ANALYTICAL STUDY OF THE EFFECT OF LOCAL THERMAL NON-EQUILIBRIUM INTENSITY IN POROUS FOAMS

As a kind of compact heat transfer media, open-cell foams offer a combination of advantages that include high porosity, high surface density, low pressure drop, and long-term service, making them well suited for advanced heat recovery applications. The study of fluid flow and heat transfer in porous foams is generally carried out using two models, namely the Local Thermal Equilibrium (LTE) and the Local Thermal non-Equilibrium (LTNE). The LTE model assumes no temperature difference between the local fluid and solid struts. However, when there is a significant temperature difference between solid and fluid phases, then the local phases must be treated individually by the LTNE model. The LTNE model is more difficult to apply because of more information required on the interfacial heat transfer coefficient, which is usually obtained through experimental investigations. In this study, by defining Nusselt deviation obtained by the LTE and LTNE models, operational and geometrical conditions in which the effect of LTNE model is significant are determined. As a matter of fact, based on this analysis, one can choose a suitable model for heat transfer analysis in porous open-cell foams based on the solid foam properties (such as porosity, pore density, and thermal conductivity of the foam) and the desired operating conditions (such as fluid velocity, pipe diameter, and thermal conductivity of the fluid). Results show that among the properties of porous foams, lower pore density and higher thermal conductivity (metallic foams against the ceramic foams) have the greatest impact on the importance of the LTNE model. From the viewpoint of flow operating conditions, the lower flow velocity, smaller pipe diameter, and lower thermal conductivity of the fluid increase the importance of the LTNE model in heat transfer analysis of porous foams. Among the parameters mentioned above, thermal conductivity of porous foam has the greatest impact on the local thermal non-equilibrium intensity.