Compact Augmented Spark Impinging Igniters for Liquid Rocket Engines

The primary objective of this project was to characterize augmented spark impinging (ASI or torch) igniters for liquid rocket engines. Torch igniters inject propellants into a small prechamber (torch tube) where a spark igniter imparts the activation energy to initiate combustion. The flame stabilizes within the torch tube and exits into the main combustion chamber where it ignites the main flow directly or by mixing with a coaxial-flow to create a secondary flame external to the torch tube. The underlying phenomena that promote reliable ignition are multifaceted and thus required a broad scope of investigations. Initial efforts focused on spark discharges (equilibrium plasmas) created by electrical arcs jumping across an annulus. Gaseous propellent flowed through the annulus and projected the electrical arc toward the prechamber. The arc and the thermal energy imparted to the surrounding fluid then met with a combustible mixture to initiate combustion. Spark discharges were parametrically examined against pressure, spark gap size, and exciter types. A variety of pressures were used to emulate the transient engine startup. Schlieren imaging was used to determine the approximate velocity and distance traveled by the exhaust plume from the spark discharge. High voltage measurements were taken to determine the voltage, current, and energy imparted to the flow. The importance of spark quenching (no energy deposited to the fluid) and its effects on perceived versus actual measurements were shown. Results from this experimental phase were used to outline the target zone where a combustible mixture would be needed for torch ignition. After evaluation of the spark discharge event, three-dimensional, time-accurate, and nonreacting computational fluid dynamic simulations were performed to assess effects of geometric and mass flow differences on the size, location, and composition of a combustible mixture. Insight to fluid dynamic phenomena (under-expanded jets, shock wave formation, local recirculation zones, and pressure variations) was elucidated. Correlations of mixture ratios with geometric variables were found as well as correlations of the mixing length with flow rate (Reynolds number) and geometric variables. Results from the computational campaign were coupled with a conservation-law analysis that was used to produce novel objective functions for igniter design. The final phase of the project was full-scale testing of a modular torch igniter. Injector sizes, mixture ratios, and injector momentum ratios were systematically varied to map ignition probability and ignition delay. Measurements of the high-voltage spark igniter were correlated to the first study of spark discharges to connect the theoretical results to a closer real-world application. Results from this study showed the importance of local mixture ratios, provided guidelines for acceptable injector momentum ratios, and demonstrated reliable ignition of core mixture ratios typically outside of normal flammability limits. Page 2 – Revised 02/2018 The primary deliverable of this project was not in fact a torch igniter or derivative thereof. Instead, this project produced novel objective functions for igniter design. These equations are functions of geometric and fluid dynamic variables that may be constrained by engine-level requirements, physics (i.e. flame speed), and experimental results from the current studies. The objective functions provide a foundation for future igniter design and will minimize iterative processes from the design cycle for next-generation ignition hardware. Results of this work are directly applicable to NASA Technology Taxonomy TX01.1.3 Cryogenic under TX01.1 Chemical Space Propulsion and help advance development of liquid-oxygen/liquid-methane combustion devices. Results are applicable to ignition needs including, but not limited to, small in-space thrusters to torch igniters used in core-stage engines that have thrust capabilities similar to the space shuttle main engine. Auxiliary applications of this technology are terrestrial ignition needs such as scramjets or gas turbines. Results are available in public literature, conference proceedings, the JANNAF journal of Propulsion and Energetics, and the dissertation of the NASA Space Technology Research Fellow Darren C. Tinker through Vanderbilt University.