Study on flow characteristics in an open two-phase natural circulation loop

Highlights: • Six circulation modes were identified in an open loop. • Flashing and geysering are the determinants of flow characteristics. • The evolution of circulation modes was discussed. • Suppressing geysering facilitates the emergence of continuous flashing. • By taking into account subcooling, flashing and heat dissipation, the average circulation flowrate was predicted well. - Abstract: Flow characteristics are relevant to the safe operation of natural circulation systems. Experimental research on flow characteristics in an open natural circulation loop was conducted. It was shown that flashing and geysering are the two fundamental phenomena influencing the flow characteristics; in addition, flashing and geysering interact reciprocally: geysering weakens the impact of flashing on the flow, while flashing can suppress geysering. Six circulation modes were identified based on the weight of flashing and geysering, as well as typical inlet flowrate curve and experimental observations. As the heating power varies, so do the weights of flashing and geysering, which result in variable flow behavior and the evolution of circulation modes. Based on the theoretical analysis and the calculation of average circulation flowrate by taking into account the effects of subcooling, flashing and heat dissipation, suggestions for the operating and improvement of the system were proposed.