ANALYSIS OF STRESSES IN TURBINE
ROTOR USING FINITE ELEMENT
METHOD (FEM)-A PAST REVIEW
Homeshwar G. Nagpure1*
PG-Student M.Tech (M.E.D), Mechanical Engineering Department, KDK College of Engineering, Nandanwan, Nagpur (India)-440009
Dr.C.C.Handa2
Prof. & Head, Mechanical Engineering Department, KDK College of Engineering, Nandanwan, Nagpur (India)-440009
Dr.A.V.Vanalkar3
Prof. Mechanical Engineering Department, KDK College of Engineering, Nandanwan, Nagpur (India)-440009
Dr.S.K.Nath4
Engineering Officer, Mechanical Engineering Division, CENTRAL POWER RESEARCH INSTITUTE, Thermal Research Centre (A Govt.of India Society, Min. of Power), Koradi, Nagpur-441111 Abstract:
The present paper is a review of the past work done in the field of Stress analysis in turbine rotor using Finite Element Method (FEM).The analysis of stress values that are produced while the turbine is running are the key factors of study while designing the next generation turbines. A turbine rotating system is loaded with time by the changes in stress levels as a result of start-up and shutdown procedures. The temperature gradients that can be established in the transient state are generally higher than those that occur in the steady-state and hence thermal shock is important factor to be considered relative to ordinary thermal stress. The “heart” of these versatile machines is made by the blades and vanes , which are subjected during operation to very high thermal and mechanical stresses (combined effects of centrifugal force and thermal gradient), in aggressive environment The turbine rotor is subjected to temperature variations in short periods of time due to the start and stop cycles of the turbine. This causes sudden changes in the temperature with transient thermal stresses being induced into the turbine rotor. The transient effect is due to the changes in the material properties like Density, Specific heat and Young’s Modulus. The estimate of thermal stresses induced in the turbine rotor is important in determining the start up cycle of a steam turbine. Thermal gradients developed during thermal transients are the key source of stress generation in the rotor.
Keywords: Stresses, FEM, Turbine rotor, start up cycles, Ansys
1. Introduction
before it attains an equilibrium temperature. During this interim period, the temperature varies with time and the disk is said to be in a transient state. During this unsteady state, it is subjected to different temperature gradients. Transient thermal analysis indicates the thermal shocks that are induced in the disk. Thermal stress and thermal shock may be distinguished by the fact that in thermal shock, the stresses are produced by transient temperature gradients that are applied abruptly. The temperature gradients that can be established in the transient state are generally higher than those that occur in the steady-state and hence thermal shock is important relative to ordinary thermal stress. Another distinction between thermal stress and thermal shock is the rate of application of stress is very rapid and many materials are affected by the rate at which the load is applied. The effects of the cyclic plastic flow resulting from cyclic heating and cooling indicate that the number of starts and stops is an important factor in determining the turbine disk life. The steady state running time has less effect on the length of service.
2. Review of past work carried out on Stress Analysis
Sukhvinder Kaur Bhatti,Shyamala Kumari,M.L.Neelappu,C.Kedarinath,Dr.I N Niranjan Kumar [1] Their paper has primarily focused on the Transient heat transfer characteristics, centrifugal and the thermal stresses arising in the disk. Interesting results obtained in terms of maximum operational radial stress, maximum operational hoop stress, maximum operational Vonmises stress, the temperature field etc. So the disk is expected to perform well in spite of all the stringent operating conditions. The object is to provide understanding and information for designers to improve the life and efficiency of future generations of engines. The finite element model used in this analysis. The time steps are varied and the results are stored in load step files.ANSYS allows the thermal analysis to be conducted first and then the results are automatically transferred to the structural model. This approach is taken. The analysis is run assuming a uniform initial metal temperature of 30 deg C. The heat transfer coefficients are applied on the model along with the gas bulk temperatures. The initial time step size for transient analysis is quite important from the accuracy and the convergence standpoint.
