Final Report Summary - COBRA (Innovative counter rotating fan system for high bypass ratio aircraft engine)
Executive Summary:
The philosophy of COBRA project is to use technology breakthroughs and the complementarity of the EU-Russia expertise, to overcome the insufficient noise performance of the counter-rotating fan tested in VITAL. Higher by-pass ratio will be explored resulting in much lower blade tip speed and blade count while at least maintaining and perhaps even increasing the very good achieved aerodynamic efficiency.
Aviation is an essential element of today's global society, bringing people and cultures together (2.2 billion passengers yearly) and creating economic growth (425 billion of world GDP - gross domestic product).
The air transport industry is paying a lot of attention to growing public concern about the environmental issues of air pollution, noise and climate change. Although today air transport only produces 2 % of man-made CO2 emissions, this is expected to increase to 3 % by 2050 with the continuous and steady growth of traffic.
To address this great challenge, the European Commission promotes the international cooperation. COBRA is a small collaborative project based on the cooperation between Europe and Russia and deals with Innovative Counter rOtating fan system for high Bypass Ratio Aircraft engine. This project is a 3.5-year FP7 collaborative research project between Europe and Russia (under grant agreement № 605379), started on 1st October 2013. Coordinated by Onera, it associates 5 European and 4 Russian organizations with an overall budget of 3.5 M€.
In the continuity of previous European projects (such as VITAL, DREAM, etc.) and in a perfect complementarity with Clean Sky 2 and current FP7/H2020 projects (ENOVAL, EBREAK, etc.), COBRA has the ambition to find the most suitable intersection between UHBR (Ultra High Bypass Ratio) configuration and CRTF (Counter Rotating TurboFan) architecture. In the past CRTF architecture was studied for lower bypass ratio (maximum of 11) values implying the non-necessity of a gear-box.
Project Context and Objectives:
The purpose of COBRA is to use technology breakthroughs to overcome the insufficient noise performance of the counter-rotating fan tested in VITAL FP6 Program. This will be achieved through exploring higher by-pass ratio (15-25) resulting in much lower blade tip speed and blades count. The designs shall comply also with higher or equal aerodynamic efficiency compare to the VITAL outputs. COBRA is structured to benefit from the existing skills of both EU and Russian partners. It results in multidisciplinary design conception/optimization of the Ultra High Bypass-Ratio (UHBR) Counter Rotating Turbo Fan architecture. COBRA is an ambitious project that aims at meeting the ACARE environmentally objectives, where strong improvements on new engine architecture is required. Based on the current state of the art, and on the complementary skills we gather in COBRA, on the actions planned in the project, the 9 partners take on ambitious and measurable objectives: to reduce noise by 9 EPNdB per operation vs. year 2000 in service engine with the same efficiency improvement as the one achieved in VITAL for lower BPR (~2 / 2.5 points) vs. year 2000 single fan state of the art.
The COBRA consortium has been chosen to provide a multidisciplinary expertise, coming from different backgrounds from Europe and Russia Federation: industrial entities, research centres, university and SME. Coordinated by ONERA for the EU side and CIAM for the Russian side, COBRA´s consortium is largely built with former partners from the VITAL WP2.4 project. Following the success of this project, a natural cooperation has been created between partners and consolidated within DREAM FP6 Programme.
Project Results:
In order to reach objectives mentioned above, design and experimental validation are key steps. But before, it is necessary to define high and low level engine specifications in order to define the main parameters. This task was mainly performed by industrial partners (Safran Aircraft Engine as European representative and Kuznetsov as Russian representative). Based on these specifications provided at engine level an adaptation at rig level is mandatory. This task was carried out prior to design activities, where a fine multidisciplinary optimisation was performed by involved partners. According to industrial process of design, proposed geometries were presented during PDR (Preliminary Design Review) and CDR (Critical Design Review). These reviews were carried out with the participation of advisory board experts. Based on their requirements and remarks, designs were adjusted and then analysed with advanced multidisciplinary numerical tools and methods. After ultimate validation, designs were proposed to detail drawing process. Based on these drawings, the manufacturing process could start. At the end, parts will be controlled; assembled and designed module could be tested. This was the common thread of COBRA project. We propose here to highlight the main results of each process.
Specifications
In order to enable a relevant multidisciplinary optimization of the counter rotating fans modules, topic of interest of COBRA, a careful attention is to be paid to its relevance with respect to powerplant system and aircraft needs. To do so, a breakdown of high level specifications at aircraft level, then at powerplant level is to be conducted in order to extract correct boundaries for CRTF module definition. These overall boundaries are then to be translated at module level into guidelines for module optimization and design.
These requirements activities are shared between EU and RU sides, in such a way that the high level aircraft specification is understood by all partners, while two different overall powerplant design logics are followed by each partner, enabling an exploration of the full range of bypass ratio available for the CRTF concept.
• On the European side, a top down approach has been conducted, from aircraft overall requirements down to fan module specifications, through an architectural and cycle parametric studies enabling to understand the implications of the low pressure drive system on the counter-rotating fan module. This approach was focused on a bypass ratio 16 configuration, consistent with UHBR engine studies conducted in other European research projects such as ENOVAL
• On the Russian side, a bypass ratio 20 configuration has been downselected to cover the upper range of available bypass ratio, following the selection of a long range aircraft specification benefiting from Russian partners past experience. An optimization process has then been conducted to take the maximum benefit of the existing configurations based on VITAL and adjust their characteristics to be representative of advanced ultra-high bypass ratio architecture
Aircraft platforms have been discussed between European and Russian partners, from which power plant system requirements are derived. Two platforms have been selected, a short/medium range aircraft making a connexion with previous and on-going EU-funded R&T projects, and a long range aircraft to take the expected maximum benefit from ultra-high bypass ratio configuration.
On European side, short/medium range aircraft has been chosen to keep consistency with other EU-funded FP7 projects, namely DREAM (previously dealing with short/medium range aircrafts powered by turbofans and open rotors), and LEMCOTEC (in which a short/medium range aircraft platform powered by open rotor engines is considered). This aircraft is characterized by a challenging time to climb constraint, representative of traffic insertion constraints envisaged for Year 2020 and beyond. This constraint results in a rather demanding thrust level at top of climb condition.
On Russian side, long range aircraft to keep consistency with past Russian experience regarding counter-rotating turbofan, more particularly the NK-93 engine. In early of 1990s JSC KUZNETSOV had developed an experimental ultra-high bypass ratio turbofan engine of NK-93 with reduction gearbox driven fan. NK-93 is the three-spool turbofan engine with adjustable pitch fan.
(Figure 1: Selected aircraft and associated requirements)
Based on these basis and associated requirements at aircraft level, powerplant level requirements are investigated. CRTF cycle design studies based on different configurations of engine architecture associated to performance, aerodynamic, mechanical and integration constraints were conducted. The whole study previously detailed has enabled to understand impact of engine architecture (high / low speed booster) and gearbox configuration (reversed / direct) on counter rotating fans and overall on engine performances. The final choice between the three architectures specified at the beginning of this study, considering the best compromise between performance, noise, installation constraints and mechanical preliminary feasibility.
On European side, high speed booster with reversed differential gearbox was selected and several parameters were frozen at design point: bypass-ratio =16, fan diameter, fans speed ratio, fans torque ratio, etc. On Russian side, this same process allowed to converge toward a different set of specifications and parameters . Resulting from parametrical study a candidate allowing a bypass-ratio of 20 was selected.
Based on these high level studies, identification of low level specifications can start. Finest exploration of design space was performed allowing to identify some recommendations based on score table methodology. Then, a zone of aero-acoustic compromise was identified taking into account speed ratio, torque ratio, tip speed, etc. At the end, specification table and baseline geometry at engine scale was proposed for both sides.
Design and optimisation
The objective is to deliver a CRTF blading design accommodating the top level specifications coming from previous activities. Before the start the design itself, it is mandatory to derive CRTF specifications from engine scale to rig and making sure that the resulting design is also compliant with existing testing hardware at CIAM facilities. This work was performed by DLR/CIAM (European configuration/Russian configuration) in tight cooperation with industrial partners involved in specifications activities and CIAM as partner in charge of experimental validation. In parallel and prior to the design activities, ONERA was involved in the development of an acoustic criterion to be used for the design of a CRTF model with respect to sound level emission. A rather simple and fast-running method is expected so that it can be easily implemented in a CFD optimization loop with mechanic and aerodynamic constraints. At the end of this two parallel pre-design activities, baseline geometry and specifications at rig scale as well as validated acoustic criterion were proposed.
Proper design activities started end of 2014.
