Characterization of Twisted Tape Induced Helical Swirl Flows for Enhancement of Forced Convective Heat Transfer in Single Phase and Two Phase Flows

Abstract

By generating helical swirling motion inside a tube with a twistedtape insert, forced convective heat transfer is significantly enhanced. The primary mechanism entails imparting a centrifugal force component to the longitudinal fluid motion, which superimposes secondary circulation over the main axial flow to promote crossstream mixing. Based on experimental flow visualization and computational modeling of singlephase laminar flows, a fundamental scaling of the crosssectional vortex structure and a parametric analysis of the primary enhancement mechanisms in singlephase flows are delineated. Heat transfer coefficient and friction factor correlations for both laminar and turbulent regimes are presented, and the damping effect of swirl on the transition region is highlighted. In flow boiling with net vapor generation, tapetwistinduced helical swirl pushes liquid droplets from the core to the wall to enhance heat transfer and delay dryout. In subcooled boiling, the radial pressure gradient due to the swirl promotes vapor removal from the heated surface to retard vapor blanketing and accommodate higher heat fluxes. The scaling and phenomenological descriptions of the underlying vaporliquid transport in these different boiling modes and regimes are presented along with any available predictive correlations.