the fluid saturating the fin and radiation heat loss from the top and bottom surfaces of the fin. The HAM results for the temperature distribution and base heat flux are compared with the direct numerical results and found to be very accurate.
Energy Technology Data Exchange (ETDEWEB)
Forson, F.K.; Nazha, M.A.A. [De Montfort University, Leicester (United Kingdom). Dept. of Mechanical and Manufacture Engineering; Akuffo, F.O. [UST Kumasi (Ghana). Dept. of Mechanical and Manufacture Engineering
1996-07-01
Traditional open sun-drying, the predominant method of food preservation in Ghana, has serious drawbacks leading to significant post-harvest losses (between 20 and 30%). Mechanical dryers can provide high quality products with minimum losses but they require sources of electrical or chemical energy which, in the case of Ghana, are costly or scarce. This makes the development of an effective natural convection solar dryer an attractive alternative. A mixed-mode natural convection solar crop-dryer (a dryer in which the crop is dried by a combination of the direct absorption of solar radiation and by natural convection where air, heated by solar energy, is passed over the crop) has been designed. Two commercial dryers, based on this design and capable of handling between 1,000 and 1,500 kg of various agricultural products (such as cassava, pepper, maize, okra, etc.) have been constructed and partially tested at Agona-Asafo in the central region of Ghana. The results of these preliminary tests indicate that it takes 3-5 days to dry 1,000 kg of pepper from moisture content level of 78% to a safe storage level of 8%. The design, construction details, and the results of the preliminary tests are presented in this paper. (author)
Energy Technology Data Exchange (ETDEWEB)
Triplett, C.E.
1996-12-01
This thesis presents the results of an experimental investigation of natural convection heat transfer in a staggered array of heated cylinders, oriented horizontally within a rectangular enclosure. The main purpose of this research was to extend the knowledge of heat transfer within enclosed bundles of spent nuclear fuel rods sealed within a shipping or storage container. This research extends Canaan`s investigation of an aligned array of heated cylinders that thermally simulated a boiling water reactor (BWR) spent fuel assembly sealed within a shipping or storage cask. The results are presented in terms of piecewise Nusselt-Rayleigh number correlations of the form Nu = C(Ra){sup n}, where C and n are constants. Correlations are presented both for individual rods within the array and for the array as a whole. The correlations are based only on the convective component of the heat transfer. The radiative component was calculated with a finite-element code that used measured surface temperatures, rod array geometry, and measured surface emissivities as inputs. The correlation results are compared to Canaan`s aligned array results and to other studies of natural convection in horizontal tube arrays.
Solar drying of whole mint plant under natural and forced convection.
Sallam, Y I; Aly, M H; Nassar, A F; Mohamed, E A
2015-03-01
Two identical prototype solar dryers (direct and indirect) having the same dimensions were used to dry whole mint. Both prototypes were operated under natural and forced convection modes. In the case of the later one the ambient air was entered the dryer with the velocity of 4.2 m s(-1). The effect of flow mode and the type of solar dryers on the drying kinetics of whole mint were investigated. Ten empirical models were used to fit the drying curves; nine of them represented well the solar drying behavior of mint. The results indicated that drying of mint under different operating conditions occurred in the falling rate period, where no constant rate period of drying was observed. Also, the obtained data revealed that the drying rate of mint under forced convection was higher than that of mint under natural convection, especially during first hours of drying (first day). The values of the effective diffusivity coefficient for the mint drying ranged between 1.2 × 10(-11) and 1.33 × 10(-11) m(2) s(-1).
Natural air convection for cooling of particle accelerator electromagnets: case studies at CERN
Moreira, Mariana
Cooling by natural convection is investigated on two air-cooled corrector magnets at CERN. The heat transfer coefficient (HTC) for each magnet is estimated analytically using established empirical correlations for certain geometries. The HTC is also estimated numerically with three-dimensional steady-state finite element simulations. Air convection around the respective coils as well as heat conduction inside the coils are modelled. Different formulas for the HTC are tested in the post-processing of the simulation results. The HTC for each magnet is then determined experimentally by measuring surface temperatures on the coils through time at constant currents. A method to extract the HTC from these temperature curves is developed, in which the curves are fitted to a function that is derived from a thermodynamical analysis of the heating process. Some plausible ranges for the HTC for each magnet for different currents are obtained. The results of the different estimation methods are compared with the experimen...
Effect of an External Oriented Magnetic Field on Entropy Generation in Natural Convection
Directory of Open Access Journals (Sweden)
Atef El Jery
2010-05-01
Full Text Available The influence of an external oriented magnetic field on entropy generation in natural convection for air and liquid gallium is numerically studied in steady-unsteady states by solving the mass, the momentum and the energy conservation equations. Entropy generation depends on five parameters which are: the Prandtl number, the irreversibility coefficients, the inclination angle of the magnetic field, the thermal Grashof and the Hartmann numbers. Effects of these parameters on total and local irreversibilities as well as on heat transfer and fluid flow are studied. It was found that the magnetic field tends to decrease the convection currents, the heat transfer and entropy generation inside the enclosure. Influence of inclination angle of the magnetic field on local irreversibility is then studied.
Heat transfers in a double-skin roof ventilated by natural convection in summer time
Biwole, Pascal; Pompeo, C
2013-01-01
The double-skin roofs investigated in this paper are formed by adding a metallic screen on an existing sheet metal roof. The system enhances passive cooling of dwellings and can help diminishing power costs for air conditioning in summer or in tropical and arid countries. In this work, radiation, convection and conduction heat transfers are investigated. Depending on its surface properties, the screen reflects a large amount of oncoming solar radiation. Natural convection in the channel underneath drives off the residual heat. The bi-dimensional numerical simulation of the heat transfers through the double skin reveals the most important parameters for the system's efficiency. They are, by order of importance, the sheet metal surface emissivity, the screen internal and external surface emissivity, the insulation thickness and the inclination angle for a channel width over 6 cm. The influence of those parameters on Rayleigh and Nusselt numbers is also investigated. Temperature and air velocity profiles on seve...
Directory of Open Access Journals (Sweden)
Kang Luo
2014-05-01
Full Text Available Transitions and bifurcations of transient natural convection in a horizontal annulus with radiatively participating medium are numerically investigated using the coupled lattice Boltzmann and direct collocation meshless (LB-DCM method. As a hybrid approach based on a common multi-scale Boltzmann-type model, the LB-DCM scheme is easy to implement and has an excellent flexibility in dealing with the irregular geometries. Separate particle distribution functions in the LBM are used to calculate the density field, the velocity field and the thermal field. In the radiatively participating medium, the contribution of thermal radiation to natural convection must be taken into account, and it is considered as a radiative term in the energy equation that is solved by the meshless method with moving least-squares (MLS approximation. The occurrence of various instabilities and bifurcative phenomena is analyzed for different Rayleigh number Ra and Prandtl number Pr with and without radiation. Then, bifurcation diagrams and dual solutions are presented for relevant radiative parameters, such as convection-radiation parameter Rc and optical thickness τ. Numerical results show that the presence of volumetric radiation changes the static temperature gradient of the fluid, and generally results in an increase in the flow critical value. Besides, the existence and development of dual solutions of transient convection in the presence of radiation are greatly affected by radiative parameters. Finally, the advantage of LB-DCM combination is discussed, and the potential benefits of applying the LB-DCM method to multi-field coupling problems are demonstrated.
Performance analysis of greenhouse dryer by using insulated north-wall under natural convection mode
Directory of Open Access Journals (Sweden)
Prashant Singh Chauhan
2016-11-01
Full Text Available A prototype north wall insulated greenhouse dryer has been fabricated and tested in no-load conditions under natural convection mode. Experimentation has been conducted in two different cases. Case-I is when solar collector placed inside the dryer and Case-II is North wall insulated greenhouse dryer without solar collector. Coefficient of performance, heat utilisation factor, convective heat transfer coefficient and coefficient of diffusivity have been evaluated in thermal performance analysis. The difference of the highest convective heat transfer coefficient of both cases is 29.094W/m2°C which is showing the effectiveness of insulated north wall and solar collector. The maximum coefficient of diffusivity (0.0827 was achieved during the third day of experiment in Case-II. The inside room temperature of wall insulated greenhouse dryer for Case-I is 4.11%, 5.08 % and 11.61 % higher than the Case-II during the day 1, day 2 and day 3 respectively. This result is also showing the effectiveness of solar collector and insulated north wall. The highest heat utilisation factor (0.616 is obtained during the second day for Case-I while for Case-II it is 0.769 during the third day of experimentation. Maximum coefficient of performance achieved is 0.892 during the third day of the experiment for Case-I whereas 0.953 is obtained on the first day of experimentation for Case-II.
Analysis of ginger drying inside a natural convection indirect solar dryer: An experimental study
Directory of Open Access Journals (Sweden)
S. K. Sansaniwal
2015-12-01
Full Text Available In this paper, a natural convection indirect solar cabinet dryer has been fabricated to study the drying behaviour of ginger rhizomes in terms of its convective heat transfer coefficient and moisture removing rate (% db. Various experiments were conducted during the months of March and April 2014 at Guru Jambheshwar University of Science and Technology, Hisar (29o5’5’’N, 75o45’55’’E, India. Experimental data obtained were used to evaluate the Nusselt number constants using linear regression method. Considering these constants, the average value of convective heat transfer coefficient was obtained and observed to decrease with increase in mass of ginger samples and progression of drying days with variation from 0.59 to 5.42 W/m2˚C for different mass of ginger samples. The moisture removing rate was reported to increase with increase in mass of ginger samples and decreases significantly with the progression of drying days. The average collector efficiency was also observed to vary from 14.97 to 16.14% under increasing and decreasing trends of solar radiations from morning to noon and noon to evening respectively. Modified page model was reported best for describing the drying behaviour of different mass of ginger samples. The experimental error in terms of percent uncertainty ranged from 29.19 to 46.25%.
Directory of Open Access Journals (Sweden)
Perović Bojan D.
2017-01-01
Full Text Available The main purpose of this paper is to show how the inclination angle affects natural convection from a flat-plate photovoltaic module which is mounted on the ground surface. In order to model this effect, novel correlations for natural convection from isothermal flat plates are developed by using the fundamental dimensionless number. On the basis of the available experimental and numerical results, it is shown that the natural convection correlations correspond well with the existing empirical correlations for vertical, inclined, and horizontal plates. Five additional correlations for the critical Grashof number are derived from the available data, three indicating the onset of transitional flow regime and two indicating the onset of flow separation. The proposed correlations cover the entire range of inclination angles and the entire range of Prandtl numbers. This paper also provides two worked examples, one for natural convection combined with radiation and one for natural convection combined with forced convection and radiation. [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. TR33046
Decoupled Scheme for Time-Dependent Natural Convection Problem II: Time Semidiscreteness
Directory of Open Access Journals (Sweden)
Tong Zhang
2014-01-01
stability and the corresponding optimal error estimates are presented. Furthermore, a decoupled numerical scheme is proposed by decoupling the nonlinear terms via temporal extrapolation; optimal error estimates are established. Finally, some numerical results are provided to verify the performances of the developed algorithms. Compared with the coupled numerical scheme, the decoupled algorithm not only keeps good accuracy but also saves a lot of computational cost. Both theoretical analysis and numerical experiments show the efficiency and effectiveness of the decoupled method for time-dependent natural convection problem.
Simulation of natural convection heat transfer using nanofluid in a concentric annulus
Directory of Open Access Journals (Sweden)
Fallah Keivan
2017-01-01
Full Text Available In the present study, natural convection of nanofluids in a concentric horizontal annulus enclosure has been numerically simulated using the lattice Boltzmann method. A water-based nanofluid containing Al2O3 nanoparticle has been studied. Simulations have been carried while the Rayleigh number ranges from 103 to 105 and solid volume fraction varies between 0 and 0.04. The effects of solid volume fraction of nanofluids on hydrodynamic and thermal characteristics such as average and local Nusselt numbers, streamlines, and isotherm patterns for different values of solid volume fraction, annulus gap width ratio and Rayleigh number are investigated and discussed in detail.
Numerical study of natural melt convection in cylindrical cavity with hot walls and cold bottom sink
Directory of Open Access Journals (Sweden)
Ahmanache Abdennacer
2013-01-01
Full Text Available Numerical study of natural convection heat transfer and fluid flow in cylindrical cavity with hot walls and cold sink is conducted. Calculations are performed in terms of the cavity aspect ratio, the heat exchanger length and the thermo physical properties expressed via the Prandtl number and the Rayleigh number. Results are presented in the form of isotherms, streamlines, average Nusselt number and average bulk temperature for a range of Rayleigh number up to 106. It is observed that Rayleigh number and heat exchanger length influences fluid flow and heat transfer, whereas the cavity aspect ratio has no significant effects.
Fully developed natural convective micropolar fluid flow in a vertical channel with slip
Directory of Open Access Journals (Sweden)
E.A. Ashmawy
2015-10-01
Full Text Available The problem of fully developed natural convective micropolar fluid flow is investigated. The slip boundary conditions for fluid velocity are applied. Non-dimensional variables are introduced. The closed form solutions of the field equations are represented graphically. As expected, it can be seen that the increase in micropolarity parameter results in a decrease in the velocity and an increase in the microrotation. Also, it is observed that the increase in the slip parameter increases the velocity and decreases the microrotation. The no slip case can be recovered as a limiting case of this work when the slip parameter goes to infinity.
Directory of Open Access Journals (Sweden)
S. Das
2016-03-01
Full Text Available The transient natural convection in a vertical channel filled with nanofluids has been studied when thermal radiation is taken into consideration. The equations governing the flow are solved by employing the Laplace transform technique. Exact solutions for the velocity and temperature of nanofluid are obtained in cases of both prescribed surface temperature (PST and prescribed heat flux (PHF. The numerical results for the velocity and temperature of nanofluid are presented graphically for the pertinent parameters and discussed in detail. The fluid velocity is greater in the case of PST than that of PHF.
A study of natural convection cooling of multiple discrete heat sources in a vertical channel
Willson, Thomas D.
1988-06-01
Natural convection liquid cooling of simulated electronic components in a vertical channel was investigated. The test surface contained a single column of eight rectangular, protruding heated elements, each simulating a 20 pin dual-in-line package. Temperature measurements and flow visualization were performed for a number of power dissipation levels and channel widths. Collectively, this information was used in interpreting the flow and transport characteristics. A correlation to predict the heat transfer rates was developed based on the component surface temperatures. Optimum channel widths were determined from these surface temperature measurements for the range of power levels investigated. Temperature distributions in the fluid were measured using a traversing thermocouple probe.
Mayor, T S; Couto, S; Psikuta, A; Rossi, R M
2015-12-01
The ability of clothing to provide protection against external environments is critical for wearer's safety and thermal comfort. It is a function of several factors, such as external environmental conditions, clothing properties and activity level. These factors determine the characteristics of the different microclimates existing inside the clothing which, ultimately, have a key role in the transport processes occurring across clothing. As an effort to understand the effect of transport phenomena in clothing microclimates on the overall heat transport across clothing structures, a numerical approach was used to study the buoyancy-driven heat transfer across horizontal air layers trapped inside air impermeable clothing. The study included both the internal flow occurring inside the microclimate and the external flow occurring outside the clothing layer, in order to analyze the interdependency of these flows in the way heat is transported to/from the body. Two-dimensional simulations were conducted considering different values of microclimate thickness (8, 25 and 52 mm), external air temperature (10, 20 and 30 °C), external air velocity (0.5, 1 and 3 m s(-1)) and emissivity of the clothing inner surface (0.05 and 0.95), which implied Rayleigh numbers in the microclimate spanning 4 orders of magnitude (9 × 10(2)-3 × 10(5)). The convective heat transfer coefficients obtained along the clothing were found to strongly depend on the transport phenomena in the microclimate, in particular when natural convection is the most important transport mechanism. In such scenario, convective coefficients were found to vary in wavy-like manner, depending on the position of the flow vortices in the microclimate. These observations clearly differ from data in the literature for the case of air flow over flat-heated surfaces with constant temperature (which shows monotonic variations of the convective heat transfer coefficients, along the length of the surface). The flow
Natural convection of the oxide pool in a three-layer configuration of core melts
Energy Technology Data Exchange (ETDEWEB)
Kim, Su-Hyeon; Park, Hae-Kyun; Chung, Bum-Jin, E-mail: bjchung@khu.ac.kr
2017-06-15
Highlights: • Natural convection of oxide pool in 3-layer configuration during IVR was investigated. • High Ra was achieved by using mass transfer experiments based on analogy concept. • Heat ratio to light metal layer was 14% higher for 3-layer configuration than 2-layer one. • Heat transfer to heavy metal layer was poor and hence heat load to side wall increased. • Angular heat loads to side wall showed strengthened heat focusing at uppermost location. - Abstract: We investigated the natural convection of the oxide layer in a three-layer configuration of core melts in a severe accident. In order to achieve high modified Rayleigh numbers of 10{sup 12}–10{sup 13}, mass transfer experiments were performed using a copper sulfate electroplating system based upon the analogy between heat and mass transfer. Four different cooling conditions of the top and the bottom plates were tested. The upward heat ratios were 14% higher for three-layer than for two-layer due to the reduced heights and the downward heat ratios were lower the same amount. The local Nusselt numbers for the top and the bottom plates were measured and compared with the two layer configuration. To explore the heat load to the reactor vessel, the angle-dependent heat fluxes at the side wall, were measured and compared with the two-layer configuration. Heat load to the side wall and peak heat at the uppermost location were intensified for the three-layer configuration.
Energy Technology Data Exchange (ETDEWEB)
Ambrosini, Dario [DIMEG, Universita dell' Aquila, Loc. Monteluco di Roio, I-67040 Roio Poggio (AQ) (Italy); Tanda, Giovanni [DIPTEM, Universita di Genova, Via all' Opera Pia 15a, I-67100 Genova (Italy)
2006-01-01
In this work, natural convection heat transfer in vertical channels is experimentally investigated by applying different optical techniques, namely holographic interferometry and schlieren. Both these techniques are based on the temperature dependence of the air refractive index but they detect different optical quantities and their use involves different instrumentation and optical components. Optical methods, non-intrusive in nature, are particularly suitable for the visualization of flow and thermal fields as witnessed by their increasing use in a range of scientific and engineering disciplines; for this reason, the introduction of these experimental tools into a laboratory course can be of high value. Physics and engineering students can get familiarized with optical techniques, grasp the basics of thermal phenomena, usually elusive, which can be more easily understood if they are made visible, and begin to master digital image analysis, a key skill in laboratory activities. A didactic description of holographic interferometry and schlieren is provided and experimental results obtained for vertical, smooth and rib-roughened channels with asymmetrical heating are presented. A comparison between distributions of the local heat transfer coefficient (or its dimensionless counterpart, the Nusselt number) revealed good agreement between the results separately obtained by the two techniques, thus proving their suitability for investigating free convection heat transfer in channels.
A numerical study on the conjugate natural convection in a circular pipe containing water
Energy Technology Data Exchange (ETDEWEB)
Choi, Myoung-Young; Choi, Hyoung-Gwon [Seoul National University of Science & Technology, Seoul (Korea, Republic of)
2017-07-15
In this paper, the effect of material property of pipe on the conjugate natural convection in a circular pipe containing water was investigated by solving the unsteady incompressible Navier-Stokes equations coupled with energy equations of the water and pipe. Natural convection and conduction of water inside the pipe was coupled with the conduction of the pipe whose bottom was subject to uniform heat source. From the present grid resolution and time-step independent solutions, it has been confirmed that the water temperature inside a PVC pipe was higher than that inside a steel pipe due to the smaller heat capacity of PVC and that the streamline patterns of the two cases were found to be opposite because the thermal diffusivity of steel (PVC) is larger (smaller) than that of water such that steel (PVC) pipe is heated faster (slower) than water. Furthermore, a quantitative comparison of heat flux to water was performed by examining the distributions of the heat flux along the inside walls of steel/PVC. The average temperature of water inside steel was found to be higher than that inside PVC at the initial stage of heating. On the other hand, PVC provided a larger heat flux to water when it reached a steady value.
Directory of Open Access Journals (Sweden)
A. K. A. SHATI
2012-04-01
Full Text Available The effects of natural convection with and without the interaction of surface radiation in square and rectangular enclosures have been studied, numerically and theoretically. The analyses were carried out over a wide range of enclosure aspect ratios ranging from 0.0625 to 16, including square enclosures in sizes from 40cm to 240cm, with cold wall temperatures ranging from 283 to 373 K, and hot to cold temperature ratios ranging from 1.02 to 2.61. The work was carried out using four different fluids whose properties are varying with temperature. FLUENT software was used to carry out the numerical study. Turbulence was modelled using the RNG k-ε model with a non-uniform grid. The Discrete Transfer Radiation Model (DTRM was used for radiation simulation. A correlation equation for the new dimensionless group represented by the ratio of natural convection to radiation, as a function of Nusselt, Grashof, Prandtl numbers and temperature ratio also, the average Nusselt number without radiation as a function of Grashof and Prandtl numbers have been provided along with the constants needed to use them as a function of temperature ratio. This provides a generalised equation for heat transfer in square and rectangular enclosures both with and without radiation.
Thin-layer solar drying characteristics of rough rice under natural convection
Energy Technology Data Exchange (ETDEWEB)
Basunia, M.A. [Bangladesh Agricultural University, Mymensingh (Bangladesh). Department of Farm Power and Machinery; Abe, T. [Ehime University, Matsuyama (Japan). Department of Biomechanical Systems, Laboratory of Agricultural Process Engineering
2001-07-01
Thin-layer solar drying experiments were conducted at Matsuyama, Japan, with medium grain rough rice. The range of average drying air temperature was 22.3 - 34.9 deg C, and the relative humidities were between 34.5 per cent and 57.9 per cent. The initial moisture contents were in the range of 37.07 - 37.69 per cent dry-basis. A mixed-mode natural convection solar grain dryer was used for this experiment. The data of sample weight and dry- and wet-bulb temperatures of the drying air were recorded continuously from morning to evening for each test. The drying data were then fitted to the Page model, based on the ratios of the differences between the initial and final moisture contents and the equilibrium moisture content (EMC). The model gave a good fit for the moisture content with an average standard error of 0.00387 dry-basis. Both the drying parameters N and K are linear functions of temperature and relative humidity. This single-layer drying equation can be used for the simulation of deep bed drying of rough rice in mixed-mode type natural convection solar dryers. (author)
Three dimensional natural convection analysis in the VENUS vertex chamber at TRISTAN
Energy Technology Data Exchange (ETDEWEB)
Maekawa, Isamu [Kawasaki Heavy Industries, Ltd., Kobe (Japan)
1997-03-01
The trace of elementary particles produced by collision of electrons and positrons was determined by VENUS Vertex Chamber. To measure the accurate trace, the correct temperature of gases in the chamber had to be estimated under the various conditions of incident electrons and positrons. There dimensional natural convection analysis was carried out in this paper in order to solve the above problem. The change of temperature of gas and structure materials of Vertex Chamber was analyzed for 160 min by ZEPHYRUS (three dimensional Navier-Stokes code). The results of analysis made clear that the internal mixture gases in Macor showed the characters of horizontal annular natural convection flow by heating the inner skin and cooling the outer skin. The temperature of gas space increased up to 25degC at inner skin. Stratification phenomena of temperature was observed in the lower part of section. The difference of temperature between upper and lower part was 0.6degC (at 60min) and 0.4degC (at 160min) which had tendency to decrease. (S.Y.)
Natural and mixed convection in the cylindrical pool of TRIGA reactor
Henry, R.; Tiselj, I.; Matkovič, M.
2017-02-01
Temperature fields within the pool of the JSI TRIGA MARK II nuclear research reactor were measured to collect data for validation of the thermal hydraulics computational model of the reactor tank. In this context temperature of the coolant was measured simultaneously at sixty different positions within the pool during steady state operation and two transients. The obtained data revealed local peculiarities of the cooling water dynamics inside the pool and were used to estimate the coolant bulk velocity above the reactor core. Mixed natural and forced convection in the pool were simulated with a Computational Fluid Dynamics code. A relatively simple CFD model based on Unsteady RANS turbulence model was found to be sufficient for accurate prediction of the temperature fields in the pool during the reactor operation. Our results show that the simple geometry of the TRIGA pool reactor makes it a suitable candidate for a simple natural circulation benchmark in cylindrical geometry.
Directory of Open Access Journals (Sweden)
Md. Jashim Uddin
2012-01-01
Full Text Available Two-dimensional, steady, laminar and incompressible natural convective flow of a nanofluid over a connectively heated permeable upward facing radiating horizontal plate in porous medium is studied numerically. The present model incorporates Brownian motion and thermophoresis effects. The similarity transformations for the governing equations are developed by Lie group analysis. The transformed equations are solved numerically by Runge-Kutta-Fehlberg fourth-fifth order method with shooting technique. Effects of the governing parameters on the dimensionless velocity, temperature and nanoparticle volume fraction as well as on the dimensionless rate of heat and mass transfer are presented graphically and the results are compared with the published data for special cases. Good agreement is found between numerical results of the present paper and published results. It is found that Lewis number, Brownian motion and convective heat transfer parameters increase the heat and mass transfer rates whilst thermophoresis decreases both heat and mass transfer rates.
Natural convection of Al2O3-water nanofluid in a wavy enclosure
Leonard, Mitchell; Mozumder, Aloke K.; Mahmud, Shohel; Das, Prodip K.
2017-06-01
Natural convection heat transfer and fluid flow inside enclosures filled with fluids, such as air, water or oil, have been extensively analysed for thermal enhancement and optimisation due to their applications in many engineering problems, including solar collectors, electronic cooling, lubrication technologies, food processing and nuclear reactors. In comparison, little effort has been given to the problem of natural convection inside enclosures filled with nanofluids, while the addition of nanoparticles into a fluid base to alter thermal properties can be a feasible solution for many heat transfer problems. In this study, the problem of natural convection heat transfer and fluid flow inside a wavy enclosure filled with Al2O3-water nanofluid is investigated numerically using ANSYS-FLUENT. The effects of surface waviness and aspect ratio of the wavy enclosure on the heat transfer and fluid flow are analysed for various concentrations of Al2O3 nanoparticles in water. Flow fields and temperature fields are investigated and heat transfer rate is examined for different values of Rayleigh number. Results show that heat transfer within the enclosure can be enhanced by increasing surface waviness, aspect ratio or nanoparticles volume fraction. Changes in surface waviness have little effect on the heat transfer rate at low Rayleigh numbers, but when Ra ≥ 105 heat transfer increases with the increase of surface waviness from zero to higher values. Increasing the aspect ratio causes an increase in heat transfer rate, as the Rayleigh number increases the effect of changing aspect ratio is more apparent with the greatest heat transfer enhancement seen at higher Rayleigh numbers. Nanoparticles volume fraction has a little effect on the average Nusselt number at lower Rayleigh numbers when Ra ≥ 105 average Nusselt number increases with the increase of volume fraction. These findings provide insight into the heat transfer effects of using Al2O3-water nanofluid as a heat
Natural Convection Heat and Mass Transfer from Falling Films in Vertical Channels
Buck, Gregory Allen
1990-01-01
In the design of solar collector/regenerators for use in open cycle absorption refrigeration (OCAR) units, the problem of predicting evaporation rates and solution temperatures is of paramount importance in determining overall cycle performance. This transport of heat and mass is dominated by natural convection with buoyant forces primarily generated as a result of film heating by the solar flux, but aided by the evaporation of water (the lighter species) into the rising moist air stream. In order to better understand the mechanism of these combined buoyant interactions, the governing equations for natural convection flow in a vertical channel bounded by a heated falling film (simulating a glazed collector/regenerator) were solved using several different finite difference techniques. The numerical results were validated against existing experimental and numerical results for simplified boundary conditions. The appropriate nondimensionalization for the falling film boundary condition was established, ostensibly for the first time, and a parametric study for an air-water vapor mixture has been presented. Curve fits to the numerical results were determined for engineering design applications. To further confirm the validity of the numerical solutions, an experimental apparatus was constructed using electric resistance heat to simulate the constant heat flux of the solar source. Water was introduced at the top of this heated vertical surface at various flow rates and under various supplied heat fluxes, and a natural convection channel flow generated between the heated falling film and a parallel, plexiglass surface. Film temperatures and moist air velocity profiles were measured at various streamwise (vertical) locations for comparison with the numerical results. In general, measured film temperatures were 15 to 20 percent lower than the predicted values, but came to within 3 percent of the predictions when experimental uncertainty was incorporated into the numerical
Directory of Open Access Journals (Sweden)
R Mehdaoui
2016-09-01
Full Text Available Two-dimensional, double diffusion, natural convection in a partially porous cavity satured with a binary fluid is investigated numerically. Multiple motions are driven by the external temperature and concentration differences imposed across vertical walls. The wavy interface between fluid and porous layer is horizontal. The equations which describe the fluid flow and heat and mass transfer are described by the Navier-Stokes equations (fluid region, Darcy-Brinkman equation (porous region and energy and mass equations. The finite element method was applied to solve the governing equations. The fluid flow and heat and mass transfer has been investigated for different values of the amplitude and the wave number of the interface and the buoyancy ratio. The results obtained in the form of isotherms, stream lines, isoconcentrations and the Nusselt and Sherwood numbers; show that the wavy interface has a significant effect on the flow and heat and mass transfer.
Directory of Open Access Journals (Sweden)
I. Alhama
2014-01-01
Full Text Available The nondimensionalization of the equations governing a given problem is a direct, relatively easy, and low-cost way to provide interesting information, the dimensionless groups that rule the problem and define its final patterns of solution. In complex problems, however, this technique frequently does not provide the precise and complete set of monomials we are looking for. The use of discrimination in the process of nondimensionalization, the detailed application of which is explained in this paper, always leads to a minimum set of parameters, which, separately, determine the solution of the problems. In addition, the physical meaning and order of magnitude of these discriminated monomials are also provided by the discrimination. The technique is applied to the coupled problem of natural convection between horizontal plates heated from below, containing an anisotropic porous medium.
Simulation on Natural Convection of a Nanofluid along an Isothermal Inclined Plate
Mitra, Asish
2017-08-01
A numerical algorithm is presented for studying laminar natural convection flow of a nanofluid along an isothermal inclined plate. By means of similarity transformation, the original nonlinear partial differential equations of flow are transformed to a set of nonlinear ordinary differential equations. Subsequently they are reduced to a first order system and integrated using Newton Raphson and adaptive Runge-Kutta methods. The computer codes are developed for this numerical analysis in Matlab environment. Dimensionless velocity, temperature profiles and nanoparticle concentration for various angles of inclination are illustrated graphically. The effects of Prandtl number, Brownian motion parameter and thermophoresis parameter on Nusselt number are also discussed. The results of the present simulation are then compared with previous one available in literature with good agreement.
Unsteady natural convection flow of nanofluids past a semi-infinite isothermal vertical plate
Tippa, Sowmya; Narahari, Marneni; Pendyala, Rajashekhar
2016-11-01
Numerical analysis is performed to investigate the unsteady natural convection flow of a nanofluid past a semi-infinite isothermal vertical plate. Five different types of water based nanofluids are considered in this investigation where Silver (Ag), Copper (Cu), Copper Oxide (CuO), Alumina (Al2O3) and Titanium Oxide (TiO2) are the nanoparticles. The governing non-dimensional partial differential equations are solved by employing an implicit finite-difference method of Crank-Nicolson type. Numerical results are computed for different values of pertinent parameters. The results for nanofluid temperature, velocity, local Skin friction and Nusselt number, average Skin friction and Nusselt number are discussed through graphs. The present numerical results for local Nusselt number have been compared with the well-established pure fluid correlation results for the limiting case and the comparison shows that the results are in excellent agreement.
Temperature and velocity fields in natural convection by PIV and LIF
DEFF Research Database (Denmark)
Meyer, Knud Erik; Larsen, Poul Scheel; Westergaard, C. H.
2002-01-01
Natural convection in a cubical cavity (L = 250 mm) filled with water is created by heating a square plate (0.5 L) centred in the bottom wall and by cooling the sidewalls, while the remaining walls are insulated. The Rayleigh number based on cavity side length and temperature difference between...... plate and cooled walls is 1.4×10^10. The flow is turbulent and is similar to some indoor room flows. Combined Particle Image Velocimetry (PIV) and Planar Light Induced Fluorescence (LIF) are used to measure local velocities and temperatures. Data measured in a symmetry plane parallel to a sidewall...... are presented in terms of mean velocities and temperature and in terms turbulent quantities including Reynolds fluxes. The flow consists a plume rising above the heated plate into an almost stagnant fluid with a weakly stratified temperature field, as well as thin buoyancy driven boundary layers down...
Design, development and performance testing of a new natural convection solar dryer
Energy Technology Data Exchange (ETDEWEB)
Pangavhane, D.R. [K.K. Wagh College of Engineering, Nashik (India). Department of Mechanical Engineering; Sawhney, R.L.; Sarsavadia, P.N. [Devi Ahilya Vishwa Vidhyalaya, Indore (India). School of Energy and Environmental Studies
2002-06-01
Mechanical drying of agricultural products is an energy consuming operation in the post-harvesting technology. Greater emphasis is given to using solar energy sources in this process due to the high prices and shortages of fossil fuels. For these purposes, a new natural convection solar dryer consisting of a solar air heater and a drying chamber was developed. This system can be used for drying various agricultural products like fruits and vegetables. In this study, grapes were successfully dried in the developed solar dryer. The qualitative analysis showed that the traditional drying, i.e. shade drying and open sun drying, dried the grapes in 15 and 7 days respectively, while the solar dryer took only 4 days and produced better quality raisins. (author)
Fully developed MHD natural convection flow in a vertical annular microchannel: An exact solution
Directory of Open Access Journals (Sweden)
Basant K. Jha
2015-07-01
Full Text Available An exact solution of steady fully developed natural convection flow of viscous, incompressible, electrically conducting fluid in a vertical annular micro-channel with the effect of transverse magnetic field in the presence of velocity slip and temperature jump at the annular micro-channel surfaces is obtained. Exact solution is expressed in terms of modified Bessel function of the first and second kind. The solution obtained is graphically represented and the effects of radius ratio (η, Hartmann number (M, rarefaction parameter (βvKn, and fluid–wall interaction parameter (F on the flow are investigated. During the course of numerical computations, it is found that an increase in Hartmann number leads to a decrease in the fluid velocity, volume flow rate and skin friction. Furthermore, it is found that an increase in curvature radius ratio leads to an increase in the volume flow rate.
Experimental investigation of chimney-enhanced natural convection in hexagonal honeycombs
Directory of Open Access Journals (Sweden)
Xiaohu Yang
2014-01-01
Full Text Available The natural convective heat transfer performance of an aluminum hexagonal honeycomb acting as a novel heat sink for LED cooling is experimentally investigated. The concept of adding an adiabatic square chimney extension for heat transfer enhancement is proposed, and the effects of chimney shape, height, and diameter are quantified. The average N uav of a heated honeycomb with straight chimney is significantly higher than that without chimney, and the enhancement increases with increasing chimney height. At a given chimney height, honeycombs with divergent chimneys perform better than those with convergent ones. For a fixed divergent angle, the N uav number increases monotonically with increasing chimney height. In contrast, with the convergent angle fixed, there exists an optimal chimney height to achieve maximum heat transfer.
Entropy Generation Analysis of Natural Convection in Square Enclosures with Two Isoflux Heat Sources
Directory of Open Access Journals (Sweden)
S. Z. Nejad
2017-04-01
Full Text Available This study investigates entropy generation resulting from natural convective heat transfer in square enclosures with local heating of the bottom and symmetrical cooling of the sidewalls. This analysis tends to optimize heat transfer of two pieces of semiconductor in a square electronic package. In this simulation, heaters are modeled as isoflux heat sources and sidewalls of the enclosure are isothermal heat sinks. The top wall and the non-heated portions of the bottom wall are adiabatic. Flow and temperature fields are obtained by numerical simulation of conservation equations of mass, momentum and energy in laminar, steady and two dimensional flows. With constant heat energy into the cavity, effect of Rayleigh number, heater length, heater strength ratios and heater position is evaluated on flow and temperature fields and local entropy generation. The results show that a minimum entropy generation rate is obtained under the same condition in which a minimum peak heater temperature is obtained.
Validation of system codes RELAP5 and SPECTRA for natural convection boiling in narrow channels
Energy Technology Data Exchange (ETDEWEB)
Stempniewicz, M.M., E-mail: stempniewicz@nrg.eu; Slootman, M.L.F.; Wiersema, H.T.
2016-10-15
Highlights: • Computer codes RELAP5/Mod3.3 and SPECTRA 3.61 validated for boiling in narrow channels. • Validated codes can be used for LOCA analyses in research reactors. • Code validation based on natural convection boiling in narrow channels experiments. - Abstract: Safety analyses of LOCA scenarios in nuclear power plants are performed with so called thermal–hydraulic system codes, such as RELAP5. Such codes are validated for typical fuel geometries applied in nuclear power plants. The question considered by this article is if the codes can be applied for LOCA analyses in research reactors, in particular exceeding CHF in very narrow channels. In order to answer this question, validation calculations were performed with two thermal–hydraulic system codes: RELAP and SPECTRA. The validation was based on natural convection boiling in narrow channels experiments, performed by Prof. Monde et al. in the years 1990–2000. In total 42 vertical tube and annulus experiments were simulated with both codes. A good agreement of the calculated values with the measured data was observed. The main conclusions are: • The computer codes RELAP5/Mod 3.3 (US NRC version) and SPECTRA 3.61 have been validated for natural convection boiling in narrow channels using experiments of Monde. The dimensions applied in the experiments were performed for a range that covers the values observed in typical research reactors. Therefore it is concluded that both codes are validated and can be used for LOCA analyses in research reactors, including natural convection boiling. The applicability range of the present validation is: hydraulic diameters of 1.1 ⩽ D{sub hyd} ⩽ 9.0 mm, heated lengths of 0.1 ⩽ L ⩽ 1.0 m, pressures of 0.10 ⩽ P ⩽ 0.99 MPa. In most calculations the burnout was predicted to occur at lower power than that observed in the experiments. In several cases the burnout was observed at higher power. The overprediction was not larger than 16% in RELAP and 15% in
Numerical investigation of natural convection in two enclosures separated by anisotropic solid wall
Salama, Amgad
2014-10-28
Purpose: The problem of natural convection in two cavities separated by an anisotropic central solid wall is considered numerically. When the thermal conductivity of the central wall is anisotropic, heat flux and temperature gradient vectors are no longer coincidence. This apparently has interesting influences on the heat and fluid flow patterns in this system. The paper aims to discuss these issues.Design/methodology/approach: In this work, several flow patterns have been investigated covering a wide range of Rayleigh number up to 108. Several thermal conductivity anisotropy scenarios of the central wall have been investigated including 0, 30, 60, 120 and 1501 principal anisotropy directions. The governing equations have been solved using control volume approach.Findings: Probably the most intriguing is that, for some anisotropy scenarios it is found that the temperature at the same elevation at the side of the central wall which is closer to the colder wall is higher than that at the side closer to the hot wall. Apparently this defies intuition which suggests the reverse to have happened. However, this behavior may be explained in light of the effect of anisotropy. Furthermore, the patterns of streamlines and temperature fields in the two enclosures also changes as a consequence of the change of the central wall temperatures for the different anisotropy scenarios.Originality/value: This work discusses a very interesting topic related to heat energy exchange among two compartments when the separating wall is anisotropic. In some anisotropy scenarios, this leads to more uniform distribution of Nusselt number than the case when the wall is isotropic. Interesting patterns of natural convection is investigated.
Natural convection in a water tank with a heated horizontal plate facing downward
Energy Technology Data Exchange (ETDEWEB)
Yang, Sun Kyoo; Jung, Moon Kee [Korea Atomic Energy Research Institute, Taejon (Korea, Republic of); Helmut Hoffmann [Kemforschungszentrum Karlsruhe/IATF, (Germany)
1995-06-01
Experimental and computational studies were carried out to investigate the natural convection of the single phase flow in a tank with a heated horizontal plate facing downward. This is a simplified model for investigations of the influence of a core melt at the bottom of a reactor vessel on the thermal hydraulic behavior in a water filled cavity surrounding the vessel. In this case the vessel is simulated by a hexahedron insulated box with a heated plate horizontally mounted at the bottom of the box. The box with the heated plate is installed in a water filled hexahedron tank. Coolers are immersed in the U-type water volume between the box and the tank. Although the multicomponent flows exist more probably below the heated plate in reality, present study concentrates on the single phase flow in a first step prior to investigating the complicated multicomponent thermal hydraulic phenomena. In the present study, in order to get a better understanding for the natural convection characteristics below the heated plate, the velocity and temperature are measured by LDA(Laser Doppler Anemometry) and thermocouples, respectively. And flow fields are visualized by taking pictures of the flow region with suspended particles. The results show the occurrence of a very effective circulation of the fluid in the whole flow area as the heater and coolers are put into operation. In the remote region below the heated plate the flow is nearly stagnant, and a remarkable temperature stratification can be observed with very thin thermal boundary. Analytical predictions using the FLUTAN code show a reasonable matching of the measured velocity fields. 18 figs., 2 tabs., 18 refs. (Author).
Directory of Open Access Journals (Sweden)
Pensiri Sompong
2014-01-01
Full Text Available The effects of wavy geometry on natural convection in an enclosure with two wavy vertical walls and filled with fluid saturated porous media are investigated numerically by using finite element method. The wavy enclosure is transformed to a unit square in the computational domain and the finite element formulations are solved in terms of ξη-coordinate based on iterative method. In order to investigate the effects of interested parameters, the values of wave amplitude (λ = 0.05 and 0.1 and number of undulations (n = 1 and 2 are chosen with constants Ra = 105, Da = 10−3, and Pr = 0.71. It is found that the increase in number of undulations has small effect on natural convection inside the enclosure whereas the increase in wave amplitude reduces the strength of convection because higher wave volume plays a barricade role.
Energy Technology Data Exchange (ETDEWEB)
Cho, Jae Seon; Suh, Kune Yull; Chung, Chang Hyun [Seoul National University, Seoul (Korea, Republic of); Paark, Rae Joon; Kim, Sang Baik [Korea Atomic Energy Research Institute, Taejon (Korea, Republic of)
1997-12-31
This paper presents results of experimental studies on the heat transfer and solidification of the molten metal pool with overlying coolant with boiling. The metal pool is heated from the bottom surface and coolant is injected onto the molten metal pool. Ad a result, the crust, which is a solidified layer, may form at the top of the molten metal pool. Heat transfer is accomplished by a conjugate mechanism, which consists of the natural convection of the molten metal pool, the conduction in the crust layer and the convective boiling heat transfer in the coolant. This work examines the crust formation and the heat transfer rate on the molten metal pool with boiling coolant. The simulant molten pool material is tin (Sn) with the melting temperature of 232 deg C. Demineralized water is used as the working coolant. The crust layer thickness was ostensibly varied by the heated bottom surface temperature of the test section, but not much affected by the coolant injection rate. The correlation between the Nusselt number and the Rayleigh number in the molten metal pool region of this study is compared against the crust formation experiment without coolant boiling and the literature correlations. The present experimental results are higher than those from the experiment without coolant boiling, but show general agreement with the Eckert correlation, with some deviations in the high and low ends of the Rayleigh number. This discrepancy is currently attributed to concurrent rapid boiling of the coolant on top of the metal layer. 10 refs., 4 figs., 1 tab. (Author)
Siahaan, A. S.; Ambarita, H.; Kawai, H.; Daimaruya, M.
2017-01-01
In an oil refinery unit, coke drum is subjected cyclic thermal stress and mechanical loads due to cyclic heating and cooling loads. Thus, the useful life of a coke drum is much shorter than other equipment. One of the most severe locations due to thermal stress is shell to skirt junction. Here, a hot box is proposed. In this study effectiveness of a hot box will be analyzed numerically. The addition of hot box (triangular cavity) was expected to generate natural convection, which will enhance heat transfer. As for the result show that heat flux conduction and natural convection have the same trend. The peak of conduction heat flux is 122 W/m2 and for natural convection is 12 W/m2. In the heating stage of coke drum cycle it found that the natural convection only provide approximately 10 % of heat transfer compare to conduction heat transfer. In this study it was proved that in the heating stage, the addition of triangular enclosure is less effective to enhance the heat transfer than previously thought.
Kori, Francis K. K.
2017-01-01
The objective of this work was to determine the effects of blanching and two drying methods, open-sun drying and natural convection solar drying, on the quality characteristics of red pepper. A 2 × 3 factorial design with experimental factors as 2 drying methods (open-sun drying and use of solar dryer) and 3 levels of pepper blanching (unblanched, blanched in plain water, and blanched in 2% NaCl) was conducted. Dried pepper samples were analysed for chemical composition, microbial load, and consumer sensory acceptability. Blanching of pepper in 2% NaCl solution followed by drying in a natural convection solar dryer reduced drying time by 15 hours. Similarly, a combination of blanching and drying in the solar dryer improved microbial quality of dried pepper. However, blanching and drying processes resulted in reduction in nutrients such as vitamin C and minerals content of pepper. Blanching followed by drying in natural convection solar dryer had the highest consumer acceptability scores for colour and overall acceptability, while texture and aroma were not significantly (p > 0.05) affected by the different treatments. Therefore, natural convection solar dryer can be used to dry pepper with acceptable microbial and sensory qualities, as an alternative to open-sun drying. PMID:29082236
Saatadjian, Esteban; Lesage, Francois; Mota, Jose Paulo B.
2013-01-01
A project that involves the numerical simulation of transport phenomena is an excellent method to teach this subject to senior/graduate chemical engineering students. The subject presented here has been used in our senior/graduate course, it concerns the study of natural convection heat transfer between two concentric, horizontal, saturated porous…
Directory of Open Access Journals (Sweden)
James Owusu-Kwarteng
2017-01-01
Full Text Available The objective of this work was to determine the effects of blanching and two drying methods, open-sun drying and natural convection solar drying, on the quality characteristics of red pepper. A 2 × 3 factorial design with experimental factors as 2 drying methods (open-sun drying and use of solar dryer and 3 levels of pepper blanching (unblanched, blanched in plain water, and blanched in 2% NaCl was conducted. Dried pepper samples were analysed for chemical composition, microbial load, and consumer sensory acceptability. Blanching of pepper in 2% NaCl solution followed by drying in a natural convection solar dryer reduced drying time by 15 hours. Similarly, a combination of blanching and drying in the solar dryer improved microbial quality of dried pepper. However, blanching and drying processes resulted in reduction in nutrients such as vitamin C and minerals content of pepper. Blanching followed by drying in natural convection solar dryer had the highest consumer acceptability scores for colour and overall acceptability, while texture and aroma were not significantly (p>0.05 affected by the different treatments. Therefore, natural convection solar dryer can be used to dry pepper with acceptable microbial and sensory qualities, as an alternative to open-sun drying.
Owusu-Kwarteng, James; Kori, Francis K K; Akabanda, Fortune
2017-01-01
The objective of this work was to determine the effects of blanching and two drying methods, open-sun drying and natural convection solar drying, on the quality characteristics of red pepper. A 2 × 3 factorial design with experimental factors as 2 drying methods (open-sun drying and use of solar dryer) and 3 levels of pepper blanching (unblanched, blanched in plain water, and blanched in 2% NaCl) was conducted. Dried pepper samples were analysed for chemical composition, microbial load, and consumer sensory acceptability. Blanching of pepper in 2% NaCl solution followed by drying in a natural convection solar dryer reduced drying time by 15 hours. Similarly, a combination of blanching and drying in the solar dryer improved microbial quality of dried pepper. However, blanching and drying processes resulted in reduction in nutrients such as vitamin C and minerals content of pepper. Blanching followed by drying in natural convection solar dryer had the highest consumer acceptability scores for colour and overall acceptability, while texture and aroma were not significantly (p > 0.05) affected by the different treatments. Therefore, natural convection solar dryer can be used to dry pepper with acceptable microbial and sensory qualities, as an alternative to open-sun drying.
DEFF Research Database (Denmark)
Hosseini, R.; Kolaei, Alireza Rezania; Alipour, M.
2012-01-01
In this work, the natural convection heat transfer from a long vertical electrically heated cylinder to an adjacent air gap is experimentally studied. The aspect and diameter ratios of the cylinder are 55.56 and 6.33, respectively. The experimental measurements were obtained for a concentric...
Betz, J; Straub, J
2002-10-01
In the presence of a temperature gradient at a liquid-gas or liquid-liquid interface, thermocapillary or Marangoni convection develops. This convection is a special type of natural convection that was not paid much attention in heat transfer for a long time, although it is strong enough to drive liquids against the direction of buoyancy on Earth. In a microgravity environment, however, it is the remaining mode of natural convection and supports heat and mass transfer. During boiling in microgravity it was observed at subcooled liquid conditions. Therefore, the question arises about its contribution to heat transfer without phase change. Thermocapillary convection was quantitatively studied at single gas bubbles in various liquids, both experimentally and numerically. A two-dimensional mathematical model described in this article was developed. The coupled mechanism of heat transfer and fluid flow in pure liquids around a single gas bubble was simulated with a control-volume FE-method. The simulation was accompanied and compared with experiments on Earth. The numerical results are in good accordance with the experiments performed on Earth at various Marangoni numbers using various alcohols of varying chain length and Prandtl numbers. As well as calculations on Earth, the numerical method also allows simulations at stationary spherical gas bubbles in a microgravity environment. The results demonstrate that thermocapillary convection is a natural heat transfer mechanism that can partially replace the buoyancy in a microgravity environment, if extreme precautions are taken concerning the purity of the liquids, because impurities accumulate predominantly at the interface. Under Earth conditions, an enhancement of the heat transfer in a liquid volume is even found in the case where thermocapillary flow is counteracted by buoyancy. In particular, the obstructing influence of surface active substances could be observed during the experiments on Earth in water and also in
Model Based Analysis of Forced and Natural Convection Effects in an Electrochemical Cell
Directory of Open Access Journals (Sweden)
D Brunner
2017-03-01
Full Text Available High purity copper, suitable for electrical applications, can only be obtained by electro-winning. The hallmark of this process is its self-induced natural convection through density variations of the electrolyte at both anode and cathode. In order to do this, first the full dynamic complexity of the process needs to be understood. Thus an OpenFoam®-based 2D model of the process has been created. This finite-volume multiphysics approach solves the laminar momentum and copper-ion species conservation equations, as well as local copper-ion conversion kinetics. It uses a Boussinesq approximation to simulate the species-momentum coupling, namely natural draft forces induced by variations of the spatial copper concentration within the fluid. The model shows good agreement with benchmark-cases of real-life electrochemical cells found in literature. An additional flow was imposed at the bottom of a small scale electrochemical cell in order to increase the ionic transport and thereby increase the overall performance of the cell. In a small scale electrochemical cell in strictly laminar flow, the overall performance could be increased and stratification decreased.
Energy Technology Data Exchange (ETDEWEB)
Ribando, R.J.
1979-01-01
A comparison is made between computed results and experimental data for a single-phase natural convection test in an experimental sodium loop. The test was conducted in the Thermal-Hydraulic Out-of-Reactor Safety (THORS) facility, an engineering-scale high temperature sodium loop at the Oak Ridge National Laboratory (ORNL) used for thermal-hydraulic testing of simulated Liquid Metal Fast Breeder Reactor (LMFBR) subassemblies at normal and off-normal operating conditions. Heat generation in the 19 pin assembly during the test was typical of decay heat levels. The test chosen for analysis in this paper was one of seven natural convection runs conducted in the facility using a variety of initial conditions and testing parameters. Specifically, in this test the bypass line was open to simulate a parallel heated assembly and the test was begun with a pump coastdown from a small initial forced flow. The computer program used to analyze the test, LONAC (LOw flow and NAtural Convection) is an ORNL-developed, fast-running, one-dimensional, single-phase, finite-difference model used for simulating forced and free convection transients in the THORS loop.
