Home / Examples / Coupled Analysis / Fluid-Thermal Analysis [Bernoulli/Watt] / Example 18: Natural Convection Under Reduced-Pressure Environment

Example 18: Natural Convection Under Reduced-Pressure Environment

General

• This is the modified example of Example 6: Cooling of IC by Natural Convection. The same model is solved under a reduced-pressure environment.
   

• Compare the heat dissipation characteristics under ambient pressure and reduced pressure.
   

• Unless specified in the list below, the default conditions are applied.
   

• The above analysis can be performed by modeling a substrate and an IC and applying [Automatic Fluid Creation].
  The analysis can also be performed by creating a fluid manually. The setting for the analysis is described at the end of this explanation.
  

• Obtain this session's project file. (Right-click and choose 'Save link as')
  The project has four analysis models.
  Ambient temperature and pressure 100 kPa: Analysis with automatic fluid creation
  Reduced pressure 50 kPa: Analysis with automatic fluid creation
  Ambient temperature and pressure 100 kPa: ic_board_with_air: Analysis with manual fluid creation
  Reduced pressure 50 kPa: ic_board_with_air: Analysis with manual fluid creation




• 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	Fluid Analysis [Bernoulli] Thermal Analysis [Watt]
Analysis Type	Steady-state Analysis
Laminar Flow/Turbulent Flow	Select Turbulent Flow
Flow Type	Select [External Flow]. Select Take into Account Buoyancy (Natural Convection)
Ambient Value Setting	Select [Analyze using the material properties depending on ambient values] Ambient Temperature: 25℃ Ambient Pressure: 51200 Pa (refer to Ambient Value Setting)
Automatic Fluid Creation Setting	External Flow: Natural convection in the +Z direction
Layer Mesh Setting	Parameters for Automatic Creation Expected Temperature Difference: 50 [deg]
Detailed Setting	Deselect [Output main fields only]. (To display buoyancy and fluid density)
Meshing Setup	General Mesh Size: 2 [mm]

Model

The model is a box solid body. The material is Air (000_Air).

Set the inlet/outlet type of the outer boundary condition to natural inflow/outflow.

The substrate (VOL1) and IC (VOL2) are solid bodies.

 

  

 

Setting of Body Attributes, Materials, and Mesh Sizes

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

Fluid material 000_Air * is set in the [Automatic Flow Creation] setting.

 

* Available from the material DB

 

IC (VOL2) is set as follows on the Heat Quantity tab.

Body Attribute Name	Tab	Settings
VOL2	Heat Quantity	1W

Boundary Condition

Set the inlet/outlet type of the outer boundary condition to natural inflow/outflow.

Since the temperature of the incoming fluid is the ambient temperature, the ambient temperature is applied In the [Ambient Setting] dialog box.

 

Results

The temperature distribution is shown below.

As in Example 6, the cross section at x = 0 is shown below. The air body is shown as well.

There is a high-temperature area above the IC. You can observe that heat is transferred mainly upward.

It is observed that the IC has higher temperatures under reduced pressure, indicating reduced heat dissipation efficiency.

 

Ambient Pressure (100 kPa)	Reduced Pressure (50 kPa)

 

 

 

Next, switch the solver type from thermal to fluid analysis to display buoyancy.

The buoyancy under reduced pressure is about half of that under normal pressure.

Ambient Pressure (100 kPa)	Reduced Pressure (50 kPa)

 

 

Lastly, the fluid density is shown below.

The density under reduced pressure is about half of that under normal pressure.

This is because the ambient pressure is set to about half of the normal pressure and the density is calculated using it. (refer to Density Tab)

Since buoyancy is calculated using density, the buoyancy under reduced pressure is about half of that under normal pressure. (refer to Section "4 External Force" in Differential Equations in Fluid Analysis/Fluid-Thermal Analysis)

Ambient Pressure (100 kPa)	Reduced Pressure (50 kPa)

 

 

 

Do not apply [Automatic Fluid Creation]

For reference, the settings for analysis with [Automatic Fluid Creation] disabled are described below.

 

Analysis Conditions [Manual Fluid Creation]

Item	Settings
Solver	Fluid Analysis [Bernoulli] Thermal Analysis [Watt]
Analysis Type	Steady-state Analysis
Laminar Flow/Turbulent Flow	Select Turbulent Flow
Flow Type	Select [External Flow]. Select Take into Account Buoyancy (Natural Convection)
Ambient Value Setting	Select [Analyze using the material properties depending on ambient values] Ambient Temperature: 25℃ Ambient Pressure: 51200 Pa (refer to Ambient Value Setting)
Layer Mesh Setting	Parameters for Automatic Creation Expected Temperature Difference: 50 [deg]
Detailed Setting	Deselect [Output main fields only]. (To display buoyancy and fluid density)
Meshing Setup	General Mesh Size: 10 [mm]

Model [Manual Fluid Creation]

The model is a box solid body. The material is Air (000_Air).

Set the inlet/outlet type of the outer boundary condition to natural inflow/outflow.

The substrate (VOL1) and IC (VOL2) are solid bodies.

 

  

 

Setting of Body Attributes, Materials, and Mesh Sizes [Manual Fluid Creation]

Body Number/Type	Body Attribute Name	Material Name	Mesh Size
0/Solid	VOL1	006_Glass_epoxy *	2.0
1/Solid	VOL2	001_Alumina *	2.0
2/Solid	Air	000_Air *	-

* Available from the material DB

 

IC (VOL2) is set as follows on the Heat Quantity tab.

Body Attribute Name	Tab	Settings
VOL2	Heat Quantity	1W

Boundary Condition [Manual Fluid Creation]

Boundary Condition Name/Topology	Tab	Boundary Condition Type	Settings
Outer Boundary Condition	Fluid-Thermal	Inlet/Outlet	Natural Inflow/Outflow Inflow Temperature: Use ambient temperature (25 [deg])