【作者】 黄钰期；

【导师】 俞小莉；

【作者基本信息】 浙江大学 ， 车辆工程， 2010， 博士

【摘要】 汽车冷却系统是保障车辆稳定运行的重要辅助系统,对节能减排具有直接影响。在当前环境下,随高密度、大功率发动机的出现以及众多新技术、新系统在汽车上的应用,车辆散热环境日趋恶劣,对冷却系统的体积、重量、工作效率等也提出了新的需求。而各热交换器组件的性能优化是实现冷却系统整体优化设计的基础。如何在不借助辅助设备,不改变使用条件并合理控制阻力能耗的前提下,实现流动传热过程的优化,是现代汽车冷却系统设计和研究的核心问题。本文以强化传热的场协同原理为理论依托,以计算流体力学(CFD)方法为主要研究手段,与风洞实验技术相结合,对车用冷却系统从单元到组合模块,从局部到整场,从微观到宏观的流动传热特性分别进行研究。主要研究内容包括：1.对车用冷却系统的局部换热单元、单个热交换器、热交换器组件以及热交换器与风扇组合分别建立CFD三维仿真模型,研究换热过程的流动传热特性。2.采用多尺度耦合法对单个热交换器进行微观分析和整体仿真。3.利用风洞实验平台,采集实验数据以验证仿真模型的有效性。4.根据实际工作时可能遇到的状况对原始模型和边界条件进行调整,分析冷热介质流动不均匀性对单元换热过程的影响。5.研究单个热交换器内外侧的流动特性,分析结构、冷却介质分布规律等因素对整体换热效果的影响,提出可行的结构优化方案。6.分析两相邻热交换器之间的相对位置、距离以及热介质流动方式对组合模块换热过程的影响。7.研究吸气式风扇驱动时空气的流动特性。将热交换器和风扇进行组合,比较不同风扇配置方案对热交换器换热效果的影响。8.结合热交换器中的场协同原理,对以上研究结果进行印证和解释,设计相应的强化传热方法或结构优化方案。通过以上一些研究,发现：对以空气(气体)为冷热介质的换热过程,如果在计算时考虑介质的物性变化,与常物性并参照入口温度设置物性参数的仿真模型相比,会使冷热侧温差略为降低,冷空气压降略有增加,而热空气的压降明显减少。对局部单元的换热,如果热介质流速分布不均匀,尤其是热介质流速沿冷却介质流动方向降低时,会使单元换热能力降低。如果冷却介质的温度分布沿热介质流向降低,冷热介质温度场协同性相对更好,能适当提高换热效果,反之则使换热效果变差；如果冷却介质的流速沿热介质流动方向升高,流速升高导致温度降低,也能改善冷热介质温度场的协同性,使换热效果略好,反之则换热较差。对单个热交换器,如果采用多尺度耦合分析方法对热交换器进行详解,能显著提高计算精度,但操作过程较为复杂。在车用热交换器中,热介质在各通道的不均匀分布是降低整体工作效率的一个主要因素。因此,改良热交换器结构,使热介质在各通道分布更均匀,是有效的优化方向。此外,增大热交换器低温区的冷却介质流量,能降低低温区冷却介质的温度,改善冷热流体温度场的协同性,实现强化换热的目的。对相邻热交换器组合模块,在不考虑热辐射因素时,变换芯部上下的相对位置会对换热有轻微的影响,而当芯部距离达到一定范围且四周密封时,再增大芯部间距也并不会影响前后热交换器的换热效果。改变上游热交换器热介质流动方向,使热通道中的流量和温度分配更均匀,能显著提高该热交换器的换热效率,但由于冷却风在流经该热交换器后的温升更高,将导致下游热交换器的换热效果变差。在吸气式风扇的驱动下,风扇上游速度分布并不均匀,叶片尤其是叶尖正前方的来流速度较高,轴心和边角处速度较低。将热交换器及风扇区域密封,对提高风扇的工作效率十分关键。对典型的乘用车水散热器,采用一个较大风扇和两个小风扇的组合,和传统风扇配置方案相比,在静压(总和)相等的情况下,能实现更好的换热效果。更多还原

