Numerical and experimental investigations on efficient design and performance of hydrokinetic Banki cross flow turbine for rural areas

Ocean Engineering 159 (2018) 437–456

Contents lists available at ScienceDirect

Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng

Numerical and experimental investigations on efficient design and performance of hydrokinetic Banki cross flow turbine for rural areas A.H. Elbatran a, b, *, O.B. Yaakob a, c, Yasser M. Ahmed a, d, Ahmed S. Shehata b a

Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310, Skudai, Johor, Malaysia Faculty of Engineering and Technology, Arab Academy for Science and Technology and Maritime Transport, 1029, Alexandria, Egypt Marine Technology Center, Universiti Teknologi Malaysia, 81310, Skudai, Johor, Malaysia d Dept. of Naval Architecture and Marine Engineering, Faculty of Engineering, Alexandria University, Alexandria, Egypt b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Hydrokinetic energy CFT Inlet angle Outlet angle Performance Flow characteristics Power coefficient

Micro hydrokinetic energy scheme presents an attractive, environmentally-friendly and efficient electric generation in rural, remote and hilly areas. However, this scheme is yet to be fully discovered, as researchers are still searching for solutions for the main problems of low velocity of current in the open flow channels and low efficiency of hydrokinetic turbines. This research proposes a novel system configuration to capture as much kinetic energy as possible from stream water current. This system, known as bidirectional diffuser augmented (BDA) channel, functions by utilizing dual directed nozzles in the flow and is surrounded by dual cross flow/Banki turbines. It is also important to obtain the efficient design parameters of the turbines to use in the current configuration. The appropriate angle is important in order to guide the flow to touch the blades more perpendicularly to capture as much torque and power as possible. Hence, experimental and numerical investigations have been carried out in this research paper to study the performance characteristics of the CFT configuration applied in BDA system and investigate the effects of blades' inlet and outlet angles of CFT runners on the internal flow characteristics and efficiency. In this study, four different runners with various inlet and outlet angles of two CFT have been investigated. The CFD results have been validated with the experimental work and proven acceptable with flow pattern and performance characteristics. The results of the current study conclude that the maximum power coefficients (Cp) of 0.612 and 0.473 for lower and upper turbines are recorded for best runner angles of Case 3.

1. Introduction Rural electrification of many developing countries is very costly, especially in areas with economic problems (Elbatran et al., 2015a). Using micro hydropower can be the perfect solution to overcome the economical and operational problems (Laghari et al., 2013). Micro hydropower plants can be used to produce suitable electrical power for homes, plantations and farms in small villages (Elbatran et al., 2015b). They are more predictable when the supply of water is enough (Mohibullah et al., ), and they also have positive environmental impacts (Teuteberg, 2010). Hydrokinetic is a new type of micro hydro-power, which extracts kinetic energy from the flow of water in open channels, rivers or canals by deploying hydrokinetic turbines without any facilities like weirs, barrages or falls; this scheme is not deployed with any kind of reservoir (Okot, 2013; Yaakob et al., 2014; Herman Jacobus et al., 2014;

Kumar et al., 2011; Elbatran et al., 2015c; Chamorro et al., 2013; Shabara et al., 2015). This application attracts investments in hilly and isolated areas' electrification due to its easy construction and low cost. It also exploits small hydrological areas (Rojanamon and Taweep, 2009). Many researches focus on studying the water stream technology from both flow and turbines systems perspectives by considering improvements on the open channel flow and suitable turbine systems utilized in these micro channels to be used in the micro power production. The free stream flow systems normally need a higher amount of mass flow with low pressure to be able to extract energy, but the conventional current turbines are more suitable for high pressure and flow rate (Ki-Pyoung et al., 2012). Hence, many studies tried to develop unique and new technology designs and configurations to capture as much kinetic energy as possible. Using nozzles is the most efficient choice to accelerate the flow because it can increase the harnessed power. Nozzles can be utilized in

* Corresponding author. Faculty of Engineering and Technology, Arab Academy for Science and Technology and Maritime Transport, 1029, Alexandria, Egypt. E-mail address: [email protected] (A.H. Elbatran). https://doi.org/10.1016/j.oceaneng.2018.04.042 Received 29 December 2017; Received in revised form 9 March 2018; Accepted 13 April 2018 0029-8018/© 2018 Elsevier Ltd. All rights reserved.

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

studied the helical channel flow properties from the hydraulic perspective in order to utilize these channels in the renewable energy field. Moreover, Elbatran et al. (2017) proposed a ducted nozzle configuration around Savonius turbine in water channel to increase the efficiency of the turbine. 1.1. Cross flow turbines/Banki turbines The turbine is one of the most costly parts in the budget of micro power scheme; it can even reach up to 30% of the total budget, where the cost depends on the type of turbine (Elbatran et al., 2015b). Cross flow or Banki turbine is more preferable in micro hydropower scales compared with other turbines, based on its performance and cost options (Olgun, 1998). Since it can be familiarized with various ranges of flow rate and lower head, it is more proper for low head schemes (Ghosh and Prelas, 2011; Ossberger GmbH Co, 2011). The cross flow turbine performance depends on geometrical parameters, such as runner diameter ratio, nozzle entry arc, guide vanes, number of blades, angle of attack and the inlet and exit blade angles as a described in Fig. 1 α is the angle between the inlet velocity and the tangent direction of the turbine runner inlet, and are the outer and inner diameters of the runner, and are the inlet and outlet blade angles with respect to the tangent direction of the outer and the inner diameters, Z is the blade number and t is the blade thickness, λ is the angle of the arc available for the discharge inlet along the runner outer circumference, and δ is the central angle of the blade. Mockmore and Merryfield (1949) study is one of the oldest and the most important studies on cross flow turbines. They defined the hydraulic efficiency of the turbine and the relation between the angle of attack (α) and the inlet blade angle (β). Their study also suggested a value of 16
for the angle of attack, and the maximum efficiency of 68% was obtained in this study. Nakase et al. (1982) studied the effects of the nozzle shape on the performance of cross flow turbines. The maximum efficiency of this study was 82%, with the recommended number of blades as 26. Khosrowpanah et al. (Shahram et al., 1988) experimentally studied the CFT performance through different geometrical parameters under various flow and head conditions. Their results demonstrated that the maximum efficiency of the CFT increased as the nozzle entry arc increased or the aspect ratio of the runner decreased. In their research, the recommended number of blades, diameter ratio and angle of attack were 15, 0.68 and 16
, respectively. Furthermore, the peak value of efficiency occurred at 0.52 speed ratio. Moreover, Durgin and Fay (Durgin, 1984), Hothersall (1985) and Otto and Chappel (Ott and Chappel, 1989) reported that the maximum efficiencies were 66%, 75% and 89%, respectively. Fiuzat et al. (Fiuzat and Akerkar, 1991) proved that the turbine's second stage contributes more significantly to the power production than reported in the analytic literature. Desai and Aziz (1994) concluded that the maximum efficiency of CFT decreased with the increase in the angle of attack in the range of 22
–32
. It also increased when the number of blades increased from 15 to 30. In their study, the recommended diameter ratio was 0.6 and the exit angle was 55
. The peak value of efficiency was also almost 88% at speed ratio of near 0.55. The results of Hara et al. (Totapally and Aziz, 1994) presented that the turbine was more efficient by using nozzles that were narrower than the runner. It is possible when the angles of attack are between 22
and 24
, the number of blades is 35 and the exit angle is smaller than 90
. According to Olgun (1998) study, the highest efficiency of 72% was obtained at 0.67 runner diameter ratio. Moreover, the study selected 30
and 90
for the blade inlet and outlet angles, respectively. Kokubu et al. (2013) proved that the CFT efficiency was improved in the presence of guide vane with current plate. Kaunda et al. (2014a) experimentally investigated the performance of CFT to enhance the design of a Cross flow turbine, as an appropriate technology for small-scale power generation. Hence, their study was depending on conditions other than the ‘best efficiency point’. It also explored the influence of nozzle opening parameter as well as the characteristics of the

