New mode to operate centrifugal pump as impulse turbine

Accepted Manuscript New mode to operate centrifugal pump as impulse turbine K. Sengpanich, Erik L.J. Bohez, P. Thongkruer, K. Sakulphan PII:

S0960-1481(19)30428-8

DOI:

https://doi.org/10.1016/j.renene.2019.03.116

Reference:

RENE 11388

To appear in:

Renewable Energy

Received Date: 19 January 2018 Revised Date:

16 March 2019

Accepted Date: 25 March 2019

Please cite this article as: Sengpanich K, Bohez ELJ, Thongkruer P, Sakulphan K, New mode to operate centrifugal pump as impulse turbine, Renewable Energy (2019), doi: https://doi.org/10.1016/ j.renene.2019.03.116. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

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NEW MODE TO OPERATE CENTRIFUGAL PUMP AS IMPULSE TURBINE

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Nowadays large numbers of potential hydropower resources are still underutilized, such as existing medium and small irrigation dam, weir, and waterfalls. These potential sites, called “Micro hydropower”, have a power capacity less than 200 kW and require different hydro generating system compared to large hydro system to operate efficiently. Major obstruction for development of these Micro hydropower resources is high cost of hydro turbine system. For Micro hydropower project, the cost of electro-mechanical components will be varied between 35 – 40 % of total project cost [1]. One method to overcome this problem is using centrifugal pump operated in reverse operation mode called “Pump-as-Turbine” (PaT). PaT is the conventional pump which run in the reverse direction to normal pump [2]. The advantage for using PaT in Micro hydropower is mainly due to low cost since the number of centrifugal pumps produced in the world is much larger than the turbines [3]. Cost of PaT is 50% less than the cost of corresponding turbines [4]. PaT is available for various ranges of flow and heads, and it is available locally and abroad with a wide range of standard sizes, which make spare parts easily available [5] [6] [7]. PaT has been gradually developed from many researchers to improve both output performance and pump selection procedure [8] [9] [10] [11] [12]. However, main drawback for PaT when compared with purpose built turbine is the lack of flow regulation device. The absence of flow regulation device causes the problem of output power control where inflow condition change. This drawback, combined with the nature of pump which is designed for only one best efficiency point (BEP) condition, lead to rapid performance drop when operate at part-load condition [4]. There are some methods to correct this deficiency such as using multiple PaTs with different capacity connected to generator coaxially [13] or replace pump impeller with new forward-curved blades impeller [14] and insert guide vane inside PaT volute [15] but these proposed methods increase investment cost and complexity of PaT system. The new proposed concept is to inject the water as a jet by water injector into the pump under atmospheric pressure as done in the Pelton turbine, as shown in Figure 1. Turbine power adjustment in this mode is much easier and can be done by flow control through spear valves like the Pelton turbines. The benefit of a large operating range with good efficiency can be obtained since inlet velocity is constant in magnitude and direction, thus inlet velocity triangle can be kept over large range of flowrates. Turbine damage from cavitation is also eliminated. Since this concept is new and never explored by other researcher, theoretical study of this new concept will be adapted from study of performance of inward-flow jet wheel in [16] as shown in Figure 2. This paper will show the optimization by one dimensional calculation for pump in turbine mode with a broad operating range with high efficiency. Turbine performance validation will be performed by computational fluid dynamics (CFD) technique, which widely used to prove new concept of turbomachinery [17] [18].

