# Finite Element Approach for Dynamic Analysis of a Gas Turbine and Flutter Analysis of an Aero File

Topics: Damping, Vibration, Harmonic oscillator Pages: 43 (12682 words) Published: January 15, 2013
CHAPTER 1
INTRODUCTION
Rotor dynamics is the study of rotating machines and has a very important part to play throughout the modern industrial world. Rotating machinery is used in many applications such as Turbo machinery, Power stations, Machine tools, Medical equipment, etc. Failure of machinery in applications such as aero engines, turbo-machines, space vehicles, etc. creates enormous repair costs and more importunately may put human life in danger. Rotor dynamics is a collective terms for rotating machines and can be split into the sub groups that make it up. These are rotating shafts, bearings, seals, out of balance systems, instability and condition monitoring. The Contact of fluid with the rotating shaft leads to unwanted Vibrations and several other defects. Vibration is considered with the oscillating motions of the bodies and the forces associated with them. Objectionable vibrations in a machine may cause the loosening of the parts, its malfunctioning or its eventual failure. The study of the vibration is to determine its effect on the performance and safety of the system under consideration. The performance of many instruments depends on the proper control of the vibration characteristics of the devices. Energy may be stored in the mass and spring, and dissipated in the damper in the form of heat. The mass may gain or lose kinetic energy in accordance with the velocity change of the body. The spring possesses elasticity and is capable of storing the potential energy under deformation. When the excitation is oscillatory, the system is forced to vibrate at the excitation frequency. If the frequency of excitation coincides with one of the natural frequencies of the system, a condition of resonance is encountered, and dangerously large oscillations may result. The failure of the major structures such as bridges, buildings or airplane wings is an awesome possibility under resonance. Thus, the calculation of the natural frequencies is of major importance in the study of the vibration.

1.1 GAS TURBINE
A gas turbine is an engine where fuel is continuously burnt with compressed air to produce a stream of hot, fast moving gas. This gas stream is used to power the compressor that supplies the air to the engine as well as providing excess energy that may be used to do other work. The engine consists of three main parts viz., compressor, combustor and turbine. The compressor usually sits at the front of the engine. There are two main types of compressor, the centrifugal compressor and the axial compressor. The compressor will draw in air and compress it before it is fed into the combustion chamber. It is driven by a shaft that passes through the middle of the engine and is attached to the turbine. The combustor is where fuel is added to the compressed air and burnt to produce high velocity exhaust gas. Down the middle of the combustor runs the flame tube. The flame tube has a series of holes in it to allow in the compressed air. It is inside the flame tube that fuel is injected and burnt. There will be one or more igniters that project into the flame tube to start the mixture burning. Air and fuel are continually being added into the combustor once the engine is running. Combustion will continue without the use of the igniters once the engine has been started Fig. 1.1.1 Simplified gas turbine diagram.

The turbine extracts energy from the exhaust gas. The turbine can, like the compressor, be centrifugal or axial. In each type the fast moving exhaust gas is used to spin the turbine. Since, the turbine is attached to the same shaft as the compressor at the front of the engine the turbine and compressor will turn together. The rest of the exhaust gas is left to exit the rear of the engine to provide thrust as in a pure jet engine. Cold air is drawn in from the left into the compressor (blue). The compressed air (light blue) then goes into the combustor. From the outside of the combustor the air goes through holes...