Effects of recess ratio on combustion characteristics of LOX/methane swirl coaxial injectors

Abstract

To explore the effects of recess ratio on the flame spatial structure and combustion efficiency of LOX/methane swirl coaxial injectors, visual combustion experiments with injectors featuring varied recess ratios were carried out based on an optically accessible combustor. The instantaneous spray and flame images were obtained using the red background light imaging technique. Liquid oxygen was tangentially injected into the center post of the injector at a temperature of 121.5 K, while gaseous methane was axially introduced into the coaxial annulus at a temperature of 290.5 K. With the recess ratio increasing from 0 to 0.83, the flame transforms from a steady state to an oscillating state at the frequency of 3144 Hz to 3281 Hz. A strong relationship between flame length and combustion efficiency is identified firstly. The combustion efficiency of stable flame is higher than that of the oscillating flame. There is a specific recess ratio between 0.33 and 0.54, corresponding to the maximum combustion efficiency, which causes the transition of the flame from a steady state to an oscillating state simultaneously. The combustion efficiency of the injector critical recess ratio (RR = 0.73) is the lowest. Both the spray and flame without recess appear to exhibit axial and transverse coupled oscillation. The transverse oscillation of spray and flame is gradually suppressed with larger recess ratio. As opposed to the oscillation region area, the amplitude of spray oscillation may be the main influence factor of flame oscillation scale. In addition, the relationship between self-pulsation, flame oscillation and combustion instability has been further verified.