过渡流下分离与再附不稳定性及其传热强化机理的研究

The transition flow has stronger instability and its instability has periodicity which can produce the periodical heat transfer enhancement to the wall surface. Especially in the separation and reattachment, instability has a three-dimensional structure, therefore, to make heat transfer enhancement mechanisms to be more complex. In addition, with a compactness trend of heat transfer equipments, internal flow in some of heat transfer equipments has large probability to be in transition flow region. Therefore, the research on instability of flow separation and reattachment and heat transfer enhancement mechanisms in the transition flow has higher academic significance and practical value. In this project, firstly, the theoretical analysis and mechanism researches in the transition flow are studied to establish a corresponding mathematical model. Secondly, a heat transfer numerical model of the three-dimensional backward-facing step, which is validated by PIV experiments and heat transfer experiments, is established. Finally, flow characteristics of separation and reattachment in the transition flow are analyzed systematically and the three-dimension characteristics of instability are researched particularly. And then the heat transfer enhanced mechanisms on the wall are concluded to find the key factors, which reduce flow resistance and improve heat transfer efficiency. It will provide theoretical basis for the design of the high efficiency heat transfer enhancement equipment.

过渡流有着较强的不稳定性，这种不稳定性具有周期性，能对壁面产生周期性强化作用。尤其在分离和再附中，不稳定性具有三维结构特征，壁面的传热强化机理变得更为复杂。此外，随着换热设备向小型化的发展趋势，部分换热设备内部流动处于过渡流状态的概率变大。因此，研究过渡流下分离与再附不稳定性、传热强化机理具有很高的学术意义及实用价值。本项目首先对过渡流进行理论分析和机理研究，建立相应的数学模型；其次建立相关三维后台阶流动传热数值模型，通过PIV实验及相关传热实验验证上述过渡流传热数值模型；最后系统分析过渡流下分离与再附的流动特性，着重研究过渡流区湍流结构不稳定性的三维特性，继而研究相关壁面的传热强化机理，找到降低流场阻力和提高传热效率的关键因素及有效手段，为高效壁面传热强化换热设备的设计开发提供理论依据。

分离再附流动在工程中具有广泛的应用，多年来一直是国内外学者研究的热点。过渡流存在着周期变化的流动不稳定性，能够强化壁面传热。因此，研究过渡流下的分离再附流动不稳定性及其强化传热机理具有一定的科学意义和工程价值。本项目通过数值模拟结合实验的方法对过渡流下的分离再附流动传热问题进行了研究。取得了以下成果：1.建立了二维和三维的台阶绕流流动传热数值计算模型，并通过实验验证了数值模型的准确性。2.搭建了后向台阶绕流闭式循环水槽实验台，开展了PIV实验和传热实验，根据实验结果研究了固体侧壁边界引起的三维效应。3.研究了二维和三维后向台阶再附着区下游的流动不稳定性与旋涡发展规律，重点考察了该区域流动的低频脉动特征。4.研究了二维和三维后向台阶绕流流动不稳定性与壁面传热的关联性，重点考察了流动不稳定性与传热不稳定性的耦合关系。基于上述研究结果，代表性的成果有：发表SCI收录的论文6篇，申请发明专利4项（授权4项），培养青年教师2名，研究生10名。总的来看，本项目的研究成果能够为高效换热设备的设计开发提供一定的理论依据。