numerical and experimental analysis of ......three radial discharge mono block centrifugal pumps...

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International Journal of Advanced Research in Engineering and Technology (IJARET) Volume 11, Issue 2, February 2020, pp. 186-199, Article ID: IJARET_11_02_018

Available online at http://www.iaeme.com/IJARET/issues.asp?JType=IJARET&VType=11&IType=2

ISSN Print: 0976-6480 and ISSN Online: 0976-6499

© IAEME Publication Scopus Indexed

NUMERICAL AND EXPERIMENTAL ANALYSIS

OF CENTRIFUGAL PUMP IN REVERSE MODE

OPERATION

Ajit Singh Aidhen

Research Scholar, Mechanical Engineering Department, Raffles University,

Neemrana, India

Sandeep Malik

Assistant Professor, Computer Science and Engineering Department, Raffles University

Neemrana, India

Chavan Dattatraya Kishanrao

Principal, Siddhant College of Engineering, Sudumbare, Pune, India

ABSTRACT

Hydropower has been the cheapest renewable energy source for years. Large

hydropower plants however require huge initial capital, relocation of population and

also cause changes to the aquatic habitat. High terrain remote villages where grid

supply is not economically feasible can be provided electric supply by installation of

standalone power generation plants. Pico hydro power generation is economical as it

does not require construction of large dams. Custom designed hydro turbines for a

particular site are definitely efficient but are expensive. In the past few decades

researchers have explored possibility of using a centrifugal pump in reverse mode as

hydro turbine. The challenge lies in correct selection of a pump to be used as hydro

turbine for a particular site. Characteristic curves for pumps to be operated in pump

mode are provided by manufacturers but the same are not provided by the

manufacturers for turbine mode operation of pumps. Researchers have proposed

performance prediction methods and correlations for predicting turbine mode

performance of the pumps but review of literature suggests scope of further research

for more accurate results. This paper presents numerical and experimental analysis of

radial discharge centrifugal pump operated in turbine mode. New correction factors

are proposed for performance prediction of pump as turbine.

Key words: Pump as Turbine (PAT), Pico hydro, Renewable energy

Ajit Singh Aidhen, Sandeep Malik and Chavan Dattatraya Kishanrao


Cite this Article: Ajit Singh Aidhen, Sandeep Malik and Chavan Dattatraya

Kishanrao, Numerical and Experimental Analysis of Centrifugal Pump in Reverse

Mode Operation, International Journal of Advanced Research in Engineering and

Technology (IJARET), 11 (2), 2020, pp 186-199.

http://www.iaeme.com/IJARET/issues.asp?JType=IJARET&VType=11&IType=2

1. INTRODUCTION

Electricity availability plays an important role in upgrading life standards for any community.

There are yet many rural communities which do not have access to electricity. The gird

supply of electricity to high terrain rural villages is not economically feasible. Compared with

other renewable energy sources such as wind, solar, energy from waste the cheapest and

simple technology is small scale run off river type hydro power generation. [1].There are

many benefits of electricity for rural communities some of which are presented in Table 1.1.

Table 1.1: Benefits of electricity for rural communities

(Source: https://www.ruralelec.org/benefits-clean-rural-electrification)

Energy utility Benefits

Lighting

Recharging and power for

communication devices Radio

repeaters

Receivers

Remote weather measuring

Transmission systems

Refrigeration

Water Pumps

Grinding, milling, husking

Increased education possibilities

Clinics and hospitals

Improved health conditions

Increased comfort

Improved connectivity and communication

Possibility of distance learning

Better social connectivity

Mobile banking possible

Better disaster management

Updates for agriculture and geological possible

Food storage possible with refrigerated units

Clean drinking water, minimizing risk of diseases

Produce refined oil from seeds

Create value-added product from raw agricultural

commodity

Numerical and Experimental Analysis of Centrifugal Pump in Reverse Mode Operation


The hydro power plants are classified as presented in Figure 1.1. As compared to large

scale hydro power projects in MHPs (Micro hydro power) the cost of electro mechanical

machinery may vary from 35% to 70%. [2]. The cost of electro mechanical machinery can be

reduced by using a centrifugal pump as turbine. [3–6].The present study involves

investigation of pump as turbine performance through numerical and experimental analysis.

Three radial discharge mono block centrifugal pumps with specific speed 35.89, 21.66, 19.26

(m, m3/s) were analyzed.

