An Experimental Investigation of Sweeping Mist Jet Film Cooling With Droplet Analysis

Abstract The sweeping jet shows significant potential for use in film cooling over gas turbine airfoils due to its ability to spread over a wider area, resulting in more uniform lateral film cooling effectiveness. These sweeping jets were generated through fluidic oscillators without the need for any moving parts, making them an attractive option as a passive and efficient cooling method. Furthermore, mist was injected to provide additional cooling enhancement. This paper presents an experimental study on utilizing two-phase (air/mist) flow through a row of passive fluidic oscillators in film cooling applications for gas turbine airfoil cooling. The research focused on two main objectives: (a) measuring the potential enhancement of mist film cooling with sweeping jets and (b) analyzing droplet dynamics. Three blowing ratios (BR = 0.66, 1.28, 2.40) and two mist ratios (MR = 2% and 10%) were studied. Infrared thermography and E-type thermocouples measured wall temperature, while a phase Doppler particle analyzer (PDPA) was employed. The results indicated that a higher mist ratio of 10% provided significantly better film cooling enhancement along the centerline, with a 76% improvement compared to 40% with a lower mist ratio of 2%. The highest droplet concentration, represented by the maximum data rate, occurred at the upper boundary of the droplet distribution. This boundary was consistently located between 1.5d and 2d. Beyond this region, the droplet density sharply decreased, demonstrating a significant drop-off in the distribution. A secondary-flow sweeping vortex appears in the spanwise direction. Due to centrifugal forces, larger droplets were pushed to the periphery, while smaller droplets remained near the vortex center and evaporated quickly, resulting in significantly lower droplet density than at the top of the periphery vortex, which coincides with the coolant film upper boundary. The mist sweeping jets achieved approximately 270% enhancement over steady round hole mist jets, and over 475% over air-steady jets in the region downstream of 10-hole diameter (x > 10d).