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

摘要

Abstract Twisted-tape-induced swirl-flow heat transfer due to exponentially increasing heat inputs with various exponential periods ( Q  =  Q ~( 0 ) exp(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/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 δ ~( 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–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 ΔT ~( L )[ = T ~( s,av ) - T ~( L ), T ~( L ) = ( T ~( in ) +  T ~( out ))/2] considered in this study. The theoretical values of the local surface temperature T ~( s ), local average liquid temperature T ~( f,av ), and local liquid pressure drop ΔP ~( x )were found to be within almost 15% of the corresponding experimental ones. The thickness of the conductive sub-layer δ ~( CSL )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 u ~( sw )ranging from 5.23 to 21.18 m/s. Correlations between the conductive sub-layer thickness δ ~( CSL )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.