| The development of high power density and high speed is an important way to realize energy saving and emission reduction of diesel engines and adapt to variable environments.This also puts forward higher requirements for the injection pressure and atomization control of the fuel supply system of diesel engines.At present,the maximum injection pressure of electronically controlled common rail injectors has increased from 160～180MPa to 250 MPa,and will reach 300 MPa in the future;however,while enjoying the uniform spray and sufficient combustion brought by the pursuit of high pressure parameters.It is also inevitable that new challenges are presented to the stability and reliability of key components such as spool and nozzles.And the cavitation and erosion caused by the rapid pressure drop formed by the position of the variable flow cross-section of these components is one of the typical problems.According to the theoretical analysis and actual failure investigation of such problems,the description of the dynamic evolution behavior of cavitation development is the key to the analysis.Therefore,in order to explore the transient behavior of cloud cavitation shedding in the injection nozzle,this paper obtains the evolution law of cavitation flow inside the injection system from two aspects: numerical simulation and visual observation.In this paper,based on the flow characteristics of the internal flow channel of the diesel injector nozzle,a visual observation model of the asymmetric nozzle structure is designed,and a visualization experiment table is established to capture the transient characteristics of the developing,shedding and collapsing of cavitation cloud generation.At the same time,the data image method was used to analyze the data of the nozzle flow channel,and finally combined with the large eddy simulation method to carry out model verification and numerical simulation analysis of the cavitation cloud shedding of the designed flow channelIn terms of numerical simulation,the interaction relationship between cavitationvortex-pressure fluctuations was explored and verified by experiments.Through the analysis of the relative vorticity transport equation describing the characteristics of the cavitation cloud,it can be seen that the relative vortex stretch term and the relative vortex expansion term have a great influence on the shedding of the cavity.The peak of the intensity of the pressure fluctuation appears at the location where the cloud cavitation collapses behind the cavitation cavity.At the same time,it is also found that there is a phase difference between the vapor volume fraction,vorticity and pressure fluctuations.The change of the gas volume fraction is the main cause of the cavitationvortex-pressure interaction.In terms of experiments,the principal component analysis method is used to decompose the cavitation cloud shedding process captured by the visualization experiment.The results show that the flow field after time-space decoupling reconstruction can clearly show the space distribution of the cavitation cloud,and can show the location of re-entrant jet breakdown,cavitation cloud shedding and collapse.The analysis of the cavitation cloud shedding frequency according to the extracted time evolution sequence shows that the cavitation cloud shedding frequency has the characteristics of increasing with the increase of inlet and outlet pressure and temperature. |
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