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 liqu
id) as energy reflectors. This technique was then utilized to measure the a
xial velocity of Xanthan Gum solutions behind cooling coil banks in two lar
ge (300 and 20 000 gal) fermentors, agitated by multiple down-pumping A315
impellers. The results indicate that, at both scales, the fluid in the annu
lar region behind the cooling coil banks becomes completely stagnant at mod
erate 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 greater than or
equal to0.1 vvm. A computational fluid dynamics (Fluent) model predicted re
asonably 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 resu
lts 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), DIT (0.36-0.48) and the cooling
coil bank location relative to the tank wall (BIT: 0.06-0.09) and the tank
bottom (O-b/D: 0.45-0.90).