Scaling criteria for LWR's under single-phase and two-phase natural circulation

Scaling criteria for a natural circulation loop under single-phase and two-phase flow conditions have been derived. For a single-phase case the continuity, integral momentum, and energy equations in one-dimensional area average forms have been used. From this, the geometrical similarity groups, friction number, Richardson number, characteristic time constant ratio, Biot number, and heat source number are obtained. The Biot number involves the heat-transfer coefficient which may cause some difficulties in simulating the turbulent-flow regime. For a two-phase-flow case, the similarity groups obtained from a perturbation analysis based on the one-dimensional drift-flux model have been used. The physical significance of the phase-change number, subcooling number, drift-flux number, friction number are discussed and conditions imposed by these groups are evaluated. In the two-phase-flow case, the critical heat flux is one of the most-important transients which should be simulated in a scale model. The above results are applied to the LOFT facility in case of a natural-circulation simulation