Effect of self-excited instability on single liquid swirl injector flow characteristics

Owing to their simple structure and high performance, swirl injectors have been used as propellant injectors for rocket engines in various countries, starting from Russia. However, when a swirl injector is used in combustion, flow instability becomes important because of its relation to combustion instability. Because the external flow of a swirl injector is a thin liquid sheet, wave-forming self-excited instability could occur. In this study, the effect of self-excited instability on the flow characteristics of a swirl injector was investigated. Moreover, through the use of a mechanical pulsator, the effect of self-excited instability frequency on the dynamic characteristics of the swirl injector was examined. As a result, self-excited instability was determined to cause the periodic breakup of the spray sheet. In addition, large droplets generated immediately after breakup are confirmed to be periodically concentrated with a specific frequency equal to the self-excited instability frequency. With external pulsation, breakup length was observed to be shortened with pulsation and had minimum value when pulsation of frequency equal to the self-excited instability frequency was applied. Droplet characteristics with pulsation were also investigated, and external pulsation was determined to cause mean droplet diameter (D10) and Sauter mean diameter (SMD) to be smaller. When SMDs at pulsations of different frequencies were compared, the SMD at pulsation with self-excited instability frequency was observed to be larger because this frequency causes droplet distribution to be more non-uniform. Through this study, self-excited instability was confirmed to be able to change the static and dynamic characteristics of a swirl injector. This result can serve as a reminder of the importance of the self-excited instability of a sheet in the design of a swirl injector.