Home / Examples / Thermal Analysis [Watt] / Example 8: Heat Radiation by Forced Convection (Transient Analysis)

Example 8: Heat Radiation by Forced Convection (Transient Analysis)

General

The model is the same as Example 7. A heat source is placed on a substrate, and there is a forced air flow for cooling in parallel to the substrate. The heat dissipation is analyzed under the transient condition.
The heat transfer coefficient is acquired manually.
To acquire it automatically, see [Ex.1 of Simple Fluid-Thermal Analysis].
The temperature distribution and the heat flux vectors are solved.
Unless specified in the list below, the default conditions will be applied.

Results will vary depending on Femtet version and the PC environment.

Analysis Space

Item	Settings
Analysis Space	3D
Model Unit	mm

Analysis Conditions

Item	Settings
Solver	Thermal Analysis [Watt]
Analysis Type	Transient Analysis
Options	N/A

The transient analysis tab is set up as follows. The number of calculation steps is 20. The timestep is 30 sec.

Therefore, the temperature distributions for 600 sec are solved.

Tabs

Setting Item

Settings

Transient analysis

Table

Number	Calculation Steps	Output Interval	Timestep [s]
1	20	1	30

Initial Temperature

25 [deg]

Model

The same as Example 7. The body attributes, the material properties and the boundary conditions are the same as well.

The substrate (VOL1) and the heat source (VOL2) are created as solid body box, and the heat source is defined in the body attribute of VOL2.

The heat transfer coefficients for the top and bottom faces of the substrate and the top face of the heat source are calculated basede on the simplified equation.

Body Attributes and Materials

Body Number/Type	Body Attribute Name	Material Name
0/Solid	VOL1	006_Glass_epoxy *
1/Solid	VOL2	001_Alumina *

* Available from the material DB

The heat source of VOL2 is set up as follows.

Body Attribute Name	Tab	Settings
VOL2	Heat Source	1 [W]

Boundary Conditions

The heat transfer coefficients for the forced convection are calculated as follows. See[Heat Transfer Coefficient for Forced Convection] for more information.

To acquire it automatically, see [Ex.1 of Simple Fluid-Thermal Analysis].

h = 3.86 x (V/L)0.5 x C [W/m2/deg]

where

Air flow V=1 [m/s]

Top and Bottom Faces of Substrate (VOL1): typical length L=0.05, C=1 -> h=17.26
Top Face of Heat Source (VOL2): typical length L=0.02, L'=0.015, C=1 * -> h=27.3

The thickness (d) of the speed boundary layer at the edges of the heat source is calculated as follows

D=0.0182 x (L’/V)0.5 = 2.3 [mm]

This is close enough to the thickness of heat source, so we set C=1.

Boundary Condition Name/Topology

Tab

Boundary Condition Type

Settings

BC1/Face

Thermal

Heat Transfer: Convection

Heat Transfer Coefficient: 17.26 [W/m2/deg]

Ambient Temperature: 25 [deg]

BC2/Face

Thermal

Heat Transfer: Convection

Heat Transfer Coefficient: 27.3 [W/m2/deg]

Ambient Temperature: 25 [deg]

Results

The temperature distributions at each time are shown below.

At Minimum/Maximum Value on the Contour tab of [Graphics Setup], deselect [Automatic] and set 25 => 62.