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.