Investigation of Liquid Film Behavior on the Surface of an Airfoil in a High-Speed Flow and Subsequent Atomization From the Trailing Edge - Effect of Airfoil Shape

Abstract

Abstract The breakup of liquid films in high-speed flows is found in many applications. These include pre-filming air-blast atomization found in fuel injectors and shedding from airfoils. In this work, the effect of airfoil shape on the liquid film flow on the surface and subsequent atomization from the trailing edge of the airfoil is investigated. Specifically, a symmetric airfoil (NACA0012) and an asymmetric airfoil (NACA2312) are studied and are placed in a high-speed flow of air at velocities up to 175 m/s. Water was introduced onto one side of each airfoil via 26 0.5 mm diameter holes spaced 1 mm apart. These holes are positioned 35 mm downstream from the leading edge and 65 mm upstream from the trailing edge. Water flow rates between 1.4cm2/s and 2.6cm2/s are used. Liquid film behavior is characterized by 1) point measurements of dynamic liquid film thickness using a confocal arrangement of laser induced fluorescence and 2) high-speed video to visualize liquid accumulation and measure ligament breakup length at the trailing edge. Laser diffraction is used to measure line of sight average droplet sizes. Finally, Phase Doppler interferometry is used to measure spatially and temporally resolved droplet size and velocity in the pitchwise direction at 70mm downstream trailing edge distance. The film thickness formed on the suction surface of NACA2312 is thicker than on the pressure surface and on NACA0012 which is attributed to larger adverse pressure gradientand thicker air boundary layer. Ligament lengths are almost identical for films on either surface of the NACA2312 vane. This confirms that the trailing edge vortices have minute influence on the atomization at an immediate distance from the trailing edge. Phase Doppler results indicate that the spray produced from the suction side of NACA2312 and NACA0012 airfoils are symmetric about the center of the vane while introducing water from the pressure surface of NACA2312 shifts the spray 3 mm on the same side. This is confirmed from the droplet size distribution specifically at higher velocities. This is consistent with the stronger trailing edge vortices on the pressure surface at high air velocity (175m/s) and at a distance well below the trailing edge (approximately 0.7c) The results demonstrate that the shape of the airfoil can have an impact on spray distribution from the trailing edge.