A Study of Combustion Augmentation in Supersonic Flows via a Pulsed-Detonation Device

Abstract

Robust ignition and stable flame holding are challenging feats in scramjet combustors because of their susceptibility to large variations in flow, pressure, temperature, and composition. In an effort to help resolve some of these challenges, the development of the Michigan Hypersonic Expansion Tube (MHExT) Facility was undertaken. The 14 meter long expansion tube provides an experimental platform for well controlled studies of various supersonic mixing and combustion phenomena. MHExT was designed with the capability of generating a wide range of aerothermal flow properties representative of combustor entry conditions for flight Mach numbers ranging between 4 and 11. More specifically, the theoretical range of achievable conditions with air as the test gas is as follows; 80 < T [K] < 2500, 0.5 < H0 [MJ/kg] < 8.1, 0.6 < P [kPa] < 180, 1 < M < 11.2, and 600 < U [m/s] < 3800. A portion of this dissertation analyzes the capacity of the facility through a combined experimental, analytical, and computational analysis, providing the necessary foundational groundwork for future supersonic mixing and combustion studies.

New and creative solutions are crucial in achieving robust control of supersonic combustion devices. Since the fluid residence times within non-premixed supersonic combustion devices are short, ignition typically cannot be achieved by solely relying on diffusion and heat conduction alone. The work of this dissertation explores a new technique of active combustion augmentation, where a hydrogen-oxygen detonation is used to generate and deliver a high-temperature radical-rich exhaust to the wake of a (primary) reacting jet in supersonic crossflow (JISCF), here taken to be representative of a scramjet combustion system. The new technique provides several advantages over conventional forms of energy deposition such as those produced by deflagration waves, spark igniters, and plasma discharges, which are localized and therefore have a limited spatial influence on the reacting system. By forming intermediate species of combustion through a detonation wave (constant volume process) as opposed to a deflagration wave (isobaric process), larger temperatures and pressures are achieved, providing additional opportunities and mechanisms for combustion enhancement.

The impact of the transient blowdown process on the JISCF system, which takes place over a 300 microsecond period, is studied using schlieren imaging, OH* chemiluminescence imaging, and OH PLIF imaging. The imaging techniques provide insight to several hydrodynamic and chemical processes, by revealing properties such as shear layer penetration, modification in shock structures and recirculation regions, stability of reactions along the shear layer, net (enhanced) heat release distribution, and the evolution of the instantaneous reaction and post-reaction zone structures. With requisite concentrations of radical addition, macroscopic properties such as ignition delay and reaction time are shown to be reduced by several orders of magnitude, while extending the flammability limits over a significantly wider range of mixture equivalence ratios. The consequences are of critical significance in non-premixed compressible flow environments which rely heavily on the autoignition mode of combustion. A combined theoretical and experimental analysis, intended to decouple the complex interplay between chemical and hydrodynamic effects, is conducted to elucidate the governing processes. A series of analytical models are developed to understand the experimental observations, and thereby shed light on the potentials and limitations of the technique.