Natural Convection in Binary Gases Due to Horizontal Thermal and Solutal Gradients

1991 ◽  
Vol 113 (1) ◽  
pp. 141-147 ◽  
Author(s):  
J. A. Weaver ◽  
R. Viskanta
Keyword(s):  
Natural Convection ◽  
Unsteady Flow ◽  
Buoyancy Force ◽  
Measured Temperature ◽  
Concentration Gradients ◽  
Thermal Buoyancy ◽  
Smoke Flow ◽  
Numerical Predictions ◽  
Body Forces ◽  
Thermal Buoyancy Force

The influence of augmenting and opposing thermal and solutal buoyancy forces on natural convection of binary gases due to horizontal temperature and concentration gradients is examined through comparison of smoke flow visualization and measured temperature and concentration distributions with numerical predictions. The observed flow at the cold wall was unsteady for opposing body forces. The same basic flow structure was observed, but the unsteady flow intensifies as the opposing solutal buoyancy force increases as compared to the thermal buoyancy force. Comparison of predicted and measured temperatures and concentrations is fair overall, but the steady-state analytical model fails to predict the unsteady flow and heat and mass transport for opposing body forces.

scholarly journals Natural Convection Experiments in a Liquid-Saturated Porous Medium Bounded by Vertical Coaxial Cylinders

1983 ◽  
Vol 105 (4) ◽  
pp. 795-802 ◽  
Author(s):  
D. C. Reda
Keyword(s):  
Natural Convection ◽  
Surface Temperature ◽  
Fluid Layer ◽  
Saturated Porous Medium ◽  
Outer Cylinder ◽  
Power Input ◽  
Surface Boundary ◽  
Measured Temperature ◽  
Heater Surface ◽  
Numerical Predictions

An experimental effort is presently underway to investigate natural convection in liquid-saturated porous media utilizing a geometry and hydrodynamic/thermal boundary conditions relevant to the problem of nuclear-waste isolation in geologic repositories. During the first phase of this research program, detailed measurements were made of the steady-state thermal field throughout an annular test region bounded by a vertical, constant-heat-flux, inner cylinder and a concentrically placed, constant-temperature, outer cylinder. An overlying, constant-pressure fluid layer was utilized to supply a permeable upper surface boundary condition. Results showed the heater surface temperature to increase with increasing vertical distance due to the buoyantly driven upflow. The measured temperature difference (ΔT) between the average heater surface temperature and the constant outer-surface temperature was found to be progressively below the straight-line/conduction-only solution for ΔT versus power input, as the latter was systematically increased. Comparisons between measured results and numerical predictions obtained using the finite element code MARIAH showed very good agreement, thereby contributing to the qualification of this code for repository-design applications.

Combined Heat and Mass Transfer in Mixed Convection over a Horizontal Flat Plate

1980 ◽  
Vol 102 (3) ◽  
pp. 538-543 ◽  
Author(s):  
T. S. Chen ◽  
F. A. Strobel
Keyword(s):  
Mass Transfer ◽  
Flat Plate ◽  
Heat And Mass Transfer ◽  
Transfer Rate ◽  
Buoyancy Force ◽  
Schmidt Number ◽  
Thermal Buoyancy ◽  
Buoyancy Forces ◽  
Thermal Buoyancy Force ◽  
Species Diffusion

The combined effects of buoyancy forces from thermal and species diffusion on the heat and mass transfer characteristics are analyzed for laminar boundary layer flow over a horizontal flat plate. The analysis is restricted to processes with low concentration levels such that the interfacial velocities due to mass diffusion and the diffusion-thermo/thermo-diffusion effects can be neglected. Numerical results for friction factor, Nusselt number, and Sherwood number are presented for gases having a Prandtl number of 0.7, with Schmidt numbers ranging from 0.6 to 2.0. In general, it is found that, for the thermally assisting flow, the surface heat and mass transfer rates as well as the wall shear stress increase with increasing thermal buoyancy force. These quantities are further enhanced when the buoyancy force from species diffusion assists the thermal buoyancy force, but are reduced when the two buoyancy forces oppose each other. While a higher heat transfer rate is found to be associated with a lower Schmidt number, a higher mass transfer rate occurs at a higher Schmidt number.

