Investigation of Liquid Film Atomization From the Trailing Edge of an Airfoil in a High-Speed Flow-Effect of Trailing Edge Thickness

Abstract

Abstract. Liquid film atomization from trailing edges is found in numerous applications. These include air blast atomization in prefilming type fuel injectors and shedding from airfoils. This study investigates the effect of trailing edge thickness on liquid sheet breakup and droplet formation using NACA0012 airfoils with rounded trailing edges of 0.5 mm, 1.0 mm, and 2.0 mm, in high-speed airflow of up to 175 m/s. Water was introduced on the airfoil surface through 26 holes, each 0.5 mm in diameter and spaced 1 mm apart. These holes were located at 65 mm upstream of the trailing edge along the approximately 100 mm chord length. Three liquid flow rates, i.e., 1.4 cm3/(s·cm), 2.0 cm3/(s·cm), and 2.6 cm3/(s·cm) were tested. A variety of experimental techniques were implemented to analyze liquid film breakup and droplet behavior. High-speed video imaging captured film flow on the surface, the accumulation of liquid, ligament formation, and breakup at the trailing edge. Microscopic high-speed imaging was used to observe droplet behavior as a function of distance downstream of the trailing edge. This imaging reveals the transition from ligament formation to secondary atomization to the final droplet size distribution with downstream distance. At 70 mm downstream, atomization is essentially complete, and Laser Diffraction is also used to measure droplet Sauter mean diameters (SMD). The results show that thicker trailing edges and higher liquid mass flow rates promote liquid accumulation at the trailing edge. This, in turn, leads to increased ligament length and sheet breakup time. In the region just downstream of the ligament breakup, droplet sizes increase with increasing trailing edge thickness and liquid volume flowrate. Once the secondary atomization is complete (∼70 mm from the trailing edge), the drop sizes were found to be less affected by the trailing edge thickness and liquid flow and mainly determined by the air velocity. An analysis of variance (ANOVA) was performed and a model to predict SMD as a function of weber number, air to liquid mass flowrate (MFR), downstream distance and trailing edge thickness was developed. Liquid film thickness as a characteristics length scale was selected as it directly influences the liquid accumulation and maximum ligament breakup length.