Definitions
With the next steps you define the variable T_in according to Equation 1.
Average 1
First, create an average operator to evaluate the average temperature at the outlet.
1
In the Definitions toolbar click Nonlocal Couplings  and choose Average .
2
In the Settings window for Average locate the Source Selection section, and from the Geometric entity level list choose Boundary.
3
From the Selection list, choose Outlet.
Variables 1
1
In the Model Builder window, right-click Component 1 > Definitions and choose Variables .
2
In the Settings window for Variables, locate the Variables section and enter the following expressions
The expressions ht.Cp and ht.rho refer to the heat capacity and density of water as defined by the Heat Transfer in Solids and Fluids interface. The expression aveop1 refers to the average operator defined previously.