Abstract:

The heat transfer surface of the nuclear reactor steam generator consists of helical tube bundles. As the spatial spiral structure of the tubes, the slip velocity between phases increases due to the combined effect of gravity, centrifugal force and buoyancy. Accordingly, the distribution of the bubbles in bubbly flow and plug flow shows an asymmetric profile, leading to a significant effect on the heat transfer performance and even the occurrence of departure from nucleate boiling (DNB). In this paper, we employ the developed conductive wire mesh sensor and data post-processing algorithm to investigate the flow field and bubble behaviors in both bubbly and plug flow in a helically coiled tube. Reconstruction of the time and space distribution of the flow field based on the proposed algorithm provides an in-depth knowledge of the characteristics of the bubbles. Based on the present study, the geometric structure of the helically coiled tube can be optimized to avoid heat transfer deterioration, which provides basic experimental data and optimization for the design of the helically coiled tube evaporator. The results indicate that both the increase of the superficial gas velocity and superficial liquid velocity can promote the bubble coalescence. The increase of the gas velocity increases the instability of the gas-liquid interface, and the higher liquid velocity splits the gas plug into several small bubbles in the plug flow.

Key words: steam generator, helically coiled tube, wire mesh sensor, gas-liquid two-phase flow, bubble, particle size distribution

摘要：

核反应堆蒸汽发生器的传热面由螺旋管束组成。螺旋管的三维螺旋结构使得泡状流和塞状流等气液两相流中的气泡在重力、离心力和浮力等作用下在管道内部呈现不对称的相分布状态，两相滑移速度增大，显著影响换热性能并导致DNB型传热恶化难以预测。实验介质为空气-水，结合自主开发的电导式丝网探针技术并发展先进的数据后处理算法，实现了复杂流场的三维时空重构和离散气泡粒径的精细测量，获得了螺旋管内泡状流和塞状流的截面空泡分布规律。基于研究结果，可根据气泡分布规律对螺旋管道的几何结构进行调整以避免传热恶化，为螺旋管式蒸发器的安全设计提供了基础数据和优化思路。

关键词: 蒸汽发生器, 螺旋管, 丝网探针, 气液两相流, 气泡, 粒度分布