Longitudinal combustion instability of a pintle injector for a liquid rocket engine combustor

The longitudinal combustion instability characteristics of a pintle injector for a liquid rocket engine combustor are investigated experimentally. The effects of the propellant total momentum ratio (TMR) on the combustion oscillation characteristics and the characteristic exhaust velocity (C*) efficiency are investigated. The time scales of the combustion oscillations are compared with various phenomena in the combustion chamber in order to identify dominating phenomena of each combustion oscillation mode. The target combustion pressure and the propellant mixture ratio are 0.5 MPa and 1.45, respectively. High-speed optical measurements for the CH* chemiluminescence and backlit spray images are also conducted to investigate the oscillating combustion behaviors of the pintle injector. The amplitude of the pressure oscillation (P') increases with decreasing TMR. When the TMR is 0.73, the P'is approximately 50 % of the average combustion pressure, whereas the P' decreases to 20 % of the average combustion pressure when the TMR is increased to 2.35. Frequency characteristics of the combustion oscillations are affected by the TMR. When the TMR is smaller than unity, the pressure oscillations at the chamber acoustic natural frequencies are dominant, and the coupling between the heat release rate and the combustion chamber acoustics could occur. When the TMR is increased, low frequency oscillations whose frequencies are smaller than the fundamental natural frequency of the combustion chamber are observed. The time scale of the observed oscillation is close to the residence time of the combustion gas in the combustion chamber, and the convective process could cause the low frequency oscillations.