Fig. 1. Main geometrical parameters of CFT (Sammartano et al., 2013).

Table 1 The recommended dimensional and performance parameters of CFT can be drawn from the literature review (Referring to Fig. 1). Main parameters

Recommended values

Diameter ratio 12-04-18

0.6–0.68 (Olgun, 1998; Shahram et al., 1988; Desai and Aziz, 1994; Choi et al., 2009, 2010; Prasad et al., 2014; Kim et al., 2015) 16
-22
(Sammartano et al., 2013; Mockmore and Merryfield, 1949; Shahram et al., 1988;

Angle of attack (α)

Desai and Aziz, 1994; Totapally and Aziz, 1994) Inlet blade angle (β1 ) Exit blade angle (β2 )

Central angle of blade (δ) Number of blades (Z)

CFT efficiency (η) (hydropower stations)

CFT efficiency (η) (wave, tidal and hydrokinetic generation)

30
(Olgun, 1998; Desai and Aziz, 1994; Totapally and Aziz, 1994; Choi et al., 2008) 90
or below (Olgun, 1998; Desai and Aziz, 1994; Totapally and Aziz, 1994; Choi et al., 2008) 61.5
(Sammartano et al., 2013) 25-30 (Sammartano et al., 2013; Nakase et al., 1982; Shahram et al., 1988; Desai and Aziz, 1994; Totapally and Aziz, 1994) 68%–88% (Olgun, 1998; Mockmore and Merryfield, 1949; Nakase et al., 1982; Durgin, 1984; Hothersall, 1985; Ott and Chappel, 1989; Desai and Aziz, 1994; Totapally and Aziz, 1994; Kaunda et al., 2014a; De Andrade et al., 2011) 40%–55% (Elbatran et al., 2015a; Ki-Pyoung et al., 2012; Choi et al., 2009, 2010; Prasad et al., 2014; Kim et al., 2015)

streams of micro channel flow or ducted around turbines (Elbatran et al., 2016). Deploying nozzles in channels accelerates the flow and increases the water's kinetic energy. Khan et al. (2013) conducted analytical and numerical investigations to enhance the flow by utilizing convergent nozzles for flow in open water channels. The power extracted from channels mainly depends on the velocity of the in-stream water flow; thus, geometrical parameters of the nozzles will have major effects on the flow patterns and velocity. By focusing only on one parameter which was the inlet angle of the convergence nozzle, Khan et al. (2013) studied the flow patterns through the velocity and pressure behavior contours. The convergence nozzle succeeded in accelerating the water flow by increasing velocity at the nozzle plan. It is important to investigate the effects of nozzle geometrical parameters, such as diameter ratio, nozzle configuration and nozzle edges shape on the flow characteristics in micro scale channels through parametric study. Consequently, Elbatran et al. (2015d) concentrated on determining the flow field pattern, water velocity values, pressure distribution, turbulence effects, volume flow rate and the amount of power that could be captured with respect to different geometrical parameters of the deploying nozzles. These also included the effects of the free surface. Elbatran et al. (Elbatran et al., 2015), also 438

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 2. Augmentation channel surrounding CFT runner (Elbatran et al., 2015a).

obtained in this study. Elbatran et al. (2015a) proposed zero-head cross flow turbine to extract the hydrokinetic power in micro scale channels. Hence, CFT is indeed suitable for other applications, such as current and wave power generation, besides hydropower applications. From the current literature review results, the following recommended runner main geometrical and effectiveness parameters are summarized in Table 1:

torque transfer in the two stages of the turbine. It was reported that the highest turbine efficiency was 79%. Recently, the researchers paid more attention to the use of CFD, which is becoming an important tool to investigate and design the cross flow turbines. Choi et al. (2008) investigated the effect of the turbine's configuration on the CFT performance and internal flow properties by using CFD analysis. It was found that the maximum efficiency occurred with 25
angle of attack, 87
blade exit angle and 26 as the number of blades. The efficiency also leads to an increase in narrow nozzle passage. Andrade et al. (De Andrade et al., 2011) proved, numerically by using ANSYS CFX, that 68.5% percent of the energy transmission occurs in the 1st stage, and the remaining 31.5% was transferred in the 2nd stage; they concluded that there was a good agreement between the numerical and experimental results. Sammartano et al. (Sammartano et al., 2013) used CFD simulations in order to design the main geometrical parameters of the cross flow turbines. In this study, a turbine with 35 blades and an attack angle of 22
, central angle of blade (δ) of 61.5
displayed at the design point a high efficiency equal to 86%. Meanwhile, Kaunda et al. (2014b) numerically studied the flow profile in the CFT at different operating conditions by means of ANSYS CFX to calculate the turbine performance. The cross flow turbine is more suitable for run-of-river and wave generation applications because its efficiency is mainly dependent on the flow rate compared to the other types of hydro power turbines. Choi et al. (2009), Choi et al. (2010) and Prasad et al. (Prasad et al., 2014) experimentally and numerically studied the performance of cross flow turbine as Direct Drive Turbine (DDT) for wave power generation. Highest efficiencies of 51.7%, 51.6% and 55% were reported for the above-mentioned works, respectively. Recently, Kim et al. (2015) integrated CFT in a power-take off (PTO) system of new floating wave energy converter, and the experimental maximum efficiency was nearly 40%. In addition, Kim et al. (Ki-Pyoung et al., 2012) numerically proposed a new configuration of cross flow turbine for harnessing tidal energy by utilizing a larger area of the channel. The maximum efficiency of 52% was

