Turbulent burning velocity of lean premixed hydrogen/air flames at engine conditions: Effects of turbulence intensity and length scale

Abstract

For turbulent lean premixed hydrogen flames with strong thermodiffusively instabilities, most previous studies have focused on the influence of turbulence intensity, whereas the role of turbulence length scale is less well understood. This study addresses this gap by conducting direct numerical simulations (DNS) of statistically planar turbulent premixed flames for a lean ($\varphi$=0.35) hydrogen/air mixture under independently varied turbulence intensity ($u^{\prime }$) and length scale ($l_T$) at engine-relevant thermodynamics conditions. Results show that as $u^{\prime }$ increases, the flame front becomes increasingly wrinkled, forming smaller cellular structures. In contrast, $l_T$ variations do not significantly alter the size of these structures. For the turbulent burning velocity ($S_T$), the normalized $S_T$ (i.e., $S_T/S_L$, where $S_L$ is the laminar flame speed) increases linearly with $u^{\prime }$, driven by both enhanced flame surface wrinkling (i.e., increased $A_T/A_L$) and enhanced local burning rate (i.e., increased $I_0$). However, increasing $l_T$ reduces $I_0$, despite a continued increase in $A_T/A_L$, resulting in only a marginal increase in $S_T/S_L$. To reveal the underlying mechanisms, especially the decreasing trend of $I_0$ with $l_T$, local flame dynamics analyses are performed. It is found that as $l_T$ increases, the interaction between thermodiffusive effects and turbulence weakens due to the reduced tangential strain rate, while the flame curvature remains largely unchanged. This suppresses local reactivity enhancement and thus decreases $I_0$, In contrast, an increase in $u^{\prime }$ enhances the interaction by amplifying both curvature fluctuation and tangential strain rate, leading to increased local reactivity (increased $I_0$). Finally, based on the DNS data, new scaling models are proposed for the three global properties, $S_T/S_L$, $A_T/A_L$, and $I_0$, and show improvements compared to existing models. These findings provide new insights into the flame-turbulence interactions in thermodiffusively unstable hydrogen flames. The DNS dataset is also useful for the development of turbulent combustion models applicable to practical engine simulations.

Novelty and Significance Statement Despite extensive research on the effects of turbulence intensity on the turbulent burning velocity of lean premixed hydrogen flames, the influence of turbulence length scale remains largely unexplored. To the authors’ knowledge, this study is the first to perform a series of 3-D Direct Numerical Simulations of lean premixed hydrogen flames with independently and systematically varied turbulence intensity and length scale at engine-relevant conditions. The analysis of flame morphology, global flame properties, and local flame dynamics provides novel insights into the influence of turbulence length scale on thermodiffusive effects. In addition, new empirical scaling models are proposed for the turbulent burning velocity and its contributors. These findings and the new DNS data set can also benefit the modeling community for model development and validation. Therefore, this study is of critical importance in both fundamental research and practical applications for hydrogen engines.