Thermal conductivity air to metal

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Discussion Overview

The discussion centers on the thermal conductivity and heat transfer mechanisms between a heat-generating processor chip and a metal case, specifically examining the roles of conduction, convection, and radiation in different configurations. Participants explore the efficiency of heat transfer when air is present as an intermediary and consider experimental observations related to temperature differences in various setups.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant inquires about the efficiency of heat transfer from a processor to air and from air to metal in two different sizes of metal boxes, noting that direct contact would be more efficient.
  • Another participant emphasizes the importance of convection and radiation in the heat transfer process, suggesting that it cannot be considered solely as conduction.
  • A participant describes an experiment with a small camera processor and questions why the sides of the metal case remain hot despite the top being open, proposing that heat transfer may involve conduction to the metal sides and questioning the roles of convection and radiation.
  • Some participants express uncertainty about the split between radiative, convective, and conductive cooling, noting that accurate calculations are difficult and often require experimental approaches.
  • One participant questions the significance of radiative heat transfer between two metals that are one inch apart, suggesting that infrared radiation is primarily relevant in larger scale scenarios like the sun-earth interaction.
  • Another participant presents a specific experimental setup with a small pinhole camera and discusses unexpected temperature readings, raising the possibility that infrared radiation contributes to heating the metal case.
  • There is a suggestion that the black surfaces of both the chip and the metal case may enhance heat transfer through infrared radiation, questioning the effectiveness of conduction through air.
  • Some participants recommend consulting textbooks on heat transfer for methods of calculation, while others seek clarification on whether the observed heating is primarily due to convection or infrared radiation.
  • One participant notes that convection often dominates in similar configurations and mentions that the orientation of the plates may affect the heat transfer dynamics.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the mechanisms of heat transfer, with no consensus on the dominant factors influencing the heating of the metal case. The discussion remains unresolved with varying hypotheses presented.

Contextual Notes

Participants acknowledge the complexity of accurately calculating the contributions of conduction, convection, and radiation, with many factors potentially influencing the outcomes, such as the orientation of surfaces and the specific experimental conditions.

themaea
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Let's say you have a small metal case that is 3 inches by 3 inches versus 2 inches by 2 inches. Inside at center is a heat generating source (let's say a small processor chip). How efficient is the transfer of heat from the processor to the air... and from air to metal between the 2 different size metal box? How do you compute for this? I know directly gluing the metal to the processor would be more efficient.. but I want to know the thermal transfer if air is in the middle between them where the heat sink is not directly connected to the source but has air medium between them like that occurring in many enclosure housing electronic devices with processor chip inside (like in cctv camera). Thank you.
 
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Convection and radiation are playing important roles, and one cannot consider the mechanism to be exclusively conduction.
 
Dr. Courtney said:
Convection and radiation are playing important roles, and one cannot consider the mechanism to be exclusively conduction.

Here's an actual setup I tried. The square metal case is 1 inch high and 1.6 inch in the 4 of its sides. There is a small camera processor (13mm x 13mm) at bottom with temperature of 70 degree Celsius.. I opened the top metal cover to test if the temperature just rise straight up to outside from convection but what really happened is the sides of the metal case are still hot (even if the top cover is open). So what really goes on microscopically? When the processor gets hot.. it conducts heat to the air.. but since the top cover is opened.. why is its least resistance is not the open air above (and outside) but the metal at the sides? Before doing the experiment. I was guessing the metal sides would remain cold since the heat from the hot cpu at bottom would just rise up. You also mentioned about radiation. In this experiment, how many percentage (approximately) of the heat from the cpu goes to the metal at the sides in form of radiation and in form of conduction and convection? The cpu and metal case are all colored black so let's assume an emissivity of 0.95. Thank you.
 
The split between radiative, convective, and conductive cooling are notoriously hard to calculate with confidence.

Most designers use several prototypes and approach the problem experimentally, adding heat sinks, vents, and fans if needed.
 
or let's just use a very simple setup... just two metals let's say one inch apart.. when one is heated 70 degree celsius.. is there significant radiative heat transfer from one metal to the other? i thought IR radiative heat transfer is only important for the sun-earth heat transfer bec the sun is great concentrated source of ir
 
http://imageshack.com/a/img913/9240/RH8zTD.jpg
http://imageshack.com/a/img537/8577/NIdMHw.jpg
http://imageshack.com/a/img537/563/m7ER9p.jpg

The above picture is a small pinhole camera that you can buy for $30. The processor inside is so tiny no heatsink can't be put inside. The black aluminum metal case is sized 1.6 inches at the sides and 1 inch height. Here's the mystery. When put on table such that the top cover is open. The thermograph shows the processor to be 71.8 Celsius.. but the metal case is measuring about 44 degrees Celsius. Convection should make the heat go straight up without touching the sides. Conduction through air is said to be poor. Is it really possible the IR radiation from the processor is what heat up the metal case? This is just for theoretical understanding and not about putting heatsink to the cheap camera. I thought only the sun IR radiation can bestow heat to earth. I'd like to understand the role of IR radiation in terrestrial object.. usually how much does it contribute to heat versus conduction and convection in daily objects.. can you give another example? Thank you.
 
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notice the black metal case is like a blackbody (because it's painted black) and the chip is black too.. so the question is.. would there be significant transfer of heat from the chip to the metal case by plain ir radiation? I think it should, isn't it. because I can't see how conduction by air can increase the temperature of the metal case by 10 degrees Celsius, could it?
 
The methods for calculating the things you're looking for are explained in any number of textbooks on heat transfer. You should read one of these books and then ask questions on the parts you don't understand. I like this one,
https://www.amazon.com/dp/0070479801/?tag=pfamazon01-20
 
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gmax137 said:
The methods for calculating the things you're looking for are explained in any number of textbooks on heat transfer. You should read one of these books and then ask questions on the parts you don't understand. I like this one,
https://www.amazon.com/dp/0070479801/?tag=pfamazon01-20

I just want to know whether the metal case heating up is caused by convection or ir radiation.. what is usually the case.. that's all.
 
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  • #10
Convection tends to dominate many similar configurations. You didn't mention if the plates are vertical or horizontal. The aspect ratio of the problem may also matter. If the plates are very large in relation to the gap between them (like a double glazed window) then conduction might dominate over convection. Exactly where that cross over is I couldn't say.
 

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