M. Fathi, S. Hickel, N.A.K. Doan, I. Langella (2025)
Combustion and Flame 282: 114459. doi: 10.1016/j.combustflame.2025.114459
This work examines for the first time in detail the coupled effects of strain and turbulence in hydrogen flames, for various conditions spanning different signs of the Markstein length and increasing applied strain levels. In particular, it clarifies the different roles of applied strain, turbulence-driven strain, and curvature on both flame structure and NOx generation. Results show for the first time that both in-flame and post-flame NOx can be suppressed at high strain levels under turbulent combustor-relevant conditions by straining the flame.
Direct numerical simulations (DNS) of hydrogen/air flames have been performed for increasing levels of applied strain on a turbulent reactants-to-products counterflow configuration to investigate how tangential strain affects the general flame behaviour and NOx emissions in particular. The analysis has been repeated for two lean equivalence ratios for which the Markstein length in the limit of low strain is respectively negative and positive.
It is found that the mean tangential strain on the flame causes a redistribution of radicals such as OH which result in an overall decrease of NO. This decrease with increased strain is found to be faster under turbulent conditions than for laminar flow. One reason for this is that turbulence limits the occurrences of super-adiabatic temperatures within the flame, thus limiting the in-flame NO production by the thermal route. Moreover, turbulence yields an effective increase of mean positive tangential flame on the flame which, unlike for the laminar case, triggers an inversion of the Markstein length at the highest strain rate from negative to positive in the leaner of the two configurations examined. Since a negative Markstein length is observed to limit the decrease of NOx, this inversion leads to a faster reduction of this species. Furthermore, the analysis conducted in the present study brought the following conclusions:
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overall NOx production within the domain mainly decreases due to the redistribution of radicals, in particular OH radicals. This decrease is primarily due to post-flame NOx, which always decreases with strain independently of the Markstein length
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curvature does not seem to have a significant effect on the production of NO, despite its contribution to the overall stretch rate being significant as compared to that of strain;
- little correlation was found between the local fluctuations of flame-tangential strain rate and NOx production, which suggests the NOx reduction with high strain comes mostly from bulk effects.
The results mentioned above pertain to turbulent conditions characterized by moderate turbulent Reynolds numbers. They indicate that high levels of mean applied strain can be used to significantly suppress NOx emissions at practical combustor conditions, thus paving the way for the design of novel combustors based on this principle.