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

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suddenly seemed to become more viscous when a large diameter
bob 'i;'JaS used at high speeds. The problem was easily eliminated
by using a smaller diameter bob, thus greatly reducing
the linear velocity of the bob at the expense of a small
increase in clearance.
T.emJ2erature effects..
In very viscous fluids the heat
generated by fluid friction is enough to cause a temperature
rise. The un1tJary investigator may then mistakenly report
pseudoplasticity or thixotropy due to the decrease in
apparent viscosity which in actuality is caused by the increase
in temperature.
McKelvey (114) reports that the heat
generation per unit volume is the product of the shear stress,
shear rate,
Heat generation = "~i J (2-16
and convers ion factor to heat units. WeI tmann (206) has
derived a complex equation giving the temperature variation
with radius caused by viscous heating. Heat generation is
not a problem for lOH viSCOSity materials and good temperature
control.
Time dependency.
Thixotropic fluids may best be measured
in a rotational viscometer.
The shearing stress is determined
for various times of shear at a constant shear rate. The
apparent viscosity is plotted versus the natural logarithm
of the time of shear and the slope is called the I1 coefficient