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

16
can be s~p1i~ied to (100~ 125, 213).
'2fw = 47r N/:t!l.u (2-12
Fer any cup rond bob a~-amgement t~ eup radius~ the bob
radius, and the equivalent height of the bob &re eonst~t.
The sneaP stress and she&r rate are thus evaluated by
taking readings o~ torque versus angular veloCity or the
bob. These values are plotted on log-log graph paper to
evaluate slo~e,' ntT. The shear stress and shear rate are
then ealeulated using equation 2-5 and 2-11. ~e power
law eonstaats, R and K, e~ be evaluated from a logarithmic
plot or shear stress versus shear rate. (~e slope n shonld
be equa.l to 11. 11 ).
E,nd e~fe_et.~.
Equation 2-5 is an expression ror the
shear stress on the curved surface of the bob •. However,
when the lengtli of the bob is not very large im comparison
with the bob diameter tae torees on the ends of the cylinder
m~ ~ot be neglected. These are usually taken into account
by eonsiderimgthe m ds as extensions at.' the cylindrical
height ~d ealling the total the effective height. Ir
more bobs of equal diameter but different lemgths are
available th~ value of the et.'feetive height f0~ that bob
di~eter oaE be evaluated by plotting the to~que ~t CORstant
speed versus bob height ~d extrapolating to zero
torque. The absolute value or height at zero torque
(this height will be a negative value) should b$ added to
the aetual height to get the effective height (54, 107).