Advanced Building Physics
4
Outline
 Heat conduction in a timber-framed envelope
 3 D Thermal conduction
 Geometry
 Subdomain settings
 Boundary settings
 Mesh
 Solution and obtaining results
 Results
Example
 The analysed structure represents junction of external wall-corner and a floor of a timberframed house. The horizontal cross-section geometry of the model is illustrated in the following
figure including material definition.
 Simulate heat conduction in this case and evaluate temperature and heat flux distribution.
3D Problem definition
 Start Comsol to get Startup Comsol window and choose Model Wizard.
 In the Select Space Dimension window. Choose 3D.
 In Select Physics choose Heat Transfer → Heat Transfer in Solids and click the Add button.
Then, click the Study button.
 In the Select Study click Stationary and confirm by clicking Done button.
Geometry
Geometry: Model Builder → Component 1 → right click on Geometry 1 and choose Work
Plane. Then right-click on Model Builder → Component 1 → Geometry 1 → Work Plane →
Plane Geometry and select Import.
In Import 1 settings click Browse and find your dxf. file and click Import. Be careful of size of
the imported geometry. In the case, of needed size change: Model Builder → Component 1 →
Geometry 1 → Work Plane 1 right click on Plane geometry and choose Transforms - Scale.
Assign all the geometry and define Scale Factor.
Model Builder → Component 1 → Geometry 1 right click on Work Plane 1 and choose
Extrude. In the Extrude Settings define height of layers in z-direction → 1, 1.15, 1.2, 2.2m.
Define a new work plane by right-click on Model Builder → Component 1 → Geometry 1.
Locate the Work plane at 1m of z-coordinate. Click Plane Geometry and define rectangles
representing wooden beams.
 Rectangle 1:
width:
height:
x:
y:
0.7
0.05
0.009
0.3
 Rectangle 2:
width:
height:
x:
y:
0.7
0.05
0.009
0.6
Model Builder → Component 1 → Geometry 1 right click on Work Plane 2 and choose
Extrude. In the Extrude Settings define height of layer in z-direction → 0.15m.
Geometry
Define a new work plane by right-click on Model
Builder → Component 1 → Geometry 1.
Locate the Work plane at 1.15m of z-coordinate.
Click Plane Geometry and define rectangle
representing floor.
 Rectangle 1:
width:
height:
x:
y:
0.487
0.487
0.222
0.222
Model Builder → Component 1 → Geometry 1 right
click on Work Plane 3 and choose Extrude. In the
Extrude 3 Settings define height of layer in z-direction
→ 1m.
Click Build All
Mesh
Tetrahedral
 In Model Builder → Component 1 → right click on Mesh 1 and choose Free
Tetrahedral.
 In Model Builder → Component 1 → Mesh 1 → right click on Free Tetrahedral 1 and
choose Size.
 In Model Builder → Component 1 → Mesh 1 → Free Tetrahedral 1 → Size define
Element Size. Mark Custom and in the Element Size Parameters define 0.01 in Minimum
element size and 2 in Resolution of narrow regions.
 In Model Builder → Settings → click Build All.
Selection of entities
 Right click Model Builder → Component 1 → Definitions and choose Selections - Explicit. In
the Input Entities – Geometric entity level: select Domains and define domains representing
wooden elements. Rename the Explicit 1 to wood, or so.
Do the same for the other material elements.
Right click Model Builder → Component 1 → Definitions and choose Selections - Explicit. In the
Input Entities – Geometric entity level: select Boundary and define boundaries representing
exterior surface of the model. Rename the Explicit X to exterior, or so.
Do the same for the interior surface.
Subdomain settings
 Material properties needed for stationary heat conduction solution is only thermal conductivity
k.
Wood:
k=0.6 W/m*K
Insulation:
k=0.04 W/m*K
Gypsum board:
k=0.12 W/m*K
Windshield:
k=0.05 W/m*K
 In Model Builder → Component 1 → Heat Transfer in Solids (ht) → Heat Transfer in Solids
1. There are marked all subdomains in Domain Selection. In Heat Conduction, Solid → Heat
conductivity choose User defined and enter 0.04.
