Study of local heat transfer in a spirally fluted tube

Local Nusselt numbers in a spirally fluted circular tube are presented. Thermochromic liquid crystals and shroud-transient technique are used to measure spatially-resolved surface temperature distributions, which are used to deduce local Nusselt numbers. The Reynolds number Re ranges from 30,000 to 70,000, a spiral angle θ is fixed at 60° and the dimensionless streamwise distance z/d ranges from 0 to 4.5. The results show that in general, the local Nusselt numbers monotonically decrease with a distance away from both windward and leeward crests of the spiral flute and reach a minimum value near its valley for all Re's and z/d's tested. The local Nusselt numbers in the spirally fluted tube are maximum 179–89.3% higher for Re = 30,000–70,000 near the windward crest at z/d = 4.5 than the fully developed values in the smooth tube. It is also found that the average Nusselt numbers are 52–23.7% higher for Re = 30,000–70,000 than the fully developed values in the smooth tube.

► Nusselt numbers monotonically decrease from the crests of the spiral flute.
► Minimum Nusselt numbers occur near the valley for all Re's and z/d's.
► Average Nusselt numbers are 52–23.7% higher than fully developed values.