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Quantitative Assessment of the Overall Heat Transfer Coefficient U

机译：整体传热系数U的定量评估

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This investigation was performed in order to quantify the validity of the assumed constancy of the overall heat transfer coefficient U in heat exchanger design. The prototypical two-fluid heat exchanger, the double-pipe configuration, was selected for study. Heat transfer rates based on the U = constant model were compared with those from highly accurate numerical simulations for 60 different operating conditions. These conditions included: (a) parallel and counter flow, (b) turbulent flow in both the pipe and the annu-lus, (c) turbulent flow in the pipe and laminar flow in the annulus and the vice versa situation, (d) laminar flow in both the pipe and the annulus, and (e) different heat exchanger lengths. For increased generality, these categories were further broken down into matched and unmatched Reynolds numbers in the individual flow passages. The numerical simulations eschewed the unrealistic uniform-inlet-velocity-profile model by focusing on pressure-driven flows. The largest errors attributable to the U = constant model were encountered for laminar flow in both the pipe and the annulus and for laminar flow in one of these passages and turbulent flow in the other passage. This finding is relevant to microchannel flows and other low-speed flow scenarios. Errors as large as 50% occurred. The least impacted were cases in which the flow is turbulent in both the pipe and the annulus. The general level of the errors due to the U = constant model were on the order of 10% and less for those cases. This outcome is of great practical importance because heat-exchanger flows are more commonly turbulent than laminar. Another significant outcome of this investigation is the quantification of the axial variations of the temperature and heat flux along the wall separating the pipe and annulus flows. It is noteworthy that these distributions do not fit either the uniform wall temperature or uniform heat flux models.
机译：进行这项研究是为了量化热交换器设计中总体传热系数U的假设恒定性的有效性。选择了典型的双流体热交换器双管结构进行研究。将基于U =常数模型的传热速率与来自60种不同工况的高精度数值模拟的传热速率进行了比较。这些条件包括：（a）平行和逆流，（b）管道和环面中的湍流，（c）管道中的湍流，环面的层流，反之亦然，（d）管道和环空中的层流，以及（e）不同的热交换器长度。为了提高通用性，这些类别在各个流道中进一步细分为匹配和不匹配的雷诺数。数值模拟通过关注压力驱动的流，避免了不切实际的均匀入口速度剖面模型。对于管道和环空中的层流以及其中一个通道的层流和另一通道的湍流，遇到了归因于U =常数模型的最大误差。这一发现与微通道流量和其他低速流量场景有关。发生高达50％的错误。受影响最小的是管道和环空中的湍流。对于这些情况，由于U =常数模型而导致的一般错误级别约为10％，甚至更低。由于热交换器流通常比层流更湍急，因此这一结果具有重要的实际意义。这项研究的另一个重要成果是量化了沿分隔管道和环流的壁的温度和热通量的轴向变化。值得注意的是，这些分布既不适合均匀的壁温也不适合均匀的热通量模型。

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