Experimental investigation of turbulent flow through a circular-to-rectangular transition duct

An experimental study was conducted to investigate incompressible turbulent flow through a circular-torectangular transition duct having the same inlet and outlet cross-sectional areas, an overall length-to-diameter ratio of 4.5, and an aspect ratio of 3.0 at the exit plane. Downstream of the inlet, the local cross-sectional area increased to a value 15% above the inlet value and then decreased to the exit value. Mean flow and turbulence data were obtained by means of pressure probe and hot-wire instrumentation for an operating bulk Reynolds number of 3.9 x 10 5. The results show that curvature of the duct walls induces a relatively strong pressuredriven crossflow that develops into a contrarotating vortex pair near the diverging side walls of the duct. This vortex pair significantly distorts both the primary mean velocity and Reynolds stress fields. Law-of-the-wall behavior was observed in the near-wall region at the duct exit plane and, to a lesser extent, immediately after the end of transition. Analysis of the results shows that caution must be observed if conventional wall functions are used to predict the present flow situation.