Experimental investigation of primary breakup in close-coupled gas atomization

Abstract

Primary breakup of a liquid water jet in close-coupled gas atomization (CCGA) was studied using digital inline holography. Different nozzles with constant liquid protrusion length and characterized by three different apex angles, θ = 14o, 24o and 34o were used. Measurements were conducted at two Weber numbers, Weg = 57.5 and 82.5. At each Weg, five different momentum flux ratios, M, were studied. A detailed analysis of the instantaneous liquid jet interfaces indicated that the “filming” occurred at a lower critical M with increasing θ. Furthermore, with increasing M, peak probabilities of interface lengths shifted to larger values while increasing θ led to increased maximum lengths. Fractal dimensions increased with downstream distance. Distributions of area-based droplet diameters spanned a broad size range up to 3 mm and were well described by least-squares fitted power laws, including an exponential cut-off. The highest number of droplets was generated at θ = 24o for M = 1.67 and 2.40 for Weg = 57.5 and 82.5, respectively. The percentage of circular droplets (based on a circularity-based threshold) was highest at θ = 14o and decreased with increasing M.