K. Sengpanich, Erik L.J. Bohez , P. Thongkruer, K. Sakulphan Industrial System Engineering, School of Engineering and Technology, Asian Institute of Technology, Thailand

ABSTRACT

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Keywords: Pump as Turbine, Impulse Turbine, Computational Fluid Dynamics

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Centrifugal pumps can be used as “Pump-as-Turbine (PaT)” by reversing the flow and operating as a Francis turbine. The proposed concept “Impulse Pump-as-Turbine (Impulse PaT)” will use centrifugal pump impeller to be used as hydro turbine by pairing with spear valve injector from impulse hydro turbine. Spear valve injector regulate water inlet flow rate, thus regulate power output of our new concept turbine. Additional benefit from utilized spear valve injector is low loss of turbine efficiency when operate at part-load condition, therefore this new concept turbine can be operated at wide range of flow condition without losing good efficiency while also eliminate risk of turbine damage from cavitation. Flow regulation through spear valve will also simplify control system for this proposed turbine compared to guide vanes. The validation methods from 1-D calculation based on Euler’s turbine equation and numerical simulation by commercial CFD package shows that this new proposed concept is feasible with efficiency around 40% and wide operating range from 25% to maximum inlet flow rate. Results from numerical calculation shows that efficiency of this new concept of turbine is limited by number of blades in commercially available pump.

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1. Introduction

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Side View

Top View

Figure 1 Concept of new model for operate pump as impulse turbine

o

w2 α2

A

55

Water Injector

u2 w1

v1

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β2

v2

r1

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r2

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53 54

β1

α1 u1

Figure 2 Inward-flow jet wheel [16]

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2. Impulse PaT Optimization by 1-D calculation

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The Impulse PaT optimization by 1-D calculation is performed to find the maximum efficiency of pump operated as Impulse turbine based on Euler’s turbine equation. Assume that losses in pump and flow regulation device is neglected, the peripheral speed at the inlet of impeller of Impulse PAT will be half of velocity of water jet ( = 0.5 ). This is a heuristic rule adapted from Pelton turbine design. The optimization can be divided in 2 modes. First mode is reverse mode with rotating direction opposite to pump rotation direction as shown in Figure 3 (a). The second mode is normal mode with rotating direction the same as pump rotation direction as shown in Figure 3 (b). Reverse mode can be considered when the velocity of the water jet smaller than the peripheral speed of Impulse turbine at the inlet ( < ). Normal mode can be considered when the velocity of the water jet larger than the peripheral of Impulse turbine at the inlet ( > ). The subscribe 3 and 4 represent inlet and outlet of impeller when pump operated as Impulse PaT. The parameters blade inlet angle (β3 ), blade outlet angle (β4 ), impeller inner diameter (d4 ), impeller outer diameter (d3 ) and impeller inlet width ( ) will be obtained from impeller geometries while Head () depended on site condition. Turbine rotational speed () will be varied for Impulse PaT performance optimization. The maximum inlet flow rate (Q) has to be considered since diameter of inflow water jet (  ) should not larger than impeller width ( ) to prevent power loss due to non-impact waterjet. Maximum inlet flow rate can be calculated from;

73 74 75 76 77

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Q=

"#$ %&.'($ )* +

(1)

Absolute velocity at inlet ( ) will be equal to velocity of water jet and calculated from;  = ,2-

(2)

The performance of Impulse PaT can be calculated by using Euler’s turbine equation as follow; .=

#$* /#0* #$*

× 100 %

(3)

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2

ACCEPTED MANUSCRIPT (a) α3 v 3

w3

v4

β3

w4

α4

β4

N

d3

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d4

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u4

u3

v3r

79

(b)

w3 β3

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v3r

v3

α3

u3

Turbine

u4

82 83 84 85 86 87 88 89 90

v4

Pump

d4

d3

Figure 3 Vector Diagram of Impulse PaT in (a) reverse mode (b) normal mode Power of Impulse PaT will be calculated from;

EP

81

< = =>

#$* /#0* ?

(4)

Absolute velocity at impeller outlet (+ ) can be calculated by solving quadratic equation of cosine law from vector diagram at impeller outlet. The reverse mode will use equation (5) and the normal mode use equation (6).

