The influence of catalytic activity on the ignition of boundary layer flows. Part III: Hydroxyl radical measurements in low-pressure boundary layer flows

Abstract

The boundary layer ignition of fuel/air mixtures by heated catalytic (platinum) and noncatalytic (quartz) surfaces was studied through spatially resolved measurement of hydroxyl radicals at total pressures from 40 to 175 torr. Subatmospheric pressure operation increases both length scales and the ratio of surface/gas-phase reaction contribution to ignition, facilitating the study of the effect of surface catalytic activity on the initiation of gas-phase combustion. Fuels studied include ethane, acetylene, hydrogen, and methane, usually premixed in air. Ethane ignition was promoted by catalytic reaction at all equivalence ratios studied. This was a wider range of equivalence ratios evidencing catalytic enhancement than noted at atmospheric pressure in previous studies. At 70 torr, this enhancement was observed from 0.015 < Φ ≤ 0.5. Surface hydroxyl radical production rates were estimated for preignition conditions in the gas phase for ethane in air over both quartz and platinum surfaces. Combustion of hydrogen or acetylene in air at these conditions was not significantly affected by the catalytic activity of the surface, because for the reactions studied, gas-phase reaction was significant near the leading edge of the plate. N 2O substitution for O 2 severely inhibited the gaseous ignition of the ethane, hydrogen, and acetylene over platinum surfaces, and suppressed preignition OH concentrations to below our detection limits.