A numerical analysis of the axial and secondary flow behavior in a curved annular sector duct is presented in the paper. The flow is considered to be fully developed laminar flow with constant physical properties. Five parameters have been identified as major variables in controlling the flow behavior. The study indicates that with a moderate Dean number and when the sector angle is smaller than π/2, only two vortices will appear in the cross section of the curved annular sector duct. When the sector angle is larger than π/2, the vortex structure can be very complex, and is often determined by other parameters, especially by the angle between the annular sector duct centerline and the curvature radius direction. The friction coefficient of the curved annular sector duct is affected mainly by the radius ratio, curvature, and axial pressure gradient. The radius ratio of the inner/outer walls can affect the vortex structure only when the radius ratio is very small. When the radius ratio is larger than 0.6, the friction coefficient is only slightly higher than that of a straight annular sector duct. Nevertheless, for the small radius ratio duct, the friction coefficient can be tripled, as compared with a straight annular sector duct. Although the holding pipe curvature and the axial pressure gradient cannot significantly change the vortex structure of the secondary flow, they can however, remarkably increase the friction coefficient by increasing the velocity gradient near the solid boundary.

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