The Minimum Recording Time for PIV Measurements in a Vessel Agitated by a High-Shear Tooth Impeller

Abstract

The flow field in a mechanically agitated vessel has often been investigated by Particle Image Velocimetry (PIV). In published studies, the recording time ranges between tenths of seconds and units of seconds. The question arises: What is the minimum recording time that is needed to obtain relevant data? Our study was aimed at determining the minimum recording time that is required to get consistent velocity field results, using the PIV technique, in a vessel agitated by a high-shear tooth impeller within the impeller flow discharge zone. Our results help to adjust PIV experimental conditions that guarantee correct velocity data. The tests were performed in a fully-baffled cylindrical flat-bottom vessel 400 mm in inner diameter agitated by a tooth impeller 133 mm in diameter, with three different liquids, using 2-D Time-Resolved PIV in the impeller Reynolds number range from 68 000 to 221 000. A statistical analysis of the radial and axial components of the mean and fluctuation velocities measured in the impeller discharge flow showed that the dimensionless minimum recording time is independent of the impeller Reynolds number. The value is N.tRmin = 62 for the mean radial velocity and fluctuation velocities in both the radial direction and the axial direction, while a longer measuring time, N.tRmin = 174, is required to obtain the consistent mean axial velocity for ±2% volatility. The dimensionless criterion N.tRmin = const is recommended as the scale-up rule for an estimate of the minimum recording time.