Laser Induced Spark Ignition Characteristics of a Methane-Oxygen Rocket Combustor*

The mechanics of laser induced spark ignition of non-premixed gaseous methane and oxygen are investigated. The reactants are injected into an optically accessible combustion chamber from an oxidizer centered shear coaxial injector. High-speed schlieren imaging and deposited laser energy measurements allow for the characterization of ignition behavior at various spark locations throughout the chamber. A two dimensional map of ignition probability is generated from 199 tests completed at 22 different locations. The rate of pressure rise from successful tests reveals two location dependent ignition methods, referred to as "direct" and "indirect". Physical processes occurring over multiple timescales throughout the direct ignition method are first discussed. Ignition outcomes associated with the indirect method are determined by the hydrodynamic ejection protruding from the laser spark, the behavior of which is explored in detail using flow statistics extracted from images taken at a single spark location. The spatial-temporal progression of this jet is found to be dependent on deposited laser energy, leading to a relationship between the amount of energy added to the flow and ignition outcome. General bounds for spark location, deposited laser energy, and ejection behavior are created in order to predict ignition outcomes. Cases that are an exception from these bounds are investigated in detail to understand the cause of unique behavior. These cases lie within regions of the variable space that are susceptible to stochastic elements of the flow.