Abstract: In the range of pressure from 11.5 to 28 MPa,mass flow rate from 450 to 1550 kg·m^-2·s^-1,and heat flux at inner wall from 50 to 585 kW·m^-2,an experiment was conducted to investigate the heat transfer characteristics of water in vertical downward tubes.The temperature distribution in the tube wall was obtained.The effects of heat flux at inner wall on heat transfer coefficient and temperature at inner wall were analyzed and corresponding empirical correlations were presented.The heat transfer coefficient of vertical downward flow was compared with that of vertical upward flow.The experimental results show that at subcritical and near critical pressures,dryout and departure from nucleate boiling occur in vertical downward tube.With the increase of inner wall heat flux,heat transfer deterioration occurs ahead and the peak wall temperature increases obviously after heat transfer deterioration.At supercritical pressures,the wall temperature increases slowly with fluid enthalpy when the bulk temperature is lower than the pseudo-critical temperature and increases considerably when the bulk temperature is higher than the pseudo-critical temperature.In the pseudo-critical enthalpy region,the heat transfer is enhanced.

Key words: vertical downward tube, heat transfer deterioration, wall temperature, heat transfer coefficient

摘要： 在压力11.5~28 MPa,质量流速450~1550 kg·m^-2·s^-1,内壁热负荷50~585 kW·m^-2的范围内,对水在垂直下降管内的传热特性进行了实验研究,得到了垂直下降管内水的传热特性,分析了热负荷对内壁温度和传热系数的影响,给出了能用于工程实际的传热实验关联式,并且对垂直下降流动和垂直上升流动的传热特性进行了比较。实验结果表明:在亚临界和近临界压力区,垂直下降管会发生第一类传热恶化——膜态沸腾和第二类传热恶化——干涸。热负荷的增大,会导致传热恶化的提前发生和传热恶化发生后的壁温飞升值增大。在超临界压力区,壁温在低焓值区随焓值平缓增加,而在高焓值区随焓值明显升高,表明在拟临界点附近发生了传热强化现象。

关键词: 垂直下降管, 传热恶化, 壁温, 传热系数