Analysis of particle cloud height dynamics in a stirred tank

Local and temporal variations of the particle cloud formed in a cylindrical mixing vessel were investigated experimentally. Different particle sizes (0.5, 1, and 2 mm) and volumetric concentration up to 20 vol % were evaluated at different impeller speeds. The time-averaged cloud height was linear with impeller frequency and with volume concentration. Suspensions with larger particles had a lower average cloud height, while the standard deviation for the temporal cloud height variation was larger. Two strong periodic phenomena were identified to be dominating the particle cloud height variations. The frequencies were linear with impeller speed, resulting in dimensionless frequencies of S-1 = 0.02-0.03 and S-2 = 0.05-0.06. The frequencies were affected by neither the particle size nor the volumetric concentration. The amplitude showed no dependency on the particle size, but the S-2 amplitude significantly decreases and S-1 increases with increasing solid concentration. The results were compared to LES/discrete element method simulations and showed a fair agreement. (C) 2015 American Institute of Chemical Engineers