Transient Thermo-Structural Analysis-Displacement Plot Thermal Analysis-Von Mises stress Distribution (Pa)
Zvonimir Guzoviæ, Branimir Matijaševiæ, Tihomir Mihaliæ [2]In this paper the algorithm and the results of non-stationary thermal stresses modeling in steam turbine rotor by means of the users software package are shown. Non-stationary thermal stresses are stipulated by pressure change on turbine exit. The results of non-stationary thermal stress calculations (i.e. of the numerical modeling) show on several characteristic regions of the rotor thermal stressed state: a) the rotor central bore; b) the low-pressure rotor; c) the disc of the last turbine stage, and d) the rear-end labyrinth gland. As in the low-pressure part of the rotor the high gradients of thermal and mechanical quantities (temperature, heat flux, deformation, stresses) are determined, so this region of steam turbine rotor is analyzed in detail. In this paper the results of this analysis are presented. Regardless of the nature of the distribution of equivalent von Mises stress in the individual sections, it can be generally concluded that the value of these stresses in the area of low pressure rotor increases downstream of the turbine from section a) to section c). Thus, the maximum of about 9.7 MPa at the periphery of section a) is amended to approximately 34 MPa in the bore of section c). Generally, on the basis of the obtained thermal and mechanical values it is possible to conclude that the thermal-stressed state of the low pressure part of the steam turbine rotor is acceptable: maximal equivalent von Mises stress is below 50 MPa. These values of equivalent von Mises stress in the low pressure part of the rotor are specific for the majority of steam turbines due to the nature of the thermal state and conditions which exist in this region.
Chunlin Zhang, Niansu Hu, Jianmei Wang, Qiping Chen,Feng He,Xiaoli Wang[3] This paper used a 600MW supercritical steam turbine’s rotor as the research object, and analyzed the variation of thermal stress in the warm starting up process. In this paper the analysis based on the operating data measured from actual operation and calculate the variations of the temperature field and stress field during the process of warm starting-up with the method of thermal-structure direct interaction analysis by Ansys. By analyzing the results, it proves that the maximum stress of the rotor is in the first stage of the intermediate pressure casing, and it is the main factor restricting the velocity of steam turbine’s starting-up process. This result which calculated and analyzed by finite element method can be a theoretical basis for the optimal operation and online monitoring of turbine units.
3 D Modeling
Temperature variation after 12000 secs
Stress variation with respect to time Temperature variation with respect to time
Deepak Dhar, A.M.Sharan [4]. This paper is concerned with life estimation of a turbine blade taking into account the combined effects of centrifugal stresses, vibratory stresses and thermal stresses. The stresses are determined by accounting for the rotor acceleration. The blades are subjected to aerodynamic excitation force obtained from thin cambered aerofoil theory under incompressible flow. The thermo-elastic forces are obtained from the three-dimensional non-linear heat transfer equations using the finite element analysis. The fatigue life is estimated using two well known theories, from the number of cycles in various blocks during start-up and shut-down periods of the turbine operation when the stresses peak. The turbine blade material is subjected to thermal expansion. The system analyzed consists of 12 blades. The rotor is accelerated to a steady state value, maintained at that speed, and then brought to rest. The analysis includes the acceleration and deceleration stages where the transients occur. The blades are excited by gas forces during their rotation. The stresses obtained are normalized with respect to yield stress which is a function of temperature.
Vibratory stresses at different rotor speeds Centrifugal stresses at different rotor speeds
Yong Li, Haoran Sun, Yuhuo Nie [5]. In this paper thermal stresses in 600MW steam turbine in different governing modes is discussed. The computational object is intercepted from hp-ip rotor of the 600MW steam turbine in this paper. The distribution of the temperature and thermal stress in the governing stage and the first stage of high pressure cylinder is calculated two-dimensionally. The changes of the temperature and stress in different governing modes are analyzed. The result of calculation provides a reliable basis for on-line monitoring of the rotor thermal stress, which ensures the security and economical operation of steam turbine units. Transforming the primary throttling governing into nozzle governing reduces the throttling loss with the part load, but increases the steam temperature variation in the same part of steam turbine. The temperature field and stress field after the governing stage of 600MW steam turbine are calculated, which comes to a conclusion that the axial thermal stress of nozzle governing is larger than that of the throttling governing. The results of calculation provide a reliable basis for on-line monitoring of the rotor thermal stress and determining the most critical area in the system of life time management. In order to
Ensure the security and the economical efficiency of the units after transforming the primary throttling governing into nozzle governing; it is necessary to re-account the velocity of load change.
