Mitigation of the Bifurcation Impact in low Fan Pressure Ratio Propulsion Systems by Individual OGV Designs

Abstract

The integration of bifurcations in the bypass duct of turbofan engines to mount the engine structure may significantly impact fan performance and acoustics due to the upstream potential field caused by stagnating the flow at the bifurcation leading edge. This effect can lead to flow separations on the outlet guide vanes (OGV) because of increased flow incidence imposed by the additional streamline curvature. It may also disturb the rotor outlet flow homogeneity across the circumference and impose periodic forcing, which all in all may have a negative impact on the engine overall performance in terms of flow quality, stability or compromised off-design performance, as well as noise. To mitigate these negative effects, one approach is to re-design individual OGVs in the vicinity of the bifurcation in terms of their stagger angles and clocking positions in relation to the bifurcation positions. Therefore, local adjustments of the OGVs are made within the bifurcation potential field, depending on the size of the influenced potential field upstream of the bifurcations. The engine configuration used for the underlying studies is based on a scaled and aerodynamically reshaped version of the ASPIRE fan, which was adapted and downscaled for wind tunnel testing with integrated bifurcations in the bypass duct. The investigated fan configuration to be used in the SA2FIR-Integration-Rig consists of a bypass and a core section, including a fan stage of rotor and OGVs, as well as two different bifurcation geometries downstream of the OGVs in the bypass duct. Additionally, there are engine section stators (ESS) placed in the core part of the engine. The configuration is simulated with the computational fluid dynamics (CFD) solver TRACE by means of steady 3D-RANS calculations. Within the CFD setup the rotor and ESS section are simulated in a single passage approach with mixing plane interfaces. For the OGV and bifurcation sections the domain covered the full annulus in order to determine the effect of the potential field on the OGVs. During the design, most efforts were concentrated on a specified near sideline operating point. In the present engine configuration this point is relevant for acoustic certification and is also operating close to the stability boundary of the fan. Thus, the configuration benefits from improved flow quality by the redesigned OGVs. The results being presented in this paper summarize some important aspects that arise when re-designing the OGV to mitigate the upstream effect the bifurcations have on the OGV. To isolate the effect of different geometric modifications a parametric study with different sets of individually applied stagger angles, clocking positions and the different OGV hub lean angles in the vicinity of the bifurcations are used. All design efforts comprised of axisymmetric as well as non-axisymmetric modifications of the outlet guide vanes. One of the main conclusions from the study is that due to the high maturity of the OGV baseline design, any change in passage area distribution as introduced e.g. by the local re-staggering has to be carefully applied in order to lead to a desired reduction in bifurcation inflow velocity, without overloading the vanes due to the additional diffusion of those ones affected by the bifurcation.