Research on the cavitation characteristic of Kaplan turbine under sediment flow condition

Transcription

1 IOP Conference Series: Earth and Environmental Science Research on the cavitation characteristic of Kaplan turbine under sediment flow condition To cite this article: L Weili et al 2010 IOP Conf. Ser.: Earth Environ. Sci Recent citations - CFD Based Analysis of Combined Effect of Cavitation and Silt Erosion on Kaplan Turbine Dinesh Kumar and P.P. Bhingole View the article online for updates and enhancements. This content was downloaded from IP address on 08/10/2018 at 17:46

2 Research on the cavitation characteristic of Kaplan turbine under sediment flow condition 1. Introduction L Weili 1, L Jinling 2, L Xingqi 3 and L Yuan 4 1 Institute of water resources and hydro-electric engineering, Xi'an University of Technology No.5 South Jinhua Road, Xi'an, Shaanxi, , China 2 1Institute of water resources and hydro-electric engineering, Xi'an University of Technology No.5 South Jinhua Road, Xi'an, Shaanxi, , China liaoweili2004@163.com Abstract. The sediment concentration in many rivers in our world is very high, and the Kaplan turbine running in these rivers are usually seriously abraded. Since the existence of sand, the probability of cavitation is greatly enhanced. Under the joint action and mutual promotion of cavitation and sand erosion, serious abrasion could be made, the hydraulic performance of the Kaplan turbine may be descended, and the safety and stability of turbine are greatly threatened. Therefore, it is very important and significant to investigate the cavitation characteristic of Kaplan turbine under sediment flow condition. In this paper, numerical simulation of cavitation characteristic in pure water and solid-liquid two-phase flow in Kaplan turbine was performed. The solid-liquid two-fluid model were adopted in the numerical simulation, and the pressure, velocity and particle concentration distributive regularity on turbine blade surface under different diameter and concentration was revealed. Particle trajectory model was used to investigate the region and degree of runner blade abrasion in different conditions. The results showed that serious sand abrasion could be found near the blade head and outlet in large flow rate working condition. Relatively slight abrasion may be found near blade flange in small flow rate working condition. The more the sediment concentration and the large the sand diameter, the serious the runner is abraded, and the greater the efficiency is decreased. further analysis of the combined effects of wear and abrasion was performed. The result shows that the cavitation in silt flow is more serious than in pure water. The runner cavitation performance become worse under high sand concentration and large particle diameter, and the efficiency decrease greatly with the increase of sediment concentration. Most of the rivers in our country contain a lot of sand. When the sediment flow passes through the turbines, it can cause abrasion on the surfaces of turbine components. Sand may cause earlier and worse cavitation on the turbines which run in the hyper-concentration sediment flow than in clean flow, thus the existence of sand increases the probability of cavitation of turbine components [1]. As a rule, cavitation is specific to clean flow, while wear is specific to the damage of the sand in flow, and abrasion refers to the damage under the joint effect of cavitation and abrasion. Abrasion damage is much severer than pure wear or cavitation. The research in reference [2] is about the influence of sediment content on cavitation and wear, and the result shows that the increase of the sediment content may cause the interaction between cavitation and wear when the sediment content is quite small. In reference [3], abrasion test is done on different test equipment, and the result shows that great damage is caused to the material under abrasion condition. Thus we conclude that it s not the pure wear but the joint action of wear and cavitation, i.e. abrasion causes the great damage on the surface of turbine components. Reference [4] is about the numerical analysis of the interaction of wear and cavitation in turbine components, where the result qualitatively explains the principle of joint action of sand wear and cavitation under the condition of small particle size sediment. And the research in reference [5] also shows that the joint action of wear and cavitation is usually several times higher than separate action of each one. Due to the clearance between blade rim and chamber, leakage and vortices inevitably exist for Kaplan turbines. Thus, clearance cavitation and vortex cavitation happen easily over the blade rim even under the condition of clean c 2010 Ltd 1

