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To reduce the specific fuel consumption of future turbofan engines via increased propulsion efficiency, the bypass ratio is increased by lowering the fan hub-to-tip ratio. Based on a comparison of fan configurations for an ultrahigh bypass ratio (UHBR) engine the effects of an increasing bypass ratio on the downstream near hub flow are presented. In order to evaluate the increasing amount of near hub flow associated with a reduced fan hub-to-tip ratio, the splitter is lowered at radial height. Numerical simulations of three different UHBR configurations of the fan were conducted and compared with fan configurations of the V2500 fan rotor. The results of the numerical flow simulations show, that the near hub flow is dominated by secondary flow effects. By increasing the bypass ratio, the fan wake is affected negligibly for both fan configurations. The interaction of the secondary flow and the core stator of the UHBR fan, combined with a reducing annulus cross section, is found to have a detrimental effect on the downstream stator. The findings of this paper provide a better understanding of the near hub flow sensitivities towards increasing bypass ratios of future turbofan engines. INTRODUCTION Recent design trends are proposing to increase the engine bypass ratio by reducing the hub-to-tip ratio. Near hub fan flow dictates the core compressor inflow and thereby the core efficiency. The goal is to minimize the loss of the near hub fan section, keeping the pressure ratio high. Lowering the hub radius leads directly to decreasing hub circumferential speeds. While trying to maintain high work, the required flow turning of the rotor at the hub increases. In practise a compromise between high pressure ratio and reasonable flow turning is mostly chosen, leading to a nonuniform flow distribution. The downstream low-pressure compressor or booster is thereby subjected to a flow dominated by stronger fan wakes and secondary flow phenomena, resulting in higher stagnation pressure losses. Zamboni and Xu (2012) showed via a simple cycle analysis that these near hub stagnation pressure losses become more significant with increasing bypass ratio. The uniformity of radial distribution of the near hub flow plays a decisive role as the flow interacts with the downstream splitter and the core inlet guide vane (IGV). General theories of secondary flow effects can be found in literature (Horlock and Lakshminarayana, 1973). Furthermore, the core flow performance is strongly impacted by the interaction between fan wake and splitter. Goyal and Dawes (1993) present the effect of a splitter on the comparison between measured and predicted flow. They state that near hub flow of the fan wake gets affected by the presence of a splitter. The fan total pressure ratio differs around 2.6% in the near hub region at design point (DP) and a bypass ratio of 4.6. Investigations on simulations of fan-bypass configurations made clear, that the splitter flow incidence caused by variation in bypass ratio can affect the downstream flow significantly. Depending on the incidence flow, the boundary layer is increased and the core flow is subjected to a highly non-uniform flow (Dawes, 1991). In this paper the effect of increasing bypass ratio on the near hub flow is investigated by increasing the hub-to-tip ratio of the core flow section. In particular, the interaction between the near hub flow and the splitter and the influence of the partial increasing near hub flow in the core section will be analysed. To determine the major causes for the effects, the UHBR configuration is compared with the V2500 fan configuration. FAN CONFIGURATIONS For research of the fundamentals on an environmental friendly future regional aircraft, an UHBR engine was designed within the framework of the Coordinated Research Centre 880 (Giesecke, 2018). The UHBR engine was designed at top of climb conditions with a bypass ratio of 17. The design process and off-design behaviour of the fan stage can be found in (Giesecke, 2017). In total, the UHBR fan

Effects of Increasing Bypass Ratio of UHBR Fans on Near Hub Flow