Observations of Transition Phenomena on a Controlled Diffusion Compressor Stator With a Circular Arc Leading Edge

Laminar-turbulent transition behavior is studied near the leading edge of an outlet stator blade in a low-speed 1.5-stage axial-flow research compressor. The stator is a typical controlled diffusion design with a circular arc leading edge profile. Slow-response surface pressure distribution measurements are compared with numerical predictions from the quasi-two-dimensional flow solver, MISES. These both show a strong flow acceleration around each side of the circular arc, followed by a rapid deceleration near each blend point of the arc to the main surface profile. The relative magnitude of the localized overspeeds varies significantly over the wide range of stator flow incidence investigated. The unsteady boundary layer behavior on the stator is studied using a midspan array of surface-mounted hot-film sensors. On the suction surface, wake-induced transitional and turbulent strips are observed to originate close to the leading edge. The boundary layer approaches separation near the leading edge blend point on the suction surface, but this does not always lead to localized turbulent breakdown or continuous turbulent flow: a significant portion of the flow on the forward part of the surface remains laminar between the wake-induced transitional strips. At high positive incidence the wake-induced transitional strips originate near the leading edge blend point, but their growth is suppressed by the strong flow acceleration. On the pressure surface, a small separation bubble forms near the leading edge blend point resulting in almost continuous turbulent flow over the whole incidence range studied.