Test and Analysis on High-pressure Turbine Tip Clearance Measurement of Turbofan Engine

The tip clearance of high-pressure turbine rotor is a key parameter for the performance and control of turbofan engine. It is of great significance to master the variation law of tip clearance for evaluating the working efficiency and performance attenuation of turbofan engine. The high-frequency dynamic analysis method based on time-frequency analysis is established. The variation law of high-pressure turbine tip clearance with engine state is obtained, and the influencing factors are analysed by simulation. The test results show that the variation law of the tip clearance of the high-pressure is consistent with the design, that is about 0∼3mm. The tip clearance of the high-pressure turbine is greatly affected by the rotor vibration. It means that we can monitor vibration of the rotor by monitoring changes in tip clearance. Real-time measurement and control adjustment should be carried out in the flight test to improve the working efficiency of the engine.


Introduction
Tip clearance of turbine blade refers to the radial distance between the turbine blade and the engine casing.Small clearance can reduce gas leakage and improve engine turbine efficiency, but may cause the rotor and the casing rub.The active tip clearance control technology can provide a better clearance with the program.So it is widely used in the design of modern advanced high bypass ratio turbofan engine.The tip clearance measurement technology can provide data support for the active tip clearance control algorithm and feedback parameters for the control system, which is the core technology of the closed-loop control system.In addition, the tip clearance signal can identify a variety of fault information such as rotating disc cracks, blade cracks, and external damage.It is an effective gas turbine rotating disc health monitoring parameter.The research on tip clearance test technology of high-pressure turbine can provide technical basis for the design and revision of engine installation clearance, and provide technical support for the realization of efficient tip clearance active control technology.It also can be applied to engine health management system to ensure flight safety [1].At present, due to the high temperature environment of the turbine and the easy carbon deposition of the turbine components, the multi-selection capacitance method is used to measure the tip clearance of the high-pressure turbine [2] - [3].In the turbine tip clearance test, there have been a lot of research results in China.Cao Suzhi [4] studied the circuit design of capacitive tip clearance measurement system.Zhang Jian et al. [5] carried out a single-stage turbine variable tip clearance test on the tester, and explored the influence of tip clearance on turbine performance through simulation calculation and test comparison.Chen Hongmin et al. [6]  In this paper, the measurement principle of capacitive tip clearance is introduced, and the influencing factors are calculated and analysed.The dynamic calibration test process and precautions are introduced.Finally, the engine test was carried out to obtain the measured data of the tip clearance of the high-pressure turbine, and the law of the tip clearance of the high-pressure turbine changing with the engine state was analysed.

Capacitive tip clearance measurement principle
The measuring probe is fixed in the casing forms one pole of the capacitance, and the blade tip of the engine blade forms the other pole of the capacitance.The measured positive area of the two poles of the capacitor is proportional to the distance and inversely proportional to the distance.In the case of a certain test object, the area is basically unchanged, the capacitance value is inversely proportional to the distance, and the distance between the blade and the sensor can be converted by measuring the capacitance.In engineering, the relationship equation between the voltage and the tip clearance is obtained by pre-calibration test.
Since the sensor is fixed on the casing, during the rotation of the turbine rotor, the sensor measures the tip clearance between the blade and the sensor when different blades sweep through the sensor, and the obtained time-domain signal is a series of pulses.Rotating a circle can produce the number of blade pulse signal.

Analysis of influencing factors
The principle of capacitive tip clearance measurement is based on the assumption that after the test object is determined, the area between the sensor and the blade is unchanged; the relative dielectric constant of high temperature and high-pressure gas after combustion remains unchanged.The variation range of relative dielectric constant is very small, and the influence on the measurement results can be ignored [7].So we will focuses on the impact of area in the following discussion.

