Turbulent ignition of non-premixed hydrogen by heated counterflowing atmospheric air

Experiments were conducted to determine the effects of turbulence on the temperature of a heated air jet required to ignite a counterflowing cold hydrogen/nitrogen jet at atmospheric pressure. At high fuel concentrations, the ignition temperature was found to be insensitive to the turbulent intensity. This is consistent with previous results on laminar flows, which found the dominant chemistry governing the second explosion limit, around atmospheric pressure, to be sufficiently rapid when compared with the transport rates for the ignition temperature to be insensitive to the local strain rate of the flow. The present results indicate that this is true whether the strain rate is manifested through the steady bulk strain rate of the flow or the unsteady straining due to turbulent eddies. At lower hydrogen concentrations, however, ignition was found to be intermittent in that the flow is repeatedly ignited and extinguished over a range of temperatures. With decreasing fuel concentration, increasing turbulent intensity, or increasing bulk strain rate, the difference between the air temperatures when ignition events were first observed and when extinction was totally inhibited is increased, widening the range of intermittent ignition. This behavior can be explained using laminar flow calculations with detailed chemical kinetics and transport. These calculations show that there is only minimal S-curve hysteresis between ignition and extinction for low fuel concentrations. In turbulent flows, there is a fluctuating instantaneous strain rate experienced by the ignition kernel, which is the sum of the bulk and eddy strain rates. Consequently, in these flows the strain rate can alternately traverse beyond the ignition and extinction turning points, causing the flow to intermittently ignite and extinguish.