Experimental Study On Steady-State Scavenging Flow Of Low-Speed Two-Stroke Marine Diesel Engines

The process of scavenging flow at low-speed two-stroke marine diesel engine has the functions of providing fresh charge,exhausting the exhaust gas in the cylinder and enhancing the vortex to promote combustion.So it has an important influence on the power,emissions and mixing of gas and oil of the diesel engine.It is very important to study the scavenging process.In this paper,a scavenging flow test bed is built for a low-speed two-stroke marine diesel engine.The device maintains the same dimensional structure characteristics as the original machine in the design of key components.On this basis,the Stereo Particle Image Velocimetry(Stereo PIV)is used to capture the flow field at different positions in the cylinder.The structure of different scavenging ports and the scavenging flow under different working conditions are studied,and the analysis and evaluation of the scavenging flow of the two-stroke marine diesel engine is realized.The results show that increasing the radial cutting angle of the scavenging port can increase the swirl ratio significantly.As the angle increases from 10° to 30°,the swirl ratio increases by about 329.2%.In the case of high swirl ratio,there will be a phenomenon of backflow at the center of the vortex,resulting in poor scavenging uniformity.In addition,the in-cylinder flow capacity is first enhanced and then weakened;The increase of the number of scavenging ports will slightly increase the swirl ratio.When increasing from 18 to 30,the swirl ratio increases by about 35.3%.Influenced by the interference of more jet flows,the kinetic energy decays rapidly and the axial velocity is uniform.The average scavenging non-uniformity coefficient of the 30 scavenging ports is 76.9% of the 24 scavenging ports;The swirl ratio of the composite scavenging port is increased by 13.8% compared with its average angle of 10°,and the scavenging uniformity is increased.With the decrease of the opening of the scavenging port,the influence of the opening degree on the airflow motion is gradually increased.the swirl ratio change from 100% opening to 75% opening and 75% to 50% opening is not more than 5%.However,when the opening degree is gradually reduced to 25%,the swril ratio is increased by 19.5% compared with the 50% opening.And in the process of 50% to 25% opening,the flow capacity is reduced by 8.3%,while the scavenging uniformity is reduced by 51.9%;The main difference between the sweep pressure difference and the lift of the exhaust valve is the flow capacity.As the scavenging pressure increases and the lift of the exhaust valve increases,the flow capacity increases,but the change of the vortex shape has little effect.Under this condition,the swirl ratio does not change by more than 5%.It can be seen that the main cause of the vortex shape in the cylinder is the structural of the scavenging port.