On European side, ONERA and DLR worked – each of them separately – on the optimisation of the baseline geometry (rig scale) taking into account adapted requirements and recommendations. Advanced optimisation process based on low and high level definition calculations were performed by both partners. Parallel design activities were conducted until the PDR (Preliminary Design Review – October 2015) where both of them proposed designs for review. Analyse of proposed design was made by Safran Aicraft Engines with the participation of an advisory board, assembled for that purpose. The PDR allowed noticing that considerable effort was made by involved partners in order to provide the most suitable design with respect to aerodynamic performances, acoustic behaviour and mechanical resistance but some actions and recommendations were noticed: difficulties to reach the specified torque ratio/ pressure ratio with acceptable acoustic level, consolidation of flutter margin, consolidate mechanical status. Taking into account remaining time and agenda constraints, it was decided to gather ONERA/DLR efforts in order to work together to tackle raised actions and recommendations.
(Figure 2: ONERA results of advanced aero-acoustic optimisation)
(Figure 3: DLR result of advanced aero-acoustic optimisation)
From PDR to CDR several designs were proposed in order to fit to specifications and experts actions. For each design a strong multidisciplinary assessment was performed (this part will be described in a dedicated section) taking into account aerodynamic, acoustic and structural purposes of geometries with and without structural modifications of the blades under pressure loads during the operations.
(Figure 4: Geometry proposed to CDR)
After validation by the experts of the geometry proposed by the partners, there was only one action to be taken into account in order to fulfil all recommendations. This activity was to redesign the blade root. Indeed, in order to increase the axial spacing in the tip region – which is directly linked to the acoustic behaviour of the CRTF module interaction – a sweep angle on leading edge is imposed. A high value of this angle induces a high value of stress located in the blade root region. A special effort was made in order to redesign this region allowing reducing the stress level to be compliant with provided requirements.
(Figure 5: Re-design of the rotor 2 root section for stress peak reduction)
(Figure 6: Resulting stresses after re-design of the rotor 2 root section)
On Russian side, joint design activities were performed by CIAM, AEROSILA and KUZNETSOV. Based on the NK-93 engine, design activities were firstly performed by AEROSILA and KUZNETSOV using 1D/2D approach. Then, CIAM performed optimization of the CRTF module based on the 3D-inverse method. The objective of this optimization was to ensure the behaviour of the module through the following variant: efficiency, stall margin. After, CIAM performed mechanical optimization based on its LOPATKA software and acoustic optimization based on its 3DFS and 3DAS software. CIAM, based on mechanical calculations, built Campbell diagrams for the Russian CRTF configuration, ensure absence of resonance in the operating range, as well as ensure flutter-free operation. (Figure 7)
At the end, an 11x9 blades count for European CRTF module was selected. With an efficiency of 0.940 for fan1 and 0.893 for fan2. Based on a bypass-ratio of 20, a 12x8 blades count was proposed for the Russian variant of the CRTF module. Before start the manufacturing process, a remaining activity is to verify the compliance of the proposed designs with the available rig requirements. In that respect, rig integration effort was made by CIAM and COMOTI (respectively in charge of the experimental validation and the manufacturing). Detailed FEM analysis (based on blisk approach) including the blade root was performed by COMOTI, while CIAM has been responsible of the identification of parts to be remanufactured and nozzle designs.
CIAM's С179-2 fan model adapted for testing the counter-rotating fan with a by-pass ratio of about 15÷17. The model has a 4-stage booster which provides the air pumping through along the core duct. Change of the by-pass ratio is realized with the help of a throttle in the internal duct. A capability has been envisaged for replacing a nozzle of the bypass duct to conduct acoustic tests and to map performance at several operating lines. The bypass duct casing is supported by two rings of struts. Both rotors of the counter rotating fan have been made dismountable (they are not blisks). The test rig parts available, not to be fabricated, are marked with black color. The parts being newly designed are marked with red color.
(Figure 8: Arrangement of the counter-rotating fan model with by-pass ratio of 15÷17 (European arrangement))
The model includes a one-staged technological booster which provides pumping of the air through the core duct. Change of the by-pass ratio is realized with the help of a throttle in the core duct. A capability has been envisaged for replacing a nozzle of the bypass duct to conduct acoustic tests and to map performance at several operating lines. The bypass duct casing is supported by two rings of struts. Both rotors of the counter rotating fan have been made dismountable (they are not blisks). The test rig parts available, not to be fabricated, are marked with black color. The parts being newly designed are marked with red color.
(Figure 9: Arrangement of the counter-rotating fan model with by-pass ratio of ~ 20 (Russian arrangement))
Manufacturing
As described above, in addition to the blades and disks other parts were manufactured. Before starting manufacturing process, CIAM covered all detailed drawings activities in order to provide to COMOTI (in charge of manufacturing process) required technical drawings. After validation of these drawings, COMOTI started the manufacturing. The Russian CRTF variant was the first to be produced. COMOTI started from early 2015 the production of more than 1500 parts. These parts are covering essential parts (such as blades, disks, casing, nozzles, retainers, casing) and subsidiaries bodies (such as bolts, nuts, struts, pylons, rings, rods, insulators, etc.). Blades and disks have been carved from a solid block of titanium while other parts are made of high quality of alloyed steel, stainless steel and aluminium alloy.
(Figure 10: Example of Raw materials ordered and used for COBRA parts)
The manufacturing processes were designed in accordance with COMOTI machine tools endowment. For raw machining (turning and milling) conventional machine tools have been used, as well as for grinding operations. The turning and milling finishing operations and complex shaped surfaces were performed on CNC machines. Special fixtures were designed and manufactured when standard fixtures and clamping devices were not appropriate.
(Figure 11: Machines used for COBRA manufacturing process)
After completion of manufacturing process, dimension inscpetion was performed by COMOTI. In accordance to the quality system, all the parts manufactured should be inspected. The inspection is carried out during manufacturing process and also when the part is completed. All the COBRA Program parts have been submitted to the inspection process. The inspection techniques were tailored to the precision requirements. The most complex measurements have been carried out on the 3D measuring machine DEA Delta. For all manufactured parts, inspection certificates have been issued. More than 95% of the parts have been inspected by CIAM during a control meetinf dedicated to this task. Using this process, Russian CRTF variant manufacturing and controls were completed durig Q3 2016.
For european CRTF variant, first delivery of the detailed drawings was carried out on Q1 2017. Due to considerable efforts made by COMOTI, only seven months was necessary to complete the manufacturing and control process covering almost 1000 parts. The control meeting took place in October 2017.
For both cases, the preparation for shipment was done very carefully in order to avoid any damage during transportation from Bucharest to Moscow. The following images show some representative parts prepared for shipment. Taking into account that the shipment is outside European Union, additional approval were requested. COMOTI obtained from Romanian government officials all the necessary additional documents.
(Figure 12: Shipment preparation)
In order to proceed to the shipment administrative folders have to be prepared on both side. COMOTI have to prepare necessary papers for export while CIAM have to prepare all customs papers allowing to obtain clearances for receipt. Durint the first shipment process now issue was noticed, while for the second shipment covering European CRTF variant, an administrative issue on Romanian side delayed the process.
Experimental campaigns
After reception of the Russian variant strain gages were glued, fan rotors balanced, modules mounted, intrumentaion connected, measurement chanel checked, acoustic measurements verified and anechohic chamber prepared. After validation during the test rediness review, the experimental capaign could start. Experimental matrix was prepared in advance. Two parts characterizes that matrix:
• Aerodynamic tests are conducted within the range of average rotary speed from 54% to 100% at nominal rotary speed ratio with four removable nozzles in the bypass duct as well as without a nozzle. The tests result in estimation of the nozzle diameter corresponding to the design ground operation line. Then a nozzle of smaller diameter is chosen from the four ones and processed to the estimated diameter. This nozzle is used in one aerodynamic test where the rotary speed is varied within +/- 5% of the nominal value, and after that it is used in acoustic tests as a basic (nominal) nozzle.
• Acoustic tests are carried out with the nominal nozzle within the range of rotor speeds from 54% to 100%. Acoustic characteristics are estimated within the range of arc from 0̊ up to 160̊. Acoustic response of COBRA fan module is estimated in the frequency range between 400 Hz and 80 kHz.
