Local entropy generation model for numerical CFD analysis of fluid flows through porous media, under laminar and turbulent regimes

Abstract

Porous media structures have been proposed as an interesting solution on the design of high-temperature volumetric heat exchangers and sensible thermal energy storage devices. The wide exchange area between the solid matrix and the fluid offers the possibility to reach higher conversion efficiencies, particularly on applications of high-temperature (similar to 1000 degrees C) gases. Nevertheless, the presence of the solid matrix increases the hydrodynamic resistance on the flow, and consequently, generates irreversibilities. The entropy generation can assess in the same figure of merit the different irreversibilities generation mechanisms. In this context, this work presents a physical and mathematical model to determine the local entropy generation (LEG) rate and recognizes its different generation mechanisms for porous media. The proposed model defines a useful expression to determine the LEG as a post-process variable from the usual CFD scalar and vectorial results (temperature, velocity, TKE, and epsilon), without the necessity of solving an additional entropy transport equation. A numerical experiment was implemented showing inflection points where the porous hydrodynamic resistance forces exceed the heat transfer in the LEG rate. The Forchheimer hydrodynamic resistance effect can domine the LEG in comparison to the volumetric heat transfer for high porous Reynolds regimes (Re-D >100) when the porosity is under 0.6.