| Diesel engines are widely used in transportation,industrial and agricultural production,and national defense equipment due to their excellent thermal efficiency,economic benefits,and high torque.However,they are facing challenges in further energy conservation and emission reduction.Various new energy-saving emission reduction technologies,clean combustion modes and alternative fuel blended fuel technologies put forward higher requirements for fuel injection system performance.The cavitation transient flow characteristics of the nozzles in the fuel injection system directly determines the fuel injection quantity and the precise control of spray that determine the key parameters of the fuel injection system performance.The cavitation transient flow characteristics in the diesel engine nozzle have three main influencing factors: geometric structure factor,fluid dynamic factor and fuel properties factor.At present,the influence of geometry and dynamic factors on cavitation flow has been widely studied and can be well mastered,while the influence of fuel temperature and fuel type(especially mixed fuel)on the influence of fuel properties on cavitation flow and spray is very limited,and the effect mechanism is not yet clear.Therefore,this paper mainly focuses on the high-pressure common rail original size order transparent nozzle inflow and spray visualization test bench,focusing on the fuel medium factor on the vortex cavitation characteristics in the diesel engine nozzle and its influence on the spray.The main research contents and the innovative research results obtained are as follows:(1)Based on the high-pressure common rail injection system,a visual test bench for transient cavitation flow and spray in the real size transparent nozzles was built,and the material screening and processing assembly of the real size transparent nozzles were carried out.The dimensionless parameters that the vortex cavitation intensity in the nozzle were defined,and the test data are processed to obtain the needle lift,spray cone angle,vortex cavitation intensity and other data for analysis.Based on the shadow method,a high-speed digital microscopic imaging acquisition system was used to study the transient cavitation flow characteristics in the nozzle and its effect on the spray at different fuels and different fuel temperatures.(2)Based on the real size nozzle internal flow and spray visualization test bench,using the background light shadow method to study the influence and difference of different fuels on the vortex cavitation development and spray characteristics in thediesel engine nozzle.The results show that mixing a certain proportion of gasoline in diesel fuel helps to enhance the vortex cavitation intensity in the nozzle.The coexistence and intertwining of two vortex cavitation were discovered.The addition of gasoline makes even weak vortex flow induce vortex cavitation.However,due to its high viscosity and low fuel injection rate,the gasoline / HCB mixed fuel is not easy to cause vortex cavitation in the nozzle.In addition,in the mixed fuel,the degree of influence of adding gasoline on the vortex cavitation is related to the specific range of another fuel viscosity and saturated vapor pressure.Within an appropriate range,the addition of gasoline will significantly affect the development intensity of cavitation,and once the viscosity is too large and the saturated vapor pressure is too low,the effect of gasoline addition will be greatly reduced,The overall development of cavitation is mainly dominated by fuels with high viscosity and low saturated vapor pressure.(3)A simplified nozzle test bench was built to investigate the cavitation flow characteristics of the different tapers nozzle with different fuel temperature.The incipient point of cavitation in the divergent hole is obviously earlier than that of the straight and convergent nozzle,and the development speed of cavitation is also faster.But the convergent nozzle has the highest flow coefficient.When the fuel temperature rises,it obviously promotes the incipient point of cavitation and accelerates the development of cavitation.The fuel temperature has the greatest influence on the primary cavitation in the straight hole nozzle.When the geometry-induced cavitation in the nozzle gradually increases,the cloud cavitation vortex will be separated.When the fuel temperature increases,the cloud cavitation area become larger and deeper.(4)The cavitation flow characteristics of different fuels in the nozzle were compared and studied.With the increase of fuel temperature,the vortex cavitation intensity of diesel in the nozzle will increase.The phenomenon of geometrically induced cavitation and vortex cavitation coexist in the straight hole nozzle.When the fuel temperature is increased,the vortex cavitation intensity in the nozzle will become stronger and last longer.At this time,the vortex cavitation will gather the geometry-induced cavitation in the nozzle hole around it,forming agglomeration phenomenon.Due to the coupling of flash and cavitation,the vortex cavitation of gasoline / diesel mixed fuel in the nozzle will increase significantly as the fuel temperature increases.The gasoline / HCB mixed fuel is relatively insensitive to fuel temperature due to its high viscosity.(5)Three kinds of needle valves with different shapes were designed to form nozzles with different sac chamber flow structures.The results show that the matching relationship among sac chamber,needle valve and hole are the key factors to determines the intensity and distribution of the vortex flow field in the nozzle,which strongly affects the vortex cavitation and spray characteristics in the nozzle.(6)The particle size distribution characteristics of diesel nozzles were studied by using Malvern particle size analyzer on a high-pressure common rail test bench.The SMD decreases with the increase of fuel temperature.Large-diameter droplets in the spray began to transfer to smaller-diameter droplets. |
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