Technology For Measurement Of Blade Tip Clearance In The Gas Turbine

Blade tip clearance is an important parameter that influences the performance of gas turbine. If the blade tip is too big, it will decrease the efficiency of gas turbine, even lead to engine surge. If blade tip clearance is too small, it will lead to friction between the blade and the casing, and result in failure of gas turbine's operative safety. The gas turbine is working in high temperature, high pressure and large libration enviroment, so there are many factors affecting the blade tip clearance. They include the elongation of the wheel and blades caused by centrifugal force and thermal expasion; the elongation of the rotor and the contraction of the casing by high temperature working, and the changes of the gas turbine conditions will influence the blade tip clearance. Therefore, it is very necessary to create a real time and on-line measurement.The conventional measurement technique mostly comprises discharging probe, capacitance measurement, eddy current measurement, ultrasonic measurement, and fiber optical measurement. In order to measurement of the blade tip clearance reached the pratical level, this dissertation uses laser triangulation method to measure the blade tip clearance. The laser triangulation measurement can be extempted from the blade material, as long as transient is enough it can measure blade tip clearance continuously.Laser triangulation system consists of laser sources, probes, optical fiber, synchronized sensor, line CCD camera, computer, monitor. The laser is chose to light on the blade tip, and the high accurate line CCD imaging sensor was used to receive reflected light. In the static condition, the system demarcate the initial blade tip clearance value. According to the imaging position changed on line CCD, we can determine the change of blade tip clearance.This dissertation built the the blade tip clearance measurement system, and collected the CCD image to measure the tip clearance. To improve the resolution, this dissertation use sub-pixel technology to accurate positon of spot. This paper analyze center of gravity method, polynomial fitting and polynomial interpolation method which can increase the CCD resolution. These methods have both the advantages and disadcantages. On the basis of these methods, the improved polynimial interpolation method is put forth to enhance the tip clearance measurement precision. Finally, the paper analyze the affects of measurement accuracy made by the tip end characteristics and the environment. The system fullfills measurement by collecting the energy in the dispersion photic area of the laser spots. The difference of the blade tip surface roughness, color and material of the measured objects result in the alteration of the photic area so that the affects on the measurement come into being. Meanwhile, the gas turbine work in haigh temperature, high pressure and large shock enviroment, so the enviroment will also have some impact on measurement. By the. experiment, the affects about these factors was added to measure. The reasons why the errors take place was discovered, and several methods to reduce these influences ara explained.