Prediction of batch heat transfer coefficients for pseudoplastic fluids ...

'7
ft~
easier method of calibrating the bob with a Newtonian
flllid has been sh01.Jn to be accurate.
!1 NeHtonian fluid of
knovm viscosity is placed in the viscometer and the torque,
angular velocity, and appropriate dimension data taken.
The shear rate is calculated using equation 2-11 laJi th n tl
equal to Ul~ity.
The shear stress is then calculated using
the shear stI'ess-ra te relationship for Nev.rtonian fluids
/ =/1 r (2-13
The effective height is then calculated using equation 2-5.
Metzner and otto (125) have sho~~
that viscometer bobs calibrated
in this manner give accurate values for shear stress
even Hhen using highly pseudoplastic fluids.
Turbulence.
for laminar flmeJ.
Equations 2-5 through 2-12 are only valid
Above a critical Reynolds Number the
flov! becomes turbulent and the shear stress increases at a
mOI'e than proportionate rate, resulting in an increasing
apparent viscosity. Van Wazer (196) reports that the
critical Reynolds nu~ber
is a function of the linear velocity
of bob surface, Vb' the clearance bet",reen bob and cup, and
the viscosity of the fluid.
NReCCRIT.) = "i (Re -R 6 )f
~ = 41.3/Rc/th?c-H2)
/It = 2100
(2-14
{2-l5
It was found in this work that the less viscous fluids