This invention relates to a runner blade for low specific speed Francis turbine.
Conventionally the reaction hydraulic turbine consists of spiral casing (or scroll case), stay vane, guide vane, runner and draft tube in that order. Runner also known as impeller is the only rotating passage. Mixed flow hydraulic turbine where the runner inlet flow is radial and runner exit flow is predominantly axial in direction is known as Francis turbine. The runner which is characterized by a large number of blades (usually 11–19), equally spaced circumferentially around the turbine axis; converts hydraulic energy (water head) into mechanical energy. It is called reaction turbine by definition if the pressure drop across the runner alone is more than 60% of the pressure drop occurring across the total water path (spiral casing to draft tube). Francis turbines with various kind of water path are used in practice for a range of specific speed ns, m=70–400 and head range H=50–500 m; or alternatively for a range of unit discharge q11=150–1350 liter/second, and unit speed n11=40–130 rpm. The hydraulic efficiency η (or eta) for prototype runner is expected to be in the range of 90–95%. A runner is designed to be site specific i.e. for a limited range of unit discharge q11: design unit discharge +−100 liters per second (lps) and fixed unit speed n11 based on runner diameter (D or d1), machine speed (n) rpm and rated head H (m); to provide efficient and cavitation free characteristics. The characteristic dimensions are defined as:
Where q is discharge in lps (liter/second), Pkw is power developed (in KW) by the turbine, rho (998.2) and g (9.81) are water density and acceleration due to gravity.
A typical turbine characteristics are plotted as shown in
George E. Hecker & Willem Jansen have described turbine having 2 or more runner blades each having a Cork Screw configuration (U.S. Pat. No. 5,997,242; Dec. 7, 1999). One wicket gate configuration for hydraulic turbine is proposed by A Gokhman (U.S. Pat. No. 5,441,384; Aug. 15, 1995). David G. Homes et. al has proposed blade configuration for Francis runner for improved cavitation-free performance (U.S. Pat. No. 4,379,757; Oct. 30, 1984). Benno Buchelt has proposed a blade for Kaplan turbine (U.S. Pat. No. 6,007,297, Dec. 28, 1999). The reported work by Kurokawa et al. (U.S. Pat. No. 6,217,285 B1; Apr. 17, 2001) and Strycek et al. (U.S. Pat. No. 3,964,841; Jun. 22, 1976) concerning the design of centrifugal blower, fan and pump for flow discharge as output. They are not meant for turbine application where power is output. The work by Billdal et al. (U.S. Pat. No. 6,135,716; Oct. 24, 2000) proposed impeller for turbine with leaning of leading and trailing edges with respect to hub and shroud, respectively, in the direction of rotation. Harada et al. (U.S. Pat. No. 6,338,610 B1; Jan. 15, 2002) propose impeller for discharge as output with secondary flow loss reduction as goal by providing leaned blade. Nishikawa (U.S. Pat. No. 4,274,810; Jun. 23, 1981) proposes blade for fan application. Centrifugal compressor for discharge as output is proposed by Seleznev et al. (U.S. Pat. No. 3,973,872; Aug. 10, 1976). Swearingen (U.S. Pat. No. 3,610,775; Oct. 5, 1971) is for impeller to work with flow motion in gaseous medium entrained with liquid and solid particles. Kugel (U.S. Pat. No. 2,042,064; May 26, 1936), Kaplan (U.S. Pat. No. 1,509,653; Sep. 23, 1924) and Konda M. (U.S. Pat. No. 3,440,969; Apr. 29, 1969) deal with various constructional features of runners for centrifugal flow machines. The present invention relates to design of new runner blade consisting of blade profiles with reduced plan angle for retrofit work to existing mixed flow machines Francis turbine) water paths, to suit deteriorated head.
An object of the present invention is to propose a new runner blade with 9 profile sections suitable to meet new unit discharge and speed more efficiently than the known art.
Another object is to propose an efficient runner blade to fit with existing water path, i.e. retaining crown, skirt, inlet and outlet boundaries.
