Transient simulation of the combustion of fuel-lean hydrogen/air mixtures in platinum-coated channels

The start-up of platinum-coated, hydrogen-fuelled planar channels with heights of 1mm is investigated numerically using 2-D transient simulations with detailed hetero-/homogeneous chemistry, heat conduction in the solid wall and surface radiation heat transfer. Simulations encompass pressures of 1 and 5bar and fuel-lean H-2/air equivalence ratios of 0.10 to 0.28. Catalytic ignition is inhibited by rising pressure and increasing hydrogen concentration. However, at temperatures above the catalytic ignition temperature T-ign, the dependencies of the heterogeneous reactivity reverse, showing a positive order similar to 1.5 with respect to hydrogen concentration and an overall positive pressure order of similar to 0.97. Despite the longer catalytic ignition times for the larger equivalence ratios, the times required to reach steady state are shorter at larger stoichiometries due to their enhanced catalytic reactivity at T>T-ign and the resulting higher exothermicity. Following catalytic ignition, the wall temperatures eventually attain superadiabatic values due to the diffusional imbalance of hydrogen. Homogeneous chemistry considerably moderates the superadiabatic surface temperatures at 5bar, as the gaseous combustion zone extends parallel to the channel wall and thus shields the catalyst surface from the hydrogen-rich channel core. Furthermore, gas-phase chemistry reduces the steady-state times and substantially increases the hydrogen conversion.