Forced convection heat transfer in narrow annulus with bilateral heating
International Nuclear Information System (INIS)
Experiments were conducted to study the single-phase forced convection heat transfer with water in vertical narrow annulus which is simultaneously heated from inside and outside by primary water. A comparative analysis on heat transfer performance between bilateral and unilateral heating was carried out. The results indicate that double side heating enhances the heat transfer on inside surface when Reynolds number is higher and reduces the heat transfer on outside surface when Reynolds number is small, compared with single-side heating. For both heating modes, the varying trend of Nusselt number is similar and the total heat transfer rate keeps almost equal in value
Forced heat convection in annular spaces
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This report deals with the experimental study of forced heat convection in annular spaces through which flow of air is passing when a uniform heat flux is dissipated across the inner wall. These observations took place chiefly in the region where thermal equilibrium are not yet established. Amongst other things it became apparent that, both in the region where thermal equilibrium conditions are on the way to establishment and where they are already established, the following relationship held good: the longitudinal temperature gradient, either on the wall or in the fluid stream, is proportional to the heat flux dissipated q, and inversely proportional to the average flow rate V: dT/dx = B (q/V). From this result the next step is to express the variations of the local convection coefficient ? (or of the Margoulis number M) in a relationship of the form: 1/M = ?(V) + F(x). If this relationship is compared with the classical empirical relationship ? = KVn (where n is close to 0.8), the relationship: 1/M = ?V1-n + F(x) is obtained (? is a constant for a given annular space); from this it was possible to coordinate the whole set of experimental results. (author)
Theory of heat transfer with forced convection film flows
Shang, Deyi
2010-01-01
Developing a new treatment of ""Free Convection Film Flows and Heat Transfer"" began in Shang's first monograph and is continued in this monograph. The current book displays the recent developments of laminar forced convection and forced film condensation. It is aimed at revealing the true features of heat and mass transfer with forced convection film flows to model the deposition of thin layers. The novel mathematical similarity theory model is developed to simulate temperature - and concentration - dependent physical processes. The following topics are covered in this book: Mathematical meth
Forced convective post-CHF heat transfer and quenching
International Nuclear Information System (INIS)
This paper discusses mechanisms in the postcritical heat flux region that provide understanding and qualitative prediction capability for several current forced-convective heat transfer problems. In the area of nuclear reactor safety, the mechanisms are important in the prediction of fuel rod quenches for the reflood phase, blowdown phase, and possibly some operational transients with dryout. Results using the mechanisms to investigate forced-convective quenching are presented. 26 refs
Approximate techniques for heat transfer description for forced convective boiling
International Nuclear Information System (INIS)
Empirical ratios for heat transfer coefficients prediction during the boiling of one-component liquid under the forced convection are presented. Convection without nucleate boiling is studied, an additional contribution of nucleate boiling of saturated and subcooled liquid is also considered
Pressure drop, forced convective, free convective and radiant heat transfer of pebble beds
International Nuclear Information System (INIS)
For safety studies low Reynold numbers, free convective heat transfer and high temperature radiation are considered when assuming failure of the total forced convective cooling system of the gas-cooled high temperature reactor with pebble bed core. A computer programme (THERMIX) has been developed to calculate the fluid dynamics and heat transfer of a pebble bed. The Nusselt number for natural convective heat transfer was found to be independent of the void fraction. (DG)
Transient heat transfer for forced convection flow of helium gas
International Nuclear Information System (INIS)
The knowledge of forced convection transient heat transfer at various periods of exponential increase of heat input to a heater as a database for understanding the transient heat transfer process in a high temperature gas cooled reactor (HTGR) due to an accident in excess reactivity. The transient heat transfer coefficients for forced convection flow of helium gas over a horizontal cylinder were measured using a forced convection test loop. The platinum heater with a diameter of 1.0 mm was heated by electric current with an exponential increase of Q0 exp(t/ ). It was clarified that the heat transfer coefficient approaches the steady-state one for the period over 1 s, and it becomes higher for the period of shorter than 1 s. The transient heat transfer shows less dependent on the gas flowing velocity when the period becomes very shorter. Semi-empirical correlations for steady state and transient heat transfer were developed based on the experimental data. (authors)
Subcooled forced convection film boiling heat transfer
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In this article the theoretical calculation of film boiling heat transfer to subcooled liquid flowing upwards vertically is presented. Four heat transfer models have been proposed, analysis and comparison have been made in these models. The author takes into account both laminar and turbulent flow patterns for vapor film, different thermal boundary conditions and hydraulic conditions at the liquid-vapor interface, etc. The prediction is compared with reflooding experimental data, the steady state film boiling data. The agreement is satisfactory. At the same time, the heat transfer in self-modelling region is discussed
Forced-convective post-CHF heat transfer and quenching
International Nuclear Information System (INIS)
Mechanisms in the postcritical heat flux region that provide understanding and qualitative prediction capability for several current force-convective heat-transfer problems are discussed. In the area of nuclear reactor safety, the mechanisms are important in the prediction of fuel rod cooldown and quenches for the reflood phase, blowdown phase, and possibly some operational transients with dryout. Results using the mechanisms to investigate forced-convective quenching are presented. Data reduction of quenching experiments is discussed, and the way in which the quenching transient may affect the results of different types of quenching experiments is investigated. This investigation provides an explanation of how minimum wall superheats greater than the homogeneous nucleation temperature result, as well as how these may be either hydrodynamically or thermodynamically controlled
Steady state forced convection heat transfer in He II
International Nuclear Information System (INIS)
A study of forced convection heat transfer in superfluid helium (He II) is initiated to better understand the physical behavior of this process and to compare it with the more familiar He II heat transfer mechanism of internal convection. An experimental assembly is designed to achieve fluid flow by a motor-driven hydraulic pump which utilizes two stainless steel bellows. Each bellows is connected to one end of a copper tube, 3 mm in diameter and 2 m long. The system allows measurements of one dimensional heat and mass transfer where the measured quantities include: temperature profile and pressure drop. The variable quantities are the helium bath temperature, flow velocity and heat input. The helium bath is held at 1.8 K and under saturation pressure. The flow tube is heated at the middle and the flow velocity is varied up to 97 cm/s. The helium pressure is monitored at both ends of the tube and a friction factor is estimated for He II. Temperature measurements are made at seven evenly spaced locations along the tube. The experimental temperature profile is compared with a numerical solution of an analytical model developed for the problem under study
Single phase channel flow forced convection heat transfer
Energy Technology Data Exchange (ETDEWEB)
Hartnett, J.P.
1999-04-01
A review of the current knowledge of single phase forced convection channel flow of liquids (Pr > 5) is presented. Two basic channel geometries are considered, the circular tube and the rectangular duct. Both laminar flow and turbulent flow are covered. The review begins with a brief overview of the heat transfer behavior of Newtonian fluids followed by a more detailed presentation of the behavior of purely viscous and viscoelastic Non-Newtonian fluids. Recent developments dealing with aqueous solutions of high molecular weight polymers and aqueous solutions of surfactants are discussed. The review concludes by citing a number of challenging research opportunities.
Forced convection film boiling heat transfer on spheres
International Nuclear Information System (INIS)
This paper presents an experimental study of forced convection film boiling on sphere. To measure the film boiling heat transfer coefficients, a heated sphere with a thermocouple embedded at the center is dropped in (1) a vertical tube filled with water, or (2) a rotating water pool. The rotating pool is for providing higher fluid velocity. The falling speed of the sphere in the vertical stainless steel tube is measured using a set of magnet pickup coils distributed along the tube. The rotating pool is made of Pyrex to make a visualization. The ranges of the experimental conditions are: sphere speed 0.1-1.6 m/s, sphere surface temperature 500 and 700degC. The results show that the forced convection film boiling heat transfer coefficients were 200?400 W/m2K in the present experimental conditions and also show that there are large differences between the data sets or correlations due mainly to experimental difficulties. (author)
Comparative analysis of heat transfer correlations for forced convection boiling
International Nuclear Information System (INIS)
A critical survey was conducted of the most relevant correlations of boiling heat transfer in forced convection flow. Most of the investigations carried out on partial nucleate boiling and fully developed nucleate boiling have led to the formulation of correlations that are not able to cover a wide range of operating conditions, due to the empirical approach of the problem. A comparative analysis is therefore required in order to delineate the relative accuracy of the proposed correlations, on the basis of the experimental data presently available. The survey performed allows the evaluation of the accuracy of the different calculating procedure; the results obtained, moreover, indicate the most reliable heat transfer correlations for the different operating conditions investigated. This survey was developed for five pressure range (up to 180bar) and for both saturation and subcooled boiling condition
Forced convective heat transfer in a porous plate channel
Jiang, Peixue; Wang, Zhan; Ren, Zepei; Wang, Buxuan
1997-09-01
Forced convective heat transfer in a plate channel filled with metallic spherical particles was investigated experimentally and numerically. The test section, 58 mm×80 mm×5 mm in size, was heated by a 0.4 mm thick plate electrical heater. The coolant water flow rate ranged from 0.015 to 0.833 kg/s. The local wall temperature distribution was measured along with the inlet and outlet fluid temperatures and pressures. The results illustrate the heat transfer augmentation and increased pressure drop caused by the porous medium. The heat transfer coefficient was increased 5-12 times by the porous media although the hydraulic resistance was increased even more. The Nusselt number and the heat transfer coefficient increased with decreasing particle diameter, while the pressure drop decreased as the particle diameter increased. It was found that, for the conditions studied (metallic packed bed), the effect of thermal dispersion did not need to be considered in the physical model, as opposed to a non-metallic packed bed, where thermal dispersion is important.
Heat transfer enhancement by using nanofluids in forced convection flows
International Nuclear Information System (INIS)
In the present paper, the problem of laminar forced convection flow of nanofluids has been thoroughly investigated for two particular geometrical configurations, namely a uniformly heated tube and a system of parallel, coaxial and heated disks. Numerical results, as obtained for water-?Al2O3 and Ethylene Glycol-?Al2O3 mixtures, have clearly shown that the inclusion of nanoparticles into the base fluids has produced a considerable augmentation of the heat transfer coefficient that clearly increases with an increase of the particle concentration. However, the presence of such particles has also induced drastic effects on the wall shear stress that increases appreciably with the particle loading. Among the mixtures studied, the Ethylene Glycol-?Al2O3 nanofluid appears to offer a better heat transfer enhancement than water-?Al2O3; it is also the one that has induced more pronounced adverse effects on the wall shear stress. For the case of tube flow, results have also shown that, in general, the heat transfer enhancement also increases considerably with an augmentation of the flow Reynolds number. Correlations have been provided for computing the Nusselt number for the nanofluids considered in terms of the Reynolds and the Prandtl numbers and this for both the thermal boundary conditions considered. For the case of radial flow, results have also shown that both the Reynolds number a shown that both the Reynolds number and the distance separating the disks do not seem to considerably affect in one way or another the heat transfer enhancement of the nanofluids (i.e. when compared to the base fluid at the same Reynolds number and distance)
Dag, Yusuf
Forced convection over traditional surfaces such as flat plate, cylinder and sphere have been well researched and documented. Data on forced convection over airfoil surfaces, however, remain very scanty in literature. High altitude vehicles that employ airfoils as lifting surfaces often suffer leading edge ice accretions which have tremendous negative consequences on the lifting capabilities and stability of the vehicle. One of the ways of mitigating the effect of ice accretion involves judicious leading edge convective cooling technique which in turn depends on the accuracy of convective heat transfer coefficient used in the analysis. In this study empirical investigation of convective heat transfer measurements on asymmetric airfoil is presented at different angle of attacks ranging from 0° to 20° under subsonic flow regime. The top and bottom surface temperatures are measured at given points using Senflex hot film sensors (Tao System Inc.) and used to determine heat transfer characteristics of the airfoils. The model surfaces are subjected to constant heat fluxes using KP Kapton flexible heating pads. The monitored temperature data are then utilized to determine the heat convection coefficients modelled empirically as the Nusselt Number on the surface of the airfoil. The experimental work is conducted in an open circuit-Eiffel type wind tunnel, powered by a 37 kW electrical motor that is able to generate subsonic air velocities up to around 41 m/s in the 24 square-inch test section. The heat transfer experiments have been carried out under constant heat flux supply to the asymmetric airfoil. The convective heat transfer coefficients are determined from measured surface temperature and free stream temperature and investigated in the form of Nusselt number. The variation of Nusselt number is shown with Reynolds number at various angles of attacks. It is concluded that Nusselt number increases with increasing Reynolds number and increase in angle of attack from 0° to 20° on the upper and lower surface of the airfoil.
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Ghadimi, Mohammad; Ghadamian, Hossein [Islamic Azad Univ., Tehran (Iran, Islamic Republic of). Dept. of Energy Engineering, Science and Research Branch; Hamidi, Aliasghar A. [Tehran Univ. (Iran, Islamic Republic of). Dept. of Chemical Engineering; Fazelpour, Farivar [Islamic Azad Univ. of South Tehran Branch, Tehran (Iran, Islamic Republic of). Dept. of Energy System Engineering; Behghadam, Mehdi [Islamic Azad Univ. of Roudehen Branch, Tehran (Iran, Islamic Republic of). Dept. of Mechanical Engineering
2012-11-01
The present paper describes a two-dimensional finite volume numerical simulation of flow and heat transfer in airflow windows by free and forced convection techniques. The governing equations are the fully elliptic, Reynolds-averaged Navier-Stokes equations. The simple algorithm is employed to correct the pressure term. The second-order upwind scheme is used to discretize the convection terms. The (k-{epsilon}/RNG) turbulence model is applied for the flow simulation. The mesh used is the body-fitted, multi-plane grid system. Results on the variations of velocity and temperature profiles with geometrical parameters, at different temperature and velocity, for heat transfer by free and forced convection techniques are presented. Comparisons of the present results on temperature distribution for forced convection and for free convection with the available experimental forced convection data indicate that the airflow-influenced forced convection methods are considerably enhanced. (orig.)
International Nuclear Information System (INIS)
For predicting the fully developed upward flow in a uniformly heated vertical pipe by taking account of the buoyancy force, the k-? models of turbulence for low Reynolds number flows were adopted. The regime map for forced, mixed and natural convections as well as for laminar and turbulent flows was plotted from the numerical calculations. At the same time, an experiment was carried out at Reynolds numbers of 3000 and 5000 with the Grashof number varied over a wide range by using pressurized nitrogen gas as a test fluid. In agreement with the prediction, buoyancy-induced impairment of heat transfer was measured right in the mixed convection region. Further, from hot-wire measurement, complete laminarization was demonstrated in the mixed convection region at a Reynolds number of 3000. (author)
Transient forces convection heat transfer to helium during a step in heat flux
International Nuclear Information System (INIS)
Transient forced convection heat transfer coefficients for both subcritical and supercritical helium in a rectangular flow channel heated on one side were measured during the application of a step in heat flow. Zero flow data were also obtained. The heater surface which served simultaneously as a thermometer was a fast response carbon film. Operating conditions covered the following range: Pressure, 1.0 x 105 PAia (1 bar) to 1.0 x 106 Pa (10 bar); Temperature, 4 K-10 K; Heat flux, 0.1 W/cm2-10 W/cm2; Reynolds number, 0-8 x 105. The experimental data and a predictive correlation are presented
Effect of Buoyancy on Forced Convection Heat Transfer in Vertical Channels - a Literature Survey
International Nuclear Information System (INIS)
This report contains a short resume of the available information from various sources on the effect of free convection flow on forced convection heat transfer in vertical channels. Both theoretical and experimental investigations are included. Nearly all of the theoretical investigations are concerned with laminar flow with or without internal heat generation. More consistent data are available for upward flow than for downward flow. Curves are presented to determine whether free convection or forced convection mode of heat transfer is predominant for a particular Reynolds number and Rayleigh number. At Reb > 105 free convection effects are negligible. Downward flow through a heated channel at low Reynolds number is unstable. Under similar conditions the overall heat transfer coefficient for downward flow tends to be higher than that for upward flow
Uddin, Md Jashim; Khan, Waqar A.; Ismail, A. I. Md
2013-01-01
A two-dimensional steady forced convective flow of a Newtonian fluid past a convectively heated permeable vertically moving plate in the presence of a variable magnetic field and radiation effect has been investigated numerically. The plate moves either in assisting or opposing direction to the free stream. The plate and free stream velocities are considered to be proportional to whilst the magnetic field and mass transfer velocity are taken to be proportional to where is the distance along ...
Forced convection heat transfer from heated surface in a cavity formed by two high ribs
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The work under consideration is devoted to experimental study on forced convection heat transfer from heated surface between two high ribs. Great attention is paid to the flow visual picture. The experiments were carried out in the channel of a subsonic aerodynamic tube with 200x200 mm cross section. Ribs of material with low heat conductivity (organic glass) 3 mm thick were located at the channel lower wall. The ribs were of similar height (h=60 mm) and the distance between them was equal to L/h=1; 2; 3 calibers. The most interesting fact from practical point of view is that average heat transfer between the ribs appeared to be by 1.5-3 times lower as compared to streamlining smooth surface
Forced convective heat transfer enhancement with perforated pin fins
Chin, Swee-Boon; Foo, Ji-Jinn; Lai, Yin-Ling; Yong, Terry Kin-Keong
2013-10-01
Increasing miniaturization of high speed multi-functional electronics demands ever more stringent thermal management. The present work investigates experimentally and numerically the use of staggered perforated pin fins to enhance the rate of heat transfer in these devices. In particular, the effects of the number of perforations and the diameter of perforation on each pin are studied. The results show that the Nusselt number for the perforated pins is 45 % higher than that for the conventional solid pins and it increases with the number of perforation. Pressure drop with perforated pins is also reduced by 18 % when compared with that for solid pins. Perforations produce recirculations in the x- y as well as the x- z planes downstream of the pins which effectively increase convective heat transfer. However, thermal dissipation decreases significantly when the ratio of pin diameter to perforation diameter exceeds 0.375. This is due to both a reduction in the number of perforation per pin and the decrease in the axial heat conduction along the pin.
Studies of Forced-Convection Heat Transfer Augmentation in Large Containment Enclosures
International Nuclear Information System (INIS)
Heat transfer enhancement due to jet mixing inside a cylindrical enclosure is discussed. This work addresses conservative heat transfer assumptions regarding mixing and condensation that have typically been incorporated into passive containment design analyses. This research presents the possibility for increasing decay heat removal of passive containment systems under combined natural and forced convection. Eliminating these conservative assumptions could result in a changed containment design and reduce the construction cost. It is found that the ratio of forced- and free-convection Nusselt numbers can be predicted as a function of the Archimedes number and a correlated factor accounting for jet orientation and enclosure geometry
NUMERICAL ANALYSIS OF FORCED CONVECTIVE HEAT TRANSFER THROUGH HELICAL CHANNELS
Reby Roy, Dr K. E.; Shenoy, Rohit N.; Bibin Prasad
2012-01-01
Helical ducts are used in a variety of applications including food processing, thermal processing plants and refrigeration. They are advantageous due to their high heat transfer coefficient and compactness compared to straight tubes. The curvature of the coil governs the centrifugal force resulting in development of secondaryflow i.e. the fluid stream in the outer side of the pipe moves faster than the fluid streams in the inner side of the pipe. In the present study, Computational Fluid Dyna...
NUMERICAL ANALYSIS OF FORCED CONVECTIVE HEAT TRANSFER THROUGH HELICAL CHANNELS
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Dr. K. E. Reby Roy
2012-07-01
Full Text Available Helical ducts are used in a variety of applications including food processing, thermal processing plants and refrigeration. They are advantageous due to their high heat transfer coefficient and compactness compared to straight tubes. The curvature of the coil governs the centrifugal force resulting in development of secondaryflow i.e. the fluid stream in the outer side of the pipe moves faster than the fluid streams in the inner side of the pipe. In the present study, Computational Fluid Dynamics (CFD simulations using Fluent 6.3.26 are carried out for helical rectangular ducts wound over a cylindrical passage. The cylindrical passage is oriented horizontallyand acts as a counterflow heat exchanger. The analysis is done by changing the flow rates of four different fluids like Ethylene Glycol, Kerosene, Nano Fluid and Water. The fluid flow and heat transfer characteristics of the fluids are studied and Nusselt Number correlations with Dean Number are developed.
Effect of vapor condensation on forced convection heat transfer of moistened gas
Liang, Yongbin; Che, Defu; Kang, Yanbin
2007-05-01
The forced convection heat transfer with water vapor condensation is studied both theoretically and experimentally when wet flue gas passes downwards through a bank of horizontal tubes. Extraordinarily, discussions are concentrated on the effect of water vapor condensation on forced convection heat transfer. In the experiments, the air steam mixture is used to simulate the flue gas of a natural gas fired boiler, and the vapor mass fraction ranges from 3.2 to 12.8%. By theoretical analysis, a new dimensionless number defined as augmentation factor is derived to account for the effect of condensation of relatively small amount of water vapor on convection heat transfer, and a consequent correlation is proposed based on the experimental data to describe the combined convection condensation heat transfer. Good agreement can be found between the values of the Nusselt number obtained from the experiments and calculated by the correlation. The maximum deviation is within ±6%. The experimental results also shows that the convection condensation heat transfer coefficient increases with Reynolds number and bulk vapor mass fraction, and is 1˜3.5 times that of the forced convection without condensation.
International Nuclear Information System (INIS)
Combined forced and free convective heat transfer characteristics were experimentally investigated for water flowing under about 1 atm in a narrow vertical rectangular (750 mm long, 50 mm wide and 18 mm in gap) channel heated from both sides. Experiments were carried out for both downward and upward forced convective flows for Reynolds number Re of 4 x 101 ? 5 x 105 and Grashof number Gr of 5 x 104 ? 4 x 1011, where the distance x from the inlet of the channel is adopted as the characteristic length in Re and Gr. As the results, the following were revealed : (1) Heat transfer coefficients for the combined convective flow region can be expressed in simple forms with a dimensionless parameter Gr/(Re21/8 Pr1/2) which is constituted by Gr, Re and Prandtl number Pr. (2) From the above-mentioned simple expressions it can be identified that the combined convective heat transfer is significant for the condition of 2.5 x 10-4 21/8 Pr2/1) -3. (3) In the present experiment, little significant differences were observed in heat transfer characteristics between the combined convective flow region with upward forced convective flow and that with downward forced convective flow. (author)
Theoretical solution for forced convection film boiling heat transfer from a horizontal cylinder
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The theoretical solutions of forced convection film boiling heat transfer from horizontal cylinders were obtained based on a two-phase laminar boundary layer film boiling model. It was clarified that authors' previous experimental data for the cylinders with the non dimensional diameters, D', of around 1.3 in water and in Freon-113 agreed with the values of theoretical numerical solutions based on the two-phase laminar boundary layer model with the smooth vapor-liquid interface except those for low flow velocities. A forced convection film boiling heat transfer correlation for the cylinders with non dimensional diameters of around 1.3 under saturation conditions was developed based on forced convection two-phase laminar boundary layer film boiling model and the experimental data
International Nuclear Information System (INIS)
Heat transfer and CHF (Critical heat flux) characteristics of flow boiling of R-113 in helically coiled tubes were experimentally investigated. Two coiled tubes with coil diameters of 0.165 and 0.32 m, and 10 mm I.D. were tested at a pressure of 0.39 MPa. In the nucleate boiling region, circumferential difference in heat transfer was not clarified qualitatively as well as quantitatively. The ratio of circumferential average boiling heat transfer coefficient to that of the single-phase flow in a curved tube, hTP/hLo, was found to be rather close to the correlations of Pujol-Stenning and Ueda-Kim for a straight tube flow. In the high quality region, the heat transfer coefficient was highest at the coil outside and lowest at the inside, and the average heat transfer coefficient ratio hTP/hLo was expressed in terms of Martinelli parameter, Xtt, only. A correlation applicable to both the nucleate boiling and forced convective evaporation regions was proposed. The CHF of the coiled tube was higher than that of the straight tube when the coil diameter was small, but it became considerably low at low mass velocity when the coil diameter was large. (author)
Experiment of forced convection heat transfer using microencapsulated phase-change-material slurries
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The present study describes an experiment on forced convective heat transfer using a water slurry of Microencapsulated Phase-change-material. A normal paraffin hydrocarbon is microencapsulated by melamine resin, melting point of 28.1degC. The heat transfer coefficient and pressure drop in a circular tube were evaluated. The heat transfer coefficient using the slurry in case with and without phase change were compared to in case of using pure water. (author)
Convective Heat Transfer Coefficients of the Human Body under Forced Convection from Ceiling
DEFF Research Database (Denmark)
Kurazumi, Yoshihito; Rezgals, Lauris
2014-01-01
The average convective heat transfer coefficient for a seated human body exposed to downward flow from above was determined. Thermal manikin with complex body shape and size of an average Scandinavian female was used. The surface temperature distribution of the manikin’s body was as the skin temperature distribution of an average person. The measurements were performed in a room with controlled thermal environment. Air temperature was set at 26ºC for cooling and at 20ºC for heating. The radiant temperature asymmetry in horizontal and vertical direction was close to zero, i.e. mean radiant temperature was equal to the air temperature. The air velocity of the isothermal downward flow from the ceiling at height of 1.5 m above the floor (above the top of the head) was set in a range between still air and 0.73 m/s. Based on the analyses of the results relationships for determination of the convective heat transfer coefficient of the whole body (hc [W/(m2•K)]) was proposed: hc=4.088+6.592V1.715 for a seated naked body at 20ºC and hc=2.874+7.427V1.345 for a seated naked body at 26ºC. Differences in the convective heat transfer coefficient of the whole body in low air velocity range, V<0.3 m/s, due to the natural convection were found. The results may be useful during design of air distribution in rooms, e.g. low impulse ventilation, diffuse ventilation, etc.
Forced-convection heat transfer in a spherical annulus heat exchanger
International Nuclear Information System (INIS)
Results are presented of a combined numerical and experimental study of steady, forced-convection heat transfer in a spherical annulus heat exchanger with 53 0C water flowing in an annulus formed by an insulated outer sphere and a 0 0C inner sphere. The inner sphere radius is 139.7 mm, the outer sphere radius is 168.3 mm. The transient laminar incompressible axisymmetric Navier-Stokes equations and energy equation in spherical coordinates are solved by an explicit finite-difference solution technique. Turbulence and buoyancy are neglected in the numerical analysis. Steady solutions are obtained by allowing the transient solution to achieve steady state. Numerically obtained temperature and heat-flux rate distributions are presented for gap Reynolds numbers from 41 to 465. Measurements of inner sphere heat-flux rate distribution, flow separation angle, annulus fluid temperatures, and total heat transfer are made for Reynolds numbers from 41 to 1086. The angle of separation along the inner sphere is found to vary as a function of Reynolds number. Measured total Nusselt numbers agree with results reported in the literature to within 2.0 percent at a Reynolds number of 974, and 26.0 percent at a Reynolds number of 66
Pacio, J.; Marocco, L.; Wetzel, Th.
2015-02-01
Within the present work the dataset of experimental points and the heat transfer correlations available in literature for liquid-metal fully-developed, forced-convective heat transfer in pipes are reviewed and critically analyzed. Over 1,100 data points from 21 different sources are considered for constant heat flux, covering a wide range of operating conditions (velocity, heat flux, diameter, among others). Among 15 evaluated correlations, four appropriate ones are recommended for forced turbulent convection: one covering all the data points and the other three respectively related to alkali liquid metals, lead alloys and mercury. Moreover, a new correlation has been derived as a best fit of the limited number of available data points for constant wall temperature, while an alternative evaluation method is also described for this boundary condition.
LAMINAR FORCED CONVECTION HEAT TRANSFER OVER STAGGERED CIRCULAR TUBE BANKS: A CFD APPROACH
Tahseen, Tahseen A.; Ishak, M.; Rahman, M. M.
2013-01-01
This paper presents the numerical study of two-dimensional forced convection heat transfer for staggered tube banks in cross flow under incompressible, steady-state conditions. This system is solved for body-fitted coordinates using the finite volume method for flow over a bundle of cylindrical tubes. A constant heat flux is imposed on the surface of the tubes as the thermal boundary condition. The type of arrangement considered is a set of staggered tubes. Ratios of longitudinal pitch to tub...
Parwani, Ajit K.; Talukdar, Prabal; Subbarao, P. M. V.
2015-03-01
Heat flux at the boundary of a duct is estimated using the inverse technique based on conjugate gradient method (CGM) with an adjoint equation. A two-dimensional inverse forced convection hydrodynamically fully developed turbulent flow is considered. The simulations are performed with temperature data measured in the experimental test performed on a wind tunnel. The results show that the present numerical model with CGM is robust and accurate enough to estimate the strength and position of boundary heat flux.
Experimental and numerical study of steady forced-convection heat transfer in a spherical annulus
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The results of a combined experimental and numerical study of steady forced-convection heat transfer in a spherical annulus with 500C heated water flowing in the annulus, an insulated outer sphere, and a 00C cooled inner sphere are presented. The inner sphere radius is 139.7 mm, the outer sphere radius is 168.3 mm and the radius ratio is 1.2. Measurements of inner sphere heat- flux rate distribution, flow separation angle, annulus fluid temperatures and total heat transfer were made for gap Reynolds numbers from 41 to 1086. The angle of separation along the inner sphere was found to vary as a function of Reynolds number. Measured total Nusselt numbers agreed with results reported in the literature to within 2.0% at a gap Reynolds number of 974 and 26.0% at a gap Reynolds number of 66. Results of numerical calculations of laminar incompressible fluid flow and heat transfer in a spherical annulus are presented. Velocity, temperature, pressure and heat-flux rate distributions are presented for gap Reynolds numbers from 4.4 to 440. It is believed that this is the first experimental study of spherical annulus convective heat transfer in which inner sphere heat-fux distribution has been measured and in which sepration angle was measured by non-visual methods and found to vary as a function of Reynolds number. Also, the numerical analysis is the first solution of the full laminar Navier-Stokes equations for forced-convection heat transfer in a sphericaled-convection heat transfer in a spherical annulus. The computations predict well the experimental trends and qualitative aspects of the flow and heat transfer while underpredicting heat-flux rates by a factor of two or more. It is felt that a turbulence model is necessary to predict more accurately the experimental results
Forced convection heat exchange inside porous sintered metals
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Methods and results of investigating heat exchange in the process of liquid flow inside porous sintered metals have been analyzed. It has been shown that experimental data available include extremely conflicting correlations between heat transfer coefficient and Reynolds number, porosity, and relative wall thickness. Scattering of the data can attain one order of magnitude. The volume coefficient of heat transfer inside pores determined in papers does not correspond to its real value in the initial equations of the inner problem of porous cooling. Calculating and experimental method of determining the heat transfer coefficient has been developed and realized on the unit of radiation heating. More accurate experimental data on intraporous heat exchange have been obtained. It has been established that relative wall thickness does not affect the intensity of heat transfer inside pores
Heat transfer enhancement for single phase forced convection
Fiebig, Martin
Goals for heat exchanger design are outlined and performance evaluation criteria are discussed. The flow geometries in heat exchangers, which can be classified as channel or tube flow, are described. The use of finned plates or tubes for channel flow and internally finned tubes or wire coil and twisted tape inserts for tube flow are considered.
Forced convection heat transfer in a bundle of 12 rods
International Nuclear Information System (INIS)
The paper describes the heat transfer experiments performed with two bundles of 12 electrically heated rough rods. The fundamentals of the SAGAPO code are given. SAGAPO calculates the friction and heat transfer coefficients in turbulent flow by integrating the logarithmic universal laws of the wall for velocity and temperature in the various coolant channels confined by rough surfaces. The code accounts for turbulent mixing and cross flow among the channels, for spacer effects on wall temperatures and pressure drop, for fin efficiency effects due to the roughness ribs, and for inlet effects on wall temperatures in case of smooth rods. Also laminar flow can be calculated. Comparisons between computed and experimental results on pressure and temperature distributions are presented
International Nuclear Information System (INIS)
In a high-level waste (HLW) repository, heat is generated by the radioactive decay of the waste. This can affect the safety of the repository because the surrounding environment can be changed by the heat transfer through the rock. Thus, it is important to determine the heat transfer coefficient of the atmosphere in the underground repository. In this study, the heat transfer coefficient was estimated by measuring the indoor environmental factors in the Korea Atomic Energy Research Institute Underground Research Tunnel (KURT) under forced convection. For the experiment, a heater of 5 kw capacity, 2 meters long, was inserted through the tunnel wall in the heating section of KURT in order to heat up the inside of the rock to 90 .deg. C, and fresh air was provided by an air supply fan connected to the outside of the tunnel. The results showed that the average air velocity in the heating section after the provision of the air from outside of the tunnel was 0.81 m/s with the Reynolds number of 310,000 ? 340,000. The seasonal heat transfer coefficient in the heating section under forced convection was 7.68 W/m2 K in the summer and 7.24 W/mm2 K in the winter
Uddin, Md Jashim; Khan, Waqar A; Ismail, A I Md
2013-01-01
A two-dimensional steady forced convective flow of a Newtonian fluid past a convectively heated permeable vertically moving plate in the presence of a variable magnetic field and radiation effect has been investigated numerically. The plate moves either in assisting or opposing direction to the free stream. The plate and free stream velocities are considered to be proportional to x(m) whilst the magnetic field and mass transfer velocity are taken to be proportional to x((m-1)/2) where x is the distance along the plate from the leading edge of the plate. Instead of using existing similarity transformations, we use a linear group of transformations to transform the governing equations into similarity equations with relevant boundary conditions. Numerical solutions of the similarity equations are presented to show the effects of the controlling parameters on the dimensionless velocity, temperature and concentration profiles as well as on the friction factor, rate of heat and mass transfer. It is found that the rate of heat transfer elevates with the mass transfer velocity, convective heat transfer, Prandtl number, velocity ratio and the magnetic field parameters. It is also found that the rate of mass transfer enhances with the mass transfer velocity, velocity ratio, power law index and the Schmidt number, whilst it suppresses with the magnetic field parameter. Our results are compared with the results existing in the open literature. The comparisons are satisfactory. PMID:23741295
The effect of internal ribbing on forced convective heat transfer in circular-sectioned tubes
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This paper presents the results of an experimental examination of the effect of internal circumferential ribs on forced convection in circular-sectioned tubes. The work is relevant to the internal cooling of gas turbine rotor blades. The influence of rib geometry is investigated for three different rib configurations and simple design-type, empirical equations are developed for estimating heat transfer at rib and mid-rib locations. It is demonstrated that heat transfer may be improved by up to three fold in relation to fully developed forced convection in smooth-walled tubes. The geometric parameters which have been used for the experiments are typical of those currently applied to gas turbine blade cooling designs
Munir, Asif; Shahzad, Azeem; Khan, Masood
2014-01-01
The major focus of this article is to analyze the forced convective heat transfer in a steady boundary layer flow of Sisko fluid over a nonlinear stretching sheet. Two cases are studied, namely (i) the sheet with variable temperature (PST case) and (ii) the sheet with variable heat flux (PHF case). The heat transfer aspects are investigated for both integer and non-integer values of the power-law index. The governing partial differential equations are reduced to a system of nonlinear ordinary...
Boiling and bubbling heat transfer under the conditions of free and forced convection
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The present paper gives the main results of studying the hydrogasdynamic aspects of heat transfer and blow-off effect at the developed nucleate boiling by analogy with bubbling under the conditions of free convection. For the first time it proved possible to detect the explicit influence of gas (vapour) phase compressibility on the heat transfer intensity and the effect of liquid blow-off from the surface of heating (bubbling). The existence of regimes automodel with respect to liquid phase viscosity has been found. A description is given of the first forced-circulation loop to study heat transfer and blow-off effect at forced gas-liquid flow in tubes and channels
Forced Convection Heat Transfer of He I and He II at Pressures up to Supercritical
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The forced convection heat transfer coefficients were measured on a horizontal flat plate heater located on inner wall of a rectangular duct for pressures from 1 atm to 2.8 atm, flow velocities up to 2.1 m/s and for liquid temperatures from 1.8 to 6.5 K. Critical heat fluxes (CHFs) in He II under subcritical and supercritical pressures are higher for higher flow velocity and lower liquid temperature. In He I under supercritical pressures, there are two distinct ranges in heat transfer curves. The curve for lower heat flux range has a steeper gradient and the curve for higher heat flux has a lower gradient like film boiling regime. The peak heat fluxes of the former curves are treated as pseudo CHFs in this study and compared with the CHFs for He II
Kim, Jeong-Hun; Arima, Hirofumi; Ikegami, Yasuyuki
In the present study, the fundamental experiments that investigate characteristics of local heat transfer in forced convective boiling on vertical flat plate with 2-mm channel height are taken to realize plate type compact evaporator for OTEC or STEC. The experiments are performed with ammonia as the working fluid. The experiments are also carried out with the following test conditions; saturated pressure = 0.7, 0.8, 0.9 MPa, mass flux = 7.5, 10, 15 kg/(m2•s), heat flux = 15, 20, 25 kW/m2 and inlet quality = 0.1 ~ 0.4 [-]. The result shows that the wall superheated temperature of forced convective boiling is lower than that of pool boiling. And the heat transfer coefficient increases with an increase in quality and the decrease in the local heat flux and saturated pressure for prescribed experimental conditions. However, local heat transfer coefficients are not affected by mass fluxes in the prescribed experimental conditions. An empirical correlation that can predict the local heat transfer coefficient on vertical flat plate within experimental conditions is also proposed.
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For development of new reactor, supercritical water is expected to be used as coolant to improve thermal efficiency. However, the thermal characteristics of supercritical fluid is not revealed completely because its difficulty for experiment. Specific phenomena tend to occur near the pseudo-boiling point which is characterised by temperature corresponding to the saturation point in ordinary fluid. Around this point, the physic properties such as density, specific heat and thermal conductivity are drastically varying. Although there is no difference between gas and liquid phases in supercritical fluids, phenomena similar to boiling (with heat transfer deterioration) can be observed round the pseudo-boiling point. Experiments of heat transfer have been done for supercritical fluid in forced convective condition. However, these experiments were mainly realised inside stainless steel cylinder pipes, for which flow visualisation is difficult. Consequently, this work has been devoted to the development of method allowing the visualisation of supercritical flows. The experiment setup is composed of main loop and test section for the visualisation. Carbon dioxide is used as test fluid. Supercritical carbon dioxide flows upward in rectangular channel and heated by one-side wall to generate forced convection heat transfer. Through window at mid-height of the test section, shadowgraphy was applied to visualize density gradient distribution. The behavior of the density wave in the channel is visualized and examined through the variation of the heat transfer coefficient. (author)
Forced convection heat transfer from banks of helical coiled resistance wires
Energy Technology Data Exchange (ETDEWEB)
Comini, G.; Savino, S. [University of Udine, Department of Energy and Fluid Machinery, Via delle Scienze 208, 33100 Udine (Italy); Bari, E.; Bison, A. [Electrolux Home Products Italy Spa, Corso Lino Zanussi 30, 33080 Porcia (PN) (Italy)
2008-04-15
Forced convection heat transfer from banks of helical coiled resistance wires in a cross flow of air was investigated experimentally, with reference to various operative conditions and several coil geometries. The resistance banks were subdivided in two branches, independently powered by single-phase alternating current. Average convection heat transfer coefficients were determined for each branch. No significant interactions were detected between adjacent coils nor between upstream and downstream coils, possibly because of the high percentage of inter-spaces between wires. The experimentally measured Nusselt numbers were thus correlated by a modified version of the Churchill and Bernstein model, valid for a single cylinder in cross flow. To this purpose, an equivalent wire diameter was defined in order to take into account the influences of diameter and pitch of the coils. The average deviation between measured and calculated values of the Nusselt number is of the order of {+-}7% in the range of Reynolds numbers from 70 to 400. (author)
Forced convection heat transfer of subcooled liquid nitrogen in a vertical tube
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Experimental research on forced convection heat transfer of subcooled liquid nitrogen ranging from the pressures of 0.3 MPa to its supercritical pressure is carried out for wide ranges of inlet temperature and flow velocity. A stainless steel tube heater with the inner diameter of 5.4 mm and the length of 100 mm is mounted vertically. The heat transfer coefficients in non-boiling region and the DNB (departure from nucleate boiling) heat fluxes are higher for higher flow velocity and higher subcooling. The trend of the heat transfer coefficients in the non-boiling region agree with those by the Dittus-Boelter correlation, although they are unaffected by the flow velocity for Re w, exceeds its pseudo-critical temperature, Tc', although they are similar to those under subcritical pressure for Tw c'. The heat transfer characteristics of supercritical nitrogen can be predicted by authors' correlation.
Maximal heat transfer density: Plates with multiple lengths in forced convection
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Bello-Ochende, T.; Bejan, A. [Department of Mechanical Engineering and Materials Science, Duke University, Box 90300, Durham, NC (United States)
2004-12-01
This paper shows that in a space filled with heat generating parallel plates and laminar forced convection, the heat transfer density can be increased beyond the level known for parallel plates with optimal spacing. The technique consists of inserting in every entrance region new generations of smaller plates, because smaller plates have thin boundary layers that fit in the unused (isothermal) entrance flow. This technique can be repeated several times, and the result is a sequence of multi-scale flow structures that have progressively higher heat transfer densities. The work consists of numerical simulations in a large number of flow configurations, one differing slightly from the next. The complete optimized architecture and performance of structures with one, two and three plate length scales are reported. Diminishing returns are observed as the number of length scales increases. This method can be used to develop multi-scale nonuniform flow structures for heat exchangers and cooled electronic packages. (authors)
Combined free and forced convection heat transfer in magneto fluid mechanic pipe flow
International Nuclear Information System (INIS)
A study is made of fully developed, laminar, free-and-forced convection heat transfer in an electrically conducting fluid flowing in an electrically insulated, horizontal, circular pipe in a vertical transverse magnetic field. The normalized magnetofluidmechanic and energy equations are reduced to three coupled partial differential equations by the introduction of a stream function of the secondary flow. A perturbation solution is generated in inverse powers of the Lykoudis number, Ly = M2/?Gr, which yields the influence of the magnetic field on the stream function of the secondary flow, axial velocity profiles, temperature profiles, and Nusselt number. 6 figures, 1 table
Numerical study of forced convection in a vertical channel filled with heat-generating porous medium
International Nuclear Information System (INIS)
Steady laminar non-Darcian forced convection in a vertical channel filled with heat-generating porous medium is studied numerically by using the local thermal non-equilibrium model. The heat source generated by solid framework is uniform and kept constant; and the temperature of vertical walls is kept at constant temperature T0. The flow inside porous medium is modelled by using Forchheimer-Brinkman extended Darcy model. The effects of Reynolds number (0.5 ?Re ? 50), effective fluid-to-solid thermal conductivity ratio ?(0.001 ? ? ? 1.0)and Darcy number (10-3 ? Da ? 10-5) are analyzed in detail. It is found that, the effects of Re, ? and Da are remarkable; at low values of Re and ?, and at high value of Da, the effect of local thermal non-equilibrium is significant and the local thermal non-equilibrium model should be adopted for predicting the heat transfer characteristics exactly. (authors)
Effect of uncertainties in physical properties on forced convection heat transfer with nanofluids
International Nuclear Information System (INIS)
Nanofluids are considered to offer important advantages over conventional heat transfer fluids. However, at this early stage of their development, their thermophysical properties are not known precisely. As a result, the assessment of their true potential is difficult. This fact is illustrated by analyzing their thermohydraulic performance for both laminar and turbulent fully developed forced convection in a tube with uniform wall heat flux. Two different models from the literature are used to express these properties in terms of particle loading and they lead to very different qualitative and quantitative results in two types of problems: replacement of a simple fluid by a nanofluid in a given installation and design of an elementary heat transfer installation for a simple fluid or a nanofluid
International Nuclear Information System (INIS)
In the first part, free-convection and nucleate pool boiling heat transfer (up to burn-out heat flux) between a platinum wire of 0.15 mm in diameter in neon, deuterium and hydrogen has been studied at atmospheric pressure. These measurements were continued in liquid neon up to 23 bars (Pc ? 26.8 b). Film boiling heat transfer coefficients have been measured in pool boiling liquid neon at atmospheric pressure with three heating wires (diameters 0.2, 0.5, 2 mm). All the results have been compared with existing correlations. The second part is devoted to measurements of the critical heat flux limiting heat transfer with small temperature differences between the wall and the liquid neon flowing inside a tube (diameters 3 x 3.5 mm) heated by joule effect on 30 cm of length. Influences of flow stability, nature of electrical current, pressure, mass flow rate and subcooling are shown. In conclusion, the similarity of the heat transfer characteristics in pool boiling as well as in forced convection of liquid neon and hydrogen is emphasized. (author)
Enhancement of forced convective heat transfer in narrow concentric annulus by turbulence promoters
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Enhancement of forced convective heat transfer in a narrow concentric annulus was performed by turbulence promoters in order to improve the heat removal from High Temperature Gas-cooled Reactor, Gas-cooled Nuclear Fusion Reactor and other narrow flow passages. Present experiments, which were different from the conventional research, were carried out to examine the effect of the turbulence promoters on the inner insulated wall opposite the outer smooth heated wall by changing the ratio of the pitch and the height P/epsilon (from 5 to 60), the ratio of the height and the space epsilon/epsilon' (from 0.2 to 2) and the type of turbulence promoters. Experimental results were arranged for the local heat transfer coefficient distribution on the smooth outer tube, the average heat transfer coefficient, the friction factor and the thermal performance. Five kinds of evaluation for thermal performance were presented: The ratio of heat transfer coefficient with and without turbulence promoters at three conditions, (1) constant Reynolds number, (2) constant pressure drop and (3) constant pumping power, additionally, (4) pumping power reduction at constant heat transfer coefficient and (5) working ratio. An example of a fuel element of HTGR with turbulence promoters was indicated as an application of present results. (author)
Numerical prediction of the turbulent forced heat convection in a two-dimensional driven cavity flow
Abadie, P.; Schiestel, R.
1986-03-01
Turbulent flow and associated heat transfer in driven two-dimensional closed cavity flow has been studied using a finite-difference numerical method. Turbulence modeling is based on one point closures derived from the classical turbulent kinetic energy dissipation rate model. Calculated mean velocity and turbulent kinetic energy are compared with available experimental data, and the model is shown to successfully predict global quantities despite limitations. The problem of forced heat convection with fixed wall temperature is considered, and mean temperature field and overall thermal properties of the cavity flow are studied. Numerically deduced correlations giving Nusselt numbers for each face of the cavity as a function of Reynolds number, sum up the mean transfer properties of such a flow configuration. The study has applications to the problem of cooling of turbomachine rotors.
LAMINAR FORCED CONVECTION HEAT TRANSFER OVER STAGGERED CIRCULAR TUBE BANKS: A CFD APPROACH
Directory of Open Access Journals (Sweden)
Tahseen A. Tahseen
2013-06-01
Full Text Available This paper presents the numerical study of two-dimensional forced convection heat transfer for staggered tube banks in cross flow under incompressible, steady-state conditions. This system is solved for body-fitted coordinates using the finite volume method for flow over a bundle of cylindrical tubes. A constant heat flux is imposed on the surface of the tubes as the thermal boundary condition. The type of arrangement considered is a set of staggered tubes. Ratios of longitudinal pitch to tube diameter (ST/D of 1.25, 1.5, and 2 are considered. Reynolds numbers are varied from 25 to 250 and the Prandtl number is taken as 0.71. Velocity field vectors, temperature contours, and the local and average Nusselt numbers are analyzed in this paper. It can be seen that the predicted results are in good agreement with experimental and numerical results obtained previously. The obtained results show that the heat transfer rate increases with a reduction in the step of the longitudinal tube diameter. The local heat transfer depends strongly on the Reynolds number. The highest values are obtained at the surface opposite to the direction of flow. The heat transfer rate is insignificant in the areas of recycling.
Directory of Open Access Journals (Sweden)
P. MOHAJERI KHAMENEH,
2010-11-01
Full Text Available Three dimensional simulations of the single-phase laminar flow and forced convective heat transfer of water in round tube and straight microchannel heat exchangers were investigated numerically. This numerical method was developed to measure heat transfer parameters of round tube and straight microchannel tube geometries. Then, similarities and differences were compared between different geometries. The geometries and operating conditions of those indicated heat exchangers were created using a finite volume-based computational fluid dynamics technique. In this article, at each Z-location variation of dimensionless local temperature, nondimensional local heat flux variation and dimensionless local Nusselt number distribution along the tube length were compared between round tube and straight microchannel heat exchangers. Consequently, averaged computational Nusselt number was obtained for those indicated models and then validation study was performed for round tube counter flow type heat exchanger model. Finally, all of these numerical results for both kind of geometries in counter flow heat exchangers were discussed in details.
Improved modelling of turbulent forced convective heat transfer in straight ducts
International Nuclear Information System (INIS)
Ducts with non-circular cross sections are frequently encountered in industrial heat transfer equipment, e.g., compact heat exchangers, cooling channels in gas turbine blades, nuclear reactors, etc. This investigation concerns numerical calculation of turbulent forced convective heat transfer and fluid flow in their fully developed state at low Reynolds number. The authors have developed a low Reynolds number version of the non-linear ?-? model combined with the heat flux models of SED, low Reynolds number version of GGDH and WET in general three dimensional geometries. The wall function approach is abandoned and the friction factor is predicted in agreement with experiments. The Nusselt numbers are also predicted very well. Fully developed condition is achieved by imposing cyclic boundary conditions in the main flow direction. The numerical approach is based on the finite volume technique with a non-staggered grid arrangement and the SIMPLEC algorithm. Results have been obtained with the linear and the nonlinear ?-? model, combined with the Lam-Bremhorst damping functions for low Reynolds numbers. The secondary flow patterns are also of major concern
Improved modelling of turbulent forced convective heat transfer in straight ducts
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Rokni, M.; Sunden, B.
1997-07-01
Ducts with non-circular cross sections are frequently encountered in industrial heat transfer equipment, e.g., compact heat exchangers, cooling channels in gas turbine blades, nuclear reactors, etc. This investigation concerns numerical calculation of turbulent forced convective heat transfer and fluid flow in their fully developed state at low Reynolds number. The authors have developed a low Reynolds number version of the non-linear {kappa}-{epsilon} model combined with the heat flux models of SED, low Reynolds number version of GGDH and WET in general three dimensional geometries. The wall function approach is abandoned and the friction factor is predicted in agreement with experiments. The Nusselt numbers are also predicted very well. Fully developed condition is achieved by imposing cyclic boundary conditions in the main flow direction. The numerical approach is based on the finite volume technique with a non-staggered grid arrangement and the SIMPLEC algorithm. Results have been obtained with the linear and the nonlinear {kappa}-{epsilon} model, combined with the Lam-Bremhorst damping functions for low Reynolds numbers. The secondary flow patterns are also of major concern.
Conceptual Design of Forced Convection Molten Salt Heat Transfer Testing Loop
Energy Technology Data Exchange (ETDEWEB)
Manohar S. Sohal; Piyush Sabharwall; Pattrick Calderoni; Alan K. Wertsching; S. Brandon Grover
2010-09-01
This report develops a proposal to design and construct a forced convection test loop. A detailed test plan will then be conducted to obtain data on heat transfer, thermodynamic, and corrosion characteristics of the molten salts and fluid-solid interaction. In particular, this report outlines an experimental research and development test plan. The most important initial requirement for heat transfer test of molten salt systems is the establishment of reference coolant materials to use in the experiments. An earlier report produced within the same project highlighted how thermophysical properties of the materials that directly impact the heat transfer behavior are strongly correlated to the composition and impurities concentration of the melt. It is therefore essential to establish laboratory techniques that can measure the melt composition, and to develop purification methods that would allow the production of large quantities of coolant with the desired purity. A companion report describes the options available to reach such objectives. In particular, that report outlines an experimental research and development test plan that would include following steps: •Molten Salts: The candidate molten salts for investigation will be selected. •Materials of Construction: Materials of construction for the test loop, heat exchangers, and fluid-solid corrosion tests in the test loop will also be selected. •Scaling Analysis: Scaling analysis to design the test loop will be performed. •Test Plan: A comprehensive test plan to include all the tests that are being planned in the short and long term time frame will be developed. •Design the Test Loop: The forced convection test loop will be designed including extensive mechanical design, instrument selection, data acquisition system, safety requirements, and related precautionary measures. •Fabricate the Test Loop. •Perform the Tests. •Uncertainty Analysis: As a part of the data collection, uncertainty analysis will be performed to develop probability of confidence in what is measured in the test loop. Overall, the testing loop will allow development of needed heat transfer related thermophysical parameters for all the salts, validate existing correlations, validate measuring instruments under harsh environment, and have extensive corrosion testing of materials of construction.
Munir, Asif; Shahzad, Azeem; Khan, Masood
2014-01-01
The major focus of this article is to analyze the forced convective heat transfer in a steady boundary layer flow of Sisko fluid over a nonlinear stretching sheet. Two cases are studied, namely (i) the sheet with variable temperature (PST case) and (ii) the sheet with variable heat flux (PHF case). The heat transfer aspects are investigated for both integer and non-integer values of the power-law index. The governing partial differential equations are reduced to a system of nonlinear ordinary differential equations using appropriate similarity variables and solved numerically. The numerical results are obtained by the shooting method using adaptive Runge Kutta method with Broyden's method in the domain[Formula: see text]. The numerical results for the temperature field are found to be strongly dependent upon the power-law index, stretching parameter, wall temperature parameter, material parameter of the Sisko fluid and Prandtl number. In addition, the local Nusselt number versus wall temperature parameter is also graphed and tabulated for different values of pertaining parameters. Further, numerical results are validated by comparison with exact solutions as well as previously published results in the literature. PMID:24949738
Lee, Chi M.; Schock, Harold J.
1988-01-01
Currently, the heat transfer equation used in the rotary combustion engine (RCE) simulation model is taken from piston engine studies. These relations have been empirically developed by the experimental input coming from piston engines whose geometry differs considerably from that of the RCE. The objective of this work was to derive equations to estimate heat transfer coefficients in the combustion chamber of an RCE. This was accomplished by making detailed temperature and pressure measurements in a direct injection stratified charge (DISC) RCE under a range of conditions. For each specific measurement point, the local gas velocity was assumed equal to the local rotor tip speed. Local physical properties of the fluids were then calculated. Two types of correlation equations were derived and are described in this paper. The first correlation expresses the Nusselt number as a function of the Prandtl number, Reynolds number, and characteristic temperature ratio; the second correlation expresses the forced convection heat transfer coefficient as a function of fluid temperature, pressure and velocity.
International Nuclear Information System (INIS)
Highlights: ? Superposition of forced and thermal convection is studied in a rectangular cavity. ? For pure forced convection the mean wind exhibits a solid body rotation. ? Four buoyancy induced convection rolls are formed for mixed convection at Ar ? 3.3. ? The enthalpy flux difference between out- and inflowing air has a maximum at Ar ? 0.6. - Abstract: Results of an experimental study of flow structure formation and heat transport in turbulent forced and mixed convection are presented. The experiments were conducted in a rectangular cavity with a square cross section, which has an aspect ratio between length and height of ?xz = 5. Air at atmospheric pressure was used as working fluid. The air inflow was supplied through a slot below the ceiling, while exhausting was provided by another slot, which is located directly above the floor. Both vents extend over the whole length of the cell. In order to induce thermal convection the bottom of the cell is heated while the ceiling is maintained at a constant temperature. This configuration allows to generate and study mixed convection under well defined conditions. Results of forced convection at Re = 1.07 x 104 as well as mixed convection at 1.01 x 104 ? Re ? 3.4 x 104 and Ra = 2.4 x 108 (3.3 ? Ar ? 0.3), which were obtained by means of Particle Image Velocimetry and local temperature measurements, are presented. For purely forced convection a 2Dorced convection a 2D mean wind, which can be approximated by a solid body rotation, is found. With increasing Archimedes number this structure becomes unstable, leading to a transition of the solid body rotation into additional smaller convection rolls. Proper orthogonal decomposition of the instantaneous velocity fields has been performed for further analysis of these coherent large-scale structures. Their fingerprint is found in the spatial temperature distribution of the out flowing air at the end of the outlet channel, which reveals a temporally stable profile with two maxima over the length of the outlet. Moreover a maximum in the global enthalpy transport by the fluid is found at Ar ? 0.6.
