Advanced Research Journals of Science and Technology
ADVANCED RESEARCH JOURNALS OF SCIENCE AND TECHNOLOGY
(ARJST)
FINITE ELEMENT ANALYSIS OF A GAS TURBINE ROTOR BLADE
2349-1845
K.Rajesh1, N.V.R.BOSE 2, 1 Research Scholar, Department of Mechanical Engineering,Eswar College of Engineering, Narasaraopet, Guntur,India. 2 Associate professor, Department of Mechanical Engineering, Eswar College of Engineering, Narasaraopet, Guntur,India.
Abstract In the present work the first stage rotor blade of a two-stage gas turbine has been analyzed for structural, thermal using ANSYS 9.0, which is a powerful Finite Element Software. In the process of getting the thermal stresses, the temperature distribution in the rotor blade has been evaluated using this software. The design features of the turbine segment of the gas turbine have been taken from the preliminary design of a power turbine for maximization of an existing turbojet engine. It was observed that in the above design, the rotor blades after being designed were analyzed only for the mechanical stresses but no evaluation of thermal stress was carried out. As the temperature has a significant effect on the overall stress on the rotor blades, it has been felt that a detail study can be carried out on the temperature effects to have a clear understanding of the combined mechanical and thermal stresses. In the present work, the first stage rotor blade of the gas turbine has been analyzed using ANSYS 9.0 for the mechanical and radial elongations resulting from the tangential, axial and centrifugal forces. The gas forces namely tangential, axial were determined by constructing velocity triangles at inlet and exist of rotor blades. The rotor blade was then analyzed using ANSYS 9.0 for the temperature distribution. For obtaining temperature distribution, the convective heat transfer coefficients on the blade surface exposed to the gas have to feed to the software. The convective heat transfer coefficients were calculated using the heat transfer empirical relations taken from the heat transfer design dada book. After containing the temperature distribution, the rotor blade was then analyzed using ANSYS 9.0 for the combined mechanical and thermal stresses. The radial elongations in the blade were also evaluated. The material of the blade was specified as N155 but its properties were not given. This material is an iron based super alloy and structural and thermal properties at gas room and room temperatures were taken from the design data books that were available in the library of BHEL(R & D), Hyderabad. The turbine blade along with the groove is considered for the static, thermal, modal analysis. The blade is modeled with the 3D-Solid Brick element. The geometric model of the blade profile is generated with splines and extruded to get a solid model. The first stage rotor blade of a two-stage gas turbine has been analyzed for structural, thermal using ANSYS 9.0 Finite Element Analysis software. The thermal boundary conditions such as convection and operating temperatures on the rotor blade are applied on theoretical modeling. Analytical approach is used to estimate the tangential, radial and centrifugal forces
*Corresponding Author: K.Rajesh, Research Scholar, Department Of Mechanical Engineering, Eswar College of Engineering, Narasaraopet, Guntur,India. Published: January 04, 2016 Review Type: peer reviewed Volume: III, Issue : I Citation: K.Rajesh,Research Scholar (2016) FINITE ELEMENT ANALYSIS OF A GAS TURBINE ROTORBLADE
INTRODUCTION The finite element method (FEM) has now become a very important tool of engineering analysis. Its versality is reflected in its popularity among engineers and designers belonging to nearly all the engineering disciplines. Whether a civil engineer designing bridges, dams or a me-
chanical engineers designing auto engines, rolling mills, machine tools or an aerospace engineer interested in the analysis of dynamics of an aeroplane or temperature rise in the heat shield of a space shuttle or a metallurgist concerned about the influence of a rolling operation on the microstructure of a rolled product or an electrical engineer interested in analysis of the electromagnetic field in electrical machinery-all find the finite element method handy and useful. It is not that these problems remained unproved before the finite element method came into vogue; rather this method has become popular due to its relative simplicity of approach and accuracy of results. Traditional methods of engineering analysis, while attempting to solve an engineering problem mathematically, always try for simplified formulation in order to overcome the various complexities involved in exact mathematical formulation. In the modern technological environment the conventional methodology of design cannot compete with the modern trends of Computer Aided Engineering (CAE) techniques. The constant search for new innovative design in the en71
