低压自然循环复合流量脉动实验研究

在低压流动沸腾不稳定性实验中，研究了自然循环流动在不同入口过冷度下的演化过程。对实验中的流动沸腾不稳定性入口流量信号进行快速傅里叶变换，基于振幅和频率特性区分了3种流动脉动模式：小幅流量脉动、复合流量脉动和逆流。分析了加热功率和入口过冷度对自然循环不稳定性的影响。根据加热段出口水温变化得到了出口的流型变化，当流量波动振幅较小时加热段出口流体始终是饱和状态，而当流量波动振幅较大时，加热段出口为单相液体和两相混合物交替通过。给出了这3种流量脉动的边界图，分析了热流密度和入口过冷度对流量脉动模式的影响。结果表明：出口含气率大于0时发生流动不稳定性，热流密度达到间歇干涸型临界热流密度时发生逆流。

Experimental Investigation of Compound Flow Oscillation under Low Pressure Natural Circulation

The evolution of natural circulation flow under different inlet sub-coolings was studied in the low pressure flow boiling instability experiment. Fast Fourier transform was applied to the inlet flow rate signal under flow boiling instability. Three flow oscillation modes were found based on the characteristics of amplitude and frequency: Small amplitude flow oscillation, compound flow oscillation and flow reversal. The effects of heating power and inlet sub-cooling on natural circulation flow instability were analyzed. Flow patterns at the exit of the heated section were obtained from the change of exit water temperature. Fluid can be kept at saturated state during the whole oscillation period when the amplitude of flow oscillation is small. However, single phase liquid and two-phase mixture appear alternatively through the exit of the heated section when the amplitude of flow oscillation is large. At last, the stability map indicating the boundary of these three flow oscillation modes was given. Effects of heat flux and inlet sub-cooling on flow oscillation modes were analyzed. The results show that flow instability happens when exit quality exceeds zero and flow reversal happens when heat flux exceeds periodic dry-out CHF.