Transient forced convection heat transfer from a circular cylinder embedded in a porous medium
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Thevenin, J. [Universite Pierre et Marie Curie, Paris (France). Modelisation des Transferts Thermiques
1995-07-01
Studies of the transient heat transfer past a circular cylinder in a steady-state viscous flow are presented for some fluid saturated fibrous porous media. Numerical results have been obtained according to the Darcy-Brinkman model by means of the finite element method. Analysis of the influence of the Darcy and Peclet numbers on the mean Nusselt number exhibits the successive conduction, transition and convection regimes. The duration necessary to reach the steady-state convection heat transfer appears as a function of the Peclet and Darcy numbers.
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This paper presents the unsteady laminar forced convection heat transfer from a row of five isothermal square cylinders placed in a side-by-side arrangement at a Reynolds number of 150. The numerical simulations are performed using a finite volume code based on the PISO algorithm in a collocated grid system. Special attention is paid to investigate the effect of the spacing between the cylinders on the overall transport processes for the separation ratios (spacing to size ratio) between 0.2 and 10. No significant interaction between the wakes is observed for spacing greater than four times the diameter at this Reynolds number. However, at smaller spacing, the wakes interact in a complicated manner resulting different thermo-hydrodynamic regimes. The vortex structures and isotherm patterns obtained are systematically presented and discussed for different separation ratios. In addition, the mean and instantaneous drag and lift coefficients, mean and local Nusselt number and Strouhal number are determined and discussed for various separation ratios. A new correlation is derived for mean Nusselt number as a function of separation ratio for such flows.
Bouchenafa, Rachid; Saim, Rachid; Abboudi, Said
2015-01-01
Forced convection is a phenomenon associated with the heat transfer fluid flows. The presence of convection affects simultaneously the thermal and hydrodynamic fields, the problem is thus coupled. This form of heat transfer inside ducts occurs in many practical applications such as solar collectors, heat exchangers, cooling of electronic components as well as chemical and nuclear. In this work, we are interested primarily for a numerical study of thermo-hydraulic performances of an incompressible turbulent flow of air through a heat sink composed of several rows of bars of square section. Profiles and the axial velocity fields, as well as profiles and the distribution of the Nusselt number are plotted for all the geometry considered and chosen for different sections. The effects of geometrical parameters of the model and the operating parameters on the dynamic and thermal behavior of the air are analyzed.
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In this study, numerical investigations are conducted for forced convective heat transfer in an annular helicoidal tube under uniform wall temperature condition for laminar flow including developing region. The numerical computations reveal the developments and distributions of heat transfer and flow fields in the annular helicoidal tube when the outer tube wall is heated and the inner tube wall is insulated. The effects of Reynolds number, curvature ratio, and coil pitch on the circumferential average friction factor and Nusselt number at different axial locations, and the non-dimensional entropy generation number of laminar convection in an annular helicoidal tube are investigated. In addition, the differences of flow and heat transfer characteristics between the annular helicoidal tube and circular helicoidal tube are also described
Mancin, Simone; Zilio, Claudio; Diani, Andrea; Rossetto, Luisa
2012-05-01
In this paper, copper and aluminum foams with different porosity, number of pores per inch (PPI) and foam core height, are experimentally studied during air forced convection. The experimental measurements permit to understand how each parameter (i.e. porosity, PPI, material, and foam thickness) affects the heat transfer and fluid flow behavior of the metal foams. The paper presents the experimental heat transfer coefficients, permeability and inertia coefficients; moreover, it reports the normalized mean wall temperature as a function of the pumping power per unit of heat transfer area: two meaningful parameters that allow quantitative comparisons of different enhanced surfaces, which can be considered suitable for electronic thermal management.
Design of Test Loops for Forced Convection Heat Transfer Studies at Supercritical State
Balouch, Masih N.
Worldwide research is being conducted to improve the efficiency of nuclear power plants by using supercritical water (SCW) as the working fluid. One such SCW reactor considered for future development is the CANDU-Supercritical Water Reactor (CANDU-SCWR). For safe and accurate design of the CANDU-SCWR, a detailed knowledge of forced-convection heat transfer in SCW is required. For this purpose, two supercritical fluid loops, i.e. a SCW loop and an R-134a loop are developed at Carleton University. The SCW loop is designed to operate at pressures as high as 28 MPa, temperatures up to 600 °C and mass fluxes of up to 3000 kg/m2s. The R-134a loop is designed to operate at pressures as high as 6 MPa, temperatures up to 140 °C and mass fluxes in the range of 500-6000 kg/m2s. The test loops designs allow for up to 300 kW of heating power to be imparted to the fluid. Both test loops are of the closed-loop design, where flow circulation is achieved by a centrifugal pump in the SCW loop and three parallel-connected gear pumps in the R-134a loop, respectively. The test loops are pressurized using a high-pressure nitrogen cylinder and accumulator assembly, which allows independent control of the pressure, while simultaneously dampening pump induced pressure fluctuations. Heat exchangers located upstream of the pumps control the fluid temperature in the test loops. Strategically located measuring instrumentation provides information on the flow rate, pressure and temperature in the test loops. The test loops have been designed to accommodate a variety of test-section geometries, ranging from a straight circular tube to a seven-rod bundle, achieving heat fluxes up to 2.5 MW/m2 depending on the test-section geometry. The design of both test loops allows for easy reconfiguration of the test-section orientation relative to the gravitational direction. All the test sections are of the directly-heated design, where electric current passing through the pressure retaining walls of the test sections provides the Joule heating required to heat up the fluid to supercritical conditions. A high-temperature dielectric gasket isolates the current carrying parts of the test section from the rest of the assembly. Temperature and pressure drop data are collected at the inlet and outlet, and along the heated length of the test section. The test loops and test sections are designed according to American Society of Mechanical Engineers (ASME) Pressure Piping B31.1, and Boiler and Pressure Vessel Code, Section VIII-Division 1 rules. The final test loops and test sections assemblies are certified by Technical Standards and Safety Authority (TSSA). Every attempt is made to use off-the-shelf components where possible in order to streamline the design process and reduce costs. Following a rigorous selection process, stainless steel Types 316 and 316H are selected as the construction materials for the test loops, and Inconel 625 is selected as the construction material for the test sections. This thesis describes the design of the SCW and R-134a loops along with the three test-section geometries (i.e., tubular, annular and bundle designs).
Subcooled forced convection boiling of trichlorotrifluoroethane
Dougall, R. S.; Panian, D. J.
1972-01-01
Experimental heat-transfer data were obtained for the forced-convection boiling of trichlorotrifluoroethane (R-113 or Freon-113) in a vertical annular test annular test section. The 97 data points obtained covered heat transfer by forced convection, local boiling, and fully-developed boiling. Correlating methods were obtained which accurately predicted the heat flux as a function of wall superheat (boiling curve) over the range of parameters studied.
Low Reynolds number forced convection steam cooling heat transfer in rod bundles
International Nuclear Information System (INIS)
A series of forced convection steam cooling tests at low Reynolds numbers were conducted in the rod bundle test facility of the FLECHT-SEASET program. The data was reduced using a rod-centered subchannel energy balance to obtain the vapor temperature and by modeling the bundle with the COBRA-IV-I computer code. The comparisons between the COBRA-IV-I vapor temperatures and subchannel energy balance vapor temperatures were quite good. 5 refs
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Kubitschek, Joseph P.; Weidman, Patrick D. [University of Colorado, Department of Mechanical Engineering, Boulder, CO (United States)
2006-07-15
A linear stability analysis determining the critical Rayleigh number R{sub c} for onset of convection in a bounded vertical cylinder containing a fluid-saturated porous medium is performed for insulated sidewalls, isothermal top surface, and bottom surface heated by forced convection. This Newtonian heating of the bottom surface involves a Biot number Bi that allows consideration of the continuum of boundary conditions ranging from constant heat flux, with global minimum R{sub min}=27.096 found as Bi{yields}0, to isothermal, with global minimum R{sub min}=4{pi}{sup 2} found as Bi{yields}{infinity}. In both cases and for most cylinder aspect ratios, incipient convection sets in as an asymmetric mode, though islands of aspect ratio exist where the onset mode is symmetric. Sample three-dimensional renderings of disturbance temperature distributions showing preferred modes at onset of convection for fixed Bi are provided and an analytical fit to R{sub min} as a function of Bi is given. (orig.)
Transient convective heat transfer
Scientific Electronic Library Online (English)
J., Padet.
2005-03-01
Full Text Available SciELO Brazil | Language: English Abstract in english In nature, as well as within the human-made thermal systems, the time-variable regimes are more commonly encountered, if not always, than the permanent regimes. Nevertheless, studies in convection are still more frequent in the permanent regimes, undoubtedly due to the related difficulties in calcul [...] ation in terms of time and cost of computation. One may distinguish two categories of time-dependent transfers: those which are due to external causes (variable boundary conditions) and those that are due to internal causes (sources of variable power, instabilities, turbulence), and the combination of these two types may also be encountered. In this presentation, we shall analyze some situations which belong to the first category. These are concerned with: - a group of boundary layer flows in forced, natural or mixed convection, where the wall is subjected to time-variable conditions in temperature or flux. - another group of fluid flows within ducts, in laminar mixed convection regime, where the entry conditions (mass flow rate, temperature) are time-dependent. The techniques of analysis are mainly extensions to the differential method and to the integral method of Karman-Polhausen in boundary layer flows, and the finite differences solution of the vorticity and energy equations for internal flows. The results presented in the transient state are caused by steps of temperature, heat flux or velocity, and in particular show the time evolution of the dynamic and thermal boundary layers, as well of the heat transfer coefficients. Three examples of applications will then be treated: the active control of convective transfers, the measurement of heat transfer coefficients, and the analysis of heat exchangers. The main idea in the active control is that of managing the temperatures or heat fluxes by employing a variable regime. Under certain conditions, this procedure may reveal itself quite interesting. The measurement of transfer coefficients by the photothermal impulse method possesses a great interest since it is performed in a non-intrusive way without contact. However, in order to be precise, it needs to account for the thermal boundary layer perturbation due to the radiative flux sent over the surface, which means to know the evolution of the transfer coefficient during the measurement. Previous studies therefore provide essential information. Within the domain of heat exchangers, we shall present a different global method, which allows for the evaluation of the time constant of an equipment in response to sample variations of temperature or mass flow rates at the entrance. In conclusion, a brief balance of the ICHMT Symposium "Transient heat and mass transfer", Cesme, Turkey, August 2003, will be presented.
Transient convective heat transfer
Directory of Open Access Journals (Sweden)
J. Padet
2005-03-01
Full Text Available In nature, as well as within the human-made thermal systems, the time-variable regimes are more commonly encountered, if not always, than the permanent regimes. Nevertheless, studies in convection are still more frequent in the permanent regimes, undoubtedly due to the related difficulties in calculation in terms of time and cost of computation. One may distinguish two categories of time-dependent transfers: those which are due to external causes (variable boundary conditions and those that are due to internal causes (sources of variable power, instabilities, turbulence, and the combination of these two types may also be encountered. In this presentation, we shall analyze some situations which belong to the first category. These are concerned with: - a group of boundary layer flows in forced, natural or mixed convection, where the wall is subjected to time-variable conditions in temperature or flux. - another group of fluid flows within ducts, in laminar mixed convection regime, where the entry conditions (mass flow rate, temperature are time-dependent. The techniques of analysis are mainly extensions to the differential method and to the integral method of Karman-Polhausen in boundary layer flows, and the finite differences solution of the vorticity and energy equations for internal flows. The results presented in the transient state are caused by steps of temperature, heat flux or velocity, and in particular show the time evolution of the dynamic and thermal boundary layers, as well of the heat transfer coefficients. Three examples of applications will then be treated: the active control of convective transfers, the measurement of heat transfer coefficients, and the analysis of heat exchangers. The main idea in the active control is that of managing the temperatures or heat fluxes by employing a variable regime. Under certain conditions, this procedure may reveal itself quite interesting. The measurement of transfer coefficients by the photothermal impulse method possesses a great interest since it is performed in a non-intrusive way without contact. However, in order to be precise, it needs to account for the thermal boundary layer perturbation due to the radiative flux sent over the surface, which means to know the evolution of the transfer coefficient during the measurement. Previous studies therefore provide essential information. Within the domain of heat exchangers, we shall present a different global method, which allows for the evaluation of the time constant of an equipment in response to sample variations of temperature or mass flow rates at the entrance. In conclusion, a brief balance of the ICHMT Symposium "Transient heat and mass transfer", Cesme, Turkey, August 2003, will be presented.
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....
Freche, John C; Schum, Eugene F
1951-01-01
Blade-to-coolant convective heat-transfer coefficients were obtained on a forced-convection water-cooled single-stage turbine over a large laminar flow range and over a portion of the transition range between laminar and turbulent flow. The convective coefficients were correlated by the general relation for forced-convection heat transfer with laminar flow. Natural-convection heat transfer was negligible for this turbine over the Grashof number range investigated. Comparison of turbine data with stationary tube data for the laminar flow of heated liquids showed good agreement. Calculated average midspan blade temperatures using theoretical gas-to-blade coefficients and blade-to-coolant coefficients from stationary-tube data resulted in close agreement with experimental data.
Kahani, M.; Zeinali Heris, S.; Mousavi, S. M.
2014-05-01
Coiled tubes and nanofludics are two significant techniques to enhance the heat transfer ability of thermal equipments. The forced convective heat transfer and the pressure drop of nanofluid inside straight tube and helical coiled one with a constant wall heat flux were studied experimentally. Distilled water was used as a host fluid and Nanofluids of aqueous TiO2 nanoparticles (50 nm) suspensions were prepared in various volume concentrations of 0.25-2 %. The heat transfer coefficient of nanofluids is obtained for different nanoparticle concentrations as well as various Reynolds numbers. The experiments covered a range of Reynolds number of 500-4,500. The results show the considerable enhancement of heat transfer rate, which is due to the nanoparticles present in the fluid. Heat transfer coefficient increases by increasing the volume concentration of nanoparticles as well as Reynolds number. Moreover, due to the curvature of the tube when fluid flows inside helical coiled tube instead of straight one, both convective heat transfer coefficient and the pressure drop of fluid grow considerably. Also, the thermal performance factors for tested nanofluids are greater than unity and the maximum thermal performance factor of 3.72 is found with the use of 2.0 % volume concentration of nanofluid at Reynolds number of 1,750.
Babalola, David
In this study we investigate the flow of a Boussinesq fluid contained in a rotating, differentially heated spherical shell. Previous work, on the spherical shell of Boussinesq fluid, differentially heated the shell by prescribing temperature on the inner boundary of the shell, setting the temperature deviation from the reference temperature to vary proportionally with -cos 2?, from the equator to the pole. We change the model to include an energy balance equation at the earth's surface, which incorporates latitudinal solar radiation distribution and ice-albedo feedback mechanism with moving ice boundary. For the fluid velocity, on the inner boundary, two conditions are considered: stress-free and no-slip. However, the model under consideration contains only simple representations of a small number of climate variables and thus is not a climate model per se but rather a tool to aid in understanding how changes in these variables may affect our planet's climate. The solution of the model is followed as the differential heating is changed, using the pseudo arc-length continuation method, which is a reliable method that can successfully follow a solution curve even at a turning point. Our main result is in regards to hysteresis phenomenon that is associated with transition from one to multiple convective cells, in a differentially heated, co-rotating spherical shell. In particular, we find that hysteresis can be observed without transition from one to multiple convective cells. Another important observation is that the transition to multiple convective cells is significantly suppressed altogether, in the case of stress-free boundary conditions on the fluid velocity. Also, the results of this study will be related to our present-day climate.
International Nuclear Information System (INIS)
Heat transfer characteristics of mainly combined forced and free convective flow in a vertical rectangular flow channel with a gap of 25 mm, which was quite narrow compared with those investigated in previous experiments, were studied experimentally for water. As a result, the following heat transfer characteristics were made clear, using a non-dimensional parameter Grx/Rex21/8Pr1/2 similarly to the case for the 18 mm gap which was already reported by the authors. (1) When the Grx/Rex21/8Pr1/2 is less than 10-4, both upward flow and downward flow show the nature of forced convective heat transfer. (2) When the Grx/Rex21/8Pr1/2 is between 10-4 and 10-2, heat transfer coefficients for both upward flow and downward flow are higher than any of those predicted by the previous correlations for turbulent forced convection along a flat plate and turbulent free convection along a vertical flat plate. This is, differently from the case of 18 mm gap, due to the effect of the acceleration of main flow induced by the development of the boundary layer along the channel. (3) When the Grx/Rex21/8Pr1/2 is larger than 10-2, the upward flow shows the nature of free convective flow even with the gap as narrow as 2.5 mm in the vertical rectangular flow channel. Heat transfical rectangular flow channel. Heat transfer correlations which have been developed for the 18 mm gap channel, are also available for the described-above regions of 2.5 mm gap channel. (author)
Forced- and natural-convection studies on solar collectors for heating and cooling applications
Pearson, J. T.
1983-03-01
Convection in air heating solar collectors for heating and cooling applications was studied. It was determined that improvement in the overall conductance between the absorber and the flowing air was an area that needed much improvement. Studies were performed to obtain several absorber convector configurations which have superior heat transfer performance, modest drop penalties, and a high potential for economical manufacturing. Four surfaces which may be fabricated from aluminum or steel are recommended. Three utilize corrugated sheets bonded to the backplate and/or the back side of the absorber. These three surfaces are recommended for applications where airflow behind the absorber is appropriate. For those applications where airflow above the absorber is appropriate, a louvered surface which can be fabricated from metal or plastic is recommended.
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The heat transfer resistance of a porous deposit can be expressed as the sum of two components: one associated with conduction through the fluid-filled deposit and a second associated with surface roughness. This simple relationship appears to hold under both single-phase forced-convection and boiling heat-transfer conditions. The conductive component of the total deposit resistance is always positive, whereas the roughness component is negative. Values for ? and Rroughness measured in this investigation are as follows. Under single-phase forced-convection heat-transfer conditions, ? = 1.3 ± 0.2 W/mK and Rroughness = -4 x 10-6 m2K/W for magnetite deposits. Under flow-boiling heat-transfer conditions, ? = 0.2 to 0.9 W/mK and Rroughness = -36 x 10-6 m2K/W for magnetite deposits, whereas ? = 2.0 W/mK and Rroughness = -43 x 10-6 m2K/W for deposits composed of approximately equal proportions of copper and magnetite. (author)
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An experimental determination was made of heat transfer critical conditions in a circular channel, uniformly heated, and internally cooled by water in ascending forced convection, under a pressure slightly above atmospheric pressure. Measurements were made of water flow, pressure, electric power temperature and heating, and a systematic analysis was made of the system's parameters. The values obtained for the heat critical flux are circa 50% lower than those predicted by Becker and Biasi and this is accounted to flowing instabilities of thermo-hydrodynamic nature. It is suggested that the flowing channels of circuits aiming at the study of the boiling crisis phenomenon be expanded in its upper extremity, and that the coolant circulation be kept through a pump with a pressure X flow characteristic as vertical as possible
Tatsumoto, H.; Shirai, Y.; Shiotsu, M.; Naruo, Y.; Kobayashi, H.; Inatani, Y.
2014-12-01
Forced convection heat transfer from a PtCo wire with a length of 120 mm and a diameter of 1.2 mm that was inserted into a vertically-mounted pipe with a diameter of 8.0 mm to liquid hydrogen flowing upward was measured with a quasi-steady increase of a heat generation rate for wide ranges of flow rate under saturated conditions. The pressures were varied from 0.4 MPa to 1.1 MPa. The non-boiling heat transfer characteristic agrees with that predicted by Dittus-Boelter correlation. The critical heat fluxes are higher for higher flow rates and lower pressures. Effect of Weber number on the CHF was clarified and a CHF correlation that can describe the experimental data is derived based on our correlation for a pipe.
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Emphasizing the integration of mathematical expressions with clear physical associations, this textbook on convective heat and mass transfer reviews the laws of thermodynamics and fluid motions, behavior of laminar and turbulent flows in a variety of conditions, natural free convection in space, and flows through porous media
Forced convection heat transfer from rib-roughened pins tested singly and in a cluster of seven
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Rib roughened surfaces are used to improve forced convection heat transfer. The basic data on roughened surface performance is usually obtained from tests using a roughened cylinder in a smooth cylindrical flow channel. The practical applications, in particular nuclear fuel elements, usually involve a cluster of 'pins' with roughened surfaces. Methods have been developed to apply the basic data to clusters and this report presents experimental results which can be used to check the accuracy of these methods. A single roughened pin has been tested in two smooth-walled channels of different diameters. An identical pin surrounded by a ring of six pins in a smooth channel has been tested with all the pins uniformly heated and only the centre pin heated. All the tests were repeated for a range of coolant flow rates. (author)
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A theoretical study was performed to investigate turbulent forced-convective momentum and heat transport in a rectangular channel under fully developed flow and heat transfer conditions. Main emphasis has been devoted to analyse the effects of the anisotropic turbulent transport properties and the turbulence-induced secondary flow on the main flow field and the temperature distribution. The effects of peripheral wall conduction as well as radiation within the channel are included in the analysis. The analysis reveals that only a single secondary current occurs in the trapezoidal symmetry element of the rectangular duct. Furthermore, when the heating extends over one or two oppositely located sides only, the location of the maximum wall temperature is shifted from the corner to the center of the wall. (orig.)
International Nuclear Information System (INIS)
A theoretical study was performed to investigate turbulent forced-convective momentum and heat transport in a rectangular channel of arbitrary aspect ratio under fully developed flow and heat transfer conditions. Main emphasis has been devoted to analyse the effects of the anisotropic turbulent transport properties and the turbulence-induced secondary flow on the main flow field and the temperature distribution. The effects of peripheral wall conduction as well as radiation within the channel are included in the analysis. Numerical results are presented for the turbulent velocity and temperature field and compared with experimental data. The analysis reveals that only a single secondary current occurs in the trapezoidal symmetry element of the rectangular duct. Furhtermore, when the heating extends over one or two oppositely located sides only, the location of the maximum wall temperature is shifted from the corner to the center of the wall. (orig.)
Kozlova, Sofya V; Ryzhkov, Ilya I
2014-09-01
In this paper, laminar convective heat transfer of water-alumina nanofluid in a circular tube with uniform heat flux at the tube wall is investigated. The investigation is performed numerically on the basis of two-component model, which takes into account nanoparticle transport by diffusion and thermophoresis. Two thermal regimes at the tube wall, heating and cooling, are considered and the influence of nanoparticle migration on the heat transfer is analyzed comparatively. The intensity of thermophoresis is characterized by a new empirical model for thermophoretic mobility. It is shown that the nanoparticle volume fraction decreases (increases) in the boundary layer near the wall under heating (cooling) due to thermophoresis. The corresponding variations of nanofluid properties and flow characteristics are presented and discussed. The intensity of heat transfer for the model with thermophoresis in comparison to the model without thermophoresis is studied by plotting the dependence of the heat transfer coefficient on the Peclet number. The effectiveness of water-alumina nanofluid is analyzed by plotting the average heat transfer coefficient against the required pumping power. The analysis of the results reveals that the water-alumina nanofluid shows better performance in the heating regime than in the cooling regime due to thermophoretic effect. PMID:25260328
International Nuclear Information System (INIS)
Forced convective boiling heat transfer coefficients were predicted for an annular flow inside a horizontal tube for pure refrigerants and nonazeotropic binary refrigerant mixtures. The heat transfer coefficients were calculated based on the turbulent temperature profile in liquid film and vapor core considering the composition difference in vapor and liquid phases, and the nonlinearity in mixing rules for the calculation of mixture properties. The heat transfer coefficients of pure refrigerants were estimated within a standard deviation of 14% compared with available experimental data. For nonazeotropic binary refrigerant mixtures, prediction of the heat transfer coefficients was made with a standard deviation of 18%. The heat transfer coefficients of refrigerant mixtures were lower than linearly interpolated values calculated from the heat transfer coefficients of pure refrigerants. This degradation was represented by several factors such as the difference between the liquid and the overall compositions, the conductivity ratio and the viscosity ratio of both components in refrigerant mixtures. The temperature change due to the concentration gradient was a major factor for the heat transfer degradation and the mass flux itself at the interface had a minor effect
Ashjaee, Mehdi; Goharkhah, Mohammad; Khadem, Leila Azizi; Ahmadi, Reza
2014-12-01
The effect of an external magnetic field on the forced convection heat transfer and pressure drop of water based Fe3O4 nanofluid (ferrofluid) in a miniature heat sink is studied experimentally. The heat sink with the dimensions of 40 mm (L) × 40 mm (W) × 10 mm (H) consists of an array of five circular channels with diameter and length of 4 and 40 mm, respectively. It is heated from the bottom surface with a constant heat flux while the other surfaces are insulated. The heat sink is also influenced by an external magnetic field generated by an electromagnet. The local convective coefficients are measured at various flow rates (200 < Re < 900), magnetic field intensities (B < 1,400 G), and particle volume fractions (? = 0.5, 1, 2 and 3 %). Results show that using ferrofluid results in a maximum of 14 % improvement in heat transfer compared to the pure water, in the absence of magnetic field. This value grows up to 38 % when a magnetic field with the strength of 1,200 G is applied to the ferrofluid. On the other hand, it is observed that the significant heat transfer enhancement due to the magnetic field is always accompanied by a pressure drop penalty. The optimum operating condition is obtained based on the maximum heat transfer enhancement per pressure loss.
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Oosthuizen, P.H.
1999-07-01
Two-dimensional forced convective flow over a circular cylinder embedded in a saturated porous medium has been numerically studied. The axis of a cylinder is arranged at right angles to the oncoming forced flow. The cylinder is heated to a uniform surface temperature which is different from than that of the fluid in the forced flow ahead of the cylinder. There is an impermeable plane surface above the cylinder, the surface being parallel to the forced flow. This surface is either adiabatic or at a uniform specified temperature that is between the temperature of the liquid flowing over the cylinder and the temperature of the cylinder surface. The study is based on the Darcy type assumptions. The governing equations, written in dimensionless form, have been solved using the finite element method. The solution has as parameters the dimensionless depth of the cylinder from the impermeable surface, the Peclet number and the dimensionless thermal boundary condition at the impermeable surface. Solutions have been obtained for a range of values of all these parameters. The results indicate that the dimensionless impermeable surface temperature has a strong influence of the heat transfer rate from the cylinder at smaller values of the dimensionless cylinder-to-surface distance.
Directory of Open Access Journals (Sweden)
Thamir K. Jassem
2013-05-01
Full Text Available An experimental forced laminar study was presented in this research for an air flowing through a circular channel for different angles ( ,30o,45o,60o, the channel was heated at constant heat flux , the channel also was packed with steel and glass spheres respectively . The tests were done for three values of Peclets number (2111.71,3945.42,4575.47 with changing the heat flux for each case and five times for each number.The results showed that the dimensionless temperature distribution will decrease with increasing the dimensionless channel length for all cases with changing Peclet number, heat flux and inclination angles, and its lowest value will be for glass spheres at highest flux, while at lower flux for , and the decreasing in dimensionless temperature was closed for both types of packed at other inclination angles.The study declared that the local Nusselt number decreases with increasing the dimensionless length of the channel for both packeds and for different applied heat flux, also through this study it was declared that the average Nusselt increases as Peclet number increases for both packed. Its value for the glass spheres is greater than the steel spheres with percentage (98.3% at small Peclet, and percentage (97.2% at large Peclet number for the horizontal tube, and (98.3% at small Peclet number and (97.8% at large Peclet number at .Through this study its was found that average Nusselt number increases along the channel as the heat flux increases, because the bulk temperature will increase as the flow proceeds toward the end of the channel , so the heat transfer coefficient will increase. It was declared from this study that in the case of the steel packed the heat transfer will occur mainly by conduction, while in the case of glass packed the heat transfer will occur mainly by laminar forced convection, where the lowest Nusselt number (Nu=3.8 was found when the pipe is horizontal and lowest heat flux and lowest Peclet number.
Bejan, Adrian
2013-01-01
Written by an internationally recognized authority on heat transfer and thermodynamics, this second edition of Convection Heat Transfer contains new and updated problems and examples reflecting real-world research and applications, including heat exchanger design. Teaching not only structure but also technique, the book begins with the simplest problem solving method (scale analysis), and moves on to progressively more advanced and exact methods (integral method, self similarity, asymptotic behavior). A solutions manual is available for all problems and exercises.
International Nuclear Information System (INIS)
An analytical study on laminar and fully developed forced convection heat transfer in a parallel-plate horizontal channel filled with an anisotropic permeability porous medium is performed. The principal axis of the anisotropic porous medium is oriented from 0 to 90 degrees. A constant heat flux is applied on the outer wall of the channel. Both clear (Newtonian) fluid and Darcy viscous dissipations are considered in the energy equation. Directional permeability ratio parameter A* is defined to combine both the effect of the dimensionless permeability ratio parameter K*=(K1/K2) and orientation angle ? into one parameter. The effects of the parameter A*, the Darcy number Da and the modified Brinkman number Br* on the heat transfer and fluid flow characteristics in the channels are investigated and presented in graphs. The obtained results show that the parameters A*, Da and Br* have strong effects on the dimensionless normalized velocity and temperature profiles as well as on the Nusselt number. It is found that for a particular value of A*, called as critical value Acr*, the external heat applied to the surface of the channel is balanced by the internal heat generation due to viscous dissipation and the bulk mean temperature approaches the wall temperature. Hence, the Nusselt number approaches infinity for the critical values Acr*cr*. (authors)
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Variation of forced convective heat transfer in a rectangular duct flow of a magnetic fluid under a magnetic field was investigated experimentally. Experiments were performed changing the magnetic field intensity, and this magnetic field could be varied from 0 mT to 600 mT. The Reynolds number based on the hydraulic diameter was set to 960, 1900 (laminar flow), and 2830 (turbulent flow). The results of the experiments show that in the case of laminar flow of the magnetic fluid, when a magnetic field is applied to a magnetic fluid flow, heat transfer locally increases in the region where the magnetic field exists. In contrast, in the turbulent flow of the magnetic fluid, heat transfer is not enhanced but reduced. In order to better understand this heat transfer phenomenon, we measured the velocity distribution of magnetic fluid flow by the Ultrasonic Velocity Profile (UVP) method. In the case of laminar flow, the result shows that the flow velocity at the center of the rectangular duct decreases and the velocity gradient in the near-wall region increases. Moreover, we calculated the flow resistance under a magnetic field by measurement of the pressure gradient, and the relationship between heat transfer and flow resistance was discussed.
Motozawa, M.; Sekine, T.; Sawada, T.; Kawaguchi, Y.
2013-02-01
Variation of forced convective heat transfer in a rectangular duct flow of a magnetic fluid under a magnetic field was investigated experimentally. Experiments were performed changing the magnetic field intensity, and this magnetic field could be varied from 0 mT to 600 mT. The Reynolds number based on the hydraulic diameter was set to 960, 1900 (laminar flow), and 2830 (turbulent flow). The results of the experiments show that in the case of laminar flow of the magnetic fluid, when a magnetic field is applied to a magnetic fluid flow, heat transfer locally increases in the region where the magnetic field exists. In contrast, in the turbulent flow of the magnetic fluid, heat transfer is not enhanced but reduced. In order to better understand this heat transfer phenomenon, we measured the velocity distribution of magnetic fluid flow by the Ultrasonic Velocity Profile (UVP) method. In the case of laminar flow, the result shows that the flow velocity at the center of the rectangular duct decreases and the velocity gradient in the near-wall region increases. Moreover, we calculated the flow resistance under a magnetic field by measurement of the pressure gradient, and the relationship between heat transfer and flow resistance was discussed.
Forced convective heat transfer with freezing from tandem cylinders in a porous medium
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This paper reports on heat transfer from two circular cylinders of equal diameter buried in a porous medium with a forced flow over them. The cylinders are assumed to be cooled to the same uniform temperature that is, in general, lower than the freezing temperature of the liquid flowing through the porous medium, while the temperature of this liquid in the forced flow ahead of the cylinders is, of course, above this freezing temperature. The cylinders are arranged in tandem such that the approaching forced flow is at right angles to the cylinder axes. The flow has been assumed to be steady and two-dimensional and the usual Darcy type assumptions have been adopted. The fluid properties have been assumed to be constant except that the conductivity of the frozen material is, in general, different from that of the unfrozen material. The fluid velocity and temperature well upstream of the cylinders are assumed to be uniform. The governing equations written in dimensionless form have been solved, subject to the boundary conditions using the finite element method
Cucchi, M.; Fustinoni, D.; Gramazio, P.; Colombo, L. P. M.; Niro, A.
2014-04-01
In this paper we present new experimental results of investigation on average heat transfer characteristics of a forced air-flow through a rectangular channel with the lower and upper surfaces roughened by ribs; data for a rectangular channel with flat surfaces are presented for comparison as well. The channel cross-section is 120 mm wide and 12 mm high; the channel is operated with the lower and upper walls kept at fixed temperature whereas the sides are adiabatic. The ribs have a square cross section and are mounted 60° parallel-tilted (the angle is respect to main stream) in astaggered arrangement. The tested configurations differ each other for the rib side dimension, namely, 2 or 4 mm, and for their pitch-to-side ratio equal to 10, 20 and 40. Upstream the test channel, there is an entry-section consisting of a 800 mm long, rectangular duct with the same transverse dimensions as the test section but with flat and adiabatic walls. Air flow rates have been varied in order to have Reynolds numbers, based on the duct hydraulic diameter, ranging between 700 and 7500. The average Nusselt numbers are evaluated on the basis of the air-flow bulk-temperature at entrance and exit from the heated zone, as well as of the surface temperature measured by eight T-type thermocouples plugged into the heated walls. The test section is also equipped with static pressure taps placed at the heated zone ends. Results show an increase of the average Nusselt number, calculated as the ratio Nu/Nu0, for the all tested ribbed channels ranging between 1.0 and 5.0.
Optimal spacing of parallel boards with discrete heat sources cooled by laminar forced convection
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Morega, A.M.; Bejan, A. (Duke Univ., Durham, NC (United States). Dept. of Mechanical Engineering and Materials Science)
1994-04-01
This paper shows numerically how to select the optimal spacing between boards mounted in a stack of specified volume, so that the overall thermal conductance between the stack and the forced coolant is maximum. Several configurations are considered: boards with uniform flux, flush-mounted discrete sources, and protruding heat sources. The flow is laminar and the pressure difference across the stack is fixed ([Delta]p). It is shown that for all the board geometries and thermal boundary conditions studied, the optimal board-to-board spacing is correlated by (D[sub opt]/l)[approximately] 2.7 ([Delta]pl[sup 2]/[mu][alpha])[sup [minus]1/4], where l is the effective longitudinal (flow) distance occupied by the discrete sources and the unheated patches contained between them, and [mu] and [alpha] are the viscosity and thermal diffusivity of the fluid. If U[sub [infinity
Combined convective heat transfer from short cylinders
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Considerable experimental evidence has been produced recently showing that the free convective heat transfer rate from horizontal circular cylinders becomes influenced by the length to diameter ratio L/D. The major aim of the present study was to determine the influence of the L/D ratio on the conditions under which buoyancy forces cause the heat transfer rate to start to deviate significantly from that existing in purely forced convection
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An investigation of the thermal hydraulic characteristics in the passive residual heat removal system of the System integrated Modular Advanced ReacTor-P (SMART-P) has been carried out using the MARS code, which is a best estimate system analysis code. The SMART-P is designed to cool the system during accidental conditions by a natural convection. The dominant heat transfer in the steam generator is a boiling mode under a forced convection condition, and it is a single-phase liquid and a boiling heat transfer under a natural convection condition. Most of the heat is removed in the heat exchanger of the passive residual heat removal system by a condensation heat transfer. The passive residual heat removal system can remove the energy from the primary side as long as the heat exchanger is submerged in the refueling water tank. The mass flow is stable under a natural circulation condition though it oscillates periodically with a small amplitude. The parameter study is performed by considering the effects of an effective height between the steam generator and the heat exchanger, a hydraulic resistance, an initial pressure, a non-condensable gas fraction in the compensating tank, and a valve actuation time, which are useful for the design of the passive residual heat removal system. The mass flow in the passive residual heat removal system has been affected by the height between the steam generator and the heat exchanger, and the hydraulic resistance of the loopydraulic resistance of the loop
Optimal discrete distribution of heat flux elements for in-tube laminar forced convection
International Nuclear Information System (INIS)
Highlights: ? Effect of an insulated segment on the peak temperature of a heated pipe is studied. ? Peak temperatures in a pipe wall heated by step-wise heat flux are minimized. ? Effects of Graetz number, fraction and number of insulated segments are studied. ? Use of upper unheated section(s) enhance(s) the thermal performance. -- Abstract: A new technique is proposed to enhance the heat transfer from a discretely heated pipe to a developing laminar fluid flow. Unlike the common heating situation where the fluid is continuously heated along the pipe wall with uniform heat flux, the proposed technique consists in heating the fluid with stepwise distributed heat flux, namely by placing insulated segments between the heated segments. Applying this technique, the effective length of the thermal entrance region is enlarged and as a result, the average heat transfer is invigorated. In order to maximize the heating performance, an optimal placement of the insulated segments between the heated segments is calculated according to constructal design. This serves to describe the optimal stepwise distribution of the heat flux. Owing that the total heat load is considered fixed, the maximization of the heating performance translates into the minimization of the peak temperature (‘hot spot’) of the pipe wall. The analytical results demonstrate that the optimal location of the insulated segments along with the reduction of the peak temperature strongly depend on the Graetz number. It is also shown that for intermediate values of the Graetz number, the peak temperatures are remarkably reduced in response to the optimal placement of the insulated/heated segments
Convective heat transfer with a periodic longitudinal heat flux variation
International Nuclear Information System (INIS)
The effects of longitudinally periodic heat flux profile on forced convection heat transfer are considered in a single phase liquid, subcooled boiling, and low-quality two-phase flow boiling conditions. Tests were made at low pressure in a Joule-heated container with an exterior wall of thickness varied in a stepwise fashion. The variation in thickness was sufficient to modify the resistance generated wall heat flux when direct current passed through the tube walls. The overall average forced convection boiling heat transfer coefficient was about half of the predicted value for flow boiling water under constant heat flux boundary conditions
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Kousksou, T.; El Rhafiki, T.; El Omari, K.; Zeraouli, Y.; Le Guer, Y. [Laboratoire de Thermique Energetique et Procedes, Universite de Pau et des Pays de l' Adour (UPPA), Campus Universitaire, 64000 Pau (France)
2010-12-15
A model has been developed for the simulation of laminar hydrodynamic and heat transfer characteristics in a horizontal circular tube for a suspension flow with micro-size phase-change material (PCM) particles. The energy equation is formulated by taking into consideration the heat absorption (or release) due to the phase-change process. The heat source or heat generation term in the energy equation is derived from solutions for freezing or melting inside the particles. The supercooling breakdown induced by the stochastic character of the crystallization is taken into account in the modeling with nucleation laws. The solutions were compared to experimental results obtained by different authors. (author)
Study of the equivalent diameter concept for heat transfer by forced convection in annular channels
International Nuclear Information System (INIS)
This work describes a comparative analysis between experimental values of heat transfer coefficients in fully developed turbulent flow for a concentric annular channel, and those calculated with the empirical correlations obtained for tubes by Dittus-Boelter, Sieder and Tate, a modified Colburn equation, and that proposed by Gnielinski which applies the analogy between friction and heat transfer. The coefficients were calculated by means of two different equivalent diameters: 1) The hydraulic equivalent diameter; and 2) The heated equivalent diameter. It was concluded that the hydraulic equivalent diameter gives much better results than the heated equivalent diameter. (Author)
Directory of Open Access Journals (Sweden)
Sundus Hussein Abd
2012-01-01
Full Text Available In this research, an experimental study was conducted to high light the impact of the exterior shape of a cylindrical body on the forced and free convection heat transfer coefficients when the body is hold in the entrance of an air duct. The impact of changing the body location within the air duct and the air speed are also demonstrated. The cylinders were manufactured with circular, triangular and square sections of copper for its high thermal conductivity with appropriate dimensions, while maintaining the surface area of all shapes to be the same. Each cylinder was heated to a certain temperature and put inside the duct at certain locations. The temperature of the cylinder was then monitored. The heat transfer coefficient were then calculated for forced convection for several Reynolds number (4555-18222.The study covered free convection impact for values of Rayleigh number ranging between (1069-3321. Imperical relationships were obtained for all cases of forced and free convection and compared with equations of circular cylindrical shapes found in literature. These imperical equations were found to be in good comparison with that of other sources.
Experiments on a forced convection heat transfer at supercritical pressures - 6.32 mm ID tube
International Nuclear Information System (INIS)
The size of a sub-channel of the conceptual SCWR core design studied at KAERI is 6.5 mm. In order to provide heat transfer information in such a narrow sub-channel at supercritical pressure, an experiment was performed with a test section made of Inconel 625 tube of 6.32 mm ID. The test pressures were 7.75 and 8.12 MPa corresponding to 1.05 and 1.1 times the critical pressure of CO2, respectively. The mass flux and heat flux, which were in the range of 285 ? 1200 kg/m2s and 30 ? 170 kW/m2, were changed at a given system pressure. The corresponding Reynolds numbers are 1.8 x 104 ? 7.5 x 104. The effect of mass flux and heat flux was dominant factor in the supercritical pressure heat transfer while the effect of pressure was negligible. The Bishop's correlation predicted the test result most closely and Bae and Kim's recent correlation was the next. The heat transfer deterioration occurred when GR)b/Reb2.7 > 2.0 x 10-5. As soon as the heat transfer was deteriorated, it entered a new regime and did not recover the normal heat transfer nevertheless Grb/Reb2.7 reduced below 2.0 x 10-5. It may mean that the correlation must be developed for the normal and deterioration regime separately
International Nuclear Information System (INIS)
Highlights: • A systematical study on contact modifications is performed for structured packed beds. • The bridges modification is found to give the most reasonable macroscopic results. • The overlaps and bridges methods are suitable for predicting local heat transfer. • Reasonable bridge diameter is found in a range from 16% dp to 20% dp. - Abstract: The present paper systematically investigated the appropriateness of different contact point modification approaches for forced convective heat transfer analysis in structured packed beds of spheres. The three-dimensional Navier–Stokes equations and RNG k–? turbulence model with scalable wall function are adopted to model the turbulent flow inside the pores. Both macroscopic and local flow and heat transfer characteristics for different packing forms (simple cubic, body center cubic and face center cubic packing forms) and contact treatments (gaps, overlaps, bridges and caps modifications) are carefully examined. In particular, the effects caused by the bridge size for the bridges treatment are discussed, and the numerical results are compared with available experiments in literature. It is found that the effects of contact treatments on the pressure drops are remarkable for different structured packing forms, especially when the porosity is relatively low, while such effects on the Nusselt numbers are relatively small. Among the four different contact modifications, the bridges method would give the most reasonable pressure drops for all the structured packing forms studied and this method is also proved to be suitable for predicting the Nusselt numbers. The local flow and heat transfer characteristics in the structured packed bed are sensitive to the methodology of contact modifications. The gaps and caps treatments would distort the local flow and temperature distributions in the packed bed, especially near the contact zones. While the local flow and temperature distributions from the overlaps and bridges treatments would be more reasonable and close to those in the original packing with points contact. Based on both the macroscopic and local flow and heat transfer analyses, the bridges treatment is recommended. The effects caused by the bridge size in the bridges treatment are also remarkable. It is noted that too small or too large bridge size would lead to unreasonable results for both the macroscopic and local flow and heat transfer analyses. A reasonable range of bridge diameter is found to be from 16% dp to 20% dp
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Bu, S.S.; Yang, J.; Zhou, M.; Li, S.Y. [Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049 (China); Wang, Q.W., E-mail: wangqw@mail.xjtu.edu.cn [Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049 (China); Guo, Z.X. [Department of Mechanical and Aerospace Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854 (United States)
2014-04-01
Highlights: • A systematical study on contact modifications is performed for structured packed beds. • The bridges modification is found to give the most reasonable macroscopic results. • The overlaps and bridges methods are suitable for predicting local heat transfer. • Reasonable bridge diameter is found in a range from 16% d{sub p} to 20% d{sub p}. - Abstract: The present paper systematically investigated the appropriateness of different contact point modification approaches for forced convective heat transfer analysis in structured packed beds of spheres. The three-dimensional Navier–Stokes equations and RNG k–? turbulence model with scalable wall function are adopted to model the turbulent flow inside the pores. Both macroscopic and local flow and heat transfer characteristics for different packing forms (simple cubic, body center cubic and face center cubic packing forms) and contact treatments (gaps, overlaps, bridges and caps modifications) are carefully examined. In particular, the effects caused by the bridge size for the bridges treatment are discussed, and the numerical results are compared with available experiments in literature. It is found that the effects of contact treatments on the pressure drops are remarkable for different structured packing forms, especially when the porosity is relatively low, while such effects on the Nusselt numbers are relatively small. Among the four different contact modifications, the bridges method would give the most reasonable pressure drops for all the structured packing forms studied and this method is also proved to be suitable for predicting the Nusselt numbers. The local flow and heat transfer characteristics in the structured packed bed are sensitive to the methodology of contact modifications. The gaps and caps treatments would distort the local flow and temperature distributions in the packed bed, especially near the contact zones. While the local flow and temperature distributions from the overlaps and bridges treatments would be more reasonable and close to those in the original packing with points contact. Based on both the macroscopic and local flow and heat transfer analyses, the bridges treatment is recommended. The effects caused by the bridge size in the bridges treatment are also remarkable. It is noted that too small or too large bridge size would lead to unreasonable results for both the macroscopic and local flow and heat transfer analyses. A reasonable range of bridge diameter is found to be from 16% d{sub p} to 20% d{sub p}.
Energy Technology Data Exchange (ETDEWEB)
Raji, A. [Faculty of Sciences and Technologies, Department of Physics, University Sultan Moulay Slimane, Team of Flows and Transfers Modeling (EMET), Laboratory of Physics and Mechanics of Materials, B.P. 523, Beni-Mellal (Morocco)], E-mail: abderaji@fstbm.ac.ma; Hasnaoui, M. [Faculty of Sciences Semlalia, Department of Physics, University Cadi Ayyad, UFR TMF, B.P. 2390 Marrakesh (Morocco); Bahlaoui, A. [Faculty of Sciences and Technologies, Department of Physics, University Sultan Moulay Slimane, Team of Flows and Transfers Modeling (EMET), Laboratory of Physics and Mechanics of Materials, B.P. 523, Beni-Mellal (Morocco)
2008-08-15
Mixed convection heat transfer in a ventilated cavity is numerically studied by solving the mixed convection equations with the Boussinesq approximation. Results are presented in terms of streamlines, isotherms and heat transfer for different combinations of the governing parameters namely, the Reynolds number (10 {<=} Re {<=} 5000), the Rayleigh number (10{sup 4} {<=} Ra {<=} 10{sup 6}) and the relative height of the openings (B = h'/H' = 1/4). The numerical results show the presence of a maximum interaction between the effects of the forced and natural convection and the existence of different flow regimes. The latter are delineated in the Ra-Re plane and the values of Re separating the different regions are determined and correlated versus Ra.
Enhancement of forced-convection cooling of multiple heated blocks in a channel using porous covers
Energy Technology Data Exchange (ETDEWEB)
Huang, P.C.; Yang, C.F. [National Taipei Univ. of Technology, Taiwan (China). Dept. of Air Conditioning and Refrigerating Engineering; Hwang, J.J.; Chiu, M.T. [National Sun Yat Sen Univ., Kaohsiung, Taiwan (China). Dept. of Mechanical Engineering
2005-02-01
A numerical study was carried out for enhanced heat transfer from multiple heated blocks in a channel by porous covers. The flow field is governed by the Navier-Stokes equation in the fluid region, the Darcy-Brinkman-Forchheimer equation in the porous region, and the thermal field by the energy equation. Solution of the coupled governing equations is obtained using a stream function-vorticity analysis. This study details the effects of variations in the Darcy number, Reynolds number, inertial parameter, and two pertinent geometric parameters, to illustrate important fundamental and practical results. The results show that the recirculation caused by porous-covering block will significantly enhance the heat transfer rate on both top and right faces of second and subsequent blocks. (Author)
Forced convective heat transfer in vertical air-water bubble flow
International Nuclear Information System (INIS)
The dependence of heat transfer on the cross-sectional distribution of void fractions in a diabatic two-phase bubble flow has been explored experimentally. A vertical tube of 16.9 mm ID was employed in the investigation. Two different types of the mixer were installed to realize the flows with different distributions of void fractions even at the same flow rates of both phases. It was found that these two types of flows present different heat transfer coefficients due to the difference in the void fraction distributions. Increasing the void fraction near the tube wall causes an increase in heat transfer coefficient. The effect of bubble agitation on the thermal diffusivity in the radial direction is discussed based on the distributions of both void fractions and liquid temperatures. (author)
Sensitivity studies of heat transfer: forced convection across a cylindrical pipe and duct flow
Ferrantelli, Andrea; Viljanen, Martti
2013-01-01
We consider two common heat transfer processes and perform a through sensitivity study of the variables involved. We derive and discuss analytical formulas for the heat transfer coefficient in function of film velocity, air temperature and pipe diameter. The according plots relate to a qualitative analysis of the multi-variable function $h$, according to functional optimization. For each process, we provide with graphs and tables of the parameters of interest, such as the Reynolds number. This method of study and the specific values can constitute a useful reference for didactic purposes.
Forced heat convection in semi-open channels with equilateral triangle lattice rod array
International Nuclear Information System (INIS)
The nature of nuclear physics and nuclear technology led to unusual channel shapes and geometric designs far different from that known as closed cross-section. This is, for instance the case of fuel rod cluster in a nuclear reactor core. Fuel units of this kind are composed of parallel rod bundles, arranged in a proper lattice position. Heat, which is originated in these fuel elements, as a result of nuclear fissions, is removed by coolant flowing in axial direction. An experimental study was carried out, in which the influence of flow condition, geometry of semi-open channels and thermal conditions on heat transfer were investigated. Three equilateral triangle lattice array test sections were designed and constructed, having pitch to diameter ratios of 1.09, 1.15 and 1.30. Fuel elements were simulated by stainless-steel-304 and brass heaters. They supplied constant and uniform heat fluxes of 3.5x104 watt/m2 to 3.4x105 watt/m2. The hydraulic system supplied steady flow demineralized water, in Reynolds Number from 2.5x104 to 1.5x105. Prandtl Numbers were in range of 2.7 to 5. All measurements were carried out on steady state condition. Experimental results were investigated to find the influence of geometry on local heat transfer values and were analyzed by computer programming to get the heat transfer coefficient by means of Nusselt Number, which was found to be dependent on geometry of the channel and thermohydraulic conditions. The correlation found was: Nusub(b) = 0.0177[1+151.58(De/L)2] Resub(b)sup(0.814) Prsub(b)sup(0.44) (Prsub(b)/Prsub(w))sup(0.14) The results correspond to other experimental and theoretical investigations carried out in different ways. (author)
Energy Technology Data Exchange (ETDEWEB)
Asirvatham, L. G.; Vishal, N.; Gangatharan, S. K.; Lal, D.M. [Department of Mechanical Engineering, Anna University, Chennai (India)
2009-07-01
The present work is an experimental study of steady state convective heat transfer of de-ionized water with a low volume fraction (0.003% by volume) of copper oxide (CuO) nanoparticles dispersed to form a nanofluid that flows through a copper tube. The effect of mass flow rate ranging from (0.0113 kg/s to 0.0139 kg/s) and the effect of inlet temperatures at 10 {sup o}C and 17 {sup o}C on the heat transfer coefficient are studied on the entry region under laminar flow condition. The results have shown 8% enhancement of the convective heat transfer coefficient of the nanofluid even with a low volume concentration of CuO nanoparticles. The heat transfer enhancement was increased considerably as the Reynolds number increased. Possible reasons for the enhancement are discussed. Nanofluid thermo-physical properties and chaotic movement of ultrafine particles which accelerate the energy exchange process are proposed to be the main reasons for the observed heat transfer enhancement. A correlation for convective heat transfer coefficient of nanofluids, based on transport property and D/{chi} for 8 mm tube has been evolved. The correlation predicts variation in the local Nusselt number along the flow direction of the nanofluid. A good agreement ({+-}10%) is seen between the experimental and predicted results. (author)
International Nuclear Information System (INIS)
Degradation of the thermal performance of steam generators(SGs) is a serious problem in nuclear power stations throughout the world (Lovett and Dow, 1991). In pressurized-heavy-water reactors (PWHRs), the reduced thermal performance of the SGs is manifested by an increase of the primary coolant reactor inlet header temperature (RIHT). In pressurized-light-water reactors(PWRs), which operate with fixed primary coolant temperature, the loss of thermal performance is manifested by a reduction of the steam pressure. Degradation mechanisms that may contribute to the loss of SG thermal performance include: fouling of the boiler tube inner surfaces (primary-side fouling); fouling of the boiler tube outer surfaces (secondary-side fouling); divider and thermal plate leakage that causes the coolant to bypass either the SG or the integral preheater and fouling of the steam separators. The relative contribution of these various degradation mechanisms to the overall loss of thermal performance is still under investigation. Soulard et al. (1990) examined the relative contributions of tube bundle fouling, divider plate leakage, and thermal plate leakage to the increase in RIHT at the Point Lepreau Generating Station, and concluded that tube fouling contributes to a significant fraction of the loss of thermal performance. Corrosion products deposit on both the inner and outer surfaces of the boiler tubes. Thus a complete understanding of the reasons fro the loss of thermal performance and the development of strategies to mitigate this loss requires a knowledge of the thermal resistance of tube deposits under primary and secondary side heat transfer conditions. We present here the results of measurements of the thermal resistance of primary-side and secondary-side boiler tube deposits performed under single-phase forced convection and flow-boiling conditions, respectively. The results are discussed in terms of the physical properties of the deposit and the mode of heat transfer
Two-phase pressure drop and heat transfer of sodium at forced convection
International Nuclear Information System (INIS)
Experiments with sodium for the two-phase pressure drop in vertical tubes with upward flow (internal diameters 6 and 9 mm) performed at the Joint Research Centre (JRC) of the European Communities in Ispra, Italy, and at the Nuclear Research Centre in Karlsruhe (KfK) were evaluated and analysed. Furthermore, experiments for the single-phase and two-phase heat transfer in the grid spaced twelve-rod bundle (pd/d =1.3, rod diameter 8 mm) with flow in axial direction performed at the JRC were evaluated and analysed. The pressure drop measurements were carried out at moderate to high mass flow rates (30 to 4500 kg/(m2s)) and at moderate pressures (50 to 300 kPa, density ratio ?f/?g = 950 to 5400). The measurements for the single-phase heat transfer at high heat fluxes (0.16 to 1.6 MW/m2) were carried out in the Reynolds number region (3100 2s)) and at high heat fluxes (0.46 to 1.6 MW/m2) within the temperature range from 870 to 9700C. For the subsequent calculation of the experiments relating to the two-phase pressure drop a computer program was developed, which is based on the so-called slip model. It requires a friction pressure loss correlation and a slip correlation. The tested correlations were not suitable for describing the experimental measurements. Accordingly, simplified equations of momentum were used to develop a new slip correlation for the case of annular flow together with the annular-mist flow, the most important two-phase flow regimes for sodium in the measurement range. After the inception of the entrainment - transition from the annular flow to the annular-mist flow - an even larger fraction of liquid enters the vapour core in the form of droplets, as the vapour quality increases. An equation was formulated for the slip in this region and adapted to the experiments via coefficients. (orig./GL)
Energy Technology Data Exchange (ETDEWEB)
Ahmed, Sameh E., E-mail: sameh_sci_math@yahoo.com [Mathematics Department, Faculty of Sciences, South Valley University, 83523 Qena (Egypt); Mansour, M.A. [Department of Mathematics, Assuit University, Faculty of Science, Assuit (Egypt); Mahdy, A., E-mail: mahdy4@yahoo.com [Mathematics Department, Faculty of Sciences, South Valley University, 83523 Qena (Egypt)
2013-12-15
Highlights: • We model MHD mixed convection in an inclined lid-driven cavity. • Increasing the Hartmann number leads to increase the heat transfer rate. • Increasing the inclination angle leads to the increase of the heat transfer rate. • Nusselt number at the left wall, for forced convection case, increases as the amplitude ratio increases. - Abstract: A numerical study of laminar magnetohydrodynamic mixed convection in an inclined lid-driven square cavity with opposing temperature gradients is presented. The vertical sidewalls are assumed to have non-uniform temperature variation while the top and bottom walls are kept insulated with the top surface moving at a constant speed. The transport equations are given in terms of the stream functions-vorticity formulation and are non-dimensionalized and then solved numerically by an accurate finite-volume method. The computation is carried out for wide ranges of the inclination angle (0 ? ? ? ?/2), the Richardson number (0.01 ? Ri ? 100), the Hartmann number (0 ? Ha ? 100), the amplitude ratio (0 ? ? ? 1) and the phase deviation (0 ? ? ? ?). The results indicate that the rate of heat transfer along the heated walls is enhanced on increasing either Hartmann number or inclination angle. Average Nusselt number is also, increased with increasing of the amplitude ratio for all values of the phase deviation. The non-uniform heating on both walls provides higher heat transfer rate than non-uniform heating of one wall.
