EFFECTS OF INLET CONDITIONS ON THE FLOW IN A FISHTAIL CURVED DIFFUSERWITH STRONG CURVATURE

The paper presents detailed measurements of the incompressible flow at the exit of a large-scale 90-degree curved diffuser with strong curva ture and significant streamwise variation in the cross-section aspect ratio. The diffuser flow path approximates the so-called fish-tail dif fuser utilized on small gas turbine engines for the transition between the centrifugal impeller and the combustion chamber. Five variations of the inlet boundary layer are considered. The results provide insigh t into several aspects of the diffuser flow including: the effect of f low turning on diffusion performance; the dominant structures influenc ing the flow development in the diffuser; and the effect of the inlet boundary layer integral parameters on the diffusion performance and th e exit velocity field. The three-dimensional velocity distribution at the diffuser exit is found to be sensitive to circumferentially unifor m alterations to the inlet boundary layer. In contrast, circumferentia l variations in the inlet boundary layer are observed to have only sec ondary effects on the velocity field at the diffuser exit. The static pressure recovery is observed to be comparable to the published perfor mance of conical diffusers with equivalent included angle and area rat ios. Furthermore, both the static pressure recovery and the total pres sure losses are observed to be relatively insensitive to variations in the inlet boundary layer. The physical mechanisms dominating the flow development in the diffuser are discussed in light of these observati ons.