Thermodynamics Analyses of Porous Microchannels With Asymmetric Thick Walls and Exothermicity: An Entropic Model of Microreactors

Abstract

This paper presents a study of the thermal characteristics and entropy generation of a porous microchannel with thick walls featuring uneven thicknesses. Two sets of asymmetric boundary conditions are considered. The first includes constant temperatures at the surface of the outer walls, with the lower wall experiencing a higher temperature than the upper wall. The second case imposes a constant heat flux on the lower wall and a convection boundary condition on the upper wall. These set thermal models for microreactors featuring highly exothermic or endothermic reactions such as those encountered in fuel reforming processes. The porous system is considered to be under local thermal nonequilibrium (LTNE) condition. Analytical solutions are, primarily, developed for the temperature and local entropy fields and then are extended to the total entropy generation within the system. It is shown that the ratio of the solid to fluid effective thermal conductivity and the internal heat sources are the most influential parameters in the thermal and entropic behaviors of the system. In particular, the results demonstrate that the internal heat sources can affect the entropy generation in a nonmonotonic way and that the variation of the total entropy with internal heat sources may include extremum points.

Non-Equilibrium Thermodynamic Analysis of Double Diffusive, Nanofluid Forced Convection in Catalytic Microreactors with Radiation Effects

“…The temperature and Nusselt number solutions developed in Section 4 were compared with those developed by Elliott et al [15] for the case with ordinary fluid and no thermal radiation. This resulted in observing no difference between the two datasets and hence confirmed the validity of the mathematical model developed in Sections 2-4.…”

“…For example, the small dimensions of microreactors warrants inclusion of the solid walls of microchannels within the thermophysical calculations of the system [12,13]. Further, due to the internal heat generation within the microreactor, the local thermal non-equilibrium (LTNE) approach should be applied to the thermal analyses of porous microreactors [14,15]. Owing to these complexities, thermodynamic analysis of microreactors is amongst the state-of-the-art research categories in the broad field of thermo-chemical systems.…”

“…It was shown that by choosing a specific value for the thicknesses of the walls, it is possible to minimise the entropy generation rate within the microchannel [14]. In another recent study, Elliott et al [15] extended the investigation of Hunt et al [14] to fully filled porous microchannels. Wang et al [20] have considered a microchannel with thick walls in premixed micro-combustor.…”

“…As internal heat generation is an intrinsic feature of many microreactors, the use of LTNE model instead of LTE approach provides more accurate predictions of the temperature field [24,25]. Further, using a non-equilibrium model influences the entropy generation and species concentration field of the system [14,15]. This paper has been divided into five sections.…”

The effects of exothermic catalytic reactions upon combined transport of heat and mass in porous microreactors

“…As opposed to classical reaction vessels, where the vessel walls comprise a relatively small component of the total volume of the system, the walls of a microchannel are of comparable volume to that of the microchannel itself. Recent studies of microreactors have revealed the essential necessity of taking into account the enclosing structure when examining the transport of heat [18,19] and mass [17]. It is common for microreactors to feature multiple microchannels inside which chemical reactions take place [5,8].…”