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

Abstract

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 13570kg/m2/s, inlet liquid temperature Tin=300.13 to 305.78K and inlet pressure Pin=866.52 to 945.86kPa by an experimental water loop flow. Measurements were made on a 59.2mm effective length and its three sections (upper, mid and lower positions), which were spot-welded four potential taps on the outer surface of a 6mm inner diameter, a 69.6mm heated length and a 0.4mm thickness of Platinum circular test tube with the twisted-tape insert. The SUS304 twisted-tape of width w=5.6mm, thickness δT=0.6mm, total length l=372mm, pitch of 180° rotation H=20.34mm 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 6mm in diameter and a 636mm long with the twisted-tape insert were numerically solved for heating of water with heated section of a 6mm in diameter and a 70mm 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=5.6mm, δT=0.6mm, l=370mm, H=20mm 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 18.03m/s. The correlations of δCSL and y+CSL for twisted-tape-induced swirl flow heat transfer in a vertical circular tube are derived.