This paper presents the results from a number of studies conducted in an effort to gain insight into how to control bubble size during gas injection through a porous media into a liquid cross-flow, what effect bubble size has on the spray characteristics from an effervescent atomizer, and to provide input for future effervescent atomizer designs and studies. Experiments were performed in a specially designed atomizer which allowed for manipulation of the air injector geometry in order to vary bubble size from sizes much smaller than the nozzle exit diameter to much larger than the exit diameter.
A parametric study was conducted to examine how three different bubble control mechanisms affect the average bubble size, bubble size standard deviation, and gas to liquid mass flow ratio (GLR) at the transition point between bubbly flow and slug flow. It was found that changing the channel hydraulic diameter at the air injection site had the largest effect while pore size and electrolyte concentration had smaller, though still significant, effects. A dimensional analysis was performed which arrived at a similarity parameter which correlates to bubble size for air injected through a porous media into a cross-flow.
Bubble size was seen to have an effect on the stability, spray half cone angle, and Sauter mean diameter of the liquid droplets produced by an effervescent atomizer. The effect of bubble size on improving spray characteristics was shown to be optimal for bubble sizes on the order of the exit diameter. The mechanism by which bubble size has an affect is suggested to be due to liquid velocity fluctuations at the exit of the nozzle as opposed to the often cited mechanism of causing the flow to choke thus allowing gas to expand beyond the exit. A comparison of bubbly and annular flows at identical conditions further suggests that effervescent atomizer design and operation may benefit from trying to produce annular flow conditions at low GLRs rather than bubbly flow conditions at high GLRs.