Experimental study on flow modes and transient characteristics in low-pressure equal-height-difference natural circulation system.

• The four flow modes were observed and analyzed of an EHDNCS. • The stability map of EHDNCS has been obtained based on 134 experimental cases. • The regions of stability and CIWH can be well divided by different equilibrium qualities. • Discrete and amplitude characteristics of the flow rate were analyzed for the four flow modes. • Increasing the inlet temperature and resistance is more easily induce flow instability of EHDNCS. As nuclear power plants tend to be miniaturized and integrated, equal-height-difference natural circulation systems (EHDNCSs) have potential applications in floating nuclear power plants. This work experimentally investigated the flow modes of an EHDNCS at low pressure. It has observed four quintessential flow modes in the experiments. The transient characteristics of each flow mode were analyzed by the visualization results. In addition, the dispersion and amplitude characteristics of flow rate were compared in different flow modes based on 134 experimental cases. The experimental results reveal that the flow oscillation is related to unsteady flow pattern for intermittent and condensation oscillations. The condensation induced water hammer (CIWH) is found in the condensation oscillation. According to the stability map, it is found that the regions of stability and CIWH can be well divided by equilibrium quality of −0.23% and 0.47%, and the unstable range is enlarged by increasing the inlet temperature and resistance. [ABSTRACT FROM AUTHOR]