PIV measurements in the vicinity of a steam sparger in the PANDA facility

To study the thermal behaviour, e.g. the stratification build-up and the homogenization, steam is injected through a sparger into a large pool containing 45 m³ of water in the PANDA facility; a scenario which could take place in the wetwell of a BWR in the case of a postulated accident. Depending on the steam flow rates vented in the wetwell two regimes can develop in the water pool (i) a thermal stratification build-up and (ii) thermal mixing or homogenization. If thermal stratification build-up occurs, the temperature of the upper part of the water pool is much higher than the bottom of the pool which would decrease the rate at which steam condenses since the heat sink capability of the pool decreases. Conversely if mixing occurs the pool temperature remains homogeneous throughout the pool and provides a higher heat sink capability for steam condensation. As a complement to temperature recordings, the use of Particle Image Velocimetry (PIV) is needed to validate theoretical models and CFD codes. However, in a two-phase flow of water and condensing steam one is faced with two main challenges. (1) as the steam condenses around the sparger pipe, the temperature increases locally in the pool, consequently, the solubility of air in water decreases and as a result the local heating forces dissolved air out of the water which forms small bubbles rising to the surface. These bubbles might act as additional scattering centres disturbing the velocity measurements in the liquid phase. (2) for high air bubble densities the dense bubble cloud might even block the light passage from the illumination plane to the camera such that the sparger becomes invisible. Thus, a degasification of the water pool is absolutely necessary. To eliminate the bubbles as scattering centres we used Rhodamine-B seeding particles for the PIV measurements. These particles emit light in the red spectrum and with a red filter in front of the camera, only those particles contribute to the velocity calculation. Using degassed water together with Rhodamin-B particles it was finally possible to record velocity fields even in this demanding environment for high steam mass flow rates. The paper contains a description of the methods developed to perform PIV measurements in the vicinity of the sparger and a selection of the results obtained.