Understanding the collapse of flash-boiling sprays formed by multi-hole injectors operating at low injection pressures

Abstract

Sprays consisting of initially separated plumes, which are generated by multi-hole injectors in strong flash-boiling conditions, may collapse and form a single spray cloud. It has been shown that for high-pressure injection systems (≥5 MPa), such behaviour is dependent on both the superheat and the geometrical configuration of the injector; while the spray collapse in low-pressure systems (≤1 MPa) is not yet understood. To the best of the author's knowledge, this is the first study aimed at filling this information gap. The principal novelty of this work is related to understanding the collapse in low-pressure systems, in terms of the different numbers of nozzles and the difference in the effects from flash boiling between low- and high-pressure sprays.The results showed that the flash-boiling sprays formed by the two- and six-hole injectors were substantially different, which is similar behaviour as for high-pressure sprays. The major difference from the sprays formed at high pressure is that the flash boiling does not result in an increased penetration (in relation to the subcooled conditions). This distinct behaviour was linked with a different jet break-up regime, and different droplet size reduction due to flash boiling, both depending on the injection pressure. This work discusses that the importance of the aerodynamic drag force due to a stronger droplet size reduction in the case of low-pressure sprays can be increased more than twofold compared to the high-pressure sprays.The lack of increased penetration leads to an important finding in terms of practical applications. In contrast to high-pressure systems, flash boiling will not lead to enhanced wall wetting regardless of the number of nozzles.This study has revealed that the interaction of the spray plumes is not sufficient to cause a strong spray collapse and the formation of the single jet-like spray structure. The necessary additional factor is the flow acceleration in the axis of the injector. The results highlighted the crucial role of the circular pattern, which already in subcooled cases led to the creation of flow conditions promoting axial propagation of a gaseous medium; while in the event of flash boiling, it prevented the surrounding air from entering the centre region of the spray cloud.