Large-scale stochastic dynamics of the afterburning in a supersonic fuel-rich plume: Possible evidence of unconfined deflagration-to-detonation transition

The afterburning of a fuel-rich supersonic hot plume exhausting from a model rocket engine exhibited a complex dynamics, whereby the combustion of an unreacted fuel blend—containing hydrogen and carbon monoxide—with atmospheric air neither stabilized nor blew out. High-speed chemiluminescence of the excited hydroxyl radical (OH*) revealed significant stochastic variations of the flame's leading edge location, extending over approximately 25 nozzle diameters in the axial direction. By tracking its apparent motion, we show that the propagation speed of the flame front, relative to the flow, is most of the time well framed by the Chapman–Jouguet (CJ) detonation and deflagration velocities. As such, we infer that the afterburning dynamics consists of an aperiodic cycle of stochastic time-varying phases, including autoignition, deflagration propagation, transition to detonation, detonation propagation, and failure. The mechanism of turbulence-induced spontaneous deflagration-to-detonation transition proposed by [Poludnenko et al., “Spontaneous transition of turbulent flames to detonations in unconfined media,” Phys. Rev. Lett. 107, 054501 (2011)], combined with stationary shock cells at regularly spaced intervals and an overall hot state of the flow, is suspected to be responsible for the repeated initiation of detonations observed in such unusual unconfined medium. We develop a stochastic model, which is able to reproduce the experimental probability density distribution of the flame's leading edge axial position. Overall, this case study constitutes a challenging benchmark for the validation of large eddy simulation of scramjet combustors or rocket plumes.