Experimental Investigation of the Effect of the Internal Two-Phase Flow and the Bubble Size on the Effervescent Spray Characteristics

Abstract

Reducing the pressurized atomizing gas in the effervescent atomizers with keeping fine spray is a desirable aim. This experimental study focuses on improving the characteristics of the effervescent spray (i.e. drop size and spray cone angle); while an "in-out" gas injection configuration atomizer operates under low gas-to-liquid ratios (GLR). These important characteristics are mainly affected by the internal two-phase flow pattern. Wherefore, shadowgraphy is utilized for visualizing and investigating the internal two-phase flow pattern directly upstream the exit orifice. The atomizer is operated at air gauge pressure of 0.6 Mpa and GLRs of (0.71%-5.74%). For controlling the internal flow at the operating GLRs, two novel bubble breakers (BB) are designed and tested. The bubble breakers are fixed in the mixing chamber to affect the bubble size. The shadowgraph technique is utilized for visualizing and quantifying bubbles inside the atomizer, spray characteristics and the near nozzle spray. From the visualization and results, the internal two-phase flow patterns inside the mixing chamber vary from bubbly flow at the lowest three GLRs to a bubble-annular flow at the largest GLR, passing through an unsteady transition stage between these two patterns. Also, it is observed that the bubble breakers efficiently disintegrate large bubbles; the largest reduction percentage in the bubble size is 75.63% for the case of flat end BB at GLR of 2.21%. Also, the results showed that the drop size and spray cone angle are strongly affected by the bubble size (i.e. the internal flow structure) subsequently the presence of the bubble breakers. The largest reduction percentage in the droplet size is 68.88% for the case of the cone end BB at GLR of 0.77% and the largest increment percentage in the spray cone angle is 69.97% for the cone end BB at GLR of 0.77 %, all relative to the case of without BB.