Recent experience of IFFM PAS in the design process of lowhead propeller hydraulic turbines for Small Hydro

Transcription

1 IOP Conference Series: Earth and Environmental Science Recent experience of IFFM PAS in the design process of lowhead propeller hydraulic turbines for Small Hydro To cite this article: M Kaniecki and Z Krzemianowski 2010 IOP Conf. Ser.: Earth Environ. Sci Related content - Counter-rotating type axial flow pump unit in turbine mode for micro grid system R Kasahara, G Takano, T Murakami et al. - Design of large Francis turbine using optimal methods E Flores, L Bornard, L Tomas et al. - Flow simulation and efficiency hill chart prediction for a Propeller turbine T C Vu, M Koller, M Gauthier et al. View the article online for updates and enhancements. This content was downloaded from IP address on 11/01/2018 at 06:12

2 Recent experience of IFFM PAS in the design process of low head propeller hydraulic turbines for Small Hydro 1. Introduction M Kaniecki 1 and Z Krzemianowski 1 1 Center for the Mechanics of Liquids, Institute of Fluid Flow Machinery of the Polish Academy of Sciences Fiszera 14 st., Gdańsk, , Poland kaniecki@imp.gda.pl Abstract. The paper contains the short description of the design process of the axial flow turbines for Small Hydro. The crucial elements of the process are: ARDES programme for 1D inverse problem (containing the statistic information of the well performed hydraulic units, applying the lifting aerofoil theory); determination of universal hill diagram and optimization of the runner blades geometry by utilization of the 3D CFD codes. As the result of design process with utilization of both design steps, the generated runner blades geometry (1D inverse problem) and some computational results of 3D CFD solver have been presented. As the conclusion some crucial remarks of the designed process have been brought forward. Nowadays the propeller hydraulic turbines seem to be the most popular solutions for the minimal low head turbine applications. In Poland the hydraulic potential concentrated in lowland rivers, with relatively high meanyear flows. Most of small hydro power plants are situated on such lowland rivers, thus the low head hydraulic turbines are becoming the objects of investors interest. Relatively the most expensive applications of that turbines are the small capacity machines with the range of power output limited to 3-4 hundreds KW. To reduce the costs of manufactory and installation of propeller turbines in SH, the designers often decide to equip the turbine only with single regulation mechanism (in most cases only the runner blades regulation) and single or double (s-shape) bended draft tube. The Institute of Fluid Flow Machinery of the Polish Academy of Sciences (IFFM PAS) has some experiences in designed, tests and installation of the amount of such simplified designed low head turbines for various SH in Poland [5]. Since eighties and nineties of last century the 1D design methodology and related programmes for axial flow turbines have been developed in the institute (mainly by Janusz Steller and Jacek Świderski authors of ARDES programme, based on 1D designed method). Nowadays, the design process in its optimization terms is supported by 3D Computation Fluid Dynamics, which has reached high level of reliability. Authors utilized the commercial code Numeca for academic studies of design process of newly designed TSN turbine (newly design low head axial flow turbines). The paper shortly characterizes the complete design process of TSN turbine, since the very beginning definition of main geometrical and operating parameters of the turbine, through the 1D inverse task of runner blade creation, up to the final step optimization of the runner blades geometry by utilization 3D numerical analyses. 2. Design method 2.1 Definition of the main geometrical and operating parameters of the TSN turbine The initial step of the design is realized by ARDES package of programme. The design process is initialized with proper selection of main geometrical and operating dimensions like: net head H, rotational speed n, the diameter of the runner D, diameter of the hub d, amount of runner blades, kinematics specific speed n q (wildly nq np applied in the literature as n q =, n[min -1 m ]) and dynamic specific speed n 0.75 s =. The selection is 1.25 H H based on the literature information and author s own experiences. Some of the upper parameters are absolutely imposed (head), the others (like n q, n s and the runner diameter D) could be imposed or approximately determined c 2010 Ltd 1

