Worldwide economic growth requires stable performance and sustainable power supply system. Energy consumption worldwide is increasing about 1.2percent a year, while emission standards define stricter regulations. The gas turbine is a widely used power generating system in these days due to its huge capacity, from 100MW to 1000MW, so low emission and reliable performance. Recent technological improvement enabled manufacturers to develop high efficiency gas turbine. Its efficiency is directly proportional to operating temperature which is called the turbine entry temperature (TET). A lot of effort was made to increase the TET, which is always a design goal for a gas turbine system. Typically, TET is far above the permissible metal temperature. Composites are employed to protect the metallic blades in extreme thermal environment. Functionally graded materials (FGMs) are a state-of-the-art composite that can withstand high-temperature environment without delamination phenomena. The FGMs are microscopically non-homogeneous composite materials characterized by the smooth and continuous variation of material properties from one interface to the other. Temperature distribution within the FGM blades having temperature-dependent thermal properties, which cause changes of material properties, is forced to alter modal characteristics and dynamic responses of blades. Such undesirable variations of modal characteristics and dynamic responses can cause an unexpected resonance phenomenon and the excessive stress concentration phenomenon that leads to turbine system critical failure. Therefore, the modal characteristics and dynamic responses of a gas turbine blade undergoing extreme temperature environment should be precisely identified for designing the gas turbine blades.
The purpose of the present study is to propose a modeling method to analyze temperature-dependent modal characteristics of rotating FGM-gas turbine blades under extreme temperature field. The temperature distribution within the blade is imposed by solving the one-dimensional nonlinear heat transfer equation which regards temperature dependency of thermal conductivity. Looking at the heat transfer mechanism, internal cooling passages are used to control temperature of metal blades. In these surfaces, convection boundary condition is applied to the heat equation. Next, external gas temperature allows the heat not only to convect but also to radiate into blades, so radiation boundary condition is added to the heat equation. In addition, a mathematical model by Every et al. which develops Bruggeman’s equation is used to predict the effective thermal conductivity. The model took into account the effect of decreasing particle size.
The purpose of the present study is to propose a modeling method to analyze temperature-dependent modal characteristics and dynamic responses of rotating FGM gas turbine blades under extreme temperature field. The temperature distribution within the blade is imposed by solving the one-dimensional nonlinear heat transfer equation which regards temperature dependency of thermal conductivity. Looking at the heat transfer mechanism, internal cooling passages are used to control temperature of metal blades. In these surfaces, convection boundary condition is applied to the heat equation. Next, external gas temperature allows the heat not only to convect but also to radiate into blades, so radiation boundary condition is added to the heat equation. In addition, a mathematical model by Every et al. which develops Bruggeman’s equation is used to predict the effective thermal conductivity. The model took into account the effect of decreasing particle size. This heat transfer analysis was used to determine the stiffness of gas turbine blade system.
In this work, FGM-gas turbine blades are modeled as hollow rectangular blades mounted on a rigid turbine rotor disk, rotating with an angular speed. The accuracy of the model proposed in this study is validated by comparing its temperature-dependent modal characteristics to those obtained by using a commercially available finite element code. Using our model, we can obtain temperature-dependent modal characteristics and dynamic responses which can be used for designing the blades to avoid an unexpected resonance phenomenon and excessive stress concentration phenomenon for rotating gas turbine blades undergoing extreme thermal environment.