Step 5: Boundary Conditions

Boundary Conditions

1.   Read the boundary conditions profile file.

File $\rightarrow$ Read $\rightarrow$ Profile...

(a)   Select berl.prof from the Select File dialog box.

(b)   Click OK.

  The CFD solution for reacting flows can be sensitive to the boundary conditions, in particular the incoming velocity field and the heat transfer through the walls. Here, you will use profiles to specify the velocity at air-inlet-4, and the wall temperature for wall-9. The latter approach of fixing the wall temperature to measurements is common in furnace simulations, to avoid modeling the wall convective and radiative heat transfer. The data used for the boundary conditions was obtained from experimental data [ 1].

2.   Set the boundary conditions for the pressure outlet ( poutlet-3).

Boundary Conditions poutlet-3 Edit...

(a)   Select Intensity and Hydraulic Diameter from the Specification Method drop-down list in the Turbulence group box.

(b)   Enter 5% for Backflow Turbulent Intensity.

(c)   Enter 600 mm for Backflow Hydraulic Diameter.

(d)   Click the Thermal tab and enter 1300 K for Backflow Total Temperature.

(e)   Click OK to close the Pressure Outlet dialog box.

  The exit gauge pressure of zero defines the system pressure at the exit to be the operating pressure. The backflow conditions for scalars (temperature, mixture fraction, turbulence parameters) will be used only if flow is entrained into the domain through the exit. It is a good idea to use reasonable values in case flow reversal occurs at the exit at some point during the solution process.

3.   Set the boundary conditions for the velocity inlet ( air-inlet-4).

Boundary Conditions air-inlet-4 Edit...

(a)   Select Components from the Velocity Specification Method drop-down list.

(b)   Select vel-prof u from the Axial-Velocity drop-down list.

(c)   Select vel-prof w from the Swirl-Velocity drop-down list.

(d)   Select Intensity and Hydraulic Diameter from the Specification Method drop-down list in the Turbulence group box.

(e)   Enter 17% for Turbulent Intensity.

(f)   Enter 29 mm for Hydraulic Diameter.

  Turbulence parameters are defined based on intensity and length scale. The relatively large turbulence intensity of 17% may be typical for combustion air flows.

(g)   Click the Thermal tab and enter 312 K for Temperature.

  For the non-premixed combustion calculation, you have to define the inlet Mean Mixture Fraction and Mixture Fraction Variance in the Species tab. In this case, the gas phase air inlet has a zero mixture fraction. Therefore, you can retain the zero default settings.

(h)   Click OK to close the Velocity Inlet dialog box.

4.   Set the boundary conditions for the velocity inlet ( fuel-inlet-5).

Boundary Conditions fuel-inlet-5 Edit...

(a)   Select Components from the Velocity Specification Method drop-down list.

(b)   Enter 157.25 m/s for Radial-Velocity.

(c)   Select Intensity and Hydraulic Diameter from the Specification Method drop-down list in the Turbulence group box.

(d)   Enter 5% for Turbulent Intensity.

(e)   Enter 1.8 mm for Hydraulic Diameter.

  The hydraulic diameter has been set to twice the height of the 2D inlet stream.

(f)   Click the Thermal tab and enter 308 K for Temperature.

(g)   Click the Species tab and enter 1 for Mean Mixture Fraction for the fuel inlet.

(h)   Click OK to close the Velocity Inlet dialog box.

5.   Set the boundary conditions for wall-6.

Boundary Conditions wall-6 Edit...

(a)   Click the Thermal tab.

i.   Select Temperature in the Thermal Conditions list.

ii.   Enter 1370 K for Temperature.

iii.   Enter 0.5 for Internal Emissivity.

(b)   Click OK to close the Wall dialog box.

6.   Similarly, set the boundary conditions for wall-7 through wall-13 using the following values:

Zone Name	Temperature	Internal Emissivity
wall-7	312	0.6
wall-8	1305	0.5
wall-9	temp-prof t (from the drop-down list)	0.6
wall-10	1100	0.5
wall-11	1273	0.6
wall-12	1173	0.6
wall-13	1173	0.6

7.   Plot the profile of temperature for the wall furnace ( wall-9).

Plots Profile Data Set Up...

(a)   Select temp-prof from the Profile selection list.

(b)   Retain the selection of t and x from the Y Axis Function and X Axis Function selection lists respectively.

(c)   Click Plot (Figure  15.6).

Figure 15.6: Profile Plot of Temperature for wall-9

8.   Plot the profiles of velocity for the swirling air inlet ( air-inlet-4).

(a)   Plot the profile of axial-velocity for the swirling air inlet.

Plots Profile Data Set Up...

i.   Select vel-prof from the Profile selection list.

ii.   Retain the selection of u from the Y Axis Function selection list.

iii.   Select y from the X Axis Function selection list.

iv.   Click Plot (Figure  15.7).

Figure 15.7: Profile Plot of Axial-Velocity for the Swirling Air Inlet ( air-inlet-4)

(b)   Plot the profile of swirl-velocity for swirling air inlet.

Plots Profile Data Set Up...

i.   Retain the selection of vel-prof from the Profile selection list.

ii.   Select w from the Y Axis Function selection list.

iii.   Retain the selection of y from the X Axis Function selection list.

iv.   Click Plot (Figure  15.8) and close the Plot Profile Data dialog box.

Figure 15.8: Profile Plot of Swirl-Velocity for the Swirling Air Inlet ( air-inlet-4)