[Bldg-sim] exterior convection coefficient for tilted roof

Luis Miguel Candanedo luismiguel_78 at hotmail.com
Mon Feb 13 01:16:34 PST 2012



Hi Leen,

I did my PhD work on Building Integrated Photovoltaic/Thermal systems.

My
 work was mostly focused at the interior convective heat transfer 
coefficients in the cavity/gap between PV modules and the next layer of 
building envelope in the roof where the air flowed. 

However I looked numerically the influence of different wind correlations:



Test et al. (1981), Sharples and Charlesworth (1998), McAdams (McAdams, W.H.,
1954; Duffie, J.A. & Beckman, W.A.,

2006) as a function of the wind speed in m/s:

ho = 8.55
+ 2.56
⋅Vwind 

ho =11.9+2.2⋅Vwind


h=5.7+3.8Vwind 


and also I used a correction factor for the Wind speed at the height of my experimental setup.



 (Hutcheon, Handegord, 1995)


Vwind=Vg(Z/Zg)^alphawind


 In general I found that the Sharples and Charlesworth correlations gave me the best results.

You can look at my thesis and this paper

Candanedo, L. M., Athienitis, A., Candanedo, J., O’Brien, W., Chen, Y., 
(2010). Transient and Steady State Models for Open-Loop Air-Based BIPV/T
 Systems. ASHRAE Transactions. Vol 116. P. 600-612.

I hope it helps,

Luis

Research Assistant
University of Mons
Belgium

From: luismiguel_78 at hotmail.com
To: bldg-sim at lists.onebuilding.org
Subject: RE: Bldg-sim Digest, Vol 51, Issue 10
Date: Mon, 13 Feb 2012 04:09:07 -0500







Hi Leen,

I did my PhD work on Building Integrated Photovoltaic/Thermal systems.

My work was mostly focused at the interior convective heat transfer coefficients in the cavity/gap between PV modules and the next layer of building envelope in the roof where the air flowed. 

However I looked numerically the influence of different wind correlations:




From: bldg-sim-request at lists.onebuilding.org
Subject: Bldg-sim Digest, Vol 51, Issue 10
To: bldg-sim at lists.onebuilding.org
Date: Sun, 12 Feb 2012 13:30:46 -0800

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--Forwarded Message Attachment--
From: jhaberl at tamu.edu
To: l.f.r.peeters at gmail.com; bldg-sim at lists.onebuilding.org
Date: Sat, 11 Feb 2012 21:55:59 +0000
Subject: Re: [Bldg-sim] exterior convection coefficient for tilted roof










Leen,

 

This depends on whether or not the wind is blowing... or not. ASHRAE used to have tabluated values (still in some textbooks) of R = .17 Btu/ft2-hr-F for winter (15 mph wind) and .25 Btu/ft2-hr-F for summer (7 mph wind). Otherwise, for a vertical surface
 it would be .68 for free convection, probably less for a tilted surface, .9 if horizontal heat flow down.


 

Also, radiation can be significant if the surface is facing a cloudless sky. I think Kreider's solar textbook has a equation for sky temperature...which is somewhat complicated by cloud cover, etc.

 

Jeff


 

8=!  8=)  :=)  8=)  ;=)  8=)  8=(  8=)  8=()  8=)  8=|  8=)  :=')  8=)8=?



Jeff S. Haberl, Ph.D.,P.E., FASHRAE..............jhaberl at tamu.edu



Professor............................................................Office Ph: 979-845-6507



Department of Architecture.............................Lab Ph:979-845-6065



Energy Systems Laboratory.............................FAX: 979-862-2457



Texas A&M University.....................................77843-3581



College Station, Texas, USA, 77843..................URL:www.esl.tamu.edu



8=/  8=)  :=)  8=)  ;=)  8=)  8=()  8=)  :=)  8=)  8=!  8=)  8=? 8=)8=0





From: bldg-sim-bounces at lists.onebuilding.org [bldg-sim-bounces at lists.onebuilding.org] on behalf of leen peeters [l.f.r.peeters at gmail.com]

Sent: Saturday, February 11, 2012 1:59 PM

To: bldg-sim at lists.onebuilding.org

Subject: [Bldg-sim] exterior convection coefficient for tilted roof






Dear,
 
can anyone point me to some reference values for exterior convection coefficients for tilted surfaces like a roof?
 
