The Fluid-structure Conjugate Heat Transfer And Strength Analysis In Combustion Chamber Components Of Marine Diesel Engine

Nowadays, improving reliability, high specific volume power, low emission and intellectualization are the development trend of marine diesel engine. The increase in power density of diesel engine leads to the increase of mechanical and thermal loads. High thermal-mechanical load causes the increase of failure rate, and reduces the reliability of diesel engine. To study the effects on strength of mechanical and thermal loads, the fluid-solid conjugate heat transfer and thermal-structure strength in combustion chamber components were calculated. To reduce the destruction of high mechanical and thermal load, the optimization design of piston was proposed.The main content of the paper is described as follows:(1) According to the characteristics of heat transfer in marine diesel engine, the fluid-solid coupling heat transfer model of fuel gas, combustion chamber components and cooling medium were built. To investigate the heat transfer of combustion chamber components, the fluid-solid conjugate heat transfer under steady-state of rated power was analyzed. The accuracy of the simulation in terms of temperature estimation within the combustion chamber components, as well as its thermo-mechanical behavior, heavily depends on the choice of proper thermal boundary conditions. In particular, boundary conditions accounting for the combustion and gas/solid heat fluxes were derived from a combination of three-dimensional simulations of the in-cylinder processes. The boundary conditions for the components/coolant interface, where heat transfer coefficients were precisely mapped from the CFD solution, were calculated. To obtain the temperature distribution, a static fluid-solid conjugate thermal finite element calculation was performed. The highest temperature of the combustion chamber components was about526℃which was located in the center of the bottom surface of the exhaust valve spindle.(2)The transient heat transfer of combustion chamber components under starting and stopping condition were calculated. The thermal boundary conditions under25%,50%,75%,85%,100%loads, accounting for the combustion and gas/solid heat fluxes were derived from three-dimensional simulations of the in-cylinder processes. Under starting condition, the temperature of the combustion chamber components was raised along with the increase of the load. It tended to stable within15minutes after the change of load. Under stopping condition, the temperature lowered along with the decrease of load. For a better cooling, the temperature of piston, cylinder cover and cylinder liner decreased faster than exhaust components. For a higher temperature in combustion chamber after the engine shut-down., the cooling system should continue to run after stopping.(3)Under rated power condition, periodic transient heat transfer simulation in an operating cycle was calculated. The primary distribution principles of temperature field of combustion chamber components were worked out. In an operating cycle, temperature fluctuations of combustion chamber components occurred only in the shallow surface of the combustion chamber wall. With increasing depth from the wall, the amplitude of temperature fluctuation decreased. The temperature fluctuation of components was consistent with the trend of the thermal state of the gas, but lagged behind the gas temperature change at some crank angle.(4)Based on the fluid-solid conjugate heat transfer simulation, the strength analyses under steady and periodic transient condition were obtained. The stress distribution of combustion chamber components was mainly caused by the thermal load, and the stress fluctuation was mainly caused by the mechanical load. The results suggest that stress concentration is mainly caused by thermal load. Enhancing cooling and improving structural design were used to reduce destructive effect, and to improve the reliability of piston.