fluent 教程icepak资料icepak_helppdfugchp25

1、Chapter 25.Radiation ModelingIcepak uses a surface-to-surface radiation model for modeling radiation. Heating or cool- ing of surfaces due to radiation can be included in your model.This chapter describes how to include radiation in your Icepak model. See Section 33.4 for details about the theory be

2、hind Icepaks radiation model.Information about radiation modeling is presented in the following sections:Section 25.1: When to Include Radiation Section 25.2: Modeling RadiationSection 25.3: User Inputs for Radiation Modeling25.1When to Include RadiationTypical applications well suited for simulatio

3、n using radiation heat transfer include the following: Surface-to-surface radiant heating or cooling Coupled radiation, convection, and/or conduction heat transferYou should include radiation heat transfer in your simulation when the radiant heat flux, T 4 ), is large compared to the heat transfer r

4、ate due to convectionQrad= (T 4maxminor conduction. Typically this will occur at high temperatures where the fourth-orderdependence of the radiation heat flux on temperature implies that radiation will domi- nate. Also, radiation is typically more important for natural convection problems than for f

5、orced convection problems of interest in electronics cooling.If you need to include radiation heat transfer from exterior walls of your model, you can include an external radiation boundary condition for walls.25-1c Fluent Inc. August 6, 2003Radiation Modeling25.2Modeling RadiationThe surface-to-sur

6、face radiation model in Icepak provides an economical way to account for radiation effects in most applications. The surface-to-surface model uses form factors(also known as view factors) that are calculated for surfaces of Icepak objects.Themethod of computing form factors in Icepak does not accoun

7、t specifically for symmetryboundaries when Icepak computes form factors to or from surfaces that are adjacent to symmetry boundaries.25.2.1Radiation Modeling for ObjectsIcepak allows you to specify that any or all objects exchange radiative energy with other objects in the model or with a specified

8、remote temperature if you are using the surface- to-surface radiation model. For radiation, the heat transfer rate is defined asq = eF (T 4 T 4)(25.2-1)surfaceremotewhere Tsurface is the temperature of the object surface, Tremote is the temperature of the surface to which the object radiates heat, i

9、s the Stefan-Boltzmann constant, F is a view factor specifying the fraction of radiant energy that is intercepted by the surface of the object, and e is the emissivity of the surface of the object (defined as part of the properties of the surface material specified for the object).If you specify an

10、object as radiating energy to and from a specified temperature, you must specify a remote temperature and an appropriate view factor.If you specify an object as radiating energy to other objects in the model, Icepak automat- ically calculates the view factors and computes the radiative heat flux bas

11、ed on calculated object temperatures. If the sum of the view factors for a radiating object is less than 1, Icepak will define the remainder to radiate to the radiation temperature (Radiation temp) specified under Ambient values in the Basic parameters panel (see Section 6.4.5).25-2c Fluent Inc. Aug

12、ust 6, 200325.3 User Inputs for Radiation Modeling25.3User Inputs for Radiation ModelingTo solve a problem involving the surface-to-surface radiation model in Icepak (the default radiation model), you will follow the procedure outlined below:Enable the calculation of radiation in the Basic parameter

13、s panel. To open the Basic parameters panel (Figure 25.3.1), double-click the Basic parameters item under the Problem setup node in the Model manager window.1.Problem setupBasic parametersEnable the calculation of radiation by selecting On next to Radiation. Note that On is selected by default, but

14、if you do not specify any surfaces to be included in the radiation calculation, it will be ignored and radiation will not be computed. If you have defined some radiating surfaces in your model, you can turn off the calculation of radiation by selecting Off next to Radiation in the Basic parameters p

15、anel.Specify the objects to be included in the radiation calculation. You can do this in two ways:2.Select the radiation option in each object panel (i.e., the Walls panel, the Blocks panel, etc.) and specify the radiation parameters for each object or surface (described below). The view factors can

16、 then be calculated in one of two ways: Open the Form factors panel and click Compute. You can also display,save, and edit the view factors, as describedection 25.3.2. If you do not compute the view factors before you start the calculation,Icepak will compute them for you at the start of the calcula

17、tion.If you have already calculated the view factors for your model and you do not want Icepak to recalculate them, select Dont recompute in the Form factors panel (Figure 25.3.4). Specify the objects in the Form factorspanel (described Define the rest of your problem in the usual way.Start the calc

18、ulation using the Solve panel. To open this panel, click on Run solutionin the Solve menu. Click Accept to start the calculation.If you have enabled radiation on surfaces in your model, you can turn off the calculation of radiation by selecting the Disable radiation calculations option in the Solve

19、panel.ection 25.3.2).3.4.25-3c Fluent Inc. August 6, 2003Radiation ModelingFigure 25.3.1: The Basic parameters Panel25-4c Fluent Inc. August 6, 200325.3 User Inputs for Radiation Modeling25.3.1User Inputs for Specification of Radiation in Individual Object PanelsThe calculation of radiation for the

