Rosario Spataro

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

© 2014 by ASME. The paper 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 in order to simulate the flow behavior of a transonic turbine followed by a counter-rotating low pressure (LP) stage like the spools of a modern high bypass aeroengine. The meridional flow path of the machine is characterized by a diffusing S-shaped duct between the two rotors. The role of turning struts placed into the mid turbine frame is to lead the flow towards the LP rotor with appropriate swirl. Experimental and numerical investigations performed on the setup over the last years, which were used as baseline for this paper, showed that wide chord vanes induce large wakes and extended secondary flows at the LP rotor inlet flow. Moreover, unsteady interactions between the two turbines were observed downstream of the LP rotor. In order to increase the uniformity and to decrease the unsteady content of the flow at the inlet of the LP rotor, the mid turbine frame was redesigned with two zero-lifting splitters embedded into the strut passage. In this first part of the paper the design process of the splitters and its critical points are presented, while the time-averaged field is discussed by means of five-hole probe measurements and oil flow visualizations. The comparison between the baseline case and the embedded design configuration shows that the new design is able to reduce the flow gradients downstream of the turning struts, providing a more suitable inlet condition for the low pressure rotor. The improvement in the flow field uniformity is also observed downstream of the turbine and it is, consequently, reflected in an enhancement of the LP turbine performance. In the second part of this paper the influence of the embedded design on the time-resolved field is investigated.

EXPERIMENTAL INVESTIGATION OF THE NOISE GENERATION AND PROPAGATION FOR DIFFERENT TURNING MID TURBINE FRAME SETUPS IN A TWO-STAGE TWO-SPOOL TEST TURBINE

The paper deals with the investigation of the noise generation in the two-stage two-spool test turbine located at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) at Graz University of Technology. The rig went into operation within the EU-project DREAM, where the target was to investigate the aerodynamics of interturbine flow ducts. The facility is a continuously operating cold-flow open-circuit plant which is driven by pressurized air. The flow path contains a transonic turbine stage (HP) followed by a low pressure turbine stage consisting of a turning mid turbine frame and a counter-rotating LP-rotor.Downstream of the low pressure turbine a measurement section is instrumented with acoustic sensors. The acquisition system consists of a fully circumferentially traversable microphone array located at the outer casing, while at the hub endwall a stationary flush mounted microphone is placed as a reference.Additionally a new embedded concept for the turning mid turbine frame was tested. Here, two zero-lift splitters were located into the vane passage.In order to evaluate the noise emission of the turbine the facility was instrumented with a new acoustic measurement setup which is presented in the paper. Therefore the emitted sound pressure level and the microphones signal spectra are compared for both configurations. The acoustic field was characterized by azimuthal modes by means of a microphone array traversed over 360 degrees.In the multisplitter configuration, the propagating modes due to the HP turbine are found suppressed by 5 dB, while the increase in amplitude of the modes related to the LP turbine is negligible. The overall effect is a reduction of the acoustic emission for the turning mid turbine frame with embedded design.Copyright

Experimental Investigation of the Unsteady Flow Field Downstream of a Counter-Rotating Two-Spool Turbine Rig

The paper presents an experimental investigation of the unsteady flow field in the two-spool counter-rotating transonic turbine at Graz University of Technology. The test setup consists of a high pressure (HP) stage, a diffusing mid turbine frame with turning struts (TMTF) and a shrouded low pressure (LP) rotor. The two rotors are mounted on mechanically independent shafts in order to provide engine-representative operating conditions. The rig was designed in cooperation with MTU Aero Engines and Volvo Aero within the EU project DREAM (ValiDation of Radical Engine Architecture SysteMs).A two-sensor fast response aerodynamic pressure probe (2S-FRAP) has been employed to provide time-resolved aerodynamic area traverses downstream of the LP turbine. Such measurement allows estimating the total deterministic unsteadiness transported through the duct. In particular, it has been possible to isolate the structures induced by each rotor by means of the encoders mounted on the two shafts. A global ensemble averaged field, which takes into account the rotor-rotor interactions, is also provided. The time resolved distributions of the flow quantities are then discussed in details. The perturbations due to the HP rotor in terms of velocity and flow angle are negligible in this downstream plane. Indeed, the largest fluctuations of velocity are due to the TMTF-LP rotor interaction, they occur in the wake and secondary flows of the TMTF. Large fluctuations of static and total pressure are instead due to both rotors to the same extent.Copyright

On the Flow Evolution Through a LP Turbine With Wide-Chord Vanes in an S-Shaped Channel

The paper discusses the time averaged flow field in a test facility located at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) of Graz University of Technology. The rig was designed in order to reproduce the flow leaving a transonic turbine through a following counter rotating low pressure stage. This configuration is common in modern multi-shaft jet engines and will become a standard in the future.The discussion on the flow field is based on numerical results obtained by a commercial CFD code and validated by aerodynamic measurements and oil flow visualization performed on the facility itself. The meridional flow path of the machine is characterized by a diffusing S-shaped duct between the two rotors. Within the duct turning struts lead the flow to the following rotor. The LP stage inlet condition is given by the outlet flow of the high pressure turbine whose spanwise distribution is strongly affected by the shape of the downstream S-channel. A special focus is concentrated on the generation and propagation of secondary flows in such a turning mid turbine frame (TMTF). The aim of the present work is to isolate the flow structures moving from the outlet of the transonic stage through the low pressure stage and identify their effect on the time-averaged flow.The main outcome of this paper is that, whenever a TMTF is placed between counter-rotating high pressure and low pressure turbines, the structures coming from the upstream rotor will not decay (like in a co-rotating setup), but they will be convected and transported towards the downstream rotor. Moreover, the turning of the struts will enhance the vorticities generated by the upstream turbine. The application of technical solutions such as embedded TMTF designs or endwall contouring should be aimed to reach LP rotor uniform inlet conditions, minimize the TMTF secondary flows and thus to damp the rotor-rotor interaction.© 2012 ASME

