Understanding the Gap Radiation type of Thermal Coupling — Radiation

The Gap Radiation type of Thermal Coupling — Radiation models radiative heat paths between close parallel surfaces with known emissivity.

With Gap Radiation, the view factor is effectively 1 so radiative conductance is based solely on emissivity.

Typical applications for thermal modeling are:

• Multi-layer insulating panels.

• Honeycomb panels.

• Any pair of broad thin surfaces separated by a narrow gap, across which radiative conductances are significant.

Choosing a Gap Radiation type

The conductances in a Thermal Coupling — Radiation, of Gap Radiation type depends upon the on the Type option you select in the Parameters group:

• With the Close Parallel Plates option, the software uses the optical property emissivity values of both the primary and secondary elements to compute the Gray Body View Factor. See Understanding the Close Parallel Plates type for more information.

• With the Gray Body View Factor type, you must specify a value for the Gray Body View Factor. See Understanding the Gray Body View Factor type for more information.

• With the Effective Emissivity type, the software calculates the couplings based on the elements' effective emissivity value. See Understanding the Effective Emissivity type for more information.

Understanding the Close Parallel Plates type

Given the assumption that the view factor between the elements set is 1.0, the gray body view factor is computed by using the emissivity of the primary elements and secondary elements only as shown with the following equation:

GBVF = 1/(1 + ε1/ε2-ε1)

where ε1 and ε2 are the emissivity of the primary and secondary elements, respectively.

Both the primary element and secondary element optical property emissivity values are used to compute the gray body view factor and magnitude of the radiative conductance. See Thermo-optical properties for more information.

Understanding the Gray Body View Factor type

With the Gray Body View Factor type, you must specify the Gray Body View Factor (GBVF) value. For closely spaced parallel plates, the above equation may be used to evaluate the GBVF value.

You can also use the Gray Body View Factor type to create a radiative coupling between a set of primary elements and a large environment. In that case the GBVF value would be 1.0. This option is also compatible with any Radiation type of Thermal Couplings that you import from NX I-deas TMG models.

The primary element optical property emissivity value is used to compute the magnitude of the radiative conductance.

Note:

When the solver calculates a Thermal Coupling — Radiation on 2D elements with the Gray Body View Factor, it always uses the top side emissivity, regardless of the element's normal direction. If you want to create a Thermal Coupling — Radiation on the bottom side but are unable to change the top side emissivity because another radiation object is using the same mesh and needs top side emissivity defined differently, then you must either specify a Effective Emissivity value, or adjust the specified Gray Body View Factor value to account for the incorrect emissivity.

Understanding the Effective Emissivity type

With the Effective Emissivity type, the software calculates the couplings based on the elements' effective emissivity value, which is defined as the emissivity of the primary elements multiplied by the Gray Body View Factor. The elements' optical property emissivity values are not used. With this option, you specify a value for Emissivity * Gray Body View Factor. For closely spaced parallel plates, the above equation may be used to evaluate the Gray Body View Factor value and effective emissivity.

You can use the Effective Emissivity type to create a radiative coupling between a set of primary elements and a large environment. In that case the GBVF value would be 1.0. This option is also compatible with any effective emissivity Radiation type of Thermal Couplings imported from I-DEAS TMG models.

With this option, the optical property emissivity of the primary elements is ignored since the emissivity is embedded in the effective emissivity value specified.

The Effective Emissivity type is useful for modeling surfaces that are covered in multilayer insulation MLI without additional elements to represent the insulation.

How do I

Create a Thermal Coupling — Radiation

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Understanding the Object to Object type of Thermal Coupling — Radiation

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Specifying a Coupling Resolution

Using the Only Connect Overlapping Elements option

Using the Overlap Projection Direction option

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Understanding the Gap Radiation type of Thermal Coupling — Radiation, Simcenter 3D 2021.1 Series

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