Dynamic characteristics of in-nozzle flash boiling bubbles and corresponding temporal responses of external spray

Abstract

Flash boiling spray is believed to have the potential in further improving the efficiency of practical liquid fuel combustors. However, the complex physics of this transient, multiphase process are not fully understood. Despite improved spray breakup and atomization performance, near-nozzle flash boiling sprays also faced the challenge in cycle-to-cycle variation and fluctuation. This work aims to explore the dynamic formation, development, and aggregation of in-nozzle flash boiling bubbles to establish the connection between bubble features and external spray behaviors. In this investigation, a transparent two-dimensional nozzle was utilized for high-speed shadowgraph visualization and flash boiling sprays at various fuel temperatures (superheat degrees) were measured and investigated. Theoretical analysis was incorporated to interpret the nucleation and growth of the flash boiling bubbles. Near-field external sprays were also captured to reveal the temporal response with regard to in-nozzle flash boiling bubble structures. Results showed that the layered structure inside the nozzle had a direct impact on the oscillation on the external spray. Fast Fourier transform (FFT) was also adopted in quantifying the dominant frequency of the oscillating flash boiling sprays. Growth of flash boiling bubble is accelerated in axial direction of the nozzle. Wavy structure of flash boiling bubbles can be found inside of the nozzle. Number of bubble layers increases with increasing fuel temperature. Fluctuation of external spray is fiercer with greater number of bubble layers.