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β4

N

v4r

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w4

α4

+ = ,@+? + +? − 2@+ + CDE F+

(5)

+ = ,@+? + +? + 2@+ + CDE F+

(6)

Relative velocity at impeller outlet (@+ ) can be calculated by using formula for inward-flow jet wheel; @+ = ,@? − %? − +? )

(7)

Impeller peripheral velocity at inlet ( ) will be calculated from; "G$ H

91

 =

92

Impeller peripheral velocity at outlet (+ ) will be calculated in the same way as follow;

93

+ =

I&

"G0 H I&

(8)

(9)

3

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The relative velocity at impeller inlet (@ ) can be calculated by solving quadratic equation of cosine law from vector diagram at impeller inlet. The reverse mode will use equation 10 and the normal mode use equation 11.

97

@ =  CDE F ± ,% CDE F )? + ? − ?

(10)

98

@ = − CDE F ± ,%− CDE F )? + ? − ?

(11)

The angle between peripheral velocity and absolute velocity at impeller inlet (K ) can be calculated from vector diagram as;

102

(12)

(13)

The angle between peripheral velocity and absolute velocity at impeller outlet (K+ ) can be calculated from vector diagram as; K+ = ELM/N O

#0P #0

Q

(14)

When absolute velocity in radial direction at impeller outlet (+R ) can be calculated from; +R = @+ ELM F+

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Q

R = @ ELM F

106 107

#$

When absolute velocity in radial direction at impeller inlet (R ) can be calculated from;

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#$P

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K = ELM/N O

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99 100

(15)

These equations assumed that inflow jet contact impeller with angle K . However, if the jet is parallel to the impeller as show in Figure 4, we can consider this as approximately a liner cascade and we find that the force on the blade by the momentum law is;

v3

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112

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u3

Figure 4 Vector diagram of jet parallel to impeller in reverse mode

116

When

117

Then

118 119 120 121

S=

G%TU #) GV

=

GTU GV

% −  )

(16)

S ∗  = XD@YZ = [U%  − ? ) \]^_`R

=0

\a

 =

(17)

#$

(18)

?

From equation 18, Impeller peripheral velocity at inlet which gives the maximum power is half of absolute velocity at inlet. The maximum power that can obtain is;
#$ ?

−

#$* +

Q = [U O

#$* ?

−

#$* +

Q = [U

#$* +

(19)

And maximum efficiency will be;

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122

TU 0

*

e TU $

= 50%

(20)

*

The optimization by varies turbine rotational speed () was performed by using MatLab. The pump impeller used in this optimization was selected from centrifugal pump with parameters as shown in Table 1 and performance of this pump is shown in Figure 5. Maximum inlet flow rate of 0.2 kg/s has been calculated from equation (1). Optimization result show that, for the reverse mode, impeller outlet absolute velocity + is higher than inlet absolute velocity  , for example at turbine rotational speed of 1,500 RPM  is 14.007 m/s while + is 17.390 m/s. This value will only happen when operate in pump mode since velocity of waterjet can be increased after pass through impeller by adding power into system. So Impulse PaT in reverse mode cannot produce realistic result. Thereby, this mode is not satisfied.

131 132 133 134 135 136 137 138 139 140 141 142 143

For the normal mode, performance of Impulse PaT can be derived in the rotational speed range from 0 to 1,550 RPM. At rotational speed higher than 1550 RPM, the results from calculating is not realistic because from equation (7), peripheral inlet velocity  and peripheral outlet velocity + values are increase relative to rotational speed, in contrast to relative velocity at impeller inlet @ value which decrease, resulting in complex number for relative velocity at the outlet @+ . From Figure 6, inlet peripheral velocity  is nearly half of velocity of water jet  at rotational speed of 1,500 RPM, confirming our heuristic. From Figure 7, angle between peripheral velocity and absolute velocity at impeller inlet and outlet decrease when rotational speed increase. Angle K decreases linearly while angle K+ decreases in a more curved pattern which rapidly decrease when rotational speed is over 1,200 RPM. The calculation has shown that angle K is 14.8 degree at 1,550 RPM and 15.39 degree at 1,500 RPM, while angle K+ is 28.01 degree at 1,550 RPM and 34.71 degree at 1,500 RPM. The optimization has shown that efficiency will be increased when rotational speed was increased and increase in quadratic pattern. The maximum efficiency of Impulse PaT in this option is 90.11% at maximum rotational speed of 1,550 RPM and 85.76% at 1,500 RPM as shown in Figure 8.

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Table 1 Main parameters of centrifugal pump Parameter Pump design flow rate Pump design head

Impeller blade inlet angle Impeller blade outlet angle Impeller outer diameter

Unit

Value

[g]

(liter/s)

2.4

[h]

(m.)

10

[ij ]

(RPM)

3,000

[kl ]

(deg)

30

[km ]

(deg)

60

[nl ]

(mm.)

90

[nm ]

(mm.)

33.2

[ol ]

(mm.)

5.34

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Impeller inlet width

EP

Impeller inner diameter

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Pump nominal operation speed

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123 124 125 126 127 128 129 130

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Head (m)

15.15 10.15 5.15

200 100

Efficiency (%)

0

65.00%

15.00% 0

145

0.5

1

1.5

2

2.5

3

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Flow Rate (l/s)

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Power (Watt)

0.15 300

3.5

Figure 5 Centrifugal pump performance curve for pump mode

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200

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Velocity (m/s)

146

400

600

800

1000

1200

ql

pl qm rm 1400

1600

1800

Rotational Speed (RPM)

147

Figure 6 Velocity components of Impulse PaT when increasing rotational speed

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EP

70 60

α (degree)

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50 40

sm

30 20

sl

10 0 0

149 150

200

400

600

800

1000

1200

1400

1600

1800

Rotational Speed (RPM) Figure 7 Impact Angle of Impulse PaT when increase rotational speed

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0

200

400

600

800

1000

1200

Rotational Speed (RPM)

1400

1600

1800

SC

151

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Efficiency (%)

Power (Watt.)

18 16 14 12 10 8 6 4 2 0 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%

Figure 8 Power and Efficiency of Impulse PaT when increase rotational speed

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3. Impulse PaT Optimization by numerical simulation

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A 3D model of the impulse PaT was generated by CFturbo® software. ANSYS CFX®-14 was used for the numerical simulation since this program has been widely implemented for impulse turbine CFD simulation [14] [19] [20] [21] [22]. Based on mentioned literatures of free surface flow simulation, the multiphase methods used was homogeneous two phases of fluid consisted of ideal water at 25 °C and Air. Air-water interaction has been considered as water droplet in the air. Water surface tension is the major interface force. The advection scheme was set to high resolution, which uses a fourth-order numerical model and blend to second-order near pressure extrema. The convergence criteria were a residual target of 1 × 10/+ .

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Turbulence model was using k-ω SST turbulence model. The sketch of computational domains is shown in Figure 9 (a). Domain computational fluid was split into two component parts; injector and impeller as shown in Figure 9 (b). This separation allows each component to generate mesh individually. The injector domain was set in stationary frame and impeller domain was set in rotary frame. The interfaces between rotary and stationary components was set to general grid and frozen rotor interface. The boundary condition detail for each location is shown in Table 2.

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Jet

168

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Impeller

(a)

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Figure 9 (a) sketch of computational domain (b) CFD domains and boundary conditions

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(b)

Table 2 Boundary conditions for CFX ® Location Injector inlet

Boundary Inlet

Detail Mass and momentum: bulk mass flow rate Value: 0.2 kg/s

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Injector surfaces and impeller surfaces

Wall

Opening

Impeller outlet and opening

Mass and momentum: no slip wall Wall roughness: smooth wall Mass and momentum: opening pressure and direction. Value: 0 Pa.

Unstructured tetrahedral mesh was implemented due complex curvature of model. Mesh independence study was performed in order to evaluate the discretization error at turbine rotational speed of 1,500 RPM. The densest mesh simulated contained around 9.6 × 10I elements (Mesh 5). Table 3 shows the error of turbine efficiency difference related to mesh density when compared with efficiency from the densest mesh (Mesh 5). The smallest difference error was at -1.87% at meshes around 5.2 × 10I elements (Mesh 3). This mesh size has been selected for simulate turbine efficiency in this study.

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Table 3 Turbine efficiency error vs mesh element Mesh 2 2431538 556135 2987673 4.54%

Mesh 3 4497551 718177 5215728 -1.87%

Mesh 4 6025879 1998031 8023910 -3.24%

Mesh 5 8330063 1302996 9633059 0%

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The optimization by numerical simulation was performed at constant flow rate of 0.2 kg/s and head at 10 meters, jet width (  ) can be calculated as 4.27 mm. Theoretical power output of turbine will be 19.67 watts. Average turbine torque results from numerical simulation will be used to calculated actual turbine power output. The normal mode of Impulse PaT operation will perform numerical simulation with jet impact angle (K ) calculated by 1-D calculation with value of 25.17, 22.75, 20.32, 17.86 and 15.39 degree with turbine rotational speed of 500, 750, 1,000, 1,250 and 1,500 RPM respectively. The results from numerical simulation will be compared with results from 1-D calculation. Figure 10 shows the results from 1-D calculation and numerical calculation. The results from numerical calculation complied to results from 1-D calculation at turbine rotational speed lower than 500 RPM. At rotational speed over 500 RPM, the error between results from 1-D calculation and numerical calculation increased proportion to rotational speed that increased. The effect of jet impact angle to performance of Impulse PaT also investigated by numerical calculation at varying impact angle from 15.39 to 45 degrees for turbine rotational speed of 750 and 1,500 RPM. Figure 11 shows the comparison results of turbine performance at turbine rotational speed of 750 and 1,500 RPM. The result has shown that the jet impact angle which has maximum efficiency point for turbine rotational speed of 750 RPM was 30 degrees for 32.82% efficiency, and 38.5 degrees for 38.29% efficiency at rotational speed of 1,500 RPM. The numerical calculation result has shown that turbine maximum efficiency depends on good combination of rotational speed and jet impact angle when inlet jet velocity is constant. 100.00%

60.00%

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Efficiency (%)

80.00%

EP

180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199

Mesh 1 594520 501516 1096036 26.54%

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Item Impeller domain Injector domain Whole domain Eff. diff. error

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172 173 174 175 176 177 178

1D

40.00%

CFD

20.00% 0.00%

400

200 201

600

800

1,000

1,200

1,400

1,600

Rotational Speed (RPM) Figure 10 Comparison between results from 1-D calculation and numerical

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50.00%

Efficiency (%)

40.00%

N1500 30.00%

N750

20.00% 10.00% 0.00% 25

30

35

sl (degree)

40

45

50

Figure 11 Effect of jet impact angle to Impulse PaT efficiency

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To investigate relation between water jet and output torque of Impulse PaT, instantaneous torque at each rotation angle is compared with flow visualization at jet impact angle of 30 and 38.5 degrees for turbine rotational speed of 750 and 1,500 RPM, respectively. These angles yield the highest efficiency for each turbine rotational speed. From graph in Figure 12 ,which compare output torque at each rotation angle of Impulse PaT between turbine speed of 750 RPM and 1,500 RPM, has shown that output torque of Impulse PaT behave periodically in every 60 degrees of rotation angle v, which conform to angle between impeller blades. During this period, maximum torque has been observed at rotation angle of 0 and 60 degrees, which is the point that water jet impact at impeller tip. Between this interval, value of second highest output torque is approximately same as average output torque of turbine. In this cases, output torque value of 0.0832 N.m. at rotation angle of 26.87 degrees is closed to average torque value of 0.0820 N.m. for turbine speed of 750 RPM, while output torque value of 0.0498 N.m. at rotation angle of 38.5 degrees is also nearly the same as average torque value of 0.0478 N.m. for turbine speed of 1,500 RPM. Meanwhile, minimum output torque between each impeller blade has occurred at nearly the same rotation angle for both turbine speed of 750 RPM and 1,500 RPM at around 51 degrees, with another minimum output torque at rotation angle 12.54 and 26.87 degrees for turbine speed of 750 and 1,500 RPM, respectively. From Figure 14, which show the flow visualization for iso-surface of water volume fraction at 0.5 with jet impact angle at 30 degrees for turbine speed of 750 RPM, has shown that after water jet impact tip of impeller blade at 0 degrees rotation angle (maximum output torque), water jet impact point will gradually inside impeller blade, which reduced torque radius and output torque value. Output torque will reduce to minimum point at rotation angle of 12.54 degrees and rebound due to the fact that minimum torque radius has been reached. Increase in torque value reach the second highest point at rotation angle of 26.87 degrees and start to decrease due to impact force reduction when distance from injector to impact point is more than 10 times of jet diameter. This decline will reach the minimum torque value at rotation angle around 51 degrees and sharply increase due to water jet start to impact another following impeller blade. The same trend in instantaneous output torque of impeller blade rotate at speed of 1,500 RPM as shown in Figure 15, except between turbine rotation angle of 12 to 25 degrees, which water jet does not impact with impeller blade result in rapid reduction in torque value. Flow visualization in Figure 15 also shows that some portion of water that hit Impulse PaT impeller at rotational speed of 1,500 RPM does not pass through outlet side, this portion of water instead bounce of turbine blade at inlet side by the effect of rotation (pumping back effect). This phenomenon does not occur when observe at 750 RPM turbine rotational speed.

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20

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15

202

0.1000

A30-N750

Torque (N.m)

0.0800

A38.5-N1500

0.0600 0.0400 0.0200 0.0000

0

235 236

10

20

30

40

50

60

70

w (degree) Figure 12 Instantaneous torque at each rotation angle of Impulse PaT

9

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The performance of Impulse PaT when operate at part-load condition has been investigated by numerical calculation at flow rate between 25% to 100% of inlet flow at 0.2 kg/s while inlet pressure is constant and jet impact angle is 38.5 degrees. Turbine rotational speed at 1,500 RPM was applied. Figure 13 has shown result from numerical calculation compared with result from 1-D calculation. The result has shown that Impulse PaT can maintain constant efficiency for wide range of inlet flow quantity. Efficiency around 40% can be achieved from quarter to maximum flow rate. The results from CFD also show that, although error between CFD result and 1-D calculation result is still high, trend of both results is agreeable. 20.15

10.15

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Power (Watt)

1-D 15.15

CFD

5.15 0.15

100.00% 90.00% 80.00% 70.00% 60.00% 50.00% 40.00% 30.00% 20.00%

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1-D

0.00

0.20

0.30

0.40

0.50

0.60

0.70

0.80

CFD 0.90

1.00

1.10

EP

TE D

Q/Qmax Figure 13 Power and Efficiency of Impulse PaT when operate at part-load condition

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245 246

0.10

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Efficiency (%)

244

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v = 0°

v = 30°

AC C

EP

v = 26.87°

v = 20°

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v = 10°

v = 40°

247

v = 50°

Figure 14 Iso-surface shown water volume fraction at 0.5 for 1 injector Impulse PaT rotate at 750 RPM

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v = 0°

v = 38.5°

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v = 30°

v = 20°

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v = 10°

v = 40°

248

v = 50°

Figure 15 Iso-surface shown water volume fraction at 0.5 for 1 injector Impulse PaT rotate at 1,500 RPM

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The error of maximum efficiency point between numerical calculation and 1-D calculation for rotational speed of 750 and 1,500 RPM is 22.92% and 69.69%, respectively. This phenomenon occurred because from Figure 7, angle between peripheral velocity and absolute velocity at impeller inlet or jet impact angle (K ) decrease when increase rotational speed. Low impact angle will decrease jet impact duration with current impeller blade when impeller rotation angle (v) has increased. This situation will cause water jet to leave from current blade before impact to succeeding blade as shown in Figure 16. The discontinuation of power transfer cause lower power output of turbine since water jet has shorter time to contact with each impeller blade and less energy to transfer. Comparison between result from CFD and 1-D calculation in Figure 10 confirm this phenomenon since efficiency of impulse PaT rotating at 1,500 RPM will be maximum at impact angle of 38.5 degrees instead of 15.39 degrees which calculated from 1-D calculation.

