Research On Cativation Inside Nozzle And Fuel Atomization In The Marine Diesel Engine

Marine diesel engine is widely used in large ships because of its large power.Spray characteristics effected by the upstream cavitation inside the injector are the important factors of the combustion in Marine diesel engine.In order to improve the atomization which benefits dynamic performance,economy,emission of marine diesel engine,the cavitation phenomenon,atomization characteristics and mechanism must be studied clearly.Currently,most researches focus on the injection system of general diesel engine instead of marine diesel engine.However,the high-power marine diesel engine is equipped with the larger nozzle size,the higher chamber pressure and the longer injection duration which in turn influence atomization and combustion in the cylinder.So,it is necessary to investigate cavitation and atomization in high-power-marine diesel engine,and thus becoming an objective of this study.Firstly,the marine injector is modeled and meshed.Combined with single bubble dynamics,the diesel liquid-gas two-phase cavitation model is established to simulate the cavitation inside the injector of marine diesel engine.The simulated results are analyzed by Fast Fourier Transform.It is found that with increasing the injection pressure,the pressure fluctuations become more intense and also increased gradually from the nozzle inlet to the outlet along the axis direction of which all the peaks range in 300-500 Hz.As the injection pressure increased,the cavitation becomes more obvious,and flow coefficient is decreased while the mass flow rate increases.Then,based on the simulated cavitation results,the initial spray parameter equation is first proposed to be applied in numerical simulation in this study.The large eddy simulation model based on Samagorinsky-Lilly,the O’Rourke droplets collision model and WAVE droplet-breakage model are built to study the influence of injection pressure on atomization characteristics.The results shows that the spray penetration is proportional to time at the beginning of injection while the growth of penetration slows down gradually in the middle and later periods of injection.There are larger spray cone angle and penetration in marine diesel engine because of the larger orifice diameter and injection pressure.The volume of spray head becomes larger as the injection pressure increasing.The massive structure on spray edge can return to the spray body while the massive structure on spray head remains unchanged for its lifetime.The sauter mean diameter decreased with the injection pressure increasing,and each additional 20 MPa of injection pressure reduces the sauter mean diameter by approximately 9%.Finally,by comparing the simulated results with experimental data,the spray shape and penetration are in agreement with experimental results,which indicates that the initial parameters equation and numerical model of cavitation and spray are proved high accuracy in this paper.