 In Model Builder → Component 1 → right click on Heat Transfer in Solids (ht) and choose
Heat Transfer in Solids. Select domains representing wooden elements in the Selection window.
They are domains: 5,6,7,8,13,14,15,16,25,26,27,28,30,31,33,34,35,36,43,45,47,49,50,51. In Heat
Conduction, Solid → Heat conductivity choose User defined and enter 0.6.
In Model Builder → Component 1 → right click on Heat Transfer in Solids (ht) and choose
Heat Transfer in Solids. Select domains representing gypsum board in the Selection window. They
are domains: 41,42,44,46,48. In Heat Conduction, Solid → Heat conductivity choose User
defined and enter 0.12.
 In Model Builder → Component 1 → right click on Heat Transfer in Solids (ht) and choose
Heat Transfer in Solids. Select domains representing gypsum board in the Selection window. They
are domains: 1,2,3,4. In Heat Conduction, Solid → Heat conductivity choose User defined and
enter 0.05.
Boundary conditions
 Exterior conditions: In Model Builder → Component 1 → right click on Heat Transfer
in Solids (ht) and choose Heat Flux. In Heat Flux 1 → Settings → Boundary Selection
choose boundaries 1,2,4,5,7,8,10,11. Mark Convective heat flux. Than fill fields for Heat
transfer coefficient and External temperature as following:
h
Text
1/0.04
-10[degC]
 Interior conditions: In Model Builder → Component 1 → right click on Heat Transfer
in Solids (ht) and choose Heat Flux. In Heat Flux 2 → Settings → Boundary Selection
choose boundary
198,199,200,201,204,205,206,207,209,210,212,213,214,217,219,220,221,240,241. Mark
Convective heat flux. Than fill fields for Heat transfer coefficient and External
h
1/0.13
temperature as following:
Text
20[degC]
Results
 Important quantity in Heat conduction solution are Temperature and Heat Flux.
 Just after solution is shown temperature in the
model in Graphic field in Kelvins. Units of
temperature is possible to change in Model Builder
→ Results → Temperature (ht) → Surface 1. In
Surface 1 settings field change in Unit list K to
degC and click the Plot button.
 In Model Builder → Results → Temperature
(ht) → Surface 1 choose Rainbow from the Color
table list.
Results
Physical phenomena at exact point
 To get Temperature value at any point of the model: Model Builder → Results →
right click the Data Sets and choose Cut Point 3D. In Cut Point 3D 1 Settings window
define coordinates of the point. Let’s define inner corner on the top of the floor with
x=0.222, y=0.222 and z=1.2m. Click Plot to display location of the point.
 In Model Builder → Results → right click on Derived Values and choose Point
Evaluation.
 In Model Builder → Results → Derived Values → Point Evaluation 1 Settings
choose Cut Point 3D 1 in the Data set and change units from K to degC in the Unit.
 Click Evaluate at the top of the Settings window. The temperature is 13.526 0C.
Results
Temperature distribution along line
 To get Temperature distribution along a line: Model Builder → right click on Results and
choose 1D Plot Group.
 Then: Model Builder → Results → right click on 1D Plot Group 3 and choose Line Graph.
In Model Builder → Results → 1D Plot Group 3 → Line Graph 1 Settings define lines 226
and 227 representing inner corners between walls and floor and choose Expression: T and Unit:
degC.
Click Plot at the top of the Settings window.
Results
Heat flux distribution along line
 In Model Builder → Results → 1D Plot Group 3 → Line Graph 1 Settings click Replace
Expression and choose Model – Component 1 – Heat Transfer in Solids – Domain Fluxes –
ht.tfluxMag-Total Heat Flux Magnitude
Click Plot at the top of the Settings window.
 To get Heat flux along an edge: Model Builder → Results → right click the Derived Values
and choose Integration – Line Integration. In Line Integration 1 Settings window define
expression by replacing as: Model – Component 1 – Heat Transfer in Solids – Domain Fluxes
– ht.tfluxMag-Total Heat Flux Magnitude.
 In Model Builder → Results → Derived Values → Line Integration 1 Settings define lines
226 and 227. Click Evaluate to display heat flux along inner surface of the model.