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Showing papers on "Combined forced and natural convection published in 2001"


Book
23 Feb 2001
TL;DR: Free and mixed convection boundary-layer flow on non-Newtonian fluids in porous media has been studied in this article for convective flow in buoyant plumes and jets.
Abstract: Chapter Headings. I Convective flows: viscous fluids. Free convection boundary-layer over a vertical flat plate. Mixed convection boundary-layer flow along a vertical flat plate. Free and mixed convection boundary-layer flow past inclined and horizontal plates. Double-diffusive convection. Convective flow in buoyant plumes and jets. Conjugate heat transfer over vertical and horizontal flat plates. Free and mixed convection from cylinders. Free and mixed convection boundary-layer flow over moving surfaces. Unsteady free and mixed convection. II Convective flows: porous media. Free and mixed convection boundary-layer flow on non-Newtonian fluids. Free and mixed convection boundary-layer flow over vertical surfaces in porous media. Free and mixed convection past horizontal and inclined surfaces in porous media. Conjugate free and mixed convection over vertical surfaces in porous media. Free and mixed convection from cylinders and spheres in porous media. Unsteady free and mixed convection in porous media. Non-Darcy free and mixed convection boundary-layer flow in porous media.

664 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of viscous dissipation, the boundary condition assumptions, thermal dispersion, particle diameters and the variable properties of oil on convection heat transfer are analyzed using a numerical model including thermal non-equilibrium assumption.

169 citations


Journal ArticleDOI
TL;DR: In this article, the authors measured heat transfer in a low-Prandtl-number (Pr = 0.023), electrically conducting fluid as a function of the applied temperature difference, rotation rate, applied magnetic field strength and fluid-layer aspect ratio.
Abstract: Thermal convection experiments in a liquid gallium layer subject to a uniform rotation and a uniform vertical magnetic field are carried out as a function of rotation rate and magnetic field strength. Our purpose is to measure heat transfer in a low-Prandtl-number (Pr = 0.023), electrically conducting fluid as a function of the applied temperature difference, rotation rate, applied magnetic field strength and fluid-layer aspect ratio. For Rayleigh-Benard (non-rotating, non-magnetic) convection we obtain a Nusselt number-Rayleigh number law Nu = 0.129 Ra 0.272±0.006 over the range 3.0 x 10 3 < Ra < 1.6 x 10 4 . For non-rotating magnetoconvection, we find that the critical Rayleigh number Ra c increases linearly with magnetic energy density, and a heat transfer law of the form Nu ∼ Ra 1/2 . Coherent thermal oscillations are detected in magnetoconvection at ∼ 1.4Ra c . For rotating magnetoconvection, we find that the convective heat transfer is inhibited by rotation, in general agreement with theoretical predictions. At low rotation rates, the critical Rayleigh number increases linearly with magnetic field intensity. At moderate rotation rates, coherent thermal oscillations are detected near the onset of convection. The oscillation frequencies are close to the frequency of rotation, indicating inertially driven, oscillatory convection. In nearly all of our experiments, no well-defined, steady convective regime is found. Instead, we detect unsteady or turbulent convection just after onset.

160 citations


Journal ArticleDOI
TL;DR: In this article, numerical simulations of thermal convection in rapidly rotating spherical fluid shells have been carried out with and without magnetic fields generated by the dynamo process Relaxation oscillations and localized convection activity represent coherent phenomena of turbulent convection.

101 citations


Journal ArticleDOI
TL;DR: In this paper, it is suggested that the decrease in convective activity above 30°C could result from a self aggregation, whereby the occurrence of convection makes future convection more likely to occur in the same location.
Abstract: Tropical observations show convective activity increasing sharply above sea surface temperatures (SSTs) of around 26°C and then decreasing as the SST exceeds 30°C, with maximum observed SSTs of around 32°C.Although some aspects of this relationship are reasonably well understood, as of yet no theory has explained the decrease in convective activity above 30°C. Here it is suggested that this aspect of the relationship could result from a organizational positive feedback, sometimes termed “self aggregation,” whereby the occurrence of convection makes future convection more likely to occur in the same location. Using cloud-resolving simulations, it is shown that the feedback between convection and the water vapor field is a good candidate for this role.