At C-3A test bench total and static pressures are measured with pressure scanners ESP 32 HD/DTC of 0.06% accuracy produced by “Pressure Systems” (USA). The test bench system for total and static pressure measurements contains 384 channels of 0.15% UL accuracy (12 pressure scanners have 32 channels each) with measurement limits from 5.0 kPa to 100 kPa. Flow temperatures are measured in the range between 30ºС and +120ºС by means of thermocouples of ХК-type with measurement complex MIC-036 (MC-114), produced and arranged at the test bench by MERA company. Total temperatures of the flow are measured with accuracy of no more than 1.3ºС. In addition one of eight 7-points radial rakes for measurements total temperatures of flow in the fan outlet is equipped by individually calibrated platinum resistance thermometer of type "HEL-707" with measurement accuracy not more than 0.10̊ C. Rotary speed is measured with induction sensors with accuracy of no more than 0.1% of measured value. Torques on the shafts of the drive turbine and rotational speed of the shafts are measured by the torque ratio sensors T32FNA produced by HBM company. The error of the torque measurements is 0.1% from the upper value, the error of measurement of rotor rotational speed is 0.1% from measured value.
The atmospheric parameters are measured in three points inside the anechoic chamber. Atmospheric pressure is measured by barometers of BRS-1M-2 type with accuracy class 0.02% from the upper value. The air temperature inside the anechoic chamber is measured by platinum resistance thermometers with the accuracy of 0.3̊C. Relative humidity inside the anechoic chamber is measured by the sensors (measuring device for temperature and humidity measurements) with an accuracy of 2%. Adiabatic efficiency of the fan is determined in two ways: by flow temperature at the inlet and outlet of the fan and by torque ratios on drive shafts of turbines taking into account the losses in the transmission stand. The error in determining the efficiency of the fan in both ways does not exceed 0.5%.
System for measurements of acoustic characteristics of the test article has 24 channels which provide the recording of the acoustic signals in the frequency range between 200 and 80000 Hz. The system contains microphones, preamplifiers and amplifiers produced by “Bruel & Kjar” company (Denmark) and 2 recorder-analyzers MIC-300M (8 and 16 channels) by MERA company. ¼ inch free-field microphones of 4139 model have frequency range from 4 to 100000 Hz (+/- 2 dB) and dynamic range of 35 -164 dB. The preamplifiers are of 2670 model.
Russian variant capaign was performed end of 2016. Concerning European variant, as descibed above, an administrative issue delayed the reception by CIAM of the manufactured parts. These parts were receieved only end of January 2018. Test will be conduct during Q1 2018, implying that European variant campaign will be perfomed out of forme of the official European COBRA project. Concerning this capaign, there will be only to major modifications:
• During aerodynamic tests, variable fan nozzle system will be used in order to change the secondary massflow range.
• Contraty to Russian campaign (constant speed ratio), European capaign will be perfomed by using a constant torque ratio.
(Figure 13: COBRA-Variant1 fan model installed at the C-3A test bench in CIAM)
Update on 01.05.2018
After reception of the rejection letter and iterations with the project officer an additional paragraph is proposed in order to describe the current situation (Cf. Figure 20).
As raised by the conclusion part, post-test activities were not conducted because experimental campaign was not concluded. But at this time (01.05.2018) the Test Readiness Review (TRR) was successfully conducted early April 10th 2018 and the two phases of the test matrix were performed. Indeed, as agreed with involved partners, the test matrix is made by four phases (1. mechanical check-out, 2. Aero-flow correlation, 3. Acoustic evaluation, 4. Aero fan mapping). Mechanical check-out aims to check the test rig initial operation under a constant torque ratio mode, the aerodynamic and acoustic instrumentations, acquisition and monitoring quality. This phase, will also allow identifying mechanical safe operation speed ranges with open nozzle. The second phase – Aero-flow correlation – aims to test the nozzles (corresponding to nominal and closed configurations) and verification of possible instabilities, determine the appropriate core massflow to reach the targeted BPR for acoustic points, correlate exact acoustic test points positions in the fan map. Both phases raised very promising results and good numerical-experimental correlation (Cf. Figure 19). Next step will be the third phase of the test matrix with a planned start on 07.05.2018. The end of the test campaign is planned on 08.06.2018. The entire duration of the test campaign was extended because COBRA test had to cohabit with ENOVAL campaign. Both cannot operate on the same time a suitable update of the agenda was proposed by CIAM.
(Figure 19: First aerodynamic measurements, comparison to numerical results)
(Figure 20: Mounted CRTF European variant on CIAM C-3A test rig)
Experimental Results
At the C-3A test facility assembly and experimental studies of the aerodynamic characteristics of the Russian CRTF fan model with different nozzles in the outlet section were carried out within the range of average rotors speed (n = (n1 + n2)/2) from 54 % up to 100 %. At the design mode were determined the following main results of experimental studies, which are in good agreement with the calculated data:
- mass air flow is equal to G=82.5 kg (1.86 % more than the calculated value 81.04 kg/s);
- the total pressure ratio π*= 1.264 which is 0.24 % lower than the calculated value (1.267);
- the fan adiabatic efficiency is equal to η*ad = 0.933 which is almost identical with the calculated value (0.934);
- the torque ratios on shafts of fan model M2/M1 = 1.479 which is 4.15 % above the estimated value (1.42).
At nominal ratio of rotors rotational speed of the fan with the nominal nozzle the ratio of the torques on the fan shafts is not kept constant. It changed from M2/M1= 1.608 and n = 54 % up to M2/M1 = 1.479 at n = 100 %.
The experimental studies of the acoustic response of the ducted counter rotating fan model COBRA-Variant1 of the ultra-high bypass ratio equal to 20 was completed in the C-3A test facility with anechoic chamber at different operating modes. The Russian CRTF fan model (fan diameter is equal to 700 mm) was tested without noise reduction system. The sum of the three modes total sound power level BTW COBRA1 12 dB lower than an axial fan, C180-2, with the bypass ratio m = 8.5.
Multidisciplinary Assessment
During the entire duration of the project in parallel of design/optimization activities and during pre-tests and post-tests phases, a multidisciplinary assessment was conducted by involved partners. This assessment was essentially made to evaluate finely the aerodynamic, acoustic and mechanical performances and characteristics of the proposed CRTF module.
Aerodynamic
A large code-to-code benchmark was conducted by the partners in order to highlight some differences regarding the used CFD codes and numerical approaches. Significant numerical campaign was performed at several steps of the design/optimization phase. This campaign was focused on the global performance of this geometry, by using:
• elsA, (http://elsa.onera.fr/) [1] ONERA in-house CFD code, is a simulation platform dealing with internal and external aerodynamics from the low subsonic to the high supersonic flow regime. The compressible 3-D Reynolds averaged Navier-Stokes equations for arbitrary moving bodies are solved by a cell centered finite-volume method with second order upwind or central space discretization with scalar or matrix artificial dissipation on multi-block structured meshes. The discrete equations are integrated either by multistage Runge-Kutta schemes with implicit residual smoothing, or, which in general leads to a better efficiency, by backward Euler integration with implicit LU schemes. For time accurate computations, the implicit dual time stepping method or the Gear integration scheme are employed. Preconditioning is used for low speed flow simulations. A large variety of turbulence models are available, ranging from eddy viscosity to full differential Reynolds stress models.
• TRACE [2] (Turbomachinery Research Aerodynamic Computational Environment), DLR’s Institute of Propulsion Technology in-house CFD code, is a set of tools dedicated to the turbomachinery applications. This code It can be used for both grid types, structured and unstructured. The different solver modules interact with a conservative hybrid-grid interfacing algorithm to allow mismatched abutting interfaces between the structured and unstructured grid blocks. The numerical features of the hybrid-grid CFD solver are its second-order-accurate Roe’s upwind spatial discretization to the convective fluxes with MUSCL or linear reconstruction approaches and its first- or second-order accurate implicit predictor corrector formulation. A wide variety of models are integrated, e. g. implicit steady and unsteady nonlinear solvers, implicit non-reflecting boundary conditions, a two equation turbulence model based on Wilcox k-ε model, including special extension for rotating, compressible flows and streamline curvature, and transition models.
The conducted simulations was performed by using steady-Mixing plane approach in order to simulate the counter rotating fans at engine scale for several operating points. An example of this benchmark study is presented below.
[1] L. Cambier, S. Heib, S. Plot, The Onera elsA CFD software : input from research and feedback from industry, Mechanics & Industry, 14(3): 159-174, doi:10.1051/meca/2013056 2013.
[2] Becker, Kai and Heitkamp, Kathrin and Kügeler, Edmund (2010) Recent Progress In A Hybrid-Grid CFD Solver For Turbomachinery Flows. In: Proceedings Fifth European Conference on Computational Fluid Dynamics ECCOMAS CFD 2010. V European Conference on Computational Fluid Dynamics ECCOMAS CFD 2010, 14.-17. Juni 2010, Lisabon, Portugal. ISBN 978-989-96778-1-4.
