Natural convection question

[smukthiHT]

Hi All,

I hope all are doing good!


I had our lab do a test for heat transfer for a simple scenario. It is heat transfer from a top(outside) surface of a horizontal plate. The temperature of the ambient is around 23 C and temperature measured at 2 thermo couple points on the top surface of part is 165 C for a heat source of 15 watts on the inside of the horizontal flat plate. The other side of the horizontal plate has a thick insulation to make sure heat flow only towards the top surface of the part.

I have used analytical equation from standard literature for a horizonatl plate with HT from the top surface to find out the heat transfer due to convection for this configuration for the above parameters and I got a h_conv=13.1 W/m2 and heat transfed_convection=8.5 Watts by using Newton’s law of cooling.
The heat transfer because of radiation is 0.75W/m2 as the emissivity of polished aluminum is 0.1.
The sum of heat transfers because of convection and radiation(8.5+0.75) does not even come close to the input in the test=15 Watts.
I am trying to unsderatnd the what could hav lead to this devaition?
Is it because, I am computing the heat transfer_covection using an average film coefficient of convection value but not localized one?
Can you please comment and share your thoughts?

Thanks
Sridhar

 

[IRstuff]

The setup should match your application in as many aspects as practical. The support structure should obviously be non-conductive and non-interfering, so you need to at least qualitatively map out how the air will be moving, and make sure the supports are out of the way. There should be an external shield, sufficiently far away enough to not interfere with the air flow. Otherwise, the external environment should match the expected installation, e.g., height above floor, ceiling height, etc.

TTFN
faq731-376

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[btrueblood]

All I can tell you is, the theoretical analysis (typically a semi-infinite plane) is never realized in practice, and the difference has been shown by numerical studies to be fairly significant. Like IRstuff said, try to match the expected installation as best as possible, or at least bound the limits of same.

 

[btrueblood]

Going back to your post of 1 Apr, "1. The heat source is a set of resistors that are attached on the back of the plate and the resistors are glued to the plate and a thick foam of 5 cm is placed on the other side to make sure heat trasnsfer happens through the top surface of the part. There is nothing perfect, It think it a reasonable assumption to say that heat is trasfred only through the top surface of the part."

I'm not sure you understood a part of what I was saying: do you account for heat transfer at the sides (thickness dimension) of the plate? If these edges are insulated, did you do a 2d or 3d study to determine the area of insulation near the contact zone which is heated above ambient, and the effect of convection off the top of that foam? If the edges are not insulated, did you look at the convection of heat along those edges? The difference is probably slight, but it sounds like you are looking for a ~10% difference, and those types of differences are in the details...

 

[Dougt115]

You never said but I hope you let your test reach equilibrium before taking your measurements.