Numerical study of secondary flows in curved ducts.

摘要

The occurrence of secondary flow in curved ducts due to the centrifugal forces as well as wall temperature effects can often significantly influence the volume flux and heat transfer rates. In the present work, the secondary flow of an incompressible viscous fluid in a curved duct with and without sidewall heating is studied by using a finite volume method. It is known that for low Dean numbers, the secondary flow is characterized by a pair of counter-rotating vortices. The present study shows that as the Dean number is increased the secondary flow structure evolves into a double vortex pair for low aspect ratio ducts and roll-cells for ducts of high aspect ratio.;A stability diagram is obtained in the domain of curvature ratio and Reynolds number. It is found that for ducts of high curvature the onset of instability depends on the Dean number and the curvature ratio, while for ducts of small curvature the onset can be characterized by the Dean number alone. A comparison with the available theoretical and experimental results indicates good agreement. A correlation for the friction factor as a function of the Dean number and aspect ratio is developed. It is found to be in good agreement with the available experimental and computational results for a wide range of parameters.;When there is sidewall heating, the interaction between the centrifugal and the buoyancy forces characterizes the secondary flow structure. It is shown that as the Grashof number is increased, the friction factor can decrease due to the transition from centrifugally dominant flow to buoyancy dominated flow. It is also found that for curved ducts the inertial effect dominates and the heat transfer may be enhanced at lower Grashof numbers. In addition, over the range in which the computations were performed the curvature effect is shown to enhance the heat transfer rate.;Computations for turbulent flow were carried out using a non-linear two-equation turbulence model. The model is used to predict turbulent secondary flows in straight and curved ducts. Comparison with available experimental results indicates good agreement.