Characterizing the influence of thermodiffusive effects on turbulent burning velocity of lean hydrogen/air mixtures using critically stretched laminar flames

This work focuses on characterizing the influence of thermodiffusive effects on turbulent burning velocity (S_T) of lean hydrogen/air mixtures, employing the so-called leading point concept. To this end, the flame characteristics of 1-D critically stretched laminar flames, including the flame consumption velocity and flame thickness, are extracted from two flame configurations: counter-flow twin premixed flames and spherically expanding flames. The former configuration is solely subject to strain-related stretch, whereas the latter configuration is subject to both strain-related and curvature-related stretches. Lean hydrogen/air mixtures are considered at a wide range of temperatures and pressures, addressed in recent direct numerical simulations (DNS) of turbulent, premixed, lean hydrogen/air flames (Wang et al., 2024). It is found that the critically stretched flame consumption velocities obtained from these two configurations are closely aligned, while the flame thickness obtained from the spherically expanding flames is substantially greater than that from the counter-flow twin premixed flames. Capabilities of these flame characteristics for capturing the thermodiffusive effects on S_T are demonstrated by incorporating these characteristics into fits to S_T dataset obtained from the aforementioned DNS study. Various definitions of flame thickness are also examined, with the thickness of fuel consumption zone showing the best performance. These findings support the leading point concept and imply that both critically strained planar flames and highly curved spherically expanding flames could be used to characterize the local burning state at the leading edges of turbulent lean premixed hydrogen flames. Novelty and Significance Statement The novelty of this research consists of comparing characteristics of critically stretched laminar flames, extracted from two flame configurations, i.e., twin planar counterflow flames and highly curved spherical flames. The results show that flame consumption velocities are approximately equal in these two extreme cases if complex chemistry of lean hydrogen burning is taken into account. Furthermore, for the first time to the authors’ knowledge, the results (i) validate the leading point concept under conditions of fixed Karlovitz and Damköhler numbers, but different pressures and temperatures, (ii) indicate utility of characteristics of highly curved spherically expanding flames within the framework of this concept, and (iii) imply that the width of fuel consumption zone is better suited for evaluating thicknesses of critically stretched laminar flames (again within the framework of this concept). It is significant because prediction of high turbulent burning velocities, well documented in lean hydrogen/air mixtures, challenges the combustion community from the fundamental perspective and is of critical importance for the design and development of hydrogen-fueled engines.