Advanced Technology and Manufacturing Institute (ATAMI)

Experiments on a novel, microscale hydrogen combustor heat exchanger (uCHX) are presented in this thesis. The uCHX has been designed to efficiently transfer thermal energy to a heat transfer fluid for use in desorption of hydrogen from a metal hydride storage system of a hydrogen-powered fuel cell vehicle. The experimental study demonstrates proof of concept for the design and examines the effect of varying selected fluidic and geometric conditions on the overall efficiency of the uCHX. The small-scale device characterized in this thesis is a unit cell consisting of a combustion channel, a recuperator channel and an oil heat exchange channel. This unit cell device has the potential to be scaled up to a reactor in the tens of kilowatts range required for a vehicle scale system. Combusted gases flow back through the device in a counter-flow configuration between the combustion channel and the oil channel, preheating the inlet gas stream and transferring heat to the oil. Platinum has been selectively deposited on the stainless steel inner wall of the combustion channel to facilitate heterogeneous combustion of hydrogen and air. The performance of the uCHX is characterized based on a global efficiency, defined as the ratio of the energy transferred to the oil stream compared to the energy content of the supplied hydrogen gas. The effects of residence time, equivalence ratio and average oil temperature have been investigated. Repeated testing of the catalyst over 80 hours indicated stable and repeatable results. Maximum hydrogen conversion in excess of 92 % was achieved for residence times greater than 20 ms at a stoichiometric equivalence ratio. Efficiencies above 90 % were obtained for equivalence ratios greater than 0.5 and for average oil temperatures between 63 °C and 105 °C. Experiments were conducted with catalyst deposited over 87 % and 20 % of the channel length. The longer catalyst demonstrated the highest efficiency and hydrogen conversion. This work was performed concurrently with the development of a CFD model by Ghazvini and Narayanan [1] with preliminary comparisons showed qualitatively good agreement. A preliminary design and financial analysis of a full size combustor was developed based on the demonstrated performance of the uCHX.