Turbulent flow and heat transfer of helical coils with twisted section

Abstract

An innovative heat transfer enhancement method that integrates helical coil and twisted channel to augment swirl intensity is newly proposed. The characteristics of turbulent flow and heat transfer of five twisted helical coils with identical helical geometries but different twist pitches are numerically explored using ANSYS-Fluent code. Acting by the twist forces along the twisted helical coils, the swirl number, Sw, and turbulent kinetic energy of the flows are enhanced from those in the untwisted helical coils. Swirl enhancements in the twisted helical coils are derived as the function of channel twist ratio. The correlative flow and heat transfer analysis confirms the enhanced swirl intensity is the dominant flow physic for heat transfer enhancement due to the channel twist. The ratios of Nusselt number and friction factor between the twisted and untwisted helical coils at different Reynolds numbers and twist ratios are converged into the Sw functions. With all the thermal performance factors above than unity for the present twisted helical coils, the twisted coil with the twist ratio of three channel hydraulic diameters offers the maximum 19% of heat transfer elevation and 69.8% of friction augmentation from the untwisted-coil references. Based on the numerical heat-transfer and pressure-drop results, two sets of correlations that permit the evaluations of averaged Nusselt number at developed flow regime and the Fanning friction factor of the twisted helical coils are developed for relevant applications.