Abstract

The breakup of liquid films in high-speed flows are found in many applications. These include pre-filming air blast atomization found in fuel injectors and shedding from airfoils. In this work, the behavior of a liquid film on the surface of a NACA 0012 airfoil placed in a high-speed air flow is investigated. The results complement previous results obtained on time averaged ligament behavior and droplet sizes generated by the same airfoil. A combination of computational fluid dynamic (CFD) simulations and experimental studies were carried out to assess features of water films, including the film thickness, wave speed, droplet shedding, and the dynamics of the sheet. In the present work, air velocities up to 175 m/s were used with water films flows 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 1mm at a location 35 mm downstream of the leading edge of the vane. Experimental results were obtained using four primary 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 is used to document the spray dynamics downstream of the vane. In addition to the experimental results, CFD simulations of the film behavior are also carried out using ANSYS Fluent. The results illustrate that the average film thickness decreases with air velocity and increases with the water flow rate. The results are consistent with the previous study and the current CFD analysis. Frequency analysis suggests 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 flow rate. 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.

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