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.

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.

A Study of the Nusselt-Rayleigh and Sherwood-Rayleigh Number Relations for Water Undergoing Free-Surface Natural Convection

2007 ◽  
Author(s):  
S. M. Bower ◽  
J. R. Saylor
Keyword(s):  
Free Surface ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Power Law ◽  
Fluid Layer ◽  
Power Laws ◽  
Surface Boundary ◽  
Free Surface Boundary Condition ◽  
Transfer Case ◽  
Solid Plate

An experimental study is presented of the Nusselt-Rayleigh and Sherwood-Rayleigh number relations for water undergoing free-surface natural convection, which is natural convection beneath an air/water interface. The focus of this work is on the Nu-Ra relationship. This relationship is typically studied using the traditional Rayleigh-Be´nard convection experiment where a fluid layer is bounded above and below by solid plates of different, but constant, temperatures. Hence, the boundary conditions are of the no-slip, constant-temperature type. Power laws are typically used in these studies to correlate the Nu-Ra data, and existing studies have given power law exponents that are usually close to 1/3. The experimental data obtained in this study yields a power law relation of the form: Nu=(0.0016)Ra0.328(1) for 107 < Ra < 1011. This result is surprising in that the effect of the free-surface boundary condition on the power law exponent is quite small when compared to the solid plate case. However, the prefactor in Eq. (1) is significantly smaller than for the solid plate case. The Sh-Ra data obtained in this study are also fit to a power law, giving: Sh=(0.0019)Ra0.329(2) where Sh is the dimensionless mass transfer coefficient for evaporation. The exponent of this power law differs from that which has been observed by prior researchers. However, the prior research on evaporation that utilizes this form for scaling the data is considerably smaller than for the heat transfer case. Possible explanations for the observed behavior are presented.

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.

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.

Natural Convection in a Horizontal Layer of Water Cooled From Above to Near Freezing

1975 ◽  
Vol 97 (1) ◽  
pp. 47-53 ◽  
Author(s):  
R. E. Forbes ◽  
J. W. Cooper
Keyword(s):  
Heat Transfer ◽  
Boundary Condition ◽  
Natural Convection ◽  
Hydrodynamic Instability ◽  
Convective Heat Transfer Coefficient ◽  
Free Water ◽  
Ambient Air ◽  
Maximum Density ◽  
Surface Boundary ◽  
Initial Water

Natural convection in horizontal layers of water cooled from above to near freezing was studied analytically. The water was confined laterally and underneath by rigid insulators, and the upper horizontal surface was subjected to: (1) a constant 0C temperature, rigid conducting boundary, and (2) a free, water to air convection boundary condition, in which the convective heat transfer coefficient was held constant at values of 5.68 W/m2 · K and 284 W/m2 · K (1.0 and 50.0 Btu/hr ft2F) and the temperature of the ambient air was maintained at 0C. The ratios of the width to the depth of the rectangular water layers under consideration were W/D = 1, 3, and 6. Initially the water is assumed to be at a uniform temperature of either 4C or 8C, and then the upper surface boundary condition was suddenly applied. It was observed in all cases for which the initial water temperature was 4C, that the water remained stagnant and became thermally stratified. Heat transfer application of either of the surface boundary conditions to water initially at 8C produced large convective eddies extending from the bottom to the top of the layer of water. As the liquid layer cooled further, two distinct horizontal regions appeared, the 4C isothermal line separating the two. This produces a region of hydrodynamic instability in the fluid since the maximum density fluid (4C) is physically located above the less dense fluid in the lower portion of the cavity. The large eddies which appeared initially were confined to the hydrodynamically unstable region bounded by the 4C isotherm and the bottom of the cavity. The action of viscous shearing forces upon the stable water above the 4C isotherm produced a second “layer” of eddies. An alternating direction implicit finite difference method was used to solve the coupled system of partial differential equations. The paper presents transient isotherms and streamlines and a discussion of the effect of maximum density on the flow patterns.

Sign in / Sign up

Export Citation Format

Share Document