1.2. Problem statement and objectives of the study In addition to hydropower applications, CFT is indeed suitable for other applications, such as current and wave power generation. However, the application of cross flow turbines-as tested in these studies-still poses a question: how can the cross flow runner design be further modified to optimize the turbine performance, if special configuration of new channel design is given as in the present work? It is noticed that many researchers use the recommended geometrical parameters as used in hydropower applications, but with different configurations and arrangements. Accordingly, Ref (Elbatran et al., 2015a). numerically proposed surrounding the CFT by directed nozzle diffuser system with recommended CFT geometrical parameters as used in hydropower applications. They, however, do not accurately signify appropriate parameters, especially the inlet and outlet angles of the runner blades. Thus, the concluded results in Ref (Elbatran et al., 2015a) recommended that it was important to get the suitable inlet and outlet angles for using in the current configuration. Hence, there is a lack of theoretical understanding on changing these angles to be suitable in the present hydrokinetic applications, which is strongly manifested in all previous literature. Such outcome of the literature review has driven the objectives of the current step. Therefore, this study intends to discover the effects of inlet and outlet angles of the CFT runner blades on determining the performance characteristics of the dual CFT/Banki turbines configuration and flow characteristics through system, which is designed for application in hydrokinetic micro channel scheme. Numerical and experimental 439

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 3. CFT runner cases.

incoming water has a free surface at 0.4 m from the bottom. The inlet of the channel occupies 0.25 m of the inlet water level, with the clearance between the blade tip and casing set at 5 mm. The free surface at the special system inlet enables the deployment of current arrangement in shallow water or at the surface of river applications as well as channels. Inlet nozzle is not necessarily symmetrical because one of the main purposes of the current study is to investigate the upper turbine efficiency under free surface effect. The lower turbine also is in a normal mode of operation; thus, it is better if the lower region occupied more area of water than the upper part to fully exploit the system and achieve higher performance. The use of CFT runner in the present augmentation channel system is a newly designed concept proposed by the authors in Ref. (Elbatran et al., 2015a).

Table 2 Four tested cases of inlet and outlet angles. Runner case

Blade inlet angle (β1 )

Blade outlet angle (β2 )

Outer diameter (Do)

Inner diameter (Di)

Diameter ratio (Di/ Do)

Case Case Case Case

15
30
45
60


75
90
105
120


0.1930 m 0.1995 m 0.2000 m 0.2052 m

0.1422 m 0.1392 m 0.1370 m 0.1390 m

0.736 0.692 0.68 0.677

1 2 3 4

investigations had been performed in the present study. This numerical work was conducted using a finite volume of RANSE code Ansys CFX. The main objective of this research is to 1) improve the flow characteristics through CFT runner by defining the appropriate inlet and outlet blade angles, 2) assess the overall power coefficient of the turbine to extract as much energy as possible from flow in a channel.

2.1. Test cases Fig. 3a presents the descriptions of the lower and upper parts of the turbine. Its length (L) was 0.38 m. A total of 26 blades were formed with an arc camber having 2.7 mm thickness (t). Four test cases with different inlet (β1 ), outlet (β2 ) angles and diameter ratio of turbine models were adopted in this study. The blade inlet and outlet angles of the turbine models are summarized in Table 2 and described in Fig. 3b.

2. Augmentation channel and CFT cases model Fig. 2 shows models of two CFT runners surrounded by augmentation channel, composed of an inlet nozzle and a diffuser at the exit side of channels and cross flow turbine runner. In this configuration, the nozzle

440

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 4. Velocity triangles of four tested cases.

peripheral velocity and relative velocity, respectively. Referring to Fig. 4, the increase of inlet and outlet angles of the runner (β1; β2 ) decreased the angle of attack, which theoretically led to the increase in the magnitude of the velocity in the first stage; consequently, the efficiency of this stage could be increased. Furthermore, it resulted in the increase of the cross flow through the runner due to the increase of the direction of the exit velocity of the first stage as in Cases 3 & 4. Subsequently, the efficiency of the second stage could be developed. In

2.2. Velocity diagrams The velocity diagram theoretically proposes the magnitude and direction of the velocity of the water in the two stages of cross flow turbine. No flow losses through the inside of the runner were assumed, which should give equal diagrams at the exit of the first stage and input of the second stage. Fig. 4 shows the velocity triangles through the runner for four studied cases, where V, U and represent the absolute velocity, the 441

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 5. Schematic diagram of the present experimental apparatus.

Fig. 6. Meshing for (a) model inlet and outlet channel, (b) augmentation channel and (c) the turbine runner.

diffuser-augmented channel, including two cross flow turbines with connection to the channel system. The current channel had a total length of 3.2 m, by which the present model system-as shown in Fig. 1- was positioned in the middle of the channel plan and had a length of 1.2 m. The overall length of the front and rear channels (Lc) was 2 m, with a width of the channel, W ¼ 0.6 m, and the channel depth, h ¼ 0.6 m. The water level at the channel inlet was 0.4 m, as shown in Fig. 5. The main aim of the present study is to investigate the flow and

conclusion, based on the current configuration, the efficiency of the turbine analytically tended to increase with the decrease of angle of attack and increase of the inlet and outlet angles. 3. Experimental apparatus The test rig consisted of water pump (preference to variable speed pump), loop channel system from and to water reservoir, bidirectional 442

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

camera was used to record videos at the plane of the channel system from the side view. High speed camera was used as a particle-tracking method, where small lighter rings were used as tracking particles. Then, the videos recorded by the high speed camera were imported to Phantom Camera Control software to measure the velocities. Futek TRS 605 torque meter was used to measure the turbine shaft torque and rotational speed. Torque sensors functioned to measure the torque with the aid of strain gages, which were applied to the torsion section to undergo a change in impedance that depended on the torque. This caused a voltage change that was proportional to the change in impedance and reached the Evaluation Instrument (IBT 100). The load was changed by a brake system, by which the rotational speed (RPM) of the turbine shaft was controlled. The uncertainties for the experimental results in different parameters, TSR, torque coefficient and power coefficient were around 2.8%, 4% and 4.5% respectively.

