The pintle injector has various attractive features as a propellant injector of a liquid rocket engine such as the simple structure and the combustion stability. Since few fundamental studies on the pintle injector have been conducted, most of the design parameters of the pintle injector are determined based on the empirical knowledge. Therefore, combustion experiments with a planar pintle injector, which is designed for optical measurements of a pintle injector, are conducted to clarify combustion behaviors under various operating conditions. Another combustion experiment is also conducted using an axisymmetric combustor with a pintle injector to compare the combustion characteristics of the planar pintle injector and the axisymmetric pintle injector. The target combustion pressure and O/F is 0.5 MPa and 1.45, respectively. Denatured ethanol and liquid oxygen are used as the propellant. The injection configuration is oxidizer-centered in both tests. The effects of total momentum ratio (TMR) on the combustion characteristics are investigated by changing the propellant injection velocity. It is clarified that characteristic exhaust velocity efficiency of the planar pintle injector decreases with an increase in TMR due to the impingement of the propellant droplets on the combustor wall, which is observed with a high-speed imaging technique. The same tendency is observed in the axisymmetric pintle injector, therefore, the test method with the planar pintle injector is a promising testing method for a pintle injector to investigate fundamental combustion characteristics. Longitudinal combustion instability is observed during the tests with the axisymmetric pintle injector. The amplitude of the pressure fluctuation increases with decreasing LOX injection pressure. Not only the acoustic modes but also injection coupling combustion instability is observed during the unstable combustion. The amplitude of the pressure fluctuation during the stable combustion is less than 50 % of that of the unstable case, however, the pressure fluctuation with the frequency of 400 Hz, which is not dominant in the unstable case, is observed. The frequency corresponds to the convective time scale of the combustion gas, which means that the oscillation at the frequency can be caused by the entropy wave.
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