dynamic modeling of natural convection solar energy collectors for ...
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terms for the plates' temperatures showed the transient contribution to be very ... conversion of solar energy is so low. (about 1 ... ovens, and low temperature and industrial ..... White, G.K. Experimental Techniques in Low Temperature Physics.
The Effect of Circulation Fan on Energy Consumption in Nature Convective Refrigerator-Freezer
Institute of Scientific and Technical Information of China (English)
LU Zhi-li; DING Guo-liang
2007-01-01
The effect of the circulation fan installed in fresh food compartment on energy consumption of natural convective refrigerator/freezers (RFs) was experimentally studied. Five different RF models with different cycles were tested. The experimental results showed that the energy consumption of the single-loop cycle RF increased by 2.4%～3.8%, that of the bypass two-circuit cycle RF decreased by 1.0%, and that of the two-circuit cycle RF with its evaporators in parallel when the geometry parameters of refrigeration system and the refrigerant charge were not changed after the circulation fan was installed decreased by 3.3%. When the optimization on the refrigerant charge and the evaporator was carried out, the energy consumption of the single-loop cycle RF , the bypass two-circuit cycle RF and the two-circuit cycle RF with its evaporators in parallel, decreased by 1.0%～ 6.4%,3.25% and 3.23% respectively. The present conclusions will provide a guideline to the optimum design for the RF with the circulation fan.
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.
Natural convection between concentric spheres
Garg, Vijay K.
1992-01-01
A finite-difference solution for steady natural convective flow in a concentric spherical annulus with isothermal walls has been obtained. The stream function-vorticity formulation of the equations of motion for the unsteady axisymmetric flow is used; interest lying in the final steady solution. Forward differences are used for the time derivatives and second-order central differences for the space derivatives. The alternating direction implicit method is used for solution of the discretization equations. Local one-dimensional grid adaptation is used to resolve the steep gradients in some regions of the flow at large Rayleigh numbers. The break-up into multi-cellular flow is found at high Rayleigh numbers for air and water, and at significantly low Rayleigh numbers for liquid metals. Excellent agreement with previous experimental and numerical data is obtained.
Topology optimisation for natural convection problems
Alexandersen, Joe; Andreasen, Casper Schousboe; Sigmund, Ole
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 in thermal conductivity of the solid and fluid phases. The governing equations are discretised using stabilised finite elements and topology optimisation is performed for two different problems using discrete adjoint sensitivity analysis. The study shows that topology optimisation is a viable approach for designing heat sink geometries cooled by natural convection and micropumps powered by natural convection.
NATURAL CONVECTION IN PASSIVE SOLAR BUILDINGS: EXPERIMENTS, ANALYSIS AND RESULTS
Energy Technology Data Exchange (ETDEWEB)
Gadgil, A.; Bauman, F.; Kammerud, R.
1981-04-01
Computer programs have been developed to numerically simulate natural convection in two- and three-dimensional room geometries. The programs have been validated using published data from the literature, results from a full-scale experiment performed at the Massachusetts Institute of Technology, and results from a small-scale experiment performed at LBL. One of the computer programs has been used to study the influence of natural convection on the thermal performance of a single zone in a direct-gain passive solar building. It is found that the convective heat transfer coefficients between the air and the enclosure surfaces can be substantially different from the values assumed in the standard building energy analysis methods, and can exhibit significant variations across a given surface. This study implies that the building heating loads calculated by standard building energy analysis methods may have substantial errors as a result of their use of common assumptions regarding the convection processes which occur in an enclosure.
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)
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 equation...
Natural convective heat transfer from square cylinder
Novomestský, Marcel; Smatanová, Helena; Kapjor, Andrej
2016-06-01
This article is concerned with natural convective heat transfer from square cylinder mounted on a plane adiabatic base, the cylinders having an exposed cylinder surface according to different horizontal angle. The cylinder receives heat from a radiating heater which results in a buoyant flow. There are many industrial applications, including refrigeration, ventilation and the cooling of electrical components, for which the present study may be applicable
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...... (Ekofiber Vind) from Ekofiber. The density of the sheep's wool was 28 kg/m3 and the paper insulation was 65 kg/m3. The temperature on the hot side of the test specimen was 30°C and on the cold side ca. -10°C, that is a temperature difference over the test specimen of ca. 40 K.The measurements on the two...... 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....
Natural convection in eccentric spherical annuli
Gallegos, A D
2015-01-01
A fluid between two spheres, concentric or not, at different temperatures will flow in the presence of a constant gravitational force. Although there is no possible hydrostatic state, energy transport is dominated by diffusion if temperature difference between the spheres is small enough. In this conductive regime the average Nusselt number remains approximately constant for all Rayleigh numbers below some critical value. Above the critical Rayleigh number, plumes appear and thermal convection takes place. We study this phenomenon, in particular the case where the inner sphere is displaced from the centre, using a two-component thermal lattice Boltzmann method to characterize the convective instability, the evolution of the flow patterns and the dependence of the Nusselt number on the Rayleigh number beyond the transition.
Unsteady natural convection in micropolar nanofluids
Directory of Open Access Journals (Sweden)
Rup Kazimierz
2014-09-01
Full Text Available This paper presents the analysis of momentum, angular momentum and heat transfer during unsteady natural convection in micropolar nanofluids. Selected nanofluids treated as single phase fluids contain small particles with diameter size 10-38.4 nm. In particular three water-based nanofluids were analyzed. Volume fraction of these solutions was 6%. The first of the analyzed nanofluids contained TiO2 nanoparticles, the second one contained Al2O3 nanoparticles, and the third one the Cu nanoparticles.
Thermophoresis in natural convection with variable properties
Jayaraj, S.; Dinesh, K. K.; Pillai, K. L.
The present paper deals with thermophoresis in natural convection with variable properties for a laminar flow over a cold vertical flat plate. Variation of properties like density, viscosity and thermal conductivity with temperature is included in the formulation of the problem. Selection of components for the property ratio is made by fitting the property values between the desired temperature limits. For a selected fluid, Prandtl number variation with temperature is neglected and the Prandtl number corresponding to film temperature is used for the analysis. Solution is carried out by finite difference method. Variation of wall concentration and wall flux along the length of plate is studied. The effect of thermophoretic coefficient on wall concentration is also studied. Results are presented in the form of graphs. The result is compared with similarity solution by Runge-Kutta method and found to be accurate upto second decimal place.
Probing the energy cascade of convective turbulence.
Kunnen, R P J; Clercx, H J H
2014-12-01
The existence of a buoyancy-dominated scaling range in convective turbulence is a longstanding open question. We investigate this issue by considering the scale-by-scale energy budget in direct numerical simulations of Rayleigh-Bénard convection. We try to minimize the so-called Bolgiano length scale, the length scale at which buoyancy becomes dominant for scaling. Therefore, we deliberately choose modest Rayleigh numbers Ra=2.5×10(6) and 2.5×10(7). The budget reveals that buoyant forcing, turbulent energy transfer, and dissipation are contributing significantly over a wide range of scales. Thereby neither Kolmogorov-like (balance of turbulent transfer and dissipation) nor Bolgiano-Obukhov-like scaling (balance of turbulent transfer and buoyancy) is expected in the structure functions, which indeed reveal inconclusive scaling behavior. Furthermore, we consider the calculation of the Bolgiano length scale. To account for correlations between the dissipation rates of kinetic energy and thermal variance we propose to average the Bolgiano length scale directly. This gives an estimate, which is one order of magnitude larger than the previous estimate, and actually larger than the domain itself. Rather than studying the scaling of structure functions, we propose that the use of scale-by-scale energy budgets resolving anisotropic contributions is appropriate to consider the energy cascade mechanisms in turbulent convection.
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 convection in sheep's wool and paper insulation
DEFF Research Database (Denmark)
Kristiansen, Finn Harken; Rode, Carsten
1999-01-01
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.......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...... (Ekofiber Vind) from Ekofiber. The density of the sheep's wool was 28 kg/m3 and the paper insulation was 65 kg/m3. The temperature on the hot side of the test specimen was 30°C and on the cold side ca. -10°C, that is a temperature difference over the test specimen of ca. 40 K.The measurements on the two...
Nee Alexander
2016-01-01
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....
A numerical study of natural convection in a narrow annulus
Energy Technology Data Exchange (ETDEWEB)
Sahai, V.
1991-12-01
Various numerical models were used to predict the natural convection of a solidifying liquid metal in a narrow annulus. Previous work in this area do not consider the temperature variation that exists in the fluid and the resulting heat conduction in the solid mold material. The finite element fluid dynamics code FIDAP was sued to solve these models. The results indicate that the natural convective effects are small. 6 refs.
A numerical study of natural convection in a narrow annulus
Sahai, V.
1991-12-01
Various numerical models were used to predict the natural convection of a solidifying liquid metal in a narrow annulus. Previous work in this area does not consider the temperature variation that exists in the fluid and the resulting heat conduction in the solid mold material. The finite element fluid dynamics code FIDAP was used to solve these models. The results indicate that the natural convective effects are small.
Analysis of natural convection in a low gravity environment
Mattor, Ethan E.; Durgin, William W.; Bloznalis, Peter; Schoenberg, Richard
1992-01-01
Natural convection inside a spherical container was studied experimentally with two apparatuses at low buoyancy levels. The data generated by these experiments, plotted nondimensionally as the Nusselt versus Rayleigh numbers, give correlations for Rayleigh numbers between 1000 and 10 exp 8, a range previously untested. These results show that natural convection has significant effects at a Rayleigh number of 1000 and higher, although the behavior of the Nusselt number as the conduction limit is approached is still unknown for a spherical geometry.
M. Ghalambaz; Noghrehabadi,A.; Ghanbarzadeh, A.
2014-01-01
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 param...
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) equilibrium with the medium), cubic boxes and closed-top conditions. Checks were performed on an unfractured porous medium and the convection cells do start for the theoretical value of Ra, namely 4p². 2D convection was verified up to Ra=800. The influence of parameters such as 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 50
Institute of Scientific and Technical Information of China (English)
Xiao－BoWu; Zeng－YuanGuo
1996-01-01
A full solution of two-dimensional Navier-Stokes and energy equation was conducted numerically to analyze the natural convection of the horizontal strip with an adiabatic substrate.The main features of such convection are:(i) the leading and trailing edge effect,(ii) the non-boundary layer effect,and (iii)the side edge effect .The results are compared with the boundary layer theory and exprimental data.
A decoupled monolithic projection method for natural convection problems
Pan, Xiaomin; Kim, Kyoungyoun; Lee, Changhoon; Choi, Jung-Il
2016-06-01
We propose an efficient monolithic numerical procedure based on a projection method for solving natural convection problems. In the present monolithic method, the buoyancy, linear diffusion, and nonlinear convection terms are implicitly advanced by applying the Crank-Nicolson scheme in time. To avoid an otherwise inevitable iterative procedure in solving the monolithic discretized system, we use a linearization of the nonlinear convection terms and approximate block lower-upper (LU) decompositions along with approximate factorization. Numerical simulations demonstrate that the proposed method is more stable and computationally efficient than other semi-implicit methods, preserving temporal second-order accuracy.
Adjoint optimization of natural convection problems: differentially heated cavity
Saglietti, Clio; Schlatter, Philipp; Monokrousos, Antonios; Henningson, Dan S.
2016-06-01
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
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.
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.
Turbulent natural and mixed convection along a vertical plate
Energy Technology Data Exchange (ETDEWEB)
Abu-Mulaweh, H.I.; Armaly, B.F.; Chen, T.S.; Zhao, J.Z.
1997-07-01
Measurements of turbulent boundary-layer air flow in natural and mixed convection adjacent to an isothermal vertical flat plate are reported. Laser-Doppler velocimeter and cold wire anemometer were used, respectively, to measure simultaneously the mean turbulent velocity and temperature distributions were measured for a temperature difference, {Delta}T, of 30 C between the heated wall and the free stream air at a fixed location x = 3 m (with a corresponding Grashof number Gr{sub x} = 8.55 x 10{sup 10}), and for a range of free stream velocities 0 m/s {le} U{sub {infinity} } {le} 0.41 m/s. The effect of small free stream velocity on the turbulent natural convection is examined. These results reveal that the introduction of small free stream velocity on turbulent natural convection flow suppresses turbulence and decreases the heat transfer rate from the heated wall.
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)
Directory of Open Access Journals (Sweden)
M. Ghalambaz
2014-06-01
Full Text Available 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.
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.
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.
Inverse boundary design of square enclosures with natural convection
Energy Technology Data Exchange (ETDEWEB)
Payan, S.; Sarvari, S.M.H.; Ajam, H. [The University of Sistan and Baluchestan, Mechanical Engineering Dept. (Iran, Islamic Republic of)
2009-04-15
An optimization technique is applied to design of heat transfer systems in which the natural convection is important. The inverse methodology is employed to estimate the unknown strengths of heaters on the heater surface of a square cavity with free convection from the knowledge of the desired temperature and heat flux distributions over a given design surface. The direct and the sensitivity problems are solved by finite volume method. The conjugate gradient method is used for minimization of an objective function, which is expressed by the sum of square residuals between estimated and desired heat fluxes over the design surface. The performance and accuracy of the present method for solving inverse convection heat transfer problems is evaluated by comparing the results with a benchmark problem and a numerical experiment. (authors)
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.
Talebi, Maryam; Setareh, Milad; Saffar-Avval, Majid; Hosseini Abardeh, Reza
2017-04-01
Application of ultrasonic waves for heat transfer augmentation has been proposed in the last few decades. Due to limited researches on acoustic streaming induced by ultrasonic oscillation, the effect of ultrasonic waves on natural convection heat transfer is the main purpose of this paper. At first, natural convection on up-ward-facing heating surface in a cylindrical enclosure filled with air is investigated numerically by the finite difference method, then the effect of upper surface oscillation on convection heat transfer is considered. The conservation equations in Lagrangian approach and compressible fluid are assumed for the numerical simulation. Results show that acoustic pressure will become steady after some milliseconds also pressure oscillation amplitude and acoustic velocity components will be constant therefore steady state velocity is used for solving energy equation. Results show that Enhancement of heat transfer coefficient can be up to 175% by induced ultrasonic waves. In addition, the effect of different parameters on acoustic streaming and heat transfer has been studied.
Temporal response of laser power standards with natural convective cooling.
Xu, Tao; Gan, Haiyong; Yu, Jing; Zang, Erjun
2016-01-25
Laser power detectors with natural convective cooling are convenient to use and hence widely applicable in a power range below 150 W. However, the temporal response characteristics of the laser power detectors need to be studied in detail for accurate measurement. The temporal response based on the absolute laser power standards with natural convective cooling is studied through theoretical analysis, numerical simulations, and experimental verifications. Our results show that the response deviates from a single exponential function and that an ultimate response balance is difficult to achieve because the temperature rise of the heat sink leads to continuous increase of the response. To determine the measurement values, an equal time reading method is proposed and validated by the laser power calibrations.
Novel natural convection heat sink design concepts from first principles
Fletcher, Derek E.
2016-01-01
Approved for public release; distribution is unlimited This was a two-part numerical study using ANSYS Fluent to develop novel heat sink concepts from first principles. The objective of this research was to highlight geometric structures that incorporate the principles of the stack effect to improve the heat transfer capability of a heat sink under natural convection. The first part investigated the heat transfer/fluid flow characteristics of vertically aligned tubes. The gaps between tube...
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.
A numerical study of natural convection in eccentric spherical annuli
Gallegos, Angel; Malaga, Carlos
2016-11-01
A fluid between two spheres, concentric or not, at different temperatures will flow in the presence of a constant gravitational force. Although there is no possible hydrostatic state, energy transport is dominated by diffusion if temperature difference between the spheres is small enough. By the use of a full three-dimensional thermal lattice Boltzmann model we study the transition between the conductive, the steady convective, and the unsteady convective regimes. We use the concentric case to validate the results by comparing with experiments and numerical simulations found in the literature, and then we extend our numerical experiments to the eccentric case to observe the general behavior of the different regimes. We analyze the energy transport characterized by the relation between Nusselt and Rayleigh numbers as well as the arising flow patterns. This work was partially supported by UNAM-DGAPA-PAPIIT Grant Number IN115216.
Natural convective boundary layer flow of a nano-fluid past a convectively heated vertical plate
Energy Technology Data Exchange (ETDEWEB)
Aziz, A. [Department of Mechanical Engineering, School of Engineering and Applied Science, Gonzaga University, Spokane, WA 99258 (United States); Khan, W.A. [Department of Engineering Sciences, PN Engineering College, National University of Sciences and Technology, Karachi 75350 (Pakistan)
2012-03-15
Natural convective flow of a nano-fluid over a convectively heated vertical plate is investigated using a similarity analysis of the transport equations followed by their numerical computations. The transport model employed includes the effect of Brownian motion and thermophoresis. The analysis shows that velocity, temperature and solid volume fraction of the nano-fluid profiles in the respective boundary layers depend, besides the Prandtl and Lewis numbers, on four additional dimensionless parameters, namely a Brownian motion parameter Nb, a thermophoresis parameter Nt, a buoyancy-ratio parameter Nr and convective parameter Nc. In addition to the study of these parameters on the boundary layer flow characteristics (velocity, temperature, solid volume fraction of the nano-fluid, skin friction, and heat transfer), correlations for the Nusselt and Sherwood numbers have been developed based on a regression analysis of the data. These linear regression models provide a highly accurate (with a maximum standard error of 0.004) representation of the numerical data and can be conveniently used in engineering practice. (authors)
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
Topological analysis of a mixing flow generated by natural convection
Contreras, Pablo Sebastián; de la Cruz, Luis Miguel; Ramos, Eduardo
2016-01-01
We use topological tools to describe the natural convective motion and the Lagrangian trajectories of a flow generated by stepwise, alternating heating and cooling protocol of opposite vertical walls of a cubic container. The working fluid considered is Newtonian and the system is in presence of the acceleration of gravity but the nonlinear terms are neglected, i.e., we study the piece-wise steady and linear problem. For this convective mixing flow, we identify invariant surfaces formed by the Lagrangian orbits of massless tracers that are topologically equivalent to spherical shells and period-1 lines with elliptic and hyperbolic segments that are located on symmetry planes. We describe the previous features as functions of the Rayleigh number in the range 3 × 104 ≤ Ra ≤ 5 × 105. We show that this system shares properties with other systems with non-toroidal invariant surfaces.
Natural convection between a vertical cylinder and a surrounding array
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McEligot, D.M.; O`Brien, J.E.; Stoots, C.M.; Larson, T.K.; Christenson, W.A.; Mecham, D.C.; Lussie, W.G.
1992-09-01
The generic situation considered is natural convection between a single heated, vertical cylinder and a surrounding array of cooler vertical cylinders in a triangular pattern. The ratio of the test section temperature to the cooling tube temperature was varied up to 2.6 by adjusting the electrical power. The Rayleigh number, based on test section diameter and air properties evaluated at cooling tube temperature, ranged from 2.9 x 10{sup 4} to 4.6 x 10{sup 5}. Results indicate that the convective heat transfer data could be approximated as Nu{sub D} (T{sub ts}/T{sub ct}){sup 0.14} = 0.156 Ra{sub D}{sup 1/3} in the apparent turbulent region for Ra{sub L} > 1.2 x 10{sup 11.}
Natural convection between a vertical cylinder and a surrounding array
Energy Technology Data Exchange (ETDEWEB)
McEligot, D.M.; O' Brien, J.E.; Stoots, C.M.; Larson, T.K.; Christenson, W.A.; Mecham, D.C.; Lussie, W.G.
1992-01-01
The generic situation considered is natural convection between a single heated, vertical cylinder and a surrounding array of cooler vertical cylinders in a triangular pattern. The ratio of the test section temperature to the cooling tube temperature was varied up to 2.6 by adjusting the electrical power. The Rayleigh number, based on test section diameter and air properties evaluated at cooling tube temperature, ranged from 2.9 x 10{sup 4} to 4.6 x 10{sup 5}. Results indicate that the convective heat transfer data could be approximated as Nu{sub D} (T{sub ts}/T{sub ct}){sup 0.14} = 0.156 Ra{sub D}{sup 1/3} in the apparent turbulent region for Ra{sub L} > 1.2 x 10{sup 11.}
Three-dimensional natural convection in a narrow spherical shell
Liu, Ming; Egbers, Christoph
The convective motions in a shallow fluid layer between two concentric spheres in the presence of a constant axial force field have been studied numerically. The aspect ratio of the fluid layer to inner radius is beta =0.08, the Prandtl number Pra =37.5. A three-dimensional time-dependent numerical code is used to solve the governing equations in primitive variables. Convection in the sphe rical shell has then a highly three-dimensional nature. Characteristic flow patterns with a large number of banana-type cells, oriented in north-south direction and aligned in the azimuthal direction, are formed on the northern hemisphere, which grow gradually into the equatorial region accompanied by the generation of new cells as the Rayleigh number is increased. Various characteristics of these flows as well as their transient evolution are investigated for Rayleigh numbers up to 20 000.
Tropical Cyclone Signatures in Atmospheric Convective Available Potential Energy
Studholme, Joshua; Gulev, Sergey
2016-04-01
Tropical cyclones play an important role in the climate system providing transports of energy and water vapor, forcing the ocean, and also affecting mid-latitude circulation phenomena. Tropical cyclone tracks experience strong interannual variability and in addition, longer term trend-like changes in all ocean basins. Analysis of recent historical data reveal a poleward shift in the locations of tropical cyclone tracks in both the Northern and Southern Hemispheres (Kossin et al. 2014, Nature, 509, 349-352). The physical consequences of these alterations are largely unconstrained. For example, the increasing encroachment of tropical cyclone activity into the extra-tropical environment presents a novel and still poorly understood paradigm for tropical-extratropical interactions. In this respect, the role that the atmospheric convective available potential energy (CAPE) plays in the dynamics of tropical cyclones is highly interesting. The two characteristic global-scale spatial patterns in CAPE are identified using EOF analysis. The first pattern shows an abundance of CAPE in the centre of the Pacific and corresponds to the El Nino Southern Oscillation. The second one is capturing positive CAPE anomalies in the oceanic tropics and negative anomalies over equatorial Africa. Associated with these buoyancy patterns, alterations in tropical cyclone activity occur in all basins forming both zonal and meridional patterns. Atmospheric buoyancy is the trigger for deep convection, and subsequently cyclone genesis. This is the mechanism of impact upon location at the start of cyclone tracks. It is found to have less impact upon where cyclones subsequently move, whether or not they undergo extratropical transition and when and where they experience lysis. It is shown that CAPE plays a critical role in the general circulation in the tropics which in turn is the larger steering context for embedded systems within the Walker and Hadley cells. So this lack of `latter life' impact
Natural Convection Heat Transfer in Concentric Horizontal Annuli Containing a Saturated Porous Medi
Directory of Open Access Journals (Sweden)
Ahmed F. Alfahaid, R.Y. Sakr
2012-10-01
Full Text Available Natural convection in horizontal annular porous media has become a subject receiving increasing attention due to its practical importance in the problem of insulators, such as ducting system in high temperature gas-cooled reactors, heating systems, thermal energy storage systems, under ground cable systems, etc. This paper presents a numerical study for steady state thermal convection in a fully saturated porous media bounded by two horizontal concentric cylinders, the cylinders are impermeable to fluid motion and maintained at different, uniform temperatures. The solution scheme is based on two-dimensional model, which is governed by Darcy-Oberbeck-Boussinesq equations. The finite element method using Galerkin technique is developed and employed to solve the present problem. A numerical simulation is carried out to examine the parametric effects of Rayleigh number and radius ratio on the role played by natural convection heat transfer in the porous annuli. The numerical results obtained from the present model were compared with the available published results and good agreement is observed. The average Nusselt number at the heating surface of the inner cylinder is correlated to Rayleigh number and radius ratio.Keywords: Natural convection, numerical investigation, saturated porous media, finite element method, concentric horizontal annuli.
Effect of an External Oriented Magnetic Field on Entropy Generation in Natural Convection
Atef El Jery; Nejib Hidouri; Mourad Magherbi; Ammar Ben Brahim
2010-01-01
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 ir...
Double-diffusive natural convection in an enclosure filled with nanofluid using ISPH method
Abdelraheem M Aly; Zehba A.S. Raizah
2016-01-01
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 config...
Institute of Scientific and Technical Information of China (English)
LIU Guo-qing; ZHOU Zhi-fang
2012-01-01
By combining sand tank tests with numerical simulations,this paper studies the temperature fields around constant heat sources to reveal the mechanism of the natural convection and its influence on the temperature fields in the process of energy storage.Using the “24-channel temperature auto acquisition system” developed by our research group in the tests,the temperatures are recorded at measuring points within the research area in the tests,the revised Brinkman equation and a transfer-convection balance model are used for solving the aquifer water-thermal coupling problems,and through comparison of the test results with the calculation results,it is discovered that the influence ranges and the variations of the two temperature fields are consistent,which validates the mathematical model.On the basis of this,we also study the influences of the heat source positions and the boundary conditions on the temperature fields,and the results show that,under the natural convection,the heat source positions may influence the distribution of the temperature fields,thus affect the energy storage.For the same energy storage layer,the temperature field for the top energy storage is characterized by a smaller heat influence range and a relatively concentrated temperature distribution.However,when the heat source is at the bottom,the range of a temperature field,and the temperature is relatively dispersed,which is not favorable to heat recycle,with the same heat sourcc position,the boundary conditions determine the size of the critical Rayleigh number,and thus have an influence on the occurrence and the strength of the natural convection,and accordingly,on the temperature fields.
Natural convection heat transfer along vertical rectangular ducts
Ali, M.
2009-12-01
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.
The flow patterning capability of localized natural convection.
Huang, Ling-Ting; Chao, Ling
2016-09-14
Controlling flow patterns to align materials can have various applications in optics, electronics, and biosciences. In this study, we developed a natural-convection-based method to create desirable spatial flow patterns by controlling the locations of heat sources. Fluid motion in natural convection is induced by the spatial fluid density gradient that is caused by the established spatial temperature gradient. To analyze the patterning resolution capability of this method, we used a mathematical model combined with nondimensionalization to correlate the flow patterning resolution with experimental operating conditions. The nondimensionalized model suggests that the flow pattern and resolution is only influenced by two dimensionless parameters, and , where Gr is the Grashof number, representing the ratio of buoyancy to the viscous force acting on a fluid, and Pr is the Prandtl number, representing the ratio of momentum diffusivity to thermal diffusivity. We used the model to examine all of the flow behaviors in a wide range of the two dimensionless parameter group and proposed a flow pattern state diagram which suggests a suitable range of operating conditions for flow patterning. In addition, we developed a heating wire with an angular configuration, which enabled us to efficiently examine the pattern resolution capability numerically and experimentally. Consistent resolutions were obtained between the experimental results and model predictions, suggesting that the state diagram and the identified operating range can be used for further application.
Energy Technology Data Exchange (ETDEWEB)
Luo, Kang; Yi, Hong-Liang, E-mail: yihongliang@hit.edu.cn; Tan, He-Ping, E-mail: tanheping@hit.edu.cn [School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001 (China)
2014-05-15
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.
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.
Institute of Scientific and Technical Information of China (English)
LU; YuanWei; LI; XiaoLi; LI; Qiang; WU; YuTing; MA; ChongFang
2013-01-01
In order to get the natural convection heat transfer mechanism of molten salt, the experimental investigation of natural convective heat transfer of LiNO3was studied after it was simulated by numerical calculation. Experiment was carried out on the nat-ural convection heat transfer of air and water around the fine wire using the method of Joule heating. The results showed that the natural convection heat transfer of air and water around the fine wire agreed well with Fand’s correlation. Based on the aforementioned experiment, the natural convection heat transfer of molten salt LiNO3was studied by experiment and the same results were got. Therefore, the natural convection heat transfer of molten salt can be calculated by Fand’s correlation, which takes into consideration the effect of viscosity dissipation.
Scott, Douglas
2007-01-01
It is now well accepted that both Dark Matter and Dark Energy are required in any successful cosmological model. Although there is ample evidence that both Dark components are necessary, the conventional theories make no prediction for the contributions from each of them. Moreover, there is usually no intrinsic relationship between the two components, and no understanding of the nature of the mysteries of the Dark Sector. Here we suggest that if the Dark Side is so seductive then we should not be restricted to just 2 components. We further suggest that the most natural model has 5 distinct forms of Dark Energy in addition to the usual Dark Matter, each contributing precisely equally to the cosmic energy density budget.
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.
PARALLEL LATTICE BGK SIMULATION OF NATURAL CONVECTION IN A CAVITY
Institute of Scientific and Technical Information of China (English)
SHI Bao-chang; LIU Hong-juan
2005-01-01
A Coupled Lattice Bhatnagar-Gross-Krook (CLBGK) model was proposed with a robust boundary scheme to model the Bossinesq incompressible flows. The LBGK method is a powerful approach, but is also computationally demanding. Therefore, parallel computing was implemented, and the codes were run on the parallel computer "Lenovo DeepComp 1800" with 24 nodes written in C++ using the Message Passing Interface (MPI) library. Numerical results for natural convection in a cavity with the Rayleigh number (Ra) ranging from 106 to 4×1010 were presented, and found to agree well with the previous work. And with the increase of the resolution rate, the accuracy has been greatly improved. In addition, new models were set up by applying different equilibriums to the CLBGK model to simulate the same problem, which all yield sound results.
THE UNSTABLE MODES OF NATURAL CONVECTION BOUNDARY LAYER
Institute of Scientific and Technical Information of China (English)
Tao Jianjun; Zhuang Fenggan; Yan Dachun
2000-01-01
The instability of natural convection boundary layer around a vertical heated flat plate is analyzed theoretically in this paper. The results illustrate that the “loop” in the neutral curve is not a real loop but a twist of the curve is the frequencywave number-Grashof number space, and there is only one unstable mode at small Prandtl numbers. Specially, when the Prandtl number is large enough two unstable modes will be found in the “loop” region. Along the amplifying surface intersection the two unstable modes have the same Grashof number, wave number and frequency but different amplifying rates. Their instability characteristics are analyzed and the criterion for determining the existence of the multi-unstable modes is also discussed.
Modeling natural convection heat transfer from perforated plates
Institute of Scientific and Technical Information of China (English)
Zan WU; Wei LI; Zhi-jian SUN; Rong-hua HONG
2012-01-01
Staggered pattern perforations are introduced to isolated isothermal plates,vertical parallel isothermal plates,and vertical rectangular isothermal fins under natural convection conditions.The performance of perforations was evaluated theoretically based on existing correlations by considering effects of ratios of open area,inclined angles,and other geometric parameters.It was found that staggered pattern perforations can increase the total heat transfer rate for isolated isothermal plates and vertical parallel plates,with low ratios of plate height to wall-to-wall spacing (H/s),by a factor of 1.07 to 1.21,while only by a factor of 1.03 to 1.07 for vertical rectangular isothermal fins,and the magnitude of enhancement is proportional to the ratio of open area.However,staggered pattern perforations are detrimental to heat transfer enhancement of vertical parallel plates with large H/s ratios.
Li, Zheng; Zhang, Yuwen
2016-01-01
The purposes of this paper are testing an efficiency algorithm based on LBM and using it to analyze two-dimensional natural convection with low Prandtl number. Steady state or oscillatory results are obtained using double multiple-relaxation-time thermal lattice Boltzmann method. The velocity and temperature fields are solved using D2Q9 and D2Q5 models, respectively. With different Rayleigh number, the tested natural convection can either achieve to steady state or oscillatory. With fixed Rayleigh number, lower Prandtl number leads to a weaker convection effect, longer oscillation period and higher oscillation amplitude for the cases reaching oscillatory solutions. At fixed Prandtl number, higher Rayleigh number leads to a more notable convection effect and longer oscillation period. Double multiple-relaxation-time thermal lattice Boltzmann method is applied to simulate the low Prandtl number fluid natural convection. Rayleigh number and Prandtl number effects are also investigated when the natural convection...
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.
Natural convection of ferrofluids in partially heated square enclosures
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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.
Natural convection heat transfer on surfaces of copper micro-wires
Guan, Ning; Liu, Zhigang; Zhang, Chengwu; Jiang, Guilin
2014-02-01
The natural convection heat transfer characteristics and mechanism for copper micro-wires in water and air were investigated experimentally and numerically. The wires with diameters of 39.9, 65.8 and 119.1 μm were placed horizontally in water inside of a sealed tube and in air of a large room, respectively. Using Joule heating, the heat transfer coefficients and Nusselt numbers of natural convection for micro-wires in ultra pure water and air were obtained. A three dimensional incompressible numerical model was used to investigate the natural convection, and the prediction with this model was in reasonable accordance with the experimental results. With the decrease of micro-wire diameter, the heat transfer coefficient of natural convection on the surface of micro-wire becomes larger, while the Nu number of natural convection decreases in water and air. Besides, the change rate of Nu number in water decreases apparently with the increase of heat flux and the decrease of wire diameter, which is larger than that in air. The thickness of boundary layer on the wall of micro-wire becomes thinner with the decrease of diameter in both water and air, but the ratio of boundary layer thickness in water to the diameter increases. However, there is almost no change of this ratio for natural convection in air. As a result, the proportion of conduction in total heat transfer of natural convection in water increases, while the convective heat transfer decreases. The velocity distribution, temperature field and the boundary layer in the natural convection were compared with those of tube with conventional dimension. It was found that the boundary layer around the micro-wire is an oval-shaped film on the surface, which was different from that around the conventional tube. This apparently reduces the convection strength in the natural convection, thus the heat transfer presents a conduction characteristic.
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”.
Natural Convection and Boiling for Cooling SRP Reactors During Loss of Circulation Conditions
Energy Technology Data Exchange (ETDEWEB)
Buckner, M.R.
2001-06-26
This study investigated natural convection and boiling as a means of cooling SRP reactors in the event of a loss of circulation accident. These studies show that single phase natural convection cooling of SRP reactors in shutdown conditions with the present piping geometry is probably not feasible.
Institute of Scientific and Technical Information of China (English)
Yoshihiromochimaru
2000-01-01
A steady-state two-dimensional natural convection in a rectangular equlateral triangle cavity is analyzed numercally,using a spectral finite difference scheme,where a conformal mapping coordinate system is adopted with a unit circle for the boundary.Vorticity-stream function formulation is used in conjunction with an energy equation.Time marching algorithm in a diagonal dominant form under a Fourier series decomposition is used to give a steady-state field for a mixed(Neumann and Dirichlet) thermal boundary condition even at a Grashof number of 106.
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.
Energy Technology Data Exchange (ETDEWEB)
Beltran, Jorge I. LLagostera; Trevisan, Osvair Vidal [Universidade Estadual de Campinas, SP (Brazil). Faculdade de Engenharia Mecanica. Dept. de Energia
1990-12-31
Natural convection flow induced by heating from below in a irregular porous cavity is investigated numerically. The influence of the modified Rayleigh number and geometric ratios on heat transfer and fluid flow is studied. Global and local Nusselt for Rayleigh numbers covering the range 0 - 1600 and for several geometric ratios. The fluid flow and the temperature field are illustrated by contour maps. (author) 6 refs., 10 figs., 7 tabs.
Mode-to-mode energy transfers in convective patterns
Indian Academy of Sciences (India)
Mahendra K Verma; Krishna Kumar; Bhaskar Kamble
2006-12-01
We investigate the energy transfer between various Fourier modes in a low-dimensional model for thermal convection. We have used the formalism of mode-to-mode energy transfer rate in our calculation. The evolution equations derived using this scheme is the same as those derived using the hydrodynamical equations for thermal convection in Boussinesq fluids. Numerical and analytical studies of this model show that convective rolls appear as the Rayleigh number is raised above its critical value c. Further increase of Rayleigh number generates rolls in the perpendicular directions as well, and we obtain a dynamic asymmetric square pattern. This pattern is due to Hopf bifurcation. There are two sets of limit cycles corresponding to the two competing asymmetric square patterns. When the Rayleigh number is increased further, the limit cycles become unstable simultaneously, and chaotic motion sets in. The onset of chaos is via intermittent route. The trajectories wander for quite a long time almost periodically before jumping irregularly to one of the two ghost limit cycles.
Universality of energy spectrum in turbulent Rayleigh-Benard convection
Bai, Kunlun; Hoeller, Judith; Brown, Eric
2016-11-01
We present study of energy spectrum in turbulent Rayleigh-Benard convection, in both cylindrical and cubic containers, tilting and non-tilting conditions, and with Rayleigh number ranging from 0 . 5 ×109 to 1 ×1010 . For these different conditions of geometry, tilt, and Rayleigh number, the temperature spectra measured on the system side walls are significantly different from each other. Even for the same condition, the spectrum varies depending on whether the sensors locate in the path of large-scale circulations. However, quite interestingly, once the signals of large-scale circulations are subtracted from the raw temperature, all spectra display a universal shape, regardless of system geometry, tilt, Rayleigh number, and location of sensors. It suggests that one could model the large-scale circulations and small-scale fluctuations separately in turbulent Rayleigh-Benard convection.
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.
Magnetohydrodynamic stability of natural convection in a vertical porous slab
Shankar, B. M.; Kumar, Jai; Shivakumara, I. S.
2017-01-01
The stability of the conduction regime of natural convection in an electrically conducting fluid saturated porous vertical slab is investigated in the presence of a uniform external transverse magnetic field. The flow in the porous medium is described by modified Brinkman-extended Darcy equation with fluid viscosity different from effective viscosity. The boundaries of the vertical porous slab are assumed to be rigid-isothermal and electrically non-conducting. The resulting stability equations are solved numerically using Galerkin method. The critical Grashof number Gc, the critical wave number αc and the critical wave speed cc are computed for a wide range of porous parameter σp, the ratio of effective viscosity to the fluid viscosity Λ, the Prandtl number Pr and the Hartmann number M. Based on these parameters, the stability characteristics of the system are discussed in detail. The presence of advective inertia is to instill instability on the flow in a porous medium and found that the magnetic field, porous parameter and ratio of viscosities have a stabilizing effect on both stationary and oscillatory wave instabilities. Besides, the value of Pr at which transition occurs from stationary to oscillatory mode of instability decreases with increasing M ,σp and Λ .
Directory of Open Access Journals (Sweden)
K Rahmani
2013-01-01
Full Text Available This work concerns the study of heat transfer by means of natural convection with fluids circulating in enclosures. These topics are largely studied both experimentally and numerically due to their wide industrial application in various fields such as nuclear energy, the heating and cooling of buildings, solar collectors, etc. A great deal of relevant research work consists in numerical simulations of natural convection mechanisms with laminar flows in closed cavities. In this context, the present study comes as a contribution in numerical form investigating the turbulent natural convection in vertical enclosure which presents sinusoidal protuberances on one of its vertical walls. Both the top and bottom of the enclosure are open to allow the fluid flow. The horizontal walls are supposed adiabatic. We are interested in determining for various amplitudes and periods. The influence of geometry on several factors such as: temperature, the number of local Nusselt, the turbulent kinetic energy k and its dissipationï¥. Based on the Navier-Stokes equations and Boussinesq approximation, the equations were solved by the CFD technique using the Finite Volume Method In the case of enclosures having the form ratio equal to 0.6 (A=0.6. Given the steady conditions of heat flow on the vertical walls and the pressures at the entry and exit of the cavity, the results show that when we gradually increase the amplitudes of the protuberance wall (say a=0.005 m, a=0.010, a=0.015, a= 0.02, and a=0.025, the maximal temperature increases with the increase of amplitude. This is due to the rise of the heat transfer surface of the modified wall. Regarding heat transfer parameters, the results show that the number of local Nusselt varies relatively with the amplitudes. This explains that the modified wall is affected locally by a pure conduction. The results obtained in this study are in agreement with recent works of several authors.
Experimental study of natural convective heat transfer in a vertical hexagonal sub channel
Tandian, Nathanael P.; Umar, Efrizon; Hardianto, Toto; Febriyanto, Catur
2012-06-01
The development of new practices in nuclear reactor safety aspects and optimization of recent nuclear reactors, including the APWR and the PHWR reactors, needs a knowledge on natural convective heat transfer within sub-channels formed among several nuclear fuel rods or heat exchanger tubes. Unfortunately, the currently available empirical correlation equations for such heat transfer modes are limited and researches on convective heat transfer within a bundle of vertical cylinders (especially within the natural convection modes) are scarcely done. Although boundary layers around the heat exchanger cylinders or fuel rods may be dominated by their entry regions, most of available convection correlation equations are for fully developed boundary layers. Recently, an experimental study on natural convective heat transfer in a subchannel formed by several heated parallel cylinders that arranged in a hexagonal configuration has been being done. The study seeks for a new convection correlation for the natural convective heat transfer in the sub-channel formed among the hexagonal vertical cylinders. A new convective heat transfer correlation equation has been obtained from the study and compared to several similar equations in literatures.
Neshat, E.; Hossainpour, S.; Bahiraee, F.
2014-06-01
Both of experimental and numerical investigations were performed to understand unsteady natural convection from outer surface of helical coils. Four helical coils with two different curvature ratios were used. Each coil was mounted in the shell both vertically and horizontally. The cold water was entered the coil and the hot water in the shell was cooling by unsteady natural convection. A CFD code was developed to simulate natural convection heat transfer. Equations of tube and shell are solved simultaneously. Statistical analyses have been done on data points of temperature and natural convection Nusselt number. It was revealed that shell-side fluid temperature and the Nusselt number of the outer surface of coils are functions of in-tube fluid mass flow rate, specific heat of fluids and geometrical parameters including length, inner diameter of the tube and the volume of the shell, and time.
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.
Pitz, DB; Chew, JW
2015-01-01
Natural convection in differentially heated enclosures is a benchmark problem used to investigate the physics of buoyant flows and to validate numerical methods. Such configurations are also of interest in engineering applications such as cooling of electronic components and air flow around buildings. In this work a spectral element method is used to carry out direct numerical simulations of natural convection in a tall enclosure of aspect ratio 4 with isothermal vertical walls and adiabatic ...
Natural convection gas pendulum and its application in accelerometer and tilt sensor
Institute of Scientific and Technical Information of China (English)
ZHANG Fuxue
2005-01-01
It is discovered that the natural convection gas has the pendulum characteristic, which leads to the introduction of the new concept of gas pendulum. In this paper, the buoyancy lift of natural convection gas is analyzed in a hermetic chamber, and the relationship between the buoyancy lift and the change of temperature is formulated. The experimental results show that the gas pendulum,similar to the solid pendulum and liquid pendulum, can be utilized to sense the acceleration and the tilt angle.
