For this work, reality effects, more precisely backward and forward facing steps, and their influence on the flow through a two-stage two-spool turbine rig under engine-relevant conditions were experimentally investigated. The test rig consists of a HP and a LP stage, with the two rotors rotating in opposite direction with two different rotational speeds. An S-shaped transition duct, which is equipped with turning struts (so-called turning mid turbine frame or TMTF) and making therefore a LP stator redundant, connects both stages and leads the flow from a smaller to a larger diameter.
This test setup allows the investigation of a TMTF deformation, which occurs in a real aero-engine due to non-uniform warming of the duct during operation — especially during run up — and causes backward and forward facing steps in the flow path. This happens for non-segmented ducts, which are predominantly part of smaller engines. In the case of the test rig, steps were not generated by varying temperature but by shifting the TMTF in horizontal direction while the rotor and its casing were kept in the same position. In this way, both backward and forward facing steps between duct endwalls and rotor casing could be created. In order to avoid steps further downstream of the interface between HP rotor and TMTF, the complete aft rig was moved laterally too. In this case, the aft rig incorporates amongst others the LP rotor, the LP rotor casing and the deswirler downstream of the LP stage.
In order to catch the influence of the steps on the whole flow field, 360 deg rake traverses were performed downstream of the HP rotor, downstream of the duct and downstream of the LP rotor with newly designed, laser-sintered Combi-Rakes for the measurement of total pressure and total temperature. Only the compact design of the rakes, which can be easily realised by additive manufacturing, makes the aforementioned 360 deg traverses in this test rig possible and allows a number of radial measurements positions, which is comparable to those of a five hole probe. To get a more detailed information about the flow, also five hole probe measurements were carried out in three measurement planes and compared to the results of the Combi-Rakes.