Ngamchuea, Kamonwad; Eloul, Shaltiel; Tschulik, Kristina; Compton, Richard G
2015-07-21
Understanding mass transport is prerequisite to all quantitative analysis of electrochemical experiments. While the contribution of diffusion is well understood, the influence of density gradient-driven natural convection on the mass transport in electrochemical systems is not. To date, it has been assumed to be relevant only for high concentrations of redox-active species and at long experimental time scales. If unjustified, this assumption risks misinterpretation of analytical data obtained from scanning electrochemical microscopy (SECM) and generator-collector experiments, as well as analytical sensors utilizing macroelectrodes/microelectrode arrays. It also affects the results expected from electrodeposition. On the basis of numerical simulation, herein it is demonstrated that even at less than 10 mM concentrations and short experimental times of tens of seconds, density gradient-driven natural convection significantly affects mass transport. This is evident from in-depth numerical simulation for the oxidation of hexacyanoferrate (II) at various electrode sizes and electrode orientations. In each case, the induced convection and its influence on the diffusion layer established near the electrode are illustrated by maps of the velocity fields and concentration distributions evolving with time. The effects of natural convection on mass transport and chronoamperometric currents are thus quantified and discussed for the different cases studied.
Karvinkoppa, M. V.; Hotta, T. K.
2017-11-01
The paper deals with the numerical investigation of natural and mixed convection heat transfer on optimal distribution of five non-identical protruding discrete heat sources (Aluminium) mounted on a substrate (Bakelite) board. The heat sources are subjected to a uniform heat flux of 2000 W/m2. The temperature of heat sources along with the effect of thermal interaction between them is predicted by carrying out numerical simulations using ANSYS Icepak, and the results are validated with the existing experimental findings. The results suggest that mixed convection is a better method for cooling of discrete heat source modules. Also, the temperature of heat sources is a strong function of their shape, size, and positioning on the substrate. Effect of radiation is studied by painting the surface of heat sources by black paint. The results conclude that, under natural convection heat transfer, the temperature of heat sources drops by 6-13% from polished to black painted surface, while mixed convection results in the drop by 3-15%. The numerical predictions are in strong agreement with experimental results.
Directory of Open Access Journals (Sweden)
Sonam Singh
2014-01-01
Full Text Available This paper presents a numerical study of natural convection within a wavy enclosure heated via corner heating. The considered enclosure is a square enclosure with left wavy side wall. The vertical wavy wall of the enclosure and both of the corner heaters are maintained at constant temperature, Tc and Th, respectively, with Th>Tc while the remaining horizontal, bottom, top and side walls are insulated. A penalty element-free Galerkin approach with reduced gauss integration scheme for penalty terms is used to solve momentum and energy equations over the complex domain with wide range of parameters, namely, Rayleigh number (Ra, Prandtl number (Pr, and range of heaters in the x- and y-direction. Numerical results are represented in terms of isotherms, streamlines, and Nusselt number. It is observed that the rate of heat transfer depends to a great extent on the Rayleigh number, Prandtl number, length of the corner heaters and the shape of the heat transfer surface. The consistent performance of the adopted numerical procedure is verified by comparison of the results obtained through the present meshless technique with those existing in the literature.
Experimental investigation of a solar dryer with natural convective heat flow
Energy Technology Data Exchange (ETDEWEB)
Gbaha, P.; Yobouet Andoh, H.; Kouassi Saraka, J. [Laboratoire d' Energies d' Energies Nouvelles et Renouvelables, Institut National Polytechnique Felix Houphoeuet-Boigny, B.P. 1526 Yamoussoukro (Ivory Coast); Kamenan Koua, B.; Toure, S. [Laboratoire d' Energie Solaire, Universite de Cocody, 22 B.P.: 582, Abidjan 22 (Ivory Coast)
2007-09-15
A direct type natural convection solar dryer is designed. It is constructed in local materials (wood, blades of glass, metals) then tested experimentally in foodstuffs drying (cassava, bananas, mango). It is about an experimental approach which consists in analyzing the behavior of the dryer. The study relates mainly kinetics and establishment of drying heat balances. The influence of significant parameters governing heat and mass transfers, such as solar incident radiation, drying air mass flow and effectiveness, is analyzed in order to evaluate its thermal performances. Experimental data can be represented by empirical correlations of the form M(t)=M{sub i}exp(-kt) for representation of drying process. The resolution of these drying equations makes-possible to predict total drying time of each product. Moreover, this drying process allows to reduce the moisture content of cassava and sweet banana approximately to 80% in 19 and 22 h, respectively to reach the safety threshold value of 13%. This value permits the conservation of these products about one year without deterioration. The determination of parameters, like ambient temperature, drying chamber temperature, drying air mass flow and incident heat fluxes, allow to predict the drying effectiveness for modeling and refining the dimensioning of the elaborate prototype. (author)
Mostafa, Golam; Munshi, M. Jahirul Haque; Hossain, Sumon; Ali, M.
2017-06-01
Analysis of hydro-magnetic natural convection flow in a square cavity with internal elliptic shape cold block at the centre with Prandtl number of 0.711 has been investigated numerically. The governing equations, mass, momentum, energy and induction equations are applied to the cavity. The governing differential equations are solved by using finite element method (Galerkin weighted residual method). The top wall, left wall, right wall and elliptic obstacle are kept at cold Tc. The bottom wall is kept at heated Th. The study is performed for different Rayleigh numbers(103 ≤ Ra ≥ 106) and Hartmann numbers(0 ≤ Ha ≥ 100). A cold elliptic block is located at the centre of the cavity. The object of this study is to describe the effects of MHD on the field of buoyancy-driven and flow in presence of such cold block by visualization of graph. The obtained results showed that temperature distribution and flow pattern inside the cavity depend on both strength of the magnetic field and Rayleigh number. For all cases, two or more counter rotating eddies were formed inside the cavity. The results are illustrated with the streamlines, isotherms, velocity and temperature fields. Numerical results show good accuracy and stability of the proposal strategy.
Directory of Open Access Journals (Sweden)
Ahmed Kadhim Hussein
2016-06-01
Full Text Available Numerical computation of unsteady laminar three-dimensional natural convection and entropy generation in an inclined cubical trapezoidal air-filled cavity is performed for the first time in this work. The vertical right and left sidewalls of the cavity are maintained at constant cold temperatures. The lower wall is subjected to a constant hot temperature, while the upper one is considered insulated. Computations are performed for Rayleigh numbers varied as 103 ⩽ Ra ⩽ 105, while the trapezoidal cavity inclination angle is varied as 0° ⩽ Φ ⩽ 180°. Prandtl number is considered constant at Pr = 0.71. Second law of thermodynamics is applied to obtain thermodynamic losses inside the cavity due to both heat transfer and fluid friction irreversibilities. The variation of local and average Nusselt numbers is presented and discussed, while, streamlines, isotherms and entropy contours are presented in both two and three-dimensional pattern. The results show that when the Rayleigh number increases, the flow patterns are changed especially in three-dimensional results and the flow circulation increases. Also, the inclination angle effect on the total entropy generation becomes insignificant when the Rayleigh number is low. Moreover, when the Rayleigh number increases the average Nusselt number increases.
Design and Fabrication of a Direct Natural Convection Solar Dryer for Tapioca
Directory of Open Access Journals (Sweden)
Diemuodeke E. OGHENERUONA
2011-06-01
Full Text Available Based on preliminary investigations under controlled conditions of drying experiments, a direct natural convection solar dryer was designed and fabricated to dry tapioca in the rural area. This paper describes the design considerations followed and presents the results of MS excel computed results of the design parameters. A minimum of 7.56 m2 solar collector area is required to dry a batch of 100 kg tapioca in 20 hours (two days drying period. The initial and final moisture content considered were 79 % and 10 % wet basis, respectively. The average ambient conditions are 32ºC air temperatures and 74 % relative humidity with daily global solar radiation incident on horizontal surface of 13 MJ/m2/day. The weather conditions considered are of Warri (lat. 5°30, long. 5°41, Nigeria. A prototype of the dryer so designed was fabricated with minimum collector area of 1.08 m2. This prototype dryer will be used in experimental drying tests under various loading conditions.
Directory of Open Access Journals (Sweden)
Md. Mamun Molla
2014-01-01
Full Text Available The purpose of this study is to investigate the natural convection laminar flow along an isothermal vertical flat plate immersed in a fluid with viscosity which is the exponential function of fluid temperature in presence of internal heat generation. The governing boundary layer equations are transformed into a nondimensional form and the resulting nonlinear system of partial differential equations is reduced to a convenient form which are solved numerically using an efficient marching order implicit finite difference method with double sweep technique. Numerical results are presented in terms of the velocity and temperature distribution of the fluid as well as the heat transfer characteristics, namely, the wall shear stress and the local and average rate of heat transfer in terms of the local skin-friction coefficient, the local and average Nusselt number for a wide range of the viscosity-variation parameter, heat generation parameter, and the Rayleigh number. Increasing viscosity variation parameter and Rayleigh number lead to increasing the local and average Nusselt number and decreasing the wall shear stress. Wall shear stress and the rate of heat transfer decreased due to the increase of heat generation.
Directory of Open Access Journals (Sweden)
Sourtiji Ehsan
2012-01-01
Full Text Available A numerical study of natural convection heat transfer through an alumina-water nanofluid inside L-shaped cavities in the presence of an external magnetic field is performed. The study has been carried out for a wide range of important parameters such as Rayleigh number, Hartmann number, aspect ratio of the cavity and solid volume fraction of the nanofluid. The influence of the nanoparticle, buoyancy force and the magnetic field on the flow and temperature fields have been plotted and discussed. The results show that after a critical Rayleigh number depending on the aspect ratio, the heat transfer in the cavity rises abruptly due to some significant changes in flow field. It is also found that the heat transfer enhances in the presence of the nanoparticles and increases with solid volume fraction of the nanofluid. In addition, the performance of the nanofluid utilization is more effective at high Rayleigh numbers. The influence of the magnetic field has been also studied and deduced that it has a remarkable effect on the heat transfer and flow field in the cavity that as the Hartmann number increases the overall Nusselt number is significantly decreased specially at high Rayleigh numbers.
HOT AIR TREATMENT, IN NATURAL CONVECTION CONDITIONS, FOR EGG SURFACE DECONTAMINATION
Directory of Open Access Journals (Sweden)
Chiara Cevoli
2012-06-01
Full Text Available In order to improve the animal welfare, the Council Directive 1999/74/EC (defining minimum standards for the welfare of laying hens will ban conventional cage systems from 2012, in favor of enriched cages or floor systems. As a consequence an increased risk of bacterial contamination of eggshells is expected. This has lead toward the development of efficient techniques to reduce eggshell contamination by bacterial pathogens, and thus to prevent any potential or additional food safety risk for Human health. The objectives of this research were to develop and validate a numerical model useful to study the physics of treatment with hot air, in natural convection conditions, and then to assess the impact of the treatment on egg quality and Salmonella Enteritidis inactivation on the shell. As regards the decontamination effectiveness, a reduction in microbial load up to 2.6 log10 after 200 minutes was observed. A linear correlation between the time of treatment and the microbial decimal reduction (R2 0.92 was also observed. This suggests the opportunity of further study with longer treatment. Moreover the results about weak action on quality parameters suggest the practical feasibility of a transportation system for eggs, from production to sorting plant, in heated container.
Directory of Open Access Journals (Sweden)
Constantin Fetecau
2017-03-01
Full Text Available The studies of classical nanofluids are restricted to models described by partial differential equations of integer order, and the memory effects are ignored. Fractional nanofluids, modeled by differential equations with Caputo time derivatives, are able to describe the influence of memory on the nanofluid behavior. In the present paper, heat and mass transfer characteristics of two water-based fractional nanofluids, containing nanoparticles of CuO and Ag, over an infinite vertical plate with a uniform temperature and thermal radiation, are analytically and graphically studied. Closed form solutions are determined for the dimensionless temperature and velocity fields, and the corresponding Nusselt number and skin friction coefficient. These solutions, presented in equivalent forms in terms of the Wright function or its fractional derivatives, have also been reduced to the known solutions of ordinary nanofluids. The influence of the fractional parameter on the temperature, velocity, Nusselt number, and skin friction coefficient, is graphically underlined and discussed. The enhancement of heat transfer in the natural convection flows is lower for fractional nanofluids, in comparison to ordinary nanofluids. In both cases, the fluid temperature increases for increasing values of the nanoparticle volume fraction.
Natural Convection in a Differentially Heated Square Enclosure with a Solid Polygon
Directory of Open Access Journals (Sweden)
R. Roslan
2014-01-01
Full Text Available The aim of the present numerical study is to analyze the conjugate natural convection heat transfer in a differentially heated square enclosure containing a conductive polygon object. The left wall is heated and the right wall is cooled, while the horizontal walls are kept adiabatic. The COMSOL Multiphysics software is applied to solve the dimensionless governing equations. The governing parameters considered are the polygon type, 3≤N≤∞, the horizontal position, 0.25≤X0≤0.75, the polygon size, 0≤A≤π/16, the thermal conductivity ratio, 0.1≤Kr≤10.0, and the Rayleigh number, 103≤Ra≤106. The critical size of the solid polygon was found exists at low conductivities. The heat transfer rate increases with the increase of the size of the solid polygon, until it reaches its maximum value. Here, the size of the solid polygon is reaches its critical value. Further, beyond this critical size of the solid polygon, will decrease the heat transfer rate.
A Numerical Study of Natural Convection Heat Transfer in Fin Ribbed Radiator
Directory of Open Access Journals (Sweden)
Hua-Shu Dou
2015-01-01
Full Text Available This paper numerically investigates the thermal flow and heat transfer by natural convection in a cavity fixed with a fin array. The computational domain consists of both solid (copper and fluid (air areas. The finite volume method and the SIMPLE scheme are used to simulate the steady flow in the domain. Based on the numerical results, the energy gradient function K of the energy gradient theory is calculated. It is observed from contours of the temperature and energy gradient function that the position where thermal instability takes place correlates well with the region of large K values, which demonstrates that the energy gradient method reveals the physical mechanism of the flow instability. Furthermore, the effects of the fin height, the fin number, and the fin shape on the heat transfer rate are also investigated. It is found that the thermal performance of the fin array is determined by the combined effect of the fin space and fin height. It is also observed that the effect of fin shape on heat transfer is insignificant.
Numerical and Experimental Studies of Transient Natural Convection with Density Inversion
Mizutani, Satoru; Ishiguro, Tatsuji; Kuwahara, Kunio
1996-11-01
In beer manufacturing process, we cool beer in storage tank down from 8 to -1 ^circC. The understanding of cooling process is very important for designing a fermentation tank. In this paper, flow and temperature distribution in a rectangular enclosure was studied. The unsteady incompressible Navier-Stokes equations were integrated by using the multi-directional third-order upwind finite difference method(MUFDM). A parabolic density-temperature relationship was assumed in water which has the maximum density at 3.98 ^circC. Cooling down from 8 to 0 ^circC of water in 10 cm cubical enclosure (Ra=10^7) was numerically done by keeping a vertical side wall at 0 ^circC. Vortex was caused by density inversion of water which was cooled bellow 4 ^circC, and it rose near the cold wall and reached water surface after 33 min from the start of cooling. Finally, cooling proceeded from upper surface. At the aim of verifing the accuracy of the numerical result, temperature distribution under the same condition was experimentally visualized using temperature sensitive liquid crystal. The results will be presented by using video movie. Comparison between the computation and the experiment showed that the present direct simulation based on the MUFDM was powerful tool for the understanding of the natural convection with density inversion and the application of cooling phenomenon to the design of beer storage tanks.
Analysis and Testing of a Natural Convection Solar Dryer for the Tropics
Directory of Open Access Journals (Sweden)
A. O. Adelaja
2013-01-01
Full Text Available Solar dryers are imperative for the tropical and sub-Saharan African countries, which are faced with the duo challenges of inadequate electrical energy supply, which has severely limited the application of conventional refrigeration as a means of preservation of agricultural produce, and the need to make produce competitive in the international market. In this study, a cost-effective natural convection solar dryer was developed; the thermal and drying analyses were done and tested to obtain some performance evaluation parameters for the system in order to examine its efficiency and effectiveness by drying some plantain fillets. The collector and system efficiencies are found to be 46.4% and 78.73%, respectively, while a percentage moisture removal of 77.5% was achieved at the 20th hour in order to give final moisture contents of 15.75% in the product, which still maintained its integrity. With a cost of about $195.00, it has been affordable for the small- and medium-scale enterprises as well as for private use in domestic applications.
Design and Testing of a Natural Convection Solar Tunnel Dryer for Mango
Directory of Open Access Journals (Sweden)
Isaac Nyambe Simate
2017-01-01
Full Text Available A natural convection solar tunnel dryer comprising three major units, a solar collector unit, a drying unit, and a vertical bare flat-plate chimney, was constructed. No-load tests with a horizontal configuration of air entry into the collector resulted in a bidirectional air flow in the dryer. To correct this undesirable situation, an air guide at the collector was incorporated to ensure that air entered in a vertical direction. To investigate its performance, drying experiments with mango were carried out at the University of Zambia, Department of Agricultural Engineering. Uncertainties in the parameters measured in the experiment were analysed and quantified. The results showed that, under solar radiation between 568.4 and 999.5 W/m2, air temperature of up to 65.8°C was attained at the collector unit. The average relative humidity values were 30.8%, 6.4%, and 8.4% for the ambient, collector, and drying unit, respectively. Under these conditions, mango with an initial moisture content of 85.5% (wet basis was dried to 13.0% (wet basis in 9.5 hours. The collector, drying, and pick-up efficiencies were found to be 24.7%, 12.8%, and 35.0%, respectively. The average temperature difference between the chimney air and ambient air was 12.1°C, and this was sufficient in driving the flow of air through the dryer.
Jagadeesha, R. D.; Prasanna, B. M. R.; Younghae, D.; Sankar, M.
2017-10-01
A numerical study is conducted to understand the effect of magnetic field on the natural convection in a tilted parallelogrammic porous enclosure. The two opposing side walls are differentially heated with a temperature difference specified, while the upper and lower walls are adiabatic. Using the Darcy model to formulate the problem, a finite difference scheme consisting of the Alternating Direction Implicit and Successive Line over Relaxation methods are used to solve the coupled non-linear governing equations. Computations are carried out for a wide range of Rayleigh number ranging from 100 to 5 × 103, inclination of the walls of the parallelogram ϕ from -600 to 600, angle of inclination of the enclosure α from -600 to 600, aspect ratio from 0.5 to 2 and Hartmann number from 0 to 20. The flow structure and the corresponding heat transfer characteristics inside the enclosure are presented in detail. The results revealed that both the magnetic force and the inclination angle have significant effect on the flow field and heat transfer in porous medium. The reported results are in good agreement with the available published work in the literature.
Natural convection in a differentially heated square enclosure with a solid polygon.
Roslan, R; Saleh, H; Hashim, I
2014-01-01
The aim of the present numerical study is to analyze the conjugate natural convection heat transfer in a differentially heated square enclosure containing a conductive polygon object. The left wall is heated and the right wall is cooled, while the horizontal walls are kept adiabatic. The COMSOL Multiphysics software is applied to solve the dimensionless governing equations. The governing parameters considered are the polygon type, 3 ≤ N ≤ ∞, the horizontal position, 0.25 ≤ X 0 ≤ 0.75, the polygon size, 0 ≤ A ≤ π/16, the thermal conductivity ratio, 0.1 ≤ K r ≤ 10.0, and the Rayleigh number, 10(3) ≤ Ra ≤ 10(6). The critical size of the solid polygon was found exists at low conductivities. The heat transfer rate increases with the increase of the size of the solid polygon, until it reaches its maximum value. Here, the size of the solid polygon is reaches its critical value. Further, beyond this critical size of the solid polygon, will decrease the heat transfer rate.
From cat's eyes to disjoint multicellular natural convection flow in tall tilted cavities
Energy Technology Data Exchange (ETDEWEB)
Nicolas, Alfredo, E-mail: anc@xanum.uam.mx [Depto. Matematicas, 3er Piso Ed. AT-Diego Bricio, UAM-I, 09340 Mexico D.F. (Mexico); Baez, Elsa [Depto. Matematicas Aplicadas y Sistemas, UAM-C, 01120 Mexico D.F. (Mexico); Bermudez, Blanca [Facultad de C. de la Computacion, BUAP, 72570 Puebla, Pue. (Mexico)
2011-07-11
Numerical results of two-dimensional natural convection problems, in air-filled tall cavities, are reported to study the change of the cat's eyes flow as some parameters vary, the aspect ratio A and the angle of inclination φ of the cavity, with the Rayleigh number Ra mostly fixed; explicitly, the range of the variation is given by 12≤A≤20 and 0{sup o}≤φ≤270{sup o}; about Ra=1.1x10{sup 4}. A novelty contribution of this work is the transition from the cat's eyes changes, as A varies, to a disjoint multicellular flow, as φ varies. These flows may be modeled by the unsteady Boussinesq approximation in stream function and vorticity variables which is solved with a fixed point iterative process applied to the nonlinear elliptic system that results after time discretization. The validation of the results relies on mesh size and time-step independence studies. -- Highlights: → Fixed point iterative method for solving symmetric linear elliptic problems. → Robust method to study effects as aspect ratio and angle of inclination vary. → Interest on the dynamics and evolution of the fluid flow and on heat transfer. → Study of transition from cat's eyes instability to a disjoint multicellular flow.
Directory of Open Access Journals (Sweden)
Sieres Jaime
2016-01-01
Full Text Available This paper presents an analytical and numerical computation of laminar natural convection in a collection of vertical upright-angled triangular cavities filled with air. The vertical wall is heated with a uniform heat flux; the inclined wall is cooled with a uniform temperature; while the upper horizontal wall is assumed thermally insulated. The defining aperture angle φ is located at the lower vertex between the vertical and inclined walls. The finite element method is implemented to perform the computational analysis of the conservation equations for three aperture angles φ (= 15º, 30º and 45º and height-based modified Rayleigh numbers ranging from a low Ra = 0 (pure conduction to a high 109. Numerical results are reported for the velocity and temperature fields as well as the Nusselt numbers at the heated vertical wall. The numerical computations are also focused on the determination of the value of the maximum or critical temperature along the hot vertical wall and its dependence with the modified Rayleigh number and the aperture angle.
Energy Technology Data Exchange (ETDEWEB)
Yang, Fan; Yang, Haicheng; Guo, Xueyan; Ren Dai [University of Shanghai for Science and Technology, Shanghai (China); Yan, Yonghua [Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power Engineering, Shanghai (China); Liu, Chaoqun [University of Texas at Arlington, Arlington (United States)
2017-06-15
Natural convection heat transfer in an inclined polar cavity was studied using a Finite-difference lattice Boltzmann method (FDLBM) based on a double-population approach for body-fitted coordinates. A D2G9 model coupled with the simplest TD2Q4 lattice model was applied to determine the velocity field and temperature field. For both velocity and temperature fields, the discrete spatial derivatives were obtained by combining the upwind scheme with the central scheme, and the discrete temporal term is obtained using a fourth-order Runge-Kutta scheme. Studies were carried out for different Rayleigh numbers and different inclination angles. The results in terms of streamlines, isotherms, and Nusselt numbers explain the heat transfer mechanism of natural convection in an inclined polar cavity due to the change of Rayleigh number and inclination angle.
Energy Technology Data Exchange (ETDEWEB)
Jobert, Patrice; Beghein, Claudine; Sergent, Anne [LEPTAB, Universite de La Rochelle (France); Le Quere, Patrick [LIMSI, CNRS, Orsay (France); Collignan, Bernard; Couturier, Stephane [CSTB, Marne La Vallee (France); Glockner, Stephane; Vincent, Stephane [MASTER, ENSCPB, Pessac (France); Groleau, Dominique; Lubin, Pierre [CERMA, CNRS, Nantes (France)
2005-04-01
We present the results of a numerical exercise aimed at comparing the predictions of different conventional turbulent modelling approaches for natural convection at Rayleigh numbers characteristic of applications such as energy savings, fire safety or thermal comfort. A two-dimensional configuration was considered that consists of two adjacent rooms separated by a lintel in which natural convection is induced through heating on their opposite sides and subjected to diffusion of a pollutant from one room to the other. Seven contributions are available. The comparison is carried out, in terms of local or global quantities, for the mean thermal and dynamic fields and for the unsteady diffusion of the pollutant from one room to the other. Characteristic differences between steady RANS and unsteady two-dimensional DNS and LES approaches are observed and discussed. (authors)
Energy Technology Data Exchange (ETDEWEB)
Colomer, G. [Lab. de Termotecnia i Energetica, Centre Tecnologic de Transferencia de Calor (CTTC), Universitat Politecnica de Catalunya (UPC), ETSEIAT, c/Colom 11, 08222 Terrassa (Spain)]. E-mail: cttc@cttc.upc.edu; Consul, R. [Lab. de Termotecnia i Energetica, Centre Tecnologic de Transferencia de Calor (CTTC), Universitat Politecnica de Catalunya (UPC), ETSEIAT, c/Colom 11, 08222 Terrassa (Spain); Oliva, A. [Lab. de Termotecnia i Energetica, Centre Tecnologic de Transferencia de Calor (CTTC), Universitat Politecnica de Catalunya (UPC), ETSEIAT, c/Colom 11, 08222 Terrassa (Spain)]. E-mail: oliva@cttc.upc.edu
2007-09-15
The coupling between non-gray radiation heat transfer and convection-conduction heat transfer is studied. The spectral line weighted sum of gray gases model (SLW) is used to account for non-gray radiation properties. The aim of this work is to analyze the influence of the different approaches used when calculating the parameters of the SLW model. Such strategies include the use of optimized model coefficients to reduce the number of operations, and the interpolation of the distribution function instead of the use of mathematical correlations. Non-gray calculations are also compared to gray solutions using the Planck mean absorption coefficient, which can be also calculated with the SLW model. The radiative transfer equation (RTE) is solved by means of the discrete ordinates method (DOM). A natural convection driven cavity is chosen to couple radiation and conduction-convection energy transfer. Several cases, with a significant variation of the ratio between radiation to convection heat transfer, as well as the ratio between radiation to conduction heat transfer, are discussed.
Energy Technology Data Exchange (ETDEWEB)
Bondareva, Nadezhda S. [Laboratory on Convective Heat and Mass Transfer, Tomsk State University, 634050 Tomsk (Russian Federation); Sheremet, Mikhail A., E-mail: Michael-sher@yandex.ru [Laboratory on Convective Heat and Mass Transfer, Tomsk State University, 634050 Tomsk (Russian Federation); Department of Nuclear and Thermal Power Plants, Tomsk Polytechnic University, 634050 Tomsk (Russian Federation)
2016-12-01
MHD natural convection melting in a square cavity with a local heater has been analyzed numerically. The domain of interest is an enclosure bounded by isothermal vertical walls of low constant temperature and adiabatic horizontal walls. A heat source of constant temperature is located on the bottom wall. An inclined uniform magnetic field affects the natural convective heat transfer and fluid flow inside the melt. The governing equations formulated in dimensionless stream function, vorticity and temperature with corresponding initial and boundary conditions have been solved using implicit finite difference method of the second-order accuracy. The effects of the Rayleigh number, Stefan number, Hartmann number, magnetic field inclination angle and dimensionless time on streamlines, isotherms and Nusselt number at the heat source surface have been analyzed. - Highlights: • Natural convection melting in a square cavity with a local heater under the effect of megnetic field is analyzed. • The finite difference method with enthalpy formulation is used to solve the dimensionless governing equations. • Magnetic field intensity and orientation define the shape of phase transition line. • High values of Hartmann number reflect more homogeneous melting of the phase change material.
Kamajaya, Ketut; Umar, Efrizon; Sudjatmi, K. S.
2012-06-01
This study focused on natural convection heat transfer using a vertical rectangular sub-channel and water as the coolant fluid. To conduct this study has been made pipe heaters are equipped with thermocouples. Each heater is equipped with five thermocouples along the heating pipes. The diameter of each heater is 2.54 cm and 45 cm in length. The distance between the central heating and the pitch is 29.5 cm. Test equipment is equipped with a primary cooling system, a secondary cooling system and a heat exchanger. The purpose of this study is to obtain new empirical correlations equations of the vertical rectangular sub-channel, especially for the natural convection heat transfer within a bundle of vertical cylinders rectangular arrangement sub-channels. The empirical correlation equation can support the thermo-hydraulic analysis of research nuclear reactors that utilize cylindrical fuel rods, and also can be used in designing of baffle-free vertical shell and tube heat exchangers. The results of this study that the empirical correlation equations of natural convection heat transfer coefficients with rectangular arrangement is Nu = 6.3357 (Ra.Dh/x)0.0740.
Energy Technology Data Exchange (ETDEWEB)
Bucknor, Matthew; Hu, Rui; Lisowski, Darius; Kraus, Adam
2016-04-17
The Reactor Cavity Cooling System (RCCS) is an important passive safety system being incorporated into the overall safety strategy for high temperature advanced reactor concepts such as the High Temperature Gas- Cooled Reactors (HTGR). The Natural Convection Shutdown Heat Removal Test Facility (NSTF) at Argonne National Laboratory (Argonne) reflects a 1/2-scale model of the primary features of one conceptual air-cooled RCCS design. The project conducts ex-vessel, passive heat removal experiments in support of Department of Energy Office of Nuclear Energy’s Advanced Reactor Technology (ART) program, while also generating data for code validation purposes. While experiments are being conducted at the NSTF to evaluate the feasibility of the passive RCCS, parallel modeling and simulation efforts are ongoing to support the design, fabrication, and operation of these natural convection systems. Both system-level and high fidelity computational fluid dynamics (CFD) analyses were performed to gain a complete understanding of the complex flow and heat transfer phenomena in natural convection systems. This paper provides a summary of the RELAP5-3D NSTF model development efforts and provides comparisons between simulation results and experimental data from the NSTF. Overall, the simulation results compared favorably to the experimental data, however, further analyses need to be conducted to investigate any identified differences.
Numerical study of natural turbulent convection of nanofluids in a tall cavity heated from below
Directory of Open Access Journals (Sweden)
Mebrouk Ridha
2016-01-01
Full Text Available In the present paper a numerical study of natural turbulent convection in a tall cavity filled with nanofluids. The cavity has a heat source embedded on its bottom wall, while the left, right and top walls of the cavity are maintained at a relatively low temperature. The working fluid is a water based nanofluid having three nanoparticle types: alumina, copper and copper oxid. The influence of pertinent parameters such as Rayleigh number, the type of nanofluid and solid volume fraction of nanoparticles on the cooling performance is studied. Steady forms of twodimensional Reynolds-Averaged-Navier-Stokes equations and conservation equations of mass and energy, coupled with the Boussinesq approximation, are solved by the control volume based discretisation method employing the SIMPLE algorithm for pressure-velocity coupling. Turbulence is modeled using the standard k-ε model. The Rayleigh number, Ra, is varied from 2.491009 to 2.491011. The volume fractions of nanoparticles were varied in the interval 0≤φ≤ 6% . Stream lines, isotherms, velocity profiles and Temperature profiles are presented for various combinations of Ra, the type of nanofluid and solid volume fraction of nanoparticles. The results are reported in the form of average Nusselt number on the heated wall. It is shown that for all values of Ra, the average heat transfer rate from the heat source increases almost linearly and monotonically as the solid volume fraction increases. Finally the average heat transfer rate takes on values that decrease according to the ordering Cu, CuO and Al2O3.
National Research Council Canada - National Science Library
Mohammad Mahfuzul Islam; Md. M. Alam; M. M. Parvez; M. A. Rahman
2015-01-01
Abstract In this paper is presented to study conjugate effects of stress work and heat generation on MHD natural convection flow along a vertical flat plate with power law variation of surface temperature...
Directory of Open Access Journals (Sweden)
Alok Kumar Pandey
2017-03-01
Full Text Available The purpose of the present work is to examine the collective influence of thermal radiation and convection flow of Cu-water nanofluid due to a stretching cylinder in a porous medium along with viscous dissipation and slip boundary conditions. The governing non-linear ODEs and auxiliary boundary conditions those obtained by applying assisting similarity transformations have been handled numerically with shooting scheme through Runge-Kutta-integration procedure of fourth-fifth order. The non-dimensional velocity and temperature distribution are designed and also skin friction coefficient as well as heat transfer rate are tabulated for various values of relatable parameters. The results explain that Nusselt number depreciates with boost in radiation parameter, thermal slip parameter and Eckert number. Moreover, it is accelerated with increase in velocity slip parameter and natural convection parameter. The results are distinguished via published ones and excellent accord has been detected.
Energy Technology Data Exchange (ETDEWEB)
Michener, T. [Pacific Northwest National Laboratory, Richland WA (United States); Guttmann, J.; Bajwa, C. [United States Nuclear Regulatory Commission, One White Flin North, Rockville MD (United States)
2001-07-01
The purpose of this study is to identify the importance of natural convection cooling within a nuclear dry spent fuel storage system. In the past, applicants submitting requests to the United States Nuclear Regulatory Commission (USNRC) for a license for a dry spent fuel storage system design did not rigorously treat natural convection within the fuel package of the dry storage system. Typically, the applicant applies heat transfer correlations that raise the thermal conductivity of the materials (gas and solid structures) to account for the impact of convection on the thermal performance of the system. (author)
Sman, van der R.G.M.
2006-01-01
In this paper we present lattice Boltzmann (LB) schemes for convection diffusion coupled to fluid flow on two-dimensional rectangular lattices. Via inverse Chapman-Enskog analysis of LB schemes including source terms, we show that for consistency with physics it is required that the moments of the
Adiabatic partition effect on natural convection heat transfer inside a square cavity
DEFF Research Database (Denmark)
Mahmoudi Nezhad, Sajjad; Rezaniakolaei, Alireza; yousefi, Tooraj
2017-01-01
A steady state and two-dimensional laminar free convection heat transfer in a partitioned cavity with horizontal adiabatic and isothermal side walls is investigated using both experimental and numerical approaches. The experiments and numerical simulations are carried out using a Mach-Zehnder int...
Energy Technology Data Exchange (ETDEWEB)
Simon, Berno
2015-09-04
Passive Autocatalytic Recombiners (PAR) are installed inside the containment of nuclear power plants in order to prevent the build-up of flammable mixtures and to mitigate the effects of hydrogen deflagrations, which can occur in the event of a severe accident combined with the release of hydrogen. In order to simulate the operating behaviour of PARs, the computer program REKO-DIREKT is being developed at the Forschungszentrum Juelich in collaboration with the Institute for Reactor Safety and Reactor Technology at the RWTH Aachen. For the validation of the code, data from experimental facilities operated at Juelich are used. This work focusses on the analysis of the chimney effect through the PAR housing as well as the optimization of the chimney model of REKO-DIREKT. Therefore experimental investigations are carried out in the REKO-4 facility under natural convection conditions. This facility is equipped with numerous measuring devices, e.g. katharometers for in-situ measuring of the hydrogen concentration and the optical flow measurement technique Particle-Image-Velocimetry. In preliminary assessments the equipment is being qualified in order to determine the measurement accuracy. In the following experimental investigations, a small-scale PAR is used, that is built in a modular way allowing it to be equipped with different chimney geometries. The experimental results produce a database that shows the central correlation between the hydrogen concentration, the catalyst temperature and the inlet velocity. The results include the variation of the recombiner's chimney height and experiments at different operating pressures. After optimization of the chimney model, the simulation program is validated against experiments in the large-scale facility THAI in Eschborn, which have been performed subsequent to this thesis in the context of the OECD/NEA-THAI project. Finally, the influence of a downward-directed, near-wall flow on the operational behaviour of the small
Rasmussen, Kristen L.; Zuluaga, Manuel D.; Brodzik, Stella R.
2015-01-01
Abstract For over 16 years, the Precipitation Radar of the Tropical Rainfall Measuring Mission (TRMM) satellite detected the three‐dimensional structure of significantly precipitating clouds in the tropics and subtropics. This paper reviews and synthesizes studies using the TRMM radar data to present a global picture of the variation of convection throughout low latitudes. The multiyear data set shows convection varying not only in amount but also in its very nature across the oceans, continents, islands, and mountain ranges of the tropics and subtropics. Shallow isolated raining clouds are overwhelmingly an oceanic phenomenon. Extremely deep and intense convective elements occur almost exclusively over land. Upscale growth of convection into mesoscale systems takes a variety of forms. Oceanic cloud systems generally have less intense embedded convection but can form very wide stratiform regions. Continental mesoscale systems often have more intense embedded convection. Some of the most intense convective cells and mesoscale systems occur near the great mountain ranges of low latitudes. The Maritime Continent and Amazonia exhibit convective clouds with maritime characteristics although they are partially or wholly land. Convective systems containing broad stratiform areas manifest most strongly over oceans. The stratiform precipitation occurs in various forms. Often it occurs as quasi‐uniform precipitation with strong melting layers connected with intense convection. In monsoons and the Intertropical Convergence Zone, it takes the form of closely packed weak convective elements. Where fronts extend into the subtropics, broad stratiform regions are larger and have lower and sloping melting layers related to the baroclinic origin of the precipitation. PMID:27668295
Natural convection at an indoor glazing surface with different window blinds
Energy Technology Data Exchange (ETDEWEB)
Cuevas, Cristian; Fissore, Adelqui [Departamento de Ingenieria Mecanica, Facultad de Ingenieria, Universidad de Concepcion, Casilla 160-C, Concepcion (Chile); Fonseca, Nestor [Facultad de Ingenieria Mecanica, Universidad Tecnologica de Pereira, AA. 97 Pereira, Risaralda (Colombia)
2010-10-15
In the present study, an empirical model to determinate the convective heat loss, at an indoor glazing surface, is proposed. This model allows calculating the convective heat transfer coefficient and the air flow rate entering to the window cavity formed between the glazing surface and the protection device. The window blind is first studied experimentally by using a rigid paper, which is installed at four different distances from the window frame. This configuration is used as reference to determinate a global model, which is mainly composed of two correlations: one for the Nusselt number and other one for the air mass flow rate incoming to the window cavity. Then, more realistic configurations are tested: single curtains, double curtains, PVC blinds, wood blinds, Venetian blinds or polyester blinds. In general, heat transfer coefficients for these configurations are equal or higher than that obtained with a free plate. Several correlations are proposed for each configuration. (author)
Qi, Cong; Wang, Guiqing; Ma, Yifeng; Guo, Leixin
2017-12-01
The stability and natural convection heat transfer characteristics of TiO2-water nanofluid in enclosures with different rotation angles (α = -45°, α = 0°, α = 45°, and α = 90°) are experimentally investigated. The effects of different pH values and doses (m) of dispersant agent on the stability of TiO2-water nanofluid are investigated. It is found that TiO2-water nanofluid with m = 6 wt% and pH = 8 has the lowest transmittance and has the best stability. The effects of different rotation angles (α = -45°, α = 0°, α = 45°, and α = 90°), nanoparticle mass fractions (wt% = 0.1%, wt% = 0.3%, and wt% = 0.5%) and heating powers (Q = 1 W, Q = 5 W, Q = 10 W, Q = 15 W, and Q = 20 W) on the natural convection heat transfer characteristics are also studied. It is found that the enclosure with rotation angle α = 0° has the highest Nusselt number, followed by the enclosure with rotation angles α = 45° and α = 90°, the enclosure with rotation angle α = -45° has the lowest Nusselt number. It is also found that natural convection heat transfer performance increases with the nanoparticle mass fraction and heating power, but the enhancement ratio decreases with the heating power.
Energy Technology Data Exchange (ETDEWEB)
Kang, Sarah; Bang, In Cheol [Ulsan National Institute of Science and Technology, Ulsan (Korea, Republic of)
2014-10-15
These advantages make the MSR attractive and to be one of the six candidates for the Generation IV Reactor. Therefore, the researches related to the MSR are being conducted. To analyze the molten salt-cooled systems in the laboratory, this study generated the properties of molten salt using MARS-LMR. In this research, the implemented salts were Flibe (LiF-BeF{sub 2}) in a molar mixture that is 66% LiF and 34% BeF{sub 2}, respectively. Table 1 indicates the comparison of thermal properties of various coolants in nuclear power plants. Molten salt was added to the MARS-LMR code to support the analysis of Flibe-cooled systems. The molten salt includes LiF-BeF{sub 2} in a molar mixture that is 66% LiF and 34% BeF{sub 2}, respectively. MARS-LMR code for liquid metals uses the soft sphere model based on Monte Carlo calculations for particles interacting with pair potentials. Although MARS was originally intended for a safety analysis of light water reactor, Flibe properties were newly added to this code as so-called MARS-FLIBE which is applicable for Flibe-cooled systems. By using this thermodynamic property table file, the thermal hydraulic systems of Flibe can be simulated for numerical and parametric studies. In this study, the natural convection phenomena in the rectangular natural convection loop and IVR-ERVC in APR 1400 were simulated. Through the simulations in Flibe-cooled systems, the temperature distribution and mass flowrate of Flibe can be calculated and the heat transfer coefficients of Flibe in natural convection loop will be calculated by adding the related heat transfer correlations in the MARS-FLIBE code. MARS-FLIBE code will be used to predict and design of Flibe-cooled systems.
On the episodic nature of derecho-producing convective systems in the United States
Ashley, Walker S.; Mote, Thomas L.; Bentley, Mace L.
2005-11-01
Convectively generated windstorms occur over broad temporal and spatial scales; however, one of the larger-scale and most intense of these windstorms has been given the name derecho. This study illustrates the tendency for derecho-producing mesoscale convective systems to group together across the United States - forming a derecho series. The derecho series is recognized as any succession of derechos that develop within a similar synoptic environment with no more than 72 h separating individual events. A derecho dataset for the period 1994-2003 was assembled to investigate the groupings of these extremely damaging convective wind events. Results indicate that over 62% of the derechos in the dataset were members of a derecho series. On average, nearly six series affected the United States annually. Most derecho series consisted of two or three events; though, 14 series during the period of record contained four or more events. Two separate series involved nine derechos within a period of nine days. Analyses reveal that derecho series largely frequent regions of the Midwest, Ohio Valley, and the south-central Great Plains during May, June, and July. Results suggest that once a derecho occurred during May, June, or July, there was a 58% chance that this event was the first of a series of two or more, and about a 46% chance that this was the first of a derecho series consisting of three or more events. The derecho series climatology reveals that forecasters in regions frequented by derechos should be prepared for the probable regeneration of a derecho-producing convective system after an initial event occurs. Copyright
Directory of Open Access Journals (Sweden)
R. J. Moitsheki
2008-10-01
Full Text Available In this article, the heat transfer characteristics of natural convection about a vertical permeable flat surface embedded in a saturated porous medium are studied by taking into account the thermal radiation effect. The plate is assumed to have a power-law temperature distribution. Similarity variables are employed in order to transform the governing partial differential equations into a nonlinear ordinary differential equation. Both Adomian decomposition method (ADM and He's variational iteration method (VIM coupled with PadÃƒÂ© approximation technique are implemented to solve the reduced system. Comparisons with previously published works are performed, and excellent agreement between the results is obtained.
Directory of Open Access Journals (Sweden)
Dr. G. Prabhakara Rao,
2015-04-01
Full Text Available We consider a two-dimensional MHD natural convection flow of an incompressible viscous and electrically conducting fluid through porous medium past a vertical impermeable flat plate is considered in presence of a uniform transverse magnetic field. The governing equations of velocity and temperature fields with appropriate boundary conditions are solved by the ordinary differential equations by introducing appropriate coordinate transformations. We solve that ordinary differential equations and find the velocity profiles, temperature profile, the skin friction and nusselt number. The effects of Grashof number (Gr, Hartmann number (M and Prandtl number (Pr, Darcy parameter (D-1 on velocity profiles and temperature profiles are shown graphically.
Directory of Open Access Journals (Sweden)
M. Sheikholeslami
2013-01-01
Full Text Available This investigation reports the magnetic field effect on natural convection heat transfer in a curved-shape enclosure. The numerical investigation is carried out using the control volume-based-finite element method (CVFEM. The numerical investigations are performed for various values of Hartmann number and Rayleigh number. The obtained results are depicted in terms of streamlines and isotherms which show the significant effects of Hartmann number on the fluid flow and temperature distribution inside the enclosure. Also, it was found that the Nusselt number decreases with an increase in the Hartmann number.
Ahmad, Bakhtiar; Ali Shah, Syed Inayat; Ul Haq, Sami; Ali Shah, Nehad
2017-09-01
In this paper the exact solution of the unsteady natural convection radiating flow in an open ended vertical channel is studied. The channel is stationary with non-uniform temperature. The governing equations are fractional differential equations with the Caputo time-fractional derivative. Closed form analytical solutions for the temperature and velocity fields are obtained by using the Laplace transform technique. These solutions are expressed with the Wright function, the Robotnov and Hartley function. The effects of the fractional order and physical parameters on temperature and fluid velocity are presented graphically.
Energy Technology Data Exchange (ETDEWEB)
Hirata, T. (Faculty of Engineering, Shinshu Univ. (JP)); Hanaoka, C. (Nihon Tabaco Sangyo Co., Ltd. (JP))
1990-01-01
Laminar-flow heat transfer and ice formation phenomena in a horizontal tube with freezing have been examined experimentally. The effect of flow acceleration accompanied by ice growth on local ice thickness was studied, and it was found that the ice thickness can be correlated in terms of a flow acceleration parameter. The effect of natural convection in a horizontal tube with internal freezing on the heat transfer is also investigated. It is shown that the average Nusselt number in a horizontal tube with internal freezing is correlated with a modified form of Oliver's empirical formula for a horizontal tube without ice.
Houze, Robert A.; Rasmussen, Kristen L.; Zuluaga, Manuel D.; Brodzik, Stella R.
2015-01-01
Abstract For over 16?years, the Precipitation Radar of the Tropical Rainfall Measuring Mission (TRMM) satellite detected the three?dimensional structure of significantly precipitating clouds in the tropics and subtropics. This paper reviews and synthesizes studies using the TRMM radar data to present a global picture of the variation of convection throughout low latitudes. The multiyear data set shows convection varying not only in amount but also in its very nature across the oceans, contine...
Adesanya, S. O.; Oluwadare, E. O.; Falade, J. A.; Makinde, O. D.
2015-12-01
In this paper, the free convective flow of magnetohydrodynamic fluid through a channel with time periodic boundary condition is investigated by taking the effects of Joule dissipation into consideration. Based on simplifying assumptions, the coupled governing equations are reduced to a set of nonlinear boundary valued problem. Approximate solutions are obtained by using semi-analytical Adomian decomposition method. The effect of pertinent parameters on the fluid velocity, temperature distribution, Nusselt number and skin friction are presented graphically and discussed. The result of the computation shows that an increase in the magnetic field intensity has significant influence on the fluid flow.
Directory of Open Access Journals (Sweden)
Abdullah A.A.A Al-Rashed
2017-09-01
Full Text Available Natural convection and entropy generation due to the heat transfer and fluid friction irreversibilities in a three-dimensional cubical cavity with partially heated and cooled vertical walls has been investigated numerically using the finite volume method. Four different arrangements of partially active vertical sidewalls of the cubical cavity are considered. Numerical calculations are carried out for Rayleigh numbers from (103 ≤ Ra ≤ 106, various locations of the partial heating and cooling vertical sidewalls, while the Prandtl number of air is considered constant as Pr=0.7 and the irreversibility coefficient is taken as (φ=10−4. The results explain that the total entropy generation rate increases when the Rayleigh number increases. While, the Bejan number decreases as the Rayleigh number increases. Also, it is found that the arrangements of heating and cooling regions have a significant effect on the fluid flow and heat transfer characteristics of natural convection and entropy generation in a cubical cavity. The Middle-Middle arrangement produces higher values of average Nusselt numbers.
Energy Technology Data Exchange (ETDEWEB)
Huh, Seon Jeong; Lee, Hee Joon [Kookmin University, Seoul (Korea, Republic of); Kim, Myoung Jun; Moon, Joo Hyung; Bae, Youngmin; Kim, Young-In [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)
2016-10-15
Recently emergency cooldown tank(ECT) is a great concern of passive cooling system for the safety of nuclear reactor. After the operation of a conventional passive cooling system for an extended period, however, the water level falls as a result of the evaporation from the ECT, as steam is emitted from the open top of the tank. In this study, the effect of heat transfer area at the air cooled condensing heat exchanger was investigated by changing 5×5 tube banks into 4×4 and 3×3. Moreover, each of air-side natural convective heat transfer coefficient of tube banks was compared to existing correlations. This study presents the effect of heat transfer area at air-cooled condensing heat exchanger. As heat transfer area decreased, the temperature of outlet increased. In other words, the cooling performance got lower with the decrease of heat transfer area. In addition, the average natural convective heat transfer coefficient was 15.3 W/m{sup 2}/K from the 4×4 tube banks, and 4.92 W/m{sup 2}/K from the 3×3 tube banks, which had quite a large error more than 46% especially with the value of 4×4 tube banks compared to the value from correlation equation. Therefore, according to this result, it is needed to measure the local heat transfer coefficient of vertical cylinder more elaborately in further study.
Directory of Open Access Journals (Sweden)
Rasoul Nikbakhti
2016-03-01
Full Text Available This paper deals with a numerical investigation of double-diffusive natural convective heat and mass transfer in a cavity filled with Newtonian fluid. The active parts of two vertical walls of the cavity are maintained at fixed but different temperatures and concentrations, while the other two walls, as well as inactive areas of the sidewalls, are considered to be adiabatic and impermeable to mass transfer. The length of the thermally active part equals half of the height. The non-dimensional forms of governing transport equations that describe double-diffusive natural convection for two-dimensional incompressible flow are functions of temperature or energy, concentration, vorticity, and stream-function. The coupled differential equations are discretized via FDM (Finite Difference Method. The Successive-Over-Relaxation (SOR method is used in the solution of the stream function equation. The analysis has been done for an enclosure with different aspect ratios ranging from 0.5 to 11 for three different combinations of partially active sections. The results are presented graphically in terms of streamlines, isotherms and isoconcentrations. In addition, the heat and mass transfer rate in the cavity is measured in terms of the average Nusselt and Sherwood numbers for various parameters including thermal Grashof number, Lewis number, buoyancy ratio and aspect ratio. It is revealed that the placement order of partially thermally active walls and the buoyancy ratio influence significantly the flow pattern and the corresponding heat and mass transfer performance in the cavity.
Buonomo, B.; Diana, A.; Manca, O.; Nardini, S.
2017-11-01
Natural convection gets a great attention for its importance in many thermal engineering applications, such as cooling of electronic components and devices, chemical vapor deposition systems and solar energy systems. In this work, a numerical investigation on steady state natural convection in a horizontal channel partially filled with a porous medium and heated at uniform heat flux from above is carried out. A three-dimensional model is realized and solved by means of the ANSYS-FLUENT code. The computational domain is made up of the principal channel and two lateral extended reservoirs at the open vertical sections. Furthermore, a porous plate is considered near the upper heated plate and the aluminium foam has different values of PPI. The numerical simulations are performed with working fluid air. Different values of assigned wall heat flux at top surface are considered and the configuration of the channel partially filled with metal foam is compared to the configuration without foam. Results are presented in terms of velocity and temperature fields, and both temperature and velocity profiles at different significant sections are shown. Results show that the use of metal foams, with low values of PPI, promotes the cooling of the heated wall and it causes a reduction of Nusselt Number values with high values of PPI.
