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90-degree elbow pipe/duct is one of the piping element which is used frequently in industrial apparatus. The separated flow which occurs at inner corner of elbow part and the secondary flow which occurs at the elbow and its downstream cause significant pressure loss especially in the case of reducing elbow. Generally the inner corner is rounded or guide vane is mounted near the corner in order to reduce the losses by controlling this separation and secondary flow, however, the authors newly propose the mounting weir-shaped obstacles on the inner wall upstream of the corner. This method is easier to introduce to the duct/pipe which has a sharp edged inner corner, because machining the corner is not needed and the shape of the object mounted is simpler than guide vane. In this study, in order to reduce the pressure loss of reducing elbow ducts by installing weir-shaped obstacle and to investigate the effect of the cross-sectional reduction ratio on the loss reduction by the obstacle, pressure measurement and flow visualization using water were carried out. Experiments by varying the position (distance from the corner) and height of the obstacle made clear that the obstacle conditions which makes the flow resistance of reducing elbow duct smaller and the loss is reduced by about 30 -60%.

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