TURBULENCE FLOW CHARACTERISTICS IN A SPECIAL ATOMIZER

A special atomizer was developed to study the effects of gas turbulence on atomization. This paper presents the flow characteristics produced by the special atomizer as well as breakup and drop size measurements. The special air assist atomizer can produce a variety of turbulence intensities while maintaining a constant mean velocity in both gas and liquid streams. The atomizer consists of four transverse flow tubes mounted around the periphery of the outer tube, perpendicular to the main flow direction. Flow visualization studies were made in transparent glass nozzles using water instead of air. Nozzles geometry and liquid velocities were carefully scaled to achieve Reynolds numbers similitude between the atomizer and the glass nozzles. A tracer dye, a stroboscope, and a CCD camera were used to study the flow structures in the region of interaction between the axial and the transverse streams. The study shows that initially large coherent vortex rings propagate along each one of the transverse jets. The propagation rate of vortices increases with the total flow rate and decreases as the axial to transverse flow ratio increases. Due to further interaction with the other streams, the vortices break down into smaller and smaller structures while maintaining traces of the original structures which increase in size along the nozzle axis. As expected it was found that the axial location of impingement of the transverse jets with each other is dependent on the axial to transverse flow ratio but insensitive to the total flow rate. Flow characteristics of the air jet generated by the special atomizer are presented. Mean and rms velocities, shear Reynolds stress, and turbulence time scale were measured and correlated with the flow properties inside the glass nozzles. Finally the effect of the gas flow characteristics on the breakup and atomization of liquid jets is demonstrated.