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The article presents the influence of diamond-shaped rings (DRs) mounted repeatedly in a uniform heat-fluxed heat exchanger tube on flow resistance and thermal behaviors. In the current study, the DR elements used for producing the longitudinal vortex flows were placed in the tube by employing two small rods to connect the DRs together. Effects of three relative ring-pitches or pitch ratios (RP = P/D = 0.5, 1.0, 2.0) and three ring blockage ratios (RB = b/D = 0.1, 0.15 and 0.2) at the attack angle of 45° on Nusselt number (Nu), friction factor (f) and thermal enhancement factor (TEF) in turbulence regimes, Reynolds number (Re) between 4220–25,800 are examined. The results have shown that the heat transfer and pressure loss increase considerably with raising RB but with the decline of RP. The maximum Nu and f obtained at RB = 0.2 and RP = 0.5 are, respectively, about 3.21–4.18 and 11.73–32.85 times above the smooth tube. The highest TEF around 1.57 is seen at RB = 0.10 and RP = 1.0. Furthermore, empirical correlations for Nu, f and TEF were established and found to fit the experimental data with deviations within ±8%, ±9% and ±5%, respectively.
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