Please read Read before posting: How do I get my question answered?
That is not a reproducible example, since, for instance, we don’t have access to the mesh. You could have easily used a builtin mesh. You must also include the imports in your code.
From what I gather, in
def mu(T):
return 19302.7-0.23786*T-2.2448e-5*T**2
is unnecessary to use a python function. Simply define a Function (not TrialFunction) T, and use it in
mu = 19302.7-0.23786*T-2.2448e-5*T**2