Analysis of the Unsteady Flow Field in Turbines by Means of Modal Decomposition

This paper presents the results of a modal decomposition method applied to the time resolved data of two different test turbines. The analysis is carried out on the measurements performed by fast response aerodynamic pressure probes as well as on CFD simulations. As shown in the earlier aeroacoustic theory, a plurality of rotating patterns, also called spinning modes, are generated by the rotor-stator interactions. The modes may be computed from the flow quantities, such as total pressure, velocity and flow angles, through Fourier decompositions performed in time and space. The deterministic unsteadiness is then simplified to a limited number of Fourier coefficients. At a fixed radial position, circumferential lobes are identified for any multiple of the blade passing frequency. Therefore, the flow may be described as the superposition of rotating patterns, the spatial characteristics of which are correlated to the linear combinations of blade/vane number.This analysis has been applied to a one and a half stage low pressure turbine and to a two-stage counter-rotating transonic turbine. In the former test case there is a limited number of modes that characterize the flow field. Hence, the decomposition in modes simplifies considerably the evaluation of the sources of unsteadiness and deterministic stresses. The second test case presents more complex interactions. In fact, the presence of two rotors induces oscillations at frequencies that corresponds to the linear combinations of the two blade passing frequencies. Circumferential modes are identified for the most characteristic frequencies and their physical meaning is discussed.Copyright

Unsteady flow evolution through a turning midturbine frame Part 2: Spectral analysis

This paper analyzes the propagation of the aerodynamic deterministic stresses through a two-spool counter-rotating transonic turbine at Graz University of Technology. The test setup consists of a high-pressure stage, a diffusing midturbine frame with turning struts and a counter-rotating low-pressure rotor. The discussion of the data is carried out in this second part paper on the basis of spectral analysis. The theoretical framework for a double Fourier decomposition, in time and space, is introduced and discussed. The aim of the paper is the identification of the sources of deterministic stresses that propagate through the turbine. A fast-response aerodynamic pressure probe has been employed to provide time-resolved data downstream of the high-pressure rotor and of the turning strut. The fast-response aerodynamic pressure probe measurements were acquired together with a reference signal (a laser vibrometer) downstream of the high-pressure rotor to identify different sources of deterministic fluctuations...

Flow Evolution Through a Turning Mid Turbine Frame With Embedded Design

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The Influence of Blade Sweep Technique in Linear Cascade Configuration

One key issue in the advanced aerodynamic optimization of turbomachinery involves the application of 3D blade design techniques. The complex shape of the resulting blades is often a combination of simple techniques such as sweep. Such a blade arrangement is often imposed to the designer by structural constrains, space reduction needs, diameter optimization or spanwise blade loading control. This work aims to study the aerodynamic effects produced on turbine passages by blade sweep; with this term we refer to a configuration where the flow mainstream direction and the blade stacking axis are not orthogonal. A linear cascade of turbine blades, obtained by stacking the same profile with a sweep angle of 20 degrees, was investigated in a blow down facility at an isentropic downstream Mach number of 0.65. Standing the low aspect ratio of the cascade, the blade was built by simply shifting in axial direction the 2D profile originally used in the reference prismatic blade. The choice to build the swept blade keeping the same 2D section parallel to the incident flow was considered taking into account the blade low aspect ratio. Measurements were performed by means of blade surface pressure taps and five holes probe traversing downstream of the cascade; oil and dye flow visualizations were also performed to study the effects on the secondary vortices evolution inside of the passage. Moreover, a commercial CFD code was applied to provide information on the flow field all along the passage. The same profile was already extensively investigated both by measurements and CFD calculations [1, 2] in order to clarify the effects of blade lean and bowing. This additional paper gives a final contribution addressed to deeply understand the aerodynamic effects produced on turbine cascade flow field by the separate application of each one of the typical 3D design techniques. Results from both the experimental and computational investigations are presented and discussed in the paper where a phenomenological approach has been preferred. Measurements of the blade surface pressure distribution, performed at several blade heights, support the analysis of the pressure field inside of the passage which is mainly based on numerical results. In particular, the paper shows the influence of pressure contours shape on streamwise vorticity inside and downstream of the passage focusing the attention on secondary structures. The downstream vorticity field is then discussed together with the loss distribution in the same region to provide a more exhaustive description.Copyright

Flow evolution through a Turning Mid Turbine Frame with embedded design

The paper discusses the time-averaged flow of a new-concept turbine transition duct placed in a two-stage counter-rotating test turbine. As a possible architecture for the turbine transition duct o...

Unsteady flow evolution through a turning midturbine frame Part 1: Time-resolved flow

This paper identifies and analyzes the propagation of aerodynamic deterministic stresses through a two-spool counter-rotating transonic facility representative of modern and future turbine aeroengine sections. The test setup consists of a high-pressure stage, a diffusing turning midturbine frame with turning struts, and a counter-rotating low-pressure rotor. The flowfield downstream of the high-pressure stage is strongly influenced by the stator–rotor interaction. Such a mechanism interacts again with the downstream turning midturbine frame leading to a vane–rotor–vane interaction, which affects the behavior of the low-pressure stage. The results presented were obtained using a fast-response aerodynamic pressure probe for unsteady measurements as well as three-dimensional unsteady Reynolds-averaged Navier–Stokes calculations. The work is presented in two parts. This first part focuses on the explanation of the flow physics that governs the convection of unsteady three-dimensional flow through the midturbi...