Improved Closure Relations for the Two-Fuid Model in Flashing Flows
Abstract: The two-fluid model is a widely used approach for the modeling of two-phase flows, consisted of two separate sets of mass, momentum, and energy conservation equations for the liquid and gas phases where the interaction between the phases is determined by the mass, momentum, and energy transfer terms. This work focuses on the modeling of the interfacial mass generation rate of the two-fluid model in flashing flows by studying the modeling of the interfacial area concentration and the driving flux. Three sets of experiments are performed in this work with a closed-loop test facility to obtain measurements of two-phase parameters. The first set is a natural circulation experiment carried out with an annulus test section, while the second and third sets are forced convective flashing experiments carried out with an annulus and an annulus-to-pipe test section, respectively.
The forced convection flashing data are used to benchmark the systems code, RELAP5/MOD3.3 where the void fraction is observed to be underpredicted. The benchmarking of the static correlations used by RELAP5/MOD3.3 and TRACE to calculate the interfacial area concentration also shows inaccuracy, suggesting that the existing model is underpredicting the interfacial mass generation rate. On the other hand, from the benchmarking of the decoupled one-group Interfacial Area Transport Equation (IATE), it is observed that small and large bubbles need to be treated separately to improve the prediction of the IATE. Similar analysis is carried out for the coupled two-group model with IATE where the simulation results suggest the need of a stronger interfacial mass generation model.
Interfacial mass generation models are derived from the mass-energy balance equation for group-2 bubbles with the spherical and cylindrical growth assumptions. A different Nusselt number correlation is suggested to the group-1 interfacial mass generation model. The models are implemented to the coupled two-group model with IATE for benchmarking and favorable predictions are obtained. Sensitivity studies are performed to investigate the effects of group-2 shape coefficient and relative velocities. Lastly, modifications are proposed to the interfacial area concentration correlation. The modified correlation is coupled to the one-group void transport equation and benchmarked with the newly acquired dataset where improved predictions are obtained.
