Experimental Study of Liquid Spray Mode of Twin Fluid Atomizer Using Optical Diagnostic Tool

Abstract

Twin fluid atomizers allow for two different spray forming modes, flow focusing and flow blurring, depending on the operating, geometric, and thermophysical properties of the working liquids. In flow focusing mode, the liquid jet breaks outside the injector, whereas in flow blurring mode, the liquid jet breaks inside the atomizer. Operating conditions are believed to play an essential role in determining the size and velocity of droplets with specific geometric and fluid properties. This work investigates the effect of air flow rate and liquid flow rate on spray characteristics by varying them independently. The geometric parameters, height (H = 0.3 mm) and orifice diameter (D = 1.5 mm) of the atomizer are kept constant (with H/D = 0.2). Spray mode is identified using high-speed images. Phase Doppler particle analyzer is used to measure droplet size and velocity simultaneously. Regardless of liquid flow rate, flow focusing mode is observed at low air flow rate and flow blurring mode is observed at high air flow rate. In general, flow blurring mode produced smaller droplets at higher velocities than flow focusing mode. The Joint probability distribution function of droplet size and velocity shows unimodal near the exit of the atomizer and bimodal distribution around 50 mm downstream due to the completion of the secondary breakup around that position and the subsequent loss of momentum due to the resistance of the surrounding air from the injector outlet. Flow focusing mode showed that sauter mean diameter and mass median diameter tend to increase initially due to coalescence of the spray and begin to decrease due to secondary breakup. The joint probability distribution of droplet Weber number and the droplet Reynolds number indicate that the flow blurring spray produced finer and faster droplets than the flow focusing spray. Overlapping of two Gaussians representing smaller and larger droplets best fits the droplet probability distribution as compared to lognormal or gamma or beta distribution.