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.

This dissertation details the development and application of mid-infrared laser absorption spectroscopy sensing methods towards advancing low-carbon reciprocating engines for high-efficiency and low-emission power generation in a decarbonized energy sector. The scope of this work includes advancement in methods for fundamental spectroscopic studies, integration of advanced sensors into production reciprocating engines for characterization of combustion of low-carbon fuel blends, and computational methods advancement for high-speed real-time signal processing. A high-temperature, high-pressure optical gas cell is designed to enable controlled studies of molecular absorption spectra at high temperatures (>1200 K) and high pressures (>200 atm) to validate spectroscopic parameters at the elevated conditions in combustion engines. A novel optical approach provides access to the mid-wave infrared wherein lies the fundamental rovibrational absorption bands of combustion species critical to characterization of combustion process and emissions formation. Laser absorption sensors are developed and utilized for experimental measurements in the exhaust of a production Honda single-cylinder spark-ignition engine through design of an in-line exhaust sensor module to gain optical access to exhaust gases close-coupled to the exhaust valve. High-temperature opto-mechanical design and laser fiber-coupling assist in achieving robust measurements of cycle-resolved temperature and species (CO and NO) concentration at a rate of 10 kHz. The exhaust sensor is demonstrated by capturing cycle-to-cycle and intra-cycle emissions dynamics and characterizing emissions response to low-carbon fuel blends incorporating natural gas, hydrogen, and ammonia. To enable real-time measurement output at 10 kHz, computational time of the sensor data processing is reduced to sub-ms scales through the use of machine learning algorithms on an embedded processing platform. Compact neural network and ridge regression models are developed to calculate species concentration and temperature directly from transmitted laser signals, removing the need for computationally-intensive nonlinear fitting methods. The machine learning algorithms are deployed to a field-programmable gate array (FPGA) for further acceleration. Hardware-in-the-loop demonstration yields computational time and latency below 100 µs to expand use of the 10 kHz exhaust sensor for real-time sensing applications. Complementary to the sensor development work, a time-resolved chemical-kinetic model is constructed within Cantera to evaluate reciprocating engine performance and emissions during fueling with low- and non-carbon blends. The simulation model provides insights into strategies for optimization of low-carbon combustion and serves as a foundation for sensor interpretation and future work in engine optimization. Discussion of ongoing work includes the design and development of an electro-hydraulic camless valvetrain for future integration into a reciprocating engine architecture to enhance adaptability for fuel-flexible operation.

Additive manufacturing (AM) has matured significantly over the past decade and become a highly attractive tool for reducing manufacturing complexity and removing traditional design constraints. This is particularly desirable for rocket combustion devices which often feature hundreds of individual parts with precise tolerances. The degree to which AM can be used to improve combustion device performance, however, has been less rigorously explored. In this work, hydraulic performance impacts associated with and enabled by AM are assessed for a liquid bi-propellant swirl coaxial injector. Specifically, a single-element liquid oxygen/kerosene injector based on a canonical design was manufactured from inconel using Direct Metal Laser Sintering at two different coaxial recess depths. Cold-flow testing for the as-manufactured baseline injector found a reduction in the designed discharge coefficients, which is primarily attributed to increased surface roughness inherent in the AM process. Quantifying this difference provides data for tuning surface roughness effects in Computational Fluid Dynamics (CFD) models, which may be used to inform the design stage. In addition, CFD simulations were leveraged to identify hydraulic losses in the baseline design and redesign a more hydraulically efficient injector geometry utilizing reductions in dramatic flow constrictions. For the modified injector geometry, computational results predict a 19% and 8% increase in the fuel and oxidizer discharge coefficients, respectively, as well as an 18% increase in angular momentum relative to the baseline. These results in addition to recommendations made for minimizing frictional losses that resulted from the printing process demonstrate how AM and analysis-driven design may be utilized to develop a more hydraulically efficient liquid rocket engine injector, thus highlighting a pathway to higher thrust-to-weight ratio propulsion systems and increased launch vehicle payload capacity. This work was presented in part at the 2019 AIAA Propulsion and Energy Conference.

