Flow of Viscous Shear-Thinning Fluids Behind Cooling Coil Banks in Large Reactors

Abstract

Through the use of a laminar flow loop, an ultrasound pulsed doppler device was found to accurately (±5%) measure the velocity of a viscous, opaque fluid (Xanthan Gum solution), using the entrained air (moving with the liquid) as energy reflectors. This technique was then utilized to measure the axial velocity of Xanthan Gum solutions behind cooling coil banks in two large (300 and 20 000 gal) fermentors, agitated by multiple down-pumping A315 impellers. The results indicate that, at both scales, the fluid in the annular region behind the cooling coil banks becomes completely stagnant at moderate to low agitation power (3.7 HP/1000 gal) when air is not sparged into the tank. Stagnant flow was observed at all airflow rates ≥0.1 vvm. A computational fluid dynamics (Fluent) model predicted reasonably well the actual liquid flow rate in the annular region behind the coil bank(s) of both tanks for the cases when air was not sparged. The results from an experimental design, using this Fluent model for the 20 000 gal tank, indicate that the axial velocity in the annulus is linearly related to the impeller Reynolds number (500−2000), D/T (0.36−0.48) and the cooling coil bank location relative to the tank wall (B/T: 0.06−0.09) and the tank bottom (Ob/D: 0.45−0.90).