UCLA

This dissertation details novel experimental methods and analytical techniques developed to characterize hybrid combustion of solid fuels with gaseous oxidizers. The primary focus of this work is on hybrid rocket motors, which are less developed than liquid and solid fuel chemical propulsion systems. Historically poor performance of hybrid propulsion systems is attributed, in part, to combustion efficiencies below theoretical limits, hindering hybrid rocket development and motivating experimental and modeling studies to explain shortcomings. Combustion in such systems is typified by a turbulent reacting boundary layer above the fuel surface that involves a convolution of fluid dynamics, heat transfer, and chemical kinetics. Although modeling efforts have advanced significantly in recent years, there remains a lack of quantitative data, particularly in-situ in the reacting flow regions, that are necessary to validate combustion models and make definitive assessments of the reacting boundary layer flow-field. Optical diagnostics have become invaluable tools for obtaining such data due to their non-intrusive nature and their capability in harsh combustion environments. For much of the research detailed herein, an axisymmetric solid fuel burner was used to examine the near-surface reaction layer via spatially-resolved measurements using laser absorption tomographic methods. The data obtained from these experiments were compared to relevant multi-physics combustion models.

The hybrid rocket motor experiments discussed in this dissertation primarily involve polymethyl methacrylate (PMMA) as the fuel with gaseous oxygen. The solid fuel burner and laser diagnostic sensors were used to assess hybrid PMMA/GOx combustion as influenced by differing oxidizer injector geometries, those including both axial and swirl varieties. Two-dimension quantitative measurements of species and temperature provided crucial comparisons to theorized and modelled thermochemical structure evolution. Additionally, the injector specific findings highlight the sensitivity of the combustion performance to motor geometry and quantify the utility of introducing incipient swirling flow into the combustion chamber. Significant discrepancies were observed between reactive flow modeling and the 2D experimental results for hybrid combustion of polymethyl methacrylate (PMMA) in gaseous oxidizer cross-flow, prompting a fundamental shock-tube chemical kinetic studies of the monomer, methyl methacrylate (MMA), that involved measuring time evolution of intermediate and product species. This additional data was used to improve the existing chemical kinetic mechanisms by modifying Arrhenius rate parameters of sensitive reactions via genetic algorithm optimization anchored to the speciation measurements. The aforementioned spectroscopic, experimental, and analysis techniques designed for hybrid rocket propulsion were also extended to study hybrid combustion of the fire-resistant polymer polytetrafluorethylene (PTFE) in oxidizer-cross flow to help inform toxicant predictions in structural fires and develop useful sensors for fire safety.

It is envisioned that the data in this dissertation will be used to anchor and improve reacting flow models relevant to both hybrid rocket propulsion and fire safety. The sensors, experimental facilities, and analysis procedures can also be further employed in the future to study a wide range of solid fuels across combustion applications.