Effect of blade tip geometry.
The typical turbine blade tip geometry is shown in Figure 1.The tip of the blade is concave, and the tip clearance signal detected by the sensor is the distance between the edge of the blade and the sensor.Because the design of the engine turbine blade only has the quality distribution requirement in the radial direction, there is no clear requirement for the thickness of the blade tip.Therefore, the edge thickness of different blades on the turbine is different, and the thickness of the same blade edge is also different.When the engine works, the turbine disk will move along the axial direction of the engine, and the range of the blade tip actually detected by the sensor is shown in the dotted line of Figure 1.The change of blade edge thickness in this range and the difference of different blade edge thickness will make errors to the measurement results.In addition, if the blade twists, or the sensor installation error causes the sensor to be not parallel to the tip of the blade, as shown in Figure 2, it will also bring errors to the measurement results.The actual measurement of a certain type of blade is carried out.In the sensor detection area, the thickness variation range of the blade tip edge is about 1.1 mm ~0.8mm, and the depression depth is about 2.7mm.If the calibration test is obtained when the thickness of the tip edge is 1mm, then the two influencing factors are simulated and analysed according to the geometric relationship, and the tip clearance measurement deviation estimation results caused by different tip edge thicknesses are obtained, as shown in Figure 3.It can be seen from Figure 3 that the greater the thickness deviation of the blade tip edge, the greater the deviation of the measurement results.When the tip clearance is 1mm and the thickness error of the tip edge is 0.05mm, the relative error reaches 5 %.Therefore, the influence of the uneven thickness of the blade tip edge on the measurement results cannot be ignored.In the calibration test, it is necessary to ensure that the blade tip geometry of the test piece is consistent with the real blade, and the relative position of the sensor should be consistent with the position in engine test.

Effect of installation error or blade torsion.
The installation error of the sensor or the torsion of the blade will lead to a certain angle between the tip of the blade and the electrode plane of the sensor.In the calibration test and the installation test, the angle cannot be guaranteed to be completely consistent, which brings errors to the measurement results of the tip clearance.The angel is calculated with a step size of 0.5°, and the tip clearance corresponding to the area is calculated.The real value of the voltage is simulated and calculated, and finally the blade tip gap measurement deviation is obtained, as shown in Fig. 4. It can be seen from Figure 4 that when the tip clearance is 1mm, the thickness of the blade tip edge is 1mm, and the angle between the sensor and the blade tip is 1.5°, the tip clearance measurement deviation is about 0.001mm.With the angle increases, the deviation of the small tip clearance measurement results will increase rapidly.However, compared with the influence   .Summary.According to the above simulation, it is considered that errors caused by the difference in the geometric shape of the turbine blade cannot be ignored.In the calibration test process, the calibration test can be considered in the detection area of the sensor, and the corresponding calibration curve can be selected according to the design value of the moving distance along the axis of turbine.By measuring the geometric shape of each blade on the turbine disk, we can correct the test results.

Dynamic calibration test
The calibration test was completed at a speed of 2700 rpm using a real turbine blade.Since the voltage is inversely proportional to the tip clearance, the principle of selecting the test point is that more point in fast curve slope change areas, and less point in other regional.Twelve points were selected in the range of 0.75mm ~3.5 mm, and the measurement system was calibrated in three regions.The schematic diagram of the three regions is shown in Figure 5

Tip clearance signal processing method
The dynamic tip clearance signal is a series of pulse signals with two peaks, so it is necessary to extract the peak accurately.The steps of the tip clearance signal extraction method used in this paper are as follows: (1) Spectrum analysis of tip clearance time-domain signal is carried out to obtain the frequency band range of tip clearance signal; (2) According to experience, Cut-off frequency of low pass filter is set to the product of the rotational speed ten times the frequency and the number of blades; After low-pass filtering we can obtain a relatively smooth signal; (3) All peaks are obtained by calculating the maximum value, which contains invalid peaks; (4) The reference value is calculated by averaging the regions between two pulses; (5) According to the experience, 0.4 times of the maximum value of the peak is taken as the threshold, The maximum value greater than the threshold value is the results that we need; (6) The measured tip clearance is obtained by subtracting the reference value from the voltage signal; (7) The tip clearance value is calculated according to different calibration equations.
For the tip clearance data in the steady state of the engine, the average tip clearance is obtained by arithmetic average.For the tip clearance data of the transition state, the transient tip clearance change curve is given.

