THE ROLE OF CAPILLARY WAVES IN 2-FLUID ATOMIZATION

A mechanistic study of two-fluid atomization has been carried out usin g a new spray technique called ultrasound-modulated two-fluid (UMTF) a tomization. This technique is based on resonance between the liquid ca pillary waves generated by ultrasound and those generated by high-velo city air. Specifically, capillary waves are established on the surface of a liquid jet as it issues from a coaxial two-fluid atomizer, the n ozzle tip of which vibrates at the same frequency as the ultrasound wh ile the frequency of the capillary waves is only half of the ultrasoun d frequency. As these capillary waves travel downstream in the directi on of air flow, their amplitude is further amplified by the air flowin g around them. Atomization occurs when the wave amplitude becomes too great to maintain wave stability; the resulting drop sizes are proport ional to the wavelength of the resonant capillary waves which is deter mined by the harmonic frequency of the ultrasound in accordance with t he Kelvin equation. Theoretical calculations of the amplitude growth r ate are based on two models of temporal instability of wind-generated capillary waves: Taylor's dispersion relation and Jeffreys' one-parame ter (sheltering factor) model. Good agreements between the theoretical predictions by these models and the experimental results of how drop- size and size distributions are influenced by air velocity and surface tension led to the conclusion that Taylor-mode breakup of capillary w aves plays a very important role in two-fluid atomization. Furthermore , all peak drop diameters can be accounted for by the harmonic frequen cies of the ultrasound. Hence, it is further concluded that secondary atomization is negligible in co-flow two-fluid atomization of a water jet at air velocities up to 170 m/s and air-to-water mass ratio up to 5.6. In addition, uniform drops with diameters predetermined by the ul trasound frequency can be accomplished by adjusting the air velocity. (C) 1997 American Institute of Physics.