Davide Lengani

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

Identification of Spinning Mode in the Unsteady Flow Field of a Low Pressure Turbine

This paper presents an experimental investigation of the vane-blade unsteady interaction in an unshrouded low pressure (LP) turbine research rig with uneven blade/vane count (72 blades and 96 vanes). The rig was designed in cooperation with MTU Aero Engines and considerable efforts were put on the adjustment of all relevant model parameters. In particular blade count ratio, airfoil aspect ratio, reduced mass flow, reduced speed, and Mach and Reynolds numbers were chosen to reproduce the full scale LP turbine at take off condition. Measurements by means of a fast-response pressure probe were performed adopting a phase-locked acquisition technique in order to provide the time resolved flow field downstream of the turbine rotor. The probe has been fully traversed both in circumferential and radial traverses. The rotor exit is characterized by strong perturbations due to the tip leakage vortex and the rotor blade wake. Circumferential nonuniformities due to the upstream vane wake and to the downstream exit guide vane potential effects are also identified. Furthermore, in the present configuration with an uneven blade/vane count the nonuniformities due to the stator and rotor row are misaligned along the whole turbine circumference and create a spinning mode that rotates in direction opposite to the rotor at a high frequency. The aeroacoustic theory is employed to explain such further unsteady pattern. The variations of the exit flow angle within a cycle of such pattern are not negligible and almost comparable to the ones within the blade passing period.

Aerodynamic Effects of an Unshrouded Low Pressure Turbine on a Low Aspect Ratio Exit Guide Vane

This paper presents the effects of an unshrouded low pressure turbine (LPT) onto the following exit guide vane row (EGV). The measurement results were obtained in the subsonic test turbine facility at Graz University of Technology by means of a fast response pressure probe in planes downstream of the rotor as well as oil flow visualisation. The test rig was designed in cooperation with MTU Aero Engines and represents the last 1.5 stages of a commercial aero engine. Considerable efforts were put into the adjustment of all relevant model parameters to reproduce the full scale LPT situation.Different tip clearances were evaluated by means of CFD obtained using a commercial Navier-Stokes code and validated with experimental results. The goal is to evaluate the effect of the varying leakage flow on the flow in the low aspect ratio EGV. Special attention is given to the impact on the development of secondary flows as well as the flow structures downstream of the EGV. The effect of the leakage flow causes a change of the flow structure of the EGV, particularly losses. Considering the largest investigated tip-clearance, the losses increased by 71% when compared to a zero-leakage case.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

Influence of Aerodynamic Loading on Rotor-Stator Aerodynamic Interaction in a Two-Stage Low Pressure Research Turbine

The unsteady flow within a two-stage low-pressure research turbine equipped with high lift profiles has been investigated in detail for three different aerodynamic loading conditions. Experiments have been carried out at low speed. Velocity and turbulence intensity in the blade-to-blade plane at midspan have been measured by means of a crossed hot-wire probe, upstream and downstream of each blade row. The probe has been traversed circumferentially over 1.5 bladings pitch and the phase-locked data acquisition and ensemble average technique have been used to reconstruct the flow in space and time. The effects of multistage configuration have been identified and analyzed by considering the velocity components and turbulence intensity. Potential interaction from the downstream blading in relative motion, periodic wake perturbations from the upstream blading and preceding stage perturbations make the flow in the second stage extremely complex. Overall the flow downstream of rotors is perturbed in space by upstream and downstream stators, while flow downstream of stators is mostly perturbed in time by rotor effects. As expected, high lift profiles are significantly sensitive to incidence variation, with this effect further enhanced by the multistage cumulative interactions.

Boundary Layer Separation Control on a Flat Plate With Adverse Pressure Gradients Using Vortex Generators

The continuous tendency in modern aeroengine gas turbines towards reduction of blade count and ducts length may lead to aerodynamic loading increase beyond the limit of boundary layer separation. For this reason boundary layer separation control methods, up to now mostly employed in external aerodynamics, begin to be experimented in internal flows applications. The present paper reports the results of a detailed experimental study on low profile vortex generators used to control boundary layer separation on a large-scale flat plate with prescribed adverse pressure gradients. Inlet turbulent boundary layer conditions and pressure gradients are representative of aggressive turbine intermediate ducts. This activity is part of a joint European research program on Aggressive Intermediate Duct Aerodynamics (AIDA). The pressure gradients on the flat plate are generated by increasing the aperture angle of a movable wall opposite to the flat plate. To avoid separation on the movable wall, boundary layer suction is applied on it. Complementary measurements (surface static pressure distributions, surface flow visualizations by means of wall mounted tufts, instantaneous and time-averaged velocity fields in the meridional and cross-stream planes by means of Particle Image Velocimetry) have been used to survey the flow with and without vortex generators. Three different pressure gradients, which induce turbulent separation in absence of boundary layer control, were tested. Vortex generators height and location effects on separation reduction and pressure recovery increase were investigated. For the most effective VGs configurations detailed analyses of the flow field were performed, that demonstrate the effectiveness of this passive control device to control separation in diffusing ducts. Particle Image Velocimetry vector and vorticity plots illustrate the mechanisms by which the vortex generators transfer momentum towards the surface, re-energizing the near-wall flow and preserving the boundary layer from separation.Copyright

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

Investigation of the Dynamics of an Ultra Low NOx Injection System by POD Data Post-Processing

The present paper is focused on the investigation of the dynamics of the flow downstream of an Ultra Low NOx (ULN) injection system, designed to reduce NOx emissions and combustor axial length. Two rectangular flame tubes have been experimentally investigated: one aimed at simulating an unconfined exit flow, and another with the same transverse dimensions of the combustor annular sector, to simulate the confined flow field. The effects induced by the realistic flame tube presence are investigated comparing the flow field with that generated in the unconfined case. Particular attention is paid to the vortex breakdown phenomena associated with the flow generated by the two co-rotating swirlers constituting the injection system.Two different and complementary measurement techniques have been adopted to characterize the aerodynamics of the vortex breakdown. The hot-wire investigation results reveal the frequencies associated with the precession motion due to the vortex breakdown. The Particle Image Velocimetry technique has been coupled with Proper Orthogonal Decomposition (POD) for data post-processing in order to reconstruct the swirling motion generated by the injection system. The property of POD, which consists of splitting temporal from spatial information of the flow field in analysis, allows the distinction between deterministic and random fluctuations without the need of an external trigger signal. This feature is fundamental for the better understanding of an highly-swirling flow.Copyright