Numerical Simulation Of The Transient Flow Fields Inside The Diesel Engine Spray Nozzle During The Needle Valve Closing Process

A large number of experiments and simulation results show that, flow in the diesel engine spray nozzle hole has a great influence on the atomization quality, which impacts on engine fuel economy and emission characteristics. There are lots of studies about the turbulent and cavitating flow inside the nozzle which is the characteristics before the end of injection, while reports about characteristics after the end of an injection pulse are very rare. Recent experiments had found, there were bubbles in some structure nozzles after the injection. In the next injection, as the needle valve opening, the bubbles would burst with impact energy, which increased the turbulence degree of the flow fields to impact on the atomization quality at this time. As the injection interval shortening, and many times injection technology widespread application, it should be given more attention, and it is necessary to further research the nozzle flow fields during this process. Due to the limitation of actual conditions, it is difficult to obtain the flow fields inside nozzle using the method of experiment. So the transient flow fields inside the diesel engine spray nozzle hole during the needle valve closing process were studied by the means of numerical simulation in this paper.First, based on the open source software OpenFOAM, this paper proposed a new algorithm of two kind of continuous mesh-moving to keep the needle valve closed completely through the design of three sets of continuous grid, with VOF method and introducing the model of contact Angle, established the three-dimensional two-phase flow model to numerically simulate the transient internal flow fields during the needle valve closing process by three steps. Comparing the calculated results with S.Jollet experiments, verified the accuracy of the established calculation model. Then comparing the changing process of the gas and liquid distribution in one nozzle with another after the needle valve was turned off, this paper analyzed the causes of the formation of those bubbles.On this basis, the paper analyzed how the nozzle hole diameter, length and taper(K > 0) impacted on the transient flow fields inside the nozzle during the needle valve closing process. Results showed that, during the whole process of needle valve closing, average pressure in pressure chamber and nozzle hole area of all nozzles reduced first, then tended to environmental pressure; when the needle valve closed, on the nozzle exit, the speed of the backflow of air increased when the nozzle hole diameter increased, the nozzle hole length was shorter and the nozzle hole taper decreased; after the needle valve was turned off, the size of bubbles inside the nozzle also increased when the nozzle diameter increased, the nozzle length was shorter and the nozzle taper decreased.In the end, the paper analyzed how the injection pressure, environmental pressure and speed of needle valve impacted on the transient flow fields inside the nozzle during the needle valve closing process. Results showed that, during the whole process of needle valve closing, average pressure in pressure chamber and nozzle hole area of all nozzle reduced first, then tended to environmental pressure; when the needle valve was closed, on the nozzle exit, the speed of the backflow of air increased when the injection pressure increased, the environmental pressure decreased and speed of needle valve increased; after the needle valve was turned off, the size of bubbles inside the nozzle also increased when the injection pressure increased, the environmental pressure decreased and speed of needle valve seat increased.