Patent ID: 8251648

Claim:
A fluid flow machine comprising: a casing; a rotating shaft; a flow path formed between the casing and the rotating shaft; rows of blades positioned in the flow path; a plurality of spaced apart recesses disposed circumferentially around a surface facing free blade tips of at least one row of blades in an area of the free blade tips, the recesses and the free blade tips being circumferentially movable relative to one another, the recesses being open to the free blade tips and otherwise closed, wherein a cross-section and position of each recess are defined as follows: an axial arrangement is configured such that a partial length of the recess extends by a maximum of 30 percent of a chord length of a blade from a leading edge of the blade against a direction of flow, and up to a maximum of 50 percent of the chord length of the blade from the blade leading edge in the direction of flow, an angle β of a leading flank of the recess is inclined by 30° to 180° to a wall of the casing, an angle α of a trailing flank of the recess is inclined by 20° to 70° to the wall of the casing, a radial depth of the recess is a maximum of 50 percent of an axial chord length of the blade, an angle γ of a forward surface of the recess lies between 30° and 60° in a circumferential direction, and an inclination angle δ of a rearward surface of the recess lies between 30° and 60° in the circumferential direction; wherein each recess extends along an axis to maintain a straight air flow through the recess.