Research On Pipe-Flow Measurement Based On Ultrasonic Doppler Methods

Ultrasonic flowmeter is one of the most promising three methods in flow measurement. But present researches on ultrasonic flowmeters mainly center on transit-time methods. Doppler flowmeters for pipe-flow measurement have poor performances. Their theories and implementations are still stay at an early stage of the 1990's. They suffered from such disadvantages as can't decide the flow direction, difficult to measure the low-velocity flow, low dynamic response speed and bad real-time performance. This paper investigated into ultrasonic Doppler approaches for pipe-flow measurement, including theories, system implementation and commercialization. At first the thesis analyzed velocity-distribution discipline of the pipe-flow, equivalent circuit of the piezoelectric transducer in thickness-mode vibration, and ultrasonic propagation rules in pipes and flows. All of these analyses provided theoretical basis for transducers design and evaluation of average flow velocity in cutaway section.In the aspect of theories, ultrasonic Doppler signal was demodulated to an intermediate frequency, establishing a novel ultrasonic Doppler method for flow measurement. This method has three advantages as: (1) can decide the flow direction; (2) resolved the key problem to measure the low-velocity flow; (3) high dynamic response speed of flow measurement, especially at a very low velocity. A high precision algorithm for frequency evaluating was employed to analyze the demodulated narrowband Doppler signal. A novel method, which picks up effective frequency value by a special threshold, was also developed to evaluate the maximum flow velocity on central axis of the pipe. It can correct the low-frequency excursion and has such advantages as lesser operation.In the aspect of system implementation, clamp-on transducers were worked out and a novel ultrasonic Doppler system for flow measurement was implemented based on virtual instrument. A fluid-cycling experimental apparatus was established for experimental researches. A simple method for deciding thresholds was also presented. Performances of the novel system were analyzed in detail, especially at low velocity. The gauged experiment was presented according to national metrological verification rules. Result indicated that accuracy class of the novel system is 1.5.In the aspect of commercialization, according to the novel ultrasonic Doppler method, a flowmeter product was implemented based on a high-speed digital signal processor. Experiment was presented to demarcate instrument coefficient, repeatability, linearity and fundamental error of the product. Result indicated that accuracy class of the novel product is 2.0.