• Flash boiling had a significant influence on sprays formed by impinging jets. • Liquid jet impingements transformed into two intersecting sprays and vapour streams. • Vapour dominated the behaviour of the spray from impinging jets under flash boiling. • The momentum exchange remained effective even for fully developed flash boiling. • The spray shape was continuous and extended perpendicularly to the jets’ plane. Liquid jets impinging on each other have been widely used in rocket engines to atomise the fuel and oxidiser. In the upper rocket stages, vacuum conditions occurring before the engine ignition evoke flash-boiling effects on the injected liquids. Consequently, the character of the interactions between the two colliding liquid jets changes significantly and impacts the atomisation process. This study examined the flash-boiling effects on symmetrical impinging water jets and resultant sprays. Two synchronised cameras were used to observe the spray simultaneously from two perpendicular directions - normal and parallel to the plane of the colliding jets. Results showed that the impingement of continuous liquid jets occurring in the subcooled regime transitioned into a collision between atomised sprays under strong flash-boiling conditions. For the first time, it was demonstrated that the post-impingement sprays generated by the two impinging flash-atomised jets are entirely different from those formed by interacting jets from multi-nozzle injectors with diverging nozzle axes. Moreover, these sprays resembled those formed by crossing gaseous jets, indicating that despite the disintegration of the jets before reaching the impingement point, momentum was effectively exchanged between the intersecting spray clouds. The resultant spray expanded primarily in the direction perpendicular to the plane of the colliding plumes, demonstrating an increase in the relevant spray angle from approximately 12° to 40° under fully developed flash-boiling conditions. These observations imply the primary significance of flash-evaporated liquid in the interaction process of two jets under flash-boiling conditions.
Kaźmierski et al. (2026) studied this question.