| With the development of today’s society,automotive industry technology has made great progress.In order to meet the requirements of diesel engine combustion and emission regulations in the future,common rail injection system has gradually become the mainstream.As an important part of fuel injection system,fuel injector plays an important role in diesel engine combustion and spray optimization.When the fuel flows through the nozzle orifice inlet,affected by its geometry,the fuel static pressure decreases rapidly and cavitation occurs.The cavitation bubble produced by cavitation collapses rapidly in the high pressure region,which will produce shock wave and micro jet in the local fluid near the collapse point.It will make the nearby solid surface bear high local transient surface stress,and the repeated pressure caused by a large number of cavitation collapse will lead to the separation and falling off of the material on the inner surface of the nozzle hole.Then cavitation erosion and wear at the nozzle will occur,which will cause the direct failure of the nozzle in the past for a long time.Therefore,it is of great significance to analyze the cavitation flow and cavitation risk in the nozzle.In this paper,the cavitation phenomenon and cavitation erosion in the nozzle hole of diesel fuel injector are studied.Firstly,based on the previous research theory of the effect of cavitation collapse on the wall,The jet velocity and pressure of cavitation bubble acting on the wall under different near wall distances are fitted by Matlab software（considering the pressure caused by the direct impact of micro jet on the wall and the pressure caused by the shock wave after bubble collapse respectively）.Combined with the traditional empirical formula,the empirical formula of multi-layer cavitation impact considering different distances from the inner wall of the jet hole is integrated and deduced,At the same time,the mathematical expression of the distance between the bubble in the three-dimensional cylindrical and conical orifice and the wall of the orifice is deduced and integrated into the above improved empirical formula to establish a set of front and back correlation suitable for the cavitation flow in the nozzle orifice to the risk of cavitation erosion on the inner wall of the orifice,Based on the improved empirical formula,the transient characteristic analysis model of cavitation flow and cavitation risk in diesel nozzle orifice is established.The mature Rrscavitation risk prediction model verified by experiments is introduced into Fluent software,and the new model proposed in this paper is verified by Rrscavitation risk prediction model and cavitation fatigue test results.The results show that the location trend of cavitation damage is basically the same,and both models are prone to cavitation damage at the entrance of the jet hole.It can prove the reliability of the new model built in this paper.Secondly,according to the basic parameters of the nozzle studied in this paper,the grid adaptive algorithm and dynamic grid technology in fluent software are used to simulate the process of oil film change under the movement of nozzle needle valve,and the improved empirical formula is compiled with UDF module in the software,The model related to the cavitation flow in the nozzle orifice and the cavitation risk prediction of the inner wall of the orifice（the cavitation flow and cavitation risk analysis model）is established.The compiled cavitation flow and cavitation risk analysis model are simulated and calculated by Fluent software The cavitation flow and cavitation risk transient characteristics of the fluid in the orifice under different working conditions are studied.The cavitation flow distribution,bubble collapse impact area distribution,bubble collapse micro jet velocity,bubble collapse micro jet impact water hammer pressure and pulse pressure caused by shock wave after bubble collapse are calculated respectively.The results show that the pulse pressure caused by shock wave is significantly higher than that caused by micro jet impact,and both are of the same order of magnitude,which should not be ignored in the analysis.With the increase of orifice conicity,cavitation is significantly restrained,and the pressure impact caused by the impact area on the wall and the impact of micro jet decreases significantly.With the increase of diameter,the cavitating flow in the orifice is obviously restrained,the impact area on the wall decreases,and the pressure impact caused by micro jet impact increases slightly.With the increase of orifice length,cavitation is restrained,and the pressure impact caused by the impact area on the wall and the impact of micro jet decreases.With the increase of the maximum needle valve lift,the cavitating flow area in the orifice is restrained,the impact area on the wall decreases,and the pressure impact produced by the micro jet impact has little change.Finally,combined with the nozzle head shell,a simple nozzle cavitation life prediction model is established.The nozzle cavitation life prediction model is established by using the transient structural module and Ncode module in Anasys software.Combined with the above cavitation flow and cavitation risk transient characteristics,the impact pressure calculated by the model is analyzed,Two kinds of loads are used to load,which are only considering the pressure caused by the direct impact of micro jet on the wall and considering the pressure caused by the direct impact of micro jet on the wall and the shock wave after bubble collapse.Their effects on the cavitation life of nozzle orifice are compared.By comparing the effects of two different analysis methods,which only consider the dimensionless distance between the bubble and the wall as a constant and consider the impact of the bubble on the wall under the multi-layer distance,on the cavitation life of the nozzle orifice,the more suitable method for the cavitation life analysis of the wall is obtained by comparing the calculation results.The life of the inner wall of the nozzle orifice under different working conditions is simply evaluated,and the influence of different working conditions on the life of the inner wall of the nozzle orifice is explored.The results show that ignoring the pressure effect caused by shock wave after bubble collapse will overestimate the cavitation life of nozzle orifice.Ignore the distance from the orifice wallδ≠1 the impact of bubbles on the wall will affect the cavitation life of the nozzle head.If the cavitation life of the nozzle head is analyzed,only the distance from the wall is consideredδ≠1,the estimated cavitation life will be too large.The change of nozzle hole taper and nozzle hole length has a great impact on the cavitation life of nozzle hole.With the increase of nozzle hole taper,the cavitation life of nozzle hole is significantly improved,and the life is increased by 21.76%and 72.41%respectively.With the increase of nozzle orifice length,the cavitation life of nozzle orifice is greatly improved.The service life was increased by7.54%and 19.03%respectively.Compared with the change of nozzle hole taper and nozzle hole length,the change of nozzle maximum needle valve lift and nozzle hole diameter has less effect on the cavitation life of nozzle hole. |
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