Is it possible to set a constant temperature in a fluid subdomain?
Yes, you can apply a source/sink term to the subdomain to maintain it at a constant temperature. The source/sink terms is applied to the Energy equation and depends on the local fluid temperature. It acts as a source when the temperature is too low and as a sink when the temperature is too high. You will never get a perfectly constant temperature since you would need an infinite source/sink term to instantaneously change the fluid temperature.
To maintain a subdomain at 325 [K], create the following expressions in CFX-Pre:
Scoeff = -1e6 [W m^-3 K^-1]
Sterm = Scoeff * (T - 325 [K])
Next, create a subdomain and apply an Energy source term. The CCL will look similar to the following:
SUBDOMAIN: Subdomain 1
Coord Frame = Coord 0
EQUATION SOURCE: energy
Option = Source
Source = Sterm
Source Coefficient = Scoeff
You may need to increase the value of Scoeff if you find that the desired temperature is not reached quickly enough.