Table 3 The grid specifications for four tested grids. Grid No.

1 2 3 4

No. of Cells Main channel

Augmentation channel

CFT turbines

Total cells

135893 164719 208506 257296

144346 176032 220401 270652

1280055 1602250 1882361 2315304

1,560,294 1,943,001 2,311,268 2,843,252

4. Numerical approach In this research, a finite volume method was used for discretizing the governing equations in Ansys CFX. The convective terms were discretized using Second Order Upwind scheme, and the pressure was interpolated using linear interpolation scheme, while the central difference scheme was utilized for diffusion terms. For the pressure-velocity coupling, SIMPLE (Semi-Implicit Methods for Pressure-Linked Equation) was utilized. Unsteady simulation was performed based on Reynolds averaged Navier–Stokes equations with (SST) k-ω, as introduced by Menter (Kim et al., 2015) for turbulence model. This turbulence model combines k–ω turbulence model and k–ε model, where k–ω turbulence model is used in the near wall region and the k–ε model is used in the free-stream flow.

Fig. 7. Power coefficient variations with different TSR of Case 3 lower turbine at different grids.

4.1. Governing equations

performance characteristics of the proposed CFT in the augmentation channels to improve the efficiency of the turbine in hydrokinetic mode by studying the influence of changing the inlet and outlet angles. This was done through measuring the free flow water velocity, mean inlet runner velocity, turbine RPM, tip speed ratio, shaft torque, output power, torque coefficient and power coefficient. Power coefficient (Cp ) and Torque coefficient (Ct ) of turbines were used to reflect the turbine performance and efficiency, as presented in Equations (1) and (2): Cp¼

T ω 0:5ρAU 3 0

CT¼

T 0:5ρArU 2 0

The governing equations for the unsteady, viscous, and incompressible turbulent flow are the Navier-Stokes equations, which can be written in the following form (Menter, 1994):

∂ui ¼ 0:0 ∂xi 

ωr V




∂ui ∂ ∂p ∂ þ uu ¼  þ ∂t ∂xi i j ∂xi ∂xi

(1)

ρ

(2)

Where  ρu'i u'j ¼ μt

Where T represents the output torque, ω is the angular velocity of the turbine, Pc ¼ 0:5ρAU03 is the maximum hydrokinetic power that can be extracted from the channel, is the density of water, (A) is the sectional area of the flow at the inlet of channel, and indicates the flow stream velocity, r represents the turbine radius. It is important to determine the effects of tip speed ratio on the efficiency of the turbine runner. TSR is the ratio of the peripheral velocity of the runner (U) to inflow jet speed (V). In order to get different values of TSR, the angular velocity (ω) was varied, whose TSR was calculated by using the equation below: TSR ¼

(4) 

  ρu'i u'j

 

þ

∂ ∂ui ∂uj μ þ ∂xj ∂xi ∂xj

 (5)



∂ui ∂uj 2 þ  ðpkÞδij 3 ∂xj ∂xi

(6)

The SST turbulent model can be expressed in the following mathematical form: 



∂ðpkÞ ∂ ∂ ∂k þ ðpkui Þ ¼ Γ þ Gk  Yk ∂t ∂xi ∂xj k ∂xj 

(7)



∂ðpωÞ ∂ ∂ ∂ω ðpωui Þ ¼ þ Γ þ Gω  Yω þ Dω ∂t ∂x i ∂xj ω ∂xj

(8)

In the previous equations, ui ¼ (u, v, w) are velocity components in the directions of xi ¼(x y z), ρ is water density (kg/m3 ), ω represents specific turbulence dissipation, μ represents water dynamic viscosity (kg/ m .s), k is turbulence kinetic energy. Γ k and Γ ω are the effective diffusivity for k and ω. Gk is the generation of turbulence kinetic energy due to mean velocity gradients, and Gω represents the generation of ω. Yk and Yω represent the dissipation of k and ω due to turbulence, while Dω is the cross-diffusion term.

(3)

Where V is the mean runner inlet velocity. Test measurement apparatus used in the current tests were a high speed camera, digital torque sensor and RPM meters, all of which had been calibrated. The velocity of the flow of water through the channel system was measured by using a high speed 10 megapixel camera, branded Phantom V7, capable of taking 1,400,000 pictures-per-second, with Ultra-fast – 7 Gpx/second throughput, 1280  800 at 7530 fps, 300 ns digital exposure and is Phantom Cine Mag compatible. High speed 443

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 8. Experimental measured data and calculated data from CFD of (a) torque coefficient and (b) power coefficient for lower turbine.

Time step (t) corresponded to 1
of rotation by the turbine. Interfaces of non-conformal mesh were used with the stationary and rotating domains in this study. Sliding mesh model was used in the current study for predicting the flow properties and performance characteristics through cross flow turbines in the various cases. The domains were meshed using hybrid (structured hexahedron and unstructured tetrahedral) grids, as shown in Fig. 6. Ultimately, structured hexahedral mesh elements were used for the main channel, while unstructured tetrahedral mesh elements were used for the CFT and the augmentation channel in the present study. Four computational grids had been tested in this study to check the solution sensitivity. Table 3 describes four grid cells specifications for all domains. Power coefficients variations with TSR for lower turbine of case 3, which were determined for four grids, are shown in Fig. 7. It was noticed that grid 3 and grid 4 gave acceptable and similar results, while grid 4 required more time for solution. Hence, grid 3 was chosen to carry out the investigations of this

4.2. Computational schemes and boundary conditions In this study, 3D models of CFT surrounding by BDA system had been built and validated by current experimental tests. The simulations were carried out in unsteady condition, and ANSYS ICEM CFD software was used for meshing the models. The computational grid was divided into three domains, which are stationary domain (main channel, augmentation system), and rotational domain (cross flow turbines) for the sliding mesh model. Banki turbines and augmentation channel were considered to be walls, where the no slip condition was assigned to these walls in the different simulations. The velocity had normal direction to the water model channel inlet. The outlet boundary condition was specified according to the outflow, and the reference pressure was set to atmospheric 1 bar. The working fluid was water, with density of 1000, and dynamic viscosity of 1x. Convergence was monitored through making dimensionless residual sum for all variables across the computational points. 444

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 9. Torque coefficient variations of the tested turbines cases at different TSR for (a) lower turbine (b) upper turbine.

5. Results and discussions

study.

