Experimental and numerical investigation of heat transfer enhancement in coiled tubes by chaotic mixing

The potential for chaotic dynamics in furthering mixing is exploited to enhance in-tube heat transfer in a coiled tube configuration. The basis for this enhancement stems from periodic spatial modulations of the base flow state through boundary perturbations effected by rotating the coil axis about the tube axis at periodic axial intervals. A simplified model based on Dean's flow is used to show that the particle pathlines for the proposed heat exchanger configuration are chaotic with a positive Lyapunov exponent.;An experimental investigation is undertaken to compare the performance of a helical coil heat exchanger to that of the proposed alternating axis coil configuration. In-tube inlet and outlet temperatures are obtained through thermocouple measurements and the pressure drop across the coil by a differential manometer. The calculated inner heat transfer coefficient is found to be enhanced by 6-8% over a Reynolds number range of between 3000-10,000.;A three-dimensional numerical investigation of steady laminar flow and heat transfer based on the control volume method is undertaken to study developing as well as fully developed thermal fields. Loosely coiled tubes are examined at large Reynolds numbers under the parabolic approximation. The Nusselt number increase in the thermal entrance region is explained by means of a simplified model and the circumferential averaged axial Nusselt numbers are scaled onto a single curve. The transverse flow field bifurcation to a four vortex structure beyond a Reynolds number of 1900 leads to little change in the Nusselt number and friction factor as compared to the stable 2-roll structure.;Introduction of spatial modulation by periodic rotation of the tube axis is shown to increase the Nusselt number over that of helical coils with little change in pressure drop. The enhancement is quantified as a function of the curvature ratio, switching angle and the flow Reynolds number.