Natural convection of a two-dimensional Boussinesq fluid does not maximize entropy production.
Bartlett, Stuart; Bullock, Seth
2014-08-01
Rayleigh-Bénard convection is a canonical example of spontaneous pattern formation in a nonequilibrium system. It has been the subject of considerable theoretical and experimental study, primarily for systems with constant (temperature or heat flux) boundary conditions. In this investigation, we have explored the behavior of a convecting fluid system with negative feedback boundary conditions. At the upper and lower system boundaries, the inward heat flux is defined such that it is a decreasing function of the boundary temperature. Thus the system's heat transport is not constrained in the same manner that it is in the constant temperature or constant flux cases. It has been suggested that the entropy production rate (which has a characteristic peak at intermediate heat flux values) might apply as a selection rule for such a system. In this work, we demonstrate with Lattice Boltzmann simulations that entropy production maximization does not dictate the steady state of this system, despite its success in other, somewhat similar scenarios. Instead, we will show that the same scaling law of dimensionless variables found for constant boundary conditions also applies to this system.
Directory of Open Access Journals (Sweden)
Nee Alexander
2016-01-01
Full Text Available This paper presents the results of mathematical modelling of turbulent natural convection under conditions of the conjugate heat transfer in an open cavity heated by infrared emitter. Two-dimensional problem of heat transfer was formulated in the vorticity - stream function - temperature dimensionless variables and solved by means of the finite difference method on a uniform grid. According to the results of numerical analysis, the fields of differential and integral heat transfer characteristics, illustrating the unsteady nature of the thermophysical process under study, were obtained. The nonlinear dependence of the average Nusselt number versus dimensionless time (τ was established. It was shown that the average Nusselt number monotonically increased in a range of 800<τ<2000
Energy Technology Data Exchange (ETDEWEB)
Fox, E.; Visser, A.; Bridges, N.
2011-07-18
This paper presents an experimental study of natural convection heat transfer for an Ionic Liquid. The experiments were performed for 1-butyl-2, 3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, ([C{sub 4}mmim][NTf{sub 2}]) at a Raleigh number range of 1.26 x 10{sup 7} to 8.3 x 10{sup 7}. In addition to determining the convective heat transfer coefficients, this study also included experimental determination of thermophysical properties of [C{sub 4}mmim][NTf{sub 2}] such as, density, viscosity, heat capacity, and thermal conductivity. The results show that the density of [C{sub 4}mmim][NTf{sub 2}] varies from 1.437-1.396 g/cm{sup 3} within the temperature range of 10-50 C, the thermal conductivity varies from 0.105-0.116 W/m.K between a temperature of 10 to 60 C, the heat capacity varies from 1.015 J/g.K - 1.760 J/g.K within temperature range of 25-340 C and the viscosity varies from 18cp-243cp within temperature range 10-75 C. The results for density, thermal conductivity, heat capacity, and viscosity were in close agreement with the values in the literature. Measured dimensionless Nusselt number was observed to be higher for the ionic liquid than that of DI water. This is expected as Nusselt number is the ratio of heat transfer by convection to conduction and the ionic liquid has lower thermal conductivity (approximately 18%) than DI water.
Gelfgat, Alexander
2015-01-01
Transition from steady to oscillatory buoyancy convection of air in a laterally heated cubic box is studied numerically by straight-forward time integration of Boussinesq equations using a series of gradually refined finite volume grids. Horizontal and spanwise cube boundaries are assumed to be either perfectly thermally conducting or perfectly thermally insulated, which results in four different sets of thermal boundary conditions. Critical Grashof numbers are obtained by interpolation of numerically extracted growth/decay rates of oscillations amplitude to zero. Slightly supercritical flow regimes are described by time-averaged flows, snapshots, and spatial distribution of oscillations amplitude. Possible similarities and dissimilarities with two-dimensional instabilities in laterally heated square cavities are discussed. Arguments for grid and time step independence of the results are given.
Directory of Open Access Journals (Sweden)
Vanita
2016-06-01
Full Text Available In the present paper, an analysis has been performed to study the influence of induced magnetic field on the transient free convective flow of an electrically conducting and viscous incompressible fluid over a vertical cone. The coupled nonlinear partial differential equations governing the transient flow have been solved numerically by using the implicit finite difference method of Crank–Nicolson type. The influence of magnetic parameter, magnetic Prandtl number and semi-vertical angle of the cone on the velocity and induced magnetic field profiles has been illustrated graphically. Also, the local as well as average skin-friction and Nusselt number has been presented graphically. For result validation, we have done a comparative study and the present results are found to be in very good agreement with available results.
Ullah, Imran; Khan, Ilyas; Shafie, Sharidan
2016-12-01
In the present work, the effects of chemical reaction on hydromagnetic natural convection flow of Casson nanofluid induced due to nonlinearly stretching sheet immersed in a porous medium under the influence of thermal radiation and convective boundary condition are performed numerically. Moreover, the effects of velocity slip at stretching sheet wall are also examined in this study. The highly nonlinear-coupled governing equations are converted to nonlinear ordinary differential equations via similarity transformations. The transformed governing equations are then solved numerically using the Keller box method and graphical results for velocity, temperature, and nanoparticle concentration as well as wall shear stress, heat, and mass transfer rate are achieved through MATLAB software. Numerical results for the wall shear stress and heat transfer rate are presented in tabular form and compared with previously published work. Comparison reveals that the results are in good agreement. Findings of this work demonstrate that Casson fluids are better to control the temperature and nanoparticle concentration as compared to Newtonian fluid when the sheet is stretched in a nonlinear way. Also, the presence of suspended nanoparticles effectively promotes the heat transfer mechanism in the base fluid.
Hoteit, Hussein
2017-12-29
Computation of the distribution of species in hydrocarbon reservoirs from diffusions (thermal, molecular, and pressure) and natural convection is an important step in reservoir initialization. Current methods, which are mainly based on the conventional finite difference approach, may not be numerically efficient in fractured and other media with complex heterogeneities. In this work, the discontinuous Galerkin (DG) method combined with the mixed finite element (MFE) method is used for the calculation of compositional variation in fractured hydrocarbon reservoirs. The use of unstructured gridding allows efficient computations for fractured media when the crossflow equilibrium concept is invoked. The DG method has less numerical dispersion than the upwind finite difference (FD) methods. The MFE method ensures continuity of fluxes at the interface of the grid elements. We also use the local discontinuous Galerkin (LDG) method instead of the MFE calculate the diffusion fluxes. Results from several numerical examples are presented to demonstrate the efficiency, robustness, and accuracy of the model. Various features of convection and diffusion in homogeneous, layered, and fractured media are also discussed.
Energy Technology Data Exchange (ETDEWEB)
Javaherdeh, Korosh; Moslemi, Mehdi; Shahbazi, Mona [University of Guilan, Rasht (Iran, Islamic Republic of)
2017-04-15
A numerical analysis has been performed to investigate the laminar natural convection heat characteristics in a wavy cavity filled with CuO/water nanofluid. One of the sinusoidal walls (BC) is at the volatile high temperature and the opposite wavy surface is at a stable low temperature and the two other walls are considered flat and insulated while the uniform magnetic field is considered. Performing the analysis, the governing equations are given in terms of the stream function-vorticity formulation. In order to solve the nondimensionalized equations, discretizing with second-order accurate central difference method is performed then the successive under relaxation method with appropriate boundary conditions is considered. To validate the numerical model, various comparisons with previously published studies have been conducted and the results are in a good agreement. The main objective is to survey the effects of the Rayleigh number, Hartmann number, and nanoparticles volume fraction on the fluid flow and heat transfer characteristics. The results are illustrated in contours of stream function, constant temperature, and Nusselt number. The results show that the presence of the magnetic field the local Nusselt number decreases at the hot wall. Moreover, the enhancement in the heat transfer performance increases with an increasing nanoparticle concentration. However, for all values of Rayleigh number, the presence of nanoparticles leads to significant enhancement in heat transfer and the increase of Rayleigh number causes the heat transfer mechanism to change from conduction to convection.
Directory of Open Access Journals (Sweden)
Nee Alexander
2016-01-01
Full Text Available Mathematical modelling of turbulent natural convection in a semi-open cavity with a heat-conducting walls of finite thickness with radiant heating of internal boundaries was performed. Two-dimensional problem of the conjugate heat transfer was solved by means of the finite difference method. Scale influence of open boundaries and radiant heating of the gas – wall interfaces on the formation of differential and integral heat transfer characteristics was established. An increase in the dimensionless time (τ led to displacement of extremum temperatures in the typical cross section (Y = 0.5 to an open vertical boundary. The average Nusselt number monotonically increased at the gas – a wall interfaces in a range of 400<τ<1200.
Kozhevnikov, Danil A.; Sheremet, Mikhail A.
2018-01-01
The effect of surface tension on laminar natural convection in a vertical cylindrical cavity filled with a weak evaporating liquid has been analyzed numerically. The cylindrical enclosure is insulated at the bottom, heated by a constant heat flux from the side, and cooled by a non-uniform evaporative heat flux from the top free surface having temperature-dependent surface tension. Governing equations with corresponding boundary conditions formulated in dimensionless stream function, vorticity, and temperature have been solved by finite difference method of the second-order accuracy. The influence of Rayleigh number, Marangoni number, and aspect ratio on the liquid flow and heat transfer has been studied. Obtained results have revealed that the heat transfer rate at free surface decreases with Marangoni number and increases with Rayleigh number, while the average temperature inside the cavity has an opposite behavior; namely, it growths with Marangoni number and reduces with Rayleigh number.
Natural convection flow of Cu-H2O nanofluid along a vertical wavy surface with uniform heat flux
Habiba, Farjana; Molla, Md. Mamun; Khan, M. A. Hakim
2016-07-01
A numerical study on natural convection flow of Cu-Water nanofluid along a vertical wavy surface with uniform heat flux has been carried out. The governing boundary layer equations are transformed into parabolic partial differential equations by applying a suitable set of variables. The resulting nonlinear system of equations are then mapped into a regular rectangular computational domain and solved numerically by using an implicit finite difference method. Numerical results are thoroughly discussed in terms of velocity and temperature distributions, surface temperature distribution, skin friction coefficient and Nusselt number coefficient for selected key parameters such as solid volume fraction of nanofluid (ϕ) and amplitude (α) of surface waviness. In addition, velocity vectors, streamlines and isotherms are plotted to visualize momentum and thermal flow pattern within the boundary layer region.
Kulacki, F. A.; Emara, A. A.
1975-01-01
Natural convection energy transport in a horizontal layer of internally heated fluid was measured for Rayleigh numbers from 1890 to 2.17 x 10 to the 12th power. The fluid layer is bounded below by a rigid zero-heat-flux surface and above by a rigid constant-temperature surface. Joule heating by an alternating current passing horizontally through the layer provides the uniform volumetric energy source. The overall steady-state heat transfer coefficient at the upper surface was determined by measuring the temperature difference across the layer and power input to the fluid. The correlation between the Nusselt and Rayleigh numbers for the data of the present study and the data of the Kulacki study is given.
Kozhevnikov, Danil A.; Sheremet, Mikhail A.
2017-07-01
The effect of surface tension on laminar natural convection in a vertical cylindrical cavity filled with a weak evaporating liquid has been analyzed numerically. The cylindrical enclosure is insulated at the bottom, heated by a constant heat flux from the side, and cooled by a non-uniform evaporative heat flux from the top free surface having temperature-dependent surface tension. Governing equations with corresponding boundary conditions formulated in dimensionless stream function, vorticity, and temperature have been solved by finite difference method of the second-order accuracy. The influence of Rayleigh number, Marangoni number, and aspect ratio on the liquid flow and heat transfer has been studied. Obtained results have revealed that the heat transfer rate at free surface decreases with Marangoni number and increases with Rayleigh number, while the average temperature inside the cavity has an opposite behavior; namely, it growths with Marangoni number and reduces with Rayleigh number.
Directory of Open Access Journals (Sweden)
M. B. K. Moorthy
2012-01-01
Full Text Available The heat and mass transfer characteristics of natural convection about a vertical surface embedded in a saturated porous medium subject to variable viscosity are numerically analyzed, by taking into account the diffusion-thermo (Dufour and thermal-diffusion (Soret effects. The governing equations of continuity, momentum, energy, and concentrations are transformed into nonlinear ordinary differential equations, using similarity transformations, and then solved by using Runge-Kutta-Gill method along with shooting technique. The parameters of the problem are variable viscosity, buoyancy ratio, Lewis number, Prandtl number, Dufour effect, Soret effect, and Schmidt number. The velocity, temperature, and concentration distributions are presented graphically. The Nusselt number and Sherwood number are also derived and discussed numerically.
Directory of Open Access Journals (Sweden)
G.S. Seth
2014-06-01
Full Text Available An investigation of the effects of Hall current and rotation on unsteady hydromagnetic natural convection flow with heat and mass transfer of an electrically conducting, viscous, incompressible and optically thick radiating fluid past an impulsively moving vertical plate embedded in a fluid saturated porous medium, when temperature of the plate has a temporarily ramped profile, is carried out. Exact solution of the governing equations is obtained in closed form by Laplace transform technique. Exact solution is also obtained in case of unit Schmidt number. Expressions for skin friction due to primary and secondary flows and Nusselt number are derived for both ramped temperature and isothermal plates. Expression for Sherwood number is also derived. The numerical values of primary and secondary fluid velocities, fluid temperature and species concentration are displayed graphically whereas those of skin friction are presented in tabular form for various values of pertinent flow parameters.
Sahebi, S. A. R.; Pourziaei, H.; Feizi, A. R.; Taheri, M. H.; Rostamiyan, Y.; Ganji, D. D.
2015-12-01
In this paper, natural convection of non-Newtonian bio-nanofluids flow between two vertical flat plates is investigated numerically. Sodium Alginate (SA) and Sodium Carboxymethyl Cellulose (SCMC) are considered as the base non-Newtonian fluid, and nanoparticles such as Titania ( TiO2 and Alumina ( Al2O3 were added to them. The effective thermal conductivity and viscosity of nanofluids are calculated through Maxwell-Garnetts (MG) and Brinkman models, respectively. A fourth-order Runge-Kutta numerical method (NUM) and three Weighted Residual Methods (WRMs), Collocation (CM), Galerkin (GM) and Least-Square Method (LSM) and Finite-Element Method (FEM), are used to solve the present problem. The influence of some physical parameters such as nanofluid volume friction on non-dimensional velocity and temperature profiles are discussed. The results show that SCMC- TiO2 has higher velocity and temperature values than other nanofluid structures.
Energy Technology Data Exchange (ETDEWEB)
Nazaroff, W.W.; Kong, D.; Gadgil, A.J.
1992-02-01
We report numerical predictions of the deposition to enclosure surfaces of unattached {sup 218}Po and {sup 212}Pb, short-lived decay products of {sup 222}Rn and {sup 220}Rn, respectively. The simulations are conducted for square and rectangular two-dimensional enclosures under laminar natural convection flow with Grashof numbers in the range 7 x 10{sup 7} to 8 x 10{sup 10}. The predictions are based upon a finite-difference natural-convection fluid-mechanics model that has been extended to simulate the behavior of indoor radon decay products. In the absence of airborne particles, the deposition velocity averaged over the enclosure surface was found to be in the range (2-4) x 10{sup -4} m s{sup -1} for {sup 218}Po and (1-3) x 10{sup -4} m s{sup -1} for {sup 212}Pb. In each simulation, the deposition rate varied by more than an order of magnitude around the surface of the enclosure with the largest rates occurring near corners. Attachment of decay products to airborne particles increased the deposition velocity; for example, attachment of {sup 218}Po at a rate of 50 h{sup -1} increased the predicted average deposition velocity by 30-70% over values in the absence of attachment. The simulation results have significance for assessing the health risk associated with indoor exposure to {sup 222}Rn and {sup 220}Rn decay products and for investigating the more general problem of the interaction of air pollutants with indoor surfaces.
Directory of Open Access Journals (Sweden)
Mohammad Mahfuzul Islam
2015-08-01
Full Text Available Abstract In this paper is presented to study conjugate effects of stress work and heat generation on MHD natural convection flow along a vertical flat plate with power law variation of surface temperature. Stress work and heat generation effects on magneto-hydrodynamics natural convection flows are considered in this investigation. With a goal to attain similarity solutions of the problem the developed equations are made dimensionless by using suitable transformations. The non-dimensional equations are then transformed into non-similar forms by introducing non- similarity transformations. The resulting non-similar equations together with their corresponding boundary conditions based on conduction and convection are solved numerically by using the shooting method of Nachtsheim-swigert iteration technique and finite difference method together with Keller box Scheme. Numerically calculated velocity profiles and temperature profiles skin friction and the rate of heat transfer coefficient are shown on graphs for different values of the parameters entering into the problem.
Lin, Wenxian; Armfield, S. W.
2013-12-01
It is of fundamental significance, especially with regard to application, to fully understand the flow behavior of unsteady natural convection boundary layers on a vertical plate heated by a time-dependent heat flux. Such an understanding is currently scarce. In this paper, the scaling analysis by Lin et al. [Phys. Rev. E 79, 066313 (2009), 10.1103/PhysRevE.79.066313] using a simple three-region structure for the unsteady natural convection boundary layer of a homogeneous Newtonian fluid with Pr >1 under isothermal heating was substantially extended for the case when the heating is due to a time-varying sinusoidal heat flux. A series of scalings was developed for the thermal boundary thickness, the plate temperature, the viscous boundary thicknesses, and the maximum vertical velocity within the boundary layer, which are the major parameters representing the flow behavior, in terms of the governing parameters of the flow, i.e., the Rayleigh number Ra, the Prandtl number Pr, and the dimensionless natural frequency fn of the time-varying sinusoidal heat flux, at the start-up stage, at the transition time scale which represents the ending of the start-up stage and the beginning of the transitional stage of the boundary-layer development, and at the quasi-steady stage. These scalings were validated by comparison to 10 full numerical solutions of the governing equations with Ra, Pr, and fn in the ranges 106≤Ra≤109, 3≤Pr≤100, and 0.01≤fn≤0.1 and were shown in general to provide an accurate description of the flow at different development stages, except for high-Pr runs in which a further, although weak, Pr dependence is present, which cannot be accurately predicted by the current scaling analysis using the simple three-region structure, attributed to the non-boundary-layer nature of the velocity field with high-Pr fluids. Some scalings at the transition time scale and at the quasi-steady stage also produce noticeable deviations from the numerical results when
Energy Technology Data Exchange (ETDEWEB)
Farmer, M. T.; Kilsdonk, D. J.; Tzanos, C.P.; Lomperski, S.; Aeschlimann, R.W.; Pointer, D.; Nuclear Engineering Division
2005-09-01
As part of the Department of Energy (DOE) Generation IV roadmapping activity, the Very High Temperature gas cooled Reactor (VHTR) has been selected as the principal concept for hydrogen production and other process-heat applications such as district heating and potable water production. On this basis, the DOE has selected the VHTR for additional R&D with the ultimate goal of demonstrating emission-free electricity and hydrogen production with this advanced reactor concept. One of the key passive safety features of the VHTR is the potential for decay heat removal by natural circulation of air in a Reactor Cavity Cooling System (RCCS). The air-cooled RCCS concept is notably similar to the Reactor Vessel Auxiliary Cooling System (RVACS) that was developed for the General Electric PRISM sodium-cooled fast reactor. As part of the DOE R&D program that supported the development of this fast reactor concept, the Natural Convection Shutdown Heat Removal Test Facility (NSTF) was developed at ANL to provide proof-of-concept data for the RVACS under prototypic natural convection flow, temperature, and heat flux conditions. Due to the similarity between RVACS and the RCCS, current VHTR R&D plans call for the utilization of the NSTF to provide RCCS model development and validation data, in addition to supporting design validation and optimization activities. Both air-cooled and water-cooled RCCS designs are to be included. In support of this effort, ANL has been tasked with the development of an engineering plan for mechanical and instrumentation modifications to NSTF to ensure that sufficiently detailed temperature, heat flux, velocity and turbulence profiles are obtained to adequately qualify the codes under the expected range of air-cooled RCCS flow conditions. Next year, similar work will be carried out for the alternative option of a water-cooled RCCS design. Analysis activities carried out in support of this experiment planning task have shown that: (a) in the RCCS, strong
Development of an ex-vessel corium debris bed with two-phase natural convection in a flooded cavity
Energy Technology Data Exchange (ETDEWEB)
Kim, Eunho; Lee, Mooneon; Park, Hyun Sun, E-mail: hejsunny@postech.ac.kr; Moriyama, Kiyofumi; Park, Jin Ho
2016-03-15
Highlights: • For ex-vessel severe accidents in LWRs with wet-cavity strategy, development of debris bed with two-phase natural convection flow due to thermal characteristics of prototypic corium particles was investigated experimentally by using simulant particles and local air bubble control system. • Based on the experimental results of this study, an analytical model was established to describe the spreading of the debris bed in terms of two-phase flow and the debris injection parameters. • This model was then used to analyze the formation of debris beds at the reactor scale, and a sensitivity analysis was carried out based on key accident parameters. - Abstract: During severe accidents of light water reactors (LWRs), the coolability of relocated corium from the reactor vessel is a significant safety issue and a threat to the integrity of containment. With a flooded cavity, a porous debris bed is expected to develop on the bottom of the pool due to breakup and fragmentation of the melt jet. As part of the coolability assessment under accident conditions, the geometrical configuration of the debris bed is important. The Debris Bed Research Apparatus for Validation of the Bubble-Induced Natural Convection Effect Issue (DAVINCI) experimental apparatus facility was constructed to investigate the formation of debris beds under the influence of a two-phase flow induced by steam generation due to the decay heat of the debris bed. Using this system, five kilograms of stainless steel simulant debris were injected from the top of the water level, while air bubbles simulating the vapor flow were injected from the bottom of the particle catcher plate. The airflow rate was determined based on the quantity of settled debris, which will form a heat source due to the decay of corium. The radial distribution of the settled debris was examined using a ‘gap–tooth’ approach. Based on the experimental results of this study, an analytical model was established to
Convective adjustment timescale (τ) for cumulus clouds is one of the most influential parameters controlling parameterized convective precipitation in climate and weather simulation models at global and regional scales. Due to the complex nature of deep convection, a pres...
Directory of Open Access Journals (Sweden)
Goyal M.
2017-12-01
Full Text Available In this paper, triple diffusive natural convection under Darcy flow over an inclined plate embedded in a porous medium saturated with a binary base fluid containing nanoparticles and two salts is studied. The model used for the nanofluid is the one which incorporates the effects of Brownian motion and thermophoresis. In addition, the thermal energy equations include regular diffusion and cross-diffusion terms. The vertical surface has the heat, mass and nanoparticle fluxes each prescribed as a power law function of the distance along the wall. The boundary layer equations are transformed into a set of ordinary differential equations with the help of group theory transformations. A wide range of parameter values are chosen to bring out the effect of buoyancy ratio, regular Lewis number and modified Dufour parameters of both salts and nanofluid parameters with varying angle of inclinations. The effects of parameters on the velocity, temperature, solutal and nanoparticles volume fraction profiles, as well as on the important parameters of heat and mass transfer, i.e., the reduced Nusselt, regular and nanofluid Sherwood numbers, are discussed. Such problems find application in extrusion of metals, polymers and ceramics, production of plastic films, insulation of wires and liquid packaging.
Alsabery, Ammar I.; Hussain, Salam H.; Saleh, Habibis; Hashim, Ishak
2015-09-01
The problem of inclination angle effect on natural convection in a square cavity partially filled with non-Newtonian fluid layer is studied numerically using The Finite Volume Method. Governing equations are solved over wide range of Darcy number (10-5 ≤ Da ≤ 10-1), power-law index(0.6 ≤ n ≤ 1.4), the inclination angle of the cavity (0° ≤ ω ≤ 90°), Rayleigh number (Ra = 105) and porous layer thickness (S = 0.5). The results presented for values of the governing parameters in terms of streamlines in both porous/non-Newtonian fluid-layer, isotherms in both porous/non-Newtonian fluid-layer and average Nusselt number. It is shown that the heat transfer has maximum value when the power-law index is less than one (pseudoplastic fluid), and then decreases remarkably as the power-law index increases. The results have possible applications in heat-removal and heat-storage non-Newtonian fluid-saturated porous systems.
Warda, Boudaoud; Amina, Sabeur; Souad, Morsli
2017-05-01
The aim of this work is to analyze the natural convection phenomena and entropy generation of water-based Al2O3 nanofluids in square enclosure. The simulated domain corresponds to a square cavity heated from below and cooled from the top. The left and right walls are heated up to a height H = (3/4 W) and are adiabatic in the remaining part (1-H). Numerical investigations have been carried out based on coupled partial differential equations of momentum and energy which are solved using finite volume method. The effective thermal conductivity of the nanofluid was expressed by the Maxwell-Garnetts model however the dynamic viscosity was calculated according to the Brinkman formula. The obtained results were presented by average Nusselt number, streamlines, isotherms and entropy generation with various pertinent parameters, namely, Rayleigh number (100 ≤ Ra ≤ 106), volumetric fraction of nanoparticles (1% ≤ ϕ ≤ 4% ). It was found that the heat transfer increases with the increase of Rayleigh number and volume fraction. The choice of these parameters is important to obtain maximum enhancement of heat transfer with minimum entropy generation. Contribution to the topical issue "Materials for Energy harvesting, conversion and storage II (ICOME 2016)", edited by Jean-Michel Nunzi, Rachid Bennacer and Mohammed El Ganaoui
Directory of Open Access Journals (Sweden)
A. A. Altawallbeh
2013-01-01
Full Text Available The effect of magnetic field on double-diffusive natural convection in a square cavity filled with a fluid-saturated porous medium is studied numerically. The bottom wall is fully heated at a constant temperature, and the top wall is maintained at a constant cold temperature. The right wall is fully salted to a high concentration, while the left wall is fully salted at a lower concentration than the right one. A magnetic force is applied on the cavity along the gravity force direction. The Darcy model is used for the mathematical formulation of the fluid flow through porous media. The governing equations for heat and mass transfer are solved using the finite volume method. The governing parameters of the present study are Rayleigh number (Ra, Lewis number (Le, buoyancy ratio (N, and Hartmann number (Ha. The numerical solutions were studied in the range of −10 ≤ N ≤ 10, 0 ≤ Ha ≤ 10, 50 ≤ Ra ≤ 500, and 10−4 ≤ Le ≤ 10. The results were discussed considering the effect of these parameters on the heat and mass transfer processes. The results were presented in terms of streamlines, isotherms, isoconcentration, average Nusselt number, and average Sherwood number for different values of the governing parameters. In general, it has been found that the increase of magnetic force has an effect to retard the strength of the flow inside the cavity and reduce the heat and mass transfer processes. For high Hartmann number, the flow is almost suppressed.
Goyal, M.; Goyal, R.; Bhargava, R.
2017-12-01
In this paper, triple diffusive natural convection under Darcy flow over an inclined plate embedded in a porous medium saturated with a binary base fluid containing nanoparticles and two salts is studied. The model used for the nanofluid is the one which incorporates the effects of Brownian motion and thermophoresis. In addition, the thermal energy equations include regular diffusion and cross-diffusion terms. The vertical surface has the heat, mass and nanoparticle fluxes each prescribed as a power law function of the distance along the wall. The boundary layer equations are transformed into a set of ordinary differential equations with the help of group theory transformations. A wide range of parameter values are chosen to bring out the effect of buoyancy ratio, regular Lewis number and modified Dufour parameters of both salts and nanofluid parameters with varying angle of inclinations. The effects of parameters on the velocity, temperature, solutal and nanoparticles volume fraction profiles, as well as on the important parameters of heat and mass transfer, i.e., the reduced Nusselt, regular and nanofluid Sherwood numbers, are discussed. Such problems find application in extrusion of metals, polymers and ceramics, production of plastic films, insulation of wires and liquid packaging.
Baghaei Lakeh, Reza; Lavine, Adrienne S.; Kavehpour, H. Pirouz; Wirz, Richard E.
2013-11-01
Heat transfer can be a limiting factor in the operation of thermal energy storage, including sensible heat and latent heat storage systems. Poor heat transfer between the energy storage medium and the container walls impairs the functionality of the thermal storage unit by requiring excessively long times to charge or discharge the system. In this study, the effect of turbulent, unsteady buoyancy-driven flow on heat transfer in vertical storage tubes containing supercritical CO2 as the storage medium is investigated computationally. The heat transfer from a constant-temperature wall to the storage fluid is studied during the charge cycle. The results of this study show that turbulent natural convection dominates the heat transfer mechanism and significantly reduces the required time for charging compared to pure conduction. Changing the L/D ratio of the storage tube has a major impact on the charge time. The charge time shows a decreasing trend with RaL. The non-dimensional model of the problem shows that Nusselt number and non-dimensional mean temperature of the storage fluid in different configurations of the tube is a function Buoyancy-Fourier number defined as of FoL * RaLm* L/D. This study was supported by award No. DE-AR0000140 granted by U.S. Department of Energy under Advanced Research Projects Agency - Energy (ARPA-E) and by award No. 5660021607 granted by Southern California Gas Company.
Directory of Open Access Journals (Sweden)
Cianfrini Marta
2015-01-01
Full Text Available Laminar natural convection of Al2O3 + H2O nanofluids inside square cavities differentially heated at sides is studied numerically. A computational code based on the SIMPLE-C algorithm is used for the solution of the system of the mass, momentum and energy transfer governing equations. Assuming that the nanofluid behaves like a single-phase fluid, these equations are the same as those valid for a pure fluid, provided that the thermophysical properties appearing in them are the nanofluid effective properties. The thermal conductivity and dynamic viscosity of the nanofluid are calculated by means of a couple of empirical equations based on a wide variety of experimental data reported in the literature. The other effective properties are evaluated by the conventional mixing theory. Simulations are performed for different values of the nanoparticle volume fraction in the range 0-0.06, the diameter of the suspended nanoparticles in the range 25-100 nm, the temperature of the cooled sidewall in the range 293-313 K, the temperature of the heated sidewall in the range 298-343 K, and the Rayleigh number of the base fluid in the range 103-107. All computations are executed in the hypothesis of temperature-dependent effective properties. The main result obtained is the existence of an optimal particle loading for maximum heat transfer, that is found to increase as the size of the suspended nanoparticles is decreased, and the nanofluid average temperature is increased.
Energy Technology Data Exchange (ETDEWEB)
Ahmed, Sameh E., E-mail: sameh_sci_math@yahoo.com [Department of Mathematics, Faculty of Sciences, South Valley University, Qena (Egypt); Hussein, Ahmed Kadhim, E-mail: ahmedkadhim7474@gmail.com [College of Engineering, Mechanical Engineering Department, Babylon University, Babylon City—Hilla (Iraq); Mohammed, H.A. [Department of Thermofluids, Faculty of Mechanical Engineering, University Teknologi Malaysia (UTM), 81310 UTM Skudai, Johor Bahru (Malaysia); Adegun, I.K. [Department of Mechanical Engineering, University of Ilorin, Ilorin (Nigeria); Zhang, Xiaohui [School of Physics Science and Technology, School of Energy—Soochow University, Suzhou 215006, Jiangsu (China); Kolsi, Lioua [Unite de Metrologie en Mecanique des Fluides et Thermique, Ecole Nationale d’Ingenieurs, Monastir (Tunisia); Hasanpour, Arman [Department of Mechanical Engineering, Babol University of Technology, PO Box 484, Babol (Iran, Islamic Republic of); Sivasankaran, S. [Institute of Mathematical Sciences, University of Malaya, Kuala Lumpur 50603 (Malaysia)
2014-01-15
Highlights: • Ha decelerates the flow field. • Ha enhances conduction. • Magnetic field orientation is important. • Radiation parameter important. • Nu decreases as Ha increases. -- Abstract: Numerical two-dimensional analysis using finite difference approach with “line method” is performed on the laminar magneto-hydrodynamic natural convection in a square enclosure filled with a porous medium to investigate the effects of viscous dissipation and radiation. The enclosure heated from left vertical sidewall and cooled from an opposing right vertical sidewall. The top and bottom walls of the enclosure are considered adiabatic. The flow in the square enclosure is subjected to a uniform magnetic field at various orientation angles (φ = 0°, 30°, 45°, 60° and 90°). Numerical computations occur at wide ranges of Rayleigh number, viscous dissipation parameter, magnetic field orientation angles, Hartmann number and radiation parameter. Numerical results are presented with the aid of tables and graphical illustrations. The results of the present work explain that the local and average Nusselt numbers at the hot and cold sidewalls increase with increasing the radiation parameter. From the other side, the role of viscous dissipation parameter is to reduce the local and average Nusselt numbers at the hot left wall, while it improves them at the cold right wall. The results are compared with another published results and it found to be in a good agreement.
On the stability of natural convection in a porous vertical slab saturated with an Oldroyd-B fluid
Shankar, B. M.; Shivakumara, I. S.
2017-06-01
The stability of the conduction regime of natural convection in a porous vertical slab saturated with an Oldroyd-B fluid has been studied. A modified Darcy's law is utilized to describe the flow in a porous medium. The eigenvalue problem is solved using Chebyshev collocation method and the critical Darcy-Rayleigh number with respect to the wave number is extracted for different values of physical parameters. Despite the basic state being the same for Newtonian and Oldroyd-B fluids, it is observed that the basic flow is unstable for viscoelastic fluids—a result of contrast compared to Newtonian as well as for power-law fluids. It is found that the viscoelasticity parameters exhibit both stabilizing and destabilizing influence on the system. Increase in the value of strain retardation parameter Λ _2 portrays stabilizing influence on the system while increasing stress relaxation parameter Λ _1 displays an opposite trend. Also, the effect of increasing ratio of heat capacities is to delay the onset of instability. The results for Maxwell fluid obtained as a particular case from the present study indicate that the system is more unstable compared to Oldroyd-B fluid.
Directory of Open Access Journals (Sweden)
Mohammad Yaghoub Abdollahzadeh Jamalabadi
2016-04-01
Full Text Available In this article, we present a comprehensive analysis of the flow and heat transfer characteristics of a fully developed incompressible, electrically conducting, and radiatively active fluid flow in micro-channel in the presence of transverse magnetic field. The Navier–Stokes and energy governing equations for magnetohydrodynamic flow, including thermal radiation and rarefaction effects, are considered to examine the wall properties (friction and heat transfer and the flow properties (temperature and velocity. Two rarefaction effects of velocity slip and temperature jump at the wall are modeled as the product of characteristic slip/jump length and the first derivatives of velocity and temperature, respectively. Since the natural convection of magnetohydrodynamic flow in channel is resulted from the competition between deriving forces by pressure gradient, temperature gradient, and magnetic field, its flow and heat transfer characteristics should be understood systematically. First, we obtain the system parameters representing thermal radiation, buoyancy, magnetic field, temperature difference, velocity slip length, and temperature jump length through the non-dimensionalization process, and then their influences are rigorously evaluated by solving the governing equations numerically using Runge–Kutta algorithm with shooting method.
Lee, Seung-Hyun; Jang, Seok Pil
2012-07-01
In this paper, numerical and experimental investigations are systematically performed to identify the effect of the tilting angle of the wire on the onset of natural convection in the transient hot wire method (THWM), a widely accepted technique for measuring the thermal conductivity of various media, especially nanofluids. To validate our numerical simulation code, the numerical results are compared with theoretical solutions as well as with experimental results. Based on the results, we show that the onset time of natural convection in THWM decreases rapidly with the increase of the wire's tilting angle from vertical position. Also, we systematically show the effect of the wire's tilting angle on the linear region, which is a suitable measurement interval, and on the measurement error of THWM.
Energy Technology Data Exchange (ETDEWEB)
Veloso, Maria Auxiliadora Fortini
2004-07-01
The STHIRP-1 computer program, which fundamentals are described in this work, uses the principles of the subchannels analysis and has the capacity to simulate, under steady state and transient conditions, the thermal and hydraulic phenomena which occur inside the core of a water-refrigerated research reactor under a natural convection regime. The models and empirical correlations necessary to describe the flow phenomena which can not be described by theoretical relations were selected according to the characteristics of the reactor operation. Although the primary objective is the calculation of research reactors, the formulation used to describe the fluid flow and the thermal conduction in the heater elements is sufficiently generalized to extend the use of the program for applications in power reactors and other thermal systems with the same features represented by the program formulations. To demonstrate the analytical capacity of STHIRP-l, there were made comparisons between the results calculated and measured in the research reactor TRIGA IPR-R1 of CDTN/CNEN. The comparisons indicate that the program reproduces the experimental data with good precision. Nevertheless, in the future there must be used more consistent experimental data to corroborate the validation of the program. (author)
Energy Technology Data Exchange (ETDEWEB)
Lee, Jae Ryong [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of); Park, Il Seouk [School of Mechanical Engineering, Kyungpook National University, Daegu (Korea, Republic of)
2012-04-15
The natural convection in a horizontal enclosure heated from the bottom wall, cooled at the top wall, and having a square adiabatic body in the center is studied. Three different Prandtl numbers (0.01, 0.7 and 7) are considered for the investigation of the effect of the Prandtl number on natural convection. Adiabatic boundary conditions are employed for the side walls. A two dimensional solution for unsteady natural convection is obtained, using an accurate and efficient Chebyshev spectral methodology for different Rayleigh numbers varying over the range of 103 to 106. It had been experimentally reported that the heat transfer mode becomes oscillatory when Pr is out of a specific Pr band beyond the critical Ra. In this study, we reproduced this phenomenon numerically. It was found that when Ra=106, only the case for intermediate Pr (=0.7) reached a non-changing steady state and the low and high Pr number cases (Pr=0.01 and 7) showed a periodically oscillatory fashion hydro dynamically and thermally. The variation of time- and surface-averaged Nusselt numbers on the hot and cold walls for different Rayleigh numbers and Prandtl numbers are presented to show the overall heat transfer characteristics in the system. Further, the isotherms and streamline distributions are presented in detail to compare the physics related to their thermal behavior.
Dhote, Yogesh; Thombre, Shashikant
2016-10-01
This paper presents the thermal performance of the proposed double flow natural convection solar air heater with in-built liquid (oil) sensible heat storage. Unused engine oil was used as thermal energy storage medium due to its good heat retaining capacity even at high temperatures without evaporation. The performance evaluation was carried out for a day of the month March for the climatic conditions of Nagpur (India). A self reliant computational model was developed using computational tool as C++. The program developed was self reliant and compute the performance parameters for any day of the year and would be used for major cities in India. The effect of change in storage oil quantity and the inclination (tilt angle) on the overall efficiency of the solar air heater was studied. The performance was tested initially at different storage oil quantities as 25, 50, 75 and 100 l for a plate spacing of 0.04 m with an inclination of 36o. It has been found that the solar air heater gives the best performance at a storage oil quantity of 50 l. The performance of the proposed solar air heater is further tested for various combinations of storage oil quantity (50, 75 and 100 l) and the inclination (0o, 15o, 30o, 45o, 60o, 75o, 90o). It has been found that the proposed solar air heater with in-built oil storage shows its best performance for the combination of 50 l storage oil quantity and 60o inclination. Finally the results of the parametric study was also presented in the form of graphs carried out for a fixed storage oil quantity of 25 l, plate spacing of 0.03 m and at an inclination of 36o to study the behaviour of various heat transfer and fluid flow parameters of the solar air heater.
Taji, S. G.; Parishwad, G. V.; Sane, N. K.
2014-07-01
This paper presents results of the experimental study conducted on heated horizontal rectangular fin array under natural convection. The temperature mapping and the prediction of the flow patterns over the fin array with variable fin spacing is carried out. Dimensionless fin spacing to height (S/H) ratio is varied from 0.05 to 0.3 and length to height ratio (L/H) = 5 is kept constant. The heater input to the fin array assembly is varied from 25 to 100 W. The single chimney flow pattern is observed from 8 to 12 mm fin spacing. The end flow is choked below 6 mm fin spacing. The single chimney flow pattern changes to sliding or end flow choking at 6 mm fin spacing. The average heat transfer coefficient (ha) is very small (2.52-5.78 W/m2 K) at 100 W for S = 5-12 mm. The ha is very small (1.12-1.8 W/m2 K) at 100 W for 2-4 mm fin spacing due to choked fin array end condition. The end flow is not sufficient to reach up to central portion of fin array and in the middle portion there is an unsteady down and up flow pattern resulting in sliding chimney. The central bottom portion of fin array channel does not contribute much in heat dissipation for S = 2-4 mm. The ha has significantly improved at higher spacing as compared to lower spacing region. The single chimney flow pattern is preferred from heat transfer point of view. The optimum spacing is confirmed in the range of 8-10 mm. The average heat transfer results are compared with previous literature and showed similar trend and satisfactory agreement. An empirical equation has been proposed to correlate the average Nusselt number as a function of Grashof number and fin spacing to height ratio. The average error for this equation is -0.32 %.
Energy Technology Data Exchange (ETDEWEB)
Dyrboel, Susanne
1998-05-01
Fibrous materials are some of the most widely used materials for thermal insulation. In this project the focus of interest has been on fibrous materials for building application. Interest in improving the thermal properties of insulation materials is increasing as legislation is being tightened to reduce the overall energy consumption. A knowledge of the individual heat transfer mechanisms - whereby heat is transferred within a particular material is an essential tool to improve continuously the thermal properties of the material. Heat is transferred in fibrous materials by four different transfer mechanisms: conduction through air, conduction through fibres, thermal radiation and convection. In a particular temperature range the conduction through air can be regarded as a constant, and conduction through fibres is an insignificant part of the total heat transfer. Radiation, however, constitutes 25-40% of the total heat transfer in light fibrous materials. In Denmark and a number of other countries convection in fibrous materials is considered as non-existent when calculating heat transmission as well as when designing building structures. Two heat transfer mechanisms have been the focus of the current project: radiation heat transfer and convection. The radiation analysis serves to develop a model that can be used in further work to gain a wider knowledge of the way in which the morphology of the fibrous material, i.e. fibre diameter distribution, fibre orientation distribution etc., influences the radiation heat transfer under different conditions. The convection investigation serves to examine whether considering convection as non-existent is a fair assumption to use in present and future building structures. The assumption applied in practically is that convection makes a notable difference only in very thick insulation, at external temperatures below -20 deg. C, and at very low densities. For large thickness dimensions the resulting heat transfer through the
Directory of Open Access Journals (Sweden)
Nee Alexander E.
2017-01-01
Full Text Available The numerical simulation results of three-dimensional natural convection in a closed cavity were presented under conditions of the bottom horizontal solid-fluid interface radiant heating and conjugate heat exchange. Conservation equations of mass, momentum, and energy were formulated in terms of vorticity vector – vector potential – temperature dimensionless variables and solved by means of the finite difference method. It was found that the heat transfer process under study had a significant unsteady nature. According to the results of conjugate heat exchange integral analysis, it was shown that similar trends of mean Nusselt numbers versus dimensionless time were formed for both two and three dimensional problem formulations.
Energy Technology Data Exchange (ETDEWEB)
Martins, G.; Pereira, J. T. V.
1996-11-01
Natural convection flow and heat and mass transfer resulting from the combined effects of thermal and mass diffusion occurring in the evaporator of a triple-fluid (NH{sub 3}-H{sub 2}O-H{sub 2}) absorption refrigeration system was studied by modelling. Significant similarities between the boundary conditions of constant heat flux and convective condition with similar heat flux densities were observed. Overall results led to the conclusion that the geometric parameters of the tube, such as length and radius, are by far the most important influencing parameters in the characteristics of the buoyancy flow induced inside the tube. The absorber of this this type of refrigeration system also has been studied by modelling. In this case the attention was focused on counter-current fluid flow. It was suggested that the model described here could be used as an efficient tool in designing, simulating and optimizing evaporators in absorption refrigeration systems. 22 refs., 14 figs.
Bower, S. M.; Saylor, J. R.
2007-11-01
An experimental study is presented of free-surface natural convection, with a focus on the Nu-Ra relationship. This relationship is typically studied using the traditional Rayleigh-Benard convection setup, where a fluid layer is bounded above and below by solid plates of different temperatures. Power laws of the form Nu = A Ra^n are typically used in these studies to correlate the data, giving exponents that are usually close to n = 1/3. The experimental data obtained in this study yields values of n that do not deviate significantly from 1/3 for 10^7 transfer coefficient for evaporation. The exponent m differed from that obtained by prior researchers. However, the prior research on evaporation that utilizes this scaling law is considerably smaller than for the heat transfer case. The effect of the tank aspect ratio on both scaling laws is also discussed.
Energy Technology Data Exchange (ETDEWEB)
Neitzel, H.J.
1992-06-01
A new containment concept for future pressurized water reactors should ensure that the containment remains intact even in severe accidents and that the decay heat can be removed by natural air convection in a passive way. The composite containment proposed consists of an inner steel shell of 60 m diameter with a 38 mm wall thickness and an outer reinforced concrete shell of about 2 m wall thickness. The annulus of about 80 cm radial width is bridged by longitudinal support ribs placed at intervals of about 50 cm on the circumference forming in this way individual chimneys. In these chimneys the decay heat is removed by natural air convection to the environment. Filters prevent eventual fission gas leakages into the annulus from being released to the environment. Estimates have been performed to assess the coolability by natural air convection. It has been shown that a decay heat of about 8 MW can be removed which seems to be sufficient for a 1300 MWe PWR. (orig.). [Deutsch] Durch ein neuartiges Containmentkonzept fuer zukuenftige Druckwasserreaktoren soll gewaehrleistet werden, dass auch nach schwersten Unfaellen das Containment intakt bleibt und die Nachwaerme passiv durch Naturkonvektion an die Luft abgefuehrt wird. Das vorgeschlagene Verbund-Containment besteht aus einer inneren Stahlschale von 60 m Durchmesser und 38 mm Wandstaerke sowie einem aeusseren etwa 2 m starken Stahlbetonmantel. Dazwischen befindet sich ein Ringraum von etwa 80 cm radialer Tiefe, der durch laengslaufende Rippen in kaminartige etwa 50 cm breite Kanaele unterteilt ist. In diesen Kanaelen wird die Nachwaerme durch Naturkonvektion an die Luft abgegeben. Durch Einsatz von Filtern wird verhindert, dass die durch eventuelle Leckagen in den Ringraum austretenden Spaltgase in die Umgebung gelangen. Fuer die Waermeuebertragung auf der Luftseite wurden rechnerische Abschaetzungen durchgefuehrt. Es ergab sich, dass eine Nachwaermeleistung von 8 MW abgefuehrt werden kann, was fuer einen 1300 MWe
Hyland, P.; Biggerstaff, M. I.; Uman, M. A.; Hill, J. D.; Krehbiel, P. R.; Rison, W.
2012-12-01
During the summers of 2011-2012, a C-band polarimetric Shared Mobile Atmospheric Research and Teaching (SMART) radar from the University of Oklahoma was deployed to Keystone Heights, FL to study the relationship between cloud structure and the propagation of triggered and natural lightning channels. The radar was operated in Range-Height-Indicator (RHI) volume scanning mode over a narrow azimuthal sector that provided high spatial vertical resolution every 90 seconds over the rocket launch facility at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL. In this presentation, we will focus on observations collected in 2011. Seven successful triggers (with return strokes) out of 20 attempts were sampled by the SMART-R from June to August. Most of the trigger attempts occurred during the dissipating stages of convection with steady ground electric field values. Specific differential phase (KDP) showed evidence of ice crystal alignment due to strong electric fields within the upper portions of the convection over ICLRT around the time of launch attempts. Consecutive RHI sweeps over ICLRT revealed changes in KDP that suggested the building of electric fields and subsequent relaxation after a triggered flash. KDP signatures relative to other radar variables will also be investigated to determine the microphysical and convective nature of the storms in which natural and triggered lightning strikes occurred. Lightning Mapping Array (LMA) sources of the triggered flash channels showed a preference for horizontal propagation just above the radar bright band associated with the melting layer. This finding agrees with several past studies that used balloon soundings and found intense layers of charge near the 0°C isotherm. The propagation path also seemed to be related to the vertical distribution of KDP in some of the triggered flashes. A preferred path through areas of generally positive values of KDP suggests that triggered lightning
Abramzon, B.; Edwards, D. K.; Sirignano, W. A.
1986-01-01
A numerical study has been made of transient heat transfer and fluid flow in a cylindrical enclosure containing a two-layer gas-and-liquid system. The geometric configuration and the boundary conditions of the problem are relevant to the analysis of the preignition processes during the fire accident situation involving a pool of liquid fuel in the vicinity of an ignition source. It is demonstrated that the effects of the natural and thermocapillary convection, radiative transfer, thermal inertia and conduction of the walls bounding the enclosure, as well as, the magnitude of the gravity field play important roles in the development of the temperature and velocity fields in the container.
Energy Technology Data Exchange (ETDEWEB)
Lisowski, D. D. [Argonne National Lab. (ANL), Argonne, IL (United States); Farmer, M. T. [Argonne National Lab. (ANL), Argonne, IL (United States); Lomperski, S. [Argonne National Lab. (ANL), Argonne, IL (United States); Kilsdonk, D. J. [Argonne National Lab. (ANL), Argonne, IL (United States); Bremer, N. [Argonne National Lab. (ANL), Argonne, IL (United States); Aeschlimann, R. W. [Argonne National Lab. (ANL), Argonne, IL (United States)
2014-06-01
The Natural convection Shutdown heat removal Test Facility (NSTF) is a large scale thermal hydraulics test facility that has been built at Argonne National Laboratory (ANL). The facility was constructed in order to carry out highly instrumented experiments that can be used to validate the performance of passive safety systems for advanced reactor designs. The facility has principally been designed for testing of Reactor Cavity Cooling System (RCCS) concepts that rely on natural convection cooling for either air or water-based systems. Standing 25-m in height, the facility is able to supply up to 220 kW at 21 kW/m^{2} to accurately simulate the heat fluxes at the walls of a reactor pressure vessel. A suite of nearly 400 data acquisition channels, including a sophisticated fiber optic system for high density temperature measurements, guides test operations and provides data to support scaling analysis and modeling efforts. Measurements of system mass flow rate, air and surface temperatures, heat flux, humidity, and pressure differentials, among others; are part of this total generated data set. The following report provides an introduction to the top level-objectives of the program related to passively safe decay heat removal, a detailed description of the engineering specifications, design features, and dimensions of the test facility at Argonne. Specifications of the sensors and their placement on the test facility will be provided, along with a complete channel listing of the data acquisition system.
Javed, Tariq; Mehmood, Z.; Abbas, Z.
2017-02-01
This article contains numerical results for free convection through square enclosure enclosing ferrofluid saturated porous medium when uniform magnetic field is applied upon the flow along x-axis. Heat is provided through bottom wall and a square blockage placed near left or right bottom corner of enclosure as a heat source. Left and right vertical boundaries of the cavity are considered insulated while upper wall is taken cold. The problem is modelled in terms of system of nonlinear partial differential equations. Finite element method has been adopted to compute numerical simulations of mathematical problem for wide range of pertinent flow parameters including Rayleigh number, Hartman number, Darcy number and Prandtl number. Analysis of results reveals that the strength of streamline circulation is an increasing function of Darcy and Prandtl number where convection heat transfer is dominant for large values of these parameters whereas increase in Hartman number has opposite effects on isotherms and streamline circulations. Thermal conductivity and hence local heat transfer rate of fluid gets increased when ferroparticles are introduced in the fluid. Average Nusselt number increases with increase in Darcy and Rayleigh numbers while it is decreases when Hartman number is increased.
