flow simulation (cfd) & fatigue analysis (fea) of a centrifugal pump

7/30/2019 Flow Simulation (Cfd) & Fatigue Analysis (Fea) of a Centrifugal Pump

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 6340(Print), ISSN 0976 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) IAEME

67

FLOW SIMULATION (CFD) & FATIGUE ANALYSIS (FEA) OF A

CENTRIFUGAL PUMP

Manish Dadhich1, Dharmendra Hariyani

2and Tarun Singh

3

1(Department of Mechanical Engineering, SKIT, Jaipur, RTU Kota, India

Email: [email protected])2

(Department of Mechanical Engineering, SKIT, Jaipur RTU Kota, IndiaEmail: [email protected])

3 (Department of Mechanical Engineering, SKIT, Jaipur RTU Kota, India

Email:[email protected])

ABSTRACT

In the present work the CFD analysis of a centrifugal pump is done using k- turbulent modelingand SIMPLEC algorithm. In this work the mass flow rate is varied three times for two different

fluids that are water and fuel oil. Firstly the mass flow rate is decreased by 10% from the initial

value of mass flow rate and then increased by 10% from the initial value of mass flow rate.

Using the results of varying mass flow rate the efficiency at different mass flow rate is

calculated. The results are compared with the theoretical results. Then the theoretical andsoftware results are plotted in the operating characteristics curves and are verified. The deviation

produced among them nearly 10% to 15 % is permissible. Secondly the number of RPM hasbeen changed by decreasing and increasing by 10% and results are compared with the

experimental results and verified through operating characteristics curves. After the complete

flow analysis the maximum pressure acting on the blades for the maximum efficiency in case ofwater and fuel oil is calculated and the fatigue (FEA) analysis of a centrifugal pump is done.

Analysis is linear static structural and the model used is Gerber zero based. From fatigue analysis

we can determine whether the pump is safe to run in the operating conditions by plotting various

curves of the fatigue analysis and design can be improved.

Key words: CFD, Fatigue, k- turbulent model and SIMPLEC algorithm.

1. INTRODUCTION

Turbo machinery in mechanical engineering describes machines that transfer energy between a

rotor and a fluid, machines are governed by the same basic relationships including Newton's

second Law of Motion and Euler's energy equation [3] for fluids. Centrifugal pump are also

turbo machines that transfer energy from a rotor to a fluid usually a liquid. Pump has very wide

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ISSN 0976 6340 (Print)

ISSN 0976 6359 (Online)

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range of turbo machinery applications; it plays an important role in various sectors of the

industries. Many centrifugal pumps types have been constructed e.g. centrifugal slurry pump

[20] and are used in different applications in industry and other technical sectors.However, their design and performance prediction process is still a difficult task, mainly due to

the great number of free geometric parameters, the effect of which cannot be directly evaluated.

For this reason CFD analysis is currently being used in the design and construction stage ofvarious pump types. Using CFD analysis the internal flow conditions of the pump can be

predicted and can be analyzed. From the CFD analysis software and advanced post processing

tools the complex flow inside the impeller can be analyzed.FEA analysis helps us to know thefatigue life of the pump.

Numerical simulation of centrifugal pumps is not easy due to the usual CFD difficulties:

turbulence, separation, boundary layer [1] etc. Although there are also specific problems:

Complex geometry, Energy transfer is generated mainly by the centrifugal force in the impelleretc.CFD and FEA has probed to be a very useful tools in the analysis of these turbo machines,

both in design and performance prediction. Many researches have been done on the flow analysis

of the centrifugal pump.

The purpose of the present study is to show the flow simulation (CFD) & fatigue analysis (FEA)of a centrifugal pump using the commercial softwares like Catia (V5R20) and ANSYS 12. From

the past researches it was seen that only the CFD analysis of the pump was done. In the presentwork the CFD and FEA analysis were coupled to carry out the fatigue analysis of the centrifugal

pump. In the CFD analysis the mass flow rate and number of RPM [2] were varied for the flow

analysis and results obtained from the CFD analysis were implemented to carry out fatigue(FEA) analysis. From the fatigue (FEA) analysis we can determine whether the pump is safe torun on the operating conditions if not further design can be improved for the long life of the

pump.

In the CFD analysis results were compared with the experimental results. The results calculatedfrom the CFD analysis were implemented to carry out the FEA analysis. In the CFD analysis two

working fluid water and fuel oil were taken and the comparative study was also done. BasicallyCFD involves the numerical solution of the equations like Navier Stokes equation [20], energy

equation etc.

