[TRNSYS-users] Variable global thermal conductivity

David BRADLEY d.bradley at tess-inc.com
Mon Nov 7 07:41:17 PST 2011


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
>>>
>>> _______________________________________________
>>> TRNSYS-users mailing list
>>> TRNSYS-users at cae.wisc.edu
>>> https://mailman.cae.wisc.edu/listinfo/trnsys-users
>>>
>> --
>> ***************************
>> 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 at 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 at tess-inc.com

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




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