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259 260

Figure 16 Situation that water jet leave current blade before impact to succeed blade

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θ

Table 4 Summary of Impulse PaT performance for inflow type Impulse PaT mode

Numerical Efficiency

 2

80%

30 – 40%

 ≈ 2

80%

30 – 40%

 ≅

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Normal

Reverse

Theoretical Efficiency

Optimal pl

Parallel Normal

 ≈

 2

50%

20%

Parallel Reverse

 ≈

 2

50%

45%

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4. Conclusion & Discussion

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In this study, a new concept of using injector as flow control device for centrifugal Pump as Turbine called “Impulse Pump as Turbine” was investigated by 1-D calculation and numerical simulation. The results from both methods confirm that this new propose concept was feasible. To investigate the concept, an existing centrifugal pump was selected, 1-D calculation and computational fluid dynamic (CFD) simulation was done to investigate the maximum efficiency of turbine at inlet flow rate of 0.2 kg/s and head of 10 meters was 38.29% at rotational speed of 1,500 RPM. Impulse PaT can maintain good efficiency at wide operation range from 25% to maximum flow rate in the same manner of impulse hydro turbine. The difference in the 1-D calculation and CFD is due to the fact the in the 1-D calculation we assume the jet to be tangent to the blade tip. But this happens only a fraction of time during one revolution. If the number of blades would be very large the 1-D calculation and CFD would be much closer but there would be more loss through friction. Although difference of results from 1-D calculation and numerical simulation is high, further investigate in turbine operation behavior and loss associated with this new concept of turbine will improve the accuracy of 1-D calculation. Operation mode of Impulse PaT that is investigated in this research is only inflow type normal mode as shown in Table 4. Results from numerical simulation in Figure 12 has shown that further study in position of waterjet impact with turbine impeller in radial direction (inward impact point) should be performed. Other turbine operation mode should also be investigated by numerical simulation. Furthermore, waterjet injected from inner section of turbine impeller should be studied because it could reduce water backflow from impeller that reduce efficiency of recent studied turbine. Number and cross section shape of waterjet should be further studied in order to improve efficiency of this new turbine concept. Further research in turbine sizing to site required flow rate and head, which also include centrifugal pump selection criteria for using as Impulse PaT. Relationship between turbine required head (V ) and centrifugal pump discharge pressure (~ ) should also studied. Multiple jets distributed along the impeller could also improve the operation.

293 294 295 296 297 298 299

This research is funded by Department of Research Fund and Technology Development, Provincial Electricity Authority (PEA), Thailand. The authors would like to thank the Department of Industrial Systems Engineering, School of Engineering and Technology, Asian Institute of Technology to provide invaluable support for this project.

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6. References

[4] [5]

[6]

[7] [8] [9] [10] [11] [12] [13]

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[3]