Advanced Research Journals of Science and Technology
gineering field is a common trend. To build highly optimized product, which is the basic requirement of today for survival in the global market today. All round efforts were put forward in this direction. Software professional and technologists have developed various design packages. NEED AND OBJECTIVE: The stress analysis in the fields of civil, mechanical and aerospace engineering, nuclear engineering is invariably complex and for many of the problems it is extremely difficult and tedious to obtain analytical solutions. In these situations engineers usually resort to numerical methods to solve the problems. With the advent of computers, one of the most powerful techniques that have been developed in the engineering analysis is the finite element method and the method being used for the analysis of structures/ solids of complex shapes and complicated boundary conditions. Due to development of computers and subsequent development of numerical methods, it is now possible to model the components, simulate the conditions and perform testing on computer without actual model making, One of the most popular numerical methods used is the Finite Element (FEM) offered by the existing CAD/CAM/CAE. The most popular software, which is based on Finite Element Analysis, is “ANSYS�package, which is used in this work. PRODUCTION OF BLADES: Blades may be considered to be the heart of turbine and all other member exist for the sake of the blades. Without blading there would be no power and the slightest fault in blading would mean a reduction in efficiency and costly repairs. The following are some of the methods adopted for production of blades. 1) Rolling: - Sections are rolled to the finished size and used in conjunction with packing pieces. Blades manufactured by this method do not fail under combined bending and centrifugal force. 2) Machining: - Blades are also machined from rectangular bars. This method has more or less has the same advantage as that of first. Impulse blading are manufactured by this technique. 3) Forging: - Blade and vane sections having airfoil sections are manufactured by specialist techniques. The simplest way is to determine the profile required at the hub and tip and join then by straight ruled lines. Once the geometry of the ruled line is established they may be machined by a milling machine, rest carefully by each line to generate the shape required in a master block from which the forging die may be copy machined. This method ensures the accurate forging of blades to their finished size, requiring only finishing. Broaching often does the machining of the fir-tree root and electrochemical machining may be used in some parts to avoid the conventional cutting processes. In advance methods, computers are used to determine the blade shape required by aerodynamic and stress criteria. The computer may then instruct a numerically controlled milling machine to prepare the dies. 4) Extrusion: - Blades are sometimes extruded and the roots are left on the subsequent machining. This method is not reliable as rolled sections, because of narrow limits imposed on the composition of blade material.
BLADE MATERIALS Proper selection of blade material plays an n important role in blade design. The factors that influence the selection of blade materials are: 1) Method of manufacture 2) Ease of machining 3) The ability to produce blade sections free from flaws. 4) Ductility both allow of rolling of shapes. 5) The capacity for being welded. 6) Ease of forging easily. 7) Condition of operations. 8) Suitable tensile strength at high temperature. 9) Resistance to creep. 10) Cost. The commonly used blade materials are: Brass: - Brass (70 to 72 % Cu and 28 to 30 % Zn) is suitable for temperature upto 230 deg. This is rarely used now days. Copper Nickel: - This is an alloy containing about 80 % Cu, 19 % Ni and fraction of iron and magnesium. Nickel Brass: - It is suitable for temperature range up to 230 and contains 50 % Cu, 40 % Zn and 10 % Ni.It may be cold drawn. Manganese copper: - Its composition is 95 to 96 % Cu, 4 to 5 % Mn and small amounts of iron, carbon and leads. It is not suitable for high stress and temperature .It may be cold drawn and cold rolled. Phosphor Bronze: - This is copper tin alloy with a small amount of phosphorous. Its composition is 86 % Cu, 14 % tin and 1 % phosphorous for hard bronze. Monel Metal: - The composition of Monel metal is 67 % Ni and 28 % Cu with a small amount