A Research Of Microwave Testing Technology For Blade Tip Clearence

The engine is the core of the aircraft, it is a mechanical device which is used to generate kinetic energy. The fuel consumption of the engine's compressor and turbine is proportional to the efficiency of the engine, so improving the efficiency of the engine becomes particularly important. The traditional method of improving the efficiency of the engine with aerodynamic design has come to the bottleneck. In order to further improve the efficiency of the engine, now the main method is reducing the loss of the end wall in the flow channel; and the loss of the end wall is mainly caused by the gap between the blade tip of engine's rotor and the casing wall (tip clearance). The study shows that smaller tip clearance makes higher engine efficiency. However, too small tip clearance may cause the collision between the blade and the casing, and endanger the safety of the engine. Therefore, through measuring the tip clearance in real time,dynamic control of the clearance can be achieved, so that the engine is always in the best running state.Today, the main technologies for blade tip clearance measurement in engine include discharge probe method, eddy current method, capacitance method, optical fiber method and microwave tip clearance measurement method. In this paper, the microwave tip clearance measurement method based on the phase modulation principle were discussed, and the phase shift of the reflected wave signal with respect to the incident wave signal were analyzed to obtain the measured tip clearance.The core of the microwave sensing system is the high temperature microwave probe installed in the casing. This paper analyzes the characteristics and design method of the microwave probe. Based on the structure of the existing microwave probe,several other kinds of microwave probes were designed. The feasibility of these structures was verified by simulation and experiment. Secondly, in view of the fact that the microwave probe needs to work in the high temperature environment of thousands of degrees, the high temperature condition will bring many problems to the test. In this paper, the high temperature probe structure and the high temperature coaxial structure were also designed. Finally, based on the rotation test, the calculation formula of the sampling frequency used in the rotation test is analyzed theoretically, furthermore, using this system to test the rotating speed of the engine blade speed was suggested.After simulation and experiment, it is found that the phase decreases with the increase of the clearance, which is consistent with the result obtained by using the transmission line model. When the clearance is fixed at a certain value, so that the blade swept the probe front; for different width of the blade, in the interaction area between the probe and the blade, phase and amplitude have the same trend on the displacement.If the blades are narrow, The phase is greatly affected by the amount of displacement in the interaction region of the probe and the blade. If the blades are wide, the phase is slightly affected by the displacement near the center of probe. Even if the blade tip clearance and the position of the blade relative to the probe are same, different blade sizes result in different phases; even if the blade tip clearance and blade sizes are the same, different position makes different phases. This may be a result of the spatial filtering effect. It is also found that when the blade length is in the direction of the excited electric field in the microwave probe, we can get the lower error caused by the axial vibration of the blade.