Evolution of the flow through a turning mid turbine frame applied between a transonic hp-turbine stage and a counter-rotating LP-Turbine

Abstract

To enhance the performance of modern high-bypass jet engines a promising approach is to replace the low pressure-vane row arranged downstream of a strutted intermediate turbine diffuser (mid turbine frame) by the application of turning struts and to integrate functionalities such as bearing supports, lubrication pipes, engine mounts and aggressive duct designs. Thus the weight may be reduced which results in a lower specific fuel consumption and reduced CO2-emissions. The additional functions of the mid turbine frame (MTF) structures lead to constraints like a minimum required strut thickness leading to a maximum possible number of struts. Therefore a highly three-dimensional design of the airfoils and the duct, respectively, is necessary to reduce strong secondary flow effects and further to provide suitable flow angle and Mach number for the following low pressure turbine (LPT). A future area of application for mid turbine frames with turning struts, so-called turning mid turbine frames (TMTF), may also be in direct-drive open-rotor engine architectures. There the LPT with contra-rotating stator and rotor blades is arranged at relatively large radii and the rear bearings of these shafts are supported in the TMTF. The present paper examines a configuration consisting of a transonic HPT stage followed by an S-shaped TMTF and a counter-rotating LPT rotor at larger diameter. The test arrangement was applied in the newly designed continuously operating two-spool-two-stage turbine rig at Graz University of Technology. To gain a better understanding of the turbulent and unsteady flow field through the MTF which is dominated by strong secondary flows as well as the complex interactions between the components, detailed experimental investigations were performed by means of five-hole-probes at duct inlet and exit as well as downstream the LPT. The results from this unique HPT-Duct-LPT configuration will allow the designers to investigate how to reach optimal designs. This paper is part of the EU-project DREAM (ValiDation of Radical Engine Architecture SysteMs, contract No. ACP7-GA-2008-211861).