Sundus Hussein Abd
2012-01-01
In this research, an experimental study was conducted to high light the impact of the exterior shape of a cylindrical body on the forced and free convection heat transfer coefficients when the body is hold in the entrance of an air duct. The impact of changing the body location within the air duct and the air speed are also demonstrated. The cylinders were manufactured with circular, triangular and square sections of copper for its high thermal conductivity with appropriate dimensions, while ...
International Nuclear Information System (INIS)
Some correlations of forced convection burn-out data are based on the approximate linearity of the relationship between burn-out heat flux and the channel-averaged quality at the burn-out point. These correlations perform satisfactorily on data obtained from uniformly heated configurations. Therefore the further inference is sometimes made that the burn-out heat flux is uniquely related to the quality, and that the burn-out in non-uniformly heated configurations can be calculated from measurements made with uniform heating. This report presents burn-out data for Freon 12 flowing vertically upwards through both uniformly and non-uniformly heated round tubes. This data shows that the quality at burn-out does depend on the heat flux profile, and that the inference mentioned above is not justified. (author)
International Nuclear Information System (INIS)
A model describing the thermal behaviour of a slurry of phase change material flow in a circular duct is presented. Reactors connected in series are considered for the representation of the circular duct with constant wall temperature. A phenomenological equation is formulated to take account of the heat generation due to phase change in the particles. Results of the simulation present a plateau of temperature along the longitudinal direction, characteristic of the phase change. The effect of different parameters such as the Reynolds number, the weight fraction and the temperature of the cold spring on the length of the plateau is analysed. A correlation resulting from numerical results is proposed for use in the determination of the characteristics of the exchanger for a phase change material slurry
International symposium on transient convective heat transfer: book of abstracts
International Nuclear Information System (INIS)
The international symposium on convective heat transfer was held on 19-23 August 1996, in Cesme, Izmir, Turkey. The spesialists discussed forced convection, heat exchangers, free convection and multiphase media and phase change at the meeting. Almost 53 papers were presented in the meeting
International Nuclear Information System (INIS)
This paper explores the bearing that a non-uniform distribution of heat flux used as a wall boundary condition exerts on the heat transfer improvement in a round pipe. Because the overall heat load is considered fixed, the heat transfer improvement is viewed through a reduction in the maximum temperature (‘hot spot’) by imposing optimal distribution of heat flux. Two cases are studied in detail 1) fully developed and 2) developing flow. Peak temperatures in the heated pipe wall are calculated via an analytical approach for the fully developed case, while a numerical simulation based on CFD is employed for the developing case. By relaxing the heat flux distribution on the pipe wall, the numerical results imply that the optimum distribution of heat flux, which minimizes the peak temperatures corresponds with the ‘descending’ distribution. Given that the foregoing approach is quite different from the ‘ascending’ heat flux distribution recommended in the literature by means of the entropy generation minimization (EGM) method, it is inferred that the optimization of heat transfer and fluid flow, in comparison with the thermodynamic optimization, may bring forth quite different guidelines for the designs of thermal systems under the same constraints and circumstances. -- Highlights: • Considered the bearing of non-uniform distribution of heat flux on the hot spots. • Determined the optimal distribution of heat flux that minimizes the hot spots. • Results are compared with those obtained by EGM method
International Nuclear Information System (INIS)
Highlights: • We model MHD mixed convection in an inclined lid-driven cavity. • Increasing the Hartmann number leads to increase the heat transfer rate. • Increasing the inclination angle leads to the increase of the heat transfer rate. • Nusselt number at the left wall, for forced convection case, increases as the amplitude ratio increases. - Abstract: A numerical study of laminar magnetohydrodynamic mixed convection in an inclined lid-driven square cavity with opposing temperature gradients is presented. The vertical sidewalls are assumed to have non-uniform temperature variation while the top and bottom walls are kept insulated with the top surface moving at a constant speed. The transport equations are given in terms of the stream functions-vorticity formulation and are non-dimensionalized and then solved numerically by an accurate finite-volume method. The computation is carried out for wide ranges of the inclination angle (0 ? ? ? ?/2), the Richardson number (0.01 ? Ri ? 100), the Hartmann number (0 ? Ha ? 100), the amplitude ratio (0 ? ? ? 1) and the phase deviation (0 ? ? ? ?). The results indicate that the rate of heat transfer along the heated walls is enhanced on increasing either Hartmann number or inclination angle. Average Nusselt number is also, increased with increasing of the amplitude ratio for all values of the phase deviation. The non-uniform heating on both walls provides higher heat transfer rate than non-uniform
Convective Flow under Rotating Force.
Directory of Open Access Journals (Sweden)
Meenu Hooda
2013-09-01
Full Text Available A finite difference code using the primitive variables is used to simulate the mixed convection in air (Pr=0.7 and liquid metals (Pr=0.015. The present study involves numerical simulation of momentum and energy equations in order to analyze two dimensional mixed convection in air and liquid metals in a differentially heated square cavity subjected to rotation for a broad range of operating parameters i.e. Rayleigh number (Ra , Taylor number (Ta and rotational Raleigh number (Raw.
Lazarus Godson Asirvatham; Nandigana Vishal; Senthil Kumar Gangatharan; Dhasan Mohan Lal
2009-01-01
The present work is an experimental study of steady state convective heat transfer of de-ionized water with a low volume fraction (0.003% by volume) of copper oxide (CuO) nanoparticles dispersed to form a nanofluid that flows through a copper tube. The effect of mass flow rate ranging from (0.0113 kg/s to 0.0139 kg/s) and the effect of inlet temperatures at 100C and 17 0C on the heat transfer coefficient are studied on the entry region under laminar flow condition. The results have shown 8% e...
Steady, three-dimensional, internally heated convection
Schubert, G.; Glatzmaier, G. A.; Travis, B.
1993-01-01
Numerical calculations have been carried out of steady, symmetric, 3D modes of convection in internally heated, infinite Prandtl number, Boussinesq fluids at a Rayleigh number of 1.4 x 10 exp 4 in a spherical shell with inner/outer radius of 0.55 and in a 3 x 3 x 1 rectangular box. Multiple patterns of convection occur in both geometries. In the Cartesian geometry, the patterns are dominated by cylindrical cold downflows and a broad hot upwelling. In the spherical geometry the patterns consist of cylindrical cold downwellings centered either at the vertices of a tetrahedron or the centers of the faces of a cube. The cold downflow cylinders are immersed in a background of upwelling within which there are cylindrical hot concentrations (plumes) and hot halos around the downflows. The forced hot upflow return plumes of internally heated spherical convection are fundamentally different from the buoyancy-driven plumes of heated from below convection.
Steady, three-dimensional, internally heated convection
International Nuclear Information System (INIS)
Numerical calculations have been carried out of steady, symmetric, three-dimensional modes of convection in internally heated, infinite Prandtl number, Boussinesq fluids at a Rayleigh number of 1.4x104 in a spherical shell with inner/outer radius of 0.55 and in a 3x3x1 rectangular box. Multiple patterns of convection occur in both geometries. In the Cartesian geometry the patterns are dominated by cylindrical cold downflows and a broad hot upwelling. In the spherical geometry the patterns consist of cylindrical cold downwellings centered either at the vertices of a tetrahedron or the centers of the faces of a cube. The cold downflow cylinders are immersed in a background of upwelling within which there are cylindrical hot concentrations (plumes) and hot halos around the downflows. The forced hot upflow return plumes of internally heated spherical convection are fundamentally different from the buoyancy-driven plumes of heated from below convection
Tasawar Hayat; Hendi, Awatif A.; Gbadeyan, Jacob A.; Olanrewaju, Philip O.
2011-01-01
In this paper we analyse the effects of internal heat generation, thermal radiation and buoyancy force on the laminar boundary layer about a vertical plate in a uniform stream of fluid under a convective surface boundary condition. In the analysis, we assumed that the left surface of the plate is in contact with a hot fluid whilst a stream of cold fluid flows steadily over the right surface; the heat source decays exponentially outwards from the surface of the plate. The similarity variable m...
International Nuclear Information System (INIS)
This study investigates the role of latent heat transfer, in connection with vaporization of a thin liquid film on the tube wall, in the laminar mixed convection flows under the simultaneous influences of combined buoyancy effects of thermal and mass diffusion. Major non-dimensional groups identified are Gr/sub T/, Gr/sub M/, Re, Pr, Sc and phi. Results are specifically presented for an air-water system under various conditions. The effects of the liquid film temperature, the Reynolds number and the relative humidity of the moist air in the ambient on the momentum, heat and mass transfer in the flow are examined in great detail. Tremendous enhancement in heat transfer due to the transport of the latent heat of vaporization is clearly demonstrated
Laminar Mixed Convection Heat Transfer Correlation for Horizontal Pipes
International Nuclear Information System (INIS)
This study aimed at producing experimental results and developing a new heat transfer correlation based upon a semi-empirical buoyancy coefficient. Mixed convection mass transfers inside horizontal pipe were investigated for the pipe of various length-to-diameters with varying Re. Forced convection correlation was developed using a very short cathode. With the length of cathode increase and Re decrease, the heat transfer rates were enhanced and becomes higher than that of forced convection. An empirical buoyancy coefficient was derived from correlation of natural convection and forced convection with the addition of L/D. And the heat transfer correlation for laminar mixed convection was developed using the buoyancy coefficient, it describes not only current results, but also results of other studies. Mixed convection occurs when the driving forces of both forced and natural convections are of comparable magnitude (Gr/Re2?1). It is classical problem but is still an active area of research for various thermal applications such as flat plate solar collectors, nuclear reactors and heat exchangers. The effect of buoyancy on heat transfer in a forced flow is varied by the direction of the buoyancy force. In a horizontal pipe the direction of the forced and buoyancy forces are perpendicular. The studies on the mixed convections of the horizontal pipes were not investigated very much due to the lack of practical uses compared to those of vertical pipes. Even the definitions on the buoyancy coefficient that presents the relative influence of the forced and the natural convections, are different by scholars. And the proposed heat transfer correlations do not agree
Laminar Mixed Convection Heat Transfer Correlation for Horizontal Pipes
Energy Technology Data Exchange (ETDEWEB)
Chae, Myeong Seon; Chung, Bum Jin [Kyung Hee Univ., Yongin (Korea, Republic of)
2013-10-15
This study aimed at producing experimental results and developing a new heat transfer correlation based upon a semi-empirical buoyancy coefficient. Mixed convection mass transfers inside horizontal pipe were investigated for the pipe of various length-to-diameters with varying Re. Forced convection correlation was developed using a very short cathode. With the length of cathode increase and Re decrease, the heat transfer rates were enhanced and becomes higher than that of forced convection. An empirical buoyancy coefficient was derived from correlation of natural convection and forced convection with the addition of L/D. And the heat transfer correlation for laminar mixed convection was developed using the buoyancy coefficient, it describes not only current results, but also results of other studies. Mixed convection occurs when the driving forces of both forced and natural convections are of comparable magnitude (Gr/Re{sup 2}?1). It is classical problem but is still an active area of research for various thermal applications such as flat plate solar collectors, nuclear reactors and heat exchangers. The effect of buoyancy on heat transfer in a forced flow is varied by the direction of the buoyancy force. In a horizontal pipe the direction of the forced and buoyancy forces are perpendicular. The studies on the mixed convections of the horizontal pipes were not investigated very much due to the lack of practical uses compared to those of vertical pipes. Even the definitions on the buoyancy coefficient that presents the relative influence of the forced and the natural convections, are different by scholars. And the proposed heat transfer correlations do not agree.
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Forced convective heat transfer coefficient and pressure drop of SiO2- and Al2O3-water nanofluids were characterized. The experimental facility was composed of thermal-hydraulic loop with a tank with an immersed heater, a centrifugal pump, a bypass with a globe valve, an electromagnetic flow-meter, a 18 kW in-line pre-heater, a test section with band heaters, a differential pressure transducer and a heat exchanger. The test section consists of a 1000 mm long aluminium pipe with an inner diameter of 31.2 mm. Eighteen band heaters were placed all along the test section in order to provide a uniform heat flux. Heat transfer coefficient was calculated measuring fluid temperature using immersed thermocouples (Pt100) placed at both ends of the test section and surface thermocouples in 10 axial locations along the test section (Pt1000). The measurements have been performed for different nanoparticles (Al2O3 and SiO2 with primary size of 11 nm and 12 nm, respectively), volume concentrations (1% v., 5% v.), and flow rates (3 103Re5). Maximum heat transfer coefficient enhancement (300%) and pressure drop penalty (1000%) is obtained with 5% v. SiO2 nanofluid. Existing correlations can predict, at least in a first approximation, the heat transfer coefficient and pressure drop of nanofluids if thermal conductivity, viscosity and specific heat were properly modelled.eat were properly modelled.
Julia, J. E.; Hernández, L.; Martínez-Cuenca, R.; Hibiki, T.; Mondragón, R.; Segarra, C.; Jarque, J. C.
2012-11-01
Forced convective heat transfer coefficient and pressure drop of SiO2- and Al2O3-water nanofluids were characterized. The experimental facility was composed of thermal-hydraulic loop with a tank with an immersed heater, a centrifugal pump, a bypass with a globe valve, an electromagnetic flow-meter, a 18 kW in-line pre-heater, a test section with band heaters, a differential pressure transducer and a heat exchanger. The test section consists of a 1000 mm long aluminium pipe with an inner diameter of 31.2 mm. Eighteen band heaters were placed all along the test section in order to provide a uniform heat flux. Heat transfer coefficient was calculated measuring fluid temperature using immersed thermocouples (Pt100) placed at both ends of the test section and surface thermocouples in 10 axial locations along the test section (Pt1000). The measurements have been performed for different nanoparticles (Al2O3 and SiO2 with primary size of 11 nm and 12 nm, respectively), volume concentrations (1% v., 5% v.), and flow rates (3 103Re<105). Maximum heat transfer coefficient enhancement (300%) and pressure drop penalty (1000%) is obtained with 5% v. SiO2 nanofluid. Existing correlations can predict, at least in a first approximation, the heat transfer coefficient and pressure drop of nanofluids if thermal conductivity, viscosity and specific heat were properly modelled.
Convection driven by internal heating
Goluskin, David; Spiegel, Edward A.
2012-01-01
Two-dimensional direct numerical simulations are conducted for convection sustained by uniform internal heating in a horizontal fluid layer. Top and bottom boundary temperatures are fixed and equal. Prandtl numbers range from 0.01 to 100, and Rayleigh numbers (R) are up to 5x10^5 times the critical R at the onset of convection. The asymmetry between upward and downward heat fluxes is non-monotonic in R. In a broad high-R regime, dimensionless mean temperature scales as R^{-1...
Direct Numerical Simulations of turbulent mixed convection in enclosures with heated obstacles
Shishkina, Olga; Wagner, Claus
Investigation of turbulent mixed convection flows past heated obstacles has physical as well as engineering objectives, since such problems as climate control in buildings, cars or aircrafts, where the temperature must be regulated to maintain comfortable and healthy conditions, can be formulated as mixed convection problems. In this type of convection the flows are determined both by the buoyancy force like in natural convection and by inertia forces like in forced convection, while neither of these forces dominates.
Design of an R-134a loop for subcritical and supercritical forced-convection heat transfer studies
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The R-134a test loop is a forced-flow experimental facility for the study of heat transfer properties of R-134a under subcritical and supercritical thermodynamic conditions. The loop is designed to operate with pressures as high as 6 MPa and temperatures up to 140 °C. The intended mass flux is in the range of 500-6000 kg/m2s for the experiments with subcritical thermodynamic states and 500-4000 kg/m2s for supercritical conditions. The loop has been designed to accommodate a variety of test-section geometries, ranging from a straight circular tube to a 7-rod bundle, achieving heat fluxes up to 2.5 MW/m2 depending on the test section geometry. The design of the loop allows for easy reconfiguration of the test-section orientation relative to the gravitational direction and adjustment to the length of the test section. (author)
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Astruc, J.M. [Commissariat a l' Energie Atomique, Grenoble (France). Centre d' Etudes Nucleaires
1967-12-01
In the first part, free-convection and nucleate pool boiling heat transfer (up to burn-out heat flux) between a platinum wire of 0.15 mm in diameter in neon, deuterium and hydrogen has been studied at atmospheric pressure. These measurements were continued in liquid neon up to 23 bars (Pc {approx_equal} 26.8 b). Film boiling heat transfer coefficients have been measured in pool boiling liquid neon at atmospheric pressure with three heating wires (diameters 0.2, 0.5, 2 mm). All the results have been compared with existing correlations. The second part is devoted to measurements of the critical heat flux limiting heat transfer with small temperature differences between the wall and the liquid neon flowing inside a tube (diameters 3 x 3.5 mm) heated by joule effect on 30 cm of length. Influences of flow stability, nature of electrical current, pressure, mass flow rate and subcooling are shown. In conclusion, the similarity of the heat transfer characteristics in pool boiling as well as in forced convection of liquid neon and hydrogen is emphasized. (author) [French] Dans la premiere partie, on a etudie les echanges thermiques en regime de convection libre et d'ebullition nucleee (jusqu'au flux critique de l'ebullition nucleee) entre un fil de platine (diametre 0,15 mm) chauffe electriquement et un bain de liquide (neon, deuterium, ou hydrogene) bouillant a la pression atmospherique. On a poursuivi ces mesures dans le neon jusqu'a une pression de 23 bars (Pc {approx_equal} 26,8 b). On a mesure en outre les coefficients d'echanges thermiques en regime de calefaction dans le neon liquide a pression atmospherique. Tous ces resultats sont compares a diverses correlations existantes. La deuxieme partie est axee sur la mesure des flux de chaleur critiques limitant les echanges thermiques avec faible ecart de temperatures entre la paroi et le neon liquide s'ecoulant dans un tube (diam. 3 x 3,5 mm) chauffe par effet joule sur 30 cm de longueur. On a etudie l'influence de la stabilite de l'ecoulement de la nature du courant electrique de chauffage, de la pression, du debit et du sous-refroidissement du liquide. On conclut en soulignant la ressemblance des caracteristiques d'echanges thermiques du neon et de l'hydrogene liquides tant en convection libre qu'en convection forcee. (auteur)
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Stability boundaries of a helium flow at forced and free convection in a vertical channel are calculated for different hydraulic resistance ratios of throttling units and channel lengths. Results are presented and stability dependence on the flow direction shown
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This paper provides information on heat transfer enhancement due to jet mixing inside a cylindrical enclosure. The work addresses conservative heat transfer assumptions regarding mixing and condensation that have typically been incorporated into passive containment design analyses. The current research presents an interesting possibility for increasing decay heat removal of passive containment systems under combined natural and forced convection. Eliminating these conservative assumptions could provide the basis for a change of containment design and reduce the construction cost. It is found that the ratio of forced- and free convection Nusselt numbers can be predicted as a function of the Archimedes number and a correlated factor accounting for jet orientation and enclosure geometry. To use the small-scale tests for large containment design, scale-up methods and criteria are important for matching the key governing parameters and fluid properties. In the present experiment, a cylindrical enclosure was constructed with a vertical wall of 2.29-m diameter and 0.8-m height and a vertically adjustable ceiling. A horizontal copper plate was installed at the bottom to provide an isothermal heating surface. Cold air was injected at several positions with varying pipe diameters and injecting orientations and was removed from the top of the enclosure. The experiment was performed with an extensive set of tests to study the combined natural- and forced convection heat transfer ural- and forced convection heat transfer in a cylindrical enclosure mixed by an injected jet. The goals are to evaluate the key parameters governing the heat transfer augmentation by a forced jet and to investigate the effect of geometric factors, including jet diameter, jet injection orientation, and enclosure geometry (aspect ratio). Flow velocity measurement further provides a better understanding of the flow patterns developed inside the enclosure, which will determine the effectiveness of the whole volume mixing process. An additional experiment with a vertically cooled wall, similar to the condition of a passive containment cooling surface, is under construction. It will be used for further investigation of similar phenomena for steam condensation in the presence of non-condensable gases. From the current study with the cylindrical enclosure, it was found that the augmentation of natural-convection heat transfer by a forced jet is primarily determined by the jet Reynolds number, while the injecting nozzle diameter has a relatively weak effect. The jet orientation also plays an important role in determining the augmentation ratio, and of the four different jet orientations studied here, vertical downward injection at the center of the enclosure gives the highest augmentation, while azimuthal injection gives the lowest. The enclosure geometry and aspect ratio are also important factors, depending on the jet orientation. A combining rule is employed with a weighted relation to balance the contributions from separate heat-transfer correlations representing natural and forced dominated convections. It was found that under natural convection without the jet, the mean Nusselt number inside the large enclosure can be correlated by the enclosure Rayleigh number, RaD1/3, and under forced convection with a strong jet, the data are well represented as a function of the jet Reynolds number, Rej2/3. According to these relationships, a correlation form predicting mixed convection inside the enclosure was developed by a function that uses the ratio of forced- and free-convection heat transfer coefficients. This form can then be further reduced to predict the augmentation ratio as a function of the Archimedes number and a correcting factor accounting for jet orientation and enclosure geometry. Figure 1 presents the heat transfer augmentation as the Nusselt number ratio of mixed convection to natural convection (NuD/Nunc) versus the Archimedes number. The data, including four injecting orientations and three different jet diameters, are well correlated for each injecti
Terminal project heat convection in thin cylinders
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Heat convection in thin cylinders and analysis about natural convection for straight vertical plates, and straight vertical cylinders submersed in a fluid are presented some works carry out by different authors in the field of heat transfer. In the part of conduction, deduction of the equation of heat conduction in cylindrical coordinates by means of energy balance in a control volume is presented. Enthalpy and internal energy are used for the outlining of the equation and finally the equation in its vectorial form is obtained. In the convection part development to calculate the Nusselt number for a straight vertical plate by a forces analysis, an energy balance and mass conservation over a control volume is outlined. Several empiric correlations to calculate the Nusselt number and its relations with other dimensionless numbers are presented. In the experimental part the way in which a prototype rode is assembled is presented measurements of temperatures attained in steady state and in free convection for working fluids as air and water are showed in tables. Also graphs of Nusselt numbers obtained in the experimental way through some empiric correlations are showed (Author)
Convective heat transfer in rotating cylindrical cavity
Owen, J. M.; Onur, H. S.
1983-04-01
In order to gain an understanding of the conditions inside air-cooled, gas-turbine rotors, flow visualization, laser-doppler anemometry, and heat-transfer measurements have been made in a rotating cavity with either an axial throughflow or a radial outflow of coolant. For the axial throughflow tests, a correlation has been obtained for the mean Nusselt number in terms of the cavity gap ratio, the axial Reynolds number, and rotational Grashof number. For the radial outflow tests, velocity measurements are in good agreement with solutions of the linear (laminar and turbulent) Ekman layer equations, and flow visualization has revealed the destabilizing effect of buoyancy forces on the flow structure. The mean Nusselt numbers have been correlated, for the radial outflow case, over a wide range of gap ratios, coolant flow rates, rotational Reynolds numbers and Grashof numbers. As well as the three (forced convection) regimes established from previous experiments, a fourth (free convection) regime has been identified.
Experimental study of turbulent forced convection in vertical eccentric annulus
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Forced convection in an open-ended vertical eccentric annulus with different eccentricities has been experimentally studied with several heat fluxes and inlet air velocities. Constant heat flux outer tube and insulated inner tube boundary conditions were used. The wall temperature of the outer tube was measured along the length for different eccentricities. Results indicate that with the increase of eccentricity, the convection heat transfer coefficient increases. Also, as the Reynolds number is increased a crest is formed in the temperature variation along the length. An empirical relation is derived for the Nusselt number as a function of the Reynolds number and eccentricity.
Experimental study of turbulent forced convection in vertical eccentric annulus
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Hosseini, R.; Ramezani, M.; Mazaheri, M.R. [Mechanical Engineering Dept., Amirkabir Univ. of Technology, 424 Hafez Ave., P.O. Box 15875-4413, Tehran (Iran)
2009-09-15
Forced convection in an open-ended vertical eccentric annulus with different eccentricities has been experimentally studied with several heat fluxes and inlet air velocities. Constant heat flux outer tube and insulated inner tube boundary conditions were used. The wall temperature of the outer tube was measured along the length for different eccentricities. Results indicate that with the increase of eccentricity, the convection heat transfer coefficient increases. Also, as the Reynolds number is increased a crest is formed in the temperature variation along the length. An empirical relation is derived for the Nusselt number as a function of the Reynolds number and eccentricity. (author)
Scientific Electronic Library Online (English)
Philip O., Olanrewaju; Jacob A., Gbadeyan; Tasawar, Hayat; Awatif A., Hendi.
2011-10-01
Full Text Available SciELO South Africa | Language: English Abstract in english In this paper we analyse the effects of internal heat generation, thermal radiation and buoyancy force on the laminar boundary layer about a vertical plate in a uniform stream of fluid under a convective surface boundary condition. In the analysis, we assumed that the left surface of the plate is in [...] contact with a hot fluid whilst a stream of cold fluid flows steadily over the right surface; the heat source decays exponentially outwards from the surface of the plate. The similarity variable method was applied to the steady state governing non-linear partial differential equations, which were transformed into a set of coupled non-linear ordinary differential equations and were solved numerically by applying a shooting iteration technique together with a sixth-order Runge-Kutta integration scheme for better accuracy. The effects of the Prandtl number, the local Biot number, the internal heat generation parameter, thermal radiation and the local Grashof number on the velocity and temperature profiles are illustrated and interpreted in physical terms. A comparison with previously published results on similar special cases showed excellent agreement.
Directory of Open Access Journals (Sweden)
Tasawar Hayat
2011-09-01
Full Text Available In this paper we analyse the effects of internal heat generation, thermal radiation and buoyancy force on the laminar boundary layer about a vertical plate in a uniform stream of fluid under a convective surface boundary condition. In the analysis, we assumed that the left surface of the plate is in contact with a hot fluid whilst a stream of cold fluid flows steadily over the right surface; the heat source decays exponentially outwards from the surface of the plate. The similarity variable method was applied to the steady state governing non-linear partial differential equations, which were transformed into a set of coupled non-linear ordinary differential equations and were solved numerically by applying a shooting iteration technique together with a sixth-order Runge–Kutta integration scheme for better accuracy. The effects of the Prandtl number, the local Biot number, the internal heat generation parameter, thermal radiation and the local Grashof number on the velocity and temperature profiles are illustrated and interpreted in physical terms. A comparison with previously published results on similar special cases showed excellent agreement.
Forced and natural convection in aggregate-laden nanofluids
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A number of experimental and theoretical studies of convective heat transfer in nanofluids (liquid suspensions of nanoparticles, typically with features below 100 nm in size) reveal contrasting results; nanoparticles can either enhance or reduce the convective heat transfer coefficient. These disparate conclusions regarding the influence of nanoparticles on convective heat transfer may arise due to the aggregation of nanoparticles, which is often not considered in studies of nanofluids. Here, we examine theoretically forced and natural convective heat transfer of aggregate-laden nanofluids using Monte Carlo-based models to determine how the aggregate morphology influences the convective heat transfer coefficient. Specifically, in this study, it is first shown that standard heat transfer correlations should apply to nanofluids, and the main influence of the nanoparticles is to alter suspension thermal conductivity, dynamic viscosity, density, specific heat, and thermal expansion coefficient. Aggregated particles in suspension are modeled as quasi-fractal aggregates composed of individual primary particles described by the primary particle radius, number of primary particles, fractal (Hausdorff) dimension, pre-exponential factor, and degree of coalescence between primary particles. A sequential algorithm is used to computationally generate aggregates with prescribed morphological descriptors. Four types of aggregates are considered; spanning the range of aggregate morphred; spanning the range of aggregate morphologies observed in nanofluids. For each morphological type, the influences of aggregates on nanofluid dynamic viscosity and thermal conductivity are determined via first passage-based Brownian dynamics calculations. It is found that depending on both the material properties of the nanoparticles as well as the nanoparticle morphology, the addition of nanoparticles to a suspension can either increase or decrease both the forced and natural convective heat transfer coefficients, with both a 51% increase and a 32% decrease in the heat transfer coefficient achievable at particle volume fractions of 0.05. This study shows clearly that the influence of particle morphology needs to be accounted for in all studies of heat transfer in nanofluids.
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To clarify thermohydrodynamic processes in a heat exchanger with a low relative length of heated sections (l/d < 50), convective heat transfer in a heated tube is analytically investigated. Equations of heat transfer in the wall and in liquid are considered along with motion equations, i.e. a conjugate problem is solved. Iterations over the joint of temperature fields of wall inner surface and the external current surface is carried out for boundary conditions of the 1st kind from the wall and the second kind from the current. The model adequacy to real physical processes is experimentally established. The information obtained permits to calculate practically any termohydrodynamic characteristic of the model considered. Results of investigations permit to recommend the above mathematical model to investigate analogous problems connected with the calculation of convection heat exchange in porced liquid currents with variable properties under conditions of considerable nonisothermality characteristic of small sections of active heating
Natural Convection Heat Transfer from a Heated Fine Wire in Nanofluids
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Recent research on nanofluids under forced convection experiment shows that there is little relationship between convective heat transfer and thermal conductivity increase of nanofluids. This kind of new findings are totally different from the traditional theory of nanofluids, which says that the higher thermal conductivity is a prerequisite for convective heat transfer enhancement. To elucidate this controversial issue in a very comprehensible manner, simple natural convection experiment has been carried out for the water- and oil-based nanofluids. (water-Al2O3, transformer oil-Al2O3) Present research shows that there exists strong dependence between natural convection performance and thermal conductivity increase of nanofluids
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Square-ribbed coolant channels with helium gas for fusion power reactors were designed to enhance the turbulent heat transfer in comparison with smooth coolant channels. Heat transfer coefficients and friction factors in the square-ribbed annular channels were investigated quantitatively under high pressure. The turbulent heat transfer coefficients in the square-ribbed annulus, with a ratio of the rib pitch to a height of ten, were 200-300% higher than those in the smooth annulus and then the thermal-hydraulic empirical correlations were derived. The average heat fluxes from the heated wall were predicted using the present empirical correlations and the maximum heat flux at p/h=10 reached ?1.5 MW/m2 at Re=65000. From the results of the present study, it can be expected that the proposed rib-roughened coolant channels are available to remove surface heat fluxes at the first wall and blanket in the fusion power reactors
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Within the project 'Convection in a Cylinder' (CiC) heat transfer enhancement is studied for the case of two concentric, vertically aligned cylinders. The cylindrical gap is filled with a dielectric liquid, which viscosity is just few times higher than that of water. The inner cylinder is heated and the outer one is cooled. This setup in a gravitational buoyancy field leads to a fluid movement in a single convective cell with hot fluid rising at the inner boundary and cold fluid sinking at the outer boundary. The top and bottom part of the system shows horizontal movement, again in boundary layers. The strengthening of temperature gradient induces instabilities of that convective motion. If we vary the buoyancy force by means of electro-hydrodynamic effects, the patterns of convection differ from those instabilities rising only from variation of the temperature gradient.
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Distributions of heat transfer coefficients in axial heat exchangers, consisting of closely spaced tube bundles, were measured with a holographic interferometer. For the transition from laminar to turbulent flow (Re 500 to 5000) six different spacings were examined. In addition to the local differences, temporal variations could be observed. A theoretical calculation of the local heat transfer, using the finite difference method, is in good agreement with the experimental results. (U.S.)
Spatial forcing of Rayleigh-Benard Convection
McCoy, Jonathan; Shaughnessy, Gabe; Hagedorn, Charlie; Bodenschatz, Eberhard; Lipson, Steve
2004-11-01
We report experimental results on spatial forcing of thermally driven convection in a large aspect ratio fluid layer. In anisotropic systems, such as electroconvection in nematic liquid crystals (Lowe et. al, 1983), wavelength competition caused by spatially periodic forcing was found to generate novel pattern transitions. Thermally driven fluid convection offers a number of interesting possibilities arising from the isotropy of the system and from the presence of spatiotemporally chaotic states with multiple length scales of interest. We will discuss how broken translational symmetry caused by lithographically micromachining a forcing pattern onto the bottom plate of the convection cell affects (i) pattern selection at onset and (ii) the observed bistability of straight rolls and spiral defect chaos. This work is supported by the National Science Foundation grant DMR 0305151.
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Chang, Shyy Woei [Thermal Fluids Laboratory, National Kaohsiung Marine University, No. 142, Haijhuan Road, Nanzih District, Kaohsiung City 81143 (China); Yang, Tsun Lirng [Department of Marine Engineering, National Kaohsiung Marine University, No. 142, Haijhuan Road, Nanzih District, Kaohsiung City 81143 (China)
2009-10-15
This experimental study comparatively examined the two-phase flow structures, pressured drops and heat transfer performances for the cocurrent air-water slug flows in the vertical tubes with and without the spiky twisted tape insert. The two-phase flow structures in the plain and swirl tubes were imaged using the computerized high frame-rate videography with the Taylor bubble velocity measured. Superficial liquid Reynolds number (Re{sub L}) and air-to-water mass flow ratio (AW), which were respectively in the ranges of 4000-10000 and 0.003-0.02 were selected as the controlling parameters to specify the flow condition and derive the heat transfer correlations. Tube-wise averaged void fraction and Taylor bubble velocity were well correlated by the modified drift flux models for both plain and swirl tubes at the slug flow condition. A set of selected data obtained from the plain and swirl tubes was comparatively examined to highlight the impacts of the spiky twisted tape on the air-water interfacial structure and the pressure drop and heat transfer performances. Empirical heat transfer correlations that permitted the evaluation of individual and interdependent Re{sub L} and AW impacts on heat transfer in the developed flow regions of the plain and swirl tubes at the slug flow condition were derived. (author)
Convective cooling of three discrete heat sources in channel flow
Thiago Antonini Alves; Altemani, Carlos A. C.
2008-01-01
A numerical investigation was performed to evaluate distinct convective heat transfer coefficients for three discrete strip heat sources flush mounted to a wall of a parallel plates channel. Uniform heat flux was considered along each heat source, but the remaining channel surfaces were assumed adiabatic. A laminar airflow with constant properties was forced into the channel considering either developed flow or a uniform velocity at the channel entrance. The conservation equations were solved...
Mechanistic modeling of CHF in forced-convection subcooled boiling
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Because of the complexity of phenomena governing boiling heat transfer, the approach to solve practical problems has traditionally been based on experimental correlations rather than mechanistic models. The recent progress in computational fluid dynamics (CFD), combined with improved experimental techniques in two-phase flow and heat transfer, makes the use of rigorous physically-based models a realistic alternative to the current simplistic phenomenological approach. The objective of this paper is to present a new CFD model for critical heat flux (CHF) in low quality (in particular, in subcooled boiling) forced-convection flows in heated channels
The optimal spacing for cylinders in crossflow forced convection
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In this note I draw attention to a new fundamental aspect of the heat transfer performance of a bundle of parallel cylinders with crossflow forced convection, namely, the maximization of the thermal contact between the bundle and the fluid, when the volume occupied by the bundle is fixed. In the experiments described by Jubran et al. we have seen empirical evidence that the total heat transfer rate is maximum when the cylinder-to-cylinder spacing S has a certain value. This finding is important because it has been overlooked for decades, while forced convection from cylinders in crossflow grew into one of the most researched topics in heat transfer. 8 refs., 3 figs
The optimal spacing for cylinders in crossflow forced convection
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Bejan, A. [Duke Univ., Durham, NC (United States)
1995-08-01
In this note I draw attention to a new fundamental aspect of the heat transfer performance of a bundle of parallel cylinders with crossflow forced convection, namely, the maximization of the thermal contact between the bundle and the fluid, when the volume occupied by the bundle is fixed. In the experiments described by Jubran et al. we have seen empirical evidence that the total heat transfer rate is maximum when the cylinder-to-cylinder spacing S has a certain value. This finding is important because it has been overlooked for decades, while forced convection from cylinders in crossflow grew into one of the most researched topics in heat transfer. 8 refs., 3 figs.
Effects of rib size and arrangement on forced convective heat transfer in a solar air heater channel
Skullong, Sompol; Thianpong, Chinaruk; Promvonge, Pongjet
2015-02-01
The article presents an experimental investigation on turbulent heat transfer and friction loss behaviors of airflow through a constant heat-fluxed solar air heater channel fitted with rib turbulators. The experiment was conducted for the airflow rate in terms of Reynolds numbers based on the hydraulic diameter of the channel in a range of 5000-24,000. In the present work, a comparative study between square and thin ribs (90°-rib) with three rib arrangements, namely, one ribbed wall (or single rib), in-line and staggered ribs on two opposite walls was first introduced. The study shows a significant effect of the presence of the ribs on the heat transfer rate and friction loss over the smooth wall channel. The comparison made at a single rib pitch and height also revealed that the thin rib performs better than the corresponding square one. Among the three arrangements, the in-line rib array provides higher heat transfer and friction loss than the staggered and the single one. However, the staggered thin rib provides the highest thermal performance. With this reason, only the staggered thin ribs at four different relative heights (BR = 0.1, 0.2, 0.3 and 0.4) and three relative pitches (PR = 0.5, 0.75 and 1.33) are investigated further. It is found that the staggered rib at BR = 0.4 and PR = 0.5 yields the highest heat transfer and friction factor but the maximum thermal performance is at BR = 0.2 and PR = 0.75.
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Sridharan, Kumar; Anderson, Mark; Allen, Todd; Corradini, Michael
2012-01-30
The goal of this NERI project was to perform research on high temperature fluoride and chloride molten salts towards the long-term goal of using these salts for transferring process heat from high temperature nuclear reactor to operation of hydrogen production and chemical plants. Specifically, the research focuses on corrosion of materials in molten salts, which continues to be one of the most significant challenges in molten salts systems. Based on the earlier work performed at ORNL on salt properties for heat transfer applications, a eutectic fluoride salt FLiNaK (46.5% LiF-11.5%NaF-42.0%KF, mol.%) and a eutectic chloride salt (32%MgCl2-68%KCl, mole %) were selected for this study. Several high temperature candidate Fe-Ni-Cr and Ni-Cr alloys: Hastelloy-N, Hastelloy-X, Haynes-230, Inconel-617, and Incoloy-800H, were exposed to molten FLiNaK with the goal of understanding corrosion mechanisms and ranking these alloys for their suitability for molten fluoride salt heat exchanger and thermal storage applications. The tests were performed at 850Ã?Â?Ã?Â?Ã?Â?Ã?Â?C for 500 h in sealed graphite crucibles under an argon cover gas. Corrosion was noted to occur predominantly from dealloying of Cr from the alloys, an effect that was particularly pronounced at the grain boundaries Alloy weight-loss due to molten fluoride salt exposure correlated with the initial Cr-content of the alloys, and was consistent with the Cr-content measured in the salts after corrosion tests. The alloysÃ?Â?Ã?Â¢Ã?Â?Ã?Â?Ã?Â?Ã?Â? weight-loss was also found to correlate to the concentration of carbon present for the nominally 20% Cr containing alloys, due to the formation of chromium carbide phases at the grain boundaries. Experiments involving molten salt exposures of Incoloy-800H in Incoloy-800H crucibles under an argon cover gas showed a significantly lower corrosion for this alloy than when tested in a graphite crucible. Graphite significantly accelerated alloy corrosion due to the reduction of Cr from solution by graphite and formation on Cr-carbide on the graphite surface. Ni-electroplating dramatically reduced corrosion of alloys, although some diffusion of Fe and Cr were observed occur through the Ni plating. A pyrolytic carbon and SiC (PyC/SiC) CVD coating was also investigated and found to be effective in mitigating corrosion. The KCl-MgCl2 molten salt was less corrosive than FLiNaK fluoride salts for corrosion tests performed at 850oC. Cr dissolution in the molten chloride salt was still observed and consequently Ni-201 and Hastelloy N exhibited the least depth of attack. Grain-boundary engineering (GBE) of Incoloy 800H improved the corrosion resistance (as measured by weight loss and maximum depth of attack) by nearly 50% as compared to the as-received Incoloy 800H sample. Because Cr dissolution is an important mechanism of corrosion, molten salt electrochemistry experiments were initiated. These experiments were performed using anodic stripping voltammetry (ASV). Using this technique, the reduction potential of Cr was determined against a Pt quasi-reference electrode as well as against a Ni(II)-Ni reference electrode in molten FLiNaK at 650 oC. The integrated current increased linearly with Cr-content in the salt, providing for a direct assessment of the Cr concentration in a given salt of unknown Cr concentration. To study heat transfer mechanisms in these molten salts over the forced and mixed convection regimes, a forced convective loop was constructed to measure heat transfer coefficients, friction factors and corrosion rates in different diameter tubes in a vertical up flow configuration in the laminar flow regime. Equipment and instrumentation for the forced convective loop was designed, constructed, and tested. These include a high temperature centrifugal pump, mass flow meter, and differential pressure sensing capabilities to an uncertainty of < 2 Pa. The heat transfer coefficient for the KCl-MgCl2 salt was measured in two different diameter channels (0.083Ã?Â?Ã?Â¢Ã?Â?Ã?Â?Ã?Â
Free convection heat transfer to supercritical helium
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The study of cryogenic free convective heat transfer from a sphere to supercritical helium is reported. The free convective heat transfer coefficient has been measured within the region of 4.2 to 25 K and 3 to 35 atmospheres. Measurements were made for sphere to helium temperature difference of 0.1 to 7 K. (author)
Natural, forced and mixed convection in fibrous insulation
International Nuclear Information System (INIS)
A numerical solution of flow and temperature distribution in fibrous insulation has been obtained. Rectangular and cylindrical geometries have been modeled. Boundary conditions included permeable hot wall and convective heat transfer at the walls. Good agreement has been obtained with published experimental and numerical values on rectangular cavities. The computed velocity and temperature profiles gave a better understanding of flow and heat transfer phenomena in fibrous insulation. Local cold wall and average Nusselt numbers presented, provide useful information in the design of the fibrous insulation for concrete reactor vessel and primary coolant piping of the gas cooled nuclear power plants. Average Nusselt number has been correlated with Rayleigh number when only natural convection is present, and with Rayleigh and Reynolds numbers when a combination of natural and forced convection is present
Laminar free-convective heat transfer in vertical uniform heat flux ducts
International Nuclear Information System (INIS)
A numerical analysis is made of laminar free convection in vertical ducts with Uniform Heat Flux (UHF), in consideration of temperature-dependence of fluid thermophysical properties. The local Nusselt numbers of Constant Property Solutions (CPS) can be well correlated with the dimensionless parameters derived by the authors, regardless of the exit Rayleigh number, Prandtl number, and circular tube or parallel plates. The CPS of free-convective heat transfer is close to that of forced convective heat transfer in UHF-ducts. The local Nusselt numbers of Variable Property Solutions (VPS) can also be well correlated by the usual correction method used for forced convective heat transfer in duct. The difference in the heat transfer characteristics of UHF-ducts between CPS and VPS is not so large as those of Uniform Wall Temperature ducts. (author)
Forced-convection boiling tests performed in parallel simulated LMR fuel assemblies
Energy Technology Data Exchange (ETDEWEB)
Rose, S.D.; Carbajo, J.J.; Levin, A.E.; Lloyd, D.B.; Montgomery, B.H.; Wantland, J.L.
1985-04-21
Forced-convection tests have been carried out using parallel simulated Liquid Metal Reactor fuel assemblies in an engineering-scale sodium loop, the Thermal-Hydraulic Out-of-Reactor Safety facility. The tests, performed under single- and two-phase conditions, have shown that for low forced-convection flow there is significant flow augmentation by thermal convection, an important phenomenon under degraded shutdown heat removal conditions in an LMR. The power and flows required for boiling and dryout to occur are much higher than decay heat levels. The experimental evidence supports analytical results that heat removal from an LMR is possible with a degraded shutdown heat removal system.
Turbulent mixed convection in asymmetrically heated vertical channel
Directory of Open Access Journals (Sweden)
Mokni Ameni
2012-01-01
Full Text Available In this paper an investigation of mixed convection from vertical heated channel is undertaken. The aim is to explore the heat transfer obtained by adding a forced flow, issued from a flat nozzle located in the entry section of a channel, to the up-going fluid along its walls. Forced and free convection are combined studied in order to increase the cooling requirements. The study deals with both symmetrically and asymmetrically heated channel. The Reynolds number based on the nozzle width and the jet velocity is assumed to be 3 103 and 2.104; whereas, the Rayleigh number based on the channel length and the wall temperature difference varies from 2.57 1010 to 5.15 1012. The heating asymmetry effect on the flow development including the mean velocity and temperature the local Nusselt number, the mass flow rate and heat transfer are examined.
International Nuclear Information System (INIS)
This paper reports an experimental study on flow boiling of pure refrigerants R134a and R123 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10 mm located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6MPa in the heat flux ranges of 5-50kW/m2, vapor quality 0-100 percent and mass velocity of 150-600 kg/m2s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen's superposition model, a new correlation is presented for heat transfer coefficients of mixture
Free-jet-induced mixed convection with internal heat sources
International Nuclear Information System (INIS)
The thermohydraulic conditions in fluids with internal heat sources and forced convection have been experimentally investigated with the aid of holographic interferometry and laser Doppler anemometry. The measurements were carried out in plane fluid layers of segmental cross section onto which a fluid of the same composition flows from above in the symmetry axis in the form of a laminar, plane free jet. The local heat transfer on the curved bottom of the fluid layer, which is cooled on all sides, have been determined from the interference pictures. The convection behaviour of the fluid, including the beam, has been represented by the measured velocity fields and profiles. (orig./GL)
Latent Heating Processes within Tropical Deep Convection
van den Heever, S. C.; Mcgee, C. J.
2013-12-01
It has been suggested that latent heating above the freezing level plays an important role in reconciling Riehl and Malkus' Hot Tower Hypothesis (HTH) with observational evidence of diluted tropical deep convective cores. In this study, recent modifications to the HTH have been evaluated through the use of Lagrangian trajectory analysis of deep convective cores simulated using the Regional Atmospheric Modeling System (RAMS), a cloud-resolving model (CRM) with sophisticated microphysical, surface and radiation parameterization schemes. Idealized, high-resolution simulations of a line of tropical convective cells have been conducted. A two-moment microphysical scheme was utilized, and the initial and lateral boundary grid conditions were obtained from a large-domain CRM simulation approaching radiative convective equilibrium. As the tropics are never too far from radiative convective equilibrium, such a framework is useful for investigating the relationships between radiation, thermodynamics and microphysics in tropical convection. Microphysical impacts on latent heating and equivalent potential temperature (?e) have been analyzed along trajectories ascending within convective regions. Changes in ?e along backward trajectories are partitioned into contributions from latent heating due to ice processes and a residual term that is shown to be an approximate representation of mixing. It is apparent from the CRM simulations that mixing with dry environmental air decreases ?e along ascending trajectories below the freezing level, while latent heating due to freezing and vapor deposition increase ?e above the freezing level. The along-trajectory contributions to latent heating from cloud nucleation, condensation, evaporation, freezing, deposition, and sublimation have also been quantified. Finally, the source regions of trajectories reaching the upper troposphere have been identified. The analysis indicates that while much of the air ascending within convective updrafts originates from above the lowest 2 km AGL, the strongest updrafts are composed of air from closer to the surface. Thus, both the boundary layer and mid-level inflow appears to play an important role in deep convection developing within moist environments.
SIMULATION OF MIXED CONVECTIVE HEAT TRANSFER USING LATTICE BOLTZMANN METHOD
Rosdzimin, A. R. M.; Zuhairi, S. M.; Azwadi, C. S. N.
2010-01-01
In this paper, mixed (forced–natural) convective heat transfer around a heated square cylinder located inside a lid driven cavity has been studied numerically using the lattice Boltzmann method in the range of 100? Re ? 1000 with the corresponding Richardson number 0.01?Ri?10. The double-population lattice Boltzmann formulation is used as the governing equation. Two dimensional nine-velocity models are used for the computation of the velocity field while a four-velocity model is use...
Experimental study of mixed convective heat transfer in narrow vertical rectangular channel, 1
International Nuclear Information System (INIS)
This report describes experimental results and discussions of mixed convective heat transfer experiments which were carried out in order to utilize the experimental data to the thermo-hydraulic design and the safety analysis of upgraded JRR-3. During some anticipated operational occurrences and accident conditions, which were posturated in the JRR-3 safety assessment, there is a case that core flow decreases from steady-state downward forced convective flow to zero flow and at last becomes upward flow due to the natural convective flow induced between the core and the reactor pool, that is, core flow reversal occurs. During the core flow reversal, the mixed convective heat transfer becomes significant and important because natural convection coexists with forced convection. In this case, mixed heat transfer correlations are need to evaluate heat transfer coefficients. In this study, therefore, the local heat transfer coefficients of narrow vertical rectangular channel were obtained and investigated for laminar to turbulent flow in the viewpoint of both forced convection and natural convection. As the results, the region for mixed convective heat transfer were made clear, and the heat transfer correlations for mixed convection were proposed. (author)
Energy Technology Data Exchange (ETDEWEB)
Kodama, Shigeo, E-mail: skodama@neltd.co.jp [Nuclear Engineering Ltd., 1-3-7, Tosabori, Nishi Ward, Osaka 550-0001 (Japan); Yoshida, Kenji; Kataoka, Isao [Department of Mechanical Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
2014-03-15
Thermal stratification and mixing under single- and two-phase flow natural convection are studied experimentally in relation to the safety of nuclear reactors. Flow structure and temperature distribution were measured for a rod bundle with axially distributed heat flux wherein the upper parts of the heaters are heated and the lower parts are unheated. In this scenario, under conditions of single-phase flow, thermal stratification is quite appreciable. A drastic temperature change was observed at the interface of thermal stratification. In the heated region, upward flow occurred in the rod bundle and downward flow occurred at the peripheral region of the rod bundle. As the heat flux increased, radial mixing was observed between subchannels in the rod bundle. At the interface of thermal stratification, however, almost no mixing was observed. Under conditions of boiling two-phase flow, on the other hand, thermal stratification also occurred but mixing at the interface of thermal stratification was promoted due to agitation of flow induced by bubbles. As the heat flux increased and the void fraction in the heated section increased, the interface of thermal stratification gradually advanced toward the unheated section.
Mutuku-Njane, Winifred Nduku; Makinde, Oluwole Daniel
2013-01-01
We examine the effect of magnetic field on boundary layer flow of an incompressible electrically conducting water-based nanofluids past a convectively heated vertical porous plate with Navier slip boundary condition. A suitable similarity transformation is employed to reduce the governing partial differential equations into nonlinear ordinary differential equations, which are solved numerically by employing fourth-order Runge-Kutta with a shooting technique. Three different water-based nanofluids containing copper (Cu), aluminium oxide (Al2O3), and titanium dioxide (TiO2) are taken into consideration. Graphical results are presented and discussed quantitatively with respect to the influence of pertinent parameters, such as solid volume fraction of nanoparticles (?), magnetic field parameter (Ha), buoyancy effect (Gr), Eckert number (Ec), suction/injection parameter (f w ), Biot number (Bi), and slip parameter ( ? ), on the dimensionless velocity, temperature, skin friction coefficient, and heat transfer rate. PMID:24222749
Convective Heat Transfer and Infrared Thermography (IRTh
Directory of Open Access Journals (Sweden)
J.M. Buchlin
2010-01-01
Full Text Available The paper deals with the application of the infrared thermography to the determination of the convective heat transfer coefficient in complex flow configurations. The fundamental principles upon which the IRTh relies are reviewed. The different methods developed to evaluate the heat exchange are described and illustrated through applications to the aerospace and aeronautical field as well as to the industrial processes.
An assessment on forced convection in metal foams
International Nuclear Information System (INIS)
Metal foams are a class of cellular structured materials with open cells randomly oriented and mostly homogeneous in size and shape. In the last decade, several authors have discussed the interesting heat transfer capabilities of these materials as enhanced surfaces for air conditioning, refrigeration, and electronic cooling applications. This paper reports an assessment on the forced convection through metal foams presenting experimental and analytical results carried out during air heat transfer through twelve aluminum foam samples and nine copper foam samples. The metal foam samples present different numbers of pores per linear inch (PPI), which vary between 5 and 40 with a porosity ranging between 0.896–0.956; samples of different heights have been studied. From the experimental measurements two correlations for the heat transfer coefficient and pressure drop calculations have been developed. These models can be successfully used to optimize different foam heat exchangers for any given application.
An assessment on forced convection in metal foams
Mancin, S.; Rossetto, L.
2012-11-01
Metal foams are a class of cellular structured materials with open cells randomly oriented and mostly homogeneous in size and shape. In the last decade, several authors have discussed the interesting heat transfer capabilities of these materials as enhanced surfaces for air conditioning, refrigeration, and electronic cooling applications. This paper reports an assessment on the forced convection through metal foams presenting experimental and analytical results carried out during air heat transfer through twelve aluminum foam samples and nine copper foam samples. The metal foam samples present different numbers of pores per linear inch (PPI), which vary between 5 and 40 with a porosity ranging between 0.896-0.956 samples of different heights have been studied. From the experimental measurements two correlations for the heat transfer coefficient and pressure drop calculations have been developed. These models can be successfully used to optimize different foam heat exchangers for any given application.
Influence of forced convection on unidirectional growth of crystals
Energy Technology Data Exchange (ETDEWEB)
Dinakaran, S. [Department of Physics, Loyola College, University of Madras, Chennai 600 034 (India); Verma, Sunil, E-mail: sverma1118@gmail.co [Laser Materials Development and Devices Division, Raja Ramanna Centre for Advanced Technology, Indore 452013 (India); Jerome Das, S. [Department of Physics, Loyola College, University of Madras, Chennai 600 034 (India); Kar, S.; Bartwal, K.S. [Laser Materials Development and Devices Division, Raja Ramanna Centre for Advanced Technology, Indore 452013 (India)
2010-09-15
Influence of forced convection on the growth rate of KDP crystal grown by unidirectional method has been investigated. The results were compared with the crystal grown under free convection conditions. To the best of our knowledge the effect of forced convection on unidirectional growth has been reported for the first time. An apparatus was designed and developed for growth of crystals by cooling under forced convection conditions. The growth rate achieved under forced convection was double to that under free convection conditions. Transmittance in the visible region for the crystals grown under the two types of convection regimes was {approx}90%. Birefringence and Mach-Zehnder interferometry shows good refractive index homogeneity of the grown crystals.
Terekhov, V. I.; Kalinina, S. V.; Lemanov, V. V.
