Experimental investigation of primary breakup in close-coupled gas atomization

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, $\theta$ = 14${}^o$, 24${}^o$ and 34${}^o$ were used. Measurements were conducted at two Weber numbers, We${}_g$ = 57.5 and 82.5. At each We${}_g$, 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 $\theta$. Furthermore, with increasing $M$, peak probabilities of interface lengths shifted to larger values while increasing $\theta$ 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 $\theta$ = 24${}^o$ for $M$ = 1.67 and 2.40 for We${}_g$ = 57.5 and 82.5, respectively. The percentage of circular droplets (based on a circularity-based threshold) was highest at $\theta$ = 14${}^o$ and decreased with increasing $M$.