In recent years, it is desired in the field of a power plant including an axial-flow turbine to improve power generation efficiency and also to suppress the amount of generated gas such as CO2, SOx, and NOx, to suppress global warming.
In a thermal power plant using an axial-flow turbine such as a steam turbine or a gas turbine, as an effective technique to improve the power generation efficiency, the temperature of working fluid supplied to the axial-flow turbine, that is, the temperature of the turbine inlet, is raised.
Further, when a volumetric flow rate of the working fluid flowing into the axial-flow turbine is small, a throat area (minimum passage area) between nozzles (stator vanes) needs to be reduced. Consequently, the airfoil height of the nozzle needs to be designed small. In this case, the secondary flow becomes significant between the nozzles and the secondary loss increases, which may cause a drop in performance of the axial-flow turbine. For this reason, in the case when a volumetric flow rate of the working fluid is small, suppressing the drop in performance of the turbine as well as suppressing the secondary flow may be difficult.
In a steam turbine, when a volumetric flow rate of the working fluid is small, a so-called partial admission structure is applied to suppress the secondary flow. Generally, a plurality of nozzles is arranged in the whole circumference of an annular opening portion formed between an outer ring diaphragm and an inner ring diaphragm of the steam turbine. The working fluid flows into the whole circumference of the annular opening portion. On the contrary, the partial admission structure is a structure in which the working fluid flows into a portion of a region of the annular opening portion in the circumferential direction and another portion of the region is closed. That is, a plurality of nozzles is arranged in one portion of the region of the annular opening portion in the circumferential direction, and no nozzle is provided in second portion of the region. The second region is closed so that the working fluid will not flow there into. In this case, by changing the ratio of the closed region to the whole region of the annular opening portion, the height of the nozzle can be adjusted to be larger. Therefore, in the case when the volumetric flow rate of the working fluid is small, the secondary flow can be suppressed and the drop in performance of the axial-flow turbine can be suppressed. Generally, such partial admission structure is preferably used in a speed control stage of a steam turbine.