An experimental study of vortex generators in boundary layer ingesting diffusers with a centerline offset

A fundamental experimental investigation was conducted on the flow characteristics and performance of a diffuser ingesting a thick boundary layer of a high shape factor. The duct is characterized by a S-shaped offset and a cross-sectional area change going from a semi-circle at the entrance to a full circle at the exit. The performance of the diffuser was quantified by the two parameters of primary concern in jet engine applications: Total pressure recovery and total pressure distortion at the fan face (i.e. the exit) calculated from the total pressure data measured inside the duct. Flat plate vortex generators (vg’s) were deployed in an attempt to alter the flow structure inside the diffuser and eliminate separation in order to realize improvements in the diffuser performance descriptors. It was readily observed that the presence of the offset had a very strong effect on the behavior of the flow, causing separation and strong secondary flows in the Sduct accompanied by total pressure recovery of approximately 90% and total pressure distortion at the fan face of 77% (normalized to dynamic pressure) for the base case without flow control. Installation of vg’s in the Sduct helped to decrease the distortion to about 11% in some cases and prevented flow separation. However, in all cases, it was not possible to improve the pressure recovery in the S-duct much beyond the 90% range for the base flow with no vg’s.