TURBULENT CHEMICAL AND THERMONUCLEAR FLAMES: INTRINSIC INSTABILITY AND ANISOTROPIC TURBULENCE AMPLIFICATION

Abstract

In our previous study (Poludnenko, 2015), we presented the analysis of the intrinsic stability of high-speed turbulent reacting flows. A systematic survey of a wide range of turbulent intensities and system sizes showed that turbulent flames in the regimes considered are intrinsically unstable even in the absence of the surrounding combustor walls or obstacles, which can support the thermoacoustic feedback. In particular, three effects were observed. 1) The turbulent flame speed, ST, can develop pulsations with the observed peak-to-peak amplitude S max /S min > 10. 2) Unstable burning results in the periodic pressure build-up and the formation of pressure waves or shocks, when ST approaches or exceeds the speed of a Chapman-Jouguet deflagration. 3) Coupling of pressure gradients formed during pulsations with density gradients across the flame leads to the anisotropic amplification of turbulence inside the flame volume and flame acceleration. In this work we extend prior analysis, which relied on a simplified single-step reaction model, by demonstrating existence of the pulsating flame instability in two realistic reactive systems: chemical flames in atmospheric H2-air mixtures and thermonuclear flames in degenerate, relativistic plasmas found in stellar interiors. Finally, we also consider the dependence of the instability on the system size by performing a direct numerical simulation containing 32 billion cells in a domain twice larger than considered by Poludnenko (2015). No significant change in the instability dynamics is observed, though further analysis of this question