Energy Technology Data Exchange (ETDEWEB)
Minocha, Nitin [Homi Bhabha National Institute, Anushaktinagar, Mumbai 400 094 (India); Joshi, Jyeshtharaj B., E-mail: jbjoshi@gmail.com [Homi Bhabha National Institute, Anushaktinagar, Mumbai 400 094 (India); Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 400 019 (India); Nayak, Arun K. [Reactor Engineering Division, Bhabha Atomic Research Center, Trombay, Mumbai 400 085 (India); Vijayan, Pallippattu K., E-mail: vijayanp@barc.gov.in [Reactor Engineering Division, Bhabha Atomic Research Center, Trombay, Mumbai 400 085 (India)
2016-08-15
Highlights: • Investigation of three-dimensional natural convection and thermal stratification inside large water pool. • Effect of inclination (α) of condenser tube on fluid flow and heat transfer. • The heat transfer was found to be maximum for α = 90° and minimum for α = 15°. • Laminar-turbulent natural convection and heat transfer in the presence of longitudinal vortices. - Abstract: Many advanced nuclear reactors adopt methodologies of passive safety systems based on natural forces such as gravity. In one of such system, the decay heat generated from a reactor is removed by isolation condenser (ICs) submerged in a large water pool called the Gravity Driven Water Pool (GDWP). The objective of the present study was to design an IC for the passive decay heat removal system (PDHRS) for advanced nuclear reactor. First, the effect of inclination of IC tube on three dimensional temperature and flow fields was investigated inside a pilot scale (10 L) GDWP. Further, the knowledge of these fields has been used for the quantification of heat transfer and thermal stratification phenomenon. In a next step, the knowledge gained from the pilot scale GDWP has been extended to design an IC for real size GDWP (∼10,000 m{sup 3}). Single phase CFD simulation using open source CFD code [OpenFOAM-2.2] was performed for different tube inclination angles (α) (w.r.t. to vertical direction) in the range 0° ⩽ α ⩽ 90°. The results indicate that the heat transfer coefficient increases with increase in tube inclination angle. The heat transfer was found to be maximum for α = 90° and minimum for α = 15°. This behavior is due to the interaction between the primary flow (due to pressure gradient) and secondary flow (due to buoyancy force). The primary flow enhanced the fluid sliding motion at the tube top whereas the secondary flow resulted in enhancement in fluid motion along the circumference of tube. As the angle of inclination (α) of the tube was increased, the
Directory of Open Access Journals (Sweden)
Timchenko V.
2015-01-01
Full Text Available Numerical and experimental investigations of the flow and heat transfer in open-ended channel formed by the double skin façade have been undertaken in order to improve understanding of the phenomena and to apply it to passive cooling of building integrated photovoltaic systems. Both uniform heating and non-uniform heating configurations in which heat sources alternated with unheated zones on both skins were studied. Different periodic and asymmetric heating modes have been considered for the same aspect ratio 1/15 of wall distance to wall height and for periodicity 1/15 and 4/15 of heated/unheated zones and heat input, 220 W/m2. In computational study three dimensional transient LES simulation was carried out. It is shown that in comparison to uniformly heating configuration, non-uniformly heating configuration enhances both convective heat transfer and chimney effect.
Mathematical models of convection
Andreev, Victor K; Goncharova, Olga N; Pukhnachev, Vladislav V
2012-01-01
Phenomena of convection are abundant in nature as well as in industry. This volume addresses the subject of convection from the point of view of both, theory and application. While the first three chapters provide a refresher on fluid dynamics and heat transfer theory, the rest of the book describes the modern developments in theory. Thus it brings the reader to the ""front"" of the modern research. This monograph provides the theoretical foundation on a topic relevant to metallurgy, ecology, meteorology, geo-and astrophysics, aerospace industry, chemistry, crystal physics, and many other fiel
Energy Technology Data Exchange (ETDEWEB)
Bau, H.H. [Univ. of Pennsylvania, Philadelphia, PA (United States)
1995-12-31
Using stability theory, numerical simulations, and in some instances experiments, it is demonstrated that the critical Rayleigh number for the bifurcation (1) from the no-motion (conduction) state to the motion state and (2) from time-independent convection to time-dependent, oscillatory convection in the thermal convection loop and Rayleigh-Benard problems can be significantly increased or decreased. This is accomplished through the use of a feedback controller effectuating small perturbations in the boundary data. The controller consists of sensors which detect deviations in the fluid`s temperature from the motionless, conductive values and then direct actuators to respond to these deviations in such a way as to suppress the naturally occurring flow instabilities. Actuators which modify the boundary`s temperature/heat flux are considered. The feedback controller can also be used to control flow patterns and generate complex dynamic behavior at relatively low Rayleigh numbers.
Directory of Open Access Journals (Sweden)
Meraj Mustafa
Full Text Available The problem of natural convective boundary layer flow of nanofluid past a vertical plate is discussed in the presence of nonlinear radiative heat flux. The effects of magnetic field, Joule heating and viscous dissipation are also taken into consideration. The governing partial differential equations are transformed into a system of coupled nonlinear ordinary differential equations via similarity transformations and then solved numerically using the Runge-Kutta fourth-fifth order method with shooting technique. The results reveal an existence of point of inflection for the temperature distribution for sufficiently large wall to ambient temperature ratio. Temperature and thermal boundary layer thickness increase as Brownian motion and thermophoretic effects intensify. Moreover temperature increases and heat transfer from the plate decreases with an increase in the radiation parameter.
Energy Technology Data Exchange (ETDEWEB)
Capdevila, R; Trias, F X; Perez-Segarra, C D [Centre Tecnologic de Transferencia de Calor (CTTC), Lab.Termotecnia i Energetica, Universitat Politecnica de Catalunya (UPC), C/Colom, 11, E08222 Terrassa, Barcelona (Spain); Lehmkuhl, O; Colomer, G, E-mail: cttc@cttc.upc.edu, E-mail: termofluids@termofluids.com [Termofluids, S. L., Magi Colet 8, E08204 Sabadell, Barcelona (Spain)
2011-12-22
In the present work, turbulent natural convection in a tall differentially heated cavity of aspect ratio 5:1, filled with air (Pr = 0.7) under a Rayleigh number based on the height of 4.5 {center_dot} 10{sup 10}, is studied numerically. Two different situations have been analysed. In the first one, the cavity is filled with a transparent medium. In the second one, the cavity contains a grey participating gas. The turbulent flow is described by means of Large Eddy Simulation (LES) using symmetry-preserving discretizations. Simulations are compared with experimental data available in the literature and with Direct Numerical Simulations (DNS). Surface and gas radiation have been simulated using the Discrete Ordinates Method (DOM). The influence of radiation on fluid flow behaviour has also been analysed.
Energy Technology Data Exchange (ETDEWEB)
Richard C. Martineau; Ray A. Berry; Aurélia Esteve; Kurt D. Hamman; Dana A. Knoll; Ryosuke Park; William Taitano
2009-01-01
This report illustrates a comparative study to analyze the physical differences between numerical simulations obtained with both the conservation and incompressible forms of the Navier-Stokes equations for natural convection flows in simple geometries. The purpose of this study is to quantify how the incompressible flow assumption (which is based upon constant density advection, divergence-free flow, and the Boussinesq gravitational body force approximation) differs from the conservation form (which only assumes that the fluid is a continuum) when solving flows driven by gravity acting upon density variations resulting from local temperature gradients. Driving this study is the common use of the incompressible flow assumption in fluid flow simulations for nuclear power applications in natural convection flows subjected to a high heat flux (large temperature differences). A series of simulations were conducted on two-dimensional, differentially-heated rectangular geometries and modeled with both hydrodynamic formulations. From these simulations, the selected characterization parameters of maximum Nusselt number, average Nusselt number, and normalized pressure reduction were calculated. Comparisons of these parameters were made with available benchmark solutions for air with the ideal gas assumption at both low and high heat fluxes. Additionally, we generated body force, velocity, and divergence of velocity distributions to provide a basis for further analysis. The simulations and analysis were then extended to include helium at the Very High Temperature gas-cooled Reactor (VHTR) normal operating conditions. Our results show that the consequences of incorporating the incompressible flow assumption in high heat flux situations may lead to unrepresentative results. The results question the use of the incompressible flow assumption for simulating fluid flow in an operating nuclear reactor, where large temperature variations are present. The results show that the use of
Energy Technology Data Exchange (ETDEWEB)
Jiji, L.M. [City Univ. of New York, NY (United States). Dept. of Mechanical Engineering
2006-07-01
Professor Jiji's broad teaching experience lead him to select the topics for this book to provide a firm foundation for convection heat transfer with emphasis on fundamentals, physical phenomena, and mathematical modelling of a wide range of engineering applications. Reflecting recent developments, this textbook is the first to include an introduction to the challenging topic of microchannels. The strong pedagogic potential of Heat Convection is enhanced by the following ancillary materials: (1) Power Point lectures, (2) Problem Solutions, (3) Homework Facilitator, and, (4) Summary of Sections and Chapters. (orig.)
Zannouni, K.; El Abrach, H.; Dhahri, H.; Mhimid, A.
2017-06-01
The present paper reports a numerical study to investigate the drying of rectangular gypsum sample based on a diffusive model. Both vertical and low sides of the porous media are treated as adiabatic and impermeable surfaces plate. The upper face of the plate represents the permeable interface. The energy equation model is based on the local thermal equilibrium assumption between the fluid and the solid phases. The lattice Boltzmann method (LBM) is used for solving the governing differential equations system. The obtained numerical results concerning the moisture content and the temperature within a gypsum sample were discussed. A comprehensive analysis of the influence of the mass transfer coefficient, the convective heat transfer coefficient, the external temperature, the relative humidity and the diffusion coefficient on macroscopic fields are also investigated. They all presented results in this paper and obtained in the stable regime correspond to time superior than 4000 s. Therefore the numerical error is inferior to 2%. The experimental data and the descriptive information of the approach indicate an excellent agreement between the results of our developed numerical code based on the LBM and the published ones.
Yadollahi, A.; Khalesidoost, A.; Kasaeipoor, A.; Hatami, M.; Jing, D.
2017-08-01
The effects of a magnetic field on a free convection regime of silver-water nanofluid are investigated. The considered geometry is an F-shaped cavity under the influence of a constant magnetic field. The left vertical walls temperature is Th, while the middle and right walls are at a constant temperature Tc, and the other walls are insulated. A FORTRAN program is developed for the numerical simulation of the considered problem. The governing equations are solved using the FVM with the SIMPLE algorithm. The effect of important physical parameters such as the Rayleigh number, the Hartmann number, AR and φ on the problem are discussed in detail. We have concluded that the increase in the Hartmann number causes a decrease in vertical velocity and heat transfer. By increasing the Rayleigh number, the influence of the Hartmann number will be increased. An increase in the dimensional ratio of the cavity causes a decrease in the Nusselt number except in AR = 0.4. The AR has the maximum impact on the local Nusselt number, at the bottom of the hot wall. The effect of the dimensional ratio of the cavity on the Nusselt number is reversed on top of the wall. The maximum value of the Nusselt number is observed at AR = 0.4.
Prueitt, Melvin L.
1994-01-01
Convection towers which are capable of cleaning the pollution from large quantities of air and of generating electricity utilize the evaporation of water sprayed into the towers to create strong airflows and to remove pollution from the air. Turbines in tunnels at the skirt section of the towers generate electricity. Other embodiments may also provide fresh water, and operate in an updraft mode.
Directory of Open Access Journals (Sweden)
Soraia Vilela Borges
2008-12-01
Full Text Available A abóbora (Cucurbita moschata, L. é uma importante fonte de provitamina A, de baixo custo, e sob a forma desidratada oferece diferentes opções de utilização e consumo. Secagens por convecção natural e forçada foram comparadas quanto ao grau de secagem atingido e encolhimento, em função da temperatura, velocidade de ar e dimensões do produto. Os resultados obtidos mostraram que o uso de fatias com volume de 6,25 cm³ em secador por convecção forçada a 50 °C e à velocidade de 5,5 x 10-4 ms-1 resultaram em produtos de menor encolhimento, sendo recomendadas estas condições.Besides its low price, Pumpkin (Cucurbita moschata, L. is an important source of provitamin-A, and when dehydrated it offers different options of utilizations and consumption. Natural and forced convection drying were compared according to the drying degree shrinkage as a function of temperature, and air velocity and product dimensions. The obtained results showed that slices with the volume of 6.25 cm³ in forced convection oven at 50 °C and at the velocity of 5.5 x 10-4 ms-1 resulted in a lower shrinkage products, so these conditions were recommended.
Temperature Programming of the Second Dimension in Comprehensive Two-Dimensional Gas Chromatography.
Chow, Hei-Yin J; Górecki, Tadeusz
2017-08-15
Comprehensive two-dimensional gas chromatography (GC × GC) provides a significant increase in selectivity and peak capacity for the separation of complex mixtures. Optimization of the system is often complicated, with many interconnected parameters between the two dimensions and additional problems like peak wraparound that need to be eliminated or minimized. Wraparound peaks are compounds with retention times in the second dimension that are longer than the modulation period. This results in broad peaks that elute in subsequent modulation cycles, potentially coeluting with separated compounds. The use of a secondary oven is often the solution to the problem. By applying a constant positive temperature offset from the main oven temperature, the retention of all analytes can be reduced so that they elute within their respective modulation periods. However, this reduces the separation of less retained compounds, a classical consequence of the general elution problem due to the isothermal conditions during the limited separation time in the second dimension. To overcome this problem, the second dimension was temperature-programmed by resistively heating an electrically conductive secondary column using constant current. The column was cooled through forced convection inside the GC oven within the time frame of a single modulation period. Temperature programming in the second dimension of GC × GC was able to improve separation while eliminating wraparound peaks and reducing peak widths, leading to significantly increased second dimension peak capacity.
Parameterizing convective organization
Directory of Open Access Journals (Sweden)
Brian Earle Mapes
2011-06-01
Full Text Available Lateral mixing parameters in buoyancy-driven deep convection schemes are among the most sensitive and important unknowns in atmosphere models. Unfortunately, there is not a true optimum value for plume mixing rate, but rather a dilemma or tradeoff: Excessive dilution of updrafts leads to unstable stratification bias in the mean state, while inadequate dilution allows deep convection to occur too easily, causing poor space and time distributions and variability. In this too-small parameter space, compromises are made based on competing metrics of model performance. We attempt to escape this “entrainment dilemma” by making bulk plume parameters (chiefly entrainment rate depend on a new prognostic variable (“organization,” org meant to reflect the rectified effects of subgrid-scale structure in meteorological fields. We test an org scheme in the Community Atmosphere Model (CAM5 with a new unified shallow-deep convection scheme (UW-ens, a 2-plume version of the University of Washington scheme. Since buoyant ascent involves natural selection, subgrid structure makes convection systematically deeper and stronger than the pure unorganized case: plumes of average (or randomly sampled air rising in the average environment. To reflect this, org is nonnegative, but we leave it dimensionless. A time scale characterizes its behavior (here ∼3 h for a 2o model. Currently its source is rain evaporation, but other sources can be added easily. We also let org be horizontally transported by advection, as a mass-weighted mean over the convecting layer. Linear coefficients link org to a plume ensemble, which it assists via: 1 plume base warmth above the mean temperature 2 plume radius enhancement (reduced mixing, and 3 increased probability of overlap in a multi-plume scheme, where interactions benefit later generations (this part has only been implemented in an offline toy column model. Since rain evaporation is a source for org, it functions as a time
Directory of Open Access Journals (Sweden)
David Aquino da Costa
2017-07-01
Full Text Available Abstract The natural drying of in-shell Brazil nuts carried out by the extractivists is not effective in reducing contamination by aflatoxin-producing fungi. Thus the use of an artificial heater could prove to be a favourable method to bring about a rapid reduction in the moisture content of the nuts and thereby prevent fungal growth. Hence the objective of this study was to evaluate the efficiency of a natural convection-type drier with respect to the physical, physicochemical and microbiological quality of nuts after drying for 6 hours at 45 °C. A random block experimental design with two treatments (nuts before and after drying was used, using 10 replications of 3 kg. The nuts were analysed for their moisture, ash, protein, dietary fibre, total carbohydrates and lipid contents, water activity, total count of filamentous, potentially aflatoxin-producing fungi, and also the quantification of aflatoxins B1, B2, G1, G2 and the total aflatoxins. There was no effect of drying on the Aspergillus flavus and Aspergillus parasiticus counts or on the physicochemical composition of the nuts, except for the ash content. However the moisture content of the nuts was reduced by 39.7% and there was a decrease in the contamination by pre-existing total filamentous fungi. The dryer was effective in reducing the average time taken for drying as compared to the traditional method used by extractivists.
Entropy Generation Due to Natural Convection in a Partially Heated Cavity by Local RBF-DQ Method
DEFF Research Database (Denmark)
Soleimani, S.; Qajarjazi, A.; Bararnia, H.
2011-01-01
The Local Radial Basis Function-Differential Quadrature (RBF-DQ) method is applied to twodimensional incompressible Navier-Stokes equations in primitive form. Numerical results of heatlines and entropy generation due to heat transfer and fluid friction have been obtained for laminar natural...
Equilibrium adsorption data from temperature-programmed desorption measurements
Foeth, F.; Mugge, J.M.; van der Vaart, R.; van der Vaart, Rick; Bosch, H.; Reith, T.
1996-01-01
This work describes a novel method that enables the calculation of a series of adsorption isotherms basically from a single Temperature-Programmed Desorption (TPD) experiment. The basic idea is to saturate an adsorbent packed in a fixed bed at a certain feed concentration and temperature and to
Fast temperature programming in gas chromatography using resistive heating
Dallüge, J.; Ou-Aissa, R.; Vreuls, J.J.; Brinkman, U.A.T.; Veraart, J.R.
1999-01-01
The features of a resistive-heated capillary column for fast temperature-programmed gas chromatography (GC) have been evaluated. Experiments were carried out using a commercial available EZ Flash GC, an assembly which can be used to upgrade existing gas chromatographs. The capillary column is placed
Design and fabrication of an automated temperature programmed ...
Indian Academy of Sciences (India)
Unknown
*For correspondence. Design and fabrication of an automated temperature programmed reaction system to evaluate 3-way catalysts ... Since the design of the first desorption system by Cvetonovic and Amenomiya,1 .... tored by a software program through PC via RS232 interface. Typical heating rates range from 5°C min–1.
Temperature-programmed desorption of water and ammonia on ...
Indian Academy of Sciences (India)
Home; Journals; Journal of Chemical Sciences; Volume 115; Issue 4. Temperature-programmed desorption of water and ammonia on sulphated zirconia catalysts for ... Author Affiliations. Vasant R Choudhary1 Abhijeet J Karkamkar1. Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India ...
Design and fabrication of an automated temperature programmed ...
Indian Academy of Sciences (India)
A completely automated temperature-programmed reaction (TPR) system for carrying out gas-solid catalytic reactions under atmospheric flow conditions is fabricated to study CO and hydrocarbon oxidation, and NO reduction. The system consists of an all-stainless steel UHV system, quadrupole mass spectrometer SX200 ...
Temperature-programmed desorption of water and ammonia on ...
Indian Academy of Sciences (India)
Unknown
Abstract. Temperature-programmed desorption (TPD) of water and ammonia over. ZrO2 and sulphated ZrO2 prepared by different methods has been investigated for measuring strong acidity and acidity distribution on sulphated zirconia-type solid super-acid catalysts. The TPD of water provides a simple reliable method for ...
Hammond, R.P.; King, L.D.P.
1960-03-22
An homogeneous nuclear power reactor utilizing convection circulation of the liquid fuel is proposed. The reactor has an internal heat exchanger looated in the same pressure vessel as the critical assembly, thereby eliminating necessity for handling the hot liquid fuel outside the reactor pressure vessel during normal operation. The liquid fuel used in this reactor eliminates the necessity for extensive radiolytic gas rocombination apparatus, and the reactor is resiliently pressurized and, without any movable mechanical apparatus, automatically regulates itself to the condition of criticality during moderate variations in temperature snd pressure and shuts itself down as the pressure exceeds a predetermined safe operating value.
Energy Technology Data Exchange (ETDEWEB)
Tzanos, C.P.; Farmer, M.T.; Nuclear Engineering Division
2007-08-31
-normal operating conditions. The standpipes are headered (in groups of four in the prototype) to water supply (header) tanks that are situated well above the reactor vessel to facilitate natural convection cooling during a loss of forced flow event. During normal operations, the water is pumped from a heat sink located outside the containment to the headered inlets to the standpipes. The water is then delivered to each standpipe through a centrally located downcomer that passes the coolant to the bottom of each pipe. The water then turns 180{sup o} and rises up through the annular gap while extracting heat from the reactor cavity due to a combination of natural convection and radiation across the gap between the reactor vessel and standpipes. The water exits the standpipes at the top where it is headered (again in groups of four) into a return line that passes the coolant to the top of the header tank. Coolant is drawn from each tank through a fitting located near the top of the tank where it flows to the heat rejection system located outside the containment. This completes the flow circuit for normal operations. During off-normal conditions, forced convection water cooling in the RCCS is presumed to be lost, as well as the ultimate heat sink outside the containment. In this case, water is passively drawn from an open line located at the bottom of the header tank. This line is orificed so that flow bypass during normal operations is small, yet the line is large enough to provide adequate flow during passive operations to remove decay heat while maintaining acceptable fuel temperatures. In the passive operating mode, water flows by natural convection from the bottom of the supply tank to the standpipes, and returns through the normal pathway to the top of the tanks. After the water reaches saturation and boiling commences, steam will pass through the top of the tanks and be vented to atmosphere. In the experiment system shown in Fig. 4, a steam condensation and collection system is
Sarper, Bugra; Saglam, Mehmet; Aydin, Orhan; Avci, Mete
2017-10-01
In this study, natural convection in a vertical channel is studied experimentally and numerically. One of the channel walls is heated discretely by two flush-mounted heaters while the other is insulated. The effects of the clearance between the heaters on heat transfer and hot spot temperature while total length of the heaters keeps constant are investigated. Four different settlements of two discrete heaters are comparatively examined. Air is used as the working fluid. The range of the modified Grashof number covers the values between 9.6 × 105 and 1.53 × 10.7 Surface to surface radiation is taken into account. Flow visualizations and temperature measurements are performed in the experimental study. Numerical computations are performed using the commercial CFD code ANSYS FLUENT. The results are represented as the variations of surface temperature, hot spot temperature and Nusselt number with the modified Grashof number and the clearance between the heaters as well as velocity and temperature variations of the fluid.
National Convective Weather Diagnostic
National Oceanic and Atmospheric Administration, Department of Commerce — Current convective hazards identified by the National Convective Weather Detection algorithm. The National Convective Weather Diagnostic (NCWD) is an automatically...
Convection in complex shaped vessel; Convection dans des enceintes de forme complexe
Energy Technology Data Exchange (ETDEWEB)
NONE
2000-07-01
The 8 november 2000, the SFT (Societe Francaise de Thermique) organized a technical day on the convection in complex shaped vessels. Nine papers have been presented in the domains of the heat transfers, the natural convection, the fluid distribution, the thermosyphon effect, the steam flow in a sterilization cycle and the transformers cooling. Eight papers are analyzed in ETDE and one paper dealing with the natural convection in spent fuels depository is analyzed in INIS. (A.L.B.)
Guha, Abhijit; Nayek, Subhajit
2017-10-01
A compulsory element of all textbooks on natural convection has been a detailed similarity analysis for laminar natural convection on a heated semi-infinite vertical plate and a routinely used boundary condition for such analysis is u = 0 at x = 0. The same boundary condition continues to be assumed in related theoretical analyses, even in recent publications. The present work examines the consequence of this long-held assumption, which appears to have never been questioned in the literature, on the fluid dynamics and heat transfer characteristics. The assessment has been made here by solving the Navier-Stokes equations numerically with two boundary conditions—one with constrained velocity at x = 0 to mimic the similarity analysis and the other with no such constraints simulating the case of a heated vertical plate in an infinite expanse of the quiescent fluid medium. It is found that the fluid flow field given by the similarity theory is drastically different from that given by the computational fluid dynamics (CFD) simulations with unconstrained velocity. This also reflects on the Nusselt number, the prediction of the CFD simulations with unconstrained velocity being quite close to the experimentally measured values at all Grashof and Prandtl numbers (this is the first time theoretically computed values of the average Nusselt number N u ¯ are found to be so close to the experimental values). The difference of the Nusselt number (Δ N u ¯ ) predicted by the similarity theory and that by the CFD simulations (as well as the measured values), both computed with a high degree of precision, can be very significant, particularly at low Grashof numbers and at Prandtl numbers far removed from unity. Computations show that within the range of investigations (104 ≤ GrL ≤ 108, 0.01 ≤ Pr ≤ 100), the maximum value of Δ N u ¯ may be of the order 50%. Thus, for quantitative predictions, the available theory (i.e., similarity analysis) can be rather inadequate. With
Energy Technology Data Exchange (ETDEWEB)
Das, S.S. [Department of Physics, K B D A V College, Nirakarpur, Khurda-752 019 (Orissa) (India); Tripathy, R.K. [Department of Physics, D R Nayapalli College, Bhubaneswar-751 012 (Orissa) (India); Padhy, R.K. [Department of Physics, D A V Public School, Chandrasekharpur, Bhubaneswar-751 021 (Orissa) (India); Sahu, M. [Department of Physics, Jupiter +2 Women’s Science College, IRC Village, Bhubaneswar-751 015 (Orissa) (India)
2012-07-01
This paper theoretically investigates the combined natural convection and mass transfer effects on unsteady flow of a viscous incompressible fluid past an infinite vertical porous plate embedded in a porous medium with heat source. The governing equations of the flow field are solved analytically for velocity, temperature, concentration distribution, skin friction and the rate of heat transfer using multi parameter perturbation technique and the effects of the flow parameters such as permeability parameter Kp, Grashof number for heat and mass transfer Gr, Gc; heat source parameter S, Schmidt number Sc, Prandtl number Pr etc. on the flow field are analyzed and discussed with the help of figures and tables. The permeability parameter Kp is reported to accelerate the transient velocity of the flow field at all points for small values of Kp (£1) and for higher values the effect reverses. The effect of increasing Grashof numbers for heat and mass transfer or heat source parameter is to enhance the transient velocity of the flow field at all points while a growing Schmidt number retards its effect at all points. A growing permeability parameter or heat source parameter increases the transient temperature of the flow field at all points, while a growing Prandtl number shows reverse effect. The effect of increasing Schmidt number is to decrease the concentration boundary layer thickness of the flow field at all points. Further, a growing permeability parameter enhances the skin friction at the wall and a growing Prandtl number shows reverse effect. The effect of increasing Prandtl number or permeability parameter leads to increase the magnitude of the rate of heat transfer at the wall.
Directory of Open Access Journals (Sweden)
Fakher Oueslati
2013-12-01
Full Text Available Double-diffusive natural convection with entropy generation in a two-dimensional enclosure with partial vertical heating and salting sources for an aspect ratio Ar = 4 is investigated in the current study. A numerical methodology based on the finite volume method and a full multigrid technique is employed. The effects of various parameters; the Rayleigh number (103 ⩽ Ra ⩽ 2 x 105, the buoyancy ratio (−8 ⩽ N ⩽ 8, the source location (0 ⩽ η ⩽ 3, the Lewis number (10 ⩽ Le ⩽ 100, and, the source length (0.25 ⩽ d ⩽ 2 on the flow patterns are analyzed. Correlations of average Nusselt and Sherwood numbers are established as a function of two parameters (Ra, d and (Le, d, respectively. The numerical outcome of the present study shows that, by increasing the Lewis number in the range 10–100, the heat and mass transfer rates are significantly enhanced. In terms of irreversibility phenomena at the steady case, comparison of total entropy profiles with respect to N shows that irreversibility criterion is dominated by entropy generation due to fluid friction. Especial attention is attributed to the periodic flow behavior that appears for N in the range (0.87–0.99 outside of which it remains steady. According to the entropy generation phenomena, total entropy generation Stot and Bejan number Be were observed to oscillate with the same frequency but in opposing phases and with different amplitudes.
EXPERIMENTAL STUDY IN NATURAL CONVECTION
African Journals Online (AJOL)
PROF. BARTH EKWEME
climatic conditions are favorable for the development of solar chimneys for the production of electrical energy or the drying of agro-food products. 1.1 Experimental models. The first prototype tower was built in Manzanares in. 1981, thanks to the team of Professor Schlaich, Haaf et al., (1983); Haaf et al., (1984), in Spain.
EXPERIMENTAL STUDY IN NATURAL CONVECTION
African Journals Online (AJOL)
PROF. BARTH EKWEME
The study of thermal and ventilation parameters, obtained in a transient, laminar solar chimney of reduced dimensions, (1 < m <3) m with a square collector (side = 2m) is presented. Experimental measurements has been made to determine the temperature of the absorber and the fluid in the collector, it is shown that at the ...
McNamara, Allen; Li, Mingming; Garnero, Ed; Marin, Nicole
2017-04-01
Seismic observations of the lower mantle infer multiple scales of compositional heterogeneity. The largest-scale heterogeneity, observed in seismic tomography models, is in the form of large, nearly antipodal regions referred to as the Large Low Shear Velocity Provinces (LLSVPs). In contrast, diffracted wave and core-reflection precursor seismic studies reveal small-scale Ultra Low Velocity Zones (ULVZs) at the base of the mantle that are almost two orders of magnitude smaller than the LLSVPs. We hypothesize that ULVZs provide insight into the nature of LLSVPs, and the LLSVPs, in turn, provide clues to the nature of global-scale mantle convection and compositional state. However, both LLSVPs and ULVZs are observations, and it remains unclear what is causing them. Here, we examine several related questions to aid in understanding their cause and the dynamical processes associated with them. Can we use seismic observations of ULVZ locations to differentiate whether they are caused by compositional heterogeneity or simply partial melting in otherwise normal mantle? Can we use the map-view shape of ULVZs to tell us about lowermost mantle flow directions and the temporal stability of these flow directions? Can the cross-sectional morphology of ULVZs tell us something about the viscosity difference between LLSVPs and background mantle? We performed geodynamical experiments to help answer these questions. We find that ULVZs caused by compositional heterogeneity preferentially form patch-like shapes along the margins of LLSVPs. Rounded patches indicate regions with long-lived stable mantle flow patterns, and linear patches indicate changing mantle flow patterns. Typically, these ULVZ patches have an asymmetrical cross-sectional shape; however, if LLSVPs have a larger grain-size than background mantle, their increased diffusion creep viscosity will act to make them more symmetrical. Alternatively, ULVZs caused simply by partial melting of normal mantle are preferentially
Temperature-programmed desorption for membrane inlet mass spectrometry
DEFF Research Database (Denmark)
Ketola, R.A.; Grøn, C.; Lauritsen, F.R.
1998-01-01
We present a novel technique for analyzing volatile organic compounds in air samples using a solid adsorbent together with temperature-programmed desorption and subsequent detection by membrane inlet mass spectrometry (TPD-MIMS). The new system has the advantage of a fast separation of compounds...... to diffuse through the membrane into the mass spectrometer in a few seconds. In this fashion we could completely separate many similar volatile compounds, for example toluene from xylene and trichloroethene from tetrachloroethene. Typical detection limits were at low or sub-nanogram levels, the dynamic range...
National Convective Weather Forecast
National Oceanic and Atmospheric Administration, Department of Commerce — The NCWF is an automatically generated depiction of: (1) current convection and (2) extrapolated signficant current convection. It is a supplement to, but does NOT...
Directory of Open Access Journals (Sweden)
Soraia Vilela Borges
2011-04-01
Full Text Available The influence of variables as temperature, solid shape, temperature, branching and cultivar on drying of bananas were studied. Bananas from cv. Prata and D'água, on disk and cylindrical shape, blanched or not, were dehydrated in a tray dryer in natural convection at 40 and 70ºC. Drying behavior was analyzed by using a mathematical model. The exponential model showed good agreement to experimental data (r² 0.93 - 0.99 and standard error: 0.01- 0.05. Temperature presented positive influence on drying rate in all the tested conditions. With respect to the sample shape, the disk shape carried out to significantly higher drying rates only for D'água cultivar without blanching. Blanching was significantly influent, with positive influence, on drying rate of banana cv. Prata at 40º C. The influence of the cultivar did not present a defined tendency.No presente trabalho, estudou-se a influência de variáveis como temperatura, formato do sólido, branqueamento e cultivar na secagem de bananas. Bananas cultivares Prata e D'água, nos formatos disco e cilindro, branqueadas ou não, foram desidratadas em secador de bandejas com convecção natural nas temperaturas de 40 e 70ºC. O comportamento da secagem foi analisado utilizandose modelagem matemática. O modelo exponencial mostrou bom ajuste aos resultados experimentais (r²: 0,93 - 0,99 e erro de estimativa padrão: 0,01 - 0,04. A temperatura apresentou influência positiva na taxa de secagem em todas as condições testadas. Com relação ao formato das amostras, o formato disco conduziu a taxas de secagem significativamente maiores somente para o cultivar D'água e sem branqueamento. O branqueamento se mostrou significativamente influente, com influência positiva na taxa de secagem da banana Prata a 40º C. A influência da cultivar não apresentou uma tendência definida.
CSIR Research Space (South Africa)
Grobler, Carla
2015-07-01
Full Text Available stream_source_info Grobler_2015.pdf.txt stream_content_type text/plain stream_size 23883 Content-Encoding UTF-8 stream_name Grobler_2015.pdf.txt Content-Type text/plain; charset=UTF-8 EXPERIMENTAL STUDY ON CAVITY FLOW.... THEORY Analytical Prediction for Heat Transfer Different heat transfer regimes have been defined for nat- ural convection in a 2D porous rectangular cavity which is heated from the sides. Figure 1 shows under which circum- stances each of these heat...
Using Jupiter's gravitational field to probe the Jovian convective dynamo.
Kong, Dali; Zhang, Keke; Schubert, Gerald
2016-03-23
Convective motion in the deep metallic hydrogen region of Jupiter is believed to generate its magnetic field, the strongest in the solar system. The amplitude, structure and depth of the convective motion are unknown. A promising way of probing the Jovian convective dynamo is to measure its effect on the external gravitational field, a task to be soon undertaken by the Juno spacecraft. We calculate the gravitational signature of non-axisymmetric convective motion in the Jovian metallic hydrogen region and show that with sufficiently accurate measurements it can reveal the nature of the deep convection.
Internal Wave Generation by Convection
Lecoanet, Daniel Michael
In nature, it is not unusual to find stably stratified fluid adjacent to convectively unstable fluid. This can occur in the Earth's atmosphere, where the troposphere is convective and the stratosphere is stably stratified; in lakes, where surface solar heating can drive convection above stably stratified fresh water; in the oceans, where geothermal heating can drive convection near the ocean floor, but the water above is stably stratified due to salinity gradients; possible in the Earth's liquid core, where gradients in thermal conductivity and composition diffusivities maybe lead to different layers of stable or unstable liquid metal; and, in stars, as most stars contain at least one convective and at least one radiative (stably stratified) zone. Internal waves propagate in stably stratified fluids. The characterization of the internal waves generated by convection is an open problem in geophysical and astrophysical fluid dynamics. Internal waves can play a dynamically important role via nonlocal transport. Momentum transport by convectively excited internal waves is thought to generate the quasi-biennial oscillation of zonal wind in the equatorial stratosphere, an important physical phenomenon used to calibrate global climate models. Angular momentum transport by convectively excited internal waves may play a crucial role in setting the initial rotation rates of neutron stars. In the last year of life of a massive star, convectively excited internal waves may transport even energy to the surface layers to unbind them, launching a wind. In each of these cases, internal waves are able to transport some quantity--momentum, angular momentum, energy--across large, stable buoyancy gradients. Thus, internal waves represent an important, if unusual, transport mechanism. This thesis advances our understanding of internal wave generation by convection. Chapter 2 provides an underlying theoretical framework to study this problem. It describes a detailed calculation of the
Bednarz, Tomasz Piotr; Lei, Chengwang; Patterson, John C.
2009-07-01
The present experimental investigation is concerned with the transient flow response in a reservoir model to periodic heating and cooling at the water surface. The experiment reveals a stable stratification of the water body during the heating phase and an unsteady mixing flow in the reservoir during the cooling phase. It is shown that thermal instabilities play an important role in breaking up the residual circulation and initiating a reverse flow circulation in deep waters after the switch of thermal forcing from heating to cooling. Moreover, the heating from the water surface results in a stable large-scale convective roll that is clearly observed in the experiment. The present flow visualization is carried out with the application of thermo-chromic liquid crystals. Quantitative temperature and velocity fields are extracted using Particle Image Thermometry and Particle Image Velocimetry techniques. Understanding of the flow mechanisms pertinent to this problem is important for predicting the transport of nutrients and pollutants across reservoirs.
Stochastic Convection Parameterizations
Teixeira, Joao; Reynolds, Carolyn; Suselj, Kay; Matheou, Georgios
2012-01-01
computational fluid dynamics, radiation, clouds, turbulence, convection, gravity waves, surface interaction, radiation interaction, cloud and aerosol microphysics, complexity (vegetation, biogeochemistry, radiation versus turbulence/convection stochastic approach, non-linearities, Monte Carlo, high resolutions, large-Eddy Simulations, cloud structure, plumes, saturation in tropics, forecasting, parameterizations, stochastic, radiation-clod interaction, hurricane forecasts
Joseph J. Charney; Brian E. Potter
2017-01-01
Convection and downbursts are connected meteorological phenomena with the potential to affect fire behavior and thereby alter the evolution of a wildland fire. Meteorological phenomena related to convection and downbursts are often discussed in the context of fire behavior and smoke. The physical mechanisms that contribute to these phenomena are interrelated, but the...
Gelfgat, Alexander Yu.
2017-08-01
Transition from steady to oscillatory buoyancy convection of air in a laterally heated cubic box is studied numerically by straight-forward time integration of Boussinesq equations using a series of gradually refined finite volume grids. Horizontal and spanwise cube boundaries are assumed to be either perfectly thermally conducting or perfectly thermally insulated, which results in four different sets of thermal boundary conditions. Critical Grashof numbers are obtained by interpolation of numerically extracted growth/decay rates of oscillation amplitude to zero. Slightly supercritical flow regimes are described by time-averaged flows, snapshots, and spatial distribution of the oscillation amplitude. Possible similarities and dissimilarities with two-dimensional instabilities in laterally heated square cavities are discussed. Break of symmetries and sub- or supercritical character of bifurcations are examined. Three consequent transitions from steady to the oscillatory regime, from the oscillatory to the steady regime, and finally to the oscillatory flow, are found in the case of perfectly insulated horizontal and spanwise boundaries. Arguments for grid and time-step independence of the results are given.
Directory of Open Access Journals (Sweden)
Nordlund Åke
2009-04-01
Full Text Available We review the properties of solar convection that are directly observable at the solar surface, and discuss the relevant underlying physics, concentrating mostly on a range of depths from the temperature minimum down to about 20 Mm below the visible solar surface.The properties of convection at the main energy carrying (granular scales are tightly constrained by observations, in particular by the detailed shapes of photospheric spectral lines and the topology (time- and length-scales, flow velocities, etc. of the up- and downflows. Current supercomputer models match these constraints very closely, which lends credence to the models, and allows robust conclusions to be drawn from analysis of the model properties.At larger scales the properties of the convective velocity field at the solar surface are strongly influenced by constraints from mass conservation, with amplitudes of larger scale horizontal motions decreasing roughly in inverse proportion to the scale of the motion. To a large extent, the apparent presence of distinct (meso- and supergranulation scales is a result of the folding of this spectrum with the effective “filters” corresponding to various observational techniques. Convective motions on successively larger scales advect patterns created by convection on smaller scales; this includes patterns of magnetic field, which thus have an approximately self-similar structure at scales larger than granulation.Radiative-hydrodynamical simulations of solar surface convection can be used as 2D/3D time-dependent models of the solar atmosphere to predict the emergent spectrum. In general, the resulting detailed spectral line profiles agree spectacularly well with observations without invoking any micro- and macroturbulence parameters due to the presence of convective velocities and atmosphere inhomogeneities. One of the most noteworthy results has been a significant reduction in recent years in the derived solar C, N, and O abundances with
Observing Convective Aggregation
Holloway, Christopher E.; Wing, Allison A.; Bony, Sandrine; Muller, Caroline; Masunaga, Hirohiko; L'Ecuyer, Tristan S.; Turner, David D.; Zuidema, Paquita
2017-11-01
Convective self-aggregation, the spontaneous organization of initially scattered convection into isolated convective clusters despite spatially homogeneous boundary conditions and forcing, was first recognized and studied in idealized numerical simulations. While there is a rich history of observational work on convective clustering and organization, there have been only a few studies that have analyzed observations to look specifically for processes related to self-aggregation in models. Here we review observational work in both of these categories and motivate the need for more of this work. We acknowledge that self-aggregation may appear to be far-removed from observed convective organization in terms of time scales, initial conditions, initiation processes, and mean state extremes, but we argue that these differences vary greatly across the diverse range of model simulations in the literature and that these comparisons are already offering important insights into real tropical phenomena. Some preliminary new findings are presented, including results showing that a self-aggregation simulation with square geometry has too broad distribution of humidity and is too dry in the driest regions when compared with radiosonde records from Nauru, while an elongated channel simulation has realistic representations of atmospheric humidity and its variability. We discuss recent work increasing our understanding of how organized convection and climate change may interact, and how model discrepancies related to this question are prompting interest in observational comparisons. We also propose possible future directions for observational work related to convective aggregation, including novel satellite approaches and a ground-based observational network.
Stein, Robert F
2012-07-13
Convection is the transport of energy by bulk mass motions. Magnetic fields alter convection via the Lorentz force, while convection moves the fields via the curl(v×B) term in the induction equation. Recent ground-based and satellite telescopes have increased our knowledge of the solar magnetic fields on a wide range of spatial and temporal scales. Magneto-convection modelling has also greatly improved recently as computers become more powerful. Three-dimensional simulations with radiative transfer and non-ideal equations of state are being performed. Flux emergence from the convection zone through the visible surface (and into the chromosphere and corona) has been modelled. Local, convectively driven dynamo action has been studied. The alteration in the appearance of granules and the formation of pores and sunspots has been investigated. Magneto-convection calculations have improved our ability to interpret solar observations, especially the inversion of Stokes spectra to obtain the magnetic field and the use of helioseismology to determine the subsurface structure of the Sun.
Observing Convective Aggregation
Holloway, Christopher E.; Wing, Allison A.; Bony, Sandrine; Muller, Caroline; Masunaga, Hirohiko; L'Ecuyer, Tristan S.; Turner, David D.; Zuidema, Paquita
2017-06-01
Convective self-aggregation, the spontaneous organization of initially scattered convection into isolated convective clusters despite spatially homogeneous boundary conditions and forcing, was first recognized and studied in idealized numerical simulations. While there is a rich history of observational work on convective clustering and organization, there have been only a few studies that have analyzed observations to look specifically for processes related to self-aggregation in models. Here we review observational work in both of these categories and motivate the need for more of this work. We acknowledge that self-aggregation may appear to be far-removed from observed convective organization in terms of time scales, initial conditions, initiation processes, and mean state extremes, but we argue that these differences vary greatly across the diverse range of model simulations in the literature and that these comparisons are already offering important insights into real tropical phenomena. Some preliminary new findings are presented, including results showing that a self-aggregation simulation with square geometry has too broad distribution of humidity and is too dry in the driest regions when compared with radiosonde records from Nauru, while an elongated channel simulation has realistic representations of atmospheric humidity and its variability. We discuss recent work increasing our understanding of how organized convection and climate change may interact, and how model discrepancies related to this question are prompting interest in observational comparisons. We also propose possible future directions for observational work related to convective aggregation, including novel satellite approaches and a ground-based observational network.
Topology Optimisation for Coupled Convection Problems
DEFF Research Database (Denmark)
Alexandersen, Joe
This thesis deals with topology optimisation for coupled convection problems. The aim is to extend and apply topology optimisation to steady-state conjugate heat transfer problems, where the heat conduction equation governs the heat transfer in a solid and is coupled to thermal transport in a sur......This thesis deals with topology optimisation for coupled convection problems. The aim is to extend and apply topology optimisation to steady-state conjugate heat transfer problems, where the heat conduction equation governs the heat transfer in a solid and is coupled to thermal transport...... in a surrounding uid, governed by a convection-diffusion equation, where the convective velocity field is found from solving the isothermal incompressible steady-state Navier-Stokes equations. Topology optimisation is also applied to steady-state natural convection problems. The modelling is done using stabilised...... finite elements, the formulation and implementation of which was done partly during a special course as prepatory work for this thesis. The formulation is extended with a Brinkman friction term in order to facilitate the topology optimisation of fluid flow and convective cooling problems. The derived...
Competing kinematic dynamo mechanisms in rotating convection with shear
Proctor, Michael R. E.; Hughes, David W.
2011-08-01
Following earlier work by Hughes & Proctor (2009) on the role of velocity shear in convectively driven dynamos, we present preliminary results on the nature of dynamo action due to modified flows derived by filtration from the full convective flow. The results suggest that filtering the flow fields has surprisingly little effect on the dynamo growth rates.
Measurements of convective and radiative heating in wildland fires
David Frankman; Brent W. Webb; Bret W. Butler; Daniel Jimenez; Jason M. Forthofer; Paul Sopko; Kyle S. Shannon; J. Kevin Hiers; Roger D. Ottmar
2012-01-01
Time-resolved irradiance and convective heating and cooling of fast-response thermopile sensors were measured in 13 natural and prescribed wildland fires under a variety of fuel and ambient conditions. It was shown that a sensor exposed to the fire environment was subject to rapid fluctuations of convective transfer whereas irradiance measured by a windowed sensor was...
Kakac, Sadik; Pramuanjaroenkij, Anchasa
2014-01-01
Intended for readers who have taken a basic heat transfer course and have a basic knowledge of thermodynamics, heat transfer, fluid mechanics, and differential equations, Convective Heat Transfer, Third Edition provides an overview of phenomenological convective heat transfer. This book combines applications of engineering with the basic concepts of convection. It offers a clear and balanced presentation of essential topics using both traditional and numerical methods. The text addresses emerging science and technology matters, and highlights biomedical applications and energy technologies. What’s New in the Third Edition: Includes updated chapters and two new chapters on heat transfer in microchannels and heat transfer with nanofluids Expands problem sets and introduces new correlations and solved examples Provides more coverage of numerical/computer methods The third edition details the new research areas of heat transfer in microchannels and the enhancement of convective heat transfer with nanofluids....
Directory of Open Access Journals (Sweden)
Prasad Ramachandra V.
2014-01-01
Full Text Available In the present paper we examine the effects of viscous dissipation, Joule heating and heat source/sink on non-Darcy MHD natural convection heat transfer flow over permeable horizontal circular cylinder in a porous medium. The boundary layer equations, which are parabolic in nature, are normalized into non-similar form and then solved numerically with the well-tested, efficient, implicit, stable Keller-box finite difference scheme. A parametric study illustrating the influence of Darcy parameter (Da, Forchheimer parameter (Λ, Grashof number(Gr, heat source/sink parameter (Ω and viscous dissipation parameter (Ec on the fluid velocity, temperature as well as local skin-friction and Nusselt numbers is conducted Increasing Forchheimer inertial drag parameter (Λ retards the flow considerably but enhances temperatures. Increasing viscous dissipation parameter(Ec is found to elevate velocities i.e. accelerate the flow and increase temperatures. Increasing heat source/sink parameter (Ω is found to elevate velocities and increase temperatures. Increasing the Grashof number (Gr is found to elevate the velocity and decrease the temperatures. Local skin friction number is found to be increases with increasing heat source/sink parameter (Ω where as Local Nusselt number is found to decrease with increasing heat source/sink parameter (Ω.
Belachsen, Idit; Marra, Francesco; Peleg, Nadav; Morin, Efrat
2017-04-01
Space-time patterns of rainfall are important climatic characteristics that influence runoff generation and flash flood magnitude. Their derivation requires high-resolution measurements to adequately represent the rainfall distribution, and is best provided by remote sensing tools. This need is further emphasized in dry climate regions, where rainfall is scarce and, often, local and highly variable. Our research is focused on understanding the nature of rainfall events in the dry Dead Sea region (Eastern Mediterranean) by identifying and characterizing the spatial structure and the dynamics of convective storm cores (known as rain cells). To do so, we take advantage of 25 years of corrected and gauge-adjusted weather radar data. A statistical analysis of convective rain-cells spatial and temporal characteristics was performed with respect to synoptic pattern, geographical location, and flash flood generation. Rain cells were extracted from radar data using a cell segmentation method and a tracking algorithm and were divided into rain events. A total of 10,500 rain cells, 2650 cell tracks and 424 rain events were elicited. Rain cell properties, such as mean areal and maximal rain intensity, area, life span, direction and speed, were derived. Rain events were clustered, according to several ERA-Interim atmospheric parameters, and associated with three main synoptic patterns: Cyprus Low, Low to the East of the study region and Active Red Sea Trough. The first two originate from the Mediterranean Sea, while the third is an extension of the African monsoon. On average, the convective rain cells in the region are 90 km2 in size, moving from West to East in 13 ms-1 and living 18 minutes. Several significant differences between rain cells of the various synoptic types were observed. In particular, Active Red Sea Trough rain cells are characterized by higher rain intensities and lower speeds, suggesting a higher flooding potential for small catchments. The north
Phenomenology of turbulent convection
Verma, Mahendra; Chatterjee, Anando; Kumar, Abhishek; Samtaney, Ravi
2016-11-01
We simulate Rayleigh-Bénard convection (RBC) in which a fluid is confined between two thermally conducting plates. We report results from direct numerical simulation (DNS) of RBC turbulence on 40963 grid, the highest resolution hitherto reported, on 65536 cores of Cray XC40, Shaheen II, at KAUST. The non-dimensional parameters of our simulation are: the Rayleigh number Ra = 1 . 1 ×1011 (the highest ever for a pseudo-spectral simulation) and Prandtl number of unity. We present energy flux diagnostics of shell-to-shell (in wave number space) transfer. Furthermore, noting that convective flows are anisotropic due to buoyancy, we quantify anisotropy by subdividing each wavenumber shell into rings and quantify ring energy spectrum. An outstanding question in convective turbulence is the wavenumber scaling of the energy spectrum. Our pseudo-spectral simulations of turbulent thermal convection coupled with novel energy transfer diagnostics have provided a definitive answer to this question. We conclude that convective turbulence exhibits behavior similar to fluid turbulence, that is, Kolmogorov's k - 5 / 3 spectrum with forward and local energy transfers, along with a nearly isotropic energy distribution. The supercomputer Shaheen at KAUST was utilized for the simulations.
Anomalously Weak Solar Convection
Hanasoge, Shravan M.; Duvall, Thomas L.; Sreenivasan, Katepalli R.
2012-01-01
Convection in the solar interior is thought to comprise structures on a spectrum of scales. This conclusion emerges from phenomenological studies and numerical simulations, though neither covers the proper range of dynamical parameters of solar convection. Here, we analyze observations of the wavefield in the solar photosphere using techniques of time-distance helioseismology to image flows in the solar interior. We downsample and synthesize 900 billion wavefield observations to produce 3 billion cross-correlations, which we average and fit, measuring 5 million wave travel times. Using these travel times, we deduce the underlying flow systems and study their statistics to bound convective velocity magnitudes in the solar interior, as a function of depth and spherical- harmonic degree l..Within the wavenumber band l convective velocities are 20-100 times weaker than current theoretical estimates. This constraint suggests the prevalence of a different paradigm of turbulence from that predicted by existing models, prompting the question: what mechanism transports the heat flux of a solar luminosity outwards? Advection is dominated by Coriolis forces for wavenumbers l convection may be quasi-geostrophic. The fact that isorotation contours in the Sun are not coaligned with the axis of rotation suggests the presence of a latitudinal entropy gradient.
