An Experimental Investigation of Sweeping Air/Mist Jet Film Cooling Through a Row of Passive Fluidic Oscillators

Abstract

Sweeping jet fluidic oscillators can be a promising candidate in film cooling applications since they have a large lateral jet spreading with more uniform cooling, which can be considered an advantage when compared to the regular steady jet film cooling. Sweeping jet fluidic oscillators are passive and can generate a sweeping movement without the need for any moving parts. In addition, they can be more conveniently manufactured by additive manufacturing techniques. This paper presents an experimental study of employing two-phase (air/mist) flow through a row of passive fluidic oscillators in film cooling applications for gas turbine airfoil cooling. The objective of this study is to investigate the potential enhancement of film cooling by using sweeping jets injected with water mist. Three blowing ratios (BR = 0.85, 1.66, 2.40) and two mist ratios (MR = 2.75% and 6.92%) are used (i.e., a total of nine cases including the air-only cases). Infrared thermography and E-type thermocouples were used to measure the wall temperature, while a phase Doppler particle analyzer (PDPA) was used to measure the droplets' size distributions. The initial mean droplet size is 11.6 μm. An interesting observation was found where thin water liquid streaks were formed at the exit of the fluidic oscillators and traveled for very long distances (about 140D) providing excellent local film cooling under the laboratory low heating condition. The forming of liquid streaks is due to the significant droplets' coalescence phenomenon caused by large oscillating vortical flows inside the fluidic oscillator. Adding small mist ratios provided better cooling effectiveness with an overall time-averaged cooling enhancement of 15–90% and a maximum local cooling enhancement of 300–350% in all the cases studied. The sweeping mist jet provided better film cooling effectiveness at the centerline and spanwise direction (87% and 76%, respectively) when compared to steady jets under the laboratory conditions with low temperatures and low pressure.