开孔翼型涡流发生器诱导相干结构动力学特性的TRPIV研究

In the turbulent flow, the vortex generator induces coherent structure, which can enhance the fluid’s transport capacity and promote flow mixing, to lead to the heat transfer enhancement. It has been widely used because of the advantages on the expenditure and practicality. However, at present the theoretical research on the flow field induced by vortex generator is still not deep enough and lacking for the comprehensive analysis of coherent structures. As the important turbulent structures, the coherent structures have significant impact on the heat transfer phenomenon accompanied with flow. This project employs time-resolved particle image velocimetry (TRPIV), from the side and top side, to measure the flow flied induced by an improved winglet with punched hole. Based on this experimental database in a water channel, the mechanism of enhancement of the overall performance factor is explored from the viewpoint of space-time influence the dynamic characteristics of the coherent structure. In Eulerian coordinate system, the shape, strength and shedding frequency of vortical structures in the wake of vortex generator are studied by the use of correlation function, conditional samplingand and linear stochastic estimation (LSE). Furthermore, the spatial topological mode of coherent structures during bursts is extracted by the new mu-level criteria based on locally averaged velocity structure function to research the burst intensity and frequency. In Lagrange coordinate system, the direct Lyapunov exponent (DLE) method is utilized to describe the Lagrangian hairpin vortices and to analyze their spatial evolution when convecting down streamwise. From the perspective of coherent structure, not only the effect of the shape, location and area of punched hole on coherent structures’ dynamics behavior, but also the accompanying change of internal momentum and energy transport mechanism are systematically investigated.

涡流发生器能诱导湍流产生相干结构，增强流体输运能力，促进流动混合，实现强化换热，已被广泛应用。但现有研究对于涡流发生器诱导的流场分析仍不够深入，尤其缺乏对湍流相干结构的全面分析，而湍流相干结构对流动中伴有的换热现象有着非常重要的作用。项目拟用TRPIV技术，以相干结构的动力学特性为切入点，从时空维度详细研究开孔翼型涡流发生器所诱导的流场。在欧拉坐标系下，运用相关函数、条件平均及线性随机估计等多种方法，分析开孔涡流发生器后所形成涡结构的空间形态、旋转强度、脱落频率的变化，并利用基于平均速度结构函数的mu-level法研究条带结构在喷射、扫掠过程中的拓扑形态、猝发强度和频率。在拉格朗日坐标系下，运用李雅普诺夫指数对相干结构进行辨识，分析相干结构在向下游迁移时的空间演化过程。从相干结构角度，系统分析开孔形状、位置和面积对不同形态相干结构动力学行为的作用，及其引起的湍流内在动量能量输运机制的变化。

涡流发生器作为被动强化换热元件，因其换热性能优越、加工成本低廉，广泛应用于化工、冶金、电力等领域。目前对涡流发生器的研究多停留于对其传热、流阻性能的评价，缺乏内在强化换热机理的分析。湍流流动中，相干结构对壁面阻力和传热传质有非常重要的作用。本项目从湍流相干结构角度详细研究了翼型涡流发生器所诱导的流场。结果表明涡流发生器后方存在不同的涡结构，空间位置不同，对强化换热的贡献也不同。在涡流发生器上边缘产生的发卡涡周期性脱落，向高处运动，形成发卡涡涡包结构。发卡涡的存在使流向平均速度剖面的上方具有拐点特征。在涡流发生器高度处，发现发卡涡腿特征，流向平均速度在展向上出现拐点，并且随着发卡涡向下游发展，涡腿的展向距离逐渐增加，对应的拐点也在展向上扩展移动。在发卡涡的诱导作用下，下方的低速流体向后喷射，携带近壁区的高温流体至更高处，与当地低温流体发生热交换。在涡流发生器的中后部，由于逆压梯度的存在，在两侧产生了准流向的反旋涡对。它对周围流体的诱导作用使得近壁面高温流体向上运动，将热量传递给高处的低温流体。由于流体的诱导作用，发卡涡和反旋涡对都会产生二次结构。翼型涡流发生器的几何形状对其综合换热性能的分析表明，梯形底边宽度和高度、开孔的大小及位置对综合换热因子影响最大。在Re=12000-36000范围内，梯形的下底和高分别为矩形通道宽和高的3/8和1/10，上底为下底的1/10，发生器的倾角为60°，开孔圆心位于梯形的半高处，开孔面积占梯形面积的0.03时，综合换热性能最佳。通过比较发现，最优涡流发生器诱导的涡旋结构更接近边界层，对边界层的破坏作用更大，能有效地提升换热效果。拟合出了不同雷诺数下，综合换热因子与主要几何影响因素之间的预测关联式，96%的预测值偏差在±12%以内，为强化换热领域翼型涡流发生器的设计和优化提供了理论依据。