Investigation of breakup mode and oscillation characteristics in liquid–liquid pintle injection element

Pintle injectors have been widely adopted in variable-thrust liquid rocket engines. However, the breakup modes and spray oscillation characteristics of the discrete liquid–liquid pintle injector remain unclear. This study utilizes a high-speed camera and backlighting to capture spray images of the liquid–liquid pintle injection element. By processing and analyzing these images, the breakup modes and oscillation characteristics were investigated. The results revealed three distinct breakup modes. As the local momentum ratio (LMR) increases, the breakup modes transition in sequence: liquid film dominating with multi-finger, impact wave, and liquid jet penetration and stretching. The mode boundaries have been identified: when the LMR reaches 0.386–0.504, the liquid film dominating with multi-finger shifts to impact wave, and further shifts to liquid jet penetration and stretching as the LMR increases to 2.222–2.564. In the impact wave mode, with increasing jet and film Weber numbers, the maximum amplitude initially rises and then decreases. Conversely, the location of maximum amplitude and breakup length trends are opposite, with the breakup length being larger. Furthermore, the oscillation wavelength trend mirrors that of the maximum amplitude. When the Weber number exceeds 3000, the wavelength stabilizes between 5 and 7 mm, with only slight variation. Unlike other quantities, the oscillation frequency is primarily influenced by the liquid film Weber number (Wef) and increases as Wef rises. A strong core zone exists in the impact wave mode, in which the spray oscillates intensely with larger amplitude, higher frequency, and shorter breakup length. These findings offer valuable insights into the breakup modes and oscillation characteristics, enhancing the understanding of breakup mechanisms.