Computational Analysis of Gas Flow and Heat Transport Phenomena in Ducts Relevant for Fuel Cells

Abstract: The thesis concerns modeling, simulation and numerical analysis of heat and mass transport, and fluid flow in ducts of both solid oxide fuel cells (SOFCs) and proton exchange membrane fuel cells (PEMFCs). The unique fuel cell boundary conditions (thermal, mass) for the flow ducts in fuel cells are identified and applied. Various duct configurations have been studied, such as rectangular and trapezoidal cross sections, composite geometries including porous layer, flow duct and solid current inter-collector (-connector). Numerical simulations of fully developed / developing laminar flow and heat transfer have been presented with various Gr', Re, Rem, aspect ratio (b/h) and base angle at electrolyte supported SOFC boundary conditions. It was verified that the mass injection through one wall increases the friction factor f and decreases the Nusselt number Nu; the mass suction decreases f but Nu increases. Onset of deviation of friction factor and heat transfer from pure forced convection is caused by the formation of a vortex. It was verified that the number of vortices and enhancement of f and Nu depends mainly on the magnitude of Ratio of Gr'/Re2. For the same design, combined effects of mass transfer and buoyancy force have been simulated for various Grashof number Gr', mass suction rate Rem and Reynolds number Re. It was found that onset of gas flow and heat transfer from pure forced convection is caused by combined effects of the formation of vortices associated with buoyancy and downward flow with mass suction. The mass suction can advance the onset of instability for the combined flow For the same design, combined effects of mass transfer and buoyancy force have been simulated for various Grashof number Gr', mass suction rate Rem and Reynolds number Re. It was found that onset of gas flow and heat transfer from pure forced convection is caused by combined effects of the formation of vortices associated with buoyancy and downward flow with mass suction. The mass suction can advance the onset of instability for the combined flow relative to pure forced convection flow at all Reynolds numbers. The effect of combined flow on the friction factor is less significant than on the heat transfer. From the simulated velocity distribution in the PEMFC composite ducts, it was found that the velocity in the porous diffusion layer is only about a few percent of that in the gas flow duct, and secondary flows caused by mass transfer can be clearly found in both diffusion layer and gas flow duct. It was also found, among the parameters studied, that permeability and thermal conductivity ratio have a significant impact on f and Nu for both cathode and anode ducts at a fixed porous layer configuration. It has been revealed that the values of fRe and Nu vary widely in anode-supported SOFC ducts, where the thickness of the porous layer is large. fRe decreases from that of pure duct flow and has a low value due to the permeation effects in the entrance. By varying one or more of the characteristic ratios identified in this study, effects of permeation and secondary flow on the gas flow and heat transfer have been investigated in terms of fRe and Nu. A uniform fRe in the main flow direction can be achieved through appropriate adjustment of the flow parameters or / and duct configuration. Based on the concentration prediction, it has been found that gas species transport from/to the reaction site is mainly dominated by mass convection in the entrance region and by species mass concentration associated diffusion after a certain distance downstream the inlet; The position for occurrence of this change depends on several parameters, e.g., the current density.