I've been looking into turbulators, following an article I read, (I'll post a pic next week). Here's some other things I've found:
A link to Hayden industries
(not new to us, but an interesting place to start)
A Finnish site
with an interesting idea about throwing in a coil. (would help break up boundary layer)
A tiny bit of theory, with an Excel sheet
A PDF from Trane about turbulator (again!)