convective heat transfer coeficient of air

[Plasmech]

What is a good number to use for the convective heat transfer coeficient of air? I'm assuming *relatively* "stagnant" air...although I guess if the air really was 100% stagnant it wouldn't convect at all...

Plastics Industry

 

[prex]

For nearly ambient conditions, take 10 W/m²/°C, the double for moving air (windy, though not in a hurricane).

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

: Air bearing pads

 

[Plasmech]

So if I have an insulated vessel, say 4" of fiberglass in a double wall arrangement, I can I assume that between the outside surface of the *inner* wall and the fiberglass that convection is pretty much shut down (because of the fiberglass), but that between the outer skin and the ambient air, I'll have convection, that will be dependant upon the temperature of said outer skin and the ambient temperature...?



Plastics Industry

 

[IRstuff]

That would be the basic assumption


TTFN

FAQ731-376

 

[25362]

Holman's Heat Transfer (McGraw-Hill) 1976, pp 263, gives for air natural convection on vertical plain or cylindrical surfaces, at atmospheric pressure, the following estimates for the convection coefficient h:

laminar : h = 1.42([Δ]T/L)^0.25
turbulent: h = 0.95([Δ]T)^0.33

h, W/m.K
[Δ]T, K
L, vertical dimension, m

Turbulence defined when Gr.Pr > 10⁹

 

[IRstuff]

Depends quite a bit on geometry, temperature difference:

http://www.frigprim.com/online/natSinglePlate.html

For temperature differences of less than 5ºC, the htc is less than 2.5 W/m^2-K


TTFN

FAQ731-376