Abstract
In this paper, a new thermal-hydraulic model is developed to analyze both, the density wave oscillation and flashing processes, in a core comprised of a heated flow channel, an adiabatic unheated riser above the channel and a downcomer. A linear correlation between saturation enthalpy and local pressure is assumed. The homogeneous equilibrium model (HEM) is used in the two-phase region. The heated channel and the riser are divided into three connected regions from the channel inlet to the riser exit. The two lower regions are single phase regions, and the upper most region is a two-phase region. Each of the three regions is divided into a number of equal-size nodes. Piece-wise linear profiles of phase variables including subcooled enthalpy in the single phase region, and saturation enthalpy, mixture flow speed and steam quality in the two phase region, are assumed in each node. Classical weighted-residual approach is then applied to energy, momentum and mass conservation equations to derive the set of ODEs comprising this model. The steady states or fixed points of the set of ODEs are analyzed and compared with the results of stability tests carried out in a natural circulation BWR (Dodewaard, the Netherlands). Results are also compared with those calculated using another model developed by van Bragt et al. Direct numerical simulations of the ODEs are also performed to study the dynamical behavior of the model.
