Wind Adjusted Heat Transfer Coefficient

for 60 mph winds, that does not sound unreasonable. what is the minimum air speed?

TTFN

FAQ731-376

IRstuff - there is not a minimum air speed as this is more concerned with heating the inside of the enclosure.

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.

No minimum speed? You're not going to get 250W/m^2-K at 0 mph wind speed, it'll be more like 4 W/m^2-K. I think I overspoke with my first answer.

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.

TTFN

FAQ731-376

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.

Oh, would have helped if you had mentioned that you're trying to keep the enclosure warm. In that case, wind alone is not sufficient to determine the required heat load. What's the temperature of the wind? Is there going to be ice formation, etc.?

TTFN

FAQ731-376

Rain and ice/snow, depending on your climate, are going to pull insane amounts of heat away from the enclosure unlike any hurricane-force wind. Depending on how hot you need it, boiling can be approximated between 10 and 100 kW m^-2 K^-1.

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?

That might be, but if those were the constraints, then the problem's worst case would have always been forcing the outer surface at -20ºC, regardless of whether wind was present or not. Seems like the wind issue is a bit of a red herring.

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.

TTFN

FAQ731-376

EngAP

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

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

Often you will se weatehr reports referring to "wind chill facotr". The definition of these should be avaiable i will guess. As far as i know they refer to heat loss from your body at a given wind speed and temperature referred to a "standard" temperature" at no wind.

Maybe this could help?

Best regards

Morten

vpl - Thanks for the interest.

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.