The influence of Soret effect on the prediction of flame characteristics and NO_x emissions in lean premixed hydrogen flames is studied in a reactant-to-product counterflow configuration under high strain conditions. By means of one-dimensional detailed chemistry simulations, the impact of Soret effect on the response of the flame to strain is first analyzed. The results show that leaner mixtures exhibit a stronger sensitivity to strain, and modeling thermal diffusion further intensifies this behaviour by affecting the prediction of temperature, peak of radicals, and consumption speed. Moreover, the Markstein length prediction is found to be affected by the thermal diffusion, with the main effect being to shift the point of sign inversion to a richer equivalence ratio as compared to the case where Soret effect is not considered. Isolating the hydrogen preferential diffusion and Lewis number effect, it is found that the response to strain is mainly driven by the Lewis number effect. Nevertheless, preferential diffusion behaviour is still observed to play a significant role in the leaning of the mixture ahead of the flame when Soret effect is taken into account. In terms of NO_x emissions, including thermal diffusion in the modeling causes an increase in both the peaks of NO mass fraction and its formation rate, especially under ultra-lean conditions where NO formation is primarily through the NNH pathway. The profiles of NO production rate with strain are also influenced, with prediction discrepancies ranging from 10 % in moderately lean conditions to 30 % in ultra-lean conditions. These effects are observed to be mainly associated to the preferential diffusion (as opposed to non-unity Lewis number effect) and its coupling with strain. Effect of pressure is also investigated, showing that the thermal diffusion can significantly alter the rate of production of NO even at high pressure conditions.

The interplay between strain and preferential diffusion in lean premixed hydrogen flamelets is investigated numerically. Lean conditions are established at an equivalence ratio of 0.5. Detailed chemistry, one-dimensional simulations are performed on a reactants-to-products counterflow configuration, both including and artificially excluding preferential diffusion effects. A comprehensive analysis of the flame physical properties is performed, showing that preferential diffusion tends to weaken the flame as compared to the case where it is artificially suppressed, as it triggers a local leaning of the mixture ahead of the flame front. Counterintuitively, strain is observed to counteract or limit this preferential diffusion effect, with the peaks of radicals and reaction rate, flame thickness, and consumption speed, progressively approaching and in some cases overtaking the corresponding solution obtained with equal diffusivities as strain increases. This is shown to be a consequence of the fluid elements being increasingly preferentially transported in the flame tangential direction rather than diffusing in the flame normal direction. Hence, the flame weakening effect due to different diffusive fluxes of fuel and oxidizer across the flame front is progressively compensated by their differential transport on the flame tangential direction triggered by increasing applied strain rate, which instead enables an overall enrichment of the burning mixture. This analysis provides a different view as compared to previous studies attributing to strain an enhancing influence on the effects of preferential diffusion. In this work the opposite interpretation is proposed instead, where strain acts as a limiting factor to the weakening effect of preferential diffusion on lean hydrogen flames.

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.