The Study On Cavitating Pattern Characteristics And Their Effects On Pressure Fluctuations And Spray In The Diesel Nozzle

For a long time,the cavitating two-phase flow characteristics in the injector nozzle has been widely concerned by scholars.The focus of the attention is mostly on the geometry-induced cavitation with irregular patterns,but the research on the string cavitation induced by vortex is rarely mentioned.At the same time,the research conditions involved in two cavitating patterns are mostly the nozzle geometric parameters,and the effects of dynamic factors such as fuel temperature,pressure fluctuations and ventilation contents are rarely reported.Based on this,a large number of experimental studies have been carried out on geometry-induced cavitation and string cavitation characteristics of the nozzle and their effects on spray based on the visual test rig of scaled-up transparent nozzles and spray.The main research contents are as follows:The development of near-field geometry-induced cavitation was weaker than that of vortexinduced string cavitation at different fuel temperatures.The string cavitation at the nozzle outlet was easy to incept at higher temperature,but it became difficult to develop to the nozzle inlet when the injection pressure increased.With the increase of fuel temperature,the viscosity decreased and the frequency of cloud cavitation shedding increased.It was found that the increase in fuel temperature could also promote the development of string cavitation and the stability of the super-cavitation state.In addition,due to the micro scale of the nozzle,there was a strong interaction and influence between the geometry-induced cavitation and the hole-to-hole string cavitation.The hole-to-hole string cavitation in the nozzle's entrance was easily interrupted by the geometry-induced cavitation,and the cloud shedding phenomenon of geometry-induced cavitation could also break the string cavitation break.The effects of undissolved single bubble and aeration content on cavitating development in the nozzle.Cavitating inception occured on the wall of the undissolved single bubble after it entered the nozzle,which was promoted by the fuel temperature increasing.Compared with the former,the string cavitation was more stable.In the study of ventilation,it was found that too much ventilation would destroy the structure of string cavitation.During the single injection,when there was no ventilation in the hole,the hole-to-hole cavitation was intermittent,and it was difficult to maintain a stable state.With the increase of ventilation,the intensity of hole-to-hole string cavitation increased and tended to be stable.However,for the hole-to-hole string cavitation in SAC,it was the most stable in the case of no ventilation.With the bubble group content increasing,the position of string cavitation changed greatly,the bubble group content increased to a certain extent,and the string cavitation had bypassed the tip of needle valve.For the needle-originated string cavitation,a small number of bubble groups had no obvious effect on its shape due to the strong eddy current.When the needle valve was eccentric,the cavitation morphology in the two holes would be significantly different,and the stability of the string cavitation morphology in the orifice on the side of the needle valve was poor.It was also found that the bifurcation string cavitation in the SAC cavity was a pair of non-isotropic string cavitation induced by a pair of vortices.In this paper,the bubble was used as a special "tracer particle" for the first time to capture the "cloud cluster" in the SAC cavity of the nozzle.The pressure fluctuation increased significantly during hole-to-hole string cavitation changing to needle-originated string cavitation.Due to the instability of cavitating pattern,the string cavitation at low pressure could be easily transformed into geometry-induced cavitation,and the disturbance of spray cone angle was enhanced.The cavitation phenomenon of high injection pressure was more intense,and the pressure fluctuation became stronger.Under the same injection pressure,although the string cavitation intensity was smaller than the geometryinduced cavitation intensity,the pressure fluctuation amplitude and the spray cone angle were larger,and the peak pressure of thestring cavitation was higher than the peak pressure of the geometry-induced cavitation.The pressure fluctuation amplitude increased significantly during the disappearance,rupture and fusion of string cavitation.It was concluded that the main factor affecting the pressure fluctuation was the "transient change process" of cavitating pattern and intensity.This conclusion had a guiding role for the effective control of pressure fluctuation in the injection system.The spray characteristics corresponding to the geometry-induced cavitation and string cavitation were contrasted based on the PIV particle image velocimetry system and a slit plate.The distribution of the spray velocity field was closely related to the cavitating phenomenon in the nozzle hole.The distribution of the spray velocity field corresponding to the two cavitating patterns was obviously different.At the same time,the needle valve lift and injection pressure caused significant differences in the flow and cavitating pattern of the nozzle,which would eventually affect the final spray pattern.In this paper,the development of two cavitating patterns,the geometry-induced cavitation and string cavitation was revealed in high injection pressure and small-size hole by using the internal flow and spray visualization testbed of scale-up transparent nozzle.It was clarified the interaction characteristics between fuel temperature,pressure fluctuations,internal structure of spray,aeration content and the cavitating pattern,and new progress achievements have been made.The data obtained in the experimental research are abundant and reliable,which can provide real and effective data support for the future cavitating model and the improvement of spray atomization mechanism,and provide a new theoretical background for the design of the new generation of high fuel pressure and micro injector.