High efficiency blade configuration for steam turbine

A steam turbine that passes more turbine driving steam by off-setting the turbine moving blade throat.pitch ratio before operation and, when a blade untwist is generated during operation, causing more turbine driving steam to flow by maintaining an appropriate value, and, at the same time causing the turbine moving blade throat.pitch ratio to swell by giving the blade untwisting angle to the blade cross-sections in regions where the aerodynamic loss is small. The steam turbine is one in which the throat.pitch ratio (S/T) distribution of a turbine moving blade is offset by forming a curve providing at least one minimal value and maximal value by giving blade twist angle to the blade cross-sections in the blade height direction from blade root to blade tip and, at the same time, the distribution of throat.pitch ratio (S/T) taking into consideration blade untwist generated during operation.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT A preferred embodiment of turbine moving blades and turbine nozzle blades assembled into a turbine relating to the present invention will be described below with reference to the drawings and the reference numerals assigned in the drawings. In the steam turbine relating to this embodiment, as shown in FIG. 1, a turbine stage 22 is composed of an assembly of turbine nozzle blades 20 , which are supported at their ends by an inner diaphragm 23 and an outer diaphragm 24 , and an assembly of turbine moving blades 21 , which are embedded in the turbine shaft 25 . A plurality of such turbine stages 22 are arranged along the turbine shaft 25 . The blades are made of an alloy of about 88% to about 92% titanium, about 4% to about 8% aluminium and about 2% to about 6% vanadium by weight percent. A rotation speed of 3000 rpm is used in 50 Hz areas and a rotation speed of 3600 rpm is used in 60 Hz areas. Each turbine moving blade 21 has a blade embedded part 26 and a blade effective portion 27 . Also, each turbine moving blade 21 is provided with a blade tip connector 28 at the blade tip, and an intermediate connector 29 at the blade intermediate part. The diameter of the blade root of the blade effective portion 27 is 1.4 m or more, and the blade height is 1.0 m or more. The intermediate connector 29 is installed in a position in the about 50% to about 70% range of normalized blade height and is designed to reduce vibration of the turbine moving blades 21 during operation and, simultaneously, to suppress any untwisting of the turbine moving blade 21 to a low level. The blade tip connector 28 and the intermediate connector 29 are respectively of the same configurations as shown in FIG. 1 1 and FIG. 12 , and described above in reference to those figures. The turbine moving blade 21 has a blade row performance distribution shown in FIG. 2 . This blade row performance distribution shows aerodynamic loss (turbine moving blade loss) on the vertical axis and normalized blade height on the horizontal axis, respectively, and shows that aerodynamic loss becomes small in the normalized blade height range of about 15 to about 45%. This blade row performance distribution was obtained by numeric analysis of the turbine driving steam flow, and agrees well with experimental data for model turbines and, as such, is effective data when carrying out three-dimensional design of a blade row. Referring to FIG. 3 , with the turbine moving blades 21 subject to such design requirements and provided with such a blade row performance distribution, the three-dimensional flow pattern of the turbine blade row can be optimized by the appropriate setting of throat.pitch ratio (S/T), where the pitch between one blade effective portion 27 a and the adjacent blade effective portion 27 b is taken as T, and the width of the flow throat (the narrowest passage) formed by the back 30 of the one blade effective portion 27 a and the belly of the adjacent blade effective portion 27 b is taken as S. In FIG. 3 , when blade cross-sections are taken at arbitrary positions along the blade height from the blade root to the blade tip (for example, when the blade cross-section at the blade root (blade height 0%) is taken as A 0 , the blade cross-section at blade height about 15% as A 15 , the blade cross-section at blade height about 30% as A 30 , the blade cross-section at blade height about 85% as A 85 , and the blade cross-section at blade tip (blade height 100%) as A 100 ), then, if a greater twist angle is given to each cross-section A 0 , A 15 , . . . than in the prior art, the prior art trailing edge ridge line TERL (shown by the broken line) that joins each trailing edge 31 , 31 , . . . shifts to off-set trailing edge ridge line OTERL (shown by the solid line). In practice, the twist angle is given in the clockwise direction so that cross-section A 0 shifts from point P 0 to point Q 0 , cross-section A 15 shifts from point P 15 to point Q 15 and cross-section A 85 shifts from point P 85 to point Q 85 , and also the twist angle is given in the anti-clockwise direction so that cross-section A 30 shifts from point P 30 to point Q 30 and cross-section A 100 shifts from point P 100 to point Q 100 . Offset leading edge ridge line OLERL is formed by the solid line that joins a leading edges 32 , 32 , . . . of each cross-section A 0 , A 15 , . . . The twist angles given to each cross-section A 0 , A 30 , . . . are in the clockwise or anti-clockwise direction when viewed with the leading edges on the left and, at the same time, with the backs facing upwardly. If