Freezing of Water-Saturated Porous Media in the Presence of Natural Convection: Experiments and Analysis

1989 ◽  
Vol 111 (2) ◽  
pp. 425-432 ◽  
Author(s):  
S. Chellaiah ◽  
R. Viskanta
Keyword(s):  
Porous Media ◽  
Natural Convection ◽  
Convection Flow ◽  
Liquid Region ◽  
Measured Temperature ◽  
Density Inversion ◽  
Flow And Heat Transfer ◽  
Numerical Predictions ◽  
Darcy Equations ◽  
Water Saturated

Freezing of superheated water-porous media (glass beads) contained in a rectangular test cell has been studied both experimentally and numerically. The effects of liquid superheat and imposed temperature difference were investigated. When the superheat across the liquid region was small the flow in the porous media was weak, and the interface was almost planar. For larger superheats, natural convection flow and the solidification front shape and velocity were found to depend on the imposed temperature and the permeability of the porous medium. Due to the density inversion of water, the rate of freezing was higher, either at the top or at the bottom of the cell, depending on the amount of superheat. The measured temperature distributions were compared with predictions of numerical model that considered both conduction in the solid and natural convection in the liquid region. This model is based on volumetric averaging of the macroscopic transport equations, with phase change assumed to occur volumetrically over a small temperature range. Both Brinkman and Forchheimer extensions were added to the Darcy equations. The effect of density inversion of water on the fluid flow and heat transfer has been modeled. Good agreement has been found between the experimental data and numerical predictions.

Thermal Assessment of a Latent Heat Energy Storage Module Using a High Temperature Phase Change Material With Enhanced Radiative Properties

2014 ◽  
Author(s):  
Antonio Ramos Archibold ◽  
Abhinav Bhardwaj ◽  
Muhammad M. Rahman ◽  
D. Yogi Goswami ◽  
Elias L. Stefanakos
Keyword(s):  
Heat Transfer ◽  
High Temperature ◽  
Natural Convection ◽  
Phase Change ◽  
Temperature Response ◽  
High Temperature Phase ◽  
Radiative Properties ◽  
Temperature Phase ◽  
Measured Temperature ◽  
Numerical Predictions

This paper presents a comprehensive analysis of the heat transfer during the melting process of a high temperature (> 800°C) PCM encapsulated in a vertical cylindrical container. The energy contributions from radiation, natural convection and conduction have been included in the mathematical model in order to capture most of the physics that describe and characterize the problem and quantify the role that each mechanism plays during the phase change process. Numerical predictions based on the finite volume method has been obtained by solving the mass, momentum and energy conservation principles along with the enthalpy porosity method to track the liquid/solid interface. Experiments were conducted to obtain the temperature response of the TES-cell during the sensible heating and phase change regions of the PCM. Continuous temperature measurements of porcelain crucibles filled with ACS grade NaCl were recorded. The temperature readings were recorded at the center of the sample and at the wall of the crucible as the samples were heated in a furnace over a temperature range of 700 °C to 850 °C. The numerical predictions have been validated by the experimental results and the effect of the controlling parameters of the system on the melt fraction rate, total and radiative heat transfer rates at the inner surface of the cell have been evaluated. Results showed that the natural convection is the dominant heat transfer mechanism. In all the experimental study cases, the measured temperature response captures the PCM melting trends with acceptable repeatability. The uncertainty analysis of the experiment yielded an approximate error of ±5.81°C.

Combined Free and Forced Convection Couette-Hartmann Flow in a Rotating Channel with Arbitrary Conducting Walls and Hall Effects

2016 ◽  
Vol 32 (5) ◽  
pp. 613-629 ◽  
Author(s):  
G. S. Seth ◽  
S. Sarkar ◽  
O. D. Makinde
Keyword(s):  
Magnetic Field ◽  
Buoyancy Force ◽  
Hall Current ◽  
Fluid Velocity ◽  
Lower Wall ◽  
Thermal Buoyancy ◽  
Free And Forced Convection ◽  
Rotating Channel ◽  
Thermal Buoyancy Force ◽  
Secondary Fluid

AbstractCombined free and forced convection Couette-Hartmann flow of a viscous, incompressible and electrically conducting fluid in rotating channel with arbitrary conducting walls in the presence of Hall current is investigated. Boundary conditions for magnetic field and expressions for shear stresses at the walls and mass flow rate are derived. Asymptotic analysis of solution for large values of rotation and magnetic parameters is performed to highlight nature of modified Ekmann and Hartmann boundary layers. Numerical solution of non-linear energy equation and rate of heat transfer at the walls are computed with the help of MATHEMATICA. It is found that velocity depends on wall conductance ratio of moving wall and on the sum of wall conductance ratios of both the walls of channel. There arises reverse flow in the secondary flow direction near central region of the channel due to thermal buoyancy force. Thermal buoyancy force, rotation, Hall current and wall conductance ratios resist primary fluid velocity whereas thermal buoyancy force and Hall current favor secondary fluid velocity in the region near lower wall of the channel. Magnetic field favors both the primary and secondary fluid velocities in the region near lower wall of the channel.

