Study On The Influence Of Pressure Chamber On Nozzle Cavitation And After Start Of Injection Process

Diesel engines act as a power source in the field of construction machinery.Faced with the deterioration of the atmospheric environment and increasingly strict emission regulations,the diesel engine’s combustion emission performance poses greater challenges.The quality of the spray directly affects the combustion and emission performance of the diesel engine.The research shows that the spray atomization process is closely related to the flow field in the nozzle,besides the influence of aerodynamic factors.With the wide application of diesel engine multiple injection technology,the injection specific gravity of the unsteady stage increases correspondingly,and the injection dynamic process of the needle valve opening and closing stage is worthy of further study.Most of the research on the unsteady(starting and oil-breaking)stage focuses on the structure of the nozzle hole and the injection conditions.For the pressure chamber with a large volume,there is still a lack of more detailed analysis,such as the pressure chamber structure for the start-up phase and the oil cut.The influence of stage cavitation,especially the pressure chamber,has a more significant effect on the gas backflow caused by cavitation in the oil cut stage.Based on the transparent nozzle and the visual test bench,the high-speed camera combined with the backlight imaging technology was used to visualize the nozzles of different pressure chamber structures(spherical pressure chamber,conical pressure chamber).Under the condition of injection pressure of 30 MPa,the nozzles of the two pressure chambers do not cavitation during the process of the needle valve lifting up to the full lift of the needle valve.During the needle valve lifting process,the residual air bubbles at the end of the last injection period Broken by the impact.During the seating process of the needle valve,the two nozzles have significant differences in cavitation evolution.The cavitation development process of the spherical pressure chamber nozzle has a hierarchical structure.The cavitation position and sequence are as follows: the nozzle inlet corner and the nozzle hole are completely empty.Cavitation in the pressure chamber;the cavitation in the conical pressure chamber is more rapid,and the cavitation and the pressure chamber are basically cavitation at the same time.It is difficult to capture the sequence of cavitation.In addition,the nozzle of the conical pressure chamber is empty.Both the collapse time and the flow field stabilization time are later than the spherical pressure chamber nozzle.On the OpenFOAM computing platform under Linux system,a two-phase three-component cavitation solver based on VOF(Volume of Fluid)method is constructed.The numerical reliability verification is carried out based on Sou’s quasi-two-dimensional nozzle test data.Subsequently,the model is used to numerically simulate the influence of the pressure chamber turning angle and the pressure fluctuation in the pressure chamber on the cavitation in the hole.The results show that under the same conditions,the pressure chamber turning angle is more convenient at the 90 degree.In the first birth,the larger and smaller pressure chamber turning angle will increase the nozzle shrinkage coefficient and inhibit the cavitation initial growth and development;under the pressure chamber pressure cycle fluctuation condition,the mass flow will have a periodic fluctuation of amplitude gradually increasing,followed by Stable fluctuations,the cavitation intensity in the pores fluctuates in a rising form,and then fluctuates steadily.The fluctuation periods of the two have a certain phase difference with the pressure fluctuations at all times.Based on the same mathematical model,the influence of the pressure chamber on the transient flow of the nozzle during the oil cut process was analyzed and compared with the results of the oil cut test.The analysis indicates that for the standard spherical pressure chamber nozzle,cavitation occurs in the hole and pressure chamber in the oil cut phase,and the cavitation volume of the pressure chamber is large.The collapse of the cavitation causes the outside air to flow backward,forming a bubble column or discrete in the nozzle.bubble.After the flow breaks,there is a strong vorticity in the pressure chamber,which forms a vortex cavitation form.There is a pressure chamber space volume limit value to maximize the cavitation volume in the pressure chamber.Finally,the cavitation and gas backflow of the spherical pressure chamber and the conical pressure chamber structure nozzle are compared in the oil cut stage.The spherical pressure chamber mainly exists in the form of vortex cavitation caused by the vortex motion.The conical pressure chamber is mainly pulled by the fluid.The strip-shaped cavitation dominated by the extension exists,and its maximum cavitation volume is larger than the cavitation volume of the spherical pressure chamber.For the spherical pressure chamber,the frictional dissipation is more dissipated due to the vortex motion than the tensile action,so that the cavitation collapse is more rapid,the fuel inertial outflow volume is more,and the final gas backflow is larger.