Simulating deep convection with a shallow convection scheme
Directory of Open Access Journals (Sweden)
C. Hohenegger
2011-10-01
Full Text Available Convective processes profoundly affect the global water and energy balance of our planet but remain a challenge for global climate modeling. Here we develop and investigate the suitability of a unified convection scheme, capable of handling both shallow and deep convection, to simulate cases of tropical oceanic convection, mid-latitude continental convection, and maritime shallow convection. To that aim, we employ large-eddy simulations (LES as a benchmark to test and refine a unified convection scheme implemented in the Single-column Community Atmosphere Model (SCAM. Our approach is motivated by previous cloud-resolving modeling studies, which have documented the gradual transition between shallow and deep convection and its possible importance for the simulated precipitation diurnal cycle.
Analysis of the LES reveals that differences between shallow and deep convection, regarding cloud-base properties as well as entrainment/detrainment rates, can be related to the evaporation of precipitation. Parameterizing such effects and accordingly modifying the University of Washington shallow convection scheme, it is found that the new unified scheme can represent both shallow and deep convection as well as tropical and mid-latitude continental convection. Compared to the default SCAM version, the new scheme especially improves relative humidity, cloud cover and mass flux profiles. The new unified scheme also removes the well-known too early onset and peak of convective precipitation over mid-latitude continental areas.
Intermittent flow regimes near the convection threshold in ferromagnetic nanofluids.
Krauzina, Marina T; Bozhko, Alexandra A; Putin, Gennady F; Suslov, Sergey A
2015-01-01
The onset and decay of convection in a spherical cavity filled with ferromagnetic nanofluid and heated from below are investigated experimentally. It is found that, unlike in a single-component Newtonian fluid where stationary convection sets in as a result of supercritical bifurcation and where convection intensity increases continuously with the degree of supercriticality, convection in a multicomponent ferromagnetic nanofluid starts abruptly and has an oscillatory nature. The hysteresis is observed in the transition between conduction and convection states. In moderately supercritical regimes, the arising fluid motion observed at a fixed temperature difference intermittently transitions from quasiharmonic to essentially irregular oscillations that are followed by periods of a quasistationary convection. The observed oscillations are shown to result from the precession of the axis of a convection vortex in the equatorial plane. When the vertical temperature difference exceeds the convection onset value by a factor of 2.5, the initially oscillatory convection settles to a steady-state regime with no intermittent behavior detected afterward. The performed wavelet and Fourier analyses of thermocouple readings indicate the presence of various oscillatory modes with characteristic periods ranging from one hour to several days.
Infrared thermography for convective heat transfer measurements
Energy Technology Data Exchange (ETDEWEB)
Carlomagno, Giovanni Maria; Cardone, Gennaro [University of Naples Federico II, Department of Aerospace Engineering, Naples (Italy)
2010-12-15
This paper deals with the evolution of infrared (IR) thermography into a powerful optical tool that can be used in complex fluid flows to either evaluate wall convective heat fluxes or investigate the surface flow field behavior. Measurement of convective heat fluxes must be performed by means of a thermal sensor, where temperatures have to be measured with proper transducers. By correctly choosing the thermal sensor, IR thermography can be successfully exploited to resolve convective heat flux distributions with both steady and transient techniques. When comparing it to standard transducers, the IR camera appears very valuable because it is non-intrusive, it has a high sensitivity (down to 20 mK), it has a low response time (down to 20 {mu}s), it is fully two dimensional (from 80 k up to 1 M pixels, at 50 Hz) and, therefore, it allows for better evaluation of errors due to tangential conduction within the sensor. This paper analyses the capability of IR thermography to perform convective heat transfer measurements and surface visualizations in complex fluid flows. In particular, it includes the following: the necessary radiation theory background, a review of the main IR camera features, a description of the pertinent heat flux sensors, an analysis of the IR image processing methods and a report on some applications to complex fluid flows, ranging from natural convection to hypersonic regime. (orig.)
Convection and stellar oscillations
DEFF Research Database (Denmark)
Aarslev, Magnus Johan
2017-01-01
of stars. For stars like the sun, energy transport in the outer layers occurs mainly through turbulent convection. Here, pressure mode oscillations are essentially propagating sound waves, whose properties can be altered by interaction with the turbulent motion of the gas. This has always been a problem...... for asteroseismology, because of the challenges inherent in modelling turbulent convection in 1D stellar models. As a result of oversimplifying the physics near the surface, theoretical calculations systematically overestimate the oscillation frequencies. This has become known as the asteroseismic surface effect. Due...... to lacking better options, this frequency difference is typically corrected for with ad-hoc formulae. The topic of this thesis is the improvement of 1D stellar convection models and the effects this has on asteroseismic properties. The source of improvements is 3D simulations of radiation...
Energy Technology Data Exchange (ETDEWEB)
Rocha, Alan Carlos Bueno da
1997-07-01
A heat transfer (condenser) of a domestic freezer was tested in a vertical channel in order to study the influence of the chimney effect in the optimization of the heat transfer coefficient. The variation of the opening of the channel, position and the heating power of the heat exchanger in the heat transfer coefficient was considered. The influence of the surface emissivity on the heat transfer by thermal radiation was studied with the heat exchanger testes without paint and with black paint. The air velocity entering the channel was measured with a hot wire anemometer. In order to evaluate the chimney effect, the heat exchanger was testes in a open ambient. This situation simulates its operational conditions when installed on the freezer system. The variables collected in the experimental procedures was gathered in the form of dimensionless parameters as Nusselt, Rayleigh, Grashof and Prandtl numbers, and dimensional parameters of the convection. The results showed that the highest heat transfer value occurred when both a specific position and a specific channel opening were used. The experiments pointed out that the radiation contribution must be considered in heat transfer calculations. The conclusions showed that different channel openings can improve the heat transfer coefficient in this heat transfer exchanger. (author)
CDM Convective Forecast Planning guidance
National Oceanic and Atmospheric Administration, Department of Commerce — The CDM Convective Forecast Planning (CCFP) guidance product provides a foreast of en-route aviation convective hazards. The forecasts are updated every 2 hours and...
Two-dimensional turbulent convection
Mazzino, Andrea
2017-11-01
We present an overview of the most relevant, and sometimes contrasting, theoretical approaches to Rayleigh-Taylor and mean-gradient-forced Rayleigh-Bénard two-dimensional turbulence together with numerical and experimental evidences for their support. The main aim of this overview is to emphasize that, despite the different character of these two systems, especially in relation to their steadiness/unsteadiness, turbulent fluctuations are well described by the same scaling relationships originated from the Bolgiano balance. The latter states that inertial terms and buoyancy terms balance at small scales giving rise to an inverse kinetic energy cascade. The main difference with respect to the inverse energy cascade in hydrodynamic turbulence [R. H. Kraichnan, "Inertial ranges in two-dimensional turbulence," Phys. Fluids 10, 1417 (1967)] is that the rate of cascade of kinetic energy here is not constant along the inertial range of scales. Thanks to the absence of physical boundaries, the two systems here investigated turned out to be a natural physical realization of the Kraichnan scaling regime hitherto associated with the elusive "ultimate state of thermal convection" [R. H. Kraichnan, "Turbulent thermal convection at arbitrary Prandtl number," Phys. Fluids 5, 1374-1389 (1962)].
Sustained shear flows in Rayleigh-Bénard convection
Quist, Tayler; Anders, Evan; Brown, Benjamin; Oishi, Jeffrey
2016-11-01
Zonal shear flows play important roles in both the solar and geo dynamos. In two dimensional simulations, and at relatively narrow aspect ratios, Rayleigh-Bénard convection naturally achieves zonal shear flows. These zonal flows are driven by the convection and modify it, significantly altering the heat transport and convective structures. Here we study shear flows in two and three-dimensional simulations of Rayleigh-Bénard convection using the Dedalus pseudospectral framework. At small aspect ratios and at Prandtl number 1, a large horizontal shear naturally occurs. At larger aspect ratios, we find that shearing is naturally prevented unless manually induced; there is a bistability between states dominated by "flywheel" modes and states dominated by large scale shear. We explore these states and the possibilities of sustained large scale shear in 3-D simulations.
Response of reverse convection to fast IMF transitions
DEFF Research Database (Denmark)
Taguchi, S.; Tawara, A.; Hairston, M. R.
2015-01-01
The nature of the transition that high-latitude reverse convection makes in response to fast interplanetary magnetic field (IMF) changes is investigated using observations from multiple spacecraft and a ground magnetometer array. We focused on two fast IMF-transition events on 22 April 2006......-aligned current or reverse convection, was in a transition state for about 10 min. For the second event, the Greenland magnetic perturbations also showed that a transition state occurred in the near-noon sector for 10-15 min. Three DMSP spacecraft that traversed the polar cap provided evidence showing...... convection region to the near-noon sector at the speed of 0.5-1 km s-1, which is coupled with the moving away of the old convection region at a similar speed. For the higher-latitude sunward flow region, it is found that the convection takes a transition state almost simultaneously (within 1 min...
From convection rolls to finger convection in double-diffusive turbulence
Yang, Yantao; Verzicco, Roberto; Lohse, Detlef
2015-01-01
Double-diffusive convection (DDC), which is the buoyancy-driven flow with fluid density depending on two scalar components, is ubiquitous in many natural and engineering environments. Of great interests are scalars’ transfer rate and flow structures. Here we systematically investigate DDC flow
Natural convection in cavities containing internal sources
Barozzi, G. S.; Corticelli, M. A.
Numerical predictions are reported for buoyancy-induced circulations in a 2D closed cavity with internal heat sources. Two cases are considered: (A) two vertical plates with uniform heat generation, forming a short vertical channel within the enclosure; and, (B) a rectangular heating block with uniform wall temperature, placed central in the enclosure. Air, with a Prandtl number 0.71, is considered as the working fluid. The vertical enclosure walls are isothermal, while the horizontal enclosure walls are adiabatic. Results are presented for two values of the Grashof number, one below the stability limit for laminar flow, and one well above it. In those latter cases, the long-term behaviour of the numerical solution is time-dependent, i.e. no steady-state can be reached. Heat transfer results are compared with predictions from standard correlations for isolated surfaces.
Using Jupiter’s gravitational field to probe the Jovian convective dynamo
Kong, Dali; Zhang, Keke; Schubert, Gerald
2016-01-01
Convective motion in the deep metallic hydrogen region of Jupiter is believed to generate its magnetic field, the strongest in the solar system. The amplitude, structure and depth of the convective motion are unknown. A promising way of probing the Jovian convective dynamo is to measure its effect on the external gravitational field, a task to be soon undertaken by the Juno spacecraft. We calculate the gravitational signature of non-axisymmetric convective motion in the Jovian metallic hydrogen region and show that with sufficiently accurate measurements it can reveal the nature of the deep convection. PMID:27005472
Mixed thermal convection: fundamental issues and analysis of the planar case
Directory of Open Access Journals (Sweden)
JACQUES PADET
2015-09-01
Full Text Available This paper aims to renew interest on mixed thermal convection research and to emphasize three issues that arise from the present analysis: (i a clear definition of the reference temperature in the Boussinesq approximation; (ii a practical delimitation of the three convective modes, which are the forced convection (FC, mixed convection (MC and natural (or free convection (NC; (iii and, finally, a uniform description of the set FC/MC/NC in the similarity framework. The planar case, for which analytical solutions are available, allows a detailed illustration of the answers here advanced to the above issues.
Thermal Convection and Emergence of Isolated Vortices in Soap Bubbles
Seychelles, F.; Amarouchene, Y.; Bessafi, M.; Kellay, H.
2008-04-01
A novel thermal convection cell consisting of half a soap bubble heated at the equator is introduced to study thermal convection and the movement of isolated vortices. The soap bubble, subject to stratification, develops thermal convection at its equator. A particular feature of this cell is the emergence of isolated vortices. These vortices resemble hurricanes or cyclones and similarities between our observed structures and these natural objects are found. This is brought forth through a study of the mean square displacement of these objects showing signs of superdiffusion.
Thermal convection and emergence of isolated vortices in soap bubbles.
Seychelles, F; Amarouchene, Y; Bessafi, M; Kellay, H
2008-04-11
A novel thermal convection cell consisting of half a soap bubble heated at the equator is introduced to study thermal convection and the movement of isolated vortices. The soap bubble, subject to stratification, develops thermal convection at its equator. A particular feature of this cell is the emergence of isolated vortices. These vortices resemble hurricanes or cyclones and similarities between our observed structures and these natural objects are found. This is brought forth through a study of the mean square displacement of these objects showing signs of superdiffusion.
Bejan, Adrian
2013-01-01
Written by an internationally recognized authority on heat transfer and thermodynamics, this second edition of Convection Heat Transfer contains new and updated problems and examples reflecting real-world research and applications, including heat exchanger design. Teaching not only structure but also technique, the book begins with the simplest problem solving method (scale analysis), and moves on to progressively more advanced and exact methods (integral method, self similarity, asymptotic behavior). A solutions manual is available for all problems and exercises.
Nonlinear Interaction of Convective Cells in Plasmas
DEFF Research Database (Denmark)
Pécseli, Hans; Juul Rasmussen, Jens; Thomsen, Kenneth
1985-01-01
The nonlinear interaction of externally excited convective cells was investigated experimentally. Two cells of the same polarity coalesced into one large cell provided their relative distance was sufficiently short, while cells of opposite polarity interacted through a mutual perturbation of orbi...... only. The nonlinear nature of the coalescence was explicitly demonstrated. The implications of the observations for interpreting the cascade in a turbulent spectrum in two-dimensional systems are pointed out....
Rossby numbers of fully convective and partially convective stars
Landin, Natália R.; Mendes, Luiz T. S.
2017-10-01
In this work, we investigate the stellar magnetic activity in the theoretical point of view, through the use of stellar structure and evolution models. We present theoretical values of convective turnover times and Rossby numbers for low-mass stars, calculated with the ATON stellar structure and evolution code. We concentrate our analysis on fully convective and partially convective stars motivated by recent observations of X-ray emission of slowly rotating fully convective stars, which suggest that the presence of a tachocline is not a central key for magnetic fields generation. We investigate the behavior of the convective turnover time evolution, as well as its radial profile inside the star. A discussion about the location where the convective turnover time is calculated in the stellar interior is also addressed. Our theoretical results are compared to observational data from low-mass stars.
Convection patterns in end-heated inclined enclosures.
Delgado-Buscalioni, R
2001-07-01
The natural convection in inclined side-heated rectangular boxes with adiabatic walls is theoretically and numerically investigated. The study is focused on the characterization of the convection patterns arising at the core of the basic steady unicellular flow and covers the whole range of Prandtl numbers (0enclosures illustrate each type of multicellular flow and examples of instability interactions near the codimension-2 lines predicted by the theory.
Energy Technology Data Exchange (ETDEWEB)
Navarro, J. A.; Madariaga, J. A.; Santamaria, C. M.; Saviron, J. M.
1980-07-01
10 refs. Flow pattern calculations in natural convection between two vertical coaxial cylinders are reported. It is assumed trough the paper. that fluid properties, viscosity, thermal conductivity and density, depend no-linearly on temperature and that the aspects (height/radius) ratio of the cylinders is high. Velocity profiles are calculated trough a perturbative scheme and analytic results for the three first perturbation orders are presented. We outline also an iterative method to estimate the perturbations on the flow patterns which arise when a radial composition gradient is established by external forces in a two-component fluid. This procedure, based on semiempirical basis, is applied to gaseous convection. The influence of the molecules gas properties on tho flow is also discussed. (Author) 10 refs.
Titan Balloon Convection Model Project
National Aeronautics and Space Administration — This innovative research effort is directed at determining, quantitatively, the convective heat transfer coefficients applicable to a Montgolfiere balloon operating...
Modelling of stellar convection
Kupka, Friedrich; Muthsam, Herbert J.
2017-07-01
The review considers the modelling process for stellar convection rather than specific astrophysical results. For achieving reasonable depth and length we deal with hydrodynamics only, omitting MHD. A historically oriented introduction offers first glimpses on the physics of stellar convection. Examination of its basic properties shows that two very different kinds of modelling keep being needed: low dimensional models (mixing length, Reynolds stress, etc.) and "full" 3D simulations. A list of affordable and not affordable tasks for the latter is given. Various low dimensional modelling approaches are put in a hierarchy and basic principles which they should respect are formulated. In 3D simulations of low Mach number convection the inclusion of then unimportant sound waves with their rapid time variation is numerically impossible. We describe a number of approaches where the Navier-Stokes equations are modified for their elimination (anelastic approximation, etc.). We then turn to working with the full Navier-Stokes equations and deal with numerical principles for faithful and efficient numerics. Spatial differentiation as well as time marching aspects are considered. A list of codes allows assessing the state of the art. An important recent development is the treatment of even the low Mach number problem without prior modification of the basic equation (obviating side effects) by specifically designed numerical methods. Finally, we review a number of important trends such as how to further develop low-dimensional models, how to use 3D models for that purpose, what effect recent hardware developments may have on 3D modelling, and others.
Bidispersive-inclined convection
Mulone, Giuseppe; Straughan, Brian
2016-01-01
A model is presented for thermal convection in an inclined layer of porous material when the medium has a bidispersive structure. Thus, there are the usual macropores which are full of a fluid, but there are also a system of micropores full of the same fluid. The model we employ is a modification of the one proposed by Nield & Kuznetsov (2006 Int. J. Heat Mass Transf. 49, 3068–3074. (doi:10.1016/j.ijheatmasstransfer.2006.02.008)), although we consider a single temperature field only. PMID:27616934
Numerical Analysis of Convection/Transpiration Cooling
Glass, David E.; Dilley, Arthur D.; Kelly, H. Neale
1999-01-01
An innovative concept utilizing the natural porosity of refractory-composite materials and hydrogen coolant to provide CONvective and TRANspiration (CONTRAN) cooling and oxidation protection has been numerically studied for surfaces exposed to a high heat flux, high temperature environment such as hypersonic vehicle engine combustor walls. A boundary layer code and a porous media finite difference code were utilized to analyze the effect of convection and transpiration cooling on surface heat flux and temperature. The boundary, layer code determined that transpiration flow is able to provide blocking of the surface heat flux only if it is above a minimum level due to heat addition from combustion of the hydrogen transpirant. The porous media analysis indicated that cooling of the surface is attained with coolant flow rates that are in the same range as those required for blocking, indicating that a coupled analysis would be beneficial.
Confinement and dynamical regulation in two-dimensional convective turbulence
DEFF Research Database (Denmark)
Bian, N.H.; Garcia, O.E.
2003-01-01
In this work the nature of confinement improvement implied by the self-consistent generation of mean flows in two-dimensional convective turbulence is studied. The confinement variations are linked to two distinct regulation mechanisms which are also shown to be at the origin of low......-frequency bursting in the fluctuation level and the convective heat flux integral, both resulting in a state of large-scale intermittency. The first one involves the control of convective transport by sheared mean flows. This regulation relies on the conservative transfer of kinetic energy from tilted fluctuations...... to the mean component of the flow. Bursting can also result from the quasi-linear modification of the linear instability drive which is the mean pressure gradient. For each bursting process the relevant zero-dimensional model equations are given. These are finally coupled in a minimal model of convection...
Energy Technology Data Exchange (ETDEWEB)
Hedayat, Afshin [Reactor and Nuclear Safety School, Nuclear Science and Technology Research Institute (NSTRI), Tehran (Iran, Islamic Republic of)
2017-08-15
In this paper, a complete station blackout (SBO) or complete loss of electrical power supplies is simulated and analyzed in a downward cooling 5-MW pool-type Material Testing Reactor (MTR). The scenario is traced in the absence of active cooling systems and operators. The code nodalization is successfully benchmarked against experimental data of the reactor's operating parameters. The passive heat removal system includes downward water cooling after pump breakdown by the force of gravity (where the coolant streams down to the unfilled portion of the holdup tank), safety flapper opening, flow reversal from a downward to an upward cooling direction, and then the upward free convection heat removal throughout the flapper safety valve, lower plenum, and fuel assemblies. Both short-term and long-term natural core cooling conditions are simulated and investigated using the RELAP5 code. Short-term analyses focus on the safety flapper valve operation and flow reversal mode. Long-term analyses include simulation of both complete SBO and long-term operation of the free convection mode. Results are promising for pool-type MTRs because this allows operators to investigate RELAP code abilities for MTR thermal–hydraulic simulations without any oscillation; moreover, the Tehran Research Reactor is conservatively safe against the complete SBO and long-term free convection operation.
Interaction Between Convection and Pulsation
Directory of Open Access Journals (Sweden)
Günter Houdek
2015-12-01
Full Text Available This article reviews our current understanding of modelling convection dynamics in stars. Several semi-analytical time-dependent convection models have been proposed for pulsating one-dimensional stellar structures with different formulations for how the convective turbulent velocity field couples with the global stellar oscillations. In this review we put emphasis on two, widely used, time-dependent convection formulations for estimating pulsation properties in one-dimensional stellar models. Applications to pulsating stars are presented with results for oscillation properties, such as the effects of convection dynamics on the oscillation frequencies, or the stability of pulsation modes, in classical pulsators and in stars supporting solar-type oscillations.
Energy Technology Data Exchange (ETDEWEB)
Hirata, Silvia C. [Laboratoire de Mecanique de Lille, UMR CNRS 8107 - Universite Lille I, Bld. Paul Langevin, 59655 Villeneuve d' Ascq cedex (France); Ouarzazi, Mohamed Najib, E-mail: najib.ouarzazi@univ-lille1.f [Laboratoire de Mecanique de Lille, UMR CNRS 8107 - Universite Lille I, Bld. Paul Langevin, 59655 Villeneuve d' Ascq cedex (France)
2010-06-07
By using the mathematical formalism of absolute and convective instabilities we study the nature of unstable three-dimensional disturbances of viscoelastic flow convection in a porous medium with horizontal through-flow and vertical temperature gradient. Temporal stability analysis reveals that among three-dimensional (3D) modes the pure down-stream transverse rolls are favored for the onset of convection. In addition, by considering a spatiotemporal stability approach we found that all unstable 3D modes are convectively unstable except the transverse rolls which may experience a transition to absolute instability. The combined influence of through-flow and elastic parameters on the absolute instability threshold, wave number and frequency is then determined, and results are compared to those of a Newtonian fluid.
Energy Technology Data Exchange (ETDEWEB)
Bruehwiler, D.
1995-08-15
With 163 MJ/m{sup 2}a, the heating energy characteristic of the Gwadt house for the heating period 1993/94, is approximately half of the SIA target value. The investigations which were carried out show that this was achieved with a basically good degree of thermal comfort due to good heat insulation, heating on requirement, direct heat saving and the convective collector. (author) 42 figs., 8 tabs., 24 refs.
Nield, Donald A
1992-01-01
This book provides a user-friendly introduction to the topic of convection in porous media The authors as- sume that the reader is familiar with the basic elements of fluid mechanics and heat transfer, but otherwise the book is self-contained The book will be useful both as a review (for reference) and as a tutorial work, suitable as a textbook in a graduate course or seminar The book brings into perspective the voluminous research that has been performed during the last two decades The field has recently exploded because of worldwide concern with issues such as energy self-sufficiency and pollution of the environment Areas of application include the insulation of buildings and equipment, energy storage and recovery, geothermal reservoirs, nuclear waste disposal, chemical reactor engineering, and the storage of heat-generating materials such as grain and coal Geophysical applications range from the flow of groundwater around hot intrusions to the stability of snow against avalanches
Nield, Donald A
2013-01-01
Convection in Porous Media, 4th Edition, provides a user-friendly introduction to the subject, covering a wide range of topics, such as fibrous insulation, geological strata, and catalytic reactors. The presentation is self-contained, requiring only routine mathematics and the basic elements of fluid mechanics and heat transfer. The book will be of use not only to researchers and practicing engineers as a review and reference, but also to graduate students and others entering the field. The new edition features approximately 1,750 new references and covers current research in nanofluids, cellular porous materials, strong heterogeneity, pulsating flow, and more. Recognized as the standard reference in the field Includes a comprehensive, 250-page reference list Cited over 2300 times to date in its various editions Serves as an introduction for those entering the field and as a comprehensive reference for experienced researchers Features new sections on nanofluids, carbon dioxide sequestration, and applications...
Directory of Open Access Journals (Sweden)
Elías Gómez Macías
2006-01-01
Full Text Available Partiendo de óxido de magnesio comercial se preparó una suspensión acuosa, la cual se secó y calcinó para conferirle estabilidad térmica. El material, tanto fresco como usado, se caracterizó mediante DRX, área superficial BET y SEM-EPMA. El catalizador mostró una matriz de MgO tipo periclasa con CaO en la superficie. Las pruebas de actividad catalítica se efectuaron en lecho fijo empacado con partículas obtenidas mediante prensado, trituración y clasificación del material. El flujo de reactivos consistió en mezclas gas natural-aire por debajo del límite inferior de inflamabilidad. Para diferentes flujos y temperaturas de entrada de la mezcla reactiva, se midieron las concentraciones de CH4, CO2 y CO en los gases de combustión con un analizador de gases tipo infrarrojo no dispersivo (NDIR. Para alcanzar conversión total de metano se requirió aumentar la temperatura de entrada al lecho a medida que se incrementó el flujo de gases reaccionantes. Los resultados obtenidos permiten desarrollar un sistema de combustión catalítica de bajo costo con un material térmicamente estable, que promueva la alta eficiencia en la combustión de gas natural y elimine los problemas de estabilidad, seguridad y de impacto ambiental negativo inherentes a los procesos de combustión térmica convencional.
Frequency of Deep Convective Clouds and Global Warming
Aumann, Hartmut H.; Teixeira, Joao
2008-01-01
This slide presentation reviews the effect of global warming on the formation of Deep Convective Clouds (DCC). It concludes that nature responds to global warming with an increase in strong convective activity. The frequency of DCC increases with global warming at the rate of 6%/decade. The increased frequency of DCC with global warming alone increases precipitation by 1.7%/decade. It compares the state of the art climate models' response to global warming, and concludes that the parametrization of climate models need to be tuned to more closely emulate the way nature responds to global warming.
Heat transport in bubbling turbulent convection.
Lakkaraju, Rajaram; Stevens, Richard J A M; Oresta, Paolo; Verzicco, Roberto; Lohse, Detlef; Prosperetti, Andrea
2013-06-04
Boiling is an extremely effective way to promote heat transfer from a hot surface to a liquid due to numerous mechanisms, many of which are not understood in quantitative detail. An important component of the overall process is that the buoyancy of the bubble compounds with that of the liquid to give rise to a much-enhanced natural convection. In this article, we focus specifically on this enhancement and present a numerical study of the resulting two-phase Rayleigh-Bénard convection process in a cylindrical cell with a diameter equal to its height. We make no attempt to model other aspects of the boiling process such as bubble nucleation and detachment. The cell base and top are held at temperatures above and below the boiling point of the liquid, respectively. By keeping this difference constant, we study the effect of the liquid superheat in a Rayleigh number range that, in the absence of boiling, would be between 2 × 10(6) and 5 × 10(9). We find a considerable enhancement of the heat transfer and study its dependence on the number of bubbles, the degree of superheat of the hot cell bottom, and the Rayleigh number. The increased buoyancy provided by the bubbles leads to more energetic hot plumes detaching from the cell bottom, and the strength of the circulation in the cell is significantly increased. Our results are in general agreement with recent experiments on boiling Rayleigh-Bénard convection.
Convection in Type 2 supernovae
Energy Technology Data Exchange (ETDEWEB)
Miller, Douglas Scott [Univ. of California, Davis, CA (United States)
1993-10-15
Results are presented here from several two dimensional numerical calculations of events in Type II supernovae. A new 2-D hydrodynamics and neutrino transport code has been used to compute the effect on the supernova explosion mechanism of convection between the neutrinosphere and the shock. This convection is referred to as exterior convection to distinguish it from convection beneath the neutrinosphere. The model equations and initial and boundary conditions are presented along with the simulation results. The 2-D code was used to compute an exterior convective velocity to compare with the convective model of the Mayle and Wilson 1-D code. Results are presented from several runs with varying sizes of initial perturbation, as well as a case with no initial perturbation but including the effects of rotation. The M&W code does not produce an explosion using the 2-D convective velocity. Exterior convection enhances the outward propagation of the shock, but not enough to ensure a successful explosion. Analytic estimates of the growth rate of the neutron finger instability axe presented. It is shown that this instability can occur beneath the neutrinosphere of the proto-neutron star in a supernova explosion with a growth time of ~ 3 microseconds. The behavior of the high entropy bubble that forms between the shock and the neutrinosphere in one dimensional calculations of supernova is investigated. It has been speculated that this bubble is a site for γ-process generation of heavy elements. Two dimensional calculations are presented of the time evolution of the hot bubble and the surrounding stellar material. Unlike one dimensional calculations, the 2D code fails to achieve high entropies in the bubble. When run in a spherically symmetric mode the 2-D code reaches entropies of ~ 200. When convection is allowed, the bubble reaches ~60 then the bubble begins to move upward into the cooler, denser material above it.
Internally heated convection and Rayleigh-Bénard convection
Goluskin, David
2016-01-01
This Brief describes six basic models of buoyancy-driven convection in a fluid layer: three configurations of internally heated convection and three configurations of Rayleigh-Bénard convection. The author discusses the main quantities that characterize heat transport in each model, along with the constraints on these quantities. This presentation is the first to place the various models in a unified framework, and similarities and differences between the cases are highlighted. Necessary and sufficient conditions for convective motion are given. For the internally heated cases only, parameter-dependent lower bounds on the mean fluid temperature are proven, and results of past simulations and laboratory experiments are summarized and reanalyzed. The author poses several open questions for future study.
Vanhoenacker, Gerd; Sandra, Pat
2006-08-01
Temperature, as a powerful variable in conventional LC is discussed from a fundamental point of view and illustrated with applications from the author's laboratory. Emphasis is given to the influence of temperature on speed, selectivity, efficiency, detectability, and mobile phase composition (green chromatography). The problems accompanying the use of elevated temperature and temperature programming in LC are reviewed and solutions are described. The available stationary phases for high temperature operation are summarized and a brief overview of recent applications reported in the literature is given.
Note: Heated sample platform for in situ temperature-programmed XPS.
Samokhvalov, Alexander; Tatarchuk, Bruce J
2011-07-01
We present the design, fabrication, and performance of the multi-specimen heated platform for linear in situ heating during the Temperature-Programmed XPS (TPXPS). The platform is versatile, compatible with high vacuum (HV) and bakeout. The heater platform is tested under in situ linear heating of typical high surface area sorbent∕catalyst support--nanoporous TiO(2). The platform allows the TPXPS of multiple samples located on specimen disk that can be transferred in and out of the TPXPS chamber. Electric characteristics, temperature and pressure curves are provided. Heating power supply, PID temperature controller, data-logging hardware and software are described.
Energy Technology Data Exchange (ETDEWEB)
Tonks, James P., E-mail: james.tonks@awe.co.uk [Department of Mechanical Engineering Sciences, University of Surrey, Guildford, Surrey GU2 7XH (United Kingdom); AWE Plc, Aldermaston, Reading, Berkshire RG7 4PR (United Kingdom); Galloway, Ewan C., E-mail: ewan.galloway@awe.co.uk; King, Martin O. [AWE Plc, Aldermaston, Reading, Berkshire RG7 4PR (United Kingdom); Kerherve, Gwilherm [VACGEN Ltd, St. Leonards-On-Sea, East Sussex TN38 9NN (United Kingdom); Watts, John F. [Department of Mechanical Engineering Sciences, University of Surrey, Guildford, Surrey GU2 7XH (United Kingdom)
2016-08-15
A dual purpose mass spectrometer chamber capable of performing molecular beam scattering (MBS) and temperature programmed desorption (TPD) is detailed. Two simple features of this design allow it to perform these techniques. First, the diameter of entrance aperture to the mass spectrometer can be varied to maximize signal for TPD or to maximize angular resolution for MBS. Second, the mass spectrometer chamber can be radially translated so that it can be positioned close to the sample to maximize signal or far from the sample to maximize angular resolution. The performance of this system is described and compares well with systems designed for only one of these techniques.
Generalized drying curves in conductive/convective paper drying
Directory of Open Access Journals (Sweden)
O.C. Motta Lima
2000-12-01
Full Text Available This work presents a study related to conductive/convective drying of paper (cellulose sheets over heated surfaces, under natural and forced air conditions. The experimental apparatus consists in a metallic box heated by a thermostatic bath containing an upper surface on which the paper samples (about 1 mm thick are placed. The system is submitted to ambient air under two different conditions: natural convection and forced convection provide by an adjustable blower. The influence of initial paper moisture content, drying (heated surface temperature and air velocity on drying curves behavior is observed under different drying conditions. Hence, these influence is studied through the proposal of generalized drying curves. Those curves are analyzed individually for each air condition exposed above and for both together. A set of equations to fit them is proposed and discussed.
Convection From a Slender Cylender in a Ventilated Room
DEFF Research Database (Denmark)
Heiselberg, Per Kvols; Sandberg, M.
1990-01-01
The equations, based on an integral formulation, for turbulent natural boundary layer flow are solved in both the constant temperature case and in the constant heat flux case. Solutions are found. for convection along both flat plates and cylinders. Theoretical predictions are compared with measu......The equations, based on an integral formulation, for turbulent natural boundary layer flow are solved in both the constant temperature case and in the constant heat flux case. Solutions are found. for convection along both flat plates and cylinders. Theoretical predictions are compared...... uniformly from floor to ceiling. It was found that the convective flow at the heat source as well as the flow at the walls was influencing the level of stratification....
Heat Transfer Correlations for Free Convection from Suspended Microheaters
Directory of Open Access Journals (Sweden)
David GOSSELIN
2016-08-01
Full Text Available Portability and autonomy for biomedical diagnostic devices are two rising requirements. It is recognized that low-energy heating of such portable devices is of utmost importance for molecular recognition. This work focuses on screen-printed microheaters based on on Joule effect, which constitute an interesting solution for low-energy heating. An experimental study of the natural convection phenomena occurring with such microheaters is conducted. When they are suspended in the air, and because of the thinness of the supporting film, it is shown that the contributions of both the upward and downward faces have to be taken into account. A total Nusselt number and a total convective heat transfer coefficient have been used to describe the natural convection around these microheaters. In addition a relation between the Nusselt number and the Rayleigh number is derived, leading to an accurate prediction of the heating temperature (MRE< 2 %.
A low-power pressure-and temperature-programmed separation system for a micro gas chromatograph.
Energy Technology Data Exchange (ETDEWEB)
Sacks, Richard D. (University of Michigan, Ann Arbor, MI); Robinson, Alex Lockwood (Advanced Sensor Technologies, Albuquerque, NM); Lambertus, Gordon R. (University of Michigan, Ann Arbor, MI); Potkay, Joseph A. (University of Michigan, Ann Arbor, MI); Wise, Kensall D. (University of Michigan, Ann Arbor, MI)
2006-10-01
This thesis presents the theory, design, fabrication and testing of the microvalves and columns necessary in a pressure- and temperature-programmed micro gas chromatograph ({micro}GC). Two microcolumn designs are investigated: a bonded Si-glass column having a rectangular cross section and a vapor-deposited silicon oxynitride (Sion) column having a roughly circular cross section. Both microcolumns contain integrated heaters and sensors for rapid, controlled heating. The 3.2 cm x 3.2 cm, 3 m-long silicon-glass column, coated with a non-polar polydimethylsiloxane (PDMS) stationary phase, separates 30 volatile organic compounds (VOCs) in less than 6 min. This is the most efficient micromachined column reported to date, producing greater than 4000 plates/m. The 2.7 mm x 1.4 mm Sion column eliminates the glass sealing plate and silicon substrate using deposited dielectrics and is the lowest power and fastest GC column reported to date; it requires only 11 mW to raise the column temperature by 100 C and has a response time of 11s and natural temperature ramp rate of 580 C/min. A 1 m-long PDMS-coated Sion microcolumn separates 10 VOCs in 52s. A system-based design approach was used for both columns.
Topology Optimization for Convection Problems
DEFF Research Database (Denmark)
Alexandersen, Joe
2011-01-01
This report deals with the topology optimization of convection problems.That is, the aim of the project is to develop, implement and examine topology optimization of purely thermal and coupled thermomechanical problems,when the design-dependent eects of convection are taken into consideration...... problems, steady-state thermal problems and coupled thermomechanical problems, for a range of objective functions subjected to a volume-fraction constraint. The programme utilises the GCMMA algorithm, by K. Svanberg, to optimize the given problems. Design-dependent side convection has been formulated...... and implemented, where the convection is interpolated into the design-domain and applied upon the boundaries of the design using a density-based interpolation function.The implementation has been tested for both pure thermal analysis, along with thermal and thermomechanical optimization. The solutions to the test...
National Research Council Canada - National Science Library
McWilliams, James
1999-01-01
... mechanism of water mass transformation. The resultant newly mixed deep water masses form a component of the thermohaline circulation, and hence it is essential to understand the deep convection process if the variability of the meridional...
Novel bimetallic dispersed catalysts for temperature-programmed coal liquefaction. Final report
Energy Technology Data Exchange (ETDEWEB)
Chunshan Song; Schobert, H.H.; Parfitt, D.P. [and others
1997-11-01
Development of new catalysts is a promising approach to more efficient coal liquefaction. It has been recognized that dispersed catalysts are superior to supported catalysts for primary liquefaction of coals, because the control of initial coal dissolution or depolymerization requires intimate contact between the catalyst and coal. This research is a fundamental and exploratory study on catalytic coal liquefaction, with the emphasis on exploring novel bimetallic dispersed catalysts for coal liquefaction and the effectiveness of temperature-programmed liquefaction using dispersed catalysts. The primary objective of this research was to explore novel bimetallic dispersed catalysts from organometallic molecular precursors, that could be used in low concentrations but exhibit relatively high activity for efficient hydroliquefaction of coals under temperature-programmed conditions. We have synthesized and tested various catalyst precursors in liquefaction of subbituminous and bituminous coals and in model compound studies to examine how do the composition and structure of the catalytic precursors affect their effectiveness for coal liquefaction under different reaction conditions, and how do these factors affect their catalytic functions for hydrogenation of polyaromatic hydrocarbons, for cleavage of C-C bonds in polycyclic systems such as 4-(1-naphthylmethyl)bibenzyl, for hydrogenolysis of C-O bond such as that in dinaphthylether, for hydrodeoxygenation of phenolic compounds and other oxygen-containing compounds such as xanthene, and for hydrodesulfurization of polycyclic sulfur compounds such as dibenzothiophene. The novel bimetallic and monometallic precursors synthesized and tested in this project include various Mo- and Fe-based compounds.
Scales of Free Convection around a Vertical Cylinder
Lira, Ignacio
2008-01-01
The natural scales of the laminar steady-state free convection flow regime surrounding an isothermal vertical cylinder are established. It is shown that nondimensionalizing the momentum and energy equations in terms of the Rayleigh or Boussinesq numbers allows the use of the Prandtl number as a criterion to establish whether the motive buoyancy…
Project "Convective Wind Gusts" (ConWinG)
Mohr, Susanna; Richter, Alexandra; Kunz, Michael; Ruck, Bodo
2017-04-01
Convectively-driven strong winds usually associated with thunderstorms frequently cause substantial damage to buildings and other structures in many parts of the world. Decisive for the high damage potential are the short-term wind speed maxima with duration of a few seconds, termed as gusts. Several studies have shown that convectively-driven gusts can reach even higher wind speeds compared to turbulent gusts associated with synoptic-scale weather systems. Due to the small-scale and non-stationary nature of convective wind gusts, there is a considerable lack of knowledge regarding their characteristics and statistics. Furthermore, their interaction with urban structures and their influence on buildings is not yet fully understood. For these two reasons, convective wind events are not included in the present wind load standards of buildings and structures, which so far have been based solely on the characteristics of synoptically-driven wind gusts in the near-surface boundary layer (e. g., DIN EN 1991-1-4:2010-12; ASCE7). However, convective and turbulent gusts differ considerably, e.g. concerning vertical wind-speed profiles, gust factors (i.e., maximum to mean wind speed), or exceedance probability curves. In an effort to remedy this situation, the overarching objective of the DFG-project "Convective Wind Gusts" (ConWinG) is to investigate the characteristics and statistics of convective gusts as well as their interaction with urban structures. Based on a set of 110 climate stations of the German Weather Service (DWD) between 1992 and 2014, we analyzed the temporal and spatial distribution, intensity, and occurrence probability of convective gusts. Similar to thunderstorm activity, the frequency of convective gusts decreases gradually from South to North Germany. A relation between gust intensity/probability to orography or climate conditions cannot be identified. Rather, high wind speeds, e.g., above 30 m/s, can be expected everywhere in Germany with almost
Thermal computations for electronics conductive, radiative, and convective air cooling
Ellison, Gordon
2010-01-01
IntroductionPrimary mechanisms of heat flowConductionApplication example: Silicon chip resistance calculationConvectionApplication example: Chassis panel cooled by natural convectionRadiationApplication example: Chassis panel cooled only by radiation 7Illustrative example: Simple thermal network model for a heat sinked power transistorIllustrative example: Thermal network circuit for a printed circuit boardCompact component modelsIllustrative example: Pressure and thermal circuits for a forced air cooled enclosureIllustrative example: A single chip package on a printed circuit board-the proble
Heat Flux Sensors for Infrared Thermography in Convective Heat Transfer
Carlomagno, Giovanni Maria; de Luca, Luigi; Cardone, Gennaro; Astarita, Tommaso
2014-01-01
This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR) thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors' research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dynamic flows. Several applications to streams, which range from natural convection to hypersonic flows, are also described. PMID:25386758
Heat flux sensors for infrared thermography in convective heat transfer.
Carlomagno, Giovanni Maria; de Luca, Luigi; Cardone, Gennaro; Astarita, Tommaso
2014-11-07
This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR) thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors' research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dynamic flows. Several applications to streams, which range from natural convection to hypersonic flows, are also described.
Heat Flux Sensors for Infrared Thermography in Convective Heat Transfer
Directory of Open Access Journals (Sweden)
Giovanni Maria Carlomagno
2014-11-01
Full Text Available This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors’ research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dynamic flows. Several applications to streams, which range from natural convection to hypersonic flows, are also described.
TEMPERATURE-PROGRAMMED DESORPTION: PRINCIPLES, INSTRUMENT DESIGN, AND DEMONSTRATION WITH NAALH4
Energy Technology Data Exchange (ETDEWEB)
Stowe, A; Ragaiy Zidan, R
2006-11-07
This article is a brief introduction to temperature-programmed desorption (TPD), an analytical technique devised to analyze, in this case, materials for their potential as hydrogen storage materials. The principles and requirements of TPD are explained and the different components of a generic TPD apparatus are described. The construction of a modified TPD instrument from commercially available components is reported together with the control and acquisition technique used to create a TPD spectrum. The chemical and instrumental parameters to be considered in a typical TPD experiment and the analytical utility of the technique are demonstrated by the dehydrogenation of titanium-doped NaAlH{sub 4} by means of thermally programmed desorption.
NATO Advanced Study Institute on Buoyant Convection in Geophysical Flows
Fedorovich, E; Viegas, D; Wyngaard, J
1998-01-01
Studies of convection in geophysical flows constitute an advanced and rapidly developing area of research that is relevant to problems of the natural environment. During the last decade, significant progress has been achieved in the field as a result of both experimental studies and numerical modelling. This led to the principal revision of the widely held view on buoyancy-driven turbulent flows comprising an organised mean component with superimposed chaotic turbulence. An intermediate type of motion, represented by coherent structures, has been found to play a key role in geophysical boundary layers and in larger scale atmospheric and hydrospheric circulations driven by buoyant forcing. New aspects of the interaction between convective motions and rotation have recently been discovered and investigated. Extensive experimental data have also been collected on the role of convection in cloud dynamics and microphysics. New theoretical concepts and approaches have been outlined regarding scaling and parameteriz...
Structures, profile consistency, and transport scaling in electrostatic convection
DEFF Research Database (Denmark)
Bian, N.H.; Garcia, O.E.
2005-01-01
that for interchange modes, profile consistency is in fact due to mixing by persistent large-scale convective cells. This mechanism is not a turbulent diffusion, cannot occur in collisionless systems, and is the analog of the well-known laminar "magnetic flux expulsion" in magneiohydrodynamics. This expulsion process...... involves a "pinch" across closed streamlines and further results in the formation of pressure fingers along the-separatrix of the convective cells. By nature, these coherent structures are dissipative because the mixing process that leads to their formation relies on a finite amount of collisional...... diffusion. Numerical simulations of two-dimensional interchange modes confirm the role of laminar expulsion by convective cells, for profile consistency and structure formation. They also show that the fingerlike pressure structures ultimately control the rate of heat transport across the plasma layer...
Lattice Boltzmann simulations of convection heat transfer in porous media
Liu, Qing; He, Ya-Ling
2017-01-01
A non-orthogonal multiple-relaxation-time (MRT) lattice Boltzmann (LB) method is developed to study convection heat transfer in porous media at the representative elementary volume scale based on the generalized non-Darcy model. In the method, two different LB models are constructed: one is constructed in the framework of the double-distribution-function approach, and the other is constructed in the framework of the hybrid approach. In particular, the transformation matrices used in the MRT-LB models are non-orthogonal matrices. The present method is applied to study mixed convection flow in a porous channel and natural convection flow in a porous cavity. It is found that the numerical results are in good agreement with the analytical solutions and/or other results reported in previous studies. Furthermore, the non-orthogonal MRT-LB method shows better numerical stability in comparison with the BGK-LB method.
Statistical characteristics of convective wind gusts in Germany
Mohr, Susanna; Kunz, Michael; Richter, Alexandra; Ruck, Bodo
2017-06-01
Due to the small-scale and non-stationary nature of the convective wind gusts usually associated with thunderstorms, there is a considerable lack of knowledge regarding their characteristics and statistics. In an effort to remedy this situation, we investigated in this study a set of 110 climate stations of the German Weather Service between 1992 and 2014 to analyze the temporal and spatial distribution, intensity, and occurrence probability of convective gusts. Similar to thunderstorm activity, the frequency of convective gusts decreases gradually from southern to northern Germany. No further spatial structures, such as a relation to orography or climate conditions, can be identified regarding their strength or likelihood. Rather, high wind speeds of above 30 m s-1 can be expected everywhere in Germany with almost similar occurrence probabilities. A comparison of the 20-year return values of convective gusts with those of turbulent gusts demonstrates that the latter have higher frequencies, especially in northern Germany. However, for higher return periods, this effect can be reversed at some stations. The values of the convective gust factors are mainly in a range between 1 and 4 but can even reach values up to 10. Besides the dependency from the averaging time period of the mean wind, the values of the gust factors additionally depend on the event duration and the storm type, respectively.
State of the Magnetotail: Steady and Bursty Magnetospheric Convection
Tanskanen, E. I.; Slavin, J. A.
2004-12-01
Geotail and Cluster observations are used to examine the state of the Earth's magnetotail. In this paper we are particularly interested about the plasma sheet convection and the tail "stress level". Tail static pressure (magnetic + thermal pressure) is used to characterize different convection modes, and earthward bulk velocity is used to identify burstyness of the plasma sheet. Four basic convection modes are identified: loading, unloading, steady magnetospheric convection (SMC) and continuous magnetospheric dissipation (CMD). Bulk ion velocity is used to characterize the nature of the convection in the plasma sheet during each of these states. Bursty bulk flows (BBFs) were found to be basic building blocks of the all the tail states. When solar wind drives magnetosphere over several substorm cycle (> 8 hours) the plasma sheet was observed to be highly active and non-steady. The tail stress level was studied by using magnetic and plasma measurements from four Cluster spacecraft. Tail total current was computed, and x-component of the J cross B was used as a "tail stress index". Examples of slightly and strongly stretched magnetotail will be presented and the validity of the tail stress index will be evaluated.
Kinetic energy budgets in areas of convection
Fuelberg, H. E.
1979-01-01
Synoptic scale budgets of kinetic energy are computed using 3 and 6 h data from three of NASA's Atmospheric Variability Experiments (AVE's). Numerous areas of intense convection occurred during the three experiments. Large kinetic energy variability, with periods as short as 6 h, is observed in budgets computed over each entire experiment area and over limited volumes that barely enclose the convection and move with it. Kinetic energy generation and transport processes in the smaller volumes are often a maximum when the enclosed storms are near peak intensity, but the nature of the various energy processes differs between storm cases and seems closely related to the synoptic conditions. A commonly observed energy budget for peak storm intensity indicates that generation of kinetic energy by cross-contour flow is the major energy source while dissipation to subgrid scales is the major sink. Synoptic scale vertical motion transports kinetic energy from lower to upper levels of the atmosphere while low-level horizontal flux convergence and upper-level horizontal divergence also occur. Spatial fields of the energy budget terms show that the storm environment is a major center of energy activity for the entire area.
Presentation on Tropical Mesoscale convective Systems and ...
Indian Academy of Sciences (India)
IAS Admin
Exchange. Dynamics of deep convection. Microphysics of deep convection. Fountain. (Water Vapor). Intrusion. (Ozone). Circulation. Mass and Energy. Budget ... km (wet spell). 厂 Inversion more during dry spell. 厂 Buoyancy - more during the wet spell. Mon. Wea. Rev, 2010 & 2011. Convection in wet and dry spells.
Solar Surface Magneto-Convection
Directory of Open Access Journals (Sweden)
Robert F. Stein
2012-07-01
Full Text Available We review the properties of solar magneto-convection in the top half of the convection zones scale heights (from 20 Mm below the visible surface to the surface, and then through the photosphere to the temperature minimum. Convection is a highly non-linear and non-local process, so it is best studied by numerical simulations. We focus on simulations that include sufficient detailed physics so that their results can be quantitatively compared with observations. The solar surface is covered with magnetic features with spatial sizes ranging from unobservably small to hundreds of megameters. Three orders of magnitude more magnetic flux emerges in the quiet Sun than emerges in active regions. In this review we focus mainly on the properties of the quiet Sun magnetic field. The Sun’s magnetic field is produced by dynamo action throughout the convection zone, primarily by stretching and twisting in the turbulent downflows. Diverging convective upflows and magnetic buoyancy carry magnetic flux toward the surface and sweep the field into the surrounding downflow lanes where the field is dragged downward. The result is a hierarchy of undulating magnetic Ω- and U-loops of different sizes. New magnetic flux first appears at the surface in a mixed polarity random pattern and then collects into isolated unipolar regions due to underlying larger scale magnetic structures. Rising magnetic structures are not coherent, but develop a filamentary structure. Emerging magnetic flux alters the convection properties, producing larger, darker granules. Strong field concentrations inhibit transverse plasma motions and, as a result, reduce convective heat transport toward the surface which cools. Being cooler, these magnetic field concentrations have a shorter scale height and become evacuated. The field becomes further compressed and can reach strengths in balance with the surrounding gas pressure. Because of their small internal density, photons escape from deeper in
Assessing Intraseasonal Variability Produced by Several Deep Convection Schemes in the NCAR CCM3.6
Maloney, E. D.
