Turbine

1. A turbine comprising:

a turbine rotor blade; and

a turbine casing having,

an introducing part, and

a scroll part extending along a circumferential direction of the turbine rotor blade, an outer peripheral wall of the scroll part extending continuously from an outer peripheral wall of the introducing part,

wherein the outer peripheral wall of the scroll part has a tongue part in contact with a virtual circle around an axis of the turbine rotor blade, and

wherein when a flow passage of an exhaust gas is defined by the outer peripheral wall of the scroll part and the virtual circle, an A/R ratio of a flow passage area A to a distance R between an axis of the turbine rotor blade and a flow passage center of the scroll part has a concave distribution at least in a part of a graph where an abscissa represents a circumferential position around the axis of the turbine rotor blade and an ordinate represents the A/R ratio.

2. The turbine according to claim 1 ,

wherein, provided that the circumferential position is 0° at an inlet of the scroll part and a positional value of the circumferential position increases from the inlet toward an end of the scroll part, the scroll part is configured such that a rate of change of the A/R while the circumferential position changes from 0° to 90° is at least 1.2 times higher than a case where the A/R linearly decreases.

3. The turbine according to claim 2 ,

wherein the scroll part is configured such that the rate of change of the A/R while the circumferential position changes from 0° to 90° is at least 1.4 times higher than the case where the A/R linearly decreases.

4. The turbine according to claim 1 ,

wherein the scroll part is configured such that the rate of change of the A/R while the circumferential position changes from 0° to 90° is up to three times higher than a case where the A/R linearly decreases.