Institute of Scientific and Technical Information of China (English)
罗振东; 朱江; 谢正辉; 张桂芳
2003-01-01
The non-stationary natural convection problem is studied. A lowest order finite difference scheme based on mixed finite element method for non-stationary natural convection problem, by the spatial variations discreted with finite element method and time with finite difference scheme was derived, where the numerical solution of velocity, pressure, and temperature can be found together, and a numerical example to simulate the close square cavity is given, which is of practical importance.
Beukema, K.J.
1980-01-01
Three different models of bulk-stored agricultural products with air flow through the bulk, predicting the temperature profiles or the velocity of natural convection, are developed. The temperature distribution in a cylindrical container with insulated walls and open top and bottom, filled with heat-generating model material is studied experimentally, used to calculate the velocity of natural convection and compared with model predictions. A two-dimensional two-phase model of temper...
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.
Energy Technology Data Exchange (ETDEWEB)
Barthold, W.P. [Barthold and Associates, Inc., Albuquerque, NM (United States)
1984-08-01
The scope of work is to summarize inherent safety advantages that are unique to the use of a carbide based fuel system and to summarize the technical issues regarding natural convection flow in LMFBR cores. As discussed in this report, carbide fuel provides the designer with far greater flexibility than oxide fuel. Carbide fuel systems can be designed to eliminate major accident initiators. They turn quantitative advantages into a qualitative advantage. The author proposed to LANL a series of core design and component concepts that would greatly enhance the safety of carbide over oxide systems. This report cites a series of safety advantages which potentially exist for a carbide fuel system. Natural convection issues have not been given much attention in the past. Only during the last few years has this issue been addressed in some detail. Despite claims to the contrary by some of the LMR contractors, the author does not think that the natural convection phenomena is fully understood. Some of the approximations made in natural convection transient analyses have probably a greater impact on calculated transient temperatures than the effects under investigation. Only integral in-pile experimental data and single assembly out-of-pile detailed data are available for comparisons with analytical models and correlations. Especially for derated cores, the natural convection capability of a LMR should be far superior to that of a LWR. The author ranks the natural convection capability of the LMR as the most important inherent safety feature.
Optimization of natural convection heat transfer of Newtonian nanofluids in a cylindrical enclosure
Institute of Scientific and Technical Information of China (English)
Hamid Moradi; Bahamin Bazooyar; Ahmad Moheb; Seyed Gholamreza Etemad
2015-01-01
This study characterizes and optimizes natural convection heat transfer of two Newtonian Al2O3 and TiO2/water nanofluids in a cylindrical enclosure. Nusselt number (Nu) of nanofluids in relation to Rayleigh number (Ra) for different concentrations of nanofluids is investigated at different configurations and orientations of the enclosure. Results show that adding nanoparticles to water has a negligible or even adverse influence upon natural convec-tion heat transfer of water:only a slight increase in natural convection heat transfer of Al2O3/water is observed, while natural convection heat transfer for TiO2/water nanofluid is inferior to that for the base fluid. Results also reveal that at low Ra, the likelihood of enhancement in natural convection heat transfer is more than at high Ra:at low Ra, inclination angle, aspect ratio of the enclosure and nanoparticle concentration influence natural convec-tion heat transfer more pronouncedly than that in high Ra.
The nature of near-wall convection velocity in turbulent channel flow
Institute of Scientific and Technical Information of China (English)
Yuhui Cao; Jun Chen; Zhensu She
2008-01-01
A novel notion of turbulent structure-the local cascade structure-is introduced to study the convection phenomenon in a turbulent channel flow. A space-time cross-correlation method is used to calculate the convection velo-city. It is found that there are two characteristic convection speeds near the wall, one associated with small-scale streaks of a lower speed and another with streamwise vortices and hairpin vortices of a higher speed. The new concept of tur-bulent structure is powerful to illustrate the dominant role of coherent structures in the near-wall convection, and to reveal also the nature of the convection-the propagation of patterns of velocity fluctuations-which is scale-dependent.
Double-diffusive natural convective boundary-layer flow of a nano-fluid past a vertical plate
Energy Technology Data Exchange (ETDEWEB)
Kuznetsov, A.V. [Department of Mechanical and Aerospace Engineering, North Carolina State University, Campus Box 7910, Raleigh, NC 27695-7910 (United States); Nield, D.A. [Department of Engineering Science, University of Auckland, Private Bag 92019, Auckland 1142 (New Zealand)
2011-05-15
The double-diffusive natural convective boundary-layer flow of a nano-fluid past a vertical plate is studied analytically. The model used for the binary nano-fluid incorporates the effects of Brownian motion and thermophoresis. In addition the thermal energy equations include regular diffusion and cross-diffusion terms. A similarity solution is presented. Numerical calculations were performed in order to obtain correlation formulas giving the reduced Nusselt number as a function of the various relevant parameters. (authors)
Energy transport by thermocapillary convection during Sessile-Water-droplet evaporation.
Ghasemi, H; Ward, C A
2010-09-24
The energy transport mechanisms of a sessile-water droplet evaporating steadily while maintained on a Cu substrate are compared. Buoyancy-driven convection is eliminated, but thermal conduction and thermocapillary convection are active. The dominant mode varies along the interface. Although neglected in previous studies, near the three-phase line, thermocapillary convection is by far the larger mode of energy transport, and this is the region where most of the droplet evaporation occurs.
Directory of Open Access Journals (Sweden)
Ali Hamza Altaee
2017-07-01
Full Text Available Natural convection heat transfer phenomenon in a square enclosure containing equilateral triangle cylinder on its centroid is numerically investigated. Two dimensional for air (Pr=0.71 with outer cold walls of square enclosure and hot inner walls of the triangle cylinder by using CFD techniques is implemented. (Ansys Fluent 16 is used to simulate this problem. TheRayleigh number varied from 104 to 106 and orientation angles from 00 to 1050 step 150 for each case. The laminar boussinesq approximation is used with the convergence criteria less than 10-5 for continuity, momentum and for energy equation 10-8. Fluid and temperature are exhibited by stream and isothermal lines while heat transfer presented by the average Nusselt number. According to orientation angle, results can be divided into two groups; (0o-45o and (60o-105o. Maximum heat transfer obtained at orientation angle of 300 and Ra=106.
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.
Analysis of Natural Convection in an Elliptical Cavity,Using a Dini Expansion
Institute of Scientific and Technical Information of China (English)
Y.Mochimaru
1998-01-01
Analyzed is natural transient convection followed by steady-state convection in an elliptical cavity heated from below,using a spectral finite difference scheme in terms of a Dini expansion.The major or minor axis of the elliptic section of the Cavity is assumed to be placed horizontally.As a thermal boundary condition,Dirichlet or Neumann condition is specifice.Not only for nearly pure-heat conduction case at low Grashof numbers but also for strong convection flow with cellular patterns at larger Grashof numbers are found to be attained at least for some combination of Prandtl numbers and geometrical parameters.
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.
Numerical simulation of magnetic nanofluid natural convection in porous media
Sheikholeslami, Mohsen
2017-02-01
Free convection of magnetic nanofluid in a porous curved cavity is investigated. Influence of external magnetic source is taken into account. Innovative numerical approach, namely CVFEM, is applied. Impacts of Darcy number (Da), Rayleigh (Ra), Hartmann (Ha) numbers and volume fraction of Fe3O4 (ϕ) on hydrothermal characteristics are examined. Results indicate that heat transfer augmentation augments with rise of Ha and reduces with rise of Da , Ra . Lorentz forces make the nanofluid motion to decrease and enhance the thermal boundary layer thickness. Temperature gradient enhances with increase of Da , Ra , ϕ, but it reduces with rise of Ha.
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.
Natural convection heat transfer in vertical triangular subchannel in Zirconia-water nanofluid
Tandian, N. P.; Alkharboushi, A. A. K.; Kamajaya, K.
2015-09-01
Natural convection heat transfer in vertical triangular sub-channel has important role in cooling mechanism of the APWR and the PHWR nuclear reactors. Unfortunately, natural convection correlation equations for such geometry are scarcely available. Recent studies showed that ZrO2-water nanofluid has a good prospect to be used in the nuclear reactor technology due to its low neutron absorption cross section. Although several papers have reported transport properties of ZrO2-water nanofluids, practically there is no correlation equation for predicting natural convection heat transfer in a vertical triangular sub-channel in ZrO2-water nanofluid. Therefore, a study for finding such heat transfer correlation equation has been done by utilizing Computational Fluid Dynamics software and reported in this paper. In the study, natural convection heat transfer in a vertical triangular sub-channel has been simulated at several values of heat transfer flux within 9.1 to 30.9 kW/m2 range and ZrO2 concentrations of 0 (pure water), 0.27, and 3 volume-% of ZrO2. The study shows that the ZrO2 concentration has no significant influence to the natural convection heat transfer at those concentration levels. The obtained theoretical heat transfer correlation equations were verified through experiment, and they showed very similar results. The correlation equations are reported in this paper.
Miranda Fuentes, Johann; Kuznik, Frédéric; Johannes, Kévyn; Virgone, Joseph
2014-01-01
This article presents a new model to simulate melting with natural convection of a phase change material. For the phase change problem, the enthalpy formulation is used. Energy equation is solved by a finite difference method, whereas the fluid flow is solved by the multiple relaxation time (MRT) lattice Boltzmann method. The model is first verified and validated using the data from the literature. Then, the model is applied to a tall brick filled with a fatty acid eutectic mixture and the results are presented. The main results are (1) the spatial convergence rate is of second order, (2) the new model is validated against data from the literature and (3) the natural convection plays an important role in the melting process of the fatty acid mixture considered in our work.
Some Properties of the Kinetic Energy Flux and Dissipation in Turbulent Stellar Convection Zones
Meakin, Casey
2010-01-01
We investigate simulated turbulent flow within thermally driven stellar convection zones. Different driving sources are studied, including cooling at the top of the convectively unstable region, as occurs in surface convection zones; and heating at the base by nuclear burning. The transport of enthalpy and kinetic energy, and the distribution of turbulent kinetic energy dissipation are studied. We emphasize the importance of global constraints on shaping the quasi-steady flow characteristics, and present an analysis of turbulent convection which is posed as a boundary value problem that can be easily incorporated into standard stellar evolution codes for deep, efficient convection. Direct comparison is made between the theoretical analysis and the simulated flow and very good agreement is found. Some common assumptions traditionally used to treat quasi-steady turbulent flow in stellar models are briefly discussed. The importance and proper treatment of convective boundaries are indicated.
Zhang, Guang; Jiang, Shaohui; Yao, Wei; Liu, Changhong
2016-11-16
Owing to the outstanding properties of thermal conduction, lightweight, and chemical durability, carbon nanotubes (CNTs) have revealed promising applications in thermal management materials. Meanwhile, the increasingly popular portable electronics and the rapid development of space technology need lighter weight, smaller size, and more effective thermal management devices. Here, a novel kind of heat dissipation devices based on the superaligned CNT films and underlying microchannels is proposed, and the heat dissipation properties are measured at the natural condition. Distinctive from previous studies, by combining the advantages of microchannels and CNTs, such a novel heat dissipation device enables superior natural convection heat transfer properties. Our findings prove that the novel CNT-based devices could show an 86.6% larger total natural heat dissipation properties than bare copper plate. Further calculations of the radiation and natural convection heat transfer properties demonstrate that the excellent passive cooling properties of these CNT-based devices are primarily caused by the reinforcement of the natural convection heat transfer properties. Furthermore, the heat dissipation mechanisms are briefly discussed, and we propose that the very high heat transfer coefficients and the porous structures of superaligned CNT films play critical roles in reinforcing the natural convection. The novel CNT-based heat dissipation devices also have advantages of energy-saving, free-noise, and without additional accessories. So we believe that the CNT-based heat dissipation devices would replace the traditional metal-finned heat dissipation devices and have promising applications in electronic devices, such as photovoltaic devices, portable electronic devices, and electronic displays.
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.
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.
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.
Natural Gas Energy Educational Kit.
American Gas Association, Arlington, VA. Educational Services.
Prepared by energy experts and educators to introduce middle school and high school students to natural gas and its role in our society, this kit is designed to be incorporated into existing science and social studies curricula. The materials and activities focus on the origin, discovery, production, delivery, and use of natural gas. The role of…
Budgets of divergent and rotational kinetic energy during two periods of intense convection
Buechler, D. E.; Fuelberg, H. E.
1986-01-01
The derivations of the energy budget equations for divergent and rotational components of kinetic energy are provided. The intense convection periods studied are: (1) synoptic scale data of 3 or 6 hour intervals and (2) mesoalphascale data every 3 hours. Composite energies and averaged budgets for the periods are presented; the effects of random data errors on derived energy parameters is investigated. The divergent kinetic energy and rotational kinetic energy budgets are compared; good correlation of the data is observed. The kinetic energies and budget terms increase with convective development; however, the conversion of the divergent and rotational energies are opposite.
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.
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 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.
Natural convection characteristics of flat plate collectors. Progress report
Energy Technology Data Exchange (ETDEWEB)
Randall, K.R.; Wl-Wakil, M.M.; Mitchell, J.W.
1977-09-01
The results of an experimental investigation into the convective heat losses in large aspect ratio flat-plate solar collectors are described. An experimental study has been undertaken on a specially designed test cell using a 3 inch Mach-Zehnder interferometer. Air at atmospheric pressure was used as the heat-transfer fluid. The experimental results include interferograms which show the thermal boundary layer formations and the temperature profiles. Local temperature profiles have been analyzed through the use of an optical comparator to determine local Nusselt number profiles, which have, in turn, been integrated to give average heat-transfer results. Angles of inclination from the horizontal of 45, 60, 75 and 90 degrees have been investigated. Aspect ratios from 9 to 36 were examined over a Rayleigh number range of 4,000 to 310,000. Finally, heat-transfer correlations have been developed for the prediction of local Nusselt numbers in the starting and departure corners and for the average heat-transfer results as a function of collector tilt angle.
Horizontal velocity field near the hot plate in turbulent natural convection
Koothur, Vipin
2014-01-01
We study the velocity field in a horizontal (x-y) plane 1.5 mm above the hot plate in turbulent natural convection using PIV at a Rayleigh number Raw=106 and Prandtl number Pr=5.2. The plane of measurement is inside the velocity boundary layer estimated from the natural convection boundary layer equations[7] as well as inside the velocity boundary layer due to the large scale flow[2, 5].The boundary layer comprises of line plumes with sinking fluid between them. The instantaneous velocity variation from the center of the sinking fluid to the line plumes is found to deviate with the classical Prandtl-Blasius laminar boundary layer profile, which is assumed to be the nature of boundary layer by the GL theory [2, 5]. Our results agree well with the natural convection boundary layer profile. The time averaged mean velocity variation deviates from both natural convection and Blasius type profiles as expected as it depends on the orientation of the line plumes. Our measurement result is a proof to the theory of the...
Directory of Open Access Journals (Sweden)
Missoum Abdelkrim
2016-01-01
Full Text Available This study focuses on the numerical simulation of heat transfer by natural convection in a rectangular enclosure, filled with a liquid metal (low Prandtl number partially heated from below with a sinusoidal temperature. The value of the study lies in its involvement in the crystal growth for the manufacture of semiconductors and electronics cooling. Indeed, the occurrence of convection during crystal growth can lead to in homogeneities that lead to striations and defects that affect the quality of the crystals obtained by the Bridgman techniques or Chochrawlski. Temperature of the oscillations, due to the instabilities of the convective flow in the liquid metal, also induces non-uniform cooling in the solidification front. Convection is then studied in order to reduce it. A modelling of the problem in two dimensions was conducted using Comsol computer code that is based on the finite element method, by varying the configuration of the control parameters, namely, the Rayleigh number, the nature of fluid (Prandtl number and amplitude of temperature on heat transfer rate (Nusselt number on convective structures that appear.
Energy Technology Data Exchange (ETDEWEB)
Moufekkir, F.; Moussaoui, M.A.; Mezrhab, A. [Laboratoire de Mecanique and Energetique, Faculte des sciences, Departement de physique 60000 Oujda (Morocco); Lemonnier, D. [Institut Pprime, CNRS-ENSMA-Univ. Poitiers, ENSMA, BP 40109, 86961 Futuroscope Chasseneuil cedex (France); Naji, H. [Universite Lille Nord de France, F-59000 Lille (France); Laboratoire Genie Civil and geo-Environnement - LGCgE- EA 4515, UArtois/FSA Bethune, F-62400 Bethune (France)
2012-04-15
A numerical analysis is carried out for natural convection while in an asymmetrically heated square cavity containing an absorbing emitting medium. The numerical approach adopted uses a hybrid thermal lattice Boltzmann method (HTLBM) in which the mass and momentum conservation equations are solved by using multiple relaxation time (MRT) model and the energy equation is solved separately by using the finite difference method (FDM). In addition, the radiative transfer equation (RTE) is treated by the discrete ordinates method (DOM) using the S8 quadrature to evaluate the source term of the energy equation. The effects of parameters such as the Rayleigh number Ra, the optical thickness {tau} and the inclination angle {phi}, are studied numerically to assess their impact on the flow and temperature distribution. The results presented in terms of isotherms, streamlines and averaged Nusselt number, show that in the absence of the radiation, the temperature and the flow fields are centro-symmetric and the cavity core is thermally stratified. However, radiation causes an overall increase in temperature and velocity gradients along both thermally active walls
Adiabatic partition effect on natural convection heat transfer inside a square cavity
DEFF Research Database (Denmark)
Mahmoudinezhad, S.; Rezania, A.; Yousefi, T.
2017-01-01
. The results are performed for the various Rayleigh numbers over the cavity side length, and partition angles ranging from 1.5 × 105 to 4.5 × 105, and 0° to 90°, respectively. The experimental verification of natural convective flow physics has been done by using FLUENT software. For a given adiabatic...
Asymptotic solution of natural convection problem in a square cavity heated from below
Grundmann, M; Mojtabi, A; vantHof, B
1996-01-01
Studies a two-dimensional natural convection in a porous, square cavity using a regular asymptotic development in powers of the Rayleigh number. Carries the approximation through to the 34th order. Analyses convergence of the resulting series for the Nusselt number in both monocellular and multicell
Numerical Simulation of Natural Convection in Heterogeneous Porous media for CO2 Geological Storage
Ranganathan, P.; Farajzadeh, R.; Bruining, J.; Zitha, P.L.J.
2012-01-01
We report a modeling and numerical simulation study of density-driven natural convection during geological CO2 storage in heterogeneous formations. We consider an aquifer or depleted oilfield overlain by gaseous CO2, where the water density increases due to CO2 dissolution. The heterogeneity of the
A computational fluid dynamics model for designing heat exchangers based on natural convection
Dirkse, M.H.; Loon, van W.K.P.; Walle, van der T.; Speetjens, S.L.; Bot, G.P.A.
2006-01-01
A computational fluid dynamics model was created for the design of a natural convection shell-and-tube heat exchanger with baffles. The flow regime proved to be turbulent and this was modelled using the k¿¿ turbulence model. The features of the complex geometry were simplified considerably resulting
Asymptotic solution of natural convection problem in a square cavity heated from below
Grundmann, M; Mojtabi, A; vantHof, B
1996-01-01
Studies a two-dimensional natural convection in a porous, square cavity using a regular asymptotic development in powers of the Rayleigh number. Carries the approximation through to the 34th order. Analyses convergence of the resulting series for the Nusselt number in both monocellular and multicell
Beukema, K.J.
1980-01-01
Three different models of bulk-stored agricultural products with air flow through the bulk, predicting the temperature profiles or the velocity of natural convection, are developed. The temperature distribution in a cylindrical container with insulated walls and open top and bottom, filled
Directory of Open Access Journals (Sweden)
A. Atia
2016-01-01
Full Text Available In this paper, we study the combination between the inclinations of the enclosure and the magnetic field orientation on the oscillatory natural convection. For this, a cylindrical enclosure filled with electrically conducting fluid, has an aspect ratio equal to 2, and subjected to a vertical temperature gradient and different uniform magnetic field orientations were considered. The finite volume method is used to discretize the equations of continuity, momentum and energy. Our computer program based on the SIMPLER Algorithm has a good agreement with available experimental and numerical results. The time-dependent flow and temperature field are presented in oscillatory state, for different cases: inclination of the cylinder, under the effect of magnetic field in different orientations (δ = 0°, 30°, 45° and 90° and the combination between them. The results are presented at various inclinations of the cylinder (φ = 0°, 30° and 45°, and the Hartmann numbers Ha ≤ 50. The stability diagrams of the dependence between the complicated situations with the value of the critical Grashof number Grcr and corresponding frequency Frcr, are established according to the numerical results of this investigation. The combination between the studied state has a significant effect on the stabilization of the convective flow, and shows that the best stabilization of oscillatory natural convection is obtained at the inclination of the cylinder φ = 30°, and the applied of radial magnetic field (δ = 0°.
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.
Spiral plume structures in turbulent natural convection between two vertical walls
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
By means of direct numerical simulation, coherent structures are investigated in turbulent natural convection between two vertical differentially heated walls. It is observed that large-scale spanwise vortices and spiral plume structures exist together in the flow. Spiral plume structures appear at the positions with relatively large helicity, large normal vorticity and high fluctuating temperature. In this note, the shape, the characteristics and formation of the spiral structures are studied and compared with those in Rayleigh-Bénard convection. The conditional sampling analysis indicates the main properties of the spiral structures.
Wong, Teresa; Solomatov, Viatcheslav S.
2016-05-01
We perform numerical simulations of lithospheric failure in the stagnant lid regime of temperature-dependent viscosity convection, using the yield stress approach. We find that the time of failure can vary significantly for the same values of the controlling parameters due to the chaotic nature of the convective system. The general trend of the dependence of the time of lithospheric failure on the yield stress can be explained by treating lithospheric failure as a type of Rayleigh-Taylor instability. This study suggests that it is important to address not only the question of whether plate tectonics can occur on a planet but also when it would occur if conditions are favorable.
Institute of Scientific and Technical Information of China (English)
CAI; Ruixian(蔡睿贤); ZHANG; Na(张娜)
2002-01-01
Some algebraically explicit analytical solutions are derived for the anisotropic Brinkman model an improved Darcy model describing the natural convection in porous media. Besides their important theoretical meaning (for example, to analyze the non-Darcy and anisotropic effects on the convection), such analytical solutions can be the benchmark solutions to promoting the develop ment of computational heat and mass transfer. For instance, we can use them to check the accuracy,convergence and effectiveness of various numerical computational methods and to improve numerical calculation skills such as differential schemes and grid generation ways.
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
Shateyi, Stanford
2017-07-01
The spectral relaxation method is employed to examine natural convective heat and mass transfer, MHD flow over a permeable moving vertical plate with convective boundary condition in the presence of viscous dissipation, thermal radiation and chemical reaction. The governing partial differential equations were transformed into a system of nonlinear ordinary differential equations by using a similarity approach. The pertinent results are then displayed in tabular form and graphically.
Institute of Scientific and Technical Information of China (English)
ABD ELMABOUD Y; MEKHEIMER Kh S; MOHAMED Mohamed S
2015-01-01
An analysis has been achieved to study the natural convection of a non-Newtonian fluid (namely a Carreau fluid) in a vertical channel with rhythmically contracting walls. The Navier-Stokes and the energy equations are reduced to a system of non- linear PDE by using the long wavelength approximation. The optimal homotopy analysis method (OHAM) is introduced to obtain the exact solutions for velocity and temperature fields. The convergence of the obtained OHAM solution is discussed explicitly. Numerical calculations are carried out for the pressure rise and the features of the flow and temperature characteristics are analyzed by plotting graphs and discussed in detail.
MR-6 type fuel elements cooling in natural convection conditions after the reactor shut down
Energy Technology Data Exchange (ETDEWEB)
Pytel, K.; Bykowski, W.; Moldysz, A. [Institute of Atomic Energy, Otwock Swierk (Poland)
2002-07-01
Natural cooling conditions of the nuclear fuel in the channel type reactor after its shut down are commonly determined with relatively high uncertainty. This is not only to he lack of adequate measurements of thermal parameters i.e. the residual power generation, the coolant flow and temperatures, but also due to indeterminate model of convection mechanism. The numerical simulation of natural convection in multitube fuel assembly in the fuel channel leads to various convection modes including evidently chaotic behaviour. To determine the real cooling conditions in the MARIA research reactor a series of experiments has been performed with fuel assembly equipped with a set of thermocouples. After some forced cooling period (the shortest was half an hour after the reactor shut down) the reactor was left with the only natural convection. Two completely different cooling modes have been observed. The MARIA core consists of series of individual fuel channel and so called bypasses, maintaining the hydraulic properties of the fuel channel, connected in parallel. Initially, the convection cells were established trough few so-called bypasses providing a very effective mode of cooling. In this mode the flow charts were identical to those existing in forced cooling mode. After certain period the system switched on the second cooling mode with natural circulation within the individual fuel cells. Higher temperatures and temperature fluctuations were characteristic for this mode approaching 30 deg in amplitude. In almost all the cases the system was switching few times between modes, but eventually remained in the second mode. The switching times were not regular and the process has a chaotic behaviour. (author)
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
Natural life styles library: energy
Energy Technology Data Exchange (ETDEWEB)
1974-01-01
Three new sections appear in this issue. Information from the readers on good products, books, catalogues, places to eat and sleep, good doctors, foods, farms, natural cosmetics, or nutritionists is provided. Another section is devoted to housing to serve as an information exchange. This section is open to comments from readers on house construction, about materials, or alternate energy sources. The third new section deals with wild food and natural food recipes. Articles in this issue deal with wood stoves; solar energy in use; wood dome planner for NLS or communal living; farming information; the thriftchanger, a device for heating and cooling; maintaining the old truck; and how to build a hotbed for all seasons. Information on Jerusalem artichokes, pond lilies, pumpkin pies, persimmons, rose hips, sunflowers, and seed gathering is included in the food section. Information on Alternate Energy Access and Consumers in a Food Economy are two additional articles. Two nature poems are included. Reviews on an organic primer, shelter and survival data, the mountain people, an herbal guide, super beauty and natural cosmetics, a review of six cookbooks, and a healthy family cookbook are presented. (MCW)
On Identical Problems of Natural Convection in Enclosures and APplications of the Identity Character
Institute of Scientific and Technical Information of China (English)
无
1993-01-01
In this paper,a concept of “identical problems”of natural convection in enclosures is presented.A pair of problems of natural conyection in enclosures is said to be identical if they look different in appearance but with appropriate selection of coordinates and appropriate definition of dimensionless variables for each problem;they will have identical geometric configurations and identical dimensionless governing equations and related boundary conditions,The identical characteristics of a pair of problems of natural convection in enclosures with an interanl isolated island are demonstrated via dimensionless mathematical formulation and flow visualization results.Numericasl computations are performed and the predicted streamlines agree with the flow visualization results.A number of other possible pairs of identical problems are also presented.Applications of the identical character are provided and assumptions under which the identical problems may exist are discussed.
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.
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)
Mahmood Husain Ali
2013-05-01
Full Text Available In this paper, numerical solution is presented for the steady state, two dimensional natural convection heat transfer from two parallel horizontal cylinders enclosed by circular cylinder. The inner cylinders are heated and maintained at constant surface temperature, while the outer cylinder is cooled at constant surface temperature. Boundary fitted coordinate system is used to solve governing equations. The vorticity-stream function and energy equations is solved using explicit finite deference method and stream function equation solved by successive iteration method. (20Deferent cases are studied cover rang of Rayleigh number from (1,000 to (25,000 based on the inner cylinder diameter. These cases study the effect of the varying inner cylinders position horizontally and vertically within outer cylinder on the heat transfer and buoyancy that causes the flow. Outputs are displayed in terms of streamline, isothermal contours and local and average Nusselt number. The results showed that the position of the inner cylinders highly affects the heat transfer and flow movements in the gap. At low Rayleigh numbers the average Nusselt number increases with increase of horizontal distance between inner cylinders but the state is reversed at high Rayleigh numbers, while the average Nusselt number is increases with inner cylinder moving down at all Rayleigh numbers. The optimal position of inner cylinders for maximum and minimum heat transfer is located at each Rayleigh number so can be employed in isolation process or cooling process.
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.
Energy Technology Data Exchange (ETDEWEB)
Tournier, C.; Genthon, P.; Rabinowicz, M. [UMR 5562 Observatoire Midi-Pyrenees, Toulouse (France)
1999-07-01
Very few results are available on free convection in fractured zones, although this is a major mechanism for heat and mass transfer in crystalline rocks. Murphy (1979) has shown, using analytical stability analysis, that the critical Rayleigh number for the onset of free convection in a fracture greatly exceeds the value of 4{pi}{sup 2}, which is the value for an infinite porous medium, and even for a subcritical Rayleigh number, convection may occur after a time delay. Murphy proposed that this delayed convection results from a blanketing effect of the fracture induced by the progressive development of a thermal skin inside the fracture walls. The present paper extends Murphy's results by means of numerical modelling. Our numerical method involves a 2-D computation of convection in the fracture plane, and a 3-D solution of the conduction problem inside the fracture wall. The coupling of the codes is achieved by imposing a common temperature at the mid-fracture plane, together with the conservation of energy at the fracture-wall interface. We use two kinds of initial perturbation, which are assumed to constitute end-members for natural or application cases. For an A-type initial condition the thermal field is disrupted in the fracture only, while for a B-type initial condition the perturbation is introduced in the fracture and in the walls. For a given perturbation wavenumber, three distinct domains can be defined according to the Rayleigh number (R). In the first domain, convection takes place immediately; in the second one, convection starts after a delay; and in the third one, convection is damped. These three domains are therefore termed the instantaneous convection (R > R{sub s}), delayed convection (R{sub d} < R < R{sub s}), and conduction (R < R{sub d}) domains, respectively. It is noteworthy that these three domains are bounded by the same values of the Rayleigh number for both A-type and B-type perturbations. Except for R close to R{sub d}, the time
Critical heat flux in natural convection cooled TRIGA reactors with hexagonal bundle
Energy Technology Data Exchange (ETDEWEB)
Yang, J.; Avery, M.; De Angelis, M.; Anderson, M.; Corradini, M. [Univ. of Wisconsin-Madison, 1500 Engineering Drive, Madison, WI 53706 (United States); Feldman, E. E.; Dunn, F. E.; Matos, J. E. [Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439 (United States)
2012-07-01
A three-rod bundle Critical Heat Flux (CHF) study at low flow, low pressure, and natural convection condition has been conducted, simulating TRIGA reactors with the hexagonally configured core. The test section is a custom-made trefoil shape tube with three identical fuel pin heater rods located symmetrically inside. The full scale fuel rod is electrically heated with a chopped-cosine axial power profile. CHF experiments were carried out with the following conditions: inlet water subcooling from 30 K to 95 K; pressure from 110 kPa to 230 kPa; mass flux up to 150 kg/m{sup 2}s. About 50 CHF data points were collected and compared with a few existing CHF correlations whose application ranges are close to the testing conditions. Some tests were performed with the forced convection to identify the potential difference between the CHF under the natural convection and forced convection. The relevance of the CHF to test parameters is investigated. (authors)
Energy Technology Data Exchange (ETDEWEB)
Wu, Y.; Welty, J.R. (Oregon State Univ., Corvallis, OR (United States). Dept. of Mechanical Engineering)
1994-08-01
A two-dimensional finite difference computer program in cylindrical coordinates has been developed to solve the case of laminar natural convection in a vertical tube open to large reservoir. Such a device, the open thermosyphon, is used in a number of applications, including the cooling of gas turbines, geothermal energy extraction, and thermosyphon solar water heaters. The objective of this work were to study the nature of fluid flow and the heat transfer rate along the tube wall. A semi-implicit, time marching, finite difference solution procedure was used, satisfying continuity, momentum, and energy equations for incompressible flow. Results show three well-defined flow regimes appearing as functions of the tube length-to-radius (aspect) ratio. Fluid motion in the tube and heat transfer rates became oscillatory at long time intervals. Plots of streamlines and isotherms at selected times for different aspect ratio tubes are also presented to show the transition behavior of fluid motion.
Institute of Scientific and Technical Information of China (English)
无
2010-01-01
Numerical investigation using SIMPLE algorithm with QUICK scheme for natural convection and heat transfer in the enclosure bounded by a solid wall and with heat transfer and radiation coupled in natural convection has been conducted.The various parameters are:Rayleigh number(from 103 to 105),dimensionless conductivity of bounding wall(from 0 to 100),dimensionless wall thickness(from 0 to 0.6) and radiation emissivity of all surfaces(from 0 to 1).The results suggest that flow and heat transfer are influenced by radiation.Radiation is a dominant action on flow and heat transfer.With increase of the thermal conductivity of wall,flow and heat transfer turn stronger.The temperature distribution changes obviously.When the thermal conductivity of wall is over a certain critical number,the increasing trend of flow and heat transfer may disappear.With increase of enclosure wall thickness,flow and heat transfer turn slighter.When the enclosure wall thickness is over a certain critical number,the flow and heat transfer will turn slow.
Institute of Scientific and Technical Information of China (English)
Xiaofan LI
2009-01-01
A lag correlation analysis is conducted with a 21-day TOGA COARE cloud-resolving model simulation data to identify the phase relation between surface rainfall and convective available potential energy (CAPE) and associated physical processes. The analysis shows that the maximum negative lag correlations between the model domain mean CAPE and rainfall occurs around lag hour 6. The minimum mean CAPE lags mean and convective rainfall through the vapor condensation and depositions, water vapor convergence, and heat divergence whereas it lags stratiform rainfall via the transport of hydrometeor concentration from convective regions to raining stratiform regions, vapor condensation and depositions, water vapor storage, and heat divergence over raining stratiform regions.
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.
Effect of natural convection heat transfer during polymer optical fiber drawing
Reeve, Hayden Matane
The quality of polymer optical fiber is dependent on the diametral uniformity of the fiber and the applied drawing force. In this study, the force required to draw a polymer preform into optical fiber is predicted and measured as it is heated in an enclosed cylindrical furnace. The draw force is a function of the highly temperature dependent polymer viscosity. Therefore accurate prediction of the drawing force requires a detailed investigation of the heat transfer within the furnace. In this investigation, the full axi-symmetric conjugate problem (including both natural convection and thermal radiation) was solved. In addition, the location of the polymer/air interface was solved for as part of the problem and was not prescribed beforehand. Numerical results compared well with the experimentally measured draw tension and neck-down profiles for several preform diameters, draw speeds, and furnace temperatures. The experimental investigation also found that as the buoyant potential of the air within the furnace was increased the natural convection transitioned from time-invariant to oscillatory, and finally, to chaotic flow. The time-varying heating caused by the oscillatory and chaotic regimes alters the rheology of the elongating polymer preform, causing detrimental variations in the fiber diameter. When subjected to oscillatory and chaotic natural convection the standard deviation of the fiber diameter variations was up to 2.5 to 10 times greater, respectively, than that measured under time-invariant heating conditions. Experimental visualization of the unsteady natural convection flow indicates that the instability occurs at the interface between two counter-rotating cells. Numerical simulations of natural convection within a tall non-isothermal axi-symmetric annular cavity with an aspect ratio of 10 and a radius ratio of 0.6 predicted unsteady phenomena. At low Rayleigh numbers a steady bi-cellular flow was predicted. As the Rayleigh number was increased the
Chen, Bingyan; Fowler, Alex; Bhowmick, Sankha
2006-06-01
Trehalose is believed to offer desiccation protection to mammalian cells by forming stable glassy matrices. The goal of the current study was to explore the desiccation kinetics of thin films of trehalose-water solution under forced and natural convective conditions and to investigate the thermophysical state of mammalian cells at the bottom of the thin film. We developed a finite difference model based on the mass and energy conservation equations coupled to the water transport model from the cells. The boundary conditions were obtained from correlations or experimental measurements and the Gordon-Taylor equation was used to predict the glass transition temperature at every location. Results indicated that there are three distinct regimes for drying for both forced and natural convection, characterized by the slope of the moisture content plot as a function of time. Our results also indicate that the surface of the solution reached the glassy state in less than 10 min for the Reynolds (forced) numbers explored and approximately 30 min for some Rayleigh (natural convective) numbers; however, significant water was trapped at this instant. Larger drying force hastened quicker glass formation but trapped more water. The numerical model was capable of predicting the drying kinetics for the dilute region accurately, but deviated while predicting the other regimes. Based on these experimental validations of the model, the osmotic response of different cells located at the bottom of the solution with orders of magnitude difference in their membrane permeability (Lp) was predicted. The results suggested that extracellular glass formed around cells at the bottom of a trehalose-water solution by the propagation of glass into the solution; however it takes more than an order of magnitude time (approximately 7 min to >100 min for forced convective drying) to remove sufficient water to form glass around cells from the time when the first surface glass is formed. This is
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)
Indian Academy of Sciences (India)
Ke Wu; Le Wang; Yi-Bo Yu; Zhi-Yi Huang; Pei Liang
2013-12-01
Heat dissipation enhancement of LED luminaries is of great significance to the large-scale application of LED. Luminaries-level structure improvement by the method of boring through-hole is adopted to intensify heat dissipation. Furthermore, the natural convection heat transfer process of LED luminaries is simulated by computational fluid dynamics (CFD) model before and after the structural modification. As shown by computational results, boring through-hole is beneficial to develop bottomto-top natural convection, eliminate local circumfluence, and finally form better flow pattern. Analysis based on field synergy principle shows that boring through-hole across LED luminaries improves the synergy between flow field and temperature field, and effectively decreases the thermal resistance of luminaries-level heat dissipation structure. Under the same computational conditions, by luminaries-level structure improvement the highest temperature of heat sink is decreased by about 8° C and the average heat transfer coefficient is increased by 45.8%.
Algebraically explicit analytical solutions of two-buoyancy natural convection in porous media
Institute of Scientific and Technical Information of China (English)
CAI Ruixian; ZHANG Na; LIU Weiwei
2003-01-01
Analytical solutions of governing equations of various physical phenomena have their own irreplaceable theoretical meaning. In addition, they can also be the benchmark solutions to verify the outcomes and codes of numerical solution, and to develop various numerical methods such as their differencing schemes and grid generation skills as well. In order to promote the development of the discipline of natural convection, three simple algebraically explicit analytical solution sets are derived for a non-linear simultaneous partial differential equation set with five dependent unknown variables, which represents the natural convection in porous media with both temperature and concentration gradients. An extraordinary method separating variables with addition is applied in this paper to deduce solutions.
Wang, Wei-Hsiang; Fu, Wu-Shung; Tsubokura, Makoto
2016-11-01
Unstable phenomena of low speed compressible natural convection are investigated numerically. Geometry contains parallel square plates or single heated plate with open boundaries is taken into consideration. Numerical methods of the Roe scheme, preconditioning and dual time stepping matching the DP-LUR method are used for low speed compressible flow. The absorbing boundary condition and modified LODI method is adopted to solve open boundary problems. High performance parallel computation is achieved by multi-GPU implementation with CUDA platform. The effects of natural convection by isothermal plates facing upwards in air is then carried out by the methods mentioned above Unstable behaviors appeared upon certain Rayleigh number with characteristic length respect to the width of plates or height between plates.
Potential of enhancing a natural convection loop with a thermomagnetically pumped ferrofluid
Aursand, Eskil; Lervåg, Karl Yngve; Lund, Halvor
2016-01-01
The feasibility of using a thermomagnetically pumped ferrofluid to enhance the performance of a natural convection cooling loop is investigated. First, a simplified analytical estimate for the thermomagnetic pumping action is derived, and then design rules for optimal solenoid and ferrofluid are presented. The design rules are used to set up a medium-scale (1 m, 10-1000 W) case study, which is modeled using a previously published and validated model (Aursand et al. [1]). The results show that the thermomagnetic driving force is significant compared to the natural convection driving force, and may in some cases greatly surpass it. The results also indicate that cooling performance can be increased by factors up to 4 and 2 in the single-phase and two- phase regimes, respectively, even when taking into the account the added heat from the solenoid. The performance increases can alternatively be used to obtain a reduction in heat-sink size by up to 75 %.
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.
Novak, Milos H.; Nowak, Edwin S.
1993-12-01
To analyze the laminar natural convection heat transfer and fluid flow distribution in vertical rectangular cavities with or without inner partitions, the personal computer finite difference program entitled CAV is used. The CAV program was tested successfully for slender cavities with aspect ratios as high as R = H/ L = 90 and for the Grashof numbers, based on the cavity height, up to GrH = 3 x10 9. To make the CAV program useful for a number of applications, various types of boundary conditions can also be imposed on the program calculations. Presented are program applications dealing with the 2-D numerical analysis of natural convection heat transfer in very slender window cavities with and without small inner partitions and recommendations are made for window design.
Design and Fabrication of a Direct Natural Convection Solar Dryer for Tapioca
Diemuodeke E. OGHENERUONA; Momoh O.L. YUSUF
2011-01-01
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 consider...
On the onset of turbulence in natural convection on inclined plates
Rodríguez Sevillano, Angel; Pérez Grande, María Isabel; Meseguer Ruiz, José
2011-01-01
The problem of determination of the turbulence onset in natural convection on heated inclined plates in an air environment has been experimentally revisited. The transition has been detected by using hot wire velocity measurements. The onset of turbulence has been considered to take place where velocity fluctuations (measured through turbulence intensity) start to grow. Experiments have shown that the onset depends not only on the Grashof number defined in terms of the temperature difference ...
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.
Magnetic energy cascade in spherical geometry: I. The stellar convective dynamo case
Strugarek, A; Mathis, S; Sarazin, Y
2013-01-01
We present a method to characterize the spectral transfers of magnetic energy between scales in simulations of stellar convective dynamos. The full triadic transfer functions are computed thanks to analytical coupling relations of spherical harmonics based on the Clebsch-Gordan coefficients. The method is applied to mean field $\\alpha\\Omega$ dynamo models as benchmark tests. From the physical standpoint, the decomposition of the dynamo field into primary and secondary dynamo families proves very instructive in the $\\alpha\\Omega$ case. The same method is then applied to a fully turbulent dynamo in a solar convection zone, modeled with the 3D MHD ASH code. The initial growth of the magnetic energy spectrum is shown to be non-local. It mainly reproduces the kinetic energy spectrum of convection at intermediate scales. During the saturation phase, two kinds of direct magnetic energy cascades are observed in regions encompassing the smallest scales involved in the simulation. The first cascade is obtained through ...
