Film-droplet split correlation at the onset of annular-mist flow

Boiling transition in BWR fuels occurs in the annular-mist flow regime. Many subchannel analysis methods have been developed to predict critical power, as well as flow and void distributions, in BWR fuel bundles. Two-phase annular-mist flow can be better simulated with two-fluid, three-field formulations for liquid film flow on surfaces, vapor and entrained droplet flows inside the core of the subchannel. To predict dryout of BWR fuels in the annular flow regime, the liquid film mass balance is analyzed using appropriate droplet entrainment and deposition models along with evaporation to determine the condition when film dries out or a dry patch forms (i.e. boiling transition). It is of great importance to define a correct boundary condition for the liquid flow split between the liquid film and entrained droplets at the onset of annular flow to accurately model the entrainment and deposition at the downstream annular flow, predict dryout location and corresponding critical power. No reliable information on the film-droplet split at onset of annular flow have been found in the literature and most analyses have been performed with an assumption that a single value is valid for wide ranges of operating conditions. In this study, a new film-droplet split correlation in the form of film fraction is developed based on available data with steam-water experiments. The liquid split correlation is then applied to simulate film flow measurement data over a wide range of exit quality.