Linear thermal transmittance guided calculation.
Angular connection clay block walls with concrete pilaster.
How to use right now one of the most interesting two-dimensional conduction heat-transfer analysis free software based on the finite-element method, which can model the complicated geometries of building components.
The guided exercise will consider the analysis of a building’s clay block angular connection walls featuring a concrete pilaster, the aim is to check the existence of a thermal bridge and relative thermal dispersion flow.
Let’s consider an exterior wall corner, the two wall sections are made in clay blocks with cladding plaster. A concrete pilaster is positioned into the junction of this the construction node.
The considered horizontal cut-off plane is positioned from the interior corner at dmin = 1035 mm (where dmin is the greater of 1 ml and three times the walls thickness 345 mm)
It will be a bi-dimensional drawing so does not matter the walls hight, anyway conventionally it is considered to be 1000 mm.
Summarising the whole wall layers, the cross section includes a 20 mm thick cement plaster layer following by the first clay blocks wall which thickness is 100 mm, before the second clay blocks layer wall (thickness 150 mm) is there a 50 mm wide cavity filled by mineral wall insulation the cross section ends with the external surface 25 mm thick cement plaster. The armed concrete pilaster dimensions are 300 mm x 300 mm.
DEFINITION OF CUT-OFF PLANES – NORM UNI EN ISO 10211:2018*
*this is the Italian standard Norm but as you can see it refers to ISO International Standard Organisation so depending which country you live in probably there is a national norm you can look at.
During the representation of the cross-sections that the software will use to run the calculation, It is important to get confident with cut-off planes.
In simple words, considering the hypothesis that the thermal bridge will be located at the corner, I will have to know at what distances to cut the wall’s drawing and consider in that point the adiabatic boundary condition.
In order to set out adiabatic cut-off planes we learn from the norm UNI EN ISO 10211:2018 that the there is a minimal distance dmin from the considered node (corner in our case) where cut off planes shall be positioned:
where dmin is:
- at a symmetry plane if this is less than dmin from the central element (node);
- at least dmin from the central element (node) if there is no nearer symmetry plane;
dmin = is the greater of 1 m and three times the thickness of the flanking element concerned (thickness of the walls in our case).
PRELIMINARY STUDY OF THE CROSS SECTION
Having identified the need to verify the presence or not of a thermal bridge in a part of the building envelope, if you want to use the software Therm, the starting point will be to draw the geometry.
Underestimating the importance to follow a proper drawing process could lead to frustrated situations with relative waste of time. For a for a better knowledge about drawing technique and useful drawing tips take a look here.
I want to mention in particular the importance to follow the national technical standards relatively to the cut-off planes (as seen above) in order to have reliable results. In addiction, focus on how the various layers are drawn, otherwise will be easy encounter potential errors in the processing phase. For example, It is not recommended to draw too thin material layers (polygons) and too complex geometries.The drawing should be as simple as possible.
In this exercise a DXF file is provided with the geometry already drawn which will be sufficient to import. A little tip could be prepare in advance a check list including all steps and use it in order to avoid some apparently trivial but crucial step.
THE DATA WE NEED FOR THE CALCULATION
Some data are needed in order to process the calculation, at first we have to focus on the site where our building is because how we will see next, boundary conditions require temperature and surface resistances
he building site is Trento
Heat-Flux direction is horizontal
Reason for the simulation is to know the thermal bridge value in order to calculate de thermal loss.
Data related to external temperature are indicated from the Italian norm UNI 10349-1 (you need to check to which norm correspond in your own country) considering that we need the medium monthly temperature of the coldest month in the year*
*External Temperature = 1 °C
Internal Temperature = 20°C
Being the purpose of the simulation discover the value of a thermal bridge and its linear transmittance, the data about the film coefficient as seen for the temperature are provided in Italy from a specific standard UNI EN ISO 6946.
Taking in mind that the heat flow will be horizontal we will have on the surface of the wall:
External Film coefficient horizontal flow = 25 W/mq.k
Internal Film coefficient horizontal flow = 7,69 W/mq.k
Material data will be easily find out thought technical documentation and products certification, we need to know:
- Conductivity W/m.k
Once our cross section geometry will be defined (this mean to set up thickness of all layers) and owning data seen so far we will be ready to perform the analysis with the help of therm.
