Wind Adjusted Heat Transfer Coefficient
Wind Adjusted Heat Transfer Coefficient
(OP)
I would like a rough estimate of the heat transfer coefficient on the outside of an enclosure exposed to the wind.
In (relatively) static air the coefficient has been approximated to 7W/m^2-K through experiment, with total surface area 0.2m^2.
Would a value of 250W/m^2 be a useful starting point, assuming gale force winds up to 60mph? I am not after an exact value so any information willl be welcomed.
Thanks.





RE: Wind Adjusted Heat Transfer Coefficient
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RE: Wind Adjusted Heat Transfer Coefficient
Extending the external heat transfer coefficient up to around 250 shows that the heat transfer reaches an asymptote well within the suggested upper limit of forced convection. I expect this to occur at a relatively normal wind speed.
The geometry could be represented by a cylinder, with the wind direction perpendicular to the axis of the cylinder.
I expect that there is a modified heat transfer coefficientv equation to match this geometric condition - where is the best place to find this.
I know the equations for flat plates in forced convection and for fully developed flow in circular tubes but do not want to apply these to my geometry at the risk of being too confidentin the results.
If anyone knows where I can find modified equations I would be grateful.
Thanks.
RE: Wind Adjusted Heat Transfer Coefficient
250 W/m^2-K is on the high side. I'm now thinking it's more like 25-50 W/m^2. There are equations for calculating this, but I don't have them at this instant.
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RE: Wind Adjusted Heat Transfer Coefficient
I agree. Calculations based on a air flowing normal to the axis of a cylinder give h as approximately 30W/m^2-K at 70mph.
I estimated about 7 at 0 mph wind speed but in this condition far less heat is required to heat the enclosure. The key is at maximum wind speed.
Thanks for the help - shows I am in the right ball-park.
RE: Wind Adjusted Heat Transfer Coefficient
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RE: Wind Adjusted Heat Transfer Coefficient
RE: Wind Adjusted Heat Transfer Coefficient
Thanks.
I need to keep the internals of the enclosure above 0degC, preferably at 5degC in temperatures as low as -20degC.
Therefore there will be ice formation, however I believe that increasing the coefficent beyond 250W/m-K on the outer surface does not increase the heat transfer as the outer surface thermal resistance is now negligible in comparison to the inner surface thermal resistance.
Is this correct?
RE: Wind Adjusted Heat Transfer Coefficient
That said, -20ºC air temperature doesn't even mean that the surface isn't lower than -20ºC. Evaporative cooling could drive the surface below that temperature.
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RE: Wind Adjusted Heat Transfer Coefficient
I've been following this thread, and now your subsequent thread, with some interest.
You've mentioned that there will be 20° temperature difference, with the outside at -20°C and the inside at 0°C. In your other thread, you also mentioned that the structure is thin-walled aluminum.
I'm wondering how you know that you will always have just those two temperatures, especially given the aluminum material. I can't think of many places that will have a constant -20°C temperature be be subject to varying wind levels.
Would you mind sharing a bit more information on your project?
Patricia Lougheed
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RE: Wind Adjusted Heat Transfer Coefficient
Maybe this could help?
Best regards
Morten
RE: Wind Adjusted Heat Transfer Coefficient
Basically it is a sealed enclosure exposed to ambient temperatures as low as -20°C that has to have in internal ambient temperature guranteed above 0°C.
Without a fan there are large differentials in the internal air temperature distribution but this is pretty much evened out with the use of a fan.
Condensation on the inner surface must be avoided but this is achievable with internal air humidity limited at Rh=14% even in external ambient RH of 100% at -20degC. Limiting the use to ambient RH of 80% will allow a higher internal RH.
I am in the process of determining the heating required to guarantee the internal temperature in ambient air of -20degC.
Snow and ice build up cannot be avoided. Therefore I am assuming worst case is when the outer surface is equal to the air temperature at -20°C. This may not be true when ice/water phase change (and latent heat gain) is taken into account however I feel that adding in a 5°C buffer would overcome this.
With access to CFD I would be able to target the heating at the critical parts but this is currently not an option so have to assume the entire bulk of internal air is to be heated.