Icepak: Computation of View Factors for S2S Radiation Heat Transfer
1. How can we compute radiation view factors within an enclosure, in Icepak.
2. How can we compute radiation view factors between exposed surfaces and the ambient envirobment, in Icepak.
Icepak uses the same interface and calculation mechanism to compute the view factors between surfaces both, inside enclosures and surfaces that are exposed to the ambient environment. The following basic steps should be followed to compute the view factor:
1. Generate a satisfactory mesh
2. Go to main menu > Model > Radiation. This will open the Form Factors (View Facrors) computation panel.
3. In the "Use geometry" column (top left), select all the objects which must participate in the View Factor computation. In the "Radiation enabled" column (top middle) select all the objects that must participate in radiation heat transfer. Typically you could select all the objects in the model in both these columns.
4. If you have very few objects in the model (~ 20 or less), then select "Adaptive" in the method box located approximately at the bottom center of the Form Factors panel. If you have many objects, select "Hemicube" method. The Coarse/Refine option under Adaptive, and Ref level 1 to 7 under Hemicube, specify the accuracy of view factor computation. In Adaptive methos, Refine is more accurate. Likewise, in Hemicube method, Ref level = 7 will be most accurate. View factor computation time will increase with the accuracy requested.
5. Click on "Compute" button in the Left center of the panel. Icepak will compute the view factors. When finished computing the Icepak messages window will have a message similar to this: "Loading file <file path>/ <Solution ID>.ff_out"
In the "Display values" column (top right), you may check the view factors from any surface of the selected objects to all other ubjects' surfaces.
Icepak uses the surface mesh and the mesh in fluid regions when computing view factors. This automatically takes care of all manners/angles of views from a given surface to all other surfaces in the model. For example, as a result of the above mesh usage, a surface that is inside an enclosure, will aoutomatically have no view factors to surfaces that are outside the enclosure.
Note: Simply turning on the "Radiation" in the model tree/ Problem setup/ Basic parameters panel does not ensure radiation heat transfer computations in the model. This is because, the suer has still not specified what objects in the model are to participate in radiation heat transfer. This is done in step 3 above. Once the objects that participate in (a) radiation view factor computation, and (b) radiation heat transfer, are specified, unless othewise stated, Icepak will first compute the view factors whenever the solver is started.
When objects are exposed to the ambient, this situation may be modeled in two ways:
1. A flat wall that forms the boundary of the domain may be facing the outside environment
2. You may model a portion of the ambient fluid. This ambient fluid would extend to an Icepak opening/grille object from where free convection flow is exchanged between the Icepak CFD domain and the outside ambient.
In both cases, the boundary objects are also available for selection in the Form factors panel. The summation of view factor values from a given surface to all other surfaces "in the universe" is 1. Internally, after the view factor computation, Icepak will do a summation of view factors from each surface to all other surfaces in the model. If this does not add to 1, the balance will be assigned as view factor to the outside ambient. Eventually, all the above will ensure that view factor sums to 1 for all internal surfaces in the model. For external faces, such as the side of a wall that faces only the outside envirnoment, the view factor to the ambient is 1. In other words these faces can only see the ambient.