Need help Surface Temperature Increase due to Sun Loading 1

[bamawres]

Assuming worst case; the military 99-percentile, the standard value is 1120 W/m^2. Surface is painted white so, the radiation absorpity is 0.14. What is the temperature increase on the surface? Assuming the object is at ambient temperature.

 

[IRstuff]

It depends. You have a convective component and a radiative component. You have to assume values for:

> Air temp
> Wind speed
> Radiative background temperature
> emissivity

You can pretty much get anything from 82°C up to 200°C, depending on your assumptions

TTFN

FAQ731-376

 

[bamawres]

Surface area of Box is 30 x 8 x 8ft, contains helium.

Assume worst case, so no wind and direct sunlight-no shade.

Box is stored outside at 110F, relative humidity 70%-worst case.

Use stainless-301, assume 0.5 emissivity.

I need the formulas and source to change variables and compute the different temperatures.

thanks

 

[IRstuff]

The formulas are pretty much in every heat transfer textbook. A free one is here:

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

http://www/ahtt.html

TTFN

FAQ731-376

 

[bamawres]

Thanks.

I used the example on page 577. I assumed no wind so the h value is 0, alpha for white = .26, e for white =.9. sky temp = 60 deg C, qsolar = 1120 w/m^2.

Calculated 93 deg C. Sounds right?

thanks

 

[bamawres]

How about internal? I used Hoffman's caluclation program and the internal was 132 deg F. That sounds low.

 

[IRstuff]

zero is probably overly conservative, and only applicable if your box was inside a vacuum chamber. A typically conservative value would be more like 2.5 W/m^2-K, while a more aggressive value might be more like 7 W/m^2-K. But, since you're using a 60°C sky temp, that might compensate for the lack of h. Generally speaking, it would be very unusual for the sky temp to exceed the 49°C that is commensurate with the 1120 W/m^2 solar load.

Luckily, MIL-HDBK-310 claims the maximum solar load only exists for about 2 hrs a day, so, it might be possible that a transient analysis would show a slightly lower surface temp than calculated.

Your internal temp cannot be lower than your box's surface temp, otherwise, heat will flow INTO the box from the outside to compensate. If additional heat is being generated inside the box, then the interior temperature MUST be higher than the surface temperature.

TTFN

FAQ731-376

 

[bamawres]

what does omega stand for in the equation?

 

[bamawres]

I can not solve the equation when I use 8 for h, slight breeze. Any ideas?

thanks

 

[IRstuff]

omega??

Should be solvable. Can you post your equation? What are you using to solve the equation?

TTFN

FAQ731-376

 

[bamawres]

Used a quartic equation solver. Using h value = 8 reduces the surface temp to 67.9 deg C.

 

[bamawres]

Does anybody have a link to a solar radiation key for cities. Need to calculate 41, 47, and 51 deg C solar radition = qsolar.

thanks

 

[racookpe1978]

Mark for later.