quote: It's because it's a non-optimal configuration which makes airflow perpendicular to heat flow, instead of parallel to it
quote: components higher up your chimney will have less cooling because they're getting air that's been warmed up by components lower in the chimney
quote: Ideally you want airflow to be parallel to the flow of heat. e.g. heatsink fan sucks in air through the sides of the heatsink and blows it out the top. So the heatsink is hottest at the bottom (closest to CPU), coolest at the top. Air is coolest at the bottom, hottest at the top. That makes the heat gradient consistent across the air temperature gradient (a certain temperature air always sees the same temperature metal), providing uniform cooling.
quote: Also, I'd need to do some calcs but my intuition says a circular shape is the worst shape for this. To maximize heat exchange, you want to maximize surface area presented to the airflow. A circular cross-section minimizes surface area encountering a given volume of airflow.