Axial Flow Turbine

An axial flow turbine comprising:

• a casing;
• a rotating body;
• a plurality of stator blades that are provided on an inner-circumference side of the casing and arrayed in a circumference direction;
• a plurality of rotor blades that are provided on an outer-circumference side of the rotating body and arrayed in the circumference direction;
• a main flow path in which the plurality of stator blades are arranged and the plurality of rotor blades are arranged on a downstream side of the plurality of stator blades, and through which a working fluid is distributed;
• a shroud provided on an outer-circumference side of the plurality of rotor blades;
• an annular groove portion that is formed on the casing and houses the shroud;
• a gap flow path into which part of the working fluid flows as a leakage flow from a downstream side of the stator blades in the main flow path, and out of which the leakage flow flows to a downstream side of the rotor blades in the main flow path, the gap flow path being formed between the groove portion and the shroud;
• and
• a plurality of stages of seal fins provided in the gap flow path,
• wherein the axial flow turbine further comprises:
• an annular first protruding portion that is formed on the shroud and protrudes toward an upstream side in an axis direction of the rotating body;
• an annular second protruding portion that is formed on the shroud so as to be positioned on an outer side in a radius direction of the rotating body relative to the first protruding portion, protrudes toward the upstream side in the axis direction of the rotating body, and has a length longer than or equal to the first protruding portion;
• a circulating-flow generating chamber formed between the first protruding portion and the second protruding portion;
• and
• a plurality of guide plates formed on the shroud so as to be positioned in the circulating-flow generating chamber,
• wherein the circulating-flow generating chamber is configured to generate a circulating flow such that part of the leakage flow is allowed to collide with a tip surface of the first protruding portion to be directed to the outer side in the radius direction of the rotating body and thereafter collide with an inner circumferential surface of the second protruding portion to be directed to an inner side in the radius direction of the rotating body, and
• the plurality of guide plates are inclined opposite a rotation direction of the rotating body relative to an inward direction in the radius direction of the rotating body.