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Optimal High-Lift Turbine Blade Aero-Mechanical Design

Final Report Summary - ITURB (Optimal High-Lift Turbine Blade Aero-Mechanical Design)

Executive Summary:
The increase of the engine by-pass ratios leads to high speed LPTs for which the rotational speed is significantly increased with potential benefits in performance, weight and overall dimensions. As a drawback, the designer has to face additional critical issues during the design phases, with the consequent need for the application of optimization procedures. Indeed, the high speed low pressure turbine modules are characterized by critical mechanical constraints due to the large hub stresses which the rotor blades are subjected to, and represent a real challenge for the design. In order to assist the designer with reliable tools it is mandatory to assess the performance of turbine rotor blades of innovative concept with both numerical and experimental investigations.
The ITURB project has been devoted to the numerical design and experimental investigation of an LPT rotor row optimized under the aero-mechanical point of view.
Starting from a baseline configuration, representative of the state-of-the-art of LPT high-speed rotor blades, a multi-objective optimization, aimed at fulfilling the mechanical and geometrical constraints without threatening the rotor aerodynamic performance, has been performed at the University of Firenze and University of Padova.
Successively, the baseline and optimized rotor rows have been also experimentally investigated in a large scale cold-flow installed at the Aerodynamics and Turbomachinery Laboratory of the University of Genova. The facility has been properly upgraded to allow detailed investigations of the flow within the two rotor rows, and in particular the measurement of the rotor aerodynamic loadings. The experimental results confirmed the numerical predictions, giving the evidence that the mechanically optimized rotor is able to maintain the aerodynamic performance of the baseline rotor.
Moreover, detailed phase-locked investigations carried out downstream of the rotor allowed the understanding of the flow physics and of rotor/stator interaction.

Project Context and Objectives:
The ITURB project develops in the context of the innovative turbofan engines, where high speed LPTs should be adopted to move the large by-pass ratio fans, limiting the engine weight increase.
The main objectives of the project deal with the design and validation of an innovative rotor row able to hold up the large mechanical stresses associated with the high-speed rotation without any decrease of the aerodynamic performance.

The main objectives of the project have been fulfilled. In particular the following activities have been carried out during the project:
- Numerical analyses on baseline configuration;
- Definition of high speed to low speed scaling procedure adopted to provide experimental geometries to be tested in the low speed facility;
- Definition of the rig 3D scaled geometry of the baseline rotor rows;
- Definition of optimization procedure for the aero-mechanical optimization of the baseline profile;
- Numerical investigation of optimized geometry and aero-mechanical numerical comparisons with baseline profile;
- Definition of rig 3D scaled geometry of the optimized rotor rows;
- Design, manufacturing and installation of experimental facility upgrades, in order to improve the test-rig capabilities and the quality of the results;
- Manufacturing and installation of baseline vanes and blades within the rig;
- Instrumentation of the test article;
- Definition of the test matrix;
- Experimental analysis of the baseline rotor: measurement of rotor aerodynamic loadings to verify the numerical scaling procedure, investigation of total pressure and flow angles in the frontal planes upstream and downstream of stator and rotor rows to assess their aerodynamic performance, phase-locked investigations in the frontal plane downstream of the baseline rotor to study the time-varying flowfield
- Experimental analysis of the optimized rotor: measurement of rotor aerodynamic loadings to verify the numerical scaling procedure, investigation of total pressure and flow angles in the frontal planes upstream and downstream of stator and rotor rows to assess their aerodynamic performance, phase-locked investigations in the frontal plane downstream of the optimized rotor to study the time-varying flowfield
- Comparison of numerical and experimental results

Project Results:
The main results and achievements of the project are summarized in the following, divided by workpackages.

WP1: Airfoil Optimization for Rotor High Speed Blades
The baseline configuration has been numerically analyzed, and the 3D experimental geometry for the stage has been defined by a high-speed to low-speed scaling procedure. The scaling activity was aimed at obtaining an “equivalent” low speed geometry which matches the blade load distributions of the high speed one, developing a similar boundary layer on the blade surface and resulting in similar performance. The aerodynamic redesign of the blade at low speed was handled using a response surface approach that coupled CFD analyses with a neural-network-based optimization method (ANN) and accounted both for compressibility effects and experimental rig geometrical constraints.
The aerodynamic performance of the rotor blade at low speed (LS) in terms of losses as a function of the exit Reynolds number ranging from 0.30x105 to 2.0x105 have been compared to the ones obtained in high-speed (HS) flow conditions. The agreement was found to be very good on the whole range of investigated Reynolds number. The performance of the high speed geometry operated at low speed are reported for comparison purposes (ORIG) to highlight the effect of scaling. The 3D baseline rotor blade has been aero-mechanically optimized in high-speed flow conditions and the new design solution has been numerically investigated. The efficiency gain with respect to the baseline geometry is about 0.15% for the 3rd rotor and 0.10% for the 3rd stage.
Following the approach used for the baseline geometry, the rig 3D scaled geometry has been defined at low-speed.
A preliminary numerical assessment of the new rig upgrade on the designed low-speed geometries was carried out to account for the redesign of the casing to allow a smaller tip clearance and the increase of the blade aspect ratio.