2. MODELLING AND MESHING OF PUMP

To study the numerical analysis on the pump, the dimension data of the pump was required to

generate a model in the software. The assembly consist the casing and impeller. The modelling

was done in two different softwares Catia and ANSYS 12. This was because Catia is modellingsoftware and has sophisticated modelling tools that helped in modelling the complex geometries

[12]. Apart from the modelling tools the software has features that helped in creating cross-

sections at different locations which helped in better visualization and understanding of theactual pump geometry and ANSYS 12 software helps us in designing the complex geometry like

impeller and other parts of the turbo machinery.

2.1 Dimensions of the pump1. Head [4] on which the pump is working =

15m

2. Inlet diameter of impeller = 170mm

3. Outlet diameter of impeller = 280mm


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4. No. of RPM on which pump is running =

1150

5. Inlet blade angle = 16[12]

6. Outlet blade angle = 27[12]

7. Width of impeller at inlet = 84mm

8. Width of impeller at outlet = 45mm

9. Shaft power of the pump = 25 kW

10. Number of blades (Vanes) on impeller =

6

11. Blade (Vanes) thickness= 8mm

12. Blade height = 47 mm

Fig. 2.1 model of impeller

Fig. 2.2 assembly model of pump components

Fig. 2.3 final assembly model of pumpcomponents


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Fig. 2.4 meshing of pump assembly (tetrahedral)

Meshing of the pump assembly is done by using ANSYS 12. Meshing of the pump consist oftetrahedral elements [9]. Meshing consists of following numeric values:

Number of elements=1033878

Number of nodes=19858

3. COMPUTATIONAL EVALUATION OF PUMP PERFORMANCE

3.1 Flow Simulation of PumpAfter meshing of the model of pump assembly commercial CFD code FLUENT [13] is used for

simulation of the pump performance .The boundary conditions of mass flow rate given at pump

inlet. The performance results are obtained at different mass flow rate conditions with constant

operating rotational speed and second case is by varying the operating rotational speed. In boththe above mentioned cases the operating fluid is changed from water to fuel oil. Numerical

(Software) performance results compared with the theoretical results at the same operating

conditions [16].3.1.1 Assumptions

The following assumptions were taken for simulation:

The walls of the casing were assumed to be smooth hence any disturbances in flow due toroughness of the surface were neglected and pump is vibration free [10].

The friction co-efficient for all surfaces were set to 0, hence friction between the wallsand fluid was neglected and wall boundary with no slip boundary condition [19].

Steady state conditions and incompressible fluid flow [15].3.1.2 Solution parameters

3-D double precision solver used to solve for simulation [17]. Multiple reference frame [8] technique used to simulate the pump performance. Clear water [11] and fuel oil are taken as working fluid. Standard K-Epsilon [6] simulation model is used for turbulence modelling. Convergence criteria [5] for continuity, velocity and turbulence parameter was set

to10.


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Second order scheme is used for pressure correction as well as for solving momentum,turbulent kinetic energy and turbulence dissipation rate [18].

SIMPLEC [7] scheme is used for pressure velocity coupling.3.2 Streamline and Vector Plot of Pump in Case of Water

Fig. 3.1 streamline plot of pump in case of water Fig. 3.2 vector plot of pump in case of water

3.3 Velocity and Pressure Distribution of Pump in Case of Water (When changing the mass

flow rate)

3.3.1 Velocity and pressure distribution of pump at mass flow rate 130.9 kg/s (initial)

Fig. 3.3 velocity distribution at mass flow

rate=130.9 kg/s

Fig. 3.4 pressure distribution at mass flow

rate=130.9kg/s


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3.3.2 Velocity and pressure distribution of pump at mass flow rate 117.8 kg/s (10% decrease

from initial)

Fig. 3.5 velocity distribution at mass flowrate=117.8kg/s

Fig. 3.6 pressure distribution at mass flowrate=117.8kg/s

3.3.3 Velocity and pressure distribution of pump at mass flow rate 143.9 kg/s (10% increase

from initial)


rate=143.9kg/s


rate=143.9kg/s


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3.4 Velocity and Pressure Distribution of Pump in Case of Water (Changing the RPM of pump)

3.4.1 Velocity and pressure distribution of pump at 1150 RPM (initial)

Fig. 3.9 velocity distribution at 1150 RPM Fig. 3.10 pressure distribution at 1150 RPM

3.4.2 Velocity and pressure distribution of pump at 1035 RPM (10% decrease)



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3.4.3 Velocity and pressure distribution of pump at 1265 RPM (10% increase)


3.5 Performance Characteristic of Pump using Water

The performance characteristic of the centrifugal pump has been predicted experimentally and

through software handling water. Efficiency, discharge, mass flow rate and number of RPMcharacteristics of the pump are predicted by CFD analysis [14]. The numerical simulation on

pump performance handling water by varying the mass flow rate and number of RPM three

times and the efficiency and discharge are calculated by using turbulent modelling namely as K-

Epsilon. The parameters namely efficiency and discharge rate are tabulated for a constant headpump of 15m.