Shen, Chunyun; Zhang, Yuwen; Li, Zheng
2016-01-01
Natural convection in a cylinder with an internally slotted annulus was solved by SIMPLE algorithm, and the effects of different slotted structures on nonlinear characteristics of natural convection were investigated. The results show that the equivalent thermal conductivity Keq increases with Rayleigh number, and reaches the maximum in the vertical orientation. Nonlinear results were obtained by simulating the fluid flow at different conditions. With increasing Rayleigh number, heat transfer is intensified and the state of heat transfer changes from the steady to unsteady. We investigated different slotted structures effects on natural convection, and analyze the corresponding nonlinear characteristics.
Institute of Scientific and Technical Information of China (English)
无
1996-01-01
A numerical study of natural convection beat transfer in an inclined cylindrical annulus has been conducted.The inner cylinder of the annulus is maintained at uniform heat flux and the outer cylinder at constant temperature.the two end walls are assumed to be insulated.A numerical code has been developed to calculate the steady state three-dimensional natural convection in an inclined cylindrical annulus,and the research emphasis is placed on the influences of inclination angle α and modified Rayleigh number Ra on the natural convection heat transfer in the annulus.Computations were carried out in the ranges of 0°≤α≤90*,2.5×105≤Ra*≤7and Pr=0.7 with fixed aspect ratio of H=28.97 and radius ratio of K=3.33,The numerical results are compared with the experimental correlations from the literature and the inclination angle effect on heat transfer is found to be insignificant.Detailed results of heat transfer rate,temperature,and velocity fields are presented for the case of α=45° and discussion is also made concerning the comparison between the numerical and experimental results for the specific case of α=90°。
Effects of natural convection on thermal explosion in a closed vessel.
Liu, Ting-Yueh; Campbell, Alasdair N; Cardoso, Silvana S S; Hayhurst, Allan N
2008-09-28
A new way of ascertaining whether or not a reacting mixture will explode uses just three timescales: that for chemical reaction to heat up the fluid containing the reactants and products, the timescale for heat conduction out of the reactor, and the timescale for natural convection in the fluid. This approach is developed for an nth order chemical reaction, A --> B occurring exothermically in a spherical, batch reactor without significant consumption of A. The three timescales are expressed in terms of the physical and chemical parameters of the system. Numerical simulations are performed for laminar natural convection occurring; also, a theoretical relation is developed for turbulent flow. These theoretical and numerical results agree well with previous experimental measurements for the decomposition of azomethane in the gas phase. The new theory developed here is compared with Frank-Kamenetskii's classical criterion for explosion. This new treatment has the advantage of separating the two effects inhibiting explosion, viz. heat removal by thermal conduction and by natural convection. Also, the approach is easily generalised to more complex reactions and flow systems.
Energy Technology Data Exchange (ETDEWEB)
Aksenova, A.E.; Chudanov, V.V.; Strizhov, V.F.; Vabishchevich, P.N. [Institute of Nuclear Safety Russian Academy Science, Moscow (Russian Federation)
1995-09-01
Unsteady natural convection of a heat-generating fluid with phase transitions in the enclosures of a square section with isothermal rigid walls is investigated numerically for a wide range of dimensionless parameters. The quasisteady state solutions of conjugate heat and mass transfer problem are compared with available experimental results. Correlation relations for heat flux distributions at the domain boundaries depending on Rayleigh and Ostrogradskii numbers are obtained. It is shown that generally heat transfer is governed both by natural circulation and crust formation phenomena. Results of this paper may be used for analysis of experiments with prototypic core materials.
Solar drying of whole mint plant under natural and forced convection
Directory of Open Access Journals (Sweden)
Y.I. Sallam
2015-03-01
Full Text Available 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 m2 s−1.
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.
Energy Technology Data Exchange (ETDEWEB)
Freitag, M., E-mail: freitag@becker-technologies.com; Schmidt, E.; Gupta, S.; Poss, G.
2016-04-01
Highlights: . • We studied the generation and dissolution of steam stratification in natural convection. • We performed a computer code benchmark including blind and open phases. • The dissolution of stratification predicted only qualitatively by LP and CFD models during the blind simulation phase. - Abstract: Locally enriched hydrogen as in stratification may contribute to early containment failure in the course of severe nuclear reactor accidents. During accident sequences steam might accumulate as well to stratifications which can directly influence the distribution and ignitability of hydrogen mixtures in containments. An international code benchmark including Computational Fluid Dynamics (CFD) and Lumped Parameter (LP) codes was conducted in the frame of the German THAI program. Basis for the benchmark was experiment TH24.3 which investigates the dissolution of a steam layer subject to natural convection in the steam-air atmosphere of the THAI vessel. The test provides validation data for the development of CFD and LP models to simulate the atmosphere in the containment of a nuclear reactor installation. In test TH24.3 saturated steam is injected into the upper third of the vessel forming a stratification layer which is then mixed by a superposed thermal convection. In this paper the simulation benchmark will be evaluated in addition to the general discussion about the experimental transient of test TH24.3. Concerning the steam stratification build-up and dilution of the stratification, the numerical programs showed very different results during the blind evaluation phase, but improved noticeable during open simulation phase.
Chen, Yan-Jun; Wang, Ping-Yang; Liu, Zhen-Hua
2016-11-01
The natural convective heat transfer and flow characteristics of nanofluids in an enclosure are numerically simulated using the multiphase-flow model and single phase model respectively. The simulated results are compared with the experimental results from the published papers to investigate the applicability of these models for nanofluids from a macro standpoint. The effects of Rayleigh number, Grashof number and volume concentration of nanoparticles on the heat transfer and flow characteristics are investigated and discussed. Comparisons of the horizontal and vertical central dimensionless velocity profiles between nanofluid and water for various Grashof numbers are studied. In addition, both streamline contours and isotherms lines for different volume concentrations of nanofluids are analyzed as well. The study results show that a great deviation exists between the simulated result of the single phase model and the experimental data on the relation of Nusselt number and Rayleigh number, which indicates that the single phase model cannot reflect the heat transfer characteristic of nanofluid. While the simulated results using the multiphase-flow model show a good agreement with the experimental data of nanofluid, which means that the multiphase-flow model is more suitable for the numerical study of nanofluid. For the natural convection, the present study holds the point that using Grashof numbers as the benchmark would be more appropriate to describe the heat transfer characteristics of nanofluid. Moreover, the simulated results demonstrate that adding nanoparticles into the base fluid can enhance both the motion of fluid and convection in the enclosure significantly.
Directory of Open Access Journals (Sweden)
Saad Najeeb Shehab
2016-09-01
Full Text Available In this work an experimental simulation is made to predict the performance of steady-state natural heat convection along heated finned vertical base plate to ambient air with different inclination angles and configurations of fin array. Two types of fin arrays namely vertical fins array and V-fins array on heated vertical base plate are used with different heights and spaces. The influence of inclination angle of the plate , configuration of fins array and fin geometrical parameters such as fin height and fin spacing on the temperature distribution, base convection heat transfer coefficient and average Nusselt number have been plotted and discussed. The experimental data are correlated to a formula between average Nusselt number versus Rayleigh number for vertical plate and vertical fins array. The results indicate that the configuration of V-fins array gave best natural-convection heat transfer performance as base heat transfer coefficient about 20% greater compared with vertical fins array. Experimental simulation data and correlations of the present work are compared with a previous works shows good agreement
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 convection in a room with two opposite heated vertical walls
Directory of Open Access Journals (Sweden)
Ameer Saad, Abdul Jabbar N. Khalifa
2015-01-01
Full Text Available In this study, investigation of radiation and natural convection in cubic enclosure has been carried out. A model of an enclosure representing a room was constructed from polystyrene boards. Two vertical walls are supplied with constant heat flux in the range of 9.4-47.8 W/m2. Temperatures of walls, ceiling, floor and air inside enclosure were measured using a 26 K-type thermocouples under steady state condition. Heat transfer was investigated for Rayleigh numbers in the range 4.4e7 - 1.2e8 with Prandtl number of 0.71. Detailed results including temperature profiles and correlation equations for convection heat transfer coefficient in terms of temperature difference between the heated surface temperature and the temperature of the air have been obtained for the walls of the enclosure.
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...... and several optimised designs are presented for Grashof numbers between 103 and 106. Interestingly, it is observed that the number of branches in the optimised design increases with increasing Grashof numbers, which is opposite to two-dimensional topology optimised designs. Furthermore, the obtained...... 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....
Large scale three-dimensional topology optimisation of heat sinks cooled by natural convection
Alexandersen, Joe; Aage, Niels
2015-01-01
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 the Bousinessq approximation. The fully coupled non-linear multiphysics system is solved using stabilised trilinear equal-order finite elements in a parallel framework allowing for the optimisation of large scale problems with order of 40-330 million state degrees of freedom. The flow is assumed to be laminar and several optimised designs are presented for Grashof numbers between $10^3$ and $10^6$. Interestingly, it is observed that the number of branches in the optimised design increases with increasing Grashof numbers, which is opposite to two-dimensional optimised designs.
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...
Model for natural convective flow of visco-elastic nanofluid past an isothermal vertical plate
Mustafa, M.; Mushtaq, Ammar
2015-09-01
The present article addresses the classical problem of the natural convection flow past a vertical plate by considering visco-elastic nanofluid. The mathematical model is constructed by following the constitutive equations of the upper-convected Maxwell (UCM) fluid. The novel aspects of Brownian motion and thermophoresis are taken into account. The recently proposed condition of passively controlled wall nanoparticle volume fraction is used. The shooting approach combined with the fourth-fifth-order Runge-Kutta integration procedure is utilized for computing the numerical solutions. The results are in agreement with the available studies in limiting sense. Our results indicate that the velocity profile is parabolic and it decreases with an increment in the visco-elastic parameter.
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...
MHD natural convection in an inclined square porous cavity with a heat conducting solid block
Sivaraj, C.; Sheremet, M. A.
2017-03-01
This paper deals with natural convection in an inclined porous cavity with a heat conducting solid body placed at its center under the influence of the applied magnetic field of different orientations. The left and right vertical walls of the cavity are maintained at different temperatures Th and Tc, respectively, while the horizontal walls are adiabatic. The governing coupled partial differential equations were solved using a finite volume method on a uniformly staggered grid system. The effects of the inclination angles of the magnetic field and cavity and the Hartmann number on the flow and thermal fields are investigated in detail. Numerical results are presented in terms of isotherms, streamlines and average Nusselt numbers. In general, the results indicate that the inclusion of the magnetic field reduces the convective heat transfer rate in the cavity. It is also found that an increase in the angle of the applied magnetic field produces a non-linear variation in the average Nusselt numbers.
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.
An evaluation of gas transfer velocity parameterizations during natural convection using DNS
Fredriksson, Sam T.; Arneborg, Lars; Nilsson, Hâkan; Zhang, Qi; Handler, Robert A.
2016-02-01
Direct numerical simulations (DNS) of free surface flows driven by natural convection are used to evaluate different methods of estimating air-water gas exchange at no-wind conditions. These methods estimate the transfer velocity as a function of either the horizontal flow divergence at the surface, the turbulent kinetic energy dissipation beneath the surface, the heat flux through the surface, or the wind speed above the surface. The gas transfer is modeled via a passive scalar. The Schmidt number dependence is studied for Schmidt numbers of 7, 150 and 600. The methods using divergence, dissipation and heat flux estimate the transfer velocity well for a range of varying surface heat flux values, and domain depths. The two evaluated empirical methods using wind (in the limit of no wind) give reasonable estimates of the transfer velocity, depending however on the surface heat flux and surfactant saturation. The transfer velocity is shown to be well represented by the expression, ks=A |Bν|1/4 Sc-n, where A is a constant, B is the buoyancy flux, ν is the kinematic viscosity, Sc is the Schmidt number, and the exponent n depends on the water surface characteristics. The results suggest that A=0.39 and n≈1/2 and n≈2/3 for slip and no-slip boundary conditions at the surface, respectively. It is further shown that slip and no-slip boundary conditions predict the heat transfer velocity corresponding to the limits of clean and highly surfactant contaminated surfaces, respectively. This article was corrected on 22 MAR 2016. See the end of the full text for details.
Magnetic energy dissipation and mean magnetic field generation in planar convection-driven dynamos.
Tilgner, A
2014-07-01
A numerical study of dynamos in rotating convecting plane layers is presented which focuses on magnetic energies and dissipation rates and the generation of mean fields (where the mean is taken over horizontal planes). The scaling of the magnetic energy with the flux Rayleigh number is different from the scaling proposed in spherical shells, whereas the same dependence of the magnetic dissipation length on the magnetic Reynolds number is found for the two geometries. Dynamos both with and without mean field exist in rapidly rotating convecting plane layers.
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.
Directory of Open Access Journals (Sweden)
Mohd Rozainee Taib
2011-01-01
Full Text Available Dried fish is commonly produced by convective hot air drying. Microwave technology was presented in this paper to improve both process and product quality. Catfish (order Siluriformes slices were dehydrated in a microwave assisted hot air convective dehydration (MWHA system to investigate the effects of microwave power and hot air temperature on drying time, dehydration behaviour, energy consumption and colour of dried fish. Three different microwave power outputs namely medium (373 W, medium low (217 W and low (91 W combined with convective hot air temperature of 40°C, 70°C and 130°C accordingly were employed in the drying experiments. Results show that microwave accelerates drying time up to 120 folds faster compared to drying with hot air convective alone. It was also noted that increasing the hot air temperature was not as significant as increasing the microwave power in reducing the drying time. Experiments show that drying time was reduced about 75 % when an increase of microwave power from low (91 W to medium mode (373 W combining with convective hot air. However, drying with microwave alone required longer drying time. Energy consumption analysis shows that microwave assisted drying process requires less energy usage. Drying fish with microwave assisted hot air dehydration treatment make the colour brighter, shifting towards red and yellow.
Natural energy and vernacular architecture
Energy Technology Data Exchange (ETDEWEB)
Fathy, H.
1986-01-01
This volume presents insights into the indigenous architectural forms in hot arid climates. The author presents his extensive research on climate control, particularly in the Middle East, to demonstrate the advantages of many locally available building materials and traditional building methods. He suggests improved uses of natural energy that can bridge the gap between traditional achievements and modern needs. He argues that various architectural forms in these climates have evolved intuitively from scientifically valid concepts. Such forms combine comfort and beauty, social and physical functionality. He discusses that in substituting modern materials, architects sometimes have ignored the environmental context of traditional architecture. As a result, individuals may find themselves physically and psychologically uncomfortable in modern structures. His approach, informed by a sensitive humanism, demonstrates the ways in which traditional architectural forms can be of use in solving problems facing contemporary architecture, in particular the critical housing situation in the Third World.
THE EFFECT OF MAGNETIC FIELDS ON LOW FREQUENCY OSCILLATING NATURAL CONVECTION WITH PRESSURE GRADIENT
Institute of Scientific and Technical Information of China (English)
G.C. Sharma; Madhu Jain; Mahesh Chandra
2003-01-01
The oscillating natural convection in the presence of transverse magnetic field with time depending pressure gradient is studied. The analysis of the problem is carried out by assuming that the fluid is flowing in a parallel plate configuration. The emphasis is on low frequency oscillating convective flows induced by g-jitter associated with micro gravity because of their importance to the space processing materials. A general solution for an oscillating flow in the presence of transverse magnetic field is carried out. Some special cases of the oscillating flow and its response to an applied magnetic field are performed. It was observed that the behavior of oscillating free convective flows depends on frequency, amplitude of the driving buoyancy forces, temperature gradient, magnetic field and the electric conditions of the channel walls. In the absence of magnetic field, buoyancy force plays a predominant role in driving the oscillatory flow pattern, and velocity magnitude is also affected by temperature gradients. To suppress the oscillating flow external magnetic field can be used. It is also found that the reduction of the velocity is inversely proportional to the square of the applied magnetic field with conducting wall but directly proportional to the inverse of the magnetic field with insulating wall. Detailed calculations and computational results are also carried out to depict the real situation.
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
The effect of natural and forced melt convection on dendritic solidification in Ga-In alloys
Shevchenko, N.; Roshchupkina, O.; Sokolova, O.; Eckert, S.
2015-05-01
The directional solidification of Ga-25 wt%In alloys within a Hele-Shaw cell was visualized by means of X-ray radioscopy. The experimental investigations are especially focused on the impact of melt convection on the dendritic growth. Natural convection occurs during a bottom up solidification because lighter solute is rejected at the solid-liquid interface leading to an unstable density stratification. Forced convection was produced by a rotating wheel with two parallel disks containing at their inner sides a set of permanent NdFeB magnets with alternating polarization. The direction of forced melt flow is almost horizontal at the solidification front whereas local flow velocities in the range between 0.1 and 1.0 mm/s were achieved by controlling the rotation speed of the magnetic wheel. Melt flow induces various effects on the grain morphology primarily caused by the convective transport of solute. Our observations show a facilitation of the growth of primary trunks or lateral branches, suppression of side branching, dendrite remelting and fragmentation. The manifestation of all phenomena depends on the dendrite orientation, local direction and intensity of the flow. The forced flow eliminates the solutal plumes and damps the local fluctuations of solute concentration. It provokes a preferential growth of the secondary arms at the upstream side of the primary dendrite arms, whereas the high solute concentration at the downstream side of the dendrites can inhibit the formation of secondary branches completely. Moreover, the flow changes the inclination angle of the dendrites and the angle between primary trunks and secondary arms.
Energy Technology Data Exchange (ETDEWEB)
Brasil, Cristiana S.; Valle, Ramon M.; Cortez, Marcio F.B.; Ferreira, Andre G. [Minas Gerais Univ., Belo Horizonte, MG (Brazil). Dept. de Engenharia Mecanica]. E-mail: tite@demec.ufmg.br; ramon@demec.ufmg.br; fonteboa@demec.ufmg.br; ferreira@demec.ufmg.br
2000-07-01
This paper presents a theoretical analysis of the turbulent natural convection in a solar chimney operating in steady flow, with prescribed conditions of temperature in the ground. The solar radiation heats the air under the cover, which flows to the tower without artificial pumping. The hot air produced may be used to dry several agricultural products. The numerical analysis of the natural convection in this kind of dryer has fundamental importance on the design and building of this device. The mathematical model includes the conservation laws for mass, momentum and thermal energy and the transport equations for the turbulence model variables (k and e ). The k- e model of turbulence with wall functions was used. A computational code using the Finite Volume Method in Generalized Coordinates was developed to solve the system of equations that describes thermal and hydro dynamically the flow. The velocity and temperature fields are shown to the flow in the solar chimney. With geometrical alterations on the device, one can obtain a detailed description of the flow, which allow the guideline for a suitable configuration to build an experimental prototype. (author)
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.
Institute of Scientific and Technical Information of China (English)
LI Guang-zheng; HUANG Jian-chun
2005-01-01
Numerical simulations are performed for laminar natural convection heat transfer from a centered conducting body enclosed in a square cavity. A high accuracy unsteady numerical method is used, combining the unique condition of the pressure, the convergent solutions and the stream-function value of the inside heat-conducting body are given simultaneously. Two examples are simulated with this numerical method and compared with the experimental results. The results of the numerical solutions are consistent with the experimental results. It shows that the numerical method is valid and feasible.
Passive decay heat removal by natural air convection after severe accidents
Energy Technology Data Exchange (ETDEWEB)
Erbacher, F.J.; Neitzel, H.J. [Forschungszentrum Karlsruhe Institut fur Angewandte Thermo- und Fluiddynamik, Karlsruhe (Germany); Cheng, X. [Technische Universitaet Karlsruhe Institut fur Stroemungslehre und Stroemungsmaschinen, Karlsruhe (Germany)
1995-09-01
The composite containment proposed by the Research Center Karlsruhe and the Technical University Karlsruhe is to cope with severe accidents. It pursues the goal to restrict the consequences of core meltdown accidents to the reactor plant. One essential of this new containment concept is its potential to remove the decay heat by natural air convection and thermal radiation in a passive way. To investigate the coolability of such a passive cooling system and the physical phenomena involved, experimental investigations are carried out at the PASCO test facility. Additionally, numerical calculations are performed by using different codes. A satisfying agreement between experimental data and numerical results is obtained.
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.
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...
Natural convection heat transfer of water in a horizontal circular gap
Institute of Scientific and Technical Information of China (English)
SU Guanghui; Kenichiro Sugiyama; WU Yingwei
2007-01-01
An experimental study on the natural convection heat transfer on a horizontal downward facing heated surface in a water gap was carried out under atmospheric pressure conditions. A total of 700 experimental data points were correlated using Rayleigh versus Nusselt number in various forms, based on different independent variables. The effects of different characteristic lengths and film temperatures were discussed. The results show that the buoyancy force acts as a resistance force for natural convecti on beat transfer ona downward facing horizontal heated surface in a confined space. For the estimation of the natural convection heat transfer under the present conditions, empirical correlations in which Nusselt number is expressed as a function of the Rayleigh number, or both Rayleigh and Prandtl numbers, may be used. When it is accurately predicted, the Nusselt number is expressed as a function of the Rayleigh and Prandtl numbers, as well as the gap width-to-heated surface diameter ratio; and uses the temperature difference between the heated surface and the ambient fluid in the definition of Rayleigh number. The characteristic length is the gap size and the film temperature is the average fluid temperature.
Institute of Scientific and Technical Information of China (English)
Y.M.AL-BADAWI; H.M.DUWAIRI
2010-01-01
In this study,the magnetohydrodynamics(MHD)natural convection heat transfer with Joule and viscous heating effects inside an iso-flux porous medium-filled inclined rectangular enclosure is studied numerically.An iso-heat flux is applied for heating and cooling the two opposing walls of the enclosure while the other walls are adiabatic.The Forchheimer extension of Darcy-Oberbeck-Boussinesq and energy equations is transformed into a dimensionless form using a set of suitable variables instead of a finite difference scheme.The governing parameters axe the magnetic influence number,the modified Rayleigh number,the inclination angle,and the aspect ratio of the enclosure.The results show that viscous and Joule heating effects decrease heat transfer rates.
Directory of Open Access Journals (Sweden)
Jha B.K.
2015-02-01
Full Text Available This paper investigates the role of induced magnetic field on a transient natural convection flow of an electrically conducting, incompressible and viscous fluid in a vertical channel formed by two infinite vertical parallel plates. The transient flow formation inside the channel is due to sudden asymmetric heating of channel walls. The time dependent momentum, energy and magnetic induction equations are solved semi-analytically using the Laplace transform technique along with the Riemann-sum approximation method. The solutions obtained are validated by comparisons with the closed form solutions obtained for the steady states which have been derived separately and also by the implicit finite difference method. Graphical results for the temperature, velocity, induced magnetic field, current density, and skin-friction based on the semi-analytical solutions are presented and discussed.
Directory of Open Access Journals (Sweden)
DONG-GYU LEE
2013-12-01
The predicted temperature field has indicated that the peak temperature is located in the second basket from the top along the vertical center line by effects of the natural convection. As the Rayleigh number increases, the convective heat transfer is dominant and the heat transfer due to the local circulation becomes stronger. The heat transfer characteristics show that the Nusselt numbers corresponding to 1.5×106 1.0×107.
Effect of Selected Factors on Drying Process of Tomato in Forced Convection Solar Energy Dryer
U.S. Muhammed; A.M.I. El-Okene; Isiaka, M
2012-01-01
The effect of air velocity, slice thickness and grazing materials in drying process of tomato in forced convection solar energy dryer was evaluated. The result is to serve as an input for solar energy development for drying of vegetable and fruit products in North West Ecological zone of Nigeria. In order to evaluate the effects of the above factors in drying operation, a split-split-plot experimental design was used. Differences among the treatments and their interactions were tested with or...
Cao, Y.; Faghri, A.
1991-01-01
The performance of a thermal energy storage module is simulated numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid with low Prandtl numbers are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. The numerical results show that module geometry is crucial to the design of a space-based thermal energy storage system.
Energy Technology Data Exchange (ETDEWEB)
Harzallah, H.S.; Zegnani, A.; Slimi, K.; Mhimid, A. [Ecole Nationale D' Ingenieurs de Monastir, Monastir (Tunisia)
2009-07-01
Natural convection through anisotropic porous media is of significant interest in a wide variety of applications such as in geophysics, hydrology, oil extraction, and reservoir engineering. Anisotropy is basically a consequence of a preferential orientation and/or asymmetric geometry of the grains or fibres. Despite its broad range of applications, natural problems involving anisotropic effects in the presence of porous materials coupled with the effect of the bounding walls have received relatively little attention. This paper examined the effects of both thermal and mechanical anisotropies, as well as the effect of the conductive walls on the unsteady heat transfer and fluid flow through a porous material sandwiched between two vertical finite thickness walls maintained at constant but different temperatures. The remaining two horizontal walls were thermally insulated. The influencing non-dimensional parameters governing heat and fluid flow in the presented system were the wall thickness, wall-to-porous medium thermal conductivity ratio, wall-to-porous medium heat capacity ratio, anisotropic thermal conductivity ratio and anisotropic permeability ratio according to x-axis, and Rayleigh number. The purpose of the study was to demonstrate the effects of heat conduction within solid boundaries as well as both thermal and mechanical anisotropies on flow and heat transfer rates. It was concluded that as the anisotropic thermal conductivity ratio along the x-axis increases, the conductive mode becomes stronger than the convective one. 13 refs., 7 figs.
Energy Technology Data Exchange (ETDEWEB)
Castell, Albert; Sole, Cristian; Medrano, Marc; Roca, Joan; Cabeza, Luisa F. [Departament d' Informatica i Enginyeria Industrial, Universitat de Lleida, Pere de Cabrera s/n, 25001 Lleida (Spain); Garcia, Daniel [Departament Projectes d' Enginyeria, Universitat Politecnica de Catalunya, Colom 11, 08222 Terrassa (Spain)
2008-09-15
To determine the heat transfer coefficient by natural convection for specific geometries, experimental correlations are used. No correlations were found in the literature for the geometries studied in this work. These geometries consisted of a cylindrical module of 88 mm of diameter and 315 mm height with external vertical fins of 310 mm height and 20 and 40 mm length. To determine the heat transfer coefficient by natural convection, experimental work was done. This module, containing PCM (sodium acetate trihydrate), was situated in the middle upper part of a cylindrical water tank of 440 mm of diameter and 450 mm height. The calculated heat transfer coefficient changed by using external fins, as the heat transfer surface was increased. The temperature variation of the PCM and the water are presented as a function of time, and the heat transfer coefficient for different fins is presented as a function of the temperature difference. Experimental correlations were obtained, presenting the Nusselt number as a function of different dimensionless numbers. Different correlations were analysed to find which one fit better to the experimental data. (author)
Prediction of refrigerant absorption and onset of natural convection in lubricant oil
Energy Technology Data Exchange (ETDEWEB)
Barbosa, Jader R.; Marcelino Neto, Moises A. [Department of Mechanical Engineering, Federal University of Santa Catarina, Florianopolis, SC 88040900 (Brazil); Thoma, Stefan M. [Department of Mechanical and Process Engineering, Swiss Federal Institute of Technology, Sonneggstrasse 3, 8092 Zurich (Switzerland)
2008-11-15
Refrigerant absorption and mixing in lubricant oil are important in the design of refrigeration compressors and refrigeration systems. Experimental work is reported on absorption of R-134a vapour through the top interface of an initially stagnant layer of pure lubricant oil. Since the liquid refrigerant is heavier than the oil, mixing is enhanced due to natural mass convection. In the present paper, the behaviour of the liquid temperature during absorption is described based on measurements carried out in a test rig consisting of a transparent 70 mm ID, 150 mm long, vertical glass tube through which absorption can be directly observed. Transient liquid temperatures were measured at three different heights in the test section (two in the vapour, one in the liquid). The experimental work is complemented by a theoretical analysis of the critical time for the onset of mass transfer induced Rayleigh instability. The model is based on a critical mass transfer Rayleigh number criterion widely reported in the literature and takes into account the variation of physical properties in the liquid layer. The critical time for the onset of natural mass convection increases with decreasing system pressure as a result of a lower equilibrium concentration at the vapour-liquid interface. (author)
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.
Experimental and numerical investigation on natural convection heat transfer in nanofluids
Energy Technology Data Exchange (ETDEWEB)
Nayak, A.K.; Kulkarni, P.P.; Chinchole, A.S. [Bhabha Atomic Research Centre, Mumbai (India). Reactor Engineering Div.; Mulye, S.M. [Walchand College of Engineering, Sangli (India)
2016-03-15
An experimental study has been undertaken concerning natural convection heat transfer of nanofluids over a cylindrical heater with a constant wall heat flux condition. The cylindrical heater having 7.1 mm O. D. and 0.5 mm thickness with 100 mm heated length was used as heater surface. The heat flux was varied from 0 to 50 000 W/m{sup 2} and the corresponding Rayleigh number range is varied from 3 x 10{sup 4} to 1.65 x 10{sup 5}. Different Nanofluids were used i. e. Aluminium oxide, Titanium dioxide, Silicon dioxide and Copper oxide with concentration varying from 0.0005 % to 0.05 % by weight. Results show that there was a reduction in natural convection heat transfer coefficient of nanofluids as compared to water. Experimental results were compared with existing models for similar geometry. However, the available correlation was found to be unable to predict experimental data as it does not take into account the effect of particle concentration. A new empirical model was developed based on the experimental data including the effect of nanoparticles concentration which predicts the experimental data satisfactorily.
Influence of uniform magnetic field on laminar regimes of natural convection in an enclosure
Bondareva, N. S.; Sheremet, M. A.
2015-03-01
A numerical analysis of spatial laminar regimes of natural convection in an enclosure is conducted in the presence of a uniform magnetic field. The mathematical model formulated in dimensionless natural variables "velocity-pressure-temperature" has been implemented numerically by the method of control volume. The influence of the Rayleigh number (103 ≤ Ra ≤ 105) and the Hartmann number (0 ≤ Ha ≤ 100), the orientation of the magnetic induction vector (0 ≤ φ ≤ π/2) as well as of the geometric parameter (0.2 ≤ A ≤ 5), which reflects the enclosure relative length, on the velocity and temperature distributions as well as the average Nusselt number on a typical isothermal boundary has been studied in detail. A possibility of describing the integral heat exchange in the spatial object under consideration on the basis of the two-dimensional model has been established.
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.
Energy Technology Data Exchange (ETDEWEB)
Watanabe, O. (Advanced Reactor Div., Mitsubishi Atomic Power Industries, Inc., Tokyo (Japan)); Motomiya, Y. (Advanced Reactor Div., Mitsubishi Atomic Power Industries, Inc., Tokyo (Japan)); Takeda, H. (FBR Dept., Abiko Lab., Central Research Inst. of Electric Power Industry, Chiba (Japan)); Koga, T. (FBR Dept., Abiko Lab., Central Research Inst. of Electric Power Industry, Chiba (Japan))
1994-02-01
A two dimensional thermal-hydraulic analysis of a natural circulation experiment has been performed to evaluate the effectiveness of a higher order finite difference method for solving the Navier-Stokes and the energy equations. In the method, the convection terms appearing in each equation are solved by the Method of Characteristics using the third order Lagrange type polynomial as the interpolation function, and an iterative procedure is applied to solve the time derivative terms of each equation stably with second order accuracy. The analytical results have been compared with an experiment in which the temperature and the velocity distributions in the plenum region were measured with their fluctuations, and it was shown that the higher order finite difference method could simulate natural convection phenomena involving fluctuations well. (orig.)
Featherstone, Nicholas A
2016-01-01
We investigate how rotationally-constrained, deep convection might give rise to supergranulation, the largest distinct spatial scale of convection observed in the solar photosphere. While supergranulation is only weakly influenced by rotation, larger spatial scales of convection sample the deep convection zone and are presumably rotationally influenced. We present numerical results from a series of nonlinear, 3-D simulations of rotating convection and examine the velocity power distribution realized under a range of Rossby numbers. When rotation is present, the convective power distribution possesses a pronounced peak, at characteristic wavenumber $\\ell_\\mathrm{peak}$, whose value increases as the Rossby number is decreased. This distribution of power contrasts with that realized in non-rotating convection, where power increases monotonically from high to low wavenumbers. We find that spatial scales smaller than $\\ell_\\mathrm{peak}$ behave in analogy to non-rotating convection. Spatial scales larger than $\\el...
Lin, Wenxian; Armfield, S W; Patterson, J C; Lei, Chengwang
2009-06-01
In this paper, the scalings incorporating the Prandtl number (Pr) dependence have been obtained by a scaling analysis for the unsteady natural convection boundary layer of an initially quiescent isothermal Newtonian fluid of Pr>1 produced by the sudden imposition of a higher temperature on a vertical plate. It is shown that the transient flow behavior of the resulting boundary layer can be described by a three-region structure and at the start-up stage the boundary layer development is one dimensional and independent of height due to the dominance of pure conduction; however, at steady state it becomes two dimensional and height dependent as the flow becomes dominated by convection. Numerical results demonstrate that the scalings representing the thermal boundary layer development accurately represent their Pr dependence over the whole stage of flow development. The scalings representing the viscous boundary layer development are generally in good agreement with the numerical results with the Pr variation over the whole stage of flow development, although there are small deviations from the numerical results with the Pr variation that are within acceptable limits for scaling.
Convective-absolute nature of ripple instabilities on ice and icicles
Camporeale, Carlo; Vesipa, Riccardo; Ridolfi, Luca
2017-05-01
Film hydrodynamics is crucial in water-driven morphological pattern formation. A prominent example is given by icicle ripples and ice ripples, which are regular patterns developing on freezing-melting inclined surfaces bounding open-channel flows. By a suitable mathematical model based on conservation principles and the use of the cusp map method, in this paper we address the convective-absolute nature of these two kinds of instabilities. The obtained results show that icicle ripples, which develop at inverted (overhang) conditions, have subcentimetric wavelengths which are unstable when the Reynolds number of the liquid flow (Re ) is small and the supercooling is intensive. With the increase in Re , the instability switches from absolute to convective. Ice ripples instead exhibit the opposite dependance on Re and are highly affected by the surface slope. In addition, the evaluation of the so-called absolute wave number, which is responsible for the asymptotic impulse response, suggests a different interpretation of some recent experiments about ice ripples.
Sid, Samir; Terrapon, Vincent; Dubief, Yves
2015-11-01
Results of direct numerical simulation of turbulent channel flows under unstable stratification are reported. Two Reynolds number are considered: Reτ = 180 , 395 and the Rayleigh number ranges between Ra = [106 -109 ] . The Prandtl number is set to 1. The channel is periodic in both streamwise and spanwise directions and non-slip/isothermal boundary conditions are imposed at the walls. The temperature difference between the walls is set so that the stratification is unstable and the coupling between temperature and momentum is achieved using the Boussinesq approximation. The dependency of the typical large scale convective structures on both Reynolds and Rayleigh numbers are investigated through cross flow sectional statistics and instantaneous flow field visualizations. Moreover, the effects of the natural convection on the coherent structures associated to the cycle of wall-bounded turbulence (Jimenez, et al. JFM 1999), namely velocity streaks and streamwise vortices, are examined. Finally, macroscopic quantities such as friction coefficient and Nusselt number are reported as a function of the Rayleigh number and are compared for both Reynolds numbers. The Belgian Team acknowledges computational resources from CÉCI (F.R.S.-FNRS grant No.2.5020.11) and the PRACE infrastructure. YD acknowledges the support of NSF and DOE under grant NSF/DOE 1258697.
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)
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.
Diaguila, Anthony J; Freche, John C
1951-01-01
Blade-to-coolant heat-transfer data and operating data were obtained with a natural-convection water-cooled turbine over range of turbine speeds and inlet-gas temperatures. The convective coefficients were correlated by the general relation for natural-convection heat transfer. The turbine data were displaced from a theoretical equation for natural convection heat transfer in the turbulent region and from natural-convection data obtained with vertical cylinders and plates; possible disruption of natural convection circulation within the blade coolant passages was thus indicated. Comparison of non dimensional temperature-ratio parameters for the blade leading edge, midchord, and trailing edge indicated that the blade cooling effectiveness is greatest at the midchord and least at the trailing edge.
Natural Convection of Nanofluids in a Square Enclosure with a Protruding Heater
Directory of Open Access Journals (Sweden)
J. Guiet
2012-01-01
Full Text Available This paper reports a numerical study on natural convection from a protruding heater located at the bottom of a square cavity filled with a copper-water nanofluid. The vertical walls of the cavity are cooled isothermally; the horizontal ones are adiabatic, and the heater is attached to the bottom wall. The heat source is assumed either to be isothermal or to have a constant heat flux. The effective viscosity and thermal conductivity of the nanofluid are modeled according to Brinkman and Patel, respectively. Numerical solutions of the full-governing equations, based on the lattice Boltzmann method, are obtained for a wide range of the governing parameters: the Rayleigh number, Ra; the Prandtl number, Pr; the geometrical parameters specifying the heater; the volume fraction of nanoparticles, Φ. For a particular geometry, it has been found that, for a given Ra, heat transfer is enhanced with increasing Φ, independently of the thermal boundary condition applied on the heater.
Evaluation of immersion coil designs for natural convection-driven batch cooling or heating in tanks
Energy Technology Data Exchange (ETDEWEB)
Joye, Donald D.; Smith, Michael A.
2000-09-01
Various designs of immersion coils for batch cooling or heating of liquids in tanks were investigated to determine which design was most efficient. This equipment has direct utility in chemical batch processing operations and, particularly, wort cooling - a critical step in small-scale batch brewing of beer. This operation does not use a stirrer or agitation for a variety of reasons, hence it is dependent on natural-convection mechanisms for heat transfer, yet quick cooling is desired. Four basic coil designs with several permutations were evaluated. The results fell into four groups - best, good, mediocre, and worst performers. We found that coil placement and shape were design parameters that had a critical effect on the rate of cooling. Coil spacing also had a significant effect on the cooling rate when it was less than about 2 diameters. (Author)
Katamine, Eiji; Imai, Shinya; Mathmatical design Team; Computational mechanics Team
2016-11-01
This paper presents a numerical solution to shape identification of unsteady natural convection fields to control temperature to a prescribed distribution. The square error integral between the actual temperature distributions and the prescribed temperature distributions on the prescribed sub-boundaries during the specified period of time is used as the objective functional. Shape gradient of the shape identification problem is derived theoretically using the Lagrange multiplier method, adjoint variable method, and the formulae of the material derivative. Reshaping is carried out by the traction method proposed as an approach to solving shape optimization problems. Numerical analyses program for the shape identification is developed based on FreeFem++, and the validity of proposed method is confirmed by results of 2D numerical analyses.
Transition to turbulence in strongly heated vertical natural convection boundary layers
De Larochelambert, Thierry
2008-01-01
The mechanisms governing the transition to turbulence in natural convection boundary layers along strongly heated vertical walls remain neither very clear nor well understood, because of the lack of experiments and the difficulties of physical modelling. Our measurements bring experimental data focusing on this transition in quiescent air along radiating and conducting plates in the whole range of 2000 to 8000 W/m\\^2 heating rate. The analysis of the time series obtained by sliding window cross-correlation thermoanemometry leads us to point out coherent turbulent structures on short heights throughout the thin boundary layer, which seem to be governed by heat transfer and time-microscales of turbulence through the inner sublayer. Physical interpretations are given to relate to the observed heat transfer correlation and these turbulence transition structures along with radiation and conduction.
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.
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
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......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...... 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...
From cat's eyes to disjoint multicellular natural convection flow in tall tilted cavities
Nicolás, Alfredo; Báez, Elsa; Bermúdez, Blanca
2011-07-01
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°⩽ϕ⩽270°; about Ra=1.1×10. 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.
Flow patterns of natural convection in an air-filled vertical cavity
Wakitani, Shunichi
1998-08-01
Flow patterns of two-dimensional natural convection in a vertical air-filled tall cavity with differentially heated sidewalls are investigated. Numerical simulations based on a finite difference method are carried out for a wide range of Rayleigh numbers and aspect ratios from the onset of the steady multicellular flow, through the reverse transition to the unicellular pattern, to the unsteady multicellular flow. For aspect ratios (height/width) from 10 to 24, the various cellular structures characterized by the number of secondary cells are clarified from the simulations by means of gradually increasing Rayleigh number to 106. Unsteady multicellular solutions are found in some region of Rayleigh numbers less than those at which the reverse transition has occurred.
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.
BUOYANCY INSTABILITY IN THE NATURAL CONVECTION BOUNDARY LAYER AROUND A VERTICAL HEATED FLAT PLATE
Institute of Scientific and Technical Information of China (English)
颜大椿; 张汉勋
2002-01-01
A systematic research on the buoyancy instability in the natural convection boundary layer was conducted, including the basic characteristics such as its spectral components, wave length and velocity, the location of its critical layer,and amplitude distributions of the triple independent eigenmodes with the linear instability theory, the growth rates of its temperature and velocity fluctuations and the corresponding neutral curves for the buoyancy eigenmode were also obtained.Results indicated that the neutral curve of the velocity fluctuation had a nose shape consistent with that obtained in the numerical calculation, but for the temperature fluctuation, a ring-like region could be measured at a lower Grashof number before the nose-shaped main portion of the neutral curve.
Effects of various thermal boundary conditions on natural convection in porous cavities
Cheong, H. T.; Sivasankaran, S.; Bhuvaneswari, M.; Siri, Z.
2015-10-01
The present work analyzes numerically the effects of various thermal boundary conditions and the geometry of the cavity on natural convection in cavities with fluid-saturated porous medium. Cavity of square, right-angled trapezium and right-angled triangle shapes are considered. The different temperature profiles are imposed on the left wall of the cavity and the right wall is maintained at a lower constant temperature. The top and bottom walls are adiabatic. The Darcy model is adopted for the porous medium. The finite difference method is used to solve the governing equations and boundary conditions over a range of Darcy-Rayleigh numbers. Streamlines, isotherms and Nusselt numbers are used for presenting the results. The heat transfer of the square cavity is more enhanced at high Darcy-Rayleigh number for all the thermal boundary conditions considered.
Combined effect of natural convection and non-gray gas radiation with partial heating
Indian Academy of Sciences (India)
A MAZGAR; F BEN NEJMA
2016-07-01
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 difference pre-established in the literature, between choosing a partial heating from the top and the bottom of the wall. Computations are carried out to establish flow and temperature fields of the fluid in the enclosure. The effect of enclosure dimensions and boundary conditions are analyzed. Using the computed temperature fields, the mean Nusselt number is calculated. The results show that there is no major influence of two-dimensional radiation to reduce the difference between the reported top and bottom heating for the chosen gas.
