Comments on Solver controls
-The solver settings are tuned for an overall robust solution of most situations. In this model we require the Body Force Weighted pressure scheme to account for the natural convection effects.
-Once the solution has begun to (or has) converge the momentum and energy equations should be switched to second order to improve the accuracy of the solution. You may wish to do this later if time permits.
-The momentum within the model is relatively low so reducing the momentum under-relaxation factor is also recommended.
-Solution initialization is used to provide the first guess prior to the first solver iteration; and it should be as close to the final solution as practical.
Quick post-processing
Check overall heat and mass balances.
– Reports > Fluxes > Set Up
– Select the Inlet and Outlet surfaces, then click Compute.
The net imbalance mass flux is shown under Net Results.
Note that the net imbalance is very small.
– Switch to Total Heat Transfer Rate. Select all walls, the inlet, and the outlet and click Compute.
Note that the difference roughly equals the energy source input to the package (75W).
If the model were fully converged, this value would be exact.
Create a user-defined surface:
– Create a surface in the midplane of the channel (x = 0) for post-processing:
(From top menu bar) Surface > Isosurface
Select Mesh and X-coordinate
Leave iso-value at x=0 m
Specify the name (zz-x-midplane).
Ensure that no items are selected under From Surface and/or From Zones.
Click Create.
Create Contour plot:
(Graphics and Animations > Contours > Setup)
-Select the user surface zz-x-midplane
-Check Filled
-Select contours of Temperature…Static Temperature
-Select Display
-Zoom in with the mouse to see the result
-Repeat changing the plot variable to Velocity…Velocity Magnitude
-Optional step:
– To change the number format use
Display > Colormap from the top menu, and change to float
Save Case and Data Files for Later Use
The purpose of the Workbench structure is to simplify the file structure, and reduce the risk of stray files on the hard drive.
Part of this workshop is to allow the comparison of the results both with and without radiation active. To retain this set of results use the top menu to:
– If using FLUENT standalone
File àWrite à Case & Data…
– If using FLUENT under Workbench
File àExport à Case & Data…
– Change to the working directory and label the
file logically
TIP
– Adding the .gz extension will compress the case and data files, reducing hard
disk usage!
– You do not need to uncompress the files when opening them later.
Setup Radiation Model
The temperature difference across the air space is minimal, and therefore heat transfer via thermal radiation may be significant.
Go to the Models tree item and select radiation.
– Enable the Surface-to-Surface (S2S) model.
Set Partial Enclosure temperature to 45 °C
– Click the Set button to define model parameters.
Manual Options
100 Faces Per Cluster
Apply to All Walls
Ray Tracing
OK
In the Radiation model panel Compute/Write the
S2S calculation. Enter a filename. (Calculating the view factors will take a few minutes).
Click OK when the S2S view factor calculation is complete.
Comments on S2S Radiation model
The model uses a ray tracing method, and this calculation is completed prior to the main solution. Thus whist the set-up may take slightly longer (due to view factor calculation time) than the alternatives the overall solution time is reduced.
The method determines the view factor from each wall (or boundary) surface facet (mesh cell) to every other facet. Clustering is then used to reduce the number of facet calculations needed in the actual solution. Here neighbouring facets are grouped together based on the number set and geometric factors.
The User documentation contains full details of the model.
Revise Boundary Conditions
-The surfaces surrounding the fluid region now also require an emissivity value for the radiation model.
-Open the wall_left boundary condition and under the thermal tab change the Internal Emissivity to 0.9.
-Click the Copy button and copy the boundary conditions to wall_right and wall_top.
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