According to the main factors of the accuracy of infrared temperature measurement results, the correction method, device and system are proposed to reduce the impact of emissivity, high temperature background radiation, etc., the correction method has a wide range of application, high correction accuracy, and can correct the infrared temperature measurement of curved surfaces in complex environments such as aero-engine turbine blades, and realize non-contact, multi-dimensional and online measurement of curved surfaces.
First, the principle of infrared temperature measurement of turbine blades
The temperature measurement of the infrared thermal imager is based on the received radiation energy to reverse the temperature of the surface, and the radiation energy received by the infrared detector includes the target's own radiation, the target's reflected radiation to the surrounding environment and the spontaneous radiation of the environment, and the above radiation reaches the detector through the attenuation of the radiation participation medium.
Aiming at the problems that the accuracy of traditional infrared radiation temperature measurement methods is difficult to guarantee due to factors such as uneven emissivity of turbine blade materials, serious reflection of surrounding high temperature objects and changes in detection angle, based on radiation transmission theory and infrared thermal imaging temperature measurement principle, the main factors affecting infrared temperature measurement results are analyzed, and a correction model of infrared radiation temperature measurement on three-dimensional curved surfaces under complex background is proposed.
Second, the surface physical parameters of turbine blades are measured
1. Laboratory equipment and systems
The main equipment of curved blade infrared temperature measurement system are: infrared CCD camera, thermocouple, infrared light source. The experiment uses an infrared camera
HiNet-640OEM, temperature measurement range of 0~1100°C, wavelength of 8~14μm.
The thermocouple adopts K-type high-temperature thermocouple, model WRNK-191, the specification is 3mm× 150mm×2000mm, the thermocouple is connected to 32 temperature inspectors, and the temperature measured by the thermocouple is recorded in real time.
2. Surface mesh emissivity measurement
In order to improve the service life, the current high-pressure turbine blades will spray ceramic insulation materials on the surface of the turbine blades, and the changes in the thickness and composition of this insulation material will cause a significant impact on the emissivity of the turbine blade surface.
According to the degree of wear and corrosion, the blade surface is divided into three surface types, including the surface type without wear and corrosion, the surface type with heavy wear and corrosion, and a surface type between the surface type without wear and corrosion and the surface type with heavy wear and corrosion.
3. Surface bidirectional reflection distribution measurement
Comparing the BRDF values in different incidence directions, it is found that when the exit angle is equal to the incidence angle, the reflection share of the blade is the largest, and when the incidence angle is not equal to the exit angle, there is still energy reflected, indicating that the blade surface is non-specular reflection.
At the same time, the importance of BRDF measurement is verified. At the same time, it can be seen that when measuring BRDF on the blade surface, because of the curvature of the blade surface, the value of BRDF is larger when the incidence direction is 45°, which is greater than the value when the incidence direction is 30° and 60°.
Third, the analysis of experimental results
1. Test system construction
Set the distance between the measured blades and the surrounding blades to 7.5cm, set the power supply voltage to 40V, heat the two blades for 40min at the same time, so that they reach the same temperature at the same time, and then cool at room temperature, the infrared thermal imager and the blade base at 30° to shoot the temperature distribution of the measured blade surface, record the temperature change process of the measured blade surface, and use a thermocouple to record the actual temperature of the blade in the whole process.
2. Temperature correction analysis at different temperatures
Comparison of the measurement results of two designated positions P5 and P6 at different heights at different temperatures, T0 in the table is the reference temperature measured by the thermocouple, Tm is the temperature directly measured by the infrared camera, and Tc is the corrected infrared temperature.
Compare the correction error of the model at different temperatures. After correction, the error is significantly reduced, the correction error of P5 point is less than 1%, and the correction error of P6 point is not more than 1%. This shows that through the correction method proposed in this paper, the measurement error can be effectively reduced and the measurement accuracy can be improved.
3. Temperature correction analysis under different blade grid channel sizes
When the channel spacing is 7.5cm, the temperature measured by infrared is about 19.65K lower than that measured by thermocouples, and the original error is as high as 3.91%.
After correction, the error between the temperature measured by the infrared and the temperature measured by the thermocouple is reduced to 0.63%. It can be seen that the size of the leaf grid channel is changed, but the correction effect is still relatively stable, and the correction error is maintained within 1%, which verifies the universality of the correction model for the change of the size of the leaf grid channel.
4. Temperature correction analysis under different detection directions
In the experimental system, the correction effect of temperature correction in different detection directions was explored by changing the shooting angle of the infrared camera. Get the temperature measurement results of horizontal shooting and 30° overhead shooting with infrared camera. In the obtained temperature image, the temperature measurement point of the thermocouple at the extraction point, and the temperature measurement point of the corrected temperature image are extracted at the same time, which is not lost.
The measurement results and correction results of the two designated positions P3 and P8 under 0° and 30° overhead shooting are given, T0 in the table is the reference temperature measured by the thermocouple, Tm is the temperature directly measured by the infrared camera, and Tc is the corrected infrared temperature.
IV. Conclusion
1. In view of the problems of curved surface, uneven emissivity, and the influence of high temperature background radiation in the infrared temperature measurement of turbine blades, this paper proposes a correction method for infrared radiation temperature measurement.
2. An experimental system for infrared temperature measurement of curved surface in the laboratory was built, and the basic physical parameters such as emissivity, angular coefficient and bidirectional reflection distribution function of the blade were obtained through experiments, and the distribution law of these physical parameters was summarized.
3. The proposed correction model was used to correct the direct infrared temperature measurement results of different blade temperature, blade spacing and detection angle, and the correction error was kept within 1% with the measurement temperature of the thermocouple as the reference, indicating that the method proposed in this paper has good applicability.
V. References
XU Chunlei, PAN Chengjie, WANG Liang, et al. Temperature field measurement of engine low-pressure turbine blades based on infrared technology[J].Aero engine
[2] MA Shuaikun, ZHANG Tan Gui, CAI Jing, et al. Research on temperature measurement and correction technology of turbine blade under background influence[J].Journal of Testing Technology