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
Transient natural convection and heat transfer during the storage of granular media
Energy Technology Data Exchange (ETDEWEB)
Avila-Acevedo, J.G.; Tsotsas, E. [Institute of Process Engineering, Otto-von-Guericke-University, P.O. Box 4120, 39106 Magdeburg (Germany)
2008-07-01
Transient heat transfer in an originally isothermal cylinder filled with a porous medium after sudden change of wall temperature is studied experimentally and computationally. Lab-scale experiments with water as the interstitial fluid are used in order to imitate the conditions prevailing in large, air-filled industrial silos. The proposed model assumes isotropy of the porous medium, local thermal equilibrium between the phases, Darcy flow and applicability of the Boussinesq approximation. Its predictions are in satisfactory agreement with the experimental results. Simulations reveal the role of dimensionless parameters like the modified porous media Rayleigh number and the cylinder aspect ratio. A criterion for neglecting the influence of natural convection on heat transfer is established. (author)
Directory of Open Access Journals (Sweden)
K. Alawadhi
2014-12-01
Full Text Available Computational Fluid Dynamics (CFD analysis was carried out for the convergent-divergent fins arranged inline and staggered on the base plate as per the experimental setup provided in the technical paper [1]. This paper reports on the validation of results of modeling and simulation in CFD. The simulation was carried out using the ANSYS 12.0 as the CFD modeling software. The main objective of the CFD analysis was to calculate the temperature distribution on the surface of the base plate and surface of the convergent-divergent fins for the given inline and staggered arrangement of fins due to the effect of natural convection heat transfer for different heat power inputs, and also to compare the CFD results with the experimental results.
Institute of Scientific and Technical Information of China (English)
无
2006-01-01
The influence of direct current (DC) electric field on the thickness and mass of frost on a cold vertical plate was investigated. The photos of frost layer growth were taken with and without the presence of electric field, and results showed that the electric field has a strong influence on the frost thickness. The influences of cold plate temperature and ambient temperature on frost thickness and frost mass were also investigated under the natural convection condition with electric field. Experimental results demonstrated that the cold plate temperature has very strong effect on the frost layer thickness, but its influence on frost mass is minor; the influence of ambient temperature on the frost mass is more obvious than that on the frost thickness.
A multiscale modeling study for the natural convection mass transfer in a subsurface aquifer
Alam, Jahrul M
2013-01-01
Quantitative and realistic computer simulations of mass transfer associated with CO2 disposal in subsurface aquifers is a challenging endeavor. This article has proposed a novel and efficient multiscale modeling framework, and has examined its potential to study the pen- etrative mass transfer in a CO2 plume that migrates in an aquifer. Nu- merical simulations indicate that the migration of the injected CO2 enhances the vorticity generation, and the dissolution of CO2 has a strong effect on the natural convection mass transfer. The vorticity decays with the increase of the porosity. The time scale of the vertical migration of a CO2 plume is strongly dependent on the rate of CO2 dissolution. Comparisons confirm the near optimal performance of the proposed multiscale model. These primary results with an idealized computational model of the CO2 migration in an aquifer brings the potential of the proposed multiscale model to the field of heat and mass transfer in the geoscience.
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.
Natural convection heat transfer on two horizontal cylinders in liquid sodium
Energy Technology Data Exchange (ETDEWEB)
Hata, K.; Shiotsu, M.; Takeuchi, Y. [Institute of Atomic Energy, Kyoto Univ. (Japan)] [and others
1995-09-01
Natural convection heat transfer on two horizontal 7.6 mm diameter test cylinders assembled with the ratio of the distance between each cylinder axis to the cylinder diameter, S/D, of 2 in liquid sodium was studied experimentally and theoretically. The heat transfer coefficients on the cylinder surface due to the same heat inputs ranging from 1.0 X 10{sup 7} to 1.0 x 10{sup 9} W/m{sup 3} were obtained experimentally for various setting angeles, {gamma}, between vertical direction and the plane including both of these cylinder axis over the range of zero to 90{degrees}. Theoretical equations for laminar natural convection heat transfer from the two horizontal cylinders were numerically solved for the same conditions as the experimental ones considering the temperature dependence of thermophysical properties concerned. The average Nusselt numbers, Nu, values on the Nu versus modified Rayleigh number, R{sub f}, graph. The experimental values of Nu for the upper cylinder are about 20% lower than those for the lower cylinder at {gamma} = 0{degrees} for the range of R{sub f} tested here. The value of Nu for the upper cylinder becomes higher and approaches that for the lower cylinder with the increase in {gamma} over range of 0 to 90{degrees}. The values of Nu for the lower cylinder at each {gamma} are almost in agreement with those for a single cylinder. The theoretical values of Nu on two cylinders except those for R{sub f}<4 at {gamma} = 0{degrees} are in agreement with the experimental data at each {gamma} with the deviations less than 15%. Correlations for Nu on the upper and lower cylinders were obtained as functions of S/D and {gamma} based n the theoretical solutions for the S/D ranged over 1.5 to 4.0.
Natural Gas Fires Energy Future
Institute of Scientific and Technical Information of China (English)
JANKOT
2004-01-01
On July 5, 2004, Mrs. Caoof Shanghai's Baoshan district became Shanghai's one-millionth natural gas user. "This is great!" exclaims Mrs. Cao as she points to her newly installed gas grid. "It's got a big flame perfect for Chinese stirfrying. It's easier to use and not as polluting as coal."
Model of convection mass transfer in titanium alloy at low energy high current electron beam action
Sarychev, V. D.; Granovskii, A. Yu; Nevskii, S. A.; Konovalov, S. V.; Gromov, V. E.
2017-01-01
The convection mixing model is proposed for low-energy high-current electron beam treatment of titanium alloys, pre-processed by heterogeneous plasma flows generated via explosion of carbon tape and powder TiB2. The model is based on the assumption vortices in the molten layer are formed due to the treatment by concentrated energy flows. These vortices evolve as the result of thermocapillary convection, arising because of the temperature gradient. The calculation of temperature gradient and penetration depth required solution of the heat problem with taking into account the surface evaporation. However, instead of the direct heat source the boundary conditions in phase transitions were changed in the thermal conductivity equation, assuming the evaporated material takes part in the heat exchange. The data on the penetration depth and temperature distribution are used for the thermocapillary model. The thermocapillary model embraces Navier-Stocks and convection heat transfer equations, as well as the boundary conditions with the outflow of evaporated material included. The solution of these equations by finite elements methods pointed at formation of a multi-vortices structure when electron-beam treatment and its expansion over new zones of material. As the result, strengthening particles are found at the depth exceeding manifold their penetration depth in terms of the diffusion mechanism.
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)
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
2013-01-01
Considering interaction forces (gravity and buoyancy force, drag force, interaction potential force, and Brownian force) between nanoparticles and a base fluid, a two-phase Lattice Boltzmann model for natural convection of nanofluid is developed in this work. It is applied to investigate the natural convection in a square enclosure (the left wall is kept at a high constant temperature (TH), and the top wall is kept at a low constant temperature (TC)) filled with Al2O3/H2O nanofluid. This model is validated by comparing numerical results with published results, and a satisfactory agreement is shown between them. The effects of different nanoparticle fractions and Rayleigh numbers on natural convection heat transfer of nanofluid are investigated. It is found that the average Nusselt number of the enclosure increases with increasing nanoparticle volume fraction and increases more rapidly at a high Rayleigh number. Also, the effects of forces on nanoparticle volume fraction distribution in the square enclosure are studied in this paper. It is found that the driving force of the temperature difference has the biggest effect on nanoparticle volume fraction distribution. In addition, the effects of interaction forces on flow and heat transfer are investigated. It is found that Brownian force, interaction potential force, and gravity-buoyancy force have positive effects on the enhancement of natural convective heat transfer, while drag force has a negative effect. PMID:23374509
Institute of Scientific and Technical Information of China (English)
Chen Bao-ming; Zhang Li-qiang; Wang Bu-xuan
2003-01-01
The influences of Soret effect and Dufour effect on the natural convection and heat and mass transfer for a porous enclosure were investigated by means of the penalty finite element method. Numerical results indicate that the Soret and Dufour effects have significant influences on heat and mass transfer in the presence of large temperature gradient and concentration gradient.
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…
Featherstone, Nicholas A.; Hindman, Bradley W.
2016-10-01
We investigate how rotationally constrained, deep convection might give rise to supergranulation, the largest distinct spatial scale of convection observed in the solar photosphere. While supergranulation is only weakly influenced by rotation, larger spatial scales of convection sample the deep convection zone and are presumably rotationally influenced. We present numerical results from a series of nonlinear, 3D simulations of rotating convection and examine the velocity power distribution realized under a range of Rossby numbers. When rotation is present, the convective power distribution possesses a pronounced peak, at characteristic wavenumber {{\\ell }}{peak}, whose value increases as the Rossby number is decreased. This distribution of power contrasts with that realized in non-rotating convection, where power increases monotonically from high to low wavenumbers. We find that spatial scales smaller than {{\\ell }}{peak} behave in analogy to non-rotating convection. Spatial scales larger than {{\\ell }}{peak} are rotationally constrained and possess substantially reduced power relative to the non-rotating system. We argue that the supergranular scale emerges due to a suppression of power on spatial scales larger than {\\ell }≈ 100 owing to the presence of deep, rotationally constrained convection. Supergranulation thus represents the largest non-rotationally constrained mode of solar convection. We conclude that the characteristic spatial scale of supergranulation bounds that of the deep convective motions from above, making supergranulation an indirect measure of the deep-seated dynamics at work in the solar dynamo. Using the spatial scale of supergranulation in conjunction with our numerical results, we estimate an upper bound of 10 m s‑1 for the Sun’s bulk rms convective velocity.
Electromagnetic nature of dark energy
Jimenez, Jose Beltran
2009-01-01
Out of the four components of the electromagnetic field, Maxwell's theory only contains two physical degrees of freedom. However, in an expanding universe, consistently eliminating one of the "unphysical" states in the covariant (Gupta-Bleuler) formalism turns out to be difficult to realize. In this work we explore the possibility of quantization without subsidiary conditions. This implies that the theory would contain a third physical state. The presence of such a new (temporal) electromagnetic mode on cosmological scales is shown to generate an effective cosmological constant which can account for the accelerated expansion of the universe. This new polarization state is completely decoupled from charged matter, but can be excited gravitationally. In fact, primordial electromagnetic quantum fluctuations produced during electroweak scale inflation could naturally explain the presence of this mode and also the measured value of the cosmological constant. The theory is compatible with all the local gravity test...
Energy Technology Data Exchange (ETDEWEB)
Ohk, Seung Min; Chung, Bum Jin [Kyunghee University, Yongin (Korea, Republic of)
2016-05-15
The Passive Cooling System (PCS) driven by natural forces drew research attention since Fukushima nuclear power plant accident. This study investigated the natural convection heat transfer inside of vertical pipe with emphasis on the phenomena regarding the boundary layer interaction. Numerical calculations were carried out using FLUENT 6.3. Experiments were performed for the parts of the cases to explore the accuracy of calculation. Based on the analogy, heat transfer experiment is replaced by mass transfer experiment using sulfuric acid copper sulfate (CuSO{sub 4}. H{sub 2}SO{sub 4}) electroplating system. The natural convection heat transfer inside a vertical pipe is studied experimentally and numerically. Experiments were carried out using sulfuric acid-copper sulfate (H{sub 2}SO{sub 4}-CuSO{sub 4}) based on the analogy concept between heat and mass transfer system. Numerical analysis was carried out using FLUENT 6.3. It is concluded that the boundary layer interaction along the flow passage influences the heat transfer, which is affected by the length, diameter, and Prandtl number. For the large diameter and high Prandtl number cases, where the thermal boundary layers do not interfered along the pipe, the heat transfer agreed with vertical flat plate for laminar and turbulent natural convection correlation within 8%. When the flow becomes steady state, the forced convective flow appears in the bottom of the vertical pipe and natural convection flow appears near the exit. It is different behavior from the flow on the parallel vertical flat plates. Nevertheless, the heat transfer was not different greatly compared with those of vertical plate.
Institute of Scientific and Technical Information of China (English)
Pei－XueJiang; Ze－PeiRen; 等
1993-01-01
A numerical study is reported of laminar natural convective heat and mass transfer on a vertical cooled plate for water containing metal corrosion products at super-critical pressures.The influence of variable properties at super-critical pressures on natural convertion has been analyzed.The difference between heat and mass transfer under cooling or heating conditions is also discussed and some correlations for heat and mass transfer under cooling conditions are recommended.
Stability of Natural Convection of Power-law Fluid and non-Darcy Flow in Porous Media
Institute of Scientific and Technical Information of China (English)
Kong Xiangyan; Chen Guoquan; Wu Jianbing; Li Peichao; Lu Detang; Xu Xianzhi
2001-01-01
In the present work the effect of the power law exponent of power-law fluid and non-Darcy number of non-Darcy flow on stability of natural convection in porous media are studied. The computation analysis of effect of power law exponent of power-law fluid and non-Darcy number of non-Darcy flow in the rectangular duct on the transition Rayleigh number Ra*, which means the convective model transiting from stationary state to periodic solution. The duct has filled a porous medium saturated with the power-law non-Newtonian fluid or Newtonian fluid for non-Darcy flow, in which there is uniform internal heat generation per unit volume q. In this paper the relationship between the transition Rayleigh number Ra* and the power-law exponent n, Ra* and non-Darcy number Be, are shown .To these two aspects, the transition route from steady to chaotic convection is also obtained.
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
Osnos, V. B.; Kuneevsky, V. V.; Larionov, V. M.; Saifullin, E. R.; Gainetdinov, A. V.; Vankov, Yu V.; Larionova, I. V.
2017-01-01
The method of natural thermal convection with heat agent recirculation (NTC HAR) in oil reservoirs is described. The analysis of the effectiveness of this method for oil reservoir heating with the values of water saturation from 0 to 0.5 units is conducted. As the test element Ashalchinskoye oil field is taken. CMG STARS software was used for calculations. Dynamics of cumulative production, recovery factor and specific energy consumption per 1 m3 of crude oil produced in the application of the heat exchanger with heat agent in cases of different initial water saturation are defined and presented as graphs.
Pérez Grande, Isabel; Rodriguez Sevillano, Angel; Meseguer, Jos
In June, 8th, 2009 the balloon-borne solar telescope SUNRISE was launched from the Swedish Space Corporation balloon facility Esrange. A telescope with a mirror of 1 m in diameter ob-served the Sun during six days until the mission was terminated in Canada. The design process of SUNRISE and of any optical telescope requires the analysis of the effect of surrounding air on the quality of images. The turbulence encountered in the local telescope environment de-grades its optical performance. This phenomenon called `seeing' consists of optical aberrations produced by density non-homogeneities in the air along the optical path. The refraction index of air changes due to thermal non-uniformities so that the wavefront incident on the mirror is randomly distorted, and therefore, images are altered. When telescope mirrors are heated, as it happens in solar telescopes, and therefore they are at a temperature different from the environment's, natural convection occurs. It is then crucial to know whether the flow in front of the mirror is laminar or turbulent. After reviewing the literature, it was found that the scattering of results about the onset of the transition gives only rough orders of magnitude of the values of the critical Grashof numbers. Aiming to obtain more information about it, the problem of determination of the turbulence onset in natural convection on heated inclined plates in air environment was experimentally revisited. The transition has been determined from hot wire velocity measurements. The onset of turbulence has been considered to take place where velocity perturbations start to grow. Experiments have shown that the onset depends not only on the Grashof number, but also on other parameters as the temperature difference between the heated plate and the surrounding air. A correlation between dimensionless Grashof and Reynolds numbers has been obtained, fitting extraordinarily well the experimental data. The results are obtained in terms of non
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
Energy Technology Data Exchange (ETDEWEB)
Telles, Rubens S.
1990-12-31
This work discusses the problem of hydrodynamic dispersion on natural convection in porous medium near impermeable surfaces. The study considers the convection caused by density variation due to temperature and concentration gradients combination. The parametrization of the phenomenon is obtained through scale analysis. It is also presented four possible situations according to the intensity of dispersion term. In search of similarity solutions the similarity variables and parameters associated to dispersion are found through scale analysis. The Runge-Kutta algorithm and the `shooting` method are used to solve the equations resulting from the similarity transformations. Several cases are solved covering an extensive range of the governing parameters. 22 refs., 43 figs., 13 tabs.
Nanofluid heat transfer under mixed convection flow in a tube for solar thermal energy applications.
Sekhar, Y Raja; Sharma, K V; Kamal, Subhash
2016-05-01
The solar flat plate collector operating under different convective modes has low efficiency for energy conversion. The energy absorbed by the working fluid in the collector system and its heat transfer characteristics vary with solar insolation and mass flow rate. The performance of the system is improved by reducing the losses from the collector. Various passive methods have been devised to aid energy absorption by the working fluid. Also, working fluids are modified using nanoparticles to improve the thermal properties of the fluid. In the present work, simulation and experimental studies are undertaken for pipe flow at constant heat flux boundary condition in the mixed convection mode. The working fluid at low Reynolds number in the mixed laminar flow range is undertaken with water in thermosyphon mode for different inclination angles of the tube. Local and average coefficients are determined experimentally and compared with theoretical values for water-based Al2O3 nanofluids. The results show an enhancement in heat transfer in the experimental range with Rayleigh number at higher inclinations of the collector tube for water and nanofluids.
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.
Evanescent magnetic ﬁeld effects on entropy generation at the onset of natural convection
Indian Academy of Sciences (India)
Mourad Magherbi; Atef El Jery; Nejib Hidouri; Ammar Ben Brahim
2010-04-01
This paper numerically investigates the effect of an externally evanescent magnetic ﬁeld on total entropy generation in a ﬂuid enclosed in a square cavity by using a control volume ﬁnite element method to solve the conservation equations at Prandtl number of 0·71. The values of relaxation time of the magnetic ﬁeld are chosen, so that the Lorentz force acts only in the transient state of entropy generation in natural convection. The total entropy generation was calculated for, ﬁxed value of irreversibility distribution ratio, different relaxation time varying from 0 to 1/5 and Grashof number varying from 104 to 105. The effects of the Hartman number and the magnetic ﬁeld inclination angle on the evolution of total entropy generation throughout the transient regime were investigated. Results show that the application of evanescent magnetic ﬁeld not only suppresses the ﬂuctuation of the total entropy generation in the transient state, but also reduces the gap for magnetic ﬁeld relaxation time less than 1/10.
Energy Technology Data Exchange (ETDEWEB)
N.D. Francis, Jr; M.T. Itamura; S.W. Webb; D.L. James
2002-10-01
The objective of this heat transfer and fluid flow study is to assess the ability of a computational fluid dynamics (CFD) code to reproduce the experimental results, numerical simulation results, and heat transfer correlation equations developed in the literature for natural convection heat transfer within the annulus of horizontal concentric cylinders. In the literature, a variety of heat transfer expressions have been developed to compute average equivalent thermal conductivities. However, the expressions have been primarily developed for very small inner and outer cylinder radii and gap-widths. In this comparative study, interest is primarily focused on large gap widths (on the order of half meter or greater) and large radius ratios. From the steady-state CFD analysis it is found that the concentric cylinder models for the larger geometries compare favorably to the results of the Kuehn and Goldstein correlations in the Rayleigh number range of about 10{sup 5} to 10{sup 8} (a range that encompasses the laminar to turbulent transition). For Rayleigh numbers greater than 10{sup 8}, both numerical simulations and experimental data (from the literature) are consistent and result in slightly lower equivalent thermal conductivities than those obtained from the Kuehn and Goldstein correlations.
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.
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)
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.
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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.
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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.
Directory of Open Access Journals (Sweden)
M. Narendra Kumar
2016-01-01
Full Text Available Natural convection heat transfer in a two dimensional unsteady rotating differentially heated enclosure is studied numerically in this paper. The enclosure is filled with air and executes a steady counterclockwise rotation about the centre of the enclosure. A finite volume code on a staggered grid arrangement with TDMA algorithm is developed and employed to solve the governing equations subject to Boussinesq approximation. The numerical investigation is carried out for fixed Prandtl number equal to 0.71, Rayleigh number equal to1.1×〖10〗^5 while Taylors number vary from5.2×〖10〗^4 to 3.3×〖10〗^5and Rotational Rayleigh number from 4.9×〖10〗^2 to 3.1×〖10〗^3.Results reveal that there are considerable change in heat transfer rates beyond 15 rpm. The effect of rotation on the Nusselt number for a given Rayleigh number is shown in the present work which is not normally indicated and discussed in the available literature
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.
Natural convection heat transfer of nanofluids along a vertical plate embedded in porous medium.
Uddin, Ziya; Harmand, Souad
2013-02-07
The unsteady natural convection heat transfer of nanofluid along a vertical plate embedded in porous medium is investigated. The Darcy-Forchheimer model is used to formulate the problem. Thermal conductivity and viscosity models based on a wide range of experimental data of nanofluids and incorporating the velocity-slip effect of the nanoparticle with respect to the base fluid, i.e., Brownian diffusion is used. The effective thermal conductivity of nanofluid in porous media is calculated using copper powder as porous media. The nonlinear governing equations are solved using an unconditionally stable implicit finite difference scheme. In this study, six different types of nanofluids have been compared with respect to the heat transfer enhancement, and the effects of particle concentration, particle size, temperature of the plate, and porosity of the medium on the heat transfer enhancement and skin friction coefficient have been studied in detail. It is found that heat transfer rate increases with the increase in particle concentration up to an optimal level, but on the further increase in particle concentration, the heat transfer rate decreases. For a particular value of particle concentration, small-sized particles enhance the heat transfer rates. On the other hand, skin friction coefficients always increase with the increase in particle concentration and decrease in nanoparticle size.
Study of natural convection cooling of a nanofluid subjected to a magnetic field
Mahmoudi, Ahmed; Mejri, Imen; Omri, Ahmed
2016-06-01
This paper presents a numerical study of natural convection cooling of water-Al2O3 nanofluid by two heat sinks vertically attached to the horizontal walls of a cavity subjected to a magnetic field. The left wall is hot, the right wall is cold, while the horizontal walls are insulated. Lattice Boltzmann method (LBM) is applied to solve the coupled equations of flow and temperature fields. This study has been carried out for the pertinent parameters in the following ranges: Rayleigh number of the base fluid, Ra =103 to 105, Hartmann number varied from Ha = 0 to 60 and the solid volume fraction of nanoparticles between ϕ = 0 and 6%. In order to investigate the effect of heat sinks location, three different configurations of heat sinks are considered. The effects of Rayleigh numbers, Hartmann number and heat sinks location on the streamlines, isotherms, Nusselt number are investigated. Results show that the heat transfer rate decreases with the increase of Hartmann number and increases with the rise of Rayleigh number. In addition it is observed that the average Nusselt number increases linearly with the increase of the nanoparticles solid volume fraction. Also, results show that the heat sinks positions greatly influence the heat transfer rate depending on the Hartmann number, Rayleigh number and nanoparticle solid volume fraction.
Radiation effects on an unsteady MHD natural convective flow of a nanofluid past a vertical plate
Directory of Open Access Journals (Sweden)
Parasuraman Loganathan
2015-01-01
Full Text Available Numerical analysis is carried out on an unsteady MHD natural convective boundary layer flow of a nanofluid past an isothermal vertical plate in the presence of thermal radiation. The governing partial differential equations are solved numerically by an efficient, iterative, tri-diagonal, semi-implicit finite-difference method. In particular, we investigate the effects of radiation, magnetic field and nanoparticle volume fraction on the flow and heat transfer characteristics. The nanofluids containing nanoparticles of aluminium oxide, copper, titanium oxide and silver with nanoparticle volume fraction range less than or equal to 0.04 are considered. The numerical results indicate that in the presence of radiation and magnetic field, an increase in the nanoparticle volume fraction will decrease the velocity boundary layer thickness while increasing the thickness of the thermal boundary layer. Meanwhile, an increase in the magnetic field or nanoparticle volume fraction decreases the average skin-friction at the plate. Excellent validation of the present results has been achieved with the published results in the literature in the absence of the nanoparticle volume fraction.
Non—Darcian and Anisotropic Effects on Natural Convection in Horizontal Porous Media Enclosure
Institute of Scientific and Technical Information of China (English)
ZhangJingzhou; SunRenqia
1996-01-01
Natural convection heat transfer in a horizontal enclosure filled with anisotropic porous media,being isothermally heated at bettom and cooled at top while the vertical walls being adiabatic,is numerically studied by applying the Brinkman model-a modified form of Darcy model giving consideratioin to the viscous effect.The results show that:(1)a larger permeability ratio(K*) causes a lower flow intensity in the enclosure and a smaller Nusselt number,all Nusselt numbers approach unity in the limit of K*→∞;a larger thermal conductivity ratio(λ*) causes a stranger distortion of isotherms in the enclosure and a higher flow velocity near the walls,all the Nusselt numbers approach unity in the limit of λ*-→0,the permeability and thermal conductivity ratios generally have opposing effects on the Nusselt number.(2) an increasing Darcy number decreases the flow intensity and heat tansfer rates,which is more significant at a lower permeability ratio.In particular,with K*≤0.25,the Nusselt number for Da=10-3 would differ from that of Darcy flow up to an amount of 30%,an analysis neglecting the non-Darican effect will inevitably be of considerable error.
Laminar Natural Convection of Newtonian and Non Newtonian Fluids Inside Triangular Enclosure
Directory of Open Access Journals (Sweden)
Ala?a Abbas Muhadi
2007-01-01
Full Text Available In the present work, steady two dimensional laminar natural convection heat transfer of Newtonian and non-Newtonian fluids inside isosceles triangular enclosure has been analyzed numerically for a wide range of the modified Rayleigh numbers of (103 ≤ Ra ≤ 105, with non-dimensional parameter (NE of Prandtl Eyring model ranging from (0 to 10, and modified Prandtl number take in the range (Pr* =1,10, and 100. Two types of boundary conditions have been considered. The first, when the inclined walls are heated with different uniform temperatures and the lower wall is insulated. The second, when the bottom wall is heated by applying a uniform heat flux while the inclined walls at the constant cold temperature. Also, the non-Newtonian fluids under consideration were assumed to obey the Prandtl Eyring model..The results are presented in terms of isotherms and streamlines to show the behavior of the fluid flow and temperature fields. In addition, some graphics are presented the relation between average Nusselt number and the various parameters. The results show the effect of non dimensional parameter (NE on the velocity and temperature profiles. They also show that the average Nusselt number is a strong function of modified Rayleigh number, modified Prandtl number, non-dimensional parameter, and the boundary conditions. Four different correlations have been made to show the dependence of the average Nusselt number on the non-dimensional parameter, the modified Rayleigh and Prandtl numbers.
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M.A Mansour
2016-01-01
Full Text Available Numerical investigation for heat transfer with steady MHD natural convection cooling of a localized heat source at the bottom wall of an enclosure filled with nanofluids subjected to changeable thermal boundary conditions at the sidewalls has been studied in the a presence of inclined magnetic field. Finite difference method was employed to solve the dimensionless governing equations of the problem. The effects of governing parameters, namely, Hartmann number, solid volume fraction, the different values of the heat source length and the different locations of the heat source on the streamlines and isotherms contours as well as maximum temperature, Nusselt number and average Nusselt number along the heat source were considered. The present results are validated by favorable comparisons with previously published results. The results of the problem are presented in graphical and tabular forms and discussed. It is found that an increase in the Hartmann number results in a clear reduction in the rate of heat transfer; however, the increase in Rayleigh number enhances the nanofluid flow and heat transfer rate.
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Fateh Mebarek-Oudina
2016-01-01
Full Text Available A numerical study of oscillatory magnetohydrodynamic (MHD natural convection of liquid metal between vertical coaxial cylinders is carried out. The motivation of this study is to determine the value of the critical Rayleigh number, Racr for two orientations of the magnetic field and different values of the Hartmann number (Harand Haz and aspect ratios A. The inner and outer cylinders are maintained at uniform temperatures, while the horizontal top and bottom walls are thermally insulated. The governing equations are numerically solved using a finite volume method. Comparisons with previous results were performed and found to be in excellent agreement. The numerical results for various governing parameters of the problem are discussed in terms of streamlines, isotherms and Nusselt number in the annuli. The time evolution of velocity, temperature, streamlines and Nusselt number with Racr, Har, Haz, and A is quite interesting. We can control the flow stability and heat transfer rate in varying the aspect ratio, intensity and direction of the magnetic field.
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.
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Reza Khazaeli
2015-01-01
Full Text Available In this study, a version of thermal immersed boundary-Lattice Boltzmann method (TIB-LBM is used to simulate thermal flow problems within complex geometries. The present approach is a combination of the immersed boundary method (IBM and the thermal lattice Boltzmann method (TLBM under the double population approach. The method combines two different grid systems, an Eulerian grid for the flow domain and a Lagrangian grid for the boundary points immersed in the flow. In the present method, an unknown velocity correction is considered on the boundary points to impose the no-slip boundary condition. As a similar approach, an unknown internal energy correction on the boundary points is applied to satisfy the constant temperature boundary condition. The advantages of this approach are its second-order accuracy and straightforward calculation of the Nusselt number. The natural convection in an annulus with various outer cylinder shapes for different Rayleigh numbers have been simulated to demonstrate the capability and the accuracy of present approach. In terms of accuracy, the predicted results show an excellent agreement with those predicted by other experimental and numerical approaches.
Moufekkir, F.; Moussaoui, M. A.; Mezrhab, A.; Naji, H.; Lemonnier, D.
2012-09-01
This paper deals with the numerical solution for natural convection and volumetric radiation in an isotropic scattering medium within a heated square cavity using a hybrid thermal lattice Boltzmann method (HTLBM). The multiple relaxation time lattice Boltzmann method (MRT-LBM) has been coupled to the finite difference method (FDM) to solve momentum and energy equations, while the discrete ordinates method (DOM) has been adopted to solve the radiative transfer equation (RTE) using the S8 quadrature. Based on these approaches, the effects of various influencing parameters such as the Rayleigh number (Ra), the wall emissivity (ει), the Planck number (Pl), and the scattering albedo (ω), have been considered. The results presented in terms of isotherms, streamlines and averaged Nusselt number, show that in absence of radiation, the temperature and the flow fields are centro-symmetrics and the cavity core is thermally stratified. However, radiation causes an overall increase in the temperature and velocity gradients along both thermally active walls. The maximum heat transfer rate is obtained when the surfaces of the enclosure walls are regarded as blackbodies. It is also seen that the scattering medium can generate a multicellular flow.
Ridouane, El Hassan; Hasnaoui, Mohammed; Campo, Antonio
2006-01-01
Coupled laminar natural convection with radiation in air-filled square enclosure heated from below and cooled from above is studied numerically for a wide variety of radiative boundary conditions at the sidewalls. A numerical model based on the finite difference method was used for the solution of mass, momentum and energy equations. The surface-to-surface method was used to calculate the radiative heat transfer. Simulations were performed for two values of the emissivities of the active and insulated walls (ɛ1=0.05 or 0.85, ɛ2=0.05 or 0.85) and Rayleigh numbers ranging from 103 to 2.3×106 . The influence of those parameters on the flow and temperature patterns and heat transfer rates are analyzed and discussed for different steady-state solutions. The existing ranges of these solutions are reported for the four different cases considered. It is founded that, for a fixed Ra, the global heat transfer across the enclosure depends only on the magnitude of the emissivity of the active walls. The oscillatory behavior, characterizing the unsteady-state solutions during the transitions from bicellular flows to the unicellular flow are observed and discussed.
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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.
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M.A. Taghikhani
2015-01-01
Full Text Available The magnetic field effect on laminar natural convection flow is investigated in a filled enclosure with internal heat generation using two-dimensional numerical simulation. The enclosure is heated by a uniform volumetric heat density and walls have constant temperature. A fixed magnetic field is applied to the enclosure. The dimensionless governing equations are solved numerically for the stream function, vorticity and temperature using finite difference method for various Rayleigh (Ra and Hartmann (Ha numbers in MATLAB software. The stream function equation is solved using fast Poisson's equation solver on a rectangular grid (POICALC function in MATLAB, voricity and temperature equations are solved using red-black Gauss-Seidel and bi-conjugate gradient stabilized (BiCGSTAB methods respectively. The results show that the strength of the magnetic field has significant effects on the flow and temperature fields. For the square cavity, the maximum temperature reduces with increasing Ra number. It is also observed that at low Ra number, location of the maximum temperature is at the centre of the cavity and it shifts upwards with increase in Ra number. Circulation inside the enclosure and therefore the convection becomes stronger as the Ra number increases while the magnetic field suppresses the convective flow and the heat transfer rate. The ratio of the Lorentz force to the buoyancy force (Ha2/Ra is as an index to compare the contribution of natural convection and magnetic field strength on heat transfer.
Salesky, Scott T.; Chamecki, Marcelo; Bou-Zeid, Elie
2017-04-01
Both observational and numerical studies of the convective boundary layer (CBL) have demonstrated that when surface heat fluxes are small and mean wind shear is strong, convective updrafts tend to organize into horizontal rolls aligned within 10-20° of the geostrophic wind direction. However, under large surface heat fluxes and weak to negligible shear, convection tends to organize into open cells, similar to turbulent Rayleigh-Bénard convection. Using a suite of 14 large-eddy simulations (LES) spanning a range of -z_i/L between zero (neutral) and 1041 (highly convective), where z_i is the CBL depth and L is the Obukhov length, the transition between roll- and cellular-type convection is investigated systematically for the first time using LES. Mean vertical profiles including velocity variances and turbulent transport efficiencies, as well the "roll factor," which characterizes the rotational symmetry of the vertical velocity field, indicate the transition occurs gradually over a range of -z_i/L; however, the most significant changes in vertical profiles and CBL organization occur from near-neutral conditions up to about -z_i/L ≈ 15-20. Turbulent transport efficiencies and quadrant analysis are used to characterize the turbulent transport of momentum and heat with increasing -z_i/L. It is found that turbulence transports heat efficiently from weakly to highly convective conditions; however, turbulent momentum transport becomes increasingly inefficient as -z_i/L increases.
The analysis and kinetic energy balance of an upper-level wind maximum during intense convection
Fuelberg, H. E.; Jedlovec, G. J.
1982-01-01
The purpose of this paper is to analyze the formation and maintenance of the upper-level wind maximum which formed between 1800 and 2100 GMT, April 10, 1979, during the AVE-SESAME I period, when intense storms and tornadoes were experienced (the Red River Valley tornado outbreak). Radiosonde stations participating in AVE-SESAME I are plotted (centered on Oklahoma). National Meteorological Center radar summaries near the times of maximum convective activity are mapped, and height and isotach plots are given, where the formation of an upper-level wind maximum over Oklahoma is the most significant feature at 300 mb. The energy balance of the storm region is seen to change dramatically as the wind maximum forms. During much of its lifetime, the upper-level wind maximum is maintained by ageostrophic flow that produces cross-contour generation of kinetic energy and by the upward transport of midtropospheric energy. Two possible mechanisms for the ageostrophic flow are considered.
CFD Validation Benchmark Dataset for Natural Convection in Nuclear Fuel Rod Bundles
Smith, Barton; Jones, Kyle
2016-11-01
The present study provide CFD validation benchmark data for coupled fluid flow/convection heat transfer on the exterior of heated rods arranged in a 2 × 2 array. The rod model incorporates grids with swirling veins to resemble a nuclear fuel bundle. The four heated aluminum rods are suspended in an open-circuit wind tunnel. Boundary conditions (BCs) are measured and uncertainties calculated to provide all quantities necessary to successfully conduct a CFD validation exercise. System response quantities (SRQs) are measured for comparing the simulation output to the experiment. Stereoscopic Particle Image Velocimetry (SPIV) is used to non-intrusively measure 3-component velocity fields. A through-plane measurement is used for the inflow while laser sheet planes aligned with the flow direction at several downstream locations are used for system response quantities. Two constant heat flux rod surface conditions are presented (400 W/m2 and 700 W/m2) achieving a peak Rayleigh number of 1010 . Uncertainty for all measured variables is reported. The boundary conditions, system response, and all material properties are now available online for download. The U.S. Department of Energy Nuclear Engineering University Program provided the funding for these experiments under Grant 00128493.
Somavilla Cabrillo, Raquel; Schauer, Ursula; Budeus, Gedeon; Latarius, Katrin
2015-04-01
There are only a few sites where the deep ocean is ventilated from the surface. The responsible process known as deep convection is recognized to be a key process on the Earth's climate system, but still it is scarcely observed, and its good representation by global oceanographic and climate models remains unclear. In the Arctic Ocean, the halt of deep convection in the Greenland Sea during the last three decades serves as a natural experiment to study: (1) the conditions that drive the occurrence or not of deep convection and (2) the effects of the halt of deep convection on the thermohaline properties of the deep water masses and circulation both locally and in adjacent ocean basins. Combining oceanic and atmospheric in-situ data together with reanalysis data, we observe that not only on average the winter net heat losses from the ocean to the atmosphere (Qo) have decreased during the last three decades in the Greenland Sea (ΔQo (before the 1980s- after the 1980s) = 25 Wm-2) but the intensity and number of strong cooling events (Qo ≥ 800Wm-2). This last value for convection reaching 2000 m in the Greenland Sea seems critical to make the mixed layer deepening from being a non-penetrative process to one arrested by baroclinic instabilities. Besides, changes in the wind stress curl and preconditioning for deep convection have occurred, hindering also the occurrence of deep convection. Concerning the effects of the halt of deep convection, hydrographic data reveal that the temperature between 2000 meters depth and the sea floor has risen by 0.3 °C in the last 30 years, which is ten times higher than the temperature increase in the global ocean on average, and salinity rose by 0.02 because import of relatively warm and salty Arctic Ocean deep waters continued. The necessary transports to explain the observed changes suggest an increase of Arctic Ocean deep water transport that would have compensated the decrease in deep water formation rate after the 1980s. The
Energy Technology Data Exchange (ETDEWEB)
Nickell, T.W.
1988-01-01
This study numerically analyzes combined radiative and natural or forced convective heat transfer between vertical parallel plates with two-dimensional discrete heat sources. The numerical method was verified by comparing its results with other published experimental data and the agreement was excellent. It is shown that radiative heat transfer is a significant and useful mode of heat transfer in combination with both natural and forced convection in this situation and cannot be neglected. Radiative heat transfer accounted for 50-60% or more of the total heat transfer in some cases, and usually approximately 30-35% on the top of a discrete heat source. This fact can be used to advantage in the thermal design of electronic circuit boards.
Directory of Open Access Journals (Sweden)
A. R. Rahmati
2017-02-01
0.02 and 0.03. In order to consider the effect of porous media, Darcy-Forchheimer model is used. The results show that the presence of the porous media decreases the velocity of nanofluid and consequently decreases the strength of the flow. With decreasing Darcy number and porosity coefficient, natural convection heat transfer weakens and the mechanism of natural convection of nano-fluids tends to that of thermal conduction. With increasing Rayleigh number, the strength of flow in cavity and average Nusselt number increases. In all cases studied, increase in volume fraction improves heat transfer. In constant properties model, by increasing solid volume fraction, average Nusselt number increases more than that of variable properties model. The results show that Lattice Boltzmann method has the ability to simulate flow in porous media.
Energy Technology Data Exchange (ETDEWEB)
Abbassi, Yasser, E-mail: y.abbassi@mihanmail.ir [Department of Engineering, University of Shahid Beheshti, Tehran (Iran, Islamic Republic of); Asgarian, Shahla [Department of Chemical Engineering, Isfahan University, Tehran (Iran, Islamic Republic of); Ghahremani, Esmaeel; Abbasi, Mohammad [Department of Engineering, University of Shahid Beheshti, Tehran (Iran, Islamic Republic of)
2016-12-01
Highlights: • We carried out a CFD study to investigate transient natural convection in MNSR. • We applied porous media approach to simplify the complex core of MNSR. • Method have been verified with experimental data. • Temperature difference between the core inlet and outlet has been obtained. • Flow pattern and temperature distribution have been presented. - Abstract: The small and complex core of Isfahan Miniature Neutron Source Reactor (MNSR) in addition to its large tank makes a parametric study of natural convection difficult to perform in aspects of time and computational resources. In this study, in order to overcome this obstacle the porous media approximation has been used. This numerical technique includes two steps, (a) calculation of porous media variables such as porosity and pressure drops in the core region, (b) simulation of natural convection in the reactor tank by assuming the core region as a porous medium. Simulation has been carried out with ANSYS FLUENT® Academic Research, Release 16.2. The core porous medium resistance factors have been estimated to be, D{sub ij} = 1850 [1/m] and C{sub ij} = 415 [1/m{sup 2}]. Natural Convection simulation with Boussinesq approximation and variable property assumption have been performed. The experimental data and nuclear codes available in the literature, have verified the method. The average temperature difference between the experimental data and this study results was less than 0.5 °C and 2.0 °C for property variable technique and Boussinesq approximation, respectively. Temperature distribution and flow pattern in the entire reactor have been obtained. Results have shown that the temperature difference between core outlet and inlet is about 18°C and in this situation flow rate is about 0.004 kg/s. A full parametric study could be the topic of future investigations.
Chen, Wen Ruey
2016-10-01
This paper studies the steady laminar natural convection of micropolar fluids in the complex annuli between the inner sphere and outer vertical cylinder to present a numerical analysis of the flow and heat transfer characteristics with buoyancy effects. Computations were carried out systematically by the several different parameters of geometric ratio, micropolar material parameter and Rayleigh number to determine the average Nusselt number and the skin friction coefficient on the flow and the thermal fields.
Effect of induced magnetic field on natural convection in vertical concentric annuli
Institute of Scientific and Technical Information of China (English)
R. K. Singh; A. K. Singh
2012-01-01
In the present paper,we have considered the steady fully developed laminar natural convective flow in open ended vertical concentric annuli in the presence of a radial magnetic field.The induced magnetic field produced by the motion of an electrically conducting fluid is taken into account.The transport equations concerned with the considered model are first recast in the non-dimensional form and then unified analytical solutions for the velocity,induced magnetic field and temperature field are obtained for the cases of isothermal and constant heat flux on the inner cylinder of concentric annuli.The effects of the various physical parameters appearing into the model are demonstrated through graphs and tables.It is found that the magnitude of maximum value of the fluid velocity as well as induced magnetic field is greater in the case of isothermal condition compared with the constant heat flux case when the gap between the cylinders is less or equal to 1.70 times the radius of inner cylinder,while reverse trend occurs when the gap between the cylinders is greater than 1.71 times the radius of inner cylinder.These fields are almost the same when the gap between the cylinders is equal to 1.71 times the radius of inner cylinder for both the cases.It is also found that as the Hartmann number increases,there is a flattening tendency for both the velocity and the induced magnetic field.The influence of the induced magnetic field is to increase the velocity profiles.
