Enhanced Laminar Flow Convection Heat Transfer in Longitudinally Twisted Rectangular Ducts

Abstract

Low Reynolds number fully developed swirl flows through rectangular ducts that are helically twisted along their axis are considered. The twist ratio (180 degree twist pitch-to-hydraulic diameter) and flow cross-section aspect ratio (height-to-width) characterize the channel’s geometrical attributes. Based on computational modeling and simulation results, parametric influences of flow rate (10 ≤ Re ≤ 1000) and duct geometry on swirl-flow generation and the consequent heat transfer and friction losses are discussed. The helically twisted duct-surface curvature is found to induce lateral fluid circulation, which results in the formation of axially helical swirl in the core of the duct. With increasing severity of duct twist and flow Reynolds number, or decreasing cross-section aspect ratio, the swirl structure breaks up into multiple peripheral vortices but with increased magnitude of the primary core secondary-flow cell. Consequently, in fixed pressure-gradient driven flows, the heat transfer is enhanced significantly, which is evaluated relative to the friction loss penalty and the concomitant performance in straight ducts of the same cross section.