Stall Inception in a Boundary Layer Ingesting Fan

Abstract

Jet engines with boundary layer ingestion (BLI) could offer significant reductions in aircraft fuel burn compared with podded turbofans. However, the engine fans must run continuously with severe inlet distortion, which is known to reduce stability. In this paper, an experimental study has been completed on a low-speed rig fan operating with a BLI-type inlet distortion. Unsteady casing static pressure measurements have been made at multiple locations during stall events. Steady-state, full-annulus area traverses have also been performed at rotor inlet and exit at a near-stall operating point. The reduction in stability caused by BLI is found to be small. It is found that with BLI the fan can operate stably despite the presence of localized regions where the rotor operating point lies beyond the stability boundary measured in clean flow. With the BLI-type distortion applied, the measured rotor incidence varies around the annulus due to nonuniform upstream velocity and swirl. The measured amplitude of unsteady casing pressure fluctuations just prior to stall is found to correlate with the circumferential variation of rotor incidence, suggesting that rotor incidence is a key variable affecting the creation and growth of flow disturbances. In regions of high incidence, disturbances resembling local flow separations are initiated. However, in regions of low or negative incidence, any disturbances decay rapidly. Full rotating stall with BLI occurs when high incidence regions are widespread enough to sustain disturbances which can propagate around the entire annulus.

Efficient Design Investigation of a Turbofan in Distorted Inlet Conditions

“…FIGURE 10). Besides, this definition is very conservative as it implies that a blade only needs to reach stall conditions at a specific circumferential location for the entire row to be considered in stall conditions, which has been demonstrated not to be always the case [13].…”

“…It is therefore important to take them effectively and efficiently into account, even during the first steps of the design. The impact of inlet distortion has been assessed in many studies [11,12,13]. They have in particular emphasized the importance of not only stagnation pressure [14] but also swirl distortion to account for performance and stability margin losses and have proposed relevant metrics to assess these distortions [15,16].…”

Nonaxisymmetric Stator Design for Boundary Layer Ingesting Fans

“…The experimental rig and test case used in this paper is the low-speed, single-stage fan rig shown in Figure 3 and described in [1,7]. The rig was purpose-built for measuring fan-distortion interaction, with a long intake duct that allows the rotor to interact freely with the distorted upstream flow field.…”

Effect of Forward-Swept Rotor on Stall Margin in an Axial Flow Compressor at Distorted Inflow Condition

“…However, both the stalls of rotors were caused by the spike disturbances finally. Perovic et al [3] showed that the spike disturbance is generated near the tip region and develops into the rotating cell. Therefore, it was considered that the Sweep caused some change in the flow field near the tip region at a low flow rate.…”

“…BLI engines intake the boundary layer flow of aircraft and operate at the distorted inflow condition. The experimental study using the fan with the distortion gauze to generate the inlet distortion of BLI profile has been conducted by Perovic et al [3]. It is revealed that the disturbance occurs in the region of the low-momentum fluid and the stall inception with the inlet distortion is initiated by this disturbance when it grows and propagates around the entire annulus.…”