Total Effective Thermal Conductivity Research Articles

TRANSIENT CONDUCTION-RADIATION INTERACTION IN A PLANAR PACKED BED WITH VARIABLE POROSITY

Radiation combined with conduction in a planar packed bed of opaque and diffuse-gray spheres is investigated numerically. Both constant and variable porosity distributions are considered. Kamiuto's correlated scattering theory as well as Kaviany's scaling factor are used to study the effect of dependent scattering. The discrete transfer method is used to solve the radiative part of the energy equation, and Euler's implicit finite-difference scheme is used to solve the energy equation. Effects of effective thermal conductivity, bed depth-to-particle diameter ratio, scaled scattering albedo, and conduction-radiation parameter are studied for both steady and transient cases. These parameters have been found to have significant effects on total effective thermal conductivity, temperature, and radiative and conductive heat flux distributions.

Combined Conductive and Radiative Heat Transfer through Open-Cellular Porous Plates.

The ability of the cubic lattice model for the thermal and radiative properties of open-cellular porous materials in predicting the combined effect of conduction and radiation in plane-parallel, open-cellular porous plates was investigated in comparison with available experimental data. The validities of the P1 approximation to radiative transfer and the simple adding method for the total heat flux were also addressed. It is concluded that the cubic lattice model can be utilized accurately to evaluate the temperature porfiles and heat transfer characteristics in open-cellular porous plates and that the simple adding method gives the fairly accurate total heat flux across the media. In addition, it is found that there exists only a little discrepancy between the results for the temperature profiles and mean total effective thermal conductivities obtained by the P1 approximation and those obtained by the exact numerical method.

COMBINED CONDUCTION AND RADIATION HEAT TRANSFER IN PLANE-PARALLEL PACKED BEDS WITH VARIABLE POROSITY

The primary concern of this study deals with the prediction of thermal performance of packed beds with variable porosity. Combined conduction and radiation heat transfer in plane-parallel packed beds of spherical particles was investigated numerically. The effects of variable porosity, effective thermal conductivity, and thermal radiation were taken into account. The method for radiative transfer equation was based on the P 3-spherical harmonic approximation, while the finite-difference scheme was employed to solve the energy equation. Four different models of porosity distributions were tested. It was found that variable porosity and effective thermal conductivity effects on the mean total effective thermal conductivity and radiative heat flux distributions are important and should not be ignored.

Combined conduction and correlated-radiation heat transfer in packedbeds

A quasihomogeneous model for heat transfer in a plane-parallel packed bed of opaque spheres is presented. The proposed model takes into account the variable porosity distribution within a packed bed and the correlated radiative properties of packed spheres having gray diffuse surfaces. On the basis of the proposed model, combined conductive and radiative heat transfer through comparatively thin packed layers of cordierite spheres or oxidized steel spheres are analyzed numerically and the obtained results are compared with the experimental ones. Additionally, the adequacy of a simplified analytical formula for the total effective thermal conductivities of a packed bed is discussed. It is found that the detailed theoretical model accurately predicts the heat transfer characteristics and the temperature profiles of the plane-parallel packed beds, and the predictions derived from the approximate formula well-correlate with the present experimental data for the total effective thermal conductivities.

Analytical Expression for Total Effective Thermal Conductivities of Packed Beds

Analytical Expression for Total Effective Thermal Conductivities of Packed Beds

Two-parameter formula for the total effective thermal conductivities of ceramic-fiber insulations

A comprehensive two-parameter formula for predicting the total effective thermal conductivities of alumina-silica fiber insulations is presented. The adjustable parameters involved in the formula are estimated from available experimental data for the total effective thermal conductivities of ceramic-fiber insulations. It has been found that the resulting formula reproduces the experimental data examined here within an accuracy of ±30% and well predicts the total effective thermal conductivities of Kaowool 1260 in a helium atmosphere at 0.1 MPa.

Analytical Expression for Total Effective Thermal Conductivities of Packed Beds.

Analytical Expression for Total Effective Thermal Conductivities of Packed Beds.

Effects of Correlated Scattering on Coupled Heat Transfers by Conduction and Radiation through a Packed Layer of Glass Beads

The effects of correlated scattering on the combined conductive and radiative heat transfer through a packed layer of glass beads are theoretically examined. The energy equation and the spherical harmonic equations of the first order for radiative transfer are numerically solved utilizing the correlated or uncorrelated radiative properties for packed glass spheres. The correlated scattering calculations for temperature profiles and mean total effective thermal conductivities are compared with the uncorrelated scattering calculations and our previous experimental results