Design of a counter rotating fan using a multidisciplinary and multifidelity optimisation under high level of restrictions

A model scale ducted counter-rotating fan was designed by DLR, ONERA, COMOTI and Safran Aircraft Engines within the frame of the EU-project COBRA, (Europe-Russia cooperation within FP7 program coordinated by ONERA for the European side and CIAM for the Russian side). The design was very challenging due to the multidisciplinary aspects of the problem and the high number of very restrictive constraints. This work is based on the previous EU-project VITAL (within FP6 program), objectives of which have been motivated by the ACARE-goals, namely reducing noise by 10 dB, NOx by 60% and the specific fuel consumption by 20%. The COBRA project investigated further different technical solutions to overcome the insufficient noise performance of the VITAL CRTF. An acoustic level reduction by 3 dB for all acoustic operating points has been defined as target compared to the best VITAL CRTF. To achieve these goals, a higher bypass ratio is explored resulting in much lower blade tip speeds and blade count while maintaining good aerodynamic performances. The multidisciplinary approach and the numerical tools used are described. The final CRTF geometry is based on a multiobjective and multifidelity optimisation method and derived from an initial geometry (V0) provided by Safran Aircraft Engines. During the optimisation process, all created members are evaluated with 3D RANS calculations for the CRTF aerodynamics performance, and FEM calculations for the blade structural properties. A simple acoustic evaluation provided by ONERA using steady calculation results allows to estimate the CRTF acoustic level at approach condition. The number of blades chosen fixed during the optimisation is 11 blades for the front fan and 8 for the rear fan. More than 100 free parameters are used to characterize one blade geometry, which is defined by five profiles evenly distributed in the spanwise direction. The final geometry (V4) of the counter-rotating fan achieves an efficiency similar to the previous VITAL CRTF at design point but reduces the noise of -3 dB at approach operating point, while satisfying the mechanical constraint of titanium and maintaining a very strict range of torque ratio as well as a very low residual swirl. Concerning this last point, the multifidelity optimisation method has been very helpful to respond the high level of requirements.