Experimental research on reverse flow critical point among parallel U-tubes in SG

Under weak driving force conditions, the reverse flow among U-tubes was found in steam generator (SG), which brings the negative effect on the flow and heat transfer of SG, especially on the passive safety of nuclear power plant. The reverse flow in SG is thought as the Ledinegg-type flow instability. It is of great importance to obtain the reverse flow critical point. In present work, a quasi-steady state experimental method is proposed, by which the reverse flow critical points are measured under different conditions. The experimental results show that the reverse flow occurs in the short U-tube firstly, and the inlet temperature of U-tubes will decrease and the pressure drop will increase sharply when the reverse flow occurs. The reverse flow critical point (CIT) and inlet mass flow rate (IMFR) are obtained and discussed. It is shown that the relationship between the CIT and IMFR is linear. The results are useful for the safety analysis of nuclear power plant and the optimal design of parallel U-tubes in SG.