Influence of Soret effect on flame structure and NOx emissions in highly strained lean premixed counterflow hydrogen flames

Abstract

The influence of Soret effect on the prediction of flame characteristics and NO${}_{\text{x}}$ 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${}_{\text{x}}$ 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.