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

Abstract

Cavitation in nozzles of liquid injectors is known to affect the atomization of a discharged liquid jet. To understand how cavitating flow in a nozzle enhances the liquid jet atomization, liquid velocity distribution of cavitating flow in a two-dimensional transparent nozzle was 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 separated boundary layer (SBL), where the time-averaged local velocity takes the highest value and the time-averaged pressure is almost equal to the vapor saturation pressure. (2) When the cavitation number σ is greater than 0.78 (in no cavitation and developing cavitation regimes), the reattachment of SBL occurs in the middle of the nozzle. A large velocity fluctuation, which appears just downstream of SBL, decreases near the nozzle exit. Hence the wavy jet is formed in these regimes. (3) For σ ≤ 0.65 (in super cavitation regime), the lateral flow directing from the core region toward the side walls just upstream of the nozzle exit is a major cause of the increase in the spray angle and drastic enhancement of liquid jet atomization. The strong turbulence just upstream of the exit must play an important role in the formation of ligaments on liquid jet interface.