Gas ultrasonic flowmeters are widely used in natural gas measurement. In order to achieve high accuracy, it is meaningful to study interaction mechanism between flow field and acoustic field. In this study, effects of non-ideal flow on ultrasonic propagation are discussed. Firstly, a flow-acoustic coupling model is established based on COMSOL and its feasibility is verified by experiments. In order to be more in line with actual working condition, flow field is obtained by CFD simulation instead of theoretical formula calculation. Secondly, with this method, two typical non-ideal flows which often exist in real application are mainly analysed, including vortices near transducers and bend flows. The acoustic trajectory offsets, transit time, sound pressure and measurement errors are compared with results of ideal flow field. It is shown that errors will increase 10% caused by vortices near transducers, and increase 13% caused by bend flows. Besides, when passing through vortices near transducers with negative flow, trajectory offsets are opposite to flow direction. Finally, some suggestions for flowmeter design are proposed to improve measurement performance of gas ultrasonic flowmeter.
Gas velocity as an important characteristic parameter of wet gas has become a very active research issue in engineering field. Among many gas velocity measurement techniques, the ultrasonic flowmeter technique is more suitable for actual working conditions. However, the traditional signal processing methods of ultrasonic flowmeter are only proposed for single‐phase gas. In wet gas, the ultrasonic signals fluctuate greatly, are easily distorted and even lost, so the single‐phase ultrasonic signal processing methods are not applicable. Therefore, a new ultrasonic signal DBSCAN clustering synthetic peak method is proposed to solve the above problems. To verify the feasibility of this new proposed method, three real flow experiments are carried out on horizontal pipe at ambient temperature and pressure. Additionally, comparing the gas velocity results using ultrasonic signal double threshold method and ultrasonic empirical model method for single‐phase gas and the new proposed method for wet gas, it is shown that the measurement results using new proposed method accord with the theoretical law and can accurately measure the gas velocity. Furthermore, the three real flow experimental results show that the repeatability of gas velocity in wet gas by the new proposed method is mostly <0.5%.
As a typical method for calculating time-of-flight (TOF), the double-threshold method is widely applied in many time-difference-based ultrasonic gas flowmeters. However, the cycle-skip phenomenon caused by noises and various flowrate often affects the accuracy and reliability of flowmeters. Based on traditional double-threshold method and other improved double-threshold methods, a new characteristic peaks group judgement method is proposed for calculating TOF in this work. In this method, the peak's characteristics of the received signal are used to find the feature point which is stable in different flowrates. Then, based on the feature point, zero-crossing detection is executed for calculating TOF. In order to evaluate the new method, it is compared with traditional double-threshold method and the similarity judgment-based double-threshold method through offline tests on both simulated signals and real-flow signals. Results show that the proposed method can effectively eliminate the cycle-skip phenomenon and has higher precision and reliability than other methods. Furthermore, a digital signal-processing-based ultrasonic gas flowmeter with this method was implemented. Three calibration experiments were carried out on a gas flow standard device. The calibration results show that the flowmeter can achieve accuracy of class 1 in a wide velocity range from 1 to 35 m/s.
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