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

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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.