One of the perspective schemes of air breathing engine is a scheme with Ultra High Bypass Ratio (BPR 16...25) Counter Rotating Fan. This solution potentially allows significant increase of fuel efficiency compared to modern conventional turbofans.

The model UHBR counter rotating fan named COBRA-1 was developed by CIAM within the framework of European Project COBRA (Innovative Counter rOtating fan system for high Bypass Ratio Aircraft engine). The fan was designed using up-to-date 1D, 2D and 3D methods. COBRA-1 is a 0.7 m diameter model of counter rotating fan driven by a planetary reduction gearbox. The bypass ratio of COBRA-1 is 20. The R2/R1 torque ratio was chosen to obtain 1.42-muliple prevalence in power for 2nd row. The blade numbers are 8/12 for R1/R2 correspondingly. Final geometry of airfoils was defined by 3D profiling process to achieve required aerodynamics and acoustic parameters. Application of control-diffusion airfoils allows reaching high integral performances: specific mass flow equals 211 kg/(s*m^2) and isentropic efficiency at design point is higher than 0.93.

The paper presents results of computational simulation of the flow in UHBR fan COBRA-1 based on 3D steady RANS method, 3D URANS and Non-Linear Harmonic method for different operation conditions in comparison with experimental data. Numerical simulation was carried out using Numeca FINE TURBO software package. Steady RANS approach was used during design process to make quick estimation of performances at different rpm. 3D URANS simulation was conducted to analyze unsteady wake-blade and shock-wave interaction and to make a decision about sufficient value of axial gap between rotors.

The COBRA-1 fan was tested in CIAM at C3-A test facility which allows conducting a wide range of measurements of local and integral parameters including acoustics of ducted counter rotating fan at different operating conditions.

Experimental results demonstrate a high level of integral performances and good agreement with computed values.

Significant part of numerical and experimental investigation is devoted to effect of gear-box requirements on aerodynamics. C3-A rig allows to set rotational speed of rotors independently and measure torques at each shaft to achieve required torque ratio and study the influence of small (3–5%) deviation in rpm on aerodynamic characteristics.

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