Investigation of Water Film Dynamics on the Surface of an Airfoil in a High-Speed Flow and Subsequent Ligament Formation and Breakup From the Trailing Edge

In this work, the dynamics of a liquid film on the surface of NACA 0012 airfoil placed in a high-speed air flow is investigated. The findings complement previous results obtained on time averaged ligament behavior and droplet sizes generated by the same airfoil. Experimental studies were carried out to assess the film thickness, droplet shedding, and the dynamics of the sheet. In the present work, air velocities up to 175 m/s were used with water films flowing between 1.4 and 2.6 cm²/s. The water film was introduced onto one side of the airfoil surface through a series of 0.5 mm holes separated by 1 mm at a location 35 mm downstream of the leading edge of the vane. The results were obtained using four experimental tools. The first is a point measurement of the dynamic film thickness using a confocal laser induced fluorescence method. This spatially resolved measurement provides time resolved measurement of the instantaneous liquid film thickness at specific points on the vane surface. This is complimented by time averaged images of the film thickness on the entire vane surface. Third, high speed videos are obtained to study the accumulation and breakup of the liquid at the trailing edge of vane. Finally, laser diffraction and Phase Doppler interferometry were used to document the spray dynamics downstream of the vane. The results illustrate that the average film thickness decreases with air velocity and increases with the water flowrate. The results are consistent with the previous studies and suggest that the dominant frequency of liquid film wave, ligament breakup length, drop size and spray concentration increase with the air velocity and is modestly affected by water flowrate. Finally, design tools are provided to predict the average film thickness and dominant frequencies of the film thickness, ligament breakup, spray concentration and droplet average size.