In-cylinder Flow In Scavenging Process For Low-Speed Two-Stroke Marine Diesel Engines

In low-speed two-stroke marine diesel engines,uniflow scavenging process can push the burnt gas out of cylinder and provides fresh air for the next cycle.The swirl flow generated by the scavenging port could improve the combustion performance through enhancing the fuel-air mixing.In this thesis,a steady optical experiment rig is designed to study the scavenging process in low-speed two-stroke marine diesel engines.Numerical simulations of instantaneous scavenging process in a two-stroke marine diesel engine are performed.The main contents and conclusions are as follows.Firstly,we design a steady optical experiment rig of low-speed two-stroke marine diesel engine to investigate the scavenging process using Stereo Particle Image Velocimetry(Stereo PIV)and Paddle Wheel Anemometery(PWA).Compared with similar rigs reported in the literature,a larger difference in the scavenging pressure and more realistic boundary conditions are achieved.The swirl number measured by Stereo PIV shows a similar trend with that by PWA,and this shows a good consistency between Stereo PIV and PWA in measuring the cylinder swirl number.Secondly,steady experiments of in-cylinder flow are performed based on the optical experiment rig to investigate the evolution of the in-cylinder flow,the flow capacity,the swirl number and the homogeneity of scavenging swirl.(1)When fresh air flows to the downstream,the wake-like flow of the axial velocity decreases gradually while the tangential velocity changes from Burgers vortex to solid-body swirl.(2)With increasing the radial angle of scavenging ports,the in-cylinder swirl number increases but the bulk velocity decreases,and backflow occurrs in the cross-section near the scavenging ports.(3)Regarding the influence of piston position,when the piston moves upward to block the scavenging ports,the axial velocity increases firstly and then decreases.(4)The scavenging pressure difference affects the flow capacity and fresh air mass,and a higher scavenging pressure difference could improve the uniformity of the inlet flow near the scavenging ports,even though the different scavenging pressure difference has little effect on the swirl number.(5)Regarding the influence of the exhaust valve rotational speeds,the exhaust valve makes no difference on the upstream flow field,and it only enhances the swirl number near the exhaust valve.(6)In order to study the multi-scale turbulence characteristics and the vortex procession phenomenon,the Proper Orthogonal Decomposition(POD)and statistical method are used,and the results show that the large-scale coherent structure is the primary factor which affects the vortex procession and high swirl number could maintain the stability of in-cylinder flow.This study promotes the idea of “scavenging heterogeneity coefficient” to evaluate the scavenging performance.It is found that the scavenging performance is generally low near the scavenging ports,and this tredency also occurs in the region of the exhaust port in case of high swirl.But for scavenging pressure difference,it has few influence on the scavenging performance.Thirdly,through Large-Eddy Simulation of the steady scavenging process,the backflow is occurred in the vortex core region near the scavenging ports due to the low pressure,and the precession period of scavenging vortex is quantified.In addition,the scavenging process in low-speed two-stroke marine diesel engine is simulated.It is found that the scavenging process is nonuniform.At the beginning of the scavenging process,there is a strong backflow near the scavenging ports,but when the scavenging ports open completely,the backflow disappears quickly.Meanwhile,the vorticity magnitude presents that the strong inlet jet could lead to the blends of the fresh air and exhaust gas.Finally,considering the vortex core position,the bulk velocity and the swirl number at typical moments of the scavenging process,it is found that the results of the steady scavenging experiment is consistent with that in real engines,and this indicates the significance of the steady experiment in guiding the design of real engines.