DBPIA-NURIMEDIA

Journal of Advanced Marine Engineering and Technology, Vol. 45, No. 5, pp. 252~262, 2021

ISSN 2234-7925 (Print)

J. Advanced Marine Engineering and Technology (JAMET)

ISSN 2765-4796 (Online)

https://doi.org/10.5916/jamet.2021.45.5.252

Original Paper

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0), which permits unrestricted

non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Effects of blade number and draft tube in gravitational water vortex

power plant determined using computational fluid dynamics simulations

Min-Sung Kim

ㆍ Dylan S. Edirisinghe

ㆍ Ho-Seong Yang

ㆍ S. D. G. S. P. Gunawardane

ㆍ Young-Ho Lee

†

(Received October 7, 2021； Revised October 24, 2021；Accepted October 24, 2021)

Abstract: In recent times, gravitational water vortex power (GWVP) technology has shown rapid development because of its simple

design and adaptability to a wide range of flow conditions, even for low-head hydropower applications. In this study, the performance

of a GWVP plant was investigated, particularly with respect to the blade number in the vortex turbine and draft tube added as a

modification of the conical-shaped vortex basin. Computational fluid dynamics (CFD) simulations have been extensively used to assess

hydraulic efficiency while observing the flow field. The ANSYS CFX software was used as the simulation tool, and the CFD setup

was validated prior to simulating the newly designed GWVP plant. The effect of the blade number in the vortex turbine was investigated

using, 5-,6-,8- and 10-blade turbines, while the effect of the draft tube was studied using straight and conical designs. Finally, the eight-

blade turbine achieved a maximum efficiency of 57% while maintaining a stable vortex air core. The addition of a small draft tube in

the vortex basin increased the efficiency up to 60%, as it was able to recover the pressure gradually at discharge.

Keywords: Micro hydro, Water vortex, Turbine blades, CFD, Draft tube, Efficiency

1. Introduction

In recent times, the development of gravitational water vortex

power (GWVP) technology has been very rapid, with much

research being conducted regarding optimization of the vortex

basin and turbine designs. The reason for the advanced

development of this technology is the sustainability of the GWVP

plant compared to other commercial small-scale hydropower

extraction methods. Thus, vortex power plant designs can be

adapted to suit a wide range of flow conditions involving smaller

hydraulic heads. As the power plant design is a run-of-the-river

system, no water storage reservoir is required, thus minimizing

the environmental impact [1]. In addition, it can ensure water

purification by aeration by increasing the oxygen concentration

of the water [2]. On the other hand, this technology can be

implemented in hillside areas enriched with hydro-potential to

power up rural communities that are situated far away from the

electricity distribution grid. Therefore, GWVP plants can be used

to support rural communities, thereby providing social benefits

[3]. Thus, the installation and maintenance of a GWVP plant is

simple and cost-effective [4]. Therefore, according to the three

pillars of sustainability (economic, environmental, and social),

GWVP plants are a viable solution in the hydropower extraction

sector. Franz Zotlöterer implemented the vortex power extraction

method using the basic concept of Viktor Schauberger, which

utilizes the natural energy stored in the water to form vortexes

[5]. Initially, Zotlöterer used a water vortex to aerate the water

without employing any external power. Later, he used a vertical

axis rotor at the vortex center to generate rotary energy [6].

In the GWVP plant, gravity-driven water is guided in a tan-

gential direction to a circular basin structure called a vortex basin

with a bottom center outlet. Therefore, inside the vortex basin,

water creates a strong vortex around the vertical-central axis. Us-

ing the vertical axis turbine, the vortex power generated can be

extracted as mechanical energy and later converted to electrical

energy through the use of generators. Figure 1 illustrates the

GWVP plant design used in this study, showing the main

†

Corresponding Author (ORCID: https://orcid.org/0000-0001-9598-6172): Professor, Division of Mechanical Engineering, Korea Maritime &

Ocean University

, 727, Taejong-ro, Yeongdo-gu, Busan 49112, Korea, E-mail: lyh@kmou.ac.kr, Tel: +82-51-410-4293

Ph. D. Candidate, Department of Mechanical Engineering, Korea Maritime & Ocean University, E-mail: kimms4u@naver.com

Ph. D. Candidate, Interdisciplinary Major of Ocean Renewable Energy Engineering, Department of Mechanical Engineering, Korea Maritime &

