Shear and Swirl Coaxial Injector Studies of Lox/Ch4 Rocket Combustion Using Non-Intrusive Laser Diagnostics

Seong-Young Lee1, Delphine Salgues¹, Geradine Mouis¹, Sibtosh Pal¹, and Robert Santoro¹

A comprehensive liquid rocket engine injector study has been conducted for one shear and two swirl coaxial injector cases.  Propellants are methane (CH₄) as fuel and liquid oxygen (LOX) burning at the fuel rich condition of an O/F ratio equal to 4.  Propellants were injected at subcritical conditions.  The target chamber pressure was approximately 4 MPa.  The rocket chamber features four quartz window-sections that provide optical access to the flowfield.  In the study, the swirl and shear coaxial injectors with same dimensions were tested at same inlet flow conditions in order to obtain injector characteristics.  Multiple optical diagnostics were used to visualize the combustion and atomization processes, including OH* spontaneous emission, OH-PLIF, light-scattering, and shadowgraph imaging.  Simultaneously measured techniques were carried out to capture heat release regions from chemiluminescence and PLIF, liquid areas from light-scattering and shadowgraph, and the position of liquid oxygen with respect to reaction zones.  A comparison of both injectors has shown that the swirl injector case has higher combustion efficiency than the shear injector case.  Indeed, the combustion with a swirled LOX sheet expands more radially and the liquid core breaks up earlier than combustion with a LOX jet exiting the shear injector.  C* efficiency indicated that the swirl coaxial injector provides higher combustion performance than the shear coaxial injector.  It was observed that combustion for the shear injector case allows unburned gases to be trapped in the upper (lower) corner re-circulation zone, as seen in OH-PLIF images.  For both injector cases, the region for OH-PLIF signal was located on the outward edge of the OH* chemiluminescence.  However, the OH* chemiluminescence lies very close to the light-scattering edge.  A comparison of the shadowgraph and light-scattering images provides the same information on the liquid areas, and in particular has not indicated any ligament or drop structures in the current study.  The liquid oxygen was atomized directly in a fine droplet spray, indicating that flame is burning very close to the liquid jet.