Influences of Inlet Swirl Distributions on an Inter-Turbine Duct: Part II—Hub Swirl Variation

The inter-turbine transition duct (ITD) of a gas turbine engine has significant potential for engine weight reduction and/or aerodynamic performance improvement. This potential arises because very little is understood of the flow behavior in the duct in relation to the hub and casing shapes and the flow entering the duct (e.g., swirl angle, turbulence intensity, periodic unsteadiness and blade tip vortices from upstream HP turbine blade rows). In this study, the flow development in an ITD with different inlet swirl distributions was investigated experimentally and numerically. The current paper, which is the second part of a two-part paper, presents the investigations of the influences of the hub swirl variations on the flow physics of ITD. The results show that the radial movement of the low momentum hub boundary layer and wake flow induces a pair of hub counter-rotating vortices. This pair of counter-rotating vortices merges with the upstream vorticity, forming a pair of stronger vortices, which persist until ITD exit. Due to the hub streamwise adverse pressure gradient, the hub 3D separation occurs at the exit of the ITD. The hub counter-rotating vortices are strongest with the highest inlet swirl gradient. The hub boundary layer thickness is thickest with the largest inlet hub swirl angle. The hub 3D separation is reduced by the increased hub swirl angle. Based on the studies in both parts of this paper, a detailed loss mechanism has been described. The total pressure coefficient shows that the loss increases gradually at the first bend, and then increases more rapidly at the second bend. The total pressure coefficients within the ITD are significantly redistributed by the casing and hub counter-rotating vortices.