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30120140505001 2

  1. 1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME 1 EFFECT OF DIFFUSER LENGTH ON PERFORMANCE CHARACTERISTICS OF ELBOW DRAFT TUBE WITH DIVIDING PIER Rahul Bajaj1 , Dr. Ruchi Khare2 , Dr. Vishnu prasad3 M.Tech. Student1 , Asst. Professor2 , Professor3 Department of Civil Engineering M.A. National Institute of Technology, Bhopal (M.P.) ABSTRACT The hydraulic turbines extract the energy of flowing water and converts into mechanical energy. The reaction turbine has components namely casing, stay ring, guide vane, runner and draft tube. Each component plays some role in performance of turbine. Out of above component casing, stay ring and distributor guide the flow while in runner and draft tube energy transfer and conversion takes place. In reaction turbine, significant part of input energy goes out of runner unutilized in form of kinetic energy. Draft tube are provided at exit of runner to connect turbine and tail race providing closed conduit flow of varying cross sectional area. The development in the design of turbines leads to different shape of draft tube to recover as much energy as possible. The elbow draft tube is mostly used with large reaction turbine. It is found that geometry of draft tube, flow space affects its performance to large extent but due to limitation of experimental investigation, the mesh were limited to few shapes. The growth on computational power has made it possible to investigate the many alternative design of draft tube. In the present paper, numerical simulation has been done for a large elbow draft tube with pier in diffuser. The length of diffuser has been changed to see its effect on the performance i.e. head and efficiency of draft tube. Keywords: Draft Tube, CFD, Diffuser, Pier, Kinetic Energy, Efficiency. INTRODUCTION Draft tube is an important component of hydraulic reaction turbine and improves the performance of turbine by converting the kinetic energy entering in draft tube from runner, into pressure energy. [1] In axial flow reaction turbine, the kinetic energy of water leaving the runner is INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME: www.iaeme.com/ijmet.asp Journal Impact Factor (2014): 7.5377 (Calculated by GISI) www.jifactor.com IJMET © I A E M E
  2. 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 ISSN 0976 – 6359(Online), Volume 5, Issue up to 50 % of total input energy where total energy. In beginning of 19th century, the hydrodynamic investigation was carried by F. Frankl and A.V. Milovich on straight draft tube and the shape of draft tube on the basis investigations were carried out on straight diffuser between 1909 and to design various shapes of draft tube and small diameters owing to the diameter of runner D large it is irrational to construct. To overcome these problems, recover kinetic energy of axial and rotational large capacity mixed flow hydraulic turbine, bell mouth runner diameter leads increase in dimensions and weight were overcome by elbow draft tube which is suit when diffuser length exceeds 10 to 12 m, the dividing pier is provided distribution and consequently improves the performance of draft tube and power characteristics of the turbine.[2] In hydraulic turbine, determination of optimum shape and dimension as well as prediction of flow behavior is very difficult task. To overcome these problems the numerical simulation become more informative tool.[3] At the best operating minimum whirl and radial velocity significant whirl velocity. Since from more than a decade’s Computational Fluid Dynamics (CFD) tool and widely used by researcher for predicting performance and flow behavior in a domain and optimization of design of hydraulic turbine components time consuming and to overcome this minimized laboratory testing of models.[4] GEOMETRY AND MESH GENERATION The elbow draft tube has three parts namely cone, elbow and of thickness b= 0.25 D1 are used to improve the performance of turbine. of elbow draft tube are shown in fig. 1. The modeling of elbow draft tube has been done in ICEM CFD for 3 length L/ D Model of elbow draft tube for h/D1= 2.3 and L/ mm diameter at inlet and the analysis is done for L = 5 * D length. Fig. 1: Geometric parameter of International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 e 5, Issue 5, May (2014), pp. 01-12 © IAEME 2 0 % of total input energy whereas in case of mixed flow reaction turbine it century, the hydrodynamic investigation was carried by F. Frankl and A.V. Milovich on straight draft tube and they determine the solution to the problem of determining the shape of draft tube on the basis of theoretical