(Figure 14: CRTF baseline operating lines, comparison for each operating point.)
Furthermore, unsteady aerodynamic computations were performed for acoustic and flutter evaluations. These computations were conduct for few candidates prior to PDR and CDR in order to finely evaluate the unsteady loads of the rotors allowing performing acoustic and dynamic structural analysis. An example of the one of these computations is presented below.
(Figure 15: Example of steady and unsteady convergence history.)
Aerodynamics computations were also performed in order to evaluate performances impacts after deformation of the blades during operations. After dynamic structural computations (explained below), deformed rotors blades geometry (running geometry) is obtained for each flight point. This new blade shapes is taken into account during an ultimate aerodynamic computations in order to evaluate the impact of the deformation on the aerodynamic and acoustic performances.
(Figure 16: ONERA DLR aerodynamic performances benchmark and comparison to obtained result by using running geometry at approach flight condition)
Acoustic
Based on the unsteady computations performed by ONERA, several acoustic evaluations were performed by Safran Aicraft Engines. These computations have the purpose to estimate the acoustic impact caused by a modification of the design and/or the impact of the modification of rotors shapes under an aerodynamic field during operation. Taking into account that acoustic was clearly one of the objectives of COBRA, particular attention was paid to the acoustic results ant the comparison to the previous results.
(Figure 17: Summary of obtained acoustic results and comparison to the baseline and VITAL (CRTF1 & CRTF2B) outputs)
Structural
Flutter-free operation ensured by HiFi linearized RANS computations w/ linear TRACE for first 3 Eigen modes at ADP, Approach and Cutback conditions. The figure below details the expected aerodynamic damping (always positive, so revealing a stable behavior) in Approach conditions @55%N1r.
(Figure 18: Aerodynamic damping vs. inter blade phase angle (IBPA) for several Eigen mode at 55% of N1r)
Detailed static FEM assessments were performed for several versions of the proposed designs. For the final version, a validation of the stress level was investigated and specific actions were considered when resulting levels were not acceptable. A special attention was paid considering Von Mises stress distribution and displacements of the blades under aerodynamic loads.
Integration
Based on the obtained results an integration work was performed for both CRTF configurations (European BPR=16 and Russian BPR=20).
On Russian side, NK-93 engine was used for evaluation of contribution of COBRA CRTF configuration into enhancement of turbofan engine parameters. NK-93 is a 4th generation high bypass ratio (BPR ~ 15) turbofan engine with counter rotating gear-driven variable pitch fan. It was designed at early 1990 without advanced software. Thermodynamic model of real NK93 engine was corrected by experimental data during earth certification tests. Scaled fan (Russian COBRA CRTF) map was built in thermodynamic model of real NK-93 engine instead of previous one for definition of enhanced engine parameters. Maps of all other units were the same as in the reference engine. When using binding devices thermodynamic model NK-93 turbojet held estimate of the effect of improving the aerodynamic efficiency of advanced unregulated counter rotating COBRA CRTF fan (CIAM) on the specific fuel consumption and the temperature of the gas before the turbine. NK-93 turbojet relates to 4th generation turbofan, which was developed without the use of advanced methods of 3D numerical simulations. This turbofan has a geared drive and is adjustable by changing the installation angle of the blades. Fan characteristic was scaled by providing supplies and pressure parameters of the bypass duct of the NK-93 turbojet, and is built into the thermodynamic model of NK-93 turbojet. To calculate the parameters of turbofan engines NK-93 in the working line in take-off operating mode its characteristic was extrapolated. Estimation was carried out for take-off and cruise operating modes. In opposite to the original fan the new one has fix pitch blades as the single fan map for all flight conditions. New fan engine with scaled COBRA-1 fan was named NK93-NF. Fan maps comparison is shown on Figures 1 and 2. New fan map was indifferent to bypass ratio. In NK-93 model it was extrapolated at left for calculation sea level static operation line. Concerning the noise emissions, integration on IL-96-300 aircraft was performed and compared to the measured noise levels with NK-93 engine. This integration study allows raising two major points:
• Russian COBRA CRTF model achieved target parameters including the adiabatic efficiency Eff_TT > 93% exceeded the same target parameter of UHBRTF NK-93 more than on 5%. It allows putting in compliance the proposed CRTF module to the 6th generation UHBRTF. Incorporating new CRTF fan at UHBRTF NK-93 engine allows to classify this engine as 4+ generation engine (compared to the 4th generation for UHBRTF NK-93). At the same time COBRA Project defined several new challenges in development of COBRA CRTF fan that can be addressed in the future.
• Estimated noise levels of IL-96-300 equipped by 4 Russian COBRA CRTF engines NK-93NF meet the requirements of ICAO Chapter 14 standard with the margin of 7.3 EPNdB.
On European side, the same integration work was performed. Unfortunately this work was done taking into account only numerical figures. Indeed, the experimental validation of the European CRTF module was not yet been started when this report was written. Based on the numerical results, it was highlighted the following partial conclusions:
• 10% of additional weight is resulting from weight evaluation compared to a reference UHBR engine. This evaluation was conducted taking into account the whole power plant system.
• CRTF is +5.74% worse than UHBR in terms of SFC taking into account cruise conditions. This evaluation take into account: fans efficiency, fan cowl frame pressure losses and secondary nozzle exit swirl.
• COBRA engine is significantly quieter than the VITAL engine. For instance, the COBRA engine is about 7EPNdB quieter than the VITAL engine in the absence of a liner.
Potential Impact:
COBRA participated to develop integrated, safer, greener and smarter European transport systems respecting the environmental and natural resources
Besides direct beneficiaries of the COBRA project’s scientific results like research centers, industrials, universities etc., the European society could benefit from the COBRA results project.
Indeed, the main objective of COBRA was to maintain aerodynamic efficiency raised by VITAL – CRTF configuration and reduce the noise level by 9 EPNdB relative to 2000 state of the art engines. It was highlighted that the proposed European CRTF module allows reducing by 7EPNdB the sound level comparing to VITAL engine. Taking into account the fact that the conclusion of VITAL was 2EPNdB penalty comparing to 2000 state of the art engines, at the end COBRA allows to reduce the sound level by 5EPNdB. This reduction is not fulfilling the accurate objectives of the project but is a major step forward. On Russian side, NK-93 engine was used for evaluation of contribution of the proposed Russian CRTF module. Estimated noise levels of IL-96-300 equipped by 4 COBRA-1 engines NK-93NF meet the requirements of ICAO Chapter 14 standard with the margin of 7.3 EPNdB. Beyonds this figures, COBRA Project defined several new challenges in development of proposed CRTF modules (Russian & European) that can be addressed in the future.
COBRA participated to develop and ensure the competitiveness and innovative character of the European transport industry.
To attain the goals set in ACARE regarding CO2 reduction, breakthrough innovation are now needed. The obtained results, as explained in above, should be a large step forward to new rotor design. Such achievements shall strengthen competitiveness of the European aeronautic industry and specifically shall contribute to increase the market share of European and Russian industrials of the propulsion aeronautic sector.
COBRA reinforces the cooperation in research and innovation between EU and Russian Federation in the field of civil transport aircraft.
COBRA is based on a strong cooperation which already exists between Europe and Russian Federation. It involves 4 European and 4 Russian scientific partners who have participated before in several projects within the FP7. Two coordinators are from ONERA and CIAM and, they ensured the efficient progress of the project with respect of its objectives, its schedule and its budget. ONERA and CIAM have already cooperated and were involved together in several previous R&D projects (VITAL, IFATS, ORINICO, etc.). A strong collaboration has already taken place in these R&D projects with ambitious objectives – notably VITAL in FP6 and DREAM in FP7. In this context, COBRA participated to reinforce the existing cooperation in research and innovation between EU and Russian Federation in the field of civil transport aircraft.
COBRA creates complementarity with ongoing and past projects between EU and Russian Federation.
COBRA project takes advantage of VITAL’s background and completed it for more efficiency. Furthermore, the complementarity towards ongoing projects was assured. The participation of COBRA to X-NOISE (collaborative network project in the area of aeroacoustics), FORUM AE (technical and scientific forum addressing all the issues associated to the aviation environmental concerns linked to emission) and ENOVAL workshops - to name but a few – was the perfect way to interact with ongoing projects. There was also a special attention paid to the interaction with Clean Sky activities, with a comparison of obtained results and studied technologies with concepts studied (or under study), such as Open Rotors for instance.
