Numerical simulation of a three-layered radiant porous heat exchanger including lattice Boltzmann simulation of fluid flow

This paper deals with the hydrodynamic and thermal analysis of a new type of porous heat exchanger. This system operates based on energy conversion between gas enthalpy and thermal radiation. The proposed porous heat exchanger has one high temperature and two heat recovery (HR1 and HR2) sections. In high temperature section, the enthalpy of flowing high temperature gas that is converted to thermal radiation emitted towards the two heat recovery sections where the reverse energy conversion from thermal radiation to gas enthalpy takes place. In each section, a 2-D rectangular porous segment which is assumed to be absorbing, emitting and scattering is present. For theoretical analysis of the porous heat exchanger, the gas and solid are considered in non-local thermal equilibrium and separate energy equations are used for the two phases. Besides, in the gas flow simulation, the lattice Boltzmann method is applied to obtain the velocity distribution through the porous segments. For the purpose of thermal analysis of the proposed porous heat exchanger, volume-averaged velocities through the porous matrix obtained by lattice Boltzmann method are used in the gas energy equation, and then the coupled energy equations for gas and porous medium of each section are numerically solved using finite difference method. The radiative transfer equation is solved by discrete ordinates method to calculate the distribution of radiative heat flux in the porous medium. The numerical results consist of the gas and porous media temperature distributions. The variation of radiative heat flux are also presented. Furthermore, the effects of scattering albedo, optical thickness and inlet gas temperature on the efficiency of the proposed porous heat exchanger are investigated. It is revealed that this type of heat exchanger has high efficiency in comparison to conventional one. Also, the present numerical results for a porous radiant burner are compared with theoretical finding by the other investigator and good agreement is found.