Manson, Steven James
The Pantex facility near Amarillo, Texas, is the only U.S. site charged with the disassembly of nuclear weapons. Concerns over the safety of weapons handling procedures are now being revisited, due to the enhanced safety requirements of the peace time disassembly effort. This research is a detailed examination of one possible nuclear weapons-related accident. In this hypothetical accident, a chemical explosion equivalent to over 50 kilos of TNT destroys unassembled nuclear weapons components, and may potentially result in some amount of plutonium reaching the environment. Previous attempts to simulate this accident have centered around the one-dimensional node and branch approach of the MELCOR code. This approach may be adequate in calculating pressure driven flow through narrow rampways and leak sites, however, its one-dimensionality does not allow it to accurately calculate the multi-dimensional aspects of heat transfer. This research effort uses an axi-symmetric stream function---vorticity formulation of the Navier-Stokes equations to model a Pantex cell building following a successfully contained chemical explosion. This allows direct calculation of the heat transfer within the cell room during the transient. The tool that was developed to perform this analysis is called PET (Post-Explosion Transient), and it simulates natural convection thermal hydraulics taking into account temperature-related fluid density differences, variable fluid transport properties, and a non-linear equation of state. Results obtained using the PET code indicate that previous analyses by other researchers using the MELCOR code have been overly conservative in estimating the effects of cell room heat transfer. An increase in the calculated heat transfer coefficient of approximately 20% is indicated. This has been demonstrated to significantly decrease the projected consequences of the hypothetical accident.
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.
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P. Sudarsana Reddy
2016-03-01
Full Text Available In this paper, we have presented MHD natural convection boundary layer flow, heat and mass transfer characteristics of nanofluid through porous media over a vertical cone influenced by different aspects of nanoparticles such as size, shape, type of nanoparticles and type of the base fluid and working temperature of base fluid. To increase the physical significance of the problem, we have taken dynamic viscosity and thermal conductivity as the functions of local volume fraction of nanoparticles. The drift-flux model of nanofluids, Brownian motion, thermophoresis, and enhancement ratio parameters are also considered in the present analysis. The influence of non-dimensional parameters such as magnetic field (M, buoyancy ratio parameter (Nr, conductivity parameter (Nc, viscosity parameter (Nv, Brownian motion parameter (Nb, thermophoresis parameter (Nt, Lewis number (Le on velocity, temperature and volume fraction of nanoparticles in the boundary layer region is examined in detail. Furthermore the impact of these parameters on local Nusselt number (Nux and enhancement ratio hnfhbf is also investigated. The results of present study reveal that significant natural convection heat transfer enhancement is noticed as the size of nanoparticles decreases. Moreover, type of the nanoparticles and type of the base fluid also influenced the natural convection heat transfer.
Energy Conversion in Protocells with Natural Nanoconductors
Directory of Open Access Journals (Sweden)
Jian Xu
2012-01-01
Full Text Available While much nanotechnology leverages solid-state devices, here we present the analysis of designs for hybrid organic-inorganic biomimetic devices, “protocells,” based on assemblies of natural ion channels and ion pumps, “nanoconductors,” incorporated into synthetic supported lipid bilayer membranes. These protocells mimic the energy conversion scheme of natural cells and are able to directly output electricity. The electrogenic mechanisms have been analyzed and designs were optimized using numerical models. The parameters that affect the energy conversion are quantified, and limits for device performance have been found using numerical optimization. The electrogenic performance is compared to conventional and emerging technologies and plotted on Ragone charts to allow direct comparisons. The protocell technologies summarized here may be of use for energy conversion where large-scale ion concentration gradients are available (such as the intersection of fresh and salt water sources or small-scale devices where low power density would be acceptable.
Jiang, Shaohui; Liu, Changhong; Fan, Shoushan
2014-03-12
In this work, we report our studies related to the natural-convective heat transfer properties of carbon nanotube (CNT) sheets. We theoretically derived the formulas and experimentally measured the natural-convective heat transfer coefficients (H) via electrical heating method. The H values of the CNT sheets containing different layers (1, 2, 3, and 1000) were measured. We found that the single-layer CNT sheet had a unique ability on heat dissipation because of its great H. The H value of the single-layer CNT sheet was 69 W/(m(2) K) which was about twice of aluminum foil in the same environment. As the layers increased, the H values dropped quickly to the same with that of aluminum foil. We also discussed its roles on thermal dissipation, and the results indicated that the convection was a significant way of dissipation when the CNT sheets were applied on macroscales. These results may give us a new guideline to design devices based on the CNT sheets.
Impacts of convection on high-temperature aquifer thermal energy storage
Beyer, Christof; Hintze, Meike; Bauer, Sebastian
2016-04-01
Seasonal subsurface heat storage is increasingly used in order to overcome the temporal disparities between heat production from renewable sources like solar thermal installations or from industrial surplus heat and the heat demand for building climatisation or hot water supply. In this context, high-temperature aquifer thermal energy storage (ATES) is a technology to efficiently store and retrieve large amounts of heat using groundwater wells in an aquifer to inject or withdraw hot or cold water. Depending on the local hydrogeology and temperature amplitudes during high-temperature ATES, density differences between the injected hot water and the ambient groundwater may induce significant convective flow components in the groundwater flow field. As a consequence, stored heat may accumulate at the top of the storage aquifer which reduces the heat recovery efficiency of the ATES system. Also, an accumulation of heat at the aquifer top will induce increased emissions of heat to overlying formations with potential impacts on groundwater quality outside of the storage. This work investigates the impacts of convective heat transport on the storage efficiency of a hypothetical high-temperature ATES system for seasonal heat storage as well as heat emissions to neighboring formations by numerical scenario simulations. The coupled groundwater flow and heat transport code OpenGeoSys is used to simulate a medium scale ATES system operating in a sandy aquifer of 20 m thickness with an average groundwater temperature of 10°C and confining aquicludes at top and bottom. Seasonal heat storage by a well doublet (i.e. one fully screened "hot" and "cold" well, respectively) is simulated over a period of 10 years with biannual injection / withdrawal cycles at pumping rates of 15 m³/h and for different scenarios of the temperature of the injected water (20, 35, 60 and 90 °C). Simulation results show, that for the simulated system significant convective heat transport sets in when
Qiu, Yan; Tian, Maocheng; Guo, Zhixiong
2013-03-01
A three-dimensional numerical study was made to investigate effects of fin angle, fin surface emissivity, and tube wall temperature on heat transfer enhancement for a longitudinal externally-finned tube placed vertically in a small chamber. The numerical model was first validated through comparison with experimental measurements and the appropriateness of general boundary conditions was examined. The numerical results show that the mean Nusselt number increases with Rayleigh number for all the fin angles investigated. The maximum heat transfer rate per mass occurs when the fin angle is about 60° for fin surface emissivity between 0.7 and 0.8 and 55° when the surface emissivity increases to 0.9. With increasing tube wall temperature, both the natural convection and radiation heat transfer are enhanced, but the fraction of radiation heat transfer decreases in the temperature range studied. Radiation fraction increases with increasing fin surface emissivity. Both convection and radiation heat transfer modes are important.
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M.A. Mansour
2015-11-01
Full Text Available Numerical investigation for heat transfer with natural convection and nanofluid flow subjected to changeable thermal boundary conditions and inclined magnetic field has been performed. Effect of problem’s parameters on each other has been monitored. It has been reached to that inclination angle can justify the quasi-symmetric boundary conditions to be symmetric. In addition to that as inclination angle increases, the magnetic force pointed to horizontal trend; so the convection regime dominates the cavity. In a related context, nanoparticles provide conduction regime, increase and maintenance the rate of heat transfer all over the cavity. However thermal emission at ends of heat source–sink has been found to be constant when boundary conditions change in the pure case.
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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.
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.
Cao, Y.; Faghri, A.; Juhasz, A.
1991-01-01
Latent heat energy storage systems with both annular and countercurrent flows are modeled numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. It is found that the energy storage system with the countercurrent flow is an efficient way to absorb heat energy in a short period for pulsed power load space applications.
Effects of finiteness on the thermo-fluid-dynamics of natural convection above horizontal plates
Guha, Abhijit; Sengupta, Sayantan
2016-06-01
A rigorous and systematic computational and theoretical study, the first of its kind, for the laminar natural convective flow above rectangular horizontal surfaces of various aspect ratios ϕ (from 1 to ∞) is presented. Two-dimensional computational fluid dynamic (CFD) simulations (for ϕ → ∞) and three-dimensional CFD simulations (for 1 ≤ ϕ cases, with the complex three-dimensional solutions revealed here. The present computational study establishes the region of a high-aspect-ratio planform over which the results of the similarity theory are approximately valid, the extent of this region depending on the Grashof number. There is, however, a region near the edge of the plate and another region near the centre of the plate (where a plume forms) in which the similarity theory results do not apply. The sizes of these non-compliance zones decrease as the Grashof number is increased. The present study also shows that the similarity velocity profile is not strictly obtained at any location over the plate because of the entrainment effect of the central plume. The 3-D CFD simulations of the present paper are coordinated to clearly reveal the separate and combined effects of three important aspects of finiteness: the presence of leading edges, the presence of planform centre, and the presence of physical corners in the planform. It is realised that the finiteness due to the presence of physical corners in the planform arises only for a finite value of ϕ in the case of 3-D CFD simulations (and not in 2-D CFD simulations or similarity theory). The presence of physical corners is related here to several significant aspects of the solution - the conversion of in-plane velocity to out-of-plane velocity near the diagonals, the star-like non-uniform distribution of surface heat flux on heated planforms, the three-dimensionality of the temperature field, and the complex spatial structure of the velocity iso-surfaces. A generic theoretical correlation for the Nusselt
Effect of the fins configuration on natural convection heat transfer experimentally and numerically
Directory of Open Access Journals (Sweden)
Ahmed F. Khudheyer, Zaid Hameed Hasan
2015-01-01
Full Text Available The cooling of the electronic systems, electrical systems, and CPU of the computer is very important; therefore this study is prepared to improve this aims. In this study, natural convection heat transfer from rectangular fins with five different figures (continuous fins, 1-interrupted fins, 4-interrupted fins, inclined fins and V-fins are investigated at different heat flux values (175, 350, 525, 700 and 875 Watt per square meter.The effect of base to ambient temperature difference for continuous fins, 1-interrupted fins, 4-interrupted fins, inclined fins and V-fins were determined. All types of the fins are made with different geometries by using CNC machine and wire cut machine, but it have some dimensions in common such as fins thickness (5mm, fins height (18mm, space between the fins (10mm, and the volume of the base plat of heat sink (300*95*2mm. The heat sink base plate was heated by an attached maximum electric heater 2225 W/m2 with an identical size with the base plate of the heat sink, which could supply a specific heat flux. The steady-state temperature of the base plate was measured by eleven copper-constantan (K-type thermocouples inserted into different grooves in the base plate and glued with thermal tape and epoxy to ensure good thermal contact. The mathematical model of the base plate and fins are solved numerically using COMSOL (5.0 after describing the mesh model using the COMSOL (5.0 and assuming the properties of air variation with film temperature. After finding the numerical result, the validation between experimental and numerical results has been verified. Good agreement has been found between the experimental and CFD results. Empirical correlations for the overall Nusselt number versus average Rayleigh number for these configurations are obtained and compared to other correlations sited in the literature. The range of Rayleigh number, Nusselt number and base plate temperature are, (1.7e7 - 12.5e7, (37 – 83 and (25.6
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
Energy analyses and drying kinetics of chamomile leaves in microwave-convective dryer
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Ali Motevali
2016-06-01
Full Text Available Drying characteristics and energy aspects as well as mathematical modeling of thin layer drying kinetics of chamomile in a microwave-convective dryer are reported in this article. Drying experiments were carried out at 8 microwave power levels (200–900 W, air temperature of 50 °C, and air velocity of 0.5 m/s. Increasing the microwave output power from 200 to 900 W, decreased the drying time from 40 to 10 min. The drying process took place in the falling rate period. The Midilli et al. model showed the best fit to the experimental drying data. Moisture diffusivity values increase with decreasing moisture content down to 1.70 (kg water kg−1 dry matter but decrease with a further decrease in moisture content from 1.72 to 0.96 (kg water kg−1 dry matter. The average values of Deff increased with microwave power from 5.46 to 39.63 × 10−8 (m2 s−1. Energy consumption increased and energy efficiency decreased with moisture content of chamomile samples. Average specific energy consumption, energy efficiency and energy loss varied in the range 18.93–28.15 MJ kg−1 water, 8.25–13.07% and 16.79–26.01 MJ kg−1 water, respectively, while the best energy results were obtained at 400 W, 50 °C and 0.5 m s−1.
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A. N. Ostrikov
2013-01-01
Full Text Available A mathematical model of combined radiation and convection drying of fruit and vegetable chips with pulsed energy supply is developed, the model describes the change in temperature and moisture content during the period of constant and periods of decreasing drying rate.
A Convective-like Energy-Stable Open Boundary Condition for Simulations of Incompressible Flows
Dong, Suchuan
2015-01-01
We present a new energy-stable open boundary condition, and an associated numerical algorithm, for simulating incompressible flows with outflow/open boundaries. This open boundary condition ensures the energy stability of the system, even when strong vortices or backflows occur at the outflow boundary. Under certain situations it can be reduced to a form that can be analogized to the usual convective boundary condition. One prominent feature of this boundary condition is that it provides a control over the velocity on the outflow/open boundary. This is not available with the other energy-stable open boundary conditions from previous works. Our numerical algorithm treats the proposed open boundary condition based on a rotational velocity-correction type strategy. It gives rise to a Robin-type condition for the discrete pressure and a Robin-type condition for the discrete velocity on the outflow/open boundary, respectively at the pressure and the velocity sub-steps. We present extensive numerical experiments on...
Turbulent kinetic energy generation in the convective boundary layer derived from thermodynamics
Slameršak, Aljoša; Renner, Maik; Ganzeveld, Laurens; Hartogensis, Oscar; Kolle, Olaf; Kleidon, Axel
2016-04-01
Turbulent heat fluxes facilitate the bulk of heat transfer between the surface and lower atmosphere, which results in the diurnal growth of convective boundary layer (CBL) and turbulent kinetic energy generation (TKE). Here we postulate the hypothesis that TKE generation in the CBL occurs as a result of heat transfer in a "Carnot-like" heat engine with temporal changes in the internal energy of the boundary layer. We used the Tennekes energy-balance model of CBL and extended it with the analysis of the entropy balance to derive the estimates of TKE generation in the CBL. These TKE generation estimates were compared to the turbulent dissipation from a simple dissipation model from Moeng and Sullivan, to test the validity of our heat engine hypothesis. In addition, to evaluate the performance of the dissipation model, this was independently validated by a comparison of its estimates with the turbulent dissipation calculations based on spectral analysis of eddy covariance wind measurements at a German field station. Our analysis demonstrates how a consistent application of thermodynamics can be used to obtain an independent physical constraint on the diurnal boundary layer evolution. Furthermore, our analysis suggests that the CBL operates at the thermodynamic limit, thus imposing a thermodynamic constraint on surface-atmosphere exchange.
Energy Technology Data Exchange (ETDEWEB)
Adesanya, S.O., E-mail: adesanyas@run.edu.ng [Department of Mathematical Sciences, College of Natural Sciences, Redeemer’s University (Nigeria); Oluwadare, E.O. [Department of Mathematical Sciences, College of Natural Sciences, Redeemer’s University (Nigeria); Falade, J.A., E-mail: faladej@run.edu.ng [Department of Physical Sciences, College of Natural Sciences, Redeemer’s University (Nigeria); Makinde, O.D., E-mail: makinded@gmail.com [Faculty of Military Science, Stellenbosch University, Private Bag X2, Saldanha 7395 (South Africa)
2015-12-15
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. - Highlights: • The influence of magnetic field on the free convective fluid flow is considered. • The coupled equations are solved by using Adomian decomposition method. • The Adomian series solution agreed with previously obtained result. • Magnetic field decreases the velocity maximum but enhances temperature field.
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)
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
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...... condition and six eccentricities from 0.1 to 0.92 at five different heat fluxes. The surface temperature of the heated rod is measured at different heights, and the Nusselt number is calculated at the temperature measurement locations. A correlation is suggested to determine the Nusselt number based...
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...... convection. The variations of the entropy generation for different Rayleigh numbers are also investigated. Comparison between the present results and previous works demonstrated excellent agreements which verify the accuracy and flexibility of the method in simulating the fluid mechanics and heat transfer...
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.
Torczynski, J. R.; Henderson, J. A.; Ohern, T. J.; Chu, T. Y.; Blanchat, T. K.
Three-dimensional natural convection of a fluid in an enclosure is examined. The geometry is motivated by a possible magmaenergy extraction system, and the fluid is a magma simulant and has a highly temperature-dependent viscosity. Flow simulations are performed for enclosures with and without a cylinder, which represents the extractor, using the finite-element code FIDAP (Fluid Dynamics International). The presence of the cylinder completely alters the flow pattern. Flow-visualization and PIV experiments are in qualitative agreement with the simulations.
Institute of Scientific and Technical Information of China (English)
Pei－xueJiang; Ze－peiRen; 等
1992-01-01
Corrosion products of structural materials when contained in water usually are in two states:soluble state and colloidal particles with dimeter about 10-3-10-1um,Deposits of such corrosion products on tube surfaces under high pressure will jeopardize the operating economy of power plant equipment and even esult in accidents.A numerical study is reported in this paper of the natural convective heat and mass transfer on a vertical heated plate subject to the flrst or mixed kind of boundary conditions for high-pressure water(P=17MPa) containing metal corrosion products with consideration of varialbe thermophysical properties.
Directory of Open Access Journals (Sweden)
Kazi Humayun Kabir
2015-01-01
Full Text Available An analysis is presented to investigate the influences of viscous and pressure stress work on MHD natural convection flow along a uniformly heated vertical wavy surface. The governing equations are first modified and then transformed into dimensionless non-similar equations by using set of suitable transformations. The transformed boundary layer equations are solved numerically using the implicit finite difference method, known as Keller-box scheme. Numerical results for the velocity profiles, temperature profiles, skin friction coefficient, the rate of heat transfers, streamlines and isotherms are shown graphically. Some results of skin friction, rate of heat transfer are presented in tabular form for selected values of physical parameters.
Suslov, Sergey A; Bozhko, Alexandra A; Sidorov, Alexander S; Putin, Gennady F
2012-07-01
Flow patterns arising in a vertical differentially heated layer of nonconducting ferromagnetic fluid placed in an external uniform transverse magnetic field are studied experimentally and discussed from the point of view of the perturbation energy balance. A quantitative criterion for detecting the parametric point where the dominant role in generating a flow instability is transferred between the thermogravitational and thermomagnetic mechanisms is suggested, based on the disturbance energy balance analysis. A comprehensive experimental study of various flow patterns is undertaken, and the existence is demonstrated of oblique thermomagnetic waves theoretically predicted by Suslov [Phys. Fluids 20, 084101 (2008)] and superposed onto the stationary magnetoconvective pattern known previously. It is found that the wave number of the detected convection patterns depends sensitively on the temperature difference across the layer and on the applied magnetic field. In unsteady regimes its value varies periodically by a factor of almost 2, indicating the appearance of two different competing wave modes. The wave numbers and spatial orientation of the observed dominant flow patterns are found to be in good agreement with theoretical predictions.
Effect of Selected Factors on Drying Process of Tomato in Forced Convection Solar Energy Dryer
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U.S. Muhammed
2012-09-01
Full Text Available The effect of air velocity, slice thickness and grazing materials in drying process of tomato in forced convection solar energy dryer was evaluated. The result is to serve as an input for solar energy development for drying of vegetable and fruit products in North West Ecological zone of Nigeria. In order to evaluate the effects of the above factors in drying operation, a split-split-plot experimental design was used. Differences among the treatments and their interactions were tested with orthogonal contrast test to access their significance while further analysis were done to compare all possible pairs of treatment means using Duncan’s Multiple Range Test (DMRT. The results showed that there is no significant difference, at 95% probability level, between the means of the three glazing materials used. However, the variations in mean slice thickness and in mean air flow rate are highly significant at 99% probability level. The results further revealed that drying rate increasing with decrease in slice thickness and increase in air flow rate. Drying of 15, 20 and 25 mm slice thickness of tomato was achieved in time range of 21-24, 27-29 and 30-50 h, respectively.
Cho, Jae Hyun; Batta, A.; Casamassima, V.; Cheng, X.; Choi, Yong Joon; Hwang, Il Soon; Lim, Jun; Meloni, P.; Nitti, F. S.; Dedul, V.; Kuznetsov, V.; Komlev, O.; Jaeger, W.; Sedov, A.; Kim, Ji Hak; Puspitarini, D.
2011-08-01
As highly promising coolant for new generation nuclear reactors, liquid Lead-Bismuth Eutectic has been extensively worldwide investigated. With high expectation about this advanced coolant, a multi-national systematic study on LBE was proposed in 2007, which covers benchmarking of thermal hydraulic prediction models for Lead-Alloy Cooled Advanced Nuclear Energy System (LACANES). This international collaboration has been organized by OECD/NEA, and nine organizations - ENEA, ERSE, GIDROPRESS, IAEA, IPPE, KIT/IKET, KIT/INR, NUTRECK, and RRC KI - contribute their efforts to LACANES benchmarking. To produce experimental data for LACANES benchmarking, thermal-hydraulic tests were conducted by using a 12-m tall LBE integral test facility, named as Heavy Eutectic liquid metal loop for integral test of Operability and Safety of PEACER (HELIOS) which has been constructed in 2005 at the Seoul National University in the Republic of Korea. LACANES benchmark campaigns consist of a forced convection (phase-I) and a natural circulation (phase-II). In the forced convection case, the predictions of pressure losses based on handbook correlations and that obtained by Computational Fluid Dynamics code simulation were compared with the measured data for various components of the HELIOS test facility. Based on comparative analyses of the predictions and the measured data, recommendations for the prediction methods of a pressure loss in LACANES were obtained. In this paper, results for the forced convection case (phase-I) of LACANES benchmarking are described.
Directory of Open Access Journals (Sweden)
Gauri Shanker Seth
2015-01-01
Full Text Available An investigation of unsteady hydromagnetic natural convection heat and mass trans fer flow with Hall current of a viscous, incompressible, electrically conducting, heat absorbing and optically thin radiating fluid past an accelerated moving vertical plate through fluid saturated porous medium in a rotating environment is carried out when temperature of the plate has a temporarily ramped profile. The exact solutions of momentum, energy and concentration equations are obtained in closed form by Laplace transform technique. The expressions of skin friction, Nusselt number and Sherwood number are also derived. For both ramped temperature and isothermal plates, Hall current tends to accelerate primary and secondary fluid velocities whereas heat absorption and radiation have reverse effect on it. Rotation tends to retard primary fluid velocity whereas it has a reverse effect on secondary fluid velocity. Heat absorption and radiation have tendency to enhance rate of heat transfer at the plate.
Ahmed, Mahmoud; Eslamian, Morteza
2015-01-01
Laminar natural convection in differentially heated (β = 0°, where β is the inclination angle), inclined (β = 30° and 60°), and bottom-heated (β = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and th...
Energy Technology Data Exchange (ETDEWEB)
Boudjemadi, R.
1996-03-01
The main objectives of this thesis are the direct numerical simulation of natural convection in a vertical differentially heated slot and the improvements of second-order turbulence modelling. A three-dimensional direct numerical simulation code has been developed in order to gain a better understanding of turbulence properties in natural convection flows. This code has been validated in several physical configurations: non-stratified natural convection flows (conduction solution), stratified natural convection flows (double boundary layer solution), transitional and turbulent Poiseuille flows. For the conduction solution, the turbulent regime was reached at a Rayleigh number of 1*10{sup 5} and 5.4*10{sup 5}. A detailed analysis of these results has revealed the principal qualities of the available models but has also pointed our their shortcomings. This data base has been used in order to improve the triple correlations transport models and to select the turbulent time scales suitable for such flows. (author). 122 refs., figs., tabs., 4 appends.
EXPERIMENTAL INVESTIGATION ON THE CHAOTIC PHENOMENA IN THE WAKE OF A NATURAL THERMAL CONVECTION FLOW
Institute of Scientific and Technical Information of China (English)
林贞彬; 郭大华; 余西龙; 朱进生
2000-01-01
Chaotic phenomena in the wake of thermal convection flow fields above a heating fiat plate were investigated experimentally. A newly developed electron beam fluorescence technique (EBF) was used to simultaneously measure density fluctuation at 7 points in a cross section above the plate. Correlation dimensions,intermittence coefficients, Fourier spectrum have been obtained for different Grashof numbers. Spatial distribution of correlation dimensions are presented. The experimental result shows that there is a certain relationship between the density fluctuation and the Gr number. And time-spacial characteristic of chaos evolution is also given.
Sheikholeslami, Mohsen; Rokni, Houman B.
2017-05-01
Magnetohydrodynamic nanofluid flow and convective heat transfer are studied considering thermal radiation. Effects of magnetic field and shape of nanoparticles on viscosity and thermal conductivity of the nanofluid are taken into account. The solutions of final equations are obtained by the control volume-based finite-element method (CVFEM). Roles of shape of nanoparticles, radiation parameter, ferrofluid volume fraction, Hartmann and Rayleigh numbers are presented graphically. Results demonstrate that selecting the Platelet shape for Fe3O4 nanoparticles leads to maximum Nusselt number. Rate of heat transfer increases with increasing Rayleigh number and radiation parameter but it decreases with increasing Hartmann number.
Energy Technology Data Exchange (ETDEWEB)
Vieira, Camila Braga; Romero, Gabriel Alves; Jian Su, E-mail: camila@lasme.coppe.ufrj.b, E-mail: gabrielromero@lasme.coppe.ufrj.b, E-mail: sujian@lasme.coppe.ufrj.b [Universidade Federal do Rio de Janeiro (COPPE/UFRJ), RJ (Brazil). Nuclear Engineering Program
2010-07-01
Computational simulation of natural convection in a molten core during a hypothetical severe accident in the lower head of a typical PWR pressure vessel was performed for two-dimensional semi-circular geometry with isothermal walls. Transient turbulent natural convection heat transfer of a fluid with uniformly distributed volumetric heat generation rate was simulated by using a commercial computational fluid dynamics software ANSYS CFX 12.0. The Boussinesq model was used for the buoyancy effect generated by the internal heat source in the flow field. The two-equation k-{omega} based SST (Shear Stress Transport) turbulence model was used to mould the turbulent stresses in the Reynolds-Average Navier-Stokes equations (RANS). Two Prandtl numbers, 6:13 and 7:0, were considered. Five Rayleigh numbers were simulated for each Prandtl number used (109, 1010, 1011, 1012, and 1013). The average Nusselt numbers on the bottom surface of the semicircular cavity were in excellent agreement with Mayinger et al. (1976) correlation and only at Ra = 109 the average Nusselt number on the top flat surface was in agreement with Mayinger et al. (1976) and Kulacki and Emara (1975) correlations. (author)
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)
M. Karbasi pour
2017-01-01
Full Text Available Natural convective flow over a horizontal cylinder is a phenomenon used in many industries such as heat transfer from an electrical wire, heat exchanger, pipe heat transfer, etc. In this research, fluid dynamics of natural convective flow over a horizontal rhombus cylinder, with uniform heat flux, is investigated by using two-dimensional Particle Image Velocimetry (PIV Technique. Experiments are carried out in a cubical tank full of water having an interface with air and the cylinder is placed horizontally inside the tank. The heater is turned on for 40s and the effects of heater's power and the height of water above the cylinder are surveyed. The experiments are carried out in three different heights of water and two different heater’s powers in which Rayleigh number changes from 1.33×107 to 1.76×107. The emitted heat flux causes the buoyancy force to be made and the main branch of flow to be formed. Then, moving up the main branch flow through the stationary water generates two equal anti-direction vortexes. These vortexes are developed when they reach the free surface. The results indicate that the flow pattern changes for different values of water height and heater’s power.
Institute of Scientific and Technical Information of China (English)
无
1999-01-01
The low frequency cyclical transient natural convection in a cube enclosure with an internal isolated vertical heated plate was investigated experimentally.A computer-aided experimental system was designed to generate the cyclical heating power input and also used for data reduction.The effects of the cyclic heating power input amplitude(from 0 to 8 W) and frequency(form 1/5400s-1 to 1/600s-1) as well as the per-cycle time-average power input(from 8 to 24 W)on the transient and time-average Nusselt number were parametrically studied.It was found that for such cyclical transient natural convection with low frequency,the plate heating power input amplitude and frequency have little effects on the time-average Nusselt number as long as the cyclical time-average heating power input remains the same,although the transient Nusselt number may be significantly affected.Therefore,the modified Grashof number based on the plate average heat flux can be used to characterize the time-average heat transfer process.The plate time-average Nusselt number is about 15% less than the infinite-space Nusselt number,The location of the isolated plate in enclosure does not appreciably influence the time-average heat transfer characteristics of the plate.
Institute of Scientific and Technical Information of China (English)
无
2003-01-01
Lightning flash activities on the central Tibetan Plateau have been studied by using the satellite-based Lightning Imaging Sensor (LIS) database from January 1998 to July 2002. The lightning activity shows a clear diurnal variation on the central Plateau. The peak lightning activity appears at about 17︰00 which is 3 h earlier than that in Jingzhou, Hubei in the same latitude belt nearby, indicating that the lightning activity is a sensitive indicator of solar heating on the Plateau. The lightning discharge is weaker on the Plateau than Jingzhou, Hubei and other low-altitude continental regions because of the lower convective available potential energy (CAPE) on the Plateau. The CAPE on the Plateau is 12 times lower than that in Jingzhou, Hubei, and 20 times lower than that in the sea-level region, such as Guangzhou and Florida. However, the sensitivity of lightning activity to CAPE changes on the Plateau is up to 30 times more sensitive than other prominent low-altitude regions.
Urban, Federico R
2009-01-01
The origin of the observed dark energy could be explained entirely within the standard model, with no new fields required. We show how the low-energy sector of the chiral QCD Lagrangian, once embedded in a nontrivial spacetime, gives rise to a cosmological vacuum energy density which can be can be presented entirely in terms of QCD parameters and the Hubble constant $H$ as $\\rho_\\Lambda \\simeq H \\cdot m_q\\la\\bar{q}q\\ra /m_{\\eta'} \\sim (4.3\\cdot 10^{-3} \\text{eV})^4$. In this work we focus on the dynamics of the ghost fields that are essential ingredients of the aforementioned Lagrangian. In particular, we argue that the Veneziano ghost, being unphysical in the usual Minkowski QFT, becomes a physical degree of freedom if the universe is expanding. As an immediate consequence, all relevant effects are naturally very small as they are proportional to the rate of expansion $H/ \\Lqcd \\sim 10^{-41}$. The co-existence of these two drastically different scales ($\\Lqcd \\sim 100 $ MeV and $H \\sim 10^{-33}$ eV) does not...
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.
Nathenson, Manuel
1975-01-01
This report contains background analyses for the estimates of Nathenson and Muffler (1975) of geothermal resources in hydrothermal convection systems and conduction-dominated areas. The first section discusses heat and fluid recharge potential of geothermal reservoirs. The second section analyzes the physical factors that determine the fraction of stored energy obtainable at the surface from a geothermal reservoir. Conversion of heat to electricity and the use of geothermal energy for direct-heating applications are discussed in the last two sections. Nathenson, Manuel, and Muffler, L.J.P., 1975, Geothermal resources in hydrothermal convection systems and conduction dominated areas, in White, D.E., and Williams, D.L., eds., Assessment of the Geothermal Resources of the United States--1975: U.S. Geological Survey Circular 726, p. 104-121, available at http://pubs.er.usgs.gov/usgspubs/cir/cir726
Numerical model of the solidification of alloys with natural convection of the liquid
Directory of Open Access Journals (Sweden)
E. Węgrzyn-Skrzypczak
2008-04-01
Full Text Available The paper deals with comparison of numerical analysis results obtained for binary alloys solidification process in the sand and permanent mould with motion of the fluid in the liquid and mushy zone. The partial differential equations describing mathematical model of the phenomena are presented. Finite Element Method is used for modeling process. Characteristic Based Split (CBS method is used for solving momentum equation. Such approach allows to uncouple velocities and pressure. Petrov-Galerkin formulation is employed to stabilize heat conductivity equation with convective term. The results of the numerical simulations in the 2D region are discussed. Velocity fields, cooling rates and positions of the liquid, solid-liquid and solid regions are compared.
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.
Schlieren visualization of water natural convection in a vertical ribbed channel
Fossa, M.; Misale, M.; Tanda, G.
2015-11-01
Schlieren techniques are valuable tools for the qualitative and quantitative visualizations of flows in a wide range of scientific and engineering disciplines. A large number of schlieren systems have been developed and documented in the literature; majority of applications involve flows of gases, typically air. In this work, a schlieren technique is applied to visualize the buoyancy-induced flow inside vertical ribbed channels using water as convective fluid. The test section consists of a vertical plate made of two thin sheets of chrome-plated copper with a foil heater sandwiched between them; the external sides of the plate are roughened with transverse, square-cross-sectioned ribs. Two parallel vertical walls, smooth and unheated, form with the heated ribbed plate two adjacent, identical and asymmetrically heated, vertical channels. Results include flow schlieren visualizations with colour-band filters, reconstructions of the local heat transfer coefficient distributions along the ribbed surfaces and comparisons with past experiments performed using air as working fluid.
Masiulaniec, K. Cyril; Vanfossen, G. James, Jr.; Dewitt, Kenneth J.; Dukhan, Nihad
1995-01-01
A technique was developed to cast frozen ice shapes that had been grown on a metal surface. This technique was applied to a series of ice shapes that were grown in the NASA Lewis Icing Research Tunnel on flat plates. Nine flat plates, 18 inches square, were obtained from which aluminum castings were made that gave good ice shape characterizations. Test strips taken from these plates were outfitted with heat flux gages, such that when placed in a dry wind tunnel, can be used to experimentally map out the convective heat transfer coefficient in the direction of flow from the roughened surfaces. The effects on the heat transfer coefficient for both parallel and accelerating flow will be studied. The smooth plate model verification baseline data as well as one ice roughened test case are presented.
Institute of Scientific and Technical Information of China (English)
CHEN Longxun; GAO Hui; HE Jinhai; TAO Shiyan; JIN Zuhui
2004-01-01
Zonal propagation of kinetic energy (KE) and convection in the South China Sea (SCS) and Indian summer monsoon areas are examined in present study. Results suggest that the SCS and Indian summer monsoon prevailed regions (5-15°N) are dominated by the southwesterly wind, however, the disturbances of KE at 850 hPa and convection are observed mainly coming from the western Pacific Ocean (140-150°E), after passing through the SCS, and westward propagated into the Bay of Bengal (90-100°E). In the Indian summer monsoon domain, where the disturbances of KE are found mainly coming from the Arabian Sea (AS) and eastward propagated into the Bay of Bengal. Therefore, the SCS and the Indian summer monsoon are quite different in zonal propagation of KE and convection. The SCS summer monsoon is mainly affected by the KE and convection coming from the tropical western Pacific. The Indian summer monsoon, however, can be partly influenced by the AS and the SCS summer monsoon. The analysis also suggests that the interaction region between the SCS and the Indian summer monsoon is around 90-95°E, rather than 105°E as proposed by earlier studies.
Natural radiation contribution to renewable energy searching
Energy Technology Data Exchange (ETDEWEB)
Balcazar, M.; Lopez, A. [ININ, Carretera Mexico-Toluca s/n, 52750 Ocoyoacac, Estado de Mexico (Mexico); Flores, M.; Huerta, M., E-mail: miguel.balcazar@inin.gob.mx [Comision Federal de Electricidad, Gerencia de Proyectos Geotermoelectricos, Alejandro Volta 655, 58290 Morelia, Michoacan (Mexico)
2014-08-15
High anomalies of naturally occurring radon in geothermal fields are becoming an additional geophysics tool for determining the areas of geothermal activity underground. Under close collaboration with the Federal Electricity Board in Mexico (CFE), we have study four geothermal fields (Los Azufres, Tres Virgenes, Humeros and Acoculco) for extending the energy potentially. The heat source in hydrothermal systems produces geothermal gasses, which transport radon to the surface faster than the common diffusion process in absence of a geothermal activity. This paper presents: mechanism of radon production, main physical and chemical features that make it an excellent indicator for locating heat sources of geothermal reservoirs, the detection basis of in situ radon concentration using a high sensitive radiation chamber and the planning experimental strategy for successful use of this technique. (author)
Sheikhzadeh, G. A.; Dastmalchi, M.; Khorasanizadeh, H.
2013-12-01
The effect of wall temperature variations on double diffusive natural convection of Al2O3-water nanofluid in a differentially heated square enclosure with constant temperature hot and cold vertical walls is studied numerically. Transport mechanisms of nanoparticles including Brownian diffusion and thermophoresis that cause heterogeneity are considered in non-homogeneous model. The hot and cold wall temperatures are varied, but the temperature difference between them is always maintained 5 °C. The thermophysical properties such as thermal conductivity, viscosity and density and thermophoresis diffusion and Brownian motion coefficients are considered variable with temperature and volume fraction of nanoparticles. The governing equations are discretized using the control volume method. The results show that nanoparticle transport mechanisms affect buoyancy force and cause formation of small vortexes near the top and bottom walls of the cavity and reduce the heat transfer. By increasing the temperature of the walls the effect of transport mechanisms decreases and due to enhanced convection the heat transfer rate increases.
Institute of Scientific and Technical Information of China (English)
ZhangHeng－Yun; GeXin－Shi
1997-01-01
Heat transfer in the evacuated collector tube is a three-dimensional laminar natural convection problem driven by buoyancy.Because of its complexity,no effective theoretical model is available despite of limited experimental work which is confined to one aspect.The present work aims to depict the convective heat transfer inside a two-ended inclined tube with East-West symmetric heat input using numerical methods,Based on reasonable assumptions,governing equations of the inside fluid are established.The corresponding discretizated equations are solved by emplogying numerical metholds.The calculated results are displayed for velocity and temperature profiles on different cross-sectional plasnes.which present the flow pattern characterized by upflow and downflow along the axial direction and adherent flow along the peripheral direction,and the heat transfer rpocess from the wall to the center,Furthermore,the transient Nusselt number and average temperature level are shown and discussed.Finally,the parametric effects of the tube radius and the heat input on the flow and heat transfer are also given.
Ullah, Imran; Khan, Ilyas; Shafie, Sharidan
2016-11-01
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.
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.
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.
层流自然对流传热的一种二阶投影方法%A SECOND ORDER PROJECTION METHOD FOR LAMINAR NATURAL CONVECTION
Institute of Scientific and Technical Information of China (English)
朱祚金; 解志民
2003-01-01
The paper presents a second order projection method （PmⅢ) whicn can be used to study laminar natural convection numerically by solving its relevant governing equations. Taking the natural convection occurred in an inclined parallel-walled channel as an example, the results of the application of this method to the heat transfer problem were compared with experiments, and an excellent agreement was obtained. This shows that the method given in this paper is reliable and an effective one for numerical heat transfer.
Institute of Scientific and Technical Information of China (English)
Abdullah H. AlEssa; Mohamad I. Al-Widyan
2008-01-01
This study examines the heat transfer enhancement from a horizontal rect- angular fin embedded with triangular perforations (their bases parallel and toward the fin tip) under natural convection. The fin's heat dissipation rate is compared to that of an equivalent solid one. The parameters considered axe geometrical dimensions and thermal properties of the fin and the perforations. The gain in the heat transfer enhancement and the fin weight reduction due to the perforations are considered. The study shows that the heat dissipation from the perforated fin for a certain range of triangular perforation di- mensions and spaces between perforations result in improvement in the heat transfer over the equivalent solid fin. The heat transfer enhancement of the perforated fin increases as the fin thermal conductivity and its thickness are increased.
Natural convection flow of a nano-fluid over a vertical plate with uniform surface heat flux
Energy Technology Data Exchange (ETDEWEB)
Khan, W.A. [Department of Engineering Sciences, National University of Sciences and Technology, Karachi 75350 (Pakistan); Aziz, A. [Department of Mechanical Engineering, School of Engineering and Applied Science, Gonzaga University, Spokane, WA 99258 (United States)
2011-07-15
Natural convective flow of a nano-fluid over a vertical plate with a constant surface heat flux is investigated numerically, following a similarity analysis of the transport equations. The transport model employed includes the effect of Brownian motion and thermophoresis. The analysis shows that velocity, temperature and concentration profiles in the respective boundary layers depend, besides the Prandtl and Lewis numbers, on three additional dimensionless parameters, namely a Brownian motion parameter Nb, a thermophoresis parameter Nt, a buoyancy ratio parameter Nr. In addition to the study of these parameters on the boundary layer flow characteristics (velocity, temperature, nano-particle concentration, skin friction, and heat transfer), correlations for the Nusselt and Sherwood numbers have been developed based on a regression analysis of the data. These correlations predict the numerical results with a maximum error of 5.5% for the reduced Nusselt number and 3.2% for the reduced Sherwood number. (authors)
Energy Technology Data Exchange (ETDEWEB)
Mezrhab, Ahmed; Moussaoui, M A [Laboratoire de Mecanique and Energetique, Departement de Physique, Faculte des Sciences, Universite Mohamed 1, Oujda (Morocco); Naji, H [Universite des Sciences et Technologies de Lille/Polytech' Lille/LML UMR 8107, F-59655 Villeneuve d' Ascq Cedex (France)], E-mail: mezrhab@fso.ump.ma
2008-06-07
A numerical study is carried out for conjugate natural convection-surface radiation heat transfer from a heated circular cylinder enclosed in a square cavity. A hybrid scheme with lattice Boltzmann for fluid velocity variables and finite difference for the temperature is used. The vertical walls of the enclosure are cooled with a uniform temperature while the others are adiabatic. Effects of the Rayleigh number, size and location of the heated cylinder and surface emissivities are investigated numerically. The results are reported in terms of isotherms, streamlines and the average Nusselt number. It is found that (i) the radiation exchange standardizes the temperature inside the cavity and produces an increase in the heat transfer, particularly at large Ra, (ii) the heat transfer enhances with increasing cylinder size and/or surface emissivity and (iii) the maximum rate of heat transfer occurs when the cylinder is located at the horizontal median close to the vertical wall.
