A gas turbine having an annular flowpath defined between concentric inner radial and outer radial boundaries. A plenum is positioned just outboard of the outer radial boundary that wraps circumferentially in spaced relation thereto. The height of the plenum is defined radially between a floor and ceiling. The plenum includes an inlet formed through the ceiling at a first end and spaced outlet ports formed through the floor. The plenum includes a divider baffle that spans the width of the plenum and extends between the first end and the second end of the plenum so to divide the plenum into radially stacked first and second compartments. The first compartment connects directly to the inlet, while the second compartment connects directly to the outlet ports. The height of the first compartment tapers between a greater height at the first end and a lesser height at the second end of the plenum.