Study On Oscillating Cooling And Heat Transfer Characteristics In Marine Diesel Engine Piston

Compared to the vehicle diesel engine,marine engines display some features like low speed,heavy rotate inertia,rigorous friction,bad lubrication and difficult to cool.When the thermal loading of pistons out of control in low speed high power diesel engine,some damages of engine like piston broken,cracking,cooling oil coking even cylinder friction will occur.Oscillating cooling in pistons has been widely used to solve the thermal problem in low speed high power engine.It's important for piston design to study the process of oscillating cooling and the influencing factors accurately.Here in the thesis,the work was related to the low speed high power diesel engine piston oscillating cooling method.There are described in detail in the following:In order to study the characteristics of oscillating flow and heat transfer in piston cooling gallery,a three-dimensional model was built up and meshed with grid independence check.According to the analysis of the mechanism of oscillating and heat transfer,some physical process of gas-oil two-phase flow were simplified.Considering the calculation accuracy and calculation time cost,a turbulence calculation model and a two-phase flow model CLSVOF were selected as basis.The key points focused on the process of flow and heat transfer in the cooling gallery with double cooling chamber.The variations of flow and surface heat transfer with engine speed,and the variations of flow and surface heat transfer with the oil inlet flow were studied by numerical calculation method.It was found that the oil-filled rate of the low speed diesel engine's cooling gallery is about 0.8 during the working state.There were some differences of oil flow and heat transfer characteristics between the internal cooling chamber and external cooling chamber.The internal cooling chamber reciprocating oscillation heat transfer intensity was lower than the external cooling chamber.At the low engine speed level,the total heat transfer of the large cooling gallery was mainly affected by the oil-filled rate.When the oil inlet flow was fixed,the oil-filled rate of gallery decreased with the engine speed increases,and the heat transfer also reduced.The external cooling chamber had more strong reciprocating oscillation intensity and was more sensitive with the influence of engine speed and oil-filled rate.It's surface heat transfer coefficient decreased first and then increased with the increase of the engine speed.The thesis analyzed the structural parameters of cooling gallery using the orthogonal design test,aiming at the influence of the number and diameter of the connecting pipes,the height and diameter of the outlet.The results showed that the main geometric factors influencing the heat transfer efficiency were the number and diameter of the connecting pipes.According to the simulation results,a non-dimensional correlation formula for the oscillating flow and heat transfer of outside cooling gallery of the low speed diesel engine was obtained.And the influence of the oil-filled rate and oil-flow rate were considered.In this paper,the oscillating and cooling process of the combined piston of a low speed high power diesel engine were studied,and obtained the oil flow and heat transfer characteristics of the oil gallery with separated small chamber.In order to obtain the temperature field of the piston,the boundary condition of the piston top and ring groove area and surface of gallery was calculated.A finite element analysis model of the temperature field of the piston was established by the boundary condition to obtain the accurate temperature distribution of the piston.According to the calculated results of the piston temperature field and the orthogonal design test of cooling gallery structure,the structure of the cooling gallery was optimized,increased the number of the connecting pipes.Further investigation of the flow and heat transfer shows the oscillation cooling efficiency of the cooling gallery was increased.Through the finite element analysis of the piston temperature field,it was found that the piston heat loading decreased after the optimization of the cooling gallery structure,and the maximum temperature of the piston top was reduced by 18.26 degrees Celsius.