Experimental Investigation of Choked Flow Conditions for Bubbly Flow

In an effervescent atomizer, a bubbly two-phase mixture flows through a convergent section before exhausting from an exit orifice. It is commonly believed that one of the key effects of including bubbles is in the substantial decrease in the speed of sound experienced by the two-phase flow allowing for choked flow conditions at the exit. The existence of choked conditions would result in under-expanded bubbles that would further expand upon exiting the atomizer and provide additional forces to aid in the break-up of the bulk liquid into droplets. This study examines how the homogenous two-phase flow model of speed of sound, and thus critical conditions, compare with experiments in order to better understand the fundamental physics of effervescent atomization. In these experiments, an effervescent atomizer is connected to a vacuum chamber allowing for internal atomizer pressure, liquid flow rate and air flow rate to be monitored as the post-exit pressure is decreased. Experiments reveal that the flow remains subcritical well beyond conditions that the homogenous flow theory might predict being choked. High-speed imaging is used to capture internal atomizer bubble size.