Research On Key Techniques Of Multi-path Ultrasonic Gas Flow Measurement

Multi-path ultrasonic flowmeters is an efficient method to measure gas flow in a big diameter pipe. It plays a very important role in flow measurement and trading measurement in industrial.So it is necessary to solve the key problem and technique of multi-path ultrasonic gas flow measuring. As far as big diameter pipe gas transport is concerned, gas flow distribution will influence the flow measurement performance, and cause a large measuring error. This dissertation primarily studies the modeling of multi-path ultrasonic gas flowmeter and tries to overcome the correction of flow distribution of flow rate measurement under different fluid states, in addition makes temperature and pressure compensation to output signal, and then improves the performance of multi-path ultrasonic gas flowmeter. So the studying has the academic and practical meaning.Firstly, in order to accomplish the modelling of multi-path ultrasonic gas flowmeter, measuring principle of transit-time method of ultrasonic gas flowmeter is analyzed, and many factors influencing flow measurement is also analyzed. After analyzing the empirical equations of flow velocity distribution and it's coefficient correction by Prandtl in traditional transit-time method, this dissertation studies the design procedure to flow velocity distribution on modified coefficient related to Reynolds number and pipe wall roughing, and simulates the relation between flow field distribution and Reynolds number, and investigates the measures to improve measurement accuracy of flow velocity and flow rate.Secondly, this dissertation puts the emphasis on accomplishing the modelling of multi-path ultrasonic gas flowmeter. On the foundation of measuring principle of ultrasonic flowmeter based on chord track, a definite method of multi-path ultrasonic gas flowmeter's track position by using design procedure of flow velocity distribution modified coefficient and digital integration weighting coefficients is studied, and a complete mathematical model is built. During the model building, by using Gauss-Legendre,Gauss-Yacobi and neural network numerical integrating, track position parameter and weighting coefficients are determined and three mathematical model of different characteristic are built. Through simulation and computing, all the three mathematical models could meet the requirement of flow measuring accuracy in theoretical calculation error. Gauss-Legendre modeling method is suitable for multi-path ultrasonic gas flowmeter with odd number channels; while Gauss-Yacobi modeling method is suitable for multi-path ultrasonic gas flowmeter with even number channels which avoid the problem of flow state complexity on pipeline axis. The two mathematical models built by Gauss numerical integration are integral and systemic, but the modified weighting coefficient needs to be tested at the spot before the practical application.After that, the recognition technique of acoustic signal in multi-path ultrasonic gas flowmeter is studied. Based on studying the generating and receiving circuit of ultrasonics, this dissertation analyzes that the key to improve measuring accuracy is how to seek the starting time of ultrasonic echo signal.So the improved digital polarity correlation algorithm suitable for MCU and DSP system is studied in the dissertation. Through spot test, the recognition time could reach≤0.1s, and the time measurement accuracy could also meet the requirement of the system.Finally, this dissertation analyzes and discusses the data processing method of multi-path ultrasonic gas flow measurement which includes the data fusion arithmetic of each channel flow velocity measured value and multi-path flow velocity measured value. According to the flow measured value in working condition, make compensation and amendment through temperature,pressure and gas compressibility factor, and then accomplish flow correction in the standard condition. Combining with uncertain analysis result of spot test data, it also analyzes the uncertainty of multi-path ultrasonic gas flow measuring and proofs the effects of the data processing method and uncertainty analysis mentioned above.This dissertation studies the two modeling methods of Gauss numerical integrating and neural network numerical integrating and the ultrasonic signal recognition method of digital polarity correlation algorithm based on improvement. Simulation and experimentation verification show that it has the value of increasing gas flow measuring accuracy and provides theoretical method and key techniques for the development of multi-path ultrasonic gas flowmeters.