off-setting is performed by setting the twist angles as mentioned above, throat.pitch ratio (S/T), which is determined by the distance between turbine moving blades, will have the distribution shown by the solid line in FIG. 4 when at rest, and the distribution shown by the broken line during operation. If a larger blade twist angle than in the prior art is given to each cross-section A 0 , A 15 , . . . , and the throat.pitch ratio (S/T) for each cross-section A 0 , A 15 , . . . is determined based on the blade twist angle, that throat.pitch ratio (S/T) distribution, as shown by the solid line in FIG. 4 , forms a roughly S-shaped curve having a maximum and a minimum. At the same time, the solid line is markedly shifted from the prior art throat.pitch ratio (S/T) position shown by the single-dot chain line, and is maintained, so-to-speak, off-set. Here, “maximum” and “minimum” are defined in the local sense, i.e., with reference to neighboring values, as follows: 1) If f(x)″ is negative when f(x)′&equals;0, f(x) is a maximun; 2) If f(x)″ is positive when f(x)′&equals;0, f(x) is a minimum. In other words, a “maximum” is one which is surrounded by lesser values; a “minimum” is one which is surrounded by greater values. In this way, with this embodiment, throat- pitch ratio (S/T) is determined beforehand by giving a greater twist angle than in the prior art to each cross-section A 0 , A 15 , . . . , and the determined (S/T) is off-set to the position shown by the solid line. This differential twist angle (as compare to the prior art) is defined herein as the “differential blade twist angle.” Along with the untwisting that occurs during operation, the (S/T) distribution moves from the off-set position and conforms to the throat.pitch ratio (S/T) position shown by the broken line. Therefore, more turbine driving steam can be made to flow in regions where losses are small and less in regions where losses are large, resulting in improved turbine blade row performance. The throat.pitch ratio (S/T) distribution graph for the turbine moving blade 21 shown in FIG. 4 is one in which the differential blade twist angle was set over all blade cross-sections A ) , A 15 , . . . for the entire blade from blade root to blade tip. However, whether to impart the differential blade twist angle over the entire length of the blade, or over a smaller portion of the blade, depends on whether the turbine driving steam flow is subsonic, transonic, or supersonic. When the turbine driving steam flow is subsonic or transonic, for the turbine moving blade 21 , as shown in FIG. 5 , throat.pitch ratio (S/T) is determined by giving a differential blade twist angle to each blade cross-section in the blade height range from about 10% to about 45%, taking the blade root (blade height 0%) as the reference, and the predetermined throat.pitch ratio (S/T) distribution is formed as a curve having at least one minimal value or maximal value, or forms a so-called S-shaped curve having a minimal value and a maximal value. In practice, it is preferable that the minimal value of throat.pitch ratio (S/T) should be formed in at a blade height position in the range from about 10% to about 20%, and the maximal value of throat.pitch ratio (S/T) should be formed at a blade height position in the range from about 15% to about 45%. Predetermining throat.pitch ratio (S/T) by giving a differential blade twist angle to each cross-section in the blade height range from about 10% to about 45%, and setting the throat.pitch ratio (S/T) distribution curve to have at least one minimal value or maximal value or an S-shaped curve having a minimal value and a maximal value as described above, compensates for blade untwisting that occurs during operation and, at the same time, passes more turbine driving steam in the region where turbine moving blade loss is small, as shown in FIG. 2 , thus improving turbine row performance. However, special attention must be given to giving a differential blade twist angle at blade height positions of about 10% or less. Specifically, if the throat.pitch ratio (S/T) is made smaller close to the wall surface (the turbine shaft) at the blade root, the outlet flow angle will become smaller and secondary flow loss will increase due to turbulence in the vicinity of the blade root in the corner between the blade and the embedded portion, where a root fillet is added in order to relieve stress concentration. In order to prevent the actual throat.pitch ratio (S/T) that includes the root fillet from becoming too small, it is necessary to adjust the blade twist angle of the root fillet to make the throat.pitch ratio (S/T) larger. When the turbine driving steam flows at supersonic speed, for the turbine moving blade 21 , as shown in FIG. 6 , taking the blade root as the reference in the same way as mentioned above, the throat.pitch ratios (S/T) are predetermined by giving a differential blade twist angle to each blade cross-section from a blade height of about 10% to a blade height of about 95%. The distribution of the predetermined throat.pitch ratios (S/T) thus forms an S-shaped curve which has a minimal value and a maximal value in the blade height range from about 10% to about 95% and, at the same time, is off-set in a curve having a minimal value in a blade height range from about 70% to about 95%, and preferably in the range from about 80% to about 90%. This arrangement suppresses the swollen portion (shown in FIG. 15 ) which occurs when the blades untwist during operation, and ensures that turbine driving steam flow remains in a