20、various objects can be specified in their respective object panels. You can choose the object surfaces to radiate either to a fixed temperature or to other surfaces in the model.The procedure for specifying radiation in your model using individual object panels is described below.1.Turn on radiation

21、 for the object. You will do this in different ways for different kinds of objects.Block: You can specify radiation for the whole block or for individual sides of the block. For the whole block, select the Radiation option in the Properties tab of the Blocks panel. Click Edit to access the Radiation

22、 specification panel (Figure 25.3.2). (fluid, solid, and hollow blocks only) For individual sides of the block, select the Individual sides option in the Properties tab of the Blocks panel. Click Edit to open the Individual side specification panel. Select Radiation properties to access the radiatio

23、n properties, which are the same as those shown in Figure D source: Select the Radiation option in the Properties tab of the Sourcespanel. Click Edit to access the Radiation specification panel (Figure 25.3.2).Plate: Select the Low side or High side option under Side specification in the Pro

24、perties tab of the Plates panel and click Edit to open the Low (or High) side surface properties panel. Select Radiation to access the radiation properties, which are the same as those shown in Figure 25.3.2.PCB: Select the Low side or High side option under Radiation in the Properties tab of the Pr

25、inted circuit boards panel and click Edit to open the Low (or High) side surface properties panel. Select Radiation to access the radiation properties, which are the same as those shown in Figure 25.3.2.Package: Select the Top side radiation in the Die/Mold tab of the Packages panel and click Edit t

26、o open the High side surface properties panel. Select Radiation to access the radiation properties, which are the same as those shown in Figure 25.3.2.Heat sink object: Select the Radiation option in the Properties tab of the Heat sinks panel. Click Edit to access the Radiation specification panel (

27、Fig- ure 25.3.2).Enclosure object: Select the Radiation option for the desired side of the en- closure in the Properties tab of the Enclosures panel. Click Edit to access the Radiation specification panel (Figure 25.3.2)25-5c Fluent Inc. August 6, 2003Radiation Modeling Wall: Select the Inner surfac

28、e radiation option in the Properties tab of the Wallspanel and click Edit to access the Radiation specification panel (Figure 25.3.2).Figure 25.3.2: The Radiation specification Panel2.Specify the nature of the surface(s) with which the object or surface exchanges radiation by selecting one of the op

29、tions below in the Radiation specification panel.Reference temp specifies a fixed temperature to which the selected sides of the ob- ject radiate heat. Enter a Reference temp and a View factor. The default view factor is 1.0 and the value of the ambient temperature is defined under Ambient values in

30、 the Basic parameters panel (see Section 6.4.5).All objects specifies that the object or surface exchanges radiation with all other objects and surfaces also specified as subject to radiation with heat transfer. Icepak computes the radiative heat flux to and from the object or surface based on the c

31、alculated temperatures and view factors (with respect to the object or surface) for each object or surface.Selected objects specifies that the object or surface exchanges radiation only with selected objects and surfaces in the model. Click Select to open the Radiationobject selection panel (Figure

32、25.3.3). This panel conta objects and surfaces.a list of availableThe Radiation object selection panel lists all objects and surfaces currently defined as subject to radiative heat transfer. To select one or more objects or surfaces from the list, left-click the name(s) of the object(s) and surface(

33、s) and click Done.Click Accept in the Radiation specification panel when you have finished the speci- fication.3.Specify the material to be used for the currently selected side of the object or surface. This material defines the roughness and emissivity of the object or surface, and is specified in

34、different ways for different kinds of objects.25-6c Fluent Inc. August 6, 200325.3 User Inputs for Radiation ModelingFigure 25.3.3: The Radiation object selection PanelBlock: You can specify material properties for the whole block or for individual sides of the block.For the whole block, specify the

35、 material under Surface material in theProperties tab of the Blocks panel.(fluid, solid, and hollow blocks only) For individual sides of the block, select the Individual sides option in the Properties tab of the Blocks panel. Click Edit to open the Individual side specification panel and specify the

36、 Material.2D source: Specify the material next to Material in the Sources panel.Plate: Select the Low side or High side option under Side specification in the Plates panel. Click Edit to open the Low (or High) side surface properties panel and specify the Material.PCB: Select the Low side or High si

37、de optionunder Radiation in the Printedcircuit boards panel. Click Edit to open the Low (or High) side surface propertiespanel and specify the Material.Package: Select the High side radiation option in the Die/Mold tab of the Packages panel. Click Edit to open the High side surface properties panel

38、and specify the Material.Heat sink object: Specify the material next to Surface material in the Heat sinks panel.Enclosure object: Specify the material next to Surface material in the Enclo- sures panel.Wall: Specify the material next to Internal material for a non-zero thickness wall in the Walls p

