In this example a feedback loop is used to control the outlet temperature by varying the temperature at an inlet. To illustrate the example consider the geometry shown below:

Figure 14.1: Temperature Feedback Loop

Fluid from a main and a side inlet enter at temperatures of 275 K and 375 K respectively. The temperature of the fluid entering from the third inlet depends on the outlet temperature. When the outlet temperature is greater than 325 K, the fluid from the third inlet is set to 275 K. When the outlet temperature is less than 325 K, the fluid from the third inlet is set to 375 K. In addition an expression is used to set the dynamic viscosity to be a linear function of temperature.

The LIBRARY section of the .ccl (CFX Command Language) file appears as follows. Note that the "\" character indicates a line continuation in CCL.

LIBRARY:
   MATERIAL: Water at STP Modified
      Option = Pure Substance
      PROPERTIES:
         Option = General Fluid
         Density = 9.999E2 [kg m^-3]
         Dynamic Viscosity = VisT
         Specific Heat Capacity = 4.21E3 [J kg^-1 K^-1]
         Thermal Conductivity = 5.69E-1 [W m^-1 K^-1]
      END # PROPERTIES
   END # MATERIAL Water at STP Modified
   CEL:
      EXPRESSIONS:
         Tupper = 375.0 [ K ]  # Upper temp.
         Tlower = 275.0 [ K ]  # Lower temp.
         Visupper = 0.000545 [ N s m^-2 ]  # Vis. at Tupper
         Vislower = 0.0018 [ N s m^-2 ]  # Vis. at Tlower
         VisT = Vislower+(Visupper-Vislower)*(T-Tlower)/ \
            (Tupper-Tlower)
         # Vis.-Temp. relationship
         Tm=(Tupper+Tlower)/2
         Tout=areaAve(Water at STP Modified.T)@outlet
         Tcontrol=Tlower*step((Tout-Tm)/1[K]) \
            +Tupper*step((Tm-Tout)/1[K])
      END # EXPRESSIONS
   END # CEL
END # LIBRARY

The first four expressions, Tupper, Tlower, Visupper and Vislower are simply constant values to define temperature and viscosity values. The expression VisT produces a linear function for the dynamic viscosity taking a value of Visupper at Tupper and a value of Vislower at Tlower. The expression Tm sets the desired value of the outlet temperature. In this case it is set to a mean value of the two inlet temperatures.

Tout calculates the outlet temperature using the areaAve function.

Finally the expression Tcontrol is used to set the temperature of the third inlet. Two step functions are used so that the temperature is equal to Tlower when Tout-Tm is positive (that is, the outlet temperature is greater than Tm), and is equal to Tupper when Tout-Tm is positive.