Investigation of in-tube cooling of carbon dioxide at supercritical pressure by means of direct numerical simulation
Investigation of in-tube cooling of carbon dioxide at supercritical pressure by means of direct numerical simulation
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DOI：
10.1016/j.ijheatmasstransfer.2017.06.089
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发表时间：
2017-11
影响因子：
5.2
通讯作者：
S. Pandey;X. Chu;E. Laurien
中科院分区：
文献类型：
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作者：
S. Pandey;X. Chu;E. Laurien
为了了解超临界压力下二氧化碳的冷却传热行为，在压力为8MPa、入口温度为342.05K、中等入口雷诺数为5400的条件下，对圆管内的流动与传热进行了直接数值模拟。管直径为2mm，其在紧凑型热交换器的水力直径的范围内。当管道处于垂直方向时，模拟了流动方向向上或向下的强制对流（忽略重力）和混合对流。作为热收缩的结果，主要在壁附近观察到流动减速，这与壁加热时观察到的加速相反。研究发现，在上升流中，减速和浮力的联合作用增强了传热，而在下降流中的传热恶化。进一步的研究得出，在向下流动中，当浮力方向与流动方向相同时，所有的湍流量在轴向都显著减小，这是传热恶化的原因。这里介绍了象限和八分圆分析，以了解后掠和喷射事件对湍流的影响。最后，雷诺应力张量的各向异性表明，湍流调制，特别是在近壁区在向上和向下的流动。
To understand the cooling heat transfer behavior of carbon dioxide at supercritical pressure, direct numerical simulations of the flow and heat transfer in circular tubes have been performed at a pressure of 8 MPa, an inlet temperature of 342.05 K and a moderate inlet Reynolds number of 5400. The tube diameter was 2 mm, which is in the range of the hydraulic diameter of a compact heat exchanger. Both forced (gravity neglected) and mixed convection with upward or downward direction of the flow were simulated while the pipe is in vertical orientation. As result of thermal contraction, flow deceleration was observed primarily in the vicinity of the wall, which is opposite to the acceleration observed with wall heating. It is found that combined effects of deceleration and buoyancy in the upward flow enhance the heat transfer while the heat transfer in the downward flow is deteriorated. Further investigations have educed that in downward flow, when the direction of buoyancy force and flow are the same, all turbulent quantities diminish significantly in the axial direction, which is the reason for heat transfer deterioration. Quadrant and octant analyses are presented here to understand the effects of sweep and ejection events on turbulence. Finally, the anisotropy of the Reynolds stress tensor indicates that turbulence is modulated especially in the near-wall region in both upward and downward flow.