Based on an effective energy conversion method between flowing gas enthalpy and thermal radiation, a three-layered type of porous heat exchanger (PHE) has been proposed. The PHE has one high temperature (HT) and two heat recovery (HR1 and HR2) sections. In HT section, the enthalpy of gas flow converts to thermal radiation and the opposite process happens in HR1 and HR2. In each section, a 2-D rectangular porous medium which is assumed to be absorbing, emitting and scattering is presented. For theoretical analysis of the PHE, the gas and solid phases are considered in non-local thermal equilibrium and separate energy equations are used for these two phases. Besides, in the gas flow simulation, the Fluent code is used to obtain the velocity distribution in the PHE from inlet to outlet section. For thermal analysis of the PHE, the coupled energy equations for gas and porous layer at each section are numerically solved using the finite difference method. In the computation of radiative heat flux distribution, the radiative transfer equation (RTE) is solved by the discrete ordinates method (DOM). The effects of scattering albedo, optical thickness, particle size of porous medium and inlet gas temperature on the efficiency of PHE are explored. Numerical results show that this type of PHE has high efficiency especially when the porous layers have high optical thickness. The present results are compared with those reported theoretically by other investigators and reasonable agreement is found.

The main goal of this paper is the analysis of entropy generation in a two-dimensional porous heat recovery system. This system works based on the energy conversion between fluid enthalpy and thermal radiation. The fluid phase in this system is considered to be air assuming a non-radiative medium, whilst the solid phase is regarded as a gray radiating medium with emission, absorption, and isotropic scattering. These two phases are not in thermal equilibrium and the energy equation is separately analyzed for them. To solve the radiative equations in solid phase, the discrete ordinates method is employed. For a porous heat recovery system, the local entropy generation rate is obtained by summing the entropy generation rates due to the fluid friction and conductive and radiative heat transfer mechanisms. Besides, components of radiative entropy generation rates arise from the absorption-emission, scattering, and walls influences. However, influences of radiative parameters of optical thickness and scattering albedo on the entropy generation rates in this porous system are numerically investigated with full details. Results show that the best thermal performance of the porous heat recovery system occurs in a non-scattering medium with the highest magnitudes of optical thickness.

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