Anti-vortex structures at hydropower dams

Transcription

1 International Journal of the Physical Sciences Vol. 7(28), pp , 19 July, 2012 Available online at DOI: /IJPS ISSN Academic Journals Full Length Research Paper Anti-vortex structures at hydropower dams S. M. Taghvaei, R. Roshan, Kh. Safavi and H. Sarkardeh Water Research Institute, Tehran, Iran. Accepted 17 July, 2012 Vortex formation due to power generation at hydropower dams could cause serious problems, and eliminating or reduction in its strength is one of the dam designer and manager duties. Therefore, the design and use of anti-vortex structures is usual. Design of an anti-vortex structure for each project is unique and it is therefore necessary to have knowledge about the performance of each one. In the present study, to evaluate the performance of various anti-vortex structures, physical model of power intakes were constructed and used. Intakes were projected in the reservoir and are current in many dam projects. For each anti-vortex, a comprehensive study was performed to determine better Alternative in view of dissipating vortices. Experiments were conducted by recording class of vortices in different water elevations and with the current range of discharge for 13 anti-vortex Alternatives. Experiments showed that the horizontal plate had better performance in eliminating vortices. Key words: Anti-vortex structure, physical model, vortex, power generation, dam. INTRODUTION Using hydropower dams is a way to generate green energy, which is presently commercially viable on a large scale. Formation of vortices at hydropower dams are undesirable phenomenon in lower water elevations which must be considered by dam designers and managers. Strong vortices can induce air and trash into the tunnel and reduce performance of turbine (Knauss, 1987). ased on Sarkardeh et al. (2010), vortices are classified into three general classes. Vortices of lass are considered as the safe vortices and weak rotation of flow or a small drop maybe observed in water surface. Vortices lass, the rotation of flow is extended down to the intake and may drag debris or trash into the intake. In vortices lass A, air bubbles or a stable air core are entrained from water surface and are transported down to the intake (Figure 1). To prevent formation of a strong vortex, a minimum operating level, called critical submerged depth S c is recommended for the intake. Submerged depth is defined as the distance between water surface and the axis of the intake (Figure 2). Many researchers have been carried out to find a *orresponding author. hamed_sarkardeh@yahoo.com. relationship for S c based on prototype and physical model studies (Denny and Young, 1957; erge, 1966; Gordon, 1970; Reddy and Pickford, 1972; Amphlett, 1976; hang, 1977; Anwar et al., 1978; Jain et al., 1987; Odgaard, 1986; Sarkardeh et al., 2010). It should be noted that since the operating level of power intake be reduced below the critical submerged depth, the volume of water in this region cannot be used for power generation. Therefore increasing the submerged depth of the intake for prevention of vortex formation may not always be an economical solution. Moreover, construction of deeper intakes may be more expensive. onsidering factors which has an effect on vortex strength (Sarkardeh et al., 2012), it can be concluded that vortex formation can also be prevented or its strength can be reduced if the distance between water surface and the intake is increased for example by installing a plate in the path of the vortex. Alternatively disturbing flow and increasing turbulence may have similar results. Knauss (1987) introduced various anti-vortex devices for intakes. These devices include: Vertical and half cylinder walls in front of the intakes, floating plates at reservoir water surface and horizontal plates installed on top of the intakes (Figure 3). y considering advantages of using anti-vortex devices in increasing the efficiency of hydropower plants, it seems design of them for hydropower dams is

