弹性肋片涡激振动强化圆管对流传热研究

In micro-electromechanical system (MEMS), many studies have found that the convective heat transfer of micro plates and tunnels can be enhanced significantly by the flow-induced vibration of micro-fins. However, very few work have been done on the nature of heat transfer enhancement of the flexible fin and its applications to other heat transfer structures such as tubes in many heat exchangers. Hence, the following work are carried out by this project: (1) designing a modified finned-tube structure which can take the advantages of the flow-induced vibration of the flexible fin in enhancing convective heat transfer, and establishing the corresponding mechanic and mathematical models; (2) developing a numerical method for the forced convective heat transfer of this modified finned-tube model at low Reynolds numbers, based on the characteristic-based split (CBS) finite element method (FEM), the dual time stepping method, the Galerkin FEM, the spring analogy method, the combined interface boundary condition (CIBC) method and the loosely coupled FSI method; (3) by numerical simulation, investigating in detail the influence of the flow, structure and thermal parameters on the vortex-induced vibration of the flexible fin and the heat transfer characteristics of the modified finned-tube model; (4) using the Lagrangian coherent structure, describing quantitatively the influence of the fin vibration on the convective characteristics of the flow field near the structure surface, and revealing the mechanism of heat transfer enhancement by flow-induced vibration of the flexible fin. This project will provide a new approach for the heat transfer enhancement technique using flow-induced vibration and lay a foundation for the application of the flexible fin in other heat transfer problems.

在微机电系统中，微型翅片流致振动对平板和微通道强制对流传热的显著强化作用已经为众多研究所证实。然而，弹性肋片流致振动强化传热的机理及其对于圆管等传热结构的适用性的相关研究还非常不足。因此，拟开展以下几个方面的工作：（1）提出一种可主动利用流致振动强化传热的弹性肋片圆管结构模型，建立对应力学和数学分析模型；（2）基于CBS有限元解法、双时间步方法、Galerkin有限元解法、弹簧近似法、修正结合界面边界条件方法以及流-固松耦合解法，开发层流情况下弹性肋片圆管模型强制对流传热问题的流-固耦合解法；（3）通过数值试验，揭示主要流场、结构及传热参数对弹性肋片圆管模型涡激振动、传热特性的影响规律；（4）利用拉格朗日拟序结构，定量分析肋片振动对近壁区流场对流特性的影响机制，揭示弹性肋片流致振动强化传热的机理。本项目将为流致振动强化传热技术的发展提供新的思路，为弹性肋片的推广及应用奠定理论基础。

在湍流脉动、旋涡脱落等因素影响下，流体中的弹性结构可以产生自激振动。流致振动可以增加流场湍流度、缩小“死水区”、提高近壁面流体与主流间的动量交换，因此可以用于强化对流传热。基于该思想，本项目提出了一种新型弹性肋片管结构，并利用数值方法对其流致振动特性、对流传热特性进行了研究。主要完成了以下几个方面的工作：（1）提出了一种弹性肋片管结构模型，并对相应流动、传热、振动过程建立了力学和数学模型；（2）通过将CBS有限元解法、双时间步方法、Galerkin有限元解法、弹簧近似法、流-固松耦合解法相结合，发展了一种高效稳定的流-固-热耦合解法及程序，并利用流动、对流传热和振动经典算例进行了验证；（3）通过数值模拟，研究了所提出弹性肋片管的流致振动和对流传热特性。研究结果表明：当弹性肋片结构固有频率接近肋片管旋涡脱落频率时，弹性肋片流致振动频率将“锁定”到结构固有频率上，振幅也会显著增加；弹性肋片大幅振动可使肋片管表面旋涡提前脱落，减小尾流“死水区”，从而提高对流传热效率。在计算参数范围内，与刚性肋片管相比，弹性肋片管可实现最高11.07%的强化传热效果。本项目为流致振动强化传热技术的发展提供了新的思路，为弹性肋片的推广及应用奠定了理论基础。