THE DRAWING OF THE SECTION
The first operation I recommend is to prepare the Therm file. Preparing the file means opening the program, opening a new file, setting/checking the units and adjusting some functional parameters to the calculation and saving with a name.
- Open Menu File then Save as enter the desired file name and save;
- Make sure that the units are mm and celsius, if necessary, use the Switch Unit button to change, in the main status bar or menu options -> Switch Units;
- Open the Preferences dialog box from the Options menu, activate the Therm File Options tab and set the values for the Maximun% Error Energy Norm fields by replacing the value 10 with 2 and Maximun Iterations by replacing the value 5 with 20, confirm with ok.
- Save the file.
The fields we need to modify concern two parameters:
Maximum % Error Energy Norm:
Determines the Error Energy Norm percentage. THERM will process the simulation several times until this value is reached.
Generally speaking, a lower number than the default heat (10) increases the accuracy of the calculation.
Specify the maximum number of times the software will modify the mesh and iterate the calculation. In this case, a larger value increases the precision.
NB: The explanation is deliberately simplified to provide a general functional framework for this exercise. For a more in-depth understanding of the two parameters, refer to the documentation supplied with the software.
TO IMPORT FILE DXF
After the first step of setting the file, the next step is to draw the geometry of the cross section of the construction building detail that we want to analyse. In this exercise it will be a matter of importing a previously prepared DXF file into the software (made with a third-party program) and auto-converting it to make it directly usable.
- Menu file -> Underlay from the dialog box that opens, press the browse button and select the desired file to load from the open window of windows (in our case, angle.dxf see at the end of this page);
- at this point we have pointed out to the software the path of the file to be loaded as background (underlay), wanting to take advantage of the possibility of using the drawing without tracing it, we will check the autoconvert box and then confirm.
- the loaded file (set of adjacent polygons) should appear as in the figure below;.
- We now ask the program to draw the edge line to which the boundary conditions will subsequently be associated. This operation is possible alternatively by acting directly on the BC key or from the Draw -> boundary conditions menu. This operation allows a first check that the converted geometric model is error-free. If the dxf file has been drawn correctly the border will be drawn, otherwise, a window will inform the nature of the problem encountered. In our case, for example, the software informs that there are points of the geometry closer than the tolerance value and proposes an automatic grinding solution that we will accept and confirm. At this point the edges are drawn and the model is ready.
In this specific case, the software alerts us that it is able to automatically adjust the geometry. This does not necessarily happen always, as it could be necessary to repeat the intere process of drawing the cross section in DXF format and then reload it.
ASSIGNING MATERIALS AFTER DRAWING GEOMETRY
Having obtained the geometric representation of the construction node under examination, we proceed to assigning the corresponding materials to the various polygons that characterise the stratigraphy.
To do this it will be necessary to create the materials and save them in the corresponding library then associate them with the polygons according to the following summary drawing.
We remind you that the drawing to be imported must be produced in mm on a 1: 1 scale to be usable in Therm.
- open the Material library dialog box, Libraries menu. The materials present in the library are accessible from the drop-down window and by selecting them in solids properties the values of conductivity and emissivity appear.
We need to create the materials as follow:
_Cemento armato (armed concrete)
Conductivity = 2,30 W/m.k
Conductivity = 0,90 W/m.k
Conductivity = 0,36 W/m.k
_Lana minerale035 (rock wool)
Conductivity = 0,035 W/m.k
NB: The emissivity of non-metallic construction materials is assumed to be 0.90, a value normally considered.
- we create all the materials that characterise our geometric model. Botton New to enter the desired name preceded by the underscore “_” so that in the drop-down menu they are displayed at the top of the list. For instance _armedconcrete , in the material type box we confirm that it is a solid material, then we modify the conductivity with the desired value 2.3 w/ m.k. The emissivity remains 0.9 for most construction materials. We act on the color button to associate a color with which the material will be displayed then confirm.
- we proceed this way to create all the necessary materials. At the end we save the library with the save lib as command in the program folder.
- to assign the materials simply activate the polygons of the geometric model and select the material from the drop-down menu of the command bar. Use this technique until all materials have been assigned.
It follows the drawing’s image how it should look after all materials have been associated. The colors are of course indicative as they can be customized when creating the materials.
