Similarity Investigation Of Gas Exchange Flow For Marine Diesel Engine

Marine diesel engines dominate the power plant in civilian ships and warships for their excellent performance in wide-range output power,high efficiency,low energy consumption,and easy maintenance.The gas exchange process in marine diesel engines is very important for fuel/air mixture formation and the subsequent combustion process.However,it is difficult to investigate and optimize the gas exchange process owing to its huge size and high experimental cost.In order to solve this problem,the prototype is reduced in size based on the similarity principle to study the parameter relationship of the prototype and the reduced model.In this study,the gas exchange process in a four-stroke and a two-stroke marine engine were studied using the flow experiment and large eddy simulation(LES).First,the gas exchange experiments in the prototype and the reduced model were conducted focusing on a four-stroke diesel engine.According to the similarity theory,flow coefficient(Cf)and swirl intensity(Nr)were chosen as the verification parameters.The results show that Cf and Nr of the prototype and the reduced model are in good agreement under the Re criteria.The averaged intake and exhaust Cf deviation are respectively within 2.7% and 2.5%.The averaged intake Nr deviation is within 0.084.considering that test bench precision of Cf and Nr are ±1.5% and ±0.05 respectively,it can be concluded that flow similarity between the prototype and the reduced model is satisfied,and the model with the reduced size can replace the prototype for flow experiment under the Re criterion.Then,LES was used to simulate the flow in a two-stroke uniflow scavenging platform with three dimensional(3D)Particle image velocimetry(PIV)in Technical University of Denmark.Under the Re criteria and the experimental data,the predictions of the model were validated.The result shows that the dimensionless axial velocities of the prototype and the reduced model are similar with a “wake-like” profile,indicating the high similarity.The deviation between the results from the prototype and the model is within 5% in different sections.The dimensionless tangential velocities of the prototype and the reduced model also shows high similarity with Burgers vertex profile,and the deviation is within 10% in different sections.The pressure distribution is similar too.Therefore,the prototype and the model can be considered to be flow similar.It is summarized that the scaled down model can replace the prototype for flow study under the Re criterion.