Experimental and theoretical investigations on reverse flow characteristics in vertically inverted U-tube steam generator under transient condition

Abstract

Under natural circulation conditions, the primary circuit of a steam generator (SG) has a tendency to occur flow instabilities. These instabilities due to reverse flow in some vertically inverted U-tubes will increase the flow resistance coefficient and reduce the natural circulation ability in the primary system. To address the lack of existing research on transient reversed flow, in the present work, the reverse flow characteristics in the U-tubes of vertically inverted U-tube steam generator (UTSG) are investigated experimentally and theoretically. By gradually rising the inlet plenum mass flow of steam generator simulator at a constant fluid temperature, the reverse flow characteristics are observed in a thermal hydraulic test facility. Based on the experiment, the flow and heat transfer model in U-tubes are established to study the effects of change rate of mass flow and flow distribution on the reverse flow inside the UTSG. The experimental and theoretical results demonstrate that during the transient process, some U-tubes get reverse flow with the continuous decrease of mass flow and the reverse flow U-tubes also have a significant impact on other U-tubes, which results in a maximum temperature drop of about 35%. With the decrease of mass flow in U-tubes, the critical mass flow of the UTSG will increase and the reverse flow will occur more easily during the transient process. The results can provide a basis for avoiding the occurrence of reverse flow in actual SGs and improving the ability of natural circulation.