Optical Investigation Of Fuel Spray Combustion And Soot Generation By Nozzle K-factor And Needle Lift Of Diesel Nozzle

In the face of increasingly stringent emission regulations,new combustion technologies such as low-temperature combustion(LTC)and Homogeneous Charge Compression Ignition(HCCI)have been proposed.However,these new combustion technologies require higher precision and flexible control of fuel injection.The nozzle is the key channel connecting the upstream injection system with the downstream fuel spray,which will have a direct impact on the fuel spray characteristics,and then affect the fuel combustion characteristics and final emissions.As the main factor affecting the flow characteristics of the nozzle,the nozzle structure is worth further study.This paper starts with the nozzle structure of diesel engine and takes the K-factor and needle lift as the starting point,The real-size optical internal flow nozzle based on the high-pressure common rail fuel injection system and the visualization platform of fuel spray combustion and soot based on the constant volume vessel,The effects of K-factor and needle lift on cavitation flow,near-field spray characteristics of nozzle,fuel ignition and combustion process,and soot concentration were studied by using a variety of optical testing techniques.The main results are shown as following:(1)Spray vapor penetration of diesel from K-factor=0 and K-factor=1.5 were measured by schlieren in constant volume vessel.Results show that when the spray develops to a stable form,the spray cone Angle of K0 is greater than K1.5,and the spray vapor penetration is shorter than K1.5.(2)Schlieren and Flame spontaneous luminescence were applied to investigate the flame lift-off length and ignition delay.It was found that the ignition delay and flame lift-off length of the nozzle with K=0 were slightly smaller in lower injection pressure and ambient temperature,the other working conditions are almost the same.(3)Flame spontaneous luminescence was applied to investigate the soot formation characteristics of the nozzle with K =0 and K =1.5,Results show that the soot concentration of K-factor=1.5 is larger in both the initial generation stage and the stable generation stage of soot.(4)Based on the visualization platform of real-size optical nozzle,the influence of injection pressure on the movement law of needle lift and the influence of needle lift on the cavitation characteristics in a real-size optiacl nozzle was investigated.Results show that the needle lift has been successfully regulated by means of changing the thickness of an adjusting pad between the needle and rod within the injector.The higher injection pressure,the higher speed and height of the needle lift,the time required for complete closure of the needle lift is proportional to the value of the needle lift.The string cavitation strength and stability of the real-size transparent nozzle with two holes increased first and then decreased with the increase of the needle lift.When the needle lift was 0.3mm,the cavitation strength was the largest and stability was the best.(5)The spray visualization tests of the same injector equipped with a vertical single-hole nozzle were carried out in a constant volume vessel.In order to do that,a high-speed schlieren imaging and OH* chemiluminescence techniques were applied to quantify the spray penetration,ignition delay and flame lift-off length.It was shown that the difference on the needle lifting process do not bring a significant difference on spray tip penetration,ignition delay and flame lift-off length for this vertical singlehole injector.(6)In order to have a better understanding of the effects of needle lift on spray characteristics,a series of numerical simulations of nozzle exit for a vertical singlehole injector has been performed by means of a CFD software.The simulated results show that the discharge coefficient and velocity coefficient of nozzle exit keep almost flat when needle lift is greater than 0.1 mm and decrease sharply when it is smaller than 0.05 mm because of the “throttle” effect.However,the caviation area of nozzle exit is almost kept constant within the scope of simulated needle lift.In this paper,starting from the nozzle K-factor and needle lift,exploring the differences of fuel spray characteristics,ignition performance and soot emission characteristics due to different nozzle K-factor and needle lift.It provides theoretical guidance for the optimization of nozzle structure and the research and development of efficient and clean diesel engines.