Heat transfer at altitude

[JimLad]

Without going into to much detail, I am doing some ground work on how to heat the surface area of an elevated platform. I have scoured all my heat transfer texts (not many as a mech.production engineer) and can only find equations relating to pipelines,fluid flow etc... and not a static flat structure.

Can anyone suggest a suitable text or correct formulae. Obviously there are many factors which will come into play later (wind speed,platform material and construction etc..) but at this stage I am more interested in getting the theory right.

All help greatly appreciated.

 

[quark]

why? Natural convection over falt plate is the first one which is discussed in every book.

http://www.egr.msu.edu/~somerton/Nusselt/ii/ii_a/ii_a_1.html

Follow this link and you will get plenty of Nusselt expressions for various conditions.

Regards,

 

[JimLad]

Thanks for the quick response quark !

Unfortunateley , the link given just leads to dead pages. I understand you thinking this is a simple problem but maybe I should expand slightly.

The platform I am looking at is a large surface area and will be exposed to extremeley low ambient temperatures and high (but irregular) wind chill factors. Therefore the heat losses are going to be massive, fluctuating and unevenly distributed. I don't want to use basic theory and then find that I should have used more sophisticated formulae. I was hoping that there may be some theory to cover this type of arrangement. I shall battle on bravely !

Regards.

 

[MASSEY]

I agree with quark, fluids, thermal, and heat transfer classes had these types of problems addressed. I do see your point though that real world applications don't always allow for the summary assumptions of classroom theory soooo it comes to small workable models, PI varibles, fans, and freezers. Have fun.

 

[poetix99]

Temporal variations are different from spatial variations. Without knowing how large is your "large surface", it is not clear whether the "wind chill factors" would need to be spatially discretized.

Perhaps you will need to "solve" your large surface with a Finite Element Analysis. This is especially so if you are interested to know the thermally induced stresses in your structure.

But, you should understand that the FEA tools use the same basic correlations of convective h.t. as QUARK has pointed you to. There is nothing inherently primitive or "unsophisticated" about such h.t. correlations. You can calculate local convective coefficients, or an averaged coefficient for a surface. Many effects may be arithmetically summed.

 

[Tunalover]

You should do a simple ROM calculation to gage the amount of heat ejected by radiation. Of course, forced convection with radiation is the worst case assumption. At what altitude is the platform installed? If it's installed in Colorado, then the thinner air will help heating. Air density at altitude can make a big difference. Your worst case is at sea level for heating. I recommend David S. Steinberg's "Cooling of Electronic Equipment." He discusses radiation and air density at altitude. I don't know if he discusses forced convection due to wind; he doesn't include much about heating in his book.

Did you try ASHRAE 2001 (or earlier) Fundamentals? Most college libraries will have it (and Steinberg's books) if they have an engineering school.

Bruce

 

[JimLad]

Thanks for all your interest and advice.

Project has now taken a significant step forward.

Thanks again !