An experimental study on the effect of polymers on the sustaining mechanism of turbulent heat transfer in turbulent water duct flow

我们开发了一种新技术，用于同时可视化近壁相干结构、纠缠聚合物块和湍流水管流中的热场，并将稀释聚合物溶液从横向槽引入缓冲区。主要结论如下：1。研究发现，在粘性亚层中引入荧光色微粒、氢泡和染料，可以有效地同时显示纠缠聚合物、流向涡旋和低速条纹。绿色图像包含气泡的反射光，而红色图像包含彩色颗粒和染料的荧光。因此，可以独立测量气泡、色粒和染料的运动。利用PTV技术对气泡运动进行了测量，得到了由流向涡引起的喷发流场。观察到流向涡旋的演化，从而使喷发流和低速条纹的抬升被聚合物块状物减弱。利用激光诱导荧光（PLIF）技术测量缓冲层的热场是一种很有前途的方法。结合PLIF、彩色粒子和氢泡可视化方法和双相机成像系统，可以同时观测热场、涡旋和纠缠聚合物。在缓冲层温度的降低是由于流向涡旋引起的流动。结果表明，缠结的聚合物减弱了涡旋的产生和演化，从而减弱了上述温度波动。

We developed new techniques for simultaneous visualization of near-wall coherent structure, lumps of entangled polymers and thermal field in a turbulent water duct flow with an introduction of dilute polymer solution from a transverse slot into the buffer region. The main conclusions obtained are as follows.1.The fine particles of fluorescence color, the hydrogen bubbles, dye introduced into the viscous sublayer were found to be effective for the simultaneous visualization for the entangled polymers, the streamwise vortices, and the low-speed streaks, respectively.2.The green-color images contain reflected light from the bubbles, while the red-color images contain fluorescence from the color particles and the dye. Therefore, the motions of bubbles, color particles and dye can be measured independently.3.The eruptive flow induced by the streamwise vortices was measured by applying a PTV technique to the motion of bubbles. It was observed that the evolution of streamwise vortices, and thus the eruptive flow and the lift-up of low-speed streaks were attenuated by the polymer lumps.4.The planer laser-induced fluorescence(PLIF)technique is found to be promising for the measurement of thermal filed in the buffer layer.5.The combination of PLIF, the visualization method with color particles and hydrogen bubbles, and the image-capturing system with two cameras were found to be promising to the simultaneous observation of thermal field, the vortices and the entangled polymers.6.A decrease in the temperature at the buffer layer due to the flow induced by the streamwise vortices was measured. It was observed that the entangled polymers attenuated the generation and evolution of the vortices and thus the aforementioned temperature fluctuation.