Cooled by intelligently managed convection
In concentrating on the left and right flows moving upward and merely noting a centering bias, I neglected a follow-up on the centering bias, which turns out to be more important and stronger than the up-flows to the sides. It yields more significant cooling.
The centering tendency is caused by the (at first) gently rising slope to the center. An increment of gas as it ascends is supported on all sides by neighboring packets of gas. In a simplified view, its neighbor moving to the center (on a bias) occupies a larger volume at a reduced pressure because of the increase in altitude under the slope of the back. This change in volume is necessarily accompanied by changes in the pressure/temperature values (Boyle’s Law). Our packet being faced with a (very) partial vacuum on a center bias tends to follow in its neighbor’s path. Thus we are looking at time and motion in little. In fact this only represents one active link in a chain of such links, each packet being connected to the next in their motions and defined by their motions. This overall motion is connected in time as well, registering changes in velocity.
Looked at in this way, from the time a packet becomes a part of a thread by being drawn into the iMac, until a packet in the thread is pushed out of the iMac at the vent, all packets forming the thread are interconnected: this is implicated by the effects of the motion. An increase or decrease in speed is felt all up and down the line. Any heat taken on increases the process. A change in volume follows the temperature/pressure complex; this is felt only locally, but it is implicated in the other changes. As a packet approaches the dome this process is dramatically increased. Reaching the arch of dome necessarily ends the cross-flows in their meeting. Then the increasing altitude of the dome up to the vent draws the thread flow downward. The downflows from the top merge with the side flows moving to the vent.
These effects are heightened where heat sources are encountered: the more heat, the greater the effect. Thus the heat empowers the cooling process of air movement and directs it where it is most needed. The venting to the outside provides the final added thrust moving the whole convection machine. There is no evaporative cooling, except in the heat pipes terminating at their heat sinks. There is a degree of radiant cooling, revealed in the warming of the aluminum case (note that the aluminum is not connected to the hot components to render any significant cooling by conduction). You might say the iMac is cooled by intelligently managed convection.
The teardrop shape of the lower dome below the vent has a similar centering bias, possibly with less effect until there are heat sources to encounter. The air, rising from the vents under it and near it, will tend to migrate up the dome to the vent as they encounter heat sources. The entire volume of the case experiences an overall flow to the vent that creates a drop in pressure at the intakes, drawing air in. At the vent all flows come together. The downward flow from above is discouraged from continuing downward by the dome‘s falling away, tending to generate a pressure block. The upward flow from below the vent would also tend to block the flow from above the vent. The vent itself tends to create a pressure drop in the face of all flows ending near it, enforcing a movement to the vent and out. This movement of the air to the outside creates a partial vacuum system wide: the chimney “draws,” the iMac is cooled. The fan’s action in this is primarily local, not being coupled in any special way to the vent.
I can’t wait to see the first tear-down and review of the 21.5!
I add some URLs for illustration. I have not yet been able to upload from Wikipedia to this post:
<http://en.wikipedia.org/wiki/Franklin> —the original stove using a reversed flow flue: down, then up
I was unable to find his downdraft fire place with flue under the floor leading to a chimney in the wall, which I cited in another thread
<http://www.permies.com/t/4807/stoves/rocket-mass-heater-uses-less>
a stove fired through an open top with reversed flue