Heat transfer and power consumption of Newtonian and non-Newtonian liquids in stirred tanks with vertical tube baffles

Abstract

Agitation and heating of Newtonian and non-Newtonian fluids are commonly used processes in various industrial sectors. Traditionally, heating is mediated using jackets and helical coils. Compared to these conventional techniques, a vertical tube baffles offers geometrical advantages that favor mixture quality and heat-transfer efficiency. However, the available literature on this subject is scarce. Therefore, this study aims to determining expressions for calculating the Nusselt number. To this end, plots of the power number were obtained as functions of the Reynolds number during the agitation and heating of Newtonian and non-Newtonian fluids in batch-operated tanks with vertical tube baffles. Two tanks (10 and 50 L) were used along with pitched blade turbine (PBT) and Rushton turbine (RT) mechanical impellers. The Newtonian (water and aqueous sucrose solutions) and non-Newtonian (carboxymethyl cellulose and Carbopol solutions) fluids were heated at different rotational speeds using the transient technique for heat transfer. The power consumed by the turbine was measured experimentally in terms of the torque generated during motor agitation. Non-linear regression analyses were used to develop models to plot Nusselt as a function of the Reynolds, Prandtl, and the corrective factor of viscosity. The models demonstrated good fit with the experimental results. The power number, a function of Reynolds, was found to vary in the range of 2–5 for the PBT and 5–9 for the RT. The developed heat-transfer models were found to be valid across a wide Reynolds range of 20–200,000 for both impellers in the laminar and turbulent regions, which were delimited at a critical Reynolds of 10,000.