2001-05-01
The Hack, Zhang/McFarlane, and McRAS convection schemes produce very different simulations of intraseasonal variability in the NCAR CCM3.6. A robust analysis of simulation performance requires an expanded set of diagnostics. The use of only one criterion to analyze model Madden-Julian oscillation (MJO) variability, such as equatorial zonal wind variability, may give a misleading impression of model performance. Schemes that produce strong variability in zonal winds may sometimes lack a corresponding coherent signal in precipitation, suggesting that model convection and the large-scale circulation are not as strongly coupled as observed. The McRAS scheme, which includes a parametrization of unsaturated convective downdrafts, produces the best simulation of intraseasonal variability of the three schemes used. Downdrafts in McRAS create a moister equatorial troposphere, which increases equatorial convection. Composite analysis indicates a strong dependence of model intraseasonal variability on the frictional convergence mechanism, which may also be important in nature. The McRAS simulation has limitations, however. Indian Ocean variability is weak, and anomalous convection extends too far east across the Pacific. The dependence of convection on surface friction is too strong, and causes enhanced MJO convection to be associated with low-level easterly wind perturbations, unlike observed MJO convection. Anomalous vertical advection associated with surface convergence influences model convection by moistening the lower troposphere. Based on the work of Hendon (2000), coupling to an interactive ocean is unlikely to change the performance of the CCM3 with McRAS, due to the phase relationship between anomalous convection and zonal winds. Use of the analysis tools presented here indicates areas for improvement in the parametrization of deep convection by atmospheric GCMs.
Orton, Dennis J; Doucette, Alan A
2013-03-15
As an alternative to direct UV absorbance measurements, estimation of total protein concentration is typically conducted through colorimetric reagent assays. However, for protein-limited applications, the proportion of the sample sacrificed to the assay becomes increasingly significant. This work demonstrates a method for quantitation of protein samples with high recovery. Temperature programmed liquid chromatography (TPLC) with absorbance detection at 214nm permits accurate estimation of total protein concentration from samples containing as little as 0.75μg. The method incorporates a temperature gradient from 25 to 80°C to facilitate elution of total protein into a single fraction. Analyte recovery, as measured from 1 and 10μg protein extracts of Escherichia coli, is shown to exceed 93%. Extinction coefficients at 214nm were calculated across the human proteome, providing a relative standard deviation of 21% (versus 42% at 280nm), suggesting absorbance values at 214nm provide a more consistent measure of protein concentration. These results translate to a universal protein detection strategy exhibiting a coefficient of variation below 10%. Together with the sensitivity and tolerance to contaminants, TPLC with UV detection is a favorable alternative to colorimetric assay for total protein quantitation, particularly in sample-limited applications. Copyright © 2013 Elsevier B.V. All rights reserved.
VHF/UHF radar observations of tropical mesoscale convective systems over southern India
Directory of Open Access Journals (Sweden)
K. Kishore Kumar
2005-07-01
Full Text Available Several campaigns have been carried out to study the convective systems over Gadanki (13.5° N, 79.2° E, a tropical station in India, using VHF and UHF radars. The height-time sections of several convective systems are investigated in detail to study reflectivity, turbulence and vertical velocity structure. Structure and dynamics of the convective systems are the main objectives of these campaigns. The observed systems are classified into single- and multi-cell systems. It has been observed that most of the convective systems at this latitude are multi-cellular in nature. Simultaneous VHF and UHF radar observations are used to classify the observed precipitating systems as convective, intermediary and stratiform regions. Composite height profiles of vertical velocities in these regions were obtained and the same were compared with the profiles obtained at other geographical locations. These composite profiles of vertical velocity in the convective regions have shown their peaks in the mid troposphere, indicating that the maximum latent heat is being released at those heights. These profiles are very important for numerical simulations of the convective systems, which vary significantly from one geographical location to the other. Keywords. Meteorology and atmospheric dynamics (Mesoscale meteorology; Convective processes – Radio science (Remote sensing
Statistics of Deep Convection in the Congo Basin Derived From High-Resolution Simulations.
White, B.; Stier, P.; Kipling, Z.; Gryspeerdt, E.; Taylor, S.
2016-12-01
Convection transports moisture, momentum, heat and aerosols through the troposphere, and so the temporal variability of convection is a major driver of global weather and climate. The Congo basin is home to some of the most intense convective activity on the planet and is under strong seasonal influence of biomass burning aerosol. However, deep convection in the Congo basin remains under studied compared to other regions of tropical storm systems, especially when compared to the neighbouring, relatively well-understood West African climate system. We use the WRF model to perform a high-resolution, cloud-system resolving simulation to investigate convective storm systems in the Congo. Our setup pushes the boundaries of current computational resources, using a 1 km grid length over a domain covering millions of square kilometres and for a time period of one month. This allows us to draw statistical conclusions on the nature of the simulated storm systems. Comparing data from satellite observations and the model enables us to quantify the diurnal variability of deep convection in the Congo basin. This approach allows us to evaluate our simulations despite the lack of in-situ observational data. This provides a more comprehensive analysis of the diurnal cycle than has previously been shown. Further, we show that high-resolution convection-permitting simulations performed over near-seasonal timescales can be used in conjunction with satellite observations as an effective tool to evaluate new convection parameterisations.
A transilient matrix for moist convection
Energy Technology Data Exchange (ETDEWEB)
Romps, D.; Kuang, Z.
2011-08-15
A method is introduced for diagnosing a transilient matrix for moist convection. This transilient matrix quantifies the nonlocal transport of air by convective eddies: for every height z, it gives the distribution of starting heights z{prime} for the eddies that arrive at z. In a cloud-resolving simulation of deep convection, the transilient matrix shows that two-thirds of the subcloud air convecting into the free troposphere originates from within 100 m of the surface. This finding clarifies which initial height to use when calculating convective available potential energy from soundings of the tropical troposphere.
Seismic Constraints on Interior Solar Convection
Hanasoge, Shravan M.; Duvall, Thomas L.; DeRosa, Marc L.
2010-01-01
We constrain the velocity spectral distribution of global-scale solar convective cells at depth using techniques of local helioseismology. We calibrate the sensitivity of helioseismic waves to large-scale convective cells in the interior by analyzing simulations of waves propagating through a velocity snapshot of global solar convection via methods of time-distance helioseismology. Applying identical analysis techniques to observations of the Sun, we are able to bound from above the magnitudes of solar convective cells as a function of spatial convective scale. We find that convection at a depth of r/R(solar) = 0.95 with spatial extent l Convective features deeper than r/R(solar) = 0.95 are more difficult to image due to the rapidly decreasing sensitivity of helioseismic waves.
Directory of Open Access Journals (Sweden)
Nee Alexander
2015-01-01
Full Text Available Mathematical modeling of radiant heating of a closed rectangular area under conditions of convective heat transfer at the external boundaries is passed. The fields of temperature and stream function, illustrating the unsteady nature of the heat transfer were obtained. The extent influence of convective heat transfer at the external boundaries on the circulating flows formation in the gas cavity are shown.
A numerical study of thermals as the prototype of moist convection
Hernandez-Deckers, Daniel; Sherwood, Steven C.
2015-04-01
Despite the fact that thermals'small, transient air bubbles'are known to be the main convective elements of atmospheric moist convection, current parameterizations are typically based on the steady-plume concept, mostly due to practical and historical reasons. Within the current need for new parameterizations, the thermal conceptual model for convection is one promising path. For example, thermals are highly entraining, albeit weakly damped. This would hardly fit within the typical entraining plume model, but could aid to improve key issues of current convection parameterizations, such as the lack of sensitivity to ambient humidity and the excess of condensate carried to upper levels. Furthermore, the transient nature of thermals would bring an entirely new physical ingredient to convection parameterizations. The first step towards developing such new parameterizations is to understand the dynamics of thermals. Here we will describe their dynamics based on 2D and 3D cloud resolving simulations of sea-breeze initiated convection using WRF. We analize a large sample of thermals from which we can describe statistics about their size, ascent rate, momentum budget and mixing properties. We will show how these properties define their transient nature, how they are related to each other, and how they evolve as convection deepens. Furthermore we will provide simple relations between buoyancy, vertical velocity and drag forces, which would be crucial for new parameterizations.
Convective aggregation in realistic convective-scale simulations
Holloway, Christopher E.
2017-06-01
To investigate the real-world relevance of idealized-model convective self-aggregation, five 15 day cases of real organized convection in the tropics are simulated. These include multiple simulations of each case to test sensitivities of the convective organization and mean states to interactive radiation, interactive surface fluxes, and evaporation of rain. These simulations are compared to self-aggregation seen in the same model configured to run in idealized radiative-convective equilibrium. Analysis of the budget of the spatial variance of column-integrated frozen moist static energy shows that control runs have significant positive contributions to organization from radiation and negative contributions from surface fluxes and transport, similar to idealized runs once they become aggregated. Despite identical lateral boundary conditions for all experiments in each case, systematic differences in mean column water vapor (CWV), CWV distribution shape, and CWV autocorrelation length scale are found between the different sensitivity runs, particularly for those without interactive radiation, showing that there are at least some similarities in sensitivities to these feedbacks in both idealized and realistic simulations (although the organization of precipitation shows less sensitivity to interactive radiation). The magnitudes and signs of these systematic differences are consistent with a rough equilibrium between (1) equalization due to advection from the lateral boundaries and (2) disaggregation due to the absence of interactive radiation, implying disaggregation rates comparable to those in idealized runs with aggregated initial conditions and noninteractive radiation. This points to a plausible similarity in the way that radiation feedbacks maintain aggregated convection in both idealized simulations and the real world.Plain Language SummaryUnderstanding the processes that lead to the organization of tropical rainstorms is an important challenge for weather
Midlatitude Continental Convective Clouds Experiment (MC3E)
Energy Technology Data Exchange (ETDEWEB)
Jensen, MP; Petersen, WA; Del Genio, AD; Giangrande, SE; Heymsfield, A; Heymsfield, G; Hou, AY; Kollias, P; Orr, B; Rutledge, SA; Schwaller, MR; Zipser, E
2010-04-01
Convective processes play a critical role in the Earth’s energy balance through the redistribution of heat and moisture in the atmosphere and subsequent impacts on the hydrologic cycle. Global observation and accurate representation of these processes in numerical models is vital to improving our current understanding and future simulations of Earth’s climate system. Despite improvements in computing power, current operational weather and global climate models are unable to resolve the natural temporal and spatial scales that are associated with convective and stratiform precipitation processes; therefore, they must turn to parameterization schemes to represent these processes. In turn, the physical basis for these parameterization schemes needs to be evaluated for general application under a variety of atmospheric conditions. Analogously, space-based remote sensing algorithms designed to retrieve related cloud and precipitation information for use in hydrological, climate, and numerical weather prediction applications often rely on physical “parameterizations” that reliably translate indirectly related instrument measurements to the physical quantity of interest (e.g., precipitation rate). Importantly, both spaceborne retrieval algorithms and model convective parameterization schemes traditionally rely on field campaign data sets as a basis for evaluating and improving the physics of their respective approaches. The Midlatitude Continental Convective Clouds Experiment (MC3E) will take place in central Oklahoma during the April–May 2011 period. The experiment is a collaborative effort between the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Climate Research Facility and the National Aeronautics and Space Administration’s (NASA) Global Precipitation Measurement (GPM) mission Ground Validation (GV) program. The field campaign leverages the unprecedented observing infrastructure currently available in the central United States
Infrasound generation by turbulent convection
Akhalkatsi, M.; Gogoberidze, G.; Morrison, P. J.
2004-01-01
Low frequency acoustic wave generation is studied taking into account the effect of stratification, inhomogeneity of background velocity profile and temperature fluctuations. It is shown that for the typical parameters of convective storms the dipole radiation related to temperature inhomogeneities is at least of the same order as radiation of Lighthill's quadrupole source. It is also shown that the source related to stratification could have valuable contribution whereas some other sources a...
Forced convection in the wakes of impacting and sliding bubbles
O'Reilly Meehan, R.; Williams, N. P.; Donnelly, B.; Persoons, T.; Nolan, K.; Murray, D. B.
2017-09-01
Both vapour and gas bubbles are known to significantly increase heat transfer rates between a heated surface and the surrounding fluid, even with no phase change. The cooling structures observed are highly temporal, intricate and complex, with a full description of the surface cooling phenomena not yet available. The current study uses high speed infrared thermography to measure the surface temperature and determine the convective heat flux enhancement associated with the interaction of a single air bubble with a heated, inclined surface. This process can be discretised into the initial impact, in which enhancement levels in excess of 20 times natural convection are observed, and the subsequent sliding behaviour, with more moderate maximum enhancement levels of 8 times natural convection. In both cases, localised regions of suppressed heat transfer are also observed due to the recirculation of warm fluid displaced from the thermal boundary layer with the surface. The cooling patterns observed herein are consistent with the interaction between an undulating wake containing multiple hairpin vortex loops and the thermal boundary layer that exists under the surface, with the initial nature of this enhancement and suppression dependent on the particular point on its rising path at which the bubble impacts the surface.
Energy Technology Data Exchange (ETDEWEB)
Bories, S.; Mojtabi, A.; Prat, M.; Quintard, M. [Institut de Mecanique des Fluides de Toulouse, 31 (France)
2008-10-15
Multiple physico-chemical and transport phenomena take place in porous media. The study of these phenomena requires the knowledge of fluid storage, transfer and mechanical properties of these media. Like all polyphasic heterogenous systems, these properties depend on the morphology of the matrix and of the phenomena interacting in the different phases. This makes the heat transfers in porous media a particularly huge field of researches. This article makes a synthesis of these researches. Content: 1 - classification and characterization of porous media; 2 - modeling of transfer phenomena; 3 - heat transfer by conduction: concept of equivalent thermal conductivity (ETC), modeling of conduction heat transfer, ETC determination; 4 - heat transfer by convection: modeling of convection heat transfer, natural convection (in confined media, along surfaces or impermeable bodies immersed in a saturated porous medium), forced and mixed convection; 5 - radiant heat transfer: energy status equation, approximate solutions of the radiant transfer equation, use of the approximate solutions: case of fibrous insulating materials; 6 - conclusion. (J.S.)
Energy Technology Data Exchange (ETDEWEB)
Xu, Haomiao [School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240 (China); Ma, Yongpeng [Henan Collaborative Innovation Center of Environmental Pollution Control and Ecological Restoration, Zhengzhou University of Light Industry, No. 136, Science Avenue, Zhengzhou 450001 (China); Huang, Wenjun; Mei, Jian; Zhao, Songjian; Qu, Zan [School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240 (China); Yan, Naiqiang, E-mail: nqyan@sjtu.edu.cn [School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240 (China)
2017-01-05
Highlights: • Hg-TPD method was used for speciation of mercury species. • Different elements modified MnO{sub x} have different mercury binding state. • Understanding mercury existed state was beneficial for designing novel materials. - Abstract: Mercury temperature-programmed desorption (Hg-TPD) method was employed to clarify mercury species over Mn-based oxides. The elemental mercury (Hg{sup 0}) removal mechanism over MnO{sub x} was ascribed to chemical-adsorption. HgO was the primary mercury chemical compound adsorbed on the surface of MnO{sub x}. Rare earth element (Ce), main group element (Sn) and transition metal elements (Zr and Fe) were chosen for the modification of MnO{sub x}. Hg-TPD results indicated that the binding strength of mercury on these binary oxides followed the order of Sn-MnO{sub x} < Ce-MnO{sub x} ∼ MnO{sub x} < Fe-MnO{sub x} < Zr-MnO{sub x}. The activation energies for desorption were calculated and they were 64.34, 101.85, 46.32, 117.14, and 106.92 eV corresponding to MnO{sub x}, Ce-MnO{sub x}, Sn-MnO{sub x}, Zr-MnO{sub x} and Fe-MnO{sub x}, respectively. Sn-MnO{sub x} had a weak bond of mercury (Hg-O), while Zr-MnO{sub x} had a strong bond (Hg≡O). Ce-MnO{sub x} and Fe-MnO{sub x} had similar bonds compared with pure MnO{sub x}. Moreover, the effects of SO{sub 2} and NO were investigated based on Hg-TPD analysis. SO{sub 2} had a poison effect on Hg{sup 0} removal, and the weak bond of mercury can be easily destroyed by SO{sub 2}. NO was favorable for Hg{sup 0} removal, and the bond strength of mercury was enhanced.
The convection electric field in auroral substorms
DEFF Research Database (Denmark)
Gjerløv, Jesper Wittendorff; Hoffman, R.A.
2001-01-01
Dynamics Explorer 2 (DE 2) electric field and ion drift data are used in a statistical study of the ionospheric convection electric field in bulge-type auroral substorms. Thirty-one individual DE 2 substorm crossings were carefully selected and organized by the use of global auroral images obtained...... this database enabled us to compile a model of the ionospheric convection electric field. The characteristics of the premidnight convection reversal show a pronounced local time dependency. Far west of the surge it is a fairly well defined point reversal or convection shear. Approaching the surge and within...... the surge it is a region of weak electric fields increasing in width toward midnight that separates regions of equatorward and poleward electric fields. Therefore we adopt the term Harang region rather than the Harang discontinuity for the premidnight convection reversal. A relatively narrow convection...
Samokhvalov, Alexander; Duin, Evert C.; Nair, Sachin; Tatarchuk, Bruce J.
2011-02-01
Adsorption, desorption and structure of the surface chemical compounds formed upon interaction of dibenzothiophene (DBT) in solution of n-octane with the sulfur-selective Ag/Titania sorbent for the ultradeep desulfurization of liquid fuels was characterized by the temperature-programmed X-ray photoemission spectroscopy (XPS) and Electron Spin Resonance. Adsorption of DBT proceeds via chemisorption via the oxygen-containing surface groups. Desorption of DBT and thermal regeneration of the “spent” Ag/Titania were studied by the complementary temperature-programmed XPS and ESR from 25 °C to 525 °C, in the high vacuum vs. air. The XPS spectrum of the pure DBT is reported for the first time.
Energy Technology Data Exchange (ETDEWEB)
Samokhvalov, Alexander, E-mail: alexsam@camden.rutgers.edu [Department of Chemical Engineering, Auburn University, Auburn, AL 36849 (United States); Department of Chemistry, Rutgers University, Camden, NJ 08102 (United States); Duin, Evert C. [Department of Chemistry and Biochemistry, Auburn University, Auburn, AL 36849 (United States); Nair, Sachin; Tatarchuk, Bruce J. [Department of Chemical Engineering, Auburn University, Auburn, AL 36849 (United States)
2011-02-01
Adsorption, desorption and structure of the surface chemical compounds formed upon interaction of dibenzothiophene (DBT) in solution of n-octane with the sulfur-selective Ag/Titania sorbent for the ultradeep desulfurization of liquid fuels was characterized by the temperature-programmed X-ray photoemission spectroscopy (XPS) and Electron Spin Resonance. Adsorption of DBT proceeds via chemisorption via the oxygen-containing surface groups. Desorption of DBT and thermal regeneration of the 'spent' Ag/Titania were studied by the complementary temperature-programmed XPS and ESR from 25 deg. C to 525 deg. C, in the high vacuum vs. air. The XPS spectrum of the pure DBT is reported for the first time.
Land surface modeling in convection permitting simulations
van Heerwaarden, Chiel; Benedict, Imme
2017-04-01
The next generation of weather and climate models permits convection, albeit at a grid spacing that is not sufficient to resolve all details of the clouds. Whereas much attention is being devoted to the correct simulation of convective clouds and associated precipitation, the role of the land surface has received far less interest. In our view, convective permitting simulations pose a set of problems that need to be solved before accurate weather and climate prediction is possible. The heart of the problem lies at the direct runoff and at the nonlinearity of the surface stress as a function of soil moisture. In coarse resolution simulations, where convection is not permitted, precipitation that reaches the land surface is uniformly distributed over the grid cell. Subsequently, a fraction of this precipitation is intercepted by vegetation or leaves the grid cell via direct runoff, whereas the remainder infiltrates into the soil. As soon as we move to convection permitting simulations, this precipitation falls often locally in large amounts. If the same land-surface model is used as in simulations with parameterized convection, this leads to an increase in direct runoff. Furthermore, spatially non-uniform infiltration leads to a very different surface stress, when scaled up to the course resolution of simulations without convection. Based on large-eddy simulation of realistic convection events at a large domain, this study presents a quantification of the errors made at the land surface in convection permitting simulation. It compares the magnitude of the errors to those made in the convection itself due to the coarse resolution of the simulation. We find that, convection permitting simulations have less evaporation than simulations with parameterized convection, resulting in a non-realistic drying of the atmosphere. We present solutions to resolve this problem.
Examining Chaotic Convection with Super-Parameterization Ensembles
Jones, Todd R.
This study investigates a variety of features present in a new configuration of the Community Atmosphere Model (CAM) variant, SP-CAM 2.0. The new configuration (multiple-parameterization-CAM, MP-CAM) changes the manner in which the super-parameterization (SP) concept represents physical tendency feedbacks to the large-scale by using the mean of 10 independent two-dimensional cloud-permitting model (CPM) curtains in each global model column instead of the conventional single CPM curtain. The climates of the SP and MP configurations are examined to investigate any significant differences caused by the application of convective physical tendencies that are more deterministic in nature, paying particular attention to extreme precipitation events and large-scale weather systems, such as the Madden-Julian Oscillation (MJO). A number of small but significant changes in the mean state climate are uncovered, and it is found that the new formulation degrades MJO performance. Despite these deficiencies, the ensemble of possible realizations of convective states in the MP configuration allows for analysis of uncertainty in the small-scale solution, lending to examination of those weather regimes and physical mechanisms associated with strong, chaotic convection. Methods of quantifying precipitation predictability are explored, and use of the most reliable of these leads to the conclusion that poor precipitation predictability is most directly related to the proximity of the global climate model column state to atmospheric critical points. Secondarily, the predictability is tied to the availability of potential convective energy, the presence of mesoscale convective organization on the CPM grid, and the directive power of the large-scale.
REVERSALS IN THE 6-CELLS CONVECTION DRIVEN
Directory of Open Access Journals (Sweden)
G.M. Vodinchar
2015-12-01
Full Text Available We describe the large-scale model geodynamo, which based on indirect data of inhomogeneities in the density of the Earth’s core. Convection structure is associated with spherical harmonic Y24 , which defines the basic poloidal component of velocity. Coriolis drift of this mode determines the toroidal component of velocity. Thus, 6 convective cells are formed. The model takes into account the feedback effect of the magnetic field on convection. It was ascertained that the model contains stable regimes of field generation. The velocity of convection and the dipole component of the magnetic field are close to the observed ones.
Rotating convection in a viscoelastic magnetic fluid
Energy Technology Data Exchange (ETDEWEB)
Pérez, L.M. [Departamento de Fíisica y Matemática Aplicada, Universidad de Navarra, 31080 Pamplona (Spain); Laroze, D., E-mail: dlarozen@uta.cl [Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica (Chile); Díaz, P. [Departamento de Ciencias Físicas, Universidad de La Frontera, Casilla 54 D, Temuco (Chile); Martinez-Mardones, J. [Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso (Chile); Mancini, H.L. [Departamento de Fíisica y Matemática Aplicada, Universidad de Navarra, 31080 Pamplona (Spain)
2014-09-01
We report theoretical and numerical results on convection for a magnetic fluid in a viscoelastic carrier liquid under rotation. The viscoelastic properties are given by the Oldroyd model. We obtain explicit expressions for the convective thresholds in terms of the parameters of the system in the case of idealized boundary conditions. We also calculate numerically the convective thresholds for the case of realistic boundary conditions. The effects of the rheology and of the rotation rate on the instability thresholds for a diluted magnetic suspension are emphasized. - Highlights: • Ferrofluids. • Thermal convection. • Viscoelastic model. • Realistic boundary conditions.
On sound generation by turbulent convection: A new look at old results
Musielak, Z. E.; Rosner, R.; Stein, R. F.; Ulmschneider, P.
1994-01-01
We have revisited the problem of acoustic wave generation by turbulent convection in stellar atmospheres. The theory of aerodynamically generated sound, originally developed by Lighthill and later modified by Stein to include the effects of stratification, has been used to estimate the acoustic wave energy flux generated in solar and stellar convection zones. We correct the earlier computations by incorporating an improved description of the spatial and temporal spectrum of the turbulent convection. We show the dependence of the resulting wave fluxes on the nature of the turbulence, and compute the wave energy spectra and wave energy fluxes generated in the Sun on the basis of a mixing-length model of the solar convection zone. In contrast to the previous results, we show that the acoustic energy generation does not depend very sensitively on the turbulent energy spectrum. However, typical total acoustic fluxes of order F(sub A) = 5 x 10(exp 7) ergs/sq cm/s with a peak of the acoustic frequency spectrum near omega = 100 mHz are found to be comparable to those previously calculated. The acoustic flux turns out to be strongly dependent on the solar model, scaling with the mixing-length parameter alpha as alpha(exp 3.8). The computed fluxes most likely constitute a lower limit on the acoustic energy produced in the solar convection zone if recent convection simulations suggesting the presence of shocks near the upper layers of the convection zone apply to the Sun.
Energy Technology Data Exchange (ETDEWEB)
Oreper, G.M.; Eagar, T.W.; Szekely, J.
1982-11-01
A mathematical model was developed to account for convection and temperature distributions in stationary arc weld pools driven by buoyancy, electromagnetic and surface tension forces. It is shown that the electromagnetic and surface tension forces dominate the flow behavior. In some cases, these forces produce double circulation loops, which are indirectly confirmed by experimental measurements of segregation in the weld pool. It is also shown that the surface tension driven flows are very effective in dissipating the incident energy flux on the pool surface which, in turn, reduces the vaporization from the weld pool.
A Dynamically Computed Convective Time Scale for the Kain–Fritsch Convective Parameterization Scheme
Many convective parameterization schemes define a convective adjustment time scale τ as the time allowed for dissipation of convective available potential energy (CAPE). The Kain–Fritsch scheme defines τ based on an estimate of the advective time period for deep con...
Towards a parameterization of convective wind gusts in Sahel
Largeron, Yann; Guichard, Françoise; Bouniol, Dominique; Couvreux, Fleur; Birch, Cathryn; Beucher, Florent
2014-05-01
] who focused on the wet tropical Pacific region, and linked wind gusts to convective precipitation rates alone, here, we also analyse the subgrid wind distribution during convective events, and quantify the statistical moments (variance, skewness and kurtosis) in terms of mean wind speed and convective indexes such as DCAPE. Next step of the work will be to formulate a parameterization of the cold pool convective gust from those probability density functions and analytical formulaes obtained from basic energy budget models. References : [Carslaw et al., 2010] A review of natural aerosol interactions and feedbacks within the earth system. Atmospheric Chemistry and Physics, 10(4):1701{1737. [Engelstaedter et al., 2006] North african dust emissions and transport. Earth-Science Reviews, 79(1):73{100. [Knippertz and Todd, 2012] Mineral dust aerosols over the sahara: Meteorological controls on emission and transport and implications for modeling. Reviews of Geophysics, 50(1). [Marsham et al., 2011] The importance of the representation of deep convection for modeled dust-generating winds over west africa during summer.Geophysical Research Letters, 38(16). [Marticorena and Bergametti, 1995] Modeling the atmospheric dust cycle: 1. design of a soil-derived dust emission scheme. Journal of Geophysical Research, 100(D8):16415{16. [Menut, 2008] Sensitivity of hourly saharan dust emissions to ncep and ecmwf modeled wind speed. Journal of Geophysical Research: Atmospheres (1984{2012), 113(D16). [Pierre et al., 2012] Impact of vegetation and soil moisture seasonal dynamics on dust emissions over the sahel. Journal of Geophysical Research: Atmospheres (1984{2012), 117(D6). [Redelsperger et al., 2000] A parameterization of mesoscale enhancement of surface fluxes for large-scale models. Journal of climate, 13(2):402{421.
Nonlinear Pulsations of Convective Stellar Models
Buchler, J. Robert
1999-01-01
We review the numerical modelling of the nonlinear pulsations of classical variable stars with hydrocodes that include the effects of turbulent convection. Despite their simplicity these turbulent convective recipes appear to remove many of the difficulties that radiative codes faced. In particular, the numerical modelling of double mode pulsations has become possible.
Spurious multiple equilibria introduced by convective adjustment
Toom, Matthijs den; Dijkstra, Henk A.; Wubs, Fred W.
2011-01-01
The application of bifurcation analysis to ocean climate models is substantially hampered by difficulties associated with the use of convective adjustment, i.e. a parameterisation of convection in which the vertical diffusion of heat and salt is greatly enhanced whenever the water column becomes
Convective penetration in a young sun
Pratt, Jane; Baraffe, Isabelle; Goffrey, Tom; MUSIC developers group
2018-01-01
To interpret the high-quality data produced from recent space-missions it is necessary to study convection under realistic stellar conditions. We describe the multi-dimensional, time implicit, fully compressible, hydrodynamic, implicit large eddy simulation code MUSIC. We use MUSIC to study convection during an early stage in the evolution of our sun where the convection zone covers approximately half of the solar radius. This model of the young sun possesses a realistic stratification in density, temperature, and luminosity. We approach convection in a stellar context using extreme value theory and derive a new model for convective penetration, targeted for one-dimensional stellar evolution calculations. This model provides a scenario that can explain the observed lithium abundance in the sun and in solar-like stars at a range of ages.
True polar wander on convecting planets
Rose, Ian Robert
Rotating planets are most stable when spinning around their maximum moment of inertia, and will tend to reorient themselves to achieve this configuration. Geological activity redistributes mass in the planet, making the moment of inertia a function of time. As the moment of inertia of the planet changes, the spin axis shifts with respect to a mantle reference frame in order to maintain rotational stability. This process is known as true polar wander (TPW). Of the processes that contribute to a planet's moment of inertia, convection in the mantle generates the largest and longest-period fluctuations, with corresponding shifts in the spin axis. True polar wander has been hypothesized to explain several physiographic features on planets and moons in our solar system. On Earth, TPW events have been invoked in some interpretations of paleomagnetic data. Large swings in the spin axis could have enormous ramifications for paleogeography, paleoclimate, and the history of life. Although the existence of TPW is well-verified, it is not known whether its rate and magnitude have been large enough for it to be an important process in Earth history. If true polar wander has been sluggish compared to plate tectonic speeds, then it would be difficult to detect and its consequences would be minor. I investigate rates of true polar wander on convecting planets using scaling, numerics, and inverse problems. I perform a scaling analysis of TPW on a convecting planet, identifying a minimal set of nondimensional parameters which describe the problem. The primary nondimensional numbers that control the rate of TPW are the ratio of centrifugal to gravitational forces m and the Rayleigh number Ra. The parameter m sets the size of a planet's rotational bulge, which determines the amount of work that needs to be done to move the spin axis. The Rayleigh number controls the size, distribution, and rate of change of moment of inertia anomalies, all of which affect the rate of TPW. I find that
Magnetic Fields in the Solar Convection Zone
Directory of Open Access Journals (Sweden)
Fan Yuhong
2004-07-01
Full Text Available Recent studies of the dynamic evolution of magnetic flux tubes in the solar convection zone are reviewed with focus on emerging flux tubes responsible for the formation of solar active regions. The current prevailing picture is that active regions on the solar surface originate from strong toroidal magnetic fields generated by the solar dynamo mechanism at the thin tachocline layer at the base of the solar convection zone. Thus the magnetic fields need to traverse the entire convection zone before they reach the photosphere to form the observed solar active regions. This review discusses results with regard to the following major topics: 1. the equilibrium properties of the toroidal magnetic fields stored in the stable overshoot region at the base of the convection zone, 2. the buoyancy instability associated with the toroidal magnetic fields and the formation of buoyant magnetic flux tubes, 3. the rise of emerging flux loops through the solar convective envelope as modeled by the thin flux tube calculations which infer that the field strength of the toroidal magnetic fields at the base of the solar convection zone is significantly higher than the value in equipartition with convection, 4. the minimum twist needed for maintaining cohesion of the rising flux tubes, 5. the rise of highly twisted kink unstable flux tubes as a possible origin of d -sunspots, 6. the evolution of buoyant magnetic flux tubes in 3D stratified convection, 7. turbulent pumping of magnetic flux by penetrative compressible convection, 8. an alternative mechanism for intensifying toroidal magnetic fields to significantly super-equipartition field strengths by conversion of the potential energy associated with the superadiabatic stratification of the solar convection zone, and finally 9. a brief overview of our current understanding of flux emergence at the surface and post-emergence evolution of the subsurface magnetic fields.
Marinov, I.; Cabre, A.; Gunn, A.; Gnanadesikan, A.
2016-12-01
The current generation (CMIP5) of Earth System Models (ESMs) shows a huge variability in their ability to represent Southern Ocean (SO) deep-ocean convection and Antarctic Bottom Water, with a preference for open-sea convection in the Weddell and Ross gyres. A long control simulation in a coarse 3o resolution ESM (the GFDL CM2Mc model) shows a highly regular multi-decadal oscillation between periods of SO open sea convection and non-convective periods. This process also happens naturally, with different frequencies and durations of convection across most CMIP5 models under preindustrial forcing (deLavergne et al, 2014). Here we assess the impact of SO deep convection and resulting sea surface temperature (SST) anomalies on the tropical atmosphere and ocean via teleconnections, with a focus on interannual to multi-decadal timescales. We combine analysis of our low-resolution coupled model with inter-model analysis across historical CMIP5 simulations. SST cooling south of 60S during non-convective decades triggers a stronger, northward shifted SH Hadley cell, which results in intensified northward cross-equatorial moist heat transport and a poleward shift in the ITCZ. Resulting correlations between the cross-equatorial atmospheric heat transport and ITCZ location are in good agreement with recent theories (e.g. Frierson et al. 2013; Donohoe et al. 2014). Lagged correlations between a SO convective index and cross-equatorial heat transports (in the atmosphere and ocean), as well as various tropical (and ENSO) climate indices are analyzed. In the ocean realm, we find that non-convective decades result in weaker AABW formation and weaker ACC but stronger Antarctic Intermediate Water (AAIW) formation, likely as a result of stronger SO westerlies (more positive SAM). The signals of AABW and AAIW are seen in the tropics on short timescales of years to decades in the temperature, heat storage and heat transport anomalies and also in deep and intermediate ocean oxygen. Most
Effects of gravity modulation on convection in a horizontal annulus
Energy Technology Data Exchange (ETDEWEB)
Dyko, Mark P. [Aircraft Braking Systems Corporation, Akron, Ohio (United States). Research and Technology; Vafai, Kambiz [University of California, Riverside, CA (United States). Department of Mechanical Engineering
2007-01-15
Convection in the annulus between two horizontal coaxial cylinders resulting from gravity modulation in a microgravity environment is investigated for the first time. The three-dimensional transient equations of fluid motion and heat transfer are solved to study the unsteady flow structures in a large-gap annulus. The gravity fluctuations are shown to induce recirculating flows that reverse direction of rotation in response to the gravitational reversals. Except for a short period of time following flow reversal, at low modulation frequencies the centers of rotation of these flows are below and above the horizontal angular plane when the acceleration acts in the upward and downward directions, respectively, whereas at high frequencies they are above and below this plane. The effects of gravity modulation on development of secondary flows that arise in narrow- and moderate-gap annuli owing to thermal instability are also investigated. It is found that supercritical transverse rolls repeatedly form, dissipate, and re-form in the upper and lower regions of a narrow-gap annulus as a result of the fluctuating gravity field. At the same frequency, the supercritical rolls in a moderate-gap annulus are much slower to develop between flow reversals. The results show that gravity modulation has a stabilizing effect on the secondary flows compared to the case of a constant gravity force, and this effect diminishes with reduction in either frequency or annulus radius ratio R. The effects of frequency on heat transfer in large-, moderate-, and narrow-gap annuli are also studied. It is found that the time-averaged Nusselt number approaches that of pure conduction at high frequencies and increases toward that for terrestrial natural convection as frequency is reduced. As R is decreased, the increase in time-averaged Nusselt number brought about by reducing frequency becomes a smaller percentage of the value for terrestrial natural convection. The results for large-, moderate
Sutrisno
A theoretical analysis of the effects of a gravity field on gaseous helium confined in a slender two dimensional box when boundary heating suddenly takes place at the wall is described. The gravity field is applied vertically. A heat flux, capable of producing an appreciable wall temperature increase, is applied uniformly along the longer vertical walls. A systematic analysis using various perturbation techniques is carried out in the central region away from the upper and lower insulated boundaries. A complete system of conservation equations governing viscous, heat conducting and compressible flow is employed. Analysis on the acoustic timescale shows that conduction heating takes place only in a thin, continuously growing boundary layer near the heated wall. On the conduction timescale, both acoustic and conduction phenomena occur simultaneously in the container. Multiple timescale methods are employed to separate the high frequency acoustic equations from the lower frequency conduction equations. In this fully conductive system, energy is transferred by conduction among the fluid particles which at the same time experience thermal expansion and compression.
Non-linear thermal convection in a
Directory of Open Access Journals (Sweden)
Sachin Shaw
2016-06-01
Full Text Available Casson fluid flow has many practical applications such as food processing, metallurgy, drilling operations and bio-engineering operations. In this paper, we study Casson fluid flow through a plate with a convective boundary condition at the surface and quantify the effects of suction/injection, velocity ratio, and Soret and Dufour effects. Firstly we used a similarity transformation to change the governing equations to ordinary differential equations which were then solved numerically. The effect of the rheological parameters on the velocity, temperature, and concentration with skin friction, and heat and mass transfer are shown graphically and discussed briefly. It is observed that the velocity of the fluid at the surface decreases with increase of the velocity ratio while the nature of the flow is in opposite characteristics. The local Nusselt number decreases with increase in the velocity ratio. Skin friction at the surface is enhanced by buoyancy ratio and Casson number. Due to injection of the fluid in the system, the mass transfer rate at the surface increases while it decreases with the velocity ratio parameter.
Seismic sounding of convection in the Sun
Sreenivasan, Katepalli R.
2015-11-01
Thermal convection is the dominant mechanism of energy transport in the outer envelope of the Sun (one-third by radius). It drives global fluid circulations and magnetic fields observed on the solar surface. Convection excites a broadband spectrum of acoustic waves that propagate within the interior and set up modal resonances. These acoustic waves, also called seismic waves, are observed at the surface of the Sun by space- and ground-based telescopes. Seismic sounding, the study of these seismic waves to infer the internal properties of the Sun, constitutes helioseismology. Here we review our knowledge of solar convection, especially that obtained through seismic inference. Several characteristics of solar convection, such as differential rotation, anisotropic Reynolds stresses, the influence of rotation on convection and supergranulation, are considered. On larger scales, several inferences suggest that convective velocities are substantially smaller than those predicted by theory and simulations. This discrepancy challenges the models of internal differential rotation that rely on convective stresses as a driving mechanism and provide an important benchmark for numerical simulations. In collaboration with Shravan Hanasoge, Tata Institute of Fundamental Research, Mumbai and Laurent Gizon, Max-Planck-Institut fuer Sonnensystemforschung, Goettingen.
Phenomenological Nusselt-Rayleigh Scaling of Turbulent Thermal Convection
Liu, Chien-Chia
2017-12-01
Natural convection between the hot floor and the cool ceiling, so called Rayleigh-Bénard convection, is pervasive and of both fundamental and industrial interests. One key issue is how heat transfer varies with increasing thermal potential, or equivalently how the Nusselt number (Nu) scales with the Rayleigh number (Ra). The overview of experimental findings remains to show the need of extra explanation complemental to the current theories. Here we present a model based on the phenomenological theory of turbulence, where the power-law spectral exponent of the energy spectrum is the only input parameter required. The goal aims to elucidate the unexplained aspect in the Nu-Ra scaling. We find that Kolmogorov turbulence in the current model leads to Nu ˜ Ra0.3, in good agreement with the modern experimental results. We hope that this model could stimulate the discussion as to the effects of the spectral phenomena on the Nu-Ra scaling, and thus augment our understanding of buoyancy-driven thermal convection.
Lightning-based propagation of convective rain fields
Directory of Open Access Journals (Sweden)
S. Dietrich
2011-05-01
Full Text Available This paper describes a new multi-sensor approach for continuously monitoring convective rain cells. It exploits lightning data from surface networks to propagate rain fields estimated from multi-frequency brightness temperature measurements taken by the AMSU/MHS microwave radiometers onboard NOAA/EUMETSAT low Earth orbiting operational satellites. Specifically, the method allows inferring the development (movement, morphology and intensity of convective rain cells from the spatial and temporal distribution of lightning strokes following any observation by a satellite-borne microwave radiometer. Obviously, this is particularly attractive for real-time operational purposes, due to the sporadic nature of the low Earth orbiting satellite measurements and the continuous availability of ground-based lightning measurements – as is the case in most of the Mediterranean region. A preliminary assessment of the lightning-based rainfall propagation algorithm has been successfully made by using two pairs of consecutive AMSU observations, in conjunction with lightning measurements from the ZEUS network, for two convective events. Specifically, we show that the evolving rain fields, which are estimated by applying the algorithm to the satellite-based rainfall estimates for the first AMSU overpass, show an overall agreement with the satellite-based rainfall estimates for the second AMSU overpass.
Magnetic Cycles in a Dynamo Simulation of Fully Convective M-star Proxima Centauri
Yadav, Rakesh K.; Christensen, Ulrich R.; Wolk, Scott J.; Poppenhaeger, Katja
2016-12-01
The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period ≲20 days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here, we present an anelastic dynamo simulation designed to mimic some of the physical characteristics of Proxima Centauri, a representative case for slowly rotating fully convective M-stars. The rotating convection spontaneously generates differential rotation in the convection zone that drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the “activity” cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age.
Asteroseismic Constraints on the Models of Hot B Subdwarfs: Convective Helium-Burning Cores
Schindler, Jan-Torge; Green, Elizabeth M.; Arnett, W. David
2017-10-01
Asteroseismology of non-radial pulsations in Hot B Subdwarfs (sdB stars) offers a unique view into the interior of core-helium-burning stars. Ground-based and space-borne high precision light curves allow for the analysis of pressure and gravity mode pulsations to probe the structure of sdB stars deep into the convective core. As such asteroseismological analysis provides an excellent opportunity to test our understanding of stellar evolution. In light of the newest constraints from asteroseismology of sdB and red clump stars, standard approaches of convective mixing in 1D stellar evolution models are called into question. The problem lies in the current treatment of overshooting and the entrainment at the convective boundary. Unfortunately no consistent algorithm of convective mixing exists to solve the problem, introducing uncertainties to the estimates of stellar ages. Three dimensional simulations of stellar convection show the natural development of an overshooting region and a boundary layer. In search for a consistent prescription of convection in one dimensional stellar evolution models, guidance from three dimensional simulations and asteroseismological results is indispensable.
Direct numerical simulation and statistical analysis of turbulent convection in lead-bismuth
Energy Technology Data Exchange (ETDEWEB)
Otic, I.; Grotzbach, G. [Forschungszentrum Karlsruhe GmbH, Institut fuer Kern-und Energietechnik (Germany)
2003-07-01
Improved turbulent heat flux models are required to develop and analyze the reactor concept of an lead-bismuth cooled Accelerator-Driven-System. Because of specific properties of many liquid metals we have still no sensors for accurate measurements of the high frequency velocity fluctuations. So, the development of the turbulent heat transfer models which are required in our CFD (computational fluid dynamics) tools needs also data from direct numerical simulations of turbulent flows. We use new simulation results for the model problem of Rayleigh-Benard convection to show some peculiarities of the turbulent natural convection in lead-bismuth (Pr = 0.025). Simulations for this flow at sufficiently large turbulence levels became only recently feasible because this flow requires the resolution of very small velocity scales with the need for recording long-wave structures for the slow changes in the convective temperature field. The results are analyzed regarding the principle convection and heat transfer features. They are also used to perform statistical analysis to show that the currently available modeling is indeed not adequate for these fluids. Basing on the knowledge of the details of the statistical features of turbulence in this convection type and using the two-point correlation technique, a proposal for an improved statistical turbulence model is developed which is expected to account better for the peculiarities of the heat transfer in the turbulent convection in low Prandtl number fluids. (authors)
Effects of Deep Convection on Atmospheric Chemistry
Pickering, Kenneth E.
2007-01-01
This presentation will trace the important research developments of the last 20+ years in defining the roles of deep convection in tropospheric chemistry. The role of deep convection in vertically redistributing trace gases was first verified through field experiments conducted in 1985. The consequences of deep convection have been noted in many other field programs conducted in subsequent years. Modeling efforts predicted that deep convection occurring over polluted continental regions would cause downstream enhancements in photochemical ozone production in the middle and upper troposphere due to the vertical redistribution of ozone precursors. Particularly large post-convective enhancements of ozone production were estimated for convection occurring over regions of pollution from biomass burning and urban areas. These estimates were verified by measurements taken downstream of biomass burning regions of South America. Models also indicate that convective transport of pristine marine boundary layer air causes decreases in ozone production rates in the upper troposphere and that convective downdrafts bring ozone into the boundary layer where it can be destroyed more rapidly. Additional consequences of deep convection are perturbation of photolysis rates, effective wet scavenging of soluble species, nucleation of new particles in convective outflow, and the potential fix stratosphere-troposphere exchange in thunderstorm anvils. The remainder of the talk will focus on production of NO by lightning, its subsequent transport within convective clouds . and its effects on downwind ozone production. Recent applications of cloud/chemistry model simulations combined with anvil NO and lightning flash observations in estimating NO Introduction per flash will be described. These cloud-resolving case-study simulations of convective transport and lightning NO production in different environments have yielded results which are directly applicable to the design of lightning
Transient Mixed Convection Validation for NGNP
Energy Technology Data Exchange (ETDEWEB)
Smith, Barton [Utah State Univ., Logan, UT (United States); Schultz, Richard [Idaho National Lab. (INL), Idaho Falls, ID (United States)
2015-10-19
The results of this project are best described by the papers and dissertations that resulted from the work. They are included in their entirety in this document. They are: (1) Jeff Harris PhD dissertation (focused mainly on forced convection); (2) Blake Lance PhD dissertation (focused mainly on mixed and transient convection). This dissertation is in multi-paper format and includes the article currently submitted and one to be submitted shortly; and, (3) JFE paper on CFD Validation Benchmark for Forced Convection.
High Rayleigh number convection numerical experiments
Verzicco, Roberto
2002-03-01
seem to support the idea that the observed transitional behaviors have to be attributed to the change in the topology of the mean flow rather than to a transition from a laminar to a turbulent state of the viscous boundary layers. Other issues accomplished by the simulation concern the study of the scaling properties of the turbulent quantities and length scales in terms of Ra. Finally, further details on the turbulence dynamics are obtained by the analysis of the power spectra and low order structure functions of both the temperature and the velocity components, computed from the numerical probes both within the bulk region and close to the walls. References Roche, PE; Castaing, B; Chabaud, B; Hebral, B. ``Observation of the 1/2 power law in Rayleigh-Benard convection'' Phys. Rev. E, 2001, 6304(4), p. 5303. Niemela, J.J.; Skrbek, L.; Sreenivasan, K.R. and Donnelly, R.J. ``Turbulent convection at very high Rayleigh numbers'' Nature, 405, 243-253 (11 May 2000). Verzicco, R. and Camussi, R. ``Prandtl number effects in convective turbulence'' J. of Fluid Mech., 383, (1999), 55-73.
Helium enrichment during convective carbon dioxide dissolution
Larson, T.; Hesse, M. A.
2013-12-01
Motivated by observed variations of the CO2/He ratios in natural carbon dioxide (CO2) reservoirs, such as the Bravo Dome field in northeastern New Mexico, we have performed laboratory experiments equilibrating gas mixtures containing Helium (He) and CO2 with water, at close to ambient conditions in a closed system. The experimental design allows for continuous measurement of headspace pressure as well as timed interval measurements of the CO2/He ratios and the δ13C value of CO2 in the headspace. Results from three dissolution experiments are reported: 1) pure Helium system, 2) 98% CO2 + 2% Nitrogen system, and 3) 97% CO2 and 3% Helium. Final equilibrated experimental results are compared to theoretical results obtained using Henry's Law relationships. The evolution of the amount of dissolved CO2 computed from gas pressure and gas compositions are in good agreement with Henry's Law relationships. For example, the CO2 + N2 system was initially pressurized with pure CO2 to 1323 mbar and after six days it equilibrated to a measured headspace pressure of 596 mbar. This compares very well with a calculated equilibrium headspace pressure of 592 mbar for this system. The CO2 + He system was pressurized to 1398 mbar CO2 and after six days equilibrated to a measured headspace pressure of 397 mbar. This measured pressure is slightly higher than the predicted equilibrated headspace pressure of 341 mbar, indicating a possible leak in the system during this particular experiment. In both experiments the initial pH of the water was 9.3 and the final equilibrated pH was 5.4. The δ13C value of equilibrated headspace CO2 was within 0.25‰ of its starting δ13C value, demonstrating insignificant carbon isotope fractionation at low pH. Measured Helium/ CO2 ratios throughout the CO2+Helium experiment preserve a non-linear trend of increasing He/ CO2 ratios through time that correlate very well with the measured pressure drop from CO2 dissolution. This indicates that gas composition
Directory of Open Access Journals (Sweden)
Nee Alexander
2016-01-01
Full Text Available Mathematical modeling of conjugate natural convection in a closed rectangular cavity with a radiant energy source in conditions of convective-radiative heat exchange at the external boundary was conducted. The radiant energy distribution was set by the Lambert’s law. Conduction and convection processes analysis showed that the air masses flow pattern is modified slightly over the time. The temperature increases in the gas cavity, despite the heat removal from the one of the external boundary. According to the results of the integral heat transfer analysis were established that the average Nusselt number (Nuav increasing occurs up to τ = 200 (dimensionless time. Further Nuav has changed insignificantly due to the temperature field equalization near the interfaces “gas – wall”.
DEFF Research Database (Denmark)
Le Dreau, Jerome; Heiselberg, Per; Jensen, Rasmus Lund
2015-01-01
-state and dynamic conditions. With the air-based cooling system, a dependency of the convective heat transfer on the air jet trajectory has been observed. New correlations have been developed, introducing a modified Archimedes number to account for the air flow pattern. The accuracy of the new correlations has been...... evaluated to±15%. Besides the study with an air-based cooling system, the convective heat transfer with a radiant cooling system has also been investigated. The convective flow at the activated surface is mainly driven by natural convection. For other surfaces, the complexity of the flow and the large......The complexity and diversity of airflow in buildings make the accurate definition of convective heat transfer coefficients (CHTCs) difficult. In a full-scale test facility, the convective heat transfer of two cooling systems (active chilled beam and radiant wall) has been investigated under steady...
Coexisting pulses in a model for binary-mixture convection
Energy Technology Data Exchange (ETDEWEB)
Riecke, H.; Rappel, W. [Department of Engineering Sciences and Applied Mathematics, Northwestern University, Evanston, Illinois 60208 (United States)]|[Department of Physics, Northeastern University, 111 Dana Research Center, Boston, Massachusetts 02115 (United States)
1995-11-27
We address the striking coexistence of localized waves (``pulses``) of different lengths, which was observed in recent experiments and full numerical simulations of binary-mixture convection. Using a set of extended Ginzburg-Landau equations, we show that this multiplicity finds a natural explanation in terms of the competition of two distinct, physical localization mechanisms; one arises from dispersion and the other from a concentration mode. This competition is absent in the standard Ginzburg-Landau equation. It may also be relevant in other waves coupled to a large-scale field. {copyright} {ital 1995} {ital The} {ital American} {ital Physical} {ital Society}.
