5-cm ventilated air gap

[Haalsaad]

Dear Delphin-Team,

I am simulating a highly-ventilated facade (wall + ventilated air gap + cladding). I defined the air change rate at the ventilated air gap using the source [AirChange]. I need to create a new material for the 5-cm ventilated air gap since the default air materials in Delphin are for unventilated gaps. I have Delphin 6 as well as Delphin 5, so creating a new material should not be a problem.

1. What is the thermal conductivity (lambda) of air in a 5-cm ventilated air gap? I cannot seem to find a value in the literature because all researchers refer to the standard DIN 4108 which recommends not considering the layers that are in front of the ventilated air gap when evaluating thermal performance of a structure. Instead, the standard recommends using the heat transfer resistance of Rse = 0.13 m2K/W for the exterior side (instead of the typical Rse = 0.04 m2K/W for horizontal heat transfer).

2. Similar to the first question, what is the µ-value (vapor transmission rate) of a 5-cm ventilated air gap?

Thank you very much in advance!

[fechner]

Hello,

For a ventilated air gap its difficult to say. For non-ventilated air gaps you can use EN 673. Here you can find formulas for calculating the Nusselt number and from this thermal conductivity, transfer coefficient and vapor transport value. I have an Excel sheet that can help you.
https://cloudstore.zih.tu-dresden.de/in ... JAcCd46taB
With ventilation its more difficult. You can find some hints in ISO 6946 but mainly for transfer coefficients. Otherwise, as higher the ventilation rate is as less important are the heat and vapor transfer through the gap is. Only the heat and vapor transfer coefficients at the boundaries from material into the air gap are important. You can set this by using a contact condition.

[Haalsaad]

Thank you very much!

[Haalsaad]

Dear Mr. Fechner,

To follow-up on this topic, I am wondering if I should use the airflow model (air mass balance equation) embedded in Delphin to simulate the ventilated air gap. In order to do this (please correct me if I am wrong):

1. The construction must have a 2D gird (discretization in both X and Y directions)
2. A [GasPressure] climate should be generated and defined into an [AirConvection] boundary conditions. This boundary condition should be then assigned into a Surface/Boundary.
3. The created Surface/Boundary should be assigned to the top and bottom of air cavity (or should it be assigned only to the bottom?)

As these steps made the model both slow and unstable, I was wondering if you have any suggestions/thoughts on the airflow model? do I need it to simulated my air cavity? if yes, then is there a "best practice" tips that you can give me in this regard?

Thank you very much in advance!

[fechner]

Hello,

1. Yes
2. You need at least one air pressure boundary condition (in case of bouyancy). For forced convection you need a pressure difference. That means at least two pressure boundary condition. Keep in mind we use a normal pressure of 101325Pa. All pressure differences must respect this. As example:
For 5Pa pressure difference you can use one pressure 101325pa and on with 101330Pa
3. This depends on your construction
In order to give better help i need to see your project file. We can do the further help by EMail if you like.

[KatharinaKleiss]

Hello,

are there average values ​​that can be assumed for the temperatures left and right in an air gap?
There is no new solution for the calculation of ventilated air gaps yet?

Thank you in advance!

[fechner]

Hello,

In order to have an idea for the temperature difference over a air gap you can make a simulation with standard properties first and se what differences you have. Its only a guess but better than nothing.

For a ventilated air gap you can use the source 'Air exchange with ambient air..'. Here you can set a air change rate for your gap and the properties of the inflowing air.

A remark.
This question is a bit different from the original one. It seems to be better to create a new task.

[Astridvg]

Hi,
I would like to do a simulation with an unventilated horizontal gap of 80mm.
When I download the link, the excel for air gaps you share, the macro's or formulas don't work.

1. Can i use the formulas for a bigger air gap, delphin stops at 5cm
2. Can you reupload the excel ?

Thanks in advance
Astrid

[fechner]

Hello,

I have sent you a EMail with a new link to the file.
The parameter for air gaps in the Delphin material database are set by using the air gap table from the ISO 6946 standard. If you like you can use the table in order to calculate other thicknesses.

With kind regards
Heiko