Re: [TRNSYS-users] About Convective Heat Transfer Coefficient of Wall

[David Bradley <bradley@tess-inc.com>]

Roberto,
 In addition to the conduction through the wall, there is also 
convection and radiation from the wall surface to the ambient. Changing 
the convective heat transfer coefficient only directly changes the 
convection part of the equation. Of course changing the convection will 
change the surface temperature which will in turn change the radiation 
part as well but that is an indirect effect. When you change the 
convective heat transfer coefficient you will NOT notice a change in the 
displayed wall u-value in the TRNBuild wall manager. This is because the 
convective heat transfer coefficient is dynamic (changes throughout the 
simulation) and the displayed value is static - so it assumes a combined 
radiation/convection coefficient for both the inside and outside wall 
surfaces. The only thing that you can change in TRNSYS 16 concerning the 
radiative part is the wall's absorptance to solar radiation. You cannot 
change its emissivity, which is set to 0.9 internally.

One algorithm for external convection is proposed in ASHRAE Standard 140:

(a0+a1*WindSpeed-a2*WindSpeed**2) [W/m2.K]

a0, a1, and a2 depend on the building material. For concrete, they are 
a0=8.23, a1=4.00 and a2=0.057

Kind regards,
David


caria.roberto@tiscali.it wrote:
> dear users
> I have any doubts about Convective Heat Transfer Coefficient of 
> Wall: 
> 1) if I change it does thermal trasmittance change? On the "Wall Type 
> Manager" below on value of wall trasmittance there are two standard 
> values: those will be change with a new value of Convective Heat 
> Transfer Coefficient of Wall?
> If I wanted to insert a custom value, does anyone know any formula to 
> calculate the coefficient as a function of the difference temperature 
> and wind speed? Have I also consider the radiative part?    
> thanks in advance
> Roberto
>   
>
>
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