stable state, thus suppressing the generation of shock waves. Further improvement in turbine efficiency in long-blade turbines can be realized by giving differential blade twist angles to the blade cross-sections of the turbine nozzle blades 20 , so that the steam outlet flows from the turbine nozzle blades will more effectively cooperate with the turbine moving blades in their dynamic configuration. The throat.pitch ratio (S/T) for the turbine nozzle blades is defined in the same way as (S/T) for the turbine moving blades 4 , as shown in FIG. 14 . When considering the distribution of this throat.pitch ratio (S/T) in the blade height direction from the blade root (blade height 0%) to the blade tip (blade height 100%), as shown in FIG. 7 , it appears swollen outward in the blade height range from about 20% to about 80%, taking the blade root as the reference, as if a maximal value were formed. Here, for turbine nozzle blade 20 , the blade root adjacent the inner diaphragm 23 shown in FIG. 1 , and the blade tip is adjacent the outer diaphragm 24 . This distribution of the throat.pitch ratio (S/T) results from giving differential blade twist angles to the cross-sections as if a maximal value were formed in the blade height range of about 20% to about 80%; setting throat.pitch ratio (S/T) at the blade root (blade height 0%) in the range about 0.1 to about 0.5; and setting throat.pitch ratio (S/T) at the blade tip (blade height 100%) in the range about 0.14 to about 0.5, respectively. Thus, the total loss (turbine nozzle blade loss plus turbine moving blade loss) is reduced. The about 0.1 to about 0.5 throat.pitch ratio (S/T) shown in FIG. 8 is the preferred application range obtained from a model turbine. If the throat.pitch ratios (S/T) at the blade root and the blade tip become too small, the rapid increase in loss occurs with the above-mentioned value as a boundary because the secondary (turbulent) flow loss close to the wall surface rapidly increases with this value as a boundary. Also, the flow distribution balance across the radial direction is upset causing an excessively large flow at the wall surface and rapidly increasing frictional loss close to the wall. Setting the throat.pitch ratio (S/T) at the tip (blade height 100%) to about 0.14 to about 0.5 is based on the fact that, as shown in FIG. 9 , the turbine stage loss will become smaller. This range of throat.pitch ratio (S/T) at the tip is the preferred application range, and similarly is obtained from a model turbine. Summarizing this embodiment, the throat.pitch ratio (S/T) for turbine nozzle blades 20 is determined by giving a differential blade twist angle to the blade cross-sections such that the distribution of the throat.pitch ratio (S/T) is caused to swell outward, as if the maximal value were formed, within a blade height range of about 20% to about 80%. At the same time, the throat.pitch ratio (S/T) at the blade root (blade height 0%) is set in the range of about 0.1 to about 0.5, while the throat.pitch ratio (S/T) at the blade tip (blade height 100%) is set in the range of about 0.14 to about 0.5. Thus, more turbine driving steam is concentrated and caused to flow in the region where the turbine stage loss is small. Therefore, the turbine blade row performance can be even further improved over that of the prior art. For the turbine nozzle blades, although adjustment of the blade twist angle is the most direct method for adjustment of the throat.pitch ratio (S/T), throat.pitch ratio (S/T) may also be adjusted by varying the curvature from the part that forms the suction surface throat to the trailing edge. That is, if the curvature of the part forming the back throat to the trailing edge is made smaller, the trailing edge will come closer to the back of the adjacent blade and the throat.pitch ratio (S/T) will become smaller. Conversely, if the curvature is made larger, the throat.pitch ratio (S/T) will become larger. Further, the throat.pitch ratio (S/T) can be adjusted by varying the trailing edge thickness. However, since the blade row performance will be reduced if the trailing edge is made thicker, it will be necessary to make other adjustments such that overall efficiency will be maintained. In summary, for turbine moving blades assembled in a steam turbine according to the present invention, to compensate for the blade untwisting that occurs during operation, the distribution of the throat.pitch ratio (S/T) determined according to the differential blade twist angle, which is given to the blade cross-sections, is off-set so that it becomes larger than in the prior art and, during operation, the throat.pitch ratio (S/T) thus is maintained at an optimum value. Therefore, the turbine driving steam flows in a more stable state, and turbine blade row performance is improved. For the turbine nozzle blades, the distribution of the throat.pitch ratio (S/T) determined according to the differential blade twist angle, which is given to the blade cross-sections, is made to swell in the outward direction as if the maximal value were formed. Thus, more turbine steam is concentrated and made to flow in the region where the turbine stage loss is small. Therefore, the turbine blade row performance can be even further improved over that of the prior art. Obviously, numerous modifications and variations of the present invention are possible in view of the above teachings. Japanese priority Application No. PH10-218262, filed on Jul. 31, 1998, including the specification, drawings, claims and abstract, is hereby incorporated by reference.