Research On The Main Factors Causing Error On Measurement Of Ultrasonic Gas Flowmeters: Theory And Applications

As a new coming technique in the gas flow measure area, Ultrasonic flowmeters (USF) are just becoming the most favorite selection, special for the nature gas transport project. Due to the accuracy, stability, no pressure loses, wide measure range, and low cost, the Ultrasonic flowmeters just show its competitions.USF can be used to measure the gas and liquid both, there are same theories: Ultrasonic flow meters determine the fluid velocity by transmitting pulse stream upwards and downwards and measuring the transit times. The times are used in calculating the sound and flow velocity of the fluid. When ultrasonic wave transits at the stand and moving media, according the fix coordinates (such as the meter body), the sound speed is different, and the diff is a function of the media's moving velocity. So we can get the media's velocity by measure the time of flight of the wave. Then get the flow rate. The USF's usage for liquid flow measure is developed for a long time, and got excellent success. But the usage for gas is just beginning for almost ten years, and the technique is grasped just by several companies.Because the complexity of measurement environment, the gas flow measure application will encounter more factors causing errors: the changes in velocity profiles due to installation effect and wall roughness, the pressure flaps in the pipes, ultrasonic noise produced by the regulator etc. In order to overcome those defects, the technique of fluid mechanics simulation and digital signal processing are applied to research on the flow characteristics and Laguerre filter based adaptive methods. The main content of each chapter is as following.The first chapter generally introduces the history and the actuality of the flowmeters on the basis of relevant publications search. One of the most promisingflowmeter--the gas ultrasonic flowmeter is selected as research subject. Theliterature about primary instrument and secondary instrument of the flowmeter is summarized. The project significance, background, main research contents is also provided.In the second chapter, the mechanics principle of USF is introduced. How the defects cause measure error is calculated, and draw a conclusion: the unideal velocity profile causes large error when the meter calcs flow rate by a ideal meter constant. Also, in most case of the ultrasonic meter signal processing, the pulse peak discrimination is done by a relative trigger level, set between two adjacent peaks in the acoustic pulse signal. The zero-crossing after the second positive peak is taken as the moment the pulse has been received and is clocked by a running counter. The gas flow measurement, compared to those for liquid, characterized by large signal fluctuations, is in poor tracking. So more advanced detect algorithms are needed for gas meter applications.The third chapter, a CFD software FLUENT is introduced to calc the single anddbuble bow pipe's velocity profiles. How and why such mesh is selected is introduced. For compare purposes, the laminar and turbulence flow and roughness flow are also calculated.In the fourth chapter, introduces how the meter constant is defined is explained at the beginning. Why the secondary flow and asymmetry velocity profile causes measure error are analyzed. Finally, several representative acoustic path arrangement are introduced, their error curves in single bow pipe and roughness pipe are presented also.In the fifth chapter, we face the problem of determination of pulse transit times in ultrasonic gas flow measurements at low SNR. The method is based on the application of adaptive Laguerre filter. For the experimental verification of the method, a set of acoustic sources is synthesized by submitting a high-frequency piezoelectric transducer to varying levels of electrical damping. In spite of the facts that, by visual inspection, the shapes of the signals from the synthesized sources are very similar to one another, which provide the accurate recognition of the transit times at difference flow rate and SNR while provide a real time performance.