Unsteady CFD simulation of a turning mid turbine frame with embedded design

This paper discusses the unsteady CFD simulation results of a new concept of an intermediate turbine duct between the high and low pressure turbine of a jet engine. In order to meet the requirements of future jet engines it is helpful to supply the S-shaped intermediate turbine duct with struts carrying the bearing loadings. These struts can be aerodynamically optimized to generate swirl in order to replace the first vane row of the subsequent low pressure rotor. In such a design large flow structures coming from the outlet of the transonic high pressure stage are transported towards the low pressure rotor and are superimposed by secondary flows generated by the turning struts within the duct. These effects lead to a very inhomogeneous flow reaching the downstream low pressure rotor which leads to higher losses of this rotor. In order to reduce the fluctuations behind the S-shaped duct and to homogenize the flow, the duct can be equipped with splitters. Such an embedded design can homogenize the duct outlet flow, thus improving efficiency. In this paper the results of a steady and unsteady CFD simulation of such a turning mid turbine frame with splitters as experimentally investigated in the transonic test turbine facility at the Institute for Thermal Turbomachinery and Machine Dynamics of Graz University of Technology are discussed. A special focus is laid on the interaction between the high pressure stage and the intermediate turbine duct and the thus induced unsteady effects. Expected differences between the steady and unsteady simulation are shown. Additionally the positive effect of the splitters on the uniformity of the flow is investigated.