An experimental study of natural circulation flow instability has been carried out to investigate instability modes at low pressure. Water in the primary loop is heated in a single macro-channel and condensed in a tube-shell shape condenser by circulating coolant in the secondary loop. Two kinds of flow instability modes are identified: flow instability induced by flow boiling in the heated channel and flow instability induced by loop circulation. Flow boiling instabilities are found at three exit vapor qualities: x_e < 0 (sub-cooled boiling), 0 ≤ x_e < 1 (saturated boiling) and x_e = 1 (periodic dryout). Sub-cooled boiling instability is caused by bubble growth and condensation in the long sub-cooled flow boiling region. Saturated boiling instability is classified as Pressure Drop Oscillations because it operates in the negative slope region of internal characteristic curve. Combined with the oscillations of heating surface temperature, periodic dryout is confirmed by predicting CHF from empirical correlations. Flow instability induced by loop circulation is classified as Natural Circulation Oscillations, which is governed by the imbalance between heat transfer in the heated channel and condenser. The frequency and amplitude of flow boiling instability are given. The evolution of flow instability is indicated in the stability map.

已经进行了自然循环流动不稳定性的实验研究，以研究低压下的不稳定性模式。一级回路中的水在单个宏通道中加热，并通过在二级回路中循环冷却剂在管壳式冷凝器中冷凝。确定了两种流动不稳定性模式：由加热通道中的沸腾引起的流动不稳定性和由回路循环引起的流动不稳定性。在三种出口蒸气质量下发现流沸腾不稳定性：x _e  <0（过冷沸腾），  0≤x _e  <1（饱和沸腾）和x _e = 1（周期性变干）。过冷沸腾的不稳定性是由长的过冷流沸腾区域中的气泡生长和冷凝引起的。饱和沸腾不稳定性归因于内部特性曲线的负斜率区域，因此被归类为压降振荡。结合加热表面温度的波动，可以通过经验相关性预测CHF来确认周期性变干。由回路循环引起的流动不稳定性被归类为自然循环振荡，这是由加热通道和冷凝器中传热之间的不平衡所决定的。给出了沸腾不稳定性的频率和幅度。流动不稳定性的演变在稳定性图中指示。