95 citations


Journal ArticleDOI
TL;DR: In this paper, the authors derived bounds for the bulk heat transport in Rayleigh-Benard convection for an infinite Prandtl number fluid, where the enhancement of heat transport beyond the minimal conduction value (the Nusselt number Nu) is bounded in terms of the nondimensional temperature difference across the layer (the Rayleigh number Ra) according to Nu⩽cRa2/5, where c < 1 is an absolute constant.
Abstract: Bounds for the bulk heat transport in Rayleigh–Benard convection for an infinite Prandtl number fluid are derived from the primitive equations. The enhancement of heat transport beyond the minimal conduction value (the Nusselt number Nu) is bounded in terms of the nondimensional temperature difference across the layer (the Rayleigh number Ra) according to Nu⩽cRa2/5, where c<1 is an absolute constant. This rigorous upper limit is uniform in the rotation rate when a Coriolis force, corresponding to the rotating convection problem, is included.

88 citations


Journal ArticleDOI
TL;DR: In this paper, the rate of entropy generation, S˙′G[W/mK], for forced and mixed convection in a rectangular duct heated at the bottom was examined both theoretically and numerically for a single-dimensional non-dimensional hydrodynamic and thermal parameters.
Abstract: The rate of entropy generation, S˙′G[W/mK], is examined both theoretically and numerically for forced and mixed convection in a rectangular duct heated at the bottom. Under fully-developed flow conditions S˙′G is expressed in terms of relevant non-dimensional hydrodynamic and thermal parameters. Numerically, it is demonstrated that S˙′G is a single, effective parameter to examine both thermal and hydrodynamic fields and their variations.

87 citations


Journal ArticleDOI
TL;DR: In this article, the effect of radiation on mixed convection from a vertical flat plate in a saturated porous medium is investigated and the conservation equations that govern the problem are reduced to a system of nonlinear ordinary differential equations.

80 citations


Journal ArticleDOI
TL;DR: In this paper, a numerical study of double-diffusive natural convection in a rectangular cavity filled with a saturated anisotropic porous medium is presented, where the side walls of the cavity are maintained at constant temperatures and concentrations, while the horizontal walls are adiabatic and impermeable.

78 citations


Journal ArticleDOI
TL;DR: In the special case when the conduction profile in the absence of convection has the same functional dependence on radius as the adiabatic temperature, the Rayleigh number is simply related to the entropies of magnetic and thermal diffusion as mentioned in this paper.

77 citations


Journal ArticleDOI
TL;DR: In this article, a non-local theory of convection was used to calculate the structure of the solar convection zone, paying special attention to the detailed structure of lower overshooting zone.
Abstract: Using a non-local theory of convection, we calculated the structure of the solar convection zone, paying special attention to the detailed structure of the lower overshooting zone. Our results show that an extended transition zone exists near the bottom of the convection zone, where the temperature gradient turns smoothly from adiabatic in the convection zone to radiative in solar interior. A super-radiative temperature region is found in the overshooting zone under the solar convection zone, where L-c L and >del (rad). The extension of the super-radiative region (defined by L-r/L-circle dot greater than or equal to 1.01) l is about 0.63 H-p (0.053 R-circle dot). A careful comparison of the distribution of adiabatic sound speed and density with the local one is carried out. It is found, strikingly, that the distribution of adiabatic sound speed and density of our model is roughly consistent with the results of reversion from solar oscillation observations.

Journal ArticleDOI
TL;DR: In this paper, a new technique based on use of porous or foam material inserted between the components on a horizontal board is studied, and the results show that substantial enhancement is obtained compared to the fluid case even if the permeability is low.
Abstract: A numerical investigation of electronic cooling enhancement is carried out in this study in order to determine how the operating temperature can be maintained under the allowable level. A new technique based on use of porous or foam material inserted between the components on a horizontal board is studied. One energy equation model has been adopted to analyse the thermal field. The control volume method based on finite differences with appropriate averaging for diffusion coefficients is used to solve the coupling between solid, fluid and porous regions. The effect of parameters such as Reynolds number, Darcy number and thermal conductivity ratio are considered in order to look for the most appropriate properties of the foam or porous substrate that allow optimal cooling. The results show that for high thermal conductivity of the porous substrate, substantial enhancement is obtained compared to the fluid case even if the permeability is low. In the mixed convection case, the insertion of the foam between the blocks leads to a temperature reduction of 50%.

Journal ArticleDOI
TL;DR: In this paper, the steady mixed convection flow over a vertical wedge with a magnetic field embedded in a porous medium has been investigated and the effects of the permeability of the medium, surface mass transfer and viscous dissipation on the flow and temperature fields were included in the analysis.
Abstract: The steady mixed convection flow over a vertical wedge with a magnetic field embedded in a porous medium has been investigated. The effects of the permeability of the medium, surface mass transfer and viscous dissipation on the flow and temperature fields have been included in the analysis. The coupled nonlinear partial differential equations governing the flow field have been solved numerically using the Keller box method. The skin friction and heat transfer are found to increase with the parameters characterizing the permeability of the medium, buoyancy force, magnetic field and pressure gradient. However the effect of the permeability and magnetic field on the heat transfer is very small. The heat transfer increases with the Prandtl number, but the skin friction decreases. The buoyancy force which assists the forced convection flow causes an overshoot in the velocity profiles. Both the skin friction and heat transfer increase with suction and the effect of injection is just the reverse.

