Experimental and theoretical investigations on effect of U-tube inlet and secondary temperatures on reverse flow inside UTSG

Abstract

The reverse flow phenomenon in partial tubes of vertically inverted U-tube steam generator (UTSG) during natural circulation conditions is investigated in this paper. By gradually rising the inlet plenum temperature of steam generator simulator at a constant mass flux rate, the phenomenon is 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 U-tube inlet and secondary side temperatures on the reverse flow inside the UTSG. The results show that the mass fluxes will be increased and the inlet temperatures will be decreased in the normal flow U-tubes when the reverse flow occurs, thus the normal flow in remnant tubes tends to be more stable. The space distribution of reverse flow U-tubes depends on the critical pressure drop (CPD) of each U-tube under the local physical conditions, such as the U-tube length, U-tube inlet temperature and secondary side temperature. For one whole steam generator under the natural circulation, the critical mass flow rate (CMFR) of each U-tube will be increased when the U-tube inlet temperature increases or the secondary side temperature decreases, and the larger CMFR indicates the higher possibility that reverse flow occurs in the steam generator. The relationship between the CPD or CMFR and the U-tube length is non-monotonic.