几何参数对含空气蒸汽冷凝影响的数值分析

含空气蒸汽冷凝是反应堆失水事故时安全壳内重要的热工水力现象。已有研究多关注气体压力、温度等热工参数对传热特性的影响，而对几何参数的影响及其作用原理分析较少。采用三维CFD数值模拟方法，基于扩散边界层冷凝机理模型研究了管径(4~60 mm)、管长(0.1~7 m)及倾角(0°~90°)对含空气蒸汽冷凝传热特性的影响。结果表明，管径、管长及倾角均对含空气蒸汽冷凝传热特性有显著影响。平均冷凝传热系数随管径的增大而减小；随管长的增长先减小后增大，3 m左右达到最小值；随倾角的增大而增大。局部冷凝传热系数沿管长方向先迅速减小后缓慢增大。倾斜布置时，迎流面产生明显传热强化，向两侧逐渐减弱，背流面存在一定的传热抑制。

Numerical Analysis on Effect of Geometric Parameter on Steam Condensation in Presence of Air

Steam condensation in the presence of air is an important thermal-hydraulic phenomenon in containment under loss of coolant accidents (LOCAs) or main steam line break accidents (MSLBs). Previous studies have focused on the effects of thermal parameters such as air mass fraction, gas pressure and sub-cooling on heat transfer characteristics, but less on the influence and action principle of geometric parameters such as tube length, tube diameter and inclination angle. Fully understanding the influence of geometric parameters on condensation heat transfer of steam containing air is of great significance for efficient heat transfer, so as to improve the safety and economy of related applications. The effects of tube diameter (4-60 mm), tube length (0.1-7 m) and inclination angle (0°-90°) on condensation heat transfer characteristics of air containing steam were studied based on the diffusion boundary layer condensation mechanism model by using three-dimensional CFD numerical simulation method. The results show that the tube diameter, tube length and inclination angle have significant effects on condensation heat transfer characteristics of air containing steam. However, when tube diameter exceeds 30 mm, tube length exceeds 1 m and inclination angle exceeds 60°, the average condensation heat transfer coefficient is no longer sensitive to the change of geometric parameters. The average condensation heat transfer coefficient decreases with the increase of tube diameter, and the descent speed decreases rapidly. When the tube diameter is 4 mm, the average condensation heat transfer coefficient is twice that of 60 mm. It first decreases and then increases with the increase of tube length, and reaches the minimum value at about 3 m. And it increases with the increase of inclination angle. The high concentration air layer near the wall is the main thermal resistance in the condensation process of steam-air, and its thickness makes a good negative correlation with condensation heat transfer coefficient. The high concentration air layer will first accumulate and thicken along the tube wall, and then produce a certain negative pressure due to the high density and high flow rate of mixed gas near the wall, so as to form a transverse velocity, so that the air layer will be thinned slowly after 1 m from the top of the pipe. Thus, the local condensation heat transfer coefficient decreases rapidly at first and then increases slowly along the direction of tube length. In addition, the tube circumferential local condensation heat transfer coefficient is uneven obviously under the inclined arrangement, the heat transfer on the upper surface is significantly enhanced due to the thinning of air layer by natural convection scouring and falling off, and gradually weakened to both sides, while the lower surface forms a stagnation zone due to the obstruction of the tube wall, which causes the accumulation and thickening of the air layer, resulting in a certain heat transfer inhibition. Compared with the vertical tube, about 75% of the circumferential area has different degrees of heat transfer enhancement, and only the dorsal flow surface has a certain heat transfer inhibition.