Ignition of CO/H2/N2 versus heated air in counterflow:: Experimental and modeling results

Nonpremixed ignition in counterflowing CO/H-2 vs. heated air jets is experimentally and computationally investigated. The experiments confirm the numerical modeling observation of the existence of three ignition regimes as a function of the hydrogen concentration. In all three regimes, we first detect experimentally the onset of chemiluminescent glow due to excited CO2 followed by flame ignition, as the temperature of the air jet is raised gradually. The temperature extent of the glow regime, however, is progressively reduced with increasing hydrogen addition; no glow is detected for H-2 concentrations in excess of similar to 73%. The temperatures for glow onset and flame ignition are represented by the boundary air temperatures for each threshold. The variation of these temperatures with system pressure and flow strain rate is explored, for pressures between 0.16 and 5 atm, and strain rates of 100 to 600 s(-1). The pressure variation is found to result in three p-T ignition limits, similar to the ignition limits observed in the H-2/O-2 system. This similarity is also observed on the effects of aerodynamic transport on ignition: within the second limit the ignition temperatures are found to be essentially insensitive to flow strain rate, whereas the other two limits are significantly affected by strain. The transport insensitivity is maintained even in the limit of very low H-2 concentrations, where an analogous H-2/N-2 mixture would fail to ignite. This behavior is explained computationally by the replacement of the shift reaction OH + H-2 --> H2O + H with the reaction CO + OH --> CO2 + H, thereby minimizing the effect of diminishing H-2 concentration. The experimental data are found to agree well with the calculated results, although discrepancies are noted in modeling the onset of chemiluminescence and its response to pressure variations. (C) 2000 by The Combustion Institute.