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

Laminar flow correlations for f and Nu(m) are developed based on exper imental data for water and ethylene glycol, with tape inserts of three different twist ratios. The uniform wall temperature condition is con sidered, which typifies practical heat exchangers in the chemical and process industry. These and other available data are analyzed to devis e flow regime maps that characterize twisted-tape effects in terms of the dominant enhancement mechanisms. Depending upon flow rates and tap e geometry, the enhancement in heat transfer is due to the tube partit ioning and flow blockage, longer flow path, and secondary fluid circul ation; fin effects are found to be negligible in snug- to loose-fittin g tapes. The onset of swirl flow and its intensity is determined by a swirl parameter, Sw = Re(sw)/square-root y, that defines the interacti on between viscous, convective inertia, and centrifugal forces. Buoyan cy-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 correlatio ns describe the experimental data within +/- 10 to 15 percent, and the ir generalized applicability is verified by the comparison of predicti ons with previously published data.