2010-06-01
In the second part of review, we have considered the problems related to momentum and heat transfer in nanofluids. Results on hydrodynamic friction, forced and free convection in the laminar and turbulent flows are analysed; heat transfer at boiling is considered. The available models describing heat transfer intensification and suppression in nanofluids are studied. It is shown that for some problems on convective heat transfer there is a contradiction in data of different authors; possible reasons for this contradiction are analysed.
A meshless method for modeling convective heat transfer
Energy Technology Data Exchange (ETDEWEB)
Carrington, David B [Los Alamos National Laboratory
2010-01-01
A meshless method is used in a projection-based approach to solve the primitive equations for fluid flow with heat transfer. The method is easy to implement in a MATLAB format. Radial basis functions are used to solve two benchmark test cases: natural convection in a square enclosure and flow with forced convection over a backward facing step. The results are compared with two popular and widely used commercial codes: COMSOL, a finite element model, and FLUENT, a finite volume-based model.
Numerical and experimental study of dryer in forced convection
Energy Technology Data Exchange (ETDEWEB)
Youcef-Ali, S.; Moummi, N.; Desmons, J.Y.; Abene, A.; Messaoudi, H.; Le Ray, M. [Universite de Valenciennes et du Hainaut-Cambresis, Lab. de Mecanique et Energetique, Valenciennes, 59 (France)
2001-07-01
A numerical simulation model is developed to predict the forced convection performance of a dryer. This model takes into account the shrinkage phenomenon of the products during the drying process. The experimental results of the potato drying are obtained in climatic conditions similar to conditions found in natural open-air drying when the dryer is fed by air heated by a solar air flat plate collector. After a study of the influence of parameters of the drying air on the product during the drying process, we expose the experimental results and compare them with those calculated by the theoretical model. (Author)
Prandtl Number Dependent Natural Convection with Internal Heat Sources
Energy Technology Data Exchange (ETDEWEB)
Kang Hee Lee; Seung Dong Lee; Kune Y. Suh; Joy L. Rempe; Fan-Bill Cheung; Sang B. Kim
2004-06-01
Natural convection plays an important role in determining the thermal load from debris accumulated in the reactor vessel lower head during a severe accident. Recently, attention is being paid to the feasibility of external vessel flooding as a severe accident management strategy and to the phenomena affecting the success path for retaining the molten core material inside the vessel. The heat transfer inside the molten core material can be characterized by the strong buoyancy-induced flows resulting from internal heating due to decay of fission products. The thermo-fluid dynamic characteristics of such flow depend strongly on the thermal boundary conditions. The spatial and temporal variation of heat flux on the pool wall boundaries and the pool superheat are mainly characterized by the natural convection flow inside the molten pool. In general, the natural convection heat transfer phenomena involving the internal heat generation are represented by the modified Rayleigh number (Ra’), which quantifies the internal heat source and hence the strength of the buoyancy force. In this study, tests were conducted in a rectangular section 250 mm high, 500 mm long and 160 mm wide. Twenty-four T-type thermocouples were installed in the test section to measure temperatures. Four T-type thermocouples were used to measure the boundary temperatures. The thermocouples were placed in designated locations after calibration. A direct heating method was adopted in this test to simulate the uniform heat generation. The experiments covered a range of Ra' between 1.5x106 and 7.42x1015 and the Prandtl number (Pr) between 0.7 and 6.5. Tests were conducted with water and air as simulant. The upper and lower boundary conditions were maintained uniform. The results demonstrated feasibility of the direct heating method to simulate uniform volumetric heat generation. Particular attentions were paid to the effect of Pr on natural convection heat transfer within the rectangular pool.
International Nuclear Information System (INIS)
The Muon Ionization Cooling Experiment (MICE) has three 350-mm long liquid hydrogen absorbers to reduce the momentum of 200 MeV muons in all directions. The muons are then re-accelerated in the longitudinal direction by 200 MHz RF cavities. The result is cooled muons with a reduced emittance. The energy from the muons is taken up by the liquid hydrogen in the absorber. The hydrogen in the MICE absorbers is cooled by natural convection to the walls of the absorber that are in turn cooled by helium gas that enters at 14 K. This report describes the MICE liquid hydrogen absorber and the heat exchanger between the liquid hydrogen and the helium gas that flows through passages in the absorber wall
International Nuclear Information System (INIS)
A numerical solution for laminar flow heat transfer between a flowing gas and its containing rectangular duct has been obtained for many different boundary conditions which may arise in nuclear waste repository ventilation corridors. The problem has been solved for the cases of insulation on no walls, one wall, two walls, and three walls with various finite resistances on the remaining walls. Simplifications are made to decouple the convective heat transfer problem for the far field conduction problem, but peripheral conduction is retained. Results have been obtained for several duct aspect ratios in the thermal entrance and in the fully developed regions, including the constant temperature cases. When one wall is insulated and the other three are at constant temperature, the maximum temperature occurs in the fluid rather than on the insulated wall. This maximum moves toward the insulated wall with increasing axial distance. Nusselt numbers for the same constant flux on all four walls with peripheral conduction lie in a narrow band bounded by zero and infinite peripheral conduction cases. A dimensionless wall conduction group of four can be considered infinite for the purpose of estimating fully developed Nusselt numbers to within an accuracy of 3%. A decrease in wall and bulk temperatures by finite wall conduction has been demonstrated for the case of a black body radiation boundary condition. Nusselt numbers for the case of constant temperature on the top and bottomconstant temperature on the top and bottom walls and constant heat flux on the side walls exhibited unexpected behavior
International Nuclear Information System (INIS)
A numerical solution for laminar flow heat transfer between a flowing gas and its containing rectangular duct has been obtained for many different boundary conditions which may arise in nuclear waste repository ventilation corridors. The problem has been solved for the cases of insulation on no walls, one wall, two walls, and three walls with various finite resistances on the remaining walls. Simplifications are made to decouple the convective heat transfer problem from the far field conduction problem, but peripheral conduction is retained. Results have been obtained for several duct aspect ratios in the thermal entrance and in the fully developed regions, including the constant temperature cases. When one wall is insulated and the other three are at constant temperature, the maximum temperature occurs in the fluid rather than on the insulated wall. This maximum moves toward the insulated wall with increasing axial distance. Nusselt numbers for the same constant flux on all four walls with peripheral conduction lie in a narrow band bounded by zero and infinite peripheral conduction cases. A dimensionsless wall conduction group of four can be considered infinite for the purpose of estimating fully developed Nusselt numbers to within an accuracy of 3%. A decrease in wall and bulk temperatures by finite wall conduction has been demonstrated for the case of a black body radiation boundary condition. Nusselt numbers for the case of constant temperature on the top and bottf constant temperature on the top and bottom walls and constant heat flux on the side walls exhibited unexpected behavior. (orig.)
Natural Convective Heat Transfer from Narrow Plates
Oosthuizen, Patrick H
2013-01-01
Natural Convective Heat Transfer from Narrow Plates deals with a heat transfer situation that is of significant practical importance but which is not adequately dealt with in any existing textbooks or in any widely available review papers. The aim of the book is to introduce the reader to recent studies of natural convection from narrow plates including the effects of plate edge conditions, plate inclination, thermal conditions at the plate surface and interaction of the flows over adjacent plates. Both numerical and experimental studies are discussed and correlation equations based on the results of these studies are reviewed.
Natural and mixed convection heat transfer from a horizontal heated pipe buried in a porous medium
International Nuclear Information System (INIS)
The natural and mixed convection heat transfer from horizontal heated pipe (diam. 0.646'') embedded in a water saturated a porous medium (MIT Coarse Sand, porosity 30%) for three different depth of burial to pipe diameter ratios from 11.11, 9.69 and 4.55, have been studied experimentally. A galvanized steel box, 2.5 ft long x 1.25 ft wide x 0.5 ft deep, containing the coarse sand bed measuring 1.875 ft x 1.25 ft, was used for the experimental model. The pipe was heated by an electric heating element. In the mixed convection studies, water was allowed to flow in the sand bed and past the heated cylinder in a crosswise direction. The experimental results were correlated by using the normalized parameters - Nusselt number, Rayleigh number and Reynolds number, and the aspect ratios. In the case of crossflow past the pipe, the average Reynolds number at which forced convection becomes dominant over the natural convection mode of heat transfer has been experimentally determined. In order to check the accuracy of the experimental set up and instrumentations, natural convection heat transfer from the heated cylinder to water without the porous medium was conducted and the experimental results compared favorably with those of the previous investigators
Fusion of cylindrical ice in forced convection
International Nuclear Information System (INIS)
The experimental fusion of horizontal cylindrical ice was observed by moist air jet nozzle where forced convection prevailed. Results of phortmeasurements of variational cylindrical ice were found following things. Relationship between mean Nusselt number Nu sub(m) and Reynolds number Re was described experimental formula Nu sub(m) = 0.73 resup(0.576) in the range of Re from 3,000 to 120,000 and humidity from 0.0165 to 0.0185 kg/kg'. Fusing variational form of cylindrical ice were gropped together four systematically by variation of air velocity ranges. Relationship between max. of local Nu0 and Re was described experimental formula Nu0 = 2.9 Resup(0.482). (author)
Numerical simulation of forced convection film boiling on a sphere
International Nuclear Information System (INIS)
A CFD code using improved Volume of Fluid (VOF) method to track liquid-vapor interface is developed to simulate forced convection film boiling on a sphere. The simulation results are compared with the experimental correlation, and the result show that the numerical method could simulate the physical process of forced convection film boiling on a sphere successfully. (authors)
Heat Flux Sensors for Infrared Thermography in Convective Heat Transfer
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Giovanni Maria Carlomagno
2014-11-01
Full Text Available This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors’ research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dynamic flows. Several applications to streams, which range from natural convection to hypersonic flows, are also described.
Combined convective heat transfer of liquid sodium flowing across tube banks
International Nuclear Information System (INIS)
In order to clarify the heat transfer characteristics of combined convection of liquid sodium, a numerical analysis is performed for liquid sodium which flows through a single horizontal row of tubes in the direction of gravity. The correlation of heat transfer characteristics between liquid sodium and ordinary fluids is also discussed. The heat transfer characteristics at large Reynolds numbers are improved when the Richardson number is increased, and the improvement rate is enlarged with increase in p/d value, since convection effect is relatively large. However heat transfer coefficients do not differ from those of forced convection at small Reynolds numbers even when the Richardson number reaches a high value because of conduction effect. A good consistence of heat transfer characteristics of combined convection between liquid sodium and air is obtained at the same Peclet number and Richardson number. This means that the fundamental heat transfer characteristics of combined convection of liquid sodium can be investigated with ordinary fluids. (author)
Heat transport in bubbling turbulent convection
Lakkaraju, Rajaram; Stevens, Richard J. A. M.; Oresta, Paolo; Verzicco, Roberto; Lohse, Detlef; Prosperetti, Andrea
2013-01-01
Boiling is an extremely effective way to promote heat transfer from a hot surface to a liquid due to numerous mechanisms, many of which are not understood in quantitative detail. An important component of the overall process is that the buoyancy of the bubble compounds with that of the liquid to give rise to a much-enhanced natural convection. In this article, we focus specifically on this enhancement and present a numerical study of the resulting two-phase Rayleigh–Bénard convection proce...
Boiling inception in trichlorotrifluoroethane during forced convection at high pressures
Dougall, R. S.; Lippert, T. E.
1972-01-01
The inception of bubbles during forced convection was studied experimentally by using trichlorotrifluoroethane (R-113 or Freon-113). The experiments were performed in a rectangular channel, 12.7 x 9.5 mm in cross section. Heating was from a 3.2 mm wide strip embedded in the longer side of the channel. The pressures studied ranged from 3.6 to 20.7 bar, mass velocities from 700 to 600 kg/sq m/sec, and inlet subcoolings from 26 to 97 C. Photographs of the flow were used to determine when bubbles first appeared on the heated surface. These data were compared with wall temperature measurements and inception theories. A reasonable method for calculating the complete boiling curve was found to agree with these results.
Natural convection in a uniformly heated pool
International Nuclear Information System (INIS)
In the event of a core meltdown accident, to prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head, the establishment of a coolable configuration has been proposed by flooding with water the reactor cavity. In Reference 3, it was shown that for the heavy-water new production reactor (NPW-HWR) design, this strategy, e.g., the rejection of decay heat to a containment decay heat removal system by boiling of water in the reactor cavity, could keep the reactor vessel temperature below failure limits. The analysis of Ref. 3 was performed with the computer code COMMIX-1AR/P, and showed that natural convection in the molten-corium pool was the dominant mechanism of heat transfer from the pool to the wall of the reactor vessel lower head. To determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source, in Ref. 4, the experiments of free convection in a semicircular cavity of Jahn and Reineke were analyzed with COMMIX. It was found that the Nusselt (Nu) number predicted by COMMIX was within the spread of the experimental measurements. In the COMMIX analysis of Ref. 4, the semicircular cavity was treated as symmetric. The objective of the work presented in this paper was to extend the COMMIX validation analysis of Ref. 4 by removing the assumption of symmetry and expanding the analysis up to the highest Rayleigh (Ra) number that leads to a steady state. In conclusion, this work shows that the numerical predictions of natural convection in an internally heated pool bounded by a curved bottom are in reasonably good agreement with experimental measurements
Thermal Performance Of Convective-Radiative Heat Transfer In Porous Fins
Directory of Open Access Journals (Sweden)
Majid SHAHBABAEI
2014-11-01
Full Text Available Forced and natural convection in porous fin with convective coefficient at the tip under radiation and convection effects are investigated in this letter. Aluminum and copper as materials of fins are determined. In forced and natural convection, the air and water, are applied as working fluids, respectively. In order to solution of this nonlinear equation, HPM and VIM has been used . For verifying the accuracy of the solution methods, compare them with exact solutions (BVP. In this work the effects of porosity parameter ( , Radiation parameter (? and Temperature-Ratio parameter (µ on temperature distribution for both of the flows have been shown. The results shows that the effects of (? and (µ on temperature distribution in natural convection are based on porosity and in forced convection are uniform, approximately. Also, its shown that both VIM and HPM are capables to solving this nonlinear heat transfer equation.
Convectively driven shear and decreased heat flux
Goluskin, David; Flierl, Glenn R; Spiegel, Edward A
2014-01-01
We report on direct numerical simulations of two-dimensional, horizontally periodic Rayleigh-B\\'enard convection, focusing on its ability to drive large-scale horizontal flow that is vertically sheared. For the Prandtl numbers ($Pr$) between 1 and 10 simulated here, this large-scale shear can be induced by raising the Rayleigh number ($Ra$) sufficiently, and we explore the resulting convection for $Ra$ up to $10^{10}$. When present in our simulations, the sheared mean flow accounts for a large fraction of the total kinetic energy, and this fraction tends towards unity as $Ra\\to\\infty$. The shear helps disperse convective structures, and it reduces vertical heat flux; in parameter regimes where one state with large-scale shear and one without are both stable, the Nusselt number of the state with shear is smaller and grows more slowly with $Ra$. When the large-scale shear is present with $Pr\\lesssim2$, the convection undergoes strong global oscillations on long timescales, and heat transport occurs in bursts. N...
Jet Impingement and Forced Convection Cooling Experimental Study in Rotating Turbine Blades
Li, Hsin-Lung; Chiang, Hsiao-Wei D.; Hsu, Chih-Neng
2011-06-01
Both jet impingement and forced convection are attractive cooling mechanisms widely used in cooling gas turbine blades. Convective heat transfer from impinging jets is known to yield high local and area averaged heat transfer coefficients. Impingement jets are of particular interest in the cooling of gas turbine components where advancement relies on the ability to dissipate extremely large heat loads. Current research is concerned with the measurement and comparison of both jet impingement and forced convection heat transfer in the Reynolds number range of 10,000 to 30,000. This study is aimed at experimentally testing two different setups with forced convection and jet impingement in rotating turbine blades up to 700 RPM. This research also observes Coriolis force and impingement cooling inside the passage during rotating conditions within a cooling passage. Local heat transfer coefficients are obtained for each test section using thermocouple technique with slip rings. The cross section of the passage is 10 mm × 10 mm without ribs and the surface heating condition has enforced uniform heat flux. The forced convection cooling effects were studied using serpentine passages with three corner turns under different rotating speeds and different inlet Reynolds numbers. The impingement cooling study uses a straight passage with a single jet hole under different Reynolds numbers of the impingement flow and the cross flow. In summary, the main purpose is to study the rotation effects on both the jet impingement and the serpentine convection cooling types. Our study shows that rotation effects increase serpentine cooling and reduce jet impingement cooling.
Convective cooling of three discrete heat sources in channel flow
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Thiago Antonini Alves
2008-09-01
Full Text Available A numerical investigation was performed to evaluate distinct convective heat transfer coefficients for three discrete strip heat sources flush mounted to a wall of a parallel plates channel. Uniform heat flux was considered along each heat source, but the remaining channel surfaces were assumed adiabatic. A laminar airflow with constant properties was forced into the channel considering either developed flow or a uniform velocity at the channel entrance. The conservation equations were solved using the finite volumes method together with the SIMPLE algorithm. The convective coefficients were evaluated considering three possibilities for the reference temperature. The first was the fluid entrance temperature into the channel, the second was the flow mixed mean temperature just upstream any heat source, and the third option employed the adiabatic wall temperature concept. It is shown that the last alternative gives rise to an invariant descriptor, the adiabatic heat transfer coefficient, which depends solely on the flow and the geometry. This is very convenient for the thermal analysis of electronic equipment, where the components' heating is discrete and can be highly non-uniform.
Convective cooling of three discrete heat sources in channel flow
Scientific Electronic Library Online (English)
Thiago Antonini, Alves; Carlos A. C., Altemani.
2008-09-01
Full Text Available SciELO Brazil | Language: English Abstract in english A numerical investigation was performed to evaluate distinct convective heat transfer coefficients for three discrete strip heat sources flush mounted to a wall of a parallel plates channel. Uniform heat flux was considered along each heat source, but the remaining channel surfaces were assumed adia [...] batic. A laminar airflow with constant properties was forced into the channel considering either developed flow or a uniform velocity at the channel entrance. The conservation equations were solved using the finite volumes method together with the SIMPLE algorithm. The convective coefficients were evaluated considering three possibilities for the reference temperature. The first was the fluid entrance temperature into the channel, the second was the flow mixed mean temperature just upstream any heat source, and the third option employed the adiabatic wall temperature concept. It is shown that the last alternative gives rise to an invariant descriptor, the adiabatic heat transfer coefficient, which depends solely on the flow and the geometry. This is very convenient for the thermal analysis of electronic equipment, where the components' heating is discrete and can be highly non-uniform.
Energy Technology Data Exchange (ETDEWEB)
Chen, C.H. [National Yunlin Polytechnic Inst. (Taiwan, Province of China). Dept. of Mechanical Design Engineering
1996-12-01
This article investigates numerically the heat transfer characteristics of non-Darcy mixed convection over a horizontal flat plate with nonuniform surface heat flux in a porous medium. The surface heat flux is assumed to vary as a power of the axial coordinate measured from the leading edge of the plate. The entire regime of mixed convection (including the two limits of pure forced convection and pure free convection) is divided into two regions, namely the forced convection dominated regime and the free convection dominated regime. The effects of flow inertia force, solid boundary shear, near-wall porosity variation, and thermal dispersion are considered in the present analysis. The Darcy-Brinkman-Forchheimer equation is used to model the motion of fluid through the porous medium, and the porosity variation is approximated by an exponential function. The local Nusselt numbers, valid for the entire mixed convection regime, are presented for representative values of governing parameters.
Sumon Saha; Goutam Saha; Mohammad Ali; Md. Quamrul Islam
2006-01-01
Combined free convection and forced convection from a flush-mounted uniform heat source on the bottom of a horizontal rectangular enclosure with side openings is studied numerically. The inlet opening allows an externally induced air stream at the ambient temperature to flow through the cavity and exits from another two openings placed top of the both side walls. Two-dimensional forms of Navier-Stokes equations are solved by using control volume based finite element technique. Three typical v...
Experimental study of mixed convective heat transfer in narrow vertical rectangular channel, (2)
International Nuclear Information System (INIS)
During some anticipated operational occurrences and accident conditions, which were posturated in the JRR-3 safety assessment, there is a case that core flow decreases from steady-state downward forced convective flow to zero flow and at last becomes upward flow due to the natural circulation induced between the core and the reactor pool, that is, core flow reversal occurs. During the core flow reversal, the mixed convective heat transfer becomes significant. In this case it is important to understand the heat transfer characteristics of the mixed convection to evaluate the heat transfer of the fuel plates. To investigate the heat transfer characteristics of the mixed convection, heat transfer experiments were carried out using a vertical rectangular channel with water gap of 2.5 mm, which was nearly equal to that of the subchannels of the standard fuel elements of the upgraded JRR-3. In conclusion the heat transfer correlations which can be applied for a narrow vertical channel in free, mixed and forced convections were obtained. The range of the mixed convection region was identified by the non dimensional parameter Grx/Rex21/8Pr1/2. It was confirmed that in the mixed convection in a narrow channel, the influence of the acceleration of the main flow owing to the development of the boundary layer played a important part in the promotion of the heat transfer, compared with the case along a flat plate or in a wide channel. (author)
Development of a mechanistic model for forced convection subcooled boiling
Shaver, Dillon R.
The focus of this work is on the formulation, implementation, and testing of a mechanistic model of subcooled boiling. Subcooled boiling is the process of vapor generation on a heated wall when the bulk liquid temperature is still below saturation. This is part of a larger effort by the US DoE's CASL project to apply advanced computational tools to the simulation of light water reactors. To support this effort, the formulation of the dispersed field model is described and a complete model of interfacial forces is formulated. The model has been implemented in the NPHASE-CMFD computer code with a K-epsilon model of turbulence. The interfacial force models are built on extensive work by other authors, and include novel formulations of the turbulent dispersion and lift forces. The complete model of interfacial forces is compared to experiments for adiabatic bubbly flows, including both steady-state and unsteady conditions. The same model is then applied to a transient gas/liquid flow in a complex geometry of fuel channels in a sodium fast reactor. Building on the foundation of the interfacial force model, a mechanistic model of forced-convection subcooled boiling is proposed. This model uses the heat flux partitioning concept and accounts for condensation of bubbles attached to the wall. This allows the model to capture the enhanced heat transfer associated with boiling before the point of net generation of vapor, a phenomenon consistent with existing experimental observations. The model is compared to four different experiments encompassing flows of light water, heavy water, and R12 at different pressures, in cylindrical channels, an internally heated annulus, and a rectangular channel. The experimental data includes axial and radial profiles of both liquid temperature and vapor volume fraction, and the agreement can be considered quite good. The complete model is then applied to simulations of subcooled boiling in nuclear reactor subchannels consistent with the operating conditions of the AP1000 pressurized water reactor. The effects of both axial and lateral nonuniform power distributions inside reactor fuel elements are accounted for. Boiling flows are simulated for three different computational domains of increasing complexity: a quarter-subchannel bordering a single fuel pin, two subchannels surround by an array of 2 by 3 fuel pins, and in four subchannels surrounded by an array of 3 by 3 fuel pins. The predicted behavior is consistent with expectations. In the 3 by 3 array, the two-phase coolant is predicted to flow from the hot channels to the cold channels, enhancing heat exchange between subchannels. This, in turn, demonstrates that the new model is capable of capturing the turbulence- and buoyancy-induced coolant mixing across the neighboring channels.
Natural convection in a uniformly heated pool
International Nuclear Information System (INIS)
To prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head in the event of a core meltdown accident, the establishment of a coolable configuration has been proposed by flooding the reactor cavity with water. In Ref. 3, it was shown that for the heavy-water new production reactor (NPW-HWR) design, this strategy (e.g., the rejection of decay heat to a containment decay heat removal system by boiling of water in the reactor cavity) could keep the reactor vessel temperature below failure limits. The analysis of Ref. 3 was performed with the COMMIX-IAR/P computer code and showed that natural convection in the molten-corium pool was the dominant mechanism of heat transfer from the pool to the wall of the reactor vessel lower head. COMMIX is a general-purpose thermal-hydraulics code based on finite differencing by the first-order upwind scheme. To determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source, in Ref. 4, the experiments of free convection in a semicircular cavity of Jahn and Reineke were analyzed with COMMIX in Ref. 5. It was found that the Nusselt number predicted by COMMIX was within the spread of the experimental measurements. In the COMMIX analysis of Ref. 5, the semicircular cavity was treated as symmetric. The objective of this paper was to extend the COMMIX validation analysis of Ref. 5 by removing the assumption of symmetry and expanding the analysis from the highest Rayxpanding the analysis from the highest Rayleigh number of the experiments of Ref. 4 to the highest Rayleigh number that leads to a steady state
Pattern formation without heating in an evaporative convection experiment
Mancini, Hector; Maza, Diego
2003-01-01
We present an evaporation experiment in a single fluid layer. When latent heat associated to the evaporation is large enough, the heat flow through the free surface of the layer generates temperature gradients that can destabilize the conductive motionless state giving rise to convective cellular structures without any external heating. The sequence of convective patterns obtained here without heating, is similar to that obtained in B\\'enard-Marangoni convection. This work p...
Forced-convective vitrification with liquid cryogens.
Lyu, Shaw-Ruey; Huang, Jen-Hung; Shih, Wei-Hung; Chen, Yung-Jiun; Hsieh, Wen-Hsin
2013-06-01
Cell cryopreservation by vitrification generally requires using vitrification solutions with high concentrations of cryoprotectants (CPAs), which are toxic and induce osmotic stresses associated with the addition and removal of CPAs. To increase the cooling rate and reduce the CPA concentration required for vitrification, this study proposed an innovative approach, named forced-convective vitrification with liquid cryogens, in which liquid oxygen at a temperature below its boiling point (LOX(bbp)) was used as the cryogen to reduce the generation of insulating bubbles of gaseous oxygen and the sample was subjected to a constant velocity to remove insulation bubbles from the sample. Results show that changing the cryogen from liquid nitrogen at its boiling temperature (LN(abp)) to LOX(bbp), increasing the sample velocity and reducing the test solution volume increased the cooling rate and thereby decreased the CPA concentration required for vitrification. Using the same velocity (1.2 m/s), the cooling rate achieved with LOX(bbp) was 2.3-fold greater than that achieved with LN(abp). With LOX(bbp), the increase in the sample velocity from 0.2 to 1.2 m/s enhanced the cooling rate by 1.9 times. With LOX(bbp), a velocity of 1.2m/s and a test solution volume of 1.73 ?l, the CPA concentration required for vitrification decreased to 25%. These results indicate that the new approach described here can reduce the CPA concentration required for vitrification, and thus decreases the toxicity and osmotic stresses associated with adding and removing the CPA. PMID:23545291
Analysis of laminar forced convection condensation within thin porous coatings
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Renken, K.J.; Carneiro, M.J.; Meechan, K. (Univ. of Wisconsin, Milwaukee, WI (United States))
1994-04-01
The effect of laminar forced convection on enhanced filmwise condensation within thin inclined porous coatings is numerically investigated. The model simulates two-dimensional condensation within very permeable and highly conductive porous substrates. The Darcy-Brinkman-Forchheimer model is utilized to describe the flowfield within the porous layer while classical boundary-layer equations are employed in the pure condensate region. The numerical results document the dependence of the temperature field and the heat transfer rate on the governing parameters such as the Reynolds number, the Rayleigh number, the Darcy number, the Jakob number, the Prandtl number, as well as the porous coating thickness and effective thermal conductivity. The results of this study provide valuable fundamental predictions of enhanced film condensation that can be used in a number of practical thermal engineering applications. 28 refs.
Behavior of bubble in subcooled boiling with forced convection, 2
International Nuclear Information System (INIS)
The objective of this research is to observe the bubble behavior in subcooled boiling with forced convection and to investigate the conditions of heaving test with a heater which initiates isolated bubbles and with high speed motion photography. The observation was made at three inlet subcooling of 15, 30 and 45 K with pressure of 0.3 MPa, mass flux of 1000 kg/m2·s, heat flux of 35 kW/m2. At inlet subcooling of 15 K, bubble velocity is nearly constant in the field of visions for upstream and downstream area. But at the higher subcooling, bubble velocity varies greatly and bubble collapse in the camera field. (author)
Multiscale convection in a geodynamo simulation with uniform heat flux along the outer boundary
Matsui, Hiroaki; King, Eric; Buffett, Bruce
2014-08-01
is generally expected that Earth's magnetic field, which is generated by convecting liquid metal within its core, will substantially alter that convection through the action of Lorentz forces. In most dynamo simulations, however, Lorentz forces do very little to change convective flow, which is predominantly fine-scaled. An important exception to this observation is in dynamo models that employ uniform heat flux boundary conditions, rather than the usual uniform temperature conditions, in which multiscale convection is observed. We investigate the combined influence of thermal boundary conditions and magnetic fields using four simulations: two dynamos and two nonmagnetic models, with either uniform temperature or heat flux fixed at the outer boundary. Of the four, only the fixed-heat-flux dynamo simulation produces multiscale convective flow patterns. Comparison between the models suggests that the fixed-flux dynamo generates large patches of strong azimuthal magnetic field that suppress small-scale convective motions. By allowing temperature to vary along the outer boundary, the fixed-flux dynamo generates stronger azimuthal flow and, in turn, stronger magnetic field, and the resulting Lorentz forces alter the nature of convective flow. Extrapolation of the analyses presented here suggests that magnetic fields may also suppress small-scale convection in the Earth's core.
Mixed convection in a horizontal porous duct with a sudden expansion and local heating from below
International Nuclear Information System (INIS)
Results are reported for an experimental and numerical study of forced and mixed convective heat transfer in a liquid-saturated, horizontal porous duct. The cross section of the duct has a sudden expansion with a heated region on the lower surface downstream and adjacent to the expansion. Calculated and measured Nusselt numbers for 0.1 1.5 and Ra/Pe1.5. Calculated Nusselt numbers are very close to those for the bottom-heated flat duct, and this result has several important implications for convective heat and mass transfer in geophysical systems and porous matrix heat exchangers
Daund, V. S.; Palande, D. D.
2014-01-01
Experimental and CFD analysis is conducted in order to establish effect of geometrical fin parameters for natural convection heat transfer from vertical rectangular fin arrays.Natural convective heat transfer from rectangular vertical plates has been reviewed. Study revealed that most of the work was carried out considering various configurations. Experimental work carried on steady state natural convection heat transfer from vertical rectangular fins made of aluminum. Experim...
Laminar convective heat transfer of non-Newtonian nanofluids with constant wall temperature
Hojjat, M.; Etemad, S. Gh.; Bagheri, R.; Thibault, J.
2011-02-01
Nanofluids are obtained by dispersing homogeneously nanoparticles into a base fluid. Nanofluids often exhibit higher heat transfer rate in comparison with the base fluid. In the present study, forced convection heat transfer under laminar flow conditions was investigated experimentally for three types of non-Newtonian nanofluids in a circular tube with constant wall temperature. CMC solution was used as the base fluid and ?-Al2O3, TiO2 and CuO nanoparticles were homogeneously dispersed to create nanodispersions of different concentrations. Nanofluids as well as the base fluid show shear thinning (pseudoplastic) rheological behavior. Results show that the presence of nanoparticles increases the convective heat transfer of the nanodispersions in comparison with the base fluid. The convective heat transfer enhancement is more significant when both the Peclet number and the nanoparticle concentration are increased. The increase in convective heat transfer is higher than the increase caused by the augmentation of the effective thermal conductivity.
Convective heat transfer in a closed two-phase thermosyphon
Al-Ani, M. A.
2014-08-01
A numerical analysis of heat transfer processes and hydrodynamics in a two-phase closed thermosyphon in a fairly wide range of variation of governing parameters has been investigated. A mathematical model is formulated based on the laws of mass conservation, momentum and energy in dimensionless variables "stream function - vorticity vector velocity - temperature". The analysis of the modes of forced and mixed convection for different values of Reynolds number and heat flows in the evaporation zone, the possibility of using two-phase thermosyphon for cooling gas turbine blades, when the heat is coming from the turbine blades to the thermosyphon is recycled a secondary refrigerant has been studied with different values of the centrifugal velocity. Nusselet Number, streamlines, velocity, temperature fields and temperature profile has been calculated during the investigation.
Heat transfer by natural convection in an internally heated reactor materials melt. Rev. 0
International Nuclear Information System (INIS)
The report is structured as follows: Heat flux calculations and estimates for In-Vessel Retention (IVR); Rayleigh-Benard convection and the Rayleigh number (criterion); Free convection in the horizontal layer of a liquid which is heated by internal heating sources, and a modified Rayleigh criterion; and Turbulent convection at the reactor vessel bottom during IVR. (P.A.)
International Nuclear Information System (INIS)
The numerical solution of flow and heat transfer for steady and transient laminar mixed convection near a vertical uniformly heated surface exposed to a horizontal cross-flow is presented. The transients considered include the simultaneous initiation of flow and heating and the time-varying flow generated by starting or stopping forced convection with the surface heating condition unchanged. The partial differential equations describing the conservation of mass, momentum, and energy were solved in their time-dependent forms by an explicit finite-difference technique. Calculations were performed for fluids with Prandtl numbers of 0.733 and 6.7, nominally air and water. During both steady and transient circumstances, the effects of the horizontal forced flow were dominant near the vertical leading edge, whereas natural convection dictated the flow at large values of the horizontal coordinate. The heat transfer coefficient during mixed convection was significantly higher than that with either forced or free convection alone. During simultaneous starting of flow and heating, the time to reach steady state decreased with increase in cross-flow velocity. For the transients initiated by suddenly imposing a horizontal forced flow on an existing natural convection flow, the local temperature and vertical velocity were found to undershoot before reaching their respective steady-state values. The overshoot in heat transfer coefficient in such situations was significant for fluids with smaller Prandtl number. The transient initiated by stopping the horizontal flow during mixed convection was associated with overshoot in both vertical component of velocity and local fluid temperature
Transient natural convection in heated inclined tubes
Energy Technology Data Exchange (ETDEWEB)
McEligot, D.M. (Westinghouse Electric Corp., Middletown, RI (USA). Oceanic Div.); Denbow, D.A. (Software AG of North America, Inc., Lakewood, CO (USA)); Murphy, H.D. (Los Alamos National Lab., NM (USA))
1990-05-01
To simulate natural convection flow patterns in directionally drilled wellbores, experiments and analyses were conducted for a circular tube with length-to-diameter (L/D) ratio of 36 at angles of 0{degree}, 20{degree}, and 35{degree} from the vertical. The tube was heated at the bottom and cooled at the top, and the insulation was adjusted so that approximately one- to two-thirds of the power dissipated was transferred through the tube wall to the surroundings. An aqueous solution of polyvinyl alcohol was employed as the working fluid in order to obtain low Rayleigh numbers corresponding to conditions in geothermal wellbores. Results were primarily qualitative but were useful in providing insight into the phenomena occurring. Steady-state temperature distributions were measured for the three orientations and for several heating rates to demonstrate the effects of tube angle and Rayleigh number. transient measurements of the temperature distribution were obtained during cooling from a higher temperature without a heat source to calibrate the heat losses. With the electrical heat source, temporal data were taken during heating to examine the approach to steady state. Quasi-steady flow conditions were approached rapidly, but the overall time constant of the apparatus was of the order of one-third of a day. Predictions with the three-dimensional TEMPEST code were first tested by comparison with simple conduction analyses. Comparison with actual data showed good agreement of the predicted temperature levels for the maximum inclination, 35{degree}, and slightly poorer agreement for the other limit, a vertical tube. Trends of temperature level and Nusselt number with heating rate or Rayleigh number were reasonable, but the predicted variation of the end Nusselt number versus inclination was in the opposite direction from the experiment. 75 refs., 20 figs., 8 tabs.
Turbulent forced convective flow in an an-isothermal channel
International Nuclear Information System (INIS)
The influence of variable viscosity effects on momentum and heat transfer of a non-isothermal turbulent forced convective flow is studied using thermal large-eddy simulation (LES). LES of bi-periodic channel flow with significant heat transfer at a low Mach number was performed to study the modulation in the near-wall turbulence structure due to anisotropic viscosity. The temperature ratio (R? = Thot/Tcold) is varied from 1.01 to 5 to study the isolated effect of variable viscosity with (Thot) and (Tcold) as a wall temperature. It is shown that average and turbulent fields undergo significant changes in a broad range of Reynolds number, compared to isothermal flow with constant viscosity, we observe enhanced turbulence on the cold side of the channel, characterized by locally lower viscosity whereas a decrease of turbulent kinetic energy is found at the hot wall. The turbulent structures via H criteria of high vorticity shows very short and densely populated vortices near cold wall whereas long streaky structure or large elongated vortices at the hot wall. Q invariant totally eradicate all the streaky structure at the hot wall as a consequence of re-laminarization. To further clarify this issue spectral study is conducted that reveals complete suppression of turbulence at the hot side of the channel at large temperature ratio because no inertial zone (i.e. index of Kolmogorov scaling law is zero) is obtained on the spectra in these region. (authors)
Simulation of convectively forced gravity waves in comparison with SABER satellite measurements.
Kalisch, Silvio; Trinh, Thai; Chun, Hye-Yeong; Ern, Manfred; Preusse, Peter; Kim, Young-Ha; Eckermann, Steven; Riese, Martin
2013-04-01
Gravity waves (GW) are a known coupling mechanism between lower, middle, and upper atmosphere. They are responsible for driving large scale circulations like Brewer-Dobson circulation and contribute almost 60% to the QBO of the inner tropics. Convection is the dominant source for tropical GWs, but deep convection is also one of the most difficult to understand sources of GWs. Especially, the development of atmospheric general circulation models (AGCM) suffers from improvements in the parameterization of convectively forced GWs (cGWs). In this study we present the results of GW ray-tracing calculations of cGWs. For this, we used the Gravity Wave Regional Or Global RAy-tracer (GROGRAT) and the convective source scheme from Yonsei University (South Korea). Furthermore, we used MERRA heating rates, cloud data, and background data for both the calculation of the convective forcing by deep convection and for the atmospheric background of the ray-tracing calculations afterwards. Also, we compare our results with satellite measurements of squared temperature amplitudes as well as momentum flux by the SABER instrument in order to validate our findings over a 10 years period. For the comparison the observational filter of the instrument is taken into account, the influence discussed. The modulation of GW momentum flux by the background winds and in particular the influence of the QBO is investigated. GW drag at various altitudes is calculated and compared to the drag required for the forcing of the QBO.
International Nuclear Information System (INIS)
To continue with the equipment of the thermal hydraulics laboratory, it was designed thermal and mechanically an heat exchanger, to satisfy the requirements to have circuit that allows to carry out heat transfer experiments. The heat exchanger was manufactured and proven in the workshops of the Prototypes and Models Management, and it is expected that to obtain the foreseen results once completely installed the circuit, in the laboratory of thermal hydraulics of the Management of Nuclear Systems. (Author)
International Nuclear Information System (INIS)
This paper reports on an experimental study on transitional heat transfer of water flow in a heated vertical tube under natural circulation conditions. In the experiments the local and average heat transfer coefficients were obtained. The experimental data were compared with the predictions by a forced flow correlation available in the literature. The comparisons show that the Nusselt number value in the fully developed region is about 30% lower than the predictions by the forced flow correlation due to flow laminarization in the layer induced by co-current bulk natural circulation and free convection. By using the Rayleigh number Ra to represent the influence of free convection on heat transfer, the empirical correlations for the calculation of local and average heat transfer behavior in the tube at natural circulation have been developed. The empirical correlations are in good agreement with the experimental data. Based on the experimental results, the effect of the thermal entry-length behavior on heat transfer design in the tube under natural circulation was evaluated
Numerical Simulations of Heat Explosion With Convection In Porous Media
Allali, Karam; Bikany, Fouad; Taik, Ahmed; Volpert, Vitaly
2013-01-01
In this paper we study the interaction between natural convection and heat explosion in porous media. The model consists of the heat equation with a nonlinear source term describing heat production due to an exothermic chemical reaction coupled with the Darcy law. Stationary and oscillating convection regimes and oscillating heat explosion are observed. The models with quasi-stationary and unstationary Darcy equation are compared.
Endwall convective heat transfer for bluff bodies
Wang, Lei; Salewski, Mirko; Sunde?n, Bengt; Borg, Andreas; Abrahamsson, Hans
2012-01-01
The endwall heat transfer characteristics of forced flow past bluff bodies have been investigated using liquid crystal thermography (LCT). The bluff body is placed in a rectangular channel with both its ends attached to the endwalls. The Reynolds number varies from 50,000 to 100,000. In this study, a single bluff body and two bluff bodies arranged in tandem are considered. Due to the formation of horseshoe vortices, the heat transfer is enhanced appreciably for both cases. However, for the ca...
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Dehkordi, Asghar Molaei; Mohammadi, Ali Asghar [Department of Chemical and Petroleum Engineering, Sharif University of Technology, P.O. Box 11155-9465, Tehran (Iran)
2009-04-15
A numerical investigation was conducted on the transient behavior of a hydrodynamically, fully developed, laminar flow of power-law fluids in the thermally developing entrance region of circular ducts taking into account the effect of viscous dissipation but neglecting the effect of axial conduction. In this regard, the unsteady state thermal energy equation was solved by using a finite difference method, whereas the steady state thermal energy equation without wall heat flux was solved analytically as the initial condition of the former. The effects of the power-law index and wall heat flux on the local Nusselt number and thermal entrance length were investigated. Moreover, the local Nusselt number of steady state conditions was correlated in terms of the power-law index and wall heat flux and compared with literature data, which were obtained by an analytic solution for Newtonian fluids. Furthermore, a relationship was proposed for the thermal entrance length. (author)
International Nuclear Information System (INIS)
A numerical investigation was conducted on the transient behavior of a hydrodynamically, fully developed, laminar flow of power-law fluids in the thermally developing entrance region of circular ducts taking into account the effect of viscous dissipation but neglecting the effect of axial conduction. In this regard, the unsteady state thermal energy equation was solved by using a finite difference method, whereas the steady state thermal energy equation without wall heat flux was solved analytically as the initial condition of the former. The effects of the power-law index and wall heat flux on the local Nusselt number and thermal entrance length were investigated. Moreover, the local Nusselt number of steady state conditions was correlated in terms of the power-law index and wall heat flux and compared with literature data, which were obtained by an analytic solution for Newtonian fluids. Furthermore, a relationship was proposed for the thermal entrance length
Leung, C. W.; Chan, T. L.; Chen, S.
Experimental investigation had been conducted to study the steady-state forced convection heat transfer and pressure drop characteristics of the hydrodynamic fully-developed turbulent flow in the air-cooled horizontal equilateral triangular ducts, which were fabricated with the same length and hydraulic diameter. Inner surfaces of the ducts were fixed with square ribs with different side lengths of 6.35, 9.525 and 12.7mm, respectively, and the uniform separation between the centre lines of two adjacent ribs was kept constant at 57.15mm. Both the triangular ducts and the ribs were fabricated with duralumin. The experiments were performed with the hydraulic diameter based Reynolds number ranged from 3100 to 11300. The entire inner wall of the duct was heated uniformly, while the outer surface was thermally insulated. It was found that the Darcy friction factor of the duct was increasing rather linearly with the rib size, and forced convection could be enhanced by an internally ribbed surface. However, the heat transfer enhancement was not proportional to the rib size but a maximum forced convection heat transfer augmentation was obtained at the smallest rib of 6.35mm. Non-dimensional expressions for the determination of the steady-state heat transfer coefficient and Darcy friction factor of the equilateral triangular ducts, which were internally fabricated with uniformly spaced square ribs of different sizes, were also developed.
Theoretical study of mixed convection effects on heat transfer to steam in rod bundle geometry
International Nuclear Information System (INIS)
Under certain small break LOCA scenarios, the nuclear reactor core is expected to uncover partially under a slow quasi-steady boiloff transient. In the uncovered rod bundle region, heat is transferred to low flow superheated steam by convection and radiation. Since this is the only region having potential to give thermal damage to fuel during an accident, heat transfer and fluid flow conditions need to be determined. In this steam cooling region, heat transfer is expected to be accomplished by laminar flow. However, experimental results indicated higher heat transfer rates than those expected for laminar flow below a Reynolds number equal to 2000. The possible cause for high heat transfer coefficients may be transition from forced to mixed (forced and free) convection. This project is a theoretical study of the buoyancy effects on heat transfer to superheated low flow steam flowing upward in rod bundle geometry
Laminar forced convection and flow characteristics for the multiple plate porous insulation
International Nuclear Information System (INIS)
A numerical study of steady state flow and heat transfer has been conducted for the multiple plate porous insulation used in the reactor pressure vessels of 'Magnox' nuclear power stations. The insulation pack studied, consisting of seven dimpled stainless steel sheets and six plane stainless steel sheets, was of the type installed in the Sizewell A plant. In the reactor application the fluid within the insulation pack is carbon dioxide at 20 bar but in the numerical investigation the insulation performance was examined in air at lower pressures. A three-dimensional computation model with a periodicity condition was used in the numerical investigation. Result was obtained for laminar forced convection with constant wall temperatures. Numerical results are presented to show the flow and thermal fields in a single flow passage. In forced convection it is shown that mid-dimple 'peaking' of the Nusselt number distribution may be related directly to the convective influence of distorted velocity profiles
International Nuclear Information System (INIS)
An experimental investigation has been conducted to determine the local condensation heat transfer coefficient (HTC) of steam in the presence of air or helium flowing downward inside a 46-mm-i.d. vertical tube. The gas-steam mixture flow rate was measured with a calibrated vortex flowmeter before it entered the 2.54-m-long test condenser. Cooling water flow rate in an annulus around the tube was measure with a calibrated rotameter. Temperatures of the cooling water, the gas-steam mixture, and the tube inside and outside surfaces were measured at 0.3-m intervals in the test condenser. Inlet and exit pressures and temperatures of the gas-steam mixture and of the cooling water were also measured. The local heat flux was obtained from the slope of the coolant axial temperature profile and the coolant mass flow rate. It was found that for the same mass fraction of the noncondensable gas, compared with air, helium has a more inhibiting effect on the heat transfer, but for the same molar ratio, air was found to be more inhibiting. An application where there is important is the proposed advanced passive boiling water reactor design (Simplified Boiling Water Reactor), which utilizes the isolation condenser as a main component of the passive containment cooling system (PCCS)
Dielectrophoretic force-driven thermal convection in annular geometry
Yoshikawa, Harunori; Crumeyrolle, Olivier; Mutabazi, Innocent
2013-01-01
The thermal convection driven by the dielectrophoretic force is investigated in an- nular geometry under microgravity conditions. A radial temperature gradient and a radial alternating electric field are imposed on a dielectric fluid that fills the gap of two concentric infinite-length cylinders. The resulting dielectric force is regarded as thermal buoyancy with a radial effective gravity. This electric gravity varies in space and may change its sign depending on the temperature gradient and...
Ruma Patra; Sanatan Das; . PROF. RABINDRA NATH JANA
2014-01-01
Effects of radiative heat transfer on MHD fully developed mixed convective flow of a viscous incompressible electrically conducting fluid through a vertical channel with asymmetric heating of the walls in the presence of a uniform transverse magnetic field has been studied. An exact solution of the governing equations has been obtained in closed form. It is observed that the velocity field is greatly influenced by the radiative heat transfer as well as bouyancy forces. The induced magnetic f...
Zeinali Heris, Saeed; Noie, Seyyed Hossein; Talaii, Elham; Sargolzaei, Javad
2011-01-01
In this article, laminar flow-forced convective heat transfer of Al2O3/water nanofluid in a triangular duct under constant wall temperature condition is investigated numerically. In this investigation, the effects of parameters, such as nanoparticles diameter, concentration, and Reynolds number on the enhancement of nanofluids heat transfer is studied. Besides, the comparison between nanofluid and pure fluid heat transfer is achieved in this article. Sometimes, because of pressure drop limita...
Solution of heat removal from nuclear reactors by natural convection
Zitek Pavel; Valenta Vaclav
2014-01-01
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...
Analysis of natural convection in volumetrically-heated melt pools
Energy Technology Data Exchange (ETDEWEB)
Sehgal, B.R.; Dinh, T.N.; Nourgaliev, R.R. [Royal Inst. of Tech., Stockholm (Sweden). Div. of Nuclear Power Safety
1996-12-01
Results of series of studies on natural convection heat transfer in decay-heated core melt pools which form in a reactor lower plenum during the progression of a core meltdown accident are described. The emphasis is on modelling and prediction of turbulent heat transfer characteristics of natural convection in a liquid pool with an internal energy source. Methods of computational fluid dynamics, including direct numerical simulation, were applied for investigation. Refs, figs, tabs.
SCALE ANALYSIS OF CONVECTIVE MELTING WITH INTERNAL HEAT GENERATION
Energy Technology Data Exchange (ETDEWEB)
John Crepeau
2011-03-01
Using a scale analysis approach, we model phase change (melting) for pure materials which generate internal heat for small Stefan numbers (approximately one). The analysis considers conduction in the solid phase and natural convection, driven by internal heat generation, in the liquid regime. The model is applied for a constant surface temperature boundary condition where the melting temperature is greater than the surface temperature in a cylindrical geometry. We show the time scales in which conduction and convection heat transfer dominate.
Convective boiling heat transfer of water in the transition region
International Nuclear Information System (INIS)
An experimental study of forced convective boiling heat transfer for upflow of water in a circular tube has been performed using a heat transfer system with temperature-controlled indirect Joule heating. In this way, complete boiling curves from incipience of boiling to film boiling could be measured including the transition boiling regime. Usually, the test were performed in a quasi-steady mode by increasing the set-point wall temperature average at a constant time rate. The main body of the results covers the pressure range from 0.1 to 1.0 MPa, mass flux range from 25 to 200 kg/(m2s) and inlet subcooling from 5 to 30 K. The empirical data in the transition region were correlated in terms of a heat flux/surface superheat relationship that was normalized by the maximum heat flux and its corresponding wall superheat, respectively, to anchor the transition boiling curve to its low temperature limit. The exponent n in this power law relation was assumed to depend on the ratio of phase densities, normalized wall superheat and inlet subcooling as well as a dimensionless mass flux with a rms error of 17.6 % if compared to 1094 data points within the ranges of system parameters as noted above. (author)
International Nuclear Information System (INIS)
A mechanistic model for forced convective transition boiling has been developed to investigate transition boiling mechanisms and to predict transition boiling heat flux realistically. This model is based on a postulated multi-stage boiling process occurring during the passage time of the elongated vapor blanket specified at a critical heat flux (CHF) condition. Between the departure from nucleate boiling (DNB) and the departure from film boiling (DFB) points, the boiling heat transfer is established through three boiling stages, namely, the macrolayer evaporation and dryout governed by nucleate boiling in a thin liquid film and the unstable film boiling characterized by the frequent touches of the interface and the heated wall. The total heat transfer rates after the DNB is weighted by the time fractions of each stage, which are defined as the ratio of each stage duration to the vapor blanket passage time. The model predictions are compared with some available experimental transition boiling data. The parametric effects of pressure, mass flux, inlet subcooling on the transition boiling heat transfer are also investigated. From these comparisons, it can be seen that this model can identify the crucial mechanisms of forced convective transition boiling, and that the transition boiling heat fluxes including the maximum heat flux and the minimum film boiling heat flux are well predicted at low qualities/high pressures near 10 bar. In future, this model will be improved in In future, this model will be improved in the unstable film boiling stage and generalized for high quality and low pressure situations. (orig.)
Time evolution simulation of heat removal in a small water tank by natural convection
Energy Technology Data Exchange (ETDEWEB)
Freitas, Carlos Alberto de, E-mail: carlos.freitas1950@hotmail.com [Instituto Federal do Rio de Janeiro (IFRJ), Nilopolis, RJ (Brazil); Jachic, Joao; Moreira, Maria de Lourdes, E-mail: jjachic@ien.gov.br, E-mail: malu@ien.gov.br [Instituto de Engenharia Nuclear (IEN/CNEN-RJ), Rio de Janeiro, RJ (Brazil)
2013-07-01
One of the cooling modes for any source of heat such as in a shutdown nuclear core is the natural convection. The design specifications of any cooling pool can only be done when the removal heat rate and the corresponding mass flow rate is reasonably established. In our simulation scheme, we assumed that the body forces acting in the cubic water cell are: the weight, the drag force and the integrated pressure forces on the horizontal surfaces, the viscosity shear forces on the vertical surfaces and also a special viscosity drag force due to the mass dislocation along a Bernoulli type current tube outside the motive region. For a suitable time step, the uprising convection velocity is determined by an implicit and also by an explicit solution algorithm. The resulting differential equation depends on updating specific mass, dynamic viscosity and constant pressure heat coefficient with the last known temperature in the cell that absorbed heat. Numerical calculation software was performed using MATLAB’s technical computing language and then applied for a heat generation plate simulating a spent fuel assembler from a shutdown nuclear core. The results show time evolution of convection, terminal velocity and water temperature distribution. Pool dimension as well as pool level decrement are also determined for various air exhausting system conditions and heat rate of the spent fuel plate being cooled. (author)
Time evolution simulation of heat removal in a small water tank by natural convection
International Nuclear Information System (INIS)
One of the cooling modes for any source of heat such as in a shutdown nuclear core is the natural convection. The design specifications of any cooling pool can only be done when the removal heat rate and the corresponding mass flow rate is reasonably established. In our simulation scheme, we assumed that the body forces acting in the cubic water cell are: the weight, the drag force and the integrated pressure forces on the horizontal surfaces, the viscosity shear forces on the vertical surfaces and also a special viscosity drag force due to the mass dislocation along a Bernoulli type current tube outside the motive region. For a suitable time step, the uprising convection velocity is determined by an implicit and also by an explicit solution algorithm. The resulting differential equation depends on updating specific mass, dynamic viscosity and constant pressure heat coefficient with the last known temperature in the cell that absorbed heat. Numerical calculation software was performed using MATLAB’s technical computing language and then applied for a heat generation plate simulating a spent fuel assembler from a shutdown nuclear core. The results show time evolution of convection, terminal velocity and water temperature distribution. Pool dimension as well as pool level decrement are also determined for various air exhausting system conditions and heat rate of the spent fuel plate being cooled. (author)
Enhancement of natural convection heat transfer rate by the improved heated surface
International Nuclear Information System (INIS)
The cooling system of water in spent fuel pit without electricity power is developed. The cooling system has a air radiator. The radiator size would be too large, because natural convection heat transfer is low. Therefore, enhancement of natural convection heat transfer is necessary. We applied thermal splay to heated surface, and found that thermal splay enhanced natural convection heat transfer performance. The material of thermal splay is aluminum bronze and polyester powder. Using this result, radiator size could be reduced. (author)
International Nuclear Information System (INIS)
The thermo fluid dynamic characteristics of natural convection flow depend strongly on thermal boundary condition such as the spatial and temporal variation of heat flux on the pool wall boundaries. In general the natural convection heat transfer phenomena involving the bottom heat generation are represented by the Rayleigh number, Ra, which quantifies the bottom heat source and hence the strength of the buoyancy force. This work focuses on natural convection in which the density gradient is due to a temperature gradient and the body force is gravitational. The presence of a fluid density gradient in a gravitational field does not ensure the existence of natural convection currents, however, in an apparatus enclosed by two horizontal plates of different temperature. The temperature of the lower plate exceeds that of the upper plate, and the density decreases in the direction of the gravitational force. The LIDO (Liquid Internal Dynamics Operation) tests are conducted in a horizontal circular layer 500 mm in diameter and 220 mm in height using fluid, whose thermophysical properties are typified by the Prandtl number, Pr. The tests cover the range of 3x105 10 and 0.02 < Pr < 2.22 Tests are conducted with air, water and Wood's metal (Pb-Bi-Sn-Cd) as simulant to determine the Nusselt number, Nu. The upper and side walls are cooled, while the lower wall is heated at uniform temperatures
Gorla, Rama Subba Reddy; Kumari, M.
A nonsimilar boundary layer analysis is presented for the problem of mixed convection in power-law type non-Newtonian fluids along a vertical plate with power-law surface heat flux distribution. The mixed convection regime is divided into two regions, namely, the forced convection dominated regime and the free convection dominated regime. The two solutions are matched. Numerical results are presented for the details of the velocity and temperature fields. A discussion is provided for the effect of viscosity index on the surface heat transfer rate.
