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

Abstract

Twisted-tape-induced swirl-flow heat transfer due to exponentially increasing heat inputs with various exponential periods (Q = Q 0 exp(t/),  = 6.04 to 23.07 s) and twisted-tape-induced pressure drop was systematically measured for various mass velocities (G = 4115 to 13656 kg/m 2 s), inlet liquid temperatures (T in = 285.88 to 299.09 K), and inlet pressures (P in = 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  T = 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- turbulence model for a circular 2 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

Conductive sublayer of turbulent heat transfer for heating of water in a circular tube

“…Uniform heat flux, q, was assumed at the heated pipe wall in the range 3.41  10 5 to 2.68  10 7 W/m 2 as the boundary condition, and the calculations were performed until the steady-state was obtained. The test tube surface temperature, T s , was obtained from the temperature of the outer control volume on the test tube surface, TEM, located at the center of the control volume, by solving the heat conduction equation in liquids [1][2][3][4][5]:…”

“…However, few fundamental studies have investigated the thickness of conductive sublayers, and little is known on the influence of heated length, inlet liquid temperature, and heating rate on the thickness of conductive sublayers. Knowledge of the thickness of conductive sublayers and nondimensional thickness of conductive sublayers would help advance the computational fluid dynamics (CFD) code for analyzing steady-state and transient turbulent heat transfer [1][2][3][4][5].…”