Imitating Solar Load in a Temperature Chamber
Imitating Solar Load in a Temperature Chamber
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BLUF: Is it possible to mimic a solar loading environment solely in a temperature chamber and, if so, how?
We have an 6061 Aluminum box with a known heat load inside. It needs to be subjected to a solar load IAW MIL-STD-810G, Method 505.4, Proc I. Essentially, it is a 24-hour exposure cycle with varying ranges of temperature and solid load. Our company has a thermal chamber that we'd like to pretest our units in to make sure we pass qualification testing. We can't create a solar load with our equipment, but what I'd like to do is create a temperature profile that would essentially mimic the total heat load our module will experience during solar load.
I know the following:
- Surface area of the module, A
- Reflectance level for the paint used
- Temperature and Thermal Loading (W/m2) profiles for the 24-hour cycle
What I'd like to do is convert the know Power from the solar load (W/m2 * A) at each temperature interval to a Delta T that I can add to the prescribed ambient temperature to essentially mimic the solar loading in my thermal chamber.
Any suggestions?
We have an 6061 Aluminum box with a known heat load inside. It needs to be subjected to a solar load IAW MIL-STD-810G, Method 505.4, Proc I. Essentially, it is a 24-hour exposure cycle with varying ranges of temperature and solid load. Our company has a thermal chamber that we'd like to pretest our units in to make sure we pass qualification testing. We can't create a solar load with our equipment, but what I'd like to do is create a temperature profile that would essentially mimic the total heat load our module will experience during solar load.
I know the following:
- Surface area of the module, A
- Reflectance level for the paint used
- Temperature and Thermal Loading (W/m2) profiles for the 24-hour cycle
What I'd like to do is convert the know Power from the solar load (W/m2 * A) at each temperature interval to a Delta T that I can add to the prescribed ambient temperature to essentially mimic the solar loading in my thermal chamber.
Any suggestions?





RE: Imitating Solar Load in a Temperature Chamber
RE: Imitating Solar Load in a Temperature Chamber
http://ww
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: Imitating Solar Load in a Temperature Chamber
Twoballcane - thanks for the suggestion and we were actually going to go that route, but the problem is we have no way of controlling the ambient temperature AND having the flood lights in the same space. The module takes up roughly half our thermal chamber and I would not have enough room to put a grid of flood lights in there as well.
Thanks again for the suggestions folks and any more assistance would be appreciated as this is not my area of expertise.
RE: Imitating Solar Load in a Temperature Chamber
Wiki give the resultant equation, but without the derivation.
http://en.wikipedia.org/wiki/Sol-air_temperature
RE: Imitating Solar Load in a Temperature Chamber
For most of our stuff, we add 5ºC to 10ºC to the specified ambient.
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RE: Imitating Solar Load in a Temperature Chamber
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RE: Imitating Solar Load in a Temperature Chamber
IRstuff - Interesting question. We're a pretty small outfit with a small systems engineering group (1-2 people). It seems that our systems group and what you're describing perform completely different tasks, however. Our systems guys deal more with how the different electronic modules we design and build interact with each other (power loading, Ethernet and serial connectivity, etc). They are more EE's by training than anything else.
I appreciate the 5-10oC suggestion but you are correct with your caveat. Our equipment cannot assume a high wind load for cooling. The test procedure we need to follow assume little to no air flow to expedite cooling. We were able to successfully test at 70oC for ~4hrs yesterday and our requirement is a 24hr cycle that maxes out at 49oC ambient temperature and 1120 W/m2 solar load (although, not at the same time). I think we should be OK, but I still would like empirical proof for our team and management.
RE: Imitating Solar Load in a Temperature Chamber
http://www.dtbtest.com/Contact-Us.aspx
that you can hire and do the test.
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: Imitating Solar Load in a Temperature Chamber
MintJulep - that site is very useful. I'm just curious, do you have an idea on how to calculate the value of ΔQir? The equation is:
ΔQir = Fr·hr·ΔTo-sky
I'm having trouble defining all three of those values (Fr, hr, ΔTo-sky). Are they just values from my heat exchange course in college that I'm forgetting? A quick look in the Bejan and Kraus Heat Transfer Handbook didn't result in any clarification.
