Abstract:

Helically coiled tube (HCT) steam generators are widely used in chemical engineering, aerospace industry and nuclear engineering, etc., and especially, in small modular nuclear reactors because of their high heat transfer efficiency, compact structure and free thermal expansion behavior. The special geometry of the HCT leads to complex flow characteristics such as secondary flows and the resulting special phase distribution and flow patterns in the tube and relatively high pressure drop compared to that in straight tubes. The difference of geometrical parameters usually leads to the significant difference in the characteristics of gas-liquid flow in HCTs. In the present study, the pressure drop and flow-pattern transition characteristics of subcritical pressure water-steam two-phase flow in an HCT with a special small tube diameter were investigated by experiments and numerical simulations. It can be concluded that the frictional pressure drop firstly increased with the increase in thermal equilibrium quality, and reached a peak at the quality of 0.75, and then decreased gradually. This was due to the flow regime transforms from annular flow to dispersed flow (or mist flow) when the quality equaled to 0.75, which led to the reduction of frictional pressure drop. The flow regimes of high-pressure steam-water two-phase flow in HCTs can be divided into bubble flow, intermittent flow, annular flow and dispersed flow (or mist flow). The transition criteria between bubble flow and intermittent flow was at the quality of 0.038, the transition criteria between intermittent flow and annular flow was at the quality of 0.500, the transition criteria between annular flow and dispersed flow (or mist flow) was at the quality of 0.751, and the dry-out point was at the quality of 0.93. This study can provide guidance for the design and safe operation of HCT steam generators.

Key words: helically coiled tube, gas-liquid flow, interface, frictional pressure drop, numerical simulation

摘要：

螺旋管蒸汽发生器因其传热效率高、结构紧凑、热膨胀自由等特点，被广泛应用于化工、航空航天、核工程等领域，尤其在小型模块化核反应堆中得到了广泛应用。螺旋管特殊的几何形状导致其内工质流动特性复杂，如管内二次流及由此引发的特殊气液相分布、流型转变特性以及高于直管的压降。几何结构参数的差异往往导致螺旋管内气-液流动特性显著不同。本文结合实际应用需求，通过实验和数值模拟对某一特殊细小管径的螺旋管内亚临界压力条件下水-水蒸气两相流的压降特性和流型转换特性开展研究。发现螺旋管内气-液两相摩擦压降随热平衡干度的增加而增大，在干度约为0.75处达到峰值，然后逐渐降低；该现象出现是由于在干度约为0.75时两相发生了环状流向分散流（雾状流）的流型转变，使得壁面切应力减小，从而导致两相摩擦压降减小。结合数值模拟的分析，将螺旋管内高温高压气-液两相流的流型划分为泡状流、间歇流、环状流和分散流，其中泡状流和间歇流的转换干度为0.038，间歇流和环状流的转换干度为0.500，环状流和分散流的转换干度为0.751，同时确定了蒸干点为0.93。本文的研究结果可为螺旋管式蒸汽发生器的设计和安全运行提供参考。

关键词: 螺旋管, 气-液两相流, 界面, 摩擦阻力, 数值模拟