investigation. Therefore large traight diffuser between 1909 and 1929 and emphasized draft tubes. The use of straight tubes was restricted to turbine of medium the diameter of runner D1 increases, the length of the tube becomes so large it is irrational to construct. To overcome these problems, bell mouth tubes were invented and kinetic energy of axial and rotational component of flow. With the rapid deve large capacity mixed flow hydraulic turbine, bell mouth draft tube was also absolute because large dimensions and weight of draft tube. Later on all these problems overcome by elbow draft tube which is suitable for large diameter hydraulic turbine. length exceeds 10 to 12 m, the dividing pier is provided which give consequently improves the performance of draft tube and power characteristics of , determination of optimum shape and dimension as well as prediction of flow behavior is very difficult task. To overcome these problems the numerical simulation At the best operating condition the flow enters in a draft tube but at off design condition, the flow enters with Since from more than a decade’s Computational Fluid Dynamics (CFD) widely used by researcher for predicting performance and flow behavior in a domain and ion of design of hydraulic turbine components. Conventional model testing is costly and to overcome this, CFD is an alternative tool which is a cost effective and minimized laboratory testing of models.[4] GEOMETRY AND MESH GENERATION three parts namely cone, elbow and diffuser. In large draft tube piers are used to improve the performance of turbine. The geometric dimensions draft tube are shown in fig. 1. The modeling of elbow draft tube has been done in ICEM CFD for 3 length L/ D = 2.3 and L/ D1=15 is shown in fig.2. The draft tube is having 250 mm diameter at inlet and the analysis is done for L = 5 * D1, L = 10 * D1 and L = 15 * D parameter of elbow draft tube with dividing pier [2] International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), is up to 15 % of century, the hydrodynamic investigation was carried by F. Frankl and they determine the solution to the problem of determining large numbers of and emphasized the need tubes was restricted to turbine of medium , the length of the tube becomes so bell mouth tubes were invented and flow. With the rapid development of draft tube was also absolute because large Later on all these problems able for large diameter hydraulic turbine. In case gives the better flow consequently improves the performance of draft tube and power characteristics of , determination of optimum shape and dimension as well as prediction of flow behavior is very difficult task. To overcome these problems the numerical simulation has enters mostly axially the flow enters with Since from more than a decade’s Computational Fluid Dynamics (CFD) is most effective widely used by researcher for predicting performance and flow behavior in a domain and . Conventional model testing is costly and n alternative tool which is a cost effective and . In large draft tube piers The geometric dimensions The modeling of elbow draft tube has been done in ICEM CFD for 3 length L/ D1 ratio [2]. is shown in fig.2. The draft tube is having 250 and L = 15 * D1 diffuser [2]
  3. 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME 3 Fig. 2: Isometric view of elbow draft tube In three dimensional CFD simulations, the fluid domain can be discretized by tetrahedral or hexahedral mesh. Unstructured mesh which mainly consists of tetrahedral elements while structured mesh consists of hexahedral elements.[5] Ansys CFX uses finite volume method (FVM) for the discretized domain and N-s equations is solved at every node of the cell. In the present work, meshing of draft tube is done in Ansys ICEM CFD as shown in fig. 3. For this flow domain, unstructured three dimensional tetrahedral meshing has been adopted. Fig.3: Mesh of elbow draft tube BOUNDARY CONDITIONS The numerical flow simulation is carried out for 13 whirl component varying from 3.0 m/s to -3.0 m/s at an interval of 0.5 m/s at 3 different lengths of diffuser i.e. L = 5 * D1, L = 10 * D1 and L = 15 * D1. Cylindrical velocity component at inlet of draft tube is specified as axial component = -15.41 m/s, radial component = 0 m/s and 13 whirl component at an interval of 0.5 m/s as inlet boundary condition. Average static pressure is specified at outlet of draft tube as outlet boundary condition. All the walls in the geometry is assumed as smooth and no slip. Shear Stress Transport (SST) K-w turbulence model is used with wall function as automatic in Ansys CFX code. Convergence criteria are set to be 10-6 as RMS value and 500 as maximum iteration.