Omari, Kamal El; Guer, Yves Le
2010-01-01
The present paper numerically analyzes a passive cooling system using cavities with different geometries filled with thermal conductivity-enhanced phase change material (PCM). A numerical code is developed using an unstructured finite-volume method and an enthalpy-porosity technique to solve for natural convection coupled to a solid-liquid phase change. Five geometries containing the same volume of PCM are compared while cooling the same surface. The unsteady evolution of the melting front and the velocity and temperature fields is detailed. Other indicators of cooling efficiency are monitored, including the maximum temperature reached at the cooled surface. The computational results show the high impact of varying geometry: a maximum temperature difference as high as 40 degrees Celsius is observed between two of the cavities. The best efficiency is obtained for a cavity shifted vertically relative to the cooled surface. Other findings and recommendations are made for the design of PCM-filled cavities.
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.
Directory of Open Access Journals (Sweden)
Gauri Shanker Seth
2015-01-01
Full Text Available An investigation of unsteady hydromagnetic natural convection flow of a viscous, incompressible, electrically conducting and heat absorbing fluid past an impulsively moving vertical plate with Newtonian heating embedded in a porous medium in a rotating system is carried out. The governing partial differential equations are first subjected to Laplace transformation and then inverted numerically using INVLAP routine of Matlab. The governing partial differential equations are also solved numerically by Crank-Nicolson implicit finite difference scheme and a comparison has been provided between the two solutions. The numerical solution for fluid velocity and fluid temperature are depicted graphically whereas the numerical values of skin friction and Nusselt number are presented in tabular form for various values of pertinent flow parameters. Present solution in special case is compared with previously obtained solution and is found to be in excellent agreement.
Institute of Scientific and Technical Information of China (English)
M.A.R.Akhanda
2000-01-01
Experimental study of natural convection heat transfer across air layers bounded by a lower hot rectangular and a square corrugated plates to an upper cold flat plate has been carried out.The surroundings of this space are adiabatic.The effect of the angle of inclination,the aspect ratio,the temperature potential and the Rayleigh number on average heat transfer coefficients are investigated within a range of 0°≤θ≤75°,2.33≤A≤6.33,10°≤ΔT≤35°，and 3.29×104≤RaL≤2.29×106,The developed correlation predicts well the experimental data within an error of ±15%.
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.
Natural convective heat transfer and nanofluid flow in a cavity with top wavy wall and corner heater
Institute of Scientific and Technical Information of China (English)
Mikhail A SHEREMET; Ioan POP; Hakan F ZTOP; Nidal ABU-HAMDEH
2016-01-01
A numerical analysis of natural convection of nanofluid in a wavy-walled enclosure with an isothermal corner heater has been carried out. The cavity is heated from the left bottom corner and cooled from the top wavy wall while the rest walls are adiaba- tic. Mathematical model has been formulated using the single-phase nanofluid approach. Main efforts have been focused on the effects of the dimensionless time, Rayleigh number, undulation number, nanoparticle volume fraction and length of corner heaters on the fluid flow and heat transfer inside the cavity. Numerical results have been presented in the form of streamlines, isotherms, velocity and temperature profiles, local and average Nusselt numbers. It has been found that nanoparticle volume fraction essentially affects both fluid flow and heat transfer while undulation number changes significantly only the heat transfer rate.
Energy Technology Data Exchange (ETDEWEB)
Jin, L.F.; Tou, K.W. [Nanyang Technological Univ., Singapore (Singapore). School of Mechanical and Production Engineering; Tso, C.P. [Multimedia Univ., Melaka (Malaysia). Faculty of Engineering and Technology
2005-09-01
Two-dimensional unsteady numerical studies are made on an air-filled enclosure rotated about its horizontal axis with an array of three rows of heat sources on one of the walls, revealing three physically realizable phenomena, namely, uni-periodic oscillation, multi-periodic oscillation and chaotic oscillation. The evolutionary process of flow field and natural convection characteristics from stationary to rotating situation is studied. Rotation results in an imbalance between clockwise and counter-clockwise circulations, increases heat transfer in the worst scenario, reduces the oscillation of Nusselt number, and improves or reduces mean performance in each cycle. The optimal distribution of heaters in rotating fluid is close to the results in the stationary situation if they have same dominated circulation direction. (author)
Seta, Takeshi
2013-06-01
In the present paper, we apply the implicit-correction method to the immersed-boundary thermal lattice Boltzmann method (IB-TLBM) for the natural convection between two concentric horizontal cylinders and in a square enclosure containing a circular cylinder. The Chapman-Enskog multiscale expansion proves the existence of an extra term in the temperature equation from the source term of the kinetic equation. In order to eliminate the extra term, we redefine the temperature and the source term in the lattice Boltzmann equation. When the relaxation time is less than unity, the new definition of the temperature and source term enhances the accuracy of the thermal lattice Boltzmann method. The implicit-correction method is required in order to calculate the thermal interaction between a fluid and a rigid solid using the redefined temperature. Simulation of the heat conduction between two concentric cylinders indicates that the error at each boundary point of the proposed IB-TLBM is reduced by the increment of the number of Lagrangian points constituting the boundaries. We derive the theoretical relation between a temperature slip at the boundary and the relaxation time and demonstrate that the IB-TLBM requires a small relaxation time in order to avoid temperature distortion around the immersed boundary. The streamline, isotherms, and average Nusselt number calculated by the proposed method agree well with those of previous numerical studies involving natural convection. The proposed IB-TLBM improves the accuracy of the boundary conditions for the temperature and velocity using an adequate discrete area for each of the Lagrangian nodes and reduces the penetration of the streamline on the surface of the body.
Energy Technology Data Exchange (ETDEWEB)
Rajamani, A. [Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036 (India); Sundararajan, T., E-mail: tsundar@iitm.ac.in [Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036 (India); Prasad, B.V.S.S.S. [Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036 (India); Parthasarathy, U.; Velusamy, K. [Nuclear Engineering Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102 (India)
2016-05-15
Highlights: • Transient simulations are performed for a worst case scenario of station black-out. • Inter-wrapper flow between various sub-assemblies reduces peak core temperature. • Various natural convection paths limits fuel clad temperatures below critical level. - Abstract: The 500 MWe Indian pool type Prototype Fast Breeder Reactor (PFBR) has a passive core cooling system, known as the Safety Grade Decay Heat Removal System (SGDHRS) which aids to remove decay heat after shut down phase. Immediately after reactor shut down the fission products in the core continue to generate heat due to beta decay which exponentially decreases with time. In the event of a complete station blackout, the coolant pump system may not be available and the safety grade decay heat removal system transports the decay heat from the core and dissipates it safely to the atmosphere. Apart from SGDHRS, various natural convection loops in the sodium pool carry the heat away from the core and deposit it temporarily in the sodium pool. The buoyancy driven flow through the small inter-wrapper gaps (known as inter-wrapper flow) between fuel subassemblies plays an important role in carrying the decay heat from the sub-assemblies to the hot sodium pool, immediately after reactor shut down. This paper presents the transient prediction of flow and temperature evolution in the reactor subassemblies and the sodium pool, coupled with the safety grade decay heat removal system. It is shown that with a properly sized decay heat exchanger based on liquid sodium and air chimney stacks, the post shutdown decay heat can be safely dissipated to atmospheric air passively.
Directory of Open Access Journals (Sweden)
Hussein Ahmed Mohammed
2005-01-01
Full Text Available Natural convection heat transfer is experimentally investigated for laminar air flow in a vertical circular tube by using the boundary condition of constant wall heat flux in the ranges of (RaL from (1.1*109 to (4.7*109. The experimental set-up was designed for determining the effect of different types of restrictions placed at entry of heated tube in bottom position, on the surface temperature distribution and on the local and average heat transfer coefficients. The apparatus was made with an electrically heated cylinder of a length (900mm and diameter (30mm. The entry restrictions were included a circular tube of same diameter as the heated cylinder but with lengths of (60cm, 120cm, sharp-edge and bell-mouth. The surface temperature along the cylinder surface for same heat flux would be higher values for circular restriction with length of (120cm and would be smaller values for bell-mouth restriction. The results show that the local Nusselt number (Nux and average Nusselt number are higher for bell-mouth restriction and smaller values for (120cm restriction. For all entry shape restrictions, the results show that the Nusselt number values increases as the heat flux increases. From the present work an empirical correlations were obtained in a form of (Log versus (Log for each case investigated and obtained a general correlation for all cases which reveals the effect of restriction existence on the natural convection heat transfer process in a vertical circular tube.
MHD natural convection in an inclined wavy cavity with corner heater filled with a nanofluid
Sheremet, M. A.; Oztop, H. F.; Pop, I.
2016-10-01
A mathematical modelling of MHD free convection in an inclined wavy enclosure filled with a Cu-water nanofluid in the presence of an isothermal corner heater has been carried out. The cavity is heated from the left bottom corner and cooled from the top wavy wall while the rest walls are adiabatic. Uniform magnetic field affects the heat transfer and fluid flow with an inclination angle to the axis xbar. Wavy cavity is inclined to the horizontal direction. Mathematical model formulated using the single-phase nanofluid approach in dimensionless variables stream function, vorticity and temperature has been solved by finite difference method of the second order accuracy in a wide range of governing parameters: Hartmann number (Ha=0-100), inclination angle of the magnetic field (χ = 0 - π) , undulation number (κ=0-4), inclination angle of the cavity (ζ = 0 - π) , solid volume fraction parameter of nanoparticles (φ=0.0-0.05), and dimensionless time (τ=0-0.27). Main efforts have been focused on the effects of these parameters on the fluid flow and heat transfer inside the cavity. Numerical results have been presented in the form of streamlines, isotherms and average Nusselt numbers.
Hawaii Natural Energy Institute annual report, 1984
Energy Technology Data Exchange (ETDEWEB)
1984-01-01
This tenth anniversary special reviews each project over the past 10 years, with research in progress included for FY83-84 for biomass, geothermal, ocean energy, solar energy, wind research and other renewable energy research. (PSB)
Effects of shear in the convective boundary layer: analysis of the turbulent kinetic energy budget
Pino, D.; Vilà-Guerau de Arellano, J.
2008-01-01
Effects of convective and mechanical turbulence at the entrainment zone are studied through the use of systematic Large-Eddy Simulation (LES) experiments. Five LES experiments with different shear characteristics in the quasi-steady barotropic boundary layer were conducted by increasing the value of
Directory of Open Access Journals (Sweden)
Kh. Abdul Maleque
2013-01-01
Full Text Available A local similarity solution of unsteady MHD natural convection heat and mass transfer boundary layer flow past a flat porous plate within the presence of thermal radiation is investigated. The effects of exothermic and endothermic chemical reactions with Arrhenius activation energy on the velocity, temperature, and concentration are also studied in this paper. The governing partial differential equations are reduced to ordinary differential equations by introducing locally similarity transformation (Maleque (2010. Numerical solutions to the reduced nonlinear similarity equations are then obtained by adopting Runge-Kutta and shooting methods using the Nachtsheim-Swigert iteration technique. The results of the numerical solution are obtained for both steady and unsteady cases then presented graphically in the form of velocity, temperature, and concentration profiles. Comparison has been made for steady flow ( and shows excellent agreement with Bestman (1990, hence encouragement for the use of the present computations.
Ahmed, Mahmoud; Eslamian, Morteza
2015-07-01
Laminar natural convection in differentially heated ( β = 0°, where β is the inclination angle), inclined ( β = 30° and 60°), and bottom-heated ( β = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation. The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors. Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect. Here we show that thermophoresis has a considerable effect on heat transfer augmentation in laminar natural convection. Our non-homogenous modeling approach shows that heat transfer in nanofluids is a function of the inclination angle and Ra number. It also reveals some details of flow behavior which cannot be captured by single-phase models. The minimum heat transfer rate is associated with β = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number.
Ahmed, Mahmoud; Eslamian, Morteza
2015-12-01
Laminar natural convection in differentially heated (β = 0°, where β is the inclination angle), inclined (β = 30° and 60°), and bottom-heated (β = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation. The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors. Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect. Here we show that thermophoresis has a considerable effect on heat transfer augmentation in laminar natural convection. Our non-homogenous modeling approach shows that heat transfer in nanofluids is a function of the inclination angle and Ra number. It also reveals some details of flow behavior which cannot be captured by single-phase models. The minimum heat transfer rate is associated with β = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number.
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
Energy Technology Data Exchange (ETDEWEB)
Bae, Young Min; Kim, Seong Hoon; Seo, Jae Kwang; Kim, Young In [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)
2016-08-15
In this study, laminar natural circulation and heat transfer in a tall rectangular enclosure with disconnected vertical partitions inside were investigated. Analytical expressions were developed to predict the circulation flow rate and the average Nusselt number in a partially partitioned enclosure with isothermal side walls at different temperatures and insulated top and bottom walls. The proposed formulas are then validated against numerical results for modified Rayleigh numbers of up to 106. The impacts of the governing parameters are also examined along with a discussion of the heat transfer regimes.
On the Holographic Nature Of Rindler Energy
Halyo, Edi
2014-01-01
We show that the dimensionless Rindler energy of a black hole, $E_R$, is exactly the surface Hamiltonian obtained from the Einstein--Hilbert action evaluated on the horizon. Therefore, $E_R$ is given by a surface integral over the horizon and manifestly holographic. In the context of the AdS/CFT duality, Rindler energy corresponds, on the boundary, to a dimensionless energy given by the product of the AdS radius and the extensive part of the CFT energy. We find that, beyond General Relativity, $E_R$ is still holographic but not necessarily given by the surface Hamiltonian of the theory.
Identifying the nature of high energy Astroparticles
Mora, Karen Salomé Caballero
2016-01-01
High energy Astroparticles include Cosmic Ray, gamma ray and neutrinos, all of them coming from the universe. The origin and production, acceleration and propagation mechanisms of ultrahigh-energy CR (up to $10^{20}$ eV) are still unknown. Knowledge on particle interactions taking place at those energies, useful for studying current theories on particle physics, can be obtained only from measurements of high energy astroparticles. In the present document some techniques on data analysis of mass composition of UHECR with the Pierre Auger Observatory are described. The relevance of the muon component of air showers produced by the primary CR, as well as some low energy simulations of that component, are explained.
Effect of adiabatic square ribs on natural convection in an asymmetrically heated channel
Abidi-Saad, Aissa; Kadja, Mahfoud; Popa, Catalin; Polidori, Guillaume
2017-02-01
A 2-D numerical simulation is carried out to investigate the effect of two adiabatic square ribs on laminar flow and heat transfer in an asymmetrically heated channel. The two ribs are symmetrically located on each wall, exactly above the heating zone. The computational procedure is made by solving the unsteady bi-dimensional continuity, momentum and energy equations with the finite volume method. The investigations focused more specifically on the influence of ribs sizes on the flow structure and heat transfer enhancement. The results showed that the variation of ribs sizes significantly alters the heat transfer and fluid flow distribution along the channel, especially in the vicinity of protrusions. Also, the results show that streamlines, isotherms, and the number, sizes and formation of vortex structures inside the channel strongly depend on the size of protrusions. The changes in heat transfer parameters have also been presented.
Effects of T-type Channel on Natural Convection Flows in Airflow-Path of Concrete Storage Cask
Energy Technology Data Exchange (ETDEWEB)
Kang, Gyeong Uk; Kim, Hyoung Jin; Cho, Chun Hyung [KORAD, Daejeon (Korea, Republic of)
2016-05-15
The natural convection flows occurring in airflow-path are not simple due to complex flow-path configurations such as horizontal ducts, bent tube and annular flow-path. In addition, 16 T type channels acting as the shroud are attached vertically and 16 channel supporting the canister are attached horizontally on the inner surface of over-pack. The existence and nonexistence of T type channels have influences on the flow fields in airflow- path. The concrete storage cask has to satisfy the requirements to secure the thermal integrity under the normal, off-normal, and accident conditions. The present work is aiming at investigating the effects of T type channels on the flows in airflow-path under the normal conditions using the FLUENT 16.1 code. In order to focus on the flows in airflow-path, fuel regions in the canister are regarded as a single cylinder with heat sources and other components are fully modeled. This study investigated the flow fields in airflow-path of concrete storage cask, numerically. It was found that excepting for the fuel regions, maximum temperatures on other components were evaluated below allowable values. The location of maximum velocities depended on support channels, T type channels and flow area. The flows through air inlets developed along annular flow- path with forming the hot plumes. According to the existence and nonexistence of T type channel, the plume behavior showed the different flow patterns.
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)
S. S. Das, S. Parija, R. K. Padhy, M. Sahu
2012-01-01
Full Text Available This paper investigates the natural convection unsteady magnetohydrodynamic mass transfer flow of a viscous incompressible electrically conducting fluid past an infinite vertical porous flat plate in presence of constant suction and heat sink. Using multi parameter perturbation technique, the governing equations of the flow field are solved and approximate solutions are obtained. The effects of the flow parameters on the velocity, temperature, concentration distribution and also on the skin friction and rate of heat transfer are discussed with the help of figures and table. It is observed that a growing magnetic parameter or Schmidt number or heat sink parameter leads to retard the transient velocity of the flow field at all points, while the Grashof numbers for heat and mass transfer show the reverse effect. It is further found that a growing Prandtl number or heat sink parameter decreases the transient temperature of the flow field at all points while the heat source parameter reverses the effect. The concentration distribution of the flow field suffers a decrease in boundary layer thickness in presence of heavier diffusive species (growing Sc at all points of the flow field. The effect of increasing Prandtl number Pr is to decrease the magnitude of skin-friction and to increase the rate of heat transfer at the wall for MHD flow, while the effect of increasing magnetic parameter M is to decrease their values at all points.
Zavala-Guillén, I.; Xamán, J.; Álvarez, G.; Arce, J.; Hernández-Pérez, I.; Gijón-Rivera, M.
2016-03-01
This study reports the modeling of the turbulent natural convection in a double air-channel solar chimney (SC-DC) and its comparison with a single air-channel solar chimney (SC-C). Prediction of the mass flow and the thermal behavior of the SC-DC were obtained under three different climates of Mexico during one summer day. The climates correspond to: tropical savannah (Mérida), arid desert (Hermosillo) and temperate with warm summer (Mexico City). A code based on the Finite Volume Method was developed and a k-ω turbulence model has been used to model air turbulence in the solar chimney (SC). The code was validated against experimental data. The results indicate that during the day the SC-DC extracts about 50% more mass flow than the SC-C. When the SC-DC is located in Mérida, Hermosillo and Mexico City, the air-changes extracted along the day were 60, 63 and 52, respectively. The air temperature at the outlet of the chimney increased up to 33%, 38% and 61% with respect to the temperature it has at the inlet for Mérida, Hermosillo and Mexico City, respectively.
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.
Analysis of the Natural Convection Flow in the Upper Plenum of the MONJU Reactor with Trio_U
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Ulrich Bieder
2013-01-01
Full Text Available The IAEA has coordinated a benchmark project on natural convection phenomena in the upper plenum of the MONJU reactor. JAEA has provided both detailed geometrical data of the plant and complete thermalhydraulic boundary conditions describing a pump trip transient, accomplished during the start-up experiments of the reactor. For the initial conditions of the pump trip transient, extensive sensitivity analyses have been made with the CFD code Trio_U. These calculations show a high sensitivity of the global flow pattern in the MONJU upper plenum depending on the initial order of the numerical scheme and the modelling of the geometrically complex upper core structure. During the pump trip, the formation of a thermal stratification within the plenum has been observed which persists for almost two hours. All calculations have shown a homogenization of the temperature in the plenum after about 15 minutes. A slight reduction of the mixing in the upper plenum could have been achieved by modifying the form of the flow holes in the inner barrel (fillets instead of sharp edges in order to reduce their axial pressure loss.
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Fakoor Pakdaman, M.; Lashkari, A.; Basirat Tabrizi, H.; Hosseini, R. [Department of Mechanical Engineering, Amirkabir University of Technology, Tehran (Iran, Islamic Republic of)
2011-02-15
This paper deals with an experimental investigation to evaluate different thermal characteristics of a natural-convection flat-plate solar air-heater with longitudinal rectangular fins array. Having determined the thermal performance of the system a Nusselt number correlation is presented for such finned duct devices. In the presented empirical model which may have industrial applications, solar radiation and ambient temperature have been considered as independent parameters. Other characteristics of the system such as different dimensionless variables, plates and outflow temperatures, efficiency, and mass flow rate have been empirically modeled based on these variables. The particular difference in this study in comparison with the other similar studies is the presentation of an empirical model for rectangular-finned solar air-heaters. This model proposes design concepts and rules of thumb, and demonstrates the calculations of the design parameters. Based on the order of magnitude analysis, solar radiation has been found to be the main parameter which characterizes the thermal behavior of the system. Besides, exergy analysis has been carried out, and optimum conditions in which the system has the highest performance have been determined. (author)
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Teamah, Mohamed A.; Elsafty, Ahmed F.; Massoud, Enass Z. [Mechanical Engineering Department, College of Engineering and Technology, Arab Academy for Science, Technology and Maritime Transport (Egypt)
2012-02-15
Double-diffusive natural convective flow in an inclined rectangular enclosure with the shortest sides being insulated and impermeable is investigated numerically. Constant temperatures and concentration are imposed along the longest sides of the enclosure. In addition, a uniform magnetic field is applied perpendicular to the longest sides. Laminar regime is considered under steady-state condition. The transport equations for continuity, momentum, energy and species transfer are solved using the finite volume technique. The validity of the numerical code used is ascertained and good agreement was found with published results. The numerical results are reported for the effect of thermal Rayleigh number on the contours of streamline, temperature, and concentration. In addition, results for the average Nusselt and Sherwood numbers are presented and discussed for various parametric conditions. This study is done for constant Prandtl number, Pr = 0.7; aspect ratio, A = 2 and Lewis number, Le = 2. Computations are carried out for thermal Rayleigh number ranging from 10{sup 3} to 5 x 10{sup 5}, inclination angle range of 0 deg. {<=} {gamma} {<=} 180 deg., dimensionless heat generation and absorption coefficients range of -40 {<=} {phi} {<=} 40, buoyancy ratio range of -5 {<=} N{<=} 5 and the Hartmann number range of 0{<=} Ha {<=} 70. (authors)
Zannouni, K.; El Abrach, H.; Dhahri, H.; Mhimid, A.
2016-12-01
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.
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.
Directory of Open Access Journals (Sweden)
Chamkha Ali
2011-01-01
Full Text Available Abstract A boundary layer analysis is presented for the mixed convection past a vertical wedge in a porous medium saturated with a nano fluid. The governing partial differential equations are transformed into a set of non-similar equations and solved numerically by an efficient, implicit, iterative, finite-difference method. A parametric study illustrating the influence of various physical parameters is performed. Numerical results for the velocity, temperature, and nanoparticles volume fraction profiles, as well as the friction factor, surface heat and mass transfer rates have been presented for parametric variations of the buoyancy ratio parameter Nr, Brownian motion parameter Nb, thermophoresis parameter Nt, and Lewis number Le. The dependency of the friction factor, surface heat transfer rate (Nusselt number, and mass transfer rate (Sherwood number on these parameters has been discussed.
Natural inflation and low energy supersymmetry
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Rolf Kappl
2015-06-01
Full Text Available Natural (axionic inflation provides a well-motivated and predictive scheme for the description of the early universe. It leads to sizeable primordial tensor modes and thus a high mass scale of the inflationary potential. Naïvely this seems to be at odds with low (TeV scale supersymmetry, especially when embedded in superstring theory. We show that low scale supersymmetry is compatible with natural (high scale inflation. The mechanism requires the presence of two axions that are provided through the moduli of string theory.
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Volodin, V.I.; Mikhalevich, A.A.; Nesterenko, V.B.
1983-01-01
The optimum parameters of minimum-weight finned tubes for the case of heat transfer by free convection and radiation are determined using the mathematical model with boundary conditions of the 3-type by the method of calculational experiment. The effects of the coolant and air temperatures diameter and thickness of the bimetallic wall, contamination and other factors on optimal finning parameters are investigated. It is obtained that for the case of heat transfer by free convection and radiation, the optimum finning parameters for the heat transfer surface of minimum-weight tubes by absolute value are larger than those of tube finning with forced convection.
Scherf, A.; Roth, R.
1996-12-01
During the field campaign of EFEDA II several aircraft measurements were performed in order to evaluate area mean values of turbulent energy fluxes over a relatively flat terrain in a desertification threatened area in Spain. Since earlier field experiments indicated differences between airborne measurements and surface observations, we tried to close the gap by carefully analysing the turbulence measurements. In order to evaluate the influence of the temporal variation of the convective boundary layer, the rise of the inversion, derived from simultaneously performed radiosonde ascents, was taken into account. By estimating the linear approximated fields of the meteorological parameters, it was possible to calculate the mean values of these quantities as well as the temporal and spatial derivatives, which are necessary for the evaluation of the advective terms of the energy budget. In this way is possible to examine the terms of the conservation equations in a supplementary way.
Throckmorton, D. A.
1982-01-01
Temperatures measured at the aerodynamic surface of the Orbiter's thermal protection system (TPS), and calorimeter measurements, are used to determine heating rates to the TPS surface during atmospheric entry. On the Orbiter leeside, where convective heating rates are low, it is possible that a significant portion of the total energy input may result from solar radiation, and for the wing, cross radiation from the hot (relatively) Orbiter fuselage. In order to account for the potential impact of these sources, values of solar- and cross-radiation heat transfer are computed, based upon vehicle trajectory and attitude information and measured surface temperatures. Leeside heat-transfer data from the STS-2 mission are presented, and the significance of solar radiation and fuselage-to-wing cross-radiation contributions to total energy input to Orbiter leeside surfaces is assessed.
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.
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.
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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)
Internal Wave Generation by Convection
Lecoanet, Daniel
2016-01-01
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 liq...
Zheng, Dong; Zhang, Yijun; Meng, Qing; Chen, Luwen; Dan, Jianru
2016-03-01
This study examined lightning activity and its relationship to precipitation and convective available potential energy (CAPE) in South China during 2001-12, based on data from the Guangdong Lightning Location System, the Tropical Rainfall Measuring Mission satellite, and the ERA-Interim dataset. Two areas of high lightning density are identified: one over the Pearl River Delta, and the other to the north of Leizhou Peninsula. Large peak-current cloud-to-ground (LPCCG) lightning (>75 kA) shows weaker land-offshore contrasts than total CG lightning, in which negative cloud-to-ground (NCG) lightning occurs more prominently than positive cloud-to-ground (PCG) lightning on land. While the frequency of total CG lightning shows a main peak in June and a second peak in August, the LPCCG lightning over land shows only a single peak in June. The ratio of positive LPCCG to total lightning is significantly greater during February-April than during other times of the year. Diurnally, CG lightning over land shows only one peak in the afternoon, whereas CG lightning offshore shows morning and afternoon peaks. The rain yield per flash is on the order of 107-108 kg per flash across the analysis region, and its spatial distribution is opposite to that of lightning density. Our data show that lightning activity over land is more sensitive than that over offshore waters to CAPE. The relationships between lightning activity and both precipitation and CAPE are associated with convection activity in the analysis region.
Institute of Scientific and Technical Information of China (English)
无
2010-01-01
Natural convection of air is numerically simulated in a 3-dimensional rectangular cavity heated from below using SIMPLE algorithm with a QUICK scheme.The results suggest that when all lateral walls are adiabatic,the fluid rolls occur along the long axis.When the Rayleigh number is smaller,the flow is of 2-dimensional character,and the rolls shapes are similar.The average Nusselt numbers in the central part of the cavity are similar.The average Nusselt numbers in the part near by the cavity are different.According to the comparison of 3-dimensional results with 2-dimensional results,the flow patterns and heat transfer in the central part of the cavity can be assumed as a 2-dimensional flow,While those in the part near by the cavity can not.With increasing Rayleigh number,the flow is 3-dimensional characteristic.The 3-dimensional result accords with the experimental result.When all lateral walls are adiabatic,the ten rolls occur along the long axis.But when lateral walls are heated or cooled,the rolls disappear along the long axis and two rolls occur along the short axis.The rotation direction of the rolls is reversed.When Rayleigh number is over some critical value,flow and heat transfer will be asymmetry,indicating unsteady oscillation occurs.By nonlinear analyses,it is shown that with increasing Rayleigh number,flow and heat transfer will change from steady state to unsteady state through HOPF bifurcation,and transition to chaos will occur through multi-periodical oscillation.
Soucasse, L.; Rivière, Ph.; Soufiani, A.; Xin, S.; Le Quéré, P.
2014-02-01
The transition to unsteadiness and the dynamics of weakly turbulent natural convection, coupled to wall or gas radiation in a differentially heated cubical cavity with adiabatic lateral walls, are studied numerically. The working fluid is air with small contents of water vapor and carbon dioxide whose infrared spectral radiative properties are modelled by the absorption distribution function model. A pseudo spectral Chebyshev collocation method is used to solve the flow field equations and is coupled to a direct ray tracing method for radiation transport. Flow structures are identified by means of either the proper orthogonal decomposition or the dynamic mode decomposition methods. We first retrieve the classical mechanism of transition to unsteadiness without radiation, characterized by counter-rotating streamwise-oriented vortices generated at the exit of the vertical boundary layers. Wall radiation through a transparent medium leads to a homogenization of lateral wall temperatures and the resulting transition mechanism is similar to that obtained with perfectly conducting lateral walls. The transition is due to an unstable stratification upstream the vertical boundary layers and is characterized by periodically oscillating transverse rolls of axis perpendicular to the main flow. When molecular gas radiation is accounted for, no periodic solution is found and the transition to unsteadiness displays complex structures with chimneys-like rolls whose axes are again parallel to the main flow. The origin of this instability is probably due to centrifugal forces, as suggested previously for the case without radiation. Above the transition to unsteadiness, at Ra = 3 × 108, it is shown that both wall and gas radiation significantly intensify turbulent fluctuations, decrease the thermal stratification in the core of the cavity, and increase the global circulation.
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Ngo, Ich Long; Byon, Chan [Yeungnam University, Gyeongsan (Korea, Republic of)
2015-07-15
Finite element method was used to investigate the effects of heater location and heater size on the natural convection heat transfer in a 2D square cavity heated partially or fully from below and cooled from above. Rayleigh number (5 X 10{sup 2} ≤ Ra ≤ 5X10{sup 5}), heater size (0.1 ≤ D/L ≤ 1.0), and heater location (0.1 ≤ x{sub h}/L ≤ 0.5) were considered. Numerical results indicated that the average Nusselt number (Nu{sub m}) increases as the heater size decreases. In addition, when x{sub h}/L is less than 0.4, Nu{sub m} increases as x{sub h}/L increases, and Num decreases again for a larger value of x{sub h}/L. However, this trend changes when Ra is less than 10{sup 4}, suggesting that Nu{sub m} attains its maximum value at the region close to the bottom surface center. This study aims to gain insight into the behaviors of natural convection in order to potentially improve internal natural convection heat transfer.
Batubara, Fatimah; Misran, Erni; Dina, Sari Farah; Heppy
2017-06-01
Research on potato drying using the indirect solar dryer with flat-plate finned collector and forced convection has been done. The research was conducted at the outdoor field of Laboratory of Institute for Research and Standardization of Industry on June 14th-23rd, 2016 from 9:00 am to 4:00 pm. This research aims to obtain the drying kinetics model of potato (Solanumtuberosum L.) using an indirect solar dryer's (ISD) with flat plate-finned collector and forced convection. The result will be compared to the open sun drying (OSD) method. Weather conditions during the drying process took place as follows; surrounding air temperature was in the range 27 to 34.7 °C, relative humidity (RH) 29.5 to 61.0% and the intensity of solar radiation 105.6 to 863.1 Watt/m2. The dried potato thicknesses were 1.0 cm, 1.5 cm and 2.0 cm, with the average initial water content of 76.46%. The average temperature in the collector chamber ranged from 42.2 to 57.4 °C and the drying chamber was at 46.2 °C. The best drying result was obtained from a sample size of 1 cm thickness using the IDS method with an average drying rate of 0.018 kg H2O per kg dry-weight.hour and the water content was constant at 5.02% in 21 hours of drying time. The most suitable kinetics model is Page model, equation MR = exp (-0.049 t1,336) for 1.0 cm thickness, exp (-0.066 t1,222) for 1.5 cm thickness and exp (-0.049 t1,221) for 2.0 cm thickness. The quality of potato drying using ISD method is better than using OSD which can be seen from the color produced.
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Hughes, Richard [Louisiana State Univ., Baton Rouge, LA (United States); Tyagi, Mayank [Louisiana State Univ., Baton Rouge, LA (United States); Radonjic, Mileva [Louisiana State Univ., Baton Rouge, LA (United States); Dahi, Arash [Louisiana State Univ., Baton Rouge, LA (United States); Wang, Fahui [Louisiana State Univ., Baton Rouge, LA (United States); John, Chacko [Louisiana State Univ., Baton Rouge, LA (United States); Kaiser, Mark [Louisiana State Univ., Baton Rouge, LA (United States); Snyder, Brian [Louisiana State Univ., Baton Rouge, LA (United States); Sears, Stephen [Louisiana State Univ., Baton Rouge, LA (United States)
2017-07-07
This project is intended to demonstrate the technical and economic feasibility, and environmental and social attractiveness of a novel method of heat extraction from geothermal reservoirs. The emphasis is on assessing the potential for a heat extraction method that couples forced and free convection to maximize extraction efficiency. The heat extraction concept is enhanced by considering wellbore energy conversion, which may include only a boiler for a working fluid, or perhaps a complete boiler, turbine, and condenser cycle within the wellbore. The feasibility of this system depends on maintaining mechanical and hydraulic integrity of the wellbore, so the material properties of the casing-cement system are examined both experimentally and with well design calculations. The attractiveness depends on mitigation of seismic and subsidence risks, economic performance, environmental impact, and social impact – all of which are assessed as components of this study.
Sumner, L. B. S.; Neitzel, G. P.; Fontaine, J.-P.; Dell'Aversana, P.
2001-01-01
Laboratory experimentation, numerical simulation, and energy-stability theory are used to examine the effect of interface deformation on the onset of oscillatory thermocapillary convection in half zones. Experiments are performed to map the stability boundaries marking the onset of oscillatory flow, modifying the free-surface deformation by adjusting the volume of liquid in the bridge. The stability results presented here along with those of other researchers [Monti et al., Proceedings of the 43rd Cong. Int. Artro. Fed. (1992); Hu et al., J. Cryst. Growth 142, 379 (1994)] show that free-surface curvature can have a pronounced influence on flow stability. Steady, axisymmetric flow simulations are computed using the commercial code FIDAP to model the conditions of the experiments, and reveal that flow structure near the stability boundary is sensitive to several parameters. Energy theory is applied to these simulations to determine sufficient conditions for stability. Comparisons between the theoretical and experimental results show nonconservative energy limits falling above the experimentally determined stability boundaries for bridges of various liquid volumes. While the trend of the experimental data is predicted for zones of large volume ratio (bulging zones), the same cannot be said for those with small volume ratio (necked-down zones). In addition, energy-stability limits for some undeformed-free-surface cases were determined which are above the linear-stability limits determined by other researchers, in clear contradiction of the roles of the respective theories.
Hawaii Natural Energy Institute: Annual report, 1992
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1992-01-01
This progress report from the University of Hawaii at Manoa's School of Ocean and Earth Science and Technology describes state of the art research in tapping the energy in and around the Hawaiian Islands. Researchers are seeking new ways of generating electricity and producing methanol from sugarcane waste and other biomass. They are finding ways to encourage the expanded use of methanol as a transportation fuel. They are creating innovative and cost-efficient methods of producing and storing hydrogen gas, considered the fuel of the future''. Researchers are also developing the techniques and technologies that will enable us to tap the unlimited mineral resources of the surrounding ocean. they are testing methods of using the oceans to reduce the carbon dioxide being discharged to the atmosphere. And they are mapping the strategies by which the seas can become a major source of food, precious metals, and space for living and for industry. The achievements described in this annual report can be attributed to the experience, creativity, painstaking study, perseverance, and sacrifices of our the dedicated corps of researchers.
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.
Kumar, Varun; Kumar, Manoj; Shakher, Chandra
2014-09-20
In this paper, the local convective heat transfer coefficient (h) is measured along the surface of an electrically heated vertical wire using digital holographic interferometry (DHI). Experiments are conducted on wires of different diameters. The experimentally measured values are within the range as given in the literature. DHI is expected to provide a more accurate local convective heat transfer coefficient (h) as the value of the temperature gradient required for the calculation of "h" can be obtained more accurately than by other existing optical interferometric techniques without the use of a phase shifting technique. This is because in digital holography phase measurement accuracy is expected to be higher.
Sandholt, P.; Farrugia, C. J.; Andalsvik, Y.
2012-12-01
The aim of this study is to investigate the contributions of substorm processes to temporal structure of polar cap plasma convection. The central parameter is the cross-polar cap potential (CPCP). Selecting a ten hour-long interval of stable interplanetary driving by an interplanetary CME (ICME), we are able to distinguish between the dayside and nightside sources of the convection. The event was initiated by an abrupt enhancement of the magnetopause (MP) reconnection rate triggered by a southward turning of the ICME magnetic field. This was followed by long interval (ten hours) of steady and strong driving. Under the latter condition a long series of electrojet intensifications (polar cap contractions) was observed which recurred at 50 min. intervals. The detailed temporal structure of polar cap convection in relation to the polar cap contraction events is obtained by combining continuous ground observations of convection - related magnetic deflections (including polar cap magnetic indices in the northern and southern hemispheres, PCN and PCS) and the more direct but lower resolution ion drift data obtained from a satellite (DMSP F13) in polar orbit. The observed PCN enhancements combined with satellite observations (DMSP F13 and F15 data) of polar cap contractions during the evolution of selected substorm expansions allowed us to calculate the CPCP enhancements associated with each event in the series.
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Espana Estrada, Juan M; Poujol Galvan, Federico T [Universidad Autonoma de Baja California Sur, Mexico, La Paz, B.C.S. (Mexico); Fernandez Zayas, Jose L [Instituto de Ingenieria de la UNAM, Mexico, D.F. (Mexico)
2000-07-01
A preliminary study transient on the natural convection that occurs within a laboratory still is presented. The reason for this work was to have a better understanding of the transport phenomena, to determine the manner in which it produces an effect in the operation of booth type still. The evolution of the temperature patterns and relation to the flow that is visualized in the interior of the still, with a constant temperature increase on the base of the same analyzed. The still used is composed of a stainless steel base of 1 x 0.5 m under which distilled water is made to circulate at a given temperature. The upper walls, that form an isosceles triangle of 45 Celsius degrees with a height of 0.40 m, are composed two layers of transparent glass in the middle of which the water is made to circulate at a controlled temperature. The flow patterns were visualized using smoke, illuminated with a laser sheet. Likewise, measurements of the temperatures taken with thermocouples strategically situated in the interior of the experimental device were registered. The experimental results demonstrated the initial evolution of the movement of the fluid, as well as the variation of temperature at different positions within the laboratory still throughout the course of the test. [Spanish] Se presentan los avances de un estudio preliminar sobre la conveccion natural en estado transitorio, que ocurre dentro de un destilador de laboratorio. El motivo de este trabajo fue comprender mejor los fenomenos de transporte, para determinar su influencia en la operacion de destiladores solares tipo caseta. Se analiza la evolucion de los patrones de temperatura y su relacion con el flujo que se visualiza en el interior del destilador, el cual esta llenado con agua destilada, bajo condiciones controladas en las paredes del destilador, con un incremento constante de temperatura en la base del mismo. El destilador utilizado consta de una base de acero inoxidable de 1 x 0.5 m debajo de la cual se
Conceptualising energy security and making explicit its polysemic nature
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Chester, Lynne [The John Curtin Institute of Public Policy, Curtin University, GPO Box U1987, Perth WA 6845 (Australia)
2010-02-15
Twenty-first century access to energy sources depends on a complex system of global markets, vast cross-border infrastructure networks, a small group of primary energy suppliers, and interdependencies with financial markets and technology. This is the context in which energy security has risen high on the policy agenda of governments around the world and the term 'energy security' has quietly slipped into the energy lexicon. The limited discourse about the nature of the term or its underlying assumptions has been totally eclipsed by an almost overwhelming focus on securing supplies of primary energy sources and geopolitics. An examination of explicit and inferred definitions finds that the concept of energy security is inherently slippery because it is polysemic in nature, capable of holding multiple dimensions and taking on different specificities depending on the country (or continent), timeframe or energy source to which it is applied. This 'slipperiness' poses analytical, prediction and policy difficulties but if explicitly recognised through definitional clarity, new levels of understanding will enrich the policy debate to deal with obstacles impacting on the constantly evolving nature of energy security. (author)
2010-03-22
... Maritime Administration TORP Terminal LP, Bienville Offshore Energy Terminal Liquefied Natural Gas...) for the TORP Terminal LP, Bienville Offshore Energy Terminal (BOET) Liquefied Natural Gas (LNG... Natural Gas Pipeline, Williams Natural Gas Pipeline, Destin Natural Gas Pipeline, and Viosca...
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
Energy Technology Data Exchange (ETDEWEB)
Wiser, Ryan; Bolinger, Mark; St. Clair, Matt
2004-12-21
Heightened natural gas prices have emerged as a key energy-policy challenge for at least the early part of the 21st century. With the recent run-up in gas prices and the expected continuation of volatile and high prices in the near future, a growing number of voices are calling for increased diversification of energy supplies. Proponents of renewable energy and energy efficiency identify these clean energy sources as an important part of the solution. Increased deployment of renewable energy (RE) and energy efficiency (EE) can hedge natural gas price risk in more than one way, but this paper touches on just one potential benefit: displacement of gas-fired electricity generation, which reduces natural gas demand and thus puts downward pressure on gas prices. Many recent modeling studies of increased RE and EE deployment have demonstrated that this ''secondary'' effect of lowering natural gas prices could be significant; as a result, this effect is increasingly cited as justification for policies promoting RE and EE. This paper summarizes recent studies that have evaluated the gas-price-reduction effect of RE and EE deployment, analyzes the results of these studies in light of economic theory and other research, reviews the reasonableness of the effect as portrayed in modeling studies, and develops a simple tool that can be used to evaluate the impact of RE and EE on gas prices without relying on a complex national energy model. Key findings are summarized.
Restoring Equilibrium to Natural Gas Markets: Can Renewable Energy Help?
