Lean hydrogen-air premixed flames subjected to high-frequency acoustic oscillations

The response of laminar lean H 2 -air premixed flames to acoustic oscillations has been investigated by performing simulations of 2D laminar flames at ϕ = 0 . 4 and ϕ = 0 . 7 for a range of oscillation frequencies, between 50 kHz and 500 kHz, at sound pressure levels of 120 dB and 130 dB. The flames are subjected to pressure waves by implementing a monopole-type acoustic source at the inflow, which acts to perturb the flame surface and leads to wrinkled flames for all of the conditions investigated. For all cases, a range of wave numbers appearing on the flame surface exhibit exponential growth at early times, consistent with the linear regime of flame evolution. This allowed for dispersion relations to be computed such that the flame response across acoustic conditions and equivalence ratios can be compared, and the interplay between the flame-acoustic interaction and hydrodynamic/thermodiffusive mechanisms can be investigated. Differences are noted in terms of the wave number at which the dispersion relation terminates, with the dispersion relation for the lower frequency cases (50 kHz for ϕ = 0 . 4 and 100 kHz for ϕ = 0 . 7 ) terminating at lower wave number values, indicating an immediate transition to a non-linear growth phase for high wave numbers. Higher growth rates for small wave number modes have been observed when compared with those for non-acoustically forced flames in the literature, suggesting enhancement of the thermodiffusive mechanism at these wave numbers. It is argued that the differential diffusion effects are influenced by the strong pressure gradients seen for the acoustically forced cases.