Computational fluid dynamics study on the drag and flow field differences between the single and coupled bends

Abstract

Owing to the limited installation space and duct size, coupled fittings are common in the duct systems of buildings. The coupling effect leads to changes in drag and fan energy consumption. This study investigates duct drag and flow field characteristics under coupling conditions. Experiments and numerical simulations with the Reynolds stress model are conducted. Flow field changes, flow field deformation, and drag changes in the duct are analyzed. Regardless of the coupling form, the velocity near the inner arc is fast, whereas that near the outer arc is slow. Under three different coupling connection conditions (S-shaped, L-shaped, and U-shaped), the outlet velocity gradient of the U-shaped coupling connection is the least obvious. After the fluid flows through the bend, a significant centerline velocity reduction can be observed, even greater than that in the bend. The lowest centerline velocity lies within the range of 2.5 D to 4.5 D after the bend. Coupling connection has an insignificant effect on upstream duct resistance. The resistance of single bend is less than that of the downstream bend for the coupled bend and greater than that of the upstream bend under coupling conditions. Practical application: Coupling effect is common in practical application of ventilation engineering. This effect leads to the change of fluid resistance loss of ducts and pipes. However, few researchers focus on this effect. This study finds that regardless of the coupling form, the velocity near the inner arc is fast, whereas that near the outer arc is slow. It means the guide vane should be set near inner arc. L-shaped coupling connection has the largest downstream piping resistance. The resistance of the downstream piping under S-shaped coupling is the least, thus L-shaped coupling connection should be avoided as far as possible in practical application.