Analysis of forced convective transient boiling by homogeneous model of two-phase flow
International Nuclear Information System (INIS)
Transient forced convective boiling is of practical importance in relation to the accident analysis of nuclear reactor etc. For large length-to-diameter ratio, the transient boiling characteristics are predicted by transient two-phase flow calculations. Based on homogeneous model of two-phase flow, the transient forced convective boiling for power and flow transients are analysed. Analytical expressions of various parameters of transient two-phase flow have been obtained for several simple cases of power and flow transients. Based on these results, heat flux, velocity and time at transient CHF condition are predicted analytically for step and exponential power increases, and step, exponential and linear velocity decreases. The effects of various parameters on heat flux, velocity and time at transient CHF condition have been clarified. Numerical approach combined with analytical method is proposed for more complicated cases. Solution method for pressure transient are also described. (author)
Laminar forced convection inside externally finned tubes
International Nuclear Information System (INIS)
Analytical solutions are obtained for thermal entry region problems inside ducts with axially varying heat transFer coefficient, by making use of the ideas in the recently advanced generalized integral transform technique. The analysis is applied to stepwise variations of Biot number that simulate intermitent rows of external fins. Numerical results are obtained for different fin arrangements, in a systematic manner, so as to critically examine the relative marits of wider and/or more numerous fin rows. (author)
Analytical Solution of Forced-Convective Boundary-Layer Flow over a Flat Plate
DEFF Research Database (Denmark)
Mirgolbabaei, H.; Barari, Amin
2010-01-01
In this letter, the problem of forced convection heat transfer over a horizontal flat plate is investigated by employing the Adomian Decomposition Method (ADM). The series solution of the nonlinear differential equations governing on the problem is developed. Comparison between results obtained and those of numerical solution shows excellent agreement, illustrating the effectiveness of the method. The solution obtained by ADM gives an explicit expression of temperature distribution and velocity distribution over a flat plate.
Mohammad Al-Nimr; Mohammad Abuzaid; Osamah Haddad
2004-01-01
Abstract: The entropy generation due to steady laminar forced convection fluid flow through parallel plates microchannel is investigated numerically. The effect of Knudsen, Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference on entropy generation within the microchannel is discussed. The fraction of the entropy generation due to heat transfer to the total entropy generation within the microchannel is studied in terms of Bejan number. The entropy generation within t...
Sedimentation and convective boiling heat transfer of CuO-water/ethylene glycol nanofluids
Sarafraz, M. M.; Hormozi, F.; Kamalgharibi, M.
2014-09-01
The convective boiling characteristics of dilute dispersions of CuO nanoparticles in water/ethylene glycol as a base fluid were studied at different operating conditions of (heat fluxes up to 174 kW m-2, mass fluxes range of 353-1,059 kg m-2 s-1 and sub-cooling level of 343, 353 and 363 K) inside the annular duct. The convective boiling heat transfer coefficients of nanofluids in different concentrations (vol%) of nanoparticles (0.5, 1, and 1.5) were also experimentally quantified. Results demonstrated the significant augmentation of heat transfer coefficient inside the region with forced convection dominant mechanism and deterioration of heat transfer coefficient in region with nucleate boiling dominant heat transfer mechanism. Due to the scale formation around the heating section, fouling resistance was also experimentally measured. Experimental data showed that with increasing the heat and mass fluxes, the heat transfer coefficient and fouling resistance dramatically increase and rate of bubble formation clearly increases. Obtained results were then compared to some well-known correlations. Results of these comparisons demonstrated that experimental results represent the good agreement with those of obtained by the correlations. Consequently, Chen correlation is recommended for estimating the convective flow boiling heat transfer coefficient of dilute CuO-water/ethylene glycol based nanofluids.
Natural convective heat transfer from short inclined cylinders
Oosthuizen, Patrick H
2014-01-01
Natural Convective Heat Transfer from Short Inclined Cylinders examines a heat transfer situation of significant, practical importance not adequately dealt with in existing textbooks or in any widely available review papers. Specifically, the book introduces the reader to recent studies of natural convection from short cylinders mounted on a flat insulated base where there is an “exposed” upper surface. The authors considers the effects of the cylinder cross-sectional shape, the cylinder inclination angle, and the length-to-cross sectional size of the cylinder. Both numerical and experimental studies are discussed and correlation equations based on the results of these studies are reviewed. This book is ideal for professionals involved with thermal management and related systems, researchers, and graduate students in the field of natural convective heat transfer, instructors in graduate level courses in convective heat transfer.
Details of Exact Low Prandtl Number Boundary-Layer Solutions for Forced and For Free Convection
Sparrow, E. M.; Gregg, J. L.
1959-01-01
A detailed report is given of exact (numerical) solutions of the laminar-boundary-layer equations for the Prandtl number range appropriate to liquid metals (0.003 to 0.03). Consideration is given to the following situations: (1) forced convection over a flat plate for the conditions of uniform wall temperature and uniform wall heat flux, and (2) free convection over an isothermal vertical plate. Tabulations of the new solutions are given in detail. Results are presented for the heat-transfer and shear-stress characteristics; temperature and velocity distributions are also shown. The heat-transfer results are correlated in terms of dimensionless parameters that vary only slightly over the entire liquid-metal range. Previous analytical and experimental work on low Prandtl number boundary layers is surveyed and compared with the new exact solutions.
Predictions of laminar natural convection in heated cavities
International Nuclear Information System (INIS)
Several examples of laminar, natural convection in heated cavities are discussed with illustrative calculations. These include convection in a square cavity at high Rayleigh number; in a narrow cavity at moderate aspect ratio; in a rectangular cavity heated from below; in a trapezoidal cavity, and in a rectangular cavity containing a conducting obstruction. The steady equations for the velocity, pressure and temperature are solved in the Boussinesq approximation, using a standard Galerkin formulation of the finite-element method. (author)
International Nuclear Information System (INIS)
An experimental study was performed to obtain local fluid velocity and temperature measurements in the mixed (combined free and forced) convection regime for specific flow coastdown transients. A brief investigation of steady-state flows for the purely free-convection regime was also completed. The study was performed using an electrically heated 2 x 6 rod bundle contained in a flow housing. In addition a transient data base was obtained for evaluating the COBRA-WC thermal-hydraulic computer program
Turbulent opposing mixed convection heat transfer in vertical flat channel with symmetrical heating
International Nuclear Information System (INIS)
In this paper we present the results on experimental and numerical investigation of the local turbulent mixed convection heat transfer in a vertical (?=90 degrees) flat channel with opposing flows (heated walls with down word oriented forced flow). The experiments were performed in air flow of different pressures, from 0.1 to 1 MPa, in the range of Re from 4*103 to 5.6*104 and Grq up to 1*1011 for symmetrical heating of the walls. Analysis showed that at higher pressures for Re numbers below 8000 heat transfer variation along the channel takes non monotonous character. Numerical two-dimentional simulations were performed for the same channel and for the same conditions as in experiments using FLUENT 6.0 code and low Reynolds k-? turbulence model. Modelling results show that for regimes with not monotonous variation of heat transfer along the channel, upward oriented reverse air flow at the heated wall is existing when in the core the main (forced) flow is downward oriented. (author)
Convective and radiative heat transfer coefficients for individual human body segments
de Dear, R. J.; Arens, Edward; Hui, Zhang; Oguro, Masayuki
Human thermal physiological and comfort models will soon be able to simulate both transient and spatial inhomogeneities in the thermal environment. With this increasing detail comes the need for anatomically specific convective and radiative heat transfer coefficients for the human body. The present study used an articulated thermal manikin with 16 body segments (head, chest, back, upper arms, forearms, hands, pelvis, upper legs, lower legs, feet) to generate radiative heat transfer coefficients as well as natural- and forced-mode convective coefficients. The tests were conducted across a range of wind speeds from still air to 5.0 m/s, representing atmospheric conditions typical of both indoors and outdoors. Both standing and seated postures were investigated, as were eight different wind azimuth angles. The radiative heat transfer coefficient measured for the whole-body was 4.5 W/m2 per K for both the seated and standing cases, closely matching the generally accepted whole-body value of 4.7 W/m2 per K. Similarly, the whole-body natural convection coefficient for the manikin fell within the mid-range of previously published values at 3.4 and 3.3 W/m2 per K when standing and seated respectively. In the forced convective regime, heat transfer coefficients were higher for hands, feet and peripheral limbs compared to the central torso region. Wind direction had little effect on convective heat transfers from individual body segments. A general-purpose forced convection equation suitable for application to both seated and standing postures indoors was hc=10.3v0.6 for the whole-body. Similar equations were generated for individual body segments in both seated and standing postures.
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.
Solution of heat removal from nuclear reactors by natural convection
Zitek, Pavel; Valenta, Vaclav
2014-03-01
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.
SIMULATION OF MIXED CONVECTIVE HEAT TRANSFER USING LATTICE BOLTZMANN METHOD
Directory of Open Access Journals (Sweden)
A. R. M. Rosdzimin
2010-12-01
Full Text Available In this paper, mixed (forced–natural convective heat transfer around a heated square cylinder located inside a lid driven cavity has been studied numerically using the lattice Boltzmann method in the range of 100? Re ? 1000 with the corresponding Richardson number 0.01?Ri?10. The double-population lattice Boltzmann formulation is used as the governing equation. Two dimensional nine-velocity models are used for the computation of the velocity field while a four-velocity model is used for the computation of the temperature field. We found that the combination of nine- and four-velocity models can be applied to the calculation without losing its accuracy. The results are presented in the form of streamline and isotherm plots as well as the variation of local Nusselt number at the top surface of the heated square. The computational results demonstrate that the flow pattern, formation of vortex and also the Nusselt number are influence by the Reynolds number and Richardson number.
Influence of flow with suspended particles on temperature fields and convective heat transfer
Aydin, M.; Reis, A. H.; Miguel, A. F.
2007-01-01
The present paper reports the results of a numerical study of forced convection heat transfer between two plates filled with particles. We study the influence of particles suspended in flows on heat transfer enhancement. We consider steady-state fluid flow between two flat horizontal parallel plates with both plates kept at the same temperature. Mono-disperse particles with low and high thermal conductivities injected at the entrance are entrained with the flow. Their effects on temperatur...
SRS reactor control rod cooling without normal forced convection cooling
International Nuclear Information System (INIS)
This paper describes an analytical study of the coolability of the control rods in the Savannah River site (SRS) K production reactor under conditions of loss of normal forced convection cooling. The study was performed as part of the overall safety analysis of the reactor supporting its restart. The analysis addresses the buoyancy-driven boiling flow over the control rods that occurs when forced cooling is lost. The objective of the study was to demonstrate that the control rods will remain cooled (i.e., no melting) at powers representative of those anticipated for restart of the reactor
Aerosol Radiative Effects on Deep Convective Clouds and Associated Radiative Forcing
Fan, J.; Zhang, R.; Tao, W.-K.; Mohr, I.
2007-01-01
The aerosol radiative effects (ARE) on the deep convective clouds are investigated by using a spectral-bin cloud-resolving model (CRM) coupled with a radiation scheme and an explicit land surface model. The sensitivity of cloud properties and the associated radiative forcing to aerosol single-scattering albedo (SSA) are examined. The ARE on cloud properties is pronounced for mid-visible SSA of 0.85. Relative to the case excluding the ARE, cloud fraction and optical depth decrease by about 18% and 20%, respectively. Cloud droplet and ice particle number concentrations, liquid water path (LWP), ice water path (IWP), and droplet size decrease significantly when the ARE is introduced. The ARE causes a surface cooling of about 0.35 K and significantly high heating rates in the lower troposphere (about 0.6K/day higher at 2 km), both of which lead to a more stable atmosphere and hence weaker convection. The weaker convection and the more desiccation of cloud layers explain the less cloudiness, lower cloud optical depth, LWP and IWP, smaller droplet size, and less precipitation. The daytime-mean direct forcing induced by black carbon is about 2.2 W/sq m at the top of atmosphere (TOA) and -17.4 W/sq m at the surface for SSA of 0.85. The semi-direct forcing is positive, about 10 and 11.2 W/sq m at the TOA and surface, respectively. Both the TOA and surface total radiative forcing values are strongly negative for the deep convective clouds, attributed mostly to aerosol indirect forcing. Aerosol direct and semi-direct effects are very sensitive to SSA. Because the positive semi-direct forcing compensates the negative direct forcing at the surface, the surface temperature and heat fluxes decrease less significantly with the increase of aerosol absorption (decreasing SSA). The cloud fraction, optical depth, convective strength, and precipitation decrease with the increase of absorption, resulting from a more stable and dryer atmosphere due to enhanced surface cooling and atmospheric heating.
Adegun, I. K.; Bello-ochende, F. L.
2004-01-01
A numerical study of steady state laminar forced and free convective and radiative heat transfer in an inclined rotating rectangular duct with a centered circular tube is reported for an hydrodynamically fully developed flow. The two heat transfer mechanisms of convection and radiation are treated independently and simultaneously. The coupled equations of momentum and energy transports are solved using Gauss-Seidel iteration technique subject to given boundary constraints. A thermal boundary ...
D'Orazio, A.; Karimipour, A.; Nezhad, A. H.; Shirani, E.
2014-11-01
Laminar mixed convective heat transfer in two-dimensional rectangular inclined driven cavity is studied numerically by means of a double population thermal Lattice Boltzmann method. Through the top moving lid the heat flux enters the cavity whereas it leaves the system through the bottom wall; side walls are adiabatic. The counter-slip internal energy density boundary condition, able to simulate an imposed non zero heat flux at the wall, is applied, in order to demonstrate that it can be effectively used to simulate heat transfer phenomena also in case of moving walls. Results are analyzed over a range of the Richardson numbers and tilting angles of the enclosure, encompassing the dominating forced convection, mixed convection, and dominating natural convection flow regimes. As expected, heat transfer rate increases as increases the inclination angle, but this effect is significant for higher Richardson numbers, when buoyancy forces dominate the problem; for horizontal cavity, average Nusselt number decreases with the increase of Richardson number because of the stratified field configuration.
International Nuclear Information System (INIS)
In construction, the use of Phase Change Materials (PCM) allows the storage/release of energy from solar radiation and internal loads. The application of such materials for lightweight construction (e.g., a wood house) makes it possible to improve thermal comfort and reduce energy consumption. The heat transfer process between the wall and the indoor air is convection. In this paper, we have developed a numerical model to evaluate several convective heat transfer correlations from the literature for natural, mixed and forced convection flows. The results show that the convective heat transfer highly influences the storage/release process in case of PCM walls. For the natural convection, the numerical results are highly dependent on the correlation used and the results may vary up to 200%. In the case of mixed and forced convection flows, the higher is the velocity, the more important is the storage capacity. - Highlights: ? We study effect of inside convection correlation on energy stored in PCM wall. ? We developed a 1D conduction model for multilayer walls, with phase change material. ? Correlations have been constructed for mixed convection in all flow regimes. ? Up to 200% variation of energy stored in PCM layer, depending on convection correlation. ? Ventilation can increase the energy stored in the PCM layer.
An experimental investigation of forced convection flat plate solar air heater with storage material
Aissa Walid; El-Sallak Mostafa; Elhakem Ahmed
2012-01-01
Solar air heater (SAH) is a heating device that uses the heated air in the drying of agriculture products and many engineering applications. The purpose of the present work is to study a forced convection flat plate solar air heater with granite stone storage material bed under the climatic conditions of Egypt-Aswan. Experiments are performed at different air mass flow rates ; varying from 0.016 kg/s to 0.08 kg/s, for five hot summer days of July 2008. Hourly values of global solar radi...
International Nuclear Information System (INIS)
The helical coiled tube of heat exchanger is used for the evaporator of prototype fast breeder reactor 'Monju'. This report aims at the grasp of two-phase flow phenomena of forced convective boiling of water inside helical coiled tube. A transparent double tube made of the glass is used as a heat exchanger, water flows up inside helical tube and the high temperature oil flows down in the outside tube. The oscillation of the dryout point was observed, that is mainly caused by intensive nucleate boiling near the dryout point and evaporation of thin liquid film flowing along the surface. Also, the oscillation characteristics were experimentally confirmed. (author)
International Nuclear Information System (INIS)
The void fraction in a forced convective flow boiling is very important information for understanding the characteristics of the boiling two-phase flow. Consequently, many experimental investigations have been carried out to obtain the local void fraction so far, but the detail data among the whole of the test-section has not been enough. Especially, the data under subcooled condition are quite limited. In this study, the void fraction distribution in a forced convective boiling was quantitatively measured by using the thermal neutron radiography. These results were compared with several existing void fraction correlations. Although these correlations show a good agreements with experimental results under low heat flux condition, there is no suitable correlation to estimate the void fraction under non-thermal equilibrium condition. (author)
Convective heat transfer around vertical jet fires: An experimental study
Energy Technology Data Exchange (ETDEWEB)
Kozanoglu, Bulent, E-mail: bulentu.kozanoglu@udlap.mx [Universidad de las Americas, Puebla (Mexico); Zarate, Luis [Universidad Popular Autonoma del Estado de Puebla (Mexico); Gomez-Mares, Mercedes [Universita di Bologna (Italy); Casal, Joaquim [Universitat Politecnica de Catalunya (Spain)
2011-12-15
Highlights: Black-Right-Pointing-Pointer Experiments were carried out to analyze convection around a vertical jet fire. Black-Right-Pointing-Pointer Convection heat transfer is enhanced increasing the flame length. Black-Right-Pointing-Pointer Nusselt number grows with higher values of Rayleigh and Reynolds numbers. Black-Right-Pointing-Pointer In subsonic flames, Nusselt number increases with Froude number. Black-Right-Pointing-Pointer Convection and radiation are equally important in causing a domino effect. - Abstract: The convection heat transfer phenomenon in vertical jet fires was experimentally analyzed. In these experiments, turbulent propane flames were generated in subsonic as well as sonic regimes. The experimental data demonstrated that the rate of convection heat transfer increases by increasing the length of the flame. Assuming the solid flame model, the convection heat transfer coefficient was calculated. Two equations in terms of adimensional numbers were developed. It was found out that the Nusselt number attains greater values for higher values of the Rayleigh and Reynolds numbers. On the other hand, the Froude number was analyzed only for the subsonic flames where the Nusselt number grows by this number and the diameter of the orifice.
Transient convective heat transfer to laminar flow from a flat plate with constant heat capacity
International Nuclear Information System (INIS)
Most basic transient heat transfer problem is the transient response characteristics of forced convection heat transfer in the flow along a flat plate or in a tube. In case of the laminar flow along a flat plate, the profile method using steady temperature distribution has been mostly adopted, but its propriety has not been clarified yet. About the unsteady heat transfer in the laminar flow along a flat plate, the analysis or experiment evaluating the heat capacity of the flat plate exactly was never carried out. The purpose of this study is to determine by numerical calculation the unsteady characteristics of the boundary layer in laminar flow and to confirm them by experiment concerning the unsteady heat transfer when a flat plate with a certain heat capacity is placed in parallel in uniform flow and given a certain quantity of heat generation suddenly. The basic equation and the solution are given, and the method of numerical calculation and the result are explained. The experimental setup and method, and the experimental results are shown. Both results were in good agreement, and the response of wall temperature, the response of Nusselt number and the change of temperature distribution in course of time were able to be determined by applying Laplace transformation and numerical Laplace inverse transformation to the equation. (Kako, I.)
Aerosol Optical Effects on Deep Convective Clouds and Radiative Forcing
Fan, J.; Zhang, R.; Tao, W.; Mohr, K. I.
2007-12-01
Aerosols interact directly and indirectly with the Earth's radiation budget and climate. For the direct effect, aerosols scatter and absorb solar radiation. Light scattering by aerosols changes the radiative fluxes at the top-of- atmosphere (TOA), at the surface, and within the atmospheric column, while aerosol absorption modifies the atmospheric temperature structure, decreases the solar radiation at the surface, and lowers surface sensible and latent fluxes, suppressing convection and reducing cloud fraction. Using a two-dimensional cloud-resolving Goddard Cloud Ensemble (GCE) model coupled with radiative transfer processes and the land-atmosphere interaction processes, we investigate aerosol radiative effects on deep convective clouds in an urban atmospheric environment, focussing on the radiative effects of anthropogenic aerosols containing BC. An aerosol radiative module is developed to calculate the wavelength-dependent aerosol radiative properties based on the aerosol composition, size distribution, mixing state, and ambient relative humidity. The significance of the aerosol radiative effects (ARE) is investigated by comparing with the cases excluding the ARE. The associated aerosol direct, semi-direct and indirect radiative forcing for deep convective clouds are estimated, and the sensitivity of cloud properties and radiative forcing to aerosol single-scattering albedo (SSA) are examined. The results provide insight on the coupling between the aerosol direct, semi-direct, and indirect effects on clouds.
10,000 - A reason to study granular heat convection
Energy Technology Data Exchange (ETDEWEB)
Einav, I.; Rognon, P.; Gan, Y.; Miller, T.; Griffani, D. [Particles and Grains Laboratory, School of Civil Engineering, University of Sydney, Sydney, NSW 2006 (Australia)
2013-06-18
In sheared granular media, particle motion is characterized by vortex-like structures; here this is demonstrated experimentally for disks system undergoing indefinite deformation during simple shear, as often imposed by the rock masses hosting earthquake fault gouges. In traditional fluids it has been known for years that vortices represent a major factor of heat transfer enhancement via convective internal mixing, but in analyses of heat transfer through earthquake faults and base planes of landslides this has been continuously neglected. Can research proceed by neglecting heat convection by internal mixing? Our answer is astonishingly far from being yes.
International Nuclear Information System (INIS)
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)
International Nuclear Information System (INIS)
Effect of buoyancy force in a laminar uniform forced convection flow past a semi-infinite vertical plate has been analyzed near the leading edge, taking into account the viscous dissipation. The coupled non-linear locally similar equations, which govern the flow, are solved by the method of parametric differentiation. Effects of the buoyancy force and the heat due to viscous dissipation on the flow and the temperature fields as well as on the wall shear-stress and the heat transfer at the surface of the plate are shown graphically for the values of the Prandtl number ? ranging from 10-1 to 1.0. (author). 20 refs, 3 figs, 2 tabs
Scientific Electronic Library Online (English)
Mariangela, Amendola; Saul, Dussán-Sarria; Anderson A., Rabello.
2009-04-01
Full Text Available SciELO Brazil | Language: Portuguese Abstract in portuguese Este trabalho busca, como função principal, determinar o valor do coeficiente convectivo de transferência de calor através da metodologia de modelagem matemática e simulação numérica associada ao processo experimental previamente realizado, de resfriamento rápido com ar forçado de figos "Roxo de Val [...] inhos". O objetivo foi comparar o resultado com o definido pelo ajuste entre os dados experimentais e o uso de equação empírica da literatura, já realizado. Para a predição da temperatura próximo ao centro das frutas, considerou-se o modelo da lei de Fourier em coordenadas esféricas e se implementaram algoritmos segundo os métodos de diferenças finitas e dos elementos finitos para os modelos unidimensional e tridimensional, respectivamente. O estabelecimento do referido valor se dá a partir da comparação desses dados simulados com os dados experimentais. Os valores resultantes dos dois modelos foram coincidentes e resultaram menores valores residuais que o obtido pela equação empírica; além disso, a representatividade da curva simulada mostra que a atual metodologia é mais precisa que a anteriormente realizada e, portanto, adequada para este e trabalhos futuros. Abstract in english The main purpose of this study is to determine the convective heat transfer coefficient value by using the methodology of mathematical modeling and numerical simulation associated with the experimental process previously performed, of the forced air precooling of the figs type 'Roxo de Valinhos'. Th [...] e objective was to compare the result with the one determined by fitting an empirical equation from the literature to the experimental data. For the prediction of the temperature, near to the center of fruits, the Fourier law model was considered. Algorithms employing the finite differences and finite elements methods were implemented for the one-dimensional and three-dimensional models. The establishment of the referred value was performed through the comparison of these simulated data with the experimental data. The resulting values of the two models were coincident and generated a smaller residual value than the one obtained by the empirical equation. Furthermore, the representation of the simulated curve shows that the methodology of the mathematical modeling and numerical simulation is more accurate than the previous one and suitable for this and future work.
Reynolds stress and heat flux in spherical shell convection
Käpylä, P J; Guerrero, G; Brandenburg, A; Chatterjee, P
2010-01-01
Context. Turbulent fluxes of angular momentum and heat due to rotationally affected convection play a key role in determining differential rotation of stars. Aims. We compute turbulent angular momentum and heat transport as functions of the rotation rate from stratified convection. We compare results from spherical and Cartesian models in the same parameter regime in order to study whether restricted geometry introduces artefacts into the results. Methods. We employ direct numerical simulations of turbulent convection in spherical and Cartesian geometries. In order to alleviate the computational cost in the spherical runs and to reach as high spatial resolution as possible, we model only parts of the latitude and longitude. The rotational influence, measured by the Coriolis number or inverse Rossby number, is varied from zero to roughly seven, which is the regime that is likely to be realised in the solar convection zone. Cartesian simulations are performed in overlapping parameter regimes. Results. For slow ...
Venkatachalapathy, S.; Udayakumar, M.
2010-01-01
Natural convection cooling using air as a fluid is commonly used in the cooling of electronic equipment and many other devices. In this work, a three-dimensional numerical study of natural convection heat transfer from multiple protruding heat sources simulating electronic components is conducted. Computational fluid dynamics (CFD) software, FLUENT is used in this analysis. A 4 by 5 array of heat sources are embedded in the bottom wall of an adiabatic square enclosure. The heat sources with a...
International Nuclear Information System (INIS)
Mixed forced and free convective heat transfer through a cavity with a rectangular element on one wall heated to a uniform temperature, and the opposite wall cooled to a uniform lower temperature, and with the remaining wall portions adiabatic, has been investigated. The forced flow, which is at the same temperature as the cold wall, enters through the cold vertical wall and also leaves through this wall. The study is based on the use of the two-dimensional Navier-Stokes, equations, the flow assumed steady and laminar. It is assumed that fluid properties are constant except for the density change with temperature which gives rise to the buoyancy force, this being treated using the Boussinesq approximation. These equations have been solved using the finite element method. The solution has, as parameters, the Reynolds number, the Grashof number, the fluid Prandtl number, the aspect ratio of the cavity, the angle of inclination of the cavity, the inlet and outlet duct size, and the aspect ratio of the heated element. Results have only been obtained for a Prandtl number of 0.7. 7 references
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A new experimental technique is presented that allows simultaneous measurement of convective and radiative heat flux in the underhood. The goal is to devise an easily implemented and accurate experimental method for application in the vehicle underhood compartment. The new method is based on a technique for heat-flux measurement developed by the authors (Heat flow (flux) sensors for measurement of convection, conduction and radiation heat flow 27036-2, © Rhopoint Components Ltd, Hurst Green, Oxted, RH8 9AX, UK) that uses several thermocouples in the thickness of a thermal resistive layer (foil heat-flux sensor). The method proposed here uses a pair of these thermocouples with different radiative properties. Measurements validating this novel technique are carried out on a flat plate with a prescribed constant temperature in both natural- and forced-convection flow regimes. The test flat plate is instrumented by this new technique, and also with a different technique that is intrusive but very accurate, used as reference here (Bardon J P and Jarny Y 1994 Procédé et dispositif de mesure transitoire de température et flux surfacique Brevet n°94.011996, 22 February). Discrepancies between the measurements by the two techniques are less than 10% for both convective and radiative heat flux. Error identification and sensitivity analysis of the new method are also presented
Khaled, M.; Garnier, B.; Harambat, F.; Peerhossaini, H.
2010-02-01
A new experimental technique is presented that allows simultaneous measurement of convective and radiative heat flux in the underhood. The goal is to devise an easily implemented and accurate experimental method for application in the vehicle underhood compartment. The new method is based on a technique for heat-flux measurement developed by the authors (Heat flow (flux) sensors for measurement of convection, conduction and radiation heat flow 27036-2, © Rhopoint Components Ltd, Hurst Green, Oxted, RH8 9AX, UK) that uses several thermocouples in the thickness of a thermal resistive layer (foil heat-flux sensor). The method proposed here uses a pair of these thermocouples with different radiative properties. Measurements validating this novel technique are carried out on a flat plate with a prescribed constant temperature in both natural- and forced-convection flow regimes. The test flat plate is instrumented by this new technique, and also with a different technique that is intrusive but very accurate, used as reference here (Bardon J P and Jarny Y 1994 Procédé et dispositif de mesure transitoire de température et flux surfacique Brevet n°94.011996, 22 February). Discrepancies between the measurements by the two techniques are less than 10% for both convective and radiative heat flux. Error identification and sensitivity analysis of the new method are also presented.
Correlations for heat transfer coefficients in open gaps with respect to mixed convection
International Nuclear Information System (INIS)
Published results on mixed convection phenomena have been applied to determine the convective heat transfer between the cover gas and the open gaps in the roof of SNR 2. It has been reported in the literature that heat transfer coefficients for forced flow conditions in vertical heated or cooled pipes are modified by buoyancy effects. In the ''aiding'' condition, where buoyancy and flow act in the same direction, heat transfer is enhanced. In the ''opposing'' case heat transfer is reduced. This applies for laminar flow; the reverse is true for turbulent flow. The literature indicates furthermore that: the Reynolds number indicating turbulent flow can be as low as 30 for mixed convection; the criterion for the onset of mixed convection is given by the Richardson number Ri=Gr/Re2 > 0,002. The published results have been modified in consistency with the open gaps in the SNR 2 reactor roof. Several heat transfer correlations have been evaluated and their suitability examined. (author)
International Nuclear Information System (INIS)
The objective of this paper is to numerically investigate the cooling performance of electronic devices with an emphasis on the effects of the arrangement and number of electronic components. The analysis uses a two dimensional rectangular enclosure under combined natural and forced convection flow conditions and considers a range of Raleigh numbers. Heat sources in the enclosure generate the natural convection flow and an externally sourced air stream through the enclosure generates the forced convection flow. The results show that increasing the Raleigh number significantly improves the enclosure heat transfer process. At low Raleigh numbers, placing more heat sources within the enclosure reduces the heat transfer rate from the sources and consequently increases their overall maximum temperature. The arrangement and the number of heat sources have a considerable contribution to the cooling performance. However, increasing the Raleigh number reduces this contribution. (author)
Maki, Syou; Sumitani, Mariko; Udagawa, Chikako; Morimoto, Shotaro; Tanimoto, Yoshifumi
2014-07-01
We carried out three-dimensional computations of the magnetothermal convection of diamagnetic liquids (e.g., water) in a shallow cylindrical vessel of the Rayleigh–Benard model to investigate the effects of the radial components of the magnetic force (MFR) and the vertical components of the magnetic force (MFZ) on heat transfer and pattern formation. For the purpose of examining various magnitudes of radial and vertical components of the magnetic force, we used a 1800-turn coil instead of an actual conventional magnet, and a 2-turn coil as a new proposal. In the computation, the Prandtl number was constant at 6.0 (i.e., water at room temperature), and the Rayleigh number was 1.0 × 104. As a result, an axisymmetric pattern and a spokelike pattern appeared in the convection. Subsequently, we arranged all the results with a new parameter, which is the absolute ratio of the radial component of magnetic force at the vessel sidewall to the vertical resultant force at the vessel center. This parameter represents the relative contribution of MFR to the vertical resultant force (RFZ). The RFZ is the sum of the gravitational force and MFZ, and is associated with the magnetic Rayleigh number. By using this parameter, the spoke pattern convection appeared when the parameter was within 0.11 to 0.85. This result suggests that not only the MFR but also the MFZ plays an important role in determining whether the convection flow pattern transited to the axisymmetric flow or not. This parameter range is easily satisfied in a conventional superconducting magnet. We can say that the spoke pattern convection may be a common phenomenon in the magnet. On the other hand, the flow pattern had little effect on the heat transfer in convection. Thus, the effects of MFR and MFZ on the pattern formation in the magnet are much more complicated than those previously considered.
Heat transfer enhancement in a turbulent natural convection boundary layer
International Nuclear Information System (INIS)
An experimental study on heat transfer enhancement for a turbulent natural convection boundary layer along a vertical flate plate has been performed by inserting two-dimensional flat plates (heat transfer promoters) into the boundary layer. The heat transfer rates markedly very with the position, yaw angle and size of the promoters, and it is confirmed that the promoters are effective to control turbulent heat transfer characteristics of the natural convection boundary layer. Also, the visualization of flow with smoke and laser sheet and simultaneous measurements of the flow and thermal fields with hot- and cold-wires in the downstream region of the promoter have been conducted to clarify the cause of the increase in local heat transfer rates. As a result, it is observed that the low-temperature large vortices induced by the promoter invade very near the wall and the heat transfer enhancement occurs. (author)
Heat transfer mechanisms in bubbly Rayleigh-Benard convection
Oresta, Paolo; Verzicco, Roberto; Lohse, Detlef; Prosperetti, Andrea
2008-01-01
The heat transfer mechanism in Rayleigh-Benard convection in a liquid with a mean temperature close to its boiling point is studied through numerical simulations with point-like vapor bubbles, which are allowed to grow or shrink through evaporation and condensation and which act back on the flow both thermally and mechanically. It is shown that the effect of the bubbles is strongly dependent on the ratio of the sensible heat to the latent heat as embodied in the Jacob number...
Quantification of convective heat transfer inside tree structures
International Nuclear Information System (INIS)
Convective heat transfer between a vegetal structure and its surrounding medium remains poorly described. However, for some applications, such as forest fire propagation studies, convective heat transfer is one of the main factors responsible for vertical fire transitions, from ground level to the tree crowns. These fires are the most dangerous because their rates of spread can reach high speeds, around one meter per second. An accurate characterization of this transfer is therefore important for fire propagation modelling. This study presents an attempt to formulate a theoretical modelling of the convective heat transfer coefficient for vegetal structures generated using an Iterated Function Systems (IFS). This model depends on the IFS parameters. The results obtained using this approach were compared with previously computed numerical results in order to evaluate their accuracy. The maximal discrepancies were found to be around 12% which proves the efficiency of the present model.
Convective heat transfer around vertical jet fires: an experimental study.
Kozanoglu, Bulent; Zárate, Luis; Gómez-Mares, Mercedes; Casal, Joaquim
2011-12-15
The convection heat transfer phenomenon in vertical jet fires was experimentally analyzed. In these experiments, turbulent propane flames were generated in subsonic as well as sonic regimes. The experimental data demonstrated that the rate of convection heat transfer increases by increasing the length of the flame. Assuming the solid flame model, the convection heat transfer coefficient was calculated. Two equations in terms of adimensional numbers were developed. It was found out that the Nusselt number attains greater values for higher values of the Rayleigh and Reynolds numbers. On the other hand, the Froude number was analyzed only for the subsonic flames where the Nusselt number grows by this number and the diameter of the orifice. PMID:21962859
Quantification of convective heat transfer inside tree structures
Collin, Anthony; Lamorlette, Aymeric
2012-11-01
Convective heat transfer between a vegetal structure and its surrounding medium remains poorly described. However, for some applications, such as forest fire propagation studies, convective heat transfer is one of the main factors responsible for vertical fire transitions, from ground level to the tree crowns. These fires are the most dangerous because their rates of spread can reach high speeds, around one meter per second. An accurate characterization of this transfer is therefore important for fire propagation modelling. This study presents an attempt to formulate a theoretical modelling of the convective heat transfer coefficient for vegetal structures generated using an Iterated Function Systems (IFS). This model depends on the IFS parameters. The results obtained using this approach were compared with previously computed numerical results in order to evaluate their accuracy. The maximal discrepancies were found to be around 12% which proves the efficiency of the present model.
International Nuclear Information System (INIS)
A collection of papers is presented on mixed convection, pool boiling, and flow boiling and two-phase flow. A unified similarity analysis is presented for turbulent convection next to vertical surfaces. Topics of interest include free-forced convection from a heated cone, decay of vertical buoyant jets in uniform environment, minimum heat flux during film boiling, and the dynamics of two-phase flow in a duct
Natural convection heat transfer within horizontal spent nuclear fuel assemblies
International Nuclear Information System (INIS)
Natural convection heat transfer is experimentally investigated in an enclosed horizontal rod bundle, which characterizes a spent nuclear fuel assembly during dry storage and/or transport conditions. The basic test section consists of a square array of sixty-four stainless steel tubular heaters enclosed within a water-cooled rectangular copper heat exchanger. The heaters are supplied with a uniform power generation per unit length while the surrounding enclosure is maintained at a uniform temperature. The test section resides within a vacuum/pressure chamber in order to subject the assembly to a range of pressure statepoints and various backfill gases. The objective of this experimental study is to obtain convection correlations which can be used in order to easily incorporate convective effects into analytical models of horizontal spent fuel systems, and also to investigate the physical nature of natural convection in enclosed horizontal rod bundles in general. The resulting data consist of: (1) measured temperatures within the assembly as a function of power, pressure, and backfill gas; (2) the relative radiative contribution for the range of observed temperatures; (3) correlations of convective Nusselt number and Rayleigh number for the rod bundle as a whole; and (4) correlations of convective Nusselt number as a function of Rayleigh number for individual rods within the array
Natural 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.
Heat transfer by natural convection into an horizontal cavity
International Nuclear Information System (INIS)
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)
Numerical simulation of magnetic control of heat transfer in thermal convection
Energy Technology Data Exchange (ETDEWEB)
Kenjeres, S.; Hanjalic, K
2004-06-01
We report on numerical study of effects of orientation and distribution of an external magnetic field on the reorganization of convective structures and heat transfer in thermal convection in electrically conductive fluids. The simulations were performed using a transient RANS (T-RANS) approach in which the large-scale deterministic structures are numerically resolved in time and space and the unresolved contribution is modelled using an algebraic stress-flux three-equation subscale model. For low Prandtl (Pr) fluids the subscale model was extended to include Pr-dependent molecular dissipation of heat flux. The method was first validated in natural convection in a side-heated cubical enclosure subjected to magnetic fields of different orientation, strength and penetration depth, showing good agreement with the previous benchmark studies. Subsequently, a series of simulations was performed of turbulent Rayleigh-Benard convection subjected to different magnetic fields over a range of Rayleigh (Ra) and Hartmann (Ha) numbers. The computed Nusselt number showed good agreement with the available experimental results. Numerical visualization of instantaneous flow patterns showed dramatic differences in the convective structures and local heat transfer for different orientation of the magnetic field with respect to the gravitation vector. A gradual, step-like increase in the magnetic strength revealed an interesting outcome of the 'competition' between the buoyancy and the Lorentz forces, leading first to chaotic transition and eventually to laminarization. For specific ranges of Ha, it was found that a local magnetic field confined to the wall boundary layer along the thermally active walls provides almost equal effects as the homogeneous field over the whole flow, indicating an interesting possibility for controlling thermal convection and associated heat transfer.
Numerical simulation of magnetic control of heat transfer in thermal convection
International Nuclear Information System (INIS)
We report on numerical study of effects of orientation and distribution of an external magnetic field on the reorganization of convective structures and heat transfer in thermal convection in electrically conductive fluids. The simulations were performed using a transient RANS (T-RANS) approach in which the large-scale deterministic structures are numerically resolved in time and space and the unresolved contribution is modelled using an algebraic stress-flux three-equation subscale model. For low Prandtl (Pr) fluids the subscale model was extended to include Pr-dependent molecular dissipation of heat flux. The method was first validated in natural convection in a side-heated cubical enclosure subjected to magnetic fields of different orientation, strength and penetration depth, showing good agreement with the previous benchmark studies. Subsequently, a series of simulations was performed of turbulent Rayleigh-Benard convection subjected to different magnetic fields over a range of Rayleigh (Ra) and Hartmann (Ha) numbers. The computed Nusselt number showed good agreement with the available experimental results. Numerical visualization of instantaneous flow patterns showed dramatic differences in the convective structures and local heat transfer for different orientation of the magnetic field with respect to the gravitation vector. A gradual, step-like increase in the magnetic strength revealed an interesting outcome of the 'competition' between the buoyancy and the Lompetition' between the buoyancy and the Lorentz forces, leading first to chaotic transition and eventually to laminarization. For specific ranges of Ha, it was found that a local magnetic field confined to the wall boundary layer along the thermally active walls provides almost equal effects as the homogeneous field over the whole flow, indicating an interesting possibility for controlling thermal convection and associated heat transfer
Calculational method for combined natural circulation and forced-convection flow in a channel
International Nuclear Information System (INIS)
This paper presents a finite element solution for combined natural circulation and forced convection flow in a channel. Because the buoyancy force plays an important role in a mixed convection flow, an iteration scheme was used in solving the coupled energy-momentum equations. The momentum equations and the pressure equation are solved to calculate velocity profiles instead of solving the momentum equations with the continuity equation. Though the pressure equation is obtained by using the continuity equation, the continuity principle is reinforced into the momentum equations and the pressure equation at each iteration. Calculations are performed for the combined natural circulation and forced convection case and the forced convection only case
MHD forced and free convection boundary layer flow near the leading edge
International Nuclear Information System (INIS)
Magnetohydrodynamic forced and free convection flow of an electrically conducting viscous incompressible fluid past a vertical flat plate with uniform heat flux in the presence of a magnetic field acting normal to the plate that moves with the fluid has been studied near the leading edge of the plate. The coupled non-linear equations are solved by the method of superposition for the values of the Prandtl number ranges from 0.01 to 10.0. The velocity and the temperature profiles are presented graphically and the values of the wall shear-stress as well as the heat transfer rate are presented in tabular form showing the effect of the buoyancy force and the applied magnetic field. To show the accuracy of the present method some typical values are compared with the available one. (author). 17 refs, 3 figs, 2 tabs
HEAT TRANSFER BY SIMULTANEOUS RADIATION-CONDUCTION AND CONVECTION IN A HIGH TEMPERATURE PACKED BED
Belghit, Abdelhamid
2007-01-01
A numerical model of a packed bed reactor for gasifying coal in mixed control using concentrated solar radiation is proposed. Case's normal-mode expansion technique is used to obtain solutions to the radiative transfer problem for the packed bed. The comparison between the radiative heat transfer and the exchanges by conduction and forced convection is analysed. The model permits the determination of temperature profiles for both the gas and the solid phases and the evolutions of thermal flux...
Mansour, M. A.
2013-01-01
The effects of magnetic force, acting vertically downward on natural convection within a nanofluid filled tilted trapezoidal enclosure saturated with an electrically conducting fluid have been investigated numerically. The bottom wall of the enclosure is subjected to a constant cold temperature and the top wall experiences a heat source whereas the remaining sidewalls are kept adiabatic. The physical problems are represented mathematically by different sets of governing equations along with t...
Sourtiji Ehsan; Hosseinizadeh Seyed Farid
2012-01-01
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 ...
Rahman, M. M.
2009-07-01
In this study, the steady laminar free-forced convective flow and heat transfer of micropolar fluids past a vertical radiate isothermal permeable surface with viscous dissipation and Joule heating is investigated numerically. The local similarity solutions for the flow, microrotation (angular velocity) and heat transfer characteristics are illustrated graphically for various material parameters. The effects of the pertinent parameters on the local skin friction coefficient, plate couple stress and the rate of heat transfer are also calculated. It was shown that micropolar fluids presented lower viscous drag and heat transfer values than those of the Newtonian fluids. The effect of radiation on the rate of heat transfer in a weakly concentrated micropolar fluid is higher than a strongly concentrated micropolar fluid. Results also show that full radiation has significant effect on the rate of heat transfer compared to the linear radiation.
Magnetic fields in the radiative interior of stars. I. Thermal shadows and forced convection
International Nuclear Information System (INIS)
It is pointed out that magnetic fields in the radiative interior of a star cause a slight reduction in the gas pressure, by a fraction of the order of B2/8?p. The effect is a proportionate reduction in the opacity and an increase in the effective heat transport coefficient, so that magnetic inhomogeneities cast thermal shadows in the general outflow of radiant energy. The shadows involve horizontal temperture gradients, forcing convective circulation in the otherwise stably stratified radiative zone. The associated vertical mixing may have important consequences for the thermonuclear burning of 7Li in the outer envelope of the star, taken up in the succeeding paper. The present paper provides simple quantitative illustrations of thermal shadows and the associated convection, yielding a direct relation between the strength of the magnetic inhomogeneity and the rate of circulation
Natural convection of heat generating fluid within horizontal cylinder
International Nuclear Information System (INIS)
In the fluid accompanied by nuclear fission reaction and radioactive decay, the natural convection occuring through internal heat generation, exerts large influence on the temperature distribution in the fluid and the thermal load distribution on a demarcation wall. Accordingly, for the safety design of tanks storing liquid nuclear fuel or radioactive waste solution it is important to accurately grasp the heat transfer characteristics of this natural convection. In this research, a horizontal circular pipe was taken up, and the examination of natural convection heat transfer was carried out. This system becomes practically important when the rate of heat generation of fluid is large, such as a dump tank for emergency use in a molten salt nuclear reactor. In this research, the Navier-Stokes equation and an energy equation were numerically analyzed by difference calculus, and temperature distribution and velocity distribution were theoretically determined in the range of Rayleigh number of 108, and the effect of various factors on the heat transfer was clarified. The rate of heat transfer was measured by electrically heating dilute NaCl solution, and compared with the theoretical results. (Kako, I.)
Forced convective transition boiling: review of literature and comparison of prediction methods
International Nuclear Information System (INIS)
This report reviews the published information on transition boiling heat transfer under forced convective conditions. It was found that transition boiling data have been obtained only within a limited range of conditions and many data are considered unreliable. The data do not permit the derivation of a correlation; however the parametric trends can be isolated from the data. Several authors have proposed correlations valid in the transition boiling region. Most of the correlations are valid only within a narrow range of conditions. A comparison with the data shows that in general agreement is poor. Hsu's correlation is tentatively recommended for low flows and pressures. (author)
A theoretical study of the spheroidal droplet evaporation in forced convection
Li, Jie; Zhang, Jian
2014-11-01
In many applications, the shape of a droplet may be assumed to be an oblate spheroid. A theoretical study is conducted on the evaporation of an oblate spheroidal droplet under forced convection conditions. Closed-form analytical expressions of the mass evaporation rate for an oblate spheroid are derived, in the regime of controlled mass-transfer and heat-transfer, respectively. The variation of droplet size during the evaporation process is presented in the regime of shrinking dynamic model. Comparing with the droplets having the same surface area, an increase in the aspect ratio enhances the mass evaporation rate and prolongs the burnout time.
Guedes, Rodrigo Otavio De Castro
Thermal entry laminar and turbulent forced convection problems inside ducts are solved. In Chapter 1, the steady-state conjugated turbulent forced convection heat transfer inside a parallel-plate channel with axial conduction in the wall and convection boundary conditions is studied. A lumped wall model that neglects transverse temperature gradients in the solid but takes into account the axial heat conduction along the wall is adopted. The effects of the conjugation parameter, Biot number, and the dimensionless channel length on Nusselt number and fluid bulk and wall temperatures are systematically investigated. In Chapter 2, the transient conjugated turbulent heat transfer with axial conduction in the wall and convection boundary conditions for flow in a parallel-plate channel subjected to periodically varying inlet temperature is studied. The lumped wall model adopted in Chapter 1 is used and the effects of the conjugation parameter, fluid-to-solid heat capacitance ratio, and Biot number on the periodic responses of fluid bulk and wall temperatures and wall heat flux is investigated. In Chapter 3, the transient forced convection in turbulent channel flow with a step change in inlet temperature is solved by using a hybrid analytical-numerical scheme. Numerical results are presented for the fluid bulk temperature and Nusselt number as a function of position along the channel at different times, and the propagation of the thermal wave front is examined. In Chapter 4, the transient forced convection in laminar flow inside a parallel-plate channel subjected to periodically varying inlet temperature is solved by using a hybrid analytical-numerical scheme. Semi-analytical results are presented for the variations in the amplitude of periodically varying fluid bulk temperature and wall heat flux along the channel length for different frequencies of oscillation. An approximate formula for the decay of the peak bulk temperature amplitude is developed. Finally, in Chapter 5, the transient heat transfer in forced convection for simultaneously developing laminar flow inside a parallel-plate channel is studied by solving the steady momentum equation with the generalized integral transform technique and the transient energy equation through a hybrid numerical-analytical scheme. Semi-analytical results are then presented for the fluid bulk temperature and local Nusselt number along the channel as a function of position and time.
International Nuclear Information System (INIS)
The results of an experimental study of the forced convective heat transfer of flowing gas-solid suspensions at high temperature are presented. Results are reported for helium-graphite mixtures which are electrically heated with a constant wall temperature condition up to T sub(W) = 1173K, flowing upward through a vertical circular tube; entering gas Reynolds number R sub(ei) ranged from 1.0 x 104 to 2.0 x 104, particle loading ratio GAMMA reached about 4. The following conclusions are drawn from the current study. i) In the case of R sub(ei) = 1.0 x 104, the Nusselt numbers of the forced convective heat transfer of gas solid suspensions, N sub(ub), based on local bulk properties increase monotonously with increase in the loading ratio GAMMA. On the other hand, in the runs of R sub(ei) = 1.5 x 104 and 2.0 x 104, the Nusselt numbers of suspensions, N sub(ub), have a minimum value at low solid loadings. ii) The ratio of the Nusselt number of the suspension to that of gas alone increases considerably as the wall temperatures increase. (author)
Energy Technology Data Exchange (ETDEWEB)
Mokhtari, F [Physics Department, Faculty of Science, University of Mouloud Mammeri, Tizi Ouzou (Algeria); Bouabdallah, A; Zizi, M [LTSE Laboratory, University of Science and Technology USTHB. BP 32 Elalia, Babezzouar, Algiers (Algeria); Hanchi, S [UER Mecanique/ E.M.P B.P 17, Bordj El Bahri, Algiers (Algeria); Alemany, A, E-mail: abouab2002@yahoo.f [Laboratoire EPM, CNRS, Grenoble (France)
2010-03-01
In order to understand the influence of a semispherical crucible geometry combined with different convection modes as a thermocapillary convection, natural convection and forced convection, induced by crystal rotation, on melt flow pattern in silicon Czochralski crystal growth process, a set of numerical simulations are conducted using Fluent Software. We solve the system of equations of heat and momentum transfer in classical geometry as cylindrical and modified crystal growth process geometry as cylindro-spherical. In addition, we adopt hypothesis adapted to boundary conditions near the interface and calculations are executed to determine temperature, pressure and velocity fields versus Grashof and Reynolds numbers. The analysis of the obtained results led to conclude that there is advantage to modify geometry in comparison with the traditional one. The absence of the stagnation regions of fluid in the hemispherical crucible corner and the possibility to control the melt flow using the crystal rotation enhances the quality of the process comparatively to the cylindrical one. The pressure field is strongly related to the swirl velocity.
Heat transfer mechanisms in bubbly Rayleigh-Benard convection
Oresta, Paolo; Lohse, Detlef; Prosperetti, Andrea
2008-01-01
The heat transfer mechanism in Rayleigh-Benard convection in a liquid with a mean temperature close to its boiling point is studied through numerical simulations with point-like vapor bubbles, which are allowed to grow or shrink through evaporation and condensation and which act back on the flow both thermally and mechanically. It is shown that the effect of the bubbles is strongly dependent on the ratio of the sensible heat to the latent heat as embodied in the Jacob number Ja. For very small Ja the bubbles stabilize the flow by absorbing heat in the warmer regions and releasing it in the colder regions. With an increase in Ja, the added buoyancy due to the bubble growth destabilizes the flow with respect to single-phase convection and considerably increases the Nusselt number.
Geothermal Heating, Convective Flow and Ice Thickness on Mars
Rosenberg, N. D.; Travis, B. J.; Cuzzi, J.
2001-01-01
Our 3D calculations suggest that hydrothermal circulation may occur in the martian regolith and may significantly thin the surface ice layer on Mars at some locations due to the upwelling of warm convecting fluids driven solely by background geothermal heating. Additional information is contained in the original extended abstract.
Heat convection in a set of three vertical cylinders
International Nuclear Information System (INIS)
Experimental results on temperature and heat flow in a set of three vertical cylinders with internal generation of heat, water submerged and in free convection are presented in this work . Temperature distribution, Nusselt number and convective coefficient (h) for each rod, developed for the distance between the axis of cylinders in vertical position, as a consequence of the application of power in its outside, are analyzed. Experimental information about heat transfer by free convection in vertical cylinders and surfaces is analyzed. Information of the several author who have carried out studies about the heat transfer on vertical cylinders was compiled, and the proposed equations with the experimental data obtained in the thermo fluids laboratory of National Institute of Nuclear Research (ININ) were tested. The way in which separation distance, s, distribution temperature array, Nusselt number, and convective coefficient calculated for the proposed channel with the Keyhani, Dutton and experimental equations are tabulated and they are plotted for each power value and for each separation between rods. The scheme of the used equipment and the experimentation description as well as the observations of tests and graphical results are included. (Author)
Natural convection heat transfer in moderate aspect ratio enclosures
International Nuclear Information System (INIS)
Local and average heat transfer coefficients for natural convection between parallel plates separated by slats to create enclosures of moderate aspect ratio have been experimentally determined using an interferometric technique. The effects of Rayleigh number, tilt and slat angle, and aspect ratio on the Nusselt number have been determined. The Rayleigh number range tested was up to 7 x 104, and the aspect ratio (ratio of enclosure length to plate spacing) varied between 0.25 and 4. The angles of tilt of the enclosure with respect to the horizontal were 45, 60 and 90 deg. Slat angles of 45, 60 90 and 135 deg were studied. The results obtained in a previous investigation [1] for aspect ratios of 9 to 36 are included to show continuity. The results indicate that the convective heat transfer is a strong function of the aspect ratio for aspect ratios less than 4. For aspect ratios in the range of 0.5 to 4, spacers between the plates increase, rather than decrease, natural convection heat transfer compared to that for long enclosures. Slat angles less than 90 deg (i.e., oriented downward) reduce convective heat transfer
Effect of radiation on the laminar convective heat transfer through a layer of highly porous medium
International Nuclear Information System (INIS)
A numerical investigation is reported of the coupled forced convective and radiative transfer through a highly porous medium. The porosity range investigated is high enough that the fluid inertia terms in the momentum equation cannot be neglected; i.e., the simple form of Darcy's law is invalid. The geometry studied is a plane layer of highly porous medium resting on one impermeable boundary and exposed to a two-dimensional laminar external flow field. The objective is to determine the effective overall heat transfer coefficients for such a geometry. The results are applicable to diverse situations, including insulation batts exposed to external flow, the heat loss and drying rates of grain fields and forest areas, and the drying of beds of porous material exposed to convective and radiative heating
Passive techniques for the enhancement of convective heat transfer in single phase duct flow
Rainieri, S.; Bozzoli, F.; Cattani, L.
2014-11-01
This review presents the main results of the experimental campaign on passive techniques for the enhancement of forced convective single phase heat transfer in ducts, performed in the last years at the Laboratory of the Industrial Engineering Department of the University of Parma by the Applied Physics research group. The research was mainly focused on two passive techniques, widely adopted for the thermal processing of medium and high viscosity fluids, based on wall corrugation and on wall curvature. The innovative compound heat transfer enhancement technique that couples together the effect of wall curvature and of wall corrugation has been investigated as well. The research has been mainly focused on understanding the causal relationship between the heat transfer surface modification and the convection enhancement phenomenon, by accounting the effect of the fluid Prandtl number. The pressure loss penalties were also evaluated. The principal results are presented and discussed.
Network model of free convection within internally heated porous media
International Nuclear Information System (INIS)
A hypothetical core-disruptive accident (HCDA) in a liquid metal fast breeder reactor (LMFBR) may result in the formation of an internally heated debris bed. Considerable attention has been given to postulated mechanisms by which such beds may be cooled. It is the purpose of the work described to demonstrate a method for computing the heat transfer from such a bed to the overlying sodium pool due to single-phase, free convection
Natural convection inside enclosures partially heated at one side
Corcione, M.; Fontana, L.; Habib, E.
