Abstract
Civil aviation is aiming at fuel efficient aircraft concepts. Propulsion systems using boundary layer ingestion (BLI) are promising to reach this goal. The focus of this study is on the DLR UHBR fan stage of a tube and wing aircraft with rear-integrated engines. In this integration scenario the propulsion system and especially the fan stage receives distorted inflow in steady-state flight conditions. The distortion pattern and distortion intensity are dependent on the operating conditions. Consequently, the interaction of the fan and the distortion changes over the flight envelope. The first part of the paper aims at gaining knowledge of the BLI fan performance in the operating points end of field, approach, cruise (CR) and top of climb (TOC) using high-fidelity, unsteady RANS approaches. The analysis includes fan map performance metrics and a deeper insight into the flow field at CR and TOC. The preliminary design of a fan stage requires fast turn-around times, which are not fulfilled by high-fidelity approaches. Therefore, a fast, throughflow-based methodology is developed, which enables aerodynamicists to design distortion-tolerant fans. The main characteristics of the methodology is outlined in the second part. Consequently, the methodology is taken advantage of to investigate parameter sensitivities in terms of tip speed, blade thickness, solidity, the annulus geometry and a non-axisymmetric stator. This study suggests that distortion-tolerant fans should be designed at higher tip speeds than conventional design experience recommends to limit the local operating point excursion.
Highlights
Aircraft using boundary layer ingesting propulsion systems are expected to be beneficial in terms of increasing overall efficiency [1]
Compared to the fan stages introduced above the DLR UHBR is in the same range of meridional Mach number but has the lowest total pressure ratio
As the momentum deficit, which is present in the approaching boundary layer fluid, is the main driver in terms of improving the overall engine performance, we introduce a metric, which accounts for the momentum deficit per second (MD)
Summary
Aircraft using boundary layer ingesting propulsion systems are expected to be beneficial in terms of increasing overall efficiency [1]. Except of the Whittle Fan, all fan stages are high speed and transonic They are characterized by meridional Mach number and total pressure ratio. Loaded fan stages show smaller changes in Diffusion factor around the circumference in rotor and stator compared to the baseline and low loading fans. The diffusion factor changes nearly linear with their stagnation pressure distortion (defined by a velocity ratio) and their stagnation temperature distortion (defined by a total temperature distortion) Another result of their work is the feasibility to superpose different distortions within an error range less than 10%. Defoe et al [17] investigated the aero acoustics of coupled intake with the NASA R4 transonic fan stage at BLI conditions Their findings are a 38dB higher source sound power at low-speed operation, whilst the far-field overall sound power level is only 18 dB higher due to noise-attenuating effects of the inlet duct. The high-fidelity and the low-fidelity parts, reveal that tip speed is the most important pre-design parameter in terms of a distortion-tolerant fan
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