Distortion Tolerant Fan Design
Boundary Layer Ingestion (BLI) technology could achieve a fuel burn reduction of up to 15%. However, the fans need to operate in severely distorted flow, which affects the blades’ mechanical integrity, aerodynamic efficiency and stability. The aim of this thesis is to understand the impact of strengthening rotor blades on aerodynamic performance, investigate how the performance can be improved by modifying the fan rotor radial work profile and explain the mechanism that allows BLI fans to operate stably even when there are regions where the fan operates beyond the clean flow stability limit.
Two test cases were used: a low speed rig fan and a representative transonic fan. The transonic fan rotor design was thickened to satisfy aeromechanical criteria and the flow field was simulated with a representative BLI inlet stagnation profile to investigate the impact of the design change. A combination of computational and experimental techniques was utilised to redesign and test a new low speed fan rotor, for a different purpose. The blade metal angles, chord and positions of maximum camber and thickness were modified to reduce the peak in loading, and hence improve the aerodynamic performance and the stability margin in the same BLI inlet flow field as for the transonic fan. Unsteady casing static pressure measurements were used to track the development of disturbances in the experimental rig at the rotor tip to analyse stall inception in distortion.
The necessary transonic blade design modification led to a loss of efficiency of 0.5% and reduction of mass flow of 2%, which were attributed to a rise in blockage due to blade thickening and an increase in profile loss in the hub region. This is a promising result, as it indicates that the necessary mechanical design changes only impacted slightly on the aerodynamic performance of the BLI fan.
The low speed fan redesign reduced the radial loading at the tip and hub and increased at the mid span to keep the same operating point. This helped alleviate the losses at the blade tip in distorted flow and improved blade performance, also increasing the stability margin. Casing static pressure measurements demonstrated that disturbances are created where the incidence is higher than the critical level at a near stall point in clean flow, but they decay as they propagate into the region where the incidence drops below the critical level. Stall was found to occur once the disturbances were able to propagate around the entire annulus without being suppressed, which is why the loss of stability margin due to distortion is small in BLI fans.
The results are promising as they indicate that the impact of necessary mechanical design changes is not severe and that adjusting the blade design to target the regions of peak loss in distorted flow can recover some efficiency lost due to a combination of operating in distorted flow and requiring a more robust mechanical design. Moreover, the stall inception mechanism identified supports the previous findings that loss of stability margin due to distortion is minor, confirming that stability is not an issue for a fan operating in BLI.