Journal ArticleDOI
TL;DR: In this paper, the superposition of Von Karman street and convective cells in a horizontal plane channel containing a triangular prism and heated from below constitute the principal subject of a numerical investigation.
Abstract: The superposition of Von Karman street and convective cells in a horizontal plane channel containing a triangular prism and heated from below constitute the principal subject of this numerical investigation. The numerical scheme is based on the control volume finite element method (CVFEM) adapted to the standard staggered grid with the SIM PLER algorithm for pressure-velocity coupling and an Alternating Direction Implicit (ADI) scheme for the time integration. Many standard test flows are simulated successfully. Results are obtained for a Grashof number ranging from 0 to 1.5 10 4 at Pr 0.71 and Re 100 with constant physical properties. At the outlet of the computational domain a convective boundary condition (CBC) is used. Results are presented to show the effect of development of convective cells on the flow pattern and on the Strouhal number. Regarding the heat transfer rate, we focus on the effect of the presence of the triangular prism on the heat flux transferred from the hot wall to the flow.

Journal ArticleDOI
TL;DR: In this article, a Czochralski melt is simulated in a model experiment and by numerical analysis using a transparent fluid (0.65 cSt silicone oil with Pr=6.8) which is contained in a transparent sapphire crucible.

Journal ArticleDOI
TL;DR: In this article, a suite of Cartesian models by systematically varying aspect ratios and plate geometries is investigated to determine to what extent plates affect mantle flow patterns, temperature and surface heat flux.
Abstract: SUMMARY The dynamic coupling between plate motion and mantle convection is investigated in a suite of Cartesian models by systematically varying aspect ratios and plate geometries. The aim of the study presented here is to determine to what extent plates affect mantle flow patterns, temperature and surface heat flux. To this end, we compare numerical convection models with free-slip boundary conditions to models that incorporate between one and six plates, where the geometries of the plates remain fixed while the plate velocities evolve dynamically with the flow. We also vary the widths of the plates and the computational domain in order to determine what constraint these parameters place on the mean temperature, heat flux and plate velocity of mantle convection models. We have investigated the influence of plates for three whole-mantle convection cases that differ in their heating modes (internally heated and basally heated) and rheologies (isoviscous and depth-dependent viscosity). We present a systematic investigation of over 30 models that exhibit increasingly complex behaviour in order to understand highly time-dependent systems using the insight gained from simpler models. In models with aspect ratios from 0.5 to 12 we find that for the same heating mode, variations in temperature can be as much as 40 per cent when comparing calculations with unit-width plates to models incorporating plates with widths equal to five times the model depth. Mean surface heat flux may decrease by 60 per cent over the same range of plate widths. We also find that internally heated mantle convection models incorporating plates exhibit novel behaviour that, we believe, has not been described previously in mantle convection studies. Specifically, in internally heated models, plate motion is characterized by episodic reversals in direction driven by changes in the mantle circulation from clockwise to counterclockwise and vice versa. These flow reversals occur in internally heated convection and are caused by a build-up of heat in the interiors of wide convection cells close to mantle downwellings. We find that flow reversals occur rapidly and are present in both single-plate and multiple-plate models that include internal heating. This behaviour offers a possible explanation for why the Pacific plate suddenly changed its direction some 43 Ma.

Journal ArticleDOI
TL;DR: Steady-state laminar natural convection in a cubical enclosure with a cold vertical wall and a hot square sector on the opposite wall is numerically studied in this paper.

Journal ArticleDOI
TL;DR: In this article, the steady-state and transient heat flux of small leaf-shaped model structures (area of one side = 1730mm2) were studied under zero and low (= 100mm s−1) wind velocities by using a computer simulation method.
Abstract: In order to study convective heat transfer of small leaves, the steady-state and transient heat flux of small leaf-shaped model structures (area of one side = 1730 mm2) were studied under zero and low (= 100 mm s−1) wind velocities by using a computer simulation method. The results show that: (1) distinct temperature gradients of several degrees develop over the surface of the model objects during free and mixed convection; and (2) the shape of the objects and onset of low wind velocities has a considerable effect on the resulting temperature pattern and on the time constant τ. Small leaves can thus show a temperature distribution which is far from uniform under zero and low wind conditions. The approach leads, however, to higher leaf temperatures than would be attained by ‘real’ leaves under identical conditions, because heat transfer by transpiration is neglected. The results demonstrate the fundamental importance of a completely controlled environment when measuring heat dissipation by free convection. As slight air breezes alter the temperature of leaves significantly, the existence of purely free convection appears to be questionable in the case of outdoor conditions. Contrary to the prognoses yielded by standard approximations, no quantitative effect of buoyancy on heat transfer under the considered conditions could be detected for small-sized leaf shapes.

