Dominik Broszat

Validation of turbine noise prediction tools with acoustic rig measurements

Well-defined measurement data are of key importance for the validation and calibration of numerical methods. Accordingly, an acoustic test rig resembling the last stage of a low pressure turbine (LPT) of a modern turbofan engine has been established at the Graz University of Technology in Graz, Austria within the frame of the European Research Project VITAL. It has been designed to allow for a clear separation of the contributors to turbine noise by a systematic selection of the individual blade numbers of the subsequent rows. This setup permits the classification of the measured or computed tones with respect to their origin and source mechanism. Within the VITAL project, several acoustic test campaigns have been carried out: a datum (or reference) configuration as well as variations of the axial gapping between the stator, rotor, and the EGV. These give insight into the relative weighting of the above mentioned source mechanisms and the influence of distance variations on noise generation.

Noise generation and propagation for different turning mid turbine frame setups in a two shaft test turbine

The paper deals with the investigation on the acoustics of two different turning mid turbine frames (TMTF) in the two-stage two-spool test turbine located at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) of Graz University of Technology. The facility is a continuously operating cold-flow open-circuit plant which is driven by pressurized air. The flow path consists of a transonic turbine stage (HP) followed by a low pressure turbine stage consisting of a TMTF and a counter-rotating low pressure rotor. Compared to the setup within the EU-Project DREAM, the rig was upgraded by fully circumferentially traversable measurement sections at the inlet of the TMTF as well as downstream of the LP turbine. The two TMTF setups have been investigated at engine like flow conditions. The first configuration consists of 16 highly 3D-shaped turning struts. The goal of the second design was to reduce the length of the TMTF by 10% without increasing the losses and providing the same inflow to the LP turbine rotor. This was achieved by applying 3D-contoured endwalls at the hub. Due to the fact that noise becomes more and more an issue, acoustic measurements were carried out downstream of the low pressure turbine at three different operating conditions representative for approach, cutback and sideline. In order to evaluate the noise emission of the turbine, the outflow duct of the facility was instrumented with a new acoustic measurement setup which uses traversable microphone arrays. Therefore, the emitted sound pressure level and the microphones’ spectra are compared for both configurations. The acoustic field was characterized by azimuthal and radial modes determined by traversing the microphone array over 360 degrees. By comparing the two setups in terms of noise generation, the propagating modes due to the HP turbine were found to be at the same level, while an increase of up to 9 dB in amplitude of the modes related to the LP turbine was found in the 10% shorter setup. This is in good accordance with previous studies, where reducing the distance between stator and rotor of a LPT increases the emitted sound.

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

Detailed Analysis of the Virtual Scarf Inlet (VSI) Effect and Performance

As has been presented in last year’s paper, 1 the Virtual Scarf Inlet (VSI) represents an innovative means of influencing the noise radiation of an aircraft engine via a non-uniform acoustic liner configuration. It consists of a combination of a relatively small so-called ’reflective’ liner segment exhibiting a very low impedance and a conventional absorptive liner, and aims at redirecting the acoustic directivity such that the radiated power into specific sectors is reduced. These are, in application to an aircraft engine inlet, the steeper angles directed towards the ground which contribute most significantly to the community noise during operations close to the ground, i.e. during takeo and approach. In contrast to alternative solutions, as e.g. the Negatively Scarfed Intake (NSI), 2 the solution proposed in this paper uses an alteration of the acoustic boundary conditions instead of modifying the inlet geometry (implicating additional weight and aerodynamic penalties) to achieve the desired eect. By this, the radial distribution of the sound pressure within a cross section can be manipulated and transferred to the far field if achieved at a favorable axial position within the engine inlet. The current paper contains, on the one hand, an update and extension of the results presented in the previous paper and, on the other hand, an additional study to optimize its performance. c

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.

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.

