Circular-to-rectangular transition duct flow without and with inlet swirl

Circular-to-rectangular transition ducts are used as exhaust system components of aircraft with rectangular exhaust nozzles. Often, the flow into these ducts includes a swirling velocity component remaining from the gas turbine engine. Previous transition duct studies involving detailed experimental measurements have not compared the flowfield without and with inlet swirl. The study reported in this article explored circular-to-rectangular transition duct flows without and with inlet swirl in order to document the effect of inlet swirl on the transition duct flowfield and to provide detailed duct flow data for comparison with numerical code predictions. A method was devised to develop a swirling, solid-body rotational flow with minimal associated disturbances. Coefficients based on velocities and total and static pressures measured in cross-stream planes at four axial locations within the transition duct, along with surface static pressure measurements and surface oil-film visualization, are presented and discussed for both nonswirling and swirling incoming flows. In both cases the inlet centerline Mach number was 0.35. The differences between flowfields for the two cases were striking. Two pairs of counterrotating side-wall vortices appeared in the duct flow without inlet swirl. These vortices were absent in the swirling incoming flow case.