Stereo Particle-Image-Velocimetry Measurements of an Unstable Air Stratification Due to Natural Convection in a Model Containment

Abstract

Numerical simulations of a realistic containment configuration (size of several tenths of meters) are both challenging from the physical modeling aspect and the relevant spatial/temporal resolution being necessary to capture all important mean flow and turbulence characteristics, the latter implying a correspondingly long computational time. Furthermore, full-scale experiments for model validation (CFD and lumped parameter methods; CFD – Computational Fluid Dynamics) are hardly affordable. To resolve this situation the THAI-facility (Thermal-hydraulics, Hydrogen, Aerosol and Iodine) was designed and built to enable large-scale experiments, which could then serve as a source for validation database used for the computational models improvement. This facility consists of test vessel (60 m³ volume) with a diameter of 3.4 m and a height of 9.2 m and is equipped with a broad range of measurement techniques and monitoring systems. The present study examines an unstable air stratification created by heating the lower walls and cooling the upper wall of the THAI vessel. In the center of the vessel an inner cylinder was inserted to promote establishing of a re-circulating convective flow. Velocity field data were acquired using a stereo PIV system, used to cover a flow area of about 0.5m² during the stereo PIV measurements and up to 1 m² for two-velocity component (2D) measurements. Furthermore, due to the low bulk velocities involved, the system could acquire the data of the bulk motion in a quasi time resolved manner. Long averaging times of over 1000 s allowed the estimation of the turbulent flow quantities (all velocity and Reynolds-stress components). Selected time-averaged and instantaneous flow fields illustrating mean flow and turbulence structure within the containment configuration in the case of the unstable air stratification due to natural convection will be shown and appropriately discussed.