5.1. Numerical results and discussions 4.3. Validation with experiment The concluded results in Ref (Elbatran et al., 2015a), recommended that it is important to get the most appropriate inlet and outlet angles to be used in the current configuration to increase the efficiency of the system. The numerical simulation was carried out on the four different turbine angles and diameter ratio cases. This study presents investigations on the effects of different runner inlet and outlet angles of Banki turbines, by measuring the performance and flow field characteristics of the turbines by different inlet and outlet angles.

The present numerical study was validated with the experimental analysis of the current work to verify the numerical method used in the current study. A good agreement between numerical and experimental results was reached for performance characteristics of lower turbine, as presented in Fig. 8. The maximum range of error between experimental and numerical measurements for performance curve was within 6.5–9%. The discrepancy between the numerical and experimental results tends to minor experimental factors like channel and BDA wall roughness, wall gap at the channel connection, and etc. 445

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 10. Power coefficient variations of the tested turbines cases at different TSR for (a) lower turbine (b) upper turbine.

the decrease of rotational speed (RPM). On the other hand, the power coefficient increased with increasing TSR until it reached a peak value at TSR of almost 0.44 for all cases of lower turbines, and ranged from 0.62 to 0.68 for upper turbines. It then decreased from the value onwards (see Fig. 10). Furthermore, the power coefficient increased with increasing TSR until it reached a peak value at TSR of almost 0.46 for all cases of lower turbines and ranges from 0.62 to 0.7 for upper turbines, it then decreased from here onwards (see Fig. 10). The power coefficient increased with increasing TSR until it reached a

5.1.1. Performance characteristics of four cases This section investigates the effects of different runner designs, especially inlet and exit blade angles of the CFT runner in the new current hydrokinetic flow stream in channel mode, by measuring the performance characteristics of the cross flow at various blade angles. The CFT cases were tested at different TSR and various flow rates (Q). Figs. 9 and 10 show the torque coefficient and power coefficient variations at different TSR for all cases of tested turbines. From Fig. 9, it was noticed that the coefficient of torque increased with the decrease of TSR, or with 446

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 11. Lower and upper turbines power coefficients variations with inlet runner flow rate for (a) lower turbine (b) upper turbine.

“efficiency” of 0.48 at TSR ¼ 0.62. These values of the efficiency are very considerable in comparison to the original turbine efficiency used in hydrokinetic or current applications. On the other hand, the performance curves indicated that case 2 was the second perfect configuration for inlet and outlet angles, which recorded power coefficients of 0.52 and 0.412 for lower and upper runners, respectively. This is while the cases 1 and 4 have shown very poor characteristics compared with case 2 and 3. The inlet runner flow rate (Q¼AV) is a very important factor for the input power of the current system, because the input power is proportional to the flow rate (P ¼ 0:5ρQV2 ). Hence, the inlet power will increase when the flow rate becomes larger. Meanwhile, if the efficiency is constant, the output power of the turbine will increase. However, this concept is not acceptable over time because the water at the nozzle inlet needs to be completely full so that the flow rate does not affect the peak efficiency of turbine (Fiuzat and Akerkar, 1991). The present study aims

peak value at TSR of almost 0.46 for all cases of lower turbines and ranges from 0.62 to 0.7 for upper turbines, then decreasing from here onwards. The highest power coefficient of the current CF turbines tends to increase with the increment of the inlet and outlet angle (β1; β2 ), reached to 45
and 105
for β1 and β2 respectively. After that, the power coefficients were suddenly dropped in case 4. This was the effect of the inlet and outlet blade angles which lead to the perfect direction of the flow through the runner, which resulted in increasing of the efficiency of the second stage. In the present study, regarding variations of inlet and outlet runner blade angles, there has been a point by which the turbine case has peak output power and power coefficient “efficiency”. The power coefficient of lower turbine, which was recorded for case 3, was 0.575 at adequate values of tip speed ratio of around 0.42. These values were quite acceptable and agreed with the literature and theoretical study of Balji (Balje, 1981). This is while the upper runner achieved power coefficient 447

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 12. Water velocity vectors showing the water flow through the CFT rotors and power stages.

to get an appropriate flow rate at the intake of the system to define the maximum power and efficiency. Fig. 11 illustrates the relation between the power coefficient and the inlet flow rate for all tested cases. It was not necessary that the peak value of the efficiency occurs at the maximum flow rate. Every case had a different optimized flow rate according to its configurations. The most efficient flow rates indicated for Cases 1, 2, 3 and 4 of lower turbine were 0.051, 0.0526, 0.057 and 0.053/s, respectively. For upper case, the appropriate flow rates were 0.034, 0.044, 0.045 and 0.042/s for Cases 1,

2, 3 and 4, respectively. Moreover, it was noticed that, Case 3 for upper and lower cases provided the best results for flow rate and efficiency. The efficiency of the lower turbine was higher than that of the upper turbine; this appeared for all cases of different angles. This is seen in the increase of the inlet runner velocity ðV1 Þ of lower region and flow rate through lower turbine compared to the upper region; this is due to the fact that the lower region occupied more area of water than the upper region. The velocity through runner also increased due to the effects of gravity and extensively crossed flow through the lower runner; in

448

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 13. Water velocity vectors through the runner for all cases.

produce jet flow, which contributed to added energy generation besides application of two main power stages. Hence, the blades absorbed the energy through three stages, causing them to rotate anticlockwise. On the other hand, the outlet velocity from the first stage was not the same with the inlet velocity from the second stage. This shows a disagreement with the theoretical assumption ðVr2 ¼ Vr3 ) but shows a good agreement with the current experimental results, which is important to consider through the CFT runner design. The flow pattern here proves that there is a good agreement among the analytical, experimental and numerical studies; the CFD tool has been reached at this end.

contrast, it decreased in case of upper runner as described in the following Sections of 5.2 and 5.3. 5.2. Flow field characteristics analysis The flow field characteristics through CFT runner in the present configuration were studied to evaluate the turbine efficiency. Close views of velocity vectors at a central plane through the augmentation channel and the rotor blades are displayed in Fig. 12. Entrained higher velocity flow of water between the turbine blade tips and casing was able to 449

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 14. Runner inlet velocity variations with peripheral velocity for lower and upper turbines in four cases.