Ocean University, E

-mail: dylanzenith@gmail.com

Ph. D. Candidate, Interdisciplinary Major of Ocean Renewable Energy Engineering, Department of Mechanical Engineering, Korea Maritime &

Ocean University, E

-mail: kp[email protected]mou.ac.kr

4 Professor, Department of Mechanical Engineering, University of Peradeniya, Sri Lanka, E-mail: [email protected]

Min-Sung Kim ㆍ Dylan S. Edirisinghe ㆍ Ho-Seong Yang ㆍ S. D. G. S. P. Gunawardane ㆍ Young-Ho Lee

Journal of Advanced Marine Engineering and Technology, Vol. 45, No. 5, 2021. 10

253

components. The vortex basin and vortex turbine are the main

components of a GWVP plant. Therefore, the GWVP plant de-

sign is developed mainly by optimizing the structure of the vor-

tex basin or the design of the vortex turbine. Although several

studies have been conducted using various optimization proce-

dures, GWVP technology is accepted as a relatively new technol-

ogy, as no standard design procedure is available.

Dhakal et al claimed that the design of the conical shaped vor-

tex basin performs better in terms of efficiency than the cylindri-

cal basin, in the same position of the turbine [7]. It is recom-

mended that the water flow from the inlet channel to the basin be

as tangential as possible because it causes fewer distortions and

prevents unnecessary losses [8][9]. The inlet design of most vor-

tex basins is either tangential or scroll with a flat bottom. The

scrolling inlet is more commonly used as it increases the dis-

charge gradually, spiraling along the basin towards the drain out-

let [10]. Thus, for a cylindrical-shaped basin, Choi et al suggested

a ratio for the optimal bottom outlet orifice diameter to basin di-

ameter in the range of 17-18.5% [11]. The vortex strength and

stability depend greatly on this diameter ratio, with the formation

of a central air core [12]. Smaller diameter ratios block the outlet,

thus preventing air core formation, while the ratios for larger di-

ameters release water without circulation. In both cases, it tends

to reduce the vortex strength [10].

The vortex turbine is closer to the impulse-type turbine than to

the reaction-type turbine because it does not work on the pressure

differential, but rather on the dynamic force of the water vortex

[2][13]. When the turbine is inserted into the vortex basin, the

vortex is disturbed by the turbine blades, thus decreasing the

tangential velocity component of the water while increasing the

axial velocity component [2][14]. Several studies have been con-

ducted to optimize the blade profile geometry. Dhakal et al [15]

demonstrated that a horizontal curved blade profile was the most

efficient. However, Saleem et al [14] reported low performance

for horizontal curved blades because the tangential velocity com-

ponent that was affected resolved into components, and only one

component contributed to the blade rotation. In fact, Bajracharya

et al [16] studied the effect of several blade parameters on turbine

performance in terms of the impact angle, blade angle in the ver-

tical and horizontal planes, taper angle, height, cutting of the

blade, and number of blades. The study is concluded by recom-

mending a ratio of 0.31-0.32 blade height to basin height, 20°

impact angle, and taper angle conforming to the basin cone angle

and blade at a horizontal angle of 50° to 60°.

The bottom-most position inside the vortex basin is accepted

as the best place to locate the turbine in a conical basin because

the vortex strength is at its highest at the drain orifice [13][17].

Dhakal et al [17] claimed a decline in the performance when the

number of blades in the turbine increased from six to 12; how-

ever, Christine Power et al [18] claimed heightened performance

when the number of blades was increased from two to four. The

number of blades or guide vanes is one of the most important

parameters in the design of hydro turbines [19]. The optimal

number of blades in a vortex type turbine depends on the strength

of the formed vortexes and several other factors, especially the

friction losses. Generally, in vortex phenomena, the air-core ra-

dius decreases gradually from the free surface to the bottom ori-

fice [20][21]. However, in the presence of the turbine, the air core

behavior becomes more complex. Furthermore, a larger turbine

hub diameter is not recommended because it tends to disturb the

air core formation and the vortex shape without gaining power

[14]. Finally, more complicated flow behavior occurs inside the

GWVP plant, influenced by many factors [12], especially the be-

havior of the air core and friction losses.