3 flow. During the actual operation, due to the acceleration of sand to cavitation, the damage caused is worse in sediment flow than in clean flow [6]. So it has a great important significance to research the principle of interaction of cavitation and wear of Kaplan turbine in sediment flow, as well as the abrasion performance of turbine components under different conditions, so as to improve the anti-abrasion level of turbine and stability of operation. 2. Numerical Model 2.1 Numerical Object The basic parameters of one Kaplan turbine are as follows: Diameter is D=0.34m; Hub ratio is d/d=0.428; Blade number is Z=5; Relative height of guide vane is B/D=0.3706; Guide vane number is Zb=24; Operating point: Head is H=10m; Guide opening is 40mm; Blade angle is 35 ; Speed is 140r/min. Calculation zones include two parts: guide vane zone (24 vanes) and runner zone (5 blades). For the convergence of calculation and setting of boundary condition, the calculation zone of runner downstream is extended. Due to the axially symmetrical structures of guide vanes and runner, a single passage is chosen as a calculation zone as list in Figure 1. Due to the interference between rotary part (runner) and stationary part (guide vanes) of calculation zone, sliding mesh technology is applied to connect dynamic/static faces. Fig.1 Sketch of computational domain 2.2 Control Equations Cavitation calculation in clean flow is performed by simulating cavitation flow through one intact cavitation model and one mixed uniform two-phase flow model, which were developed by Singhal [7] etc.. It regards that the timeaveraged velocities of cavity phase and current phase are equal, and both are researched as one single flow. Thus, cavitation flow is described as one group of momentum equations, of which the physical quantities have the volumeaveraged values of cavity phase and current phase. And the velocity distribution from the solution of momentum equation is applicable for both phases. Control equations are: Continuity equation: ρ + ( ρv) = 0 Mixed flow phase: τ (1) Cavity phase: ( ρ f ) + ( ρfv) = R e Rc (2) t ( ρν ) 1 Momentum equation: + ( ρυυ ) = Ρ + [ ( μ + μt ) ν ] + [( μ + μt ) υ] + ρg (3) τ 3 Where: ρis the density of two-phase mixed flow; V is quality-averaged velocity vector of two-phase mixed flow; P is pressure; μ and μ t are molecule viscosity and turbulence viscosity respectively; f is mass ratio of cavity phase; Re and Rc are formation rate and condensation rate of steam. According to the real condition of turbine, the water temperature is 25, where the vapor pressure is p=2982 N/ m2, and surface tension of cavity is σ = N/m. Provided that mass ratio of non-condensation gas in water is 15/

4 2.3 Boundary Condition and Numerical Method In the numerical simulation, the boundary conditions are: Inlet: mass flow-rate is given at the inlet, and the flow direction is perpendicular to the surface of inlet. Outlet: static pressure is given at the outlet boundary. Symmetrical boundary condition is applied at the boundary of calculation zone. Non-sliding wall boundary condition is set on both surfaces of blade and runner chamber. SIMPLEC arithmetic is applied to achieve the coupling between velocity and pressure. In order to accelerate the convergence of calculation, the steady flow calculation result of single-phase clean flow is used as initial flow for 2- phase flow cavitation calculation, and the cavitation calculation result of 2-phase clean flow is used as initial flow for cavitation calculation of sediment flow. 3. Numerical Simulation Result Analysis 3.1 Analysis of blade wear Figure 2 shows the concentration distribution of sand over the blade pressure side and suction side with sand diameter of 0.005mm, 0.024mm, 0.1mm separately under the condition of small discharge, where the sand concentration is 1%. Cv=1% d=0.005mm Cv=1% d=0.024mm Cv=1% d=0.1mm Pressure side Cv=1% d=0.005mm Cv=1% d=0.024mm Cv=1% d=0.1mm Suction side Fig.2 Sand volume fraction distribution at the blade on condition 1 As can be seen from Figure 2, when the diameter of sand is 0.005mm which is quite small, the sand concentration distribution on both sides of blade (pressure side and suction side) is quite uniform. When the diameter of sand reaches 0.024mm which is 3

