HEAT-TRANSFER AND PRESSURE-DROP CORRELATIONS FOR TWISTED-TAPE INSERTS IN ISOTHERMAL TUBES .1. LAMINAR FLOWS

Laminar flow correlations for f and Nu(m) are developed based on experimental data for water and ethylene glycol, with tape inserts of three different twist ratios. The uniform wall temperature condition is considered, which typifies practical heat exchangers in the chemical and process industry. These and other available data are analyzed to devise flow regime maps that characterize twisted-tape effects in terms of the dominant enhancement mechanisms. Depending upon flow rates and tape geometry, the enhancement in heat transfer is due to the tube partitioning and flow blockage, longer flow path, and secondary fluid circulation; fin effects are found to be negligible in snug- to loose-fitting tapes. The onset of swirl flow and its intensity is determined by a swirl parameter, Sw = Re(sw)/square-root y, that defines the interaction between viscous, convective inertia, and centrifugal forces. Buoyancy-driven free convection that comes into play at low flow rates with large y and DELTAT(w) is shown to scale as Gr/Sw2 much greater than 1. These parameters, along with numerical baseline solutions for laminar flows with y = infinity, are incorporated into correlations for f and Nu(m) by matching the appropriate asymptotic behavior. The correlations describe the experimental data within +/- 10 to 15 percent, and their generalized applicability is verified by the comparison of predictions with previously published data.