Modes of Laser Spark Ignition of a Model Rocket Combustor

Abstract

The mechanics of laser induced spark ignition of non-premixed gaseous methane and oxygen are investigated in a model rocket combustor. The reactants are injected into an optically accessible combustion chamber from an oxidizer centered shear coaxial injector. High-speed schlieren imaging and measurements of deposited laser energy are used to characterize ignition behavior at various locations in the combustion chamber. A spatial map of ignition probability is generated at multiple axial and radial locations from the point of reactant injection at an equivalence ratio (��) of 1.34. As the spark location is moved from the reactant jet to the recirculation region of the combustion chamber, two distinct modes of ignition, direct and indirect, are observed. For direct ignition, ignition is achieved with every ignition attempt and flame kernel formation is achieved within 50 ��s of spark deposition. When the spark is located outside the reactant jet in the recirculation zone, ignition is achieved by interaction between the hot gas convected by hydrodynamic ejection out of the plasma kernel and the fresh reactant core. The ignition probability with indirect ignition is dependent on the spark location and the structure of the hydrodynamic ejection.