2 5070 Int. J. Phys. Sci. Vortex lass Vortex lass Vortex lass A Figure 1. Vortex lassification (Sarkardeh et al., 2010). Figure 2. Submerged depth at an intake (D is the tunnel diameter). necessary. In the present work, by using physical model of Siahbisheh Pumped Storage Dam (scale 1:20), performance of four different types of anti-vortex structures with 13 variants was investigated. Also the performance of working two hydropower intakes together and its effect on strength of vortices was investigated. EXPERIMENTAL SETUP All experiments were conducted on a Froude-based hydraulic model in the range of hydropower intake models (Figure 4). For this purpose, two parallel intakes with rectangular inlet by dimensions 70 cm 50 cm (height width) were made of Perspex. In each intake, a transition connected the rectangular inlet to circular tunnel with 28.5 cm diameter. Such configuration is usual in the design of hydropower conveyance members. For avoiding effects of boundary condition on vortex formation, a relatively large reservoir by dimensions 16 m 9.6 m (length width) was constructed. A centrifugal pump supplied water from a canal to the reservoir of the model. A sharp crested rectangular weir was installed in the downstream of the model for discharge measurement. A precise limn meter was used to measure the water surface elevation (WRI, 2008). In order to avoid scale effects on the physical model studies of vortices and the effects of viscosity and surface tension, various minimum values were suggested for Re and We numbers as 4 follows: Re 5 10 (Daggett and Keulegan, 1974), 4 Re and We>600 (Padmanabhan and Hecker, 1984), Re and We>720 (Odgaard, 1986) and We>120 (Jain et al., 1987). Values of Re and We in the present work were more than the minimum values suggested by different researchers. HYDRAULI MODEL TESTS AND RESULTS Six different submerged depths (S/D = 2 to 4) were adjusted and regulated for tunnel Froude number of 0.7. To reach the stable condition, model was run for more

3 Taghvaei et al (a) (b) (c) (d) Figure 3. Some anti-vortex devices at horizontal intakes. (a) Vertical wall (b) Half cylender wall(c) Floating perforated plate (d) Horizontal perforated plate. Figure 4. Layout of the physical model used in the present work.

4 5072 Int. J. Phys. Sci. Table 1. lass of vortices for various submerged depths. S/D Intake lass lass A lass lass lass lass Intake lass lass lass lass lass --- Figure 5. Different types of anti-vortex structures in Alternative 1. than six hours. In each test, vortex class was recorded. Results are presented in Table 1. In prototype a trashrack structure is present at face of power intakes. Therefore a trashrack was installed at the intake entrance in the model. The net opening of trashrack was around 75% (USR, 1987). In all tests, the trashrack was kept in the model to simulate the real condition. For investigating hydraulic performance of antivortex structures under critical operations, 13 Alternatives which were categorized in four general types (horizontal plates, vertical walls, wedge shape structures and combination of them) were installed and experiments were conducted on each separately (Figures 5, 6, 7 and 8). In Alternative 1, three types of thin wedge anti-vortex plates were installed above the intake. Three variants of Alternative 1 with relative lengths, L/D=0.175, 0.35, 0.5 (where L is the length of anti-vortex plate expanded into the reservoir) and different shapes were investigated in the model (Figures 5 and 9). The distance between plates was supposed to be equal. Therefore the entrance of the left intake has been divided into three sections by about W/D=0.7 (where W is the distance between two plates). In Alternative 2, rectangular thin plates with L/D=0.175, 0.35, 0.5 were installed on the axis between two intakes (Figure 10). In Alternative 3, vertical plates with L/D=0.175, 0.35 and a combination with Alternative 2 were installed above the intakes (Figure 11).

5 Taghvaei et al Figure 6. Different types of anti-vortex structures in Alternative 2. Figure 7. Different types of anti-vortex structures in Alternative 3.

6 5074 Int. J. Phys. Sci. Figure 8. Different types of anti-vortex structures in Alternative 4. Alternative 1-1 Alternative 1-2 Alternative 1-3 Figure 9. Installed various types of Alternative 1 in the model. Alternative 2-1 Alternative 2-2 Alternative 2-3 Figure 10. Installed various types of Alternative 2 in the model.

7 Taghvaei et al Alternative 3-1 Alternative 3-2 Alternative 3-3 Alternative 3-4 Figure 11. Installed various types of Alternative 3 in the model. Alternative 4-1 Alternative 4-2 Alternative 4-3 Figure 12. Installed various types of Alternative 4 in the model. In Alternative 4, performance of horizontal anti-vortex plates with L/D=0.175, 0.35, 0.5 which were installed horizontally above both intakes were examined (Figure 12). Each described anti-vortex Alternatives were installed in the model and the relevant flow conditions over the intakes for different water elevations in the reservoir were studied. These results are presented in Table 2. Installing anti-vortex structures over the intakes by introducing more friction to the flow or cutting the flow pass lines may cause elimination or reduction in vortex strength. Two general types of anti-vortex structures in the present paper were checked and results in detail are presented in Table 2. Results showed that in Alternative 1