Engine test results
A tip clearance sensor was installed at the 6 o'clock position of a certain type of engine turbine casing along the course.The tip clearance measurement results of high-pressure turbine in different engine states and in the process of engine acceleration were carried out.Measurement results show that with the decrease of engine state, the measured value of tip clearance increases gradually.The size of the tip clearance is related to the difference in the deformation response of the casing and the blade disc, and is also related to the sequence of the engine working conditions.Therefore, the measurement results of the tip clearance under the same engine condition may not be consistent under different histories.In order to obtain more accurate rules, it is necessary to analyse the changes of the temperature field on the casing surface and the blade surface and the engine working conditions.

Conclusion
(1) In this paper, the test error caused by the geometric shape difference, torsion or installation error of H-shaped turbine blade is simulated and analysed.It is considered that the error mainly comes from the geometric shape difference of H-shaped turbine blade, which can be reduced by means of partition calibration test and correction.
(2) The variation range of the tip clearance measured by the test is consistent with the design value, that is about 0-3mm，and the variation law is consistent with the theoretical analysis, which indicates that the capacitive tip clearance measurement system can adapt to high temperature and high pressure environment.
(3) When the vibration of the engine turbine is large, the tip clearance results between individual blades are quite different.Therefore, the high-pressure turbine tip clearance control method based on the state control of the bleed air volume and bleed air temperature is not comprehensive.Real-time tip clearance measurement should be carried out, and the tip clearance measurement value should be fed back to the tip clearance control system in real time to improve the control efficiency.
studied the tip clearance measurement technology for H-shaped turbine blades and verified it on the tester.They analysed the influence of clearance change on flow loss and the change law of clearance at different rotor speeds.IOP Publishing doi:10.1088/1742-6596/2762/1/012079 2

Figure 1 .
Figure 1.Typical turbine blade tip geometry and sensor schematic diagram

Figure 2 .
Figure 2. The plate formed by the blade and the sensor is uneven

Figure 3 .
Figure 3. Deviation of measurement results caused by different thickness of blade tip edge .1088/1742-6596/2762/1/012079 4 of blade tip geometry, the installation error or blade torsion has little effect on the measurement results of blade tip clearance.

Figure 4 .
Figure 4.The tip clearance measurement deviation caused by the angle of the angle 2.2.3.Summary.According to the above simulation, it is considered that errors caused by the difference in the geometric shape of the turbine blade cannot be ignored.In the calibration test process, the calibration test can be considered in the detection area of the sensor, and the corresponding calibration curve can be selected according to the design value of the moving distance along the axis of turbine.By measuring the geometric shape of each blade on the turbine disk, we can correct the test results.

Figure 5 .Figure 6 . 4 .
Figure 5. Waveform of tip clearance output at different positions under the same clearance The obtained tip clearance calibration curve is shown in Fig. 6 .
Figure 7 shows the tip clearance measurement results of 72 blades under 5 typical working conditions of the engine.The tip clearance measurement results are normalized in the figure.The blade serial number is 1 # ~72 #, and 1.00 0.78 is the normalized high-pressure rotor speed, indicating different engine operating conditions.

Figure 7 .
Figure 7. Measurement results of tip clearance of all turbine blades under different engine conditions It can be seen that the turbine tip clearance decreases with the increase of engine speed.In the normalized speed about 0.84-0.89, the tip clearance of each leaf is quite different.Comparing the engine vibration measurement results, it is found that the turbine casing vibration also large under the same engine state, which indicates that some characteristics of the turbine tip clearance signal can reflect the vibration of the engine rotor system.Results are shown in Figure 8-9.

Figure 8 .Figure 9 .
Figure 8. Vibration measurement results at the turbine casing of the engine