The GWVP plant in this study was designed based on the find-

ings of the studies cited above, while adapting them for a selected

installation site. As one of the objectives of the current study, the

CFD flow field was observed to identify the effect exerted by the

number of blades in the vortex turbine, while improving the con-

ical basin design provided with a small draft tube. Computational

fluid dynamics (CFD) was extensively used in this study to sim-

ulate and analyze the flow field in each case, with an assessment

of the performances.

Conical

basin

Scroll

guide

Generator

Gear ar-

rangement

Turbine

Draft tube

Figure 1: GWVP plant design

Effects of blade number and draft tube in gravitational water vortex power plant determined using computational fluid dynamics simulations

Journal of Advanced Marine Engineering and Technology, Vol. 45, No. 5, 2021. 10

254

In the first section of this article, the GWVP plant is introduced

with a brief literature review on past researches; in the second

section, the fundamental concept regarding the vortex theory and

water velocity behavior in the GWVP plant is explained. In the

third section, the methodology, computational design, and simu-

lation procedure are described. In Section four, the performance

curves for different numbers of blades and draft tube cases are

presented. Validation of the CFD setup is discussed, as this study

was based on CFD simulations. Finally, in Section five, the sim-

ulated flow fields are analyzed using the water-air interface, ve-

locity vector, and pressure contour. Hence, utilizing the flow phe-

nomenon, the variations in performance are justified and dis-

cussed. In Section six, the conclusions of the study are presented,

citing the findings of the current study along with suggestions for

improvements.

2. Vortex Phenomena and Vortex Turbine

The formation of the water vortex is explained by applying the

angular momentum theory [10]. A fluid particle with a small an-

gular velocity but with a large radial position produces a signifi-

cant angular momentum. When the particle moves toward the

center, it gradually reduces the radial position while increasing

the angular velocity to conserve the angular momentum. There-

fore, at the central axis, the water flow creates a significant swirl,

called the vortex. Vortex systems are normally discussed in terms of

cylindrical coordinates, where the center axis is laid along the z axis

and the velocity vectors 



, 



correspond to the radial () , tan-

gential () and axial () directions as shown in Figure 2.

By solving the Navier-Stokes equation in continuity and three

momentum equations with the assumption of steady, asymmetric,

and inviscid flows with axial derivatives as negligibly small, the

tangential momentum equation is simplified to the relationship

given below for the potential vortex shown in Equation 1.









(1)

where Γ is the circulation, defined as the line integral of the

tangential velocity component.

Therefore, in a vortex system, the tangential velocity compo-

nent dominates the flow field, where the radial and axial velocity

components are negligible. As the potential vortex model is not

suitable for describing the center of the vortex, Rankine proposed

a more reliable model, which was later used by many profession-

als for the development of reliable vortex models. In Figure 3,

the different vortex models followed by the Rankine and potential

vortex models are depicted. Furthermore, an air core was formed

at the center of the free-water vortex. The water vortex stability

depends on the formation of a continuous stable air core [10].

When the vertical-axis turbine is placed in the water vortex

field, the vortex is distorted by the turbine blades that extract the

vortex energy. The tangential velocity component of the vortex

contributes substantially to the power extraction, as it is the dom-

inant velocity component in the vortex. Initially, the vortex is

struck on the turbine blades, reducing its tangential velocity com-

ponents, while slightly increasing the axial and radial velocity

components. The vertical twisted blade profile of the turbine was

used to extract the excess energy from the axial velocity compo-

nent just prior to discharge.

As the energy is extracted chiefly from the tangential velocity

component of the water vortex, it is important to identify the

Figure 2: Cylindrical co-ordinate system used to de-

scribe the vortex phenomena































Potential vortex

Rankine vortex

Vatistas n=2 vortex

Scully vortex

Figure 3: Tangential velocity variation for different

vortex models

Min-Sung Kim ㆍ Dylan S. Edirisinghe ㆍ Ho-Seong Yang ㆍ S. D. G. S. P. Gunawardane ㆍ Young-Ho Lee

Journal of Advanced Marine Engineering and Technology, Vol. 45, No. 5, 2021. 10

255

effect of the number of blades. The number of blades in the vor-

tex turbine is strongly dependent on the strength of the vortex,

where the distance between the blades must be effective for re-

ceiving the power of the water flow onto the blades [5]. There-

fore, as one objective of this study, an optimal number of blades

was identified in the vortex turbine while observing the behavior

of the CFD flow. Another aspect of this study involved the anal-

ysis of the effect of different draft tube designs as an extension

of the modification of the vortex basin.