Energy Technology Data Exchange (ETDEWEB)
Wiser, Ryan; Bolinger, Mark
2005-01-01
Heightened natural gas prices have emerged as a key energy-policy challenge for at least the early part of the 21st century. With the recent run-up in gas prices and the expected continuation of volatile and high prices in the near future, a growing number of voices are calling for increased diversification of energy supplies. Proponents of renewable energy technologies identify these clean energy sources as an important part of the solution. Increased deployment of renewable energy (RE) can hedge natural gas price risk in more than one way, but a recent report by Berkeley Lab evaluates one such benefit in detail: by displacing gas-fired electricity generation, RE reduces natural gas demand and thus puts downward pressure on gas prices. Many recent modeling studies of increased RE deployment have demonstrated that this ''secondary'' effect of lowering natural gas prices could be significant; as a result, this effect is increasingly cited as justification for policies promoting RE. The Berkeley Lab report summarizes recent modeling studies that have evaluated the impact of RE deployment on gas prices, reviews the reasonableness of the results of these studies in light of economic theory and other research, and develops a simple tool that can be used to evaluate the impact of RE on gas prices without relying on a complex national energy model.
Kim, Chae Bin; Janes, Dustin; Arshad, Talha; Katzenstein, Joshua; Prisco, Nathan; McGuffin, Dana; Bonnecaze, Roger; Ellison, Christopher
2015-03-01
The Marangoni effect describes how fluid flows in response to gradients in surface energy. We recently developed a method for photochemically preprograming spatial surface energy patterns in glassy polystyrene (PS) thin films. UV irradiation through a mask selectively dehydrogenates the PS, thus increasing surface energy in the UV exposed regions compared to the unexposed regions. After heating the film to the liquid state, transport of polymer occurs from regions of low surface energy to regions of high surface energy. This method can be harnessed to rapidly manufacture polymer films possessing prescribed three-dimensional topographies reflective of the original light exposure pattern. To quantify and verify this phenomenon, a theoretical model that gives a more thorough understanding of the physics of this process, its limits and ways to apply it efficiently for various target metrics will also be presented along with comparisons between theoretical predictions and experimental observations. Finally, while PS dehydrogenation can be used to produce a variety of topographical patterns, judicious selection of the photosensitizing compounds in an otherwise transparent polymer expands the use of this method to more readily available light sources.
Potential for natural evaporation as a reliable renewable energy resource.
Cavusoglu, Ahmet-Hamdi; Chen, Xi; Gentine, Pierre; Sahin, Ozgur
2017-09-26
About 50% of the solar energy absorbed at the Earth's surface drives evaporation, fueling the water cycle that affects various renewable energy resources, such as wind and hydropower. Recent advances demonstrate our nascent ability to convert evaporation energy into work, yet there is little understanding about the potential of this resource. Here we study the energy available from natural evaporation to predict the potential of this ubiquitous resource. We find that natural evaporation from open water surfaces could provide power densities comparable to current wind and solar technologies while cutting evaporative water losses by nearly half. We estimate up to 325 GW of power is potentially available in the United States. Strikingly, water's large heat capacity is sufficient to control power output by storing excess energy when demand is low, thus reducing intermittency and improving reliability. Our findings motivate the improvement of materials and devices that convert energy from evaporation.The evaporation of water represents an alternative source of renewable energy. Building on previous models of evaporation, Cavusoglu et al. show that the power available from this natural resource is comparable to wind and solar power, yet it does not suffer as much from varying weather conditions.
Yang, Xuegeng; Mühlenhoff, Sascha; Nikrityuk, Petr A.; Eckert, Kerstin
2013-03-01
Magnetic fields are well-established in electrochemistry as an attractive tool to improve both the quality of the deposit as well as the deposition rate. The key mechanism is a mass transfer enhancement by Lorentz-force-driven convection. However, during electrolysis this convection interacts with buoyancy-driven convection, which arises from concentration differences, in a sometimes intriguing way. In the case of a Lorentz force opposing buoyancy, this is due to the growth of a bubble-like zone of less-concentrated cupric ion solution at the lower part of the vertical cathode when copper electrolysis is performed. If buoyancy is strong enough to compete with the Lorentz force, this zone rises along the cathode and causes surprisingly unsteady initial transient behaviour. We explore this initial transient under galvanostatic conditions by analyzing the development of the concentration and velocity boundary layers obtained by Mach-Zehnder interferometry and particle image velocimetry. Particular attention is also paid to higher current densities above the limiting current, obtained from potentiodynamic measurements, at which a chaotic advection takes place. The results are compared by scaling analysis.
Energy Technology Data Exchange (ETDEWEB)
Sousa, Eduardo F. de [Universidade Salvador (UNIFACS), BA (Brazil)
2010-07-01
Natural gas is a major source of non-renewable energy in the Brazilian energy matrix, and the noticeable increase in demand for this energy. This can be checked with the expansion of investments in Brazil and in the state of Bahia for the various sectors. The environmental benefits of natural gas highlight the advantages of using this input to the other fossil fuels. This paper discusses the availability of natural gas in Brazil and how it occurs its participation in the national energy matrix. This issue of the vulnerability of the market by the conflict between the growing demand from various industries and the need for order of thermal. It indicates scenarios and future prospects, and limiting factors for their growth. (author)
Natural Regulation of Energy Flow in a Green Quantum Photocell
Arp, Trevor B.; Barlas, Yafis; Aji, Vivek; Gabor, Nathaniel M.
2015-01-01
Manipulating the flow of energy in nanoscale and molecular photonic devices is of both fundamental interest and central importance for applications in light harvesting optoelectronics. Under erratic solar irradiance conditions, unregulated power fluctuations in a light harvesting photocell lead to inefficient energy storage in conventional solar cells and potentially fatal oxidative damage in photosynthesis. Here, we show that regulation against these fluctuations arises naturally within a tw...
Kitiashvili, I N; Goode, P R; Kosovichev, A G; Lele, S K; Mansour, N N; Wray, A A; Yurchyshyn, V B
2012-01-01
Turbulent properties of the quiet Sun represent the basic state of surface conditions, and a background for various processes of solar activity. Therefore understanding of properties and dynamics of this `basic' state is important for investigation of more complex phenomena, formation and development of observed phenomena in the photosphere and atmosphere. For characterization of the turbulent properties we compare kinetic energy spectra on granular and sub-granular scales obtained from infrared TiO observations with the New Solar Telescope (Big Bear Solar Observatory) and from 3D radiative MHD numerical simulations ('SolarBox' code). We find that the numerical simulations require a high spatial resolution with 10 - 25 km grid-step in order to reproduce the inertial (Kolmogorov) turbulence range. The observational data require an averaging procedure to remove noise and potential instrumental artifacts. The resulting kinetic energy spectra show a good agreement between the simulations and observations, opening...
Optimal energy consumption analysis of natural gas pipeline.
Liu, Enbin; Li, Changjun; Yang, Yi
2014-01-01
There are many compressor stations along long-distance natural gas pipelines. Natural gas can be transported using different boot programs and import pressures, combined with temperature control parameters. Moreover, different transport methods have correspondingly different energy consumptions. At present, the operating parameters of many pipelines are determined empirically by dispatchers, resulting in high energy consumption. This practice does not abide by energy reduction policies. Therefore, based on a full understanding of the actual needs of pipeline companies, we introduce production unit consumption indicators to establish an objective function for achieving the goal of lowering energy consumption. By using a dynamic programming method for solving the model and preparing calculation software, we can ensure that the solution process is quick and efficient. Using established optimization methods, we analyzed the energy savings for the XQ gas pipeline. By optimizing the boot program, the import station pressure, and the temperature parameters, we achieved the optimal energy consumption. By comparison with the measured energy consumption, the pipeline now has the potential to reduce energy consumption by 11 to 16 percent.
Energy analysis of the engineering-economic optimization of convective heat-transfer surfaces
Stoyanov, N. M.
1991-07-01
The influence of the specific costs of the heat transfer surface, 1 kW of installed power of the blower and motor, 1 kW·h of electrical energy consumption by it, operating time of the surface, and other factors on the optimum specific power expenditure to force heat-transfer fluids through the ducts of heattransfer surfaces is investigated. The minimum engineering-economically justified operating time of the surface is determined.
Energy as the Mediator between Natural and Supernatural Realms
Directory of Open Access Journals (Sweden)
Kristel Kivari
2012-12-01
Full Text Available This article discusses contemporary vernacular theory about the elusive energies that emanate from the ground. These energies are reported to be the ultimate reason for different remarkable occurrences, both natural and supernatural. The hypothesis of special energies is expressed in local tourism, in ecological debates and healing practises, driving the curiosity of amateur science. In these expressions knowledge as a form of engagement with the supernatural plays an integrating role between the individual and the forces beyond. Dowsing reveals the ‘energetic’ nature of reality, which will be discussed using three examples. Tuhala Nõiakaev (the Witch’s Well as a peculiar natural sight in the north of Estonia has drawn together many reports of energy columns that are linked to underground rivers and cultic stones. Another place under discussion is also famous for healing energy points: Kirna Manor works as a centre for spreading knowledge of the interdependence of physical health and the search for a spiritual path with the help of energies that the next example, the Society of Dowsers, attempts to discover using scientific methods. In these examples ‘energy’ designates the position of the individual, in which the participative relationship with the environment works as a form of folk epistemology within the limits of cultural understanding.
Natural convection heat transfer for copper micro-wires in water%微细铜丝在水中自然对流换热
Institute of Scientific and Technical Information of China (English)
管宁; 刘志刚; 张承武
2012-01-01
The natural convection heat transfer characteristics and mechanism for copper micro-wires in water are investigated experimentally and numerically. The wires with diameters of 39. 9 μm, 65. 8 μm and 119. 1 μm are placed horizontally in a sealed tube. Using Joule heating, the heat transfer coefficients and Nusselt numbers of natural convection for micro-wires in ultra pure water are obtained. A three dimensional incompressible numerical model is used to investigate the natural convection, and the prediction with this model is in reasonable accordance with the experimental results. With the decrease of micro-wire diameter, the heat transfer coefficient of natural convection on the surface of micro-wire increases, while the Nusselt number of natural convection becomes smaller. The change rate of Nu decreases apparently with the increase of heat flux and the decrease of wire diameter, attributed to the thinner boundary layer. The thickness of boundary layer on the wall of micro-wire becomes thinner with the decrease of diameter, but the ratio of boundary layer thickness to the diameter increases. As a result, the proportion of conduction in total heat transfer increases, while the convective heat transfer decreases. The velocity distribution, temperature field and the boundary layer in the natural convection are compared with those of conventional tube. It is found that the boundary layer around the micro-wire is an oval-shaped film on the surface, which is different from that around the conventional tube. This apparently reduces the convectional intensity in the natural convection and the heat transfer presents a conduction characteristic.%采用实验与数值模拟相结合的方法分别研究了封闭空间内水平放置的直径为39.9、65.8、119.1 μm的微细铜丝(微丝)在水中的对流换热,分析了微丝表面自然对流换热特性及机理.实验通过焦耳加热的方法测量了不同直径微丝在水中自然对流的传热系数
Energy expenditure on recreational visits to different natural environments.
Elliott, Lewis R; White, Mathew P; Taylor, Adrian H; Herbert, Stephen
2015-08-01
Physical inactivity poses a significant challenge to physical and mental health. Environmental approaches to tackle physical inactivity have identified natural environments as potentially important public health resources. Despite this, little is known about characteristics of the activity involved when individuals visit different types of natural environment. Using Natural England's Monitor of Engagement with the Natural Environment Survey, we examined 71,603 English respondents' recreational visits to natural environments in the past week. Specifically, we examined the intensity of the activities they undertook on the visits (METs), the duration of their visit, and the associated total energy expenditure (MET minutes). Visits to countryside and urban greenspace environments were associated with more intense activities than visits to coastal environments. However, visits to coastal environments were associated with the most energy expenditure overall due to their relatively long duration. Results differed by the urbanity or rurality of the respondent's residence and also how far respondents travelled to their destination. Knowledge of what types of natural environment afford the highest volumes and intensities of physical activity could inform landscape architecture and exercise prescriptions. Isolating activity-supporting characteristics of natural environments that can be translated into urban design is important in providing physical activity opportunities for those less able to access expansive environments.
Puhales, Franciano Scremin; Rizza, Umberto; Degrazia, Gervásio Annes; Acevedo, Otávio Costa
2013-02-01
In this work a parametrization for the transport terms of the turbulent kinetic energy (TKE) budget equation, valid for a convective boundary layer (CBL) is presented. This is a hard task to accomplish from experimental data, especially because of the difficulty associated to the measurements of pressure turbulent fluctuations, which are necessary to determine the pressure correlation TKE transport term. Thus, employing a large eddy simulation (LES) a full diurnal planetary boundary layer (PBL) cycle was simulated. In this simulation a forcing obtained from experimental data is used, so that the numerical experiment represents a more realistic case than a stationary PBL. For this study all terms of the TKE budget equation were determined for a CBL. From these data, polynomials that describe the TKE transport terms’ vertical profiles were adjusted. The polynomials found are a good description of the LES data, and from them it is shown that a simple formulation that directly relates the transport terms to the TKE magnitude has advantages on other parameterizations commonly used in CBL numerical models. Furthermore, the present study shows that the TKE turbulent transport term dominates over the TKE transport by pressure perturbations and that for most of the CBL these two terms have opposite signs.
Gu, Wei-Min
2012-01-01
By taking into account the local energy balance per unit volume between the viscous heating and the advective cooling plus the radiative cooling, we investigate the vertical structure of radiation pressure-supported accretion disks in spherical coordinates. Our solutions show that the photosphere of the disk is close to the polar axis and therefore the disk seems to be extremely thick. However, the profile of density implies that most of the accreted matter exists in a moderate range around the equatorial plane. We show that the well-known polytropic relation between the pressure and the density is unsuitable for describing the vertical structure of radiation pressure-supported disks. More importantly, we find that the energy advection is significant even for slightly sub-Eddington accretion disks. We argue that the non-negligible advection may help to understand why the standard thin disk model is likely to be inaccurate above \\sim 0.3 Eddington luminosity, which was found by some works on the black hole spi...
law, the laws of nature and ecosystem energy services
African Journals Online (AJOL)
Office 2004 Test Drive User
Green plants use chlorophyll and solar energy (light) to convert water, carbon ... natural gas, depending on the biologic input, the cooking temperature, and ..... little more about gasoline, diesel fuel, aviation fuel and heating oil other than that a ..... conceptual model of the relevant ecosystem and the ecosystem services that it.
Quantum hoop conjecture and a natural cutoff for vacuum energy
Yang, Rong-Jia
2015-01-01
We propose here a quantum hoop conjecture which states: the de Broglie wavelength of a quantum system can not be infinitely small, otherwise it will collapse into a quantum black hole. Based on this conjecture, we find an upper bound for the wave number of a particle, which offers a natural cutoff for the vacuum energy.
Directory of Open Access Journals (Sweden)
Ahmad Reza Rahmati
2016-01-01
Full Text Available In this work, for the first time, a double multi-relaxation-time lattice Boltzmann method (2-MRT-LBM is proposed to simulate MHD natural convection of nanofluid in a two-dimensional square cavity. The cavity is filled with TiO2-water nanofluid and is get under a uniform magnetic field at different angles ϕ with respect to horizontal plane. The proposed numerical scheme is solved the flow field and the temperature field using MRT-D2Q9 and MRT-D2Q5 lattice model, respectively. So, the main objective of this work is to show the effectiveness of this model to predict the effects of pertinent parameters such as the Rayleigh number (103 < Ra < 107, the solid volume fraction (0 % < < 5 %, the Hartmann number (0 < Ha < 60 and the magnetic field angle (0 < ϕ < 90 on the flow field and temperature field and the heat transfer performance of the cavity. The obtained results indicate that the proposed method is a powerful approach to simulate the MHD natural convection of nanofluids in a square cavity. Also the numerical results show that for Ra = 105 and for the range of Hartmann number of this study, the heat transfer and fluid flow depend strongly upon the direction of magnetic field. Furthermore, the magnetic field influence on the effect of nanoparticles on the heat transfer enhancement is not significant.
Institute of Scientific and Technical Information of China (English)
Zhang Lianshan; Deng Xianhe; Liu Weitao; Yang Zhiping
2014-01-01
The natural convection heat transfer of a 60%sucrose solution in a vertical converging-diverging tube (CD) with regularly-spaced twisted tapes (RSTT) has been investigated numerically and experimentally. The effects of wall tempera-ture and number of RSTT on the Nusselt number were studied in detail. The distributions of velocity and temperature in the 60%sucrose solution were studied and the simulated results of CD with RSTT were compared with those of the smooth tube. The inlfuence of Rayleigh number and RSTT on the Nusselt number was conducted experimentally. The results indi-cate that the Nusselt number of the 60%sucrose solution obviously increased with the number of RSTT but increased in-conspicuously with 2 and more twisted tapes. The simulation shows that the distance for achieving an optimal heat transfer performance is 46 times the diameter of the tube. The mechanism of the natural convection heat transfer enhancement of the 60%sucrose solution in relationship with the CD and the RSTT was analyzed, and the change of average tangential velocity with the axial distance was presented to demonstrate that the enhancement of heat transfer was realized mainly because of the increase in tangential velocity.
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.
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....
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.
The fractal nature materials microstructure influence on electrochemical energy sources
Directory of Open Access Journals (Sweden)
Mitić V.V.
2015-01-01
Full Text Available With increasing of the world energy crisis, research for new, renewable and alternative energy sources are in growth. The focus is on research areas, sometimes of minor importance and applications, where the different synthesis methods and microstructure properties optimization, performed significant improvement of output materials’ and components’ electro-physical properties, which is important for higher energy efficiency and in the electricity production (batteries and battery systems, fuel cells and hydrogen energy contribution. Also, the storage tanks capacity improvement, for the energy produced on such way, which is one of the most important development issues in the energy sphere, represents a very promising research and application area. Having in mind, the results achieved in the electrochemical energy sources field, especially electrolyte development, these energy sources, materials fractal nature optimization analysis contribution, have been investigated. Based on materials fractal structure research field, particularly electronic materials, we have performed microstructure influence parameters research in electrochemistry area. We have investigated the Ho2O3 concentration influence (from 0.01wt% to 1wt% and sintering temperature (from 1320°C to 1380°C, as consolidation parameters, and thus, also open the electrochemical function fractalization door and in the basic thermodynamic parameters the fractal correction introduced. The fractal dimension dependence on additive concentration is also investigated. [Projekat Ministarstva nauke Republike Srbije, br. 172057: Directed synthesis, structure and properties of multifunctional materials
Can Deployment of Renewable Energy and Energy Efficiency PutDownward Pressure on Natural Gas Prices
Energy Technology Data Exchange (ETDEWEB)
Wiser, Ryan; Bolinger, Mark
2005-06-01
High and volatile natural gas prices have increasingly led to calls for investments in renewable energy and energy efficiency. One line of argument is that deployment of these resources may lead to reductions in the demand for and price of natural gas. Many recent U.S.-based modeling studies have demonstrated that this effect could provide significant consumer savings. In this article we evaluate these studies, and benchmark their findings against economic theory, other modeling results, and a limited empirical literature. We find that many uncertainties remain regarding the absolute magnitude of this effect, and that the reduction in natural gas prices may not represent an increase in aggregate economic wealth. Nonetheless, we conclude that many of the studies of the impact of renewable energy and energy efficiency on natural gas prices appear to have represented this effect within reason, given current knowledge. These studies specifically suggest that a 1% reduction in U.S. natural gas demand could lead to long-term average wellhead price reductions of 0.8% to 2%, and that each megawatt-hour of renewable energy and energy efficiency may benefit natural gas consumers to the tune of at least $7.5 to $20.
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.
Liu, Qing; He, Ya-Ling
2015-11-01
In this paper, a double multiple-relaxation-time lattice Boltzmann model is developed for simulating transient solid-liquid phase change problems in porous media at the representative elementary volume scale. The model uses two different multiple-relaxation-time lattice Boltzmann equations, one for the flow field and the other for the temperature field with nonlinear latent heat source term. The model is based on the generalized non-Darcy formulation, and the solid-liquid interface is traced through the liquid fraction which is determined by the enthalpy-based method. The present model is validated by numerical simulations of conduction melting in a semi-infinite space, solidification in a semi-infinite corner, and convection melting in a square cavity filled with porous media. The numerical results demonstrate the efficiency and accuracy of the present model for simulating transient solid-liquid phase change problems in porous media.
Liu, Qing
2015-01-01
In this paper, a double multiple-relaxation-time lattice Boltzmann model is developed for simulating transient solid-liquid phase change problems in porous media at the representative elementary volume scale. The model uses two different multiple-relaxation-time lattice Boltzmann equations, one for the flow field and the other for the temperature field with nonlinear latent heat source term. The model is based on the generalized non-Darcy formulation, and the solid-liquid phase change interface is traced through the liquid fraction which is determined by the enthalpy method. The model is validated by numerical simulations of conduction melting in a semi-infinite space, solidification in a semi-infinite corner, and convection melting in a square cavity filled with porous media. The numerical results demonstrate the efficiency and accuracy of the present model for simulating transient solid-liquid phase change problems in porous media.
Natural atomic orbital based energy density analysis: Implementation and applications
Baba, Takeshi; Takeuchi, Mari; Nakai, Hiromi
2006-06-01
We present an improvement of energy density analysis (EDA), which partitions the total energy obtained by Hartree-Fock and/or density functional theory calculations, with the use of the natural atomic orbital (NAO) [A.E. Reed et al., J. Chem. Phys. 83 (1985) 735] and Löwdin's symmetric-orthogonal orbital (LSO). The present NAO- and LSO-EDA schemes are applied to analyses of CO 2 and Li9+ with various basis sets. Numerical results confirm that NAO-EDA exhibits less basis-set dependence, while the conventional results are very sensitive to the adopted basis sets.
European Strategies for Energy Security in the Natural Gas Market
Directory of Open Access Journals (Sweden)
Boyka Stefanova
2012-08-01
Full Text Available This article examines the European Union's (EU approach to energy security on the example of its natural gas imports from Russia, the largest supplier of gas to European markets. Two major projects, Nord Stream in the Northern and Western part of the EU, and Nabucco in South-Central Europe, demonstrate opposing energy security strategies, seemingly at odds with the EU objective of achieving energy independence from Russia. The question arises: Are these strategies sustainable? How can they be reconciled and pursued under a common policy? The main argument is that such conflicting sub-regional policy initiatives are amenable to progressive realignment and serve common security objectives. The article examines the Nord Stream and Nabucco pipelines in the context of the Third Energy Package, a set of policy instruments for the creation of an EU-wide internal energy market. It demonstrates that the energy security strategies pursued through Nord Stream and Nabucco fit well with the logic of the internal market reflected in premises of flexibility and efficiency. The article concludes that the security of the EU's energy market may be pursued in practice by applying different formulas relying on a variable mix of networks, partnerships, and market integration with non-members.
The influence of an estimated energy saving due to natural ventilation on the Mexican energy system
DEFF Research Database (Denmark)
Oropeza-Perez, Ivan; Østergaard, Poul Alberg
2014-01-01
conditioning, in particular, with natural ventilation to cool residential buildings is determined. It is shown that when, as in Mexico, there is a relatively simple connection between supply and electricity demand, NV creates savings which could be used to reduce either the fossil-fuel-based generation......This article shows the impacts of the extensive use of NV (natural ventilation) in the Mexican residential sector on the Mexican energy system. By integrating a thermal-airflow simulation programme with an energy systems analysis model, the impact on the Mexican energy system of replacing air...
Izursa, Jose-Luis
Bolivia, traditionally known for being a country rich in natural resources, has suffered from a constant exploitation of its natural resources benefiting only small groups in and outside the country. The devastation of natural resources that occurred for many years was of concern to the latest government, rural communities and indigenous groups. As a result, Bolivia has a more sustainability-oriented forest law that has a strong orientation towards the utilization of natural resources at a national level and encompasses a fast-growing forestry industry than in previous years. In this dissertation, the wealth of Bolivia's national system was evaluated using solar emergy. Emergy (spelled with "m") is the sum of all energy of one form needed to develop a flow of energy of another form, over a period of time. The basic idea is that solar energy is our ultimate energy source and by expressing the value of products in solar emergy units, it becomes possible to compare different kinds of energy, allowing to express the value for the natural resources in Emergy Dollars. It was found out that Bolivia relies heavily in its natural resources and that its emergy exchange ratio with its international trading partners changed from 12.2 to 1 in 2001 to 6.2 to 1 in 2005. This means that Bolivia went from export 12.2 emdollars of goods for each 1 it received in 2001 to export 6.2 emdollars of products for each 1 it received in 2005. The study also showed that under forest certification practices less emergy is removed from forests (1.49E+19 sej/yr) compared to the amount of emergy removed (2.36E+19 sej/yr) under traditional uncertified practices, reflecting that forest ecology does better under certification. The "Ecologically-based Development for the Bolivian Industrial Forestry System" (DEBBIF) simulation model constructed during this study, compared four different scenarios: the Reference Scenario, the Increased Export Scenario, the Increased Domestic Use Scenario and the
Department of Energy power generation programs for natural gas
Energy Technology Data Exchange (ETDEWEB)
Bajura, R.A.
1995-04-01
The U.S. Department of Energy (DOE) is sponsoring two major programs to develop high efficiency, natural gas fueled power generation technologies. These programs are the Advanced Turbine Systems (ATS) Program and the Fuel Cell Program. While natural gas is gaining acceptance in the electric power sector, the improved technology from these programs will make gas an even more attractive fuel, particularly in urban areas where environmental concerns are greatest. Under the auspices of DOE`s Office of Fossil Energy (DOE/FE) and Office of Energy Efficiency and Renewable Energy (DOE/EE), the 8-year ATS Program is developing and will demonstrate advanced gas turbine power systems for both large central power systems and smaller industrial-scale systems. The large-scale systems will have efficiencies significantly greater than 60 percent, while the industrial-scale systems will have efficiencies with at least an equivalent 15 percent increase over the best 1992-vintage technology. The goal is to have the system ready for commercial offering by the year 2000.
Disruptions in energy balance: does nature overcome nurture?
Fernández, José R; Casazza, Krista; Divers, Jasmin; López-Alarcón, Mardya
2008-04-22
Fat accumulation, in general, is the result of a breakdown in the homeostatic regulation of energy balance. Although, the specific factors influencing the disruption of energy balance and why these factors affect individuals differently are not completely understood, numerous studies have identified multiple contributors. Environmental components influence food acquisition, eating, and lifestyle habits. However, the variability in obesity-related outcomes observed among individuals placed in similar controlled environments supports the notion that genetic components also wield some control. Multiple genetic regions have been associated with measures related to energy balance; however, the replication of these genetic contributors to energy intake and energy expenditure in humans is relatively small perhaps because of the heterogeneity of human populations. Genetic tools such as genetic admixture account for individual's genetic background in gene association studies, reducing the confounding effect of population stratification, and promise to be a relevant tool on the identification of genetic contributions to energy balance, particularly among individuals of diverse racial/ethnic backgrounds. Although it has been recognized that genes are expressed according to environmental influences, the search toward the understanding of nature and nurture in obesity will require the detailed study of the effect of genes under diverse physiologic and behavioral environments. It is evident that more research is needed to elucidate the methodological and statistical issues that underlie the interactions between genes and environments in obesity and its related comorbidities.
Martyushev, S. G.; Miroshnichenko, I. V.; Sheremet, M. A.
2014-01-01
Unsteady regimes of convective-radiative heat transfer in a cubic enclosure with finitely thick heat-conducting walls in the presence of a constant-temperature energy source have been modeled mathematically under the conditions of convective heat exchange with the environment. A mathematical model has been formulated in dimensionless variables "vector potential-vorticity vector-temperature;" the model was realized numerically by the finite-difference method. An analysis of radiative heat transfer has been made on the basis of the surface-radiation approximation with the balance method in Polyak's version. Three-dimensional temperature and velocity fields and dependences for the average Nusselt number have been obtained; they reflect the influence of the reduced emissivity factor of interior surfaces of enclosing walls, of the relative thermal conductivity, and of the unsteadiness factor on the flow regimes and heat transfer.
Use of natural energy in hyper-skyscrapers. Chocho koso kenchiku ni okeru shizen energy riyo
Energy Technology Data Exchange (ETDEWEB)
Unno, K. (Takenaka Komuten Co. Ltd., Osaka (Japan))
1992-05-30
Construction of large scale hyper-skyscrapers( more than 500 m high ) by next century is coming to inevitable stage, and the problems regarding supply and consumption of energy/resources together with the use of natural energy in such construction are described. Transport of water at high direction, conformity of supply and save energy are pointed out as problems for supplying water energy. Calculated amount of required electrical energy used by electrical appliances of sky-city ( 1000 m high ) has been about 440,000 KVA, and necessity of autonomous supply network system for electric power and heat supply is stressed. For sky-city, 1000 m, use of new energy( supply of 20% of total electric power is possible with optical generation, wind power generation) and the use of unused energy ( draft generations, generation from refuse incineration, pumped storage generation ) are emphasized. Effective supply of water resource like refuse of waste water is cited. 3 refs., 4 figs., 1 tab.
Kochetov, N. A.; Loktionov, V. D.; Sidorov, A. S.
2015-09-01
The possibility of using the Star CCM+ software system for analyzing the thermal state of the melt pool metal layer generated as a result of melt stratification during a severe accident in pressure-vessel nuclear reactors is considered. In order to verify and substantiate the possibility of using this software system for modeling the natural convection processes in the melt at high values of the Rayleigh number, test problems were solved. The obtained results were found to be in good agreement with the known solutions and with the experimental data. The behavior of the melt metal layer was subjected to a parametric analysis for different melt heating conditions, the results of which showed that certain parameters have a determining influence on the so-called focusing effect and on the specific features of current in this layer.
Moreno, Rafael; Ramaswamy, Balasubramaniam
2003-01-01
Using object-oriented programming (OOP) techniques and philosophies, a collection of C++ tools for the rapid development of finite element applications has been created. The object-oriented finite element analysis (OOFEA) toolkit provides both the geometrical and mathematical management tools necessary for this task in the form of useful class hierarchies. In particular, the OOFEA toolkit features methods for evaluating arbitrary weak forms provided by the user in order to solve particular problems of interest. A description of the underlying concepts, philosophies and techniques used to develop the toolkit are included. A strong effort has been made to concentrate on its possibly beneficial usage in the computational fluid dynamics area. In order to demonstrate the toolkit capabilities of managing complex projects, a simulator for laminar and turbulent natural convective flows in enclosures has been developed and a numerical study of some of these flows has been conducted.
Mustafa, Meraj; Mushtaq, Ammar; Hayat, Tasawar; Ahmad, Bashir
2014-01-01
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.
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.
Griffin, P. R.; Motakef, S.
1989-01-01
Consideration is given to the influence of temporal variations in the magnitude of gravity on natural convection during unidirectional solidification of semiconductors. It is shown that the response time to step changes in g at low Rayleigh numbers is controlled by the momentum diffusive time scale. At higher Rayleigh numbers, the response time to increases in g is reduced because of inertial effects. The degree of perturbation of flow fields by transients in the gravitational acceleration on the Space Shuttle and the Space Station is determined. The analysis is used to derive the requirements for crystal growth experiments conducted on low duration low-g vehicles. Also, the effectiveness of sounding rockets and KC-135 aircraft for microgravity experiments is examined.
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.
Griffin, P. R.; Motakef, S.
1989-01-01
Consideration is given to the influence of temporal variations in the magnitude of gravity on natural convection during unidirectional solidification of semiconductors. It is shown that the response time to step changes in g at low Rayleigh numbers is controlled by the momentum diffusive time scale. At higher Rayleigh numbers, the response time to increases in g is reduced because of inertial effects. The degree of perturbation of flow fields by transients in the gravitational acceleration on the Space Shuttle and the Space Station is determined. The analysis is used to derive the requirements for crystal growth experiments conducted on low duration low-g vehicles. Also, the effectiveness of sounding rockets and KC-135 aircraft for microgravity experiments is examined.
Directory of Open Access Journals (Sweden)
Rashidi Mohammad Mehdi
2015-01-01
Full Text Available The similar solution on the equations of the revised Cheng-Minkowycz problem for natural convective boundary layer flow of nanofluid through a porous medium gives (using an analytical method, a system of non-linear partial differential equations which are solved by optimal homotopy analysis method. Effects of various drastic parameters on the fluid and heat transfer characteristics have been analyzed. A very good agreement is observed between the obtained results and the numerical ones. The entropy generation has been derived and a comprehensive parametric analysis on that has been done. Each component of the entropy generation has been analyzed separately and the contribution of each one on the total value of entropy generation has been determined. It is found that the entropy generation as an important aspect of the industrial applications has been affected by various parameters which should be controlled to minimize the entropy generation.
Energy Technology Data Exchange (ETDEWEB)
Martineau, Richard C., E-mail: Richard.Martineau@inl.go [Fuels Modeling and Simulation, Nuclear Fuels and Materials Division, Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-3860 (United States); Berry, Ray A.; Esteve, Aurelia; Hamman, Kurt D.; Knoll, Dana A.; Park, HyeongKae; Taitano, William [Fuels Modeling and Simulation, Nuclear Fuels and Materials Division, Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-3860 (United States)
2010-06-15
This paper 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 specific force quantities and velocity and temperature 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.
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
Institute of Scientific and Technical Information of China (English)
刘俊杰; 朱学良; 曹晓东; 李佳玉; 孙贺江
2016-01-01
The flow field is actually the result of natural convection and forced convection interaction inside the cabin with narrow interior space and passengers seated intensively.To study the effect of the natural convection on the air-flow field inside occupied cabins,a full-scale 7-rows cabin mockup of Boeing737-200 has been researched with a two-dimensional particle image velocimetry(2D-PIV)system.The flow fields under isothermal and cooling conditions were compared and it was found out that the natural convection enhanced the flow field symmetry and uniformity macroscopically,and strengthened the air jet entrainment and weakened the air jet adherent effect microscopically. Then,a variety of conditions were set within a reasonable range of design parameters based on the above qualitative analysis for further measurements,so as to quantify the effects of natural convection on the air jet and the overall flow field with Archimedes number(Ar)and Reynolds number(Re).Two important conclusions can be obtained.One is that the air jet centerline velocity decay rate becomes slower with the enhancement of the natural convection, which means the air-jet velocity decay coefficientCw is in proportion toAr;the other is that a fully rotary airflow can be maintained on one side of the cabin,divided by the centerline of the aisle,whenRe>3,800 andAr3,800和Ar<6时,可在过道中线一侧形成完整涡旋流动.
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.
Energy Technology Data Exchange (ETDEWEB)
Davidson, J.H.
1998-06-01
The goals of this project are: (1) to develop guidelines for the design and use of thermosyphon side-arm heat exchangers in solar domestic water heating systems, and (2) to establish appropriate modeling and testing criteria for evaluating the performance of systems using this type of heat exchanger. The tasks for the project are as follows: (1) Develop a model of the thermal performance of thermosyphon heat exchangers in solar water heating applications. A test protocol will be developed which minimizes the number of tests required to adequately account for mixed convection effects. The TRNSYS component model will be fully integrated in a system component model and will use data acquired with the specified test protocol. (2) Conduct a fundamental study to establish friction and heat transfer correlations for conditions and geometries typical of thermosyphon heat exchangers in solar systems. Data will be obtained as a function of a buoyancy parameter based on Grashof and Reynolds numbers. The experimental domain will encompass the ranges expected in solar water heating systems.
Energy Technology Data Exchange (ETDEWEB)
Lee, Injun; Hong, Sungyull; Bai, Cheolho; Shim, Jaesool [Yeungnam Univ., Kyungsan (Korea, Republic of); Hong, Sungdeok; Kim, Chansoo; Kim, Minhwan [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)
2015-05-15
The VHTR necessarily requires an intermediate loop composed of a hot gas duct(HGD), an intermediate heat exchanger (IHX) and a process heat exchanger. The IHX is one of the important components of VHTR system because the IHX transfers the 950 .deg. high temperature massive heat to a hydrogen production plant or power conversion unit at high system pressure. Hot gas duct (HGD) is a unique component of a gas cooled reactor (GCR). The internal of a HGD is insulated using a ceramic fiber insulator (Kaowool) to prevent a mechanical failure itself from high temperature and high pressure operating conditions. The coolant temperature at the internal of a HGD can go up to 950 .deg. C. Thermo-hydraulic simulation using the COMSOL commercial solver is successfully performed at a uniform heat flux condition in a horizontal HGD. We obtained nonlinear temperature distribution from the COMSOL simulation with the assumption of the insulator in a HGD governed with conduction and convection heat transfer in porous medium.
Institute of Scientific and Technical Information of China (English)
Mohsen Nazari; Ladan Louhghalam; Mohamad Hassan Kayhani
2015-01-01
Double diffusion convection in a cavity with a hot square obstacle inside is simulated using the lattice Boltzmann method. The results are presented for the Rayleigh numbers 104,105 and 106, the Lewis numbers 0.1, 2 and 10 and aspect ratio A (obstacle height/cavity height) of 0.2, 0.4 and 0.6 for a range of buoyancy number N=0 to−4 with the effect of opposing flow. The results indicate that for|N|b 1, the Nusselt and Sherwood numbers decrease as buoyancy ratio increases, while for|N|N 1, they increase with|N|. As the Lewis number increases, higher buoyan-cy ratio is required to overcome the thermal effects and the minimum value of the Nusselt and Sherwood num-bers occur at higher buoyancy ratios. The increase in the Rayleigh or Lewis number results in the formation of the multi-cell flow in the enclosure and the vortices wil vanish as|N|increases.
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
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.
An efficiency booster for energy conversion in natural circulation loops
Energy Technology Data Exchange (ETDEWEB)
Wang, Dongqing, E-mail: wangdongqing@stu.xjtu.edu.cn [School of Nuclear Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710049 (China); Beijing Computational Science Research Center, Beijing 100084 (China); Jiang, Jin, E-mail: jjiang@eng.uwo.ca [Department of Electrical and Computer Engineering, The University of Western Ontario, London, Ontario N6A 5B9 (Canada); Beijing Computational Science Research Center, Beijing 100084 (China)
2016-08-01
Highlights: • Low driving power conversion efficiency of natural circulation loops is proved. • The low conversion efficiency leads to low heat transfer capacity of such loops. • An efficiency booster is designed with turbine to increase the efficiency. • Performance of the proposed booster has been numerically simulated. • The booster drastically enhances heat transfer capacity of such loops. - Abstract: In this paper, the capacity of a natural circulation loop for transferring heat from a heat source to a heat sink has been analyzed. It is concluded that the capacity of the natural circulation loop depends on the conversion efficiency of the thermal energy from the heat source to the driving force for the circulation of the flow. The low conversion efficiency leading to weak driving force in such loops has been demonstrated analytically and validated through simulation results. This issue has resulted in a low heat transfer capacity in the circulation loop. To increase the heat transfer capacity, one has to improve this efficiency. To meet such a need, a novel efficiency booster has been developed in this paper. The booster essentially increases the flow driving force and hence significantly improves the overall heat transfer capacity. Design and analysis of this booster have been performed in detail. The performance has been examined through extensive computer simulations. It is concluded that the booster can indeed drastically improve the heat transfer capacity of the natural circulation loop.
Energy-Efficient Train Operation Using Nature-Inspired Algorithms
Directory of Open Access Journals (Sweden)
Kemal Keskin
2017-01-01
Full Text Available A train operation optimization by minimizing its traction energy subject to various constraints is carried out using nature-inspired evolutionary algorithms. The optimization process results in switching points that initiate cruising and coasting phases of the driving. Due to nonlinear optimization formulation of the problem, nature-inspired evolutionary search methods, Genetic Simulated Annealing, Firefly, and Big Bang-Big Crunch algorithms were employed in this study. As a case study a real-like train and test track from a part of Eskisehir light rail network were modeled. Speed limitations, various track alignments, maximum allowable trip time, and changes in train mass were considered, and punctuality was put into objective function as a penalty factor. Results have shown that all three evolutionary methods generated effective and consistent solutions. However, it has also been shown that each one has different accuracy and convergence characteristics.
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.
Energy and the aged: the impact of natural gas deregulation
Energy Technology Data Exchange (ETDEWEB)
1983-01-01
Energy Secretary Donald Hodel led a group of six witnesses at a hearing held to examine the potential impact of natural gas deregulation proposals on elderly consumers. The witnesses were asked to examine the impact of current price increases on residential users, review anticipated price changes for elderly consumers, and explore whether current resources to offset these impacts on low-income and elderly consumers are adequate. Hodel's testimony is followed by that of representatives of public utility commissions and consumer groups and by an appendix with analyses by the DOE and the National Consumer Law Center.
Natural Regulation of Energy Flow in a Green Quantum Photocell
Arp, Trevor B; Aji, Vivek; Gabor, Nathaniel M
2015-01-01
Manipulating the flow of energy in nanoscale and molecular photonic devices is of both fundamental interest and central importance for applications in light harvesting optoelectronics. Under erratic solar irradiance conditions, unregulated power fluctuations in a light harvesting photocell lead to inefficient energy storage in conventional solar cells and potentially fatal oxidative damage in photosynthesis. Here, we show that regulation against these fluctuations arises naturally within a two-channel quantum heat engine photocell, thus enabling the efficient conversion of varying incident solar spectrum at Earth's surface. Remarkably, absorption in the green portion of the spectrum is avoided, as it provides no inherent regulatory benefit. Our findings illuminate a quantum structural origin of regulation, provide a novel optoelectronic design strategy, and may elucidate the link between photoprotection in photosynthesis and the predominance of green plants on Earth.
Evolution of entropic dark energy and its phantom nature
Mathew, Titus K; J, Shejeelammal
2015-01-01
Assuming the form of the entropic dark energy as arises form the surface term in the Einstein-Hilbert's action, it's evolution were analyzed in an expanding flat universe. The model parameters were evaluated by constraining model using the Union data on Type Ia supernovae. We found that the model predicts an early decelerated phase and a later accelerated phase at the background level. The evolution of the Hubble parameter, dark energy density, equation of state parameter and deceleration parameter were obtained. The model is diagnosed with $Om$ parameter. The model is hardly seems to be supporting the linear perturbation growth for the structure formation. We also found that the entropic dark energy shows phantom nature for redshifts $z<0.257.$ During the phantom epoch, the model predicts big-rip effect at which both the scale factor of expansion and the dark energy density become infinitely large and the big rip time is found to be around 36 Giga Years from now.