Enhancement of flow characteristics could be achieved using rectangular duct nozzle and NACA nozzle surrounding the CF turbine runners. Fig. 12 presents the importance of NACA augmentation for increasing the mean inlet runner velocity of the water flowing. The velocity indicated more values at the turbine inlet for lower regions than for the upper runner, where the velocity increased due to the effects of gravity and crossed flow through the lower runner; in contrast, it decreased in the case of upper runner. Consequently, the turbine indicated that the amount of kinetic energy harnessed by lower turbine was

more than in the case of upper rotor. This proves that the numerical and experimental results are comparable. 5.2.1. The effects of the blades inlet and outlet angles on the flow characteristics A numerical simulation was carried out using four different turbine angles and diameter ratio cases. Vectors of water velocity for different runner blades inlet and outlet angles (four cases) for lower and upper turbines are shown in Fig. 13. Velocity fields in the flow passage of the 450

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 15. Water velocity vectors variations with changing of TSR for case 3.

were also able to increase the area of the blades exposed to the incoming water so that the torque, power and efficiency are increased. In contrast, the large values of inlet and outlet angles of case 3 and case 4 led to the shifting of the cross flow of water towards the shaft centre and increased flow recirculation, especially for case 4, as shown in the black circle in Fig. 13. Subsequently, it was expected to reduce in the turning moment at the shaft centre due to water jet of the cross flow. Hence, there is a slight reduction in the power output and efficiency. Nevertheless, case 3 and 4 have the most effective internal flow characteristics for increasing of the efficiency of the two stages.

turbine are changed simultaneously with the variation of the inlet and outlet angles. It was observed that the velocity flow vectors crossing the runner for case 3 and case 4 were different from those for case 1 and case 2. In these cases, the blades angles made the flow more perpendicular on the blades, leading to a decrease in the angle of attack, and subsequently, it results in an increase of the velocity in the first stage and improving the efficiency of this stage. Efficiency of the second stage has also increased due to the perfect direction of the cross flow through the runner, making the cross flow more extensive than in the other two cases. These angles' configurations

451

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 16. Pressure contours through the CFT rotors and power stages for case 3.

power played an important role in the output power of the turbines. It was proven that the turbine's second stage contributed more than 30% of the total power. The differences between the power of Case 3 and Case 1 signified the amount of power represented by the second stage. This is because Case 1 had little crossed flow to the second stage. The effects of TSR on the flow field characteristics for Case 3 are presented in Fig. 15. The increase in the TSR stems from the decrease in the inlet runner velocity ( V1 ); in addition, an increase in the peripheral velocity ( U1 ) is due to the decrease of the cross flow intensity rate through the runner. This leads to a decrease in the effectiveness of the runner, especially for the second stage. Although the flow in some cases, such as TSR ¼ 0.44, has no perfect cross flow properties through the runner like in case of TSR ¼ 0.26, the cross flow of water in this case deflects away from the shaft centre. Therefore, the recirculating flow of upper part of the runner decreased. Consequently, the torque and power coefficients of the turbine improved. Water flow passed more extensively and quickly through the runner when the turbine rotated slowly or when the TSR decreased. Additionally, the maximum efficiency occurs when the interaction of the flow and the runner is maximized at appropriate

This result implies that one of these two cases is able to improve the turbine performance. Moreover, the angles of case 2 gave a good internal flow pattern, but they were not the most suitable degrees of the runner blades angles for the present configuration with respect to the flow fields. The inlet runner velocity V1 varied with the peripheral velocity U1 ¼ ωr for the four cases, as shown in Fig. 14. It was noticed that the mean intake rotor velocity for all cases considerably decreased with the increment of the peripheral velocity U1 . Consequently, the inlet velocity decreased with the increase of TSR. In other words, if the runner moved slowly, the inlet runner velocity (V1 ) would increase. Besides, the mean entrance velocity for the third case recorded higher values than did the other cases of the two turbines. Therefore, the kinetic energy available in this case was more than those in the other cases. Consequently, the power output was expected to increase as described in Figs. (9 and 10) regarding the turbine power characteristics. Moreover, it was observed that there was a significant drop in the inlet water velocity in Case 1 and Case 4 due to the flow resistance caused by angle configuration in these cases. An almost vertical angle caused some blockages at a lower flow rate. Regarding the flow analysis of the current section, the second stage 452

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

Fig. 17. Experimental results for velocity ratio V/ UO variations along the augmentation channel and CFT rotors for case 3 at TSR ¼ 0.66.

Fig. 18. Experimental results for (a) runner inlet velocity ratio and (b) peripheral velocity ratio variations at different TSR of two turbines.

453

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

to pass perfectly through the runner, leading to a larger torque, power and efficiency. Furthermore, the angles of 45
and 105
for inlet and outlet with diameter ratio of 0.68 (case 3) provide the effective results in a higher efficiency than do other angles. This runner configuration for lower and upper regions was capable of producing maximum power output and peak efficiency. Therefore, the most appropriate runner design that can be used for two turbines in the present channel configuration was in case 3. Consequently, case 3 results are presented here: the velocity measured lines, measured by using high speed cameras, for lower and upper regions were positioned at Y ¼ 0.3, Z ¼ 0.22 and Y ¼ 0.3, Z ¼ 0.3, respectively. The velocity ratio values tracking for lower and upper regionsexperimentally measured along a central plane through the BDA system-are shown in Fig. 17. It was noticed that the presented application

TSR. The pressure variations in the augmentation channel and through upstream and downstream of the turbine are presented in Fig. 16. The pressure difference created in the runner by the lower pressure in the accelerating region (upper surface) and the higher pressure in the decelerating region (lower surface) was found able to generate energy. 5.3. Experimental results The BDA system presented in the current study is planned to be utilized in micro open water channels and rivers with low velocity regions to enhance the water flow and increase the extracted hydrokinetic power. The numerical results and discussion of this study have proven that a higher value of the inlet and outlet blade angles can cause the water flow

Fig. 19. Experimental results for (a) Torque and (b) power coefficients variations with various TSR. 454