  4. 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME 4 COMPUTATION OF PARAMETARS The following formulae are used to assess performance of draft tube using the velocity and pressure distribution from numerical simulation. Head loss in draft tube 03 05( ) LD P P H γ − = Head recovery in draft tube 2 2 03 05( ) 2 RD LD V V H H g − = − Draft tube efficiency 2 03 2 100RD D gH V η = × Relative head loss 2 03 2 100LD RLD gH H V = × RESULTS AND DISCUSSIONS The 3D flow analysis has been carried out in elbow draft tube with three L / D1 ratios of 5, 10 and 15 for 13 whirl component varying from 3.0 m/s to -3.0 m/s at an interval of 0.5 m/s for constant n1’=135.6 rpm. Fig. 4 shows the various sections at which pressure and velocity contours are taken. Fig. 4: Pressure contour for different section (L=5D1)
  5. 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME 5 Fig. 5.(a): Pressure contour at elbow section for L= 5 D1 and theta component= -1 m/s Fig. 5.(b): Pressure contour at elbow section for L= 10 D1 and theta component= -1 m/s Fig. 5.(c): Pressure contour at elbow section for L= 15 D1 and theta component= -1 m/s
  6. 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME 6 Fig. 6.(a): Velocity contour at elbow section for L= 5 D1 and theta component= -1 m/s Fig. 6.(b): Velocity contour at elbow section for L= 10 D1 and theta component= -1 m/s Fig. 6.(c): Velocity contour at elbow section for L= 15 D1 and theta component= -1 m/s
  7. 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME 7 It is seen in figure 5.(a),(b) and (c) that maximum pressure variation is obtained at outer side of elbow and in fig. 6. (a),(b) and (c) that at the periphery, velocity is zero and velocity is increases as moves from outer side to inner side of elbow. Fig. 7.(a): Pressure contour at inlet of diffuser for L= 5 D1 and theta component= -1 m/s Fig. 7.(b): Pressure contour at inlet of diffuser for L= 10 D1 and theta component= -1 m/s Fig. 7.(c): Pressure contour at inlet of diffuser for L= 15 D1 and theta component= -1 m/s Fig. 8.(a): Velocity contour at inlet of diffuser for L= 5 D1 and theta component= -1 m/s Fig. 8.(b): Velocity contour at inlet of diffuser for L= 10 D1 and theta component= -1 m/s
  8. 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME 8 Fig. 8.(c): Velocity contour at inlet of diffuser for L= 15 D1 and theta component= -1 m/s From fig 7.(a),(b) and (c), it is seen that the maximum pressure zone occurs at divider and in fig 8.(a),(b) and (c) minimum velocity zone is occuer at divider and periphery of diffuser. Fig. 9.(a): Pressure contour at diffuser for L= 5 D1 and theta component= -1 m/s Fig. 9.(b): Pressure contour at diffuser outlet for L= 10 D1 and theta component= -1 m/s Fig. 9.(c): Pressure contour at diffuser outlet for L= 15 D1 and theta component= -1 m/s Fig. 10.(a): Velocity contour at diffuser outlet for L= 5 D1 and theta component= -1 m/s
  9. 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME 9 Fig. 10.(b): Velocity contour at diffuser outlet for L= 10 D1 and theta component= -1 m/s Fig. 10.(c): Velocity contour at diffuser outlet for L= 15 D1 and theta component= -1 m/s It is observed from fig 9.(a),(b) and (c) that pressure variation is nearly constant and in fig 10.(a),(b) and (c), velocity is minimum at upper side of diffuser due to sharp curvature at divider. Fig. 11.(a): Pressure Contour at L= 5 D1 and theta component= -1 m/s Fig. 11.(b): Pressure Contour at L= 10 D1 and theta component= -1 m/s
  10. 10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME 10 Fig. 11.(c): Pressure Contour at L= 15 D1 and theta component= -1 m/s The pressure contour for a draft tube is shown in Fig.11. (a), (b) and (c) at L/ D1 = 5, 10 and 15 respectively. From these contours it can be seen that after achieving a optimum pressure at the inlet of dividing pier, pressure is nearly constant throuhout the length of draft tube, there is sudden drop in pressure due to curvature at elbow section, where as . Fig. 12.(a): Velocity stream line at L= 5 D1 and theta component= -1 m/s Fig. 12.(b): Velocity stream line at L= 10 D1 and theta component= -1 m/s
