A direct connect, supersonic solid fuel combustor with a cavity is explored in the context of understanding characteristics related to ignition, regression rate, combustion, and flow fields for application in advancing solid fuel scramjet research. 3D printed, polymethylmethacrylate fuel grains are loaded into both fully enclosed and optically accessible combustors. The ignition characteristics are investigated by systematically varying the internal geometry of the fuel grain to develop a flammability map with respect to non-dimensional geometric parameters. Results reveal that a longer and larger flameholding cavity creates favorable conditions for ignition and sustained combustion. The inlet temperature is also systematically varied to extend the available literature on the supersonic combustion of solid fuels to lower temperature operating conditions and show that a higher inlet temperature is conducive to sustained combustion and higher regression rates. The regression rates of the fuel grains are measured to determine a concentration of regression in the flameholding cavity along the angle of the downstream side of the cavity. Ignition and sustained combustion rely heavily on the fuel in the flameholding cavity. A decreasing regression rate is observed as the fuel regresses by measuring the regression rate at discrete time intervals during a firing of the optical combustor. The optical combustor is also subject to various high-frequency imaging techniques. Shadowgraph imaging shows the changes in density of the flow field and finds a normal shock in the constant area section. CH* chemiluminescence imaging provides novel observations of the concentrated areas of combustion along the fuel grain wall by highlighting the heat release from combustion. A high intensity of CH* radicals is in the upstream section of the flameholding cavity. When considered in the context of the concentration of regression, this indicates that the recirculation zone pulls fuel from the downstream section of the cavity, combusts it in the upstream section of the flameholding cavity, then expels the higher enthalpy gas into the core flow. Additionally, observing the flow provides insight into the flow dynamics of opposing cavities in a supersonic flow field. The symmetry of the flow field is found to be reliant on the stability of the flameholding cavity length to depth ratio. / Master of Science / A solid fuel scramjet has the potential to be the simplest and most cost effective method of achieving hypersonic flight. A liquid fuel scramjet has been demonstrated in free flight, but liquid fuels present many issues involving safety and storage that can be eliminated by introducing solid fuels. Supersonic combustion, or burning fuel in an air flow moving faster than the speed of sound, is a complicated subject due to the irregularity of flow fields and the requirement of combustion to occur at a high rate. The research within this thesis presents many novel technologies that have never been presented in published literature in the context of the supersonic combustion of solid fuels. By conducting ground testing of a solid fuel scramjet, characteristics of the combustion can be studied to expand the available literature in the field to new fuel geometries and inlet conditions. The ignition and sustained combustion of a solid fuel scramjet is extremely reliant on the initial geometry of the fuel and the initial temperature of the flow. This research advances the field of supersonic combustion of solid fuels by developing an optically accessible combustor using quartz windows. These characteristics of supersonic combustion are investigated using highspeed video recording. The results of these techniques provide insight into favorable fuel geometries and inlet conditions. Additionally, patterns observed in the flow field explain concentrations of combustion and fuel consumption.

Cold flow inside a Solid Fueled Ramjet (SFRJ) is simulated using large eddy simulations (LES). A finite element method using a Discontinuous Galerkin bases has been implemented in the open-sourced multi-physics software SU2. Novel LES formulations of the fuel-gas boundary conditions and the heat release due to mixing are obtained using integration by parts over the discontinuous Galerkin bases. The Smagorisnki and wall-adapted subgrid stress model for the scalar variance have been implemented and investigated in twodimensions. Spectral Proper Orthogonal Decomposition is used to analyze CFD results to determine acoustic modes in the ramjet. Peak acoustic frequencies are compared between between numerical and experimental results. Comparisons are made between simulations performed with a 2D axisymmetric domain and full 3D domain. Cold-flow LES simulations show that there are two dominant acoustic modes (St ≡ f/f0 = {3, 18}) in the ramjet and their frequency appears to be invariant to the cavity configuration. The first peak corresponds to a longitudinal mode associated to the chamber fundamental oscillations (with length scale Lc). The second is characterized with radial fluctuations in the mixing chamber and features the maximum chamber radius of the ramjet as its scaling length. Mixed (radial and axial) modes in the intermediate frequency range reveal the effect of a slanted aft wall on the acoustics. Three-dimensional cold flow simulations predicted weak non-symmetric (azimuthal) modes. Hot-flow simulations show a substantial increase in the mean chamber pressure with the addition of the cavity, indicating that it enhances flame-holding in solid-fuel ramjets, in agreement with the experiments. The analysis of the ramjet acoustic modes shows the emergence of low frequency modes in the cavity cases, in agreement with the experiments. Using SPOD, these modes were associated with low frequency breathing of the recirculation region at the nozzle throat. Perturbations are localized in the throat region because of the Mach number pressure scaling. These modes do not seem to affect the pressure fluctuation and thus combustion in the chamber. Together with the emergence of low frequency vortical modes, the cavity supports a decrease in the high-wave number harmonics of the ramjet chamber acoustic mode. These fluctuations are supported by non-linear amplification of the fundamental mode, which is enhanced by the thermo-acoustic coupling. / Master of Science / Novel propulsion designs, such as solid fuel ramjets, present the opportunity of optimizing cavity shapes using additive manufacturing and three-dimensional printing to improve fuelair mixing and lowering the thermo-acoustic feedback. In this work a computational model for solid fuel ramjets is developed and applied to laboratory firing tests performed by Prof Young's group at the advanced propulsion laboratory at Virginia Tech. In order to capture the fine mixing scales a novel discretization of the reactive Navier-Stokes using discontinuous Galerkin bases is implemented in an open source CFD code popular with aerospace graduate students and researchers. Subgrid modelling is implemented to determine the effect of small scales on the PMMA combustion mechanism developed at Virginia Tech. Numerical methods are used to simulate the turbulent flow of air through an axisymmetric cavity.

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