| Spark-Ignition Direct-Injection(SIDI)engines are widely used in nowadays vehicles due to its fast transient response and improved fuel economy.However,as the emission regulation becomes more and more stringent,and the SIDI engines become miniaturization and with high compression ratio,the emission issue of SIDI engines is severer.It is documented that flash boiling spray can produce fine atomized fuel spray even at low injection pressure,which is a potential method to reduce particle emission and improve fuel economy of SIDI engines,especially under cold start conditions.However,the flash boiling fuel spray is extremely complex with significant phase transition,its atomization and evaporation mechanisms are unclear.In addition,the flash boiling spray geometry transforms significantly with the superheat degree varies.Thus,flash boiling spray is far from being properly applied in real SIDI engines.In this research,in-nozzle superheated flow is studied firstly for investigating the effect of superheat degree on in-nozzle fuel evaporation via a specially designed optical nozzle.The influence of in-nozzle fuel evaporation on near nozzle jet breakup is studied as well.Secondly,in order to investigate the primary breakup characteristics of subcooled and flash boiling sprays,near nozzle spray characteristics of various injectors are analyzed.Finally,macroscopic spray structures of many GDI injectors are studied.By combining the in-nozzle fuel characteristics,near nozzle spray characteristics,and macroscopic spray structures,the mechanisms of rapid atomization an evaporation of flash boiling sprays are revealed,and the spray collapse mechanisms of various injectors are illustrated as well.For visualizing superheated in-nozzle flow,a special two dimensional transparent slit nozzle is designed and fabricated,of which the thickness is only 0.04 mm.With such thin slit,it can enhance the visibility of tiny bubble inside the nozzle by eliminating the overlapping bubbles,and at the same time increasing the bubble surface area by squeezing small bubbles into larger ones.A fuel temperature control system is specially designed to heat the fuel,which ensures the superheated thermal state of fuel flowing the nozzle.Both internal flow and near nozzle jet can be visualized by high speed microscopic backlit imaging technique,including the flow at the nozzle inlet and exit.In addition,the nozzle configuration,such as nozzle length,inlet corner radius,can be adjusted easily to investigate the effects of nozzle configuration on internal flow and near nozzle spray.Experimental results shows that fuel already evaporates inside the nozzle under flash boiling conditions producing bubbles along the nozzle inside wall.In-nozzle fuel evaporation is enhanced under stronger superheat conditions producing more and bigger in-nozzle bubbles.The initiation position of in-nozzle bubbles varies under different superheated conditions,and stronger superheat degree results in the initiation position further away from the nozzle exit.These in-nozzle bubbles move towards the nozzle exit with flow and have the tendency to move to the central part of nozzle.As soon as these bubbles are discharged into the ambient conditions,and the constraints of nozzle wall disappear suddenly,these bubbles would rupture instantaneously,which produces strong force leading to faster disintegrate process of liquid jet.Therefore,fuel plume expands significantly near the nozzle exit under flash boiling conditions.It can be concluded that flash boiling atomization is governed by in-nozzle fuel evaporation and outside nozzle fuel boiling.For the primary breakup process,in-nozzle flash boiling plays the most vital role.Outside nozzle fuel boiling governes the macroscopic fuel atomization and evaporation.On the other hand,nozzle configuration influences the internal flow significantly.In this research,the effects of nozzle length and inlet corner radius on internal flash boiling are investigated.The relationship between cavitation and flash boiling are analyzed as well.Cavitation generates due to sudden pressure drop at the nozzle inlet corner,thus pressure inside the cavitation bubble is smaller than the static pressure of main fluid.Cavitation bubbles will disappear after vena contracta disappears due to pressure increase.However,flash boiling bubbles generate near the nozzle exit due to regular pressure decrease,and pressure inside the flash boiling bubbles is larger than the main fluid.The size of these bubbles will increase,and bubbles will rupture,which leads to enhanced fuel atomization.In order to investigate the primary breakup characteristics and mechanisms of flash boiling sprays,high speed microscopic backlit imaging method is applied to capture the magnified near nozzle spray images.The results unveil the effects of nozzle configuration and nozzle number on near nozzle flash boiling spray characteristics,which provides essential information to analyze the primary breakup process of flash boiling sprays.Under flash boiling conditions,fuel can atomize quickly soon after it is injected out of the nozzle resulting in wider near nozzle fuel plume.Thus,adjacent fuel plumes interact with each other due to compact near nozzle area.Whether the near nozzle interaction will continue or disappear is dependent on many factors,of which the superheat degree and nozzle configuration are the most important ones.On the other hand,flash boiling spray can significantly improve the end of injection performance producing well atomized fuel drops instead of large ligaments and droplets.The injector deposit issue caused by residual fuel in the nozzle can be solved.What’s more,high exhaust emission resulted from uncompleted combustion of large drops produced at end of injection can be reduced remarkably.Because under superheated conditions,innozzle bubbles generate along the nozzle wall near the nozzle exit,which separates the liquid fuel from the nozzle wall,and liquid fuel doesn’t contact the nozzle wall.Thus,it can be hypothesized that nozzle deposit caused by residual fuel trapped in the nozzle,or fuel adhered to the nozzle wall,can be reduced if flash boiling spray is utilized.Finally,flash boiling spray characteristics of a 1-hole injector and a 6-hole injector are studied systematically.In order to reveal the geometry transformation process of flash boiling spray,various injectors with different nozzle number will be investigated.The results indicate that interaction between adjacent fuel plumes is the main reason leading to spray geometry transformation of multi-hole injectors,which becomes stronger under enhanced superheated conditions.When the interaction is weak,the fuel spray gets wider and shorter,and fuel distribution is more homogeneous with the superheat degree gets stronger.However,stronger fuel plume interaction can lead to spray collapse,of which fuel spray gets very compact with small spray angle and large spray penetration.Based on the analysis of interaction length of fuel spray of the 6-hole injector,it shows the interaction length increases with elapsing injection time and increasing superheat degree.The results show that the interaction between adjacent fuel plumes is the key factor influencing the spray collapse,which is governed by the nozzle configuration,superheat degree and other injection parameters.In addition,near nozzle spray characteristics is crucial for the occurrence of interaction of adjacent fuel plumes and consequently spray collapse.In a summary,based on the analysis of internal flow characteristics,near nozzle spray structure,and macroscopic spray structure,the fast atomization and evaporation mechanisms of flash boiling spray are unveiled.Flash boiling spray collapse mechanisms of different injectors are revealed.The effects of nozzle geometry and nozzle number on flash boiling characteristics are also clarified. |
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