Calculating temperature rise 1

[MARKH1958]

Any assistance will be greatly appreciated!
How do I calculate the following?
If I apply a thermal load of 70 watts to 110 cubic inches of aluminum alloy, what is the approx temperature rise above ambient in degrees C.

 

[IRstuff]

specific heat?
time?

Is this for a class?

TTFN

 

[MARKH1958]

I have an aluminum enclosure 22" x 17" x 6" (approx).
I am dissipating 70 watts continuously within this enclosure.
What would be the temperature of the outer skin of this enclosure above the ambient air temperature of the surrounding environment?
The approx volume of the enclosure material (aluminum) is 110 cubic inches.
This is a real world problem that I’m looking for assistance on, not a class problem.
Any help would be appreciated.

 

[IRstuff]

See, right off the bat, you've changed the problem. 110 ci of aluminum is different that 22*17*6 aluminum enclosure

What is the ambient air temperature and air flow? Are there hotspots? What's it resting on? Will heat be removed through the bottom? Which dimensions correspond to W*H*D?

TTFN

 

[MARKH1958]

Ambient air temperature is 27°C.
No hot spots, uniform heat distribution.
Not resting on any surface, I would like to discount the small mounting interface from the equation.
Heat removed by natural convection.
17W*22D*6H.

 

[MARKH1958]

And all surfaces to be exposed to natural convection.

 

[ajamnia]

If you have aluminum is anodized and you take radiation into account (no solar load), your temperature rise should be about 12 deg C. If on the other hand your aluminum is nor anodized, then, your temp rise can be as high as 23 degrees about atmosphere.

I did the calculation based on a simple heat balance and a coef. of heat transfer of 1. Numbers vary slightly if you tweak h or emmissivity slightly. Also, the assumption is that the heat load is uniformly distributed on the interior surfaces.

 

[IRstuff]

A.top:17in*26in=0.2413*m^2 h.ctop:4W/m^2/ºC
A.side:2*6*in*(17in+26in)=0.3019*m^2 h.cside:2.5W/m^2/ºC
ignore bottom

input:70W deltaT=40.6978ºC

This would put the surface temperature at 67ºC, which is hot enough to be a serious skin burn hazard.

Radiative power is only about 14mW at this temperature.

TTFN

 

[ko99]

IRstuff, I agree with your natural convection estimate, but shouldn't the radiation effect be much greater?

ko (

http://www.ecooling.biz)

 

[IRstuff]

Yeah, sorry not enough T^4's.

The temp is only 39ºC if the box is perfectly emissive. A 70% emissive box raise the temperature to 59ºC

TTFN

 

[IRstuff]

rats, should be 43ºC with 59 W radiated
39ºC with 47 W radiated

TTFN

 

[MARKH1958]

Thanks to all who responded!

 

[ajamnia]

Oops!
I stand corrected! These are my corrected values:
1) e=.05, Delta T=36.8 deg C, Qrad=4.8, Qconv=65.2
2) e=.95, Delta T=18.6 deg C, Qrad=42.2, Qconv=27.8