of iron, carbon, manganese. It is resistant to corrosion and is suitable for high temperature. It is used for marine work. Mild steel: - In the past, it was used in many turbines but now it is rarely used. It corrodes very soon with wet steam but it is very inexpensive. Nickel steel: - This alloy is more resistant to corrosion than is mild steel. It may be forged or machined but not welded. Generally steel with 3 to 5 % Ni is used. It is used in many turbines. Stainless steel: - It is an alloy of iron, chromium and carbon containing 12 to 14 % chromium and normal percentage of carbon. It is very resistant to corrosion and erosion. It is also very hard. It may be rolled, easily machined and welded. TURBINE BLADE COOLING: Unlike steam turbine blading, gas turbine blading need cooling. The objective of the blade cooling is to keep the metal temperature at a safe level to ensure a long creep life and low oxidation rates. Although it is possible to cool the blades by liquid using thermosyphon and heat pipe principal, but the universal method of blade cooling is by cool air or working fluid flowing through internal passage in the blades. The mean rotor blade temperature is about 3500 c below the prevailing gas temperature after efficient blade cooling
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ANALYSIS OF STRESSES ANDS ELONGATIONS The ANSYS software then analyzed the mechanical stresses and elongations experienced by the rotor blade. The results were viewed in the post processor part of ansys. The geometry, loads and results were stored in a file named ‘X’. THERMAL BOUNDARY CONDITIONS APPLIED ON THE ROTOR BLADE MODEL
Turbine blade cooling
APPLICATIONS OF GAS TURBINE: The following are the applications of gas turbine A) Land applications 1) Locomotive propulsions. 2) Central power stations. a) Standby plants for hydro installations. b) Fully automatic booster stations at end of transmission lines. c) Standby and peak load plants for small system. d) Bomb proof power plants. e) At location where water is not available. 4) Industrial. a) Pumping stations. b) Space applicationsTurbo jet, Turbo propulsion, marine applications.
A new file was opened in ANSYS and the thermal module of ANSYS was activated. The rotor blade model was copied into this file from which the previous structural analysis files ‘X’. The structural boundary conditions that were applied previously on the rotor blade model were deleted. The element type was stitched from structural to its equivalent thermal element type. The material properties were same as those in the previous file of structural analysis. Two boundary conditions namely Heat flux and Convection were applied on the rotor blade model. The solution part of ANSYS was opened and Heat flux = 0 was applied on the areas shaded and numbered in figure Heat flux = 0 fro areas 1, 2… to 16 Areas 1, 2, 3 and 8,9,10 come in contact with similar areas on the adjacent rotor blades. Hence due to symmetry boundary conditions, these areas are assumed to be insulated. Areas 4, 5, 6 and 11,12,13,15 on account of their small dimensions are assumed to be insulated. In the convective boundary condition, the convective heat transfer coefficient (h) and temperature of surrounding gases (T) have to be specified on the areas subjected to convection.
CENTRIFUGAL FORCES EXPERIENCED BY THE ROTOR BLADES
3-D view of rotor blades showing position of centroid ‘G’
STRUCTURAL BOUNDARY CONDITIONS TO BE APPLIED ON THE ROTOR BLADE MODEL Two structural boundary conditions namely displacement and force were applied on the rotor blade model. The solution part of ANSYS was opened and the displacement constraints (U) were imposed on the areas shaded and numbered
ANALYSIS OF TEMPERATURE DISTRIBUTION The temperature distribution on the rotor blade was then analyzed by using the ANSYS software. The calculations were carried out in the solution part of Ansys.The results were viewed in the postprocessor part of software. The geometry, loads and results were stored in a separate file ‘y’. ANALYSIS OF STRESSES ANSD ELONGATIONS TAKING TEMPERATURE EFFECT INTO CONSIDERATION The thermal analysis file ‘y’ was reopened. The element type was switched from thermal to its equivalent structural element type. The structural boundary conditions namely displacements and forces were again applied on the model. In the thermal analysis the temperature distribution is stored in an .rth file. The temperature distribution was imposed on the blade by recalling it from .rth file.
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Advanced Research Journals of Science and Technology
The stresses and elongations were then analyzed using software results were viewed in the post processor.
