For T > Td, the Williams-Landel-Ferry (WLF) equation (2.30) is used:
-19(T T,)
Log a 19(T T(2.30)
\ay 92 + (T T,)
For T < Td, the Ahrennius function (2.31) is used:
Logr =-- (2.31)
a'd 2.303Rg TdT
where
aT/aTd = the shift factor relative to the defining temperature
T = Temperature at which properties are desired, ( K)
Td = Defining Temperature, ( K)
Ea = Activation energy for flow below Td, 261,000 J/mol
Rg = Ideal gal constant, 8.34 J/mol- K
K = C + 273
To make the computation easy and simple, Td was made to coincide with Tref, So, the
equation above resulted in the following equations:
For T > Tref, the Williams-Landel-Ferry (WLF) equation (2.30) is used:
Log a1 -19(T -T) (4.1)
\arre 92 + (T T )(
For T < Tref, the Ahrennius function (2.31) is used:
Ka E 1 1
Log = 3 (4.2)
Sarref 2.303R9 T ref