Thanks
 
Leen







--Forwarded Message Attachment--
From: phcjam at yahoo.com
To: bldg-sim at lists.onebuilding.org
Date: Sat, 11 Feb 2012 14:40:27 -0800
Subject: [Bldg-sim] Fw:  exterior convection coefficient for tilted roof

I've only seen 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.
 Paul Hay MBA, BA(Arch)
Managing Partner
PAUL HAY Capital Projects

Caribbean Capital Project Management & Planning



P.O.Box 3367
Constant Springs, Kgn. 8
Jamaica, W.I.

tel: 1 (876) 756-0631cel: 1 (876) 324-4274fax: 1 (876)
 756-0631  e-mail: paul.hay at phcjam.comskype name: phcjamtwitter: http://www.twitter.com/phcjamweb: http://www.phcjam.comprofile: http://www.linkedin.com/company/paul-hay-capital-projects
  


      ----- Forwarded Message -----
  From: leen peeters <l.f.r.peeters at gmail.com>
 To: bldg-sim at lists.onebuilding.org 
 Sent: Saturday, February 11, 2012 2:59 PM
 Subject: [Bldg-sim] exterior convection coefficient for tilted roof
   

Dear,
 
can anyone point me to some reference values for exterior convection coefficients for tilted surfaces like a roof?
 
Thanks
 
Leen

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--Forwarded Message Attachment--
From: chris.malcolm.yates at gmail.com
CC: bldg-sim at lists.onebuilding.org
To: phcjam at yahoo.com
Date: Sat, 11 Feb 2012 22:45:16 +0000
Subject: Re: [Bldg-sim] Fw: exterior convection coefficient for tilted roof

It's true wind does dominate external convection.

Cibse guide A will give you all the numbers that you're looking for 

Chris

On 11 Feb 2012 22:40, "Paul Hay" <phcjam at yahoo.com> wrote:

I've only seen 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.

 Paul Hay MBA, BA(Arch)
Managing Partner
PAUL HAY Capital Projects

Caribbean Capital Project Management & Planning



P.O.Box 3367

Constant Springs, Kgn. 8
Jamaica, W.I.

tel: 1 (876) 756-0631
cel: 1 (876) 324-4274fax: 1 (876)
 756-0631  e-mail: paul.hay at phcjam.com
skype name: phcjamtwitter: http://www.twitter.com/phcjam
web: http://www.phcjam.comprofile: http://www.linkedin.com/company/paul-hay-capital-projects

  


   
   ----- Forwarded Message -----
  From: leen peeters <l.f.r.peeters at gmail.com>

 To: bldg-sim at lists.onebuilding.org 
 Sent: Saturday, February 11, 2012 2:59 PM

 Subject: [Bldg-sim] exterior convection coefficient for tilted roof
   

Dear,
 
can anyone point me to some reference values for exterior convection coefficients for tilted surfaces like a roof?
 
Thanks
 
Leen

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--Forwarded Message Attachment--
From: l.f.r.peeters at gmail.com
CC: bldg-sim at lists.onebuilding.org
To: chris.malcolm.yates at gmail.com
Date: Sat, 11 Feb 2012 23:51:33 +0100
Subject: Re: [Bldg-sim] Fw: exterior convection coefficient for tilted roof

Hi Chris,
 
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 ...

 
Am I correct on this?
 
Leen



On Sat, Feb 11, 2012 at 11:45 PM, Chris Yates <chris.malcolm.yates at gmail.com> wrote:


It's true wind does dominate external convection.