List of Websites:
www.cobra-fp7.eu
Nabil Ben Nasr, Research Engineer, ONERA, Department of Applied Aerodynamics
Tel: +33 1 46 23 51 84
Fax: + 33 1 46 73 41 46
E-mail: Nabil.Ben_Nasr@onera.fr
The philosophy of COBRA project is to use technology breakthroughs and the complementarity of the EU-Russia expertise, to overcome the insufficient noise performance of the counter-rotating fan tested in VITAL. Higher by-pass ratio will be explored resulting in much lower blade tip speed and blade count while at least maintaining and perhaps even increasing the very good achieved aerodynamic efficiency.
Aviation is an essential element of today's global society, bringing people and cultures together (2.2 billion passengers yearly) and creating economic growth (425 billion of world GDP - gross domestic product).
The air transport industry is paying a lot of attention to growing public concern about the environmental issues of air pollution, noise and climate change. Although today air transport only produces 2 % of man-made CO2 emissions, this is expected to increase to 3 % by 2050 with the continuous and steady growth of traffic.
To address this great challenge, the European Commission promotes the international cooperation. COBRA is a small collaborative project based on the cooperation between Europe and Russia and deals with Innovative Counter rOtating fan system for high Bypass Ratio Aircraft engine. This project is a 3.5-year FP7 collaborative research project between Europe and Russia (under grant agreement № 605379), started on 1st October 2013. Coordinated by Onera, it associates 5 European and 4 Russian organizations with an overall budget of 3.5 M€.
In the continuity of previous European projects (such as VITAL, DREAM, etc.) and in a perfect complementarity with Clean Sky 2 and current FP7/H2020 projects (ENOVAL, EBREAK, etc.), COBRA has the ambition to find the most suitable intersection between UHBR (Ultra High Bypass Ratio) configuration and CRTF (Counter Rotating TurboFan) architecture. In the past CRTF architecture was studied for lower bypass ratio (maximum of 11) values implying the non-necessity of a gear-box.
Project Context and Objectives:
The purpose of COBRA is to use technology breakthroughs to overcome the insufficient noise performance of the counter-rotating fan tested in VITAL FP6 Program. This will be achieved through exploring higher by-pass ratio (15-25) resulting in much lower blade tip speed and blades count. The designs shall comply also with higher or equal aerodynamic efficiency compare to the VITAL outputs. COBRA is structured to benefit from the existing skills of both EU and Russian partners. It results in multidisciplinary design conception/optimization of the Ultra High Bypass-Ratio (UHBR) Counter Rotating Turbo Fan architecture. COBRA is an ambitious project that aims at meeting the ACARE environmentally objectives, where strong improvements on new engine architecture is required. Based on the current state of the art, and on the complementary skills we gather in COBRA, on the actions planned in the project, the 9 partners take on ambitious and measurable objectives: to reduce noise by 9 EPNdB per operation vs. year 2000 in service engine with the same efficiency improvement as the one achieved in VITAL for lower BPR (~2 / 2.5 points) vs. year 2000 single fan state of the art.
The COBRA consortium has been chosen to provide a multidisciplinary expertise, coming from different backgrounds from Europe and Russia Federation: industrial entities, research centres, university and SME. Coordinated by ONERA for the EU side and CIAM for the Russian side, COBRA´s consortium is largely built with former partners from the VITAL WP2.4 project. Following the success of this project, a natural cooperation has been created between partners and consolidated within DREAM FP6 Programme.
Project Results:
In order to reach objectives mentioned above, design and experimental validation are key steps. But before, it is necessary to define high and low level engine specifications in order to define the main parameters. This task was mainly performed by industrial partners (Safran Aircraft Engine as European representative and Kuznetsov as Russian representative). Based on these specifications provided at engine level an adaptation at rig level is mandatory. This task was carried out prior to design activities, where a fine multidisciplinary optimisation was performed by involved partners. According to industrial process of design, proposed geometries were presented during PDR (Preliminary Design Review) and CDR (Critical Design Review). These reviews were carried out with the participation of advisory board experts. Based on their requirements and remarks, designs were adjusted and then analysed with advanced multidisciplinary numerical tools and methods. After ultimate validation, designs were proposed to detail drawing process. Based on these drawings, the manufacturing process could start. At the end, parts will be controlled; assembled and designed module could be tested. This was the common thread of COBRA project. We propose here to highlight the main results of each process.
Specifications
In order to enable a relevant multidisciplinary optimization of the counter rotating fans modules, topic of interest of COBRA, a careful attention is to be paid to its relevance with respect to powerplant system and aircraft needs. To do so, a breakdown of high level specifications at aircraft level, then at powerplant level is to be conducted in order to extract correct boundaries for CRTF module definition. These overall boundaries are then to be translated at module level into guidelines for module optimization and design.
These requirements activities are shared between EU and RU sides, in such a way that the high level aircraft specification is understood by all partners, while two different overall powerplant design logics are followed by each partner, enabling an exploration of the full range of bypass ratio available for the CRTF concept.
• On the European side, a top down approach has been conducted, from aircraft overall requirements down to fan module specifications, through an architectural and cycle parametric studies enabling to understand the implications of the low pressure drive system on the counter-rotating fan module. This approach was focused on a bypass ratio 16 configuration, consistent with UHBR engine studies conducted in other European research projects such as ENOVAL
• On the Russian side, a bypass ratio 20 configuration has been downselected to cover the upper range of available bypass ratio, following the selection of a long range aircraft specification benefiting from Russian partners past experience. An optimization process has then been conducted to take the maximum benefit of the existing configurations based on VITAL and adjust their characteristics to be representative of advanced ultra-high bypass ratio architecture
Aircraft platforms have been discussed between European and Russian partners, from which power plant system requirements are derived. Two platforms have been selected, a short/medium range aircraft making a connexion with previous and on-going EU-funded R&T projects, and a long range aircraft to take the expected maximum benefit from ultra-high bypass ratio configuration.
On European side, short/medium range aircraft has been chosen to keep consistency with other EU-funded FP7 projects, namely DREAM (previously dealing with short/medium range aircrafts powered by turbofans and open rotors), and LEMCOTEC (in which a short/medium range aircraft platform powered by open rotor engines is considered). This aircraft is characterized by a challenging time to climb constraint, representative of traffic insertion constraints envisaged for Year 2020 and beyond. This constraint results in a rather demanding thrust level at top of climb condition.
On Russian side, long range aircraft to keep consistency with past Russian experience regarding counter-rotating turbofan, more particularly the NK-93 engine. In early of 1990s JSC KUZNETSOV had developed an experimental ultra-high bypass ratio turbofan engine of NK-93 with reduction gearbox driven fan. NK-93 is the three-spool turbofan engine with adjustable pitch fan.
(Figure 1: Selected aircraft and associated requirements)
Based on these basis and associated requirements at aircraft level, powerplant level requirements are investigated. CRTF cycle design studies based on different configurations of engine architecture associated to performance, aerodynamic, mechanical and integration constraints were conducted. The whole study previously detailed has enabled to understand impact of engine architecture (high / low speed booster) and gearbox configuration (reversed / direct) on counter rotating fans and overall on engine performances. The final choice between the three architectures specified at the beginning of this study, considering the best compromise between performance, noise, installation constraints and mechanical preliminary feasibility.
On European side, high speed booster with reversed differential gearbox was selected and several parameters were frozen at design point: bypass-ratio =16, fan diameter, fans speed ratio, fans torque ratio, etc. On Russian side, this same process allowed to converge toward a different set of specifications and parameters . Resulting from parametrical study a candidate allowing a bypass-ratio of 20 was selected.
Based on these high level studies, identification of low level specifications can start. Finest exploration of design space was performed allowing to identify some recommendations based on score table methodology. Then, a zone of aero-acoustic compromise was identified taking into account speed ratio, torque ratio, tip speed, etc. At the end, specification table and baseline geometry at engine scale was proposed for both sides.
Design and optimisation
The objective is to deliver a CRTF blading design accommodating the top level specifications coming from previous activities. Before the start the design itself, it is mandatory to derive CRTF specifications from engine scale to rig and making sure that the resulting design is also compliant with existing testing hardware at CIAM facilities. This work was performed by DLR/CIAM (European configuration/Russian configuration) in tight cooperation with industrial partners involved in specifications activities and CIAM as partner in charge of experimental validation. In parallel and prior to the design activities, ONERA was involved in the development of an acoustic criterion to be used for the design of a CRTF model with respect to sound level emission. A rather simple and fast-running method is expected so that it can be easily implemented in a CFD optimization loop with mechanic and aerodynamic constraints. At the end of this two parallel pre-design activities, baseline geometry and specifications at rig scale as well as validated acoustic criterion were proposed.
Proper design activities started end of 2014.