Convective Radio Occultations Final Campaign Summary
Energy Technology Data Exchange (ETDEWEB)
Biondi, R. [Atmospheric Radiation Measurement, Washington, DC (United States)
2016-03-01
Deep convective systems are destructive weather phenomena that annually cause many deaths and injuries as well as much damage, thereby accounting for major economic losses in several countries. The number and intensity of such phenomena have increased over the last decades in some areas of the globe. Damage is mostly caused by strong winds and heavy rain parameters that are strongly connected to the structure of the particular storm. Convection over land is usually stronger and deeper than over the ocean and some convective systems, known as supercells, also develop tornadoes through processes that remain mostly unclear. The intensity forecast and monitoring of convective systems is one of the major challenges for meteorology because in situ measurements during extreme events are too sparse or unreliable and most ongoing satellite missions do not provide suitable time/space coverage.
Layer Formation in Sedimentary Fingering Convection
Reali, J F; Alsinan, A; Meiburg, E
2016-01-01
When particles settle through a stable temperature or salinity gradient they can drive an instability known as sedimentary fingering convection. This phenomenon is thought to occur beneath sediment-rich river plumes in lakes and oceans, in the context of marine snow where decaying organic materials serve as the suspended particles, or in the atmosphere in the presence of aerosols or volcanic ash. Laboratory experiments of Houk and Green (1973) and Green (1987) have shown sedimentary fingering convection to be similar to the more commonly known thermohaline fingering convection in many ways. Here, we study the phenomenon using 3D direct numerical simulations. We find evidence for layer formation in sedimentary fingering convection in regions of parameter space where it does not occur for non-sedimentary systems. This is due to two complementary effects. Sedimentation affects the turbulent fluxes and broadens the region of parameter space unstable to the $\\gamma$-instability (Radko 2003) to include systems at l...
Transient natural convection heat and mass transfer in crystal growth
Han, Samuel S.
1990-01-01
A numerical analysis of transient combined heat and mass transfer across a rectangular cavity is performed. The physical parameters are selected to represent a range of possible crystal growth in solutions. Good agreements with measurement data are observed. It is found that the thermal and solute fields become highly oscillatory when the thermal and solute Grashof numbers are large.
Flow Visualization of Forced and Natural Convection in Internal Cavities
Energy Technology Data Exchange (ETDEWEB)
John Crepeau; Hugh M. Mcllroy,Jr.; Donald M. McEligot; Keith G. Condie; Glenn McCreery; Randy Clarsean; Robert S. Brodkey; Yann G. Guezennec
2002-01-31
The report descries innovative flow visualization techniques, fluid mechanics measurements and computational models of flows in a spent nuclear fuel canister. The flow visualization methods used a fluid that reacted with a metal plate to show how a local reaction affects the surrounding flow. A matched index of refraction facility was used to take mean flow and turbulence measurements within a generic spent nuclear fuel canister. Computational models were also made of the flow in the canister. It was determined that the flow field in the canister was very complex, and modifications may need to be made to ensure that the spent fuel elements are completely passivated.
Dynamic Modeling of Natural Convection Solar Energy Collector
African Journals Online (AJOL)
NIJOTECH
which occurred between 1800s and 3600s after solar noon where global radiation had its maximum. The shift is attributable to the heat capacities of the materials of the collector Separation of the transient terms from the steady- state terms for the plates' temperatures showed the transient contribution to be very small and ...
dynamic modeling of natural convection solar energy flat plate ...
African Journals Online (AJOL)
NIJOTECH
exposed to solar radiation, and was not given further black painting. The glazing material was clear window glass, 0.004m thick, 1.225m long, 0.95m wide, with 0.94m of the width exposed to solar radiation. The insulation material was sawdust obtained from a sawmill at Nsukka, Nigeria. The dimensions of the collector and ...
Dynamic Modeling of Natural Convection Solar Energy Collectors ...
African Journals Online (AJOL)
This was confirmed when the transient terms were completely deleted as the maximum values of the output parameters and their times of occurrence remained the same in both cases. The developed output expressions (in closed form) for the dynamic model of flat plate solar energy air heating collectors can easily be used ...
Dynamic Modeling of Natural Convection Solar Energy Collector
African Journals Online (AJOL)
NIJOTECH
ENERGY COLLECTORS FOR AGRICULTURAL DRIERS I: THEORY. AND COMPLETE RUN OF THE SOLUTIONS TO ... agricultural products like meat, fish, grains and vegetables and bring them to acceptable equilibrium moisture content to ... Sun in the forms of heat and light (two forms of electromagnetic radiations), life.
Experimental study in natural convection | Ousmane | Global Journal ...
African Journals Online (AJOL)
The study of thermal and ventilation parameters, obtained in a transient, laminar solar chimney of reduced dimensions, (1 < m <3) m with a square collector (side = 2m) is presented. Experimental measurements has been made to determine the temperature of the absorber and the fluid in the collector, it is shown that at the ...
Effect Of Natural Convection On Directional Solidification Of Pure Metal
Directory of Open Access Journals (Sweden)
Skrzypczak T.
2015-06-01
Full Text Available The paper is focused on the modeling of the directional solidification process of pure metal. During the process the solidification front is sharp in the shape of the surface separating liquid from solid in three dimensional space or a curve in 2D. The position and shape of the solid-liquid interface change according to time. The local velocity of the interface depends on the values of heat fluxes on the solid and liquid sides. Sharp interface solidification belongs to the phase transition problems which occur due to temperature changes, pressure, etc. Transition from one state to another is discontinuous from the mathematical point of view. Such process can be identified during water freezing, evaporation, melting and solidification of metals and alloys, etc.
Time-Dependent Natural Convection Couette Flow of Heat ...
African Journals Online (AJOL)
Laplace transform techniques is used to obtain the analytical solutions for the temperature and the velocity profiles while the rate of heat transfer as well the skin friction are consequently derived. The numerical simulation conducted for some saturated liquids reveled that at t ≥ Pr the steady and unsteady state velocities (as ...
Novel Natural Convection Heat Sink Design Concepts From First Principles
2016-06-01
effective f fluid ,m i mean mixing-cup at inlet ,m o mean mixing-cup at outlet open open boundary p pressure rad radiation s surface th...utilizing principles of the stack effect (i.e., using multiple inlets to create a suction effect to draw cooler ambient air into the tube). The second...of flow and heat transfer were carried out for constant temperature heat addition for single vertical tubes, multiple stacked tubes, tube system
Dynamic Modeling of Natural Convection Solar Energy Flat Plate ...
African Journals Online (AJOL)
The analytical solutions to the dynamic model of an air-heating flat plate solar energy thermal collector were validated by direct measurement from a physical model constructed for that purpose, of the temperatures of the cover and absorber plates, the inlet and outlet fluids, and the ambient air from morning to evening for ...
Controlling arbitrary humidity without convection.
Wasnik, Priyanka S; N'guessan, Hartmann E; Tadmor, Rafael
2015-10-01
In this paper we show a way that allows for the first time to induce arbitrary humidity of desired value for systems without convective flow. To enable this novelty we utilize a semi-closed environment in which evaporation is not completely suppressed. In this case, the evaporation rate is determined both by the outer (open) humidity and by the inner (semi-closed) geometry including the size/shape of the evaporating medium and the size/shape of the semi-closure. We show how such systems can be used to induce desired humidity conditions. We consider water droplet placed on a solid surface and study its evaporation when it is surrounded by other drops, hereon "satellite" drops and covered by a semi-closed hemisphere. The main drop's evaporation rate is proportional to its height, in agreement with theory. Surprisingly, however, the influence of the satellite drops on the main drop's evaporation suppression is not proportional to the sum of heights of the satellite drops. Instead, it shows proportionality close to the satellite drops' total surface area. The resultant humidity conditions in the semi-closed system can be effectively and accurately induced using different satellite drops combinations. Copyright © 2015 Elsevier Inc. All rights reserved.
An Observational Investigation of Penetrative Convection
DEFF Research Database (Denmark)
Jensen, Niels Otto; Lenschow, D. H.
1978-01-01
Data taken during the Air Mass Transformation Experiment (AMTEX) by the NCAR Electra aircraft have proven useful for investigating the structure of thermals penetrating into the turbulent inversion layer which caps the convective mixed layer. Variances, covariances, spectra and cospectra of poten......Data taken during the Air Mass Transformation Experiment (AMTEX) by the NCAR Electra aircraft have proven useful for investigating the structure of thermals penetrating into the turbulent inversion layer which caps the convective mixed layer. Variances, covariances, spectra and cospectra...
Added value of convection-permitting reanalyses
Wahl, S.; Keller, J. D.; Ohlwein, C.; Hense, A.; Friederichs, P.; Crewell, S.
2016-12-01
Atmospheric reanalyses are a state-of-the-art tool to generate consistent and realistic state estimates of the atmospheric system. They are used for validation of meteorological and hydrological models, climate monitoring, and renewable energy applications, amongst others. Current reanalyses are mainly global, while regional reanalyses are emerging for North America, the polar region, and most recently for Europe. Due to the horizontal resolution used, deep convection is still parameterized even in the regional reanalyses. However, convective parameterization is a major source of errors and uncertainties in atmospheric models. Therefore, it is expected that convection permitting reanalysis systems are able to adequately simulate the mechanisms leading to high-impact weather, notably heavy precipitation and winds related to deep moist convection. A novel convective-scale regional reanalysis system for Central Europe (COSMO-REA2) has been developed by the Hans-Ertel Center for Weather Research - Climate Monitoring Branch. The system is based on the COSMO model and uses a nudging scheme for the assimilation of observational data. In addition, radar-derived rain rates are assimilated through a latent heat nudging scheme. With a horizontal grid-spacing of 2 km, the model parameterization for deep moist convective processes is turned off. As we expect the largest benefit of the convection-permitting system for precipitation, the evaluation focuses on this essential climate variable (ECV). Furthermore, precipitation is crucial for climate monitoring purposes, e.g., in the form of extreme precipitation which is an major cause of severe damages and societal costs in Europe. This study illustrates the added value of the convective-scale reanalysis compared to coarser gridded regional European and global reanalyses.
Data Analysis and Non-local Parametrization Strategies for Organized Atmospheric Convection
Brenowitz, Noah D.
The intrinsically multiscale nature of moist convective processes in the atmosphere complicates scientific understanding, and, as a result, current coarse-resolution climate models poorly represent convective variability in the tropics. This dissertation addresses this problem by 1) studying new cumulus convective closures in a pair of idealized models for tropical moist convection, and 2) developing innovative strategies for analyzing high-resolution numerical simulations of organized convection. The first two chapters of this dissertation revisit a historical controversy about the use of convective closures based on the large-scale wind field or moisture convergence. In the first chapter, a simple coarse resolution stochastic model for convective inhibition is designed which includes the non-local effects of wind-convergence on convective activity. This model is designed to replicate the convective dynamics of a typical coarse-resolution climate prediction model. The non-local convergence coupling is motivated by the phenomena of gregarious convection, whereby mesoscale convective systems emit gravity waves which can promote convection at a distant locations. Linearized analysis and nonlinear simulations show that this convergence coupling allows for increased interaction between cumulus convection and the large-scale circulation, but does not suffer from the deleterious behavior of traditional moisture-convergence closures. In the second chapter, the non-local convergence coupling idea is extended to an idealized stochastic multicloud model. This model allows for stochastic transitions between three distinct cloud types, and non-local convergence coupling is most beneficial when applied to the transition from shallow to deep convection. This is consistent with recent observational and numerical modeling evidence, and there is a growing body of work highlighting the importance of this transition in tropical meteorology. In a series of idealized Walker cell
Convection and Mixing in Classical Novae Precursors
Dursi, L. J.; Calder, A. C.; Alexakis, A.; Truran, J. W.; Zingale, M.; Times, F. X.; Ricker, P. M.; Fryxell, B.; Olson, K.; Rosner, R.; MacNeice, P.
2002-06-01
To explain observed abundances from classical nova outbursts, and to help explain their energetics, nova models must incorporate a mechanism that will dredge up the heavier white dwarf material into the lighter accreted atmosphere. One proposed mechanism relies on the fluid motions from an early convective phase to do the mixing. We present recent work investigating two aspects of this mechanism. We examine results from two-dimensional simulations of classical nova precursor models that demonstrate the beginning of a convective phase during the `simmering' of a nova precursor. We use a new hydrostatic equilibrium hydrodynamics module recently developed for the adaptive-mesh code FLASH. The two-dimensional models are based on the one-dimensional models of Ami Glasner (Glasner et al. 1997), and were evolved with FLASH from a pre-convective state to the onset of convection. The onset of convection induces a velocity field near the C,O/H,He interface, which can then cause mixing through interactions with gravity waves. We show results from simulations of these wind-wave interactions, and estimate whether the `wind' caused by the convection could induce sufficient dredge-up to power a classical novae. This research has been supported by the US. Department of Energy under grant no. B341495 to the ASCI Flash Center at the University of Chicago
Compressible convection under hyper-gravity
Huguet, L.; Le Reun, T.; Alboussiere, T.; Bergman, M. I.; Labrosse, S. J.
2013-12-01
Convection plays an important role for heat transfer from the deep interior of planets and stars. In the Earth's core, it is responsible for the magnetic field. We often use the Boussinesq approximation for incompressible convection, and for compressible convection, we can use the anelastic liquid approximation. However, there is a lack of experimental results to check the validity of the anelastic approximation when the dissipation number is not negligible, because of the difficulty in obtaining an adiabatic gradient in the lab. Increasing the effective gravity and using a gas with a small specific heat capacity is a good way to observe a compressible convection, because for an ideal gas, the adiabatic gradient is g/Cp. We have carried out some experiments on convection in xenon gas in a cell in a centrifuge, which allows us to reach 10,000g, yielding a maximum of about 10 K across the height of the cell. In our experimental device, we measure a temperature with 11 platinum resistance thermal detectors, and the fluctuations of pressure. We can also acquire ultrasonic measurements through the cell. A Peltier module heats the bottom and PID control keeps the bottom temperature constant. The cell is insulated by perplex walls and the xenon gas in the cell is under pressure to increase the thermal inertia. We observe an adiabatic gradient at different effective gravities with different boundary conditions, and the fluctuations of temperature and pressure due to convection.
Entropy Production in Convective Hydrothermal Systems
Boersing, Nele; Wellmann, Florian; Niederau, Jan
2016-04-01
Exploring hydrothermal reservoirs requires reliable estimates of subsurface temperatures to delineate favorable locations of boreholes. It is therefore of fundamental and practical importance to understand the thermodynamic behavior of the system in order to predict its performance with numerical studies. To this end, the thermodynamic measure of entropy production is considered as a useful abstraction tool to characterize the convective state of a system since it accounts for dissipative heat processes and gives insight into the system's average behavior in a statistical sense. Solving the underlying conservation principles of a convective hydrothermal system is sensitive to initial conditions and boundary conditions which in turn are prone to uncertain knowledge in subsurface parameters. There exist multiple numerical solutions to the mathematical description of a convective system and the prediction becomes even more challenging as the vigor of convection increases. Thus, the variety of possible modes contained in such highly non-linear problems needs to be quantified. A synthetic study is carried out to simulate fluid flow and heat transfer in a finite porous layer heated from below. Various two-dimensional models are created such that their corresponding Rayleigh numbers lie in a range from the sub-critical linear to the supercritical non-linear regime, that is purely conductive to convection-dominated systems. Entropy production is found to describe the transient evolution of convective processes fairly well and can be used to identify thermodynamic equilibrium. Additionally, varying the aspect ratio for each Rayleigh number shows that the variety of realized convection modes increases with both larger aspect ratio and higher Rayleigh number. This phenomenon is also reflected by an enlarged spread of entropy production for the realized modes. Consequently, the Rayleigh number can be correlated to the magnitude of entropy production. In cases of moderate
Non-unique solution for combined-convection assisting flow over ...
Indian Academy of Sciences (India)
boundary layer equation for natural convection constitutes the self-similar solu- tion whose perturbation with respect to the small parameter (ε), which is inversely proportional to the square root of the Richardson number (Gx), provides the non- similar solution. Solutions obtained by the shooting method indicate two sets for.
Scale/Analytical Analyses of Freezing and Convective Melting with Internal Heat Generation
Energy Technology Data Exchange (ETDEWEB)
Ali S. Siahpush; John Crepeau; Piyush Sabharwall
2013-07-01
Using a scale/analytical analysis approach, we model phase change (melting) for pure materials which generate constant internal heat generation for small Stefan numbers (approximately one). The analysis considers conduction in the solid phase and natural convection, driven by internal heat generation, in the liquid regime. The model is applied for a constant surface temperature boundary condition where the melting temperature is greater than the surface temperature in a cylindrical geometry. The analysis also consider constant heat flux (in a cylindrical geometry).We show the time scales in which conduction and convection heat transfer dominate.
On the effect of laterally varying boundary heat flux on rapidly rotating spherical shell convection
Sahoo, Swarandeep; Sreenivasan, Binod
2017-08-01
The onset of convection in a rotating spherical shell subject to laterally varying heat flux at the outer boundary is considered in this paper. The focus is on the geophysically relevant regime of rapid rotation (low Ekman number) where the natural length scale of convection is significantly smaller than the length scale imposed by the boundary heat flux pattern. Contrary to earlier studies at a higher Ekman number, we find a substantial reduction in the onset Rayleigh number Rac with increasing lateral variation. The decrease in Rac is shown to be closely correlated to the equatorial heat flux surplus in the steady, basic state solution. The consistency of such a correlation makes the estimation of Rac possible without solving the full stability problem. The steady baroclinic flow has a strong cyclone-anticyclone asymmetry in the kinetic helicity only for equatorially symmetric lateral variations, with possible implications for dynamo action. Equatorially antisymmetric variations, on the other hand, break the symmetry of the mean flow, in turn negating its helicity. Analysis of the perturbation solution reveals strongly localized clusters through which convection rolls drift in and out at a frequency higher than that for the reference case with homogeneous boundary heat flux. Large lateral variations produce a marked decrease in the azimuthal length scale of columns, which indicates that small-scale motions are essential to the transport of heat in rapidly rotating, localized convection. With an equatorially antisymmetric heat flux pattern, convection in individual clusters goes through an asynchronous wax-wane cycle whose frequency is much lower than the drift rate of the columns. These continual variations in convection intensity may in turn result in fluctuations in the magnetic field intensity, an effect that needs to be considered in dynamo models. Finally, there is a notable analogy between the role of a laterally varying boundary heat flux and the role of a
Liang, Y.; DiCarlo, D. A.; Hesse, M. A.
2015-12-01
method of CO2 convective dissolution process, which will allow us have a better understanding of the convective dissolution trapping mechanism at natural geological CO2 scale.
Numerical Investigations of Convective Initiation in Barbados
Directory of Open Access Journals (Sweden)
Kim Whitehall
2013-01-01
Full Text Available Localized convection in Barbados accounts for hazardous conditions and a significant percentage of the island’s annual rainfall. The feature results in rainfall accumulations exceeding 50 mm in 3 hours or less, over isolated locations. Weather Research and Forecasting model (WRF simulations are conducted for a rapid convective initiated and heavy precipitation event of 26 August 2008 over Barbados. The simulation results from the 1 km grid resolution domain depict that the shallow topography on the island plays a significant role in enhancing convective activity under weak synoptically disturbed conditions. The model results also demonstrate that the driving forces for the development of deep convective clouds include low-level moisture convergence that form as a result of the temperature differential between the land and the ocean and forced low-level uplift as a result of the blocking by the topography. The high-resolution WRF simulations demonstrate its capability to accurately capture the low-level flow over the island, as well as the orientation of the divergence and convergence patterns throughout the depth of the atmosphere. These results are heartening to use the WRF as a resource for studying deep convection in Barbados for disaster managers and water resource managers.
Properties of convective motions in facular regions
Kostik, R.; Khomenko, E. V.
2012-09-01
Aims: We study the properties of solar granulation in a facular region from the photosphere up to the lower chromosphere. Our aim is to investigate the dependence of granular structure on magnetic field strength. Methods: We used observations obtained at the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife) using two different instruments: the Triple Etalon SOlar Spectrometer (TESOS) to measure velocity and intensity variations along the photosphere in the Ba ii 4554 Å line; and, simultaneously, the Tenerife Infrared Polarimeter (TIP-II) to the measure Stokes parameters and the magnetic field strength at the lower photosphere in the Fe i 1.56 μm lines. Results: We find that the convective velocities of granules in the facular area decrease with magnetic field while the convective velocities of intergranular lanes increase with the field strength. Similar to the quiet areas, there is a contrast and velocity sign reversal taking place in the middle photosphere. The reversal heights depend on the magnetic field strength and are, on average, about 100 km higher than in the quiet regions. The correlation between convective velocity and intensity decreases with magnetic field at the bottom photosphere, but increases in the upper photosphere. The contrast of intergranular lanes observed close to the disk center is almost independent of the magnetic field strength. Conclusions: The strong magnetic field of the facular area seems to stabilize the convection and to promote more effective energy transfer in the upper layers of the solar atmosphere, since the convective elements reach greater heights.
Scaling regimes in spherical shell rotating convection
Gastine, T; Aubert, J
2016-01-01
Rayleigh-B\\'enard convection in rotating spherical shells can be considered as a simplified analogue of many astrophysical and geophysical fluid flows. Here, we use three-dimensional direct numerical simulations to study this physical process. We construct a dataset of more than 200 numerical models that cover a broad parameter range with Ekman numbers spanning $3\\times 10^{-7} \\leq E \\leq 10^{-1}$, Rayleigh numbers within the range $10^3 < Ra < 2\\times 10^{10}$ and a Prandtl number unity. We investigate the scaling behaviours of both local (length scales, boundary layers) and global (Nusselt and Reynolds numbers) properties across various physical regimes from onset of rotating convection to weakly-rotating convection. Close to critical, the convective flow is dominated by a triple force balance between viscosity, Coriolis force and buoyancy. For larger supercriticalities, a subset of our numerical data approaches the asymptotic diffusivity-free scaling of rotating convection $Nu\\sim Ra^{3/2}E^{2}$ in ...
Importance of convective parameterization in ENSO predictions
Zhu, Jieshun; Kumar, Arun; Wang, Wanqiu; Hu, Zeng-Zhen; Huang, Bohua; Balmaseda, Magdalena A.
2017-06-01
This letter explored the influence of atmospheric convection scheme on El Niño-Southern Oscillation (ENSO) predictions using a set of hindcast experiments. Specifically, a low-resolution version of the Climate Forecast System version 2 is used for 12 month hindcasts starting from each April during 1982-2011. The hindcast experiments are repeated with three atmospheric convection schemes. All three hindcasts apply the identical initialization with ocean initial conditions taken from the European Centre for Medium-Range Weather Forecasts and atmosphere/land initial states from the National Centers for Environmental Prediction. Assessments indicate a substantial sensitivity of the sea surface temperature prediction skill to the different convection schemes, particularly over the eastern tropical Pacific. For the Niño 3.4 index, the anomaly correlation skill can differ by 0.1-0.2 at lead times longer than 2 months. Long-term simulations are further conducted with the three convection schemes to understand the differences in prediction skill. By conducting heat budget analyses for the mixed-layer temperature anomalies, it is suggested that the convection scheme having the highest skill simulates stronger and more realistic coupled feedbacks related to ENSO. Particularly, the strength of the Ekman pumping feedback is better represented, which is traced to more realistic simulation of surface wind stress. Our results imply that improving the mean state simulations in coupled (ocean-atmosphere) general circulation model (e.g., ameliorating the Intertropical Convergence Zone simulation) might further improve our ENSO prediction capability.
Convective transport resistance in the vitreous humor
Penkova, Anita; Sadhal, Satwindar; Ratanakijsuntorn, Komsan; Moats, Rex; Tang, Yang; Hughes, Patrick; Robinson, Michael; Lee, Susan
2012-11-01
It has been established by MRI visualization experiments that the convection of nanoparticles and large molecules with high rate of water flow in the vitreous humor will experience resistance, depending on the respective permeabilities of the injected solute. A set of experiments conducted with Gd-DTPA (Magnevist, Bayer AG, Leverkusen, Germany) and 30 nm gadolinium-based particles (Gado CELLTrackTM, Biopal, Worcester, MA) as MRI contrast agents showed that the degree of convective transport in this Darcy-type porous medium varies between the two solutes. These experiments consisted of injecting a mixture of the two (a 30 μl solution of 2% Magnevist and 1% nanoparticles) at the middle of the vitreous of an ex vivo whole bovine eye and subjecting the vitreous to water flow rate of 100 μl/min. The water (0.9% saline solution) was injected at the top of the eye, and was allowed to drain through small slits cut at the bottom of the eyeball. After 50 minutes of pumping, MRI images showed that the water flow carried the Gd-DTPA farther than the nanoparticles, even though the two solutes, being mixed, were subjected to the same convective flow conditions. We find that the convected solute lags the water flow, depending on the solute permeability. The usual convection term needs to be adjusted to allow for the filtration effect on the larger particles in the form (1- σ) u . ∇ c with important implications for the modeling of such systems.
Lee, Sang-Min; Byun, Hi-Ryong
2011-10-01
This paper reports a theoretical study on the possibility of inducing artificial showery rain using the convective available potential energy, which is naturally stored in the troposphere. We calculated the environmental parameters (frequency of climatic values, extreme value of stability index, etc.) in the upper troposphere using rawinsonde data from six main stations in Korea from 2001 to 2008 and examined the temporal spatial convective energy according to region. Our results showed that convective available potential energy, which can induce artificial rainfall, existed in the troposphere mainly in summer and were low in other seasons. Its value was found to be highest during late afternoon and in inland regions. We examined the vertical structure of the atmosphere using moisture convergence and vertical velocity (omega) and found that precipitation occurred under strong real latent instability conditions with high convective available potential energy (>3,000 J/kg) in summer and was characterized by moisture convergence at 1,000-400 hPa, moisture divergence at 400-300 hPa, and continuous ascending air current at 1,000-300 hPa (-ω), on average. However, precipitation still did not occur in more than half the cases with high convective available potential energy because, according to the analysis, convective rainfall is affected to a greater extent by the value of convective inhibition than by convective available potential energy. It was also verified that in spite of zero convective inhibition, if the updrafts at a lower level were not sufficient to generate high convective available potential energy at a level higher than the level of free convection, convective rainfall would not occur under real latent instability. Therefore, we suggest it might be possible during the summer to secure the water resources in regions without precipitation by inducing ascending air current artificially under unstable atmospheric conditions to induce showery rain.
Directory of Open Access Journals (Sweden)
M Safaei
2016-09-01
Full Text Available In the present study, first the turbulent natural convection and then laminar mixed convection of air flow was solved in a room and the calculated outcomes are compared with results of other scientists and after showing validation of calculations, aforementioned flow is solved as a turbulent mixed convection flow, using the valid turbulence models Standard k-ε, RNG k-ε and RSM. To solve governing differential equations for this flow, finite volume method was used. This method is a specific case of residual weighting method. The results show that at high Richardson Numbers, the flow is rather stationary at the center of the enclosure. Moreover, it is distinguished that when Richardson Number increases the maximum of local Nusselt decreases. Therefore, it can be said that less number of Richardson Number, more rate of heat transfer.
A new conceptual model of convection
Energy Technology Data Exchange (ETDEWEB)
Walcek, C. [State Univ. of New York, Albany, NY (United States)
1995-09-01
Classical cumulus parameterizations assume that cumulus clouds are entraining plumes of hot air rising through the atmosphere. However, ample evidence shows that clouds cannot be simulated using this approach. Dr. Walcek suggests that cumulus clouds can be reasonably simulated by assuming that buoyant plumes detrain mass as they rise through the atmosphere. Walcek successfully simulates measurements of tropical convection using this detraining model of cumulus convection. Comparisons with measurements suggest that buoyant plumes encounter resistance to upward movement as they pass through dry layers in the atmosphere. This probably results from turbulent mixing and evaporation of cloud water, which generates negatively buoyant mixtures which detrain from the upward moving plume. This mass flux model of detraining plumes is considerably simpler than existing mass flux models, yet reproduces many of the measured effects associated with convective activity. 1 fig.
Magnetic field generation by intermittent convection
Chertovskih, R; Chimanski, E V
2016-01-01
Magnetic field generation by convective flows in transition to weak turbulence is studied numerically. By fixing the Prandtl number at P=0.3 and varying the Rayleigh number (Ra) as a control parameter in three-dimensional Rayleigh-Benard convection of an electrically conducting fluid, a recently reported route to hyperchaos involving quasiperiodic regimes, crises and chaotic intermittent attractors is followed, and the critical magnetic Prandtl number ($P_m^c$) for dynamo action is determined as a function of Ra. A mechanism for the onset of on-off intermittency in the magnetic energy is described, the most beneficial convective regimes for dynamo action are identified, and how intermittency affects the dependence of $P_m^c$ on Ra is discussed.
Magnetic Fields in the Solar Convection Zone
Directory of Open Access Journals (Sweden)
Yuhong Fan
2009-12-01
Full Text Available Active regions on the solar surface are generally thought to originate from a strong toroidal magnetic field generated by a deep seated solar dynamo mechanism operating at the base of the solar convection zone. Thus the magnetic fields need to traverse the entire convection zone before they reach the photosphere to form the observed solar active regions. Understanding this process of active region flux emergence is therefore a crucial component for the study of the solar cycle dynamo. This article reviews studies with regard to the formation and rise of active region scale magnetic flux tubes in the solar convection zone and their emergence into the solar atmosphere as active regions.
Chaotic convection in a rotating fluid layer
Directory of Open Access Journals (Sweden)
Vinod K. Gupta
2015-12-01
Full Text Available A study of thermal convection in a rotating fluid layer is investigated based on the dynamical systems approach. A system of differential equation like Lorenz model has been obtained by using Galerkin-truncated approximation. The chaotic convection is investigated in a rotating fluid layer. A low-dimensional, Lorenz-like model was obtained using Galerkin truncated approximation. The fourth-order Runge–Kutta method is employed to obtain the numerical solution of Lorenz-like system of equations. We found that there is proportional relation between Taylor number and the scaled Rayleigh number R. This means that chaotic behavior can be delayed (for increasing value of R when we increase the scaled Taylor number. We conclude that the transition from steady convection to chaos depends on the level of Taylor number.
Mixed convection around calandria tubes in a ¼ scale CANDU-6 moderator circulation tank
Energy Technology Data Exchange (ETDEWEB)
Atkins, M.D.; Rossouw, D.J.; Boer, M. [Nuclear Science Division, School of Mechanical and Aeronautical Engineering, University of the Witwatersrand, Johannesburg (South Africa); Kim, T., E-mail: tong.kim@wits.ac.za [Nuclear Science Division, School of Mechanical and Aeronautical Engineering, University of the Witwatersrand, Johannesburg (South Africa); Rhee, B.W.; Kim, H.T. [Severe Accident and PHWR Safety Research Division, Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)
2017-05-15
Highlights: • A secondary jet is formed at a stagnation region and is directed towards the center of the MCT. • The secondary jet undergoes the significant dissipation and mixing due to calandria tubes (CTs). • Its cooling effectiveness is reduced on the CTs in the bottom of the MCT. • With forced convection dominance, peak heat transfer is on the upper CT surface. • With natural convection dominance, peak heat transfer is on the lower CT surface. - Abstract: This study experimentally characterizes mixed convection around calandria tubes (CTs) in a ¼ scale CANDU-6 moderator circulation tank (MCT) that uses air as the working fluid. In a full scale CANDU-6 reactor that undergoes a postulated dual failure with a loss-of-coolant accident without the emergency core cooling system available, mixed convection heat transfer occurs around the CTs. The cooling effectiveness of the moderator is diminished as an emergency heat sink if overheating eventually leads to film boiling. To prevent the onset of film boiling, local sub-cooling margins of the moderator needs to be maintained or else the critical heat flux should be increased. Circulating the moderator which interacts with the overheated CTs increases the heat transfer into the moderator which may suppress film boiling. The present experimental results demonstrate that the cooling effectiveness of the circulating moderator, in particular the secondary jet, is attenuated substantially as it is convected away from the inner wall towards the center of the MCT. The momentum of the secondary jet is diffused through the CTs. At a low jet Reynolds number, the secondary jet becomes ineffective so that some overheated CTs positioned in the other half of the MCT are cooled only by natural convection.
Low enthalpy convective system in Western Ohio
Energy Technology Data Exchange (ETDEWEB)
Cannon, M.S.; Tabet, C.A.; Eckstein, Y.
1980-01-01
A distinct positive anomaly in the temperatures of the shallow (Pleistocene) aquifers along the Cincinnati-Findlay Arch in Western Ohio coincides with a low geothermal gradient. A conceptual model of convective currents associated with a tensional fault and/or fracture system along the crest of the Arch is suggested as an explanation of the anomaly. Hydrochemical information indicates that various quantities of warmer ground water, with the composition characteristics of deep bedrock aquifers, is present as an admixture in the shallow aquifers. This confirms the conceptual model of convection in fractures.
Convective towers detection using GPS radio occultations
DEFF Research Database (Denmark)
Biondi, Riccardo; Neubert, Torsten; Syndergaard, S.
The tropical deep convection affects the radiation balance of the atmosphere changing the water vapour mixing ratio and the temperature of the upper troposphere and lower stratosphere. To gain a better understanding of deep convective processes, the study of tropical cyclones could play...... the GPS signals penetrate through clouds and allow measurements of atmospheric profiles related to temperature, pressure, and water vapour with high vertical resolution. Using tropical cyclone best track database and data from different GPS RO missions (COSMIC, GRACE, CHAMP, SACC and GPSMET), we selected...
Laser induced ponderomotive convection in water
Shneider, M N
2015-01-01
A new mechanism for inducing convection during IR laser interaction with water or any absorbing polar liquid is described theoretically. The numerical simulations performed using the developed model show that the ponderomotive force produces water flow in the direction of the laser beam propagation. In the later stage of interaction, when water temperature rises, the Archimedes force becomes first comparable and then dominant producing convection directed against the vector of gravitational acceleration (upward). The theoretical estimates and the numerical simulations predict fluid dynamics that is similar to the observed in the previous experiments.
Mantle Convection in a Microwave Oven: New Perspectives for the Internally Heated Convection
Limare, A.; Fourel, L.; Surducan, E.; Neamtu, C.; Surducan, V.; Vilella, K.; Farnetani, C. G.; Kaminski, E. C.; Jaupart, C. P.
2015-12-01
The thermal evolution of silicate planets is primarily controlled by the balance between internal heating - due to radioactive decay - and heat transport by mantle convection. In the Earth, the problem is particularly complex due to the heterogeneous distribution of heat sources in the mantle and the non-linear coupling between this distribution and convective mixing. To investigate the behaviour of such systems, we have developed a new technology based on microwave absorption to study internally-heated convection in the laboratory. This prototype offers the ability to reach the high Rayleigh-Roberts and Prandtl numbers that are relevant for planetary convection. Our experimental results obtained for a uniform distribution of heat sources were compared to numerical calculations reproducing exactly experimental conditions (3D Cartesian geometry and temperature-dependent physical properties), thereby providing the first cross validation of experimental and numerical studies of convection in internally-heated systems. We find that the thermal boundary layer thickness and interior temperature scale with RaH-1/4, where RaH is the Rayleigh-Roberts number, as theoretically predicted by scaling arguments on the dissipation of kinetic energy. Our microwave-based method offers new perspectives for the study of internally-heated convection in heterogeneous systems which have been out of experimental reach until now. We are able to selectively heat specific regions in the convecting layer, through the careful control of the absorption properties of different miscible fluids. This is analogous to convection in the presence of chemical reservoirs with different concentration of long-lived radioactive isotopes. We shall show results for two different cases: the stability of continental lithosphere over a convective fluid and the evolution of a hidden enriched reservoir in the lowermost mantle.
Directory of Open Access Journals (Sweden)
M. Förster
2009-08-01
Full Text Available Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for more than seven and a half years (2001–2008 have been used to derive a statistical model of the high-latitude electric potential distribution for summer conditions. Based on potential pattern for different orientations of the interplanetary magnetic field (IMF in the GSM y-z-plane, basic convection pattern (BCP were derived, that represent the main characteristics of the electric potential distribution in dependence on the IMF. The BCPs comprise the IMF-independent potential distribution as well as patterns, which describe the dependence on positive and negative IMFBz and IMFBy variations. The full set of BCPs allows to describe the spatial and temporal variation of the high-latitude electric potential (ionospheric convection for any solar wind IMF condition near the Earth's magnetopause within reasonable ranges. The comparison of the Cluster/EDI model with the IZMEM ionospheric convection model, which was derived from ground-based magnetometer observations, shows a good agreement of the basic patterns and its variation with the IMF. According to the statistical models, there is a two-cell antisunward convection within the polar cap for northward IMFBz+≤2 nT, while for increasing northward IMFBz+ there appears a region of sunward convection within the high-latitude daytime sector, which assumes the form of two additional cells with sunward convection between them for IMFBz+≈4–5 nT. This results in a four-cell convection pattern of the high-latitude convection. In dependence of the ±IMFBy contribution during sufficiently strong northward IMFBz conditions, a transformation to three-cell convection patterns takes place.
Complex Convective Thermal Fluxes and Vorticity Structure
Redondo, Jose M.; Tellez, Jackson; Sotillos, Laura; Lopez Gonzalez-Nieto, Pilar; Sanchez, Jesus M.; Furmanek, Petr; Diez, Margarita
2015-04-01
Local Diffusion and the topological structure of vorticity and velocity fields is measured in the transition from a homogeneous linearly stratified fluid to a cellular or layered structure by means of convective cooling and/or heating[1,2]. Patterns arise by setting up a convective flow generated by an array of Thermoelectric devices (Peltier/Seebeck cells) these are controlled by thermal PID generating a buoyant heat flux [2]. The experiments described here investigate high Prandtl number mixing using brine and fresh water in order to form density interfaces and low Prandtl number mixing with temperature gradients. The set of dimensionless parameters define conditions of numeric and small scale laboratory modeling of environmental flows. Fields of velocity, density and their gradients were computed and visualized [3,4]. When convective heating and cooling takes place the combination of internal waves and buoyant turbulence is much more complicated if the Rayleigh and Reynolds numbers are high in order to study entrainment and mixing. Using ESS and selfsimilarity structures in the velocity and vorticity fieds and intermittency [3,5] that forms in the non-homogeneous flow is related to mixing and stiring. The evolution of the mixing fronts are compared and the topological characteristics of the merging of plumes and jets in different configurations presenting detailed comparison of the evolution of RM and RT, Jets and Plumes in overall mixing. The relation between structure functions, fractal analysis and spectral analysis can be very useful to determine the evolution of scales. Experimental and numerical results on the advance of a mixing or nonmixing front occurring at a density interface due to body forces [6]and gravitational acceleration are analyzed considering the fractal and spectral structure of the fronts like in removable plate experiments for Rayleigh-Taylor flows. The evolution of the turbulent mixing layer and its complex configuration is studied
Knauer, Markus; Schuster, Manfred E; Su, Dangsheng; Schlögl, Robert; Niessner, Reinhard; Ivleva, Natalia P
2009-12-17
Raman microspectroscopy (RM), temperature-programmed oxidation (TPO), high-resolution transmission electron microscopy (HRTEM), and electron energy loss spectroscopy (EELS) were combined to get comprehensive information on the relationship between structure and reactivity of soot in samples of spark discharge (GfG), heavy duty engine diesel (EURO VI and IV) soot, and graphite powder upon oxidation by oxygen at increasing temperatures. GfG soot and graphite powder represent the higher and lower reactivity limits. Raman microspectroscopic analysis was conducted by determination of spectral parameters using a five band fitting procedure (G, D1-D4) as well as by evaluation of the dispersive character of the D mode. The analysis of spectral parameters shows a higher degree of disorder and a higher amount of molecular carbon for untreated GfG soot samples than for samples of untreated EURO VI and EURO IV soot. The structural analysis based on the dispersive character of the D mode revealed substantial differences in ordering descending from graphite powder, EURO IV, VI to GfG soot. HRTEM images and EELS analysis of EURO IV and VI samples indicated a different morphology and a higher structural order as compared to GfG soot in full agreement with the Raman analysis. These findings are also confirmed by the reactivity of soot during oxidation (TPO), where GfG soot was found to be the most reactive and EURO IV and VI soot samples exhibited a moderate reactivity.
Energy Technology Data Exchange (ETDEWEB)
Tewell, Craig Richmond [Univ. of California, Berkeley, CA (United States)
2002-01-01
X-ray photoelectron spectroscopy (XPS) and Temperature Programmed Desorption (TPD) have been used to investigate the surface structure of model heterogeneous catalysts in ultra-high vacuum (UHV). UV-Raman spectroscopy has been used to probe the structure of bulk model catalysts in ambient and reaction conditions. The structural information obtained through UV-Raman spectroscopy has been correlated with both the UHV surface analysis and reaction results. The present day propylene and ethylene polymerization catalysts (Ziegler-Natta catalysts) are prepared by deposition of TiCl_{4} and a Al(Et)_{3} co-catalyst on a microporous Mg-ethoxide support that is prepared from MgCl_{2} and ethanol. A model thin film catalyst is prepared by depositing metallic Mg on a Au foil in a UHV chamber in a background of TiCl_{4} in the gas phase. XPS results indicate that the Mg is completely oxidized to MgCl_{2} by TiCl_{4} resulting in a thin film of MgCl_{2}/TiCl_{x}, where x = 2, 3, and 4. To prepare an active catalyst, the thin film of MgCl_{2}/TiCl_{x} on Au foil is enclosed in a high pressure cell contained within the UHV chamber and exposed to ~1 Torr of Al(Et)_{3}.
Energy Technology Data Exchange (ETDEWEB)
S. Semancik; R. E. Cavicchi; D. L. DeVoe; T. J. McAvoy [National Institute of Standards and Technology (US)]|[University of Maryland (US)
2001-12-21
This Final Report describes efforts and results for a 3-year DoE/OST-EMSP project centered at NIST. The multidisciplinary project investigated scientific and technical concepts critical for developing tunable, MEMS-based, gas and vapor microsensors that could be applied for monitoring the types of multiple analytes (and differing backgrounds) encountered at DoE waste sites. Micromachined ''microhotplate'' arrays were used as platforms for fabricating conductometric sensor prototypes, and as microscale research tools. Efficient microarray techniques were developed for locally depositing and then performance evaluating thin oxide films, in order to correlate gas sensing characteristics with properties including composition, microstructure, thickness and surface modification. This approach produced temperature-dependent databases on the sensitivities of sensing materials to varied analytes (in air) which enable application-specific tuning of microsensor arrays. Mechanistic studies on adsorb ate transient phenomena were conducted to better understand the ways in which rapid temperature programming schedules can be used to produce unique response signatures and increase information density in microsensor signals. Chemometric and neural network analyses were also employed in our studies for recognition and quantification of target analytes.
Directory of Open Access Journals (Sweden)
Ralf Moos
2013-04-01
Full Text Available An impedimetric NOx dosimeter based on the NOx sorption material KMnO4 is proposed. In addition to its application as a low level NOx dosimeter, KMnO4 shows potential as a precious metal free lean NOx trap material (LNT for NOx storage catalysts (NSC enabling electrical in-situ diagnostics. With this dosimeter, low levels of NO and NO2 exposure can be detected electrically as instantaneous values at 380 °C by progressive NOx accumulation in the KMnO4 based sensitive layer. The linear NOx sensing characteristics are recovered periodically by heating to 650 °C or switching to rich atmospheres. Further insight into the NOx sorption-dependent conductivity of the KMnO4-based material is obtained by the novel eTPD method that combines electrical characterization with classical temperature programmed desorption (TPD. The NOx loading amount increases proportionally to the NOx exposure time at sorption temperature. The cumulated NOx exposure, as well as the corresponding NOx loading state, can be detected linearly by electrical means in two modes: (1 time-continuously during the sorption interval including NOx concentration information from the signal derivative or (2 during the short-term thermal NOx release.
Groβ, Andrea; Kremling, Michael; Marr, Isabella; Kubinski, David J.; Visser, Jacobus H.; Tuller, Harry L.; Moos, Ralf
2013-01-01
An impedimetric NOx dosimeter based on the NOx sorption material KMnO4 is proposed. In addition to its application as a low level NOx dosimeter, KMnO4 shows potential as a precious metal free lean NOx trap material (LNT) for NOx storage catalysts (NSC) enabling electrical in-situ diagnostics. With this dosimeter, low levels of NO and NO2 exposure can be detected electrically as instantaneous values at 380 °C by progressive NOx accumulation in the KMnO4 based sensitive layer. The linear NOx sensing characteristics are recovered periodically by heating to 650 °C or switching to rich atmospheres. Further insight into the NOx sorption-dependent conductivity of the KMnO4-based material is obtained by the novel eTPD method that combines electrical characterization with classical temperature programmed desorption (TPD). The NOx loading amount increases proportionally to the NOx exposure time at sorption temperature. The cumulated NOx exposure, as well as the corresponding NOx loading state, can be detected linearly by electrical means in two modes: (1) time-continuously during the sorption interval including NOx concentration information from the signal derivative or (2) during the short-term thermal NOx release. PMID:23549366
MODELING OF CONVECTIVE FLOWS IN PNEUMOBASED OBJECTS. Part 1
Directory of Open Access Journals (Sweden)
B. M. Khrustalyov
2014-01-01
Full Text Available A computer modeling process of three-dimensional forced convection proceeding from computation of thermodynamic parameters of pneumo basic buildings (pneumo supported structures is presented. The mathematical model of numerical computation method of temperature and velocity fields, pressure profile in the object is developed using the package Solid works and is provided by grid methods on specified software. Special Navier–Stokes, Clapeyron–Mendeleev, continuity and thermal-conductivity equations are used to calculate parameters in the building with four supply and exhaust channels. Differential equations are presented by algebraic equation systems, initial-boundary conditions are changed by differential conditions for mesh functions and their solutions are performed by algebraic operations. In this article the following is demonstrated: in pneumo basic buildings convective and heat flows are identical structures near the surfaces in unlimited space, but in single-multiply shells (envelopescirculation lines take place, geometrical sizes of which depend on thermal-physical characteristics of gas(airin envelopes, radiation reaction with heated surfaces of envelopes with sphere, earth surface, neighboring buildings. Natural surveys of pneumo-basic buildings of different purposes were carried out in Minsk, in different cities of Belarus and Russia, including temperature fields of external and internal surfaces of air envelopes, relative humidity, thermal (heatflows, radiation characteristics and others.The results of research work are illustrated with diagrams of temperature, velocity, density and pressure dependent on coordinates and time.
Toroidal convection rolls in the sun a challenge to theory
Ribes, E.; Laclare, F.
A large-scale circulation in the form of azimuthal (east-west oriented) rolls has been discovered in the convection zone of the Sun (Ribes et al.0, 1985; Ribes, 1986). The characteristics of the global circulation (for example orientation, number, lifetime and so forth) are quite different from those of meridional cells (oriented parallel to the solar axis) which has been predicted by theory (e.g. Glatzmaier, 1985). This raises the important question as to whether or not the newly-discovered azimuthal rolls contribute to the transport of heat from the deep interior of the Sun to its surface. Here we report new results that exhibit the convective nature of the azimuthal rolls. These are based on 22 years of observation of the roll pattern (Ribes, 1986), a 11 year sequence of solar diameter measurements (Laclare, 1987), and four years of luminosity measurements (Willson et al., 1981). The first indication is provided by the distribution of solar activity, which frames the roll pattern rather than being superimposed on it. The second hint comes from a temporal association that exists between the onset of new rolls and the luminosity and diameter oscillations with a period of 980 days.
Phenomenology of convection-parameterization closure
Directory of Open Access Journals (Sweden)
J.-I. Yano
2013-04-01
Full Text Available Closure is a problem of defining the convective intensity in a given parameterization. In spite of many years of efforts and progress, it is still considered an overall unresolved problem. The present article reviews this problem from phenomenological perspectives. The physical variables that may contribute in defining the convective intensity are listed, and their statistical significances identified by observational data analyses are reviewed. A possibility is discussed for identifying a correct closure hypothesis by performing a linear stability analysis of tropical convectively coupled waves with various different closure hypotheses. Various individual theoretical issues are considered from various different perspectives. The review also emphasizes that the dominant physical factors controlling convection differ between the tropics and extra-tropics, as well as between oceanic and land areas. Both observational as well as theoretical analyses, often focused on the tropics, do not necessarily lead to conclusions consistent with our operational experiences focused on midlatitudes. Though we emphasize the importance of the interplays between these observational, theoretical and operational perspectives, we also face challenges for establishing a solid research framework that is universally applicable. An energy cycle framework is suggested as such a candidate.
Evolution of Excited Convective Cells in Plasmas
DEFF Research Database (Denmark)
Pécseli, Hans; Juul Rasmussen, Jens; Sugai, H.
1984-01-01
Convective cells are excited externally in a fully ionized magnetized plasma and their space-time evolution is investigated by two-dimensional potential measurements. A positive cell is excited externally by control of the end losses in the 'scrape off' layer of a plasma column produced by surface...
Free convection film flows and heat transfer
Shang, Deyi
2010-01-01
Presents development of systematic studies for hydrodynamics and heat and mass transfer in laminar free convection, accelerating film boiling and condensation of Newtonian fluids, and accelerating film flow of non-Newtonian power-law fluids. This book provides a system of analysis models with a developed velocity component method.
(OLR), convective available potential energy (CAPE)
Indian Academy of Sciences (India)
mer monsoon season: ARMEX campaign and CHAMP/. COSMIC satellite observations; J. Geophys. Res. 116. D03102, doi: 10.1029/2010JD014340. Mc Bridge J L and Frank W M 1999 Relationship between stability and monsoon convection; J. Atmos. Sci. 56. 24–56. Naujokat B 1986 An update of the observed quasi- ...
Determination of the convective heat transfer coefficient
Spierings, D.; Bosman, F.; Peters, T.; Plasschaert, F.
The value of the convective heat transfer coefficient (htc) is determined under different loading conditions by using a computer aided method. The thermal load has been applied mathematically as well as experimentally to the coronal surface of an axisymmetric tooth model. To verify the assumptions
Magnetohydrodynamic mixed convection flow in vertical concentric ...
Indian Academy of Sciences (India)
This work reports an analytical solution for fully developed mixed convection flow of viscous,incompressible, electrically conducting fluid in vertical concentric annuli under the influence of a transverse magnetic field, where the outer surface of inner cylinder is heated sinusoidally and the inner surface of outercylinder is kept ...
Effect of thermosolutal convection on directional solidification
Indian Academy of Sciences (India)
The impact of thermosolutal convection during directional solidification is explored via results of numerical investigations. Results from fully transient numerical simulations of directional solidification in a differentially heated cavity under terrestrial conditions and Bridgman crystal growth in space are discussed. The pivotal ...
Salinity transfer in bounded double diffusive convection
Yang, Yantao; van der Poel, Erwin; Ostilla Monico, Rodolfo; Sun, Chao; Verzicco, Roberto; Grossmann, Siegfried; Lohse, Detlef
2015-01-01
The double diffusive convection between two parallel plates is numerically studied for a series of parameters. The flow is driven by the salinity difference and stabilised by the thermal field. Our simulations are directly compared with experiments by Hage & Tilgner (Phys. Fluids, vol. 22, 2010,
Forced Convection Heat Transfer in Circular Pipes
Tosun, Ismail
2007-01-01
One of the pitfalls of engineering education is to lose the physical insight of the problem while tackling the mathematical part. Forced convection heat transfer (the Graetz-Nusselt problem) certainly falls into this category. The equation of energy together with the equation of motion leads to a partial differential equation subject to various…
Probability distribution functions in turbulent convection
Balachandar, S.; Sirovich, L.
