Evaporation efficiency enhancement through cavitation-flash nozzle coupling

Flash evaporation is widely employed in closed-loop geothermal power generation and other low-grade heat recovery systems because of its potential to improve energy conversion efficiency. However, its performance is often limited by spontaneous vapor nucleation and the restricted liquid–vapor interfacial area available for phase change. This study investigates the enhancement of flash evaporation by introducing a cavitation nozzle upstream of a flash nozzle. High-speed visualization was combined with measurements of flash yield and evaporation efficiency under inlet temperatures of 50–80 °C in a vacuum chamber. Under identical chamber pressures, the upstream cavitation nozzle increased the mean evaporation rate by 8.34% (95% confidence interval: 6.93–9.77%), demonstrating a statistically reproducible improvement under matched thermodynamic conditions. Although flash yield increased monotonically with superheat, evaporation efficiency exhibited a non-monotonic dependence on inlet temperature, reaching a maximum at intermediate temperatures (50–70 °C) and decreasing at 80 °C. The observed reduction in efficiency at higher temperatures is likely associated with intensified bubble coalescence and a reduction in the effective liquid–vapor interfacial area, which limit further evaporation. High-speed visualizations showed that cavitation-generated two-phase structures persisted throughout the investigated temperature range, while suggesting that the performance variation is governed primarily by microscale transport processes rather than by macroscopic flow-regime transitions. To describe the combined effects of thermal loading, hydrodynamic transport, and pressure relaxation, an empirical scaling coordinate, Φ = ln [Ja/(Re·Rp)], is proposed. Mapping the experimental data using Φ reveals distinct responses for the cavitating and non-cavitating configurations, and a configuration-specific empirical correlation, E(Φ), is developed for the tested operating conditions and nozzle geometry. These results indicate that cavitation primarily enhances flash evaporation by promoting phase-change kinetics through improved nucleation and interfacial transport, rather than by increasing the thermodynamic driving force. The present findings provide new insight into the design and optimization of cavitation-assisted flash evaporation systems for low-grade heat recovery applications.