Conductive sub-layer of twisted-tape-induced swirl-flow heat transfer in vertical circular tubes with various twisted-tape inserts

Twisted-tape-induced swirl-flow heat transfer due to exponentially increasing heat inputs with various exponential periods (Q = Q0exp(t/τ), τ = 6.04 to 23.07 s) and twisted-tape-induced pressure drop was systematically measured for various mass velocities (G = 4115 to 13,656 kg/m2 s), inlet liquid temperatures (Tin = 285.88 to 299.09 K), and inlet pressures (Pin = 847.45 to 943.29 kPa) using an experimental water loop flow. Measurements were made over a 59.2-mm effective length and three sections (upper, middle, and lower positions), within which four potential taps were spot-welded onto the outer surface of a 6-mm-inner-diameter, 69.6-mm-heated length, 0.4-mm-thickness platinum circular test tube. Type SUS304 twisted tapes with a width w = 5.6 mm, a thickness δT = 0.6 mm, a total length l = 372 mm, and twist ratios y = 2.39 and 4.45 were employed in this study. The RANS equations (Reynolds Averaged Navier-Stokes Simulation) with a k–ε turbulence model for a circular tube 6 mm in diameter and 636 mm in length were numerically solved for heating of water with a heated section 6 mm in diameter and 70 mm in length using the CFD code, under the same conditions as the experimental ones and considering the temperature dependence of the thermo-physical properties concerned. The theoretical values of surface heat flux q on the circular tubes with twisted tapes with twist ratios y of 2.39 and 4.45 were found to be almost in agreement with the corresponding experimental values of heat flux q, with deviations of less than 30% for the range of temperature difference between the average heater inner surface temperature and the liquid bulk mean temperature ΔTL [ = Ts,av- TL, TL = (Tin + Tout)/2] considered in this study. The theoretical values of the local surface temperature Ts, local average liquid temperature Tf,av, and local liquid pressure drop ΔPx were found to be within almost 15% of the corresponding experimental ones. The thickness of the conductive sub-layer δCSL and the nondimensional thickness of the conductive sub-layer y+CSL on the circular tubes with various twisted-tape inserts were determined on the basis of numerical solutions for the swirl velocities usw ranging from 5.23 to 21.18 m/s. Correlations between the conductive sub-layer thickness δCSL and the nondimensional thickness of the conductive sub-layer y+CSL for twisted-tape-induced swirl-flow heat transfer in a vertical circular tube were derived.