Counter-current flow limitation experiments in a model of the hot leg of a pressurised water reactor - comparison between high pressure steam/water experiments and low pressure air/water experiments

In order to investigate the two-phase flow behaviour in a complex reactor-typical geometry and to supply suitable data for CFD code validation, a model of the hot leg of a pressurised water reactor was built at Forschungszentrum Dresden-Rossendorf (FZD). The hot leg model is devoted to optical measurement techniques, therefore, a flat test section design was chosen and equipped with large windows. In order to enable the operation at high pressures, the test section is installed in the pressure chamber of the TOPFLOW test facility of FZD, which is used to perform the experiments under pressure equilibrium with the inside atmosphere. Counter-current flow limitation (CCFL) experiments were performed, simulating the refluxcondenser cooling mode appearing in small break LOCA scenarios. The fluids used were air and water at room temperature and pressures of up to 3.0 bar, as well as steam and water at pressures of up to 50 bar and the corresponding saturation temperature of 264°C. One selected 50 bar experiment is presented in detail: the observed behaviour is analysed and illustrated by typical high-speed camera images of the flow.

Furthermore, the flooding characteristic obtained from the different experimental runs are presented in terms of the Wallis parameter and Kutateladze number, which are commonly used in the literature. However, both parameters fail to properly correlate the data: a discrepancy is observed between the air/water and steam/water series. Therefore, a modified Wallis parameter is proposed, which takes into account the effect of the fluid viscosities on the CCFL. The new parameter is validated against comparable data found in the literature, even though no data was found with such a large range of viscosities. This analysis points out that the effect of the dynamic viscosity on flooding has already been observed, but not identified. Furthermore, it is shown that the proposed modification of the Wallis parameter allows a significant improvement for experimental series with variation of the viscosities.