An Experimental Study of Ignition in Nonpremixed Counterflowing Hydrogen versus Heated Air

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Abstract
A variable pressure, counterflow combustion chamber has been built for the experimental investigation of ignition in a convective-diffusive system. In this paper we present our results on ignition of nonpremixed, counterflowing jets of 20% H2 in N2 versus heated air, within a wide range of pressures and flow strain rates. The system was brought to ignition by increasing gradually the temperature of the air stream. Each steady-state situation just prior to ignition was characterized by measuring detailed centerline axial flow velocity and temperature distributions, for ambient pressures between 0.1-6.0 atm and pressure-weighted strain rates between 50-400 s−1. The ignition temperature, defined as the maximum temperature of the air jet just prior to ignition, was found to increase with increasing flow strain rate at all pressures. Furthermore, its sensitivity to strain-rate variations was found to be much higher at elevated and reduced pressures (above ∼ 2 atm and below ∼ 0.5 atm) than it was at atmospheric pressure. Ignition temperatures at constant pressure-weighted strain rates exhibited a ‘Z’-shaped pressure dependence similar to that observed in explosion limits of homogeneous H2-air mixtures, and to the ignition limits identified in earlier numerical and asymptotic analyses. The experimental data was found to be in good agreement with computations using full chemical kinetics and transport properties.