Visual Experiment Of Transient Cavitating Flow In The Diesel Injector Nozzle And Its Influence On Spray Characteristics

As the environment problem become increasingly prominent,it's more and more important to control the emission of the diesel engine.The emission characteristics of diesel engine are largely dependent on the combustion characteristics,which are greatly affected by atomization behaviors.As the boundary condition of atomization,flow inside diesel injector has great effects on atomization behaviors.Based on this,visual experiments of transient cavitating flow characteristics in the real-size diesel injector nozzle based on the high pressure common rail system with the injection pressure as high as 110 MPa were carried out in this paper.Particular attention was focused on the string cavitation,the primary bubbles and the effects they have on atomization of flow inside diesel injector.The main achievements are as follows.(1)A transparent real-size nozzle flow and spray visualization test rig equipped with the injection pressure as high as 110 MPa based on the high pressure common rail system was set up.(2)The study on characteristics of transient cavitating flow with the needle moving quickly in the nozzle were carried out.It was found that the string cavitation was particular cavitating phenomenon which occur randomly.Two types of the string cavitation are identified,one originates from the needle tip and develops to the orifice,the other originates between the two orifice.The effects on the occurrence frequency of string cavitation of injection pressure,the location of needle and the size of sac was also investigated.(3)The tapered nozzle hole can inhibit the occurrence of geometric induced cavitation,so the string cavitation can be observed more clearly in the tapered hole.It was also found that when string cavitation occur it has little effects on the spray cone angle.However,when the string cavitation extend out the nozzle hole it had great effects on spray cone angle.Moreover,it was also found that the string cavitation has positive effects on the occurrence of geometric induced cavitation.(4)When the needle began to lift up,there would be a delay of injection.This delay consist of the bubble suction and the bubble extrusion.The bubble suction was caused by the replacement of needle space.The bubble suction from orifice exit could also be observed at the end of injection.When the needle stopped suddenly the fuel remain was injected into the air due to the huge inertia effect,thus form a low pressure area in the sac,which would cause the suction at the end of injection.Thebubbles caused by the suction at the end of the injection would be the primary bubbles at the beginning of the next injection.(5)When the cavitation extend out of the nozzle it would influence the spray and formed the umbrella spray.Higher injection pressure would cause earlier occurrence of umbrella spray.However,injection pressure had little influence on the position of the umbrella.Different primary bubbles would result in different shape of spray,and small primary bubbles would cause the mushroom spray.