RE: Ground temperature

[Michaël Kummert <kummert@xxxxxxxxxxxxx>]

Dear Helmi,

If the floor of your building is in direct contact with the ground, you
should use a "boundary" condition. The ground temperature can be
approximated in different ways, with an increasing level of complexity: 
- use a constant value
- use a typical profile Tground=f(time,depth) (e.g. Kusuda equation)
- Use a finite difference model (e.g. Lund-DST model)

The wall can also include a fictitious layer of ground (although you will
quickly run into the transfer function limitation for very thick walls, in
which case you can use a fictitious zone with an infinitely small
volume...). 
The "convection" heat transfer coefficient should be infinite since there is
direct contact between the ground and the wall. For historical reasons, the
way to specify a very high convection coefficient for "boundary" walls in
Prebid / Type56 is to use a very small value. It is somewhat confusing but
it is clearly mentioned when you set that coefficient in Prebid.

I hope this helps,

Michaël Kummert

_________________________________________________________

Michaël Kummert

Solar Energy Laboratory - University of Wisconsin-Madison
1303 Engr Res Bldg, 1500 Engineering Drive
Madison, WI 53706

Tel: +1 (608) 263-1589
Fax: +1 (608) 262-8464
E-mail: kummert@engr.wisc.edu

SEL Web Site: http://sel.me.wisc.edu 
TRNSYS Web Site: http://sel.me.wisc.edu/trnsys


-----Original Message-----
From: owner-trnsys@relay.doit.wisc.edu
[mailto:owner-trnsys@relay.doit.wisc.edu] On Behalf Of helmi
Sent: Sunday, June 01, 2003 08:08
To: trnsys group
Subject: Ground temperature


Dear Trnsys users
What is the value we must take for the external convection coefficient of
the floor, while we are describing a building in the Prebid?
And how to model the influence of the ground temperature?
Note that in the Trnsys examples, the value of Hback was very low, and the
ground temperature was fixed.
Thank you in advance.
Best regards.


Ing. Helmi Ben Rejeb
Ecole Nationale d'Ingénieurs de Tunis, BP 37, Le belvédère, 1002 Tunis,
Tunisia.




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