Experimental investigation of steam bubble condensation in vertical large diameter geometry under atmospheric pressure and different flow conditions
Experimental investigation of steam bubble condensation in vertical large diameter geometry under atmospheric pressure and different flow conditions
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DOI：
10.1016/j.ijheatmasstransfer.2013.11.049
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发表时间：
2014-03
影响因子：
5.2
通讯作者：
S. A. Issa;Patricia B. Weisensee;R. Macián‐Juan
中科院分区：
文献类型：
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作者：
S. A. Issa;Patricia B. Weisensee;R. Macián‐Juan
在低压条件下(∼为1.1~1.5bar)，对大口径垂直管内汽泡与流动的低过冷水的冷凝过程进行了实验研究。对实验研究的分析将有助于更好地理解低压下的气泡冷凝，这对核电站的安全分析以及任何具有重要意义的工业活动都是有意义的。测试段是一根1米长的透明塑料管，周围是一个18米×18厘米的长方形水族箱，里面装满了用于屈光矫正的水。利用高速摄像(HSC)记录了凝聚气泡的直径、形状和上升速度等数据。在这些实验中，蒸汽冷凝与蒸发的分离使我们能够在明确的流动条件下只专注于冷凝过程。蒸汽通过三个不同的喷嘴直接注入到测试段中心附近。本实验是在三种不同的蒸汽表观速度、水表观速度和每个喷嘴的水温下进行的。原始图像以1500帧/S和0.32万mm/像素的空间分辨率记录，每次测量的总时间为2个S。用自行开发的程序对原始图像进行处理，以识别和跟踪脱离后的凝聚气泡，并测量它们的直径、速度、位置和长径比。这些测量使气泡雷诺数和努塞尔数的计算成为可能，从而建立了努雷关联式。这些测量扩展了现有的数据库，用于验证界面热质传递关联式。例如，在CFD模拟中，蒸汽泡冷凝起着重要作用。对所给结果的分析还显示，对于高雷诺数的大气泡，如大变形、偏离球形、气泡表面粗糙和振动，以及强化的换热系数，也显示出新的有趣的观测结果。实验表明，气泡表面结构、气泡上升速度和初始喷射速度对凝结速率有很大影响。在分析计算结果的基础上，提出了气泡直径和雷诺数范围内Nusselt凝结数的新关联式。
The condensation of steam bubbles injected into a large diameter vertical pipe (100 mm) with flowing low-subcooled water at low pressure conditions (∼1.1–1.5 bar) was experimentally investigated. The analysis of the experimental investigations will contribute to a better understanding of bubble condensation at low pressures, which is of interest to the safety analysis of nuclear power plants, and any industrial activity in which bubble condensation is of importance. The test section is a transparent 1 m long plastic pipe surrounded by a 18 × 18 cm rectangular “aquarium” filled with water for refraction correction. High-speed camera (HSC) recording was used to gather data about condensing bubbles including: bubble diameter, shape and rising velocity. The isolation of the steam condensation from evaporation in these experiments enabled us to concentrate on the condensation process alone with well-defined flow conditions. Steam was injected via three different injection nozzles directly near the center of the test section. The present experiments were carried out at three different steam superficial velocities, water superficial velocities and water temperatures for each injection nozzle. Raw images were recorded at 1500 frame/s and 0.32 mm/pixel spatial resolution for a total time of 2 s in each measurement. The raw images were processed by self-developed programs in order to identify and track the condensing bubbles after detachment, and to measure their diameter, velocity, position, and aspect ratio. The measurements made possible the calculation of both the bubble Reynolds number and the Nusselt number and, thus, a Nu–Re correlation was developed. The measurements extend the already available database for the validation of interfacial heat and mass transfer correlations. These are used for instance in CFD simulations where steam bubble condensation plays an important role. The analysis of the results presented shows also new and interesting observations for large bubbles with high Reynolds number such as: large deformation, deviation from spherical form, rough and vibrating bubble surfaces, and enhanced heat transfer coefficient. The experiments show a large effect of bubble surface structure, bubble rising velocity, and initial injection velocity upon the condensation rate. Based on the analysis of the results, a new correlation of the Nusselt condensation number has been proposed for the investigated range of bubble diameter and Reynolds number.