Characterization of Flashing Phenomena with Cryogenic Fluid Under Vacuum Conditions

Flashing occurs when liquid undergoes a sudden depressurization and triggers a violent atomization and vaporization. This phenomenon may happen at the transient startup of the upper-stage rocket engine. In this paper, experimental investigation of cryogenic fluid injection into a low-pressure environment is performed using liquid nitrogen instead of the real cryogenic propellants (liquid oxygen and liquid methane). With the help of a high-speed shadowgraph technique and with the temperature measurement, the characteristics of flashing are explored, including its evolution process, the spray angle, the spray thermal behaviors along the injection centerline, and the spray phase solidification. The results show that bubble nucleation is a reasonable controlling factor for the flashing evolution process. The nondimensional energy barrier is conditionally indicative of the flashing transition process, and it works well under low injection temperature conditions. The drastic temperature drop due to flashing phenomena causes liquid jet solidification, which may pose a challenge for the engine startup process. An analysis of the spray lateral propagation indicates that superheat exerts a substantial effect on the spray angle enhancement; however, this enhanced effect is found to decrease with an increase of liquid superheat.