Thorsten Selic

Comparison of the Aerodynamics of Acoustically Designed EGVs and a State-of-the-Art EGV

Within previous EU projects, possible modifications to the engine architecture have been investigated, that would allow for an optimised aerodynamic or acoustic design of the exit guide vanes (EGV) of the turbine exit casing (TEC). However, the engine weight should not be increased and the aerodynamic performance must be at least the same.This paper compares a state-of-the art TEC (reference TEC) with typical EGVs with an acoustically optimised TEC configuration for the engine operating point approach. It is shown that a reduction in sound power level for the fundamental tone (1 blade passing frequency) for this acoustically important operating point can be achieved. It is also shown that the weight of the acoustically optimised EGVs (only bladings considered) is almost equal to the Reference TEC, but a reduction in engine length can be achieved.Measurements were conducted in the subsonic test turbine facility (STTF) at the Institute for Thermal Turbomachinery and Machine Dynamics, Graz University of Technology. The inlet guide vanes, the low pressure turbine (LPT) stage, and the EGVs have been designed by MTU Aero Engines.© 2014 ASME

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

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

Acoustic Comparison of Different Turbine Exit Guide Vane Designs Part 2: Experimental Analysis

In this paper the sound power levels of three different designs of turbine exit guide vanes (TEGV) of turbine exit casings are compared with a standard design of an TEGV. The comparison is made with respect to the LPT for the acoustically relevant operating point approach. Additionally a rough loss estimation is also given in this paper. It is shown that the acoustically optimised TEC reduces the sound power level of the main interacion modes by about 14 dB while the aeroynamically optimised TEC even increases the sound power level by 2 dB. All three TEGV designs show higher aerodynamic losses for this off design point (approach). The measurements have been conducted in the subsonic test turbine facility at the Institute for Thermal Turbomachinery and Machine Dynamics, Graz University of Technology.

Comparison of the Experimental Results Between a 2D EGV Cascade Test and a Rig Test Under Engine Representative Conditions

Today the exit guide vanes (EGV) of the turbine exit casing (TEC) of a bypass-engine have to fulfill three major functions. Firstly, they have to support the rear bearing of an aero engine and to provide space to lead through different supply lines, secondly the engine mount is supported and therefore they have to be rigid and large in thickness. Thirdly, the EGVs have to reduce swirl of the last stage LPT (carrying the aerodynamic load) in order to transform it into thrust for highest propulsive efficiency. Further, if available they have to provide the correct flow field for the following component e.g. a mixer as used in low by-pass ratio engines. Such a state-of-the-art EGV is subject to a 2D cascade test performed at the Institute for Fluid Mechanics at Technical University Braunschweig and a rig test performed at the Institute for Thermal Turbomachinery at Graz University of Technology. This work presents the differences in the results between these two tests due to three dimensional effects such as incoming wakes, turbulence and radial variations in swirl, total pressure and yaw angle over the passage height.Copyright

Verification of the Inverse Cut-off Effect in a Turbomachinery Stage - Part 1 - Numerical results

Within the scope of the author’s previous papers, the general setup of an acoustic turbine test facility at the Graz University of Technology has been presented. In addition, its significance for the validation of acoustic prediction tools and the verification of noise reduction features related to the LPT (Low Pressure Turbine) has been highlighted. In contrast to the passive noise reduction feature presented last year, an innovative integrated acoustic absorber within the Turbine Exit Case (TEC), the purpose of the present paper is the investigation of a source noise reduction technology. In this case, the TEC strut count has been selectively chosen to achieve an ’Inverse Cut-off’ effect for the (last) blade TEC interaction. This inversely cut-off design comprises a comparably high strut count at an accordingly reduced chord length. Therefore, the existing TEC strut geometry has been modified to maintain its aerodynamic functionality while allowing for the inverse cut-off design. This acoustically designed TEC has then been numerically integrated into the existing rig environment to predict the achievable acoustic effect. These predictions are based on the MTU in-house Linearized Euler tool capable of simulating coupled multistage configurations. The results show a very promising noise reduction potential of the inversely cut-off TEC at the first Blade Passage Frequency (BPF) at approach power where the inverse cut-off is effective. In contrast, the take-off conditions are only slightly affected due to modified scattering effects. For an experimental verification, the inversely cut-off TEC predictions will be compared to upcoming rig measurements in a subsequent paper.

On the influence of a five-hole-probe on the vibration characteristics of a low pressure turbine rotor while performing aerodynamic measurements

For many reasons it is essential to know and assess the flow field and its characteristics upand downstream of a turbine stage. For these purpose measurements are conducted in test rigs such as the STTF-AAAI (subsonic test turbine facility for aerodynamic, acoustic, and aeroelastic investigations) at the Institute for Thermal Turbomachinery and Machine Dynamics at Graz University of Technology. A low pressure turbine is operated in engine relevant operating conditions. The turbine is experienced high mechanical loads and is excited to vibrate (forced response). In the rotor design process forced response predictions and structural assessments are performed. However, it is not common to include instrumentation (e.g. total pressure and temperature rakes, five-hole-probes, fast response aerodynamic pressure probes) in these forced response predictions. But, these measurement devices are essential and therefore this paper investigates the influence of such an instrumentation onto the vibrational behaviour of a low pressure turbine rotor of the STTF-AAAI. Several vibration measurements at distinct circumferential and radial positions of the five-hole-probe in the flow channel are conducted. These measurement results are compared to measurements performed without a five-hole-probe in the flow channel. A clear influence of the five-hole-probe on the vibration level is shown.

