Turbine

A turbine, comprising:

• a casing;
• a rotor extending inside the casing;
• a plurality of rotor blades fixed to the rotor and arranged in a circumferential direction of the rotor;
• a surrounding portion fixed to the casing and surrounding the plurality of rotor blades via a first radial-directional gap;
• a plurality of stationary vanes fixed to the casing and arranged in the circumferential direction of the rotor so as to have a second radial-directional gap from an outer peripheral surface of the rotor;
• and
• a seal device capable of restricting a flow of a fluid in the first radial-directional gap or the second radial-directional gap,
• wherein the seal device includes:
• at least one step surface disposed on a radially outer surface of the rotor blade facing the first radial-directional gap or on the outer peripheral surface of the rotor facing the second radial-directional gap, the at least one step surface facing upstream in a flow direction of the fluid and dividing the radially outer surface of the rotor blade or the outer peripheral surface of the rotor into at least two sections in an axial direction of the rotor;
• at least two seal fins protruding toward the at least two sections, respectively, from the surrounding portion or the stationary vane, and facing the at least two sections via a seal gap, respectively, the at least two seal fins forming a cavity which extends over the at least one step surface in the axial direction of the rotor between each other;
• and
• a separation enhancing portion disposed on a downstream surface of the seal fin in the flow direction of the fluid and configured to separate the flow of the fluid along the downstream surface,
• wherein the separation enhancing portion comprises a collection of shape change points which form an annular line which extends continuously in the circumferential direction of the rotor and concentrically with the rotor.