A direct numerical simulation study of stretched laminar lean hydrogen-air flames

Analyzed in the paper are data obtained in unsteady two-dimensional complex-chemistry Direct Numerical Simulations of (i) unstable lean (the equivalence ratio is equal to 0.35, 0.5, or 0.7) H₂-air flames expanding in initially quiescent gas and (ii) counterflow twin laminar flames of the same mixtures. In the latter case, both natural development of instabilities without any imposed perturbation and flame response to large perturbations in the flow velocity or initial flame shape are explored. Time-dependent strain rates are also addressed. Results show that diffusional-thermal instability of lean hydrogen-air flames is suppressed by quite moderate (or higher) stretch rates (critical stretch rate normalized using flame time scale is substantially smaller than unity in all studied laminar flames). A higher stretch rate is required to stabilize a leaner flame. Computed values of critical strain rates that suppress instabilities of twin counterflow laminar flames (i) are weakly sensitive to time-oscillations of strain rate, (ii) are increased with increasing magnitude of inlet velocity perturbations, and (iii) are higher in the case of perturbation of flame shape when compared to perturbation of flow velocity. However, these variations in the critical strain rates are quite moderate even if perturbation magnitude is large. Novelty and significance statement: Results of new unsteady two-dimensional complex-chemistry Direct Numerical Simulations of (i) unstable flame kernels expanding in initially quiescent lean H₂-air mixtures and (ii) counterflow twin laminar flames of the same mixtures are reported. Diffusional-thermal instability is shown to be suppressed by quite moderate stretch rates. Computed values of critical strain rates that suppress flame instabilities remain low independently of perturbation type (upstream flow velocity or initial flame shape) and magnitude even if the perturbation magnitudes are high. The reported numerical results generate new fundamental knowledge about development of diffusional-thermal instability of stretched laminar premixed flames. These results and generated knowledge are significant for developing predictive models of turbulent combustion of mixtures susceptible to diffusional-thermal instability.