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Thermal Analysis

7 REPLIES 7
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Message 1 of 8
swaminathan.sekar
2795 Views, 7 Replies

Thermal Analysis

Hi,

We are doing thermal analysis using Ecotect software to select suitable materials for building envelope based on the heat load. For this, we have planned to use various materials as well as we know the following details.

Width

This defines the width of each layer in millimeters

Density

The density of each layer in kg/m³.

Sp. Heat

The specific heat of each layer in Joules per kilogram degrees Celsius (J/kg°C).

Conduct.

The conductivity of each layer in Watts per meter degrees Celsius (W/m°C).

 

If possible, I would like to know the method to calculate U – Value from the above specifications.

 

After that there is one option in Ecotect software. Please refer below for definition.

Hatch

Defines the displayed hatch pattern of the layer. Clicking the left mouse in this box will display a menu from which this value can be selected. The density of the hatch pattern can also be set by changing the second digit in the hatch code

 

What is it mean? How to use it for thermal analysis?

 

7 REPLIES 7
Message 2 of 8

Hello swaminathan.sek, 

 

Sorry for the long delayed response, I have been going through the users post to answer unanswered questions...

 

Ecotect calcultion of the U-value is pretty straight forward: 

 

U-value = 1 / R, where R = the resistance = (thickness (m) / thermal conductivity (W/mk)) + internal air resistance (0.12 m2K/W) + external air resistance (0.06 m2k/W).  

To calculate the Admittance of the material is more complex;  Ecotect uses the square root of the product of the thermal conductivity, density, specific heat capacity, and the angular velocity, where the angular velocity = 2*(3.14/24).

 

The Hatch option is mostly for graphic representation, except when using a hatch that includes air gap (air gap or stud work), whereas Ecotect will adjust the thermal conductivity of the assembly to account for the air gap.  Be careful to double check that your thermal conductivity number is correct, then press the "Calculate Thermal Properties" button to re-calculate the assembly's properties.

 

I hope that clarifies things a bit.

Let me know if you have more questions on this topic, otherwise please accept as a solution so that others can benefit from this information.

Cheers,

Olivier A. PENNETIER

SYMPHYSIS

www.symphysis.net

Message 3 of 8

Hi,

Based on the formula you given, Manually I have calculated the U Value &
admittance for single material. But the manual result is not matching with
the software result.

Please help..
Message 4 of 8

Hi Swaminathan.sek, 

The formula is correct - note that this is for homogeneous material assemblies.  If not homogeneous, you have to add the U-values for each material layers as U-values simply add up. Start testing with an assembly that has only one material layer.

I am attaching a quick spreadsheet for you here.

 

Hope that helps.

Cheers

Olivier A. PENNETIER

SYMPHYSIS

www.symphysis.net

Message 5 of 8

Hi,

 

Based on your calculation sheet, Manually i have calculated the U Value for material which has combined with some layers. But it is not matching with software U Value.

 

Please refer the attachment.

 

Note; For individual material, software u value almost matching with manual u value.

 

Please clarify & help.

Message 6 of 8

Hi swaminathan.sek, 

 

You're almost there... a few things here:

Indeed, non-homogeneous assemblies are more tricky, and adding air spaces complicates things a bit.

 

1. You are counting the inside and outside air resistance for each layers - you should only count once the outside and inside air resitance for the entire assembly. Try an assembly with NO AIR GAP, sum up the R values for each layers then add the outside and outside air resistance and you should get the correct U-value for the whole assembly.

 

2. When you add an air gap into the assembly, things get a bit more complicated because an air gap has its own resistance that cannot be calculated like the other materials (it is due to the convective flows going on in the air gap, dependant on the air gap orientation and other factors).

So when you add an air gap in your assembly, calculate its R-value like the other materials, and add 0.17 m2K/W + interior and exterior air resistance to the overall R-value of the assembly.  You will get the correct U-value.

 

I hope it clarifies things.

Let me know if you have more questions on this topic, otherwise please accept as a solution so that others can benefit from this information.

Cheers,

Olivier A. PENNETIER

SYMPHYSIS

www.symphysis.net

Message 7 of 8
jescobarsaiz
in reply to: Pennetier1

Hi, 

I'm doing thermal analysis with ecotect, but i dont understand very well how to calculate the admittance cuz I did It manually but the result is completely different, I dont know if its because o the units - square root of the product of the thermal conductivity( W/M.K ), density (Kg/M3), specific heat capacity (J/kg.K), and the angular velocity, where the angular velocity = 2*(3.14/24)(Maybe this one in seconds??). I'd like to know how to calculate it 

 

If possible, I would like to know the method to calculate admittance, more detailed.

 

Thank you!

Message 8 of 8
Pennetier1
in reply to: jescobarsaiz

Hello jescobarsaiz

 

Yes, this one has given me headaches as well and I was never able to manually get comfortably close to the Admittance value calculated by Ecotect.

By the way, yes, if you use your specific heat capacity in J/kg K, then your angular velocity must be in seconds, so not 24, but (24*3600) = 86,400.

If your specific heat is in Wh/kg k, then you can keep it in hours, using 24 in your angular velocity formula.

 

If you want to tackle the same principle for imperial units, you can look into the work and formula of Balcomb et al - attached. It uses the concept of Diurnal Heat Capacity, which is really the same as Admittance, but using the Imperial units. 

Let us know if you make any progress on this!

 

Cheers,

Olivier A. PENNETIER

SYMPHYSIS

www.symphysis.net

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