Cibse guide A will give you all the numbers that you're looking for 

Chris



On 11 Feb 2012 22:40, "Paul Hay" <phcjam at yahoo.com> wrote:




I've only seen 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.


 
Paul Hay MBA, BA(Arch)
Managing Partner
PAUL HAY Capital Projects

Caribbean Capital Project Management & Planning



P.O.Box 3367
Constant Springs, Kgn. 8

Jamaica, W.I.


tel: 1 (876) 756-0631
cel: 1 (876) 324-4274
fax: 1 (876) 756-0631
 
e-mail: paul.hay at phcjam.com
skype name: phcjam
twitter: http://www.twitter.com/phcjam
web: http://www.phcjam.com
profile: http://www.linkedin.com/company/paul-hay-capital-projects



  





----- Forwarded Message -----
From: leen peeters <l.f.r.peeters at gmail.com>

To: bldg-sim at lists.onebuilding.org 
Sent: Saturday, February 11, 2012 2:59 PM

Subject: [Bldg-sim] exterior convection coefficient for tilted roof



Dear,
 
can anyone point me to some reference values for exterior convection coefficients for tilted surfaces like a roof?
 
Thanks
 
Leen
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--Forwarded Message Attachment--
From: chris.malcolm.yates at gmail.com
To: l.f.r.peeters at gmail.com; bldg-sim at lists.onebuilding.org
Date: Sun, 12 Feb 2012 12:54:41 +0000
Subject: Re: [Bldg-sim] Fw: exterior convection coefficient for tilted roof



  


    
  
  
    Hi Leen,

    

    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?

    

    Regards

    

    Chris

    

    On 11/02/2012 22:51, leen peeters wrote:
    
      Hi Chris,
       
      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 ...
       
      Am I correct on this?
       
      Leen

        

        

      
      On Sat, Feb 11, 2012 at 11:45 PM, Chris
        Yates <chris.malcolm.yates at gmail.com>
        wrote:

        
          It's true wind does dominate external convection.

          Cibse guide A will give you all the numbers that you're
            looking for 

          
              Chris

            
          
            
              On 11 Feb 2012 22:40, "Paul Hay"
                <phcjam at yahoo.com>
                wrote:

                
                  
                    
                      I've only seen 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.

                      
                       
                      Paul Hay MBA, BA(Arch)

                        Managing Partner

                        PAUL HAY Capital Projects

                        

                        Caribbean
                          Capital Project Management & Planning

                        

                        

                        

                        P.O.Box 3367

                        Constant Springs, Kgn. 8

                        Jamaica, W.I.

                        

                      
                      tel: 1
                              (876) 756-0631
                      cel: 1
                              (876) 324-4274
                      fax: 1 (876) 756-0631
                       
                      e-mail: paul.hay at phcjam.com
                      skype name: phcjam
                      twitter: http://www.twitter.com/phcjam
                      web: http://www.phcjam.com
                      profile: http://www.linkedin.com/company/paul-hay-capital-projects
                      

                          
                        

                      
                      

                      

                      
                        
                          -----
                              Forwarded Message -----

                              From:
                              leen peeters <l.f.r.peeters at gmail.com>

                              To:
                              bldg-sim at lists.onebuilding.org
                              

                              Sent:
                              Saturday, February 11, 2012 2:59 PM

                              Subject:
                              [Bldg-sim] exterior convection coefficient
                              for tilted roof

                            
                          

                          
                            Dear,
                             
                            can anyone point me to some reference
                              values for exterior convection
                              coefficients for tilted surfaces like a
                              roof?
                             
                            Thanks
                             
                            Leen
                          
                          

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    -- 

      
        Chris Yates C Eng
            MCIBSE

        Building Physics Consultant

        Tel:   +447960731576

        Email: chris.malcolm.yates at gmail.com

        Skype: christopher.m.yates

      
     		 	   		   		 	   		  
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