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## Simulation CFD

Active Member
Posts: 7
Registered: ‎05-11-2011

# Whole wall U-Factor

408 Views, 4 Replies
02-10-2013 08:00 PM

I have been looking for a method for calculating whole wall/product U-Factors for a 3d model. I have been diving into the software looking from the Architectural side, and would like to be able to determine thermal performance of complex wall system. So far I have been able to figure out everything from modeling to simulating a wall system, but I have not been able to get an output that I can convert to a U-Factor.

Product Support
Posts: 555
Registered: ‎08-24-2011

# Re: Whole wall U-Factor

02-11-2013 01:46 PM in reply to: Rycosso

Hi Ryan,

I don't work with U-factor and R-value very much, but my understanding is that the U-factor is the equivalent to the film coefficient output in CFD.

-Royce

Royce Abel
Active Member
Posts: 7
Registered: ‎05-11-2011

# Re: Whole wall U-Factor

Typically (in calculations and software other than Sim. CFD) the film coefficiant is the insulative value that you get from the air that is touching the surface of your wall. I do not know for a fact, but I would have to assume that it is the same here. It is calculated by the velocoty of wind and the surface charictoristics of the material.

Thanks for the try,

--Ryan

Product Support
Posts: 555
Registered: ‎08-24-2011

# Re: Whole wall U-Factor

02-12-2013 07:03 AM in reply to: Rycosso

Ryan,

I wouldn't disagree with what you said, but I did leave out some details in regards to my previous comment.

Wiki: Heat transfer coefficient - 'Heat transfer coefficient is the inverse of thermal insulance. This is used for building materials (R-value) and for clothing insulation.'  This comment is more in regards to units then true equivalency.

'The overall heat transfer coefficient (U) is a measure of the overall ability of a series of conductive and convective barriers to transfer heat. It is commonly applied to the calculation of heat transfer in heat exchangers, but can be applied equally well to other problems.'

This calculation will be done by hand and will include the film coefficient along the wetted surfaces and the various wall elements properties and dimensions.

Wiki: R-value (insulation) - 'The U-factor (sometimes incorrectly referred to as "U-value"), is the overall heat transfer coefficient that describes how well a building element conducts heat. It measures the rate of heat transfer through a building element over a given area under standardised conditions.'

You could also look at this in the reverse:

Q = U*A*ΔT

Q/(A*ΔT) = U

Q = Heat Flux Through Surface of Wall

U = Overall Heat Transfer Coefficient

A = Surface Area of the Wall

ΔT = Basically your room temperature vs. outside ambient temperature if you are looking at an exterior wall.

This is the film coefficient calculation that is done in the Wall Calculator.  If you make the assumption that the interior wall temperature is close to the ambient room temperature then you can use the Wall Calculator with a reference temperature of your outside ambient temperature to get a quick estimate of your U-Factor.

You could also just use the wall calc to extract the heat flux through the wall and bring that into excel with your other knowns.

I hope that helps and adds some clarity,

Royce

Royce Abel
Active Member
Posts: 7
Registered: ‎05-11-2011

# Re: Whole wall U-Factor

I created a simple calculator that converts heat flux to various thermal conductivity units and it works great with homogeneous materials. However I have found that as my model gets more complex the difference between the interior and exterior heat flux gets larger. I applied a film coefficient boundary condition on the interior and exterior of the wall, and on all the edges I put a heat flux boundary 0.

At first I thought it was meshing, but I still had the issue when I made the mesh as small as the software allows. The problem also goes away if I apply the same material to all layers of the system.

Why am I getting a different heat flux on the interior and exterior of my model?