Experimental study on combustion stability characteristics of non-swirl and swirl coaxial injectors

Abstract

To clarify the difference of combustion stability characteristics between conventional coaxial injectors and swirl-coaxial injectors, an experimental study with liquid oxygen (LOX) /hydrogen and LOX/methane propellants was conducted using thrust chambers with nominal thrusts of 7 and 14 kN at chamber pressures from 2.3 to 10 MPa. Stability rating tests were done with a bomb disturbance technique. The results indicate that the magnitude of induced pressure disturbance with swirl-coaxial injectors is smaller than that with coaxial injectors at the same injection velocity ratio, Vf/Vo. The difference of this stability characteristic is considered to mainly result from the size of the LOX droplets produced. That is, the amplitude of induced maximum pressure disturbance with the same size of bomb appears to be smaller with injectors which produce a fine LOX spray. The increase of fuel density was found to cause a generation of acoustic mode oscillations, a trend independent of the size of LOX droplets. INTRODUCTION Since the occurrence of unstable combustion causes severe damage to rocket engines, a full understanding of this phenomenon is essential to the development of new rocket engines or improvement of existing engines. LOX/hydrogen rocket engines, in general, have employed coaxial injectors, which are known to be simple and characterized by good chamber compatibility and good combustion stability. With these conventional coaxial injectors, LOX is injected at low velocity through a center orifice, and gaseous fuel from an annulus gap around this center LOX orifice is injected with high velocity. With this configuration, atomization and mixing of propellants is achieved by the action of shearing force between gaseous fuel and LOX. Thus, the LOX stream is * Researcher, Rocket Combustion Laboratory t Senior researcher, Rocket Combustion Laboratory J Head, Rocket Combustion Laboratory, Senior Member