3. Design and Computational Modelling

3.1 Methodology

The design of the GWVP plant is highly dependent on recent

research findings and the site conditions, particularly the space

availability, flow, and hydraulic head. The design was developed

to recover the energy of wastewater discharged from a fish farm

located in the area called Namhae, on the southern coast of South

Korea. The average flow rate was recorded as 0.530 m

/s, and the

estimated hydraulic head was 2 m.

To compare the GWVP plant parameters, a three-dimensional

structural model was created for the vortex basin and turbine. Us-

ing the structural model, the fluid domains were extracted for

CFD simulations. As the CFD is based on finite element theory,

the fluid domains were meshed using ANSYS ICEM software.

Then, the fluid domains were set up in ANSYS CFX to define

the necessary boundary conditions. As this research is strongly

based on the CFD study, the validation process was initially per-

formed by comparing it with the experimental results of a recent

GWVP plant research, showing fair agreement.

In this study, two main cases related to the vortex turbine and

basin were analyzed. Under the effect of the number of blades,

four different cases were simulated having 5,6,8 and 10 blades.

Selecting the best turbine (with the most effective number of

blades) as the optimal turbine, the draft tube cases were further

studied. The effect of the draft tube was observed under four spe-

cific cases: no draft tube, straight draft, conical draft tube, and

height increased conical draft tube.

Hydraulic efficiency was used as the criterion to compare the

performance of the GWVP plant, which is denoted by the ratio

of the extracted power to the available hydraulic power, as ex-

pressed in Equation 2.

=









(2)

Generally, the available hydraulic power is calculated using

Equation 3,





= 

(3)

where ρ is the density of water (998 kg/m3), g is the gravita-

tional acceleration (9.81 m/s

), Q is the volume flow rate, and H

is the hydraulic head, which is defined as the difference between

the inflow and outflow water level but not related to the vortex

height. H was calculated using the CFD post-processing, as

shown in Figure 4.

Figure 4: Hydraulic head and the maximum vortex height inside

the designed vortex basin

Extracted power is calculated using the Equation 4.





= 

(4)

where τ is the torque on the turbine in Nm, which is monitored

during the CFD solving process. ω denotes the rotational speed

of the turbine in rad/s, which was predefined in the simulations.

3.2 Computer aided modeling

The width of the wastewater discharge channel is 1.3 m, at the

selected site. Considering it as the inflow channel to the GWVP

plant, this channel was made to converge to the vortex basin via

a scroll guide channel. Therefore, to facilitate the entry of water

into the conical-shaped part of the basin, the upper part of the

vortex basin was designed with a flat bottom. The maximum di-

ameter of the vortex basin was 3.6 m and the diameter of the bot-

tom discharge orifice was 0.6 m. Different draft tube designs

were then fitted to the drain orifice. Three types of draft tube

cases were observed: straight, conical, and increasing the conical

height, followed by no usage of the draft tube. In Figure 5, the

basic fluid model for the designed GWVP plant is illustrated, and

vortex

height

Hydrau-

lic head

Height of draft tube

Effects of blade number and draft tube in gravitational water vortex power plant determined using computational fluid dynamics simulations

Journal of Advanced Marine Engineering and Technology, Vol. 45, No. 5, 2021. 10

256

Figure 6 shows the difference between each of the simulated

draft tube designs. Although the estimated water height is 2 m,

the model was extended further, allowing it to adapt to the height

of the water level for different rotational speeds. An outlet chan-

nel was designed to observe discharge water flow after draining

out from the basin.

Figure 5: CAD model for designed vortex basin fluid domains

Figure 6: Different draft tube designs (a) no draft tube (b)

straight draft tube (c) conical draft tube- divergent angle 10

(d)

height increased conical draft tube

The basic vortex turbine model was designed to have five ver-

tical twisted blades matched with a conical shaped basin, having

0.7 m height. The twist angle was 30° between the top and bottom

blade profiles, where the twist was gradually increased from the

top to the bottom of the blade. In Figure 7, the basic turbine de-

noting the parameters of the blade profile is illustrated, whereas

in Figure 8 the 5,6,8 and 10 bladed turbines are shown. The tur-

bine hub diameter was maintained at 50 mm for every case, and

the gap between the inner wall of the basin and the blade tip was

maintained at 20 mm.

Figure 8: Different number of bladed turbines (b) five-bladed (c)

six-bladed (d) eight-bladed (e) ten-bladed

3.3 Computational fluid dynamic Simulation

The ANSYS 17.2 package was used to solve the CFD simula-

tions. The ANSYS ICEM software was used to create an unstruc-

tured hexagonal mesh for the fluid domains of the GWVP plant

provided with the necessary refinements, as shown in Figure 9.

Figure 9: (a) Hexagonal mesh of GWVP plant's fluid domains

(b) Mesh on the turbine blade surface (c) Mesh refinement near

blade wall and basin inner wall

Conical shaped

vortex basin

Turbine

Scroll guide

Pitch =0.95m

Revolution =0.76

1.3 m

0.9 m

1.15 m

3.6 m

1.5 m

0.6 m

(a)

Turbine

(b)

Turbine

0.2 m

(c)

Turbine

0.2 m

(d)

Turbine

0.4 m

(c)

(d)

(e)

(b)

30°

0.7 m

(a)

(b)

Mesh refinement

at free water surface

Figure 7: Blade profile of the vortex turbine (a) Side view (b)

top view

Min-Sung Kim ㆍ Dylan S. Edirisinghe ㆍ Ho-Seong Yang ㆍ S. D. G. S. P. Gunawardane ㆍ Young-Ho Lee

Journal of Advanced Marine Engineering and Technology, Vol. 45, No. 5, 2021. 10

257

Special attention was paid to the mesh refinement near the wall

to capture the boundary layer effect and the free water-air inter-

face to distinguish a clear interface. The O-grid concept was ap-

plied to mesh the turbine fluid to refine the mesh around the blade

surfaces. The first layer thickness around the blade wall was 0.01

mm resulting in a maximum y

of approximately 15 on the blade

surface, indicating the reliability of the mesh around the blade.

The total number of mesh elements was 2.25x10

where 1.1x10

was in the turbine domain.

CFD simulation was performed using the ANSYS CFX tool.

Steady-state simulations were conducted for all cases that were

solved using the Reynold Average Navier –Stokes (RANS) equa-

tions. Thus, a homogeneous multiphase model was introduced to

define air and water. Gravity was defined as 9.81 m/s

and a den-

sity different buoyancy model and a free surface model were se-

lected because the water and air can be separated, rather than hav-

ing a mixture model. The surface tension coefficient was intro-

duced as 0.072 N/m, indicating that water was the primary fluid.

The reference pressure was set as the atmospheric pressure

(1atm).

Only the turbine domain is defined as a rotational domain that

rotates at a given rotational speed, whereas the rest of the do-

mains are considered as stationary domains. The interface be-

tween the rotational and stationary domains is defined as the gen-

eral grid interface (GGI) connection. The 530 kg/s bulk mass

flow rate was defined as the inlet condition for the inflow chan-

nel, while the outlet was defined as having a constant water level

given by the appropriate hydraulic pressure. The top surface of

the basin was defined as an opening with the same atmospheric

pressure. The basin walls, turbine blades, and shafts indicate a

no-slip smooth wall.

Figure 10: Boundary conditions of CFD set up for GWVP plant

simulation

In Figure 10, the boundary conditions for the CFD set-up are

summarized. Based on the recommendations of several studies,

the shear stress transport (SST) turbulence model was introduced

with circular correction [10]. Thus, the SST model is highly rec-

ommended for turbomachinery simulations because it produces

a more realistic solution. In solver control, the maximum itera-

tion was set to 5000, and the residual target was fixed at 1 × 10

The torque on the turbine blades was monitored during the pro-

cess of solving the simulations.

4. Results

4.1 Validated study

The experimental results of the Yasuyuki Nishi and Terumi

Inagaki vortex turbines [2] were used to validate the ANSYS

CFX setup. In this particular study, a turbine with twenty blades,

was installed inside a cylindrical vortex basin. A maximum effi-

ciency of 35 % was recorded in this configuration, and they com-

pared the simulation and experimental performance, where the

experimental results were used to validate the current CFD setup.

The basic dimensions of the validated vortex turbine are listed in

Table 1.

