Settings for the Radiation in Participating Media Interface
The Label is the default physics interface name.
The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first physics interface in the model) is rpm.
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Radiative Heat Transfer in Finite Cylindrical Media: Application Library path Heat_Transfer_Module/Verification_Examples/cylinder_participating_media
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Radiative Heat Transfer in a Utility Boiler: Application Library path Heat_Transfer_Module/Thermal_Radiation/boiler
Participating Media Settings
Wavelength Dependence of Radiative Properties
Define the Wavelength dependence of radiative properties:
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Keep the default value, Constant, to define a gray radiation model. In this case, the radiative properties (emissivity, absorption and scattering coefficients) have the same definition for all wavelengths. These properties can still depend on other quantities, in particular on the temperature.
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Select Solar and ambient to define a spectral radiation model with two spectral bands, one for short wavelengths, [0, λsol/amb], (solar radiation) and one for large wavelengths, [λsol/amb, +∞[, (ambient radiation). It is then possible to define the Separation point between spectral bands (SI unit: m), λsol/amb, to adjust the wavelength intervals corresponding to the solar and ambient radiation. The radiative properties can then be defined for each spectral band. For integrations calculation, the upper bound of the spectral band for ambient radiation λ∞ is predefined and set to 1[mm]. This value can be modified by selecting the Specify the upper bound for integrations checkbox.
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Select Multiple spectral bands and set the value of the Right endpoint for each spectral band in the table, to define a spectral radiation model. The value of the Right endpoint must be entered without unit. Modify the Wavelength unit to set the unit of all the endpoints. Right endpoint values should be set in an ascending order. The values of the Left endpoint for the next spectral band are updated automatically. It is possible to provide a definition of the surface properties for each spectral band.
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Select Weighted Sum of Gray Gases Model to define a spectral radiation model with multiple gray gases. With this approach, the surface properties as well as the scattering coefficients and phase functions are the same for all the gases. Choose a Number of gray gases, then set their absorption coefficients and weight factors in the dedicated tables. Set Weight factors to Polynomial expansion when weight factors depend on the temperature according to a polynomial function of order inferior or equal to 5. Select Normalize with reference temperature and enter the reference temperature when the weight factors are a polynomial function of a normalized temperature. Keep Weight factors to User defined to define your own constant values or functions of the temperature.
With Multiple spectral bands, the first Left endpoint and the last Right endpoint are predefined and equal to 0 and +∞, respectively.
Parameters can be used to set the value of the Right endpoint.
With Weighted Sum of Gray Gases Model, the first gas is the transparent gas.
The first line of the absorption coefficient and weight factors tables, corresponding to the transparent gas, is grayed out. The absorption coefficient is known to be 0 m-1, and the weight factor is automatically deduced from the others since the sum should be equal to 1.
The wavelengths λ set in Solar and ambient and Multiple spectral bands are the wavelengths in vacuum.
Radiation Discretization Method
Select a Radiation discretization method: Discrete ordinates method (the default for 2D and 3D components), or P1 approximation (the default for 2D axisymmetric components).
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When Discrete ordinates method is selected, Opaque Surface (Radiation in Participating Media and Radiation in Absorbing–Scattering Media Interfaces) and Continuity on Interior Boundary (Radiation in Participating Media and Radiation in Absorbing–Scattering Media Interfaces) are automatically added as default features.
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When P1 approximation is selected, Opaque Surface (Radiation in Participating Media and Radiation in Absorbing–Scattering Media Interfaces) is automatically added as a default feature and both this and Incident Intensity (Radiation in Participating Media and Radiation in Absorbing–Scattering Media Interfaces) are made available from the Physics ribbon toolbar (Windows users), Physics context menu (Mac or Linux users), or the context menu (all users). Continuity on Interior Boundary (Radiation in Participating Media and Radiation in Absorbing–Scattering Media Interfaces) is not available.
The choice of Radiation discretization method also offers different settings for the Participating Medium (Radiation in Participating Media Interface) (all methods), Opaque Surface (Radiation in Participating Media and Radiation in Absorbing–Scattering Media Interfaces) (P1 approximation), and Incident Intensity (Radiation in Participating Media and Radiation in Absorbing–Scattering Media Interfaces) (P1 approximation) nodes.
Refractive Index
For either selection, define the Refractive index nr (dimensionless) of the participating media. The same refractive index is used for the whole model.
Quadrature Set
When Discrete ordinates method is selected, the method of angular discretization of the radiative intensity direction should be specified. In particular, it defines the method of computation of the weights wj used in the approximation of the scattering term:
Choose a Quadrature set from the list:
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The Level Symmetric Even (the default), Level Symmetric Hybrid, and Equal Weight Odd sets are SN approximations. Depending on the set and the order selected in the Discrete ordinates method list, different moment conditions are satisfied. In 3D, S2, S4, S6, and S8 generate 8, 24, 48, and 80 directions, respectively. In 2D and 2D axisymmetric, S2, S4, S6, and S8 generate 4, 12, 24, and 40 directions, respectively.
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The Quasi-uniform weight approximation discretizes the angular space by using a reference octahedron with 8 triangular faces, further discretized in function of the order of the method. This corresponds to a TN approximation, for which 8N2 weights are computed at order N in 3D and 4N2 in 2D and 2D axisymmetric.
See Discrete Ordinates Method (DOM) for details about the SN and TN quadrature sets.
Jacobian Contribution
To display the Jacobian Contribution section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog.
When the Radiation discretization method is set to Discrete ordinates method, an option to control the contributions used in the Jacobian matrix is proposed. The discrete ordinates method is intrinsically memory consuming due to the number of radiative intensities solved for. The Use reduced Jacobian option, selected by default, removes some expensive contributions from the Jacobian matrix, which is expected to greatly reduce both the requirements in memory and the computation time, especially when the number of directions of the quadrature set is large.
Provided a converged solution, the same solution is obtained within the solver tolerance with or without this option activated.
Because not all contributions are accounted for in the Jacobian matrix when this option is activated, the computation may be less stable in specific cases.
See Guidelines for Solving Radiation in Participating Media and Radiation in Absorbing-Scattering Media Problems for details about the solvers when using the discrete ordinates method.
Inconsistent Stabilization
To display the Inconsistent stabilization section, click the Show More Options button () and select Stabilization in the Show More Options dialog.
When the Radiation discretization method is set to Discrete ordinates method, a stabilization is proposed that can help with convergence. This stabilization adds an isotropic diffusion term in the equations solved by the interface. Note that this stabilization is said to be inconsistent because it does not naturally fade out in the end, that is, if it is too large it will have an impact on the final results, making them partly unphysical.
The Tuning parameter controls the amount of stabilization added, the rule being that it should be as small as possible to reduce its impact on the results, and just enough to help the solver converge. When the coefficient is set to 0, it removes entirely the contribution of the stabilization.
Discretization
Radiative Intensity
When the Radiation discretization method is set to Discrete ordinates method in Participating Media Settings, set the discretization level of the Radiative intensity: Constant, Linear (the default), Quadratic, Cubic, Quartic, or Quintic.
Incident Radiation
When the Radiation discretization method is set to P1 approximation in Participating Media Settings, set the discretization level of the Incident radiation: Linear (the default), Quadratic, Cubic, Quartic, or Quintic.