Influence of flame-to-flame interactions on statistical characteristics of premixed turbulent combustion

This work aims to explore the influence of local flame-to-flame interactions on various statistical characteristics of premixed turbulent flames. For this purpose, a Direct Numerical Simulation (DNS) database obtained from 17 complex-chemistry lean hydrogen-air flames propagating in forced turbulence in a box is screened and a single case characterized by the largest magnitude of oscillations in turbulent burning velocity UT with time t is chosen for further analysis. This newly computed case is characterized by the equivalence ratio (Formula presented), a high Karlovitz number, with Kolmogorov length scale being smaller than reaction zone thickness in the counterpart unperturbed laminar flame by an order of magnitude. Five different instants tn are selected around the highest peak of (Formula presented). Results obtained by analyzing these five snapshots show a rapid decrease in flame surface area and burning velocity from t3 to t5 due to local flame collisions, which mostly occur due to implosion of negatively curved flames (tunnel closure), with such events being mainly localized to the trailing zone of the mean flame brush. In addition to that decrease in the flame surface area and burning velocity, such flame collisions (i) can result (with a low probability) in appearance of local flame zones characterized by highly negative curvature and very large displacement speed and (ii) significantly affect statistics of flame curvature and tangential-diffusion contribution Sd,t to displacement speed Sd[jls-end-space/]. On the contrary, the influence of flame collisions on the statistics of Sd[jls-end-space/], heat release and fuel consumption rates, temperature, and mass fractions of various species is minor (if any). While flame collisions can mimic appearance of thickened reaction zones when two reaction zones merge, the statistics of the absolute value (Formula presented) of the gradient of fuel-based combustion progress variable indicate predominant thinning of reaction zones.Novelty and significance statementThe novelty of this work consists in (i) showing that flame collisions weakly affect statistics of local flame structure and displacement speed despite these collisions significantly reducing flame surface area and turbulent burning velocity and creating local flame zones with highly negative curvature and large displacement speed, (ii) showing that flame collisions can mimic appearance of thickened reaction zones despite reaction zones remaining thin under conditions of the present study (Kolmogorov length scale is smaller than reaction zone thickness by an order of magnitude), (iii) suggesting a simple method for studying the influence of flame-to-flame interactions on various statistical characteristics of premixed turbulent flames, and (iv) reporting new DNS data. Presented results are significant for advancing fundamental understanding of flame–turbulence interactions and developing high fidelity models of premixed turbulent burning. Specifically, the results call for (i) target-directed research into rare events (e.g., local flame collisions) in turbulent flames and (ii) development of high-fidelity models that pay special attention to such rare events.