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A Study on Combined Forced and Natural Convection Heat Transfer in a Vertical Pipe

立管内强制对流与自然对流相结合的传热研究

基本信息

批准号：
60460108
负责人：
TANAKA Hiroaki
金额：
$ 4.22万
依托单位：
The University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (B)
财政年份：
1985
资助国家：
日本
起止时间：
1985 至 1986
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-60460108/
关键词：
Convective Heat Transfer Combined Convection Mixed Convection Laminarization Vertical Pipe Aiding Flow Turbulence Model 温度助走区間

项目摘要

When forced and natural convection appear combined together, it is of prime interest to discriminate which convection regime is dominant as well as to resolve how much the heat-transfer coefficient contributes. This research deals with the uniformly heated upward flow in a tube, which seems to be important from theoretical as well as practical viewpoints.In experiments, the Grashof number was varied over a wide range, by using pressurized nitrogen gas as a test fluid. First, the fully developed heat transfer in a downstream enough region was investigated. As the Grashof number increased from zero under a constant Reynolds number, the heat-transfer coefficient decreased in the mixed-convection region and then turned to increase in the natural convection region. A regime map for forced, mixed, and natural convection was drawn in the plane of Grashof number vs. Reynolds number. In the mixed-convection regime, laminarization of the flow at a Reynolds number of 3000 was demonstrated by a ho … More t-wire anemometry. Secondly, streamwise variations of the heat-transfer coefficient were measured in the thermal-entry region which was following the fully developed isothermal flow. In the mixed-convection regime, the local impairment of the heat-transfer coefficient at a certain streamwise position was demonstrated. This position with the minimum heat-transfer coefficient was quantitatively grasped.Two-equation models of turbulence were applied to these problems. The predicted regime map, including discrimination between turbulent and laminar flow, as well as the fully-developed heat-transfer coefficient in each regime agreed well with the experimental results. Two-dimensional calculations in the thermal-entry region could reproduce the local impairment of heat-transfer coefficient in the mixed-convection regime as well as the longitudinal variations of heat-transfer coefficient quantitatively in good agreement with the experimental results except for the immediately downstream region of the minimum heat-transfer point.In the turbulent mixed convection, the buoyancy force makes the shear stress rapidly decrease near the wall so that the turbulence-energy production is suppressed. Resultant decrease in the turbulence energy causes the decrease in the heat-transfer coefficient and eventually the complete laminarization of the flow. Less

当强迫对流和自然对流同时出现时，区分哪种对流状态占主导地位以及解决传热系数的贡献是最重要的。本文研究了管内均匀加热的上升流动，在实验中，以加压氮气为试验流体，在很宽的范围内改变了Grashof数。首先，充分发展的传热在下游足够的区域进行了研究。当雷诺数一定时，随着Grashof数的增加，混合对流区的换热系数减小，自然对流区的换热系数增大。在格拉肖夫数与雷诺数的平面上绘制了强迫对流、混合对流和自然对流的状态图。在混合对流区，雷诺数为3000时的层流现象通过一个小的数值模拟得到了证实。 ...更多信息 T型线风速仪。其次，在充分发展的等温流动之后的热进入区域中测量了传热系数的流向变化。在混合对流区，在一定的流向位置的传热系数的局部损伤被证明。定量地把握了换热系数最小的位置，并应用双方程湍流模型对这些问题进行了研究。预测的制度地图，包括湍流和层流之间的歧视，以及充分发展的传热系数，在每个政权同意与实验结果。在混合对流区，二维计算可以定量地再现局部换热损失以及换热系数的纵向变化，除最小换热点的下游区域外，其余区域与实验结果基本一致。浮力使得壁附近的剪切应力迅速减小，从而抑制了摩擦能的产生。湍流能量的降低导致传热系数的降低，并最终导致流动的完全层流化。少