A micro/macroscale model for intermediate temperature solid oxide fuel cells with prescribed fully-developed axial velocity profiles in gas channels

A two-dimensional micro/macroscale model is proposed as an efficient numerical framework for simulating intermediate temperature solid oxide fuel cells (IT-SOFCs). This model employs a comprehensive microscale model that describes the detailed electrochemical reactions in Ni/YSZ cermet anodes and LSM/YSZ composite cathodes based on the three-phase boundary length (TPBL). A simplified macroscale model has been combined with the microscale model to consider the heat and mass transport processes in IT-SOFCs with prescribed fully-developed laminar velocity profiles in gas channels. A hydrogen-fed IT-SOFC is simulated at various operating conditions in order to demonstrate the capabilities of the proposed micro/macroscale model. The results elucidate the effects of co- and counter-flow configurations, inlet temperature, and air and fuel flow rates on the performance of the IT-SOFC.     

Three-dimensional micro/macroscale simulation of planar,anode-supported,intermediate-temperature solid oxide fuel cells: I. Model development for hydrogen fueled operation

Intermediate-temperature solid oxide fuel cells (IT-SOFCs) are promising SOFC technologies that can solve many problems of high-temperature SOFCs (HT-SOFCs), such as the stringent restriction on material selection, accelerated degradation of electrode activity, limitation in thermal cycling, and requirement for long start-up times. In this study, a comprehensive three-dimensional micro/macroscale model is developed for simulating planar, anode-supported IT-SOFCs fueled with hydrogen. Many constitutive sub-models for electrode microstructure, detailed charge-transfer processes, and heat/mass transport in three-dimensional interconnect plate/gas channel geometries are combined to investigate the performance and operating characteristics of IT-SOFCs with rather standard materials (such as nickel, YSZ, LSM, and stainless steel). The current−voltage performance curves are presented along with the contribution of activation, concentration, ohmic, and contact overpotentials to total potential loss. In addition, the spatial distributions of temperature, current density, and species concentrations are also investigated for co- and counter-flow configurations. The results clearly demonstrate the capabilities of the present three-dimensional micro/macroscale model as an accurate and efficient design tool for optimizing the operating conditions, electrode microstructures, and cell geometries of planar, anode-supported IT-SOFCs.     

CFD approaches for the simulation of hydrodynamics and heat transfer in Taylor flow

Previous studies on heat and mass transfer in the Taylor flow regime in microchannels have shown the transport (heat/mass) rates to be dependent on the length of the liquid slug. In order to understand the effect of slug length on transport rates and to have a one-to-one comparison with experimental data, a computational approach is required to simulate flows with liquid slugs and bubbles of controlled lengths.Here we describe and benchmark two approaches. The first, and conceptually simplest, is to generate bubbles and slugs in a long tube using a time-dependent boundary condition. In the second method, the flow and heat transfer in a single unit cell, consisting of a bubble surrounded by liquid slugs, is solved in a frame of reference moving with the bubble velocity. Both methods were implemented in ANSYS-Fluent.Simulations for a two-phase (liquid-only) Reynolds number of 713, Capillary number of 0.004 and void fraction of 0.366 for nitrogen-water flow were performed to compare the two techniques. There was a very large difference between the required computational mesh sizes and times for the two methods, with a wall clock time of 38 h on a single processor for the moving domain compared with 1460 h using four processors for the stationary domain approach. In addition, for a constant wall heat flux boundary condition, even with 14 bubbles present in a long tube thermal development was not achieved. The hydrodynamic and heat transfer results obtained from the two approaches were found to be very similar to each other and with results from our earlier verification and validation studies, giving a high degree of confidence in the implementation of both methods.