Study of local entropy generation in porous media under laminar and turbulent regimes

Abstract

Recently, in the last 15 years, porous structures have been proposed as an interesting solution in the design of high-temperature energy storage and exchange systems, both for applications in concentrated solar power (CSP) systems. The wide exchange area of the solid matrix allows to reaching higher conversion efficiencies, particularly in high-temperature applications (~1000°C) for compressible gases (CO2 or air). However, the presence of the solid matrix increases the hydrodynamic resistance of the flow and, consequently, generates irreversibilities to control. Similarly, the arduous operating conditions in some cases and the inherent complexity of simulating a tortuous porous medium make the task of simulation and analysis still a complex problem to solve in turbulent systems. Therefore, in this work, the generation of entropy is proposed as a comprehensive figure of merit capable of incorporating the different mechanisms of generation of irreversibilities in a single variable on the transport and management of the energy. Additionally, an analysis focused on the transport and generation of entropy to distinguish the quality of the energy potential of each energy flow interacting with a system. Despite the fact that entropy as a concept is at least 200 years old, there is no methodology in the literature that allows to determining the local entropy generation (LEG) in porous media and distinguishing its different generation mechanisms. When a porous matrix is included in the analysis, additional volumetric heat transfer and energy dissipation mechanisms appear that are not found in free internal flows. This makes it necessary to study the modeling of these additional mechanisms and their impact on the generation of entropy. This work proposes a theoretical analysis of the transport equations of mass, energy, and momentum, and from them develops a physical-mathematical expression for the transport and generation of entropy in porous media. Furthermore, a methodology is presented to apply the LEG theoretical model in CFD simulation tools. A preliminary dimensionless analysis on the LEG expression is presented to determine the dimensionless variables that define the level of significance of each LEG mechanism. Subsequently, a numerical experiment is implemented in a porous channel under 200 design and operation configurations. The results show that the irreversibilities associated with hydraulic resistance in porous media can dominate the LEG rate over volumetric heat transfer mechanisms. Similarly, inflection points are determined where the hydraulic resistance can dominate the LEG for different ReD, porosity, and inlet temperature difference. The hydrodynamic resistance effect dominate the total LEG in comparison to the volumetric heat transfer for high porous Reynolds regimes (ReD>100) when the porosity is below 0.6.