Search

Ultra-high bypass ratio turbofans offer significant reductions in fuel and pollution due to their higher propulsive efficiency. Short intakes might lead to a stronger fan-intake interaction, which creates uncertainty in stability at off-design conditions. Due to the prohibitive cost of full-scale experimental testing, subscale testing in wind tunnels is used to understand this behaviour. The low Reynolds number of subscale models results in unrepresentative laminar shock-boundary layer interactions. The boundary layer state thus needs to be conditioned to better represent full-scale transonic fans. This paper proposes the use of an inexpensive and robust flow control method for the suction side of a fan blade. Design guidelines are given for the location and height of the discrete roughness elements used to control the boundary layer state. This paper also presents a rapid experimental validation methodology to ensure and de-risk the application of the boundary layer trip to 3D rig blades. The experimental methodology is applied to a generic aerofoil representative of a fan tip section. The experimental method proves that it is possible to reproduce boundary layers and pressure distributions of a full-scale fan blade on a 1/10 subscale model. The results obtained confirm that the boundary layer trip method successfully promotes transition at the location representative of full-scale blades, avoiding unrepresentative laminar shock wave boundary layer interactions. This highlights the importance of conditioning boundary layers in low Reynolds number fan rig testing.

Boundary layer ingestion (BLI) offers potential reductions in fuel burn and emissions. The propulsive fuselage concept features an aft fan that ingests the 360 deg fuselage boundary layer. A critical condition for this configuration occurs when a sideslip angle arises between the aircraft longitudinal axis and the freestream velocity. The sideslip flow in combination with the airframe generates inlet distortion that depends on the sideslip direction. In particular, the sideslip increases flow into the windward side of the intake and reduces the flow on the opposite side, leading to either co-swirl or counter-swirl at the fan face. This article investigates the effect of sideslip direction on the aerodynamics of an aft BLI fan. Experimental measurements have been taken in a low-speed fan rig with representative total pressure and swirl distortion. These tests have been complemented by full-annulus unsteady Reynolds-averaged Navier–Stokes simulations to understand the impact of sideslip on the rotor and outlet guide vane flow fields. The results show that positive sideslip (sideslip that produces counter-swirl) increases the fan work and incidence, but the additional losses are relatively small. However, negative sideslip results in lower loading, negative incidence, and greater losses. The stage loading was 28% higher and the stage efficiency was 1.65% higher for positive versus negative sideslip. The positive sideslip case was also found to lose some operating range, equivalent to 2% of nozzle area. Overall, the fan design tolerates the sideslip distortion well and demonstrates the importance of the coupling of both swirl and total pressure distortion with the fan aerodynamics.