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Effect Of Wind Speed On Heat Loss From A Heated Building

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Posted by AC Me on December 1, 2008, 2:44 pm
 
Hi All.

Back again with another query to drive you all daft.

I can easily work out the heat loss through a building fabric if I
know the internal temperature, the external temperature and the
overall thermal resistance of the fabric and if the external air is
static.

Does anyone know how to account for wind velocity?

I know this could become very complicated - Is the flow laminar or
turbulent? What effect does wind direction have (as most buildings are
rectangular in shape)? And so on. But there might be some mechanism to
approximate the effect. And anything that could make some sort of
allowance for this would have to be more accurate than ignoring the
effect altogether.

Take care.

Mike

Posted by Morris Dovey on December 1, 2008, 3:01 pm
 
AC Me wrote:


You might try a web search for "wind chill" effect...

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/

Posted by clare on December 2, 2008, 2:29 am
 wrote:


  Wind chill only applies to warm damp "objects" like bodies

Posted by Tim Jackson on December 1, 2008, 7:35 pm
 AC Me wrote:

Well you can have my theoretical view, which is untested.

In dry conditions, this is simply forced convection.  I believe that the
convection effect accounts for about 0.1 C.m/W of wall's R-value. I
know this bit is true (and have measured it) for heat flows around 10-20
w/m through a horizontal surface in the lab.  For higher heat flows it
reduces.  For vertical surfaces I would also expect it to be less.  The
effect of forced convection can only be to reduce this.  So at worst any
amount of wind can only reduce your insulance by 0.1 C.m/W.

For wet surfaces the wind-chill effect which depends on among other
things wind speed and relative humidity, will lower the outer surface
temperature, and so will produce increased losses in proportion to the
increased temperature drop from inside to outside.  So if for example we
have 20C inside and 5C outside then we have 15 drop, but if there is
a 2 wind-chill then your will have 17 drop which will increase the
losses by 13%.

For detached houses on level ground I doubt that wind direction and
turbulence have much effect, but for terraced (row) blocks and well
sheltered houses it might make a little difference.

Not that the dry effect is an subtractive change in insulance, therefore
becomes a smaller proportion if the insulation is better, whereas the
wet effect is a simple percentage of your losses (although at least it
will dry quicker if the losses are worse).

Of course for most older buildings, the biggest effect of wind comes in
the form of draughts. which are hard to predict even if you know the
size of orifices, as it depends critically of the positioning of the
orifices, the wind direction and any labyrinth effect e.g. due to closed
internal doors.

Tim

Posted by AC Me on December 1, 2008, 8:43 pm
 
Hi Tim.

I see where you're coming from, but I hadn't thought of it this way
before.

Do you mean wind chill as we normally refer to it? Isn't this the
effect humans feel when exposed to a cooling wind? That is,
isn't this predicated on the cooling effect on a body at a
temperature of 36 degrees Celcius? I don't think we can use
the standard wind chill measuring technique when attempting
to find a wind chill effect on a structure that has an internal temp.
of, say, 20 degrees Celcius. Or am I wrong?

Take care.

Mike

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