The influences of circular-ring turbulators (CRT) and twisted tape (TT) swirl generators on the heat transfer enhancement

pressure drop and thermal performance factor characteristics in a round tube are reported. The circular-ring turbulators were individually employed and together with the twisted tape swirl generators in the heated section of the tube. Three different pitch ratios (l/D1.0

1.5

and 2.0) of the CRT and three different twist ratios (y/W 3

4

and 5) of the TT were introduced. The experiments were conducted using air as the working fluid under a uniform wall heat flux condition

for the Reynolds number between 6000 and 20000. The experimental results reveal that the heat transfer rate

friction factor and thermal performance factor of the combined CRT and TT are considerably higher than those of CRT alone. For the range examined

the increases of mean Nusselt number

friction factor and thermal performance

in the tube equipped with combined devices

respectively

are 25.8%

82.8% and 6.3% over those in the tube with the CRT alone. The highest thermal performance factor of 1.42 is found for the combined device consisting of the CRT with l/D 1.0 and TT with y/W 3. The correlations of the Nusselt number

friction factor and thermal performance factor of the tubes with combined devices are also developed in terms of Reynolds number

Prandtl number

twist ratio and pitch ratio.