Development of a Turning Mid Turbine Frame With Embedded Design—Part II: Unsteady Measurements

Abstract

This paper, the second of two parts, presents a new setup for the two-stage two-spool facility located at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) of Graz University of Technology. The rig was designed to reproduce the flow behavior of a transonic turbine followed by a counter-rotating low pressure stage such as those in high bypass aero-engines. The meridional flow path of the machine is characterized by a diffusing S-shaped duct between the two rotors. The role of wide chord vanes placed into the mid turbine frame is to lead the flow towards the low pressure (LP) rotor with appropriate swirl. Experimental and numerical investigations performed on this setup showed that the wide chord struts induce large wakes and extended secondary flows at the LP inlet flow. Moreover, large deterministic fluctuations of pressure, which may cause noise and blade vibrations, were observed downstream of the LP rotor. In order to minimize secondary vortices and to damp the unsteady interactions, the mid turbine frame was redesigned to locate two zero-lift splitters into each vane passage. While in the first part of the paper the design process of the splitters and the time-averaged flow field were presented, in this second part the measurements performed by means of a fast response probe will support the explanation of the time-resolved field. The discussion will focus on the comparison between the baseline case (without splitters) and the embedded design.

Flow Evolution Through a Turning Midturbine Frame with Embedded Design

“…Detailed information on the design of the two-spool test rig was given by Hubinka et al [15,16]. The main parameters of the HP stage and the LP stage (TMTF LP rotor) as well as the operating conditions can be found in [11,13].…”

“…The baseline duct without splitters has been investigated in previous works [8], showing a large vane passage vortex due to a strong negative incidence at the strut blades and extended flow separations at the trailing-edge region. To reduce these loss structures and to homogenize the flow downstream of the TMTF, the splitter blades were designed based on a numerical investigation undertaken on this specific baseline configuration of the TMTF [11] at the aerodynamic design point. The axial extension of the splitters as well as the splitter count represented a compromise between aerodynamic effectiveness and solid blockage, whereas the meridional flow channel was kept the same as for the baseline case.…”

“…The generation and evaluation of several designs defined the final shape, which was manufactured and tested. More details of the design were published by Spataro et al [11].…”

“…Although the splitter design had the drawbacks of a higher engine weight and increased complexity, which resulted in higher costs, the advantages of the setup in regard to efficiency may offset these disadvantages. Spataro et al already showed in steady [11] and unsteady [12] measurements the reduction of the nonuniformity of pressure fluctuations (reduced strut wakes and secondary vortices) of the embedded design. In addition to the measurements, steady [13] and unsteady [14] computational fluid dynamics (CFD) simulations have been performed at the Institute for Thermal Turbomachinery and Machine Dynamics to analyze the flow structures within the TMTF with the embedded design.…”

“…Such a concept was investigated at the von Kármán Institute (e.g., Lavagnoli et al [10]) and at the Institute for Thermal Turbomachinery and Machine Dynamics. Spataro et al [11,12] proposed a setup embedding two splitters into the strut channel of an existing turning midturbine frame placed between two counter-rotating turbines. Although the splitter design had the drawbacks of a higher engine weight and increased complexity, which resulted in higher costs, the advantages of the setup in regard to efficiency may offset these disadvantages.…”