Submerged Combustion and Two-phase Exhaust Jet Instabilities

Abstract

Compact liquid-fueled swirl-stabilized combustors using advanced fuel atomization techniques provide a potential solution to challenges associated with propulsion of submerged naval vehicles. The submerged pressurized operation of a combustor may alter the operational characteristics and performance of the combustor and also involves two-phase phenomena when the combustor exhaust gases interact with surrounding water. This work describes an investigation of swirl-stabilized flames created in a combustor featuring coannular swirling airflows under submerged conditions. A central atomization-air jet was used to atomize methanol fuel, providing great flexibility and control over fuel spray properties in a compact geometry. Direct photography was used to examine the global features of the flame. High-speed imaging was used to examine the two-phase shear-layer behavior of the exhaust jet from the combustor. Sound spectra associated with exhaust jets under different operational conditions were compared. Three nozzle geometries were examined, and the impact of nozzle geometry on the two-phase jet interaction was assessed. The two-phase interaction of the exhaust jet was found to depend heavily on the pressure drop over the exhaust nozzle. The dynamic behavior of the exhaust jet was buoyancy-driven at low-pressure drops and was affected by complex instability mechanisms at high-pressure drops. Nozzle features that have been shown to affect the behavior of single-phase jet shear-layer instability were found to have little effect in a two-phase situation. The instability mechanisms involved in the two-phase cases investigated here are shown to be significantly different from jets in single-phase cases. Evidence is presented that a pressure-wave interaction mechanism, similar to Richtmyer―Meshkov instability, may play an important role in the evolution of unstable structures at the two-phase interface when high combustor pressures are involved.