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Hi Leen,<br>
<br>
I'm not familiar with the calculation involving the Grasshof number.
The formula given is hc= 4+4v, where hc is the convection
coefficient and v is the wind velocity. The formula is probably
derived empirically (don't know for certain). Are you definitely
talking about the convection element of heat transfer or are you
talking lw radiation?<br>
<br>
Regards<br>
<br>
Chris<br>
<br>
On 11/02/2012 22:51, leen peeters wrote:
<blockquote
cite="mid:CAB_-mWNjH9oZRa7-BD+bs0XwLpG0FFd7Z=La_XJGV2iKNE3Vzg@mail.gmail.com"
type="cite">
<div>Hi Chris,</div>
<div> </div>
<div>CIBSE suggest to replace g with gcos(theta) in the Grasshof
number. That means, it accounts for the roof being tilted in
natural convection. When it comes to roofs, there is another
aspects that starts to be important ... the roof finishing in
tiles will have a 'detoriated' velocity underneath the tiles. It
is mainly that one directing the convective losses ...</div>
<div> </div>
<div>Am I correct on this?</div>
<div> </div>
<div>Leen<br>
<br>
<br>
</div>
<div class="gmail_quote">On Sat, Feb 11, 2012 at 11:45 PM, Chris
Yates <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:chris.malcolm.yates@gmail.com">chris.malcolm.yates@gmail.com</a>></span>
wrote:<br>
<blockquote style="border-left: 1px solid rgb(204, 204, 204);
margin: 0px 0px 0px 0.8ex; padding-left: 1ex;"
class="gmail_quote">
<p>It's true wind does dominate external convection.</p>
<p>Cibse guide A will give you all the numbers that you're
looking for </p>
<span class="HOEnZb"><font color="#888888">
<p>Chris</p>
</font></span>
<div class="HOEnZb">
<div class="h5">
<div class="gmail_quote">On 11 Feb 2012 22:40, "Paul Hay"
<<a moz-do-not-send="true"
href="mailto:phcjam@yahoo.com" target="_blank">phcjam@yahoo.com</a>>
wrote:<br type="attribution">
<blockquote style="border-left: 1px solid rgb(204, 204,
204); margin: 0px 0px 0px 0.8ex; padding-left: 1ex;"
class="gmail_quote">
<div>
<div style="font-family: times new roman,new
york,times,serif; font-size: 12pt;">
<div><span>I've only seen </span>reference values
of interior convection coefficients for tilted
surfaces, not exterior coefficients: which are
available for summer & winter conditions,
due to the difference in wind velocity.<br>
</div>
<div> </div>
<div>Paul Hay MBA, BA(Arch)<br>
Managing Partner<br>
PAUL HAY Capital Projects<br>
<br>
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Capital Project Management & Planning</span><br>
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<div> <br>
</div>
<br>
<br>
<div style="font-family: times new roman,new
york,times,serif; font-size: 12pt;">
<div style="font-family: times new roman,new
york,times,serif; font-size: 12pt;">
<div dir="ltr"><font face="Arial">-----
Forwarded Message -----<br>
<b><span style="font-weight: bold;">From:</span></b>
leen peeters <<a moz-do-not-send="true"
href="mailto:l.f.r.peeters@gmail.com"
target="_blank">l.f.r.peeters@gmail.com</a>><br>
<b><span style="font-weight: bold;">To:</span></b>
<a moz-do-not-send="true"
href="mailto:bldg-sim@lists.onebuilding.org"
target="_blank">bldg-sim@lists.onebuilding.org</a>
<br>
<b><span style="font-weight: bold;">Sent:</span></b>
Saturday, February 11, 2012 2:59 PM<br>
<b><span style="font-weight: bold;">Subject:</span></b>
[Bldg-sim] exterior convection coefficient
for tilted roof<br>
</font></div>
<br>
<div>
<div>Dear,</div>
<div> </div>
<div>can anyone point me to some reference
values for exterior convection
coefficients for tilted surfaces like a
roof?</div>
<div> </div>
<div>Thanks</div>
<div> </div>
<div>Leen</div>
</div>
<br>
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<div class="moz-signature">-- <br>
<div>
<p>Chris Yates <span style="color: rgb(79, 98, 40);">C Eng
MCIBSE</span></p>
<p><i style="">Building Physics Consultant<o:p></o:p></i></p>
<p>Tel:<span style=""> </span>+447960731576</p>
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