Structure for cooling turbine blade

A structure for cooling a turbine airfoil of a turbine driven by a high-temperature gas, the structure comprising:

• a cooling passage formed between a first airfoil wall of the turbine airfoil that is curved so as to be concave relative to a passage for the high-temperature gas and a second airfoil wall of the turbine airfoil that is curved so as to be convex relative to the passage for the high-temperature gas;
• a lattice structure body including a first rib set composed of a plurality of first ribs provided on a wall surface, of the first airfoil wall, that faces the cooling passage, and a second rib set composed of a plurality of second ribs provided on a wall surface, of the second airfoil wall, that faces the cooling passage, the second rib set being stacked on the first rib set so as to form a lattice pattern;
• a cooling medium discharge port provided at a downstream end portion of the cooling passage and configured to discharge the cooling medium within the cooling passage to the outside;
• an exposed wall portion in the form of a portion of the second airfoil wall extending beyond the cooling medium discharge port to the outside;
• and
• a flat surface portion that is a portion of the cooling passage from an outlet of the lattice structure body to the cooling medium discharge port and at which the wall surface of the first airfoil wall and the wall surface of the second airfoil wall are formed as flat surfaces,
• wherein the flat surface portion has a length within a range of not less than 1 time and not greater than 5 times of a height of the outlet of the lattice structure body.