| Breakup of fuel jet(atomization)is an important sub-process for combustion of diesel engines,direct injection gasoline engine and gas turbine engine.Many researches have been carried out on the breakup of fuel jet,and many valuable breakup mechanisms of liquid fuel have been proposed.However,due to the complexity of the atomization,the understanding of the breakup mechanism of fuel jet is still very limited,especially the breakup mechanism of the initial jet in the zone near the fuel atomizer.With the rapid development of computer technology and test methods,the complex problem of liquid jet breakup can gain new understanding.The focus of this article is the impact of cavitation and environmental gases on the jet disintegration process,revealing the mechanism of primary breakup of fuel jet.First,a visual test platform for the fuel injection atomization process was designed and built,and a transparent nozzle close to the actual size was produced.With a nanosecond level flash lamp being exposure light source,the digital camera was combined with a high-magnification,high-resolution long-distance microscope to capture high resolution atomization images.The data extracted from images was utilized to study the different fuel injection parameters,the development process of the cavitation in the transparent nozzle,the initial breakup process of the fuel jet in the region near atomizer,and the cavitation intensity in the transparent nozzle.The test images with higher resolution and sharpness were obtained,and the cavitation intensity in the transparent nozzle model was quantified.Secondly,using the OpenFOAM platform for open source software,Rayleigh kinetics equations for bubble growth are introduced.Based on the original two-phase flow model,the ambient gas phase(air)is added,and parameters such as pressure and volume fractions are obtained by solving conservation equations.Relevant codes are modified to reconstruct the mass transfer equation in the cavitation model,and a multiphase flow(three-phase flow)model is established,taking into account the effects of cavitation and ambient gas on the breakup of the jet.In addition,an energy equation is added to the control equation to examine the influence of temperature on the breakup process of the fuel jet,and the corresponding multiphase flow solver is modified.According to the data obtained from the experiments in this paper,the established multiphase flow model is verified and the good consistency is obtained.Therefore,the established multiphase flow model is reliable,and the numerical simulation results of the cavitated fuel jet are of high confidence.The numerical simulation and experimental results are compared and analyzed,which shows that the relevant theoretical and experimental studies,relevant conclusions obtained and the established cavitated fuel jet breakup model are reasonable.The numerical simulation results show that the initial breakup of the jet in the near-nozzle region is mainly caused by the effect of turbulence and cavitation in the nozzle hole,and the aerodynamic effects of the between and the ambient gas.Cavitation has an important influence on the initial breakup in the near-nozzle region.The collapse of cavitation bubbles at the nozzle outlet can generate strong disturbances to promote the surface wave growth of the fuel jet,causing the jet to breakup.The turbulence effect inside the nozzle produces the initial perturbation source of the jet in the near-nozzle region,and cavitation further enhances the disturbance on the surface of fuel jet.The aerodynamic effects between the jet and the ambient gas make the surface wave of the fuel jet more significant,and the ambient aerodynamic force can accelerate the initial and secondary breakup of fuel jet.The influence of fuel injection parameters on the breakup of the cavitation fuel jet is significant.The results showed that the increasing of the injection pressure could make cavitation appear,develop more rapidly,enhance the cavitation intensity,an promote the initial breakup of the fuel jet in the near atomizer region.The increase of ambient pressure is not conducive to cavitation,which is not conducive to the breaking of the jet.But due to the increase of ambient pressure,the density of ambient gas is increased,and the aerodynamic effect between jet and ambient gas is enhanced.The overall result is to promote the breaking of the jet,resulting in a smaller droplet of fuel.Increasing the fuel temperature can not only enhance the cavitation intensity,but also promote the breakup of fuel jet and improve the quality of fuel atomization.When the fuel temperature is increased to a certain value,"flash boiling" is more likely to occur and the quality of oil-gas mixing is greatly increased.Cavitation is one of the basic reasons for jet breakup.The collapse of cavitation bubbles at the exit of the nozzle produces strong perturbations that lead to the jet breakup and atomization.Therefore,the droplet formed on the surface of fuel jet is more fine.The effect of nozzle hole structure on cavitation and the breakup process of fuel jet is complicated.The convergent nozzle hole suppresses the formation of cavitation in the hole,while the divergent nozzle hole promotes the formation of cavitation in the hole.Reducing the nozzle length and increasing the inclination angle of nozzle hole are conducive to the formation of cavitation and the increase of cavitation intensity,which can significantly promote the jet breakup in the zone near the nozzle.Reducing the diameter of the nozzle hole is not conducive to the formation of cavitation.But the jet velocity is increased,the turbulence effect,the aerodynamic effect of the jet and the ambient gas are enhanced.So the breakup of the jet is promoted and the atomization quality is improved.Simultaneously,before the start of each injection,the initial bubble was observed in the injection hole.The initial bubble caused the formation of the “needle” liquid filament in the head of the mushroom near the nozzle exit.Its formation mechanism is explained through numerical simulation combined with visual test. |
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