Visualization Experiment And Numerical Simulation Of Nozzle Cavitation For High Power Density Marine Diesel Engine

Diesel engines are widely used in land transportation,ship transportation,engineering construction and even military fields for their high thermal efficiency,fuel economy and good operational stability.With the increasingly strict emission regulations in China and the requirement of energy saving and emission reduction based on carbon neutrality,improving the fuel injection characteristics of diesel engines to optimize the combustion and emission performance of diesel engines has become a major demand in the internal combustion engine industry.The cavitation characteristics of the injector is an extremely important part of the diesel engine injection characteristics.The cavitation phenomenon inside the nozzle hole will significantly reduce the fuel flow and jet velocity in the nozzle,and then affect the spray atomization characteristics,while the transient shock generated by the periodic collapse of the cavitation bubble will damage the nozzle structure,the entire diesel engine thermal efficiency and service life,Therefore it is important to study the cavitation characteristics of injectors.In this paper,a high power density marine diesel engine sub-injector is studied,which has serious cavitation phenomenon in actual use,the fuel injection performance is much lower than expected,and the nozzle inner wall structure is damaged by cavitation after long-term operation.In this paper,we study the influence of different jet parameters and nozzle structure factors on the generation and development characteristics of cavitation in the injector nozzle through a combination of visualization experiment and numerical simulation,and propose the direction of nozzle structure optimization according to the cavitation evolution mechanism.The main work and innovation points of the whole paper are as follows.(1)The nozzle cavitation visualization experiment platform equipped with a highpressure common rail fuel injection system was designed and built.A transparent nozzle based on the real nozzle size was processed by using PMMA,and a micro high-speed camera was used to observe the two-phase fuel flow field inside the nozzle,define the cavitation parameters and realize the quantitative study of the cavitation characteristics inside the nozzle through image post-processing.(2)The spatial and temporal evolution of cavitation during the whole injection process was investigated,and the generation and development of intra-hole cavitation and the twophase distribution state of the flow field under different injection pulse widths,injection pressures,and hole length-to-diameter ratios were studied in depth.The study shows that the cavitation in the nozzle hole is not affected by the injection pulse width,and the increase of injection pressure and nozzle hole L/D ratio will promote the generation and development of cavitation at the entrance of the nozzle hole,and the degree of cavitation is significantly increased,while the larger the inclination of the nozzle hole the more intense the cavitation in the hole.(3)CFD model of marine injector nozzle flow field is established,and the influence of injection pressure,dimensionless roughness,dimensionless nozzle hole rounding angle and needle valve opening on fuel flow characteristics,cavitation area volume and distribution in the nozzle hole under actual working conditions is investigated through numerical simulation,and the similarities and differences of cavitation formation mechanism are analyzed for different parameters.The study shows that the increase in injection pressure will increase the generation and flow rate of cavitation;the presence of rounded corners will weaken the magnitude of the sudden change in fuel flow at the entrance of the injection hole,which will reduce the rate of bubble generation and inhibit the development of cavitation;the increase in roughness will increase the flow resistance and at the same time will inhibit the development of cavitation in the nozzle hole,and this flow obstruction will complicate the two-phase flow characteristics of the nozzle hole;the fuel between the needle valve and the needle valve seat will be throttled due to needle valve and needle valve seat between the fuel in the sealing surface area due to the effect of throttling and local cavitation,the needle valve opening increases the pressure at the sealing surface,the cavitation completely disappears,while the cavitation characteristics of the nozzle hole get into a quasi-steady state: the larger the tilt angle of the nozzle hole,the more violent the sudden change in the flow direction of the fuel into the nozzle hole,which in turn promotes the generation of cavitation in the nozzle hole.Meanwhile,according to the cavitation simulation results,the optimization direction of nozzle structure is proposed.