Response of premixed Jet Flames to Blast Waves

Abstract

The study investigates the response dynamics of premixed jet flames when incident with blast waves along the jet axis. In the present work, blast waves are generated using the wire-explosion technique. The generated blast wave interacts with premixed jet flames that are stabilised over a thin fuel-air jet nozzle. The study is performed over a wide range of parametric space, varying the Reynolds number ($Re$) and normalised equivalence ratio ($\Phi$) of the premixed jet and the strength of the generated blast fronts ($M_{s,r}$). The blast wave imposes a decaying flow field profile characterised by a sharp discontinuity at the blast front. This is accompanied by a subsequent induced flow arising due to entertainment from the surroundings as the blast-imposed flow fields decay to sub-ambient levels. While the jet flame is observed to respond to the blast front with a jittery motion, it is found to lift off following the interaction with the induced flow. Depending on the operating conditions ($Re$, $\Phi$, and $M_{s,r}$), the flame lift-off is followed by an extinction or a re-attachment event. The flame response is further classified into two re-attachment and three extinction sub-regimes based on the response dynamics of the flame base and flame tip following the interaction process. The flame response entails flame base lift-off and flame tip distortion, potentially leading to a flame pinch-off event contingent on the operating conditions. A mathematical model was developed to explain the flame base response dynamics, yielding a scaling law for flame base lift-off height. Flame tip response trends were elucidated by extending the vorticity transport equation to estimate the vortex roll-up rate in the shear boundary surrounding the flame. Flame pinch-off was observed when shear layer vortices reached critical circulation limits and shed at length scales lower than the flame height.