Executive Summary
The goals of this research program for the period 1991 through 1994 are: (i) to identify the fundamental mechanisms responsible for effervescent atomization, (ii) to quantify the impact that variations in the two-phase flow pattern at the nozzle exit have on the mechanisms responsible for effervescent atomization, (iii) to formulate a theoretical model for the transition from a dispersed gas in liquid system, i.e. bubbly or slug flow in the nozzle, to a dispersed liquid in gas system, i.e. spray, (iv) to develop a model describing the evolution of droplets as they propagate through a series of shock and expansion fronts.
Work during 1992-1993 focused mainly on identification of the fundamental mechanisms responsible for effervescent atomization, and on the relationship between internal two-phase flow and the mechanisms responsible for effervescent atomization. Both Newtonian and non- Newtonian fluids were considered.
For the Newtonian fluids, we were able to show that spray mean drop sizes decreases rapidly & lower air-liquid ratios because the near nozzle spray structure evolves from a sequence of single bubbles undergoing rapid expansion to an annular tree as air-liquid ratio is increased. The latter results in large caps of liquid which do not break up into small drops. We were also able to show that increasing air-liquid ratio above about 10% (on a mass basis) will lead to only a slight increase in nozzle performance because no further evolution of the annular tree structure is accomplished.
For the non-Newtonian fluids, we have shown that it is the presence of fluid viscoelasticity that degrades atomizer performance and have provided a preliminary correlation between viscoelasticity and atomization. We have also developed a performance map relating polymer molecular weight, polymer concentration, and mean drop size to serve as a rough guideline when applying effervescent atomizer to non-Newtonian systems.
We also considered spray-surroundings interactions during 1992-1993. In particular, we have measured the rate at which effervescent sprays entrain surrounding air using a direct technique. It was found that the rate of entrainment increases linearly with axial distance and depends strongly on the initial gas-to-liquid mass flow ratio (GLR). Entrainment results were correlated using a dimensionless group based on spray momentum flux. The correlation indicates that spray structure may significantly affect entrainment behavior. The correlation may be used to predict entrainment in effervescent atomization of low viscosity fluids.