Internal Temperature of container

[Talos]

Hi All,

I have a box structure measuring 3000mm x 2500mm x 2100mm. This structure has 60mm walls, floor and roof with a thermal coefficient of 0.39. - 0.49 Wm/K

If I said starting point was with an internal temperture of 18 degrees centigrade and the outside was a constant -49 degrees centigrade, how easy would it be to plot the change in internal temperature against time?

I guess you could model something like this in cosmosworks but is there a formula that would work just as well if you ignored things like solar radiation

D

 

[IRstuff]

Is the box empty? full of water? full of lead?

TTFN

FAQ731-376

 

[MintJulep]

Fundamentally, for an empty box, with homogeneous wall construction and the same boundary conditions on all surfaces, it's an easy problem. Simple energy balance.

Gets more complicated if the box is partially filled with stuff, and the wall construction is non-uniform, and the floor is sitting on the ground, and if the wind is blowing...

How accurate do you need to be?

 

[dvd]

Since you are assuming constant surface temperature and no radiation, assume a very small Biot number and assume that lumped capacitance will provide a good approximation.

 

[Talos]

Thanks guys, be gentle I am a complete novice here with thermodynamics

The box is filled with air only.dvd, can you explain a little. Biot number and lumped capacitance do not mean a lot to me (being mechanical!!)


D

 

[Talos]

mintjulep,

I'd be happy with a start point, which this and worry about equipment within, wind, solar load etc later

D

 

[MintJulep]

Like I wrote earlier, fundamentally it is an energy balance problem.

At your initial condition there is some amount of energy contained in the air in the enclosure, and some amount of energy contained in the structure of the enclosure.

Over any infinitesimally small time, some energy leaves the air and enters the structure, and some energy leaves the structure to the ambient.

From there you can either solve by brute force or calculus.

 

[dvd]

Without any background in heat transfer, which is actually part of mechanical engineering, I don't think that you will understand much about how to use equations to solve your problem. If you search on terms like Biot number or lumped capacitance or transient heat transfer you will find plenty of information. You might want to buy a fundamentals of heat transfer book.

I think you might get more assistance if you explain what your setup looks like, and why you have set a constant temperature boundary condition. If this is a building that is located outdoors then it might be important to assume some wind.

 

[IRstuff]

There is a downloadable HT textbook:

http://web.mit.edu/lienhard/

http://www/ahtt.html

TTFN

FAQ731-376