Heat Flux

This excitation specifies a rate of heat flowing through object faces and is applicable to the following entities:

• Faces of solid or shell objects
• Objects (solids or shells)
Note:

Assigning a Heat Flux excitation to an object is equivalent to assigning the excitation to all faces of the object.

You can specify heat flux in either of two ways—Total Power or Surface Flux. Total Power is specified in units of Power, such as watts (W) or Btu_per_hr. See the Important note below to understand what happens when you assign Total Power to multiple faces and/or objects in a single excitation.

Surface Flux is a uniformly distributed heat flux, and it is specified in units of Irradiance (power per unit area), such as KW_per_m2 (which is, kilowatts/m²).

The Total Power or Surface Flux specified for a Heat Flux excitation may be positive or negative:

• A positive value adds heat to the model (heat flows into the body at the selected face or faces).
• A negative value removes heat from the model, cooling it (heat flows out of the body at the selected face or faces).

How to Assign a Heat Flux Excitation:

1. Select one or more faces, either graphically or using the Select by Name command on the Draw ribbon tab. In either case, the Face selection mode must be active.
2. Use one of the following three methods of accessing the Heat Flux dialog box:
• Using the menu bar, click Mechanical > Excitations > Assign > Heat Flux.
• Right-click in the Modeler window or right-click a selected item in the History Tree, and choose Assign Excitation > Heat Flux from the shortcut menu.
• Right-click Excitation in the Project Manager and choose Assign > Heat Flux.
3. Choose the desired heat flux type:
• Total Power: Specify the magnitude of the total thermal power to be applied to each selected face and/or each face of any selected objects. Choose the units of power from the adjacent drop-down menu:



• Surface Flux: Specify the uniform power per unit surface area to be applied to all selected faces and/or all faces of any selected objects. Choose the units of surface flux from the adjacent drop-down menu:



Note:

Regardless of whether you specify Total Power or Surface Flux, the value can be positive or negative and can be numeric, a variable, or an expression (consisting of constants, variables, and mathematical operators). Additionally, for transient thermal solutions, the value can vary with time by using a time-dependent dataset reference.

Example 1: Assume that ten faces and one six-sided object (a box) are selected. If you specify 1 W of Total Power, then each face will receive 1 W of heat. The model will receive a combined total thermal power of 16 W (1W x 16 faces). Because the power per face is constant but the areas of the faces differ, the heat flux density (that is, the power per unit area) will vary among the faces.



Example 2: This example shows the total combined power and the equivalent heat flux density when two faces with different areas are selected. One face has an area of 150 mm², and the other 300 mm². If you specify a Total Power of -15 W, the combined thermal power applied to the model is -30 W (-15 W x 2 faces). 15 W of heat is removed at each face (that is, the model is cooled) because the specified power is negative. The flux density of the smaller face is -0.1 W/mm² (-15 W / 150 mm²). The flux density of the larger face is -0.05 W/mm² (-15 W / 300 mm²).



Example 3: This example shows the total combined power and the power per face when two faces with different areas are selected and you choose the Surface Flux option. One face has an area of 150 mm², and the other 300 mm². If you specify a Surface Flux of 80 KW/m², the combined thermal power applied to the model is 36 W. The thermal power entering the smaller face is 12 W (80,000 W/m² * 150 mm² /1,000,000 mm²/m²). The thermal power entering the larger face is 24 W (80,000 W/m² * 300 mm² / 1,000,000 mm²/m²).



4. Optionally, change the Name of the excitation. The default name is HeatFluxx, where x is a number that increments for each heat flux excitation that you apply.
5. Click OK.

A heat flux entry appears under Excitation in the Project Manager: