COMSOL 6.2 - Heat Flux

Heat Flux

Use this node to add heat flux across boundaries. A positive heat flux adds heat to the domain. This feature is not applicable to inlet boundaries, use the Inflow condition instead.

Material Type

Select an option in the list to specify if the inputs of the section are defined in the material or spatial frame:

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The option specifies that the heat flux q0 is defined in the material frame. Because the heat transfer variables and equations are defined in the spatial frame, the inputs are internally converted to the spatial frame. See Conversion Between Material and Spatial Frames for details.

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The default option for the node is , which defines q0 in the spatial frame. No frame conversion is needed.

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The option uses the option selected in the list of the section of the material applied on the domain on which the node is active.


	This option has no effect when the component does not contain a moving frame because the material and spatial frames are identical in such cases. With a Deformed Geometry or a Moving Mesh interface, the heat transfer features automatically take into account deformation effects of the material and spatial frames on heat transfer properties. In particular the effects of volume changes on the density are considered. See Handling Frames in Heat Transfer and Material and Spatial Frames for details.

Heat Flux

Select the from the list: (default), , , or .

General Inward Heat Flux

It adds q0 to the total flux across the selected boundaries. Enter a value for q0 to represent a heat flux that enters the domain. For example, any electric heater is well represented by this condition, and its geometry can be omitted.

Convective Heat Flux

The default option is to enter a value for the h.

In addition, the following options are also available to control the type of convective heat flux to model: , , , or .

For all options except , select a : (default), , , , or .

When is selected, choose a material available on the boundary from the list.

Depending of the selected option, different parameters are needed. You can refer to the section to get an illustration of the configuration.

External Natural Convection

For select , , , , , , or from the list under . Then enter the applicable information:

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L and the ϕ. The tilt angle is the angle between the wall and the vertical direction, ϕ = 0 for vertical walls.

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L. The characteristic length is the ratio between the surface area and its perimeter.

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Internal Natural Convection

For select or from the list under . Then enter the applicable information:

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L and a H.

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D and a H.

External Forced Convection

For select , , , or from the list under . Then enter the applicable information:

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L and U.

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xpl and U.

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D and U.

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D and U.

Internal Forced Convection

For the only option is . Enter a D and a U.

If U is , enter a value or expression. Else, select a defined in the Consistent Stabilization section of a Heat Transfer or Heat Transfer in Shells interface.

External conditions

First, set the , pA. For , enter a value or expression. Else, select an defined in an node under . The pressure is used to evaluate the material properties and this setting is not available for the and options.

In addition, enter an , Text. For , enter a value or expression. Else, select an defined in an node under .

Finally, when the is , also set the , ϕw,ext, and the , ϕw,s, used to evaluate the material properties.

Heat Rate

For enter the heat rate P0 across the boundaries where the node is active. In this case q0 = P0 ⁄A, where A is the total area of the selected boundaries.

Nucleate Boiling Heat Flux

This option computes q0 with the Rohsenow’s correlation, that evaluates the heat flux due to nucleate boiling on a surface immersed in a liquid pool. See Nucleate Pool Boiling Correlation for details about the correlation.

The correlation applies to clean surfaces, and is insensitive to the shape and orientation of the surface. It relies on empirical constants Csf and s that are predefined for several combinations of fluid and surface materials, and on the boiling fluid properties at saturation temperature, also predefined for some fluids.

Select an option in the list:

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When , , , or is selected, the material properties of the liquid and vapor phases required in the correlation are predefined (at saturation temperature and pA=1 atm). Depending on the selected fluid, different materials and finishings are available in the list. For each option, the empirical constants Csf and s are predefined.

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When is selected in the list, first choose materials available on the boundary from the and lists, to define ρl, Cp,l, kl, μl, and ρv. Then set the , pA, the , Tsat, the , Lv, and the , σ. Finally, enter values or expressions for the Rohsenow’s correlation parameters: the , s, and the , Csf, which accounts for surface roughness, that tends to increase the number of active nucleation sites for boiling.


	This boundary condition will give incorrect results if T < Tsat and should only be used in a limited interval of validity Tsat + T1 < T < Tsat + T2. Typically, for water T1=10 K and T2=30 K.