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456

is likely to increase the speed of fluid. Increasing the speed can be a significant factor in designing channels low speed current appearance. In this study, the water velocity increased with the decrease of the area of the contraction along the BDA system due to the presence of two directed nozzles. The velocity values for lower and upper regions increased to 2.1 and 1.3 times, respectively, than those of the free stream inlet velocity at the end of the first nozzle. The NACA nozzle had significantly increased the water velocity by about 3.2 times and 2.3 times than that of the system free stream inlet velocity for lower and upper regions, respectively. It was observed that the velocities' ratio of lower region were more than the upper region because the lower region occupied more area of water than the upper part. Moreover, the velocity indicated almost 3.2 UO and 2.3 UO at the turbine inlet for lower and upper turbines, respectively, and it started to decrease behind the turbine, which recorded nearly 1.4 UO and 1.3 UO . The differences between the values of velocities in front of and behind the turbine presented the amount of kinetic energy harnessed by turbines. It was clearly seen that this difference through the lower turbine was more than the upper runner because the lower turbine was in a normal mode of operation and the crossed flow through the turbine was more extensive. Hence, it is expected that by this design, the kinetic power and efficiency of lower runner would be more than those of upper turbine. The variations of the mean inlet runner velocity ratio (V1 / UO ) and peripheral or circumferential velocity ratio (U1 / UO ¼ ωr/ UO ) with TSR are shown in Fig. 18. It was noticed that the mean inlet of runner velocity for two turbines decreased with the increase of TSR, while the peripheral or circumferential velocity increased with the increment of TSR. In other words, the inlet flow of water passed more quickly through the runner when the turbine rotated slowly. Furthermore, the inlet runner velocity and circumferential velocity for the lower turbine reached higher values than those of upper turbine. This is reasonable based on the reasons as in the previous section. Fig. 19 illustrates the performance characteristics curves of two tested lower and upper turbines. The torque coefficient decreased with the increase of TSR or with the increment of rotational speed (RPM). The maximum output torque coefficients' values recorded for the lower and upper turbines were about 0.76 and 0.5, respectively. These numbers were obtained at the lowest values of TSR and RPM. It was also noticed that the torque coefficient (Ct ) for lower runner was more than that in upper case, because the lower area of flow stream is more than the flow area available for upper region. The power coefficient (CP Þ of the lower and upper turbines tended to increase with the increase of TSR until it reached the peak values and then decreased. The maximum power coefficients were 0.612 and 0.473 for lower and upper turbines, respectively. These coefficients of power were obtained at TSR of 0.66 and 0.52 for lower and upper runner, respectively. The highest efficiency of overall system with the two cross flow turbines was 0.557.

obtained as 0.575 and 0.48 for lower and upper turbines, respectively. A good agreement was observed between the current numerical and experimental results for flow pattern through the BDA system and performance characteristics of the two turbines. The maximum final power coefficient of overall system, which includes two cross flow turbines, was 0.557. This system promises a sufficient performance and a higher efficiency compared to the conventional hydrokinetic turbines and CFT systems utilized in the wave and tidal power generation scheme. Acknowledgment The authors would like to express sincere gratitude to Universiti Teknologi Malaysia (UTM) for providing assistance, access to data and experimental apparatus for this publication. This project is sponsored by the Ministry of Education Malaysia under ERGS Fund No 4L.125. References Balje, O.E., 1981. Turbomachines. John Wiley & Sons Inc, New York. Chamorro, L.P., Hill, C., Morton, S., Ellis, C., Arndt, R.E.A., Sotiropoulos, F., 2013. On the interaction between a turbulent open channel flow and an axial flow turbine. J. Fluid Mech. 716, 658–670. Choi, Young-Do, Lim, Jae-Ik, Kim, You-Taek, Lee, Young-Ho, 2008. Performance and internal flow characteristics of a cross-flow hydro turbine by the shapes of nozzle and runner blade. J. Fluid Sci. Technol. 3 (3), 398–409. Choi, Young-Do, Kim, Chang-Goo, Lee, Young-Ho, 2009. Effect of wave conditions on the performance and internal flow of a direct drive turbine. J. Mech. Sci. Technol. 23 (6), 1693–1701. Choi, Young-Do, Chang-Goo, Kim, You-Taek, Kim, Jung-Il, Song, Young-Ho, Lee, 2010. A performance study on a direct drive hydro turbine for wave energy converter. J. Mech. Sci. Technol. 24, 2197–2206. De Andrade, Jesús, Curiel, Christian, Kenyery, Frank, Aguill on, Orlando, Vasquez, Auristela, Asuaje, Miguel, 2011. Numerical investigation of the internal flow in a Banki turbine. Int. J. Rotating Mach., 841214, 12 pages. Desai, V.R., Aziz, N.M., 1994. Parametric evaluation of cross-flow turbine performance. J. energy Eng. 120 (no.1), 17–34. Durgin, W., 1984. Fay W some fluid flow characteristics of a crossflow type hydraulic turbine. In: Proceedings of American Society of Mechanical Engineers (ASME) Winter Annual Meeting on Small Hydropower Fluid Machinery, New Orleans, USA. Elbatran, a, Yaakob, o, Ahmed, y, Shabara, H., 2015. Numerical investigation of curvature and torsion effects on water flow field in helical rectangular channels. J. Eng. Sci. Technol. 10 (7), 827–840. Elbatran, A.H., Yaakob, O.B., Yasser, M., Ahmed Jalal, M.R., 2015. Novel approach of bidirectional diffuser-augmented channels system for enhancing hydrokinetic power generation in channels. Renew. Energy 83, 809–819. Elbatran, A.H., Yaakob, O.B., Ahmed, Yasser M., Shabara, H.M., 2015. Operation, performance and economic analysis of low head micro-hydropower turbines for rural and remote areas: a review. Renew. Sustain. Energy Rev. 43, 40–50. Elbatran, A.H., Walid Abdel-Hamed, Mohamed, Yaakob, O.B., Ahmed, Yasser M., Arif Ismail, M., 2015. Hydro power and turbine systems reviews. Jurnal Teknologi Sci. Eng. 74 (5), 83–90. Elbatran, A.H., Yaakob, O.B., Ahmed, Yasser M., Shabara, H.M., 2015. Numerical study for the use of different nozzle shapes in microscale channels for producing clean energy. Int. J. Energy Environ. Eng. 6 (2), 137–146. Elbatran, A.H., Yaakob, O., Ahmed, Y., Abdallah, F., 2016. Augmented diffuser for horizontal Axis marine current turbine. Int. J. Power Electron. Drive Syst. 7 (1), 235. Elbatran, A.H., Ahmed, Y.M., Shehata, A.S., 2017. Performance study of ducted nozzle Savonius water turbine, comparison with conventional Savonius turbine. Energy 164, 566–584. Fiuzat, Abbas A., Akerkar, Bhushan P., 1991. Power outputs of two stages of cross-flow turbine. J. energy Eng. 117 (2), 57–70. Herman Jacobus, Vermaak, Kanzumba, Kusakana, Sandile Philip, Koko, 2014. Status of micro-hydrokinetic river technology in rural applications: a review of literature. Renew. Sustain. Energy Rev. 29, 625–633. Hothersall, R., 1985. A review of the cross-flow turbine. New York. In: Proceedings of American Society of Civil Engineers (ASCE) on Waterpower, vol. 2, p. 914. Kaunda, Chiyembekezo S., Kimambo, Cuthbert Z., Nielsen, Torbjorn K., 2014a. Experimental study on a simplified crossflow turbine. Int. J. Energy Environ. 155–182. Kaunda, Chiyembekezo S., Kimambo, Cuthbert Z., Nielsen, Torbjorn K., 2014b. A numerical investigation of flow profile and performance of a low cost Crossflow turbine. Int. J. Energy Environ. 275–296. Khan, A.A., Khan, Abdul M., Zahid, M., Rizwan, R., 2013. Flow acceleration by converging nozzles for power generation in existing canal system. Renew. Energy 60, 548–552. Ki-Pyoung, Kim, Ahmed, Rafiuddin, Lee, Young-Ho, 2012. Efficiency improvement of a tidal current turbine utilizing a larger area of channel. Renew. Energy 48, 557–564. Kim, Byung-Ha, Joji, Wata, Mohammed Asid Zullah, M., Rafiuddin, Ahmed, YoungHo, Lee, 2015. Numerical and experimental studies on the PTO system of a novel floating wave energy converter. Renew. Energy 79, 111–121.

