Define of blade natural frequency and proper nozzle number of a gas turbine in working conditions

The natural frequencies of the turbine blade operating at high temperature and high speed differ from the natural frequencies of the unloaded blade. The high temperature directly affects the material properties of turbine blades. In addition, the centrifugal forces due to rotation at very high speeds cause tensile stresses, which is called geometrical stiffening in the literature, while the fluid pressure generates bending stresses on the blade. Subject to this combined loading, i.e. bending + axial stresses, the blade undergoes some deformations. These three factors, high temperature, geometrical stiffening and bending, influence natural frequencies of the blade. In a finite element software, the ANSYS, a pre-stressed modal analysis was carried out to determine the natural frequencies of the turbine blade. Analyses show that natural frequencies under operating conditions become lower than those found in the unloaded case. Especially, the first natural frequency is as much as 9.6% for Inconel 718 and 5.6% for Inconel 738 LC smaller than that of unloaded cases, which is an important factor in turbine nozzle design. Key words: Turbine blade, Natural frequency, centrifugal force, geometrical stiffening