Characteristics of effective thermal conductivity of porous materials considering thermal radiation: A pore-level analysis

Abstract

Porous materials have attracted considerable attention in the fields of heat transfer and solar energy harvesting. Due to the complicated pore structures and multiple reflections of radiation beams at interior interfaces, the analysis of coupled radiative and conductive heat transfer in porous materials is complex and challenging. In this paper, heat transfer characteristics in porous materials considering the coupled modes of conduction and radiation are studied at pore level. A discrete ordinate ray-tracing (DORT) method is proposed to solve the radiative heat transfer in porous materials, which is then combined with the finite volume method to solve the energy balance equation. The geometry of porous structure is represented by the implicit function, which ensures the calculation accuracy of interface normal vectors during the solution of radiative heat transfer. A dimensional analysis is conducted and a governing dimensionless parameter (NSRC) is introduced, which well characterizes the scaling characteristics of the effective thermal conductivity (ETC). The proposed approach is demonstrated to be an effective way to simulate the coupled radiative-conductive heat transfer in porous materials at pore level. Due to the thermal radiation, the ETC for open-cell porous materials increases more dramatically with NSRC than that for closed-cell porous materials. A simple thermal network model is developed for both open- and closed-cell porous materials, which predicts the trends of the ETC with NSRC very well.