PREDICTION OF SWELL LEVEL AND WATER MASS INVENTORY IN STEAM GENERATORS

Abstract

A prediction of steam generator behaviour in case of a loss-of-feedwater transient and a calculation of a time period in which tubes’ dry out will occur, strongly depends on an initial water mass inventory on the secondary side of the steam generator and a two-phase mixture swell level. A reliable prediction of the swell level and water mass inventory is based on appropriate interfacial drag force correlations or steam-water slip models and a robust numerical method that is able to predict the swell level movement. A prior assumption of the swell level fixed position in a thermal-hydraulic simulation of the steam generator secondary side might lead to erroneous results. In this paper the prediction of the two-phase flow around tubes in a bundle and the calculation of the steam generator secondary side thermal-hydraulics is based on the two-fluid model and the Computational Multi Fluid Dynamics (CMFD) approach. The applied approach solves the vapourliquid multidimensional two-phase flow and it enables the determination of the swell level position and corresponding liquid mass inventory. Vapour and liquid interfacial mass, momentum and energy transfers are modelled with the closure laws, where some of them are specially developed for the condition of two-phase flow across tube bundles. The governing equations are solved by the SIMPLE type pressure-correction method that is derived for the multiphase flow conditions. The developed model is applied to the prediction of experimental conditions of refrigerant R113 boiling around tubes in a bundle at the atmospheric pressure. The numerically calculated swell level and the pressure field in the two phase mixture are compared with available measured data and a good agreement is achieved, which also indicates a good prediction of the vapour void fraction and liquid mass inventory. The developed numerical method and performed simulations are an improvement regarding previous investigations that were based on the calculation with the fixed swell level and without the validation of the water mass inventory on the steam generator secondary side.