Effect of hydrogen addition on the counterflow ignition of n-butanol at atmospheric and elevated pressures

The present experimental and computational study describes the non-premixed counterflow ignition of nitrogen-diluted pre-vaporized n-butanol/hydrogen mixtures impinging on a heated air stream for pressures of 1-4 atm and hydrogen molar percentages in the binary fuel blends ranging from ξH = 0% (pure n-butanol) to ξH = 100% (pure hydrogen). The experimental data show that hydrogen addition results in a non-linear decrease in ignition temperatures that can be broken into two regimes; a hydrogen-enhanced regime of ξH = 0-40%, where the addition of more hydrogen significantly decreases ignition temperature, and a hydrogen-dominated regime in the range of ξH = 40-100%, where ignition temperatures exhibit minimal sensitivity to further hydrogen addition. The experimental results are then simulated using n-butanol-specific skeletal mechanisms developed from two comprehensive butanol models available in the literature. The ability of these mechanisms to predict the variation of ignition temperatures associated with hydrogen addition to the n-butanol “base” fuel is assessed. Comparison between the experimental and computational results reveals that both chemical kinetic models capture the two-regime behavior associated with hydrogen addition observed in the present experiments, though both models over-predict experimental ignition temperatures. Further chemical kinetic analysis of the mechanisms reveals that the two-regime behavior is controlled by the production of hydroperoxyl radicals, with production via the reaction of formyl radicals and oxygen molecules dominating at low hydrogen addition levels, and production via the three-body H + O2+M = HO2 + M reaction dominating at high hydrogen addition levels.