1991-01-01
Results of an extensive investigation of probability distribution functions (pdf's) for Rayleigh-Benard convection, in the hard turbulence regime, are presented. It is seen that the pdf's exhibit a high degree of internal universality. In certain cases this universality is established within two Kolmogorov scales of a boundary. A discussion of the factors leading to universality is presented.
Energy Technology Data Exchange (ETDEWEB)
Rauf, A., E-mail: raufamar@ciitsahiwal.edu.pk [Department of Mathematics, Comsats Institute of Information Technology, Sahiwal 57000 (Pakistan); Siddiq, M.K. [Centre for Advanced Studies in Pure and Applied Mathematics, Department of Mathematics, Bahauddin Zakariya University, Multan 63000 (Pakistan); Abbasi, F.M. [Department of Mathematics, Comsats Institute of Information Technology, Islamabad 44000 (Pakistan); Meraj, M.A. [Department of Mathematics, Comsats Institute of Information Technology, Sahiwal 57000 (Pakistan); Ashraf, M. [Centre for Advanced Studies in Pure and Applied Mathematics, Department of Mathematics, Bahauddin Zakariya University, Multan 63000 (Pakistan); Shehzad, S.A. [Department of Mathematics, Comsats Institute of Information Technology, Sahiwal 57000 (Pakistan)
2016-10-15
The present work deals with the steady laminar three-dimensional mixed convective magnetohydrodynamic (MHD) boundary layer flow of Casson nanofluid over a bidirectional stretching surface. A uniform magnetic field is applied normal to the flow direction. Similarity variables are implemented to convert the non-linear partial differential equations into ordinary ones. Convective boundary conditions are utilized at surface of the sheet. A numerical technique of Runge–Kutta–Fehlberg (RFK45) is used to obtain the results of velocity, temperature and concentration fields. The physical dimensionless parameters are discussed through tables and graphs. - Highlights: • Mixed convective boundary layer flow of Casson nanofluid is taken into account. • Impact of magnetic field is examined. • Convective heat and mass conditions are imposed. • Numerical solutions are presented and discussed.
Probing the transition from shallow to deep convection
Energy Technology Data Exchange (ETDEWEB)
Kuang, Zhiming [Harvard Univ., Cambridge, MA (United States); Gentine, Pierre [Columbia Univ., New York, NY (United States)
2016-05-01
In this funded project we highlighted the components necessary for the transition from shallow to deep convection. In particular we defined a prototype of shallow to deep convection, which is currently being implemented in the NASA GISS model. We also tried to highlight differences between land and oceanic convection.
Convection by means of least squares projection for ALE calculations
Geijselaers, Hubertus J.M.; Huetink, Han
2001-01-01
Element result data are in general discontinuous across element boundaries. In the ALE method convection of these data with respect to the element grid is required. In this paper we present a convection method, which is based on a least squares projection. For moderate convective displacements it is
Glesener, G. B.; Aurnou, J. M.
2010-12-01
The Modeling and Educational Demonstrations Laboratory (MEDL) at UCLA is developing a mantle convection physical model to assist educators with the pedagogy of Earth’s interior processes. Our design goal consists of two components to help the learner gain conceptual understanding by means of visual interactions without the burden of distracters, which may promote alternative conceptions. Distracters may be any feature of the conceptual model that causes the learner to use inadequate mental artifact to help him or her understand what the conceptual model is intended to convey. The first component, and most important, is a psychological component that links properties of “everyday things” (Norman, 1988) to the natural phenomenon, mantle convection. Some examples of everyday things may be heat rising out from a freshly popped bag of popcorn, or cold humid air falling from an open freezer. The second component is the scientific accuracy of the conceptual model. We would like to simplify the concepts for the learner without sacrificing key information that is linked to other natural phenomena the learner will come across in future science lessons. By taking into account the learner’s mental artifacts in combination with a simplified, but accurate, representation of what scientists know of the Earth’s interior, we expect the learner to have the ability to create an adequate qualitative mental simulation of mantle convection. We will be presenting some of our prototypes of this mantle convection physical model at this year’s poster session and invite constructive input from our colleagues.
Convection surrounding mesoscale ionospheric flow channels
Rinne, Y.; Moen, J.; Baker, J. B. H.; Carlson, H. C.
2011-05-01
We evaluate data from the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) and Defense Meteorological Satellite Program (DMSP) spacecraft coupled with data from the Super Dual Auroral Radar Network (SuperDARN) polar cap convection patterns in order to study how the ionospheric convection evolves around a sequence of transient, mesoscale flow channel events in the duskside of the cusp inflow region. On a northwestward convection background for the interplanetary magnetic field (IMF) BY positive and BZ negative, a sequence of three eastward flow channels formed over the course of 1 hour in response to three sharp IMF rotations to IMF BY negative and IMF BZ positive. The first and third channels, due to IMF BY negative periods of ˜13 min and >30 min, respectively, develop in a similar manner: they span the entire ESR field of view and widen poleward with increasing time elapsed since their first appearance until the IMF rotates back. The convection patterns are consistent with the line-of-sight data from the ESR and DMSP within a 10 min adaption time. The flow lines form a twin-vortex flow, with the observed channel being the twin vortices' center flow. The fitting algorithm was pushed to its limits in terms of spatial resolution in this study. During portions of the channel events, the suggested twin-cell flow is not in agreement with our physical interpretation of the flow channels being reconnection events because cell closure is suggested across an anticipated nonreconnecting open-closed boundary. For these segments, we present simulated patterns which have been arrived at by a combination of looking at the raw data and examining the fitted convection patterns.
Convective mass transfer around a dissolving bubble
Duplat, Jerome; Grandemange, Mathieu; Poulain, Cedric
2017-11-01
Heat or mass transfer around an evaporating drop or condensing vapor bubble is a complex issue due to the interplay between the substrate properties, diffusion- and convection-driven mass transfer, and Marangoni effects, to mention but a few. In order to disentangle these mechanisms, we focus here mainly on the convective mass transfer contribution in an isothermal mass transfer problem. For this, we study the case of a millimetric carbon dioxide bubble which is suspended under a substrate and dissolved into pure liquid water. The high solubility of CO2 in water makes the liquid denser and promotes a buoyant-driven flow at a high (solutal) Rayleigh number (Ra˜104 ). The alteration of p H allows the concentration field in the liquid to be imaged by laser fluorescence enabling us to measure both the global mass flux (bubble volume, contact angle) and local mass flux around the bubble along time. After a short period of mass diffusion, where the boundary layer thickens like the square root of time, convection starts and the CO2 is carried by a plume falling at constant velocity. The boundary layer thickness then reaches a plateau which depends on the bubble cross section. Meanwhile the plume velocity scales like (dV /d t )1 /2 with V being the volume of the bubble. As for the rate of volume loss, we recover a constant mass flux in the diffusion-driven regime followed by a decrease in the volume V like V2 /3 after convection has started. We present a model which agrees well with the bubble dynamics and discuss our results in the context of droplet evaporation, as well as high Rayleigh convection.
Convection Theory and Sub-Photospheric Stratification
Arnett, David; Meakin, Casey; Young, Patrick A.
2010-02-01
As a preliminary step toward a complete theoretical integration of three-dimensional compressible hydrodynamic simulations into stellar evolution, convection at the surface and sub-surface layers of the Sun is re-examined, from a restricted point of view, in the language of mixing-length theory (MLT). Requiring that MLT use a hydrodynamically realistic dissipation length gives a new constraint on solar models. While the stellar structure which results is similar to that obtained by Yale Rotational Evolution Code (Guenther et al.; Bahcall & Pinsonneault) and Garching models (Schlattl et al.), the theoretical picture differs. A new quantitative connection is made between macro-turbulence, micro-turbulence, and the convective velocity scale at the photosphere, which has finite values. The "geometric parameter" in MLT is found to correspond more reasonably with the thickness of the superadiabatic region (SAR), as it must for consistency in MLT, and its integrated effect may correspond to that of the strong downward plumes which drive convection (Stein & Nordlund), and thus has a physical interpretation even in MLT. If we crudely require the thickness of the SAR to be consistent with the "geometric factor" used in MLT, there is no longer a free parameter, at least in principle. Use of three-dimensional simulations of both adiabatic convection and stellar atmospheres will allow the determination of the dissipation length and the geometric parameter (i.e., the entropy jump) more realistically, and with no astronomical calibration. A physically realistic treatment of convection in stellar evolution will require substantial additional modifications beyond MLT, including nonlocal effects of kinetic energy flux, entrainment (the most dramatic difference from MLT found by Meakin & Arnett), rotation, and magnetic fields.
Research of heat transfer of staggered horizontal bundles of finned tubes at free air convection
Novozhilova, A. V.; Maryna, Z. G.; Samorodov, A. V.; Lvov, E. A.
2017-11-01
The study of free-convective processes is important because of the cooling problem in many machines and systems, where other ways of cooling are impossible or impractical. Natural convective processes are common in the steam turbine air condensers of electric power plants located within the city limits, in dry cooling towers of circulating water systems, in condensers cooled by air and water, in radiators cooling oil of power electric transformers, in emergency cooling systems of nuclear reactors, in solar power, as well as in air-cooling of power semiconductor energy converters. All this makes actual the synthesis of the results of theoretical and experimental research of free convection for heat exchangers with finned tube bundles. The results of the study of free-convection heat transfer for two-, three- and four-row staggered horizontal bundles of industrial bimetallic finned tubes with finning factor of 16.8 and equilateral tubes arrangement are presented. Cross and diagonal steps in the bundles are the same: 58; 61; 64; 70; 76; 86; 100 mm, which corresponds to the relative steps: 1.042; 1.096; 1.152; 1.258; 1.366; 1.545; 1.797. These steps are standardized for air coolers. An equation for calculating the free-convection heat transfer, taking into account the influence of geometrical parameters in the range of Rayleigh number from 30,000 to 350,000 with an average deviation of ± 4.8%, has been obtained. The relationship presented in the article allows designing a wide range of air coolers for various applications, working in the free convection modes.
The Impact of Reduced Gravity on Free Convective Heat Transfer from a Finite, Flat, Vertical Plate
Lotto, Michael A.; Johnson, Kirstyn M.; Nie, Christopher W.; Klaus, David M.
2017-09-01
Convective heat transfer is governed by a number of factors including various fluid properties, the presence of a thermal gradient, geometric configuration, flow condition, and gravity. Empirically-derived analytical relationships can be used to estimate convection as a function of these governing parameters. Although it is relatively straightforward to experimentally quantify the contributions of the majority of these variables, it is logistically difficult to assess the influence of reduced-gravity due to practical limitations of establishing this environment. Therefore, in order to explore this regime, a series of tests was conducted to evaluate convection under reduced-gravity conditions averaging 0.45 m/sec2 (0.05 g) achieved aboard a parabolic aircraft. The results showed a reduction in net heat transfer of approximately 61% in flight relative to a 1g terrestrial baseline using the same setup. The average experimental Nusselt Number of 19.05 ± 1.41 statistically correlated with the predicted value of 18.90 ± 0.63 (N = 13), estimated using the Churchill-Chu correlation for free convective heat transfer from a finite, flat, vertical plate. Extrapolating this to similar performance in true microgravity (10-6 g) indicates that these conditions should yield a Nusselt Number of 1.27, which is 2.6% the magnitude of free convection at 1g, or a reduction of 97.4%. With advection essentially eliminated, heat transfer becomes limited to diffusion and radiation, which are gravity-independent and nearly equivalent in magnitude in this case. These results offer a general guideline for integrating components that utilize natural (free) convective gas cooling in a spacecraft habitat and properly sizing the thermal control system.
The Impact of Reduced Gravity on Free Convective Heat Transfer from a Finite, Flat, Vertical Plate
Lotto, Michael A.; Johnson, Kirstyn M.; Nie, Christopher W.; Klaus, David M.
2017-10-01
Convective heat transfer is governed by a number of factors including various fluid properties, the presence of a thermal gradient, geometric configuration, flow condition, and gravity. Empirically-derived analytical relationships can be used to estimate convection as a function of these governing parameters. Although it is relatively straightforward to experimentally quantify the contributions of the majority of these variables, it is logistically difficult to assess the influence of reduced-gravity due to practical limitations of establishing this environment. Therefore, in order to explore this regime, a series of tests was conducted to evaluate convection under reduced-gravity conditions averaging 0.45 m/sec2 (0.05 g) achieved aboard a parabolic aircraft. The results showed a reduction in net heat transfer of approximately 61% in flight relative to a 1 g terrestrial baseline using the same setup. The average experimental Nusselt Number of 19.05 ± 1.41 statistically correlated with the predicted value of 18.90 ± 0.63 (N = 13), estimated using the Churchill-Chu correlation for free convective heat transfer from a finite, flat, vertical plate. Extrapolating this to similar performance in true microgravity (10-6 g) indicates that these conditions should yield a Nusselt Number of 1.27, which is 2.6% the magnitude of free convection at 1 g, or a reduction of 97.4%. With advection essentially eliminated, heat transfer becomes limited to diffusion and radiation, which are gravity-independent and nearly equivalent in magnitude in this case. These results offer a general guideline for integrating components that utilize natural (free) convective gas cooling in a spacecraft habitat and properly sizing the thermal control system.
Energy Technology Data Exchange (ETDEWEB)
Noronha, F.B.; Schmal, M. [Universidade Federal do Rio de Janeiro (Brazil); Nicot, C. [Institut de Recherches sur la Catalyse, Villeurbanne (France)] [and others
1997-05-01
Graphite-supported cobalt, palladium, and cobalt-palladium systems were prepared by a simple impregnation technique and submitted to hydrogen reduction in a temperature-programmed mode. Using X-ray diffraction to define the structure of the calcined precursors, magnetic measurements to determine the amount of metallic cobalt formed after reduction, and analysis of the gaseous medium during the reduction, a general model for the reduction of the graphite supported catalysts has been suggested. At room temperature, both pure PdO and PdO associated with Co{sub 3}O{sub 4} are reduced to the metallic state. In the case of bimetallic systems, a fraction of Co{sub 3}O{sub 4} in close proximity or interfaced with PdO can be reduced to the metallic state and to COO species, demonstrating a strong catalytic effect of palladium on the reduction of cobalt oxides. At temperatures between 298 and 500 K, depending on the catalyst formulation, the presence of metallic palladium promotes the reduction of a large fraction of oxidized cobalt. At higher reduction temperature, at least two competitive phenomena were detected: direct reduction of the residual oxidized cobalt by the graphite, leading to carbon monoxide and dioxide formation, and hydrogasification of the graphite catalyzed by the supported metals, leading mainly to methane formation. Together with a possible short-range palladium-activated hydrogen migration, at low and moderate temperatures, long-range migration of particles is necessary to explain the observations. These migrations, in turn, favor the formation of bimetallic particles, after reduction at 773 K. 53 refs., 5 figs., 3 tabs.
Long-range transport of Xe-133 emissions under convective and non-convective conditions.
Kuśmierczyk-Michulec, J; Krysta, M; Kalinowski, M; Hoffmann, E; Baré, J
2017-09-01
To investigate the transport of xenon emissions, the Provisional Technical Secretariat (PTS) operates an Atmospheric Transport Modelling (ATM) system based on the Lagrangian Particle Dispersion Model FLEXPART. The air mass trajectory ideally provides a "link" between a radionuclide release and a detection confirmed by radionuclide measurements. This paper investigates the long-range transport of Xe-133 emissions under convective and non-convective conditions, with special emphasis on evaluating the changes in the simulated activity concentration values due to the inclusion of the convective transport in the ATM simulations. For that purpose a series of 14 day forward simulations, with and without convective transport, released daily in the period from 1 January 2011 to 30 June 2013, were analysed. The release point was at the ANSTO facility in Australia. The simulated activity concentrations for the period January 2011 to February 2012 were calculated using the daily emission values provided by the ANSTO facility; outside the aforementioned period, the median daily emission value was used. In the simulations the analysed meteorological input data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) were used with the spatial resolution of 0.5°. It was found that the long-range transport of Xe-133 emissions under convective conditions, where convection was included in the ATM simulation, led to a small decrease in the activity concentration, as compared to transport without convection. In special cases related to deep convection, the opposite effect was observed. Availability of both daily emission values and measured Xe-133 activity concentration values was an opportunity to validate the simulations. Based on the paired t-test, a 95% confidence interval for the true mean difference between simulations without convective transport and measurements was constructed. It was estimated that the overall uncertainty lies between 0.08 and 0.25 mBq/m3
Fournier, R.O.
1990-01-01
Much has been published about double-diffusive convection as a mechanism for explaining variations in composition and temperature within all-liquid natural systems. However, relatively little is known about the applicability of this phenomenon within the heterogeneous rocks of currently active geothermal systems where primary porosity may control fluid flow in some places and fractures may control it in others. The main appeal of double-diffusive convection within hydrothermal systems is-that it is a mechanism that may allow efficient transfer of heat mainly by convection, while at the same time maintaining vertical and lateral salinity gradients. The Salton Sea geothermal system exhibits the following reservoir characteristics: (1) decreasing salinity and temperature from bottom to top and center toward the sides, (2) a very high heat flow from the top of the system that seems to require a major component of convective transfer of heat within the chemically stratified main reservoir, and (3) a relatively uniform density of the reservoir fluid throughout the system at all combinations of subsurface temperature, pressure, and salinity. Double-diffusive convection can account for these characteristics very nicely whereas other previously suggested models appear to account either for the thermal structure or for the salinity variations, but not both. Hydrologists, reservoir engineers, and particularly geochemists should consider the possibility and consequences of double-diffusive convection when formulating models of hydrothermal processes, and of the response of reservoirs to testing and production. ?? 1990.
The influence of embedded convection on orographic precipitation
Cannon, D. J.
An in-depth analysis of the dynamical and microphysical processes by which embedded convection influences orographic precipitation, and the representation of these processes in numerical models, is presented. Benchmark idealised convection-permitting simulations are conducted to quantify the impact of embedded convection on orographic precipitation over a broad range of parameter space. Clouds that form over tall and wide mountains (e.g., the Washington Cascades) are found to be highly efficient at forming precipitations even in the absence of embedded convection. When embedded convection develops in such clouds, it produces competing effects that cancel to yield little change in the area-averaged precipitation rate. In contrast, for short and narrow mountains (e.g., the UK Pennines), stratiform precipitation formation is highly inefficient and so rapid precipitation formation within convective updraughts significantly enhances the area-averaged precipitation rate (by up to 133%). A novel analytical model of convective orographic precipitation is developed which successfully reproduces many of the benchmark simulation trends. Simplified representations of the dynamical and microphysical processes governing mixed-phase stratiform and convective clouds are developed to represent a flow with embedded convection. Finally, the representation of embedded convection in idealised simulations employing a commonly-used convection parameterisation scheme is evaluated relative to the benchmark simulations. Significant over-predictions (of up to 90%) in the area-averaged precipitation accumulations are found. Inexpensive modifications to the parameterisation such as changes to both the treatment of precipitation and the trigger function, as well as the introduction of convective memory (representing convective cell advection), may significantly improve the parameterisation of embedded convection in operational numerical weather prediction models.
Magnetic field generation by intermittent convection
Chertovskih, R.; Rempel, E. L.; Chimanski, E. V.
2017-10-01
Magnetic field generation in three-dimensional Rayleigh-Bénard convection of an electrically conducting fluid is studied numerically by fixing the Prandtl number at P = 0.3 and varying the Rayleigh number (Ra) as a control parameter. A recently reported route to hyperchaos involving quasiperiodic regimes, crises and chaotic intermittent attractors is followed, and the critical magnetic Prandtl number (Pmc) for dynamo action is determined as a function of Ra. A mechanism for the onset of intermittency in the magnetic energy is described, the most beneficial convective regimes for dynamo action in this transition to weak turbulence are identified, and the impact of intermittency on the dependence of Pmc on Ra is discussed.
Convective Flow in an Aquifer Layer
Directory of Open Access Journals (Sweden)
Dambaru Bhatta
2017-10-01
Full Text Available Here, we investigate weakly nonlinear hydrothermal two-dimensional convective flow in a horizontal aquifer layer with horizontal isothermal and rigid boundaries. We treat such a layer as a porous medium, where Darcy’s law holds, subjected to the conditions that the porous layer’s permeability and the thermal conductivity are variable in the vertical direction. This analysis is restricted to the case that the subsequent hydraulic resistivity and diffusivity have a small rate of change with respect to the vertical variable. Applying the weakly nonlinear approach, we derive various order systems and express their solutions. The solutions for convective flow quantities such as vertical velocity and the temperature that arise as the Rayleigh number exceeds its critical value are computed and presented in graphical form.
Convection and Substorms - Paradigms of Magnetospheric Phenomenology
Kennel, Charles F.
The magnetosphere is the region where cosmic rays and the solar wind interact with the Earth's magnetic field, creating such phenomena as the northern lights and other aurorae. The configuration and dynamics of the magnetosphere are of interest to planetary physicists, geophysicists, plasma astrophysicists, and to scientists planning space missions. The circulation of solar wind plasma in the magnetosphere and substorms have long been used as the principle paradigms for studying this vital region. Charles F. Kennel, a leading scientist in the field, here presents a synthesis of the convection and substorm literatures, and an analysis of convection and substorm interactions; he also suggests that the currently accepted steady reconnection model may be advantageously replaced by a model of multiple tail reconnection events, in which many mutually interdependent reconnections occur. Written in an accessible, non-mathematical style, this book introduces the reader to the exciting discoveries in this fast-growing field.
Effects of Moist Convection on Hurricane Predictability
Zhang, Fuqing; Sippel, Jason A.
2008-01-01
This study exemplifies inherent uncertainties in deterministic prediction of hurricane formation and intensity. Such uncertainties could ultimately limit the predictability of hurricanes at all time scales. In particular, this study highlights the predictability limit due to the effects on moist convection of initial-condition errors with amplitudes far smaller than those of any observation or analysis system. Not only can small and arguably unobservable differences in the initial conditions result in different routes to tropical cyclogenesis, but they can also determine whether or not a tropical disturbance will significantly develop. The details of how the initial vortex is built can depend on chaotic interactions of mesoscale features, such as cold pools from moist convection, whose timing and placement may significantly vary with minute initial differences. Inherent uncertainties in hurricane forecasts illustrate the need for developing advanced ensemble prediction systems to provide event-dependent probabilistic forecasts and risk assessment.
Circulation and Convection in the Irminger Sea
2010-02-01
area than in any other ocean ( Sabine et al., 2004). Atmospheric variabil- ity, primarily NAO-related, strongly impacts convective activity as well as...east of Cape Farewell were compromised. Turbulent heat fluxes were computed using a bulk formula (COARE 3.0, Fairall et al., 2003) with the inputs of...background levels (Fig. 2-6c). Replacing the ERA-40 wind speed with our estimate of the true wind speed in the bulk formula raised the heat fluxes in
Equilibrium Transport in Double-Diffusive Convection
2011-06-01
convection changes in other environments. External planetary systems, such as the atmospheric makeup of planets within our solar system, are...21) where ( fx ,fy) are the Floquet factors in x and y. Substituting Equation (21) in the linearized governing equations and collecting...the individual Fourier components reduces the stability problem to matrix form Equation (13). Maximizing the growth rates with respect to ( fx ,fy,m
Predicting Vertical Motion within Convective Storms
van den Heever, S. C.
2016-12-01
Convective storms are both beneficial in the fresh water they supply and destructive in the life-threatening extreme weather they produce. They are found throughout the tropics and midlatitudes, vary in structure from isolated to highly organized systems, and are the sole source of precipitation in many regions of Earth. Convective updrafts and downdrafts plays a crucial role in cloud and precipitation formation, latent heating, water vapor transport, storm organization, and large-scale atmospheric circulations such as the Hadley and Walker cells. These processes, in turn, impact the strength and longevity of updrafts and downdrafts through complex, non-linear feedbacks. In spite of the significant influence of convective updrafts and downdrafts on the weather and climate system, accurately predicting vertical motion using numerical models remains challenging. In high-resolution cloud-resolving models where vertical motion is normally resolved, significant biases exist in the predicted profiles of updraft and downdraft velocities, at least for the limited cases where observational data have been available for model evaluation. It has been suggested that feedbacks between the vertical motion and microphysical processes may be one cause of these discrepancies, however, our understanding of these feedbacks remains limited. In this talk, the results of a small field campaign conducted over northeastern Colorado designed to observe storm vertical motion and cold pool characteristics within isolated and organized deep convective storms will be described. High frequency radiosonde, radar and drone measurements of a developing through mature supercell storm updraft and cold pool will be presented and compared with RAMS simulations of the same supercell storm. An analysis of the feedbacks between the storm dynamical and microphysical processes will be presented, and implications for regional and global modeling of severe storms will be discussed.
Interferometric investigation of convection in plexiglas boxes
Koster, J. N.
1983-09-01
Real-time holographic interferometry is used to study free convection in cavities heated from below and bounded by Plexiglas windows. Advantages and shortcomings of this visualization technique applied to such Plexiglas boxes are discussed. As Plexiglas has a high temperature-dependent refractive index, temperature fields in the windows and the fluid layer are visualized. These visualizations furnished proof of a pronounced thermal influence of the walls on the flow pattern based on what we call “thermal memory” of Plexiglas.
Convective heat transfer in porous media
Cheng, P.
Recent emerging technologies on the extraction of geothermal energy, the design of insulation systems for energy conservation, the use of aquifers for hot-water storage, the disposal of nuclear wastes in sub-seabeds, the enhanced recovery of oils by thermal methods, and the design of catalyst-bed reactors have demanded an improved understanding of heat transfer mechanisms in fluid-filled porous media. Experiments have been conducted to investigate the onset of free convection in rectangular and cylindrical enclosures filled with porous media and heated from below. The Nusselt numbers determined from these experiments during steady conditions are correlated in terms of the Rayleigh number. The data for free convection in rectangular geometries show considerable scattering among investigators using different porous media and fluids. Recently, some data has been obtained for free convect on in water-filled glass beads adjacent to a heated vertical flat plate, a horizontal cylinder and between vertical concentric cylinders. The data obtained at low Rayleigh numbers is found to be in good agreement with theoretical predictions based on Darcy's law.
Patrizio, C. R.; Randall, D. A.
2016-12-01
A three-dimensional cloud-resolving model (CRM) is used to investigate the preferred separation distance between neighboring humid, rainy regions formed by convective aggregation in radiative-convective equilibrium without rotation. We performed simulations of convective aggregation with doubly-periodic square domains of widths 768 km, 1536 km and 3072 km. The simulation with the smallest domain size was run first. Then, the simulations in the larger domains are initialized using multiple copies of the equilibrated results in the smallest domain, plus a small perturbation. With all three domain sizes, the simulations eventually evolve to a single statistically steady convective cluster surrounded by a broader region of dry, subsiding air. We analyze the mechanisms that cause the initial multiple clusters in the larger domains to reorganize into a single cluster. In addition, for each domain size, we composite the vertical velocity, water vapor mixing ratio, and radiative cooling rate in the dry environmental region as functions of distance away from the single equilibrated cluster. We also explore the dependence of the results on the prescribed sea-surface temperature. An idealized model of steady-state convective aggregation is used to interpret the numerical results.
Simulating North American mesoscale convective systems with a convection-permitting climate model
Prein, Andreas F.; Liu, Changhai; Ikeda, Kyoko; Bullock, Randy; Rasmussen, Roy M.; Holland, Greg J.; Clark, Martyn
2017-10-01
Deep convection is a key process in the climate system and the main source of precipitation in the tropics, subtropics, and mid-latitudes during summer. Furthermore, it is related to high impact weather causing floods, hail, tornadoes, landslides, and other hazards. State-of-the-art climate models have to parameterize deep convection due to their coarse grid spacing. These parameterizations are a major source of uncertainty and long-standing model biases. We present a North American scale convection-permitting climate simulation that is able to explicitly simulate deep convection due to its 4-km grid spacing. We apply a feature-tracking algorithm to detect hourly precipitation from Mesoscale Convective Systems (MCSs) in the model and compare it with radar-based precipitation estimates east of the US Continental Divide. The simulation is able to capture the main characteristics of the observed MCSs such as their size, precipitation rate, propagation speed, and lifetime within observational uncertainties. In particular, the model is able to produce realistically propagating MCSs, which was a long-standing challenge in climate modeling. However, the MCS frequency is significantly underestimated in the central US during late summer. We discuss the origin of this frequency biases and suggest strategies for model improvements.
On the theory of Rayleigh-Bénard convection in homeotropic nematic liquid crystals
Feng, Q.; Decker, W.; Pesch, W.; Kramer, L.
1992-06-01
A rigorous linear and weakly nonlinear analysis of Rayleigh-Bénard convection in homeotropically aligned nematic liquid crystals is presented for both adverse convection (heating from above) and normal convection (heating from below) allowing for th presence of magnetic fields. In adverse convection we find a transition from forward to backward bifurcation with increasing vertical (stabilizing) field. The stability analysis of roll solutions explains the experimentally observed transition between roll and square patterns for weak horizontal (destabilizing) fields. In a range of high vertical magnetic fields two steady convective modes with different wavelengths and symmetries become critical at nearly the same threshold giving rise to competition and resonant interaction. For the oscillatory instability in normal convection the nature of the bifurcation for both traveling and standing rolls is determined. Comparison with previous theories and with experiments is made wherever possible. Nous reconsidérons la description théorique du comportement au seuil et près du seuil de convection de Rayleigh-Bénard dans un cristal liquide nématique en géometrie homéotrope. Lorsque le système est chauffé par le haut et soumis à un champ magnétique faiblement déstabilisant, une analyse de stabilité des rouleaux explique leur transition dans les échantillons rectangulaires connus expérimentalement. Dans la même configuration mais avec un champ vertical stabilisant, nous observons le passage d'une bifurcation directe à une bifurcation inverse. On trouve, dans un intervalle de grands champs magnetiques, un regime où deux modes stationnaires convectifs avec différentes longueurs d'onge et de symmetrie opposée ont le même seuil critique. Cela conduit à une compétition et une interaction résonnante. En cas de chauffage par le bas une instabilité oscillante se développe comme on sait. La nature de la bifurcation est analysée pour des rouleaux stationnaires et
Numerical modelling of convective heat transport by air flow in permafrost talus slopes
Wicky, Jonas; Hauck, Christian
2017-06-01
Talus slopes are a widespread geomorphic feature in the Alps. Due to their high porosity a gravity-driven internal air circulation can be established which is forced by the gradient between external (air) and internal (talus) temperature. The thermal regime is different from the surrounding environment, leading to the occurrence of permafrost below the typical permafrost zone. This phenomenon has mainly been analysed by field studies and only few explicit numerical modelling studies exist. Numerical simulations of permafrost sometimes use parameterisations for the effects of convection but mostly neglect the influence of convective heat transfer in air on the thermal regime. In contrast, in civil engineering many studies have been carried out to investigate the thermal behaviour of blocky layers and to improve their passive cooling effect. The present study further develops and applies these concepts to model heat transfer in air flows in a natural-scale talus slope. Modelling results show that convective heat transfer has the potential to develop a significant temperature difference between the lower and the upper parts of the talus slope. A seasonally alternating chimney-effect type of circulation develops. Modelling results also show that this convective heat transfer leads to the formation of a cold reservoir in the lower part of the talus slope, which can be crucial for maintaining the frozen ground conditions despite increasing air temperatures caused by climate change.
Khaled, M.; Garnier, B.; Harambat, F.; Peerhossaini, H.
2010-02-01
A new experimental technique is presented that allows simultaneous measurement of convective and radiative heat flux in the underhood. The goal is to devise an easily implemented and accurate experimental method for application in the vehicle underhood compartment. The new method is based on a technique for heat-flux measurement developed by the authors (Heat flow (flux) sensors for measurement of convection, conduction and radiation heat flow 27036-2, © Rhopoint Components Ltd, Hurst Green, Oxted, RH8 9AX, UK) that uses several thermocouples in the thickness of a thermal resistive layer (foil heat-flux sensor). The method proposed here uses a pair of these thermocouples with different radiative properties. Measurements validating this novel technique are carried out on a flat plate with a prescribed constant temperature in both natural- and forced-convection flow regimes. The test flat plate is instrumented by this new technique, and also with a different technique that is intrusive but very accurate, used as reference here (Bardon J P and Jarny Y 1994 Procédé et dispositif de mesure transitoire de température et flux surfacique Brevet n°94.011996, 22 February). Discrepancies between the measurements by the two techniques are less than 10% for both convective and radiative heat flux. Error identification and sensitivity analysis of the new method are also presented.
Convective instability and boundary driven oscillations in a reaction-diffusion-advection model
Vidal-Henriquez, Estefania; Zykov, Vladimir; Bodenschatz, Eberhard; Gholami, Azam
2017-10-01
In a reaction-diffusion-advection system, with a convectively unstable regime, a perturbation creates a wave train that is advected downstream and eventually leaves the system. We show that the convective instability coexists with a local absolute instability when a fixed boundary condition upstream is imposed. This boundary induced instability acts as a continuous wave source, creating a local periodic excitation near the boundary, which initiates waves travelling both up and downstream. To confirm this, we performed analytical analysis and numerical simulations of a modified Martiel-Goldbeter reaction-diffusion model with the addition of an advection term. We provide a quantitative description of the wave packet appearing in the convectively unstable regime, which we found to be in excellent agreement with the numerical simulations. We characterize this new instability and show that in the limit of high advection speed, it is suppressed. This type of instability can be expected for reaction-diffusion systems that present both a convective instability and an excitable regime. In particular, it can be relevant to understand the signaling mechanism of the social amoeba Dictyostelium discoideum that may experience fluid flows in its natural habitat.
Generation of magnetic fields by large-scale vortices in rotating convection
Guervilly, Celine; Jones, Chris A
2015-01-01
We propose a new self-consistent dynamo mechanism for the generation of large-scale magnetic fields in natural objects. Recent computational studies have described the formation of large-scale vortices (LSVs) in rotating turbulent convection. Here we demonstrate that for magnetic Reynolds numbers below the threshold for small-scale dynamo action, such turbulent flows can sustain large-scale magnetic fields --- i.e. fields with a significant component on the scale of the system.
Convective Heat Transfer Coefficients of the Human Body under Forced Convection from Ceiling
DEFF Research Database (Denmark)
Kurazumi, Yoshihito; Rezgals, Lauris; Melikov, Arsen Krikor
2014-01-01
The average convective heat transfer coefficient for a seated human body exposed to downward flow from above was determined. Thermal manikin with complex body shape and size of an average Scandinavian female was used. The surface temperature distribution of the manikin’s body was as the skin...... of the convective heat transfer coefficient of the whole body (hc [W/(m2•K)]) was proposed: hc=4.088+6.592V1.715 for a seated naked body at 20ºC and hc=2.874+7.427V1.345 for a seated naked body at 26ºC. Differences in the convective heat transfer coefficient of the whole body in low air velocity range, V
Convective Weather Avoidance with Uncertain Weather Forecasts
Karahan, Sinan; Windhorst, Robert D.
2009-01-01
Convective weather events have a disruptive impact on air traffic both in terminal area and in en-route airspaces. In order to make sure that the national air transportation system is safe and efficient, it is essential to respond to convective weather events effectively. Traffic flow control initiatives in response to convective weather include ground delay, airborne delay, miles-in-trail restrictions as well as tactical and strategic rerouting. The rerouting initiatives can potentially increase traffic density and complexity in regions neighboring the convective weather activity. There is a need to perform rerouting in an intelligent and efficient way such that the disruptive effects of rerouting are minimized. An important area of research is to study the interaction of in-flight rerouting with traffic congestion or complexity and developing methods that quantitatively measure this interaction. Furthermore, it is necessary to find rerouting solutions that account for uncertainties in weather forecasts. These are important steps toward managing complexity during rerouting operations, and the paper is motivated by these research questions. An automated system is developed for rerouting air traffic in order to avoid convective weather regions during the 20- minute - 2-hour time horizon. Such a system is envisioned to work in concert with separation assurance (0 - 20-minute time horizon), and longer term air traffic management (2-hours and beyond) to provide a more comprehensive solution to complexity and safety management. In this study, weather is dynamic and uncertain; it is represented as regions of airspace that pilots are likely to avoid. Algorithms are implemented in an air traffic simulation environment to support the research study. The algorithms used are deterministic but periodically revise reroutes to account for weather forecast updates. In contrast to previous studies, in this study convective weather is represented as regions of airspace that pilots
Digital Repository Service at National Institute of Oceanography (India)
RameshKumar, M.R.; Babu, A.K.; Reason, C.
been examined in relation to breaks in monsoon conditions over the Indian sub-continent during contrasting monsoon years. A see-saw nature of convection between the NWP and north Indian Ocean was found during the years with excess monsoon rainfall...
Electrical Resistivity Imaging and Hydrodynamic Modeling of Convective Fingering in a Sabkha Aquifer
Van Dam, Remke; Eustice, Brian; Hyndman, David; Wood, Warren; Simmons, Craig
2014-05-01
Free convection, or fluid motion driven by density differences, is an important groundwater flow mechanism that can enhance transport and mixing of heat and solutes in the subsurface. Various issues of environmental and societal relevance are exacerbated convective mixing; it has been studied in the context of dense contaminant plumes, nuclear waste disposal, greenhouse gas sequestration, the impacts of sea level rise and saline intrusion on drinking water resources. The basic theory behind convective flow in porous media is well understood, but important questions regarding this process in natural systems remain unanswered. Most previous research on this topic has focused on theory and modeling, with only limited attention to experimental studies and field measurements. The few published studies present single snapshots, making it difficult to quantify transient changes in these systems. Non-invasive electrical methods have the potential to exploit the relation between solute concentrations and electrical conductance of a fluid, and thereby estimate fluid salinity differences in time and space. We present the results of a two-year experimental study at a shallow sabkha aquifer in the United Arab Emirates, about 50 km southwest of the city of Abu Dhabi along the coast of the Arabian Gulf, that was designed to explore the transient nature of free convection. Electrical resistivity tomography (ERT) data documented the presence of convective fingers following a significant rainfall event. One year later, the complex fingering pattern had completely disappeared. This observation is supported by analysis of the aquifer solute budget as well as hydrodynamic modeling of the system. The transient dynamics of the gravitational instabilities in the modeling results are in agreement with the timing observed in the time-lapse ERT data. Our experimental observations and modeling are consistent with the hypothesis that the instabilities arose from a dense brine that infiltrated
Spatially Modulated Structures in Convective Systems
Kao, Hsien-Ching
This dissertation focuses on the study of spatially modulated structures in pattern forming systems. The work is motivated by recent interest in spatially localized states observed in convective systems. Weakly nonlinear analysis is applied to derive the modulation equations and systematic studies, both analytical and numerical, are then performed on the simplified equations. The following is a summary of this work: Weakly Subcritical Patterns: The transition from subcritical to supercritical periodic patterns is described by the one-dimensional cubic-quintic Ginzburg-Landau equation with cubic nonlinear gradient terms. The coefficients are real indicating that the system is spatially reversible. Properties of the equation such as well-posedness, gradient structure, and bifurcation behavior depend significantly on the coefficients of the cubic nonlinear gradient terms. In this system, periodic patterns may in turn become unstable through one of two different mechanisms, an Eckhaus instability or an oscillatory instability. Dynamics and bifurcations near the instability thresholds are analyzed. Among the stationary solutions, the front solution which connects the zero state to a spatially periodic state plays the most important role. The location of the front in the parameter mu is treated as a Maxwell point. The spatially modulated solutions which bifurcate from the periodic solutions demonstrate protosnaking behavior near this point. These results are used to shed light on both variational and non-variational systems exhibiting homoclinic snaking. Localized Patterns in Rotating Convection and Magnetoconvection: In two-dimensional rotating convection and magnetoconvection, the formation of spatially localized patterns is strongly affected by the interaction between convection and a large scale mode: zonal velocity in rotating convection and magnetic flux in magnetoconvection. A nonlocal fifth order Ginzburg-Landau theory is developed to describe the localization
Pore Water Convection in Carbonaceous Chondrite Planetesimals
Travis, B. J.; Schubert, G.
2004-12-01
Chondritic meteorites are so named because they nearly all contain chondrules - small spherules of olivine and pyroxene that condensed and crystallized in the solar nebula and then combined with other material to form a matrix. Their parent bodies did not differentiate, i.e., form a crust and a core. Carbonaceous chondrites (CCs) derived from undifferentiated icy planetesimals. Asteroids of the inner solar system are probably present-day representatives of the early planetesimals. CCs exhibit liquid water-rock interactions. CCs contain small but significant amounts of radiogenic elements (e.g., 26Al), sufficient to warm up an initially cold planetesimal. A warmed-up phase could last millions of years. During the warmed-up phase, liquid water will form, and could evolve into a hydrothermal convective flow. Flowing water will affect the evolution of minerals. We report on results of a numerical study of the thermal evolution of CCs, considering the major factors that control heating history and possible flow, namely: permeability, radiogenic element content, and planetesimal radius. We determine the time sequence of thermal processes, length of time for a convective phase and patterns of flow, amount of fluid flow throughout the planetesimals, and sensitivity of evolution to primary parameters. We use the MAGHNUM code to simulate 3-D dynamic freezing and thawing and flow of water in a self-gravitating, permeable spherical body. Governing equations are Darcy's law, mass conservation, energy conservation, and equation of state for water and ice. We have simulated the evolution of heating, melting of ice, subsequent flow and eventual re-freezing for several examples of CC planetesimals. For a reference simulation, we use typical values from meteorite analyses: 20 % porosity, 1 darcy permeability (~10-12 m2), 3x10-8 wt fraction of 26Al, rock density of 3000 kg/m3, rock specific heat of 1000 J/kg/K, body radius of 50 km, solid rock thermal conductivity of 3 W/m/K. For the
Rotating Rayleigh-Benard convection: The Kueppers-Lortz transition
Energy Technology Data Exchange (ETDEWEB)
Zhong, F.; Ecke, R.; Steinberg, V.
1990-01-01
Rayleigh-Benard convection with rotation about a vertical axis is investigated for small dimensionless rotation rates 0 < {Omega} < 50. The convection cell is cylindrical with aspect ratio {Gamma} = 10 and the convecting fluid is water with a Prandtl number of 6.8 at T = 23.8C. Comparisons are made between experimental data and linear stability theory for the onset Rayleigh number and for the wave number dependence of the convective pattern. The nonlinear Kueppers-Lortz transition is found to occur significantly below the theoretically expected rotation rate {Omega}{sub c} and to be nucleated by defects created at the lateral cell walls. 20 refs., 10 figs.
Earth's CMB topography and mantle convection
Lassak, T. M.; McNamara, A. K.; Zhong, S.; Garnero, E.
2008-12-01
Better understanding topography on Earth's core-mantle boundary (CMB) may provide important constraints on mantle dynamics, specifically the style of mantle convection, and on lower mantle heterogeneity. For example, the origin of large, lowermost mantle low shear wave velocity provinces beneath the central Pacific and Africa is not well constrained, but are likely related to both mantle dynamics and CMB topography. Two competing hypotheses for these anomalies are: thermal upwellings (e.g., plume clusters) or large intrinsically dense piles of primitive mantle material (e.g., thermochemical piles). Here we discuss the results from our current 3D investigation of CMB topography in two styles of mantle convection: 1) an isochemical mantle with plume clusters, and 2) a thermochemical mantle with large, intrinsically dense piles. In this study, we numerically investigate 3D spherical models of mantle convection and calculate maps of topography (CMB and surface, with self-gravitation included) and geoid (CMB and surface). Maps of CMB topography and geoid (CMB and surface) are produced, and compared to observed CMB topography (e.g., Morelli and Dziewonski, 1987; Boschi and Dziewonski, 2000; Sze and van der Hilst, 2003) and surface geoid (e.g., Earth Geopotential Model, 1996). Our predicted surface geoid maps provide a key image of how CMB topography, for any given model, will affect the geoid. The results of this work emphasize the importance in using a suite of observables (in this case, topography and geoid maps for CMB and surface) to constrain whole mantle dynamics and lower mantle structure.
Transition between free, mixed and forced convection
Jaeger, W.; Trimborn, F.; Niemann, M.; Saini, V.; Hering, W.; Stieglitz, R.; Pritz, B.; Fröhlich, J.; Gabi, M.
2017-07-01
In this contribution, numerical methods are discussed to predict the heat transfer to liquid metal flowing in rectangular flow channels. A correct representation of the thermo-hydraulic behaviour is necessary, because these numerical methods are used to perform design and safety studies of components with rectangular channels. Hence, it must be proven that simulation results are an adequate representation of the real conditions. Up to now, the majority of simulations are related to forced convection of liquid metals flowing in circular pipes or rod bundle, because these geometries represent most of the components in process engineering (e.g. piping, heat exchanger). Open questions related to liquid metal heat transfer, among others, is the behaviour during the transition of the heat transfer regimes. Therefore, this contribution aims to provide useful information related to the transition from forced to mixed and free convection, with the focus on a rectangular flow channel. The assessment of the thermo-hydraulic behaviour under transitional heat transfer regimes is pursued by means of system code simulations, RANS CFD simulations, LES and DNS, and experimental investigations. Thereby, each of the results will compared to the others. The comparison of external experimental data, DNS data, RANS data and system code simulation results shows that the global heat transfer can be consistently represented for forced convection in rectangular flow channels by these means. Furthermore, LES data is in agreement with RANS CFD results for different Richardson numbers with respect to temperature and velocity distribution. The agreement of the simulation results among each other and the hopefully successful validation by means of experimental data will fosters the confidence in the predicting capabilities of numerical methods, which can be applied to engineering application.
Mohr, K. I.; Slayback, D. A.; Nicholls, S.; Yager, K.
2013-12-01
The Andes extend from the west coast of Colombia (10N) to the southern tip of Chile (53S). In southern Peru and Bolivia, the Central Andes is split into separate eastern and western cordilleras, with a high plateau (≥ 3000 m), the Altiplano, between them. Because 90% of the Earth's tropical mountain glaciers are located in the Central Andes, our study focuses on this region, defining its zonal extent as 7S-21S and the meridional extent as the terrain 1000 m and greater. Although intense convection occurs during the wet season in the Altiplano, it is not included in the lists of regions with frequent or the most intense convection. The scarcity of in-situ observations with sufficient density and temporal resolution to resolve individual storms or even mesoscale-organized cloud systems and documented biases in microwave-based rainfall products in poorly gauged mountainous regions have impeded the development of an extensive literature on convection and convective systems in this region. With the tropical glaciers receding at unprecedented rates, leaving seasonal precipitation as an increasingly important input to the water balance in alpine valley ecosystems and streams, understanding the nature and characteristics of the seasonal precipitation becomes increasingly important for the rural economies in this region. Previous work in analyzing precipitation in the Central Andes has emphasized interannual variability with respect to ENSO, this is the first study to focus on shorter scale variability with respect to organized convection. The present study took advantage of the University of Utah's Precipitation Features database compiled from 14 years of TRMM observations (1998-2012), supplemented by field observations of rainfall and streamflow, historical gauge data, and long-term WRF-simulations, to analyze the intraseasonal variability of precipitating systems and their relationship regional dynamical features such as the Bolivian High. Through time series and
Surface Tension Driven Convection Experiment Completed
Jacobson, Thomas P.; Sedlak, Deborah A.
1997-01-01
The Surface Tension Driven Convection Experiment (STDCE) was designed to study basic fluid mechanics and heat transfer on thermocapillary flows generated by temperature variations along the free surfaces of liquids in microgravity. STDCE first flew on the USML-1 mission in July 1992 and was rebuilt for the USML-2 mission that was launched in October 1995. This was a collaborative project with principal investigators from Case Western Reserve University (CWRU), Professors Simon Ostrach and Yasuhiro Kamotani, along with a team from the NASA Lewis Research Center composed of civil servants and contractors from Aerospace Design & Fabrication, Inc. (ADF), Analex, and NYMA, Inc.
Mixed convection opposing flow in porous annulus
Salman, Ahmed N. J.; Kamangar, Sarfaraz; Al-Rashed, Abdullah A. A. A.; Khan, T. M. Yunus; Khaleed, H. M. T.
2016-06-01
The current work investigates the mixed convection flow in a vertical porous annulus embedded with fluid saturated porous medium. The annulus is isothermally heated discretely at 20%, 35% and 50% of the height of cylinder at the center of annulus. Darcy law with thermal non-equilibrium approach is considered. The governing partial differential equations are solved using Finite Element Method (FEM). The effects of Peclet number Pe and conductivity ratio Kr on heat transfer and fluid flow is discussed It is found that the applied velocity in the downward direction, in case of an opposing flow, does not allow the thermal energy to reach from a hot to a cold surface.
Effect of rotation on ferro thermohaline convection
Sekar, R; Ramanathan, A
2000-01-01
The ferro thermohaline convection in a rotating medium heated from below and salted from above has been analysed. The solute is magnetic oxide, which modifies the magnetic field established as a perturbation. The effect of salinity has been included in magnetisation and in the density of the ferrofluid. The conditions for both stationary and oscillatory modes have been obtained using linear stability analysis and it has been found that stationary mode is favoured in comparison with oscillatory mode. The numerical and graphical results are presented. It has been observed that rotation stabilises the system.
Convective heat transfer enhancement with nanofluids
Rahman, Md. Habibur; Abedin, Z.
2017-12-01
Nanofluids are considered to offer important advantages over conventional heat transfer fluids. Over a decade ago, researchers focused on measuring and modeling the effective thermal conductivity and viscosity of nanofluids. Recently important theoretical and experimental research works on convective heat transfer appeared in the open literatures on the enhancement of heat transfer using suspensions of nanometer-sized solid particle materials, metallic or nonmetallic in base heat transfer fluids. The purpose of this article is to summarize recent research on fluid flow and heat transfer enhancement characteristics of nanofluids and thereby identify opportunities for future research.
Convection pump and method of operation
Steinhour, Leif Alexi
2017-07-11
This disclosure provides systems, methods, and apparatus related to a convection pump. In one aspect, an apparatus includes a chamber, the chamber having an inlet at a first end of the chamber and an outlet at a second end of the chamber. The chamber further has a first surface and a second surface, the first surface being opposite to the second surface. A baffle having a substantially helical shape is disposed inside the chamber. A heating device is configured to heat the first surface of the chamber. A cooling device is configured to cool the second surface of the chamber.
Hayat, Tasawar; Ashraf, Muhammad Bilal; Alsulami, Hamed H; Alhuthali, Muhammad Shahab
2014-01-01
The objective of present research is to examine the thermal radiation effect in three-dimensional mixed convection flow of viscoelastic fluid. The boundary layer analysis has been discussed for flow by an exponentially stretching surface with convective conditions. The resulting partial differential equations are reduced into a system of nonlinear ordinary differential equations using appropriate transformations. The series solutions are developed through a modern technique known as the homotopy analysis method. The convergent expressions of velocity components and temperature are derived. The solutions obtained are dependent on seven sundry parameters including the viscoelastic parameter, mixed convection parameter, ratio parameter, temperature exponent, Prandtl number, Biot number and radiation parameter. A systematic study is performed to analyze the impacts of these influential parameters on the velocity and temperature, the skin friction coefficients and the local Nusselt number. It is observed that mixed convection parameter in momentum and thermal boundary layers has opposite role. Thermal boundary layer is found to decrease when ratio parameter, Prandtl number and temperature exponent are increased. Local Nusselt number is increasing function of viscoelastic parameter and Biot number. Radiation parameter on the Nusselt number has opposite effects when compared with viscoelastic parameter.