Flow visualization of Taylor-mode breakup of a viscous liquid jet

We recently reported a new spray technique called ultrasound-modulated two- fluid (UMTF) atomization and the pertinent "resonant liquid capillary wave (RLCW) theory" based on linear models of Taylor-mode breakup of capillary w aves. In this article, flow visualizations of liquid jets near the nozzle t ip are presented to verify the central assumption of the RLCW theory that t he resonant liquid capillary wave in UMTF atomization is initiated by the u ltrasound at the nozzle tip, Specifically, a bright band beneath the nozzle tip was seen in ultrasonic and UMTF atomization separately, but not in two -fluid atomization. The bright band can be attributed to scattering of lase r light sheet by the capillary waves generated by the ultrasound on the int act liquid jet. As the capillary wave travels downstream in the direction o f airflow, it is amplified by the air blowing around it and eventually coll apsed into drops. Therefore, the jet breakup time can be determined by divi ding the measured band length with the capillary wave velocity. The breakup times thus determined for water and glycerol/water jets are twice the valu es predicted by the modified Taylor's model with a sheltering parameter, an d are one order of magnitude shorter than those in conventional two-fluid a tomization. Furthermore, the images of the spray in the proximity of the no zzle tip obtained by 30 ns laser pulses are consistent with the drop sizes obtained 3.3-6 cm downstream from the nozzle tip by 13 s time average of co ntinuous laser light. Also reported in this article is the good agreement b etween the measured viscosity effects on the drop-size and size distributio n in UMTF atomization and those on the relative amplitude growth rates of c apillary waves at different wavelengths predicted by Taylor's model as a re sult of its inclusion of higher order terms other than the first in viscosi ty. These new findings have led to the conclusion that UMTF atomization occ urs via Taylor-mode breakup of capillary waves; secondary atomization and d rop coalescence are negligible. Further, UMTF atomization offers a means to control the drop-size and size distribution of two-fluid atomization for u niform drop formation. (C) 1999 American Institute of Physics. [S1070-6631( 99)00406-7].