Thermal Load Analysis Of The Piston Of A Low-speed Two-stroke Marine Engine By Fluid-solid Coupling Method

As one of the key heated components of diesel engine,the piston is subjected to extremely high periodic changes in thermal load and mechanical load which is a great challenge to the reliability of the piston.With the improvement of diesel engine enhancement,the thermal load of piston increasing significantly.Low speed two stroke marine diesel engine is characterized by low speed and long combustion period,which result in higher heat flux and higher thermal load.Low speed two stroke marine diesel engine usually equipped with two or three fuel injectors,which will lead to uneven distribution of equivalence ratio and flame in combustion chamber,the ultimate impact is uneven distribution of heat flux of piston head along circumference.To reduce thermal load of piston,oscillating cooling,a very high efficient cooling method is been taken in the piston cooling process.Most of the absorbed heat was taken away by lube oil in cooling gallery.The cooling mode is oscillating cooling,which is a multidimensional,multiphase and transient heat transfer process.It's obvious that the heat transfer process of piston is very complicated,zero-dimensional model and quasi-dimensional model based on empirical formula and semi-empirical formula can't reflect the real heat transfer process.In this paper,the thermal and stress analysis of a low speed two stroke marine diesel engine are carried out by means of fluid-solid coupling method.The thermal boundary of piston head and cooling gallery wall were mapped from three-dimension CFD calculation results of combustion process in cylinder and heat transfer process in cooling gallery.The main work is divided into the following three parts:(1)Based on the two-dimensional engineering drawings,the geometric model of the piston and the cooling oil chamber is established.A 3D CFD software was used to simulate the working process in the engine and multiphase flow oscillation heat transfer process of cooling gallery.The heat transfer of the piston surface and the cooling oil chamber was evaluated according to the distribution of velocity,heat transfer coefficient and temperature.(2)The thermal boundary conditions of the piston top surface and the cooling oil chamber are obtained by fluid-solid coupling method,and the temperature field calculation of the piston in 100% load is completed.By analyzing the temperature field of the piston,the rationality and distribution of thermal load for the piston are evaluated.Based on the calculated results,the possible failure modes and positions of the piston are predicted,and a solution to reduce the thermal load is proposed for areas where the heat load is too high.In addition,the heterogeneous distribution of the temperature along the circumferential direction of the piston top surface of a low speed two stroke diesel engine is studied in this paper.It has been proved that the maximum temperature fluctuation along the circumferential direction of piston head exceed 100?.(3)Based on the calculation of the temperature field,the thermal structure coupling stress field of the piston is analyzed considering the influence of the cylinder pressure,bolt preload and inertia force.The stress distribution and deformation of the piston at the maximum burst pressure are obtained.By analyzing the stress and deformation of the piston,the reliability of the piston and the rationality of the structural design were evaluated,which would provide a reference for further improvement of piston.