【Abstract】 The cooling system of vehicle is an important assistance system to ensure the stability of the vehicle, and it has a direct effect to energy-saving and emission reduction. Recently the heat dissipation environment of vehicles is gradually deteriorating due to the emergence of high-density and high-power engines, as well as the application of many new technologies and systems. As a result, new demands have been raised on the volume, weight, efficiency and intellectualization of cooling systems. Meanwhile, the profit rate of automobile industry keeps decreasing so that it becomes an important direction to work out the optimization of cooling system with economical and effective measures.Based on field synergy principle, together with the application of CFD and the combination of wind tunnel techniques, the paper completed the research on the flowing and hear transfer characteristics of vehicle cooling modules, including that of the unit and module, partial and whole, micro and macro. The main contents are as follows:1. The research on flowing and heat transfer characteristics is completed on local heat exchanger unit, single heat exchanger, heat exchanger assemble and the combination between heat exchanger and fan, according to the establishment of the CFD 3-D simulation model.2. The microscopic analysis and integral simulation of single heat exchanger was completed based on multi-scale coupling.3. The wind tunnel experiment platform was built up to examine the effectiveness of simulation model according to the experiment data.4. According to the possible conditions in real situation, the original model and boundary conditions are adjusted to analyze the effect of uneven flow of both hotter and colder medium on the heat transfer process.5. The inside and outside flowing characteristics of single heat exchanger was analyzed, the optimization method was raised based on the analysis of effect of factors such as structure and the distribution regulation coolant.6. The analysis is completed on the effect of the relative position and distance, and the flow of hotter medium on the heat transfer process of heat exchanger module.7. The air flowing characteristic is researched when driven by suction fan. The effect of different fan configuration on the heat transfer performance of cooling module was compared by the combination between cooling module and fan.8. According to the field synergy principle, the results of research mentioned above was proved and explained, and the measure to enhance the heat transfer or structure optimization were designed.Conclusions from the researches above are as follows:As for the hear transfer process with air as media, considering the media property changing during calculation, the temperature difference between hot and cold side will decrease slightly, the pressure drop on the cold side will increase, and the pressure drop on the hot side will obviously decrease, compared with constant property simulation model.For the heat transfer of partial unit, the heat transfer capacity of the unit will decrease when the velocity distribution of hot medium is not homogenous, especially the velocity of hot medium decreases along with the flowing direction of cold one. If the temperature of cold medium decreases along the flowing direction, the temperature field synergy between hot and cold media is relatively better, thus showing better heat transfer performance, otherwise the heat transfer effect will get worse. If the velocity of cooling medium increases along the flowing direction of hot medium, the temperature will decrease due to the rise of velocity, and temperature field synergy between hot and cold media could be better, thus improving the heat transfer performance, otherwise the heat transfer will also get worse.As for the single heat exchanger, the application of multi-scale coupling method could raise the calculation accuracy, however the operation process would be complex. With reference to the vehicle heat exchangers, the inhomogeneous distribution of hot medium is an important factor to lower the working efficiency of intercooler. As a result, it is an effect optimization to improve the structure of heat exchanger, thus allowing a homogeneous distribution of hot medium in channels. Besides, the temperature of cold medium in low-temperature section could be lower if the flow rate of cold medium in low-temperature was increased, and the temperature field synergy could be better, thus realizing the enhancement of heat transfer.As for the adjacent cooling module, the change of the relative position of the core would have slight effect on heat transfer performance without the consideration of heat radiation factors. However, when the distance between cores reached a certain value, the increase of core distance would not affect the heat transfer performance. Changing the hot medium flowing direction of upper heat exchanger could achieve a more homogeneous distribution of flow rate and temperature in hot channels, thus improving the efficiency evidently, but it will meanwhile worsen that of the lower exchanger.When driven by suction fan, the velocity distribution in upper area of fan is inhomogeneous, the flow velocity was high at blades, especially at blade tips, and the velocity in center and edge was relatively lower. It is a key factor for the efficiency improvement of fan if the heat exchanger and fan were sealed. For typical water tank exchanger on passenger vehicles, the combination of a larger fan and two smaller fans, compared to traditional fan configuration, could obtain better heat transfer effect under the same static pressure.更多还原