Abstract

Rewetting a hot dry surface is the establishment of wet contact between the hot surface and a liquid at a lower temperature. Rewetting occurs after destabilizing a vapor film that exists between the hot surface and the liquid. Situations involving rewetting heat transfer are encountered in a number of postulated accidents in Canada Deuterium Uranium (CANDU) reactors, such as rewetting of a hot dry calandria tube in a critical break loss-of-coolant accident (LOCA). It is also encountered in improving metals’ mechanical properties in metallurgical industries. One of the important parameters in rewetting cooling is the rewetting delay time, which is the time interval from starting to cool the surface by the liquid to the establishment of the wet contact. Determining the rewetting delay time is very important for limiting the extent of core damage during the early stages of reactor severe accidents and is essential for predicting the period after which the coolant effectively cools an overheated core. If the rewetting delay time is relatively long, an escalation in the calandria surface temperature can occur, and if the temperature was not reduced by the establishment of the wet contact, this may lead to failure of the fuel channel. Although there is increasing interest in literature in estimating the rewetting delay time of hot flat surfaces, very limited studies exist on rewetting of curved surfaces, such as tubes. In this study, experimental tests were carried out to measure the rewetting delay time at the stagnation point of hot horizontal tubes cooled by a vertical rectangular water jet. The tubes were heated to initial temperatures between 400°C and 740°C, then rapidly cooled to the jet temperature. The two-phase flow behavior was visualized using high-speed imaging, and the moment at which the vapor film collapses was captured. In addition to studying the effect of initial surface temperature on the delay time, effects of water subcooling in the range 15°C to 80°C and jet velocity in the range 0.17 to 1.43 m/s were studied and a correlation for the delay time was developed and validated. The delay time was found to strongly increase by increasing initial surface temperature and surface curvature and by decreasing water subcooling and jet velocity. The effects of solid material and tube wall thickness were also studied.

摘要

再润湿热干燥表面是在热表面和较低温度的液体之间建立湿接触。再润湿发生在使热表面和液体之间存在的蒸汽膜不稳定之后。在加拿大氘铀 (CANDU) 反应堆的许多假设事故中都遇到了涉及再润湿传热的情况，例如在临界破裂冷却剂损失事故 (LOCA) 中干热排管的再润湿。它还用于改善冶金工业中金属的机械性能。再润湿冷却的重要参数之一是再润湿延迟时间，即从液体开始冷却表面到建立湿接触的时间间隔。确定再润湿延迟时间对于限制反应堆严重事故早期阶段的堆芯损坏程度非常重要，并且对于预测冷却剂有效冷却过热堆芯的时间至关重要。如果再润湿延迟时间较长，可能会发生排管表面温度升高，如果温度没有通过建立湿接触而降低，这可能导致燃料通道失效。尽管文献对估计热平面的再润湿延迟时间越来越感兴趣，但对弯曲表面（例如管）的再润湿的研究非常有限。在这项研究中，进行了实验测试，以测量由垂直矩形水射流冷却的热水平管在停滞点处的再润湿延迟时间。将管子加热到 400°C 和 740°C 之间的初始温度，然后迅速冷却到喷射温度。使用高速成像对两相流动行为进行可视化，并捕获蒸汽膜塌陷的时刻。除了研究初始表面温度对延迟时间的影响外，还研究了 15°C 至 80°C 范围内的水过冷和 0.17 至 1.43 m/s 范围内的射流速度的影响以及延迟时间的相关性被开发和验证。发现延迟时间随着初始表面温度和表面曲率的增加以及水的过冷度和射流速度的降低而显着增加。还研究了固体材料和管壁厚度的影响。使用高速成像对两相流动行为进行可视化，并捕获蒸汽膜塌陷的时刻。除了研究初始表面温度对延迟时间的影响外，还研究了 15°C 至 80°C 范围内的水过冷和 0.17 至 1.43 m/s 范围内的射流速度的影响以及延迟时间的相关性被开发和验证。发现延迟时间随着初始表面温度和表面曲率的增加以及水的过冷度和射流速度的降低而显着增加。还研究了固体材料和管壁厚度的影响。使用高速成像对两相流动行为进行可视化，并捕获蒸汽膜塌陷的时刻。除了研究初始表面温度对延迟时间的影响外，还研究了 15°C 至 80°C 范围内的水过冷和 0.17 至 1.43 m/s 范围内的射流速度的影响以及延迟时间的相关性被开发和验证。发现延迟时间随着初始表面温度和表面曲率的增加以及水的过冷度和射流速度的降低而显着增加。还研究了固体材料和管壁厚度的影响。研究了 15°C 至 80°C 范围内的水过冷和 0.17 至 1.43 m/s 范围内的射流速度的影响，并开发并验证了延迟时间的相关性。发现延迟时间随着初始表面温度和表面曲率的增加以及水的过冷度和射流速度的降低而显着增加。还研究了固体材料和管壁厚度的影响。研究了 15°C 至 80°C 范围内的水过冷和 0.17 至 1.43 m/s 范围内的射流速度的影响，并开发并验证了延迟时间的相关性。发现延迟时间随着初始表面温度和表面曲率的增加以及水的过冷度和射流速度的降低而显着增加。还研究了固体材料和管壁厚度的影响。