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Re: [TRNSYS-users] Variable global thermal conductivity



Denis,
Okay, that helps me understand a bit better. The simple answer is that no, TNRSYS does not have a way of dynamically adjusting the thermal properties of the wall materials as a function of windspeed or anything else. I think it is worth keeping in mind that building energy simulation is very idealized. We assume that the thermal properties of the wall layers are completely uniform and that they do not change as they get near to a wall stud. We often ignore the thermal bridging that occurs in wall studs. We assume that all of the rigid foam insulation in the building has exactly the same thermal properties. We ignore the inaccuracy and non-uniformity of the installation procedures. I would guess that the change in thermal properties associated with air movement would be "in the noise" of the simulation's own uncertainty and that you might be better off looking at this problem on a micro-scale using CFD and experimentation of small wall samples in a test cell.

That said, if you have to look at this in the context of the whole building, I think the best way of estimating the effect would be to control the infiltration into the zone as a function of windspeed, ideally with the benefit of Type97/CONTAM, TRNFlow, or COMIS.
Kind regards,
 David

On 11/4/2011 07:45, Denis GARASA wrote:
Thank you David for the proposition but do you think the effect on the
internal
convective heat transfer coefficient will lead to same effect than the wind
infiltration in the insulation ?
This last effect leads to a decrease of the insulation performance of the
mineral
wool. We would like to quantify the impact on the energy consumption
taking this
effect into account or not.
Your proposition will imply to translate de global U of the roof as a
function of
the wind velocity and incidence into a modification of the external and
internal
convective heat transfer coefficient.
Is there only your proposition as a solution (except creating a new type
and link it
the Type 56 as the roof façade) ?
Best regards,
Denis


Denis,
Type56 allows you to set the outside convection coefficient of a surface
(walls, ceilings, etc.) as a constant value, as a scheduled value, or as
an input to Type56. If you select "input" then you can choose whatever
algorithm you want for the external convection from your roof. The
physical properties of the material in the roof (density, thermal
conductivity, and specific heat) are then constant but the convection on
the outside (and inside if you wish) surfaces vary with external
conditions.
Kind regards,
David


On 11/3/2011 10:47, Denis GARASA wrote:
Hello,

I'm a student in an engineering school.

I study the influence of the wind on a roof and I would like to know if
it
is possible to determine a variable global thermal conductivity using a
Type56 building?

Thanks in advance,
Denis GARASA

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--
***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI  53703 USA

P:+1.608.274.2577
F:+1.608.278.1475
d.bradley@tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com





--
***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI  53703 USA

P:+1.608.274.2577
F:+1.608.278.1475
d.bradley@tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com