An apparatus and system for injecting fluid into a boundary layer of a flow of fluid are provided. The boundary layer injection insert assembly includes an insert body and a central bore extending through the insert body from an inlet opening positioned at a first end to an outlet opening positioned at a second end of the insert body opposite the first end. The insert body is approximately cylindrical about a longitudinal axis and includes a thickness in a radial direction orthogonal to the longitudinal axis. The first end includes a plurality of injection holes extending through the thickness for a first distance, the first distance being less than the length. The boundary layer injection insert assembly also includes an annular spacer at least partially surrounding the second end and including a radially inner surface and a radially outer surface spaced apart by a thickness of the annular spacer.