TWO-PHASEWATER HAMMER SIMULATION WITH THE CATHARE CODE

Abstract

The CATHARE system code is designed for nuclear power plant safety studies and simulators. Both 1-D and 3-D modules use the 2-fluid 6-equation model. The 1-D module is fully implicit while the 3-D is nearly implicit. Both modules use a first order space discretization with a staggered grid and an upwind scheme. The experiment to be calculated consists of a vertical water tank of large dimensions connected at the bottom to a slightly inclined pipeline 36 m long with a diameter of 19.05 mm equipped with a valve at its downstream end opposite from the tank. This is an experiment of the dam with penstock type. A transient flow is initialized by fast closure of the valve. Propagation of a pressure wave is observed in the pipeline. Depending on the pressure and temperature conditions the following phenomena can be observed: propagation of pressure waves, cavitation (flashing and condensation), pressure wave reflection on pipe ends or on two-phase regions. Elastic strain of the pipeline has an influence on the speed of the pressure waves but the Cathare code does not model this fluid-structure interaction. However an equivalent problem can be calculated by modifying both initial and boundary limit conditions. Thus, the exact speed of sound in single phase liquid and the initial pressure wave magnitude can be obtained. Several calculations have shown that Cathare (1-D and 3-D) is able to predict qualitatively well the dynamics of pressure wave propagation and all types of wave reflections. A time step lower than the sonic CFL condition has been used because the characteristic time scales of source terms associated to flashing and condensation are significantly smaller than those associated to pressure wave propagation. It is shown that flashing and condensation closure laws control the quality of results more than the wave propagation itself. Then a simple first order space discretization together with a small time step are able to capture all the phenomena and can predict well the timing of events and the amplitude of the highest pressure peak. (author)