Still another object of this invention is to propose a new runner is better than prior art runner for higher unit discharge and higher unit speed in terms of efficiency; and lower pressure minima.
Yet another object of this invention is to propose a runner is more suitable for retrofit job at deteriorated available head than prior art or existing runner.
According to this invention there is provided a runner blade for low specific speed Francis turbine comprising a set of nine profiles covering crown to skirt, each profile made up of a pressure curve and suction curve; both jointed at leading edge at one end and trailing edge at other end.
The runner passage consists of flow domain bounded by inlet, outlet crown (or hub) and skirt (also known as ring, band or tip). The flow domain has an array of blades equi-spaced circumferentially and extending between and interconnecting crown and skirt. The runner rotates about the turbine center line by the action of flow causing thrust on the blade surfaces. Water path is characterized by the dimensions D, the runner inlet diameter.
The runner is expressed in global Cartesian coordinate axis (x,y,z system; z aligned with turbine axis positive toward upward i.e., guide vane side). The water path is divided into a set of quasi-streamlines (r,z system r=√(x2+y2). Each streamline is a projected view on (r,z) plane of blade profile. In (x,y) plane the blade profiles look like aerofoil sections having thicker and rounded leading edges at inlet and thinner, sharp trailing edges at outlet.
The runner consists of blade profiles with lower chord (about 15%) than that from prior art (existing runner). Invented profiles are slightly thinner (about 5%) with lower plan angle θmx−θmn (about 10°) with respect to prior art (note: θ=tan−1(−y/x). Prior art runner has S-shaped profiles.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The nature of invention vis-à-vis prior art will be apparent from the following description made with reference to non-limiting exemplary embodiments of the invention represented in the accompanying drawings:
New runner blade (
Each of the sectional profile is made of two unique curves: pressure curve (lower one) and suction curve (top one); each starting from leading edge (at inlet flow side) and ending at trailing edge (at exit flow side). The characteristics of nine invented profiles are shown in
The invented blade profiles has monotonous variation of rnorm (0.275–0.159), θmx (38.152–36.3735) and L/d1 (0.293–0.199) from crown to skirt profiles.
The value of θmn being zero; the variation of plan angles θmx-θmn is identical with that of θmx. znorm variation is between 0.102 and 0.123; with peak value is at 6th and 7th sections (
Pictorial views of typical 3 profiles (Crown, mean and skirt) for prior art and invented ones are shown in FIGS. 5,6 and 7. The assembled three dimensional view of blade (made of 9 profiles) is shown in
Performance Simulation by Computational Fluid Dynamics (CFD)
The performance of invented blade as well as prior art with existing water path is simulated by CFD for given machine rpm (500) with water as fluid medium; for various guide vane openings. Two typical results depicted pressure loading for the inlet flow angle with respect to tangential direction α tg=13° over the crown section and over the skirt are shown in
The prior art blade was to operate around a point say ‘O’ where q11=197.68 and n11=57.93. Due to aging of hydroset the available head H reduced such that the operation shifted to a point ‘R’ where q11=205.41 and n11=60.19 and efficiency is lower by 0.5%. It is obvious from the cases I–IV that invented blade for existing water path suits well for higher unit discharge and speed as needed for deteriorated head condition. The invented blade is more efficient and shows better cavitational characteristics for a range of ns,m=83–98.
Number | Date | Country | Kind |
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823/DEL/2004 | May 2004 | IN | national |
Number | Name | Date | Kind |
---|---|---|---|
1509653 | Kaplan | Sep 1924 | A |
2042064 | Kugel | May 1936 | A |
3440969 | Masukichi | Apr 1969 | A |
3610775 | Swearingen | Oct 1971 | A |
3964841 | Strycek | Jun 1976 | A |
3973872 | Seleznev et al. | Aug 1976 | A |
4274810 | Nishikawa | Jun 1981 | A |
6135716 | Billdal et al. | Oct 2000 | A |
6217285 | Kurokawa et al. | Apr 2001 | B1 |
6338610 | Harada et al. | Jan 2002 | B1 |
Number | Date | Country | |
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20050249594 A1 | Nov 2005 | US |