Abstract
The flowfield inside a rectangular-sectioned 90 deg bend was studied using laser Doppler velocimetry. The bend used in the present study was part of an auxiliary power unit for helicopter engines. The results showed strong tangential velocity gradients near the inlet of the 90 deg bend and reverse flow at the inlet corner near the top wall and the outer wall. Rapid mixing was observed as the flow traveled through the bend. Additionally, measurements were acquired at constant-theta planes with a high spatial resolution to better resolve the secondary flow structures. The geometric asymmetry in the circular-to-rectangular transition duct was measured, and the effect of this asymmetry on the downstream flowfield was characterized. Results showed that the asymmetry of the transition duct led to asymmetric flow through the bend. This was one of the few studies that showed how a small change in the upstream geometry yielded very different flow patterns, which were eventually mixed out by the exit of the bend, and thus provided some guidance on acceptable manufacturing tolerances.
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