Abstract
Auto-ignition processes of hydrogen, diluted with nitrogen, in heated air are numerically investigated by means of an unsteady laminar flamelet approach in mixture fraction space. The focus is on the auto-ignition delay time and the most reactive mixture fraction as obtained with five chemical mechanisms. Two strongly different levels of dilution, corresponding to experiments in the open literature, are considered. This concerns low-temperature chemistry at atmospheric pressure. The temperature of the air stream is much higher than the temperature of the fuel stream in the cases under study. We extensively investigate the effect of the co-flow temperature, the conditional scalar dissipation rate and the resolution in mixture fraction space for one case. With respect to the conditional scalar dissipation rate, we discuss the Amplitude Mapping Closure (AMC) model with imposed maximum scalar dissipation rate at mixture fraction equal to 0.5, as well as a constant conditional scalar dissipation rate value over the entire mixture fraction value range. We also illustrate that an auto-ignition criterion, based on a temperature rise, leads to similar results as an auto-ignition criterion, based on OH mass fraction, provided that the hydrogen is not too strongly diluted.
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