Rotating Blade’s Dynamic Strain Distributed Measurement And Vibration Estimation Research Based On FBG

Blade-like rotating mechanical is the core equipment in the field of industry. Along with the development of science, advancement of technologies and improvement of efficiency, this kind of equipment is developed continuously with a trend toward large scale, high speed and flexibility. The blade, as the core component of equipment, bored enormous unit load and the design safety factor even reaches1. In this case, the real-time and online detection and monitoring means of rotating blade becomes the key to improve equipment performance and reliability.The optical fiber bragg grating (FBG) is a kind of optical sensor and a strain measurement sensor characteristic of light weight, small volume, one line with multiple points, distributed measurement, electromagnetic interference resistance, optical signal transmission in the air. These characteristics of the FBG sensor could be utilized in measuring the dynamic strain and related parameters of rotating blades, meeting the requirements of rotating blade measurement under the extreme conditions like narrow space, high-speed, high temperature and high pressure, which is significant for understanding the mechanical behavior of the high-speed rotating blade and improving China’s blade-like rotating equipment research, design, analysis, processing and manufacturing level.In the meanwhile, rotating is a common feature of industrial equipment. This study can also be extended to the measurement of rotating parts of common equipment and improve knowledge of all industrial equipment with rotating feature, which is significant for improving the research, design, analysis, processing and manufacturing level of the industrial equipment.Firstly, the measurement method of rotating blade’s dynamic strain of jet engine is established.Based on the characteristics of optical signal can transmit in the air, the author analyzes several transmission methods of FBG signal on the rotating blade, and select the C-LENS lens method as further analysis, and design experiments by using two C-LENS lens based on the analysis results. The results show that the optical signal could be transmitted under the condition of maximum rotating shaft end run-out of0.8mm by using the C-LENS lens method. However, the C-LENS lens method is very sensitive to the variation of the included angle of rotating shaft. The result shows the dynamic strain of rotating component can transmit under the condition of4000RPM. A patent is applied for based on these studies.Subsequently, the relationship between the dynamic strain and vibration is analyzed in this paper. The equation of rotating blade’s dynamic strainâ†'vibration is established. According to the basic theories of the elastic mechanics and dynamics, approximation solution of the forced vibration equation of any structure can be obtained by using the assumed mode method. Dynamic strain at different positions can be regarded as the solution of forced vibration equation at this point; each mode can be regarded as the base of solution space of forced vibration equation; then, dynamic strain at different positions can be approximated by the modal combinations of vibrating body or the bases of solution space of vibration equation so as to estimate the vibration status of the rotating blade. Then follow the route from complexity to simplicity/from simplicity to complexity to simplify the pre-twisted rotating blade of variable cross section and variable thickness to the constant cross-section rotating beam and simplify the constant cross-section rotating beam to the constant cross-section static beam, which solve the equations of constant cross-section cantilever beam, constant cross-section rotating beam and rotating blade dynamic strainâ†'vibration in theory. Dynamic strain of constant cross-section cantilever beam, constant cross-section rotating beam and secondary rotating blade of some model of jet engine under rotating status is tested respectively on the basis of theoretical analysis of the dynamic strainâ†'vibration and experiments of rotating signal transmission, and experimental results are analyzed to estimate the experimental subject. Among which, the result of testing the dynamic strain of secondary rotating blade of some model of engine under rotating status indicates that a frequency cluster exists and this frequency cluster has significant influence on the dynamic characteristics of the engine and should be fully taken into account in the design.After the study on the dynamic strain of single rotating blade, dynamic strain at eight points of two blades are measured by utilizing FBG’s advantage of one line with multiple points and distributed measurement. The vibration characteristics of the rotating blade/hub system are studied through experiment from the point of a system consisting of multiple blades/hubs. It points out that the blade/hub system could not be regarded as a cyclic symmetric structure simply due to the inconsistency of machining error, installation error and degree of wear. Model building based on the cyclic symmetric feature will result in deviation of the calculation results, erroneous estimation of the safety factor and significant security hazards.