LDV Study of Developing Flows Through a Smooth Duct With a 180 deg Straight-Corner Turn

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Abstract
In view of the lack of velocity field data for flow through turbine blade internal cooling passages, laser-Doppler velocimetry measurements are presented for the flow development in a two-pass smooth rectangular duct of aspect ratio 1.1 with a 180 deg straight-corner turn with and without duct rotation. The test duct had a curvature axis normal to the rotational axis. The Reynolds number based on the bulk mean velocity and hydraulic diameter was 1.4 × 104 and the rotation numbers were 0 and 0.082. Characteristics such as the upstream and downstream extents of the sharp-turn effect on the main flow profile, curvature induced Dean vortices inside the turn, turning geometry-induced separating bubble immediately downstream of the turn, and the resulting double-peak mean velocity profiles in the second pass are used to describe the developing mean flow for the case without rotation. High turbulence levels and significantly more nonuniform flow after the sharp turn in the front part of the second pass explain previously reported work showing higher but nonuniform heat transfer after that turn. Rotating the duct augments and shifts the peaks of the streamwise mean velocity and turbulence intensity profiles toward the trailing and leading walls of the first and second passes, respectively. In addition, the duct rotation skews the separating bubble and reduces its size to about 75 percent of its stationary counterpart.

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