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A fluid flow machine has a main flow path 2 which is confined by a hub 3 and a casing 1 and in which at least one row of blades 5 is arranged. A gap 11 is provided on at least one blade row 5 between a blade end and a main flow path confinement, with the blade end and the main flow path confinement

A fluid flow machine has a main flow path 2 which is confined by a hub 3 and a casing 1 and in which at least one row of blades 5 is arranged. A gap 11 is provided on at least one blade row 5 between a blade end and a main flow path confinement, with the blade end and the main flow path confinement performing a rotary movement relative to each other. At least one reversing duct 7 is provided in the area of the blade leading edge in the main flow path confinement at a discrete circumferential position. The reversing duct 7 connects two openings 12, 13 arranged on the main flow path confinement.

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1. A fluid flow machine comprises: a hub;a casing;a main flow path confined by the hub and the casing;at least one row of blades arranged in the main flow path;a gap provided on the at least one row of blades between blade tips and a main flow path confinement, with the blade tips and the main flow

1. A fluid flow machine comprises: a hub;a casing;a main flow path confined by the hub and the casing;at least one row of blades arranged in the main flow path;a gap provided on the at least one row of blades between blade tips and a main flow path confinement, with the blade tips and the main flow path confinement performing a rotary movement relative to each other;at least one reversing duct provided in an area of leading edges of the blades in the main flow path confinement at a discrete circumferential position, with:a) the at least one reversing duct having and connecting an offtake opening to a supply opening, both arranged on the main flow path confinement,b) fluid flowing from the main flow path via the offtake opening into the at least one reversing duct, a centroid CGD of the offtake opening being situated downstream of the leading edges of the blade tips,c) fluid flowing from the at least one reversing duct via the supply opening into the main flow path, a centroid CGU of the supply opening being situated upstream of the leading edges of the blade tips,d) a course of the at least one reversing duct being spatially compact, such that fluid is routed upstream near the main flow path against a main flow direction and rerouted exclusively by flow reversal into the main flow path at a shallow angle,e) the at least one reversing duct having a centerline being defined as a connection of all centroids of cross-sections of the at least one reversing duct, that due to its three-dimensional shape, does not extend completely in one plane;wherein a course of lateral edges SK1 and SK2 of the offtake opening of the at least one reversing duct diverges in an inflow direction. 2. The fluid flow machine of claim 1, wherein the centerline of the at least one reversing duct has a reversing point H identifying a furthest-most meridionally upstream position of the centerline, and in that a projection of the centerline to a meridional plane x-r, in a portion of upstream fluid guidance between the points CGD and H, forms an angle α of between 135° and 225° with a tangent to the main flow path confinement in point CGD, over at least 60% of the running length of this portion. 3. The fluid flow machine of claim 2, wherein a projection of the centerline of the at least one reversing duct to the meridional plane x-r, in a portion between the reversing point H and the centroid CGU of the supply opening is exclusively oriented towards the main flow path confinement and at an increasingly shallow angle approaches the main flow path, with an inclination angle greater than 335° in point CGU. 4. The fluid flow machine of claim 2, wherein the spatial compactness of at least one reversing duct in a plane established by a circumferential direction u and a meridional direction m is provided by further characteristics, with: the projection of the centerline to the plane u-m having a point S marking a maximum extension of the centerline opposite to the relative movement direction of the blade row,the projection of the centerline to the plane u-m having a point Q marking a maximum extension of the centerline in the direction of the relative movement of the blade row,a distance d between the points S and Q being provided in the circumferential direction u,wherein a height h is provided between the points CGD and H vertically to a tangent to the main flow path confinement in point CGD and a ratio of height h to distance d is less than 1. 5. The fluid flow machine of claim 4, wherein the ratio of height h to distance d is less than 0.7. 6. The fluid flow machine of claim 1, wherein the centroid of the supply opening and the centroid of the offtake opening of the at least one reversing duct are circumferentially offset to each other opposite to the relative movement of the blade tips. 7. The fluid flow machine of claim 1, wherein the spatial compactness of the at least one reversing duct in a meridional plane x-r is provided by further characteristics, with: a) a distance “a” being provided between the points CGU and H in a direction of the tangent to the main flow path confinement in point CGD,b) a distance “b” being provided between the points CGD and H in a direction of the tangent to the main flow path confinement in point CGD,c) a height h being provided between the points CGD and H vertically to the tangent to the main flow path confinement in point CGD,d) a ratio of height h to distance b being less than 0.6. 