Table 3.1 Result of pump performance byexperimental and simulation at different

mass flow rate (kg/s)

Table 3.2 Result of pump performance byexperimental and simulation at different

number of RPM (N)

From the above mentioned theoretical and software results we see that there is only slight variation nearly

about 1% to 2% which is permissible up to 10%. So we conclude that our results are satisfactory.


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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 6340(Print), ISSN0976 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) IAEME

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3.6 Characteristic Curves of the Pump using Water

Fig. 3.15 efficiency vs. discharge curve for the

case of water

Fig. 3.16 discharge vs. number of RPM curve for

the case of waterFrom the characteristics curves we see that there is only slight variation between theoretical and software

results and characteristics curves are verified which means that simulation through software is correct.

3.7 Streamline and Vector Plot of Pump in Case of Fuel Oil

Fig. 3.17 streamline plot of pump in case of fuel oil Fig. 3.18 vector plot of pump in case of fuel oil

3.8 Velocity and Pressure Distribution of Pump in Case of Fuel Oil (When changing the mass flow rate)

3.8.1 Velocity and pressure distribution of pump at mass flow rate 125.6 kg/s (initial)


rate=125.6kg/s


rate=125.6kg/s


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3.8.2 Velocity and pressure distribution of pump at mass flow rate 113.0 kg/s (10% decrease from initial)


rate=113.0kg/s


rate=113.0kg/s

3.8.3 Velocity and pressure distribution of pump at mass flow rate 138.2 kg/s (10% increase from initial)


rate=138.2kg/s


rate=138.2kg/s

3.9 Performance Characteristic of Pump using Fuel Oil

The performance characteristic of the centrifugal pump has been predicted experimentally and through

software handling fuel oil. Efficiency, discharge and mass flow rate characteristics of the pump are

predicted by CFD analysis. The numerical simulation on pump performance handling fuel oil by varyingthe mass flow rate three times and the efficiency is calculated by using turbulent modelling namely as K-

Epsilon. The parameter namely efficiency is tabulated for a constant head pump of 15m.The results for thenumber of RPM is same as case of water.


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Table 3.3 Result of pump performance by

experimental and simulation at different mass

flow rate (kg/s)

Fig. 3.25 efficiency vs. discharge curve for the

case of fuel oil

3.10 Characteristic Curves of the Pump using Fuel Oil

From the above mentioned theoretical and software results we see that there is only slight variation nearly

about 1% to 2% which is permissible up to 10%. So we conclude that our results are satisfactory. From theabove mentioned characteristics curves we see that there is only slight variation between the theoretical and

software results and characteristics curves are also verified which means that simulation through software

is correct. But when the comparison is made between the water and fuel oil we see that is case of fuel oilthe efficiency is decreased this happen because of the decrease of the density. The discharge vs. RPM curve

remains the same in case of fuel oil because there is no impact of the density in the calculations.

3.11 Structural Simulation of Pump (Impeller and Blades)After the completion of the flow analysis the structural analysis of the pump is being done. Structural

analysis includes the fatigue testing of the impeller of the pump. The fatigue testing on the impeller of the

pump is done by applying the pressure on the impeller. The pressure applied on the impeller is calculatedfrom the pressure contour of the maximum efficiency in case of water and fuel oil respectively. The

pressure applied is somewhat less than that of the maximum pressure coming in the contour.

3.11.1 Assumption

Number of RPM is constant that is 1150. Linear analysis is being done. Static structure analysis is being done. The model taken is Gerber zero based

3.12 Meshing of the Impeller of the Pump

Meshing of the impeller of the pump is being done by using ANSYS workbench software by applying

various meshing techniques. The meshing of the impeller is shown below in the figure.


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Fig. 3.26 structural meshing of the impeller of the pump

Number of elements=46382

Number of nodes=167114

3.13 Structural Simulation in Case of WaterFrom the contour plot of pressure of maximum efficiency we calculate the pressure just below the

maximum pressure which is acting on the impeller and we apply that pressure in the structural analysis and

we get the contour of total deformation, equivalent stress, fatigue life, fatigue damage and fatigue factor ofsafety and then we analyse the results for the fatigue failure of the impeller of the pump. The pressure

acting in the case of water is 78798.4Pa

3.13.1 Total Deformation and Equivalent Stress in Case of Water

Fig. 3.27 total deformation of impeller in case of

water

Fig. 3.28 equivalent stress of impeller in case of

water


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3.13.2 Fatigue analysis results of impeller in case of water

Fig. 3.29 fatigue life of impeller in case of water Fig. 3.30 fatigue damage of impeller in case of water

Fig. 3.31 fatigue safety factor of impeller in case of water

From the above three fatigue analysis diagram we observe that when a pressure of 78798.4Pa is applied on

the impeller the impeller is safe to use and it will run through maximum life cycle (material cycle) withoutany failure. The fatigue life plot says that if the applied pressure is of constant amplitude type then from the

results from life represents the number of life cycles with the structure can withstand until it will fail due to

fatigue so from the above plot we can see that the structure will withstand up till maximum cycles. Damage

is defined as the design life divide by available life. Fatigue damage shows that no damage is there whenthe impeller is working under this operating pressure and number of RPM. Fatigue safety factor is a

contour plot of the factor of safety with respect to fatigue failure at given design life.From the above figure it is shown that factor of safety is 15.Generally the factor of safety is taken as 3 to 5

and figure shows 15 which means impeller is safe to use at particular operating conditions.