39、anel. If the wall has zero thickness, specify the External25-7c Fluent Inc. August 6, 2003Radiation Modelingmaterial (which is used for both the ide and the outside of a zero-thickness wall).By default, the material is specified as default. This means that the material specified on the object or sur

40、face is defined under Default surface in the Basic parameters panel (see Section 6.4.6). To change the material for an object or surface, select a material from the material drop-down list. See Section 7.7 for details on material properties.4. Calculate the view factors using the Form factors panel,

41、 as described below.25.3.2User Inputs for Specification of Radiation Using the Form factors PanelThe calculation of radiation for blocks, 2D sources, plates, PCBs, packages, heat sink objects, enclosure objects, and walls can be specified using the Form factors panel. To open the Form factors panel

42、(Figure 25.3.4), select Radiation in the Model menu or clickbutton in the Model and solvetoolbar.theModelRadiationFigure 25.3.4: The Form factors Panel25-8c Fluent Inc. August 6, 200325.3 User Inputs for Radiation ModelingSpecifying the Objects to Include in the Radiation CalculationAll objects can

43、receive radiant energy regardless of their ability to radiate energy them- selves. To specify that an object will receive radiant energy from other objects in the model, click on the name of the object in the Use geometry list using the left mouse but- ton. Any object that has been specified to take

44、 part in the view factor calculation (i.e., receive radiant energy) will be highlighted in blue in the Use geometry list. To include all objects in the Use geometry list in the radiation calculation, click All at the bottom of the list. To deselect an object in the Use geometry list, click on the na

45、me of the object in the Use geometry list using the left mouse button. To deselect all objects in the Use geometry list, click None at the bottom of the list.If any sides of the cabinet have been specified as wall, opening, or grille objects, you will be able to include radiation to these sides. For

46、 those sides of the cabinet that have the Default specification (i.e., an adiabatic wall), radiant energy from other objects will be lost to these sides using a partial enclosure model. View factor calculations will be disabled for these walls, which will tead be treated like a black body with the d

47、efault Radiation temperature specified in the Basic parameters panel.All the objects that can radiate energy are listed under Radiation enabled. To specify that an object will radiate, click on the name of the object in the Radiation enabled list using the left mouse button. Any object that has been

48、 specified to radiate to at least one other object or surface will be highlighted in blue in the Radiation enabled list. To include all objects in the Radiation enabled list in the radiation calculation, click All at the bottom of the list. To deselect an object in the Radiation enabled list, click

49、on the name of the object in the Radiation enabled list using the left mouse button. To deselect all objects in the Radiation enabled list, click None at the bottom of the list.!Computing View FactorsWhen you have selected the objects you want to participate in the radiation model, click Compute to

50、compute the view factors. The computation of the view factors may take a few seconds to a few minutes, depending on the size of the model.To calculate the view factors, Icepak first creates a coarse mesh. It then discards the volume mesh and coarsens the surface mesh. The Coarse tol option allows yo

51、u to specify the angular tolerance to be used during coarsening; i.e., it specifies the minimum angle between adjacent facets of the surface mesh for which coarsening will not happen. A small value allows higher accuracy, but Icepak will take more time to compute the mesh. A value of 1 (degree) is u

52、sually sufficient. To disable coarsening completely, enter a value of 1.There are two methods available in Icepak for calculating the view factors: the hemicube method and the adaptive method. The hemicube method uses a differential area-to-area method and calculates the view factors on a row-by-row

53、 basis. The view factors calculated from the differential areas are summed to provide the view factor for the whole surface.25-9c Fluent Inc. August 6, 2003Radiation ModelingThis method originated from the use of the radiosity approach in the field of computer graphics 5.To use the hemicube method t

54、o compute the view factors, select Hemicube in the Form factors panel. You can choose the refinement level to be used to calculate the view factors. To change the refinement level, click on the number to the right of Ref level in the Form factors panel (Figure 25.3.4) and select a new number from th

55、e drop-down list. There are seven refinement levels available in Icepak. A higher level allows higher accuracy, but Icepak will take more time to compute the mesh. It is recommended that you use the hemicube method for large complex models, because it is faster than the adaptive method for these typ

56、es of models.The adaptive method calculates the view factors on a pair-by-pair basis using a variety of algorithms (analytic or Gauss quadrature) that are chosen adaptively depending on the proximity of the surfaces. To maintain accuracy, the order of the quadrature increases the closer the faces ar

57、e together. For surfaces that are very close to each other, the analytic method is used. Icepak determines the method to use by performing a visibility calculation. The Gaussian quadrature method is used if none of the rays from a surface are blocked by the other surface. If some of the rays are blo

58、cked by the other surface, then either a Monte Carlo integration method or a quasi-Monte Carlo integration method is used.Two options are available in the Form factors panel for the Adaptive method: Coarse andRefined. surface. surface.If you select the Coarse option, 16 visibility samples will be taken from each If you select the Refined option, 64 visibility samples will be taken from eachThe cal