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.

Study of cavitation phenomenon using different fuels in a transparent nozzle by hydraulic characterization and visualization

“…Being a circular orifice instead of flat as in other studies [9], the images in Figure 16 show the cavitation around the whole wall of the orifice. Thus the cavitation observed in the central part of the orifice represents the cavitation from both the front and back of the orifice.…”

“…However, cavitation phenomenon has been revealed to play an important role in high pressure systems; so, other researches [8][9][10][11][12][13][14][15][16][17][18] point to the visualization of the flow inside the nozzle with the goal to observe the cavitation phenomenon and its influence in the spray development. Thereby, different nozzle models have been developed with optical materials that allow the visualization inside the hole such as quartz and methacrylate.…”

“…Some studies were performed with planar nozzles in order to better see the cavitation [8][9][10][11] and others with large-scale cylindrical nozzle [12][13][14][15] to facilitate the visualization of the phenomenon; however, there are few researches which focus on studying experimentally the phenomenon of cavitation in diesel injection orifices of smaller scale or real size [15][16][17][18]. With all this knowledge and using a small scale model and different fuels, this work targets to link the mass flow collapse with the cavitation inside the orifice and to define a new cavitation criterion.…”

Effect of geometry of real-size transparent nozzles on cavitation and on the atomizing jet in the near field

“…A significant difference in inlet edge radius was found even if the design dimensions were the same. Non-intrusive optical diagnostics are able to provide the velocity information both for the flow inside the nozzle [4,14,15,16] and for the downstream spray [17,18]. In their work, Moon et al [18] argued that the sharp inlet edge induced a higher velocity gradient than the round inlet edge in the transverse direction.…”

A Technique to Match the Refractive Index of Different Diesel Fuels with the Refractive Index of Transparent Materials to Improve the Experimental Visualization

“…However, other researches [5][6][7][8][9][10][11] point to the visualization of the flow inside the nozzle with the goal to observe the cavitation phenomenon and its influence in the spray development. In these researches, different nozzle models have been developed with optical materials that allow the visualization inside the hole such as quartz (fused silica, SiO2) and methacrylate.…”