数值研究近壁旋涡结构在强化对流换热中的作用及其局部动力学机制

Convective heat-transfer enhancement is an important component of heat transfer research and engineering application. Most of these investigations focus on the synergy between the velocity field and the temperature-gradient field based on a steady or temporal-averaged calculation, or focus on some characteristics which represent a channel's overall heat-transfer performances (e.g., the friction factor f, the Colburn j factor which measures the air-side heat-transfer coefficient, the Nusslet number Nu, and so on). In the present project, based on a general derivative-moment theory, a quantitative relationship between the near-wall flow structures and the boundary heat flux can be obtained in convective heat-transfer enhancement by performing a derivative-moment transformation, where the flow structures are usually represented by vorticity or Lamb vector. Based on the new local-dynamics principle, the dynamic mechanism between the near-wall vortices and the unsteady characteristics of the boundary heat flux can be revealed by investigating the evolution of the local vortices and the corresponding momentum-transport and heat-transport processes in convective heat-transfer enhancement. Further, we can find some special flow structures which have the dominant contributions on the boundary heat flux and the heat transport, and then analyze the physical mechanism of the production of these flow structures in convective heat-transfer enhancement. We believe that, these results should be a credible and promising principle for optimizing the design of convective heat-transfer enhancement. In this project, the numerical investigation will be performed both on wavy fins and on louvered fins as two kinds of convective heat-transfer configurations, by means of our new developed computational fluid dynamics software BXCFD.

强化对流换热技术具有广泛的工程应用背景。大量的研究工作都侧重于分析稳态换热行为的场协同性、分别表征流道整体换热特性和阻力特性的j因子和f因子、以及表征壁面对流换热效果的Nu数等特征量，对流场内部的输运特性研究相对较少。在本项目中，我们将从导数矩理论出发，利用导数矩变换等数学方法，建立强化对流换热过程中近壁流动结构（通常采用涡量或Lamb矢量等物理量进行描述）与壁面热流之间的定量关系。在此基础上，我们将以波纹翅片和百叶窗翅片为原型，利用自主开发的BXCFD软件及相关高精度计算模块，对强化对流换热问题开展精细的数值模拟研究，重点揭示强化对流换热过程中近壁旋涡结构的动力学演化规律、及其与壁面热流和近壁热量输运的非定常特性之间的内在关联机制，并试图挖掘出对强化对流换热具有关键性作用的局部流动结构、及其产生和物理演化机理，为设计高效的强化对流换热技术提供可信的、可操作的原理性依据。

本项目针对对流换热问题，首先从导数矩理论出发，推导了以涡量场及其导出量为特征的近壁流动结构与流道受力和壁面热流之间的局部动力学关系。以此为基础，我们针对传统的板翅式波纹翅片流道和项目组设计的后掠型波纹翅片流道进行了细致的数值研究，重点分析了近壁流动结构形态、旋涡演化的动力学过程及其热量输运与流道阻力特性和壁面热流之间的耦合关系。数值结果表明，传统波纹翅片流道以波峰前后的展向涡层流动为主，流动掺混效果不强，流道阻力大。而在后掠型波纹翅片流道中，展向涡层掠过波峰后将被拉伸成流向涡结构，该流动结构将诱导背风区和波谷处的热量随流体快速上升、并迅速被卷入涡核内，使得流向涡及其影响区内温升显著，并随流向涡一起沿流向输运至下游。这一流体-热量输运现象导致：（1）在流向涡形成、发展阶段（y/b=10%~50%区域），当地热流密度将显著强于非后掠型翅片流道，后掠角越大、流向涡结构越强，则强化对流换热效果越突出；（2）由于上游热量被迅速输运至下游、当地流体的温升过大，导致在流向涡破碎阶段（y/b>50%区域）的热流密度可能弱于非后掠型翅片。但由于在第1阶段的强化换热效果显著强于第2阶段的退化作用，因此后掠型波纹翅片的强化换热能力将整体高于非后掠型翅片。.在上述工作基础上，我们进一步针对后掠型波纹翅片的波长、波幅、后掠角等参数对翅片强化换热特性的影响进行了比较系统的数值研究，并设计了一个比较理想的优化方案，在保持流道尺寸、材质及边界条件不变的前提下，优化后的后掠型波纹翅片流道j因子提高了34.3%、f因子下降1.2%。.上述科研工作及成果对研究及发展强化换热新技术具有一定的指导意义。围绕本项目研究内容及成果，项目组累计申请并获得3项实用新型专利授权。