58829 - Fan-Intake Coupling With Conventional and Short Intakes 

The current trend in engine fans towards lower pressure ratio and larger diameter is accompanied by a need to shorten the engine intake length to reduce its weight and drag. This paper uses full-annulus, unsteady CFD simulations of two coupled fan-intake configurations to explain the how the impact of flow field coupling on fan and intake performance changes with intake length. All simulations were carried out in a URANS flow solver which has been extensively validated for fan performance prediction.

The coupled flow fields in both the intake and the fan are different to those observed for each component in isolation, and the importance of designing both components as a coupled system is shown to increase with decreasing intake length. On-design and off-design operating points are considered at cruise and high angle of attack, respectively.

The fan efficiency at cruise is shown to be determined by a trade-off between two effects as intake length is decreased. Efficiency is reduced with a short intake due to increased potential flow field distortion, which alters the incidence and diffusion in the rotor. This is partially offset by a reduction in casing boundary layer thickness due to lower intake wetted area.

At high angle of attack conditions, where thrust and stability are the primary considerations, a short intake leads to both increased potential flow field distortion and an earlier onset of flow separation in the intake due to a higher adverse pressure gradient approaching the fan. Both effects combine to reduce the fan thrust at such conditions. However, the fan considered here remains stable at attack angles up to 35°. The reduction in thrust is shown to be dominated by flow separations in the rotor, which increase in size and severity for a given attack angle as the intake length is decreased. The fan is also shown to have a stronger influence on the form of the intake flow field in a short intake, showing that it will be necessary in future to model the fan in the intake design process for a successful design.