Study On The Thermal Load Of The Cylinder Head In A Natural Gas Engine And The Boiling Heat Transfer Inside The Cooling Gallery

The cooling system is an important part of an engine, serving to control the engine thermal load, whose performance also affects the structure size, power, fuel economy and exhaust emission of the engine. The local heat transfer would be greatly enhanced if boiling regime is properly utilized in the development of the cooling system of an engine, which improves the work efficiency of the cooling system and the performance of the whole engine. However, excessive boiling inside the cooling gallery would cause vapor lock failures and hot spots in heated components, which further affect the normal operation of the engine. The modern design concept of cooling systems considers that the best heat transfer mechanism inside cooling galleries is convective heat transfer supplemented with moderate boiling, which promotes comprehensive utilization of stability of convective heat transfer and high heat transfer ability of boiling. Thus accurate prediction and reasonably control of the heat transfer mechanism and boiling intensity inside the cooling galleries must be achieved.A vehicle natural gas engine was taken as the research object in this thesis, a thermal balance test and cylinder head temperature measurement experiment at different cooling system working conditions was conducted. The results show that various degrees of boiling are expected inside the cooling gallery and the coolant temperature and system pressure affect different zones in the cylinder head by different level, upon which the coolant temperature has negligible effects on the high temperature zones and shows apparent effects on the low temperature zones, while the system pressure has apparent effects on the high temperature zones and shows negligible effects on the low temperature zones.In order to realize accurate prediction of the boiling intensity inside the cooling gallery, a boiling heat transfer model with improved flexibility and calculation accuracy was proposed based on Kandlikar's division description method and experiment data obtained in a cooling gallery simulator passage, otherwise, a multi-field coupled system including the flow heat transfer inside the cooling gallery, the heat conduction in the cylinder head and the combustion process inside the cylinder was established. The results show that the computational error is reduced and effective prediction of the heat transfer mechanism and intensity inside the cooling gallery is realized due to the incorporation of the proposed model in the multi-field coupled system.In order to analyze the vapor distribution inside the engine cooling gallery, a two-phase boiling flow model is established based on the framework of Euler mixture equations. On that basis, the two-phase boiling flow inside the engine cooling gallery is simulated and analyzed. The results show that bubble congestion is expected in the area located in the upper water gallery besides the exhaust port due to the presence of flow stagnation zone, suggesting that the flow distribution condition inside the engine cooling gallery need to be further optimized.The boiling condition inside the engine cooling gallery as well as its effect on the thermal load of the cylinder head was analyzed by experiment measurement and numerical simulation. The results show that both reduction in coolant temperature and rise in system pressure could increase the anti-cavitation pressure of the cooling system, which reduce the cavitation failure rate of the cooling system, however, the thermal stress on the cylinder head is increased as it weak the boiling intensity inside the cooling gallery, leading to decrease in the engine thermal fatigue strength. The results further indicate that when the metal temperature is among the allowable range of the cylinder head material, simultaneously increasing the coolant temperature and the system pressure could enhance the boiling intensity in thermal critical areas and decrease the thermal stress as well as the thermal load of the cylinder head after ensuring the anti-cavitation pressure of the cooling system.Through above study, effective prediction of the heat transfer mechanism and intensity inside the cooling gallery was realized, and guidance for reasonable control of the boiling condition was provided.