With the development of modern laser technology, optical diagnostic methods have become more and more widely used in the experimental research of engines and their components. These technologies can realize the temperature and pressure components and concentration of gas velocity in the engine, as well as the spatial distribution of the fuel liquid mist and the measurement of the two-phase flow field, and provide sufficient data for debugging the engine components and verifying and improving the numerical calculation program.
In the following years, due to the advancement of optical computer hardware and software, the most important is the increase in demand, which has led to the continuous improvement of the quality and quantity of data obtained by studying the flow field of turbomachinery with a laser speed meter. Through extensive research on turbine and compressor rotors and stator cascades and combustion chambers, there is a great deal of interest in understanding the flow of turbomachinery and combustion chambers. Due to the need to shoot the laser into the measurement area inside the machine, 3D has been subjected to certain limitations in turbomachinery testing.
With the increasing demand for three-dimensional flow data, the development and application of three-dimensional systems have been greatly enhanced, and considerable progress has been made. The Laser Doppler Velocimeter has many optical configurations that can be used to measure three-dimensional velocity components. At present, three-dimensionality is achieved by using a three-color laser to generate a three-dimensional measurement area, such as a hydrogen ion laser. The accuracy of this system for measuring the radial velocity component is directly related to the radial declination. In order to reduce the measurement error, it is recommended to adjust the direction of the system layout in accordance with the flow vector. Such three-dimensional knife systems have been successfully applied to different turbomachinery tests. Lasers usually use flat windows when entering the measurement area. Since the housing of the turbomachinery is curved, the planar window cannot conform to the outline of the housing, resulting in local distortion of the flow. Therefore, the circumferential size of the observation window is limited.
It has been shown that the system's distortion of light is not as sensitive to the system, because their light is not focused with extreme precision in the detection area. The system can tolerate sufficiently thin curved glass to produce tiny light distortions.
The research center has used detailed three-dimensional measurements of the flow field inside the turbine components to provide detailed experimental data for the development of the primary and secondary streams of open centrifugal compressors and turbines for the first year of aeroengines. In the year, a fairly complex three-dimensional system was developed. It has the same way of entering the measurement area of ​​a laser into a two-dimensional system, and it is more widely used in the flow field formed by turbomachinery. It applies a standard stripe configuration and uses fluorescent particles that can be fluoresced to measure the axial and circumferential velocity components. The radial component is obtained by scanning with a confocal-interferometer and directly analyzing the Doppler shift of the scattered light generated by the particles. Both configurations are combined into an optical system and run simultaneously. This method has been successfully applied to the measurement of turbine stator blades. However, due to the complexity of its stability and the long data acquisition time, it can not be applied to the measurement of the flow field of rotor blades. It does not occupy a very important position in the study of the flow field of turbomachinery. It has also played a significant role in the study of the combustion chamber and many meaningful measurements have been completed.
Due to the ease with which lasers enter the measurement area in non-reactive streams, most measurements are performed in non-reactive streams and the resulting measurements are accurate. The application in the reaction flow is very complex, mainly due to the different refractive index of the component particles, which makes the laser become non-uniform and the detection volume is distorted, thus affecting the quality and quantity of the measurement data. Even then, the flames with the simplified combustion chamber and the actual fan-shaped combustion chamber have been successfully tested, but the experiment was not performed at low pressure or on a small-scale experimental apparatus.
Established equipment that can perform and three-dimensional measurements. The test temperature reaches the pressure air mass flow. When the pressure reaches, due to the fluctuation of the refractive index, the measurement is seriously distorted. The standard laser two-focus speed meter is a two-dimensional measuring device that measures the direction and size of the velocity component on a plane perpendicular to the optical axis. When a standard system continuously measures point-by-point in the flow field in different directions along the optical axis, three-dimensional components can also be measured. Due to its inherent characteristics, it is mainly used in large flow and high speed turbomachinery measurements such as compressors and turbines. Under such conditions, only helium systems that operate at very low solid-focus angles can be used for three-dimensional velocity measurements. The usual system is a combination of two independent two-dimensional systems.
The systems currently on the market and in European research institutions use such systems. The newly developed three-dimensional system has been applied to analyze the unsteady flow formed by the interaction of rotor blades. It can measure the position and three-dimensional flow characteristics of shock waves. The newly developed system combines the two-dimensional measurement with the frequency analysis of scattered light. The measured frequency shift represents the velocity component of the optical axis of the optical head. Systems have been established in most turbomachinery laboratories for the measurement of compressor and turbine flow fields. For the flow field of the combustion test, this system cannot be applied due to the large disturbance intensity that exceeds the ability to resist disturbance measurements.
Coherent anti-Stokes Raman spectroscopy is a good method of measuring temperature. The detection zone is similarly generated by superimposing two or more beams of different frequencies to produce a coherent signal. The actual system usually uses twice the frequency of the laser, repeated in pulses. The application of the system has encountered the same problem, namely, how the laser enters the measurement area, and also encountered difficulties in adapting to the high density of the soot particles and the large variation of the refractive system, thereby limiting the application of gas turbine combustion equipment operating under high pressure conditions. .
In the measurement, the amount of data is greatly reduced, and even at the critical position where the mixed intensity is high, no data can be measured, indicating that it is close to the limit of the month and the measurement again. The funeral is reminiscent of measuring all areas and even the most sophisticated test equipment. In addition to the fact that the combustion chamber operating under high pressure conditions cannot be measured due to laser distortion, the data provided by these measurement techniques is sufficient to verify the validity of the calculation procedure. However, the time and cost required to apply these technologies for measurement are high. Digital Particle Image Velocimetry Digital Particle Image Velocimetry is a powerful measurement device that can be used as a substitute and supplement for laser Doppler velocimeters in a wide range of applications. Transient plane measurements can be obtained in complex flow fields created by turbomachinery, making it an attractive technology. The same problems can also be encountered in the flow field generated by rotating machinery.
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