Predicting and managing heat transfer during planetary entry is a critical engineering challenge for current and future space exploration missions. This work aims to improve the understanding of thermodynamic non-equilibrium during Mars entry via experimental studies to capture chemical kinetics and the rates of energy transfer between translation, rotation, and vibration for the dominant molecular species (CO2 and CO). The key elements of this research can be split into two distinct phases: (1) develop and demonstrate non-equilibrium high speed sensing of CO and CO2 on a high enthalpy shock tube at UCLA. (2) Deploy the sensor at NASA Ames on a representative Mars entry flow. A mid-infrared laser absorption strategy for simultaneous measurement of translational, rotational, and vibrational temperatures of carbon monoxide (CO) at high speeds was developed for application to high temperature non-equilibrium environments relevant to Mars atmospheric entry. The sensing strategy is shown to resolve each targeted transition with temporal and spectral resolution sufficient for quantitative multi-temperature measurements over a wide range of temperatures and pressures (2100 - 5500 K, 0.03 - 1.02 atm), including behind incident shock waves traveling up to 3.3 km/s. A similar strategy is employed on CO2 transitions from the ν3(00^00) and ν3(01^10) states. Vibrational relaxation times were resolved at temperatures relevant to Mars backshell heating (2,000 - 3,000 K) in various CO2 - Ar mixtures and found to be in good agreement with the Simpson rate model. The final effort of this project deployed a multi-species sensor on the Electric Arc Shock Tube facility at NASA Ames to study a recreated shock layer similar to that experienced on the Mars2020 mission. Temperature and number densities of CO2 and CO were extracted from the data and compared to various chemistry models and a simultaneous emission measurement. At shock velocities below 3.1 km/s, the agreement between the measurements and the Johnston mechanism is typically within 5\% for temperature and within 10\% for number density. At shock velocities above 3.1 km/s, the CO2 measurement becomes sensitive to a thin boundary layer and corrections of this effect are presented. On test cases with enough energy to dissociate CO2, a quantum cascade laser scanned the P(2, 20), P(0, 31), and P(3, 14) transitions of the CO fundamental band at 4.98~�m. CO formation rate is measured to be close to the Johnston kinetic mechanism at low velocities, and then trending towards the Cruden kinetic mechanism at high velocities. In summary, this work has advanced laser absorption techniques to include high speed (MHz) multi-temperature measurements of CO2 and CO on non-equilibrium flows relevant to Mars planetary entry.

As efforts to access and explore space increase, emerging rocket technologies will undeniably continue to rely on chemical propulsion, as it remains the only feasible way of providing Earth-to-orbit access and has proven invaluable in extending in-space capabilities. Accordingly, advancements are needed to improve combustion performance and the understanding of underlying chemical and physical phenomena governing chemical propulsion systems, as they have a substantial impact on the mission capabilities of flight vehicles and spacecrafts. While modeling efforts have made significant progress in recent years, empirical studies remain a necessity in the development of advanced propulsion systems; however, limits of traditional instrumentation often preclude definitive interpretation of flow-field phenomena in experimental tests and are not suitable for in-situ measurements at the extreme temperatures and pressures present in rocket propulsion systems. As such, optical diagnostics have become an attractive tool in combustion science due to the non-intrusive nature of the measurement and flexibility in the measured properties (temperature, species concentration, pressure, velocity, number density, etc.) inferred via spectroscopic interaction. Therefore, to advance the next generation of chemical propulsion systems, advanced optical diagnostic tools need be developed for characterizing propulsion test facilities and developing/validating computational models for complex chemically reacting flow-fields.

The work herein describes novel advancements in laser absorption spectroscopy for characterizing liquid- and hybrid-propellant rocket combustion systems with support of a new High-Enthalpy Shock Tube (HEST) facility at UCLA. Using the shock tube facility to emulate the high temperatures (T > 3000 K) and high pressures (P > 100 bar) present in liquid-propellant rocket combustors, a novel laser absorption spectroscopy sensor that exploits line-mixing effects in the infrared spectra was developed for temperature, carbon monoxide (CO), and carbon dioxide (CO2) measurements. This sensor was then demonstrated on a liquid-propellant rocket combustor at the Air Force Research Laboratory (AFRL) in Edwards Air Force Base with kerosene (RP-2)/oxygen and methane/oxygen propellant combinations. Successful thermochemistry measurements were obtained at pressures up to 105 bar–marking a significant improvement in the pressure capability of optical diagnostic tools. In addition to these liquid-propellant rocket combustor measurements, a unique approach was developed for investigating hybrid rocket propulsion flows. Using laser absorption tomography, spatially-resolved measurements of temperature, carbon monoxide (CO), carbon dioxide (CO2), and water (H2O) were obtained in the reaction layer of a hybrid-propellant rocket combustor with poly(methyl methacrylate) (PMMA)/oxygen and high-density polyethylene (HDPE)/oxygen propellant combinations and two injector geometries (single port and axial showerhead). These measurements highlight combustion physics and thermochemical energy conversion in the spatial domain and help identify mechanistic losses in combustion efficiency for different engine configurations. Lastly, in efforts to develop and refine combustion models for real fuels used in chemical propulsion systems, a novel time-resolved, laser absorption spectroscopy technique was developed for measuring the formation of isotopically-labeled carbon monoxide (12CO and 13CO) in shock tube oxidation experiments of isotopically- labeled fuel blends. The technique was demonstrated by examining competitive oxidation of methane (CH4) with differing C2 hydrocarbon functional groups (alkane, alkene, alkyne), namely acetylene (C2H2), ethylene (C2H4), and ethane (C2H6). By isotopically-labeling specific fuel components of the overall fuel mixture and simultaneously measuring both 12CO and 13CO, individual reaction pathways and rates are distinguishable, providing kinetic targets for reaction mechanisms used to model fuel blends present in chemical propulsion systems.