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

我们开发了一种新的技术来同时显示湍流水管流动中的近壁相干结构、纠缠聚合物块和热场，并将稀溶液从横向狭缝引入缓冲区。得到的主要结论如下：1.引入到粘性层中的荧光微粒、氢气泡和染料分别能有效地同时显示缠绕聚合物、流向涡旋和低速条纹。2.绿色图像包含气泡的反射光，而红色图像包含彩色粒子和染料的荧光。因此，可以独立地测量气泡、彩色颗粒和染料的运动。3.将PTV技术应用于气泡的运动，测量了流向涡诱导的喷发流动。4.平面激光诱导荧光(PLIF)技术被认为是一种很有前途的缓冲层内热场测量方法。5.平面激光诱导荧光技术、彩色粒子和氢泡的可视化方法以及带有两个摄像头的图像采集系统相结合，对于同时观测热场、涡流和纠缠聚合物是有希望的。6.测量了由于流向涡流诱导的流动而导致的缓冲层温度的降低。观察到，缠绕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.