Direct numerical simulation of NOx formation in turbulent lean premixed hydrogen-air flames under engine-relevant conditions

Abstract

In this study, direct numerical simulations (DNS) are employed to investigate NOx formation in turbulent lean premixed hydrogen-air flames under engine-relevant conditions. Various turbulence intensities and molecular transport models are examined to isolate the individual impacts of turbulence intensity, Lewis number, and preferential diffusion on NOx production. Results show that NOx production is significantly enhanced in all turbulent cases relative to the laminar flame, reaching approximately five times the laminar value at a mixture residence time of 0.15 ms. Increasing turbulence intensity is found to have three competing effects on NOx formation: (1) it strengthens turbulence-instability interactions by inducing local super-adiabatic hot spots and elevating key flame radical concentrations within the flame brush, thereby promoting NOx formation locally; (2) it accelerates the turbulent flame speed, reducing the flame-brush residence time and thus suppressing NOx production globally; and (3) it reduces post-flame temperature fluctuations, suppressing thermal NOx enhancement in the post-flame zone. Lewis number effects are identified as the primary mechanism driving thermodiffusive NOx enhancement, with preferential diffusion playing a secondary role, as evidenced by the nearly identical NOx reaction rates between unity Lewis number turbulent flames and their laminar counterparts. Finally, an excellent correlation between the peak conditional mean NOx reaction rate and the stretch factor is identified, and a conceptual model is proposed to predict NOx enhancement in practical engine simulations. The findings highlight that turbulence--chemistry interaction is critical for accurately predicting NOx formation in thermodiffusively unstable hydrogen flames.