Computational study of twisted-tape-induced swirl flow heat transfer and pressure drop in a vertical circular tube under velocities controlled

The twisted-tape-induced swirl flow heat transfer due to exponentially increasing heat inputs with various exponential periods and twisted-tape-induced pressure drop were systematically measured with mass velocity G = 4120 to 13 570 kg/m2/s, inlet liquid temperature Tin = 300.13 to 305.78 K and inlet pressure Pin = 866.52 to 945.86 kPa by an experimental water loop flow. Measurements were made on a 59.2 mm effective length and its three sections (upper, mid and lower positions), which were spot-welded four potential taps on the outer surface of a 6 mm inner diameter, a 69.6 mm heated length and a 0.4 mm thickness of Platinum circular test tube with the twisted-tape insert. The SUS304 twisted-tape of width w = 5.6 mm, thickness δT = 0.6 mm, total length l = 372 mm, pitch of 180° rotation H = 20.34 mm and twist ratio y = H/d = 3.39 was employed in this work. On the other hand, theoretical equations for k-ɛ turbulence model in a circular tube of a 6 mm in diameter and a 636 mm long with the twisted-tape insert were numerically solved for heating of water with heated section of a 6 mm in diameter and a 70 mm long by using PHOENICS code under the same conditions as the experimental ones considering the temperature dependence of thermo-physical properties concerned. The twisted-tape of w = 5.6 mm, δT = 0.6 mm, l = 370 mm, H = 20 mm and y = 3.33 was installed under the same experimental position. The surface heat flux q and the average surface temperature Ts,av on the circular tube with the twisted-tape of y = 3.33 obtained theoretically were compared with the corresponding experimental values on q versus the temperature difference between average heater inner surface temperature and liquid bulk mean temperature ΔTL [=Ts,av − TL, TL = (Tin + Tout)/2] graph. The numerical solutions of q and ΔTL are almost in good agreement with the corresponding experimental values of q and ΔTL with the deviations less than 0% to +20% for the range of ΔTL tested here. The numerical solutions of the local surface temperature (Ts)z, local average liquid temperature (Tf,av)z and local liquid pressure drop ΔPz were also compared with the corresponding experimental data of (Ts)z, (Tf,av)z and ΔPz versus heated length L or distance from inlet of the test section Z graph, respectively. The numerical solutions of (Ts)z, (Tf,av)z and ΔPz are within ±5% difference of the corresponding experimental data on (Ts)z, (Tf,av)z and ΔPz. The thickness of the conductive sub-layer δCSL [=(Δr)out/2] and the non-dimensional thickness of the conductive sub-layer y+CSL [=(fF/2)0.5ρluδCSL/μl] for the turbulent heat transfer on the circular tube with the twisted-tape insert are clarified based on the numerical solutions at the swirl velocity usw ranging from 5.39 to 18.03 m/s. The correlations of δCSL and y+CSL for twisted-tape-induced swirl flow heat transfer in a vertical circular tube are derived.