It was found experimentally that the three-dimensional pressure gradient developed in a circular-to-rectangular transition duct could be reduced in magnitude if the inlet boundary layer was thickened by a tripping device upstream. Accompanying with this phenomenon found was that the secondary flow formed in the corner region got diminished and the nonuniformity of the velocity distribution in the core region was increased. The condition under which the inlet flow was perturbed by a screen upstream was also investigated in this work. It was seen that the screen did not affect the inlet boundary-layer thickness appreciably, but promoted the laminar-turbulent transition process in the boundary layer and enhanced the ability of flow attaching to the diverging wall.
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