Flow and Flame Structure in Solid Fuel Combustion for Scramjets

Abstract

Scramjet technology is necessary to enable sustained hypersonic flight on an airbreathing engine architecture. Without moving mechanical parts within the engine, the scramjet already benefits from simplicity. With the use of solid fuels, a further reduction in system complexity can be obtained in addition to increased energy density, safer storage and handling, and added thermal management for combustor walls. To realize the advantages of a solid fuel scramjet (SFSJ), however, numerous challenges need to be addressed. Chief among these, the residence time of flow within an SFSJ may approach the time scale of combustion reactions, making gas phase flameholding challenging after contending with solid fuel decomposition. In line with the prior point, solid fuel combustion is inherently a complex, highly coupled process that lacks simplified crossflow experiments tailored to model validation. To address these challenges, this dissertation presents a series of experiments, beginning with an investigation of solid fuel diffusion flames in a small, low-speed crossflow burner, followed by two studies in optically accessible SFSJ combustors. The crossflow burner, in addition to serving as a useful experiment for optical diagnostic development and assessment, used O2/inert diluent (He, N2, Ar) oxidizer mixtures to explore the influence of thermodynamic gas properties on solid fuel regression rates. Results suggest interesting possibilities for future fuel design based on properties of pyrolysis products generated near the fuel surface. In a direct-connect facility, a lab-scale SFSJ combustor was used to explore flameholding stability under various flameholding cavity rear wall angles, along with a new fuel grain design that utilizes an upstream section of fuel flush with the inlet ("flush fuel grain"). The results demonstrated good agreement with mechanisms describing non-reacting supersonic flows, an increase in flame anchoring fluctuation with a steeper cavity rear wall, and that the flush fuel grain geometry can substantially improve flameholding ability and sustained combustion at lower inlet temperatures than prior SFSJ fuel geometries. The improved flammability by the flush fuel grain enabled the development of a three-sided optically accessible SFSJ by demonstrating sustained combustion of a single fuel grain on the bottom of the combustor. Laser diagnostics were applied to the three-sided SFSJ combustor for the first time and provided insight on flame structure and condensed phase behavior throughout the flow field. The work within this dissertation has improved the understanding of solid fuel combustion and expanded the operability limits of SFSJs to provide insight for future design and development efforts.