Table 1: Basic dimensions of validated vortex turbine model

Parameter

Value

Unit

Inlet channel width

0.1

Vortex basin diameter

0.49

Turbine outer diameter

0.14

Turbine inner diameter

0.09

Turbine height

0.091

Number of blades

Figure 11: Simulated water flow behavior for validated model

In Figure 11, the water flow behavior for the validated model

is illustrated,whereas in Figure 12, a comparison of the

efficiency of the validated CFD study with the experimental

study is shown. The variations in the validated efficiency showed

fair agreement with the experimental study, showing a slight

deviation in the efficiency for only two rotational speeds. As the

Bulk mass flow inlet =0.53kg/s

Up tank pressure

Up tank water VF

/Up tank air VF

Opening

Down tank pressure

Down tank water VF /

Down tank air VF

Opening, Ref. pressure =0 Pa, Water VF =0 /Air VF

Turbine

Air core prop-

agation

Cylindrical

basin

Effects of blade number and draft tube in gravitational water vortex power plant determined using computational fluid dynamics simulations

Journal of Advanced Marine Engineering and Technology, Vol. 45, No. 5, 2021. 10

258

maximum deviation was below 5%, the overall results of the

validation were recognized as suitable for simulating a new

model of the GWVP plant.

Figure 12: Validated efficiency comparison with reference ex-

perimental study

4.1 Performance of modified GWVP plant

The CFD simulations reached the steady state condition after

2000 iterations, despite being run further, up to 5000 iterations.

The values of the residuals for the u, v, and w momentums con-

verged to below 10

-3

and the mass imbalance was less than 2%.

The monitor point of the torque varied around a constant at a

value of 0.4% bandwidth. The GWVP plant performance was

measured using hydraulic efficiency, calculated using CFD sim-

ulations. The efficiency graphs were followed by variations in the

torque and hydraulic head. The variations in the torque were con-

sidered to be related to the extracted power for a given rotational

speed. The hydraulic head is related to the available power for a

constant flow rate. The effect of the number of blades in the vor-

tex turbine and the effect of the draft tube were observed for dif-

ferent rotational speeds, and the efficiency curves were con-

structed, as shown in Figures 13 and 14.

In the first part of the research, an investigation was conducted

on the effect of the number of blades in the vortex turbine. In

Figure 13, variations in the torque and hydraulic head, as well as

the CFD efficiency for different numbers of blades in the vortex

turbine are illustrated. The initial turbine had five blades and it

yielded 54% maximum efficiency, between 40-45 rpm. When the

number of blades was increased from five to eight, the peak effi-

ciency of the GWVP plant also increased, resulting in the highest

efficiency of 57.2%. However, when the number of blades was

increased from eight to ten no significant difference was ob-

served, but there was a slight reduction at higher rotational

speeds. The main reason for the increase in the efficiency of the

turbines with a higher number of blades is the increase in the

torque, accompanied by a slight decrease in the hydraulic head,

as shown in Figure 10.

Figure 13: GWVP plant performance for turbine consist with

different number of blades (a) Torque variation (b) Hydraulic

head variation (c) CFD efficiency variation

Figure 14 illustrates the variations in the torque and hydraulic

head, as well as the CFD efficiency for different draft tube de-

signs. The draft tube effect can be clearly observed, as it tends to

increase the efficiency by approximately 3%. The effects of the

straight and conical draft tubes remain almost unchanged follow-

ing the common trend curve of performance. In addition, any in-

crease in the height of the conical draft tube did not affect

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

30 60 90 120 150 180

Efficiency

Rotational speed (rpm)

Reference experiment efficiency

Validated efficiency

700

900

1100

1300

25 30 35 40 45 50 55

Torque (Nm)

Rotational speed (rpm)

1.67

1.69

1.71

1.73

25 30 35 40 45 50 55

Hydraulic head (m)

Rotational speed (rpm)

25 30 35 40 45 50 55

Efficiency (%)

Rotational speed (rpm)

NOB5 NOB6 NOB8 NOB10

Five

bladed

Six

bladed

Eight

bladed

Ten

bladed

(a)

(b)

(c)

Min-Sung Kim ㆍ Dylan S. Edirisinghe ㆍ Ho-Seong Yang ㆍ S. D. G. S. P. Gunawardane ㆍ Young-Ho Lee

Journal of Advanced Marine Engineering and Technology, Vol. 45, No. 5, 2021. 10

259

Figure 14: GWVP plant performance for different draft tube de-

signs (a) Torque variation (b) Hydraulic head variation (c) CFD

efficiency variation

the performance. The addition of the draft tube to the vortex basin

exerts its effect as it increases the torque of the turbine. This is

because it helps to maintain the low-pressure region at the bot-

tom-back side of the turbine blade, increasing the pressure drop

across the blade. This phenomenon is explained in detail in the

Discussion section.

