Response of premixed jet flames to blast waves

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 the premixed jet flame stabilized over a thin fuel-air jet nozzle. The study is performed over a wide range of parametric space, varying the Reynolds number (Re) and normalized equivalence ratio (Φ) of the premixed jet and the strength of the generated blast wave fronts (characterized by M _s,r). The blast wave imposes a characteristic flow field, marked by a sharp discontinuity at the blast front, followed by an exponential decay. After this decay phase, a subsonic bulk flow, referred to as the induced flow, follows the blast front. 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. Contingent on the operating conditions (Re, Φ, and M _s,r), an extinction or a reattachment event follows the observed flame lift-off. Additionally, the flame tip is found to exhibit stretching/distortion events, causing the tip to pinch off in a characteristic operating space of Re, M _s,r and Φ. The reattachment and extinction regimes were thus classified into sub-regimes based on the response dynamics of the flame base and the flame tip. A simplified 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 tip pinch-off was observed when shear layer vortices reached critical circulation limits and shed at length scales lower than the flame height owing to the interaction with the induced flow. Novelty and significance statement This work is the first of its kind to examine the response of premixed jet flames to blast waves that are incident along the direction of the jet axis. The study identifies qualitative and quantitative changes induced in the premixed jet flame following the interaction process and proposes a regime map classifying the flame response behaviour across the parametric space of premixed jet Reynolds number, equivalence ratio, and strength of the incident blast wave. The axial blast-flame interaction setting explored in the current work can provide valuable insights into comprehending and improving practical high-speed propulsion systems such as scram jets and RDEs, wherein flame interactions with non-linear pressure waves, such as decaying shocks and blast waves, are common.