An experimental study was conducted to examine the effect of the flow patterns inside the mixing chamber on the atomization performance of the effervescent atomizers. The mixing chamber has the rectangular cross section (8 mm × 2 mm) and made of transparent acrylic plates for flow visualization. The parameters considered were the air/liquid ratio(ALR), injection pressure, and the nozzle orifice diameter.
Three different flow regimes were observed: bubbly, annular and intermittent flows. In the bubbly flow regime, the drops are formed through the processes of the bubble expansion and the ligament breakup. On the other hand, in the annular flow regime, the liquid annulus is disintegrated into fine drops by the aerodynamic force exerted by the core gas flow. In the intermittent flow regime, the bubble-expansion/ligament-breakup and the annulus-disintegration modes appear alternatively.
The correlations representing the transition criteria between the flow regimes were proposed based on the drift flux model. The bubbly-to-intermittent flow transition was well predicted by determining the maximum void fraction for the stable bubbly flow. Due to the entrance effect, the maximum void fraction turned out to be 0.37 in the present case. The criterion of the intermittent-to-annular flow transition was determined from the force balance between the shear stresses at the wall and the interface for the vertical liquid annulus. The proposed correlations for the flow regime transitions agree satisfactorily with the experimental results.
At the same time, the drop sizes were measured by using the image processing technique. Different drop size correlations were proposed pertinent to each flow regime. In the bubbly flow regime, it was assumed that a portion of the mechanical energy concerned with the bubble expansion was consumed to atomize the two-phase mixture. On the other hand, in the annular flow regime, a correlation type for the airblast atomizers was adopted but with taking account of the liquid film thickness and the relative velocity between the phases. For the intermittent flow regime, the correlations for the bubbly and the annular flow regimes were interpolated with a weighting factor as a function of ALR taken into account. The proposed correlations represent the experimental results mostly within ±20%.
