Tip Clearance Measurements Of Rotating Blades Using Dual Frequency Laser With Large Frequency Difference

Tip clearance between rotating blades and casing is an important parameter that affects the efficiency and safety of large-sized rotating machinery such as aircraft engines, steam turbines, gas turbines and so on. Dynamic online tip clearance measurement technique is a constraints and key technology to perform health monitoring, fault diagnosis and active clearance control to improve performance. In this thesis, a new tip clearance measuring technique of rotating blades using dual frequency laser with large frequency difference is proposed to withstand the harsh industrial environments. The model of measuring system is set up, and the measuring errors are analyzed in theory and simulation. The system's hardware and software sub-modules are designed and produced, which are described in the thesis, and preliminary experiments are undertaken to test the performance of measuring system. Main works are shown as follows:(1) The mathematical model of tip clearance measuring system based on dual frequency laser is proposed. The beat-wave of laser is transmitted in fiber and its phase change, when they reflects off blade tip and returns to the collecting fiber, is only related to tip clearance, but not to blades, electromagnetic interference and other environmental factors. The system is with ability of self-calibration.(2) In-depth analysis of errors of dual frequency laser with large frequency difference based tip clearance measurement system is completed to establish the theoretical error model and influencing mechanism. Using matlab tool to simulate and verify the validity of the error model, the major factors that affect measuring accuracy are pointed out. So the potential accuracy of measuring system is thoroughly investigated.(3) A complete system prototype is designed and produced, including optic, hardware and software sub-system. The system prototype is composed of transmission fibers, sensor head, APD high voltage bias power supply, low-noise amplifier(LNA), local oscillator(LO), mixer, all phase FFT-based high-precision digital phase detection algorithm and application software.(4) Experimental tests have been completed in two stages. The first stage is phase comparison of two space optical path to detect distance between them, while the second stage is that one optical path was instead of fiber transmission. Data analysis shows technical feasibility and effectiveness of tip clearance measurements based on dual frequency laser with large frequency difference.