弹性湍流主动控制诱发及其流动换热特性研究

Elastic turbulence has received intensive interest recently due to its capability of intensifying flow and heat transfer in micro/minichannels at low Reynolds numbers. State-of-art literature review shows that the heat transfer performance was mainly characterized by a global heat transfer coefficient, which can not reveal the mechanism between flow dynamics and heat transfer. Elastic turbulence was induced passively either by the curvature of the flow channel or inserted obstacles in most of the experiments. The onset of elastic turbulence in straight channels has not been achieved, which limits its many practical applications. This project proposes an innovative method to actively trigger elastic turbulence by using nanoparticle modified polymers via an external force field (i.e. magnetic or ultrasonic field). The concept is based on the interaction of nanoparticles with the external field to trigger the onset of elastic turbulence. To validate the new concept, four tasks are planned, which include: i) Studying the influential parameters such as fluid salinity and temperature on the onset of elastic turbulence and its consequent influence on the flow and heat transfer performance; ii) Synthesizing and characterizing new fluids via nanoparticle/polymer mixture or nanoparticle-polymer conjunction that sensible to an external force field; iii) Conducing active inducing and controlling of elastic turbulence based on the new fluids under an electromagnetic field or ultrasonic field in both curved and straight channels; and iv) Investigating advanced techniques for drag reduction in the regime of elastic turbulence. This project will not only reveal the local mechanism of heat transfer intensification by elastic turbulence but also provide a new method to trigger and control elastic turbulence, both of which would promote future applications of elastic turbulence in industry.

弹性湍流微尺度强化换热技术因其低雷诺数下显著的换热潜力被国内外学者广泛关注，然而其流动换热特性的表征主要借助全局变量，局部换热机理仍没有详细的阐述；且弹性湍流的诱发主要被动依赖流道几何结构，大大限制了其应用。本项目创新性的提出利用纳米技术改性的聚合物溶液，通过施加电磁场或超声场，主动诱发弹性湍流并研究其流动换热特性。首先，研究不同聚合物敏感性因素下弹性湍流诱发机理及局部流动换热特性表征；再次，配制纳米改性聚合物溶液、表征其特性，并研究外场作用下在弯曲及直微通道内主动诱发弹性湍流的机理及流动换热特性；最后，基于主动诱发弹性湍流技术，在保障弹性湍流发生的条件下，实现粘弹性流体降粘减阻的技术研发。本项目的研究成果有助于揭示弹性湍流强化对流换热机理，建立基于改性聚合物热流体工质的主动诱发弹性湍流技术，并完善弹性湍流诱发机制，为弹性湍流大面积工业化应用提供科学依据及可靠技术支持。

本项目采用实验与数值模拟的手段，通过研究多种聚合物敏感性因素、流道结构特性下粘弹性流体的流动与传热特性，完善了对弹性湍流诱发机理及其与换热特性协同机制的认识，并利用聚合物改性思路，提出了主动诱发弹性湍流的先进技术，完成了预期目标，也为后续研究提供思路。具体内容包括：.1）针对宏观两平行圆盘间的旋转流动及蛇形微通道结构，深入研究聚合物浓度、溶剂浓度和盐度等聚合物敏感性因素对弹性湍流诱发的影响规律。并通过微粒子图像测速系统、压力温度多点测试系统综合分析了微局部换热特性和流动特性的关系、不同区域流场和流动波动场规律，并进行速度时空序列场时序分析、自相关分析和频谱分析，得到了粘弹性流体在蛇形通道中从上游到下游的流动规律，进一步解析弹性流体随沿程发展换热效果增强的现象。此外，围绕粘弹性流体开展仿真研究，兼顾初步探究聚合物敏感性因素（部分）以及几何特征（多扰流源）对弹性不稳定的影响。.2）在弹性湍流主动诱发技术研究方面，分别从聚合物工质角度及变结构角度提出主动诱发技术思路。在聚合物工质角度，通过制备纳米颗粒-聚合物共聚物改性聚合物溶液，利用纳米颗粒与激励场的反应扰动聚合物，进而诱发弹性湍流。MD模拟结果显示外加电场可以通过控制离子运动来改变聚合物构象进而诱导弹性不稳定。此外，通过在聚丙烯酰胺弹性流体中加入自主合成的碳量子颗粒的方法降低了弹性流体的粘度从而达到了流动减阻的目的。在变结构角度，通过对比不同弯曲微通道几何结构发现三维弯曲微通道有利于在压降增加较少的情况下强化传热效果，具有巨大应用前景。在直通道的弹性湍流诱发和流动减阻技术研究方面，通过在直通道内布置等距离圆柱成功诱发了弹性不稳定流态。.本项目的研究结果将对系统性揭示弹性湍流强化换热的局部流动换热机理，以及实现基于纳米改性聚合物溶液主动诱发弹性湍流的技术，解决粘弹性流体的工业化应用提供有力技术及手段的支持。