Flow and Loss Mechanisms Within an Interturbine Duct

Abstract

The present work is aimed at providing an advanced understanding of the flow and loss mechanisms in an interturbine duct subject to the upstream wakes and swirl. This interturbine duct is representative of modern engines; it has an area ratio of 1.275, a mean rise angle of 28 deg, and an axial length-to-annulus height of 3.4. The experimental results, supplemented with computations, were made inside the annulus at a Reynolds number of 150,000. The flow structures within the interturbine duct were found to be dominated by the counter-rotating vortices and boundary-layer separation in both the casing and hub regions. The combination of the casing-directed radial pressure gradient at the casing’s first bend and the upstream low-momentum wakes caused a pronounced roll-up of casing boundary layer. It was believed that this casing boundary-layer roll-up separated under the influence of the strong casing adverse pressure gradient. A pair of counter-rotating vortices was also generated at the casing’s second bend by the combination of the hub-directed radial pressure gradient and the casing boundary-layer low-momentum core resulting from the first bend casing boundary-layer roll-up. The duct loss gradually increased at the first bend and rapidly increased at the second bend. A fair agreement between the numerical and experimental results was achieved regarding the flow and loss mechanisms in the interturbine duct with the consideration of upstream wakes and swirl at the duct inlet. However, numerical investigation on the complex flow in interturbine duct by the assumption of fully mixed-out condition at duct inlet cannot capture these flow phenomena and is most likely overpredicting the duct overall losses.