Design of a Microscale Combustor-Heat Exchanger for Low Temperature Applications

A design of a compact, integrated combustor and heat exchanger for low temperature applications is presented in this paper. The design involves use of several repeating unit cells that each performs three unit operations — combustion, recuperation, and heat exchange. Heat from combustion is transferred to a cold gas stream entering the device at 200K. Heterogeneous catalytic combustion occurs on the walls of microchannels in the presence of a platinum catalyst. Two levels of numerical simulations are performed to realize the design. The first level represents a single unit cell consisting of a combustor, exhaust gas, and heat exchange fluid channels. At the unit cell level, a two-dimensional numerical model with detailed surface chemistry is used to provide high unit cell thermal efficiency. It is shown that with the help of a novel distributed catalyst arrangement, extinction of the reaction due to the cold gas stream is prevented and a high hydrogen conversion (greater than 95 percent) is achieved for a range of operating conditions. The second level of simulations is at the device scale, consisting of multiple unit cells connected together with appropriate fluidic headers. At this device level, three dimensional simulations of fluid flow were performed to ensure uniformity in flow distribution while maintaining low pressure drop through the device. Fabrication constraints were also incorporated into the device level design and simulations.