3 using the empirical formulas (1), (2) (the statistical formulas for propeller turbines up to 500 kw from the literatures data [4],[9]): 350 n q = (1) ( H ) n = s n q D q ' = ( n ) D (2) where: ' 60 D = 2gH π n D relative diameter [m], n imposed rotational speed [min -1 ], H imposed net head [m]. The diameter of the hub and the number of runner blades are determined from the empirical recommendations, frequently quote in the literature. Table 1 Literature data for number of blades and the hub diameter recommendations [8] The discharge and definite value of rotational speed is also determined using the empirical recommendations [8] of well design numbers of hydraulic units (fig.1). The figure 1 presents the relative value of crucial components of velocity triangles (C m -meridional velocity, C s outlet velocity to the draft tube, U i - circumferential velocity) obtained for wide range of dynamic specific speed n s (for different type of hydraulic turbines). Fig.1 Relative value of crucial components of velocity triangles via dynamic specific speed n s 2

4 Basing upon the relative values the designer is capable to determine the nominal discharge Q and rotational speed n of the turbine from the formulas (3) and (4). ' 30 U e n = (3) π R 2gH e where: R e outward radius of the runner [m] A - inlet surface of the runner [m 2 ] Q ' C m A = (4) 2gH The initial value of rotational speed n is generally different from the calculated one (3). The further computations are carried out with the calculated (with formula 3) value of rotational speed n. 2.2 Runner blade shaping. The next step is determination of the runner blade geometry by utilization the lifting aerofoil theory. The process is started with the determination of cylindrical cross section of the blade and the inlet and outlet velocity triangles. The Euler basic hydraulic turbine equations are being used to calculate the unknown components of velocities. The exemplary inlet and outlet velocity triangles are presented in Fig. 2. Fig.2 The draft of inlet and outlet velocity triangles on the selected runner blade profile. The results of that programme part are the averaged values of undisturbed relative velocities w i and undisturbed pitch angle β i calculated respectively form the formulas: Cm tan β i = (5) ΔCui Ui 2 Cm w = i sin β i (6) For initially imposed kinematics conditions of the flow (determined relative velocities w i and rotational speed n), one should be properly selected the aerodynamic profile, which will be realized the particular loading. The selection is realized by determination of the type, the thickness, the length and the angle of attack of the profile at each cylindrical cross section of the blade. The length of the profile l i is determined using the empirical 3

5 recommendations for density of palisade (t/l) i via dynamic specific speed (fig.3) [2]. Fig.3 The density of palisade (t/l) i via dynamic specific speed The lifting force coefficient k z, indispensable for selection of proper type of the profile, is determined form the formula: 4πηhgH 1 kzili = z ω w i where: η h approximated value of hydraulic efficiency, calculated from the empirical formula [8] 17 D η h = 0.86, 16 D + 1 z number of runner blades, ω rotational velocity [s -1 ]. Basing on the value of k z for every cross section of the blade, one should be selected the type of the profile inforced by the bigunal characteristic, cavitations features of the profile and own designer s experiences. The numbers of profile types have the characteristic formula for calculations the lifting force coefficient k z for different angles of attack. For the chosen type of profile and initially determined coefficient k z the designer is able to calculate the angle of attack α. The formula for Göttingen 428 profile is presented below: (7) hence: 4.8ymax k z = α (8) l k z 4.8ymax α = l where: y max maximal value of the profile high [m]. The pitch angle β i at each cross section of the blade must be decreased by angle of attack α i, hence the final pitch angle β i is obtained from the formula: β = β α (9) i i i 4

6 The final element of runner blade shaping process is the definition of the profile thickness. The thickness of the profiles is determined (similarly to the length), using the empirical recommendations [7] for the relative thickness via dynamic specific speed (Fig.4). Fig.4 The relative thickness λ via the relative value of particular cross section diameters. λ 1, λ 2, λ 3 distributions of relative thickness for different dynamic specific speed n s =550, n s =730, n s =850 The ARDES package, based on the presented algorithm, generates the geometry of the runner blades and calculates the result parameters of the turbine for imposed input values of operating parameters (head H and rotational speed n). The tables below presented the input and results operating parameters of newly design TSN turbine (Table2) and the geometrical parameters of TSN runner blades (Table3). Table 2 Input, results operating and main geometrical parameters of newly design TSN turbine. net head H 2.3 [m] rotational speed n 280 [rpm] discharge Q 3.1 [m 3 /s] power output P m 64 [KW] diameter of the runner D 1 [m] diameter of the hub d 0.35 [m] Number of blades z 3 kinematics specific speed n q 263 profile Göttingen 428 Table 3 Geometrical parameters of the profiles of TSN runner blade. R i radius of cylindrical cross section [m] c- chord length [m] β i - pitch angle [ ]