6. Conclusion A new cross-flow turbine based on the Banki concept was developed and deployed in the present augmentation channel. The CFT runners in the present hydrokinetic system are utilized without the need for special casing, nozzles entry arc and guide vanes. Hence, the inlet and outlet runner blades angles and diameter ratio play an important role in the direction of the cross flow through the runner in overall efficiency of the system. The most efficient runner design that can be used for the two turbines in the present channel configuration has the appropriate inlet and outlet angles of 45
and 105
, respectively, and the diameter ratio of 0.68. Turbines' operation and performance in the new channel arrangement have been analyzed and evaluated. Experiments were conducted to determine the power coefficients of the turbines, which were found to be 0.612 and 0.473 for the lower and upper turbines, respectively. Furthermore, the maximum power coefficients were numerically 455

A.H. Elbatran et al.

Ocean Engineering 159 (2018) 437–456 Ott, R., Chappel, J., 1989. Design and efficiency testing of a cross-flow turbine. In: Proceedings of American Society of Civil Engineers (ASCE) on Waterpower 89, August 23-25, pp. 1534–1539. New York, N.Y. Prasad, Deepak Divashkar, Ahmed, M.Rafiuddin, Young-HoLee, 2014. Flow and performance characteristics of a direct drive turbine for wave power generation. Ocean. Eng. 81, 39–49. Rojanamon, P., Taweep, C., 2009. Thawilwadee Bureekul Application of geographical information system to site selection of small run-of-river hydropower project by considering engineering/economic/environmental criteria and social impact. Renew. Sustain. Energy Rev. 13, 2336–2348. Sammartano, Vincenzo, Costanza, aric o, armando, carravetta, oreste, fecarotta, tullio, tucciarelli, 2013. banki-michell optimal design by computational fluid dynamics testing and hydrodynamic analysis. Energies 6 (no.5), 2362–2385. Shabara, H., Yaakob, O., Ahmed, Y., Elbatran, A., 2015. CFD simulation of water gravitation vortex pool flow for mini hydropower plants. J. Teknol. 74 (5), 77–81. Shahram, Khosrowpanah, Fiuzat, A.A., Albertson, Maurice L., 1988. Experimental study of cross-flow turbine. J. Hydraulic Eng. 114 (no3), 299–314. Teuteberg, B.H., March 2010. “Design of a Pump-as-turbine for an Abalone Farm“, Final Report for Mechanical Project 878. Department of Mechanical and Mechatronic Engineering, Stellenbosch University. Totapally, Hara GS., Aziz, N.M., 1994. Refinement of cross-flow turbine design parameters. J. energy Eng. 120 (3), 133–147. Ghosh, Tushar K., Prelas, Mark A., 2011. Energy Resources and Systems, Renewable Resources Volume 2. Springer, Netherlands chapter3. Yaakob, O.B., Ahmed, Yasser M., elbatran, A.H., Shabara, H.M., 2014. A review on micro hydro gravitational vortex power and turbine systems. Jurnal Teknologi(Sciences Eng. 69 (7), 1–7.

Kokubu, Kiyoshi, Toshiaki, Kanemoto, Keisuke, Yamasaki, 2013. Guide vane with current plate to improve efficiency of cross flow turbine. Open J. Fluid Dyn. 3 (no.2), 28. Kumar, A., Tschei, A., Ahenkorah, R. Caceves, Devernay, J.M., Freitas, M., Hall, D., Killingtveiet, A., Liu, Z., 2011. Hydropower. In: IPCC Special Report in Renewable Energy Sources and Climate Change. Cambridge University press, UK and USA. Laghari, J.A., Mokhlis, H., Bakar, A.H.A., 2013. Hasmaini Mohammad, A comprehensive overview of new designs in the hydraulic, electrical equipments and controllers of mini hydro power plants making it cost effective technology. Renew. Sustain. Energy Rev. 20, 279–293. Menter, F.R., 1994. Two-equation eddy-viscosity turbulence models for engineering applications. AIAA J. 32, 1598–1605. Mockmore, C.A., Merryfield, F., 1949. The Banki water turbine. In: Bullettin Series, Engineering Experiment Station; Oregon State System of Higher Education. Oregon State College, Corvallis, OR, USA. Mohibullah, Mohd Amran, Mohd Radzi, Mohd Iqbal Abdul Hakim, Basic design aspects of micro hydro power plant and its potential development in Malaysia. National Proceedings of Power and Energy Conference 2004, PECON, 29–30 Nov.2004 IEEE 07803-8724-4. Nakase, Y., Fukutomi, J., Watanabe, T., Suetsugu, T., Kubota, T., Kushimoto, S., 1982. A study of Cross-Flow turbine (Effects of nozzle shape on its performance). In: Small Hydro Power Fluid Machinery (Proc. The Winter Annual Meeting of the American Society of Mechanical Engineers), Phoenix, Arizona,USA, pp. 13–18. Okot, D., 2013. Review of small hydropower technology. Renew. Sustain. Energy Rev. 26, 515–520. Olgun, H., 1998. Investigation of the performance of a cross flow turbine. Int. J. Energy Res. 22, 953–964. Ossberger GmbH Co, 2011. The Ossberger turbine. Bayern, Germany. Accessed on 14/1/ 2015. http://www.ossberger.de/cms/en/hydro/the-ossberger-turbine-forasynchronous-and-synchronous-water-plants/.

456