Hetero-/homogeneous combustion of hydrogen/air mixtures over platinum at pressures up to 10 bar

摘要

The hetero-/homogeneous combustion of fuel-lean hydrogen/air premixtures over platinum was investigatedexperimentally and numerically in the pressure range 1 bar 6 p 6 10 bar. Experiments were carriedout in an optically accessible channel-flow catalytic reactor and included planar laser induced fluorescence(LIF) of the OH radical for the assessment of homogeneous (gas-phase) ignition, and 1-D Raman measurementsof major gas-phase species concentrations for the evaluation of the heterogeneous (catalytic) processes.Simulations were performed with a full-elliptic 2-D model that included detailed heterogeneousand homogeneous chemical reaction schemes. The predictions reproduced the measured catalytic hydrogenconsumption, the onset of homogeneous ignition at pressures of up to 3 bar and the diminishing gas-phasecombustion at P≥4 bar. The suppression of gaseous combustion at elevated pressures bears the combinedeffects of the intrinsic homogeneous hydrogen kinetics and of the hetero/homogeneous chemistry couplingvia the catalytically produced water over the gaseous induction zone. Transport effects, associated with thelarge diffusivity of hydrogen, have a smaller impact on the limiting pressure above which gaseous combustionis suppressed. It is shown that for practical reactor geometrical confinements, homogeneous combustionis still largely suppressed at p P 4 bar even for inlet and wall temperatures as high as 723 and 1250 K,respectively. The lack of appreciable gaseous combustion at elevated pressures is of concern for the reactorthermal management since homogeneous combustion moderates the superadiabatic surface temperaturesattained during the heterogeneous combustion of hydrogen.