| Summary: | 碩士 === 國立臺灣海洋大學 === 系統工程暨造船學系 === 106 === Laser Doppler Velocimetry (LDV) which is one of applied techniques in optical detection has become an important research direction for many researchers. It can measure fluids with different properties. Therefore, the system has lots of progress and applies it to relevant experiments. In this research work, the laser Doppler velocimetry system that developed in the past has been modified and applied to the flow rate measurement of microparticles in the tube. Laser diodes are used as the light source in this system, and the fiber optic components act as waveguides to focus the transmitted light into the tube flow more easily. In the system, scattered light, transmitted light in the fluid is collected and coupled with the reference light to generate an interference beam. When the particulate fluid flows into the tube, there will be a Doppler shift corresponding to the flow velocity in the interfering beam. High-speed fiber couplers are used to convert optical signals into electrical signals, and the data are acquired by the National Instruments DAQ cards and computed by the Fast Fourier Transform (FFT) algorithm to obtain the Doppler frequency shift. Consquently, the relationship between frequency and flow rate are developed and used to calibrate the measured flow rate. There are particles in the fluid in the tube, so it is necessary to understand the Mie scattering phenomenon. In the experiment, a polystyrene suspension with a concentration of 1:150 and a diameter of 1 micron meter was used, and the peristaltic pump was used to push the fluid at veloctities: 5 mm/s、10 mm/s、20 mm/s、30 mm/s、40 mm/s、50 mm/s、60mm/s and 70mm/s. The angle between the flow tube and the detecting direction is 82 degrees, and the velocity component along the observing direction is obtained. Finally, using the FFT algorithm programmed by matlab to obtain the frequency spectrum and obtain the frequency change from it. The measurement results confirm that the particle flow rate is linear with the frequency of the FFT analysis. In this study, an LDV system has been established, which can measure the flow rate of microparticles in the tube by Doppler shift measurement. The DAQ card is integrated to the system to access more points and increase the signal-to-noise ratio, so acquiring data also can be easier in the process.
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