Journal ArticleDOI
TL;DR: In this article, a zonal model of room air convection is formulated, which consists of a set of coupled equations determined by heat and mass balance on each of the 24 cells into which the room is divided, convection from the room surfaces and radiant exchange among the rooms surfaces.

Journal ArticleDOI
TL;DR: In this article, the authors discuss the non-linear stability of convection in a horizontal porous layer subjected to an inclined temperature gradient and a variable gravity field and show that a decrease in gravity variation has a stabilizing effect on the system.

Journal ArticleDOI
TL;DR: In this article, a new approach for modeling mixed convection within mechanically ventilated rooms is developed, which is applicable for rooms ventilated with ceiling mounted diffusers and is appropriate for both heating and cooling.

Journal ArticleDOI
TL;DR: In this paper, a nonlinear buoyancy driven convection in the melt and in cylindrical chimneys in a mushy layer during alloy solidification and subjected to an externally imposed strong magnetic field was investigated under a high-gravity environment, where the rotation axis is inclined at an angle γ to the high gravity vector.

Journal ArticleDOI
TL;DR: In this article, a volumetrically heated rectangular enclosure with a low-Prandtl number (Pr=0.0321) fluid was studied by direct numerical two-dimensional simulation.

Journal ArticleDOI
TL;DR: In this article, an analysis is made for the forced convection thermal and solute concentration Marangoni boundary layers (thin dissipative layers) that can be formed along the surface, which separates two immiscible fluids in surface driven flows when the appropriately defined Reynolds number is large enough.
Abstract: An analysis is made for the forced convection thermal and solute concentration Marangoni boundary layers (thin dissipative layers) that can be formed along the surface, which separates two immiscible fluids in surface driven flows when the appropriately defined Reynolds number is large enough. Similarity equations for the case in which an external pressure gradient is imposed are derived. These equations are then solved numerically for some values of the involved parameters using very efficient numerical schemes known as Keller-box and superposition methods and the features of the flow and transport characteristics are analysed and discussed.

Journal ArticleDOI
TL;DR: In this article, a numerical investigation of steady laminar mixed convection heat transfer in a horizontal concentric annulus using air and water as the working fluid was carried out, where the thermal boundary condition chosen is that of uniform heat flux at the inner wall and an adiabatic outer wall.

Journal ArticleDOI
TL;DR: In this paper, a numerical study was performed to investigate the evaporation cooling of liquid film falling along a vertical tube, where a marching procedure was employed for solution of the equation of mass momentum, energy and concentration in the flow.

Journal ArticleDOI
TL;DR: In this article, a numerical study has been made of the interaction of the thermal radiation with laminar mixed convection for a gray fluid (a gas that may have particulates in suspension) in a vertical square duct.

Journal ArticleDOI
TL;DR: In this article, Dongarra et al. presented an accurate numerical analysis for the onset of thermal convection in a two-layer system consisting of a saturated porous layer over which lies a layer of the same fluid.

Journal ArticleDOI
TL;DR: In this article, the interaction between mixed convection and thermal radiation in ventilated cavities with gray surfaces has been studied numerically using the Navier-Stokes equations with the Boussinesq approximation.
Abstract: The interaction between mixed convection and thermal radiation in ventilated cavities with gray surfaces has been studied numerically using the Navier‐Stokes equations with the Boussinesq approximation. The effect of thermal radiation on streamlines and isotherms is shown for different values of the governing parameters namely, the Rayleigh number (103 ≤ Ra ≤ 106), the Reynolds number (50 ≤ Re ≤ 5000) and the surfaces emissivity (0 ≤ e≤ 1). The geometrical parameters are the aspect ratio of the cavity A = L’/H’ = 2 and the relative height of the openings B = h’/H’ = 1/4. Results of the study show that thermal radiation alters significantly the temperature distribution, the flow fields and the heat transfer across the active walls of the cavities.

Journal ArticleDOI
TL;DR: In this paper, the transient skin friction coefficient and heat transfer rate in micropolar fluid flow over a vertical wavy surface were investigated. But the authors focused on the transient behavior of the laminar mixed convection.