8. The fluid flow machine of claim 7, wherein the ratio of height h to distance b is less than 0.3. 9. The fluid flow machine of claim 1, wherein, when viewing in a plane established by a circumferential direction u and a meridional direction m, a blade pitch tS is provided in the circumferential direction between two each adjacent blade tips, a distance tU is provided between two each adjacent centroids of a supply opening, and a distance tD is provided between two each adjacent centroids of an offtake opening, and at least one of: one of the two distances tU and tD coincides with tS, one of the two distances tU and tD is an integer multiple of the blade pitch tS and one of the two distances tU and tD is an integer divisor of the blade pitch tS. 10. The fluid flow machine of claim 1, wherein the spatial compactness of at least one reversing duct in a plane established by a circumferential direction u and a meridional direction m is provided by further characteristics, with: a) a projection of the centerline to the plane u-m having a point S marking a maximum extension of the centerline opposite to the relative movement direction of the blade row,b) the projection of the centerline to the plane u-m having a point Q marking a maximum extension of the centerline in the direction of the relative movement of the blade row,c) a distance a between the points CGU and H being provided in the meridional direction m,d) a distance d between the points S and Q being provided in the circumferential direction u,e) a ratio of distance a to distance d is less than 1.5. 11. The fluid flow machine of claim 10, wherein the ratio of distance a to distance d is less than 0.7. 12. The fluid flow machine of claim 1, wherein when viewing in a plane established by a circumferential direction u and a normal direction n, the at least one reversing duct is inclined in an area of the offtake opening in the direction of the relative movement of the blade tips, with an angle β between the main flow path confinement and a projection of the centerline to the plane u-n in point CGD being less than 45°. 13. The fluid flow machine of claim 1, wherein at least one tenth of a length of a confinement edge of the offtake opening is oriented essentially in a direction of a blade profile chord, so that an angle between an inclination of a tangent to the offtake opening (angle ε) and an inclination of the profile chord (angle λ) is less than 15°. 14. The fluid flow machine of claim 1, wherein, when viewing in a plane established by a circumferential direction u and a normal direction n, the at least one reversing duct is inclined in an area of the supply opening, with an angle γ between the main flow path confinement and a projection of the centerline to the plane u-n in point CGU is between 30° and 150°. 15. The fluid flow machine of claim 1, wherein, when viewing in a plane established by a circumferential direction u and a normal direction n, the centerline of the at least one reversal duct projected to this plane has a crossing such that the centerline obliquely departs from the main flow path confinement and then arcs in an opposite direction back to the main flow path confinement, thus taking a loop-type course with a crossing point outside of the main flow path. 16. The fluid flow machine of claim 1, wherein the centroid CGU of the supply opening of the at least one reversing duct, with reference to its circumferential position in a circumferential direction u, is disposed between centroids CGD of offtake openings of a next two adjacent reversing ducts each, thus providing for an overlap of adjacent reversing ducts when viewing in a plane established by the circumferential direction u and a normal direction n. 17. The fluid flow machine of claim 1, wherein the offtake opening of the at least one reversing duct is disposed completely downstream of a leading edge line LE, and that the centroid CGD of the offtake opening is provided in a meridional flow direction m between the leading edge line LE and a point at half a profile depth of the blade tip centrally between leading edge line LE and a trailing edge line TE. 18. The fluid flow machine of claim 1, wherein a plane u-n is established by a circumferential direction u and a normal direction n and the offtake opening of the at least one reversing duct is formed by a shallow ram inlet, with an inclination angle β of a projection of the centerline in the plane u-n against the main flow path confinement being less than 25°. 19. The fluid flow machine of claim 1, wherein offtake openings of adjacent reversing ducts directly adjoin each other in at least one point. 20. The fluid flow machine of claim 1, wherein the supply opening of the at least one reversing duct is provided in a groove extending downstream to behind the leading edge line. 21. The fluid flow machine of claim 1, and further comprising a two-part abradable coating, of which one part is arranged upstream and one part downstream of an offtake zone of the reversing ducts provided in the area of the running gap of the blade row, and that the blade tips at the running gap have one shallow recess each in an area not covered by the abradable coating. 