Influence of hydrodynamic instabilities on the propagation mechanism of fast flames

Abstract

The present work investigates the structure of fast supersonic turbulent flames typically observed as precursors to the onset of detonation. These high speed deflagrations are obtained after the interaction of a detonation wave with cylindrical obstacles. Two mixtures having the same propensity for local hot spot formation were considered, namely hydrogen-oxygen and methane-oxygen. It was shown that the methane mixture sustained turbulent fast flames, while the hydrogen mixture did not. Detailed high speed visualizations of nearly two-dimensional flow fields permitted to identify the key mechanism involved. The strong vorticity generation associated with shock reflections in methane permitted to drive jets. These provided local enhancement of mixing rates, sustenance of pressure waves, organization of the front in stronger fewer modes and eventually the transition to detonation. In the hydrogen system, for similar thermo-chemical parameters, the absence of these jets did not permit to establish such fast flames. This jetting slip line instability in shock reflections (and lack thereof in hydrogen) was correlated with the value of the isentropic exponent and its control of Mach shock jetting instability (Mach & Radulescu).

Mechanism of flame acceleration and detonation transition from the interaction of a supersonic turbulent flame with an obstruction: Experiments in low pressure propane–oxygen mixtures

“…The shockturbulent flame complex in our study was generated using the technique of passing a detonation wave through a perforated plate, which gives rise to a system of interacting shocks yielding a region of intense wave turbulence. This can give rise to an intense turbulent deflagration [10,11,8] approaching the Chapman-Jouguet deflagration speed [9,12] typically observed in fast flame propagation and DDT in tubes with constrictions [3]. This permits to generate flames with high levels of turbulence conducive to transition to detonation from the interaction with a single obstruction, as we will show in the present study.…”

“…1. The resulting high area blockage ratio of approximately 96% was chosen higher than in our previous studies [8,9,12], such that the strength of the transmitted shock was weaker, in order to de-couple the leading shock from the trailing flame, and the turbulent intensity higher. Two series of experiments are presented in this communication.…”

“…In the presence of obstacles, reflected shocks traversing the flame may promote turbulent mixing at the flame, leading to higher burning rates [7] conducive to a more prompt DDT. Experiments and numerical simulations usually agree that the final detonation formation involves the creation of hot-spots, where the Zel'dovich-Lee gradient mechanism [4] can be set up in a variable number of ways: shock reflections on obstacles, turbulent mixing, self-generated shocks wave turbulence [6,8,9]. These complications make it difficult to predict the eventual transition to detonation, which is found to depend on many factors: e.g., turbulence intensity, flame properties, congestion geometry, sensitivity to auto-ignition, memory of the flow field set-up during the early stage of acceleration.…”

Chapman–Jouguet deflagrations and their transition to detonation

“…The experimental configuration used to isolate these high speed waves, without relying on a prior low speed flame acceleration, is generally obtained through the interaction of a detonation wave with a porous plate [5,6,7], from which a transmitted shock -turbulent flame complex can emerge. Extending previous models of Chao [5], Radulescu et al formulated a closed form self-similar model to predict these high speed deflagrations [1].…”

Viscous solution of the triple-shock reflection problem

“…Numerous studies have examined these shock reflections, seeking insight on the phenomena responsible for the creation of locally over-driven detonations, or reinitiation in cases of detonation failure. They have come upon a handful of candidates including: adiabatic shock compression, which heats the gas and exponentially reduces induction times [9][10][11][12][13]; jet formation, which may entrain combustion radicals from reacted zones in the rear to the zone behind the Mach stem [8,12,[14][15][16], where subsequent mixing with unburnt gases may increase reaction rates; Richtmyer-Meshkov instabilities, which arise from the interaction between pressure waves and density gradients (from reactions), accelerating mixing [8]; and Kelvin-Helmholtz instabilities along shear layers, which may also accelerate mixing [8]. While these studies have been able to capture and study reactive shock reflections and have clarified the importance of certain phenomena over others, much still remains to be clarified.…”