Thermal conductance

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I'm using Excel to work out the overall u-value for a wall consisting of several layers. To do this the following formula is used:

[tex]
U = {1 \over {{1 \over {\alpha _{outside} }} + {{t_1 } \over {\lambda _1 }} + ... + {{t_n } \over {\lambda _n }} + {1 \over {\alpha _{inside} }}}}
[/tex]

It is the alpha (thermal conductance, W/m2*K) which turns out to be very difficult to determine. Apparently it is a function of many factors like air speed, density, kinematic viscosity etc. If anyone can assist me in finding a way to determine this value for a variety of circumstances, I'd really appreciate it!
 

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  • #2
Astronuc
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In air, it may be a combination of conduction and convection. Most heat transfer coefficients are determined from experiment from which a correlation may be developed or the data tabulated.

I don't believe there is a simple formula.
 
  • #3
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I've encountered values for the thermal resistance in litterature, which are 0,13 m2*K/W for inside, and 0,04 outside. The problem is that I don't know for which circumstances these are calculated. How strong winds, temp, etc. I'm hoping some kind of table excists that gives this value for varying circumstances.
 
  • #4
Astronuc
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These days, one would use computational fluid dynamics (CFD) to do calculations. I imagine inside, there is very little convection and the heat transfer is mostly conduction, whereas outside, especially with wind, convection is dominant, and the [itex]\Delta T[/itex] is greater than inside.

I'll look at my texts to see if there examples or simple formulas.

Russ Watters may some good information since (IIRC) he does heating (cooling?) systems.
 
  • #5
Heat transfer coefficients

You can use published correlations for determining these. It is not very difficult to use the the formulas, but you have to be careful as the correlations are dependent on Reynolds' number or the Rayleigh number and are valid for a limited ranges.

One of the reference books I like is:

Holman, J. P., “Heat Transfer”, 9th Edition, McGraw Hill, New York, 2002. ISBN 0-07-240655-0.

Best wishes.

I'm using Excel to work out the overall u-value for a wall consisting of several layers. To do this the following formula is used:

[tex]
U = {1 \over {{1 \over {\alpha _{outside} }} + {{t_1 } \over {\lambda _1 }} + ... + {{t_n } \over {\lambda _n }} + {1 \over {\alpha _{inside} }}}}
[/tex]

It is the alpha (thermal conductance, W/m2*K) which turns out to be very difficult to determine. Apparently it is a function of many factors like air speed, density, kinematic viscosity etc. If anyone can assist me in finding a way to determine this value for a variety of circumstances, I'd really appreciate it!
 

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