Under VITAL Project funded by the European Community, Snecma as a project leader developed a counter rotating low-speed fan concept for a high bypass ratio engine [1–4]. At first, a counter rotating fan model (hereinafter referred to as CRTF1) with solid titanium blades was designed, manufactured and tested in CIAM’s C-3A anechoic chamber. The CRTF1 fan model had blades with a conventional thickness distribution in order to provide optimal mechanical properties and weight characteristics [1, 2]. To simulate blades made of composite materials, CRTF2a [3] and CRTF2b [4] counter-rotating fan models were designed with thickened blades (by 20–40%) as compared with the CRTF1 fan model to ensure mechanical properties. The CRTF2a counter-rotating fan model with thickened blades to simulate blades made of a composite material was tested in CIAM’s C-3A anechoic chamber. Fan local and integral performances within rotational speeds from ncor = 40% to ncor = 100% were measured. The numerical investigations were based on 3DFS software package developed for RANS and URANS solutions. The solution procedure is based on a modified S.K. Godunov’s scheme of implicit finite-difference second-order approximation [5]. The numerical investigations were subdivided into three stages corresponding to various problem definitions. Numerical investigations at the first stage and second stages were carried out without flow nonuniformity at the CRTF inlet. The first stage meets the requirements of the stationary problem definition. CRTF performances at this stage were calculated in the “mixing plane” approximation and compared with test data. Numerical investigations at the second stage were completed for non-stationary problem definition without flow nonuniformity at the inlet (the same as at the first stage). Calculations at this stage were carried out for the problem definition with a common period corresponding to calculations covering 5 blade channels in Rotor 1 (R1) and 7 blade channels in Rotor 2 (R2). Results of non-stationary calculations at the second stage were in good agreement with computed data at the first stage (the stationary calculation). The third stage of numerical investigations was in line with the numerical simulation of CRTF operation with total pressure nonuniformity at the inlet. At this stage the numerical investigations were carried out for non-stationary problem definition for all blade channels in Rotor 1 and Rotor 2 (10 and 14, respectively). The computed data were in good agreement with test results for CRTF integral characteristics as well as for local flow characteristics at the CRTF outlet (instantaneous and averaged by time values of flow parameters distributed in radial and circumferential directions).

The paper presents the results of aeromechanical design of a large-scale model stage for a high-efficient low-noise fan designed for an advanced civil geared turbofan engines with ultra-low rotational speeds of rotor blades (313.4 m/s), high flow specific capacity (up to 202 kg/m2/s) and high bypass ratio (13.5). Total pressure ratio in the bypass duct of the fan model stage is 1.38. To ensure the experimental studies, characteristics are calculated from choking to a surge line within a wide range of rotational speeds. For the studies of the experimental fan model (EFM), a design project is developed and used in manufacturing a fan stage with 0.7-m rotor diameter for tests at the C-3A acoustic test facility. The manufacturing technology for blades made of polymer composite materials (PCM) is of particular importance. Rotor blades of the geared fan model are made of PCM. The analysis of experimental data and their comparison with the computation results within the range of corrected rotational speeds from 0.325 to 1.0 are presented. At first, only gas-dynamic and strength characteristics of the stage are studied. The analysis shows a good agreement of calculated integral parameters with the experimental data. Acoustic performances of the EFM will be studied later on.

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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