5 d=0.005mm d=0.024mm d=0.1mm Fig.3 Sand volume fraction distribution at the blade middle-sized, the sand concentration distribution at blade rim and outlet is a little higher. As the increase of diameter of sand, the concentration becomes much higher at the head of pressure side and the rim near to the head as well as the outlet of blade, while the sand concentrate to the head of blade at suction side. Especially when the diameter reaches 0.1mm, the sand concentration distribution on blade pressure side is obviously nonuniform, and the gradient is very big. The sand concentration at the blade hub is almost zero, which increases gradually from the hub to the rim, and the gradient near to the hub is quite big. When the sand diameter is bigger, there is a small zone of sand at the inlet part of blade pressure side, while there is a little change in the sand concentration at the rest part of blade along the flow direction from the inlet to the outlet. The sand concentration is very big in the small zone which is near to the inlet of blade suction side, which reaches the maximum of 1.6%. Then it reduces gradually, of which the distribution is quite uniform at the rest part of suction side. This is consistent with the real observation of wear part over the turbine blade. As can be seen from the above analysis, the sand concentration distribution changes obviously with the size o f sand, which is mainly because that when the sand diameter becomes smaller, the following performance of san d with the flow is relatively better, and close to the characteristic of flow. Thus, the distribution of sand on the bl ade is quite uniform, and the wear is also very small. To the contrary, when the sand diameter becomes bigger, th e following performance becomes worse, and the centrifugal force of solid phase moving with the liquid phase in creases, and the concentration range of solid particle in the runner zone also increases, thus the wear range of bla de becomes bigger. Figure 3 shows the sand concentration distribution on both the pressure side and suction over 10 cross sections from hub to the rim. Where P refers to the pressure side, S refers to the suction side, and 1-10 refers to ten constant-height cross section from hub to rim. As can be seen from the figure, the difference of concentration between different parts of blade increases with the increase of sand diameter. When the diameter is 0.005mm, the concentration range is 0.85%-1.15%; when the diameter is 0.024mm, the concentration range is 0%-2.5%; when the diameter is 0.1mm, the concentration range is 0%-7%, and the maximal concentration is 7 times of the value at the inlet. It can be concluded that under the same sand concentration at the inlet, the bigger size of the sand, the more non-uniform distribution of sand, and the maximal concentration becomes bigger, thus the wear 4

6 degree over the blade also increases. Fig.4 Efficiency change with different sand volume fraction (sand diameter of 0.024mm) Figure 4 shows the efficiency curves under different unit speed and concentration. Where, guide opening is 15mm, and blade angle is 5 in figure a; guide opening is 25mm, and blade angle is 20 in figure b; and guide opening is 40mm, and blade angle is 35 in figure c. Cv refers to the volume fraction of sand. And the diameter of sand in sediment flow is 0.024mm. As can be seen from Figure 4, due to the unconformity between actual condition and on-cam condition of runner, efficiency is quite small with the unit speed of 140r/min and small guide opening as shown in a and clean flow. For the same type of Kaplan turbine, the on-cam operating point may change when it runs in sediment flow or clean flow, and the optimal point may also change. Under the condition of clean flow and sediment flow, the change tendency of turbine performance with the increase of guide opening is uniform. As can be seen from b and c, the influence on the turbine efficiency is smaller under optimal unit parameter. The more difference between optimal point and current point, the more efficiency decreases. Under the same operating condition, the wear on the runner increases with sediment content, and thus the influence on efficiency becomes greater. Under the same unit speed and different sand concentration, efficiency decreases with increasing sediment concentration. This is consistent with the situation that turbine efficiency is greatly affected after a flood season, which is because that the sediment content in the river increases rapidly during the flood season, and high sediment concentration flow will cause severe wear to the turbine when it passes through the turbine, which should be avoided. 3.2 Influence of Sand on Cavitation Performance Research about cavitation performance in clean flow and sediment-laden flow with different concentrations is done for further study of the influence of sand on turbine cavitation performance and the interaction between wear cavitation. As can be seen from Figure 5 of cavity volume fraction distribution at the blade, the general tendencies of cavity volume fraction and pressure distribution at pressure side and suction side of blade are uniform between sediment flow and clean flow, which shows a cavitation zone at the head of blade pressure side. The cavity volume fraction in sediment flow is higher than in clean flow. Under the condition of clean flow, the maximum 5

7 of cavity volume fraction at blade pressure side and suction side are 34% and 73% separately, while the values reach 40.9% and 90% under the condition sediment flow, which means that the cavitation of runner in sediment flow is much greater than in clean flow. The result of numerical analysis shows that maximal cavity volume fraction increases gradually with diameter of sand under the same sand concentration at the inlet. Clean flow Cv=1% d=0.01mm Cv=1% d=0.024mm Cv=1% d=0.1mm Pressure surface water Cv=1% d=0.01mm 6

8 Cv=1% d=0.024mm Cv=1% d=0.1mm Suction surface Fig.5 Cavity volume fraction distribution at the blade The following introduction shows the formation process of cavitation: When the local pressure decreases the gas cores in the flow or in the clearance on the wall surface will expand rapidly to cavitys. And the cavitys will be carried to the high pressure zone, which will collapse due to the high pressure and cause cavitation damage consequently. Thus, the prerequisite of cavitation damage is the existence of gas cores in the flow. The gas cores in the clean flow are carried by the flow, while they are also carried by the sand in the sediment flow. So the cavitation is more serious in the sediment flow due to the more gas cores inside than in clean flow, which will worsen the cavitation performance of turbine components. 3.3 Influence under the same sand diameter and different concentration water Cv=1% d=0.024mm Cv=3% d=0.024mm Cv=5% d=0.024mm Fig.6 Velocity distribution on section at the blade near rim 7

