Characteristics of flame acceleration and deflagration-to-detonation transition enhanced by SF6 jet-in-cross-flow/flame interaction

Abstract

Jet-in-crossflow (JICF), as a turbulence generator, is proposed to promote the flame acceleration and deflagration-to-detonation transition (DDT) process. Previous work has suggested that JICF using CO2 or Ar was in favor of flame acceleration under optimal conditions. The competition mechanism between the positive combustion-enhancement effect caused by jet-induced turbulence and the negative combustion-inhibition caused by non-reactive gas has been demonstrated. It is worth noting that the difference in molar weight between the medium of JICF and the combustible mixture may significantly impact deflagration propagation prior to DDT. Thus, in this work, to investigate the flame propagation perturbed by the enhancement on the difference in molar weight between the JICF medium and combustible mixture, an experimental study of the flame propagation perturbed by SF6 JICF is carried out. By adjusting the injection pressure (Pj) of the SF6 JICF, the characteristics of flame propagation and DDT are analyzed. The results showed that the flame can accelerate to the onset of detonation within a short run-up distance in the cases with SF6 JICF. Schlieren images of flame evolution show that the flame disturbed by SF6 JICF rapidly transitions to a wrinkled flame. It is suggested that the SF6 JICF-enhanced turbulence promotes flame acceleration and DDT. The effect of SF6 JICF on the flame characteristics can be concluded in two aspects: one is that the inhomogeneously distributed vortices that are induced by JICF cause a nonuniform diffusion of SF6 in the tube, resulting in a nonuniform dilution of the combustible mixture near the jet; another one is that the vortices induced by JICF stretch the flame surface to increase the flame area. Increasing the injection pressure could promote the formation of complex wrinkles in the flame, however, it shows few further improvements in promoting DDT. Different jet mediums are also tested, and the results show that SF6 JICF can shorten the run-up distance significantly compared with helium and nitrogen JICF. The small and densely distributed wrinkles significantly increase the flame area to enhance heat release. The flame disturbed by SF6 JICF could accelerate to the local sound speed of the unburnt mixture, which results in the facilitation of the precursor shock wave formation. After the flame passes through the SF6 JICF, it remains a larger acceleration rate and then transitions to quasi-detonation rapidly. The conclusion of this work could help to qualitatively understand the positive effect of JICF-induced turbulence on flame acceleration and DDT. It also provides a relatively efficient and safe technology to promote detonation initiation in the design of detonation engines.