[bldg-sim] Apache uni-dimensional heat transfer assumption

[Graham & Megan]

You may find LBNL's Therm program to be of some use to understand how
U-Values may change under the influence of 2D flow compared to 1D.  Last
time I used Therm it was steady state thus no ability to account for the
transient dynamics through a thermally massive wall.  Nevertheless you may
find it useful to understand how an extended external surface area may
impact the internal heat transfer and thus effective U-Value - you could
work out your modified extrnal heat transfer coefficient from this.

With regard to some other comments made in this thread, while I cannot speak
definitively, it is my understanding that TAS and Apache handle
inter-surface radiant heat transfer very similarly and that true view
factors are not calculated.  Not having the technical manuals in front of me
I cannot comment further.  The approximations are good for square boxes and
increasingly poor with more complex and non-symmetric geometries.  If my
memory serves me correctly Energy+ also doesn't attempt to calculate view
factors precisely and also uses an approximation.

It would be worth someone clarifying this to avoid mis-information.

Regards,
Graham
----- Original Message ----- 
From: "Yates, Christopher" <christopher.yates at halcrowyolles.com>
To: <bldg-sim at gard.com>
Sent: Saturday, September 17, 2005 12:06 AM
Subject: [bldg-sim] Apache uni-dimensional heat transfer assumption


ASPAM=antispamwpk42

Thanks Gunter,

Are you aware of any guidance for adjusting this external heat transfer
coefficient (I assume you mean surface emissivity) to account for such
things as thermal bridging at corners?

I'll take the comments regarding Apache's thermal radiation model on board -
thank you very much.

Regards, Chris

-----Original Message-----
From: Pueltz, Gunter [mailto:GPueltz at MuellerBBM.de]
Sent: 16 September 2005 14:52
To: 'christopher.yates at halcrowyolles.com'; bldg-sim at gard.com
Subject: RE: [bldg-sim] Apache uni-dimensional heat transfer assumption

Dear Chris,

the error caused by the 1-D heat conduction of very thick walls can be
minimized by an adjusted (higher) external heat transfer coefficient.

But be aware of following fact:
The main problem with Apache is that the thermal radiation model is a simple
"mean-radient-model" (like TRNSYS), which is not very accurate, and not a
more sophisticated model based upon view-factors (like ESP-r, Eplus, TAS,
etc.).
Thus the errors induced by the minor radiation model will exceed the errors
caused by restricted 1D heat conduction extensively .......

Regards,
Gunter

--------------------------------------------
Dr. Gunter Pültz
Müller-BBM GmbH
Robert-Koch-Straße 11
82152 Planegg
Telefon +49 (0)89 85602 - 336
Telefax +49 (0)89 85602 - 111
http://www.MuellerBBM.de


> -----Original Message-----
> From: bldg-sim at gard.com [mailto:bldg-sim at gard.com]On Behalf Of Yates,
> Christopher
> Sent: Friday, September 16, 2005 3:35 PM
> To: bldg-sim at gard.com
> Subject: [bldg-sim] Apache uni-dimensional heat transfer assumption
>
>
> Dear All,
>
> The thermal analysis package Apache (from IES) is a multi zone model
> type analysis package, utilising finite difference methods to model
> building performance against weather data. One of the assumptions that
> it makes is that heat transfer only occurs between adjacent spaces in
> a direction normal to the adjacent surfaces. I believe that this is a
> common assumption for most building sim software (e.g. ESP-r).
>
> The assumption holds true for traditional building types with
> constructions that are relatively thin when compared to the overall
> building volume.
>
> However, we are analysing a building with wall constructions that are
> quite thick when compared to the overall volume. Common sense tells me
> that there will be a substantial 3 dimensional component to heat
> transfer.
>
> Does anybody have any experience of modelling the thermal response of
> extremely heavyweight buildings that could give me some advice on the
> limitations of this type of software (it's a nuclear waste storage
> facility)?
>
> It may be possible that this is the preserve of high end CFD,
> generating large amounts of transient data with the help of super
> computing facilities.
> However, it is difficult to know this because of the apparent lack of
> work done on modelling such facilities.
>
> Facts/ opinions???!
>
> Thank you in advance
>
> Chris Yates
>
>
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The contents of this email are confidential to
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