WP2: Experimental rig and test matrix assessment, experimental tests
The main objectives for the WP2 concerning the implementation of the rig adaptation, the manufacturing and installation of baseline and optimized rotor rows, the setting of the measurement planning, and the experimental investigation of baseline and optimized rotor rows have been all fulfilled.
The existing facility was modified to accommodate the turbine stages to be tested, in particular the adaptation was aimed at reproducing the correct geometrical parameters of the rotor rows, as well as the inlet flow angle conditions.
The experimental investigations have been performed for three different rotor incidence angles corresponding to i=0° (nominal incidence), i=-4° and i=+6°. The Reynolds number has been fixed to a value representative of the engine cruise operation (Re=80000).
In order to analyze the aerodynamic efficiency of the baseline and optimized rotors, as well as to in depth investigate the physic phenomena associated with the aerodynamic losses generated by the two rows, several measuring techniques have been implemented and set up.
To measure the rotor aerodynamic loadings a hollow shaft has been manufactured to install pressure transducer and Scanivalve instrumentation in the rotating frame. Thanks to this configuration the pressure transducer is located on the shaft axis. Thus centrifugal effects affecting the sensitive element of the pressure transducer have been minimized and accounted for.
Three instrumented blades have been manufactured with pressure taps located at midspan, at 25% and at 75% of the blade span. The radial equilibrium acting on the flow within the pipings connecting pressure taps with the transducer has been taken into account in the evaluation of the static pressures along the blades. The signal from the rotating transducer has been delivered to the static frame by means of a slip ring mounted on the shaft.
The comparison highlights that the diffusive part of the two rotors is very similar, whereas appreciable differences can be found in the frontal part of the suction side at all of the spanwise locations and in the rear part of the pressure side next to the hub and at midspan.
A five-hole probe with a very small diameter (2.5 mm) has been adopted to allow high spatial resolution and accuracy of the total pressure, mean velocity and flow angle measurements, performed upstream and downstream of each blade row.
The time-mean results obtained with the five-hole probe allowed the identification of the wake and of the secondary flows generated by the blades and the evaluation of the aerodynamic performance of the different rows tested.
The flow-field downstream of the rotor has been also investigated by means of hot-wire anemometry operated with a multiple-rotation technique, which allowed the investigation of the time-varying three-dimensional flow-field. The phase-locked investigations gave information about wake-wake interaction and vortex-vortex interaction.
The distributions of the phase-locked relative velocity and turbulence intensity measured downstream of the rotor, at a fixed circumferential position have been investigated. The rotor wake appears bowed and leaned due to the strong three dimensional geometry of the blade. The lowest values of ⟨w⟩ and the largest values of ⟨Tu⟩ are in the hub and tip regions due to the secondary flow structures.
The experimental results confirmed the numerical predictions, giving the evidence that the mechanically optimized rotor is able to maintain the aerodynamic performance of the baseline rotor.

WP3: Numerical - Experimental Comparison and Final Design Validation
All the objectives of the work package have been reached, in particular:
• The time accurate numerical analysis of both the baseline and the optimized blade were carried out considering the boundary conditions measured on the test rig (WP2). The analyses covered the Reynolds number as well as the variations in the rotational speed with respect to the design considered in the experiments.
• Detailed comparison between measured and computed spanwise distributions of total pressure loss coefficient and blade angles at both stator and rotor exit. Maps of total pressure coefficient contours on cross sections at both stator and rotor exit. Pressure coefficient distributions at 25%, 50%, and 75% span for the rotor blade.
Final assessment of the innovative blade model:
• a modern multi-objective redesign strategy was developed in order to support the design of innovative engine architectures in which high standard of performance are required in conjunction with tight mechanical and geometrical constraints.
• the redesign strategy was validated by means of a comprehensive experimental campaign carried out in a low-speed turbine facility.
• Identification and discussion of the key aspects which characterize the optimized solution with respect to the baseline blade.
The comparison between the blade loadings of the baseline and the optimized rotors considering both the numerical and the experimental results showed that the optimized blade has a similar blade loading with respect to the original one, with a very similar suction side diffusion rate. However, the optimized rotor features a smoother blade loading in the accelerating part of the suction surface.
The redesign activity allows one to define a high-speed rotor which meets the stringent mechanical and geometrical constraints with no detrimental effects on the blade performance, demonstrated by the comparison between the spanwise distributions of the rotor tensile stress for both the original and the optimized blade. The threshold value for the maximum allowable tensile strength as well as the maximum value at the hub, considered for the redesign, have been also considered.
The baseline and optimized rotor losses were evaluated considering different incidence angles i=-4°, i=0°, i=6°. Experimental data as well as numerical results clearly show that the overall losses of the baseline and the optimized blade are almost the same for all the incidence angles proving that the redesign of the rotor blade reached its target to preserve the high performance of the baseline configuration while meeting the mechanical constraints.

WP4: Project management
A detailed project plan has been prepared. The tasks of the consortium management were to organize and coordinate the work carried in the whole project, promoting collaboration between partners to ensure the fulfillment of the objectives.

Potential Impact:
The results achieved within the project have supported the preparation of Master Degree and Ph. D. University courses about the complex three-dimensional flow in an innovative design turbine having an important impact on the education.
Moreover the main results achieved have been also presented by the partners at International Conferences on turbomachinery design, in particular at the ASME Turbo Expo Conference and at the ISAIF12 Conference.

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