Countercurrent flow limitations in horizontal stratified flows of air and water
- Authors
- Publication Date
- Jan 01, 2004
- Source
- OpenGrey Repository
- Keywords
- Language
- English
- License
- Unknown
Abstract
During a postulated loss-of-coolant-accident (LOCA) in a pressurized water reactor (PWR) it is of vital importance that the reactor core remains properly cooled. The emergency core cooling system (ECCS) in German PWRs compensates the loss of coolant with injection of additional coolant into the cold legs as well as into the hot legs. While the coolant is injected in the cold legs through nozzles, the hot leg injection is performed by means of a secondary pipe placed at the bottom of the pipe of the primary circuit. The subject of this thesis concerns the latter case. The liquid injected into the hot leg flows directly into the core from its upper part and constitutes a rapid delivery of coolant into the reactor core at high mass flow rate. However, saturated steam is generated in the reactor core due to depressurization of the primary system and flows out of the reactor pressure vessel (RPV) into the hot leg. Therefore, a countercurrent stratified flow of injected coolant and saturated steam occurs along one and a half meter inside the hot leg before the coolant reaches the RPV. This horizontal stratified countercurrent flow of coolant and steam is only stable for a certain range of coolant and steam mass flow rates. Even if the coolant is injected at very high velocities and high Froude numbers, there is always a threshold steam velocity above which the cooling of the reactor core can be reduced or complete interrupted. This phenomenon is known in two-phase flow science as countercurrent flow limitation (CCFL), since there is a limitation of liquid delivery due to the presence of a gas phase flowing countercurrently to the liquid phase. CCFL in reflux condensation cooling was more investigated than in ECC in the hot leg. For this purpose, the test facility WENKA was built at Forschungszentrum Karlsruhe GmbH (Germany) to investigate for which flow conditions CCFL poses a safety risk during hot leg injection and to provide experimental data to support the analysis of such an accident scenario with CFD - Codes. The WENKA test facility models a simplified PWR hot leg geometry including the secondary pipeline placed at the bottom of the main coolant line. The countercurrent flow of coolant and saturated steam during injection by means of the ECCS was investigated with air and water in a rectangular test section. The fluid dynamics of the injection process was reproduced for a wide range of flow conditions to identify flow regimes and to derive 1-dimensional models to predict the limits of coolant delivery. On the other hand, a data base of local flow parameters was established to enhance CFD - Codes performance. Experimental local velocities of the liquid film were obtained by means of particle image velocimetry and the liquid film morphology was analyzed depending on the flow regimes. (orig.) / Available from TIB Hannover: ZA5141(7018) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische Informationsbibliothek / SIGLE / DE / Germany