Dynamic Characteristics of Open and Closed type Swirl Injectors with Varying Geometry

Abstract

Injector is one of the key component in liquid rocket engines especially when suppressing the combustion instability. Since the injector is located between the feed line and the combustor, injector plays a role as the bridge between the two components. The stable combustion can be achieved if the injector is desgined to be dynamically robust to suppress the feedback interactions among the three components. Therefore, dynamic characteristics of the injector should be investigated in order to establish criteria for injector modification. Open-type and closed-type swirl injectors have been widely used in liquid rocket engines. The type of the swirl injector should be selected based on its pros and cons. However, comparisons between the dynamic characteristics of the open and closed type swirl injectors are not thoroughly studied and further research should be done. In this study, dynamic characteristics of the each open and closed type swirl injectors with varying geometries were investigated through experiments. Influences of diameter of the manifold, diameter of the swirl chamber, length of the swirl chamber, converging angle between the swirl chamber and orifice, and length of the orifice were analyzed. Sinusoidal pressure fluctuations were generated by the hydrodynamic mechanical pulsator to simulate the external disturbances in the actual operation of liquid rocket engine. The feed line pressure measured with static and dynamic pressure sensors were selected as the input parameter and the liquid filim thickness at the injector exit measured with the titanium electrodes were selected as the output parameter since it is closely related with the mass flow rate of the swirl injector. Transfer function between the input and output parameters were obtained throughout the pulsation frequencies of 100 – 1000 Hz. The dominant geometry parameter for each open and closed type swirl injectors were verified. With the obtained experimental data of each geometry parameters sensitivity and tendency, directions for injector design modifications can be given when the combustion instability occurs during the development of a liquid rocket engine.