# 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 SOURCES: EQUATION SOURCE: energy Option = Source Source = Sterm Source Coefficient = Scoeff END ENDENDYou may need to increase the value of Scoeff if you find that the desired temperature is not reached quickly enough.