7 Fig. 5 The isometric view of the TSN runner blade. 3. Numerical analysis of the flow system of the low head TSN turbine The numerical calculation of the flow through the flow system of the TSN turbine has been carried out using the Numeca/Fine commercial software. For the initial applications of the numerical computations authors decided to defined the flow system of the turbine with the straight draft tube. The main purpose of numerical calculations was to determine the high value the hydraulic efficiency in the wide range of operating. In order to achieve the purpose two stages of CFD calculations have been carried out. The first is the determination of the proper dependence between guide vanes and runner opening (determination of cam curve) and second is reshaping the geometry of the runner blade. The initially formed geometry of runner blade has been parameterizes to facilitate the optimization process. The analysis was conducted using a 3D model of the whole flow area extending from the inlet stub pipe to the outflow section of axial turbine draft tube. The programme solves the governing integral equations for the conservation of mass and momentum for the steady state flow. The equations are reduced to their finite-volume equivalents by integration over the computational volumes into which the domain is divided. The turbulence flow has been taken in to account by application the turbulent Spalart-Almaras model. 3.1 Geometry of the flow domain The main geometrical parameters of the flow system of TSN turbine are presented in the tab.4. The dimensions of the flow channels are relevant to the runner diameter of 1m. The number of guide vanes blades was defined based on literature recommendations (17 blades). Table 4 Geometrical parameters of the flow system of TSN turbine. Diameter of the runner 1[m] Number of runner blades 3 Number of guide vanes 17 The guide vane height The draft tube length 0.45[m] 3xD ~ 3 [m] As was mentioned above, the geometry of the runner profiles has been parameterized for optimization process of runner blade shape. The mean line and the thickness line were extracted from the profiles of the runner blade. The main direction of further optimizations based on shifting the shape of mean line by increasing the value of the camber. The effect was noticeable as the increased of the profile curveting. 3.2 Boundary conditions for the flow system In the considered cases the inlet and outlet pressure conditions were respectively defined as the absolute total pressure at the inlet and the absolute static pressure at the outlet. As generally known, in case of an incompressible flow the total pressure and static pressure are related to velocity by Bernoulli equation. Thus, the 6

8 inlet velocity magnitude and consequently the mass flow rate are determined as the result of calculation. The total and static pressure were defined as beneath: p = ρgh + 1, total p amb (10) p = where: H head enlarged by the term 2, abs p amb (11) ρc 2 2 2, as the mass flow rate is unknown at the initial stage of calculation. 3.3 Grid generation The grid was generated using the AutoGrid Numeca software. Finally, the structured grid was applied in the whole area of calculation domine. The blocks of grid had been generated in single guide vane and runner blade canal and afterwards multipladed respectivaly by the number of guide vanes (x17) and runner blades (x3). The runner block contained also the draft tube geometry. The total amount of grid cells was approximately 6.5 mln. The structure of the grid was O-shape with the gradiation of the elements thickness near the walls. In the fig. 6 the grid distribution on the selected elements of TSN flow system is presented. Fig.6 Grid distribution on the guide vanes and runner blades 3.4 Definition of global parameters Besides the local flow parameters, the global performance parameters were calculated. The output P m, discharge Q and net head H net were determined using the formula: where: P m = T ω (13). m Q = ρ T torque acting on the runner blades [Nm] (14) 2 C2 H net = H (15) 2g C 2 velocity magnitude at the outflow of the draft tube [m/s] The hydraulic efficiency was defined as the ratio between the power at the shaft and the power lost by water passing through the turbine: T ω η = g H ρ Q (16) 7

