Investigations Of Cavitation In Nozzle And Characteristic Of Breakup For Diesel Spray

Fuel injection plays an extremely important role in the combustion and emission of internal combustion engine and then has a great significance for the energy reduction and environment protection. Injector, as an executor of a fuel jet breakup, is not negligible for the fuel spray, especially for primary break-up of liquid jet. The investigations of the internal flow within nozzle, especially cavitation, and the characteristic of jet breakup outside the nozzle are carefully performed in this paper.Under higher pressure, the internal flow of nozzle is a typical multi-phase flow problem.Based on the theory of computational fluid dynamics (CFD) and dependent on the Euler-Euler two-fluid method, the multi-dimension numerical simulations about cavitation, occurring in nozzle, have been widely performed. The results show that cavitation takes place more easily under the higher injection pressure and the outlet pressure of nozzle restrains the phenomenon of cavitation. The more inlet rounded radius and inclination angle of nozzle orifice can make the flow of nozzle smoother, generate less vapor bubbles and increase the fuel mass flow through the nozzle exit. The higher Reynolds number in nozzle, which means the turbulent intensity is more greatly violent, makes the cavitation phenomenon in nozzle more visible. The turbulence in nozzle makes the cavitation bubbles smash more rapidly, which is also helpful for increasing the intensity of turbulence. And so the pressure difference between the inlet and outlet of injector does, which means that the higher pressure difference can make the cavitating flow in nozzle stronger.Due to the complication of working surroundings and the real structure of nozzle, it is very difficult to observe the cavitation bubbles in nozzle orifice. Based on the theory of similarity, the large-scale nozzles, 20 times the real nozzles'greater, are redesigned in case the similarity of structure, dynamics and boundary conditions are involved. Dependent on the method of shadow-graph, the cavitating flow in the large-scale nozzles, under several different working conditions, is shot by the high-speed digital camera. Cavitation phenomenon is investigated by a series of visualizing experiments by analyzing the effect factors such as pressure, geometry dimension and turbulent in nozzle. The experimental results show a good agreement with the simulation results. The atomization of fuel jet is widely accepted to be attributed to aerodynamic forces, turbulent disturbance in nozzle orifice and cavitation in nozzle, which is also firmly confirmed in this paper. Based on the results of cavitation simulation, solved by the Euler-Euler approach of two-fluid model, the spray simulation is performed by using of the Euler-Lagrange approach of DDM model. The flow data obtained from the two-fluid nozzle calculation are transferred into the atomization model as boundary conditions. Cavitation in nozzle orifice is desired to a certain degree because the collapse of cavitation bubbles influences the turbulence intensity towards the outlet of injector. Consequently, this enhances the second atomization process of fuel in the combustion chamber. Compared with the spray of real injector and the size of liquid droplet, the results of spray simulation are reliable and then give a great significance on the mechanism of liquid jet breakup.