Suppression of NO_x emissions by intensive strain in lean premixed hydrogen flamelets

NO_x formation in premixed, highly-strained, pure hydrogen-air flamelets is investigated at lean conditions. Detailed-chemistry, one-dimensional and two-dimensional simulations are performed on a reactants-to-products configuration with varying applied strain rate. The results highlight for the first time for lean pure-hydrogen flamelets that NO_x emissions are suppressed as strain rate is increased. The most substantial decrease is observed across the thermal NO_x formation pathway. Here the suppression of NO_x is triggered by a redistribution across the flamelet of the radicals involved in the pathway reactions. A comparison with methane flamelets is further performed to understand to what extent the observed trends with strain can be specifically linked to hydrogen burning features. Similar NO_x suppression trends with strain are also observed for increased pressure and different equivalence ratios, although with different rates of decrease. A correlation is proposed based on the observed results and further theoretical considerations, to predict NO_x emissions according to applied strain rate and equivalence ratio.