Spray characteristics of free air-on-water impinging jets

Abstract

Characteristics of a water-on-air free impinging jets atomizer is investigated in this study by means of flow visualization using high speed photography with Phase Doppler Anemometry (PDA) to measure the droplet size and velocity. Spray structures and breakup process are illustrated with the aid of images captured for the water and air jets impinging at 45°. The breakup length of the water jet decreases with the increase of the air to liquid jet momentum flux ratio (ALMFR) and remains constant for values of ALMFR larger than 1. Divergence and deflection spray angles increase rapidly with the air to liquid momentum ratio (ALMR) and then remain constant for values of ALMR larger than 4. A larger impinging angle leads to a smaller breakup length and larger spray angles. PDA results indicate that the planar distribution of droplet size is symmetrical around the Y-axis, but not around the X-axis. Smaller droplets are located near the spray center, but their location varies for different experimental conditions, with the minimum value of D32 = 50 µm and increasing to around 120 µm at the outer region of the spray for conditions Q L = 100 mL/min, m˙g = 13.5 g/min and θ = 45°. The spatially-averaged Sauter mean diameter (SMD), representing the average size of droplets over a cross section plane of a spray, is defined and it remains the same at any cross section of the spray operating with the same experimental conditions. Spatially-averaged SMD is found to decrease with the increase of ALMR. Droplet mean velocity is the largest at the position downstream of the air jet exit (14 m/s at a plane of z = 75 mm in the spray with Q L = 100 mL/min, m˙g = 13.5 g/min and θ = 45°) and decreases gradually with increasing distance from the point where droplets with the maximum velocity are located. The study makes up for the spray visualization of the study of a single water jet impinging on a single air jet externally, and provides more information on the spray characteristics of this injector, which will contribute to the evaluation of improved computational models and improved injector design.