Spatio-temporal evolution of cavity ignition in supersonic flow

Successful ignition in the recirculating flow of a scramjet flame holder can be highly dependent upon the location of energy deposition because of the spatial variation of fuel concentration and flow properties. The current work experimentally investigated ignition processes when energy was deposited (～100 mJ) via a spark discharge at four locations in the base of a cavity or by laser-induced breakdown in a Mach 2 flow with a stagnation temperature and pressure of 590 K and 483 kPa, respectively. The cavity was directly fueled with ethylene injection. The time dependent heat release was imaged at 40,000 frames per second and fuel concentration and distribution measurements were taken in the cavity prior to ignition. The average fuel concentration at the lean and rich ignition limits near the energy deposition locations measured 4.4–9.3% (Φ= 0.75 to 1.47). Energy deposition near the cavity step resulted in near immediate ignition kernel development and rapid achievement of self-sustained flame propagation in the front of the cavity, often faster than the bulk recirculation time of the cavity, leading to a spike in heat release. Energy deposition away from the cavity step region led to competition between local flow velocity, fuel concentration, and flame propagation rates. Ignition kernels formed along the floor of the cavity towards the closeout ramp and were rapidly advected towards the cavity step region before flame propagation could ensue. The fastest and most robust ignition events for all fueling cases showed rapid spanwise flame propagation near the cavity step.