On European side, ONERA and DLR worked – each of them separately – on the optimisation of the baseline geometry (rig scale) taking into account adapted requirements and recommendations. Advanced optimisation process based on low and high level definition calculations were performed by both partners. Parallel design activities were conducted until the PDR (Preliminary Design Review – October 2015) where both of them proposed designs for review. Analyse of proposed design was made by Safran Aicraft Engines with the participation of an advisory board, assembled for that purpose. The PDR allowed noticing that considerable effort was made by involved partners in order to provide the most suitable design with respect to aerodynamic performances, acoustic behaviour and mechanical resistance but some actions and recommendations were noticed: difficulties to reach the specified torque ratio/ pressure ratio with acceptable acoustic level, consolidation of flutter margin, consolidate mechanical status. Taking into account remaining time and agenda constraints, it was decided to gather ONERA/DLR efforts in order to work together to tackle raised actions and recommendations.
(Figure 2: ONERA results of advanced aero-acoustic optimisation)
(Figure 3: DLR result of advanced aero-acoustic optimisation)
From PDR to CDR several designs were proposed in order to fit to specifications and experts actions. For each design a strong multidisciplinary assessment was performed (this part will be described in a dedicated section) taking into account aerodynamic, acoustic and structural purposes of geometries with and without structural modifications of the blades under pressure loads during the operations.
(Figure 4: Geometry proposed to CDR)
After validation by the experts of the geometry proposed by the partners, there was only one action to be taken into account in order to fulfil all recommendations. This activity was to redesign the blade root. Indeed, in order to increase the axial spacing in the tip region – which is directly linked to the acoustic behaviour of the CRTF module interaction – a sweep angle on leading edge is imposed. A high value of this angle induces a high value of stress located in the blade root region. A special effort was made in order to redesign this region allowing reducing the stress level to be compliant with provided requirements.
(Figure 5: Re-design of the rotor 2 root section for stress peak reduction)
(Figure 6: Resulting stresses after re-design of the rotor 2 root section)
On Russian side, joint design activities were performed by CIAM, AEROSILA and KUZNETSOV. Based on the NK-93 engine, design activities were firstly performed by AEROSILA and KUZNETSOV using 1D/2D approach. Then, CIAM performed optimization of the CRTF module based on the 3D-inverse method. The objective of this optimization was to ensure the behaviour of the module through the following variant: efficiency, stall margin. After, CIAM performed mechanical optimization based on its LOPATKA software and acoustic optimization based on its 3DFS and 3DAS software. CIAM, based on mechanical calculations, built Campbell diagrams for the Russian CRTF configuration, ensure absence of resonance in the operating range, as well as ensure flutter-free operation. (Figure 7)
At the end, an 11x9 blades count for European CRTF module was selected. With an efficiency of 0.940 for fan1 and 0.893 for fan2. Based on a bypass-ratio of 20, a 12x8 blades count was proposed for the Russian variant of the CRTF module. Before start the manufacturing process, a remaining activity is to verify the compliance of the proposed designs with the available rig requirements. In that respect, rig integration effort was made by CIAM and COMOTI (respectively in charge of the experimental validation and the manufacturing). Detailed FEM analysis (based on blisk approach) including the blade root was performed by COMOTI, while CIAM has been responsible of the identification of parts to be remanufactured and nozzle designs.
CIAM's С179-2 fan model adapted for testing the counter-rotating fan with a by-pass ratio of about 15÷17. The model has a 4-stage booster which provides the air pumping through along the core duct. Change of the by-pass ratio is realized with the help of a throttle in the internal duct. A capability has been envisaged for replacing a nozzle of the bypass duct to conduct acoustic tests and to map performance at several operating lines. The bypass duct casing is supported by two rings of struts. Both rotors of the counter rotating fan have been made dismountable (they are not blisks). The test rig parts available, not to be fabricated, are marked with black color. The parts being newly designed are marked with red color.
(Figure 8: Arrangement of the counter-rotating fan model with by-pass ratio of 15÷17 (European arrangement))
The model includes a one-staged technological booster which provides pumping of the air through the core duct. Change of the by-pass ratio is realized with the help of a throttle in the core duct. A capability has been envisaged for replacing a nozzle of the bypass duct to conduct acoustic tests and to map performance at several operating lines. The bypass duct casing is supported by two rings of struts. Both rotors of the counter rotating fan have been made dismountable (they are not blisks). The test rig parts available, not to be fabricated, are marked with black color. The parts being newly designed are marked with red color.
(Figure 9: Arrangement of the counter-rotating fan model with by-pass ratio of ~ 20 (Russian arrangement))
Manufacturing
As described above, in addition to the blades and disks other parts were manufactured. Before starting manufacturing process, CIAM covered all detailed drawings activities in order to provide to COMOTI (in charge of manufacturing process) required technical drawings. After validation of these drawings, COMOTI started the manufacturing. The Russian CRTF variant was the first to be produced. COMOTI started from early 2015 the production of more than 1500 parts. These parts are covering essential parts (such as blades, disks, casing, nozzles, retainers, casing) and subsidiaries bodies (such as bolts, nuts, struts, pylons, rings, rods, insulators, etc.). Blades and disks have been carved from a solid block of titanium while other parts are made of high quality of alloyed steel, stainless steel and aluminium alloy.
(Figure 10: Example of Raw materials ordered and used for COBRA parts)
The manufacturing processes were designed in accordance with COMOTI machine tools endowment. For raw machining (turning and milling) conventional machine tools have been used, as well as for grinding operations. The turning and milling finishing operations and complex shaped surfaces were performed on CNC machines. Special fixtures were designed and manufactured when standard fixtures and clamping devices were not appropriate.
(Figure 11: Machines used for COBRA manufacturing process)
After completion of manufacturing process, dimension inscpetion was performed by COMOTI. In accordance to the quality system, all the parts manufactured should be inspected. The inspection is carried out during manufacturing process and also when the part is completed. All the COBRA Program parts have been submitted to the inspection process. The inspection techniques were tailored to the precision requirements. The most complex measurements have been carried out on the 3D measuring machine DEA Delta. For all manufactured parts, inspection certificates have been issued. More than 95% of the parts have been inspected by CIAM during a control meetinf dedicated to this task. Using this process, Russian CRTF variant manufacturing and controls were completed durig Q3 2016.
For european CRTF variant, first delivery of the detailed drawings was carried out on Q1 2017. Due to considerable efforts made by COMOTI, only seven months was necessary to complete the manufacturing and control process covering almost 1000 parts. The control meeting took place in October 2017.
For both cases, the preparation for shipment was done very carefully in order to avoid any damage during transportation from Bucharest to Moscow. The following images show some representative parts prepared for shipment. Taking into account that the shipment is outside European Union, additional approval were requested. COMOTI obtained from Romanian government officials all the necessary additional documents.
(Figure 12: Shipment preparation)
In order to proceed to the shipment administrative folders have to be prepared on both side. COMOTI have to prepare necessary papers for export while CIAM have to prepare all customs papers allowing to obtain clearances for receipt. Durint the first shipment process now issue was noticed, while for the second shipment covering European CRTF variant, an administrative issue on Romanian side delayed the process.
Experimental campaigns
After reception of the Russian variant strain gages were glued, fan rotors balanced, modules mounted, intrumentaion connected, measurement chanel checked, acoustic measurements verified and anechohic chamber prepared. After validation during the test rediness review, the experimental capaign could start. Experimental matrix was prepared in advance. Two parts characterizes that matrix:
• Aerodynamic tests are conducted within the range of average rotary speed from 54% to 100% at nominal rotary speed ratio with four removable nozzles in the bypass duct as well as without a nozzle. The tests result in estimation of the nozzle diameter corresponding to the design ground operation line. Then a nozzle of smaller diameter is chosen from the four ones and processed to the estimated diameter. This nozzle is used in one aerodynamic test where the rotary speed is varied within +/- 5% of the nominal value, and after that it is used in acoustic tests as a basic (nominal) nozzle.
• Acoustic tests are carried out with the nominal nozzle within the range of rotor speeds from 54% to 100%. Acoustic characteristics are estimated within the range of arc from 0̊ up to 160̊. Acoustic response of COBRA fan module is estimated in the frequency range between 400 Hz and 80 kHz.