3.14 Structural Simulation in Case of Fuel Oil

The pressure acting in the case of fuel oil is 74305.8Pa


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3.14.1 Total Deformation and Equivalent Stress in Case of fuel oil

Fig. 3.32 total deformation of impeller in case of

fuel oil

Fig. 3.33 equivalent stress of impeller in case of

fuel oil

3.14.2 Fatigue analysis results of impeller in case of fuel oil

Fig. 3.34 fatigue life of impeller in case of fuel oil Fig. 3.35 fatigue damage of impeller in case of

fuel oil

Fig. 3.36 fatigue safety factor of impeller in case of fuel oil


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From the above three figure of fatigue analysis we observe that these figure resemble with the

figure of the water. So we can say that pump is safe to use in case of fuel oil under the operating

conditions of applied pressure and number of RPM.3.15 Curves of Total Deformation and Equivalent Stress

Fig. 3.37 curve of total deformation acting on hub(water and fuel oil)

Fig. 3.38 curve of total deformation acting onblade tip (water and fuel oil)

Fig. 3.39 curve of equivalent stress acting on

hub (water and fuel oil)

Fig. 3.40 curve of equivalent stress acting on

blade tip (water and fuel oil)

From the above graphs of total deformation and equivalent stress acting on the hub and blade tip

a comparison of the results of the water and fuel oil is being done and from the above graphs we

can observe that only a slight variation is there between water and fuel oil in total deformationand equivalent stress acting on the hub and blade tip respectively. In both the cases structure issafe to use.

4. CONCLUSION AND FUTURE WORK

The geometry of pump (impeller and casing) is modelled using ANSYS 12 and Catia. The mesh

is generated successfully using ANSYS 12. Complex internal flow field, pressure and velocity


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distribution investigated using FLUENT commercial computational code. The simulation results

are obtained at different operating mass flow rates and three different number of RPM for

transportation of clear water and fuel oil. The simulation was performed by using turbulentmodel k-Epsilon and SIMPLEC algorithm. Software performance results at different conditions

are compared with the theoretical results. Characteristics curves were plotted using software

results and theoretical results.Results obtained were satisfying the characteristics curves and there was only a slight variation

among the software and theoretical results nearly 1% to 2% for both the cases of water and fuel

oil. But when we compare the efficiency of water and fuel oil we see that is case of fuel oil theefficiency is decreased this happen because of the decrease of the density of the fuel oil. In the

structural analysis the fatigue analysis of the pump was done by applying the pressure on the

pump. The analysis was linear static structural and the model used was Gerber zero based. The

pressure was calculated from the pressure contour of the flow analysis.From the results of the fatigue analysis of the pump for both the cases i.e. water and fuel oil the

conclusions were that pump was safe to run on the operating conditions without any damage or

distortion. The factor of safety of the pump was also acceptable. Certain graphs of total

deformation and equivalent stress acting on the hub and blade tip were plotted and thecomparison of the results of the water and fuel oil was being done. From the graphs it was

observed that only a slight variation was there between water and fuel oil in total deformationand equivalent stress acting on the hub and blade tip respectively. In both the cases structure was

safe to use.

Future Scope

1. Pressure and velocity distribution can be calculated for a centrifugal slurry pump bymultiphase modelling.

2. Losses can be considered for the computational simulation of centrifugal pump.3.

Computational simulation models can also be used for analyzing the pressure, velocityand stress distribution of the other turbo machines like turbines, compressor, fan andblower.

4. Modal analysis of the centrifugal pump can also be done for the vibration analysis.REFERENCES[1] Blanco-Marigorta, Eduardo and Fernndez-Francos, Numerical Flow Simulation in a CentrifugalPump with Impeller -Volute Interaction, ASME 2000 Fluids Engineering Division Summer Meeting,

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Centrifugal Slurry Pump, Journal of fluid Engineering, February 2002.

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[14] Lamloumi Hedi, Kanfoudi Hatem and Zgolli Ridha, Numerical Flow Simulation in a CentrifugalPump,International Renewable Energy Congress, November 2010.

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flow simulation (cfd) & fatigue analysis (fea) of a centrifugal pump

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