内燃机冷却水腔内沸腾汽泡演化行为与强化传热机理研究

Flow and heat transfer characteristics in cooling system is the key to energy saving and emission reduction for internal combustion engines. Facing the great needs of high efficiency, energy saving and ultra-low emission, a new cooling idea of moderately introducing nucleate boiling heat transfer into local high thermal load area is put forward, to utilize its high heat transfer performance, solve the cooling problem in local high heat flux area, and realize the equilibrium temperature regulation under thermal load time and space non-uniformity of internal combustion engines. The intensive research centers around two key scientific issues, which are the dynamic evolution behavior of boiling bubbles and its mechanism of enhanced heat transfer. The boiling heat transfer visualization experimental platform is set up based on a endoscopic high speed photography system, to explore the evolution behavior of bubbles and its heat transfer characteristics in local high thermal load. Then the theoretical models about subcooled boiling heat transfer and fluid-solid coupled heat transfer for internal combustion engines are established. Through the numerical simulation and experimental investigation, the influence of operation parameters, local water jacket structure, the topography of heated surface on flow and heat transfer characteristics is studied and concluded, to reveal the mechanism of enhanced heat transfer. In view of spectrum analysis of pressure fluctuation and visualization image of vapor-liquid two-phase, a new method to identify,assess and predict boiling conditions is proposed. As a consequence, the research will lay the theoretical foundation and scientific basis for realizing the active implementation and reasonable control of boiling heat transfer with matching operation condition, cooling demand and water jacket structure in internal combustion engines.

冷却系统内的流动与传热特性是内燃机节能减排的关键之一。面对高效节能与超低排放的重大需求，项目提出在局部高热负荷区域内实施适度的核态沸腾传热冷却新思路，利用其高效换热性能，解决高热流密度空间散热冷却的问题，实现内燃机时空非均匀热负荷的均衡温度调控。围绕冷却水腔内沸腾汽泡动力学行为与强化传热机理两大关键科学问题，展开深入研究。搭建沸腾传热可视化实验平台，利用内窥镜式高速摄影系统，探明冷却水腔内汽泡演化行为特征及传热特性，建立内燃机过冷流动沸腾传热和流固耦合传热理论模型。通过数值模拟与实验研究，总结运行工况参数、局部水腔结构、传热表面形貌等因素对冷却水腔内流动特性与传热性能的影响规律，揭示沸腾强化传热机理。利用压力波动时间序列的频谱分析，结合汽液可视化图像，提出沸腾状态的识别、预测与评估方法，为最终实现与运行工况、冷却需求及局部水腔结构相匹配的沸腾传热主动利用与合理控制奠定理论基础和科学依据。

冷却系统内的流动与传热特性是内燃机节能减排的关键之一。面对高效节能与超低排放的重大需求，项目提出了在局部高热负荷区域内实施适度的核态沸腾传热冷却新思路。围绕冷却水腔内沸腾汽泡动力学行为与强化传热机理两大关键科学问题，分别搭建了水平矩形通道沸腾换热可视化试验系统和气缸盖水腔内沸腾换热可视化实验平台，利用内窥镜式高速摄影系统，探明了缸盖冷却水腔内汽泡演化行为特征及传热特性，建立了内燃机流-固耦合传热理论模型以及多种过冷流动沸腾传热模型，提高了模型预测精度。通过数值模拟与实验研究发现，当冷却介质进入到沸腾换热阶段，此时的换热效率明显高于对流换热；主流温度、流速及系统压力对沸腾换热均有不同程度的影响，其中流速对换热的影响最为明显；鼻梁区域内汽泡的成核、成长、滑移和聚合等行为促进该区域汽液两相间质量和能量的传递，打破原有的热边界状态，在局部区域内形成纵向和横向的扰动，揭示了沸腾强化传热机理；总结了运行工况参数、局部水腔结构、传热表面形貌等因素对冷却水腔内流动特性与传热性能的影响规律。尤其是发现表面形貌对冷却流道中的沸腾换热具有非常重要的影响，其中圆柱形凸起形貌具有最佳的沸腾换热效果，圆台形凹坑的换热效果较差，圆柱形凹坑能够较好地促进气泡成核，而半球形凹坑更容易形成涡流强化对流换热。另外，结合汽液可视化图像，初步提出了沸腾状态的识别、预测方法，为最终解决新一代高强化内燃机高热流密度空间散热冷却的问题，实现内燃机时空非均匀热负荷的均衡温度调控奠定理论基础和科学依据。