Helically-coiled-wire-induced swirl flow heat transfer and pressure drop in a circular tube under velocities controlled

Systematic measurements of the swirl flow heat transfer by the helical coil-wire and the pressure drop across the helical coil-wire were made by the experimental water loop flow with exponentially increasing heat input at mass velocities G = 3928 to 13,496 kg/m2s, inlet liquid temperatures Tin= 284.14 to 307.57 K and inlet pressures Pin= 763.80 to 1027.85 kPa. Systematic measurements of the pressure drop across the helical coil-wire were performed without heating circular tube. The measurements were performed on the inner surface of a SUS304 circular test tube with an inner diameter of 6 mm, a heated length of 59.7 mm, and a thickness of 0.5 mm, in which a helical coil was inserted. SUS304 helical coiled wire with wire diameter dw = 2.025 mm, coil diameter Dc= 3.4917 mm, total length l = 370 mm, coil pitch 360 degrees rotation pc= 37.222 mm, and coil pitch ratio yc =pc/d = 6.204 was employed. The relationship between swirl velocity and pump input frequency and that between fanning friction factor and Reynolds number (Red = 1.949 x 10 4 to 1.274 x 10 5 ) were clarified beforehand. On the other hand, the RANS (Reynolds mean Navier-Stokes simulation) equations for the k -s turbulence model in a circular tube of 6 mm diameter and 626 mm length with helical coil inserted, considering the temperature dependence of the thermophysi-cal properties concerned, were numerically solved for the heating of water on a heated section of 6 mm diameter and 60 mm length under the same conditions as the experiment by using the PHOENICS code. The helical coiled wire of dw = 2.025 mm, Dc= 3.4917 mm, l = 370 mm, pc= 37.222 mm and yc = 6.204 was installed at the same experimental location. The surface heat fluxes q and average surface tempera-tures Ts,av on a circular tube with a helical coil of yc = 6.204 obtained theoretically were compared with the corresponding experimental values on the graph of q versus temperature difference between average heater inner surface temperature and liquid bulk mean temperature ATL [ =Ts,av-TL, TL= (Tin+Tout)/2]. The numerical solutions for q and ATL were almost identical to the corresponding experimental values for q and ATL, with deviations from 0 to + 20% for the ATL range tested here. The numerical solutions of the local surface temperature (Ts)z, the local average liquid temperature (Tf,av)z, and the local liquid pres-sure drop APz were also compared with the corresponding experimental values of (Ts)z, (Tf,av)z, and APz against heated length L or distance from the test section inlet Z , respectively. The numerical solutions of (Ts)z, (Tf,av)z and APz differ from the corresponding experimental data of (Ts)z, (Tf,av)z and APz within +/- 5%. The thickness of the conductive sublayer delta CSL [ =(Ar)out/2] and non-dimensional thickness of the con-ductive sublayer y +CSL [= (fF /2)0.5plusw,c delta CSL/mu l] for turbulent heat transfer on a circular tube with helical coiled wire are clarified on the basis of numerical solutions in the swirl velocity usw,c ranging from 5.108 to 17.146 m/s. Correlations of Nusselt number Nud, delta CSL, and y+CSL for the swirl flow heat transfer in a vertical circular tube with helical coil wire are also derived.(c) 2023 Elsevier Ltd. All rights reserved.