【作者】 王兆文；

【导师】 黄荣华；

【作者基本信息】 华中科技大学 ， 动力机械及工程， 2008， 博士

【摘要】 随着柴油机升功率的大幅提高,柴油机的冷却问题变得越来越突出,成为制约柴油机进一步发展的关键问题之一。本文以YC6M360系列柴油机为研究对象,针对其冷却能力不足、气缸盖鼻梁区出现热裂、冷却水经常开锅等问题,开展了整机热负荷、整机水流分布、气缸盖热负荷和气缸盖水流分布等试验以及冷却水套内冷却水流动与传热的数值模拟研究。对该型柴油机冷却系统的匹配和气缸盖冷却水套的结构进行了优化设计,结果表明,各种优化设计方案能较好的提高该机整机及气缸盖的冷却能力,从而降低了该机整机热负荷和气缸盖热负荷,较好的解决了该机所出现的气缸盖鼻梁区热裂、冷却水经常开锅等故障。在发动机冷却水流动模拟中,本文提出“如试验结果缺乏,可以利用不同的软件进行数值模拟来验证另一软件的数值模拟结果,只要模拟结果差异较小,就可认为该模拟结果是正确的”这一新的验证方法。针对使用FIRE软件进行的柴油机出水管内冷却水流动数值模拟,分别使用传统的试验验证方法和用另一软件Fluent进行数值模拟验证的方法进行了验证。验证结果表明:新的验证方法可用于验证另外的软件的数值模拟结果。利用数值模拟方法进行了基于正交实验设计的上水孔扰流强化传热的研究,分析了全新设计的气缸盖冷却水套中上水孔2和上水孔3不同直径对气缸盖内各区域对流换热系数(HTC)的影响,并针对不同指标得出两个上水孔直径的最佳组合方案。随后,提出了“气缸盖内各区域的整体流速可看成是某上水孔不存在时该区域的整体流速和该上水孔存在时对该区域的整体流速贡献的叠加”,以及“上水孔对各区域HTC的贡献包括因上水孔内冷却水流动而引起该区域内流速变化对HTC的贡献和湍动变化对HTC的贡献”的分析方法。并基于此方法,成功地分离出各上水孔直径时流经其中的冷却水在各区域的扰流对该区域的HTC的强化程度,为上水孔的扰流设计提供了理论指导。进行了气缸盖沸腾传热研究。通过数值模拟,对比了考虑冷却水沸腾和不考虑冷却水沸腾时气缸盖底部换热系数的变化。另外,也对比了不同沸腾模型时换热系数的变化,为以后进行柴油机冷却水套内流动与传热的数值模拟提供参考。最后,本文进行了气缸盖内流固耦合数值模拟,通过冷却水流场和气缸盖固体传热的耦合模拟,得出了气缸盖内温度场,并通过试验数据进行了验证,验证结果表明:误差在工程许可范围以内。通过流固耦合数值模拟,可以使柴油机在初始设计阶段就能详细地了解气缸盖内温度的分布,并在此时就可以进行气缸盖结构的优化,从而可以大大缩短设计周期。更多还原

【Abstract】 With the increasing of the power density of the engine, the cooling system becomes more and more important, which turns to the key problem for further development of the diesel engine.The paper reports an experimental and simulating exploration over a series of problems, such as overheat of the complete machine and thermal cracks in the valve bridge region of cylinder head in YC6M360 series diesel engine. The studies involve heat load test of complete machine, analyzing coolant-flow distribution of each cylinder, measuring the temperature of the bottom part of cylinder head, analyzing coolant-flow distribution of upper nozzles in the bottom side of cylinder head, and three-dimensional numerical simulation on the coolant flow in the water jacket. This paper proposed some schemes to improve the matching of subassembly in the cooling system and optimized the structure within water jacket according to the test and simulation results. The follow-up numerical simulation and experimental validation identified that the temperature within valve-bridge decreased remarkably, and the heat load of complete machine could be improved significantly, by adopting these optimized schemes.Formerly, the results of simulation were always verified by the results of experiment, but there is always a great deal of the cost on the experimental study, then the experiment always hard to be achieved. But if there is no experimental validation, the mistake of numerical simulation will leads to the loss. So the verification method should be updated. In the paper, a new idea, that the results of simulation can be verified by the simulation results of another software package, was reported. In this paper, the experimental study has been done by using FIRE TM and Fluent TM software package. At last, the idea was improved by result of experiment. Results indicate that this new method can be used to verify the numerical simulation data.The disturbed flow and the boiling flow can enhanced the heat transfer of the coolant. The paper also explored the study on the influence of the flow in upper nozzles on the HTC in cylinder head by means of experimental design using orthogonal table, and the influence of boiling flow to the HTC in cylinder head. Using the numerical simulation method, the effects of the different diameter of the upper nozzles 2 and 3 of the new design cooling jacket within the cylinder head on the HTC in cylinder head have been pointed out. Through the contrast, the best scheme of the diameter has been achieved. Subsequently, the flow velocity within the cylinder head can be considered to be the sum of the velocity without the upper nozzles and the effect of the upper nozzles on the flow velocity, and the effects of the upper nozzle on the HTC within the cylinder head include the improving of the coolant flow inside the cylinder head and the effect of turbulent flow of coolant on HTC. Based on these concepts, the effects of turbulent flow of coolant which flow through the different diameter upper nozzles on the HTC within this area have been pointed out. This conclusion can be considered to be important reference for the optimized design of upper nozzle.In the research of flow boiling inside the cylinder head, the contrast of boiling heat transfer coefficient in the valve-bridge zone between taking the boiled coolant into account or not has been achieved. Besides, the contrast of boiling heat transfer coefficient in the valve-bridge zone among different boiling heat transfer models has been achieved. These contrasts can be considered to be the reference for the numerical simulation on the coolant flows and heat transferring which help to optimize the structure of water jacket.At last, the fluid-solid coupling simulation inside the cylinder head has been completed. The temperature field inside the cylinder head has been gotten by the couple simulation between the flow field of coolant and the heat transfer of the cylinder head. In the following, the results of the simulation were verified by the results from the experimental study. The contrast result indicates that the error can be accepted for engineering application. By means of the fluid-solid coupling simulation, the distribution of temperature inside the cylinder head can be acquired in detail during the initial period for the development of new diesel engine. As a result, the structure optimization of cylinder head and water jacket can also be done during this period, and then the design period is shorted significantly.更多还原