1. Field of the Invention
The present invention relates to a steam turbine stator vane.
2. Description of the Related Art
In general, a steam turbine has a plurality of stages that are each constituted by stator vanes and moving blades, while the stages are arranged in the axial direction of a turbine rotor. An exhaust hood is installed on the downstream side of the steam turbine. Steam that is a working fluid is accelerated by stator vanes that serve as a convergent passage so that kinetic energy of the steam is increased. The kinetic energy is converted into rotational energy by moving blades so that power is generated.
In the steam turbine, when the lengths of the turbine blades located at a last stage of a low-pressure turbine are increased, the area of a passage through which steam flows is increased, and the kinetic energy of the steam is reduced. Thus, kinetic energy that is not used for generation of power and is exhausted is reduced, and the turbine efficiency is improved.
However, when the lengths of the blades located at the last stage are increased, the following problems occur.
The first problem is a reduction in the efficiency due to a reduction in the degree of reaction. When the lengths of the blades of the turbine are increased, a spreading angle (flare angle) (on the outer circumferential side of the turbine) of the turbine passage through which steam flows increases. When the flare angle increases, a velocity component of steam in a radial direction (of the turbine) at a stator vane outlet increases. The velocity component in the radial direction is increased by centrifugal force of the moving blades. Intervals of an inner circumferential portion of a uniform flow diagram of a two-dimensional passage projected in a plane including a rotational axis increase. As a result, in an inner circumferential portion of the turbine passage, a substantial passage area that corresponds to the moving blade is larger than a substantial passage area that corresponds to the stator vane. Thus, the degree of reaction, which is the ratio of a reduced amount of pressure at the moving blade to a reduced amount of pressure at the stage, is reduced.
The optimal value of the degree of reaction to maximize the efficiency exists. The turbine blades are designed on the basis of the degree of reaction at which the efficiency becomes maximum. Thus, when the degree of reaction is reduced, the efficiency is reduced.
As methods for increasing the degree of reaction in the inner circumferential portion of the turbine passage and improving the efficiency, a tangential lean that causes the stator vane to be inclined toward a rotational direction of the moving blade with respect to the height direction of the stator vane, and an axial lean that causes the stator vane to be inclined toward the axial direction of the turbine, have been used, as described in U.S. Patent Application No. 2007/0071606. These leans are effective means for changing the degree of reaction. For example, JP-H10-131707-A discloses a technique for setting the degree of reaction to an appropriate degree on the basis of a Bow angle γ (that is a parameter of the shape of the tangential lean) and the ratio of a projecting amount on a tip side to a pitch on an inner circumferential side. In addition, European Patent Application No. 2075408, U.S. Pat. No. 6,099,248, JP-2009-121468-A and International Publication No. 2005/005784 each disclose a technique for adjusting the degree of reaction to an appropriate degree by means of a combination of a tangential lean and an axial lean.
The second problem with the increases in the lengths of the blades is an increase in a profile loss. This is attributable to the occurrence of a shock wave that is caused by the fact that the flow of steam into a region in which the moving blades rotate on the outer circumferential side of the moving blades is supersonic.
In a general turbine stage, when the length of the moving blade is increased, and a moving blade outer circumferential end portion Mach number, which is obtained by dividing a rotational circumferential velocity of an outer circumferential inlet portion of the moving blade by a velocity of sound in the steam flowing into a region in which an outer circumferential end portion of the moving blade rotates, exceeds 1.0, there is a possibility that a relative velocity (moving blade relative inflow velocity) of the steam flowing into a region (in which the moving blade rotates) to the rotational velocity of the moving blade may be a supersonic velocity.
When the moving blade relative inflow velocity reaches a supersonic velocity, the flow of the steam on the upstream side of the moving blade is choked. Thus, the flow rate of the steam cannot be determined on the basis of a throat (minimum distance between moving blades that are adjacent to each other in a circumferential direction of the turbine), and designed flow of the steam cannot be achieved. In addition, a large profile loss may occur due to formation of a detached shock wave on the upstream side of a leading edge of the moving blade and interference between the detached shock wave and a blade surface boundary layer.
As described above, when the length of the moving blade of the general turbine stage is increased, the moving blade relative inflow velocity reaches a supersonic velocity, and performance of the turbine stage may be significantly reduced.
As a method for suppressing a profile loss due to supersonic inflow of steam, JP-2003-27901-A discloses a turbine provided with a turbine passage having a specific shape.