Experimental Determination of the Effectiveness of a Sound Absorbing Turbine Exit Casing

This work presents results from experimental investigations conducted in the subsonic test turbine facility for aerodynamic, acoustic, and aeroelastic investigations at Graz University of Technology. The experiments have been performed for the acoustically relevant operating point approach under engine relevant conditions. The sound absorbing exit guide vanes of the turbine exit casing have been designed as Helmholtz resonators with a resonance frequency according to the blade passing frequency of an aero design point. In order to prove the effectiveness of that exit guide vanes acoustic measurements and modal decomposition have been performed and the sound power per azimuthal mode was calculated and compared with results of a conventional hard wall exit guide vane of aerodynamic design. It is shown that the sound power level can be reduced significantly by about 23 dB. However, the losses are increased dramatically.

Acoustic Comparison of Different Turbine Exit Guide Vane Designs - Part 1: Design Philosophy and Numerical Predictions

In a number of publications of the past years, the authors have presented detailed descriptions of the STTF acoustic turbine test facility at the Graz University of Technology and highlighted its importance for tool validation and technology verification. In this context, multiple configurations have been designed and tested at the cold flow LPT rig and their results with respect to aerodynamics and acoustics have been published. The present paper completes this approach by summarizing the results of a selected number of acoustic design modifications to the Turbine Exit Guide Vane (TEGV) in a comparative way and complementing it with an aerodynamically motivated design optimization. In total, a number of four TEGV designs will be presented and discussed with respect to the respective design objectives, effects on noise generation as well as scattering effects within the LPT (and TEGV) control volume, and noise reduction potential. Within this paper, the corresponding numerical predictions by the MTU in-house LEE code Lin3d will be presented. In addition, in a companion paper also to be presented at the 21st AIAA/ CEAS Aeroacoustics Conference, the experimental results of the four different TEGV designs will be assessed in detail. In addition to the modal acoustic measurements, also several aerodynamic results will be discussed.

Comparison of a State of the Art and a High Stage Loading Rotor

This paper presents measurement results of a 1½ stage LPT test rig at Graz University of Technology incorporating two different rotor geometries: one with a regular blade loading and a second rotor with a highly loaded blade geometry. 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 on the adjustment of all relevant model parameters (Mach number, blade count ratio, airfoil aspect ratio, blade loading, etc.) to reproduce the full scale LPT situation. The rig diameter is approximately half of that of a commercial aero engine LPT. The number of blades and vanes for the two investigated stages as well as the pressure ratio and power output are identical, resulting in a decrease in rotational speed of the HSL rotor. Measurement data from a fast response pressure probe (FRAPP) is used to compare the flow fields of the two different stages.The effect of the different stage designs can be seen when comparing the exit flow fields. The highly loaded stage shows a more pronounced tip leakage vortex compared to the datum stage. The highly loaded stage shows wider wakes with a lower total pressure deficit. The fluctuations of total pressure within the flow field are directly related to the upstream wake. If the measurement position is located within a stator wake, the fluctuations are significantly smaller than out of the wake.Copyright

Verification of the Inverse Cut-Off Effect in a Turbomachinery Stage; Part 2-Comparison to Experimental 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 a Low Pressure Turbine (LPT) has been highlighted. In contrast to the passive noise reduction features and means of minimizing the noise generation in a turbomachinery stage by design presented in previous papers, the author’s last year’s paper numerically investigated the ’Inverse Cut-off’ effect of an LPT last blade Turbine Exit Case (TEC) interaction. The results obtained from coupled unsteady Linearized Euler computations of the rig facility showed a very promising noise reduction potential for the approach condition at the first Blade Passage Frequency (BPF), where the Inverse Cut-off was effective. In contrast, the takeoff conditions were only slightly affected. However, for a confirmation of this effect, an experimental verification was required. Therefore, the previously modified TEC strut geometry has been manufactured and adapted to the rig environment. Aerodynamic as well as acoustic measurements of the LPT stage setup have been performed at the TU Graz using the equipment and procedures previously applied to the standard TEC struts. The postprocessed results have then been analyzed at different levels of detail and compared to the numerical predictions at the acoustic operating conditions. As well, a variation of the rotation speed has been performed to ensure the robustness of the design. Overall, the results show consistent trends relative to the numerical data but at a reduced level due to the noise floor in the experimental data.