Hydrodynamically and thermally developing laminar flow in spiral coil tubes

In this study, steady state combined developing flow and heat transfer in spiral tube coils is numerically investigated. The spiral coil is isothermal and the fluid flow is laminar. The spiral coils with four different curvature ratios of defined as the radius of the spiral at the outlet (Ro) to the radius at the inlet (Ri), Ro/Ri = 2.5, 5, 7 and 17, are simulated for Pr = 0.7 (air) and Pr = 7 (water). The cold fluid is assumed to enter the coil from the innermost turn of the spiral tube. The three-dimensional steady-state continuity, Navier-Stokes and energy equations are solved using the commercially available CFD software-Fluent v6.1.22®. The effects of the spiral tube pitch, curvature ratio, the Prandtl and the Dean number on the friction factor and the heat transfer are investigated for combined developing flow. With increasing Reynolds number, the heat transfer is enhanced 2-4 times over straight tubes of the same length due to secondary flow and centrifugal forces. The friction losses also increase to some extent. As the number of turns is increased, the normalized friction factor tends to decline towards the straight tube values. Useful correlations for the normalized apparent friction factor and the mean axial Nusselt number were generated.