SINOPSIS

Low Reynolds number fully developed swirl flows through rectangular ducts that are helically twisted along their axis are computationally modeled. The twist ratio ξ(= 180° twist pitch/hydraulic diameter) and rectangular flow cross-section aspect ratio α (= height/width) characterize the duct's geometrical attributes. A parametric numerical study delineates the influence of flow rate (10 ≤ Re ≤ 1000) and duct geometry (ξ = 3.0, 6.0, and 12.0; α = 1.0,0.75, and 0.5) on swirl-flow generation and wall friction behavior. The surface curvature of the twisted duct perturbs the axial flow tangentially, thereby inducing superimposed secondary circulation that produces helical swirl in the core of the duct. With increasing Reynolds number and severity of duct twist (ξ= 12.0 → 3.0), or decreasing cross-section aspect ratio (α = 1.0 or square → 0.5 or slender rectangle), the swirl structure is seen to break up into multiple peripheral vortices but with increased magnitude of the primary core secondary-flow cell. Consequently, infixed pressure-gradient-driven flows, the friction factor for twisted ducts increases significantly relative to that in straight ducts of the same cross section.