The Navier-Stokes equations in stream-function/vorticity form were solved numerically by over-relaxation for the case of steady state, fully developed, isothermal, incompressible viscous Newtonian flow within a rigorously treated toroidal geometry. Solutions were obtained for curvature ratios ranging from 5 to 100 and for Dean numbers as low as 1 and as high as 1000. The Dean number was demonstrated to be the principal parameter to characterize toroidal flow; however, a second-order dependence upon the curvature ratio above that expressed in the Dean number was observed. Comparisons of the numerically computed axial-velocity profiles were made with experimental data. The cross-sectional pressure distribution was calculated, and a correlation is presented for a diametral pressure drop in terms of the Dean number. Curved configurations of circular tubes-such as partial coils, single coils, helical coils, and spiral coils-have received far less attention in the literature despite their frequent use in heat exchangers, chemical reactors, rocket engines, and other apparatus, equipment, or devices. In some cases, the use of curved tubes is necessitated because of geometrical restrictions. However, it is also becoming increasingly apparent that the nature of the complex primary (axial direction) and secondary (normal to primary) flow patterns in curved tubes makes possible some definite advantages of this configuration over straight tubes for a number of situations. In fully developed curved-tube viscous flow, the primary-flow-velocity profile is distorted from its parabolic straight-tube-flow counterpart, a secondary flow is established that consists of two vortices, and the resultant combined primary and secondary flow patterns cause a fluid element to have a screw-like motion. At one instant, a fluid element may be traveling near the very center of the tube cross section. After a short period of time and a short axial distance downstream, the same fluid element may be found very near the outside wall of the tube. The nature of curved-tube viscous-fluid motion, as compared with simple straight-tube parabolic flow, causes a higher axial-pressure gradient, a higher critical Reynolds number for transition to turbulent Row, a diametral-pressure gradient, a fluid-element residence-time distribution that more closely approximates plug flow, relatively high average heat-transfer and mass-transfer rates per unit axial pressure drop, especially for high-handtlnumber and high-Schmidt-number fluids, and significant peripheral distributions of the transport rates. The latter effect can be utilized to advantage in applications where the peripheral boundary conditions are asymmetrical.Of fundamental interest to the development of a complete understanding of viscous-flow phenomena in curved or coiled tubes is the nature of the velocity and pressure distributions in the fully-developed flow region. In studies by previous investigators, these profiles have been found to depend strongly on the Dean number N D , where N D , = NRe(R/Rc)'...