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

当强迫对流和自然对流同时出现时，区分哪一种对流模式占主导地位以及确定换热系数的贡献是最有意义的。本研究以加压氮气为实验流体，对管内均匀加热的上行流动进行了研究，从理论和实践上都具有重要的意义。首先，对下游足够区域的充分发展换热进行了研究。在雷诺数不变的情况下，随着Grashof数从零开始增大，混合对流区的换热系数减小，自然对流区的换热系数逐渐增大。在Grashof数与Reynolds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。在混合对流区域，雷诺数为3000时的流动分层现象由ho…显示。更多的T形钢丝风速测定仪。其次，在完全发展的等温流动条件下，测量了热入口区换热系数的流向变化。在混合对流条件下，换热系数在某一流向位置发生了局部损伤。定量地把握了换热系数最小的位置。采用两方程湍流模型对这些问题进行了数值模拟。预测的流型图，包括湍流和层流的区分，以及在每个流型下的充分发展的换热系数与实验结果符合得很好。在热入口区的二维计算可以定量地再现混合对流中换热系数的局部损伤以及换热系数的纵向变化，但最小换热点的下游区域除外。在湍流混合对流中，浮力使壁面附近的剪应力迅速减小，从而抑制了湍流能量的产生。由此产生的湍流能量的降低导致换热系数的降低，最终导致流动的完全分层。较少

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