Experimental investigation on vibration characteristics of turbine bladed disk assemblies

• Main Author: Chua, Alvin Weiming.
• Other Authors: Lin Rongming
• Format: Final Year Project
• Language: English
• Published: 2009
• Subjects: DRNTU::Engineering::Mechanical engineering::Motors, engines and turbines
• Online Access: http://hdl.handle.net/10356/16165
• Institution: Nanyang Technological University

The literature on vibration analysis of turbine bladed disk assemblies including its mistuning and coupling effects is reviewed, and at the same time the importance of obtaining its vibration characteristics is explored. In selectively surveying the literature, emphasis is placed on the experimental techniques used to obtain a mathematical model of the structure. Since blade failures are major causes for the breakdown in turbomachineries, it is necessary to investigate the vibration characteristics of the entire assembly so as to improve the performance and reliability of turbomachineries. Having reviewed the vibration-related theories, experimental studies were carried out to investigate the complex structures which are difficult to analyze analytically. The report focused on various modal analysis methods which are based on the fundamental concept of curve-fitting frequency response measurements. In order to ensure accuracy of the results, the experimental setups and procedures were discussed in detail. Three different support conditions were considered for the experiments. Firstly, in free-free condition, six individual experiments were conducted using five blades of different shapes and sizes and a disk. Secondly, in grounded condition, these five blades were tested individually in a similar manner, and the experiment was repeated for one of the blades, measuring a different response point for each time while maintaining the same excitation point. For the third and last condition, in-situ condition, an experiment was conducted on a single-cantilevered blade assembled onto the disk, so that a single sector of the entire bladed disk assembly can be analyzed. All the results were analyzed to decide which measurement is a good indicator of the structure’s response and which to reject. Modal analysis was performed on those frequency response functions (FRFs) which were deemed to be good indicators. Methods to distinguish genuine and computational modes were discussed and were applied to the results obtained from the software analysis. Each of these modes corresponds to their respective modal frequency and modal damping for the individual components. Comparisons of the modal parameters among the different blades and bladed disk were highlighted and the significance of analyzing individual components was explained. One of the important findings was that a single-cantilevered blade assembled onto a disk had many more and quite different natural frequencies than that of the blade clamped in a grounded condition. Thus, it can be concluded that a blade when assembled onto a disk exhibits more complex vibration characteristics than that of an individual single-cantilevered blade. In addition, the mode shapes of a particular blade were determined from a set of FRFs. For illustration purposes, the first mode shape was plotted by observing the amplitude response and phase of the various points measured along the blade. During the process of modal testing, problems faced and solutions were described. Finally upon completion of the modal analysis process, various applications for the obtained mathematical model of the structure were explored. These applications include checking modal frequencies so that resonance can be avoided, and advanced trouble-shooting in the area of structural coupling and structural modification.