International Course on Small Hydropower Development, AHEC, IITR (2004). A. Tarang, Review of pump as turbine (PAT) for micro-hydropower, Int J Emerg Technol Adv Eng 2 (11) (2012) 163–169. K. Kanzumba, A survey of innovative technologies increasing the viability of micro-hydropower as a cost effective rural electrification option in South Africa, Renew. Sustain Energy 37 (2014) 370-379. G. Ranjitkar, J.X. Huang, T. Tung, Application of micro-hydropower technology for remote regions, Journal: 2006 IEEE EIC Climate Change Conference (2006) 1-10. B.H. Teuteberg, Design of a pump-as-turbine for an abalone farm, final report for mechanical report 878, Department of Mechanical and Mechatronic Engineering, Stellenbosh University, Stellenbosch, South Africa, March 2006. A.A. Williams, Pump as turbine for low cost micro-hydro power, in: Proceeding of the world renewable energy congress renewable energy (WREC), energy efficiency and the environment, 1996, 1227-1234. K.H. Motwania, S.V. Jain, R.N. Patel, Cost analysis of pump as turbine for pico hydropower plants - a case study, Proc Eng 51 (2013) 721-726. X. Su, S. Huang, X. Zhang, S. Yang, Numerical Research on unsteady flow rate characteristics of pump as turbine, Renew. Energy 94 (2016) 488-495. M. Arriaga, Pump as turbines-a pico-hydro alternative in Lao People's democratic republic, Renew. Energy 35 (2010) 1109-1115. Y. Hao, L. Tan, Symmetrical and unsymmetrical tip clearances on cavitation performance and radial force of a mixed flow pump as turbine at pump mode, Renew. Energy 127 (2018) 368-376. Y. Liu, L. Tan, Tip clearance on pressure fluctuation intensity and vortex characteristic of a mixed flow pump as turbine at pump mode, Renew. Energy 129 (2018) 606-615. M. Liu, L. Tan, S. Cao, Theoretical model of energy performance prediction and BEP determination for centrifugal pump as turbine, Energy 172 (2019) 712-732. P. Singh, Optimization of internal hydraulics and of system design for pumps as turbines with field implementation and evaluation, Ph.D. Thesis, Karlsruhe University, 2005.

AC C

[1] [2]

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[16] [17] [18] [19]

[20]

[21] [22]

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T. Wang, F. Kong, B. Xia, Y. Bai, C. Wang, The method for determining blade inlet angle of special impeller using in turbine mode of centrifugal pump as turbine, Renew. Energy 109 (2017) 518-528. V.A. Patel, S.V. Jain, K.H. Motwani, R.N. Patel, Numerical optimization of guide vanes and reducer in pump running in turbine mode, Proc. Eng. 51 (2013) 797-802. J. N. LeConte, Hydraulics, New York: McGRAW-HILL Book, 1926. L. Tan, SL. Cao, YM. Wang, BS. Zhu, Direct and inverse iterative method for centrifugal pump impellers, J. of Power and Energy 226:6 (2012) 764-775. Y. Xu, L. Tan, SL. Cao, WS. Qu, Multiparameter and multiobjective optimization design of centrifugal pump based on orthogonal method, J. of Mechanical Engineering Science 231:14 (2016) 2569-2579. D. Benzon, A. Zidonis, A. Panagiotopoulos, G.A. Aggidis, J.S. Anagnostopoulos, D.E. Papantonis, Numerical Investigation of the Spear Valve Configuration on the Performance of Pelton and Turgo Turbine Injectors and Runners, J. of Fluid Engineering 137 (2015) 111201-1 to 8. C. Zeng, Y. Xiao, W. Xu, T. Wu, J. Zhang, Z. Wang, Y. Luo, Numerical Analysis of Pelton Nozzle Jet Flow Behavior Considering Elbow Pipe, 28th IAHR symposium on Hydraulic Machinery and System (IAHR2016), IOP Conf. Series: Earth and Environmental Science 49 (2016). H. Jeon, J.H. Park, Y. Shin, M. Choi, Friction loss and energy recovery of a Pelton turbine for different spear positions, Renew. Energy 123 (2018) 273-280. C. Zeng, Y. Xiao, Y. Luo, J. Zhang, Z. Wang, H. Fan, S.H. Ahn, Hydraulic performance prediction of a prototype four-nozzle Pelton turbine by entire flow path simulation, Renew. Energy 125 (2018) 270282.

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ACCEPTED MANUSCRIPT Research Highlights

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New method of using Pump-as-Turbine (PaT) in impulse turbine mode. One-dimensional calculation to prove feasibility and performance optimization of PaT in Impulse mode. Performance validation by using Computational Fluid Dynamics (CFD) method. Broad turbine operation range with high efficiency compared with conventional PaT.

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