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Old 6th December 2014, 05:33 PM   #3791
Chartal is offline Chartal  Canada
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I think it's called "chimney effect"
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Old 7th December 2014, 12:10 AM   #3792
ostripper is offline ostripper  United States
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Maybe it's just convection. It would only be the "chimney effect" - if the semi's were
emitting "magic smoke" .

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Old 7th December 2014, 12:21 AM   #3793
nigelwright7557 is offline nigelwright7557  United Kingdom
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1600 watt peak class AB amplifier.
Click the image to open in full size.
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Old 7th December 2014, 09:50 AM   #3794
nania is offline nania
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Quote:
Originally Posted by AndrewT
A vertically aligned heatsink will always have a temperature difference along the height dimension. this does not depend on the sink being >300mm tall.
Why always? Which is hotter, top or bottom?
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Old 7th December 2014, 10:02 AM   #3795
AndrewT is offline AndrewT  Scotland
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Air entering the slots between the fins is at ambient temperature (Ta).
The fins at the BOTTOM are cooled by Ta air.
As the warmed less dense air rises it cools the adjacent fin/s
These (higher up) fins are cooled by air that is above Ta.
By the time the air has traveled up past the fins, the air is at maximum temperature Ta++
The fins at the TOP are being cooled by Ta++ air. These fins are hotter than the fins at the bottom. Because delta Ts-a is lower.

There is further complication:
The backplate is not at constant temperature, i.e. it is not isothermal.
The hottest part is directly in contact with the device emitting heat.
As the heat travels outwards (sideways as well as through the thickness) it passes through Thermal Resistance.
The further the heat travels the more the temperature drops.

Out at the sides the sink backplate can be many C degrees cooler than under the device.
At the top and the bottom, the backplate can be many C degrees cooler than under the device.
Combine these two and you will find that the corners of the heatsink are the coolest points on the backplate.
Any fins connected to these "cooler" parts will not transfer as much heat as the hotter parts of the sink.

More complication:
Radiation also cools the heatsink.
The area of the FIN END projecting DOWN radiates heat into the environment.
The area of the fin end facing up radiates heat into the environment.
The side of the fin facing sideways radiates heat into the environment.
The black body AREA of the WHOLE outside face of the heatsink (same area as the backplate) radiates heat into the environment.
The backplate area facing into the chassis radiates heat into the environment. This heats up many of the internal components. Two thin but separated layers of polished aluminium (Bakofoil) massively reduces the heating of internal components from radiated heat emitted by the heatsink.

There are at least two free Thermal Calculators that let you see the temperature variations around a heatsink.
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Last edited by AndrewT; 7th December 2014 at 10:11 AM.
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Old 7th December 2014, 12:14 PM   #3796
nania is offline nania
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All that you describe above is subject to the mounted position(s) of the devices. If there are calculators then there will be a way to distribute the heat so top and bottom is equal. This seems to refute your initial statement that prompted the question to you. Also, the implication that there will always be a circular convection with vertical fins no matter how short also seems wrong because of entropy. The temp differential and convection loop need to be large enough to force the surrounding air into an orderly flow. A tighter radius of airflow makes creating an orderly loop more difficult to achieve.
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Old 8th December 2014, 07:59 AM   #3797
AndrewT is offline AndrewT  Scotland
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Nania,
Quote:
This seems to refute your initial statement that prompted the question to you
at least one of us is misunderstanding the other.
Quote:
a circular convection
Quote:
tighter radius of airflow
what are you talking about?
warmed air rises due to density difference.
Where do circles come in?
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Old 8th December 2014, 08:01 AM   #3798
AndrewT is offline AndrewT  Scotland
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I said in post3762
Quote:
The top row of devices will run much hotter than the bottom row devices.
Nania,
do you have evidence that my statement is wrong?
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Old 9th December 2014, 03:28 AM   #3799
nania is offline nania
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When there is a temperature differential and a distance between the heat sources a circular motion of air effect can be produced (similar to all natural torque patterns-see torus). The heat dispersion is not a linear vector so it will not go straight up as you imply and depending on where the sources are mounted, the top of the heat sink does not have to be hotter. When the distance between the heat sources is short, stable vector angles cannot be formed and therefore no support for the torus so the distance matters. To say heat will rise gives an incomplete and faulty description of what is actually happening and will make someone miss an opportunity to try something innovative.
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Old 9th December 2014, 08:37 AM   #3800
AndrewT is offline AndrewT  Scotland
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I must be stupid.
I can't understand what you are trying to explain.
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