Secondary flow vortex structures and forced convection heat transfer in fluid flow through curved elliptical ducts

Fluid flow through curved ducts is essentially characterised by the secondary flow effects due to duct curvature and cross-sectional flow geometry. Such flows produce vortex structures making the fluid behaviour vastly different than those in straight ducts while intrinsically promoting forced convection through fluid mixing. Examining the unique features of secondary flow and wall heat transfer, this paper presents a numerical simulation on the fluid flow through curved elliptical ducts, including circular geometry. The study develops and validates a novel numerical model based on three-dimensional vortex structures (helicity) and a curvilinear mesh system to overcome previous modelling limitations. Considering several duct aspect ratios, flow rates and wall heat fluxes, computations are performed to obtain the flow patterns and thermal characteristics. Parametric influences on flow features and forced convection are described through physical interpretation. The onset of vortices due to secondary flow instability is carefully examined in relation to the duct aspect ratio and flow rate. Appraising their merits, two techniques are developed for accurate detection of secondary flow instability and integrated into the computational process, which was not previously feasible. An approach based on the Second Law irreversibility is evaluated for thermal optimisation of fluid flow through curved elliptical ducts.