There is currently great interest in the heat and mass transfer operative in the fuel reforming and power generating components of micro and macro scale fuel cells. In support of research and development projects at LLNL in these areas, two modeling capabilities are being added to our finite element heat transfer analysis program TOPAZ3D. First, an overall reactor rate modeling capability has been integrated into the finite element structure of TOPAZ3D. This work facilitates design and thermal management work with concepts under study today in both the Engineering Micro-technology Center and in the Energy & Environment Directorate. A plug flow reactor approach has been mated to TOPAZ3D so that both critical chemical performance parameters, such as fuel conversion efficiency, and thermal parameters, such as start up or trimming heat, can be calculated consistently. Second, a detailed surface chemical kinetic and thermal transport approach is being implemented for microscale modeling. This work uses our reentry vehicle based surface chemical kinetic model and adds detailed transport calculations using finite volume methods.
Progress with both approaches was summarized in the publication, Surface Chemical Kinetic Effects in Finite Element Modeling of Heat Transfer in Fuel Cells, presented at the International Conference on Modeling and Simulation of Microsystems MSM 2001. A surface chemical kinetic model of a methanol processing microreactor on a chip was exercised given a flow field solution. Device heat loss, reaction rates, and micro-heater power were computed. The plug flow reactor approach was also exercised with a similar reactor packed with porous fuel reforming catalyst. Device fuel conversion efficiency and the evolution of the reactant and product molar flow rates down the length of the micro-channel were computed (see Figure 1).
The plug flow reactor approach was also applied to a micro-scale device built and now in test at the Micro-technology Center (Figure 2). Performance was projected (Figure 3). Measurements of exhaust gas composition for model validation are in progress. Once agreement is demonstrated, the concept can be scaled up and heat exchange between entering and exiting streams can be optimized.
Our general and flexible surface chemical kinetic approach has been detailed in the publication, Thermo-chemical Ablation During Reentrant and High Altitude Skipping Flight, AIAA 2001-0979. The approach has also been exercised with 6 chemical kinetic systems in a second paper recently accepted for publication in the Journal of Thermophysics and Heat Transfer. The coding and input formats allow rapid comparison of the thermal consequences of varying chemical systems. Though this work is motivated by the reentry vehicle application our comparison of the 6 systems to data and to other published computations serves as a significant verification and validation effort.
Figure 1. Device fuel conversion efficiency and the evolution of the reactant and product molar flow rates.
Figure 2. Microreactor (methanol reformer for fuel cell on a silicon wafer).
Figure 3. Heated micrereactor thermal profile.
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