22. The fluid flow machine of claim 1, and further comprising a two-part abradable coating, of which one part is arranged upstream and one part downstream of an offtake zone of the reversing ducts, is provided in the area of the running gap of the blade row, and that the blade tips at the running gap have two short, shallow recesses each arranged in an area between an offtake zone and a respective part of the abradable coating. 23. The fluid flow machine of claim 1, and further comprising a shortened abradable coating provided downstream of an offtake zone of the reversing ducts in the area of the running gap of the blade row, with a number of grooves containing supply openings being located in the main flow path confinement upstream of the offtake zone, and that the blade tips at the running gap have a shallow recess extending to the leading edge. 24. The fluid flow machine of claim 1, wherein reversing ducts having at least one of different shape, position and extension are provided along a circumference of the main flow path confinement. 25. A fluid flow machine comprises: a hub;a casing;a main flow path confined by the hub and the casing;at least one row of blades arranged in the main flow path;a gap provided on the at least one row of blades between blade tips and a main flow path confinement, with the blade tips and the main flow path confinement performing a rotary movement relative to each other;at least one reversing duct provided in an area of leading edges of the blades in the main flow path confinement at a discrete circumferential position, with:a) the at least one reversing duct having and connecting an offtake opening to a supply opening, both arranged on the main flow path confinement,b) fluid flowing from the main flow path via the offtake opening into the at least one reversing duct, a centroid CGD of the offtake opening being situated downstream of the leading edges of the blade tips,c) fluid flowing from the at least one reversing duct via the supply opening into the main flow path, a centroid CGU of the supply opening being situated upstream of the leading edges of the blade tips,d) a course of the at least one reversing duct being spatially compact, such that fluid is routed upstream near the main flow path against a main flow direction and rerouted exclusively by flow reversal into the main flow path at a shallow angle,e) the at least one reversing duct having a centerline being defined as a connection of all centroids of cross-sections of the at least one reversing duct, that due to its three-dimensional shape, does not extend completely in one plane;wherein at least one tenth of a length of a confinement edge of the offtake opening is oriented essentially in a direction of a blade profile chord, so that an angle between an inclination of a tangent to the offtake opening (angle ε) and an inclination of the blade profile chord (angle λ) is less than 15°. 26. A fluid flow machine comprises: a hub;a casing;a main flow path confined by the hub and the casing;at least one row of blades arranged in the main flow path;a gap provided on the at least one row of blades between blade tips and a main flow path confinement, with the blade tips and the main flow path confinement performing a rotary movement relative to each other;at least one reversing duct provided in an area of leading edges of the blades in the main flow path confinement at a discrete circumferential position, with:a) the at least one reversing duct having and connecting an offtake opening to a supply opening, both arranged on the main flow path confinement,b) fluid flowing from the main flow path via the offtake opening into the at least one reversing duct, a centroid CGD of the offtake opening being situated downstream of the leading edges of the blade tips,c) fluid flowing from the at least one reversing duct via the supply opening into the main flow path, a centroid CGU of the supply opening being situated upstream of the leading edges of the blade tips,d) a course of the at least one reversing duct being spatially compact, such that fluid is routed upstream near the main flow path against a main flow direction and rerouted exclusively by flow reversal into the main flow path at a shallow angle,e) the at least one reversing duct having a centerline being defined as a connection of all centroids of cross-sections of the at least one reversing duct, that due to its three-dimensional shape, does not extend completely in one plane;wherein, when viewing in a plane established by a circumferential direction u and a normal direction n, the centerline of the at least one reversal duct projected to this plane has a crossing such that the centerline obliquely departs from the main flow path confinement and then arcs in an opposite direction back to the main flow path confinement, thus taking a loop-type course with a crossing point outside of the main flow path. 27. A fluid flow machine comprises: a hub;a casing;a main flow path confined by the hub and the casing;at least one row of blades arranged in the main flow path;a gap provided on the at least one row of blades between blade tips and a main flow path confinement, with