1. Blade life: A comparison by cumulative damage theories.

A turbine blade is modeled as a rotating pretwisted beam and subjected to aerodynamic excitation from the flow path interference. The resonant stresses are determined using a modal analysis. With the help of steady steam bending and centrifugal stresses and the dynamic stresses, life estimates are made using the linear and nonlinear cumulative damage theories and a comparison of the results is presented. Based on these results, recommendations are made on the usefulness of different theories for practical application

2. Analytical and Experimental Investigations of misaligned rotors.

A 3D finite element of a coupled Jeffcott rotor system is modeled using hexahedral elements. The two Jeffcott rotors are coupled by means of an automeshed coupling. Two power transmitting pins are connected across the coupling halves.  Misalignment is introduced to the driven shaft and the misalignment forces induced in the coupling pins are analyzed.

The model is rotated statically and misaligned forces are obtained at equiangular locations. It is found that the forces vary at the frequency of the shaft speed and the harmonic analysis of the force variation indicates that the system is excited by odd harmonics from 1x to 7x in case of parallel misalignment.  The axial force variation is also obtained and the corresponding harmonic analysis is also performed. The amplitudes of these forces are found to be in good agreement with the derivation of Gibbons (1976).

An Orbital analysis of the misaligned rotor system is performed. Multilobed orbits, see Bloch and Geitner (1984), are obtained and presented.

A test rig consisting of two coupled Jeffcott rotors is designed and fabricated. Parallel misalignment of 0.5mm is introduced to the driven shaft and the dynamic response of the system was analyzed for the harmonic components present in it in the direction of misalignment. The orbits are found to be multilobed around the one-third critical speed region. At around the half critical speed region, Limacon (see Lee, 1993) shaped orbits are observed. For a more severe value of misalignment (1mm), “8” shaped orbits are observed around the half-critical speed region.  The harmonic components in the direction of misalignment for the two values of parallel misalignment of the experimental rotor, the axial vibration of the disc are also reported in the paper.

The predominant radial characteristic of parallel misalignment and the predominant axial characteristic of angular misalignment are clearly identified, see Cempel and Haddad (1991). The fact that misalignment orbits are directional in nature, see Bloch and Geitner (1984) are identified and investigated. Multilobed orbits which indicate severe misalignment are obtained for severe values of misalignment around the critical speed region, see Bloch and Geitner (1984).  Backward whirl was observed between the two critical speeds, see Rao (1994). The fact that minor values of misalignment give rise largely to once per revolution component of vibration (see Ehrich (1991), is also investigated and presented in the paper.

 

1. Rao JS, Pathak A, Chawla A, Blade life: A comparison by cumulative damage theories, JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME 123 (4): 886-892 OCT 2001
    

2. A Chawla, JS Rao and A Pathak, Blade Life - a comparision by cumulative damage theories, To appear in the ASME Journal of turbo machinery.
    

3. JS Rao, R Sreenivas and A Chawla, Analytical and Experimental Investigations of mislaigned rotors, to appear in Proceedings of ISROMAC-9,
    

4. JS Rao, R Sreenivas and A Chawla, Experimental investigation of misaligned rotors, Proceedings of ASME Turbo Expo 2001.
    

5. A Chawla, JS Rao and A Pathak, Blade Life - a comparision by cumulative damage theories, ASME TURBO EXPO - 99, organized by ASME in Indianapolis in June 1999.
    

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