Rectangular bleed ports (24) are circumferentially arranged. Each port has an average radius (r) greater than 30 percent of the port width (W) throughout the reactant surface of the upstream facing edge (30), thereby avoiding flow separation from the surface. A tangent point (36) at the junction of the reactant surface (36) and the inside surface of casing (12) is located a distance (x) upstream to blades (18) greater than 40 percent of the spacing between the blades. This reduces flow losses in the ports caused by blade upwash.