Investigation of the Effect of Different Wall Boundary Conditions on the Droplet Dynamics and Mist Film Cooling Generated by Fluidic Oscillators

Sweeping jets enhance turbine film cooling by spreading wider and providing more uniform coverage. The sweeping jets are provided through fluidic oscillators without using any moving parts, making them attractive as a passive uniform cooling scheme. In this study, mist was added to the sweeping cooling flow to enhance film cooling performance further. Simulations examined mist behavior, focusing on droplet boundary conditions and phase-change dynamics. Three droplet boundary conditions (reflect, wall-film, and trap) were tested with droplet sizes of 10 and 20 µm, and Rosin-Rammler (1–40 µm, 10% mist ratio) at blowing ratios BR = 1 and 2. Adding mist significantly enhanced film cooling effectiveness along the centerline, with the wall-film and reflect cases using uniform 20 µm droplets showing average cooling effectiveness improvements of approximately 130% and 100%, respectively, over air-only cooling at BR=1. Under low-temperature lab conditions, the wall-film condition better reflected observed liquid streaks in the experiments than the reflect condition. The fluidic oscillator was evidenced to promote droplet coalescence through vortex formation, recirculation, and feedback mechanisms, producing larger droplets at the jet exit. These droplets traveled farther, extending cooling coverage downstream where air-only film cooling proved ineffective. Moreover, the sweeping vortex's centrifugal forces pushed larger droplets outward, while smaller ones stayed in the central region of the vortex, where evaporation was accelerated by negative gage pressure. Within x = 28d, 64.2% and 79% of droplets evaporated under the wall-film and reflect, respectively, with 20 µm.