DEFINING AND ASSIGNING BOUNDARY CONDITIONS
To proceed with the calculation It is now necessary to assign the boundary conditions. This means indicating to Therm the different conditions in terms of surface temperature and film coefficient present on the edges of the section (external and internal side). If we select one of the edges of our model we will see that the created edge is adiabatic. This condition must be maintained only in correspondence with the cutting planes. We then proceed to create the two boundary conditions we need which will be:
Temperature 1 °C
Film coefficient 25 W/mq.k
Site Trento average daily temperature coldest month December
1,0 °C (standard UNI 10349- 1:2016)
_interna 20 horizontal
Film coefficient 7,69 W/mq.k (standard UNI EN ISO 6946)
- access the Boundary condition library box from the Libraries menu. The boundary conditions present in the library are accessible from the drop-down window. For each of them are shown the model (simplified will be the one to be used), therefore temperature and heat transfer coefficient (Film coefficient).
- we create the two boundary conditions by acting on the new button. Enter the name _esternaTN1 in the appropriate box and confirm, enter the temperature (1) and Film Coefficient (25) values, select a color, confirm, and save the library by pressing the save lib button.
- we proceed this way to create the internal boundary condition that we will call _interna20orizzontale.
NB: Once the internal and external boundary conditions have been defined, it will be necessary to indicate to the software along which edge segment (boundary conditions) to perform the calculation. That is the surfaces affected by the calculation of thermal transmittance. To do this it is necessary to create tags (u-factor surface Tags).
Creation of U-Factor Surface tags
From the libraries menu click UFactor Names, in the appropriate field we enter first _UFesterna (the tag’s name for our exterior surface) and click ADD to add it to the library. We repeat the steps for the second tag which will be _UFinterna (the tag’s name for our interior surface) then close.
Boundary condition assignment
- To assign the conditions to the boundary it is necessary to make the desired edge segment active by double clicking on it.
- The Boundary Condition Type window appears, from the Boundary condition drop-down boxes selected from the list _externaTN1 and immediately below in the U-Factor box select _UFesterna then confirm.
- Repeat the steps for the second segment of the external condition.
- We proceed in the same way with the two inner edge segments with the difference that in this case we will choose and select _interna20horizontal and _UFinterna
Everything is ready to launch the calculation
- click on the cal button (F9)
- Once processing is complete, our geometric model will show the isotherms with the corresponding temperature values.
- Clicking on the Show U-factors button the U-Factor window appears. Let’s adjust the two drop-down boxes to Total Length.
According our previous setting with the attribution of the two tags _UFesterna and _UFinterna, the corresponding values are shown for:
- U-Factor (the thermal transmittance distributed over the length) W/mq.k
- Delta T °C
- Lenght along which the transmittance is calculated in mm
- Heat Flow Watt. Note how these values coincide since for the principle of energy conservation an incoming flow must coincide with the outgoing flow.
CALCULATING THE THERMAL BRIDGE
Now let’s see how to use the transmittance calculated by Therm in order to reach the value of the linear thermal transmittance that represents the thermal bridge.
In summary, It is a matter of calculating the heat exchange coefficient of the walls (cross section considered) assuming they are homogeneous, which means in our case, without considering the effect of the armed concrete pillar and compare it to the heat exchange coefficient calculated by Therm which otherwise will include the effect of the pillar. The difference between the two represents how is easily understandable the value of the thermal bridge sought.
Let’s proceed with the calculation of the heat transfer coefficient with the potential pt L2D.
We will take the values calculated by Therm which we will multiply by the length of the outer edge and the inner edge respectively
We will have:
_UF esternaL2D = (0,4800×2,76)=1,3248 W/m.k
_UF interna L2D = (0,6400×2,07)=1,3248 W/m.k
Let’s now calculate the thermal transmittance of the wall according to the method provided by the UNI EN 6946 standard
Uwall= 0,427 W/mq.k
Calculation of the heat exchange coefficient of surfaces considered homogeneous without a thermal bridge.
External surface He = (0,427×2,76)= 1,1785 W/m.k
Internal surface Hi = (0,427×2,07)= 0,8839 W/m.k
Calculation of linear thermal transmittance PT distinct for outer and inner edge.
Outer edge He = L2d-He =(1,3248-1,1785)= 0,1463 W/m.k
Inner edge Hi = L2d-Hi =(1,3248-0,8839)= 0,8839 W/m.k
The value of the searched thermal bridge will be 0,1463 W/m.k
File DXF you need for this guided exercise with Therm