In this study, we experimentally investigate the effect of internal flow variation on the design characteristics of a Y-jet twin-fluid nozzle and its utilization for spray droplet size prediction. For this study, a laboratory-scale twinfluid nozzle spray test system was constructed. Sauter mean diameter (SMD) measurements were made by a droplet measurement system using the laser diffraction principle, and spray images were obtained using a highspeed camera. The mass flow rate of the twin fluids under different spray conditions was expressed in terms of the gas-to-liquid mass flow rate ratio (GLR) and turn-down ratio. The GLR tended to decrease when the nozzle orifice diameter increased because the pressure of the supplied twin-fluid was the same. By contrast, increasing the nozzle mixing-tube length resulted in a negligible increase in GLR. This is because the nozzle design characteristics affect the internal pressure of the nozzle, which changes its spray characteristics. In general, the spray characteristics of Y-jet nozzles are most affected by GLR. However, in this study, a generalised GLR was devised to consider not only GLR but also the difference in the flow rate of the twin-fluid due to nozzle design factors. The generalised GLR has the advantage that it is constant under changes in twin-fluid pressure, unlike the GLR expressed by considering only the mass flow rate of the twin-fluid. Therefore, to estimate the SMD more accurately under different spray pressures for a constant GLR, we investigated the SMD estimation using the internal pressure ratio and generalised GLR.