Impact of vane clocking on the TEC loss in Rig

The 30% span profile section of the H-TEC highly loaded Turbine Exit Casing was transformed into a plain cascade configuration with identical pressure distribution and loading level. At increasing levels of complexity the incompressible flow in cascade is studied first at different inlet turbulence levels at the Technische Universitat Braunschweig. Unsteady inlet conditions were then generated using rotating bars at the cascade wind tunnel of the Armed Forces University Munich at compressible flow speeds and varying Reynolds numbers. Finally the TEC configuration is investigated in the TU Graz STTF 1.5 stage turbine rig using a conventional 5-hole-probe, a miniature Pitot-probe and a hot wire anemometer. A special test- and evaluation concept allows for highly accurate data. The results show a strong vane-TEC clocking, which for the TEC total pressure inlet profile may be approximated by a sine-function. Downstream the TEC, between the wakes, still a sinus shaped total pressure variation is found. Two loss coefficients are evaluated (i) by a classical control volume approach between TEC inlet and exit plane and (ii) by a so-called viscous wake approach. This method compares the viscous flow in the wake region with a hypothetical potential flow deduced from the flow in the same exit plane between the TEC wakes. The viscous wake method compared at 30% span quite nicely to the cascade data. However, the control volume approach yields more than twice the cascade loss, which indicates further loss sources to exist, e.g. unsteady losses to rise from a vane wake - rotor - TEC interaction or turbulence to impact the probe reading.

Progress in Experimental Research of Turbine Aeroacoustics

Modifications on the intermediate turbine duct in order to reduce noise emissions by changing interaction frequencies and/or modes capable to propagate are presented. Also different turbine exit casings are described that are optimised to reduce interaction noise that is propagating through the engine and is one of the major noise sources during landing (operating point approach). The most promising modifica‐ tions to reduce sound power levels are described. Depending on different modifica‐ tions at specific operating points, the reduction of sound power level is between 5 dB and 10 dB, which is a significant reduction. However, some of these measures show an increase in aerodynamic losses. Therefore, a compromise has to be found between higher losses during a short duration (e.g. landing) and significant noise reduction. The chapter focuses on experimental results obtained in the test facilities of the Institute for Thermal Turbomachinery and Machine Dynamics at Graz University of Technol‐ ogy.

Experimental Investigation of the Upstream Effect of Different Low Pressure Turbine Exit Guide Vane Designs on Rotor Blade Vibration

Exit guide vanes of turbine exit casings are designed to meet aerodynamic, structural and acoustic criteria. New low pressure turbine architectures of aero engines try to optimize components weight in order to decrease the fuel consumption and reduce noise emissions. For this purpose different designs of turbine exit guide vanes (TEGV) exist which vary geometry as well as the number of vanes in the casing. In the subsonic test turbine facility at the Institute for Thermal Turbomachinery and Machine Dynamics of Graz University of Technology, which represents a 1 ½ low pressure turbine stage, the upstream effect of these innovative turbine exit casings (TEC) designs is under investigation. Up to now the influence of the turbine exit casing in relation to the aerodynamic vibration excitation of the rotor blading is not well known. For rotor blade vibration measurements a telemetry system in combination with strain gauges is applied. The present paper is a report of blade vibration measurements within a rotating system in the area of low pressure turbines under engine relevant operating conditions. Within the test phase different turbine exit casings are under investigation at two different operating points (OP). These turbine exit casings represent different design goals, e.g. aerodynamically optimization was performed to reduce losses at the aero design point or an acoustically optimization was done to reduce noise emission at the operating point approach. All these different design intents lead to a changed upstream effect, thus changing rotor blade vibrations. To identify parameters affecting blade vibration attention is paid to aerodynamic measurements as well. Selected results of steady and unsteady flow field measurements are analyzed to draw conclusions. The upstream effect of different turbine exit casings can be quantified at OP1. Depending on the vane number both the potential effect of the TEGV increase and the upstream effect as well. Aerodynamic as well as acoustic improvements as wanted with H-TEC and inverse-cut-off TEC lead to unfavorable conditions and higher blade loading in comparison to the referenced TEC. OP2 provides additional information of downstream effects. Due to the stator vane number the rotor blading is excited in its 4th eigenfrequency. The comparison between all investigated turbine exit casings with respect to the referenced configuration provides a basis for numerical code validation and future developments.© 2016 ASME