A simple mathematical model following the suggestion of Smithberg and Landis has been created to predict the friction factors for the case of a fully developed turbulent flow in a spirally corrugated tube combined with a twisted tape insert. The flow field is divided into two principal regions-a helicoidal core flow modified by secondary circulation effects and a twisting boundary layer flow. The "wall roughness" has an effect simultaneously on the axial velocity, secondary fluid motion and the resulting swirl mixing. The model reflects the influence of the "wall roughness" on the axial and tangential velocity components. The calculated friction factors have been compared with 570 experimental points obtained from 57 tubes tested. Five hundred points (87.7%) have a relative difference of less than +/-15%; fifty one points (8.9%) have a difference of within +/-(15-20)% and only nineteen points (3.4%) have a relative difference greater than +/-20%. (C) 2003 Elsevier Ltd. All rights reserved.