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Drag coefficient and strouhal number analysis of cylindrical tube in two phase flow

机译：两相流中圆柱管的阻力系数和Strouhal数分析

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In many industrial equipment such as boilers and heat exchangers, the cylindrical tubes are exposed to the gas- liquid two phase flow. For any immersed body in flow field vortex shedding is created with a frequency that may be constant or variable, according to conditions such as flow rates, geometry of body, and etc. The failure will happen in the equipment, when this frequency is close to one of the natural frequencies of them. This can cause noise and flow induced vibration problem which is one of the main defects in the heat exchangers. Therefore considering these flows can play a significant role in long-term reliability and safety of industrial and laboratory equipment. In this study Eulerian–Eulerian approach is employed to simulate two-phase flow around the cylindrical tube. Since the Reynolds Stress Model (RSM) accounts for the effects of streamline curvature, swirl, rotation, and rapid changes in strain rate in a more rigorous manner than other turbulence models, it has greater potential to give accurate predictions for complex flows. So in this study the RSM is used to recognize behavior of vortex shedding in the flow. Drag coefficient, Strouhal number, vortex shedding behind cylinder, void fraction and pressure coefficient distribution were investigated in air-water two phase flows. In order to verify validity of CFD model, inlet void fraction was set to zero, Strouhal number and it’s relation with low Reynolds number (100-3000) in single phase flow were compared with experimental and numerical result of available literatures. The results show a good agreement between them. Having reasonable judgment of fluctuating lift force frequency, fast Fourier transform (FFT) was applied to lift coefficient of cylinder. The FFT produces averaged spectral coefficients that are independent of time and are useful to identify dominant frequencies in a signal. Some remedies were introduced to reduce vortex frequency of cylindrical tube subjected in gas-liquid two phase flows and protect tube from hard vibrations. For this purpose, some geometrical modifications were applied and results showed that in all cases drag coefficient and Strouhal number reduce.
机译：在许多工业设备中，例如锅炉和热交换器，圆柱管暴露于气液两相流。对于任何浸没在流场中的物体，会根据流量，物体的几何形状等条件以恒定或可变的频率产生涡旋脱落。当该频率接近零时，设备将发生故障。它们的自然频率之一。这会引起噪声和流动引起的振动问题，这是热交换器的主要缺陷之一。因此，考虑这些流量可在工业和实验室设备的长期可靠性和安全性中发挥重要作用。在这项研究中，采用欧拉-欧拉方法来模拟圆柱管周围的两相流动。由于雷诺应力模型（RSM）以比其他湍流模型更严格的方式解释了流线曲率，旋流，旋转和应变速率的快速变化的影响，因此它有很大的潜力为复杂的流动提供准确的预测。因此，在本研究中，RSM用于识别流中涡旋脱落的行为。研究了空气-水两相流中的阻力系数，斯特劳哈尔数，圆柱体后部涡旋脱落，空隙率和压力系数分布。为了验证CFD模型的有效性，将进口空隙率设为零，将Strouhal数及其与单相流中低雷诺数（100-3000）的关系与现有文献的实验和数值结果进行了比较。结果表明它们之间有很好的一致性。在合理判断起升力频率波动的情况下，对缸升力系数进行了快速傅里叶变换（FFT）。 FFT产生与时间无关的平均频谱系数，可用于识别信号中的主频。引入一些补救措施以降低圆柱管在气液两相流中的涡旋频率并保护其免受剧烈振动。为此，应用了一些几何修改，结果表明，在所有情况下，阻力系数和Strouhal数都会减小。

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