Executive Summary
1991-1992 began the first year of our three year study into the fundamental mechanisms responsible for effervescent atomization. Three issues were addressed during this period.
Early in the year we focused on spray behavior in the transition region where the two- phase flow that exits the nozzle as discrete gas bubbles in a continuous liquid is transformed into a continuous gas stream containing discrete liquid drops. Single-pulse holography was employed to observe the liquid breakup phenomena characteristic of effervescent atomization. A qualitative explanation of the mechanisms responsible for effervescent atomization is presented, based on this data. A quantitative model allowing calculation of spray mean drop size from nozzle geometry, operating conditions, and fluid rheology will be developed this year.
At mid-year we broadened our efforts to include an investigation of the interactions between the-spray and its surroundings. In particular, we were interested in methods for improving the already superior energy efficiency of effervescent atomizers by minimizing the impact of the major energy loss mechanisms. A computational study was therefor performed to identify these major loss mechanisms and suggest methods for reducing their impact. We discovered that turbulent dissipation was the largest loss, followed by transformation of bulk kinetic energy into turbulent kinetic energy, and then entrainment of surrounding air. We concluded it was unlikely that turbulent dissipation and transformation of bulk kinetic energy into.turbulent kinetic energy could be reduced. However, entrainment might be minimized by forming a more uniform distri- bution of smaller bubbles in the two-phase jet as it exits the nozzle. We will investigate spray- surroundings interactions more fully in 1992-1993.
Our most recent efforts have focused on the relationship between fluid rheological properties and spray mean drop size. This work was motivated by our previous study into the influence of polymer addition on nozzle performance. One result of that study was the conclu- sion that spray mean drop size was independent of changes in either consistency index or flow behavior index for a power law fluid. This year’s results show that it is fluid viscoelasticity that degrades nozzle perforce. We will next develop a quantitative model that describes the performance of an effervescent atom&r when operating with viscoelastic fluids.