Enhanced DDT mechanism from shock-flame interactions in thin channels

TL;DR

This paper demonstrates experimentally and numerically that shock-flame interactions in narrow channels can efficiently trigger deflagration to detonation transition via flame shape deformation and auto-ignition, without turbulence.

Contribution

It introduces a novel mechanism involving shock-induced flame elongation and boundary layer effects that accelerates flames to detonation speeds in narrow channels.

Findings

Flame surface area increases over two orders of magnitude.

Alligator-shaped flames saturate near Chapman-Jouguet conditions.

Transition to detonation occurs rapidly through auto-ignition and flame merging.

Abstract

We show experimentally and numerically that when a weak shock interacts with a finger flame in a narrow channel, an extremely efficient mechanism for deflagration to detonation transition occurs. This is demonstrated in a 19-mm-thick channel in hydrogen-air mixtures at pressures below 0.2 atm and weak shocks of Mach numbers 1.5 to 2. The mechanism relies primarily on the straining of the flame shape into an elongated alligator flame maintained by the anchoring mechanism of Gamezo in a bifurcated lambda shock due to boundary layers. The mechanism can increase the flame surface area by more than two orders of magnitude without any turbulence on the flame time scale. The resulting alligator-shaped flame is shown to saturate near the Chapman-Jouguet condition and further slowly accelerate until its burning velocity reaches the sound speed in the shocked unburned gas. At this state, the lead shock and further adiabatic compression of the gas in the induction zone gives rise to auto-ignition and very rapid transition to detonation through merging of numerous spontaneous flames from ignition spots. The entire acceleration can occur on a time scale comparable to the laminar flame time.