What is shear vs. viscosity?
A Pascal second is 1,000 cp (most viscosity is reported as mPs; milli Pascal seconds these days) So 0.5 Pascal seconds is 500 cp
Inverse second is the shear rate. So this result would imply that the viscosity is shear dependent (non Newtonian)
By taking a minimum of three readings of viscosity at different shear rates we calculate the shear rate dependence using the least squares method of curve fitting. As the relationship is exponential it can be plotted on log/log and calculated for any shear rate. Note that we use different spindle speeds for this method, we know (or can calculate) the so called shear rate constant for the known geometry of the specific spindle. Also note that only cylindrical spindles can be used for this method because the geometry is well defined. Disc spindles are not appropriate.
For mixer design the “apparent” viscosity is dependent on the impeller speed and the impeller’s own shear rate constant. Although Sharpe has not determined the shear rate constant for our impellers we do use published values, which are fine as the margin of error is no big deal when working with power law fluids.
For the HF218 the constant is 10
For Axial it is 11
For anchors and double helix it is 20
So the “apparent” viscosity of a HF218 at 350 rpm would be determined from the viscosity vs. shear rate at a shear rate of 350rpm/60 X 10 = 58/sec
Baffle recommendations need to be determined at low shear rates (as at the tank wall the viscosity is by definition infinite; the shear rate would be zero and dividing the shear stress by zero is infinite)
Typically a shear rate of 2/sec is used for baffle design. In most cases baffles need to be small or not used. For example a lime slurry may have a viscosity close to the tank wall of over 10,000 cp. Standard baffles for this would simply create dead zones and effectively reduce the tank diameter for mixing by the baffle width x 2. So for stuff like fruit slurries (ketchup, pop tart fillings, etc.) with very high “shear thinning index” on center, no baffles, slow speed to minimize cavity formation is the correct design.
By Jeremy Higginson, P.E.
Direct of Engineering
A Pascal second is 1,000 cp (most viscosity is reported as mPs; milli Pascal seconds these days) So 0.5 Pascal seconds is 500 cp
Inverse second is the shear rate. So this result would imply that the viscosity is shear dependent (non Newtonian)
By taking a minimum of three readings of viscosity at different shear rates we calculate the shear rate dependence using the least squares method of curve fitting. As the relationship is exponential it can be plotted on log/log and calculated for any shear rate. Note that we use different spindle speeds for this method, we know (or can calculate) the so called shear rate constant for the known geometry of the specific spindle. Also note that only cylindrical spindles can be used for this method because the geometry is well defined. Disc spindles are not appropriate.
For mixer design the “apparent” viscosity is dependent on the impeller speed and the impeller’s own shear rate constant. Although Sharpe has not determined the shear rate constant for our impellers we do use published values, which are fine as the margin of error is no big deal when working with power law fluids.
For the HF218 the constant is 10
For Axial it is 11
For anchors and double helix it is 20
So the “apparent” viscosity of a HF218 at 350 rpm would be determined from the viscosity vs. shear rate at a shear rate of 350rpm/60 X 10 = 58/sec
Baffle recommendations need to be determined at low shear rates (as at the tank wall the viscosity is by definition infinite; the shear rate would be zero and dividing the shear stress by zero is infinite)
Typically a shear rate of 2/sec is used for baffle design. In most cases baffles need to be small or not used. For example a lime slurry may have a viscosity close to the tank wall of over 10,000 cp. Standard baffles for this would simply create dead zones and effectively reduce the tank diameter for mixing by the baffle width x 2. So for stuff like fruit slurries (ketchup, pop tart fillings, etc.) with very high “shear thinning index” on center, no baffles, slow speed to minimize cavity formation is the correct design.
By Jeremy Higginson, P.E.
Direct of Engineering
