粘弹性减阻流近壁区发卡涡与湍流扩散传质耦合机制研究

The application of viscoelastic additives to reduce the turbulent flow resistance is one effective method to achieve energy saving and emission reduction in the engineering field of fluid flow and heat transfer. However, the mechanism of suppression of turbulent mass transport related in the method is not clear. This research will experimentally study the complicated process of turbulent mass transfer, which is coupled with the modified hairpin vortex, in the drag-reducing turbulent channel flow of inhomogeneous viscoelastic solutions. The viscoelastic solution will be injected at a low velocity to the water channel flow from the porous wall of one side of the channel wall to generate a drag-reducing flow. The techniques of particle image velocimetry and planar laser-induced fluorescence will be used to measure the instantaneous flow fields and concentration fields located at various streamwise positions, respectively. The characteristic information of the near-wall hairpin vortex will be obtained by the methods like conditional averaging. The evolution of the hairpin vortex will be achieved. The characteristics of turbulent diffusion and its change with the evolution of hairpin vortex will be obtained. A theoretical model of the hairpin vortex and the quantitative link with turbulent diffusion in the drag-reducing flow will be established. The coupling mechanism between hairpin vortex and turbulent mass transfer in the near-wall region of the drag-reducing flow will be clarified to understand the mechanism of suppression of turbulent mass transport related in the viscoelastic drag-reducing flow deeply. The findings will improve the theory of turbulent mass transfer in the viscoelastic flow.

采用粘弹性的化学添加剂溶液降低湍流流动阻力是实现流动与传热相关工程领域节能减排的有效方法之一，但应用该方法形成的减阻流中发生湍流扩散传质抑制的机理尚不明确。本研究以槽道中非均匀分布的粘弹性溶液形成的湍流减阻流动为对象，对其中耦合了近壁发卡涡变化的复杂湍流扩散传质过程进行实验研究。通过多孔性壁向槽道水流中低速注入粘弹性溶液，采用粒子成像测速技术和激光诱导荧光技术分别对槽道内处于不同流向位置的瞬时速度场和浓度场进行测试；应用条件平均等方法获取减阻流中近壁发卡涡的特征信息，揭示其空间演化规律；研究减阻流中湍流扩散传质特征及其随发卡涡的演变，揭示近壁湍流扩散传质规律；建立减阻流中发卡涡的理论模型及其与湍流扩散传质的定量关系，揭示减阻流中近壁区发卡涡与湍流扩散传质的耦合作用机制；深入认识粘弹性减阻流中湍流扩散传质抑制现象的产生机理，完善粘弹性减阻流中湍流传质理论。

在能源短缺和气候环境恶化的背景下，发展粘弹性溶液减阻技术具有较重要的现实意义，无论在内流还是外流场合对减阻均有需要。由于壁湍流近壁湍流扩散与发卡涡特征量定量的困难，使得减阻流中湍流扩散传质与发卡涡的研究还不充分，粘弹性减阻流中耦合了发卡涡变化的湍流扩散传质抑制机理尚未明确。本项目即针对粘弹性添加剂造成的减阻流动中涡结构与湍流扩散传质而展开，以壁面注入粘弹性聚合物溶液和槽道溶入表面活性剂溶液形成的粘弹性减阻流为研究对象，通过实验研究和理论分析，研究了粘弹性减阻流中近壁区发卡涡的演化规律与湍流扩散传质规律，建立了发卡涡与湍流扩散传质的定量关系，揭示了二者的耦合作用机制。研究获得了粘弹性减阻流中近壁对数层内发卡涡等相干结构的特征信息及其演变规律，随着注入的聚合物溶液的扩散、拉伸等发展，减阻率沿流向逐渐增加，近壁由准流向涡发展而来的发卡涡的生长得到抑制，猝发的高度有限，导致剪切层倾角变小；建立的发卡涡理论模型经与牛顿流体壁湍流的参数对照得到了证实；查明了浓度标量的湍流输运特征，受壁面附近发卡涡等抑制的影响，垂直于壁面的方向的湍流扩散传质通量随着减阻率的增加大幅下降，在实验工况下传质通量约为相同条件下注水扩散的1/2，表征湍流扩散相对强度的湍流施密特数增加；明确了减阻流中近壁大尺度发卡涡与浓度标量湍流输运的耦合关系，得到传质抑制后近壁对数层浓度分布函数中的冯卡门常数，在减阻率为40%的减阻流中，该常数从0.47变为0.075，且与实测值吻合较好，证实了发卡涡变化与湍流扩散抑制之间的定量关系。基于这些发现，开发了有效的多自由度促进扩散的配置高分子聚合物溶液的方法与装置，提出了用射流控制近壁发卡涡的方法，并从场协同角度研究了涡结构变化对流动与热质输运的影响。研究深入认识了粘弹性减阻流中湍流扩散传质抑制现象的产生机理，完善了粘弹性减阻流中湍流传质理论，为粘弹性溶液减阻技术在不同流动场合的应用奠定了理论基础。