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Heat Transfer and Pressure Drop Correlations for Twisted-Tape Inserts in Isothermal Tubes: Part II—Transition and Turbulent Flows
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1993
Year
EngineeringFluid MechanicsMechanical EngineeringGas-liquid FlowHeat Transfer ProcessFluid PropertiesTransport PhenomenaThermodynamicsTube BlockageLaminar-turbulent TransitionPipe FlowFlow PhysicThermal-hydraulic Design CorrelationsHeat TransferMultiphase FlowTurbulent Flow Heat TransferPressure Drop CorrelationsHeat Transfer EnhancementIsothermal TubesThermal Engineering
The study develops continuous thermal‑hydraulic correlations to predict isothermal friction factor and Nusselt number for turbulent flows in tubes with twisted‑tape inserts, extending to laminar‑to‑turbulent regimes. Experimental data for water and ethylene glycol at various tape pitch ratios were analyzed, with twist effects isolated by normalizing against asymptotic smooth‑tube predictions and correlated to Reynolds number and pitch ratio. The correlations show that tube blockage and vortex mixing dominate heat‑transfer and pressure‑drop increases, with twist effects scaling with Re and pitch ratio, and they agree with prior data within ±10 % for friction factor while Nu requires multiple curves due to transition variability.
Thermal-hydraulic design correlations are developed to predict isothermal f and Nu for in-tube, turbulent flows with twisted-tape inserts. Experimental data taken for water and ethylene glycol, with y = 3.0, 4.5, and 6.0, are analyzed, and various mechanisms attributed to twisted tapes are identified. Tube blockage and tape-induced vortex mixing are the dominant phenomena that result in increased heat transfer and pressure drop; for loose- to snug-fitting tapes, the fin effects are insignificant. The limiting case of a straight tape insert correlates with the hydraulic-diameter-based smooth tube equation. Tape twist effects are thus isolated by normalizing the data with the asymptotic predictions for y = ∞, and the swirl effects are found to correlate with Re and l/y. The validity of the final correlations is verified by comparing the predictions with previously published data, which include both gases and liquids, under heating and cooling conditions and a wide range of tape geometries, thereby establishing a very generalized applicability. Finally, correlations for laminar (presented in the companion Part I paper) and turbulent flows are combined into single, continuous equations. For isothermal f, the correlation describes most of the available data for laminar-transition-turbulent flows within ±10 percent. For Nu, however, a family of curves is needed due to the nonunique nature of laminar-turbulent transition.