Figure 1.1. Classification of hydro power plants [7]

Many researchers in past two decades have proposed methods and correlations to predict

the turbine mode performance of the pumps based on pump mode B.E.P (Best efficiency

point) and pump specific speed. Table 1.2 and Table 1.3 summarize the proposed methods by

various researchers. It is however highlighted by researchers that better performance

prediction methods are required to improve the accuracy of prediction and to increase their

generality, the proposed existing relations produce results with an error range . [8]

Researchers as Stepanoff , Childs, Sharma , Hancock, Schmiedl, Alatorre-Frenk proposed

PAT performance prediction based on pump best efficiency point (BEP) whereas

Gopalakrishnan, Hergt , Diederich , and Grover [9-14] suggested relations based on pump

specific speed. However prediction through these theoretical methods has not been very

reliable and the results are seen with large deviation when compared to experimental results.

PATs (Pump as turbine) are advantageous over custom designed turbines as they are mass

produced, lower in cost, readily available, less complicated and involve low maintenance. [3-

6, 15-18].However the peak efficiency of PAT is reported to be less than a customized hydro

turbine. Also the part load performance of PAT is reported by other researchers to be poor.

PAT based Pico hydro standalone power generation still proves to be economical and

practical as it can harness energy which otherwise would go wasted. Correct selection of

pump-turbine, based on available flow and head at the site of installation is utmost important

to achieve best performance.

Table 1.2 Head and discharge correction factors based on specific speed [19]



Table 1.3 Head and discharge correction factors based on pump mode B.E.P [19]

Two main concerns with PAT technology as highlighted by most of the researchers are

selecting correct pump for a particular site and poor part load efficiency. The turbine mode

performance curves of the pumps are not provided by the pump manufacturers [20]. Many

researchers have attempted performance prediction of pump in turbine mode through

numerical and experimental analysis. The world of centrifugal pump being large, it is not

possible to either simulate or carry out experimental analysis of every pump. It necessitates

the development of performance prediction methods which are accurate and have generalized

application across various ranges of centrifugal pumps. The present study adds to the

available literature and presents prediction correction factors the results of which are better

than earlier prediction correction factors. Numerical analysis was carried out with STAR

CCM+ CFD software. The experiments were performed on a test rig as shown later in this

paper. The results of CFD and experimentation were compared and analyzed to derive at new

proposed correction factors. Table 1.4 provides information of the tested pumps.

Table 1.4 Information of the pumps analyzed

Parameter Pump – A Pump – B Pump – C

Rated Head 13 m 22 m 30 m

Discharge 7.7 lps 6 lps 7.4 lps

Rated Power 1.5/2 (kW/HP) 2.2/3 (kW/HP) 3.7/5 (kW/HP)

Speed 2840 rpm 2840 rpm 2870 rpm

BEP 64(%) 63(%) 60.37(%)

Specific speed 35.89 (m, m3/s) 21.66 (m, m

3/s) 19.26 (m, m

3/s)

2. NUMERICAL ANALYSIS OF PUMP AS TURBINE.

CFD has proved to be a great tool in understanding the complex three dimensional flow in a

turbo machinery. There are many commercially available CFD software today in market.

Large number of pumps all cannot be experimentally investigated and hence software’s as

CFD not only help in performance prediction but also reduce design time and cost. In this

study CFD modeling and simulations were performed by STAR CCM+ and the turbulence

model adopted was K-ℇ. Grid independence study was carried out to optimize the mesh.

Figure 1.2 shows grid independence with four different meshes. The simulations were first



performed for pump mode operation at rated R.P.M and the results were compared with

manufacturer provided data and experimentally determined results. The results of pump mode

CFD simulations were in good agreement with manufacturer provided data and experimental

results.

Figure 1.2 Grid independence with four meshes

The details of the pumps simulated are provided in Table 1.4. The simulations in turbine

mode were performed for four speeds. (Rated R.P.M, Rated R.P.M plus100, Rated R.P.M

minus 200, Rated R.P. minus 400). The boundary conditions for pump and turbine mode are

shown in the Figure 2.1.