Effects of Mass Transfer and Free-Convection Currents on the Flow Near a Moving Vertical Plate With Ramped Wall Temperature

2009 ◽  
Author(s):  
M. Narahari ◽  
Binay K. Dutta
Keyword(s):  
Mass Transfer ◽  
Free Convection ◽  
Skin Friction ◽  
Heat And Mass Transfer ◽  
Buoyancy Force ◽  
Vertical Plate ◽  
Flow Temperature ◽  
Thermal Buoyancy ◽  
Laplace Transform Technique ◽  
Thermal Buoyancy Force

A theoretical analysis to the problem of free convection flow induced by an infinite moving vertical plate subject to a ramped surface temperature with simultaneous mass transfer to or from the surface is presented. The plate temperature increases linearly over a specified period of time until it reaches a constant value. Diffusional mass transfer occurs at the surface contributing to the density gradient in the boundary layer. An exact analytical solution to the governing equations for flow, temperature and concentration with coupled boundary conditions in the dimensionless form have been developed using the Laplace transform technique. Heat and mass transfer at the plate are assumed to be purely diffusive in nature. The cases of impulsive start and uniformly accelerating start of the plate are considered and solutions for the flow, temperature and concentration fields are derived. The effects of different system parameters have been studied in terms of relevant dimensionless groups such as Grashof number (Gr), Prandtl number (Pr), Schmidt number (Sc), time (t) and the mass to thermal buoyancy ratio (N). The possible cases of the last parameter, namely N = 0 (the buoyancy force is due to thermal diffusion only), N > 0 (the mass buoyancy force acts in the same direction of thermal buoyancy force) and N < 0 (the mass buoyancy force acts in the opposite direction of thermal buoyancy force) are investigated and their effects on the velocity field and skin-friction are explicitly determined. The ramped temperature boundary condition predictably has an enhancing effect on the skin friction. The mass flux to the plate influences the velocity and hence the skin friction. A critical analysis of the coupled heat and mass transfer phenomena is provided. The free convection near a ramped temperature plate has also been compared with the flow near a plate with constant temperature as a limiting case.

Comparison of Numerical and Experimental Assessment of a Latent Heat Energy Storage Module for a High-Temperature Phase-Change Material

2016 ◽  
Vol 138 (5) ◽  
Author(s):  
Antonio Ramos Archibold ◽  
Abhinav Bhardwaj ◽  
Muhammad M. Rahman ◽  
D. Yogi Goswami ◽  
Elias L. Stefanakos
Keyword(s):  
Heat Transfer ◽  
High Temperature ◽  
Natural Convection ◽  
Energy Storage ◽  
Phase Change ◽  
Phase Change Material ◽  
Temperature Response ◽  
Measured Temperature ◽  
Numerical Predictions ◽  
Change Material

This paper presents a comprehensive analysis of the heat transfer during the melting process of a high-temperature (>800 °C) phase-change material (PCM) encapsulated in a vertical cylindrical container. The energy contributions from radiation, natural convection, and conduction have been included in the mathematical model in order to capture most of the physics that describe and characterize the problem and quantify the role that each mechanism plays during the phase-change process. Numerical predictions based on the finite-volume method have been obtained by solving the mass, momentum, and energy conservation principles along with the enthalpy porosity method to track the liquid/solid interface. Experiments were conducted to obtain the temperature response of the thermal energy storage (TES) cell during the sensible heating and phase-change regions of the PCM. Continuous temperature measurements of porcelain crucibles filled with ACS grade NaCl were recorded. The temperature readings were recorded at the center of the sample and at the wall of the crucible as the samples were heated in a furnace over a temperature range of 700–850 °C. The numerical predictions have been validated by the experimental results, and the effect of the controlling parameters of the system on the melt fraction rate has been evaluated. The results showed that the natural convection is the dominant heat transfer mechanism. In all the experimental study cases, the measured temperature response captured the PCM melting trend with acceptable repeatability. The uncertainty analysis of the experimental data yielded an approximate error of ±5.81 °C.

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