Fershtman Adam, Barnea Dvora, Shemer Lev
School of Mechanical Engineering, Faculty of Engineering, Tel-Aviv University;
School of Mechanical Engineering, Faculty of Engineering, Tel-Aviv University.
J Vis Exp. 2018 Apr 30(134):57437. doi: 10.3791/57437.
This manuscript provides step by step description of the manufacturing process of a test section designed to measure the local instantaneous heat transfer coefficient as a function of the liquid flow rate in a transparent pipe. With certain amendments, the approach is extended to gas-liquid flows, with a particular emphasis on the effect of a single elongated (Taylor) air bubble on heat transfer enhancement. A non-invasive thermography technique is applied to measure the instantaneous temperature of a thin metal foil heated electrically. The foil is glued to cover a narrow slot cut in the pipe. The thermal inertia of the foil is small enough to detect the variation in the instantaneous foil temperature. The test section can be moved along the pipe and is long enough to cover a considerable part of the growing thermal boundary layer. At the beginning of each experimental run, a steady state with a constant water flow rate and heat flux to the foil is attained and serves as the reference. The Taylor bubble is then injected into the pipe. The heat transfer coefficient variations due to the passage of a Taylor bubble propagating in a vertical pipe is measured as function of the distance of the measuring point from the bottom of the moving Taylor bubble. Thus, the results represent the local heat transfer coefficients. Multiple independent runs preformed under identical conditions allow accumulating sufficient data to calculate reliable ensemble-averaged results on the transient convective heat transfer. In order to perform this in a frame of reference moving with the bubble, the location of the bubble along the pipe has to be known at all times. Detailed description of measurements of the length and of the translational velocity of the Taylor bubbles by optical probes is presented.
本手稿逐步描述了一个测试段的制造过程，该测试段旨在测量透明管道中局部瞬时传热系数与液体流速的函数关系。经过某些修正后，该方法被扩展到气液两相流，特别强调了单个细长（泰勒）气泡对传热增强的影响。应用一种非侵入式热成像技术来测量电加热薄金属箔的瞬时温度。该箔片被粘贴以覆盖管道上切割的一个窄槽。箔片的热惯性足够小，能够检测到箔片瞬时温度的变化。测试段可以沿着管道移动，并且足够长以覆盖相当一部分不断增长的热边界层。在每次实验运行开始时，达到一个具有恒定水流速和流向箔片的热通量的稳态，并将其用作参考。然后将泰勒气泡注入管道。测量由于在垂直管道中传播的泰勒气泡通过而导致的传热系数变化，作为测量点距移动泰勒气泡底部距离的函数。因此，结果代表局部传热系数。在相同条件下进行的多次独立运行允许积累足够的数据，以计算关于瞬态对流换热的可靠总体平均结果。为了在随气泡移动的参考系中进行此操作，必须始终知道气泡在管道中的位置。本文介绍了通过光学探头测量泰勒气泡长度和平移速度的详细方法。
