Cavitation in a Two-Dimensional Nozzle and Liquid Jet Atomization (2nd Report, LDV Measurement of Liquid Velocity in a Nozzle)

Abstract

Cavitation and internal flow in the nozzle of a liquid injector are known to affect the atomization of a discharged liquid jet. In order to obtain the knowledge on the mechanism how cavitating flow in a nozzle enhances the liquid jet atomization, liquid velocity distributions of cavitating flows in a two-dimensional transparent nozzle were measured using a Laser Doppler Velocimetry (LDV) system. As a result, the following conclusions were obtained : (1) The inception of cavitation occurs near the outer edge of the separated boundary layer (SBL), where the mean local velocity takes the highest value and the mean pressure is lower than the vapor saturation pressure. (2) When the cavitation number σ≥0.78 (No Cavitation or Developing Cavitation), the reattachment of SBL occurs in the middle of the nozzle. A large velocity fluctuation, which appears in and just downstream of SBL, decreases near the nozzle exit. This may be the reason of the wavy jet. (3) Whenσ≤0.65 (Super Cavitation), the lateral flow from the core region toward the side walls just upstream of the nozzle exit may be the major cause of the increase in spray angle and a drastic promotion of liquid jet atomization. The strong turbulence just upstream of the exit may also play a role on the formation of ligaments at liquid jet interface.