Hawaii Natural Energy Institute annual report, July 1981-June 1982
Energy Technology Data Exchange (ETDEWEB)
Brown, N.E. (ed.)
1982-01-01
This report includes brief progress reports on the 35 research and development projects in geothermal energy, ocean energy, biomass energy, wind energy, solar energy, and other renewable energy sources. (DLC)
Institute of Scientific and Technical Information of China (English)
黄鑫; 彭述明; 周晓松; 余铭铭; 尹剑; 温成伟
2015-01-01
ICF design requires smooth and uniform deuterium-tritium (D-T) ice layers in a spherical shell. Thermal environ-ment around the capsule is the key to reach the low-mode ice layer roughness requirement and obtain a high quality ice layer. In this paper, we present the results of three-dimensional simulation for an indirect-driven cryogenic target, focusing on the issues of heat transfer and natural convection flow inside the hohlraum. A thermal and hydrodynamic calculation is first proposed to investigate the convection heat transfer effect on the D-T ice layer. Comparing the two cases with gravity considered or neglected, we find that the temperature variation at the ice layer inner surface caused by the natural convection flow and the hohlraum’s structure are of the same order of magnitude. Then the parameters study on Rayleigh number, which is a dimensionless number associated with free convection, is carried out. Thermal simulations on different Rayleigh number are provided. Temperature variation at the D-T ice layer inner surface is to increase as soon as the Rayleigh number reaches 60. Comparisons among different gases under different operating pressure conditions are made. In order to avoid the convection heat transfer effect in a wide range of pressure, it is necessary to take pure helium or mixture gas with a small amount of hydrogen as the tamping gas. The influence of hohlraum’s orientation on the natural convection is also studied. It is found that the convective heat transfer effect in a horizontally orientated hohlraum is stronger than that in a vertical one. Based on these, we discuss the possibility to eliminate the convection flow by partitioning the hohlraum into several regions. The calculated results for several cases of different gas-region models indicate that the convection flow can be eliminated with an appropriate division in a vertically orientated hohlaum but cannot in a horizontally orientated one. The conclusions in this paper have
Institute of Scientific and Technical Information of China (English)
陈叔平; 常智新; 韩宏茵; 谢福寿; 姚淑婷
2011-01-01
The natural convective heat transfer model on the air side of air - heating cryogenic finned - tube vaporizer was established. The natural convection heat transfer of vaporizer side air was simulated 3D - numerically used by fluent software,SST model and SIMPLEC algorithm for differentstructure parameters and inside wall temperature of the vaporizer. Based on the result of numerical simulation, the influence of different structure sizes and wall temperatures on natural convection heat transfer was analysed, and a calculation correlation of Nusselt number for the air side natural convection heat transfer of vaporizer was fitted, providing a reference for engineering practice.%建立了深冷空温式星形翅片管气化器的空气侧自然对流换热模型.利用fluent软件,采用SST k-ω湍流模型和SIMPLEC算法,对多组不同结构参数、不同内壁面温度的气化器空气侧自然对流换热进行了三维数值模拟.由数值模拟结果分析了气化器各结构尺寸大小与壁温对翅片管自然对流换热的影响,并拟合了气化器空气侧自然对流换热的Nu数的计算关联式,为工程实际供参考.
QM-400 CFD 自然对流模型研究及验证%Research and Validation on CFD Natural Convection Model of QM-400
Institute of Scientific and Technical Information of China (English)
左巧林; 干富军; 朱丽兵
2016-01-01
The spent fuel dry storage facility named QM-400 module for Third Qinshan Nuclear Power Co.Ltd.(TQNPC)is the first commercial dry storage facility in opera-tion in China.The heat transfer in QM-400 mainly consists of natural convention,con-duction,conjugate heat transfer and radiation,etc.The decay heat of each fuel basket was calculated accurately at typical surrounding temperature.Mesh sensitivity analysis was performed using commercial computational fluid dynamics (CFD)code FLUENT 14.0. A set of CFD simulation models on natural convection of QM-400 were developed.The results show that the distributions of the pressure and temperature on the cylinder sur-face meet the rules of natural convection.Good agreements are achieved between the simulated temperature and the measured temperature at the measured points and the simulated temperature trend varying with surrounding temperature agree well with the measured trend,which demonstrates the correctness of the calculation method of natural convection in this paper.This work can be the reference of the further CFD simulation on temperature distributions of dry storage facility without thermal insulation panels.%秦山第三核电厂乏燃料干式贮存模块 QM-400是我国第一座投入商业运行的干式贮存设施，模块内的热量交换主要包括自然对流、热传导、耦合传热和辐射换热等。本文精确计算了典型环境温度下每个燃料篮的衰变热，运用商用计算流体动力学(CFD)软件 FLUENT 14.0开展了网格敏感性分析，并建立了 QM-400存储模块的自然对流 CFD 分析模型。结果表明，模块顶面、侧面以及贮存筒表面压力和温度分布符合自然对流规律，计算的测点温度与现场的实测温度符合良好，测点温度随环境温度的变化趋势也与实测趋势符合良好，证明了建立的 CFD 自然对流计算方法的正确性。本文结果为后续采用CFD 方法进行取消绝热板后的温度场计算奠定了基础。
Marcus, F A; Fuhr, G; Monnier, A; Benkadda, S
2014-01-01
With the resonant magnetic perturbations (RMPs) consolidating as an important tool to control the transport barrier relaxation, the mechanism on how they work is still a subject to be clearly understood. In this work we investigate the equilibrium states in the presence of RMPs for a reduced MHD model using 3D electromagnetic fluid numerical code (EMEDGE3D) with a single harmonic RMP (single magnetic island chain) and multiple harmonics RMPs in cylindrical and toroidal geometry. Two different equilibrium states were found in the presence of the RMPs with different characteristics for each of the geometries used. For the cylindrical geometry in the presence of a single RMP, the equilibrium state is characterized by a strong convective radial thermal flux and the generation of a mean poloidal velocity shear. In contrast, for toroidal geometry the thermal flux is dominated by the magnetic flutter. For multiple RMPs, the high amplitude of the convective flux and poloidal rotation are basically the same in cylindr...
Energy Technology Data Exchange (ETDEWEB)
Kasinathan, N.; Rajakumar, A.; Vaidyanathan, G.; Chetal, S.C. [Indira Gandhi Centre for Atomic Research, Kalpakkam (India)
1995-09-01
Post shutdown decay heat removal is an important safety requirement in any nuclear system. In order to improve the reliability of this function, Liquid metal (sodium) cooled fast breeder reactors (LMFBR) are equipped with redundant hot pool dipped immersion coolers connected to natural draught air cooled heat exchangers through intermediate sodium circuits. During decay heat removal, flow through the core, immersion cooler primary side and in the intermediate sodium circuits are also through natural convection. In order to establish the viability and validate computer codes used in making predictions, a 1:20 scale experimental model called RAMONA with water as coolant has been built and experimental simulation of decay heat removal situation has been performed at KfK Karlsruhe. Results of two such experiments have been compiled and published as benchmarks. This paper brings out the results of the numerical simulation of one of the benchmark case through a 1D/2D coupled code system, DHDYN-1D/THYC-2D and the salient features of the comparisons. Brief description of the formulations of the codes are also included.
Harvesting energy from the natural vibration of human walking.
Yang, Weiqing; Chen, Jun; Zhu, Guang; Yang, Jin; Bai, Peng; Su, Yuanjie; Jing, Qingsheng; Cao, Xia; Wang, Zhong Lin
2013-12-23
The triboelectric nanogenerator (TENG), a unique technology for harvesting ambient mechanical energy based on the triboelectric effect, has been proven to be a cost-effective, simple, and robust approach for self-powered systems. However, a general challenge is that the output current is usually low. Here, we demonstrated a rationally designed TENG with integrated rhombic gridding, which greatly improved the total current output owing to the structurally multiplied unit cells connected in parallel. With the hybridization of both the contact-separation mode and sliding electrification mode among nanowire arrays and nanopores fabricated onto the surfaces of two contact plates, the newly designed TENG produces an open-circuit voltage up to 428 V, and a short-circuit current of 1.395 mA with the peak power density of 30.7 W/m(2). Relying on the TENG, a self-powered backpack was developed with a vibration-to-electric energy conversion efficiency up to 10.62(±1.19) %. And it was also demonstrated as a direct power source for instantaneously lighting 40 commercial light-emitting diodes by harvesting the vibration energy from natural human walking. The newly designed TENG can be a mobile power source for field engineers, explorers, and disaster-relief workers.
Effect of Energy Efficiency Standards on Natural Gas Prices
Energy Technology Data Exchange (ETDEWEB)
Carnall, Michael; Dale, Larry; Lekov, Alex
2011-07-26
A primary justification for the establishment of energy efficiency standards for home appliances is the existence of information deficiencies and externalities in the market for appliances. For example, when a long-term homeowner purchases a new gas-fired water heater, she will maximize the value of her purchase by comparing the life-cycle cost of ownership of available units, including both total installed cost - purchase price plus installation costs - and operating cost in the calculus. Choice of the appliance with the lowest life-cycle costs leads to the most economically efficient balance between capital cost and fuel cost. However, if the purchaser's expected period of ownership is shorter than the useful life of the appliance, or the purchaser does not pay for the fuel used by the appliance, as is often the case with rental property, fuel cost will be external to her costs, biasing her decision toward spending less on fuel efficiency and resulting in the purchase of an appliance with greater than optimal fuel usage. By imposing an efficiency standard on appliances, less efficient appliances are made unavailable, precluding less efficient purchases and reducing fuel usage. The reduction in fuel demanded by residential users affects the total demand for such fuels as natural gas, for example. Reduced demand implies that residential customers are willing to purchase less gas at each price level. That is, the demand curve, labeled D{sub 0} in Figure 1, shifts to the left to D{sub 1}. If there is no change in the supply function, the supply curve will intersect the demand curve at a lower price. Residential demand is only one component of the total demand for natural gas. It is possible that total demand will decline very little if demand in other sectors increases substantially in response to a decline in the price. If demand does decrease, modeling studies generally confirm the intuition that reductions in demand for natural gas will result in reductions
Directory of Open Access Journals (Sweden)
Ahmed Kadhim Hussein
2016-03-01
Full Text Available Natural convection visualization by heatlines in three types of inclined wavy cavities filled with Al2O3–water and Ag–water nanofluids and subjected to a discrete isoflux heating from its left sidewall is investigated numerically in this work. The right sidewall together with remaining regions in the left sidewall is insulated. The solution is based on the finite volume method. The upper and lower cavity walls are maintained at a constant cold temperature and follow a profile of sine wave. Numerical computations are carried out for various values of the solid volume fraction [φ = 0, 0.05, 0.1, 0.15 and 0.2], number of undulations [N = 1, 2 and 3], Rayleigh number [Ra = 103–107], the ratio of heating element length to the cavity height [ε = 0.2, 0.4, 0.6, 0.8 and 1.0], the cavity inclination angle [Φ = 0°, 30°, 60° and 90°] and the wave amplitude [γ = 0.1, 0.15 and 0.2]. The results are presented in terms of heatlines, isotherms, streamlines together with local and average Nusselt numbers. It is found that the geometry of the wavy cavity has a cursive role on the flow and thermal fields pattern. The results also explained that streamlines and isotherms were affected significantly for high Rayleigh number and vertical cavity position [Φ = 90°]. Also, when the solid volume fractions and wave amplitudes increase, the local Nusselt number along the heat source increases. Furthermore, velocity profiles increase as [ε] increases near the left sidewall of the cavity while they decrease as [ε] increases near the right sidewall of the cavity. For all three types of horizontal wavy cavities the heat functions increase for both nano and base fluids when the wave amplitude increases. Finally, results of the present work indicated that both heatlines and heat functions techniques are applied efficiently to describe the natural convection heat transfer inside wavy cavities filled with nanofluid.
Energy Technology Data Exchange (ETDEWEB)
Khedr, A.; Abdel-Latif, Salwa H. [Nuclear and Radiological Regulatory Authority, Cairo (Egypt); Abdel-Hadi, Eed A. [Benha Univ., Cairo (Egypt). Shobra Faculty of Engineering; D' Auria, F. [Pisa Univ. (Italy)
2016-03-15
In an attempt to understand the built-up of natural circulation in MTR pool type upward flow research reactors after loss of power, an experimental test rig was built to simulate the loop of natural circulation in MTR reactors. The test rig consisting of two vertically oriented branches, in one of them the core is simulated by two rectangular, electrically heated, parallel channels. The other branch simulates the part of the return pipe that participates in the development of core natural circulation. In the first phase of the work, many experimental runs at different conditions of channel's power and branch's initial temperatures are performed. The channel's coolant and surface temperatures were measured. The measurements and their interpretation were published by the first three authors. In the present work the thermal hydraulic behavior of the test rig is complemented by theoretical analysis using RELAP5 Mod 3.3 system code. The analysis consisting of two parts; in the first part RELAP5 model is validated against the measured values and in the second part some of the other not measured hydraulic parameters are predicted and analyzed. The test rig is typically nodalized and an input dick is prepared. In spite of the low pressure of the test rig, the results show that RELAP5 qualitatively predicts the thermal hydraulic behaviour and the accompanied phenomenon of flow inversion of such facilities. Quantitatively, there is a difference between the predicted and measured values especially the channel's surface temperature. This difference may be return to the uncertainties in initial conditions of experimental runs, the position of the thermocouples which buried inside the heat structure, and the heat transfer package in RELAP5.
NUMERICAL STUDY ON MIXED CONVECTIVE FLOW IN A SOLAR COLLECTOR
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
In a solar energy heat collector forced convection and free convection will occur concurrently. In this paper, the mixed convective flow was investigated. The dimensionless equation was derived and the results was verified by experiments. The numerical solution shows that error is less than 5% if the effect of free convection is ignored.
Directory of Open Access Journals (Sweden)
Hari R. Kataria
2017-03-01
Full Text Available We study theoretically the boundary layer flow of an incompressible micropolar fluid under uniform magnetic field and motion takes place due to the buoyancy force between vertical walls. The governing unsteady boundary layer momentum, angular momentum and energy equations of micropolar fluid are nondimensionalized and solved numerically. Analytic result for steady state case is also discussed. The effects of magnetic parameter (M, vortex viscosity parameter (R, Prandtl number (Pr and material parameter (b on velocity, micro-rotation and Temperature profiles are discussed through several figures.
76 FR 63613 - Secretary of Energy Advisory Board Natural Gas Subcommittee
2011-10-13
... Doc No: 2011-26464] DEPARTMENT OF ENERGY Secretary of Energy Advisory Board Natural Gas Subcommittee... meeting of the Secretary of Energy Advisory Board (SEAB) Natural Gas Subcommittee. SEAB was reestablished... recommendations to the SEAB on how to improve the safety and environmental performance of natural gas...
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
Space and energy: relationships among architects from nature
Directory of Open Access Journals (Sweden)
Silvia Titotto
2015-07-01
Full Text Available This research reflects on the possibility of architecture eco-sustainable development from the point of view of energy, space and environmental heritage, based on the constructive process of some species of social wasps (Hymenoptera and termites (Isoptera. It aimed at understanding via computational modelling and physical prototipation how their spacial design is developed by the “architect-insects” while they build their nests in nature as a way of preserving their bio-cultural heritage besides exploring other possibilites of eco-sustainable technological innovation within low energy consumption. The present work has been made to complement and converge the researches made both in the biomimicry area and the energy field by presenting how one can adapt the solutions in the built space of wasp nests and termite mounds that can be used in many ways at human constructions. Departing from some remarkable work done by entomologists, biologists and engineers, it was possible to get to structural details and how these tiny creatures build their dwellings little by little while they work as a united civilization. After getting to these details, strenghts and weaknesses were reported and one searched similar architectural human solutions for topics such as the crafting work on the surface of the buildings, the ventilation system created by the termites and the improvement of living spaces. By following the life of these little creatures and seeing how they can behave similarly to a human society up to an extent, one notes that one might learn and find architectural solutions by better understanding wasps and termites form-function challenges.
Energy Technology Data Exchange (ETDEWEB)
Haddad, Zoubida [Department of Mechanical Engineering, Technology Faculty, Firat University, TR-23119, Elazig (Turkey); Department of Fluid Mechanics, Faculty of Physics, University of Sciences and Technology-Houari Boumediene, Algiers (Algeria); Abu-Nada, Eiyad [Department of Mechanical Engineering, King Faisal University, Al-Ahsa 31982 (Saudi Arabia); Oztop, Hakan F. [Department of Mechanical Engineering, Technology Faculty, Firat University, TR-23119, Elazig (Turkey); Mataoui, Amina [Department of Fluid Mechanics, Faculty of Physics, University of Sciences and Technology-Houari Boumediene, Algiers (Algeria)
2012-07-15
Natural convection heat transfer and fluid flow of CuO-Water nano-fluids is studied using the Rayleigh-Benard problem. A two component non-homogenous equilibrium model is used for the nano-fluid that incorporates the effects of Brownian motion and thermophoresis. Variable thermal conductivity and variable viscosity are taken into account in this work. Finite volume method is used to solve governing equations. Results are presented by streamlines, isotherms, nano-particle distribution, local and mean Nusselt numbers and nano-particle profiles at top and bottom side. Comparison of two cases as absence of Brownian and thermophoresis effects and presence of Brownian and thermophoresis effects showed that higher heat transfer is formed with the presence of Brownian and thermophoresis effect. In general, by considering the role of thermophoresis and Brownian motion, an enhancement in heat transfer is observed at any volume fraction of nano-particles. However, the enhancement is more pronounced at low volume fraction of nano-particles and the heat transfer decreases by increasing nano-particle volume fraction. On the other hand, by neglecting the role of thermophoresis and Brownian motion, deterioration in heat transfer is observed and this deterioration elevates by increasing the volume fraction of nano-particles. (authors)
Chowdhury, Raju; Parvin, Salma; Khan, Md. Abdul Hakim
2017-06-01
In the present study, natural convective heat and mass transfer and fluid flow inside a window shaped cavity filled with Cu-water nanofluid and containing multiple obstacles with a finite size heater placed in its horizontal wall is numerically investigated. Sinusoidal temperature distribution is maintained by the heater. The left and right inclined walls of the cavity are maintained at a relatively low temperature while the vertical walls are insulated. The governing equations are transformed to the dimensionless form and solved numerically using Galerkin weighted residual technique of finite element method. The influence of pertinent parameters such as thermal Rayleigh number, location of the heater and solid volume fraction of nanoparticles on the heat and mass transfer and fluid flow is studied. The results are obtained in terms of streamlines, isotherms, isoconcentrations, average Sherwood number and average Nueeslt number for the considered parameters and it is observed that the flow pattern, temperature and concentration fields are affected by the variation of the mentioned parameters.
Energy Technology Data Exchange (ETDEWEB)
Arani, A. A. Abbasian; Kakoli, E.; Hajialigol, N. [University of Kashan, Kashan (Iran, Islamic Republic of)
2014-11-15
Natural convection heat and mass transfer characteristics in a square enclosure using variable thermal conductivity and variable viscosity are numerically studied. The fluid in the enclosure is a water-based nanofluid containing Al{sub 2}O{sub 3} nanoparticles. The top and bottom horizontal walls are insulated, while a source (T{sub h}, C{sub h}) and a sink (T{sub c}, C{sub c}) are located at the vertical left and right walls as active parts, respectively, with T{sub h}>T{sub c} and C{sub h}>C{sub c}. The governing equations in the two-dimensional space are discretized using the control volume method. A proper upwinding scheme is employed to obtain stabilized solutions. The study has been carried out for the Rayleigh numbers of 10{sup 4} to 10{sup 6}, the buoyancy ratios of -5 ∼ 5, and different configurations of the source and sink. Results are presented in the form of the streamlines, isotherms and iso-concentraions as well as the average Nusselt and Sherwood numbers. It is observed that average Nusselt number is increased by adding the nanoparticles, while average Sherwood number is reduced. Moreover, both Nusselt and Sherwood number are increased as absolute value of the buoyancy ratio or Rayleigh number is increased.
Mehryan, S. A. M.; Ghalambaz, Mohammad; Ismael, Muneer A.; Chamkha, Ali J.
2017-02-01
This paper investigates numerically the problem of unsteady natural convection inside a square cavity partitioned by a flexible impermeable membrane. The finite element method with the arbitrary Lagrangian-Eulerian (ALE) technique has been used to model the interaction of the fluid and the membrane. The horizontal walls of the cavity are kept adiabatic while the vertical walls are kept isothermal at different temperatures. A uniform magnetic field is applied onto the cavity with different orientations. The cavity has been provided by two eyelets to compensate volume changes due the movement of the flexible membrane. A parametric study is carried out for the pertinent parameters, which are the Rayleigh number (105-108), Hartmann number (0-200) and the orientation of the magnetic field (0-180°). The change in the Hartmann number affects the shape of the membrane and the heat transfer in the cavity. The angle of the magnetic field orientation also significantly affects the shape of the membrane and the heat transfer in the cavity.
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.
Directory of Open Access Journals (Sweden)
Gauri Shanker Seth
2016-01-01
Full Text Available Investigation of unsteady hydromagnetic natural convection flow with heat and mass transfer of a viscous, incompressible, electrically conducting, chemically reactive and optically thin radiating fluid past an exponentially accelerated moving vertical plate with arbitrary ramped temperature embedded in a fluid saturated porous medium is carried out. Exact solutions of momentum, energy and concentration equations are obtained in closed form by Laplace transform technique. The expressions for the shear stress, rate of heat transfer and rate of mass transfer at the plate for both ramped temperature and isothermal plates are derived. The numerical values of fluid velocity, fluid temperature and species concentration are displayed graphically whereas those of shear stress, rate of heat transfer and rate of mass transfer at the plate are presented in tabular form for various values of pertinent flow parameters. It is found that, for isothermal plate, the fluid temperature approaches steady state when t 1.5 . Consequently, the rate of heat transfer at isothermal plate approaches steady state when t 1.5 .
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 bur......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...
Indian Academy of Sciences (India)
R Sapra; S K Dhaka; V Panwar; R Bhatnagar; K Praveen Kumar; Y Shibagaki; M Venkat Ratnam; M Takahashi
2011-10-01
Relationship of outgoing long-wave radiation (OLR) with convective available potential energy (CAPE) and temperature at the 100-hPa pressure level is examined using daily radiosonde data for a period 1980–2006 over Delhi (28.3° N, 77.1°E) and Kolkata (22.3°N, 88.2°E), and during 1989–2005 over Cochin (10°N, 77°E) and Trivandrum (8.5°N, 77.0°E), India. Correlation coefficient (xy) between monthly OLR and CAPE shows a significant (∼ −0.45) anti-correlation at Delhi and Kolkata suggesting low OLR associated with high convective activity during summer (seasonal variation). Though, no significant correlation was found between OLR and CAPE at Cochin and Trivandrum (low latitude region); analysis of OLR and temperature (at 100-hPa) association suggests that low OLR peaks appear corresponding to low temperature at Delhi (xy ∼ 0.30) and Kolkata (xy ∼ 0.25) during summer. However, xy between OLR and temperature becomes opposite as we move towards low latitudes (∼8° – 10°N) due to strong solar cycle influence. Large scale components mainly ENSO and quasi-biennial oscillaton (QBO) that contributed to the 100-hPa temperature variability were also analyzed, which showed that ENSO variance is larger by a factor of two in comparison to QBO over Indian region. ENSO warm conditions cause warming at 100-hPa over Delhi and Darwin. However, due to strong QBO and solar signals in the equatorial region, ENSO signal seems less effective. QBO, ENSO, and solar cycle contribution in temperature are found location-dependent (latitudinal variability) responding in consonance with shifting in convective activity regime during El Niño, seasonal variability in the tropical easterly jet, and the solar irradiance.
76 FR 67158 - Secretary of Energy Advisory Board Natural Gas Subcommittee
2011-10-31
... From the Federal Register Online via the Government Publishing Office DEPARTMENT OF ENERGY Secretary of Energy Advisory Board Natural Gas Subcommittee AGENCY: Department of Energy. ACTION: Notice of... of the Secretary of Energy Advisory Board (SEAB) Natural Gas Subcommittee. The public meeting...
Energy Technology Data Exchange (ETDEWEB)
Pless, Jacquelyn [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Arent, Douglas J. [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Logan, Jeffrey [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Cochran, Jaquelin [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Zinaman, Owen [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Stark, Camila [National Renewable Energy Laboratory (NREL), Golden, CO (United States)
2015-04-30
Ensuring the resilience, reliability, flexibility, and affordability of the U.S. electric grid is increasingly important as the country addresses climate change and an aging infrastructure. State and federal policy and actions by industry, non-profits, and others create a dynamic framework for achieving these goals. Three principle low-carbon generation technologies have formed the basis for multiple scenarios leading toward a low-carbon, resilient, and affordable power system. While there is no “silver bullet,” one avenue identified by key stakeholders is the opportunity to invest in natural gas (NG) and renewable resources, both of which offer abundant domestic resource bases and contribute to energy independence, carbon mitigation, and economic growth. NG and renewable electricity (RE) have traditionally competed for market share in the power sector, but there is a growing experience base and awareness for their synergistic use (Cochran et al. 2014). Building upon these observations and previous work, the Joint Institute for Strategic Energy Analysis (JISEA), in collaboration with the Center for the New Energy Economy and the Gas Technology Institute, convened a series of workshops in 2014 to explore NG and RE synergies in the U.S. power sector. This report captures key insights from the workshop series, Synergies of Natural Gas and Renewable Energy: 360 Degrees of Opportunity, as well as supporting economic valuation analyses conducted by JISEA researchers that quantify the value proposition of investing in NG and RE together as complements.
Law, the Laws of Nature and Ecosystem Energy Services: A Case of ...
African Journals Online (AJOL)
Law, the Laws of Nature and Ecosystem Energy Services: A Case of wilful Blindness. ... models that do not adequately account for energy ecosystem services in ... global warming; market failure; environmental externalities; energy subsidies; ...
Energy Technology Data Exchange (ETDEWEB)
Oki, Y.; Tanahashi, T. (Keio University, Tokyo (Japan). Faculty of Science and Technology)
1994-05-25
An elucidation has been made on features of two magnetic field analysis schemes in analyzing natural convection in an electromagnetically heated fluid. A quantitative discussion has also been given on the cyclicity inherent to a low-Prandtl number fluid. Twin vortices in an induced magnetic field at the center of a square cavity had Lorentz force that accelerates convection acted on them. However, as the magnetic field strength increases, the twin vortices have disappeared, with only the secondary vortices remaining on four corners of the cavity where Lorentz force that suppresses convection acts on. The motion energy and the frequency characteristics of fluid magnetic cross helicity have verified quantitatively how a flow transfers from a cyclic flow to a steady flow as the magnetic field strength increases. It was indicated that the numerical residual method has superior convergence in solution of Poisson equations except for the initial stage under a weak magnetic field to the cross helicity. It was also shown that the numerical residual method has superior solution convergence from the initial stage under a strong magnetic field. Particularly for the case with large Joule heat generation, degradation in the calculation efficiency for the numerical residual method is more remarkable than in the cross helicity method. 8 refs., 10 figs.
Wolf, Bart J.; Johnson, D. R.
1995-01-01
A kinetic energy (KE) analysis of the forcing of a mesoscale upper-tropospheric jet streak by organized diabatic processes within the simulated convective system (SCS) that was discussed in Part 1 is presented in this study. The relative contributions of the ageostrophic components of motion to the generation of KE of the convectively generated jet streak are compared, along with the KE generation by the rotational (nondivergent) and irrotational (divergent) mass transport. The sensitivity of the numerical simulations of SCS development to resolution is also briefly examined. Analysis within isentropic coordinates provides for an explicit determination of the influence of the diabatic processes on the generation of KE. The upper-level production of specific KE is due predominatly to the inertial advective ageostrophic component (IAD), and as such represents the primary process through which the KE of the convectively generated jet streak is realized. A secondary contribution by the inertial diabatic (IDI) term is observed. Partitioning the KE generation into its rotational and irrotational components reveals that the latter, which is directly linked to the diabatic heating within the SCS through isentropic continuity requirements, is the ultimate source of KE generation as the global area integral of generation by the rotational component vanishes. Comparison with an identical dry simulation reveals that the net generation of KE must be attributed to latent heating. Both the IAD and IDI ageostrophic components play important roles in this regard. Examination of results from simulations conducted at several resolutions supports the previous findings in that the effects of diabatic processes and ageostrophic motion on KE generation remain consistent. Resolution does impact the location and timing of SCS development, a result that has important implications in forecasting the onset of convection that develops from evolution of the large-scale flow and moisture
The Spectral Amplitude of Stellar Convection and Its Scaling in the High-Rayleigh-number Regime
Featherstone, Nicholas A.; Hindman, Bradley W.
2016-02-01
Convection plays a central role in the dynamics of any stellar interior, and yet its operation remains largely hidden from direct observation. As a result, much of our understanding concerning stellar convection necessarily derives from theoretical and computational models. The Sun is, however, exceptional in that regard. The wealth of observational data afforded by its proximity provides a unique test bed for comparing convection models against observations. When such comparisons are carried out, surprising inconsistencies between those models and observations become apparent. Both photospheric and helioseismic measurements suggest that convection simulations may overestimate convective flow speeds on large spatial scales. Moreover, many solar convection simulations have difficulty reproducing the observed solar differential rotation owing to this apparent overestimation. We present a series of three-dimensional stellar convection simulations designed to examine how the amplitude and spectral distribution of convective flows are established within a star’s interior. While these simulations are nonmagnetic and nonrotating in nature, they demonstrate two robust phenomena. When run with sufficiently high Rayleigh number, the integrated kinetic energy of the convection becomes effectively independent of thermal diffusion, but the spectral distribution of that kinetic energy remains sensitive to both of these quantities. A simulation that has converged to a diffusion-independent value of kinetic energy will divide that energy between spatial scales such that low-wavenumber power is overestimated and high-wavenumber power is underestimated relative to a comparable system possessing higher Rayleigh number. We discuss the implications of these results in light of the current inconsistencies between models and observations.
76 FR 31318 - Secretary of Energy Advisory Board Natural Gas Subcommittee
2011-05-31
... Secretary of Energy Advisory Board Natural Gas Subcommittee AGENCY: Department of Energy. ACTION: Notice of... (SEAB) Natural Gas Subcommittee. SEAB was reestablished pursuant to the Federal Advisory Committee Act... directed by the Secretary. The Natural Gas Subcommittee was established to provide advice...
Environmental and energy performance of the biomass to synthetic natural gas supply chain
Miedema, Jan Hessels; Moll, Henri; Benders, Reinerus
2016-01-01
A quarter of the total primary energy demand in the European Union is met by natural gas. Synthetic natural gas produced through biomass gasification can contribute to a more sustainable energy supply system. A chain analysis of the energetic performance of synthetic natural gas where the upstream,
76 FR 34070 - Secretary of Energy Advisory Board Natural Gas Subcommittee
2011-06-10
... Secretary of Energy Advisory Board Natural Gas Subcommittee AGENCY: Department of Energy, DOE. ACTION... Advisory Board (SEAB) Natural Gas Subcommittee. SEAB was reestablished pursuant to the Federal Advisory..., operational issues and other activities as directed by the Secretary. The Natural Gas Subcommittee...
Directory of Open Access Journals (Sweden)
Mahmood H. Ali
2013-05-01
Full Text Available A numerical study of non-Darcian natural convection heat transfer in a rectangular enclosure filled with porous medium saturated with viscous fluid was carried out. The effects of medium Rayleigh number, porosity, particle to fluid thermal conductivity ratio, Darcy number and enclosure aspect ratio on heat transfer were examined to demonstrate the ability of using this construction in thermal insulation of buildings walls.A modified Brinkman-Forchheimer-extended Darcy flow model was used and no-slip boundary conditions were imposed for velocity at the walls and the governing equations were expressed in dimensionless stream function, vorticity, and temperature formulation. The resulting algebraic equations obtained from finite difference discritization of vorticity and temperature equations are solved using (ADI method which uses Three Diagonal Matrix Algorithm (TDMA in each direction, while that of the stream function equation solved using successive iteration method.The study was done for the range of enclosure aspect ratio ( which is in the tall layers region at medium Rayleigh number ( , Darcy number (Da=10-3, 10-4, 10-5 , porosity (e=0.35, 0.45, 0.55, particle to fluid thermal conductivity (kS/kf=5.77, 38.5, 1385.5.The results showed that the Nusselt number is direct proportional to medium Rayleigh number and porosity and reversely proportional to Darcy number, ratio of particle to fluid thermal conductivity and enclosure aspect ratio. The variables that affect the heat transfer in the above arrangement was correlated in a mathematical equation that account better for their affects on heat transfer which is represented by mean Nusselt number (Nu.
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)
S. S. Das, R. K. Tripathy, R. K. Padhy, M. Sahu
2012-01-01
Full Text Available 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 (less than or equal 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.
底部加热长方体腔内自然对流的非线性特性%Nonlinear Characters of Natural Convection Heated From Below
Institute of Scientific and Technical Information of China (English)
战乃岩; 杨茉
2012-01-01
Nonlinear phenomena in natural convection of air in a three-dimensional rectangular cavity heated from below have been investigated numerically using SIMPLE algorithm with QUICK scheme. It is found that the different results occur when the different initial conditions are given and only one of the results is consist with the experimental result. But the influence of initial conditions appears in some range. When Rayleigh number is above a critical value, unsteady oscillation occurs. With increase of Rayleigh number, flow and heat transfer change from steady to unsteady state, and transition to chaos occurs through multi-periodical oscillation. The critical value of Rayleigh number of transition is different for different aspect ratio.%采用SIMPLE算法，QUICK差分格式，对底部加热三维长方体腔内空气的自然对流进行了数值模拟。根据模拟结果，探讨了方腔内流体流动与换热的静态分岔与振荡等非线性现象。数值结果显示，在固定的几何尺寸和不同Ra的情况下，当初始场不同时，会出现若干不同的解，即存在解的静态分岔；在固定的几何尺寸和相同的初始场情况下，低Ra时流动和换热处于稳态，当Ra超过某一临界值时，流动和换热就会随时间振荡，并通过倍周期分岔过渡到混沌；当方腔的几何尺寸不同时，分岔点的特征值Ra也发生变化。
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...
Plumes in stellar convection zones
Zahn, J P
1999-01-01
All numerical simulations of compressible convection reveal the presence of strong downwards directed flows. Thanks to helioseismology, such plumes have now been detected also at the top of the solar convection zone, on super- granular scales. Their properties may be crudely described by adopting Taylor's turbulent entrainment hypothesis, whose validity is well established under various conditions. Using this model, one finds that the strong density stratification does not prevent the plumes from traversing the whole convection zone, and that they carry upwards a net energy flux (Rieutord & Zahn 1995). They penetrate to some extent in the adjacent stable region, where they establish a nearly adiabatic stratification. These plumes have a strong impact on the dynamics of stellar convection zones, and they play probably a key role in the dynamo mechanism.
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.)
Energy efficiency as an opportunity for the natural gas industry
Energy Technology Data Exchange (ETDEWEB)
Love, P. [Canadian Energy Efficiency Alliance (Canada)
2003-07-01
Energy conservation, energy efficiency and demand side management are defined and the role played in the promotion and advancement of energy efficiency objectives by the Canadian Energy Efficiency Alliance are explained. Direct and indirect economic and environmental benefits and the potential impacts in terms of savings and jobs are discussed, with examples of successful greenhouse gas emission reduction programs by industry. The total potential for energy efficiency in Canada is estimated at 18 per cent lower energy use by 2010, and 33 per cent by 2020, assuming that specific policy recommendations and other cost effective efficiency measures are implemented. Overall conclusions are that there is a large potential for cost-effective energy savings over and above of what has been done already. Furthermore, utilities can play a leading role in realizing these efficiencies, and in the process achieve substantial benefits for themselves.
Energy Technology Data Exchange (ETDEWEB)
Knoph, Per Olaf
2006-07-01
The presentation surveys the Polytec research facility in Haugesund and some of it's programs on energy efficiency and systems, Co2 and energy source applications. The Cogen plant principle and design is described. Cogenizing with an natural gas fuelled gas engine as an example is presented. Some aspects of energy efficiency and future perspectives are discussed. (tk)
Energy efficiency evaluation of a natural gas pipeline based on an analytic hierarchy process
National Research Council Canada - National Science Library
Xie, Ying; Ma, Xiufen; Ning, Haifeng; Yuan, Zongming; Xie, Ting
2017-01-01
A long-distance natural gas pipeline system consists of considerable equipment and many pipe segments, but the conventional energy efficiency index of a natural gas pipeline is considered as a whole...
Smart procurement of naturally generated energy (SPONGE) for PHEVs
Gu, Yingqi; Häusler, Florian; Griggs, Wynita; Crisostomi, Emanuele; Shorten, Robert
2016-07-01
In this paper, we propose a new engine management system for hybrid vehicles to enable energy providers and car manufacturers to provide new services. Energy forecasts are used to collaboratively orchestrate the behaviour of engine management systems of a fleet of plug-in hybrid electric vehicle (PHEVs) to absorb oncoming energy in a smart manner. Cooperative algorithms are suggested to manage the energy absorption in an optimal manner for a fleet of vehicles, and the mobility simulator SUMO (Simulation of Urban MObility) is used to demonstrate the efficacy of the proposed idea.
Energy Technology Data Exchange (ETDEWEB)
Lee, A.; Zinaman, O.; Logan, J.
2012-12-01
Use of both natural gas and renewable energy has grown significantly in recent years. Both forms of energy have been touted as key elements of a transition to a cleaner and more secure energy future, but much of the current discourse considers each in isolation or concentrates on the competitive impacts of one on the other. This paper attempts, instead, to explore potential synergies of natural gas and renewable energy in the U.S. electric power and transportation sectors.
Modeling of heat explosion with convection.
Belk, Michael; Volpert, Vitaly
2004-06-01
The work is devoted to numerical simulations of the interaction of heat explosion with natural convection. The model consists of the heat equation with a nonlinear source term describing heat production due to an exothermic chemical reaction coupled with the Navier-Stokes equations under the Boussinesq approximation. We show how complex regimes appear through successive bifurcations leading from a stable stationary temperature distribution without convection to a stationary symmetric convective solution, stationary asymmetric convection, periodic in time oscillations, and finally aperiodic oscillations. A simplified model problem is suggested. It describes the main features of solutions of the complete problem.
QCD nature of dark energy at finite temperature: cosmological implications
Azizi, K
2015-01-01
The Veneziano ghost field has been proposed as an alternative source of dark energy whose energy density is consistent with the cosmological observations. In this model, the energy density of QCD ghost field is expressed in terms of QCD degrees of freedom at zero temperature. We extend this model to finite temperature to search the model predictions from the late time to the early universe. We depict the variations of QCD parameters entering the calculations, dark energy density, equation of state, Hubble and deceleration parameters on temperature from zero to a critical temperature. We compare our results with the observations and theoretical predictions existing at different eras.It is found that this model safely define the universe from quark condensation up to now and its predictions are not in tension with those of the standard cosmology. The finite temperature ghost dark energy predictions on the Hubble parameter slightly better fit to observations compared to those of zero temperature.
Institute of Scientific and Technical Information of China (English)
戴传山; 王珏
2015-01-01
采用铂丝自测流体温度的实验方法，利用50×10−6 m2·s−1硅油对低Rayleigh数下水平与竖直热线外自然对流传热的机理进行了研究，对比实验测得的实验数据和Tsubouchi、Fand的实验数据发现，实验结果与Tsubouchi的拟合曲线吻合度较高，低Ra下水平热线外的Nu高于竖直热线，但均远高于数值模拟的预测值。为了解释该现象，进行了低振动Re下的自然对流数值模拟，发现在低于某一振动Re下，Nu将与Re无关，但此时Nu与实验测的Nu基本一致，与Tsubouchi的数据偏差也在5%以内，说明微尺度铂丝外自然对流可能存在振动的作用机理。%The mechanism of natural convection around horizontal and vertical heated microwires in silicon oil is studied by using the wire itself for temperature measurement. The comparison between present measured Nusselt numbers and those in literature shows that the present data agreed better with those of Tsubouchi than those of Fand. The horizontal microwire presents a larger Nu than the vertical one at low Rayleigh number. However, all of these experimental Nu are much larger than the numerically simulated values. In order to give a theoretical explanation on this phenomenon, heated microwire with weak vibration was used in the numerical simulations. The simulated Nu shows a perfect match with the measured data and are in agreement with the correlation of Tsubouchi with a deviation less than 5%. Therefore, an enhanced heat transfer mode due to weak wire vibration is proposed, to fill the gap between simulated and experimental results.
OZAWA,Hisashi; SHIMOKAWA, Shinya
2015-01-01
The formation process of circulatory motion of a tropical cyclone is investigated from a thermodynamic viewpoint. The generation rate of mechanical energy by a fluid motion under diabatic heating and cooling, and the dissipation rate of this energy due to irreversible processes are formulated from the first and second laws of thermodynamics. This formulation is applied to a tropical cyclone, and the formation process of the circulatory motion is examined from a balance between the generation ...
Helioseismology challenges models of solar convection
Gizon, Laurent; 10.1073/pnas.1208875109
2012-01-01
Convection is the mechanism by which energy is transported through the outermost 30% of the Sun. Solar turbulent convection is notoriously difficult to model across the entire convection zone where the density spans many orders of magnitude. In this issue of PNAS, Hanasoge et al. (2012) employ recent helioseismic observations to derive stringent empirical constraints on the amplitude of large-scale convective velocities in the solar interior. They report an upper limit that is far smaller than predicted by a popular hydrodynamic numerical simulation.
Relationship of energy transformation and use to the natural environment
Energy Technology Data Exchange (ETDEWEB)
Horacek, P.; Razga, J.
1987-05-01
Analyzes effects of air pollution from coal combustion on living conditions, life expectance and environment. Environmental policy of Czechoslovakia is discussed. Cost of environmental protection until the year 2000 is discussed. Energy conservation planned for 1986-2000 is evaluated. Investment associated with reducing energy consumption in the national economy ranges from 40 crowns/GJ energy in 1986 to 86 crowns/GJ in 2000. Cost of sulfur dioxide control and nitrogen oxide control (investment and operating cost) in 1986-2000 is discussed. Cost of energy production by coal-fired power plants is compared to that of nuclear power plants. Environmental effects of both types of power plants are comparatively evaluated. Advantages of nuclear plant development from an environmental point of view are discussed. 12 refs.