9 For study of the influence of sand concentration at the inlet on the flow inside runner, researches are done in both clean flow and sediment flow with 3 sand concentrations, which are 1%,3% and 5% separately. The diameter of sand is 0.024mm. Figure 6 shows the velocity distribution at the cross section near to the blade rim. As can be seen from the figure, velocity tends to decrease as a whole with the increasing sand concentration. The increase of sand concentration will result to the increase of viscosity of flow and decrease of velocity. Due to the sand, pressure distribution and velocity distribution of the flow will change, which will cause cavitation and wear to turbine components. The maximal velocity is 26.27m/s at sand concentration of 1%, and the maximal velocity is m/s at sand concentration of 3%, of which the value decreases by 0.31 m/s. Velocity is m/s at sand concentration of 5%, which decreases by 0.53 m/s compared with 3%. So it s obvious that the viscosity increases with the sand concentration, while the velocity decreases inversely. a water b Cv=1% d=0.024mm c Cv=3% d=0.024mm d Cv=5% d=0.024mm Fig.7 Pressure distribution on section at the blade near rim Figure 7 shows that the pressure distribution is quite uniform at different sand concentration. The pressure on pressure side is much more than on suction side. The low pressure area locates on the suction side near to the outlet. Pressure around the blade decreases with the increasing sand concentration especially at the outlet of blade suction side, where the area of low pressure increases and minimal pressure also decreases, which will result to cavitation. Figure 8 shows the efficiency change with sand concentration. Turbine efficiency is : Mω, where, η = 100% QgH M refers to the torque on the shaft from pressure and viscous force, Q is discharge, H is actual head calculated from the total pressure difference between guide vane inlet and runner outlet sections, and ω is runner speed (rad/s). Efficiency with two phase cavitation in clean water is 89.52%, which is 87.84% in sediment flow, decreasing by 1.68%. Sand increases the probability of cavitation, and causes cavitation damages to turbine components. Plus the sand impacts the surfaces with a certain kinetic energy and causes wear on the blade, which accelerates abrasion of turbine, and finally results to the decrease of efficiency. t 8

10 3.4 Efficiency analysis 4. Conclusion efficiendy 92% 90% 88% 86% 84% 82% Cavitation with sand Water and water 80% 78% 76% 0% 1% 2% 3% 4% 5% 6% 7% sand concentration Fig.8 The efficiency change with sand concentration Conclusions can be drawn from the research on wear and cavitation of Kaplan turbine in sediment flow, which are as follows: Under the condition of two-phase flow with water and sand, the sand distribution on blade surface is nonuniform. The sand concentration is small in the area near to hub, while it is big at the inlet. The concentration changes a little for the rest parts. Under the condition of two phase flow with water and sand, the sand distribution on blade surface is closely related with its diameter. The bigger the diameter is, then the more non-uniform the distribution becomes. So the main part of wear on blade changes with the diameter of sand. The cavity volume fraction at blade pressure side or suction side in sediment flow is bigger than in clean flow. And the combined action of sand wear and cavitation causes greater abrasion, which results to the decrease of efficiency consequently. D d d R th Re Re Rc Cv Q T Diameter of runner Diameter of hub Diameter of particles Thermal resistance [ o C/W] Reynolds number (=U b D h /ν) The formation rate of steam The condensation rate of steam Density of particles at the inlet Discharge flow rate Local mean temperature [K] Nomenclature M ω σ ρ V μ μ t f torque on the shaft rotational speed surface tension of cavity The density of two-phase mixed flow the quality-averaged velocity vector of twophase mixed flow molecule viscosity turbulence viscosity mass ratio of cavity phase References [1] Li X 1995 Research on the Device for material abrasion and test result analysis Water Resources and Water Engineering (2) [2] Deng J and Yang Y 2004 The Influence to Abrasion of Sediment Concentration Journal of Sediment Research (4) [3] Du T 1994 Mechanism of pump damage in water with sand Pump technology (1) 3-9 [4] Liang J, Jiang J, Cheng L 1997 A Study on the Oscillation of the Cloud of Bubbles in a Dilute Solid Liquid Two Phase Flow Journal Of Huazhong University Of Science And Technology 25(9) [5] Li S C 2006 Cavitation enhancement of silt erosion-an envisaged micro model.wear 260(9) [6] Duan G 1981 Sand erosion of hydraulic turbine (Tsing hua university publishing company) [7] Singhal A K, Athavale M M, Li H Y 2002 Mathematical Basis and Validation of The Full Cavitation Model J. Fluids Eng 124(9)