Transient behavior and thermal efficiency of volumetric heat receivers

Abstract

Volumetric heat absorbers made of porous materials allow for a greater contact area between the porous matrix and the working fluid. In such devices, heat is collected in a volume rather than a surface. This work deals with transient behavior of Volumetric Heat Receivers using the thermal non equilibrium approach. Balances for energy amounts for the porous ceramic materials and air are solved numerically using the backward Euler discretization method and application of the SIMPLE method. After obtaining the algebraic equation system, relaxation by the SIP method is applied. We investigate here the effects of permeability, thermal conductivity ratio, inlet velocity and porosity on the temperature reached after thermal equilibrium. Higher inlet velocities attain quicker stabilization times and decreases equilibrium temperature, Teq. In addition, increasing porosity lower Teq and shorten time for temperature stabilization. Less permeable solid matrices, i.e. porous structures with lower Da, result in slightly higher Teq due to better enhancement of energy exchange between phases. Increasing kskf/ increases stabilization time as well as equilibrium temperatures. Thermal efficiency increases for lower inlet velocities and higher thermal conductivity ratios, whereas η is reduced for more permeable structures and higher porosities.