5. Discussion

The performance of each of the simulated GWVP plant cases

can be justified using flow analysis in the CFD post process.

Three main parameters related to the flow were analyzed: water-

air inter phase (at 0.5 water volume fraction), water superficial

velocity, and pressure contours.

In the case of turbines with different numbers of blades, the

performance was observed to increase when the number of

blades was increased from five to eight. When the blade number

was increased, the turbine exposed a large area to the vortex, ex-

tracting a relatively large quantity of energy. Therefore, the

torque of the turbine increased at an optimal rotational speed be-

tween 40 and 45 rpm. On the other hand, no significant change

in the performance was noted when the number of blades was

increased to ten, even though the effective area of the turbine in-

creased significantly. This is because the ten-blade turbines

would block the water outflow while avoiding air core formation.

In the presence of the vortex turbine, the air core is parted and

propagated behind the turbine blade maintaining the low-pres-

sure region, while discharging at the circumference of the drain

outlet. Therefore, without the air core, the low-pressure region

was not maintained, and hence, the pressure drop across the tur-

bine blade was reduced. In Figure 15, the propagation of the air

core behavior for turbines with different numbers of blades

shows that turbines with fewer blades tend to maintain a larger

air core, while a higher number of blades in the turbines produce

only small air cores.

Figure 15: Water and air behavior for different numbers of

bladed turbine (water-air interface is defined by 0.5 water volume

fraction) (a) five-bladed (b)six-bladed (c) eight-bladed (d) ten-

bladed

25 30 35 40 45 50 55

Efficiency (%)

Rotational speed (rpm)

DT0 DTS DTC DTC_H400

850

1050

1250

1450

25 30 35 40 45 50 55

Torque (Nm)

Rotational speed (rpm)

1.67

1.68

1.69

1.70

1.71

1.72

25 30 35 40 45 50 55

Hydraulic head (m)

Rotational speed (rpm)

draft

tube

Straight

draft

(a)

(b)

(c)

Conical

draft

Height

increased

draft tube

(a)

(b)

(c)

(d)

Effects of blade number and draft tube in gravitational water vortex power plant determined using computational fluid dynamics simulations

Journal of Advanced Marine Engineering and Technology, Vol. 45, No. 5, 2021. 10

260

Figure 16: Water superficial velocity contour and velocity vec-

tor on quarter-horizontal cross section of the turbine (red circle

area denotes the circulation) (a) five-bladed (b)six-bladed (c)

eight-bladed (d) ten-bladed

Figure 17: Pressure contour on blade surfaces for different num-

ber of bladed turbines (a) five-bladed (b)six-bladed (c) eight-

bladed (d) ten-bladed

The water superficial velocity vector fields, illustrated in Fig-

ure 16, show water circulation at the blade tip for fewer turbines

(denoted by red circles in Figure 16). This is because a large wa-

ter mass impacts on the turbine blade, resulting in splashing and

circulation. In the case of turbines with a large number of blades,

the water mass is divided among the blades and hence exerts less

impact on a single blade, resulting in less splashing of water.

Figure 17 illustrates the pressure contour on the turbine

blades. The high-pressure region can be observed at the outer

edge of the front surface of the blades, and the pressure decreases

gradually toward the center axis. A back-pressure region was also

visible at the upper tip area of the back surface of the blades. This

back pressure continues to propagate when the number of blades

is increased. Owing to the high blockage in the ten-blade

turbines, water is entrapped in a smaller space between the two

blades, while the rear blade pushes the entrapped water, thus

generating the back pressure. Hence, the performance does not

show significant improvement for the ten-blade turbine, although

the hit has the highest area of exposure.