Figure 2.1 Boundary conditions for (a) Pump mode (b) Turbine mode

The results of CFD simulations are not directly comparable to the experimental results

and need to be expressed in dimensionless form. The dimensionless parameters used to

represent head; discharge and power are expressed in equations (1-3). In carrying out

numerical analysis for performance prediction of pump in reverse mode it is assumed that the

entire discharge flows through the impeller passages, however the actual discharge due to

leakage loss is lesser and this necessitates that the measured discharge is corrected to the

actual discharge while performance is analyzed through experimentation. Numerical analysis

estimates only hydraulic power and do not include friction losses at shaft seal, bearings etc.,

whereas experimentally measured mechanical shaft power includes friction losses and so the

output measured power should be corrected to the hydraulic output power. Therefore to

compare dimensionless characteristics it requires that experimentation and CFD variables

should have a common basis.

Discharge number

(1)



Where

N = rotational speed (rps)

D = impeller diameter (m)

Head number

(2)

Where

N: rotational speed (rps)

D: impeller diameter (m)

g: acceleration due to gravity (m/s2)

H: Head (m)

Power number

(3)

Where

Po: output power (watts)

N: rotational speed (rps)

D: impeller diameter (m)

: density (kg/m3)

3. EXPERIMENTAL INVESTIGATION OF PUMP AS TURBINE

The features of experimental set up for pump and turbine mode performance are as seen in

Figure 3.1 and Figure 3.2.The service pump used in the experimental setup has 40% higher

head and 90% higher flow than the B.E.P head and flow of the tested pumps. In case of

further higher flow and head requirement the service pump motor is connected with a VFD

drive which allows for service pump R.P.M change. The details of instrumentation used in the

set up are provided in Table 3.1.

Figure 3.1 Pump mode experimental setup



Table 3.1 Details of test rig instrumentation

Instrument Type Accuracy

Flow meter EUMAG Electromagnetic flow meter + 0.5% of flow

Digital pressure gauge PG/DPG-20 Accuracy:+ 25% F.S

Rope brake Dynamometer 0-50 kg, digital display +/- 0.5 %

Speed Digital Tachometer Non Contact Photo Electric +/- 0.05%

Variable Frequency Drive Fuji VFD 10HP ±0.01% of max.

frequency

Multi-Function Meter Voltage 50 -500 VAC, Current 5/5A to

6000/5A, 0-9999kW Class 0.5

Figure 3.2 Turbine mode experimental setup

The experimentation was first conducted in pump mode to obtain the head, discharge,

power and efficiency curves. The pump was started after priming the suction side of the pump

to get rid of the air. The discharge side flow control valve initially was kept full open and

readings on pump suction pressure gauge, pump discharge pressure gauge, flow meter, volt

meter and ammeter are noted. The flow control valve was then throttled such that the flow

variation by the step of 1 lps was attained and after achieving steady state all readings were

noted. The procedure was repeated for the full flow range as indicated on the pump tally plate.

The experimental results were then compared with results of CFD to validate CFD model. In

turbine mode experimental investigation, the pump-turbine was started at no load and the flow

rate was adjusted for the desired R.P.M. The load on PAT was then gradually increased in

steps and the flow was increased to maintain the R.PM. Readings on turbine inlet pressure

gauge, turbine outlet pressure gauge, flow meter and brake dynamometer force were noted.

The mechanical power, hydraulic power and efficiency are expressed by equations (4), (5)

and (6) respectively.



( ) (4)

ω: angular velocity

τ: torque

N: speed in Rev/min.

The pressure difference across the PAT is denoted by H.

( ) (5)

Where

: is water density

g: gravitational acceleration

Q is flow rate and H is the pressure head.

The efficiency of PAT is expressed as:

(

)

(6)

4. RESULTS AND DISCUSSION

Experimental and numerical analysis results for turbine mode operation are compared and are

presented in the form of curves between head number vs discharge number, Power number vs

discharge number and efficiency vs discharge number as presented in Figure 4.1 and Figure

4.2. The curves are also plotted to compare pump and PAT mode performance as shown in

Figure 4.3 and Figure 4.4. The results of all three pumps are analyzed and theoretical

correction factors to predict head, discharge and efficiency were proposed. These correlations

are presented in equations (7), (8), (9).