2008-01-01
Steady laminar natural convection heat transfer inside airfilled square cavities cooled at one side and partially heated at the opposite side, is studied numerically. A computational code based on SIMPLE-C algorithm is used for the solution of the mass, momentum, and energy transfer equations. Simulations are performed for different values of the dimensionless size and location of the heater, and the Rayleigh number, whose effects on the temperature and velocity fields, a...
Relating Convective and Stratiform Rain to Latent Heating
Tao, Wei-Kuo; Lang, Stephen; Zeng, Xiping; Shige, Shoichi; Takayabu, Yukari
2010-01-01
The relationship among surface rainfall, its intensity, and its associated stratiform amount is established by examining observed precipitation data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). The results show that for moderate-high stratiform fractions, rain probabilities are strongly skewed toward light rain intensities. For convective-type rain, the peak probability of occurrence shifts to higher intensities but is still significantly skewed toward weaker rain rates. The main differences between the distributions for oceanic and continental rain are for heavily convective rain. The peak occurrence, as well as the tail of the distribution containing the extreme events, is shifted to higher intensities for continental rain. For rainy areas sampled at 0.58 horizontal resolution, the occurrence of conditional rain rates over 100 mm/day is significantly higher over land. Distributions of rain intensity versus stratiform fraction for simulated precipitation data obtained from cloud-resolving model (CRM) simulations are quite similar to those from the satellite, providing a basis for mapping simulated cloud quantities to the satellite observations. An improved convective-stratiform heating (CSH) algorithm is developed based on two sources of information: gridded rainfall quantities (i.e., the conditional intensity and the stratiform fraction) observed from the TRMM PR and synthetic cloud process data (i.e., latent heating, eddy heat flux convergence, and radiative heating/cooling) obtained from CRM simulations of convective cloud systems. The new CSH algorithm-derived heating has a noticeably different heating structure over both ocean and land regions compared to the previous CSH algorithm. Major differences between the new and old algorithms include a significant increase in the amount of low- and midlevel heating, a downward emphasis in the level of maximum cloud heating by about 1 km, and a larger variance between land and ocean in the new CSH algorithm.
Zhang, Nan; Park, Hyun Gyoon; Derby, Jeffrey J.
2013-03-01
Quasi-steady-state (QSS) and transient models, developed using the CrysMAS code, are employed to study the effects of transport mechanisms and cold finger design on the temperature distribution, melt flow field, and melt-crystal interface shape during the crystal growth of sapphire by a small-scale, modified heat exchanger method (HEM). QSS computations show the importance and effects of various heat transfer mechanisms in the crystal and melt, including conduction, internal radiation, and melt convection driven by buoyant and Marangoni forces. The design of the cold finger is demonstrated to have significant effects on growth states. Notably, transient computations on an idealized heat transfer model, supplemented with QSS calculations of a model with rigorous heat transfer representation, show that non-uniform growth conditions arise under uniform cooling of the system via a linear decrease in furnace set points. We suggest that more uniform HEM growth conditions may be achieved by using non-linear cool-down strategies.
International Nuclear Information System (INIS)
Numerical analysis has been conducted for combined free and forced laminar convection of liquid metals in a horizontal pipe which is isothermally heated from a certain axial location. The steady-state solutions have been obtained in consideration of axial conduction for Peclet numbers of 0.3 ? 7.5 and Rayleigh numbers of 15 ? 500. The results reveal marked effects of axial conduction on the flow and heat transfer characteristics of liquid metals. At the entrance to the heated section, the secondary flow has already developed and a reverse flow occurs near the pipe top as the buoyancy effect becomes large. Consequently, the circumferential average Nusselt number decreases with increasing secondary flow in comparison with that observed in moderate and large Prandtl-number flows. The regime of reverse flow is clearly identified in the Pe-Ra (Peclet-Rayleigh) coordinates. (author)
The effects of magnetic field on forced and free convection flow
Abdel-Khalek, M M
2003-01-01
The effects of magnetic field and permeability of the porous medium on unsteady forced and free convection flow past an infinite vertical porous plate have been studied when the temperature of the plate is oscillating with time about a constant nonzero mean value and in the presence of temperature dependent heat source. The governing equations for the hydromagnetic fluid flow and the heat transfer are solved subject to the relevant boundary conditions. The perturbation technique is used to obtain expressions for velocity field, skin friction and Nusselt number. Also, the effects of magnetic parameter, heat source parameter, suction parameter and permeability of porous medium on velocity field, skin friction and Nusselt number are discussed.
Time-dependent mixed convection heat transfer from a sphere in a micro-gravity environment
International Nuclear Information System (INIS)
A fundamental problem of interest for crystal growth in micro-gravity applications involves the mixed convection heat transfer from a sphere in a uniform flow of fluid at a differing temperature. Under the combined influence of the imposed free stream as well as an induced buoyancy force due to thermal expansion of the fluid, the heat transfer from the sphere will be different from that of either the pure forced convection flow or the pure free convection flow. For the present study, the method of matched asymptotic expansions is applied to the laminar flow problem of an impulsively heated, impulsively started sphere in an originally quiescent fluid. Time series expansions are developed for the dependent variables by acknowledging the existence of two district regions: one, an inner region, near the sphere, in which viscous effects are significant; and two, an outer region in which the fluid may be treated as inviscid. The time series expansions are developed in terms of the Reynolds number and Richardson number (Buoyancy Parameter), and the relevant heat transfer and drag coefficients are calculated and plotted
Convective heat transport in compressible fluids
Furukawa, Akira; Onuki, Akira
2002-07-01
We present hydrodynamic equations of compressible fluids in gravity as a generalization of those in the Boussinesq approximation used for nearly incompressible fluids. They account for adiabatic processes taking place throughout the cell (the piston effect) and those taking place within plumes (the adiabatic temperature gradient effect). Performing two-dimensional numerical analysis, we reveal some unique features of plume generation and convection in transient and steady states of compressible fluids. As the critical point is approached, the overall temperature changes induced by plume arrivals at the boundary walls are amplified, giving rise to overshoot behavior in transient states and significant noise in the temperature in steady states. The velocity field is suggested to assume a logarithmic profile within boundary layers. Random reversal of macroscopic shear flow is examined in a cell with unit aspect ratio. We also present a simple scaling theory for moderate Rayleigh numbers.
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)
Mixed convection heat transfer in concave and convex channels
Energy Technology Data Exchange (ETDEWEB)
Moukalled, F.; Doughan, A.; Acharya, S.
1997-07-01
Mixed convection heat transfer studies in the literature have been primarily confined to pipe and rectangular channel geometry's. In some applications, however, heat transfer in curved channels may be of interest (e.g., nozzle and diffuser shaped passages in HVAC systems, fume hoods, chimneys, bell-shaped or dome-shaped chemical reactors, etc.). A numerical investigation of laminar mixed convection heat transfer of air in concave and convex channels is presented. Six different channel aspects ratios (R/L = 1.04, 1.25, 2.5, 5, 10, and {infinity}) and five different values of Gr/Re{sup 2} (Gr/Re{sup 2} = 0, 0.1, 1, 3, 5) are considered. Results are displayed in terms of streamline and isotherm plots, velocity and temperature profiles, and local and average Nusselt number estimates. Numerical predictions reveal that compared to straight channels of equal height, concave channels of low aspect ratio have lower heat transfer at relatively low values of Gr/Re{sup 2} and higher heat transfer at high values of Gr/Re{sup 2}. When compared to straight channels of equal heated length, concave channels are always found to have lower heat transfer and for all values of Gr/Re{sup 2}. On the other hand, predictions for convex channels revealed enhancement in heat transfer compared to straight channels of equal height and/or equal heated length for all values of Gr/Re{sup 2}.
Dendrite growth under forced convection: analysis methods and experimental tests
Alexandrov, D. V.; Galenko, P. K.
2014-08-01
An analysis is given of the nonisothermal growth of a dendrite crystal under forced fluid flow in a binary system. The theoretical model utilized employs a free moving crystal–liquid interface and makes use of the Oseen approximation for the equations of motion of the liquid. A criterion for the stable growth of two-dimensional and three-dimensional parabolic dendrites is derived under the assumption of an anisotropic surface tension at the crystal–liquid interface, which generalizes the previous known results for the stable growth of a dendrite with convection in a one-component fluid and for the growth of a dendrite in a two-component system at rest. The criterion obtained within the Oseen hydrodynamic approximation is extended to arbitrary Peclet numbers and dendrite growth with convection in a nonisothermal multicomponent system. Model predictions are compared with experimental data on crystal growth kinetics in droplets processed in electromagnetic and electrostatic levitation facilities. Theoretical and simulation methods currently being developed are applied to crystallization processes under earthly and reduced gravity conditions.
Energy Technology Data Exchange (ETDEWEB)
Oosthuizen, P.H.; Chen, T.S.; Acharya, S.; Armaly, B.F.; Pepper, D.W. [eds.
1997-07-01
This volume contains a portion of the over 240 ASME papers which were presented at the conference. For over 40 years, the National Heat Transfer Conference has been the premiere forum for the presentation and dissemination of the latest advances in heat transfer. The work contained in these volumes range from studies of fundamental phenomena to applications in the latest heat transfer equipment. The following topics are covered in this volume: fundamentals of convection; turbulent heat transfer; and mixed convection heat transfer. Separate abstracts were prepared for most papers in this volume.
Energy Technology Data Exchange (ETDEWEB)
Oosthuizen, P.H.; Sun, L. [Queen' s Univ., Dept. of Mechanical Engineering, Kingston, Ontario (Canada)]. E-mail: oosthuiz@me.queensu.ca; sun@me.queensu.ca; Naylor, D. [Ryerson Univ., Dept. of Mechanical, Aerospace and Industrial Engineering, Toronto, Ontario (Canada)]. E-mail: dnaylor@ryerson.ca
2003-07-01
Natural convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat surfaces (termed 'slats') near the heated surface, these surfaces being, in general, inclined to the heated surface. There is, in general, a uniform heat generation in the slats. The slats are pivoted about their centre-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air being considered. The heat generation in the slats in this situation is the result of solar radiation passing through the window and falling on and being absorbed by the slats of the blind. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. The solution has the following parameters: (1) the Rayleigh number (2) the Prandtl number (3) the dimensionless heat generation rate in the slats per unit frontal area (4) the dimensionless distance of the slat center point (the pivot point) from the surface (5) the dimensionless slat size (6) the dimensionless slat spacing (7) the angle of inclination of the slats. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated vertical surface has been examined. (author)
International Nuclear Information System (INIS)
Natural convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat surfaces (termed 'slats') near the heated surface, these surfaces being, in general, inclined to the heated surface. There is, in general, a uniform heat generation in the slats. The slats are pivoted about their centre-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air being considered. The heat generation in the slats in this situation is the result of solar radiation passing through the window and falling on and being absorbed by the slats of the blind. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. The solution has the following parameters: (1) the Rayleigh number (2) the Prandtl number (3) the dimensionless heat generation rate in the slats per unit frontal area (4) the dimensionless distance of the slat center point (the pivot point) from the surface (5) the the pivot point) from the surface (5) the dimensionless slat size (6) the dimensionless slat spacing (7) the angle of inclination of the slats. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated vertical surface has been examined. (author)
Directory of Open Access Journals (Sweden)
Sumon Saha
2006-10-01
Full Text Available Combined free convection and forced convection from a flush-mounted uniform heat source on the bottom of a horizontal rectangular enclosure with side openings is studied numerically. The inlet opening allows an externally induced air stream at the ambient temperature to flow through the cavity and exits from another two openings placed top of the both side walls. Two-dimensional forms of Navier-Stokes equations are solved by using control volume based finite element technique. Three typical values of the Reynolds numbers, based on the enclosure height, are chosen as Re = 50, 100 and 200, and steady, laminar results are obtained in the range of Richardson number as 0 = Ri = 10 and a fixed Prandtl number of 0.71. The parametric studies for a wide range of governing parameters show consistent performance of the present numerical approach to obtain as stream functions and temperature profiles. Heat transfer rates at the heated walls are presented in terms of average Nusselt numbers. The computational results indicate that the heat transfer coefficient is strongly affected by Reynolds number and Richardson number. An empirical correlation is developed by using Nusselt number, Reynolds number and Richardson number.
A mechanistic model for forced convective transition boiling of subcooled water in vertical tubes
International Nuclear Information System (INIS)
A mechanistic model for forced convective transition boiling has been developed to predict transition boiling heat flux realistically. This model is based on a postulated multi-stage boiling process occurring during the passage time of an elongated vapor blanket specified at a critical heat flux condition. Between the departure from nucleate boiling (DNB) and the departure from film boiling (DFB) points, the boiling heat transfer is established through three boiling stages, namely, the macrolayer evaporation and dryout governed by nucleate boiling in a thin liquid film and the unstable film boiling. The total heat transfer rate during the transition boiling is the sum of the heat transfer rates after the DNB weighted by the time fractions of each stage, which are defined as the ratio of each stage duration to the vapor blanket passage time. The model predictions are compared with some available experimental transition boiling data. From these comparisons, it can be seen that the transition boiling heat fluxes including the maximum heat flux and the minimum film boiling heat flux are well predicted at low qualities/high pressures near 10 bar. 8 figs., 1 tab., 32 refs. (Author)
Gaddamwar, S. S.; Shelke, R. S.
2012-01-01
In Indian Coal mines, underground mines, mineral ore mines syngases at high pressure and high temperature are found in large quantities. This syngas is highly toxic, harmful and flammable gas which will be present in atmosphere which may cause many accidents. Hence it is necessary to reduce the content of syngas from mines in the atmosphere. This paper describes literature review of Convective heat transfer characteristics of high pressure gas used in mines. Heat transfer in convection coolin...
Extinction transition in bacterial colonies under forced convection
Neicu, T; Larochelle, D A; Kudrolli, A
2000-01-01
We report the spatio-temporal response of {\\it Bacillus subtilis} growing on a nutrient-rich layer of agar to ultra-violet (UV) radiation. Below a crossover temperature, the bacteria are confined to regions that are shielded from UV radiation. A forced convection of the population is effected by rotating a UV radiation shield relative to the petri dish. The extinction speed at which the bacterial colony lags behind the shield is found to be qualitatively similar to the front velocity of the colony growing in the absence of the hostile environment as predicted by the model of Dahmen, Nelson and Shnerb. A quantitative comparison is not possible without considering the slow dynamics and the time-dependent interaction of the population with the hostile environment.
Evaporation of a binary liquid film by forced convection
Directory of Open Access Journals (Sweden)
Nasr Abdelaziz
2011-01-01
Full Text Available This paper deals with a numerical analysis of the evaporation of a thin binary liquid film by forced convection inside a channel constituted by two parallel plates. The first plate is externally insulated and wetted by a thin water ethylene glycol film while the second is dry and isothermal. The liquid mixture consists of water (the more volatile component and ethylene glycol while the gas mixture has three components: dry air, water vapour and ethylene-glycol vapour. The set of non linear and coupled equations expressing the conservation of mass, momentum, energy and species in the liquid and gas mixtures is solved numerically using a finite difference method. Results concerns with the effects of inlet ambience conditions and the inlet liquid concentration of ethylene glycol on the distribution of the temperature, concentrations profiles and the axial variation of the evaporation rate of species i.
Heat transfer in natural convection in horisontal fuel element bundles
International Nuclear Information System (INIS)
The heat exchange in case of natural convection in horizontal fuel elements bundles enclosed in the can as applied to the conditions arising during transportation of spent fuel elements in horizontal transport containers is considered. The investigation is performed using horizontal bundles with different shape of the can and parameters of bundle spacing with different numbers of series and values of relative step of hundle location when filling the bundles space by air, carbon dioxide, water and MS-20 oils under pressure up to 1.8 MPa. The variation of distribution of rod temperatures and the form of coolant isotherms in the cross section of bundles with the development of natural convection are considered. The dependences which enable one at the designed heat release to calculate the medium temperature of rods and the temperature of the maximum heated rod in the bundle are suggested
Free and forced convective-diffusion solutions by finite element methods
International Nuclear Information System (INIS)
Several free and forced convective-diffusion examples are solved and compared to either laboratory experiment or closed-form analysis. The problems solved illustrate the application of finite element methods to both strongly-coupled and weakly-coupled velocity and temperature fields governed by the steady-state momentum and energy equations. Special attention is given to internal forced convection with temperature-dependent viscosity and free convection within an enclosure
Single-phase convective heat transfer in rod bundles
International Nuclear Information System (INIS)
The convective heat transfer for turbulent flow through rod bundles representative of nuclear fuel rods used in pressurized water reactors is examined. The rod bundles consist of a square array of parallel rods that are held on a constant pitch by support grids spaced axially along the rod bundle. Split-vane pair support grids, which create swirling flow in the rod bundle, as well as disc and standard support grids are investigated. Single-phase convective heat transfer coefficients are measured for flow downstream of support grids in a rod bundle. The rods are heated using direct resistance heating, and a bulk axial flow of air is used to cool the rods in the rod bundle. Air is used as the working fluid instead of water to reduce the power required to heat the rod bundle. Results indicate heat transfer enhancement for up to 10 hydraulic diameters downstream of the support grids. A general correlation is developed to predict the heat transfer development downstream of support grids. In addition, circumferential variations in heat transfer coefficients result in hot streaks that develop on the rods downstream of split-vane pair support grids
Sawant, S. M.; Gururaja Rao, C.
2008-10-01
The problem of combined conduction-mixed convection-surface radiation from a vertical electronic board provided with three identical flush-mounted discrete heat sources is solved numerically. The cooling medium is air that is considered to be radiatively transparent. The governing equations for fluid flow and heat transfer are converted from primitive variable form to stream function-vorticity formulation. The equations, thus obtained, are normalised and then are converted into algebraic form using a finite volume based finite difference method. The resulting algebraic equations are then solved using Gauss-Seidel iterative method. An optimum grid system comprising 151 grids along the board and 111 grids across the board is chosen. The effects of various parameters, such as modified Richardson number, surface emissivity and thermal conductivity on temperature distribution along the board, maximum board temperature and relative contributions of mixed convection and radiation to heat dissipation are studied in detail. Further, the contributions of free and forced convection components of mixed convection to board temperature distribution and peak board temperature are brought out. The exclusive roles played by surface radiation and buoyancy in the present problem are clearly elucidated.
Hsu, C. H.; Yang, S. A.
A model is developed for the study of mixed convection film condensation from downward flowing vapors onto a sphere with uniform wall heat flux. The model combined natural convection dominated and forced convection dominated film condensation, including effects of pressure gradient and interfacial vapor shear drag has been investigated and solved numerically. The separation angle of the condensate film layer, ?s is also obtained for various pressure gradient parameters, P* and their corresponding dimensionless Grashof 's parameters, Gr*. Besides, the effect of P* on the dimensionless mean heat transfer, will remain almost uniform with increasing P* until for various corresponding available values of Gr*. Meanwhile, the dimensionless mean heat transfer, is increasing significantly with Gr* for its corresponding available values of P*. For pure natural-convection film condensation, is obtained. Zusammenfassung Es wird ein Modell zur Untersuchung der Mischkonvektion bei Filmkondensation von Dämpfen an einer Kugel entwickelt, die unter gleichförmigen Wärmefluß daran abwärts strömen. Das Modell verbindet die durch natürliche und durch erzwungene Konvektion bewirkte Filmkondensation unter Einschluß von aus einem Druckgradienten resultierenden Effekten sowie von Dampfschubspannungen an der Phasengrenze. Die numerische Lösung liefert den Separationswinkel des Kondensatfilms ?s für verschiedene Druckgradienten-Parameter P* und zugehörige Grashof-Parameter Gr*. Der Einfluß von P* auf den mittleren Wärmeübergangsparameter bleibt bis ziemlich gering, auch wenn Gr* zwischen 0.01 und 100 variiert. Für reine natürliche Filmkondensation erhält man:
Numerical study of natural convection heat loss of heat pipe receiver for dish/Stirling system
Energy Technology Data Exchange (ETDEWEB)
Xu, Hui; Zhang, Hong; Zhuang, Jun [Nanjing Univ. of Technology (China). Inst. of Thermal Energy Engineering
2008-07-01
The receiver of dish/Stirling solar thermal power system is a critical component which must absorb incoming concentrated solar energy and deliver thermal power to the engine working fluid at high-flux from 40 to 80 W/cm{sup 2} and temperatures of 650-850 C. The heat loss of the receiver has serious influence upon the system performance. In this paper, a numerical study on the major convection heat loss from a heat pipe receiver of dish/stirling system is presented. The influence of the operating temperature, the inclination angle and aperture size on the total heat loss from the receiver was investigated. The effect of the temperature varying from 650 to 850 on heat loss of the receiver was simulated. The inclination angle ranges between 0 and 90 to estimate the influence of inclination upon convection heat loss of the heat pipe receiver. The receiver aperture is always optimized to be just large enough to admit most of the concentrated sunlight, but small enough to limit radiation and convection heat loss. The effect of aperture diameter to the heat loss is also investigated. The results indicate that the natural convection heat loss is decrease monotonically with the increase of the inclination angle; the aperture diameter is a critical parameter in design of a solar receiver. The investigation conclusions can be used as reference information for the heat pipe receiver design and application. (orig.)
Natural Convection Heat Transfer Experiments on an Inclined Helical Coil
International Nuclear Information System (INIS)
Research interests for the compact heat exchanger increase with growing needs on compact nuclear systems. Accordingly, the heat exchangers of helical coil types are adopted replacing the once-through type and the U-tube type ones. When the helical coil type heat exchangers are used for nuclear propulsions, the heat transfer of inclined helical coil becomes an important problem due to the shaking of the ship. This study measured the natural convection heat transfer from the outside surface of the helical coil in a circular duct varying the coil inclination and turn number. It is also conducted using a circular duct having same height of the helical coil. Based upon the analogy concept, a mass transfer system was used instead of a heat transfer system. A cupric acid-copper sulfate electroplating system was employed as the mass transfer systems
International Nuclear Information System (INIS)
The report describes an experiment on natural convective heat transfer of silicone oil and water around a horizontal cylinder in a rectangular box and on the visualization of temperature distribution by the liquid-crystal suspension method. The natural convection around the heated cylinder in the box is effected by the thermal stratification and plume. Experimental results are summarized as follows. The Nusselt numbers on the heated horizontal cylinder are correlated with the Ra numbers. The reference fluid temperature on the heat transfer is determined from the thermal stratification in the box. The swaying motion of the thermal plume is effected by the stratification. (author)
Energy Technology Data Exchange (ETDEWEB)
Hassanipour, F.; Lage, J. [Southern Methodist Univ., Dallas, TX (United States)
2009-07-01
Forced convection heat transfer in a phase change material (PCM) suspension in a rectangular duct was investigated. PCM particle sizes varied. Time-dependent moving-mesh computational fluid dynamics (CFD) models were used to simulate heat transfer processes for each particle and a series of melting and solidification processes. The effect of particle dimension on the heat transfer coefficient was also investigated. A simplified theoretical analysis was performed using a 3-D model of the heated channel. The liquid fraction was computed at each iteration in relation to an enthalpy balance. Enthalpy was computed as the sum of the sensible enthalpy and the latent heat. The finite volume method was used to derive the convection heat transfer coefficient. The model was validated by comparing it with results obtained during earlier experiments. Results of the study demonstrated that the size and fit of the particles plays a significant role in heat transfer efficiency. It was concluded that the heat transfer coefficient attains its maximum value for particles that are almost identical in size as the channel. 4 refs., 3 tabs., 7 figs.
Energy Technology Data Exchange (ETDEWEB)
Bates, J.M.; Khan, E.U.
1980-10-01
An experimental study was performed to obtain local fluid velocity and temperature measurements in the mixed (combined free and forced) convection regime for specific flow coastdown transients. A brief investigation of steady-state flows for the purely free-convection regime was also completed. The study was performed using an electrically heated 2 x 6 rod bundle contained in a flow housing. In addition a transient data base was obtained for evaluating the COBRA-WC thermal-hydraulic computer program (a modified version of the COBRA-IV code).
International Nuclear Information System (INIS)
Magnetohydrodynamic (MHD) fully developed flow of a viscous incompressible electrically conducting fluid in a vertical channel during combined convection, with asymmetric heating of the wall, under the influence of a constant pressure gradient and in the presence of an uniform transverse magnetic field, is studied. Exact solution of the governing equation is obtained in a closed form. The solution in a dimensionless form contains two pertinent flow parameters, viz. M (the Hartmann number) and Gr (the Grashof number). The limiting cases of a MHD forced and free convection are analysed, what has not been done earlier in the literature. The occurrence of flow reversal indicates that there arises a flow reversal at the cold wall when rT=1 while, for rT<1, no flow reversal is possible in the absence of magnetic forces. (author)
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.
Silk cocoon drying in forced convection type solar dryer
International Nuclear Information System (INIS)
The thin layer silk cocoon drying was studied in a forced convection type solar dryer. The drying chamber was provided with several trays on which the cocoons loaded in thin layer. The hot air generated in the solar air heater was forced into drying chamber to avoid the direct exposure of sunlight and UV radiation on cocoons. The drying air temperature varied from 50 to 75 oC. The cocoon was dried from the initial moisture content of about 60-12% (wb). The drying data was fitted to thin layer drying models. Drying behaviour of the silk cocoon was best fitted with the Wang and Singh drying model. Good agreement was obtained between predicted and experimental values. Quality of the cocoons dried in the solar dryer was at par with the cocoons dried in the conventional electrical oven dryer in term of the silk yield and strength of the silk. Saving of electrical energy was about 0.75 kWh/kg cocoons dried. Economic analysis indicated that the NPV of the solar dryer was higher and more stable (against escalation rate of electricity) as compare to the same for electrical oven dryer. Due to simplicity in design and construction and significant saving of operational electrical energy, solar cocoon dryer seems to be a viable option.
Energy Technology Data Exchange (ETDEWEB)
Schwab, A.
2002-12-01
When condensation effects with initially inexplicable causes occurred during experimental studies on ventilated facades, the team around Prof. Edgar R. F. Winter and the development engineers from the company Gartner, Gundelfingen, decided to investigate this phenomenon. As part of a research project lasting several years, studies on ventilated vertical gaps with rectangular section were therefore carried out with the aim of deriving fundamental statements about the heat transport due to convection in ventilated claddings and windows, both for winter and summer boundary conditions. The results not only contribute to the energetic evaluation of double-skin windows and facades. They are also applicable to the heat transport processes in other technical areas where free, forced or mixed convection occurs. (orig.)
A correlation for natural convection heat transfer from inclined plate-finned heat sinks
International Nuclear Information System (INIS)
Steady-state natural convection heat transfer from inclined plate-finned heat sinks to air is numerically investigated by using an experimentally validated model. The heat sinks with parallel arrangement of uniform rectangular cross section plate fins are inclined from the vertical in both forward and backward directions in order to investigate the effect of inclination on convection. Our previously validated numerical model for vertically oriented heat sinks is directly used without changing any model parameters, but only by varying the direction of the gravitational acceleration to create the effect of inclination. The flow and temperature fields are resolved using a finite volume computational fluid dynamics code. Performing a large number of simulations for the heat sink base inclination angles of ±4°, ±10°, ±20°, ±30°, ±45°, ±60°, ?65°, ?70°, ±75°, ±80°, ±85°, ±90° from the vertical, the dependence of the convective heat-transfer rate to the inclination angle and Rayleigh number is investigated. Scale analyses are performed in order to generalize estimates for the convection heat-transfer rates. A single correlation is suggested and shown to be valid for a very wide range of angles from ?60° (upward) to +80° (downward) in a wide range of Rayleigh numbers from 0 to 2 × 108. -- Highlights: ? Natural convection heat transfer from inclined plate-finned heat sinks is investigated. ? A correlation for estimating convection heat-transfer rates is suggested. ? The correlation is shown to be valid in a very wide range of angles, ?60° ? ? ? +80°. ? The correlation is verified with all available experimental data in literature. ? Flow separation and fin height play the most significant roles at high inclinations
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Schumacher, Courtney
2012-12-13
Heating associated with tropical cloud systems drive the global circulation. The overall research objectives of this project were to i) further quantify and understand the importance of heating in tropical convective cloud systems with innovative observational techniques, and ii) use global models to determine the large-scale circulation response to variability in tropical heating profiles, including anvil and cirrus cloud radiative forcing. The innovative observational techniques used a diversity of radar systems to create a climatology of vertical velocities associated with the full tropical convective cloud spectrum along with a dissection of the of the total heating profile of tropical cloud systems into separate components (i.e., the latent, radiative, and eddy sensible heating). These properties were used to validate storm-scale and global climate models (GCMs) and were further used to force two different types of GCMs (one with and one without interactive physics). While radiative heating was shown to account for about 20% of the total heating and did not have a strong direct response on the global circulation, the indirect response was important via its impact on convection, esp. in how radiative heating impacts the tilt of heating associated with the Madden-Julian Oscillation (MJO), a phenomenon that accounts for most tropical intraseasonal variability. This work shows strong promise in determining the sensitivity of climate models and climate processes to heating variations associated with cloud systems.
Energy Technology Data Exchange (ETDEWEB)
Asbik, M.; Chaynane, R.; Boushaba, H. [GMMTN, Faculty of Sciences and Technics, B.O 509, Boutalamine Errachidia (Morocco); Zeghmati, B. [GMAI-CEF, Universite de Perpignan, Avenue de villeneuve, 66860, Perpignan Cedex (France); Khmou, A. [Faculty of Sciences, B.O 1040, Meknes (Morocco)
2003-12-01
A problem of forced convection condensation in a thin porous layer is considered. The flow in the porous region is described by the Darcy-Brinkman-Forchheimer model (DBF) while classical boundary layer equations without inertia and enthalpie terms are used in the pure condensate region. In order to resolve this problem, an analytical method is proposed. Then, analytical solutions for the flow velocity, temperature distributions and for the local Nusselt number are obtained. The results are essentially presented in the form of the velocity and temperature profiles within the porous layer, the dimensionless film thickness and the heat transfer represented by the local Nusselt number. The comparison of the (DBF) model and the Darcy-Brinkman (DB) one is carried out. The effects of the effective viscosity (Reynolds numberRe{sub K}), permeability (Darcy numberDa) and dimensionless thickness of porous coating H* on the flow and the heat transfer enhancement are also documented. (orig.)
Natural Convection Heat Transfer of Two Vertically Staggered Cylinders
International Nuclear Information System (INIS)
This study measured the natural convection heat transfer of two vertically staggered cylinders varying RaD, the vertical pitch-to-diameter, Pv/D and the horizontal pitch-to-diameter, Ph/D. The geometry frequently appears in heat exchangers and other engineering applications. This study has the relevance with the design of the test facility for PDRC (Passive Decay Heat Removal System) in SFR (Sodium-cooled Fast Reactor). In the staggered arrangement, the heat transfer of the lower cylinder is unaffected by the presence of the upper cylinder. However, the heat transfer of the upper cylinder is affected by the plume developed from the lower cylinder on aligned of two vertically staggered cylinders. When the spacing between two cylinders was less than the critical distance, the upper cylinder was affected more by the preheating effect. As the distance increases, the preheating effect decreases and the velocity effect increases
Natural convection in wavy enclosures with volumetric heat sources
International Nuclear Information System (INIS)
In this paper, the effects of volumetric heat sources on natural convection heat transfer and flow structures in a wavy-walled enclosure are studied numerically. The governing differential equations are solved by an accurate finite-volume method. The vertical walls of enclosure are assumed to be heated differentially whereas the two wavy walls (top and bottom) are kept adiabatic. The effective governing parameters for this problem are the internal and external Rayleigh numbers and the amplitude of wavy walls. It is found that both the function of wavy wall and the ratio of internal Rayleigh number (RaI) to external Rayleigh number (RaE) affect the heat transfer and fluid flow significantly. The heat transfer is predicted to be a decreasing function of waviness of the top and bottom walls in case of (IRa/ERa)>1 and (IRa/ERa)<1. (authors)
A preliminary study on the mixed convection heat transfer in a horizontal pipe
International Nuclear Information System (INIS)
Mixed convection phenomena have applications in heat exchangers designed for viscous liquids, pipelines used for transporting oil, and heat exchangers for gas flows and have been investigated for numerical study. The experimental studies on the topic are rare due to the limited practical uses. The definitions on the buoyancy coefficient that represent the relative influence of the forced and the natural convection are different by scholars. When the fluid passes through a horizontal heated pipe, the secondary flow patterns of the flows moving upward along inner wall and downward through the centerline of the pipe, appear. Two symmetric counter rotating, spiraling axial flows around the vertical centerline and the heat transfer of the bottom is higher due to the unstable stratification. This study is preliminary experiments for laminar and turbulent mixed convection for horizontal pipe geometries. The diameter of the pipe was 0.026m and lengths were ranging from 0.03m to 0.50m varying the Reynolds number. Experiments were carried out using a copper sulfate electroplating system was adopted for mass transfer based on the analogy concept
Mariani, V. C.; Coelho, L. S.
2007-01-01
A numerical study was conducted to investigate steady heat transfer and flow phenomena of natural convection of air in enclosures, with three aspect ratios (H/W = 1, 2, and 4), within which there is a local heat source on the bottom wall at three different positions, Wh. This heat source occupies 1% of the total volume of the enclosure. The vertical walls in the enclosures are insulated and there is an opening on the right wall. The natural convection is influenced by the difference in temper...
Measuring convective heat transfer coefficient around a heated fine wire in cross flow of nanofluids
International Nuclear Information System (INIS)
Recent researches on nanofluids have mainly focused on the increase of thermal conductivity of nanofluids under static condition. The ultimate goal of using nanofluids, however, is to enhance the heat transfer performance under fluid flow. So it has been highly necessary to devise a simple and accurate measuring apparatus which effectively compares the heat transfer capability between the base and nanofluids. Though the convective heat transfer coefficient is not the complete index for the heat transfer capability, it might be one of useful indications of heat transfer enhancement. In this article, the working principles of experimental system for convective heat transfer coefficient around a heated fine wire in cross flow of nanofluids and its application example to three samples of nano lubrication oils are explained in detail
Al-nimr, Moh D. A.; Damseh, Renhe A.; Al-odat, Mohammed Q.
2004-01-01
Magnetic field effect on local entropy generation due to steady two-dimensional laminar forced convection flow past a horizontal plate was numerically investigated. This study was focused on the entropy generation characteristics and its dependency on various dimensionless parameters. The effect of various dimensionless parameters, such as Hartmann number (Ha), Eckert number (Ec), Prandtl number (Pr), Joule heating parameter (R) and the free stream temperature parameter (ÃŽÂ¸Ã¢ÂˆÂž)...
Convective Heat Transfer Analysis in Fluid Flow with Turbulence Promoters with Heat Pipes
Directory of Open Access Journals (Sweden)
Theodor Mateescu
2007-01-01
Full Text Available The present paper proposes the analysis and the simulation of the convection heat transfer into the fluid flow with turbulence promoters utilizing heat pipes. The study is based on the necesity of the unconventional energy forms capitalization, increasing of the energy efficiency and leads to the energy consumtion decrease in concordance with the sustainable development concept.
Thermal Performance Of Convective-Radiative Heat Transfer In Porous Fins
Shahbabaei, Majid; Saedodin, Seyfolah
2014-01-01
Forced and natural convection in porous fin with convective coefficient at the tip under radiation and convection effects are investigated in this letter. Aluminum and copper as materials of fins are determined. In forced and natural convection, the air and water, are applied as working fluids, respectively. In order to solution of this nonlinear equation, HPM and VIM has been used . For verifying the accuracy of the solution methods, compare them with exact solutions (BVP). In this work the ...
Heat transfer by natural convection between enclosed concentric vertical cylinders
International Nuclear Information System (INIS)
The effect of natural convection upon the flow pattern was investigated in the closed annular cavity formed by two concentric vertical cylinders with a part of the inner cylinder heated. From observation of the streamline in the cavity by a smoke tracer, fundamental equations are solved numerically. Steady-state isothermals and streamlines are obtained for Grashof numbers up to 2 x 107 and for various lengths of heating zone. To check the calculated results, temperature distributions are measured in this cavity. The calculated temperatures are a good approximation of the experimental ones and the calculated Nusselt number is compatible with some previous theoretical and experimental results. (author)
Heat flux intensification by vortical flow localization in rotating convection
Kunnen, R. P. J.; Clercx, H. J. H.; Geurts, B. J.
2006-11-01
The effect of rotation on turbulent convective flow between parallel plates has been assessed with direct numerical simulations. With increasing rotation-rate an interesting transition is observed in the vertical-velocity skewness. This transition indicates a localization of motion directed away from the wall and correlates well with changes observed in the heat flux, as well as in the thermal and viscous boundary layer thicknesses. The formation of localized intense vortical structures provides for intensified vertical heat transport through Ekman pumping. At higher rotation-rates this is counteracted by the inhibition of vertical motion by rotation as expressed in the geostrophic thermal-wind balance.
Convective Heat and Mass Transfer in Rotating Disk Systems
Shevchuk, Igor V
2009-01-01
The book describes results of investigations of a series of convective heat and mass transfer problems in rotating-disk systems, namely, over free rotating disks, under conditions of transient heat transfer, solid- body rotation of fluid, orthogonal flow impingement onto a disk, swirl radial flow between parallel co-rotating disks, in cone-disk systems and for Prandtl and Schmidt numbers larger than unity. Methodology used included integral methods, self-similar and approximate analytical solutions, as well as CFD. The book is aimed at the professional audience of academic researchers, industr
Dore, Valentina
2010-01-01
The motion of buoyancy driven plumes is, on all scales, the most common heat and momentum transfer mechanism in geophysical flows, well known as Free Convection. Similarly, density stratification due to heating inequalities is also an ordinary scenario in nature. Free Convection phenomenon coupled with a density stratified fluid setting leads to the so-called Penetrative Free Convection (PFC). When a fluid, in static equilibrium, is stably stratified a thermal forcing can produce an unstable ...
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.
Natural Convection heat transfer in thermally stratified liquid metal
International Nuclear Information System (INIS)
Natural convection tests were conducted using Pb-Bi alloy, with the view to evaluating the effects brought by thermal stratification in liquid metal on the natural convection heat transfer along an immersed vertical metal surface. The vertical metal surface was represented by a stainless steel plate 300 mm high, placed in a cylindrical vessel 400 mm in diameter filled with molten Pb-Bi. The experiment was performed with the heat flux of the plate surface maintained constant and uniform. The temperature distribution through the liquid metal filling the vessel was controlled by regulating a flow of air blown onto the vessel surface; temperature measurements were made by means of a traversing thermocouple. For the case of unstratified bulk fluid, the values obtained for heat transfer rate agreed well with corresponding data given by Sheriff for Na. With increasing degree of stratification, the thickness of the boundary layer decreased in keeping. The resulting increase of heat transfer rate indicated a definite dependence on the stratification parameter; this dependence was determined and expressed in terms of nondimensionalized boundary layer equations. Calculation of heat transfer rate based on numerical method yielded values agreeing fairly well with the experimentally determined data, but that based in integral approximation proved to give underestimated values. (author)
Aqueous Al2O3 nanofluids: the important factors impacting convective heat transfer
Cao, Jianguo; Ding, Yulong; Ma, Caiyun
2014-12-01
A high accuracy, counter flow double pipe heat exchanger system is designed for the measurement of convective heat transfer coefficients with different nanofluids. Both positive and negative enhancement of convective heat transfer of alumina nanofluids are found in the experiments. A modified equation was proposed to explain above phenomena through the physic properties of nanofluids such as thermal conductivity, special heat capacity and viscosity.
Numerical predictions of natural convection in a uniformly heated pool
International Nuclear Information System (INIS)
In the event of a core meltdown accident, one of the accident progression paths is fuel relocation to the lower reactor plenum. In the heavy water new production reactor (NPR-HWR) design the reactor cavity is flooded with water. In such a design, decay heat removal to the water in the reactor cavity and thence to the containment may be adequate to keep the reactor vessel temperature below failure limits. If this is the case, the accident progression can be arrested by retaining a coolable corium configuration in the lower reactor plenum. The strategy of reactor cavity flooding to prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head has also been considered for commercial pressurized water reactors. Previously, the computer code COMMIX-LAR/P was used to determine if the heat removal rate from the molten cerium in the lower plenum to the water in the cavity was adequate to keep the reactor vessel temperature in the NPR-HWR design below failure limits. It was found that natural convection in the molten pool resulted in heat removal rates that kept the peak reactor vessel temperature about 400 degrees C below the steel melting point. The objective of the work presented in this paper was to determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source. For this purpose, the experiments of free convection in a semicircular cavity of Jahn and Reeneke were analyzed with COMMIX and code predictions were compared with experimental measurements. COMMIX is a general purpose thermalhydraulics code based on finite differencing by the first order upwind scheme
Asymptotic solutions in forced convection turbulent boundary layers
Wang, Xia; Castillo, Luciano
2003-03-01
A similarity analysis has been developed for a 2D forced convection turbulent boundary layer with and without a pressure gradient. Two new inner and outer temperature scalings are derived by means of similarity analysis of the equations of motion. The new scalings will be verified by the experimental data with adverse pressure gradient, favourable pressure gradient and zero pressure gradient respectively. It will be shown that the mean temperature profiles are dependent on the external pressure gradient and the upstream conditions. However, using the new scaling in inner variables or in outer variables, the temperature profiles collapse into a single curve. Thus, the true asymptotic solution for the temperature field exists even at a finite Péclet number. These results are confirmed by using the existing experimental data and compared with the results from various scalings. The asymptotic temperature profile or the self-similar profile found in the present analysis is in agreement with the fact that an asymptotic velocity profile exists if the mean velocity deficit profile is normalized by the Zagarola and Smits scaling (Zagarola and Smits 1998 J. Fluid Mech.373 33 79).
Experimental Study of Forced Convection over Equilateral Triangle Helical Coiled Tubes
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E. El-Kashif
2012-06-01
Full Text Available
This study presents an experimental investigation of an equilateral triangular cross-sectioned helical tube under uniform heat flux boundary condition. The experiments are carried out for nine helical coiled-tubes of different parameters. Different diameter ratio (D/a ranged from 6.77 to 15.43 and pitch ratio (P/a ranged from 1.127 to 3.062 are employed in the present study, The experiments covered a range of Reynolds number from 5.3X10^{2} to 2.2X10^{3}. Uniform heat flux is applied to the inside surface of the helical coil and air is selected as tested fluid. The experimental results obtained from the equilateral triangular cross-sectioned helical tube indicated that the parameters of the coil diameter and pitch of helical coil have important effects on the heat transfer coefficient. The Nusselt number increases with the increase of Reynolds number and coil diameter at constant pitch of the helical coil. Also, Nusselt number increases with the increase of Reynolds number and Pitch of helical coil at constant coil diameter tube. A comparison between the present experimental data with a previous work with circular cross-sectioned helical tubes have the same test conditions was achieved. From this comparison, it is clear that the average enhancement of Nusselt number for equilateral triangular cross-sectioned helical is about 1.12 ~ 1.25 times the circular cross-sectioned helical for all tested conditions. A general correlation of the average Nusselt number as a function in Re, D/a and P/a ratios is obtained to describe the forced convection from the equilateral triangle cross sectioned coiled tube.
Key words: Forced convection; Helical coiled tubes; Coil diameter ratio; Pitch ratio
Directory of Open Access Journals (Sweden)
S. Venkatachalapathy
2010-01-01
Full Text Available Natural convection cooling using air as a fluid is commonly used in the cooling of electronic equipment and many other devices. In this work, a three-dimensional numerical study of natural convection heat transfer from multiple protruding heat sources simulating electronic components is conducted. Computational fluid dynamics (CFD software, FLUENT is used in this analysis. A 4 by 5 array of heat sources are embedded in the bottom wall of an adiabatic square enclosure. The heat sources with a constant heat flux source at the bottom are of square cross-section and arranged in an in-line manner. Each heat source is attached with one thermocouple, which is connected to a data acquisition system and a computer. The steady state temperatures of heat sources, air inlet, outlet and enclosure walls are measured. The analysis is carried out by varying the heat fluxes and outlet areas. The heat transfer coefficient, Nusselt number and Grashof number are obtained. Results indicate that the heat sources inside the array are hotter and the heat transfer coefficient increases almost linearly with heat source surface temperatures. Grashof number and outlet opening areas strongly influence the Nusselt number. The heat transfer coefficient for the inner heat sources in a row is lower than those near the enclosure walls. The results of numerical analysis are compared with the experiments and there is a good agreement between the two.
Reynolds stress and heat flux in spherical shell convection
Käpylä, P. J.; Mantere, M. J.; Guerrero, G.; Brandenburg, A.; Chatterjee, P.
2011-07-01
Context. Turbulent fluxes of angular momentum and enthalpy or heat due to rotationally affected convection play a key role in determining differential rotation of stars. Their dependence on latitude and depth has been determined in the past from convection simulations in Cartesian or spherical simulations. Here we perform a systematic comparison between the two geometries as a function of the rotation rate. Aims: Here we want to extend the earlier studies by using spherical wedges to obtain turbulent angular momentum and heat transport as functions of the rotation rate from stratified convection. We compare results from spherical and Cartesian models in the same parameter regime in order to study whether restricted geometry introduces artefacts into the results. In particular, we want to clarify whether the sharp equatorial profile of the horizontal Reynolds stress found in earlier Cartesian models is also reproduced in spherical geometry. Methods: We employ direct numerical simulations of turbulent convection in spherical and Cartesian geometries. In order to alleviate the computational cost in the spherical runs, and to reach as high spatial resolution as possible, we model only parts of the latitude and longitude. The rotational influence, measured by the Coriolis number or inverse Rossby number, is varied from zero to roughly seven, which is the regime that is likely to be realised in the solar convection zone. Cartesian simulations are performed in overlapping parameter regimes. Results: For slow rotation we find that the radial and latitudinal turbulent angular momentum fluxes are directed inward and equatorward, respectively. In the rapid rotation regime the radial flux changes sign in accordance with earlier numerical results, but in contradiction with theory. The latitudinal flux remains mostly equatorward and develops a maximum close to the equator. In Cartesian simulations this peak can be explained by the strong "banana cells". Their effect in the spherical case does not appear to be as large. The latitudinal heat flux is mostly equatorward for slow rotation but changes sign for rapid rotation. Longitudinal heat flux is always in the retrograde direction. The rotation profiles vary from anti-solar (slow equator) for slow and intermediate rotation to solar-like (fast equator) for rapid rotation. The solar-like profiles are dominated by the Taylor-Proudman balance. Movies and Appendix A are available in electronic form at http://www.aanda.org
Energy Technology Data Exchange (ETDEWEB)
Favre, E.
1997-09-26
coupled buoyancy and thermo-capillary convection lead to a convective motion of the interface liquid/gas which drastically changes the heat and mass transfer across the liquid layer. Two experiments were considered, depending on the fluid: oil or mercury. The liquid is set in a cooled cylindrical vessel, and heated by a heat flux across the center of the free surface. The basic flow, in the case of oil, is a torus. When the heat parameter increases, a stationary flow appears as petals or rays when the aspect ratio. The lateral confinement selects the azimuthal wavelength. In the case of petals-like flow, a sub-critical Hopf bifurcation is underlined. The turbulence is found to be `weak`, even for the largest values of the Marangoni number (Ma = 1.3 10{sup 5}). In the case of mercury, the thermo-capillary effect is reduced to zero to impurities at the surface which have special trajectories we describe and compare to a simpler experiment. Only the buoyancy forces induce a unstationary, weakly turbulent flow as soon as the heating power exceeds 4W (Ra = 4.5 10{sup 3}, calculated with h = 1 mm). The past part concerns the analysis of the effect on the flow of the boundary conditions, the geometry, the Prandtl number and the buoyancy force with the help of the literature. Results concerning heat transfer, in particular the exponent of the law Nusselt number vs. heating power, were compared with available data. (author) 115 refs.
Natural convection of heat generating fluid within horizontal cylinder
International Nuclear Information System (INIS)
The natural convection of a heat generating fluid within a horizontal cylinder has been studied analytically by solving the governing equations by a finite difference method. The rate of heat generation was assumed constant and distributed uniformly in the fluid. The surface temperature of the cylinder was considered isothermal. Computations were carried out on the case of Prandtl number Pr = 0.1, 1.0, 10 and 1000, and Rayleigh number from 102 to 108. It was revealed that the velocity and temperature fields were little affected by Prandtl number in the range of Pr ? 1.0. The effect of natural convection on the heat transfer between the wall and fluid appeared at beyond Rayleigh number Ra = 104, and became dominant at Ra ? 106. Heat transfer experiments were also performed for the range of Ra = 3 x 103 - 109 by using a NaCl solution of 0.05 mol/kg water in concentration. The calculated average Nusselt numbers were in good agreement with the experiments. (author)
Natural convection heat transfer in a stratified pool with volumetric heat generation
International Nuclear Information System (INIS)
Experiments were carried out to investigate the natural convection heat transfer behaviour in volumetrically heated two and three layer stratified pools which may exist in the lower head of a reactor vessel during a postulated severe accident. The test facility was a 1/8 scaled of a prototype PWR type reactor. The two layer stratified pool consisted of water and paraffin oil and the three layer pool consisted of chlorobenzene, water and paraffin oil. The fluids chosen were having similar thermodynamic property relations as that in prototypic melt stratified layers observed in RASPLAV and MASCA projects. The depth of the pool in all the experiments was kept constant. The main objective was to study and compare the convective heat transfer characteristics and heat flux distribution in the vessel at different Raleigh numbers in the two and three layer cases. The test results indicate that the upward heat transfer can be significantly reduced due to interfacial resistance between the layer. The heat fluxes along the pool surface were calculated from the measured wall temperature distributions, which are important for estimation of thermal load in the vessel. The convective heat transfer characteristics between the two layer and three layer pools were compared. It was found that the heat transfer behaviour are similar in two and three layers at low Raleigh number. But at high Raleigh number, the location of maximum heat flux along the vessel shifts upwards. (author)the vessel shifts upwards. (author)
Kalisch, S.; Trinh, T.; Chun, H.; Ern, M.; Preusse, P.; Kim, Y.; Eckermann, S. D.; Riese, M.
2013-12-01
Gravity waves (GW) are responsible for driving large scale circulations like Brewer-Dobson circulation, contribute to the wave driving of the QBO in the tropics, and are also known as a coupling mechanism between tropospheric sources and the upper stratosphere to mesosphere region. Convection is a dominant source for tropical GWs, but also one of the most difficult and dynamic GW sources to understand. Therefore, we present the results of GW ray-tracing calculations from tropospheric (convective) sources up to the mesosphere. We used the Gravity wave Regional Or Global RAy-Tracer (GROGRAT) to perform the GW trajectory calculations and the convective GW source scheme from Yonsei University (South Korea) to quantify the excitation by convection. Heating rates, cloud data, and atmospheric background data were provided by the MERRA dataset for the calculation of convective forcing by deep convection and for the atmospheric background of the ray-tracing calculations afterwards. In order to validate our findings we compare our simulation results with satellite measurements of temperature amplitudes and momentum flux from the SABER instrument over a 10 years period. Simulation and measurements are in good agreement for the tropics throughout the whole simulated period and show similar seasonal behavior. Additionally, the observational filter of the instrument was taken into account and its influences are discussed. The modulation of GW momentum flux by the background winds and in particular the influence of the QBO is investigated. GW drag at various altitudes is calculated and compared to the drag required for the forcing of the QBO. Further, we show the results of a non-orographic background parameterization used as start conditions for the ray-tracer to emphasize the improvements of our coupled convective GW source model over non-orographic GW parameterizations.
Convective heat transfer for viscoelastic fluid in a curved pipe
Energy Technology Data Exchange (ETDEWEB)
Norouzi, M.; Kayhani, M.H. [Shahrood University of Technology, Mechanical Engineering Department, Shahrood (Iran); Nobari, M.R.H. [Amirkabir University of Technology, Mechanical Engineering Department, Tehran (Iran); Joneidi, A.A. [Eindhoven University of Technology, Mechanical-Polymer Technology Group, Eindhoven (Netherlands)
2010-10-15
In this paper, fully developed convective heat transfer of viscoelastic flow in a curved pipe under the constant heat flux at the wall is investigated analytically using a perturbation method. Here, the curvature ratio is used as the perturbation parameter and the Oldroyd-B model is applied as the constitutive equation. In the previous studies, the Dirichlet boundary condition for the temperature at the wall has been used to simplify the solution, but here exactly the non-homogenous Neumann boundary condition is considered to solve the problem. Based on this solution, the non-axisymmetric temperature distribution of Dean flow is obtained analytically and the effect of flow parameters on the flow field is investigated in detail. The current analytical results indicate that increasing the Weissenberg number, viscosity ratio, curvature ratio, and Prandtl number lead to the increase of the heat transfer in the Oldroyd-B fluid flow. (orig.)
Turbulent free convection between vertical isothermal plates with asymmetrical heating
Terekhov, V. I.; Ekaid, A. L.
2013-06-01
Results of numerical investigation of the flow and heat transfer at turbulent free convection between the vertical parallel isothermal plates with different temperatures are presented. The temperature factor R T varied within -2 ÷ 1. The Rayleigh number changed within Ra = 107 ÷ 109, and the ratio of geometrical sizes of plates and distances between them was constant A = L/ w = 10. Numerical studies were performed via the solution to the two-dimensional Navier—Stokes equations and energy equation in Boussinesq approximation. The considered boundary-value problem has the unknown conditions at the inlet and outlet between the plates. To describe turbulence, the modified low-Reynolds k-? model was used. The effect of the temperature factor on the flow structure at the channel inlet and outlet was analyzed. Data on distributions of velocities and temperatures between the plates, local and integral heat transfer allow deeper understanding of the mechanism of transfer processes between the parallel plates with asymmetrical heating.
Convective and radiative heat transfer in MHD radiant boilers
Im, K. H.; Ahluwalia, R. K.
1981-10-01
A combined convection-gas radiation, two-zone flow model is formulated for study of the heat transfer characteristics of MHD radiant boilers. The radiative contributions of carbon dioxide, water vapor, potassium atoms, and slag particles are included in the formulation, and are determined by solving the radiation transport equation using the P1 approximation. The scattering and absorption cross sections of slag particles are calculated from Mie theory. The model is used to analyze the scale-up of heat transfer in radiant boilers with refractory thickness, wall emissivity, and boiler size under conditions of a gas composition and slag particle spectrum typical of coal-fired MHD combustion. A design procedure is suggested for sizing radiant boilers so as to achieve the required heat extraction rate and to provide a flow residence time that is adequate for decomposition of NO(x) to acceptable levels.
Natural convection in asymmetric triangular enclosures heated from below
Kamiyo, O. M.; Angeli, D.; Barozzi, G. S.; Collins, M. W.
2014-11-01
Triangular enclosures are typical configurations of attic spaces found in residential as well as industrial pitched-roof buildings. Natural convection in triangular rooftops has received considerable attention over the years, mainly on right-angled and isosceles enclosures. In this paper, a finite volume CFD package is employed to study the laminar air flow and temperature distribution in asymmetric rooftop-shaped triangular enclosures when heated isothermally from the base wall, for aspect ratios (AR) 0.2 distributions within the enclosure are analysed. Results indicate that, at low pitch angle, the heat transfer between the cold inclined and the hot base walls is very high, resulting in a multi-cellular flow structure. As the pitch angle increases, however, the number of cells reduces, and the total heat transfer rate progressively reduces, even if the Rayleigh number, being based on the enclosure height, rapidly increases. Physical reasons for the above effect are inspected.