the blade tips and the main flow path confinement performing a rotary movement relative to each other;at least one reversing duct provided in an area of leading edges of the blades in the main flow path confinement at a discrete circumferential position, with:a) the at least one reversing duct having and connecting an offtake opening to a supply opening, both arranged on the main flow path confinement,b) fluid flowing from the main flow path via the offtake opening into the at least one reversing duct, a centroid CGD of the offtake opening being situated downstream of the leading edges of the blade tips,c) fluid flowing from the at least one reversing duct via the supply opening into the main flow path, a centroid CGU of the supply opening being situated upstream of the leading edges of the blade tips,d) a course of the at least one reversing duct being spatially compact, such that fluid is routed upstream near the main flow path against a main flow direction and rerouted exclusively by flow reversal into the main flow path at a shallow angle,e) the at least one reversing duct having a centerline being defined as a connection of all centroids of cross-sections of the at least one reversing duct, that due to its three-dimensional shape, does not extend completely in one plane;wherein the centroid CGU of the supply opening of the at least one reversing duct, with reference to its circumferential position in a circumferential direction u, is disposed between centroids CGD of offtake openings of a next two adjacent reversing ducts each, thus providing for an overlap of adjacent reversing ducts when viewing in a plane established by the circumferential direction u and a normal direction n. 28. A fluid flow machine comprises: a hub;a casing;a main flow path confined by the hub and the casing;at least one row of blades arranged in the main flow path;a gap provided on the at least one row of blades between blade tips and a main flow path confinement, with the blade tips and the main flow path confinement performing a rotary movement relative to each other;at least one reversing duct provided in an area of leading edges of the blades in the main flow path confinement at a discrete circumferential position, with:a) the at least one reversing duct having and connecting an offtake opening to a supply opening, both arranged on the main flow path confinement,b) fluid flowing from the main flow path via the offtake opening into the at least one reversing duct, a centroid CGD of the offtake opening being situated downstream of the leading edges of the blade tips,c) fluid flowing from the at least one reversing duct via the supply opening into the main flow path, a centroid CGU of the supply opening being situated upstream of the leading edges of the blade tips,d) a course of the at least one reversing duct being spatially compact, such that fluid is routed upstream near the main flow path against a main flow direction and rerouted exclusively by flow reversal into the main flow path at a shallow angle,e) the at least one reversing duct having a centerline being defined as a connection of all centroids of cross-sections of the at least one reversing duct, that due to its three-dimensional shape, does not extend completely in one plane;and further comprising a shortened abradable coating provided downstream of an offtake zone of the reversing ducts in the area of the running gap of the blade row, with a number of grooves containing supply openings being located in the main flow path confinement upstream of the offtake zone, and that the blade tips at the running gap have a shallow recess extending to the leading edge. 29. A fluid flow machine comprises: a hub;a casing;a main flow path confined by the hub and the casing;at least one row of blades arranged in the main flow path;a gap provided on the at least one row of blades between blade tips and a main flow path confinement, with the blade tips and the main flow path confinement performing a rotary movement relative to each other;at least one reversing duct provided in an area of leading edges of the blades in the main flow path confinement at a discrete circumferential position, with:a) the at least one reversing duct having and connecting an offtake opening to a supply opening, both arranged on the main flow path confinement,b) fluid flowing from the main flow path via the offtake opening into the at least one reversing duct, a centroid CGD of the offtake opening being situated downstream of the leading edges of the blade tips,c) fluid flowing from the at least one reversing duct via the supply opening into the main flow path, a centroid CGU of the supply opening being situated upstream of the leading edges of the blade tips,d) a course of the at least one reversing duct being spatially compact, such that fluid is routed upstream near the main flow path against a main flow direction and rerouted exclusively by flow reversal into the main flow path at a shallow angle,e) the at least one reversing duct having a centerline being defined as a connection of all centroids of cross-sections of the at least one reversing duct, that due to its three-dimensional shape, does not extend completely in one plane;wherein reversing ducts having at least one of different shape, position and extension are provided along a circumference of the main flow path confinement.