Effects of Altitude on Convection Coefficient

[TheEggo]

I am trying to develop a spreadsheet to calculate a convection coefficient with altitude as one of the inputs. What variables should I consider? The most obvious are pressure and temperature, but I'm sure there is more to it than that.

 

[Simon Bridge]

temperature depends on altitude, however, that is usually accounted for in another part of the relation that the coefficient is used in isn't it? So you probably don't need to include that.

so I'd nix T and include density.

However - if you work out your model for convective heat flow first: work out what bits depend on altitude (sub in the dependence) then you should see what needs to be included in the coefficient.

 

[TheEggo]

After some digging I've been doing exactly that. Short answer is, it depends on a lot.

Long answer is that it depends on gravity, thermal expansion coefficient, kinematic viscosity, thermal diffusivity, absolute viscosity, and of course temperature and density.

Has anyone done this before? Can I fit curves to these dependencies to automate a spreadsheet? And what is worth considering and what isn't?

 

[Simon Bridge]

This is the Earth we are talking about right?
So a lot of those factors can be determined empirically and a curve fitted - which is pretty much what has been done.

eg. most of your factors affect the air density:
http://en.wikipedia.org/wiki/Density_of_air#Altitude

But you'll maybe want to factor humidity into your convection model due to water's specific heat. Do you want to worry about condensation and evaporation of water as a mechanism?

What you consider and what you don't depends on what you want your model to be able to do: what are you doing this for?

 

[TheEggo]

This is for an unpressurized avionics bay, making sure cables and components don't overheat at 55,000 ft.

 

[Simon Bridge]

18kms huh?
Something like this thing
Oh well - fair enough.
Cooling electronics at high altitude is much talked about, eg.
http://www.electronics-cooling.com/1998/09/cooling-electronics-at-high-altitudes-made-easy/
... and more in-depth available if you prefer.
It looks like you just need to trawl the research.

 

[TheEggo]

From the article:
"High altitude air cooling has always been somewhat of a mystery to the uninformed."

I laughed, in this case I am definitely the uninformed. Good find! I wish there was more on free convection rather than forced, but it's a great start for the properties I care about.

 

[Simon Bridge]

Yah.

Your model for convective cooling depends on what you need it to do - cooling an avionics deck is, in many ways, a simpler problem than just modelling all the possible physics that varies with altitude that could also affect convective cooling.

The article starts with something of a worst-case scenario (the big list of possible properties to be accounted for) and simplifies down. Above, I was going the other way.

Your objective restricts what you need to account for.

There are also research papers and so on all in this field - how far you need to go depends on stuff only you can know.
Have fun.