(7)

Where,

h:Head correction factor

ηp :Pump mode efficiency at B.E.P

(

√ ) (8)

Where,

q:Discharge correction factor


(9)

Where,

: Turbine mode efficiency at B.E.P


The turbine mode head and flow can then be obtained by using equations (10) and (11)

h ⁄ (10)

q ⁄ (11)



Figure 4.1 Head number and Efficiency vs Flow number (Turbine mode)

Figure 4.2 Power number and Efficiency vs Flow number (Turbine mode)



Figure 4.3 Head number and Efficiency vs Flow number (Pump and Turbine mode)

Figure 4.4 Head number and Efficiency vs Flow number (Pump and Turbine mode)

Turbine mode experimentation and CFD results for one of the three tested pumps are seen

in Figure 4.1 and Figure 4.2.It was noted that the numerical analysis results predict higher

values of head, discharge and efficiency as compared to experimental results. Close to B.E.P

it is noted that the predicted CFD values are very close to those obtained through

experimentation, however the error % slightly increases away from B.E.P. In flow range

+0.2QBEP to -0.3QBEP the performance of PAT is noted with very less drop in efficiency

and can be predicted satisfactorily through numerical simulations. The error % is noted to be

more beyond the mentioned flow range. At B.E.P for all three tested PATs and for all four test

speeds the error % in general was noted to be less than 12%.Velocity color plots for one of the



tested pump in turbine mode are presented in Figure 4.5. It was noted that beyond the rated

head and flow the shock loss increase as evident at the vane tips and also overall friction

losses also increase.

Figure 4.5 Velocity color plot turbine mode

Pump mode and turbine mode performance curve for one out of the three pump analyzed

are presented in Figure 4.3 and Figure 4.4. The efficiency in turbine mode operation is noted

to be lower than the pump mode efficiency. The B.E.P in turbine mode is attained at higher

head and flow as compared to pump mode. Shock losses due to absence of flow control

device results in mismatch of flow direction and vane angle and cause loss of efficiency. If the

turbine is operated at rated flow these losses are minimum, but as flow increases they are

proportional to square of flow rate. Friction losses including disc friction and leakage losses

also account for drop in efficiency in turbine mode operation. However the drop in turbine

mode efficiency in flow range +0.2QBEP to -0.3QBEP is very low. The results of

experimental investigation and numerical analysis were in good agreement.

The correction factors to predict turbine mode head, discharge and efficiency were

developed based on the results obtained and are presented in equations (7), (8) and (9). The

correction factors were tested by applying them to 40 other pumps found in literature and are

compared with correlations provided by other researchers. Table 4.1 shows error percentage

in estimated values obtained through proposed correction factors and experimentally obtained

values. The proposed correction factors predicted head and discharge with errors less than

15% for 36 out of 43 pumps and 37 out of 43 pumps respectively. Sharma’s proposed

correction factors were found to be close however the proposed correction factors in equation

(7), (8) and (9) were found to give better results for most of the pumps considered. Figure 4.6

and Figure 4.7 present the error percentage in head and discharge prediction through

correlations by various researchers.

Table 4.1 Estimated and experimentally determined head and discharge comparison

Estimated

Head (m)

Experimentally

Determined Head

(m)

Estimated

Discharge

(m3/h)

Experimentally

Determined

Discharge (m3/h)

% Error in

estimated

Head value

% Error in

estimated Discharge

value

23.70 26.24 40.46 36.86 -10.71 8.91

40.02 41.76 30.56 26.64 - 4.33 12.83

66.19 69.76 31.15 33.12 -5.39 - 6.33



23.79 21.40 54.16 46.66 10.04 13.85

Figure 4.6 Error percentage in head predicted through correlations by various authors

Figure 4.7 Error percentage in discharge predicted through correlations by various authors

5. CONCLUSION

This paper presents results of numerical and experimental investigation carried out on three

centrifugal pump with specific speed number 35.89, 21.66, 19.26 (m, m3/s) and rated RPM

2840, 2840 and 2870 respectively. The results of experimental and numerical analysis

revealed that the pump as turbine operation close to rated head and discharge is smooth and

the efficiency attained is slightly lower than the pump mode best efficiency. Hence in Micro

and Pico scale PAT where the pumps used usually do not have flow control devices, the PAT

must be operated at rated head, flow speed and constant load. Correct selection of pump–

turbine for a particular site is very important and requires more accurate methods and

correlations to be developed for PAT performance prediction. The expressions proposed for

head, discharge and efficiency correction factors in this paper were tested by applying them to

40 other pumps found in literature. The results were also compared with results obtained by

applying correction factors proposed by other researchers. Produced results were noted to be



with less than 15% error for about 83% of the pumps considered. The proposed correction

factors presented in this paper obtained better results for most of the considered pumps. There

is scope for exploring better and more accurate correction factors which have more general

application to the wide range of pumps existing.

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numerical and experimental analysis of ......three radial discharge mono block centrifugal pumps...

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