Convection cooling?
Nope, I don't think there are any "concealed" fans. The main heatsink sits on top of and above the processor, and there's a heat pipe from the GPU to that main heatsink. The fan you mention towards the upper left forces air across the heatsink and presumably out the vent.
Hard drives are not a significant source of heat by a long shot. They consume maybe 5-10W after startup. The hard drive in the new iMac is clearly passively cooled which is probably why some models are limited to 5400 RPM.
The only things I can't readily identify are the dark sections towards the left and right. I can't imagine they have fans since they wouldn't be tall enough and they look enclosed. I doubt they are speaker ports either since they seem much too big. Maybe they are just plastic bits for structural support.
With 40% less volume in its case how can the new iMacs deal adequately with cooling? (This decrease fails to take note that the screen mounting frees up some of this space functionally.) In older iMacs convection played a role in cooling. There were intakes at the bottom. As the air in the iMac was heated by its operation it expanded and rose toward the outlets at or near the top, drawing more air at the bottom at the same time, thus creating a chimney effect. Note that the heated air shows both a temperature change and a related pressure change. If allowed to expand, the temperature of a given volume drops; if compressed, the temperature of a given volume rises.
The new model has its outlet in a small grill below the center of the back. One member notes that the discharge of the single evident fan aligns with this vent approximately. But anyone who has attempted to use a fan placed in a window knows there is a problem of a lack of coupling: the fan must be mounted so as to block, stop up, the window opening around the fan to gain any effect. Unless there be some coupling of the fan to the vent the effect will be negligible. If there is a coupling, the heat control will be nearly 100% controlled by the fan, with little contribution by convection.
I can see, after consideration, that the shape of the back is designed to influence the effect of convection in a complex way. The extremely narrow sides and top are limiting factors in the tendency for heated air to expand freely. Thinking on an increment scale, delta T/Pair heating adjacent to any relatively hot spot off center will, in its tendency to expand, be forced to move to the center where there is more room for expansion. This is true even at the top. The movement of the heated air will tend to concentrate it in the center of the back. (The bottom is a special case because the action of the heated air overall to draw air in at the bottom.)
Thus convection cooling can play a significant role in the new iMac, continuing an outstanding design tradition. Thus the promptings of a style change are resolved in such a way that form can be truly said to follow function.
I will wait for the reviews before ordering, but I was heartened when I saw that convection cooling is part of the design.
