Transient Characteristics Of Special Flow Phenomena In Diesel Injector Nozzles And Their Effects On Spray

Diesel engine industry,an important foundation to promote the economic growth and social operation,has put forward higher requirements for the fuel injection system which is the "heart" and "brain" of diesel engines,i.e.higher injection pressure,more accurate control and greater reliability.Some enterprises come from Germany,US and Japan,however,accounted for the most of domestic diesel engine market because there are still some shortcomings in performance,reliability and consistency of domestic diesel engines.Thus,the electrically controlled high-pressure common rail system has become one of the core technologies that need to be solved urgently.Moreover,the injector nozzle of high-pressure common rail system is the key component that link between upstream fuel injection and downstream spray atomization,and its internal flows determine the evaluation index of the fuel injection system directly,including the flow performances and spray characteristics,and thus influence the accuracy,consistency and stability of control on the fuel spray process.In high-pressure fuel injection processes,in addition to the common sheet cavitation,some special flow phenomena,such as cloud cavitation shedding,air suction,residual bubbles,vortex induced string cavitation etc.,also exist inside the diesel nozzles;and there are relatively few researches on the latter at home and abroad.This thesis studies the special in-nozzle flows combined the optical experiments and numerical simulations.The main contents and innovative conclusions are as follows:(1)Different form the cavitation models based on cavitation dynamics,which are widely employed in the researches on cavitating flow,the homogeneous relaxation model is proposed based on the thermodynamic equilibrium principle,and has greater application potential in cavitation phenomena simulation of high-pressure fuel injection,especially considering the compressibility and temperature effect of fuel.Based on the verification and analysis of the applicability and accuracy of the HRM model for the simulation of cavitation flow inside the nozzle,this thesis discusses the influence of cavitation phase transition rate,initial non-condensable gas in the fuel,compressibility of fuel and turbulence model on the simulation results of cavitation flow inside the nozzle.Results show that the mass transfer rate of cavitation has an important effect on the transient evolution of cavitating flow,the fully developed cavitation region,however,is not influenced.Meanwhile,the non-condensable gas in fuel has an important influence on the stable cavitation regions but not development process.Moreover,the compressibility of the fuel accelerates the cavitation transient development process and increases the total mass of the liquid phase converted to the vapor phase after cavitation arrives to a stable state.Furthermore,the turbulent LES large eddy simulation method is more suitable for the fine capture and study of instantaneous phenomena,such as bubble suction,cloud cavitation shedding and nearnozzle jet breakup and spray formation.(2)Three different scaled-up nozzles with the same geometry are employed to explore scale effects of cavitation regimes such as sheet cavitation inception,super cavitation and hydraulic flip flow using the visualization setup,and the correlation between cloud cavitation shedding and air suction is analyzed by LES large eddy simulation.Results discover that in-nozzle cavitating flows have scaling effect.Meanwhile,numerical simulation results match well with the optical experiments and indicate that larger scaling factor or injection pressure of nozzles yield smaller averaged frequencies of cloud cavitation shedding phenomenon,the St numbers of cavitation shedding under different scaled-up nozzles and injection pressures,however,are almost stable.In conclusion,the study on properties of various flow regimes in different scaledup diesel injector nozzles provided preliminary exploration to the in-nozzle flow characteristics from the scaled-up transparent diesel nozzles to real diesel nozzles.(3)Transient characteristics of the in-nozzle vortex induced string cavitation is studied by combining the diesel nozzle internal flow and spray visualization and the three-dimensional numerical simulation of transient cavitating flow inside the multihole nozzle considering the needle motion and needle eccentricity.Results reveal that occurrence frequency,duration and intensity of the “needle-hole” type string cavitation inside the two-hole nozzle are larger.Meanwhile,the “needle-hole” type string cavitation occurs at lower needle lifts,and when it happens the spray cone angle will be increased sharply.The “hole-hole” type string cavitation,however,tends to form at higher needle lifts,and its intensity is generally weaker than “needle-hole” type string cavitation.Moreover,unlike the geometry-induced cavitation,the string cavitation origins from the vortex core and develops against the vortex flow.In addition,for multihole diesel nozzle,the needle eccentricity causes the hole-to-hole variation,that is the single-phase flow tends to occur in the hole that located at the side of needle bias,and the geometric induced cavitation tends to form at the hole that far away the nozzle tip,as well as the vortex induced string cavitation tends to be found inside the middle hole.Study of transient characteristics of string cavitation provides the theoretical guidance for the fuel injection consistency of multi-hole nozzle and shows a new idea for the design and optimization of vortex induced hollow spray nozzle.(4)Optical experiments on multi-injection processes spray patterns of the real-size transparent tapered diesel nozzle are performed based on a high-pressure common rail fuel injection system with a high-speed camera.It is found that the near-nozzle spray structures are shot-to-shot variation,as well as most of sprays can be classified as single-or double-mushroom shape with wake,single-mushroom shape,wake and columnar spray(that is,no mushroom shape or wake)in detail.Additionally,the occurrence probabilities of the five near-field spray structures are strongly influenced by the injection pressure.(5)The formation of residual bubbles/diesel inside the real-size diesel nozzle at starting and ending stages of injection processes and near-field spray structures are analyzed.Experimental and simulation results reveal that the residual bubbles and diesel origin from the ingested phenomenon at start or end stages of diesel injections,and the shot-to-shot near-nozzle spray variation can be attributed to the various distributions of residual bubbles and initial diesel.The above discussion provides a deep insight in the prediction of shot-to-shot spray variation,and thus provides a new perspective for optimizing the multiple injection strategy and accurately predicting the near-field spray characteristics as well as theoretical basis and guidance for designing the advanced injectors.In this thesis,optical experiments and high-precision numerical simulations are effectively combined to conduct systematic and in-depth study on various special flow phenomena and their influence on spray in the internal nozzle of high-pressure common rail fuel injection system of diesel engine under high injection pressure and multiple injection strategies.To sum up,this research draws some innovation conclusions in applicability using HRM cavitation model to explore the in-nozzle cavitation,cavitation scale effect,cloud cavitation shedding,air suction and the influence mechanism of residual bubbles on the near-nozzle spray patterns,as well as the effects of needle motion and eccentricity on string cavitation.Conclusions drawn provides theoretical guiding for realizing accurate,consistent and stable control of high-pressure common rail fuel injection system on the fuel injection and spray.