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，区分哪种对流占主导地位以及解决传热系数的贡献是最重要的。本研究涉及管内均匀加热向上流动，从理论和实践的角度来看，这似乎是重要的。在实验中，通过使用加压的氮气作为测试流体，可以在很大范围内改变格拉什夫数。首先，研究了下游足够区域的充分发展的传热。在一定雷诺数下，随着Grashof数从零增加，混合对流区换热系数减小，自然对流区换热系数又转为增大。在格拉索夫数与雷诺数平面上绘制了强制对流、混合对流和自然对流的状态图。在混合对流条件下，用1…More t线风速计证实了雷诺数为3000的层压化流动。其次，在等温流动完全发展后的入热区测量了换热系数的沿流变化。在混合对流状态下，传热系数在某一流向位置存在局部损伤。定量地把握了传热系数最小的位置。紊流的双方程模型应用于这些问题。预测的流态图，包括湍流和层流的区分，以及在每个流态中充分发展的换热系数，与实验结果吻合得很好。除最小换热点的直接下游区域外，入热区的二维计算可以定量再现混合对流区换热系数的局部损伤以及换热系数的纵向变化，与实验结果吻合较好。在湍流混合对流中，浮力使壁面附近的剪应力迅速减小，从而抑制了湍流能的产生。湍流能量的减少导致换热系数的降低，最终使流动完全层压化。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