Ph.D. (Engg) :Experimental Investigation of Autoignition Pathways and Shock-Train Dynamics During Mode Transition in a Dual-Mode Supersonic Cavity Combustor

Hypersonic propulsion systems capable of sustained atmospheric flight are critical enablers for future reusable launch vehicles, long-range high-speed transport, and responsive global strike platforms. Among the various air-breathing concepts, scramjet engines offer unmatched efficiency at hypersonic speeds by utilizing atmospheric oxygen and avoiding the mass penalties associated with onboard oxidizers. However, the practical realization of scramjet propulsion is fundamentally constrained by two interrelated challenges: reliable ignition and flame stabilization under extremely short residence times, and robust operation across a wide flight envelope that necessitates smooth transition between supersonic (scramjet) and subsonic (ramjet) combustion modes. Dual-mode scramjets (DMSJ) are designed to address this requirement, but their operability is limited by complex, strongly coupled interactions between shock structures, boundary-layer separation, fuel-air mixing, chemical kinetics, and unsteady pressure fields during mode transition. A central difficulty in hypersonic combustors is that global flow conditions typically yield Damköhler numbers well below unity, rendering conventional flame-holding ineffective. Localized enhancement of thermochemical coupling through elevated temperature, pressure, and residence time is therefore essential to initiate and sustain combustion. Cavity-based flameholders have emerged as a promising solution due to their passive, low-drag configuration and ability to generate recirculation zones that promote autoignition and flame anchoring. Nevertheless, cavity-stabilized combustors introduce additional challenges: strong sensitivity to geometry, concentration of thermal loads, susceptibility to unsteady shear-layer oscillations, and complex coupling with shock-train dynamics during scram-to-ram transition. Despite extensive cold-flow investigations of isolator shock trains, their behaviour under reacting, high-enthalpy conditions where heat release actively modifies the flow remains insufficiently characterized. This doctoral research discusses a systematic experimental investigation of autoignition pathways, flame stabilization mechanisms, and shock-train dynamics in a cavity-stabilized dual-mode supersonic combustor. Experiments are conducted in a direct-connect high-enthalpy facility at the Advanced Propulsion Research Laboratory (APRL), Indian Institute of Science. The combustor operates at flight relevant conditions of total temperature of 1500 ± 30 K and static pressure of 43 kPa, which corresponds to Mach 5.5 flight conditions at 28 km altitude. The experimental test article features an optically accessible supersonic combustor with a single/twin cavity configuration and is designed for an inlet Mach 2.5. Time-resolved Schlieren imaging, CH* and C2* chemiluminescence, and high-frequency wall-pressure measurements are employed to resolve unsteady flow-flame interactions governing ignition and mode transition. Two cavity geometries with identical depth but different length-to-height ratios (L/H = 5 and 8.5) were examined to quantify the influence of geometry on ignition robustness and shock–flame coupling. For the L/H = 5 configuration, ethylene ignition occurred downstream in the diverging duct at a global equivalence ratio of ϕg ≈ 0.3, followed by upstream flame propagation and eventual stabilization along the shear layer. In contrast, the L/H = 8.5 cavity enabled earlier and more robust ignition upstream, triggered by shock-assisted autoignition behind an X-type shock formed through interaction between the cavity reattachment shock and a top-wall separation bubble. The larger cavity generated stronger pressure deficits, deeper shear-layer penetration, and self-sustained oscillations at approximately 527 Hz, highlighting the critical role of cavity geometry in enhancing local Damköhler numbers. Optical diagnostics technique of two-wavelength chemiluminescence (CH* and C2*) revealed ignition kernels forming preferentially in high-temperature lean regions before stabilizing near stoichiometric zones. Shock-induced compression was shown to significantly reduce ignition delay, enabling autoignition even for fuels with substantially longer chemical timescales. Fuel-blending experiments established a limiting ignition-delay threshold, providing quantitative guidance for fuel selection in practical hypersonic combustors. The scram-to-ram mode transition occurred at ϕg ≈ 0.58 for both geometries and was marked by the formation of a pre-combustion shock train, initiated due to combustion induced boundary layer separation. The L/H = 8.5 cavity sustained stable ram-mode operation, whereas the L/H = 5 configuration frequently reverted to early scram-mode behavior, indicating weaker shock-flame coupling and reduced buffering capacity against back-pressure fluctuations. Scaling analysis of shock-train dynamics yielded Strouhal numbers (St) an order of magnitude lower than the reported values in the literature based on isothermal shock-train oscillation studies. This demonstrated the dominant influence of heat release and shock-train coupling. Proper orthogonal decomposition (POD) analysis further revealed tight coupling between shock-train motion and upstream flame propagation, identifying critical regions in the combustor with substantial heat release fluctuations. Finally, symmetric dual-cavity configurations were explored to assess coupled shear-layer dynamics. While dual cavities enhance residence time, their interaction introduces additional unsteady modes, underscoring the need for geom etry-aware stabilization strategies. Overall, this work directly addresses critical propulsion challenges for hypersonic vehicles by elucidating the mechanisms governing ignition reliability, shock-assisted autoignition, and mode-transition stability in cavity-based dual-mode scramjets. The findings provide mechanistic understanding and scalable design guidelines essential for the development of robust, operable hypersonic air-breathing propulsion systems.

Speaker :  Sumit Lonkar

Research Supervisor: Pratikash Prakash Panda