At C-3A test bench total and static pressures are measured with pressure scanners ESP 32 HD/DTC of 0.06% accuracy produced by “Pressure Systems” (USA). The test bench system for total and static pressure measurements contains 384 channels of 0.15% UL accuracy (12 pressure scanners have 32 channels each) with measurement limits from 5.0 kPa to 100 kPa. Flow temperatures are measured in the range between 30ºС and +120ºС by means of thermocouples of ХК-type with measurement complex MIC-036 (MC-114), produced and arranged at the test bench by MERA company. Total temperatures of the flow are measured with accuracy of no more than 1.3ºС. In addition one of eight 7-points radial rakes for measurements total temperatures of flow in the fan outlet is equipped by individually calibrated platinum resistance thermometer of type "HEL-707" with measurement accuracy not more than 0.10̊ C. Rotary speed is measured with induction sensors with accuracy of no more than 0.1% of measured value. Torques on the shafts of the drive turbine and rotational speed of the shafts are measured by the torque ratio sensors T32FNA produced by HBM company. The error of the torque measurements is 0.1% from the upper value, the error of measurement of rotor rotational speed is 0.1% from measured value.
The atmospheric parameters are measured in three points inside the anechoic chamber. Atmospheric pressure is measured by barometers of BRS-1M-2 type with accuracy class 0.02% from the upper value. The air temperature inside the anechoic chamber is measured by platinum resistance thermometers with the accuracy of 0.3̊C. Relative humidity inside the anechoic chamber is measured by the sensors (measuring device for temperature and humidity measurements) with an accuracy of 2%. Adiabatic efficiency of the fan is determined in two ways: by flow temperature at the inlet and outlet of the fan and by torque ratios on drive shafts of turbines taking into account the losses in the transmission stand. The error in determining the efficiency of the fan in both ways does not exceed 0.5%.
System for measurements of acoustic characteristics of the test article has 24 channels which provide the recording of the acoustic signals in the frequency range between 200 and 80000 Hz. The system contains microphones, preamplifiers and amplifiers produced by “Bruel & Kjar” company (Denmark) and 2 recorder-analyzers MIC-300M (8 and 16 channels) by MERA company. ¼ inch free-field microphones of 4139 model have frequency range from 4 to 100000 Hz (+/- 2 dB) and dynamic range of 35 -164 dB. The preamplifiers are of 2670 model.
Russian variant capaign was performed end of 2016. Concerning European variant, as descibed above, an administrative issue delayed the reception by CIAM of the manufactured parts. These parts were receieved only end of January 2018. Test will be conduct during Q1 2018, implying that European variant campaign will be perfomed out of forme of the official European COBRA project. Concerning this capaign, there will be only to major modifications:
• During aerodynamic tests, variable fan nozzle system will be used in order to change the secondary massflow range.
• Contraty to Russian campaign (constant speed ratio), European capaign will be perfomed by using a constant torque ratio.
(Figure 13: COBRA-Variant1 fan model installed at the C-3A test bench in CIAM)
Update on 01.05.2018
After reception of the rejection letter and iterations with the project officer an additional paragraph is proposed in order to describe the current situation (Cf. Figure 20).
As raised by the conclusion part, post-test activities were not conducted because experimental campaign was not concluded. But at this time (01.05.2018) the Test Readiness Review (TRR) was successfully conducted early April 10th 2018 and the two phases of the test matrix were performed. Indeed, as agreed with involved partners, the test matrix is made by four phases (1. mechanical check-out, 2. Aero-flow correlation, 3. Acoustic evaluation, 4. Aero fan mapping). Mechanical check-out aims to check the test rig initial operation under a constant torque ratio mode, the aerodynamic and acoustic instrumentations, acquisition and monitoring quality. This phase, will also allow identifying mechanical safe operation speed ranges with open nozzle. The second phase – Aero-flow correlation – aims to test the nozzles (corresponding to nominal and closed configurations) and verification of possible instabilities, determine the appropriate core massflow to reach the targeted BPR for acoustic points, correlate exact acoustic test points positions in the fan map. Both phases raised very promising results and good numerical-experimental correlation (Cf. Figure 19). Next step will be the third phase of the test matrix with a planned start on 07.05.2018. The end of the test campaign is planned on 08.06.2018. The entire duration of the test campaign was extended because COBRA test had to cohabit with ENOVAL campaign. Both cannot operate on the same time a suitable update of the agenda was proposed by CIAM.
(Figure 19: First aerodynamic measurements, comparison to numerical results)
(Figure 20: Mounted CRTF European variant on CIAM C-3A test rig)
Experimental Results
At the C-3A test facility assembly and experimental studies of the aerodynamic characteristics of the Russian CRTF fan model with different nozzles in the outlet section were carried out within the range of average rotors speed (n = (n1 + n2)/2) from 54 % up to 100 %. At the design mode were determined the following main results of experimental studies, which are in good agreement with the calculated data:
- mass air flow is equal to G=82.5 kg (1.86 % more than the calculated value 81.04 kg/s);
- the total pressure ratio π*= 1.264 which is 0.24 % lower than the calculated value (1.267);
- the fan adiabatic efficiency is equal to η*ad = 0.933 which is almost identical with the calculated value (0.934);
- the torque ratios on shafts of fan model M2/M1 = 1.479 which is 4.15 % above the estimated value (1.42).
At nominal ratio of rotors rotational speed of the fan with the nominal nozzle the ratio of the torques on the fan shafts is not kept constant. It changed from M2/M1= 1.608 and n = 54 % up to M2/M1 = 1.479 at n = 100 %.
The experimental studies of the acoustic response of the ducted counter rotating fan model COBRA-Variant1 of the ultra-high bypass ratio equal to 20 was completed in the C-3A test facility with anechoic chamber at different operating modes. The Russian CRTF fan model (fan diameter is equal to 700 mm) was tested without noise reduction system. The sum of the three modes total sound power level BTW COBRA1 12 dB lower than an axial fan, C180-2, with the bypass ratio m = 8.5.
Multidisciplinary Assessment
During the entire duration of the project in parallel of design/optimization activities and during pre-tests and post-tests phases, a multidisciplinary assessment was conducted by involved partners. This assessment was essentially made to evaluate finely the aerodynamic, acoustic and mechanical performances and characteristics of the proposed CRTF module.
Aerodynamic
A large code-to-code benchmark was conducted by the partners in order to highlight some differences regarding the used CFD codes and numerical approaches. Significant numerical campaign was performed at several steps of the design/optimization phase. This campaign was focused on the global performance of this geometry, by using:
• elsA, (http://elsa.onera.fr/) [1] ONERA in-house CFD code, is a simulation platform dealing with internal and external aerodynamics from the low subsonic to the high supersonic flow regime. The compressible 3-D Reynolds averaged Navier-Stokes equations for arbitrary moving bodies are solved by a cell centered finite-volume method with second order upwind or central space discretization with scalar or matrix artificial dissipation on multi-block structured meshes. The discrete equations are integrated either by multistage Runge-Kutta schemes with implicit residual smoothing, or, which in general leads to a better efficiency, by backward Euler integration with implicit LU schemes. For time accurate computations, the implicit dual time stepping method or the Gear integration scheme are employed. Preconditioning is used for low speed flow simulations. A large variety of turbulence models are available, ranging from eddy viscosity to full differential Reynolds stress models.
• TRACE [2] (Turbomachinery Research Aerodynamic Computational Environment), DLR’s Institute of Propulsion Technology in-house CFD code, is a set of tools dedicated to the turbomachinery applications. This code It can be used for both grid types, structured and unstructured. The different solver modules interact with a conservative hybrid-grid interfacing algorithm to allow mismatched abutting interfaces between the structured and unstructured grid blocks. The numerical features of the hybrid-grid CFD solver are its second-order-accurate Roe’s upwind spatial discretization to the convective fluxes with MUSCL or linear reconstruction approaches and its first- or second-order accurate implicit predictor corrector formulation. A wide variety of models are integrated, e. g. implicit steady and unsteady nonlinear solvers, implicit non-reflecting boundary conditions, a two equation turbulence model based on Wilcox k-ε model, including special extension for rotating, compressible flows and streamline curvature, and transition models.
The conducted simulations was performed by using steady-Mixing plane approach in order to simulate the counter rotating fans at engine scale for several operating points. An example of this benchmark study is presented below.
[1] L. Cambier, S. Heib, S. Plot, The Onera elsA CFD software : input from research and feedback from industry, Mechanics & Industry, 14(3): 159-174, doi:10.1051/meca/2013056 2013.
[2] Becker, Kai and Heitkamp, Kathrin and Kügeler, Edmund (2010) Recent Progress In A Hybrid-Grid CFD Solver For Turbomachinery Flows. In: Proceedings Fifth European Conference on Computational Fluid Dynamics ECCOMAS CFD 2010. V European Conference on Computational Fluid Dynamics ECCOMAS CFD 2010, 14.-17. Juni 2010, Lisabon, Portugal. ISBN 978-989-96778-1-4.
(Figure 14: CRTF baseline operating lines, comparison for each operating point.)
