Heat Transfer in Post Dryout and Its Augmentation

1. 测定了氟利昂11、氟利昂113及其混合物在不同压力、质量速度、进口过冷度和热流密度范围内均匀加热垂直管内流动沸腾的沸腾换热系数。现有的关系式不能很好地预测成核沸腾区和两相强制对流区的传热系数。特别是在两相强迫对流区，没有观察到传热系数随质量的增加而明显增加，而在水的情况下。提出了在有核沸腾和两相强迫对流区域换热系数的半经验相关关系。由两相强迫对流过渡到后干燥区导致管壁温度突然升高。从临界热通量的Katto相关可以很好地预测到这种转变的热通量和质量。干透后的壁面温度在干透后突然升高，然后逐渐接近纯蒸汽的壁面温度，而这一过程的壁面温度模式与质量速度、热流密度和离干点的距离有关。后干燥区换热系数明显降低。因此，从实际应用的角度来看，提高该区域的换热系数是非常重要的。

1. Boiling heat transfer coefficient for flow boiling in a uniformly heated vertical tube was measured for freon 11, freon 113 and their mixture in the various range of pressure, mass velocity, inlet subcooling, and heat flux.2. Heat transfer coefficient in the regions of nucleate boiling and two-phase forced convection was not predicted well by the available correlations proposed so far. In the two-phase forced convection region in particular, there was not observed an appreciable increase of heat transfer coefficient with increasing quality, as in the case of water.3. A semi-empirical correlation was proposed for heat transfer coefficient in the regions of nucleate boiling and two-phase forced convection.4. A transition from the two-phase forced convection to the post dry out region resulted in an abrupt increase of tube wall temperature. The heat flux and quality for this transition were well predicted from the Katto correlation for the critical heat flux.5. Wall temperature in the post dry out region increased abruptly right after the dry out and thereafter asymptotically approached to the wall temperature predicted for pure vapor, while the pattern of wall temperature in this process depended upon mass velocity, heat flux, and a distance from the dry point.6. Heat transfer coefficient was greatly degradated in the post dry out region. Thus an augmentation of heat transfer coefficient in this region is very important from the practical point of view.