Experimental study of external characteristics of a flashing water jet in water

Abstract

External characteristics of a flashing water jet released in water were studied using Particle Image Velocimetry and Shadowgraphy techniques for different pressure ratios and nozzle diameters. The flow dynamics of such a jet are dominated by vapor bubble formation due to thermal non-equilibrium and phase change processes. In order to gain insights into the physics of such complex flows, experiments were performed and measurements on velocity distribution and turbulence statistics were obtained. The bubble size distributions were also recorded to characterize the bubble disintegration mechanism. Flashing jet emanating from two different nozzle diameters, D = 5 mm and 2 mm, and at three different pressure ratios, PR = 1.5 bar, 2 bar and 2.5 bar, were studied, at a fluid inlet temperature of To = 380 K. The results revealed that the maximum value of centerline velocity occurs far downstream, at x ≥ 0.02 m from the nozzle exit, due to the presence of vapor bubble formation that leads to an interaction between the liquid and gaseous phases. This is in contrast to a canonical water jet (i.e., without phase change), where the centerline velocity attains a maximum at the nozzle exit. For D = 5 mm, the turbulent kinetic energy (TKE) was higher and the decay rate was slower as compared to D = 2 mm for PR ≥ 2 bar. From the centerline velocity and TKE measurements, a critical injection pressure ratio, PR > 1.5 bar was documented, beyond which the external flow dynamics of the jet is governed primarily by the flashing mechanism. These observations were verified from the bubble size distribution measurements and can be attributed to thermal non-equilibrium and phase change process. Lastly, the presence of coherent structures in the flow reveals that the entrainment dynamics of a flashing jet is dominated by vorticity.