Vibration characteristics and life prediction of last stage blade in steam turbine Based on wet steam model

This paper takes the last stage blade of a 600 MW steam turbine as the research object. By considering the distribution information of wet steam in the last stage flow field and the process of the droplets hitting the blade wall, the stress distribution of the blade surface under two steam models with THA and 30 % THA conditions is obtained via fluid–structure coupling calculation method. Then, the vibration characteristics of single blade and disk-blades in the last stage under wet steam model are analyzed. Finally, according to calculated displacement and stress curves, the fatigue life of the last stage blade is predicted by Smith-Watson-Topper fatigue life calculation method. The results show that the equivalent stress decreases first, and then increases along the blade height direction, and reaches maximum at 90 % of the blade height, with the maximum value 712 MPa, which is lower than the yield limit of the material. The vibration mode shapes of the last stage blade are dominated by bending and torsional vibrations. The natural frequency of the blade is much smaller than the frequency corresponding to the high-frequency excitation force. In terms of fatigue life prediction for dangerous points, under different steam models and corresponding operating conditions, the number of cycles corresponding to the trailing edge tip of the blade suction surface (DISP 2) is the smallest, followed by the leading edge tip of the blade pressure surface (DISP 3), and the number of cycles corresponding to the 90 % blade height position of the blade suction surface (DISP 4) is the highest. In addition, the number of cycles obtained by wet steam model is lower than that by ideal gas model, and this trend is more obvious under 30 % THA working condition.