9 3.5 Exemplary results of calculations The first stage of 3D numerical calculations concentrated on the determination of cam curve - optimal relation between the guide vane and the runner blades positions. It was realized consequently for the rotational speed factor for the range n 11 = 130 rpm-190 rpm. For every selected n 11 the combinatory was determined by conducting the numerous calculations of different guide vane and the runner blades positions. Basing on the final results of this analysis the part of the universal hill diagram near the optimal operating point of the turbine was created. In the Figure 7, authors present the excerpt of universal hill diagram of TSN turbine determined by utilization of the 3D steady state calculations. Fig. 7 The part of the universal hill diagram near the optimal operating point of the TSN turbine The local parameters of the flow were also consequently investigated. The second stage of numerical calculations contained the development of the runner geometry to increase the turbine efficiency. Applying the parameterization of the geometry authors decided to modify the value of maximal camber f max to investigate the tendency of local and global parameters. The foreword results of that test were presented in the Table 5. Table 5 Discharge factor and hydraulic efficiency via relative value of max camber f max Relative value of max camber f max Discharge factor Q 11 Hydraulic efficiency η Multiplication of the maximal value of camber by the imposed factor slightly increases the efficiency and simultaneously decreases the specific speed of the turbine, which effects the reduction of the discharge. The further computations of the cavitations property should be the deciding criteria of this tendency acceptance. 8

10 4 General remarks and conclusions The paper presented some crucial elements of design process of low head axial flow hydraulic turbines. The two stages of the process have been discussed. In the first part authors shortly described the 1D classic design method based on the lifting aerofoil theory, which was practically used for design the initial shape of new TSN turbine. For the designed applications the ARDES programme package has been utilised. The second part of the paper contained the description of CFD modelling of flow thought the flow system of TSN turbine presented selected results of calculations. Authors focused on the determination of shell characteristic and optimisation process of the runner blade geometry. The case of mean line modification of runner profiles has been presented as the example of optimisations directions. The calculations omitted the prediction of cavitations phenomena, which will be the next step of CFD analysis of TSN flow system. Acknowledgments The results of the presented numerical and experimental research have been obtained under the Research Project no. 3574/B of the Ministry of Sciences, Warsaw, Poland. Preparation of this paper took place within the framework of the IFFM PAS statutory activity plan. α i β i β i λ i ω A C 2 D Nomenclature angle of attack for the particular blade profile pitch angle for the particular blade profile undisturbed pitch angle for the particular blade profile relative thickness rotational velocity [s -1 ] inlet surface of the runner [m 2 ] mean value of velocity magnitude at the outflow of the draft tube [m/s] diameter of the runner [m] H k z l i m. n q n s P m Q T net head [m] lifting force coefficient length of the profile [m] mass flow rate [kg/s] kinematics specific speed dynamics specific speed output power [KW] discharge [m 3 /s] torque acting on the runner blades [Nm] References [1] Andersson U, Engström F, Gustavsson H and Karlsson R 2003 The turbine-99 workshops on draft tube flow-lessonslearned The QNET-CFD Network Newsletter 2(3) [2] Anton I 1979 Turbine hidraulice (Timisoara) Facla [3] Gordon J L 2003 Turbine selection for small low-head hydro developments Waterpower XII (Buffalo, USA) [4] Gorla R and Khan A 2003 Turbomachinery design and theory (New York, US) Marcel Dekker Inc [5] Henke A and Steller J 2003 Niskospadowe turbiny śmigłowe typu TSP i TSPu The IFFM PAS (Gdańsk) [6] Krishna R 1997 Design of hydraulic machinery Wydawnictwo Aveburg-Brookfield [7] Krzyżanowski W 1971 Turbiny wodne (Warszawa) Wydawnictwo Naukowo Techniczne [8] Nechleba M 1957 Hydraulic turbines - design and equipment (Artia, Praga) [9] Schweiger F and Gregori J 1988 Development in the design of bulb turbines Int. Water power and Dam Constr [10] Świderski J and Martin J 2000 Praktyczne zastosowanie numerycznej mechaniki płynów w projektowaniu. Wirtualne laboratorium hydrauliczne Int. Conf. HYDROFORUM (Czorsztyn) 9