Convective and radiative heat transfer in MHD radiant boilers
International Nuclear Information System (INIS)
A combined convection-gas radiation, two-zone flow model is formulated for study of the heat transfer characteristics of MHD radiant boilers. The radiative contributions of carbon dioxide, water vapor, potassium atoms, and slag particles are included in the formulation, and are determined by solving the radiation transport equation using the P1 approximation. The scattering and absorption cross section of slag particles are calculated from Mie theory. The model is used to analyze the scale-up of heat transfer in radiant boilers with refractory thickness, wall emissivity, and boiler size, under conditions of a gas composition and slag particle spectrum typical of coal-fired MHD combustion. A design procedure is suggested for sizing radiant boilers so as to achieve required heat extraction rate and to provide a flow residence time that is adequate for decomposition of NO/sub x/ to acceptable levels
International Nuclear Information System (INIS)
The conductive heat transfer in a rectangular plate with nonuniform internal heat generation, with one end convectively cooled and a part of the opposite end subjected to external heat flux is considered. The remaining part of this end as well as the other two sides are thermally insulated. The governing differential equation is solved by a finite difference scheme. The variation of the thermal resistance with Biot modulus, the plate geometry, the internal heat generation parameter and the type of profile of internal heat generation is discussed. (author)
Weber, D. D.
1980-06-01
An experimental study of natural convection heat transfer through a doorway in a two room passive solar heated building is described. Similitude modeling was the method used to measure natural convection heat transfer coefficients (h/sub NC/) in a model geometrically scaled down by a factor of 5. Freon gas was used as the working fluid to obtain dynamic similarity within the model. A temperature difference was maintained between the two rooms by a heated vertical wall which simulated a Trombe wall in one room, and by a cooled vertical wall which simulated a thermal storage wall in the other room. Heat transfer through the doorway was measured as a function of a characteristic temperature differential and the geometry of the doorway.
International Nuclear Information System (INIS)
The natural convection heat transfer from a heated vertical plate with wall temperature decreasing linearly from the lower end and embedded in a water saturated porous medium (coarse sand of 23% porosity) has been investigated experimentally. The steady state isothermal lines in the porous medium were developed with the aid of a computer program from the measured temperature data for several rates of heat input. The computed values of the local Nusselt number and modified Rayleigh number were used to obtain the empirical correlations. In order to check the accuracy of the measuring instruments and experimental procedures in simulating the thermal field, experimental investigations of heat transfer by natural convection from a vertical heated plate with constant wall temperature and embedded in the water saturated sand were conducted and the experimental results were compared with those previous investigations. The agreement was very good indicating that the instrumentation and experimental set-up used in the present study were reasonably satisfactory
Heat flux scaling in turbulent Rayleigh-B\\'enard convection with an imposed longitudinal wind
Scagliarini, Andrea; Toschi, Federico
2013-01-01
We present a numerical study of Rayleigh-B\\'enard convection disturbed by a longitudinal wind. Our results show that under the action of the wind, the vertical heat flux through the cell initially decreases, due to the mechanism of plumes-sweeping, and then increases again when turbulent forced convection dominates over the buoyancy. As a result, the Nusselt number is a non-monotonic function of the shear Reynolds number. We provide a simple model that captures with good accuracy all the dynamical regimes observed. We expect that our findings can lead the way to a more fundamental understanding of the of the complex interplay between mean-wind and plumes ejection in the Rayleigh-B\\'enard phenomenology.
A multiple-relaxation-time lattice Boltzmann model for convection heat transfer in porous media
Liu, Q; Li, Q
2013-01-01
In this paper, a multiple-relaxation-time (MRT) lattice Boltzmann (LB) model is developed for simulating convection heat transfer in porous media at the representative elementary volume scale. In the model, a MRT-LB equation is used to simulate the flow field, while another MRT-LB equation is employed to simulate the temperature field. The effect of the porous media is considered by introducing the porosity into the equilibrium moments, and adding a forcing term to the MRT-LB equation of the flow field in the moment space. The proposed MRT-LB model is validated by numerical simulations of several two-dimensional convection problems in porous media. The numerical results predicted by the present MRT-LB model agree well with those reported in the literature.
Natural Convection Heat Transfer Experiment in a Hemispherical Pool
International Nuclear Information System (INIS)
Natural convection plays an important role in determining the thermal load from molten core accumulated in the reactor vessel lower head during a severe accident. Several numerical and experimental programs were conducted to study the heat transfer in the molten pool. Previous investigations were mostly related to the rectangular and semicircular pools. Except for COPO, UCLA, ACOPO, and BALI, previous investigations suffer from inadequate representation of high modified Rayleigh number (Ra') in the hemispherical pool that may be formed in the reactor core and lower plenum. Thus, experimental work is conducted utilizing SIGMA SP (Simulant Internal Gravitated Material Apparatus Spherical Pool) producing high Ra' turbulent natural convection in a hemispherical pool up to 5.3 x 1011. The heating method has already been tested in SIGMA CP (Circular Pool). Six thin cable-type heaters, each with a diameter of 6 mm, are employed to simulate internal heating in the pool. They are uniformly distributed in the hemispherical pool to supply a maximum of 7.8 kW power to the pool. SIGMA SP has the inner and outer diameters of 500 mm and 520 mm, respectively. The upper flat plate and the curved wall of pool, with a 10 mm thick stainless steel plate, are cooled by a regulated water loop. A water-cooling system is used to maintain the temperature of water surrounding the test section nearly constant with time. This study focuses on quantifying the directional heat losses, anuantifying the directional heat losses, angular heat flux distribution, and temperature distribution inside the molten pool
Droplet combustion experiments in varying forced convection using microgravity environment
Energy Technology Data Exchange (ETDEWEB)
Mitsuya, Masaki [Tokyo Gas Corporation, 1-5-20 Kaigan, Minato-ku, Tokyo 105-0022 (Japan); Hanai, Hironao [Chiba University of Science, 3 Shiomi-cho, Choshi, Chiba 288-0025 (Japan); Sakurai, Satoru [Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577 (Japan); Ogami, Yasuhiro [Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577 (Japan); Kobayashi, Hideaki [Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577 (Japan)]. E-mail: kobayashi@ifs.tohoku.ac.jp
2005-12-15
A new microscopic model of the interaction between droplet flames and fine vortex tubes which compose a coherent structure of turbulence was developed. Three non-dimensional numbers were introduced to extend the length scale and time scale so as to be suitable for microgravity experiments using droplets of combustion of about 1 mm in diameter. An experimental apparatus for combustion of a single droplet and that of an array of two droplets in varying airflow was developed, and experiments were performed in microgravity and normal gravity at pressures up to 2.0 MPa for n-nonane and ethanol as fuels. Variations of the instantaneous burning rate constant, K {sub i}, in response to the varying flow velocity was successfully observed. At high pressure, the effects of droplet Reynolds number Re on K {sub i} was clearly seen, while the effects of natural convection, which increases K {sub i} with Re, was seen in normal gravity even in the forced airflows. As for the experiments on combustion of an array of two droplets, K {sub i} reduction of the downstream droplet became weak when the flow direction was varied. However, the K {sub i} reduction of the downstream droplet for flow direction variations was clearly seen for n-nonane droplets but almost not for ethanol droplets. The interaction mechanism between upstream and downstream droplets is considered to result from the elimination of oxidizer supply to the downstream droplet, indicating strong interaction effects of n-nonane droplets for a stoichiometric oxygen-fuel ratio of n-nonane (i.e., 14.0) greater than that of ethanol (i.e., 3.0)
Droplet combustion experiments in varying forced convection using microgravity environment
International Nuclear Information System (INIS)
A new microscopic model of the interaction between droplet flames and fine vortex tubes which compose a coherent structure of turbulence was developed. Three non-dimensional numbers were introduced to extend the length scale and time scale so as to be suitable for microgravity experiments using droplets of combustion of about 1 mm in diameter. An experimental apparatus for combustion of a single droplet and that of an array of two droplets in varying airflow was developed, and experiments were performed in microgravity and normal gravity at pressures up to 2.0 MPa for n-nonane and ethanol as fuels. Variations of the instantaneous burning rate constant, K i, in response to the varying flow velocity was successfully observed. At high pressure, the effects of droplet Reynolds number Re on K i was clearly seen, while the effects of natural convection, which increases K i with Re, was seen in normal gravity even in the forced airflows. As for the experiments on combustion of an array of two droplets, K i reduction of the downstream droplet became weak when the flow direction was varied. However, the K i reduction of the downstream droplet for flow direction variations was clearly seen for n-nonane droplets but almost not for ethanol droplets. The interaction mechanism between upstream and downstream droplets is considered to result from the elimination of oxidizer supply to the downstream droplet, indicating strong int downstream droplet, indicating strong interaction effects of n-nonane droplets for a stoichiometric oxygen-fuel ratio of n-nonane (i.e., 14.0) greater than that of ethanol (i.e., 3.0)
Development and performance evaluation of forced convection potato solar dryer
International Nuclear Information System (INIS)
This research paper deals with the design development and testing of a forced convection solar dryer, for drying and converting to flour of high moisture content vegetables like potatoes. The angle of solar collector was made adjustable for the absorption of maximum solar radiation by the absorber plate. The air flow rate was controlled by adjustable gate valve to find the optimum flow rate for dehydration of the product. The penetration of solar radiation raised the temperature of the absorber plate of the dryer to 110 deg. C during the operation under stagnation or no load conditions. The maximum air temperature attained in the solar air heater, under this condition was 80 deg. C. The dryer was loaded with 12 Kg of blanched potato chips having an initial moisture content of 89.75%, and the final desired moisture content of 6.95% was achieved within five hours without losing the color of potato chips, while the moisture contents reduction was from 89.75% to 33.75% for five hours in open sun drying under shade. The drying cost for 1 Kg of potatoes was calculated as Rs. 245 and it was Rs. 329 in the case of an electric dryer. The life span of the solar dryer was assumed to be 20 years. The cumulative present worth of annual savings over the life of the solar dryer was calculated for blanched potato chips drying, and it turned out be Rs.163177.67/- which was much higher than the capital cost of the dryer (Rs. 25000). The payback period was calculated as 0.89 years, whic period was calculated as 0.89 years, which was also very small considering the life of the system (20 years). (author)
Free convection heat transfer to mercury in vertical annuli
International Nuclear Information System (INIS)
Data on free convection heat transfer to water and mercury have been collected using a test rig in vertical annuli of three radii ratios, the walls of which were maintained at uniform temperatures. A theoretical analysis of the boundary layer equations has been attempted using local similarity transformation and double boundary layer approach. Correlations derived from the present theoretical analysis are compared with the analysis and the experimental data available in literature for non-metallic fluids and also with the present experimental data on water and mercury. Generalised correlations are set up for expressing the ratio of heat transferred by convection to the heat transferred by pure conduction and Nusselt's number, in terms of Grashof, Rayleigh and Prandtl numbers, based on the theoretical analysis and the present data on mercury and water. The present generalised correlations agree with the reported and present data for non-metallic fluids and liquid metals with an average deviation of 9% and maximum deviation of +-13.7%. (author)
Analysis of forced convection in a duct filled with porous media
International Nuclear Information System (INIS)
This paper presents a series of numerical simulations which aim to document the problem of forced convection in a channel filled with fluid-saturated porous medium. In modeling the flow in the channel, the effects of flow inertia, variable porosity and Brinkman friction are taken into account. Two channel configurations are investigated: parallel plates and circular pipe. In both cases, the channel wall is maintained at constant temperature. It is found that the general flow model predicts an overall enhancement in heat transfer between the fluid/porous matrix composite and the walls, compared to the predictions of the widely used Darcy flow model. This enhancement is reflected in the increase of the value of the Nusselt number. Important results documenting the dependence of the temperature and flow fields in the channel as well as the dependence of the thermal entry length on the problem parameters are also reported in the course of the study
Forced convection of ferro-fluids in a vented cavity with a rotating cylinder
International Nuclear Information System (INIS)
In this study, numerical investigation of the forced convection of ferro-fluid in a square cavity with ventilation ports in the presence of an adiabatic rotating cylinder is carried out. The governing equations are solved with a finite element based solver. The effects of Reynolds number (20 ? Re ? 400), angular rotational speed of the cylinder (-500 ? ? ? 500), strength and location of the magnetic dipole (0 ? ? ? 250), (0.2 ? a ? 0.8, -0.8 ? b ? -0.2) on the flow and thermal fields are numerically studied. It is observed that the length and size of the recirculation zones can be controlled with magnetic dipole strength and angular rotational speed of the cylinder. When the magnetic dipole is closer to the bottom wall of the cavity, flow is accelerated towards the bottom wall with larger influence area. The increasing values of the angular rotational speed of the cylinder in the clockwise direction enhance the heat transfer
Directory of Open Access Journals (Sweden)
Mohammad Al-Nimr
2004-12-01
Full Text Available Abstract: The entropy generation due to steady laminar forced convection fluid flow through parallel plates microchannel is investigated numerically. The effect of Knudsen, Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference on entropy generation within the microchannel is discussed. The fraction of the entropy generation due to heat transfer to the total entropy generation within the microchannel is studied in terms of Bejan number. The entropy generation within the microchannel is found to decrease as Knudsen number increases, and it is found to increase as Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference increase. The contribution of the viscous dissipation in the total entropy generation increases as Knudsen number increases over wide ranges of the flow controlling parameters.
Haddad, Osamah; Abuzaid, Mohammad; Al-Nimr, Mohammad
2004-12-01
The entropy generation due to steady laminar forced convection fluid flow through parallel plates microchannel is investigated numerically. The effect of Knudsen, Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference on entropy generation within the microchannel is discussed. The fraction of the entropy generation due to heat transfer to the total entropy generation within the microchannel is studied in terms of Bejan number. The entropy generation within the microchannel is found to decrease as Knudsen number increases, and it is found to increase as Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference increase. The contribution of the viscous dissipation in the total entropy generation increases as Knudsen number increases over wide ranges of the flow controlling parameters.
Convective heat transfer with chemical transformations in a vertical channel
Grigoruk, D. G.; Kondratenko, P. S.; Nikol'Skii, D. V.; Chizhov, M. E.
2011-06-01
A theoretical model describing laminar free convection flow of hydrogen-air mixture taking into account an exothermic heterogeneous reaction in a vertical channel with autocatalytic coating is presented. It is shown that the surface of channel walls can be subdivided into areas with different reaction flow patterns: the initial region in which the reaction rate experiences a very rapid growth and the heat flowrate on the wall has a maximum, and the region corresponding to intersection of boundary layers, in which enhancement of the hydrogen oxidation reaction is observed. The results from a numerical experiment confirmed the conclusions obtained from analytical assessments.
Convective Heat Transfer in Impinging- Gas- Jet Arrangements
Directory of Open Access Journals (Sweden)
J.M. Buchlin
2011-01-01
Full Text Available The paper deals with heat transfer by convection between impinging gas jets and solid surfaces. It considers both single and multiple jet systems. It emphasizes the flow and geometrical parameters as well as the environment conditions at which the jet emerges. In particular, it points out the effect of the jet tilting, thermal entrainment and jet confinement. ASN and ARN schemes are illustrated through industrial and aeronautical applications. Design correlations are proposed. Experimental data obtained from infrared thermography are compared to CFD simulations.
Natural convection of ferrofluids in partially heated square enclosures
Selimefendigil, Fatih; Öztop, Hakan F.; Al-Salem, Khaled
2014-12-01
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 (104?Ra?5×105), heater location (0.25H?yh?0.75H), strength of the magnetic dipole (0???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.
Mixed convection in a channel provided with heated porous blocks of various shapes
International Nuclear Information System (INIS)
The present work is a numerical simulation of laminar mixed convective in a two-dimensional parallel-plate channel provided with porous blocks of various shapes. The upper plate is thermally insulated while the blocks, heated from below, are attached on the lower one. The Brinkman-Forchheimer extended Darcy model with the Boussinesq approximation is adopted for the flow in the porous regions. The governing equations with the appropriate boundary conditions are solved by the control volume method. The influence of the buoyancy force intensity, the porous blocks shape going from the rectangular shape to the triangular shape, their height, the porous medium permeability, the Reynolds number and the thermal conductivity ratio is analyzed. The results reveal essentially, that the shape of the blocks can alter substantially the flow and heat transfer characteristics. In addition, it is shown that judicious choices of these parameters can lead to high heat transfer rates with a moderate increase of pressure drop.
Directory of Open Access Journals (Sweden)
Ruma Patra
2014-01-01
Full Text Available Effects of radiative heat transfer on MHD fully developed mixed convective flow of a viscous incompressible electrically conducting fluid through a vertical channel with asymmetric heating of the walls in the presence of a uniform transverse magnetic field has been studied. An exact solution of the governing equations has been obtained in closed form. It is observed that the velocity field is greatly influenced by the radiative heat transfer as well as bouyancy forces. The induced magnetic field decreases at any point near the left wall and it increases near the right wall of the vertical channel with increase in radiation parameter. Further, an increase in radiation parameter leads to a decrease in the temperature at any point of the channel flow. A limiting consideration of the solutions of the governing equations of the flow are analyzed for Ra<<1.
Mixed Convection Heat Transfer on the Outside of a Vertical Cylinder
International Nuclear Information System (INIS)
An experimental study was made of turbulent heat transfer from a vertical cylinder placed in a square channel. The flow medium was water flowing upwards. Basic differential equations governing the mixed flow heat transfer phenomena in a vertical annulus are presented. A dimensional analysis is done to find the dimensionless variables affecting the relative magnitude of the effect of buoyancy on forced convection heat transfer. Dimensionless equations correlating the experimental data ana incorporating a buoyancy parameter of the form Gr/Re2 are presented. Reynolds number range covered is 690 to 129,500 and the Rayleigh num- ber range covered is 109 to 4.2 x 1013 . Effect of different length parameters, like hydraulic diameter and distance of the measuring point from the inlet of the test section, on dimensionless equations are studied
IRT analysis on historic buildings: toward a controlled convection heating
Rosina, Elisabetta; Ludwig, Nicola; Redaelli, Veronica; Della Torre, Stefano; D'Ascola, Simona; Catalano, Michela; Faliva, Chiara
2005-03-01
Many applications of IRT on buildings require active approach. The solicitation has to be properly calculated, and the application has to take in account the optical characteristics of the surface and its thermal properties. In fact, non-homogeneities of the surface definitively affect the absorbance and reflectance of materials, as shown in literature. Therefore, in case of different colors like artistic paintings, dark stains and salts deposition a convection heating results more effective for IRT inspection, because it does not stimulate different localized absorption due to the colors. Using fan coil heaters, major difficulty is to obtain an even heating on the wall under inspection. The laboratory tests permitted to verify that the strength of rising warm air is higher than the one due to the heater ventilation. As a consequence, the effects of heating on the wall start from the upper part and decrease in a non-proportional way to the bottom. On the other side, thermal flux from a heater changes direction according to the geometry of the room, ambient conditions (initial temperature of the air, openings, etc), technical characteristics of the heater (power, speed of the fan, shape, etc) and its location (orientation, elevation, distance from the surface under investigation, etc). In addition, the increase of air temperature does not directly correspond to the increase of the surface temperature. The paper shows the characterization of a convective heating source, by laboratory measurements; to map the distribution of heat in time, the 14.000-26.000 kcal/h heater flux was measured following a 3D grid, by anemometers, probes, and IR Thermography.
Natural Convection Heat Transfer Experiment in a Hemispherical Pool
Energy Technology Data Exchange (ETDEWEB)
J. L. Rempe; S. D. Lee; H. M. Son; K. Y. Suh; F.B.Cheung; S. B. Kim
2005-05-19
Natural convection plays an important role in determining the thermal load from molten core accumulated in the reactor vessel lower head during a severe accident. Several numerical and experimental programs were conducted to study the heat transfer in the molten pool. Previous investigations were mostly related to the rectangular and semicircular pools. Except for COPO, UCLA, ACOPO, and BALI, previous investigations suffer from inadequate representation of high modified Rayleigh number (Ra') in the hemispherical pool that may be formed in the reactor core and lower plenum. Thus, experimental work is conducted utilizing SIGMA SP (Simulant Internal Gravitated Material Apparatus Spherical Pool) producing high Ra' turbulent natural convection in a hemispherical pool up to 5.3 x ~1011. The heating method has already been tested in SIGMA CP (Circular Pool). Six thin cable-type heaters, each with a diameter of 6 mm, are employed to simulate internal heating in the pool. They are uniformly distributed in the hemispherical pool to supply a maximum of 7.8 kW power to the pool. SIGMA SP has the inner and outer diameters of 500 mm and 520 mm, respectively. The upper flat plate and the curved wall of pool, with a 10 mm thick stainless steel plate, are cooled by a regulated water loop. A water-cooling system is used to maintain the temperature of water surrounding the test section nearly constant with time. This study focuses on quantifying the directional heat losses, angular heat flux distribution, and temperature distribution inside the molten pool.
The effect of Coriolis force on nonlinear convection in a porous medium
Riahi, D. H.
1994-01-01
Nonlinear convection in a porous medium and rotating about vertical axis is studied in this paper. An upper bound to the heat flux is calculated by the method initiated first by Howard [6] for the case of infinite Prandtl number.
Energy Technology Data Exchange (ETDEWEB)
Hsu, C.H. [Department of Mold and Die Engineering, National Kaohsiung Institute of Technology, Kaohsiung (Taiwan, Province of China); Yang, S.A. [Department of Mold and Die Engineering, National Kaohsiung Institute of Technology, Kaohsiung (Taiwan, Province of China)
1997-06-01
A model is developed for the study of mixed convection film condensation from downward flowing vapors onto a sphere with uniform wall heat flux. The model combined natural convection dominated and forced convection dominated film condensation, including effects of pressure gradient and interfacial vapor shear drag has been investigated and solved numerically. The separation angle of the condensate film layer, {phi}{sub s} is also obtained for various pressure gradient parameters, P{sup *} and their corresponding dimensionless Grashof `s parameters, Gr{sup *}. Besides, the effect of P{sup *} on the dimensionless mean heat transfer, Nu(Re){sup -1/2} will remain almost uniform with increasing P{sup *} until P{sup *}=2/9Gr{sup *} for various corresponding available values of Gr{sup *}. Meanwhile, the dimensionless mean heat transfer, Nu(Re){sup -1/2} is increasing significantly with Gr{sup *} for its corresponding available values of P{sup *}. For pure natural-convection film condensation, Nu(Re{sub w}/Gr){sup 1/3}=1.706 is obtained. (orig.). With 7 figs. [Deutsch] Es wird ein Modell zur Untersuchung der Mischkonvektion bei Filmkondensation von Daempfen an einer Kugel entwickelt, die unter gleichfoermigen Waermefluss daran abwaerts stroemen. Das Modell verbindet die durch natuerliche und durch erzwungene Konvektion bewirkte Filmkondensation unter Einschluss von aus einem Druckgradienten resultierenden Effekten sowie von Dampfschubspannungen an der Phasengrenze. Die numerische Loesung liefert den Separationswinkel des Kondensatfilms {Phi}{sub s} fuer verschiedene Druckgradienten-Parameter P{sup *} und zugehoerige Grashof-Parameter Gr{sup *}. Der Einfluss von P{sup *} auf den mittleren Waermeuebergangsparameter Nu(Re){sup -1/2} bleibt bis P{sup *}=2/9Gr{sup *} ziemlich gering, auch wenn Gr{sup *} zwischen 0.01 und 100 variiert. Fuer reine natuerliche Filmkondensation erhaelt man: Nu(Re{sub w}/Gr){sup 1/3}=1.706. (orig.)
Transition to finger convection in double-diffusive convection
Kellner, M.; Tilgner, A.
2014-01-01
Finger convection is observed experimentally in an electrodeposition cell in which a destabilizing gradient of copper ions is maintained against a stabilizing temperature gradient. This double-diffusive system shows finger convection even if the total density stratification is unstable. Finger convection is replaced by an ordinary convection roll if convection is fast enough to prevent sufficient heat diffusion between neighboring fingers, or if the thermal buoyancy force is...
Estimation of heat loss during transient study of natural convection heat transfer in path facility
International Nuclear Information System (INIS)
Post accident heat removal in Fast Breeder Reactors (FBR) is a very important issue for assuring public safety. Core meltdown accident results in fragmented debris. Accumulation of these core debris on to the main vessel bottom is to be avoided. Hence, as defense-in-depth, an in-vessel Core Catcher (CC) system is provided in FBR just below the strong back of the core support structure to collect, support and maintain the debris in coolable and sub-critical configuration. Heat generated by the debris settled on CC is removed by natural convection so that main vessel temperature does not exceed the allowable value. At SED, Safety Group, experimental and numerical studies are taken up related to core-catcher with the objectives to investigate continuous heat removal capability of core catcher plate through natural convection and to generate data for validation of mathematical models developed. To carry out the experiments on natural convection a Post Accident Thermal Hydraulic (PATH) facility is design and commissioned. In this paper the evaporative loss along with convective and evaporative heat losses from the second phase of experiments, with closed top lid have been estimated and compared with the analytical solution based on the experimental condition at different time intervals. (author)
International Nuclear Information System (INIS)
One strategy for preventing the failure of lower head of a nuclear reactor vessel is to flood the concrete cavity with subcooled water in accidents in which relocation of core material into the vessel lower head occurs. After the core material relocates into the vessel, a crust of solid material forms on the inner wall of the vessel, however, most of the pool remains molten and natural convection exists in the pool. At present, uncertainty exists with respect to natural convection heat transfer coefficients between the pool of molten core material and the reactor vessel wall. In the present work, experiments were conducted to examine natural convection heat transfer in internally heated partially filled spherical pools with external cooling. In the experiments, Freon-113 contained in a Pyrex bell jar was used as a test liquid. The pool was bounded with a spherical segment at the bottom, and was heated with magnetrons taken from a conventional microwave oven. The vessel was cooled from the outside with natural convection of water or with nucleate boiling of liquid nitrogen
Mixed-convective, conjugate heat transfer during molten salt quenching of small parts
Energy Technology Data Exchange (ETDEWEB)
Chenoweth, D.R.
1997-02-01
It is common in free quenching immersion heat treatment calculations to locally apply constant or surface-averaged heat-transfer coefficients obtained from either free or forced steady convection over simple shapes with small temperature differences from the ambient fluid. This procedure avoids the solution of highly transient, non-Boussinesq conjugate heat transfer problems which often involve mixed convection, but it leaves great uncertainty about the general adequacy of the results. In this paper we demonstrate for small parts (dimensions of the order of inches rather than feet) quenched in molten salt, that it is feasible to calculate such nonuniform surface heat transfer from first principles without adjustable empirical parameters. We use literature physical property salt data from the separate publications of Kirst et al., Nissen, Carling, and Teja, et al. for T<1000 F, and then extrapolate it to the initial part temperature. The reported thermal/chemical breakdown of NaNO{sub 2} for T>800 F is not considered to be important due to the short time the surface temperature exceeds that value for small parts. Similarly, for small parts, the local Reynolds and Rayleigh numbers are below the corresponding critical values for most if not all of the quench, so that we see no evidence of the existence of significant turbulence effects, only some large scale unsteadiness for brief periods. The experimental data comparisons from the open literature include some probe cooling-rate results of Foreman, as well as some cylinder thermal histories of Howes.
Lagrangian dispersion and heat transport in convective turbulence
Schumacher, Joerg
2008-01-01
Lagrangian studies of the local temperature mixing and heat transport in turbulent Rayleigh-Benard convection are presented, based on three-dimensional direct numerical simulations. Contrary to vertical pair distances, the temporal growth of lateral pair distances agrees with the Richardson law, but yields a smaller Richardson constant due to correlated pair motion in plumes. Our results thus imply that Richardson dispersion is also found in anisotropic turbulence. We find that extremely large vertical accelerations appear less frequently than lateral ones and are not connected with rising or falling thermal plumes. The height-dependent joint Lagrangian statistics of vertical acceleration and local heat transfer allows us to identify a zone which is dominated by thermal plume mixing.
Turbulence convective heat transfer for cooling the photovoltaic cells
Arianmehr, Iman
Solar PV (photovoltaic) is a rapidly advancing renewable energy technology which converts sunlight directly into electricity. One of the outstanding challenges of the current PV technology is the reduction in its conversion efficiency with increasing PV panel temperature, which is closely associated with the increase in solar intensity and the ambient temperature surrounding the PV panels. To more effectively capture the available energy when the sun is most intense, significant efforts have been invested in active and passive cooling research over the last few years. While integrated cooling systems can lead to the highest total efficiencies, they are usually neither the most feasible nor the most cost effective solutions. This work examines some simple passive means of manipulating the prevailing wind turbulence to enhance convective heat transfer over a heated plate in a wind tunnel.
Experimental Study of Free Convection in Coiled Tube Heat Exchanger
Directory of Open Access Journals (Sweden)
Harith Mohammed
2013-05-01
Full Text Available An experimental study has been conducted on steady-state natural convection heat transfer from helical coil tubes in vertical orientation. Water was used as a bath liquid without any mixing and cold water was used as a coolant fluid. A straight copper tube of 6 mm ID, 8 mm OD and 3 m length was bend to fabricate the helical coil. Four coils are used in this experiment has different curvature ratios and pitches. The data were correlated using tube diameter as the characteristic length. The results show that the overall heat transfer coefficient and Nusselt number increase when the flow rate of coolant and curvature ratio increase. The effect of coil pitch was investigated and the results show that when of the coil pitch (angle of inclination increases Nusselt number increase. A correlation was presented to calculate the outside average Nusselt number of coil.
Directory of Open Access Journals (Sweden)
Zeinali Heris Saeed
2011-01-01
Full Text Available Abstract In this article, laminar flow-forced convective heat transfer of Al2O3/water nanofluid in a triangular duct under constant wall temperature condition is investigated numerically. In this investigation, the effects of parameters, such as nanoparticles diameter, concentration, and Reynolds number on the enhancement of nanofluids heat transfer is studied. Besides, the comparison between nanofluid and pure fluid heat transfer is achieved in this article. Sometimes, because of pressure drop limitations, the need for non-circular ducts arises in many heat transfer applications. The low heat transfer rate of non-circular ducts is one the limitations of these systems, and utilization of nanofluid instead of pure fluid because of its potential to increase heat transfer of system can compensate this problem. In this article, for considering the presence of nanoparticl: es, the dispersion model is used. Numerical results represent an enhancement of heat transfer of fluid associated with changing to the suspension of nanometer-sized particles in the triangular duct. The results of the present model indicate that the nanofluid Nusselt number increases with increasing concentration of nanoparticles and decreasing diameter. Also, the enhancement of the fluid heat transfer becomes better at high Re in laminar flow with the addition of nanoparticles.
International Nuclear Information System (INIS)
This work studies the forced convection problem in internal flow between concentric annular ducts, with radial fins at the internal tube surface. The finned surface heat transfer is analyzed by two different approaches. In the first one, it is assumed one-dimensional heat conduction along the internal tube wall and fins, with the convection heat transfer coefficient being a known parameter, determined by an uncoupled solution. In the other way, named conjugated approach, the mathematical model (continuity, momentum, energy and K-e equations) applied to tube annuli problem was numerically solved using finite element technique in a coupled formulation. At first time, a comparison was made between results obtained for the conjugated problem and experimental data, showing good agreement. Then, the temperature profiles under these two approaches were compared to each other to analyze the validity of the one-dimensional classical formulation that has been utilized in the heat exchanger design. (author)
Heat transfer enhancement and reduction by poylmer additives in turbulent Rayleigh Benard convection
Dubief, Yves
2010-01-01
This letter confirms the existence of heat transfer enhancement (HTE) and reduction (HTR) in turbulent natural convection with polymer additives. HTE and HTR were numerically predicted by Benzi et al.(PRL, 104, 024502, 2010) in homogenous turbulent convection, but experiments by Ahlers & Nikolaenko(PRL, 104, 034503, 2010) in turbulent natural convection observed HTR only. Using direct numerical simulation of natural convection, the present study reconciles earlier numerical ...
Directory of Open Access Journals (Sweden)
Bhaskar Kalita
2012-12-01
Full Text Available This paper is devoted for the study of effects influences by heat source on unsteady free convection flow and heat transfer characteristic of a viscous incompressible and electrically conducting fluid between two heated vertical plates in the presence of a uniform magnetic field applied transversely to the flow. The leading momentum and energy equations are solved by the Laplace transform technique and solutions are presented through graphs for velocity and temperature distribution.
Bhaskar Kalita
2012-01-01
This paper is devoted for the study of effects influences by heat source on unsteady free convection flow and heat transfer characteristic of a viscous incompressible and electrically conducting fluid between two heated vertical plates in the presence of a uniform magnetic field applied transversely to the flow. The leading momentum and energy equations are solved by the Laplace transform technique and solutions are presented through graphs for velocity and temperature distribution.
Direct numerical simulation of liquid sodium droplet combustion in forced convection air flow
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In case of sodium leakage in liquid metal fast breeder reactor, the liquid sodium comes out in droplet form from a pipe accompanied with ignition and combustion Combustion heat and reaction products might affect integrity of steel liners in piping rooms. A direct numerical simulation code, COMET, is developed to simulate the combustion of a liquid sodium droplet. The extended MAC method coupled with a higher-order upwind scheme is used to calculate reacting compressible flow. Multicomponent counter diffusion of chemical species, mass and energy transfer by sodium evaporation, and heat transfer by radiation and thermal conductivity are calculated coupling with the flow. Chemical reaction of sodium, oxygen and water vapor is calculated by using the equation-solving methods of equilibrium constants. Thermodynamic properties of the mixed gas are evaluated based on the molecular transport theories. By using COMET, the single droplet combustion of liquid sodium in forced convection air flow is numerically simulated. Spatial distributions such as combustion heat, temperature, pressure, and chemical species behaviors such as formation, decomposition and transport are analyzed and discussed. The change of the droplet diameter agrees closely with the d-square law that has been experimentally observed and theoretically derived. (author)
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The helically coiled tube of heat exchanger is used for the evaporator of prototype fast breeder reactor 'Monju'. This paper aims at the grasp of two-phase flow phenomena of forced convective boiling of water inside helical coiled tube, especially focusing on oscillation phenomena of dryout point. A glass-made helically coiled tube was used to observe the inside water boiling behavior flowing upward, which was heated by high temperature oil outside the tube. This oil was also circulated through a glass made tank to provide the heat source for water evaporation. The criterion for oscillation of dryout point was found to be a function of inlet liquid velocity and hot oil temperature. The observation results suggest the mechanism of dryout point oscillation mainly consists of intensive nucleate boiling near the dryout point and evaporation of thin liquid film flowing along the helical tube. In addition, the oscillation characteristics were experimentally confirmed. As inlet liquid velocity increases, oscillation amplitude also increases but oscillation cycle does not change so much. As hot oil temperature increases, oscillation amplitude and cycle gradually decreases. (author)
Experimental investigation of turbulent mixed convection in the wake of a heated sphere
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The axisymmetric wake of a heated sphere under conditions of turbulent mixed convection is investigated in the water test section FLUTMIK. The sphere is located in a vertical channel with forced convective upward flow. The influence of buoyancy forces to the flow field is studied by comparison with the unheated wake. The theoretical fundamentals describing turbulent flows and different versions of the k-? turbulence model extended by buoyancy terms are described in detail. The quantities to be determined experimentally are derived. The temperature and the components of the velocity vector in axial and radial directions are measured simultaneously by means of a thermocouple probe and a two component, two color laser Doppler anemometer. The flow quantities are determined at axial distances between 5 and 106 sphere diameters. The functional principle and the basis of the laser Doppler anemometer are explained. The mean velocity, the mean temperature, the intensities of their fluctuations and the turbulent exchange quantities of momentum and heat transport are calculated. The decay laws of the quantities along the axis of the channel and the radial profiles are indicated and discussed. The applicability of the experimental results of the axisymmetric buoyancy influenced turbulent wake with respect to the turbulence models presented are shown. (orig.)
Experimental study of natural convective heat transfer in a vertical hexagonal sub channel
Energy Technology Data Exchange (ETDEWEB)
Tandian, Nathanael P.; Umar, Efrizon; Hardianto, Toto; Febriyanto, Catur [Faculty of Mechanical and Aerospace Engineering, Institut Teknologi Bandung, Jl. Ganesa 10, Bandung 40132 (Indonesia); Nuclear Technology Center for Materials and Radiometry, National Nuclear Energy Agency, Bandung (Indonesia); Faculty of Mechanical and Aerospace Engineering, Institut Teknologi Bandung, Jl. Ganesa 10, Bandung 40132 (Indonesia); Nuclear Energy Regulation Agency, Jakarta (Indonesia)
2012-06-06
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.
Enhanced heat transfer performances of molten salt mixed convection in a vertical annular duct
He, Shiquan; Ding, Jing; Lu, Jianfeng; Wang, Weilong; Yang, Jianping
2014-07-01
The mixed convection heat transfer of upward molten salt flow in a vertical annular duct is experimentally and numerically studied. The heat transfer performances of mixed convection are measured under Reynolds number 2,500-12,000 and inlet temperature 300-400 °C, and Nusselt number of molten salt flow with cooled inner wall monotonically increases with buoyancy number. The mixed convection is further simulated by low-Reynolds number k- ? model and variable properties, and the heat transfer tendency from numerical results agrees with that from experiments. At low Reynolds number, the natural convection plays more important role in the mixed convection. As the buoyancy number rises, the thickness of flow boundary layer near the inner wall increases, while the effective thermal conductivity remarkably rises, so the enhanced heat transfer of mixed convection is mainly affected by the effective thermal conductivity due to turbulent diffusion.
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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)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)
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.
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Highlights: ? We study the effect of radiation on natural convection in a cavity with a heated plate. ? The heated plate is placed horizontally or vertically at the center of the cavity. ? The surface radiation makes the temperature distribution uniform inside the cavity. ? The average Nusselt number increases with the emissivity and the Rayleigh number. ? The heat transfer rate is higher when the plate is oriented vertically. -- Abstract: This paper reports a fundamental theoretical study made to understand the interaction of surface radiation and natural convection in an air filled cavity with a centrally placed thin heated plate. The vertical walls of the cavity are cooled while the horizontal ones are insulated. The thin plate is assumed to be isothermal and is placed horizontally or vertically. The governing equations were solved using a finite volume method on a uniformly staggered grid system. The effects of the pertinent parameters, viz., Rayleigh number (105 ? Ra ? 107), plate length (0.25 ? D ? 0.75) and emissivity (0 ? ? ? 1) are investigated in detail. In general the results indicate a better homogenization of temperature field within the cavity by radiation. It is also found that the contribution of the convective mechanism to the overall heat transfer increases with emissivity when the plate is horizontally placed whereas decreases when it is vertically placed. This study demonstrates that any model representing a situation of this kind that ignores surface radiation leads to erroneous predictions
The Arctic Mediterranean Sea - Deep convection, oceanic heat transport and freshwater
Rudels, Bert
2014-05-01
The speculations about the driving forces behind the oceanic meridional circulation and the importance of the northward transports of oceanic heat for the ice conditions in the Arctic Ocean have a long history, but only after the Fram expedition 1893-1896 and from the studies by Nansen, Helland-Hansen and Sandström in the early 1900s did these speculations attain observational substance. In the late 1970s and onward these questions have again risen to prominence. A study of deep convection in the Greenland Sea, then assumed to drive the global thermohaline circulation, started with the Greenland Sea Project (GSP), while the investigation of the exchanges of volume and heat through Fram Strait had a more hesitant start in the Fram Strait Project (FSP). Not until 1997 with the EC project VEINS (Variation of Exchanges in the Northern Seas) was a mooring array deployed across Fram Strait. This array has been maintained and has measured the exchanges ever since. Eberhard Fahrbach was closely involved in these studies, as a secretary for the GSP and as the major driving force behind the Fram Strait array. Here we shall examine the legacy of these projects; How our understanding of these themes has evolved in recent years. After the 1980s no convective bottom water renewal has been observed in the Greenland Sea, and the Greenland Sea deep waters have gradually been replaced by warmer, more saline deep water from the Arctic Ocean passing through Fram Strait. Small-scale convective events penetrating deeper than 2500m but there less dense than their surroundings were, however, observed in the early 2000s. The Fram Strait exchanges have proven difficult to estimate due to strong variability, high barotropic and baroclinic eddy activity and short lateral coherence scales. The fact that the mass transports through Fram Strait do not balance complicates the assessment of the heat transport through Fram Strait into the Arctic Ocean and mass (volume) and salt (freshwater) balances for the entire Arctic Ocean are needed. The waters exiting the Arctic Ocean through Fram Strait are colder than those entering and with reasonable assumptions about the origin of the waters providing the net outflow it is possible to deduce the amount of the entering oceanic heat going to the atmosphere (>50%), to ice melt (20%). Almost all of this heat loss occurs in the Nansen Basin. The rest of the heat is used for heating the net outflow. It also becomes clear that freshwater, with its phase changes and its multiple transport pathways, plays a crucial role in the climate, not just of the Arctic Ocean but of the Arctic as a whole.
Studies on convective heat transfer through helical coils
Pawar, S. S.; Sunnapwar, Vivek K.
2013-12-01
An experimental investigation on steady state convection heat transfer from vertical helical coiled tubes in water was performed for laminar flow regime. Three coils with curvature ratios as 0.0757, 0.064, 0.055 and range of Prandtl number from 3.81 to 4.8, Reynolds number from 3,166 to 9,658 were considered in this work. The heat transfer data were generated from 30 experiments conducted at constant water bath temperature (60 °C) for different cold water flow rates in helical coils. For the first time, an innovative approach of correlating Nusselt number with ‘M’ number is proposed which is not available in the literature and the developed correlations are found to be in good agreement with the work of earlier researchers. Thus, dimensionless number ‘M’ was found to be significant to characterize the hydrodynamics of fluid flow and heat transfer correlations in helical coils. Several other correlations based on experimental data are developed. To cover wide range of industrial applications, suitable generalized correlations based on extended parameters beyond the range of present experimental work are also developed. All these correlations are developed by using least-squares power law fit and multiple-regression analysis of MATLAB software. Correlations so developed were compared with published correlations and were found to be in good agreement. Comparison of heat transfer coefficients, friction factor and Nusselt number for different geometrical conditions is presented in this paper.
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In this study, the problem of conjugated mixed convection-conduction heat transfer along the vertical circular pin-fin around which gases flow in the direction of fin axes is analysed by taking the radiative effect into account and the general algorithm that can be applied to both circular fin and flat-plate fin is suggested. The governing equations of cylindrical coordinates are nondimensionalized by introducing the radiation-conduction parameter M, convection-condution parameter Nc, buoyancy force parameter Ri and transverse curvature parameter ? and solved numerically by varying these parameters properly. The radiative heat flux in the energy equation is presented by using the Rosseland approximation and is calculated by the modified Killer's Box method together with the momentum equation and the finite control volume method is used to solve the heat conduction equation of the fin. The calculated results for the cases of neglecting radiative effect(M=0), forced convection alone(Ri=0) and flat-plate fin (?=0) were compared fairly well with those of previous papers. When radiation effect was not considered, the local heat transfer coefficient was almost constant from the fin root to the fin tip. But when the radiative effect was considered, in the all over the fin surface the local modified heat transfer coefficient became higher and increased as it goes from the fin tip to the fin root. (Author)
Forced convection flow boiling and two-phase flow phenomena in a microchannel
Na, Yun Whan
2008-07-01
The present study was performed to numerically analyze the evaporation phenomena through the liquid-vapor interface and to investigate bubble dynamics and heat transfer behavior during forced convective flow boiling in a microchannel. Flow instabilities of two-phase flow boiling in a microchannel were studied as well. The main objective of this research is to investigate the fundamental mechanisms of two-phase flow boiling in a microchannel and provide predictive tools to design thermal management systems, for example, microchannel heat sinks. The numerical results obtained from this study were qualitatively and quantitatively compared with experimental results in the open literature. Physical and mathematical models, accounting for evaporating phenomena through the liquid-vapor interface in a microchannel at constant heat flux and constant wall temperature, have been developed, respectively. The heat transfer mechanism is affected by the dominant heat conduction through the thin liquid film and vaporization at the liquid-vapor interface. The thickness of the liquid film and the pressure of the liquid and vapor phases were simultaneously solved by the governing differential equations. The developed semi-analytical evaporation model that takes into account of the interfacial phenomena and surface tension effects was used to obtain solutions numerically using the fourth-order Runge-Kutta method. The effects of heat flux 19 and wall temperature on the liquid film were evaluated. The obtained pressure drops in a microchannel were qualitatively consistent with the experimental results of Qu and Mudawar (2004). Forced convective flow boiling in a single microchannel with different channel heights was studied through a numerical simulation to investigate bubble dynamics, flow patterns, and heat transfer. The momentum and energy equations were solved using the finite volume method while the liquid-vapor interface of a bubble is captured using the VOF (Volume of Fluid) technique. The effects of different constant heat fluxes and different channel heights on the boiling mechanisms were investigated. The effects of liquid velocity on the bubble departure diameter were analyzed. The obtained results showed that the wall superheats at the position of nucleate boiling are relatively independent of the mass flow rates at the same channel height. The obtained results, however, showed that the heat flux at the onset of nucleate boiling strongly depends on the channel height. With a decrease of the channel height and an increase of the liquid velocity at the channel inlet, the departure diameter of a bubble was smaller. The periodic flow patterns, such as the bubbly flow, elongated slug flow, and churn flow were observed in the microchannel. Flow instabilities of two-phase flow boiling in a trapezoidal microchannel using a three-dimensional model were investigated. Fluctuation behaviors of flow boiling parameters such as wall temperature and inlet pressure caused by periodic flow patterns were studied at different heat fluxes and mass fluxes. The numerical results showed large amplitude and short period oscillations for wall temperature and inlet pressure fluctuations. Stable and unstable flow boiling regime with short period oscillations were investigated. Those flow boiling regimes were not listed in stable and unstable boiling regime map proposed by Wang et al. (2007).
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The stochastic Swift-Hohenberg equation is studied as a model of Rayleigh-Benard convection in a simple fluid. The equation has been solved numerically in two spatial dimensions to obtain the convective heat current and the roll pattern when either a bulk stochastic forcing field or different models of thermal-diffusivity mismatch at the sidewalls of the convective cell are considered. The parameters that enter the equation have been chosen to match the ramping experiments on Rayleigh-Benard convection by Meyer, Ahlers, and Cannell [Phys. Rev. Lett. 59, 1577 (1987)]. For any combination of forcing mechanisms, we are able to find values of their various amplitudes that lead to excellent fits to the experimentally measured convective current. In the case of a bulk random forcing field, we find an amplitude of F1=5x10-5, compared to Fth=1.92x10-9, the value obtained from fluctuation theory. A random, cellular pattern of rolls is observed, in agreement with experiments involving a gel sidewall designed to eliminate the influence of the sidewalls on the onset of convection. A thermal-conductivity mismatch at the sidewall has also been modeled by a variety of forcing fields. In all cases a roll-like pattern that reflects the geometry of the sidewalls is observed. Different combinations of both types of forcing fields have also been studied and found to yield patterns intermediate between cellular and roll-like, while yielding a veellular and roll-like, while yielding a very reasonable fit to the convective heat current measured experimentally
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Using a vectorized finite-difference marching technique, the steady-state continuity, momentum, and energy equations are solved numerically to evaluate the effects of buoyancy-induced secondary flow on forced flow in a horizontal rectangular duct with uniform bottom heating. Combined entry region conditions are considered, and the secondary flow is found to consist of longitudinal plumes and vortices that first develop at the vertical sidewalls and subsequently propagate to interior spanwise positions. Sequential stages of the secondary flow development are computed in detail and used to interpret the nonmonotonic longitudinal distribution of the spanwise average Nusselt number. The distribution is characterized by oscillations that, under certain conditions, are damped and yield a fully developed Nusselt number that substantially exceeds the value for pure forced convection
Numerical Simulations of Heat ExplosionWith Convection In Porous Media
Allali, Karam; Bikany, Fouad; Taik, Ahmed; Volpert, Vitaly
2013-01-01
In this paper we study the interaction between natural convection and heat explosion in porous media. The model consists of the heat equation with a nonlinear source term describing heat production due to an exothermic chemical reaction coupled with the Darcy law. Stationary and oscillating convection regimes and oscillating heat explosion are observed. The models with quasi-stationary and unstationary Darcy equation are compared.
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The effective thermal conductivity for radiative heat transfer within an optically thick fluid layer undergoing high Rayleigh number convection is derived. This result is combined with available ''pure'' free-convection heat-transfer correlations to obtain closed-form analytical descriptions of the gross properties of a radiating fluid layer heated internally or form below. These simple solutions compare favorably with recent work in which the governing energy equation incorporating both turbulant heat transport and thermal radiation is solved numerically
Neely, Aj; Ireland, Pt; Harper, Lr
1997-01-01
An experimental investigation of the performance of extended fin surfaces for the forced convective cooling of a range of engine component geometries in crossflow is reported. The experiments were undertaken to measure the surface heat transfer coefficient distributions of external finning around non-cylindrical geometries for use in aviation gas turbines in which the cooling performance/mass ratio must be maximized. The geometries examined were a box (square with rounded corners), a flute (r...
Bianco Vincenzo; Nardini Sergio; Manca Oronzio
2011-01-01
Abstract In this article, developing turbulent forced convection flow of a water-Al2O3 nanofluid in a square tube, subjected to constant and uniform wall heat flux, is numerically investigated. The mixture model is employed to simulate the nanofluid flow and the investigation is accomplished for particles size equal to 38 nm. An entropy generation analysis is also proposed in order to find the optimal working condition for the given geometry under given boundary conditions. A simple analytica...
Surducan, E.; Surducan, V.; Limare, A.; Neamtu, C.; Di Giuseppe, E.
2014-12-01
We report the design, construction, and performances of a microwave (MW) heating device for laboratory experiments with non-contact, homogeneous internal heating. The device generates MW radiation at 2.47 GHz from a commercial magnetron supplied by a pulsed current inverter using proprietary, feedback based command and control hardware and software. Specially designed MW launchers direct the MW radiation into the sample through a MW homogenizer, devised to even the MW power distribution into the sample's volume. An adjustable MW circuit adapts the MW generator to the load (i.e., the sample) placed in the experiment chamber. Dedicated heatsinks maintain the MW circuits at constant temperature throughout the experiment. Openings for laser scanning for image acquisition with a CCD camera and for the cooling circuits are protected by special MW filters. The performances of the device are analyzed in terms of heating uniformity, long term output power stability, and load matching. The device is used for small scale experiments simulating Earth's mantle convection. The 30 × 30 × 5 cm3 convection tank is filled with a water-based viscous fluid. A uniform and constant temperature is maintained at the upper boundary by an aluminum heat exchanger and adiabatic conditions apply at the tank base. We characterize the geometry of the convective regime as well as its bulk thermal evolution by measuring the velocity field by Particle Image Velocimetry and the temperature field by using Thermochromic Liquid Crystals.
On the Interaction of Internal Gravity Waves with Magnetic Field II. Convective Forcing
Rogers, T. M.; Macgregor, K. B.
2010-01-01
We present results from numerical simulations of the interaction of internal gravity waves (IGW) with magnetic fields in the radiative interior of the Sun. In this second paper, the waves are forced self-consistently by an overlying convection zone and a toroidal magnetic field is imposed in the stably stratified layer just underneath convection zone. Consistent with the results of previous analytic and simple numerical calculations, we find a strong wave-field interaction, ...
New force or thermal convection in the differential-accelerometer experiment
International Nuclear Information System (INIS)
Recently Thieberger reported that the result of his differential-accelerometer experiment is consistent with the existence of a substance-dependent medium-range repulsive force. It is shown that the effect due to thermal convection is not negligible and can be large enough to account for his data. Methods of measuring and minimizing the thermal convection effect, as well as several ways of improving the differential-accelerometer experiment, are proposed. (orig.)
Local convective heat exchanges from a rotor facing a stator
Energy Technology Data Exchange (ETDEWEB)
Harmand, S.; Desmet, B. [Universite de Valenciennes et du Hainaut-Cambresis, 59 - Valenciennes (France). Laboratoire de Mecanique et d' Energetique, ENSIMEV; Watel, B. [Universite Joseph-Fourier, Dept. Genie Thermique et Energie, Grenoble I, 38 (France)
2000-03-01
The local convective heat transfer from a rotor with a 310 mm outer radius is studied experimentally at a distance of 3 mm from a coaxial crown-shaped stator with a 176 mm inner radius and a 283 mm outer radius. The experimental technique is based on the use of a thermally thick rotor heated from behind by infrared radiation. The local heat flux distribution from the rotor surface is identified by resolving the Laplace equation by finite difference method using the experimental temperature distribution as boundary conditions. The tests are carried out with the single rotor and the stator/rotor system for local rotational Reynolds numbers ranging from 2.0.10{sup 4} to 1.47.10{sup 6} and thus sweeping across the laminar, transition and turbulent flow regimes. The local and mean Nusselt numbers for the single disc are compared with those obtained experimentally for the stator/rotor system. The flow structure in the space between the rotor and the stator is analysed by Particle Image Velocimetry. (authors)
Mixed convection heat transfer in rotating vertical elliptic ducts
Directory of Open Access Journals (Sweden)
Olumuyiwa A. Lasode
2007-06-01
Full Text Available This paper presents an investigation into the solution of laminar mixed convective heat transfer in vertical elliptic ducts containing an upward flowing fluid rotating about a parallel axis. The coupled system of normalized conservation equations are solved using a power series expansion in ascending powers of rotational Rayleigh Number, Ratau - a measure of the rate of heating and rotation as the perturbation parameter. The results show the influence of rotational Rayleigh number, Ratau and modified Reynolds number, Re m on the temperature and axial velocity fields. The effect of Prandtl number, Pr, in the range 1 to 5, and eccentricity, e on the peripheral local Nusselt number are also reported. The mean Nusselt number is observed to be highest at duct eccentricity, e=0 for a given Prandtl number. However, results indicate insensitivity of peripheral local Nusselt number to Prandtl number at eccentricity, e=0.866, which is an important result to a designer of rotating vertical heat exchanger. The effect of eccentricity on the friction coefficient is also presented. The parameter space for the overall validity of the results presented is Ratau Re mPr<820.
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 ?m 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.
Analysis of the heat transfer from horizontal pipes at natural convection
Kapjor, Andrej; Huzvar, Jozef; Ftorek, Branislav; Smatanova, Helena
2014-08-01
These article deals with heat transfer from "n" horizontal pipes one above another at natural convection. On the bases of theoretical models have been developed for calculating the thermal performance of natural convection by Churilla and Morgan, for various pipe diameters and temperatures. These models were compared with models created in CFD-Fluent Ansys the same boundary conditions. The aim of the analyze of heat and fluxional pipe fields "n" pipes one about another at natural convection is the creation of criterion equation on the basis of which the heat output of heat transfer from pipe oriented areas one above another with given spacing could be quantified.
International Nuclear Information System (INIS)
The present study investigates natural convection heat transfer from a heated cylinder cooled by a water slurry of Microencapsulated Phase Change Material (MCPCM). A normal paraffin hydrocarbon with carbon number of 18 and melting point of 27.9degC, is microencapsulated by Melamine resin into particles of which average diameter is 9.5 ?m and specific weight is same as water. The slurry of the MCPCM and water is put into a rectangular enclosure with a heated horizontal cylinder. The heat transfer coefficients of the cylinder were evaluated. Changing the concentrations of PCM and temperature difference between cylinder surface and working fluid. Addition of MCPCM into water, the heat transfer is enhanced significantly comparison with pure water in cases with phase change and is reduced slightly in cases without phase change. (author)
Kuriyama, Masafumi; Tokanai, Hideki; Li, Jianxiu; Harada, Eiji; Konno, Hirotaka
An experimental study is carried out for natural-convection heat transfer to air from upward-facing and downward-facing heated circular plates under the condition that the amount of heat dissipating through the side wall of the plates is sufficiently negligible. For the case of upward-facing plate, it is shown that in laminar flow region, the temperature distributions in air near the plate are similar when the Rayleigh numbers of the experimental runs are equal to each other. A correlation equation is proposed for the average heat transfer coefficient, with the discussion on the applicability of previously proposed equations. Another heat transfer correlation is proposed for downward-facing plate and compared with previousily proposed experimental and theoretical correlations. Moreover, a profile method is applied to investigate the functional forms of the velocity and temperature profiles in boundary layer.
