These are not so much devices as they are panels with slats. They are not elctric or anything, they just have vanes in them that cool them slightly more than everything else. They are for a thermal environment, not an IR environment. Since they are just slightly cooler than the surrounding tank, they show up as a big dark blotch in the middle of a big bright blotch on the thermal viewer. This makes the tanks identifiable in a thermal environment. As I have said elsewhere, I don't think there is a wrong way to mount them (up or down). I have several pics of them mounted up and Martin has several pics of them mounted down.
I have just added a few pics to my webshots folder that show it pointed up, and Martin has provided several that show them pointed down, so it is probably dictated by unit. Martin is also right that many of them are painted white curently, and I think this does have to do with infra red, although I see a lot where the are Desert Sand with a vehicle identifier painted or taped on. I think the current trend is white though.
Rob