A quote from the study page,
"If all other variables are held constant, the pressure drop is proportional to 1/D5. This means that the
pressure drop at a given flow rate is very sensitive to the diameter. A diameter chance in our tube of 1/100
in. (at the limit of our measurability) would increase the pressure drop by about 14%." end quote
read the pdf here
This should be a factor in keeping a rad as clean as possible. Any more comments, I thought this could make an interesting discussion.