8 5076 Int. J. Phys. Sci. Table 2. Performance of different anti-vortex structures in the present model. Alternative Anti-vortex type S/D Intake Vortex class A which is a set of vertical wedge shape thin walls (1-1, 1-2 and 1-3), increasing length of wedge shape walls has no significant effect on vortex class. However, installing a set of vertical wedge shape walls causes vortices to become unstable and by increase in the length of them, the instability was relatively increased. Installing Alternative 2 which was sloped vertical wall by different lengths had effect on the surface flow path lines by potential of circulation. This potential may be created by unsymmetrical reservoir geometry. This type of anti-vortex structures does not have any visible effect on degrading vortex classes as well as previous Alternative caused instability on formed vortices. Regarding to the effect of sloped intake head walls on reduction of the strength of vortices (Sarkardeh et al., 2010), some test were performed by installing it on the model by using two lengths (3-1 and 3-2). Also, installing them does not add any meaningful effect on the vortex class. y combination of vertical sloped wall and sloped intake head wall, experiments were conducted to see the effect of working them together. In this condition, strength of vortices was reduced and all formed vortices became unstable. y considering the research of Amiri et al. (2011), installing solid anti-vortex plates at Alternative 4 had very good effect on vortex class degradation. This showed that cutting the vortex core pass caused decreasing the

9 Taghvaei et al vortex class. y studying and comparing the flow conditions, the Alternative 4-2 had better performance. onclusions In the present study different anti-vortex structures were tested on a physical model to eliminate or reduce the vortex strength. These structures could help designer and mangers to be allowed to use more water of the dam reservoir. 13 Alternatives and 3 different S/D between 2-4 and a current power intake discharge were selected. In each test effect of different Alternatives were visually studied. Finally, Alternative 4-2 which was solid plate had better performance to degrade vortex classes. AKNOWLEDGEMENT The authors would like to thank Water Research Institute (WRI) for their kind cooperation in using data. REFERENES erge JP (1966). A Study of Vortex Formation and other Abnormal Flow in a Tank with and without a Free Surface. La Houille lanche 1. hang E (1977). Review of literature on drain vortices in cylindrical tanks. HRA Rep. TN Daggett LL, Keulegan GH (1974). Similitude in free-surface vortex formation. ASE J. Hydraul. Div. 100(11): Denny DF, Young GHJ (1957). The Prevention of Vortices and Swirl at Intakes. IAHR ong. Lissabon paper 1. Gordon JL (1970). Vortices at Intakes. J. Water Power, 22(4): Jain AK, Raju KGR, Garde RJ (1987). Vortex Formation at Vertical Pipe Intake. J. Hydraul. Eng. 100(10): Knauss J (1987). Swirling flow problems at intakes. IAHR Hydraulic Structures Manual 1. alkema, Rotterdam, The Netherlands pp Odgaard JA (1986). Free-surface air core vortex. J. Hydraul. Eng. 112(7): Padmanabhan M, Hecker GE (1984). Scale effects in pump sump models. J. Hydraulic Eng. 110(11): Reddy YR, Pickford JA (1972). Vortices at Intakes in onventional Sumps. J. Water power 24(3): Sarkardeh H, Zarrati AR, Roshan R (2010). Effect of intake head wall and trash rack on vortices. J. Hydraul. Res. 48(1): Sarkardeh H, Zarrati AR, Jabbari E, Roshan R (2012). Discussion of Prediction of Intake Vortex Risk by Nearest Neighbors Modeling. J. Hydraul. Eng. ASE 137(6): USR (1987). Design of small dams. U.S. Department of the Interior, ureau of Reclamation, Washington, D. WRI Technical Report (2008). Hydraulic Model Studies of Siahisheh Pump Storage Project. Hydraulic Structures Division. Water Research Institute (WRI) Tehran Iran. Amiri SM, Zarrati AR, Roshan R, Sarkardeh H (2011). Surface vortex prevention at power intakes by horizontal plates. J. Water Manag. (IE) 164(4): Amphlett M (1976). Air Entraining Vortices at Horizontal Intake. HR Wallingford Rep. No.OD/7. Anwar HO, Weller JA, Amphlett M (1978). Similarity of Free Vortex at Horizontal Intake. J. Hydraul. Res. 2: