Reversed flow in u-tube steam generator (SG) can result in the reduction of normal flow area and the increase of total pressure drop of SG primary side. The primary coolant flow rate will be lowered when reversed flow takes place, especially under the condition of natural circulation. In the past, the reversed flow phenomena have been observed, while most of the available publications were carried out based on one-dimensional system code or over-simplified single tube model, and very limited work has been done with 3D models. The overall 3D characteristics of flow distribution in the steam generator have not fully revealed. In this work, a fully 3D CFD model combining the porous media and open media were built to study the reversed flow in u-tube SG. The critical mass flow rate for reversed flow was obtained, so does the fraction and location of reversed flow with mass flow rate. The location of reversed flow shifts from short to long tubes at first and then shifts back to short tube with reducing the total mass flow rate.

U型管蒸汽发生器（SG）中的逆流会导致正常流动面积的减少和SG一次侧总压降的增加。发生逆流时，一次冷却剂流量会降低，特别是在自然循环条件下。过去，已经观察到逆流现象，而大多数可用的出版物都是基于一维系统代码或过于简化的单管模型进行的，并且对 3D 模型所做的工作非常有限。蒸汽发生器中流动分布的整体 3D 特性尚未完全显示。在这项工作中，建立了一个结合多孔介质和开放介质的全 3D CFD 模型来研究 U 型管 SG 中的反向流动。获得了反向流动的临界质量流量，反向流动的分数和位置与质量流量也是如此。反向流动的位置首先从短管转移到长管，然后随着总质量流量的降低而转移回短管。