HEAT-TRANSFER ENHANCEMENT IN COILED TUBES BY CHAOTIC MIXING

The present work examines chaotic mixing as a means of enhancing the in-tube convection heat transfer in helical coils. It is shown that simple modifications of coil geometry can be made to take advantage of this phenomenon. The study focuses on a geometry with the axis of the coil being turned through 90-degrees in a periodic manner, and comparisons are made with a coil without this change in axis. Particle paths are calculated using the classical perturbation solution of Dean for the secondary flow. Chaotic mixing is confirmed by a positive Lyapunov exponent. The temperature field is calculated numerically showing that chaotic mixing is responsible for considerable flattening of the temperature profile and an increase in convective heat transfer. Experiments are conducted with water on two coiled tube geometries over a Reynolds number range of 3000-10000. The coils are identical in every respect except that one is conventional with constant axis, while the other is with alternating axis. The latter shows a 6-8% higher in-tube heat transfer coefficient due to chaotic mixing, with a corresponding pressure drop increase of 1.5-2.5%.