Changing the turbulence model in use
This facility allows you to run a turbulent flow case by restarting from a simulation done for the same case but with a different turbulence model. No special user input is required to run such a case.
The table below illustrates the combinations allowed and the conversion formula adopted when STAR encounters a different turbulence model in the solution file to the one currently in force:
* k-ε, k-ε Quadratic, k-ε Cubic, k-ε RNG, k-ε CHEN, k-ε Speziale, k-ε Suga Quadratic and Cubic
Heat Transfer In Solid-Fluid Systems
The theory behind this type of heat transfer models and the manner of implementing it in STAR-CD is given inChapter 16, “Heat Transfer in Solid-Fluid Systems” of the Methodology volume. This section contains an outline of the process to be followed when setting up this type of model and includes cross-references to appropriate parts of the STAR GUIde on-line Help system. The latter contains details of the user input required and important points to bear in mind when setting up problems of this kind.
FROM (Restart field)
k-ε type* Not needed Not needed
k-ω
V2F Not needed Not needed
νt Cµk2
Heat Transfer In Solid-Fluid Systems
Setting up solid-fluid heat transfer models Step 1
Specify the model regions occupied by the solids and fluids present and define their physical properties.
Figure 3-4 Simple heat exchanger
In terms of the heat exchanger example shown inFigure 3-4, this requires the following actions (see also“Multi-Domain Property Setting” on page 3-5):
• Set up cell table entries for fluid materials 1,2 and solid material 3
• Assign all cells in the mesh to the appropriate cell type (1, 2, 3) as described in the section on“Cell indexing” on page 3-3.
• Specify the physical properties of each material Step 2
Turn onSolid-Fluid Heat Transfer in the“Thermal Options” STAR-GUIde panel.
Note that this also has the effect of switching on the temperature solver in solid materials.
Step 3
Switch on the temperature solver in each fluid material using the“Thermal Models”
panel.
Step 4
Normally, STAR-CD treats the solid-fluid interface as part of the default wall region (region 0). However, unlike other parts of this region whose default thermal condition is adiabatic, the solid-fluid interface is treated as a conducting wall.
Therefore:
• If an additional thermal resistance exists at the interface, define the latter as a separate region and use the“Define Boundary Regions” panel to specify it as a conducting wall having the required thermal resistance value (see the STAR GUIde“Wall” Help topic for more information).
• STAR uses default expressions to calculate heat transfer (film) coefficients at all solid/fluid interfaces, including those at external walls and baffles. You can supply alternative expressions for these quantities via subroutineMODSWF Step 5
If a printout of temperature distribution in the model is required, use command
Material 1 — steam
Material 2 — hot gas Heat flow Material 3 — steel
Heat Transfer In Solid-Fluid Systems
PRTEMPto specify whether the printed values are absolute or relative to the datum temperature previously defined (see topic“Reference Data” in the STAR GUIde on-line Help system).
Heat transfer in baffles
Thermal conduction along the plane of a baffle’s surface is currently neglected (see the STAR GUIde“Baffle” Help topic for more information). However, this effect may still be modelled by expanding a baffle into a single layer of solid cells using command CBEXTRUDE (see alsoChapter 2, “Extrusion” in the Meshing User Guide). The surrounding mesh is automatically adjusted to make room for the solid cells, as shown inFigure 3-5.
Figure 3-5 ‘Fully-conducting’ baffle creation Note that:
• Special cell shapes (such as prisms) are created at the edges of the solid cell layer, as shown in the exploded view of the baffle inFigure 3-5. This brings the baffle thickness down to zero and avoids the need to create coupled cells in those parts of the mesh.
• The modelling of heat conduction will be slightly in error as a result of the introduction of the above artificial cell shapes.
• A baffle of the kind described here may be attached directly to an external boundary or to internal boundaries such as solid-fluid interfaces to model a conducting fin. In the latter case, you need to make sure that the cell type assigned to baffle cells is different from that assigned to solid cells at the base of the baffle.
Alternative treatment for baffle heat transfer
It can be seen that the expansion process described above will create a disturbance in the fluid cells around the baffle and may result in a highly irregular mesh. In order to avoid this problem, a facility is provided for specifying a finite baffle thickness (to be used internally for heat conduction calculations) but without actually expanding the baffle to that thickness. Thus, the fluid flow calculations are based on an undisturbed mesh structure.
Ordinary baffle Fully-conducting baffle
Before After
Heat Transfer In Solid-Fluid Systems
To use this facility, the following steps are needed:
Step 1
Using the Cell Table Editor, create a separate baffle cell type and a separate solid cell type. The latter will be used to represent the ‘conducting baffles’.
Step 2
Create the baffle cells in the appropriate mesh location using the baffle cell type defined inStep 1.
Step 3
Apply command CBEXTRUDE to the baffle cells created inStep 2and extrude them into solid cells using the solid cell type created inStep 1. Note that:
• Upon extrusion, the baffle cells will be removed from the mesh and replaced by the solid cells that they have been extruded into.
• If no solid cell type identification, ICTID, is supplied in the CBEXTRUDE command, the solid cell identification will be set as cell type 1.
• If no solid cell thickness, DT, is supplied in the CBEXTRUDE command (this is the normal practice), the default thickness will be applied, currently set at 0.2× 10-3 m.
Step 4
Go back to the Cell Table Editor and select the solid cell type defined inStep 1.
Enter the actual conduction thickness in the box labelled Conduction Thickness Step 5
Turn onSolid-Fluid Heat Transfer in the“Thermal Options” STAR-GUIde panel.
Step 6
Apply the appropriate wall boundary condition to the solid cells created inStep 3.
If none is specified, the default wall boundary condition for region number 0 will be used. This results in a conducting, no-slip wall.
Note that:
• Conducting baffles of the same thickness DT specified in command CBEXTRUDE and of the same Conduction Thickness specified in the Cell Table Editor can share the same cell type.
• Conducting baffles that have a different DT or different Conduction Thickness must also have a different cell type.
• A conducting baffle that is attached to a solid base must have a different cell type to that of the solid to which it is attached.
Useful points on solid-fluid heat transfer
1. The On button in the Solid-Fluid Heat Transfer section of the“Thermal Options” STAR-GUIde panel must always be used to turn on the solution of the energy equation in solids, even if the entire model is made up of solid cells.
2. It is usually advisable to run solid-fluid heat transfer simulations in double precision. This helps to overcome potential convergence problems arising as a result of a large disparity in thermal conductivity between fluid and solid. The