Convection & Shapes: How Would They Affect Heat Plate?

[Hollysmoke]

Hi, I was wondering if someone could help explain how the following shapes would affect convection. What it is is a heat plate with nothing, a bundle of tubes, and just fins. It's in a vertical tube and the air flows upwards. I understand convection but I don't get how it would be affected in terms of a physical object such as these in its path.

http://img231.imageshack.us/img231/718/heatplatesf1.jpg

 

[Gokul43201]

The rule of thumb I use is that flow lines just downstream of an obstacle get very close to their far-away away structure within a distance comparable to the width of the obstacle. So if the tubes have a diameter d, then at a distance d downstream from each tube, the flow-lines are essentially homogenized again. For some shapes (tear-drop, for instance), this distance can be much smaller than the width of the object.