  11. 11. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME 11 Fig. 12.(c): Velocity stream line at L=1 5 D1 and theta component= -1 m/s The streamline patterns for a draft tube is shown in Fig.12. (a), (b) and (c) at L/ D1 = 5, 10 and 15 respectively. From the above figure it can be seen that the amount of whirl incresase with increase in diffuser length. The whirl is higher in the left of the dividing pier as compared to whirl in right of dividing pier , it may be because of eccentricity of the dividing pier with the centre line of draft tube inlet. The velocity reduces from inlet to outlet which confirms the cherecterstic of draft tube. The draft tube efficiency and relative loss in each numerical simulation are computed and graphical representation is shown in fig. 13 and fig. 14. As shown in fig.13 the maximum draft tube efficiency obtained at 0 m/s whirl component in case of L/D1 = 10. All these numerical simulation shows that when diffuser length is L > 10 D1, the highest efficiency is achieved. From fig. 14, it is seen that, minimum relative loss is for L/D1 = 5 and 10. Fig. 13: Efficiency of Draft Tube at Fig. 14: Relative Loss in Draft Tube at different L / D1 ratios different L / D1 ratios 10 20 30 40 50 60 70 80 90 -4 -3 -2 -1 0 1 2 3 4 DraftTubeEfficiency(%) Theta Component (m/s) L= 5 D1 L= 10 D1 L= 15 D1 0 10 20 30 40 50 60 70 80 -4 -3 -2 -1 0 1 2 3 4 RelativeLoss(%) Theta Component (m/s) L= 5 D1 L= 10 D1 L= 15 D1 Poly. (L= 15 D1)
  12. 12. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 01-12 © IAEME 12 CONCLUSION From numerical simulation of elbow draft tube with dividing pier it may be observed that that maximum efficiency is achieved at L= 10 * D1 length of the draft tube. The comparative study of the pressure variations and velocity contours at the inlet section of draft tube and just after the elbow section shows that location of dividing pier effects the velocity distribution significantly. Efficiency of draft tube is affected due to whirl component. REFERENCES 1. Motycak Lukas, Skotak Ales, Obrovsky Jiri, 2010, “Conditions of Kaplan Turbine CFD Analysis”, ANSYS Conference. 2. M.F Gubin, 1973, “Draft Tubes of Hydraulic Station”, Amerind Publishing Company Pvt. Ltd. 3. Khare Ruchi, Prasad Vishnu, Mittal Sushil Kumar, 2012, “Effect of Runner Solidity on Performance of Elbow Draft Tube”, Energy Procedia 14, pp 2054-2059. 4. Khare Ruchi, Prasad Vishnu, Kumar Sushil, 2010, “Derivation of Global Parametric Performance of Mixed Flow Hydraulic Turbine Using CFD”, Hydro Nepal. 5. Choi Hyen-Jun, Zullah Mohammed Asid, Roh Hyoung-Woon, Ha Pil-Su, Oh Sueg-Young, 2013, “CFD Validation of Performance of a 500 kW Francis Turbine”, Renewable Energy 54, pp 111-123. 6. Prasad Vishnu, Khare Ruchi, Abhas Chincholikar,2010, “Hydraulic Performance of Elbow Draft Tube for Different Geometric Configurations Using CFD”, IGHEN-2010, Oct.21-23, 2010, AHEC, IIT Roorkee, India. 7. U. Andersson, J. Jungstedt and M.J. Cervantes, 2008, "Model Experiments of Dynamic Loads on a Draft Tube Pier", 24th Symposium on Hydraulic Machinery and Systems. 8. S Tridon, S Barre, G D Ciocan, P Leroy and C Segoufin, 2010, "Experimental investigation of draft tube flow instability", 25th IAHR Symposium on Hydraulic Machinery and Systems. 9. ANSYS, 2010, "ANSYS CFX 13 Software Manual, ANSYS, Inc., southpointe, Canonsburg, PA. 10. P. K. Sinha, A.K.das and B. Majumdar,, “Numerical Investigation of Flow Through Annular Curved Diffuser”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 2, Issue 2, 2011, pp. 1 - 13, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. 11. Prof. N.V.Hargude and Prof. P.R.Patil, “Design and Development of Plc Operated Hydraulic System for Machining Process”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 2, 2012, pp. 366 - 373, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. 12. Tarun Singh Tanwar , Dharmendra Hariyani and Manish Dadhich, “Flow Simulation (CFD) & Static Structural Analysis (FEA) of a Radial Turbine”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 3, 2012, pp. 252 - 269, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.

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