An experimental and numerical investigation of the spontaneous ignition characteristics of hydrogen-air mixtures
The spontaneous ignition delay time of hydrogen-air mixtures has been studied experimentally and numerically. A flow reactor has been used to obtain the ignition delay times for lean hydrogen-air mixtures at atmospheric pressure. The results indicate that wall reaction effects were significant for test section diameters of less than 5 cm. Coaxial fuel injection was found to produce significant mixing delay even with multipoint injection while mixing effects were considerably reduced when the fuel was injected normal to the flow. The experimental results compared well with a numerical model of the detailed chemistry of the reaction. The numerical model was used to extend the results beyond the range of experimental conditions. In addition, the model was used to determine the underlying chemical causes of the ignition delay behavior. The effects of pressure and temperature were evaluated showing that global reaction theory could not predict the ignition delay behavior of hydrogen in the low temperature and high pressure region (P $>$ 1 atm, T $<$ 1000$\sp\circ$K). In this region the slope of the ignition delay vs. temperature curve increased dramatically. This observation was explained in terms of competition between the chain branching reaction, R1, and the chain terminating reaction, R7. The effects of adding NO and NO$\sb2$ to the flow were also studied numerically. The model results compared well with the experimental data of Slack and Grillo: both of these additives were found to significantly reduce the ignition delay times. It was discovered through this analysis that NO and NO$\sb2$ reduced the delay time through different mechanisms. Nitric oxide produced a catalytic effect converting the relatively nonreactive HO$\sb2$ radical to the more reactive OH radical. Nitrogen dioxide was found to reduce the delay time by a shift of the kinetic mechanism from one dominated by H$\sb2$-O$\sb2$ kinetics to one dominated by H$\sb2$-NO$\sb2$ kinetics. The mechanism by which each of these species reduce ignition delay is presented and the important reactions identified. The numerical model was also used to study the physical effects of flow velocity, heat loss and test section diameter on ignition delay. The assumption of convective dominated flow was checked by solving the full one dimensional transport equations for both turbulent and laminar flow.
Sojka, Purdue University.
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