Geometric effects and throttling characteristics of liquid rocket engine pintle injectors in supercritical combustion

Abstract

however, the heat flow rate decreased by 85 %. It implies that the cooling is sufficient only with fuel used as a regenerative coolant in the throttling region. The low-frequency pressure perturbation caused by the liquid oxygen boiling had the most dominant effect on combustion stability. This perturbation is weakly correlated with the TMR and BF. Instead, it was greatly influenced by the degree to which the oxygen in the manifold approached the evaporation curve. The strong low-frequency pressure perturbation in the transition state has no differentiation under each thrust condition as the liquid oxygen temperature in the manifold is stabilized in the normal section above 40 % thrust. By contrast, under 30 % thrust, oxygen boiling continued upstream of the manifold even in a steady state. Due to this, the high momentum of oxygen passing through the pintle orifice as a gaseous state continued to increase the TMR and pressure fluctuations. Therefore, rough combustion continued to occur.

Pintle injectors with intrinsic combustion stability and thrust control are promising candidates for application to reusable liquid rocket engines (LREs). Therefore, several research groups have previously investigated the shape factor of pintle injectors. However, pintle injectors have hardly been investigated under the supercritical pressure conditions in which advanced modern LREs operate. Therefore, this study investigates the geometric effects on combustion characteristics under supercritical pressure conditions. Toward this end, 70 bar combustion tests were performed using a combustion chamber with liquid oxygen and kerosene as propellants and a discontinuous oxidizer-centered two-row pintle injector . Then, the effects of the total momentum ratio (TMR) and blockage factor (BF) were experimentally explored using a combination of nine pintle injectors. In addition, a throttling combustion test was performed for 80 % , 60 %, 40 %, and 30 % thrust with a limited combination of pintle injectors. As a result of hot-firing tests in supercritical conditions, the characteristic velocity efficiency was higher than that in the subcritical combustion test at a similar BF. In addition, as in studies of subcritical combustion, a decrease in TMR improved efficiency. However, unlike under subcritical conditions, the decrease in TMR reduced the pressure perturbation ratio in the combustion chamber ($P^{'}/ P_C$). The BF determined by the aspect ratio of the second-row orifice also contributed to the change in efficiency. However, in the BF range considered in this study, the effect was insignificant compared to that of the TMR. The efficiency decreased by 1 %–2 % until 60 % thrust due to supercritical characteristic. It shifted from 40 % thrust to subcritical conditions and decreased by 3 % compared to that at 60 % thrust. At 30 % thrust, the TMR increased rapidly owing to the increase in the momentum of the oxidizer vaporized in the manifold, contributing to a 7 % decrease in efficiency compared to that at 40 % thrust. This decrease in efficiency increases the propellant mass flow rate required under each thrust condition, resulting in a pressure drop ratio of the injector to the combustion pressure ($\Delta P_{inj} ⁄ {P_C}$) that slows the reduction. This efficiency reduction allows the LRE to be throttled through a sufficient range by adjusting the mass flow rate with only the fixed-area pintle injector. Under 100 % thrust conditions, the heat flux in the combustion chamber is similar to that in the coaxial swirl injector cases. When throttling from 100 % to 30 %, the required fuel flow rate decreased by 65 %