Meshed 2 D geometry
Temperature variation over time Stress variation over time
Sudheendra V.S, S Ramamurthy, K.Murugesan [6].The Paper estimates the effects of the transient thermal stresses in the turbine rotor using Finite Element Analysis. A typical turbine rotor in the form of bladed disc called Blisk is considered for Transient Thermal Analysis using the Numerically Integrated Structural Analysis(NISA) package. The Blisk material is considered as MAR M 247 and the properties of the material is updated to the model. The appropriate boundary conditions depicting the actual environment in which the turbine rotor works in an aero engine are also updated.The result show that the maximum temperature occurs at the tip of the blade and decreases as we go into the disk. The Transient Thermal Analysis was successfully completed and the results were obtained. The thermal stresses due to large temperature gradients are higher than the steady state stresses. The large thermal stresses occur before reaching the steady state value.
any minor change in the problem, which reduces the cost and time required for manufacturing and testing of several prototypes.
Model of a steam Turbine Rotor Boundary Conditions
Hoop Stresses Radial Stresses
3. Conclusion
In this paper we have proposed FEM from a variety of aspects, such as references, features, analytical treatment and technologies. Moreover we have illustrated several representative platforms for the scope of FEM in future applications. In FEM the theoretically calculated results are compared with the measured ones (result validation) and found a good conformity. The result obtained is promising and has the potential as an alternative processing Method for stress analysis for different mechanical components. Still there is an essential need for an efficient use of FEM technique with other multi-user software so that accuracy of result could be maintained for better Analytical treatment. This study has primarily focused on the centrifugal and the thermal stresses arising in the disk. From the results obtained it is noticed that the values are in tolerable limits. Hence the disk is expected to perform well in spite of all the stringent operating conditions.
4. Future work
Our review suggests that in forthcoming efforts, analysis of stresses in turbine rotors could be best possible with Finite Element Method with ANSYS or ABAQUS as one of the helping software. In future we explore the stress analysis of turbine rotor especially the transient thermal stress analysis which is the cause of concern of various applied industries. Our future work is to propose a better analysis technique using FEM with ANSYS for analysis of Thermo-Mechanical stresses in turbine rotor.
5. References|
[1] G Sukhvinder Kaur Bhatti, Shyamala Kumari, M L Neelapu, C Kedarinath, Dr. I N Niranjan Kumar” Transient State Stress Analysis On An Axial Flow Gas Turbine Blades And Disk Using Finite Element Procedure”. in Int. Conf. on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT, Elounda, Greece, August 21-23, 2006 (pp323-330).
[2] Zvonimir Guzović, Branimir Matijašević, Tihomir Mihalić “Characteristics Of Non- Stationary Thermal Stresses In The Low-Pressure Part Of The Rotor”15th International Research/Expert Conference TMT-2011,Prague,Czech Republic 12-18 September 2011
[3] Chunlin Zhang, Niansu Hu, Jianmei Wang, Qiping,chen,Feng He,Xiaoli “ Thermal Stress Analysis for Rotor of 600MW Steam Turbine”978-1-4244-4813-5/10/&25.00c/2010/IEEE
[4] Deepak Dhar, A. M. Sharan.” Transient Stress Analysis and Fatigue Life Estimation of Turbine Blades”Journal of Vibration and Acoustics OCTOBER 2004, Vol. 126 Õ 495
[5] Yong Li, Haoran Sun, Yuhuo Nie “Thermal Stress Analysis of 600MW Steam Turbine Rotor in Different Governing Modes” 978-1-422-4813-5 28-31-March2010 IEEE
[6] Sudheendra,V.S,SRamamurthy,K.Murugesan”Transient,Thermal Analysis Of A Turbine Rotor”nal-ir.nal.res.in/8928