Line Diagram
RESULTS AND DISCUSSIONS From the post processing, the temperature variation obtained as shown in figure. From figure, it is observed that the temperature variations from leading edge to the trailing edge on the blade profile is varying from 804.567 to 786.005 at the tip of the blade and the variation is linear along the path from both inside and outside of the blade. Considerable changes are not observed fro the first 6 mm length from the leading edge and from there to next 36 mm length of blade the temperature is gradually decreasing and reaching to a temperature of 796.317 and for another 4 mm length it is almost constant. Again from this point onward it is sloping downwards and reaching795.38 at the trailing edge. Where ever maximum curvature is occurring the temperature variation is less. At the root of the blade i.e. The top flange of base, the temperature variation along X-direction is varying from 800.442 to 786.37 on front side i.e. inside and 802.505 to 790.13 on the backside of the blade. The temperature decreases gradually along X-direction. There is very small temperature gradient is occurring along Y-direction at the blade leading edge, on the suction side of blade and at the exist side of the blade and at the exist side the temperature is varying from 788.067 to 796.317. CONCLUSION
Area diagram of rotor blade
1. The finite element analysis of gas turbine rotor blade is carried out using 20 noded brick element. The static and thermal analysis is carried out. 2. The temperature has a significant effect on the overall stresses in the turbine blades. 3. Maximum elongations and temperatures are observed at the blade tip section and minimum elongation and temperature variations at the root of the blade. 4. Temperature distribution is almost uniform at the maximum curvature region along blade profile. 5. Temperature is linearly decreasing from the tip of the blade to the root of the blade section. 6. Maximum stress induced is within safe limit.
Von mises stress
7. Maximum thermal stresses are setup when the temperature difference is maximum from outside to inside. 8. Maximum stresses and strains are observed at the root of the turbine blade and upper surface along the blade roots. 9. Elongations in X-direction are observed only at the blade region in the along the blade length and elongation in Y-direction are gradually varying from different sections along the rotor axis. 10. It could be concluded that these contour maps and profiles enables us to ascertain the areas of rotor blades that are vulnerable for failure.
Deformation in Z-direction
11. For the power turbine used for marine application, the rotor blade was analyzed for mechanical stresses at only a few sections. The temperature has a significant effect on overall stresses in the turbine rotor blades. The thermal stress analysis together with the mechanical stress analysis will yield more valuable information about the actual magnitudes of overall stresses encountered by the turbine blades. Hence an attempt is made in the pre-
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sent work to use the ANSYS software to obtain the temperature distribution, thermal and mechanical stresses and radial elongations at several cross sections of the rotor blades of the two stage power turbine selected from the earlier work. The results obtained were pres, radial eloented in the form of contour maps and profiles of temperature distributionsngations and mechanical, thermal stresses for the rotors. REFERENCES: 1) S.S.Rao,”The Finite Element method in Engineering”, BH Publications New Delhi, 3rd Edition, 1999.
Author
K.Rajesh, Research Scholar, Department of Mechanical Engineering, Eswar College of Engineering, Narasaraopet, Guntur,India.
2) O.C.Zeinkiewicz,”The Finite Element method in Engineering Science”, Tata McGraw Hill, 2nd Edition, 1992. 3) T.R.Chandrupatla, Belegundu A.D.,”Finite Element Engineering”, Prentice Hall of India Ltd, 2001. 4) O.P.Gupta,”Finite and Boundary element methods in Engineering”, Oxford and IBH publishing company Pvt. Ltd.New Delhi, 1999. 5) V.Ramamurti,” Computer Aided Design in Mechanical Engineering”, Tata McGraw Hill publishing company Ltd. New Delhi, 1987.
N.V.R.Bose, Associate professor, Department of Mechanical Engineering, Eswar College of Engineering, Narasaraopet, Guntur,India.
6) C.S.Krishnamoorthy,”Finite Element Analysis, Theory and Programming, 2nd edition, Tata McGraw Hill publishing company Ltd.New Delhi, 2002. 7) P.Ravinder Reddy,”CADA Course Book”, AICTE-ISTE sponsored programme, August 1999. 8) R.Yadav,”Steam and Gas turbine”, Central Publishing House, Allahabad.
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