NONISOTHERMAL MIXING OF RHEOLOGICALLY COMPLEX FLUIDS WITH CLOSE-CLEARANCE IMPELLERS - EFFECT OF NATURAL-CONVECTION

Transient heat transfer in a mechanically agitated vessel is studied i n the case of an anchor and an helical ribbon impeller using Newtonian and shear thinning fluids. Temperature stratification is found more p ronounced with the anchor, making this impeller clearly inadequate for heat transfer. The impact of natural convection is evaluated first us ing the classical Gr/Re-2 ratio. It is shown that the use of this crit erion in viscous mixing is somewhat misleading. A new Grashof number i s then proposed to assess the significance of the viscous and buoyancy effects in non-isothermal, non-Newtonian mixing applications. It is s hown that the interpretation of this new number is strongly related to the concept of process viscosity.