RE: Imitating Solar Load in a Temperature Chamber
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RE: Imitating Solar Load in a Temperature Chamber
The research I've been doing on view factors seem to be for radiation from one body to another and don't seem to be relevant to an object and the sun. Any suggestions on resources (online, preferably) to acquire the appropriate view factor?
Also, in your post above you state
Did you mean surface temperature or ambient temperature? Everything I've been reading points to ambient temperature, not surface temperature. I'm also having a tough time understanding the value of Tsky. Any suggestions?
RE: Imitating Solar Load in a Temperature Chamber
I know the following variables:
- Tamb - The chamber temperature. This is a known/given variable that changes over time.
- A - The exposed area of the module. This variable is known and constant.
- I - The solar irradiance. This is a known/given variable that changes over time.
- ho - The convection coefficient of the module. This variable is known (or can be found/looked up) and is constant.
The only value I don't know is ΔQir which Wikipedia describes asSince I'm having trouble calculating this variable (values of Fr, hr, & ΔTo − sky seem to be fairly ambiguous), what if I essentially make ΔQir = 0.
Would this not give me a worst case scenario?
Having done that, my temperature profile is quite high, higher than most of the components prescribed temperature range. That seems to be a problem.
I guess the main question is: How much of a role does ΔQir really play in this equation?
RE: Imitating Solar Load in a Temperature Chamber
Q.sol + Q.box = Q.conv + Q.rad
i.e., the solar load and power dissipation from the box are the inputs. The outputs are the convected heat transfer and the radiated heat transfer. Zeroing the radiated heat places the entire heat burden on convection. This might be an example calculation in Mathcad (also PDF'd):
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RE: Imitating Solar Load in a Temperature Chamber
I think your equation for hr is incorrect. If you look at ΔQir = Fr⋅hr⋅ΔTo − sky and do a unit analysis, you get
ΔQir = 1 (Fr is unitless) * {1 (emissivity is unitless) * J * s * K} * K = J*s*K2. No matter how I try, I can't make that W/m2.
RE: Imitating Solar Load in a Temperature Chamber
ΔQir is in W/m^2 as
Fr is dimensionless
hr is in W/(m^2*K)
ΔTo − sky is in K
RE: Imitating Solar Load in a Temperature Chamber
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RE: Imitating Solar Load in a Temperature Chamber
Dang it... Too bad everything doesn't just have a Mathcad interface.
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RE: Imitating Solar Load in a Temperature Chamber
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RE: Imitating Solar Load in a Temperature Chamber
The convection heat transfer equation is as follows:
P = k⋅Aconv⋅ΔT where ΔT = Tcase - Tamb, Aconv is the area of the module used for convection and P = power to be dissipated
Rearranged, that equation works out to:
Tcase = Tamb + P/(h⋅Aconv)
Obviously, as the power dissipation requirement or ambient temperature rise, the case temperature will rise. The power dissipation requirements are defined by P/(h⋅Aconv). We know that, in a solar loading environment, this section of the equation will rise. However, due to our test limitations, we cannot adjust this part of the equation. Therefore, we will change the Tamb portion of the equation to mimic the change in the P/(h⋅Aconv) portion of the equation.
The new ambient temperature which will mimic solar exposure, Tsol, will be raised by the product of the solar load, W, and the exposure area, Aexp, which is defined as the area of the module exposed to the solar load. Therefore:
ΔTamb = (W⋅Aexp)/(h⋅Aconv)
and
Tsol = Tamb + ΔTamb
This solution has obvious shortcomings with the most glaring being localization of heat. In this solution we are taking a localized heat load (W⋅Aexp) and distributing it over the entire area used for convection (Aconv). This results in a more constant thermal gradient and less localized thermal stresses which may affect performance. Again, this test is being used as a 'pre-test' to a more rigid qualification test and is just a sanity check before going to the lab.
Thoughts on the approach?