Yano, Ryosuke
2015-01-01
We discuss the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, both theoretically and numerically. In the theoretical discussion, we assume that the Lorentz contraction is ignored and spacetime is flat. For understanding of the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, we solve the relativistic Boltzmann equation using the direct simulation Monte Carlo method. Numerical results indicate that strongly nonequilibrium states are formed in vicinities of two walls, which do not allow us to discuss the transition of the thermal conduction to the thermal convection of thermally relativistic fluids under the gravitational force in the framework of the relativistic Navier-Stokes-Fourier equation, when the flow-field is under the transition regime between the rarefied and continuum regimes, whereas such strongly nonequilibrium states are not formed in vicinities...
Convective heat transfer from a heated elliptic cylinder at uniform wall temperature
Kaprawi S, Dyos Santoso
2013-01-01
This study is carried out to analyse the convective heat transfer from a circular and an elliptic cylinders to air. Both circular and elliptic cylinders have the same cross section. The aspect ratio of cylinders range 0-1 are studied. The implicit scheme of the finite difference is applied to obtain the discretized equations of hydrodynamic and thermal problem. The Choleski method is used to solve the discretized hydrodynamic equation and the iteration method is applied to solve the discretiz...
International Nuclear Information System (INIS)
Highlights: ? It gives heat transfer characteristics in a rectangular heat storage tank as the basic unit for reservoir of thermal storage. ? Onset of natural convection gets easier for the MPCMS with a higher mass concentration. ? It enhances the heat transfer ability of natural convection for the MPCMS. ? Obtained the relationship between Ra and Nu of the MPCMS. - Abstract: The main purpose of this experiment is to evaluate natural convection heat transfer characteristics of microencapsulated PCM (phase change material) slurry (MPCMS) during phase change process in a rectangular heat storage tank heated from the bottom and cooled at the top. The microencapsulated PCM is several material compositions of n-paraffin waxes (mainly nonadecane) as the core materials, outside a layer of a melamine resin wrapped. In the present study, its slurry is used mixing with water. And the specific heat capacity with latent heat shows a peak value at the temperature of about T = 31 °C. We investigate the influences of the phase change process of the MPCMS on natural convection heat transfer. The experimental results indicate that phase change process of the MPCMS promote natural convection heat transfer. The local maximum heat transfer enhancement occurs at approximately TH = 34 °C corresponding to the heated plate temperature. With high mass concentration Cm, the onset of natural convection gets easier for the MPCMS. The temperature gradient is largere temperature gradient is larger near top plate and bottom plate of a rectangular heat storage tank. Heat transfer coefficient increases with the phase change of the PCM. And it summarizes that the phase change process of the PCM promote the occurrence of natural convection.
Heat Transfer Convection in The Cooking of Apple Using a Solar Cooker Box-Type
Terres, H.; Chávez, S.; Lizardi, A.; López, R.; Vaca, M.; Flores, J.; Salazar, A.
2015-01-01
In this work, experimental results to determine the convection heat transfer coefficient in the cooking process of apple using a solar cooker box-type are presented. Experimental data of temperatures for water, surface and central point of the apple were used. To determine the convection coefficient, the apple was modelled as a sphere. The temperatures evolution was defined using thermocouples located at water, surface and central point in the vegetables. Using heat transfer convection equations in transitory state and the temperatures measured, the Biot number and the convection coefficient were determined.
International Nuclear Information System (INIS)
Unsteady laminar mixed convection flow (combined free and forced convection flow) along a vertical slender cylinder embedded in a porous medium under the combined buoyancy effect of thermal and species diffusion has been studied. The effect of the permeability of the medium as well as the magnetic field has been included in the analysis. The partial differential equations with three independent variables governing the flow have been solved numerically using an implicit finite difference scheme in combination with the quasilinearization technique. Computations have been carried out for accelerating, decelerating and oscillatory free stream velocity distributions. The effects of the permeability of the medium, buoyancy forces, transverse curvature and magnetic field on skin friction, heat transfer and mass transfer have been studied. It is found that the effect of free stream velocity distribution is more pronounced on the skin friction than on the heat and mass transfer. The permeability and magnetic parameters increase the skin friction, but reduce the heat and mass transfer. The skin friction, heat transfer and mass transfer are enhanced due to the buoyancy forces and curvature parameter. The heat transfer is strongly dependent on the viscous dissipation parameter and the Prandtl number, and the mass transfer on the Schmidt number. (orig.)
Desmon, Leland G; Sams, Eldon W
1950-01-01
A heat-transfer investigation was conducted with air in an electrically heated platinum tube with long-approach entrance, inside diameter of 0.525 inch, and effective heat-transfer length of 24 inches over ranges of Reynolds number up to 320,000, average inside-tube-wall temperature up to 3053 degrees R, and inlet-air temperature up to 1165 degrees R. Correlation of data by the conventional Nusselt relation resulted in separation of data with tube-wall temperature. Good correlation was obtained, however, by use of a modified Reynolds number.
Heat Transfer Enhancement of Nanofluid in Natural Convection of an Enclosure Heated from Below
International Nuclear Information System (INIS)
The general strategy for improving the safety of nuclear power plant and its economics is to accomplish power uprates while securing sufficient thermalhydraulic margin. In order to succeed this strategy, there have been a lot of efforts in increasing the margin through the enhancement of heat transfer capability in coolants. However, despite their efforts, only about 10 ? 15 % increase of the thermal margin is possible by using the best art known well up to now with installation of mechanical engineering devices such as mixing vane or button to generating the swirl flow and turbulent mixing. The limit of the capability of the best technique has made a lot of engineers to be frustrated to do the power uprates. Nevertheless, fortunately a new innovative idea is being proposed in heat transfer community as an engineering colloidal fluid to basically change the original properties of the coolant. The fluid began to be called by Choi as a nanofluid which is a mixture of solid nanoparticles and liquid. So many researchers expressed a lot of interests in its capability. Their focuses are limited in investigations of thermal conductivity enhancement. All thermal engineers usually have learned the importance of natural convection with the convective motion driven by buoyancy in a thermal system design. Its understanding is central to investigate on flowing characteristics of the nanofluids. In particular, natural convection in enclosure has considerable interest of many engie has considerable interest of many engineers due to the characteristics of the motion derived by the interaction of a difference in density driven by thermal gradient with a gravitational field. And also, the natural convection is significantly important in an accident of loss of pumping capability in nuclear power plant and a safety analysis. Therefore, the purpose of this paper is to investigate the basic natural convection characteristics of nanofluids for which yet a lot of understanding is necessary. For this, a commercial CFD code, Fluent is used
Convective Heating Predictions of Apollo IV Flight Data
White, Molly E.
2012-01-01
It has been more than 50 years since NASA engineers have attempted to design a manned space vehicle with the capability to return from beyond low Earth orbit. In this interval, our methodologies for designing the thermal protection system (TPS) to protect humans from the extremely high temperatures of re-entry have changed significantly. With these considerations in mind, we return to the Apollo IV (AS-501) flight data. This incredible data set allows us to assess the current tools and methodologies being used to design Orion MPCV. In particular, our ability to predict the aftbody separated region convective heating environments for MPCV is critical. The design uses reusable TPS in this area, whereas Apollo designers used ablative TPS which can withstand much more severe environments. This presentation will revisit the flight data, summarize the assumptions going into the analysis, present the results and draw conclusions regarding how accurately we can currently predict the heating in the aftbody separated region of a re-entry capsule.
Natural Convection in Parabolic Enclosure Heated from Below
Directory of Open Access Journals (Sweden)
Ahmed W. Mustafa
2011-06-01
Full Text Available The effects of vertical parabolic walls on natural convection in a parabolic enclosure have investigated numerically in this paper. The bottom wall is heated isothermally, while the other vertical parabolic walls are maintained at constant cold temperature and the top wall is well insulated. The flow and temperature fields are studied numerically for three values (C = 0.1, 0.5, 1.0 of the parabolic equation constant. The laminar flow field is analyzed numerically by solving the steady, two-dimensional incompressible Navier-Stokes and energy equations. The Cartesian velocity components and pressure on a collocated (non-staggered grid are used as dependent variables in the momentum equations, which discretized by finite volume method, body fitted coordinates are used to represent the complex parabolic wall geometry accurately, and grid generation technique based on elliptic partial differential equations is employed. SIMPLE algorithm is used to adjust the velocity field to satisfy the conservation of mass. The range of Rayleigh number is (103? Ra ?105 and Prandtl number is 0.7. The results show that the heat transfer rates decrease with increase the parabolic equation constant.
Free convection heat transfer across rectangular-celled diathermanous honeycombs
International Nuclear Information System (INIS)
Experimental obtained Nusselt number-Rayleigh number plots are presented for free convective heat transfer across inclined honeycomb panels filled with air. The honeycomb cells were rectangular in shape with very long cell dimensions across the slope and comparatively short dimensions up the slope. Elevation aspect ratios, A/sub E/, investigated were 3, 5 and 10; angles of inclination, theta, measured from the horizontal, were 0, 30, 60, 75 and 90 deg. The effect on the Nusselt number, of the emissivities of the plates bounding the honeycomb, and of the emissivity of honeycomb material, was also investigated. The measurements confirmed that the critical Rayleigh number and the post-critical heat transfer depend on the radiant properties of the honeycomb cells. The critical Rayleigh numbers at theta=0 were well predicted by the methods of Sun and Edwards. For 030 deg. The theta=90 deg data were found to be closely correlated by an equation of the form recently proposed by Bejan and Tien
Coupled heat and mass transfer in a convective tunnel dryer
International Nuclear Information System (INIS)
The mechanism of drying in a convective tunnel dryer with air heated in solar collectors was approached first experimentally with a pilot laboratory unit, then numerically taking into account the coupled heat and mass transfers. In the present study, several experimental essays were conducted followed by the adoption of a simulation tool describing the opening conditions of the tunnel dryer and a behavioural model that can be of great interest in the design and the automation of such industrial units. Indeed, behavioural models of thermodynamic system are characterised by the interactions of a large number of complex phenomenon, which call for various types of energy. This dynamic feature requires a modeling approach, using physical phenomenon such as energy storage. energy transformation and energy dissipation as data. The pseudo-bond graph methodology was used in modelling the drying system. This methodology was very suitable for thermo fluid process. It accepts the use of elements that do not exist in the traditional bond graph methods. An explicit pseudo-bond graph model who describes the process of water evaporation under the tray is studies in this paper and the governing equations are determined using bond graph properties.(Author)
Directory of Open Access Journals (Sweden)
Ziyaddin RECEBL?
2008-01-01
Full Text Available In some studies, the effect of magnetic field on heat convection has been investigated given that physical properties are constant regardless of temperature. The effect of magnetic field on heat convection and fluids whose physical properties change by temperature has been investigated in this study as physical properties of fluids change by the effect of temperature. Momentum, continuity and energy equations including electromagnetic force affecting the fluid were used in the solution. Temperatures at axial and radial directions and Nusselt numbers were calculated depending on magnetic field intensity and other physical properties of fluid by solving the equation system written in cylindrical coordinates system by means of one of the numerical methods which is finite difference method. According to results, velocity and temperature of the cooled fluid decreased following an increase in the intensity of magnetic field placed vertically to flow direction. As determined in the previous one, this study also indicated that the increase in Reynolds number increases Nusselt number, and increasing the effect of magnetic field decreases Nusselt number. The theoretical results of the present study are in conformity with the results of our previous one.
Scientific Electronic Library Online (English)
V. C., Mariani; L. S., Coelho.
2007-09-01
Full Text Available SciELO Brazil | Language: English Abstract in english A numerical study was conducted to investigate steady heat transfer and flow phenomena of natural convection of air in enclosures, with three aspect ratios (H/W = 1, 2, and 4), within which there is a local heat source on the bottom wall at three different positions, Wh. This heat source occupies 1% [...] of the total volume of the enclosure. The vertical walls in the enclosures are insulated and there is an opening on the right wall. The natural convection is influenced by the difference in temperature between the left and right walls, represented by a Rayleigh number (Ra e), and by local heat source, represented by a Rayleigh number (Ra i). Numerical simulations were performed for several values of the Rayleigh number ranging between 10³ and 10(6), while the intensity of the two effects - the difference in temperature on the vertical walls and the local heat source - was evaluated based on the Ra i/Ra e ratio in the range between 0 and 2500. The analysis proceeds by observing variations in the streamlines and isotherms with respect to the different Ra e, R ratios, aspect ratios, of the radius and positions of the local heat source. The average Nusselt numbers on the hot and cold walls are influenced by different values of the parameters R, Ra e, Wh, and H/W. Results show the presence of different flow patterns in the enclosures studied. Thus, the flow and heat transfer can be controlled by external heating, and local heat source.
Directory of Open Access Journals (Sweden)
V. C. Mariani
2007-09-01
Full Text Available A numerical study was conducted to investigate steady heat transfer and flow phenomena of natural convection of air in enclosures, with three aspect ratios (H/W = 1, 2, and 4, within which there is a local heat source on the bottom wall at three different positions, Wh. This heat source occupies 1% of the total volume of the enclosure. The vertical walls in the enclosures are insulated and there is an opening on the right wall. The natural convection is influenced by the difference in temperature between the left and right walls, represented by a Rayleigh number (Ra e, and by local heat source, represented by a Rayleigh number (Ra i. Numerical simulations were performed for several values of the Rayleigh number ranging between 10³ and 10(6, while the intensity of the two effects - the difference in temperature on the vertical walls and the local heat source - was evaluated based on the Ra i/Ra e ratio in the range between 0 and 2500. The analysis proceeds by observing variations in the streamlines and isotherms with respect to the different Ra e, R ratios, aspect ratios, of the radius and positions of the local heat source. The average Nusselt numbers on the hot and cold walls are influenced by different values of the parameters R, Ra e, Wh, and H/W. Results show the presence of different flow patterns in the enclosures studied. Thus, the flow and heat transfer can be controlled by external heating, and local heat source.
HEAT TRANSFER BY NATURAL CONVECTION IN TWO VERTICAL AND ONE HORIZONTAL PLATE – AN OVERVIEW
Nimkar, Mahendra P.; Prayagi, S. V.
2011-01-01
Natural Convection flow in a vertical channel with internal objects is encountered in several technological applications of particular interest of heat dissipation from electronic circuits, refrigerators, heat exchangers, nuclear reactors fuel elements, dry cooling towers, and home ventilation etc.This study deals with the study of natural convection in horizontal plate with vertical channels. The parameters varied during the experimentation are heat input, aspect ratio (the ratio of gap of h...
ENHANCEMENT OF NATURAL CONVECTION HEAT TRANSFER FROM RECTANGULAR FINS BY CIRCULAR PERFORATIONS
Wadhah Hussein Abdul Razzaq Al- Doori
2011-01-01
The importance of heat transfer by natural convection in enclosures can be found in many engineering applications, such as energy transfer in buildings, solar collectors, nuclear reactors and electronic packaging. An experimental study was conducted to investigate heat transfer by natural convection in a rectangular fin plate with circular perforations as heat sinks. The patterns of the perforations included 24 circular perforations (holes) for the first fin; the number of perforations increa...
Numerical Modeling of Dendritic Growth in Alloy Solidification with Forced Convection
Sun, Dongke; Zhu, Mingfang; Pan, Shiyan; Raabe, Dierk
A two dimensional (2D) cellular automaton (CA) - lattice Boltzmann (LB) model is presented to investigate the effects of forced melt convection on the solutal dendritic growth. In the model, the CA approach of simulating the dendritic growth is incorporated with the kinetic-based lattice Boltzmann method (LBM) for numerically solving the melt flow and solute transport. Two sets of distribution functions are used in the LBM to model the convective-diffusion phenomena during dendritic growth. After validating the model by comparing the numerical results with the theoretical solutions, it is applied to simulate the single and multi dendritic growth of Al-Cu alloys without and with a forced convection. The typical asymmetric growth features of convective dendrite are reproduced and the dendritic morphology is strongly influenced by melt convection. The simulated convective multi dendritic features by the present model are also compared with that by the CA-NS model. The present model is found to be more computationally efficient and numerically stable than the CA-NS model.
Energy Technology Data Exchange (ETDEWEB)
Ghorbani, N. [School of Mechanical Engineering, University of Leeds, Leeds, England (United Kingdom); Taherian, H. [Department of Engineering Technology and Industrial Distribution, Texas A and M University, College Station, TX (United States); Gorji, M. [Department of Mechanical Engineering, Babol Noushirvani University of Technology, Babol (Iran); Mirgolbabaei, H. [Department of Mechanical Engineering, Islamic Azad University, Jouybar branch, Jouybar (Iran)
2010-10-15
In this study the mixed convection heat transfer in a coil-in-shell heat exchanger for various Reynolds numbers, various tube-to-coil diameter ratios and different dimensionless coil pitch was experimentally investigated. The experiments were conducted for both laminar and turbulent flow inside coil. Effects of coil pitch and tube diameters on shell-side heat transfer coefficient of the heat exchanger were studied. Different characteristic lengths were used in various Nusselt number calculations to determine which length best fits the data and several equations were proposed. The particular difference in this study in comparison with the other similar studies was the boundary conditions for the helical coils. The results indicate that the equivalent diameter of shell is the best characteristic length. (author)
Leuenberger, D.; Rossa, A.
2007-12-01
Next-generation, operational, high-resolution numerical weather prediction models require economical assimilation schemes for radar data. In the present study we evaluate and characterise the latent heat nudging (LHN) rainfall assimilation scheme within a meso-? scale NWP model in the framework of identical twin simulations of an idealised supercell storm. Consideration is given to the model’s dynamical response to the forcing as well as to the sensitivity of the LHN scheme to uncertainty in the observations and the environment. The results indicate that the LHN scheme is well able to capture the dynamical structure and the right rainfall amount of the storm in a perfect environment. This holds true even in degraded environments but a number of important issues arise. In particular, changes in the low-level humidity field are found to affect mainly the precipitation amplitude during the assimilation with a fast adaptation of the storm to the system dynamics determined by the environment during the free forecast. A constant bias in the environmental wind field, on the other hand, has the potential to render a successful assimilation with the LHN scheme difficult, as the velocity of the forcing is not consistent with the system propagation speed determined by the wind. If the rainfall forcing moves too fast, the system propagation is supported and the assimilated storm and forecasts initialised therefrom develop properly. A too slow forcing, on the other hand, can decelerate the system and eventually disturb the system dynamics by decoupling the low-level moisture inflow from the main updrafts during the assimilation. This distortion is sustained in the free forecast. It has further been found that a sufficient temporal resolution of the rainfall input is crucial for the successful assimilation of a fast moving, coherent convective storm and that the LHN scheme, when applied to a convective storm, appears to necessitate a careful tuning.
Convection heat transfer of CO2 at supercritical pressures in vertical small, mini and micro tubes
International Nuclear Information System (INIS)
Convection heat transfer of CO2 at supercritical pressures in vertical small, mini and micro tubes with inner diameters of 2 mm, 1 mm, 0.27 mm and 0.0992 mm was investigated experimentally and numerically. The effects of heat flux, property variations, buoyancy and flow acceleration on the convection heat transfer were investigated. Detailed information generated by the numerical results, such as the velocity profiles and turbulence kinetic energy near the wall varying along the tube were presented to develop a better understanding. The results show that for a small tube with an inner diameter of 2 mm, buoyancy was the dominant factor affecting the convection heat transfer rather than the flow acceleration. For cases with low inlet Reynolds numbers (less than 2500), buoyancy induces earlier transition from laminar to turbulent flow which increases the heat transfer coefficient. When heat fluxes are very high, the heat transfer in the tube is mainly controlled by the natural convection and the convection heat transfer coefficients for both upward and downward flows are very similar. For convection heat transfer in this small tube at relatively high Reynolds numbers (e.g. Rein?9730) and for the high heat fluxes, the convection heat transfer for upward flow is deteriorated by the strong buoyancy, while the convection heat transfer for downward flow is improved by the buoyancy. For convection heat transfer in a small tube with an inside diameter of 1 mm at Rebe with an inside diameter of 1 mm at Reynolds numbers of 3300?5500, the effect of flow acceleration due to heating on the heat transfer for the experimental conditions is very weak, but buoyancy significantly influences the heat transfer at high heat fluxes. For convection heat transfer in the mini tube with an inside diameter of 0.27 mm at low Reynolds numbers (less than 2900), the flow acceleration significantly influences the turbulence when the heating is relatively strong, and the buoyancy effect is relatively weak but still cannot be neglected. For convection heat transfer in the mini tube at relatively high Reynolds numbers (Rein 4000) for both low and high heat fluxes, the buoyancy and flow acceleration effects are insignificant. Numerical simulations using properly selected turbulence models accurately predict the convection heat transfer of supercritical pressure CO2 in vertical small and mini tubes when buoyancy and flow acceleration are not significant. For convection heat transfer in the micro tube with an inside diameter of 0.0992 mm, the local wall temperature varied non-linearly for both upward and downward flow when the heat flux was high. The difference of the local wall temperature between upward flow cases and downward flow cases was very small when other test conditions were held the same, which indicates that for super-critical CO2 flowing in a micro tube as employed in this study, the buoyancy effect on the convection heat transfer could be neglected, and the flow acceleration induced by the axial density variation with temperature was the main factor that lead to the abnormal local wall temperature distribution at high heat fluxes. (author)
Directory of Open Access Journals (Sweden)
Maksimov Vyacheslav I.
2015-01-01
Full Text Available Results of mathematical modeling of convective heat transfer in air area surrounded on all sides enclosing structures, in the presence of heat source at the lower boundary of the media are presented. Solved the system of differential equations of unsteady Navier-Stokes equations with the appropriate initial and boundary conditions. The process of convective heat transfer is calculated using the models of turbulence Prandtl and Prandtl-Reichard. Takes into account the processes of heat exchange region considered with the environment. Is carried out the analysis of the dimensionless heat transfer coefficient at interfaces “air – enclosures”. The distributions average along the gas temperature range are obtained.
Large-scale tomographic PIV in forced and mixed convection using a parallel SMART version
Kühn, Matthias; Ehrenfried, Klaus; Bosbach, Johannes; Wagner, Claus
2012-07-01
Large-scale tomographic particle image velocimetry (tomographic PIV) was used to study large-scale flow structures of turbulent convective air flow in an elongated rectangular convection cell. Three flow cases have been investigated, that is, pure forced convection and mixed convection at two different Archimedes numbers. The Reynolds number was constant at Re = 1.04 × 104 for all cases, while the Archimedes numbers were Ar = 2.1 and 3.6 for the mixed convection cases, corresponding to Rayleigh numbers of Ra = 1.6 × 108 and 2.8 × 108, respectively. In these investigations, the size of the measurement volume was as large as 840 mm × 500 mm × 240 mm. To allow for statistical analysis of the measured instantaneous flow fields, a large number of samples needed to be evaluated. Therefore, an efficient parallel implementation of the tomographic PIV algorithm was developed, which is based on a version of the simultaneous multiplicative reconstruction technique (SMART). Our algorithm distinguishes itself amongst other features by the fact that it does not store any weighting coefficients. The measurement of forced convection reveals an almost two-dimensional roll structure, which is orientated in the longitudinal cell direction. Its mean velocity field exhibits a core line with a wavy shape and a wavelength, which corresponds to the height and depth of the cell. In the instantaneous fields, the core line oscillates around its mean position. Under the influence of thermal buoyancy forces, the global structure of the flow field changes significantly. At lower Archimedes numbers, the resulting roll-like structure is shifted and deformed as compared to pure forced convection. Additionally, the core line oscillates much more strongly around its mean position due to the interaction of the roll structure with the rising hot air. If the Archimedes number is further increased, the roll-like structure breaks up into four counter-rotating convection rolls as a result of the increased influence of buoyancy forces. Moreover, large-scale tomographic PIV reveals that the orientation of these rolls reflects a `W'-like shape in the horizontal X- Z-plane of the convection cell.
Improvment of Free Convection Heat Transfer Rateof Rectangular Heatsink on Vertical Base Plates
HamidReza Goshayeshi; Mahdi Fahiminia; Mohammad Mahdi Naserian
2011-01-01
In this paper, the laminar heat transfer of natural convection on vertical surfaces is investigated. Most of the studies on natural convection have been considered constantly whereas velocity and temperature domain, do not change with time, transient one are used a lot. Governing equations are solved using a finite volume approach. The convective terms are discretized using the power-law scheme, whereas for diffusive terms the central difference is employed. Coupling between the velocity and ...
Sarif, Norhafizah Md; Salleh, Mohd Zuki; Tahar, Razman Mat; Nazar, Roslinda
2013-09-01
In this study, the steady magnetohydrodynamic (MHD) boundary layer flow over a stretching sheet with radiation effect under convective boundary conditions, where the heat is supplied to the convecting fluid through a bounding surface with a finite heat capacity, is considered. The governing system of partial differential equations is transformed into ordinary differential equations, which are then solved numerically via the Keller-box method. The effects of the governing parameters on the flow and heat transfer characteristics are analyzed and discussed. It is found that as the Prandtl number Pr and the radiation parameter increase, the temperature profile decreases, while as the magnetic and convective parameters decrease, the temperature profile also decreases.
Murray, Darina Bridget; Persoons, Tim
2011-01-01
This paper discusses the close interaction between local fluid dynamics and natural convection heat transfer from a pair of isothermally heated horizontal cylinders submerged in water. The presence of a second heated cylinder induces heat transfer enhancements of up to 10%, and strong fluctuations in local heat transfer rate. Therefore specific attention is focused on how the local heat transfer characteristics of the upper cylinder are affected by buoyancy induced fluid flow from the lower c...
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 fibrous material will be relatively small, which means that a relatively small increase in heat loss by convection may counterbalance part of the savings achieved by increasing the thickness. (au) 34 refs.
Energy Technology Data Exchange (ETDEWEB)
Oosthuizen, P.H.; Sun, L. [Queen' s Univ., Dept. of Mechanical Engineering, Kingston, Ontario (Canada); Naylor, D. [Ryerson Univ., Dept. of Mechanical, Aerospace and Industrial Engineering, Toronto, Ontario (Canada)
2002-07-01
Natural convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat adiabatic surfaces (termed 'slats') near the heated surface, these surfaces being, in general, inclined to the heated surface. The slats are pivoted about their center-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air, i.e. where air-conditioning is being used, being considered. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being treated by means of the Biuniqueness type approximation. Although the flow is in general three-dimensional, the flow over each slat is assumed to be the same and attention can therefore be restricted to flow over a single slat by using repeating boundary conditions. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. The solution has the following parameters: (1) the Rayleigh number (2) the Prandtl number (3) the dimensionless distance of the slat center point (the pivot point) from the surface (4) the dimensionless slat size (5) the dimensionless slat spacing (6) the angle of inclination of the slats. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated surface has been examined. (author)
International Nuclear Information System (INIS)
Natural convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat adiabatic surfaces (termed 'slats') near the heated surface, these surfaces being, in general, inclined to the heated surface. The slats are pivoted about their center-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air, i.e. where air-conditioning is being used, being considered. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being treated by means of the Biuniqueness type approximation. Although the flow is in general three-dimensional, the flow over each slat is assumed to be the same and attention can therefore be restricted to flow over a single slat by using repeating boundary conditions. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. The solution has the following parameters: (1) the Rayleigh number (2) the Prandtl number (3) the dimensionless distance of the slat center point (the pivot point) from slat center point (the pivot point) from the surface (4) the dimensionless slat size (5) the dimensionless slat spacing (6) the angle of inclination of the slats. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated surface has been examined. (author)
Fang, Pingping
1998-12-01
An extended numerical investigation of fully developed, forced convective laminar flows with heat transfer in eccentric annuli has been carried out. Both Newtonian and non-Newtonian (power-law or Ostwald-de Waele) fluids are studied, representing typical applications in petrochemical, bio-chemical, personal care products, polymer/plastic extrusion and food industries. For the heat transfer problem, with an insulated outer surface, two types of thermal boundary conditions have been considered: Constant wall temperature (T), and uniform axial heat flux with constant peripheral temperature (H1) on the inner surface of the annulus. The governing differential equations for momentum and energy conservation are solved by finite-difference methods. Velocity and temperature distributions in the flow cross section, the wall shear-stress distribution, and isothermal f Re, Nu i,T and Nu i,H1 values for different eccentric annuli (0/le?/*/le0.6,/ 0.2/le r/sp/*/le0.8) are presented. In Newtonian flows, the eccentricity is found to have a very strong influence on the flow and temperature fields. In an annulus with relatively large inner cylinder eccentricity, the flow tends to stagnate in the narrow section and has higher peak velocities in the wide section of the annulus. There is considerable flow maldistribution in the azimuthal direction, which in turn produces greater nonuniformity in the temperature field and a consequent degradation in the average heat transfer. Also, the H1 wall condition sustains higher heat transfer coefficients relative to the T boundary condition on the inner surface. For viscous, power-law type non-Newtonian flows, both shear thinning (n1) fluids are considered. Here, the non-linear shear behavior of the fluid is found to further aggravate the flow and temperature maldistribution, and once again the eccentricity is seen to exhibit a very strong influence on the friction and heat transfer behavior. Finally, the hydrodynamic characteristics of fully developed axial laminar flow of Newtonian fluids in eccentric annuli with a rotating inner cylinder are investigated. These are of significant importance to the design and operation of oil and gas drilling wells. Using finite-difference method to solve the governing flow equations in bipolar coordinates, computational results for a wide range of annulus geometry (0/le r/sp/*/le1,/ 0/le/varepsilon/sp/*/le0.8), and rotational Reynolds number (0/le Rer/le150) are presented, where the rotational speeds are restricted to the sub-critical Taylor number regime. The results delineate the effects of annuli r/sp/* and ?sp/*, and inner cylinder rotation speed on the flow structure and frictional losses.
Bieniasz, Bogumi?
2014-09-01
A forced convective mass transfer coefficient was electrochemically measured for a cylindrical bundle of transverse needle-fins ?1 × 10.9, applied as the rotor porous matrix of a rotary heat regenerator. The baffle inside the rotor was present. The technique based on the ferricyanide-ferrocyanide redox reaction controlled at the cathode, in the presence of a sodium hydroxide based electrolyte, was used in this experiment. A set of the six neighbouring fins, connected in parallel, was the cathode. The distribution of the mass transfer coefficient according to different static rotor angle position and the mean mass transfer Chilton-Colburn coefficient correlation j M = j M ( Re) for rotation numbers, Ro: 0, 0.8, 1.6 and 2.0 were stated in the mean Reynolds number, Re, range 180-985. The comparison was made between the convective heat fluxes of the pin-fins and the sheet rotor, for Ro = 0.
Burak, V. S.; Volkov, S. V.; Martynenko, O. G.; Khramtsov, P. P.; Shikh, I. A.
1995-03-01
It is observed that the temperature distribution in the boundary layer changes qualitatively and the heat transfer in the upper part of a plate intensifies substantially in comparison with a surface with a continuous heat flux. For the case of substantial heat flux density in the initial stage of formation of a free convective flow a two-dimensional vortex is found to appear.
Laboratory grey cast iron continuous casting line with electromagnetic forced convection support
Szajnar, J.; Stawarz, M.; Wro?bel, T.; Sebzda, W.
2010-01-01
The article describes the construction of a 20 mm diameter grey cast iron ingots continuous casting laboratory line. This line is made ofthree main units: melting unit (induction furnace), casting unit and the pulling unit. In order to improve the homogeneity of themicrostructure of ingots (by applying forced convection of liquid metal during the crystallization process) in this case a crystallizer system generating the forced movement of liquid metal based on a system of electrical power win...
International Nuclear Information System (INIS)
The laminar combined convection heat transfer of the liquid sodium which flows through a single horizontal row of cooling tubes in the direction of gravity are studied using numerical analysis. The heat transfer characteristics at large Reynolds numbers are improved when Richardson numbers (=Gr/Re2) are increased and the improvement rate is enlarged with an increase in p/d value. The temperature field at small Reynolds numbers does not exhibit much change even when the Richardson number reaches a high value. Consequently the Nusselt numbers do not differ from those of forced convection. In other words, in a decay heat removal system at a low velocity, there is a possibility that an improvement in the heat transfer characteristics by combined convection cannot be expected even in a system with a large Richardson number. (orig.)
Forced convection along a wall. Liquid metals application
International Nuclear Information System (INIS)
From the experimental results in pipes, heated with constant wall heat flux, the dynamical and thermal structure of the wall region of a turbulent flow is studied. We can show that, for high values of Reynolds and Peclet numbers, logarithmic profiles of velocity and temperature exist. A continuous description of the wall is obtained with the use of simple modelisation. The study of the thermal wall region structure is then made in the case of a liquid metal flow
Unsteady free convection on a vertical cylinder with variable heat and mass flux
Ganesan, P.; Rani, H. P.
The unsteady natural convection boundary layer flow over a semi-infinite vertical cylinder is considered with combined buoyancy force effects, for the situation in which the surface temperature T'w(x) and C'w(x) are subjected to the power-law surface heat and mass flux as K(T'/r) = -axn and D(C'/r) = -bxm. The governing equations are solved by an implicit finite difference scheme of Crank-Nicolson method. Numerical results are obtained for different values of Prandtl number, Schmidt number `n' and `m'. The velocity, temperature and concentration profiles, local and average skin-friction, Nusselt and Sherwood numbers are shown graphically. The local Nusselt and Sherwood number of the present study are compared with the available result and a good agreement is found to exist.
Heat transfer in the thermo-electro-hydrodynamic convection under microgravity conditions
Tadie Fogaing, Mireille; Yoshikawa, Harunori; Crumeyrolle, Olivier; Mutabazi, Innocent
2014-01-01
This article deals with the thermal convection in a dielectric fluid confined in a finite-length plane capacitor with a temperature gradient under microgravity conditions. The dielectrophoretic force resulting from differential polarization of the fluid plays the role of buoyancy force associated with an electric effective gravity. It induces the convection when the Rayleigh number based on this electric gravity exceeds a critical value. Two-dimensional numerical simulation for a geometry wit...
Simulation of Convective Heat-Transfer Coefficient in a Buried Exchanger
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Taoufik Mnasri
2008-01-01
Full Text Available This study presents analytical models allowing to study a forced convection laminar flow in non-established dynamic and thermic regimes. We treated a flow in a bitubular exchanger in permanent thermal contact with a semi-infinite medium, such as the ground. The wall temperature as well as the wall heat flux evolve in the course of time until a quasi-steady mode. The theoretical method is original because it uses Green's functions method to determine the analytical solutions of the heat propagation equation on the wall during the heating phase. These analytical solutions allow to identify the temperature distribution versus time. The complexity of the system geometry as well as the infinity of the medium surrounding the exchanger make the traditional methods of numerical resolution unable to solve the problem. We used, to solve it, the finite volume method coupled with the finite element method at the boundary. We studied the effect of Reynolds number, the fluid entry temperature and the transfer duration on the axial evolution of the heat transfer coefficient. We illustrated also the profile of the temperature field in the fluid medium.
A Numerical Solution of Natural Convection Heat Transfer in an Enclosure with a Corrugated Surface
International Nuclear Information System (INIS)
One of the most important application of corrugated surfaces in nuclear reactors is the cooling towers in the secondary loop for heat dissipation of the primary cycle. In this paper a numerical solution of natural convection heat transfer in an enclosure with a corrugated surface is presented with air as a working fluid. The enclosure is formed of three flat surfaces and a corrugated surface. The corrugated and top surfaces are respectively heated and cooled isothermally. The side flat surfaces are maintained adiabatically. The effects of geometrical parameters, such as the aspect ratio, inclination I angle and shape of corrugation on heat transfer rate are presented. The solution scheme is based on two dimensional model which is governed by continuity, momentum, and energy equation and simplified through the Boussinesque approximation for buoyancy forces. The solution scheme employs an algebraic transformation of the enclosure geometry that maps the physical domain into a rectangular domain to avoid the task of numerically generating boundary fitted coordinates. Stream function-vorticity formulation is used, a finite element technique is then developed and employed to solve the mathematical model. The numerical results obtained from the present model are compared with the available published experimental and numerical results and a good agreement is observed. Based on the numerical results, the average Nusselt number is correlated with Rayleigh number, amplitude Aspelated with Rayleigh number, amplitude Aspect ratio and number of corrugatio .
A study on sodium pool combustion phenomena under forced or natural convection airflow
Energy Technology Data Exchange (ETDEWEB)
Akira Yamaguchi [Japan Nuclear Fuel Cycle Development Institute, Ibaraki (Japan); Yuji Tajima [ENO Suri Kaiseki Research, Chiba (Japan)
2005-07-01
Full text of publication follows: Sodium pool fire is a design basis accident of sodium-cooled fast reactor. In this study, a numerical method for axisymmetric two-dimensional modeling of sodium pool fire has been developed. Flow dynamics calculation is based on SIMPLE method. It considers coupling of thermal-hydraulics, chemical reaction and aerosol dynamics equations. One assumes chemical equilibrium is valid for the chemical reaction because the reaction rate seems to be higher than mass transfer. The aerosol dynamic equation is solved for multi-group diameters classes. Also low Reynolds number two equation turbulence model for fluid flow and zero equation turbulence model for temperature field are used in the computer code. From the present two-dimensional computation, phenomena of sodium pool fire are understood such as flow and temperature fields and aerosol mass distribution of various sizes for forced and natural convection airflow situations. It has been found that the burning rate and aerosol release fraction calculated by the numerical methodology are in agreement with experimental data. The burning rate varies along with the radial direction by a factor of 20 and the mass and heat transfer around the pool edges is maximum and most influential. The thermal-hydraulic phenomena in the near-surface region are very important to determine sodium pool fire consequence such as burning rate and aerosol emission. (authors)
A study on sodium pool combustion phenomena under forced or natural convection airflow
International Nuclear Information System (INIS)
Full text of publication follows: Sodium pool fire is a design basis accident of sodium-cooled fast reactor. In this study, a numerical method for axisymmetric two-dimensional modeling of sodium pool fire has been developed. Flow dynamics calculation is based on SIMPLE method. It considers coupling of thermal-hydraulics, chemical reaction and aerosol dynamics equations. One assumes chemical equilibrium is valid for the chemical reaction because the reaction rate seems to be higher than mass transfer. The aerosol dynamic equation is solved for multi-group diameters classes. Also low Reynolds number two equation turbulence model for fluid flow and zero equation turbulence model for temperature field are used in the computer code. From the present two-dimensional computation, phenomena of sodium pool fire are understood such as flow and temperature fields and aerosol mass distribution of various sizes for forced and natural convection airflow situations. It has been found that the burning rate and aerosol release fraction calculated by the numerical methodology are in agreement with experimental data. The burning rate varies along with the radial direction by a factor of 20 and the mass and heat transfer around the pool edges is maximum and most influential. The thermal-hydraulic phenomena in the near-surface region are very important to determine sodium pool fire consequence such as burning rate and aerosol emission. (authors)
Tri–Dimensional Numerical Analysis for Forced Convection over a Forward Facing Step
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J.A. Jiménez–Berna
2010-01-01
Full Text Available A finite volume discretization technique is used to develop a numerical code to simulate the flow structures and forced convection in a forward facing step channel. The velocity field and pressure distribution inside the computacional domain are linked by the SIMPLE algorithm. The duct dimensions are defined in terms of the step height, such that the aspect and expansion ratios are four and two respectivelly. The total length in the streamflow direction is sixty times the step height, while the step edge is located twenty times the step height after the channel inlet. The boundary conditions at the channel inlet correspond to a fully developed flow at a constant temperature T0. The heating conditions are those of considereing the bottom wall at a high constant temperature (Tw>T0 and the other walls as adiabatic ones. Results for the location and size of the re–circulating zones, as well as the flow structures and temperature distributions at different planes inside the computational domain for three different Reynolds parameters are presented.
International Nuclear Information System (INIS)
Two-phase flow heat transfer has been exhaustively studied over recent years. However, in this field several questions remain unanswered. Heat transfer coefficient prediction related to nucleate and convective boiling have been studied using different approaches, numerical, analytical and experimental. In this work, an experimental analysis, data representation and heat transfer coefficient prediction on two-phase heat transfer on nucleate and convective boiling are presented. An empirical correlation is obtained based on genetic algorithms search engine over a dimensional analysis of the two-phase flow heat transfer problem. (author)
Method of cleaning convective heating surfaces from deposits
Energy Technology Data Exchange (ETDEWEB)
Keylin, I.I.; Dedikov, S.N.; Nikitin, A.I.
1982-01-01
According to the main USSR cert. of auth. 767500, sprayed water with temperature 70-100/sup 0/C is supplied to the heating surface to be cleaned having temperature of 150-350/sup 0/C. In this case the sprayed water turns into steam on the hot deposits, they are loosened and are washed away by technical water. However, in cleaning the surfaces with vertical pipes, only the front coils of the first 2-3 rows are exposed to the effect of the hot technical water. According to the new proposal, when the temperature in the gas line reaches 350/sup 0/C, exhaust fans are turned on, the slide valves are closed and sprayed water with temperature of 70-100/sup 0/C is supplied to the cleaning section, which coming into contact with the hot heating surfaces evaporates, forming a steam bath in the gas line. The deposits are separated and washed away by water from the frontal coils. Within 45 minutes after the beginning of cleaning, the slide valves are opened, and because of the thrust created by the smokestack, the boiler is ventilated for 5-15 minutes. Water passes through the entire packet of cleanable surface, sprinkling the inner coils. After this the slide valves are closed, and cleaning lasts for another 45 minutes. Then air is supplied into the convective shaft to ventilate the vertical coils in the opposite direction. Cleaning is done until the temperature in the gas line drops to 150/sup 0/C, i.e., 5-6 h. Then the surfaces are flushed with technical water from hoses. The proposed method excludes the need for deslagging and cleaning of the vertical packets by manual tool.
International Nuclear Information System (INIS)
Numerical analysis of natural convection inside a heat generated fluid was performed for four different spherical geometries that match the experimental vessels used by Asfia et al. [5-7]. The transient calculations were performed with the CFX 5.7 fluid dynamic software. The simulations show that the highest heat flux is just below the rim of the cavity and it can be 50 times higher than at the bottom. Based on the numerical results, the local values of heat transfer coefficient and the distributions of global Nusselt number were calculated. The present, three-dimensional simulation results were compared with the numerical results of Mayinger et al. [3] and Reineke et al. [4], and with the experimental data of Asfia et al. [5-7]. The agreement between the results that is well inside the experimental scatter verifies the selected modeling approach. (author)
The effect of Coriolis force on nonlinear convection in a porous medium
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D. H. Riahi
1994-09-01
Full Text Available Nonlinear convection in a porous medium and rotating about vertical axis is studied in this paper. An upper bound to the heat flux is calculated by the method initiated first by Howard [6] for the case of infinite Prandtl number.
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G. Vidyasagar
2013-03-01
Full Text Available This paper deals with the combined effect of thermal diffusion and heat absorption on the MHD free convection heat and mass transfer flow of a viscous incompressible fluid past a continuously moving infinite plate. Closed form of solution for the velocity, temperature and concentration field are obtained and discussed graphically for various values of the physical parameters present. In addition, expressions for the skin friction is also derived and finally discussed with the help of table and graphs.Keywords: Free convection, Heat and Mass transfer, Heat Sink, Thermal radiation, Magnetic Field.
Transition to finger convection in double-diffusive convection
Kellner, M
2014-01-01
Finger convection is observed experimentally in an electrodeposition cell in which a destabilizing gradient of copper ions is maintained against a stabilizing temperature gradient. This double-diffusive system shows finger convection even if the total density stratification is unstable. Finger convection is replaced by an ordinary convection roll if convection is fast enough to prevent sufficient heat diffusion between neighboring fingers, or if the thermal buoyancy force is less than 1/30 of the compositional buoyancy force. At the transition, the ion transport is larger than without an opposing temperature gradient.
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MR.S.S.GADDAMWAR
2012-04-01
Full Text Available In Indian Coal mines, underground mines, mineral ore mines syngases at high pressure and high temperature are found in large quantities. This syngas is highly toxic, harmful and flammable gas which will be present in atmosphere which may cause many accidents. Hence it is necessary to reduce the content of syngas from mines in the atmosphere. This paper describes literature review of Convective heat transfer characteristics of high pressure gas used in mines. Heat transfer in convection cooling section of pressurized coal gasifier with the membrane helical coils and membrane serpentine tubes under high pressure is studied. This review covers the status and perspectives of syngas used in heat exchanger. It also covers the work carried out by different research scholar in this promising area. Some suggestions in the form of new concept are also suggested to reduce the percentage of high pressure syngas in the atmosphere. This paper will be useful for those who are working in the field of coal mine, underground mines and mineral ore mines.
Natural convection of the Earth liquid core in the presence of internal heat sources.
Solov'ev, S. V.
1999-06-01
Results of natural-convection heat transfer in the Earth liquid core in the presence of internal sources are presented. Consideration is given to boundary conditions of the I, II, and III kind for the temperature. The effect of internal heat sources on the hydrodynamics and heat transfer of the core is studied.
Efficiency of Heat Transfer in Turbulent Rayleigh-Benard Convection.
Czech Academy of Sciences Publication Activity Database
Urban, Pavel; Musilová, V?ra; Skrbek, L.
2011-01-01
Ro?. 107, ?. 1 (2011), 014302:1-4. ISSN 0031-9007 R&D Projects: GA AV ?R KJB200650902 Institutional research plan: CEZ:AV0Z20650511 Keywords : natural convection * thermal convection Subject RIV: BK - Fluid Dynamics Impact factor: 7.370, year: 2011
Analytical study of laminar free convection heat-transfer to supercritical helium
International Nuclear Information System (INIS)
An analytical study of free convective heat transfer from a vertical plate to supercritical helium is presented. The thermophysical properties are approximately related to the enthalpy and used to solve the boundary layer equations. Quasi-linearization techniques are used to solve the resulting ordinary differential equations. The comparison made between the analytical heat transfer coefficients and the published experimental coefficients indicates that the analytical description is adequate and will be especially valid in laminar flow free convection cases. (author)
International Nuclear Information System (INIS)
Experimental data are presented on heat transfer from a horizontal cylinder 0.126 mm in diameter under conditions of free convection of helium with large temperature heads (the dimensionless temperature head, equal to the ratio of the temperature difference to the absolute temperature of the gas, ranged from 0 to 50). An empirical relation is used which reliably describes the experimental data. A condition is found for ignoring convection compared to conduction when evaluating heat transfer from small-diameter cylinders
Luo, D. D.; Leung, C. W.; Chan, T. L.
2004-09-01
The optimum rib size to enhance heat transfer had been proposed through an experimental investigation on the forced convection of a fully developed turbulent flow in an air-cooled horizontal equilateral triangular duct fabricated on its internal surfaces with uniformly spaced square ribs. Five different rib sizes (B) of 5 mm, 6 mm, 7 mm, 7.9 mm and 9 mm, respectively, were used in the present investigation, while the separation (S) between the center lines of two adjacent ribs was kept at a constant of 57 mm. The experimental triangular ducts were of the same axial length (L) of 1050 mm and the same hydraulic diameter (D) of 44 mm. Both the ducts and the ribs were fabricated with duralumin. For every experimental set-up, the entire inner wall of the duct was heated uniformly while the outer wall was thermally insulated. From the experimental results, a maximum average Nusselt number of the triangular duct was observed at the rib size of 7.9 mm (i.e. relative rib size B/D = 0.1795 ). Considering the pressure drop along the triangular duct, it was found to increase almost linearly with the rib size. Non-dimensional expressions had been developed for the determination of the average Nusselt number and the average friction factor of the equilateral triangular ducts with ribbed internal surfaces. The developed equations were valid for a wide range of Reynolds numbers of 4,000 rib sizes of 0.11 ?slant frac{B} {D} ?slant 0.21 under steady-state condition.
International Nuclear Information System (INIS)
This paper presents results of numerical investigation of turbulent natural convection in an internally-heated oxidic pool, and in a metallic layer heated from below and cooled from top and sidewalls. Emphasis is placed upon applicability of the existing heat transfer correlations (obtained from simulant-material experiments) in assessments of a prototypic severe reactor accident. The objectives of this study are (i) to improve the current understanding of the physics of unstably stratified flows, and (ii) to reduce uncertainties associated with modeling and assessment of natural convection heat transfer in the above configuration. Prediction capabilities of different turbulence modeling approaches are first examined and discussed, based on extensive results of numerical investigations performed by present authors. Findings from numerical modeling of turbulent natural convection flow and heat transfer in melt pools and metallic layers are then described. (authors)
Unsteady heat transfer by natural convection in the cavity of a passive heating room
Energy Technology Data Exchange (ETDEWEB)
Kurtbas, irfan; Durmus, Aydin [Department of Mechanical Education, University of Firat, 23119 Elazig (Turkey)
2008-08-15
In this study, three dimensional unsteady state equations of heat transfer and flow have been solved numerically for a passive heating room using values of hourly averaged radiation during winter in Elazig region in Turkey. For this purpose, a room having volume of 44.8 m{sup 3} (4 x 4 x 2.8 m) was considered. Variable heat flux boundary condition depending on time was applied on absorber surface using values of hourly averaged radiation. Convection boundary condition was used on glass surface, the walls and ceiling of the room by using overall heat transfer coefficient. Constant surface temperature (6 C) was used for floor of the room. Experiments were carried out manufacturing a model room having the size of 1 x 1.2 x 1.2 m dimensions in order to maintain the corroboration of numerical solution. Governing equations for the model room built were solved numerically and compared to experimental data. It was seen that the model predictions agreed quite well with experimental data. The effect of overall heat transfer coefficient of glass on Nusselt number was also investigated. It was revealed that the overall heat transfer coefficient for low Rayleigh number affected the average Nusselt number more than that of high Rayleigh number. (author)
Directory of Open Access Journals (Sweden)
J. Zueco
2011-01-01
Full Text Available The steady, laminar axisymmetric convective heat and mass transfer in boundary layer flow over a vertical thin cylindrical configuration in the presence of significant surface heat and mass flux is studied theoretically and numerically. The governing boundary-layer equations for momentum, energy and species conservation are transformed from a set of partial differential equations in a (x,r coordinate system to a ( system using a group of similarity transformations. The resulting equations are solved using the Network Simulation Method (NSM for the buoyancy-assisted pure free convection and also the pure forced convection cases, wherein the effects of Schmidt number, Prandtl number and surface mass parameter on velocity, temperature and concentration distributions in the regime are presented graphically and discussed. For the buoyancy-assisted pure free convection case, nondimensional velocity (f/ is found to increase with a rise in surface mass transfer (S but decrease with increasing Prandtl number (Pr, particularly in the vicinity of the cylinder surface (small radial coordinate, . Dimensionless temperature ( decreases however with increasing S values from the cylinder surface into the free stream; with increasing Prandtl number, temperature is strongly reduced, with the most significant decrease at the cylinder surface. Dimensionless concentration ( is decreased continuously throughout the boundary layer regime with an increase in S; conversely is enhanced for all radial coordinate values with an increase in Prandtl number. For the pure forced convection case, velocity increases both with dimensionless axial coordinate ( and dimensionless radial coordinate ( but decays smoothly with increasing Prandtl number and increasing Schmidt number, from the cylinder surface to the edge of the boundary layer domain. The model finds applications in industrial metallurgical processes, thermal energy systems, polymer processing, etc.
Onset of nuclear boiling in forced convection (Method of detection)
International Nuclear Information System (INIS)
Local onset of boiling in any pressure water cooling systems, as a PWR for instance, can mean a possible dangerous mismatch between the produced heat and the cooling capabilities. Its consequences can lead to serious accidental conditions and a reliable technique to detect such a phenomenon is therefore of particular need. Most techniques used up to now rely basically on local measurements and assume therefore usually the previous knowledge of the actual hot or boiling spot. The method proposed here based on externally located accelerometers appears to be sensitive to the global behaviour of the mechanical structure and is therefore not particularly bound to any exact localization of the sensors. The vibrations produced in the mechanical structure of the heated assembly are measured by accelerometers placed on the external surfaces that are easily accessible. The onset of the boiling, the growth and condensation of the bubbles on the heated wall, induces a resonance in the structure and an excitation at its particular eigen frequencies. Distinctive peaks are clearly observed in the spectral density function calculated from the accelerometer signal as soon as bubbles are produced. The technique is shown to be very sensitive even at the earliest phase of boiling and quite independent on sensor position. A complete hydrodynamic analysis of the experimental channels have been performed in order to assess the validity of the method both in steady conditions and during rapid power transients
Scale/Analytical Analyses of Freezing and Convective Melting with Internal Heat Generation
Energy Technology Data Exchange (ETDEWEB)
Ali S. Siahpush; John Crepeau; Piyush Sabharwall
2013-07-01
Using a scale/analytical analysis approach, we model phase change (melting) for pure materials which generate constant internal heat generation for small Stefan numbers (approximately one). The analysis considers conduction in the solid phase and natural convection, driven by internal heat generation, in the liquid regime. The model is applied for a constant surface temperature boundary condition where the melting temperature is greater than the surface temperature in a cylindrical geometry. The analysis also consider constant heat flux (in a cylindrical geometry).We show the time scales in which conduction and convection heat transfer dominate.
Consideration about the utilization of a heat function in problems of natural convection in cavities
International Nuclear Information System (INIS)
The heat line, as defined by Kimura and Bejan does not involve, explicitly, a reference temperature for the energy converted by the flow. As a result different flow patterns may be obtained for the same physical problem. In this paper the heat function is re-defined and the temperature of the cold wall, in natural convection flow problem inside cavities, is used as reference temperature. The heat function, as defined here, is also written in a generalized coordinate system and is employed to visualize the heat flow in some natural convection problems inside irregular cavities. (author)
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G. Pomalégni
2014-10-01
Full Text Available We investigate the combined effects of rotation, magnetic field and helical force on the onset of stationary and oscillatory convection in a horizontal electrically conducting fluid layer heated from below with free-free boundary conditions. For this investigation the linear stability analysis studied in detail by Chandrasekhar (1961 is used. We obtain the condition for the formation of a single large-scale structure. In (Pomalégni et al., 2014 it was shown the existence of a critical value of the intensity of the helical force for which the apparition of two cells at marginal stability for the oscillatory convection is obtained. Then, we have shown here how the increasing of parameter Ta influences this critical value of the helical force intensity.
Simo?es, Nuno; Tadeu, Anto?nio
2006-01-01
This paper analyses the transient heat transfer across multi-layer floors subjected to multiple heat sources. The formulation that is proposed to solve this problem uses analytical expressions handling both conduction and convection phenomena. The Green's functions (analytical expressions) for the layered formation are established by imposing the continuity of temperatures and heat fluxes at the medium interfaces between the various layers. The heat field inside a layer is obtained by adding ...
Heat transfer enhancement on thin wires in superfluid helium forced flows
Duri, Davide; Baudet, Christophe; Moro, Jean-paul; Roche, Philippe-emmanuel; Diribarne, Pantxo
2014-01-01
In this paper, we report the first evidence of an enhancement of the heat transfer from a heated wire by an external turbulent flow of superfluid helium. We used a standard Pt-Rh hot-wire anemometer and overheat it up to 21 K in a pressurized liquid helium turbulent round jet at temperatures between 1.9 K and 2.12 K. The null-velocity response of the sensor can be satisfactorily modeled by the counter flow mechanism while the extra cooling produced by the forced convection i...
International Nuclear Information System (INIS)
Natural convection in low aspect ratio rectangular enclosures is considered along with three-dimensional convection within rectangular boxes, natural convection flow visualization in irradiated water cooled by air flow over the surface, free convection in vertical slots, the stratification in natural convection in vertical enclosures, the flow structure with natural convection in inclined air-filled enclosures, and natural convection across tilted, rectangular enclosures of small aspect ratio. Attention is given to the effect of wall conduction and radiation on natural convection in a vertical slot with uniform heat generation of the heated wall, a numerical study of thermal insulation enclosure, free convection in a piston-cylinder enclosure with sinusoidal piston motion, natural convection heat transfer between bodies and their spherical enclosure, an experimental study of the steady natural convection in a horizontal annulus with irregular boundaries, three-dimensional natural convection in a porous medium between concentric inclined cylinders, a numerical solution for natural convection in concentric spherical annuli, and heat transfer by natural convection in porous media between two concentric spheres