Furthermore, unsteady aerodynamic computations were performed for acoustic and flutter evaluations. These computations were conduct for few candidates prior to PDR and CDR in order to finely evaluate the unsteady loads of the rotors allowing performing acoustic and dynamic structural analysis. An example of the one of these computations is presented below.
(Figure 15: Example of steady and unsteady convergence history.)
Aerodynamics computations were also performed in order to evaluate performances impacts after deformation of the blades during operations. After dynamic structural computations (explained below), deformed rotors blades geometry (running geometry) is obtained for each flight point. This new blade shapes is taken into account during an ultimate aerodynamic computations in order to evaluate the impact of the deformation on the aerodynamic and acoustic performances.
(Figure 16: ONERA DLR aerodynamic performances benchmark and comparison to obtained result by using running geometry at approach flight condition)
Acoustic
Based on the unsteady computations performed by ONERA, several acoustic evaluations were performed by Safran Aicraft Engines. These computations have the purpose to estimate the acoustic impact caused by a modification of the design and/or the impact of the modification of rotors shapes under an aerodynamic field during operation. Taking into account that acoustic was clearly one of the objectives of COBRA, particular attention was paid to the acoustic results ant the comparison to the previous results.
(Figure 17: Summary of obtained acoustic results and comparison to the baseline and VITAL (CRTF1 & CRTF2B) outputs)
Structural
Flutter-free operation ensured by HiFi linearized RANS computations w/ linear TRACE for first 3 Eigen modes at ADP, Approach and Cutback conditions. The figure below details the expected aerodynamic damping (always positive, so revealing a stable behavior) in Approach conditions @55%N1r.
(Figure 18: Aerodynamic damping vs. inter blade phase angle (IBPA) for several Eigen mode at 55% of N1r)
Detailed static FEM assessments were performed for several versions of the proposed designs. For the final version, a validation of the stress level was investigated and specific actions were considered when resulting levels were not acceptable. A special attention was paid considering Von Mises stress distribution and displacements of the blades under aerodynamic loads.
Integration
Based on the obtained results an integration work was performed for both CRTF configurations (European BPR=16 and Russian BPR=20).
On Russian side, NK-93 engine was used for evaluation of contribution of COBRA CRTF configuration into enhancement of turbofan engine parameters. NK-93 is a 4th generation high bypass ratio (BPR ~ 15) turbofan engine with counter rotating gear-driven variable pitch fan. It was designed at early 1990 without advanced software. Thermodynamic model of real NK93 engine was corrected by experimental data during earth certification tests. Scaled fan (Russian COBRA CRTF) map was built in thermodynamic model of real NK-93 engine instead of previous one for definition of enhanced engine parameters. Maps of all other units were the same as in the reference engine. When using binding devices thermodynamic model NK-93 turbojet held estimate of the effect of improving the aerodynamic efficiency of advanced unregulated counter rotating COBRA CRTF fan (CIAM) on the specific fuel consumption and the temperature of the gas before the turbine. NK-93 turbojet relates to 4th generation turbofan, which was developed without the use of advanced methods of 3D numerical simulations. This turbofan has a geared drive and is adjustable by changing the installation angle of the blades. Fan characteristic was scaled by providing supplies and pressure parameters of the bypass duct of the NK-93 turbojet, and is built into the thermodynamic model of NK-93 turbojet. To calculate the parameters of turbofan engines NK-93 in the working line in take-off operating mode its characteristic was extrapolated. Estimation was carried out for take-off and cruise operating modes. In opposite to the original fan the new one has fix pitch blades as the single fan map for all flight conditions. New fan engine with scaled COBRA-1 fan was named NK93-NF. Fan maps comparison is shown on Figures 1 and 2. New fan map was indifferent to bypass ratio. In NK-93 model it was extrapolated at left for calculation sea level static operation line. Concerning the noise emissions, integration on IL-96-300 aircraft was performed and compared to the measured noise levels with NK-93 engine. This integration study allows raising two major points:
• Russian COBRA CRTF model achieved target parameters including the adiabatic efficiency Eff_TT > 93% exceeded the same target parameter of UHBRTF NK-93 more than on 5%. It allows putting in compliance the proposed CRTF module to the 6th generation UHBRTF. Incorporating new CRTF fan at UHBRTF NK-93 engine allows to classify this engine as 4+ generation engine (compared to the 4th generation for UHBRTF NK-93). At the same time COBRA Project defined several new challenges in development of COBRA CRTF fan that can be addressed in the future.
• Estimated noise levels of IL-96-300 equipped by 4 Russian COBRA CRTF engines NK-93NF meet the requirements of ICAO Chapter 14 standard with the margin of 7.3 EPNdB.
On European side, the same integration work was performed. Unfortunately this work was done taking into account only numerical figures. Indeed, the experimental validation of the European CRTF module was not yet been started when this report was written. Based on the numerical results, it was highlighted the following partial conclusions:
• 10% of additional weight is resulting from weight evaluation compared to a reference UHBR engine. This evaluation was conducted taking into account the whole power plant system.
• CRTF is +5.74% worse than UHBR in terms of SFC taking into account cruise conditions. This evaluation take into account: fans efficiency, fan cowl frame pressure losses and secondary nozzle exit swirl.
• COBRA engine is significantly quieter than the VITAL engine. For instance, the COBRA engine is about 7EPNdB quieter than the VITAL engine in the absence of a liner.
Potential Impact:
COBRA participated to develop integrated, safer, greener and smarter European transport systems respecting the environmental and natural resources
Besides direct beneficiaries of the COBRA project’s scientific results like research centers, industrials, universities etc., the European society could benefit from the COBRA results project.
Indeed, the main objective of COBRA was to maintain aerodynamic efficiency raised by VITAL – CRTF configuration and reduce the noise level by 9 EPNdB relative to 2000 state of the art engines. It was highlighted that the proposed European CRTF module allows reducing by 7EPNdB the sound level comparing to VITAL engine. Taking into account the fact that the conclusion of VITAL was 2EPNdB penalty comparing to 2000 state of the art engines, at the end COBRA allows to reduce the sound level by 5EPNdB. This reduction is not fulfilling the accurate objectives of the project but is a major step forward. On Russian side, NK-93 engine was used for evaluation of contribution of the proposed Russian CRTF module. Estimated noise levels of IL-96-300 equipped by 4 COBRA-1 engines NK-93NF meet the requirements of ICAO Chapter 14 standard with the margin of 7.3 EPNdB. Beyonds this figures, COBRA Project defined several new challenges in development of proposed CRTF modules (Russian & European) that can be addressed in the future.
COBRA participated to develop and ensure the competitiveness and innovative character of the European transport industry.
To attain the goals set in ACARE regarding CO2 reduction, breakthrough innovation are now needed. The obtained results, as explained in above, should be a large step forward to new rotor design. Such achievements shall strengthen competitiveness of the European aeronautic industry and specifically shall contribute to increase the market share of European and Russian industrials of the propulsion aeronautic sector.
COBRA reinforces the cooperation in research and innovation between EU and Russian Federation in the field of civil transport aircraft.
COBRA is based on a strong cooperation which already exists between Europe and Russian Federation. It involves 4 European and 4 Russian scientific partners who have participated before in several projects within the FP7. Two coordinators are from ONERA and CIAM and, they ensured the efficient progress of the project with respect of its objectives, its schedule and its budget. ONERA and CIAM have already cooperated and were involved together in several previous R&D projects (VITAL, IFATS, ORINICO, etc.). A strong collaboration has already taken place in these R&D projects with ambitious objectives – notably VITAL in FP6 and DREAM in FP7. In this context, COBRA participated to reinforce the existing cooperation in research and innovation between EU and Russian Federation in the field of civil transport aircraft.
COBRA creates complementarity with ongoing and past projects between EU and Russian Federation.
COBRA project takes advantage of VITAL’s background and completed it for more efficiency. Furthermore, the complementarity towards ongoing projects was assured. The participation of COBRA to X-NOISE (collaborative network project in the area of aeroacoustics), FORUM AE (technical and scientific forum addressing all the issues associated to the aviation environmental concerns linked to emission) and ENOVAL workshops - to name but a few – was the perfect way to interact with ongoing projects. There was also a special attention paid to the interaction with Clean Sky activities, with a comparison of obtained results and studied technologies with concepts studied (or under study), such as Open Rotors for instance.
List of Websites:
www.cobra-fp7.eu
Nabil Ben Nasr, Research Engineer, ONERA, Department of Applied Aerodynamics
Tel: +33 1 46 23 51 84
Fax: + 33 1 46 73 41 46
E-mail: Nabil.Ben_Nasr@onera.fr
