Toward Future Installations: Mutual Interactions of Short Intakes With Modern High Bypass Fans

Abstract

In this paper, we investigate the coupled interaction between a new short intake design with a modern fan in a high-bypass ratio civil engine, specifically under the off-design condition of high incidence. The interaction is expected to be much more significant than that on a conventional intake. The performance of both the intake-alone and rotor-alone configurations are examined under isolation. Subsequently, a comprehensive understanding on the two-way interaction between intake and fan is presented. This includes the effect of fan on intake angles of attack (AoA) tolerance (FoI) and the effect of circumferential and radial flow distortion induced by the intake on the fan performance (IoF). In the FoI scenario, the rotor effectively redistributes the mass flow at the fan-face. The AoA tolerance of the short-intake design has increased by ≈4 deg when compared with the intake-alone configuration. Dynamic nature of distortion due to shock unsteadiness has been quantified. ST plots and power spectral density (PSD) of pressure fluctuations show the existence of a spectral gap between the shock unsteadiness and blade passing, with almost an order of magnitude difference in the corresponding frequencies. In the IoF scenario, both the “large” (O(360 deg)) and “small” scale distortion (O(10–60 deg)) induced by the intake results in a non-uniform inflow to the rotor. Sector analysis reveals a substantial variation in the local operating condition of the fan as opposed to its steady characteristic. Streamline curvature, upwash, and wake thickening are identified to be the three key factors affecting the fan performance. These underlying mechanisms are discussed in detail to provide further insights into the physical understanding of the fan-intake interaction. In addition to the shock-induced separation on the intake lip, the current study shows that shorter intakes are much more prone to the upwash effect at higher AoA. Insufficient flow straightening along the engine axis is reconfirmed to be one of the limiting factors for the short-intake design.

Effect of Unsteady Fan-Intake Interaction on Short Intake Design

“…Computational analysis of a fan-intake coupled conőguration under crosswind conditions showed that when the ground vortex is not taken into account then the fan is more tolerant to intake total pressure distortions when the fan rotational speed is increased [23]. Computational analysis showed that for a short intake a strong coupled interaction with the fan may be expected at both high incidence [24] and crosswind conditions [25]. For the high incidence conditions, the fan design may be tailored to improve the fan performances when the intake ŕow is separated and non homogeneous at the fan face [26].…”

Sweep Effects on Fan–Intake Aerodynamics at High Angle of Attack

“…ii) Rotor-separation interaction: RSI is the loss generated in the rotor tip and on the casing when passing through the intake separation. The rotor tip experiences high positive incidence [3],…”

Novel Method for Evaluating Intake Unsteady Flow Distortion

“…Moreover, experiments with large turbofan engine revealed that rotating stall can be triggered by unstable oscillations that are caused by the delay between the response of the inlet and of the fan to incoming fluctuations of low frequency [12]. The spectral gap between the unsteadiness of the separation at the intake lip and the blade passing frequency could ultimately provoke resonance and a substantial drop in pressure ratio [13].…”