Simulation And Experiment Study On Multi-path Ultrasonic Gas Flow Measurement

Transit-time ultrasonic flowmeters have been widely applied in gas pipeline networks. Compared with traditional mechanical flowmeters, the ultrasonic flowmetes have the advantages such as bi-directional measurement, high-accuracy, reproducibility, lack of internal moving and no added pressure drop. Hence the ultrasonic flowmeters are the first choice in fluid transportation and trade. As the legal quantity measuring meters in international natural gas trade, ultrasonic flowmeters have potential for wide application in China. While the ultrasonic flowmenters have a series of advantages, the accuracy is still influenced seriously by the flowing condition of the fluid.The study described in this thesis is focused on ultrasonic measurements influenced by the flowing fluid. Not only computational fluid dynamics (CFD) has been adopted, but the practical experiments. A validated CFD model has been established and used to simulate the flows in the circular long-straight pipe. After manufacturing a flowmeter model, the laboratory scale system is established to obtain experimental measurements. Then the measuring deviations in CFD simulations and experiments are analyzed and discussed. The main work and results are as follows.1. CFD has been used to simulate the flows. A validated model has been built and meshed to simulate how the flow distribution affects the ultrasonic flow measurement. To process the data obtained in the simulations, a novel transit-time calculation method is propounded and adopted to emulate the physical principle of ultrasound meters. The flow velocity distributions inside the long-straight circular pipe are investigated when the fluid is laminar or turbulent. Single path ultrasonic flowmeter and multi-path meters are applied respectively to measure the flow in the low rate region and the normal rate region. The simulations are also generalized to flow conditions downstream installations which disturb the flow (such as a 90° elbow or a 180° bend). The measuring results are analyzed and discussed.2. After manufacturing a flowmeter model, a series of experiments have been carried on the laboratory scale system established by our research team. The practical experiments include measurements in long-straight pipe and non-ideal flow conditions using single-path ultrasonic flowmeters and multi-path flowmeters. The experimental data demonstrate that the two different transit-time measuring schemes are suit for different flow rate regions. The scattered transit-time data phenomenon in high speed flow measuring is analyzed and advices on picking the standard receiving waves are presented. The influences caused by temperatures and pressures are discussed.3. A novel data integration method, which is based on Levenberg-Marquardt algorithm (LMA) to calculate the weight of each ultrasound path for multi-path flowmeter, is introduced and investigated in this thesis. The method based on LMA is able to reduce measurement error compared with four traditional integration methods for non-ideal flow conditions, because it adjusts the weights of each path according to the flow condition. Furthermore, measuring errors caused by the non-deal flow distribution downstream a 90° elbow are diverse according to different acoustic path installation angles. Certain installation angles lead to relatively concise and effective measurement, for the layout of paths evades the low velocity distribution.