In chatting this over with Bill and looking at his revised graphs, I'll say the following.
As a general rule, you will see improved heat transfer with increased flow rates. There exists the possibility that due to a given radiator's internal configuration, there may be a performance bump at a specific, and typically low flow rate. At the high-flow end of this bump, performance should be better than at the low-flow end of this bump. Performance outside of this bump will improve with increasing flow.
Why some radiators exhibit this performance is open for debate. I don't personally believe it is related to laminar flow. I think it's likely a result of secondary flow, mainly vortices, and how their shape varies across a narrow flow range. As I told Bill, this is the la-la land of CFD. It's also possible, though unlikely, that this is a fictional artifact due to the way Bill is testing and measuring his results. You can easily dismiss such a result in a handful of trials. Dismissal is not so easy as the number of trials grows.
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