Pulsating instability and self-acceleration of fast turbulent flames

TL;DR

This study uses 3D simulations to reveal that high-speed turbulent flames are inherently unstable, exhibiting pulsations, pressure waves, and potential transition to detonation, driven by turbulence-flame interactions and pressure-density coupling.

Contribution

It demonstrates the intrinsic pulsating instability of turbulent flames and links this behavior to pressure and turbulence interactions, advancing understanding of flame stability in turbulent regimes.

Findings

Turbulent flame speed exhibits pulsations with large amplitude.

Pressure waves or shocks form during unstable burning.

Flame acceleration can lead to detonation at higher turbulence levels.

Abstract

(Abridged) A series of three-dimensional numerical simulations is used to study the intrinsic stability of high-speed turbulent flames. Calculations model the interaction of a fully-resolved premixed flame with a highly subsonic, statistically steady, homogeneous, isotropic turbulence. We consider a wide range of turbulent intensities and system sizes, corresponding to the Damk\"ohler numbers Da = 0.1-6.0. These calculations show that turbulent flames in the regimes considered are intrinsically unstable. In particular, we find three effects. 1) Turbulent flame speed develops pulsations with the observed peak-to-peak amplitude > 10 and a characteristic time scale close to a large-scale eddy turnover time. Such variability is caused by the interplay between turbulence, which continuously creates the flame surface, and highly intermittent flame collisions, which consume the flame surface. 2) Unstable burning results in the periodic pressure build-up and the formation of pressure waves or shocks, when the flame speed 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. Such process, which is driven by the baroclinic term in the vorticity transport equation, is a reacting-flow analog of the mechanism underlying the Richtmyer-Meshkov instability. With the increase in turbulent intensity, the limit-cycle instability discussed here transitions to the regime described in our previous work, in which the growth of the flame speed becomes unbounded and produces a detonation.