Figure 18: Pressure contour on vertical cross-section plane

around draft tube (a) No draft tube (b)straight draft tube (c) con-

ical draft tube (d) height increased draft tube

Modifying the vortex basin by adding a small draft tube in-

creased the performance of the GWVP plant by approximately

3%. The main responsibility of the draft tube is to maintain a low-

pressure region at the bottom of the basin, while it gradually re-

covers. In Figure 18, the pressure contour in the vertical plane

around the draft tube is shown. In the case of no draft tube, the

low-pressure region at the circumference of the drain outlet is

recovered immediately by blocking the outlet using the down-

channel water or air. Hence, the vortex air core propagation was

reduced inside the basin, adversely affecting the performance.

The addition of a small draft tube tends to recover the pressure

gradually, maintaining the low-pressure region at the bottommost

position of the basin, while allowing the vortex air core to dis-

charge easily at the circumference of the drain outlet. On the

other hand, the low-pressure region affects the turbine bottom by

(a)

(b)

(c)

(d)

5.5

4.9

4.3

3.8

Water Superficial velocity [m/s]

3.2

2.6

2.0

1.5

0.8

0.3

(a)

(b)

(c)

(d)

High

pres-

sure

region

Back pres-

sure propa-

gation

Front side

of blade

Back side

of blade

7100

6226

5353

4479

3605

2732

1858

984

110

-763

Pressure [Pa]

8000

6842

5684

4526

3368

2210

1053

-105

-1263

-2421

Pressure [Pa]

(a)

(b)

(c)

(d)

Immediate

pressure

recovery

Gradual

pressure re-

covery

Low pressure

region

Min-Sung Kim ㆍ Dylan S. Edirisinghe ㆍ Ho-Seong Yang ㆍ S. D. G. S. P. Gunawardane ㆍ Young-Ho Lee

Journal of Advanced Marine Engineering and Technology, Vol. 45, No. 5, 2021. 10

261

supporting the maintenance of a pressure drop across the blade,

resulting in a higher torque than the no-draft tube case. The ef-

fects of the straight and conical draft tubes were similar, while

the flow diverged via the conical draft tube. The height of the

draft tube does not exert any significant effect on the perfor-

mance because the low-pressure region generated in the GWVP

plant is relatively low, and the pressure is very quickly recovered

other than the Francis or Kaplan turbines.

6. Conclusion

GWVP plant technology has gained popularity in recent times

as a micro-hydropower extraction method. Therefore, much re-

search continues, at present, involving the optimization of the

gravitational water vortex power extraction method, while devel-

oping a standard design procedure. This study was conducted to

observe the effect exerted by the number of blades in a vertical-

axis vortex turbine and the effect of the draft tube in the vortex

basin design. The study was based chiefly on CFD analysis, fol-

lowed by a validation model. The initial GWVP plant design was

performed for a selected installation site, adapting the recent re-

search findings.

Using the same blade profile, the turbines were modeled each

with 5,6,8 and ten blades. The eight-blade turbines showed good

performance for the design, yielding a hydraulic efficiency of

57 %. Thus, for the eight-blade turbines, the exposure area to the

vortex is optimal, while maintaining a stable air core propaga-

tion. Turbines with fewer blades caused water splash owing to

the impact of a massive water mass on a single blade. Turbines

with a greater number of blades tend to block water outflow, thus

propagating back pressure. Modifying the vortex basin by adding

a smaller draft tube enhanced the performance of the GWVP

plant, raising the hydraulic efficiency up to 60%. The draft tube

helps to recover the low-pressure region gradually from the drain

outlet of the basin to the down channel.

The research is supposed to continue by simulating a wide

range of flow rates. Hence, the effect of the number of blades in

the turbine can be clarified further while observing the CFD flow

field.

Acknowledgement

This work was supported by the Korea Institute of Energy

Technology Evaluation and Planning (KETEP) grant funded by

the Korean government (MOTIE) (20194210100170-

Demonstration of development of renewable energy convergence

system for fisheries).

Author Contributions

Conceptualization, Y. -H. Lee and S. D. G. S. P. Gunawardane;

Methodology, M. -S. Kim, D. S. Edirisinghe; Software, D. S.

Edirisinghe; Validation, D. S. Edirisinghe, H. -S. Yang; Formal

Analysis, M. -S. Kim; Investigation, M. -S. Kim, H. -S. Yang;

Writing—Original Draft Preparation, M. -S. Kim, D. S.

Edirisinghe; Writing—Review & Editing, S. D. G. S. P. Gun-

awardane; Supervision, Y. -H. Lee and S. D. G. S. P. Gunaward-

ane; Project Administration, H. -S. Yang.

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