Multi-path gas ultrasonic flow meters are used to measure the flow rate of natural gas in custody-transfer metering applications. Steady-flow tests were performed in the high-pressure loop (HPL) of the Southwest Research Institute (SwRI) Metering Research Facility (MRF) flowing natural gas through two 300 mm (12-inch) diameter multi-path ultrasonic flow meters with different ultrasonic path configurations. Tests were performed with both small and large temperature differences between the flowing gas temperature and the outdoor ambient temperature. This paper presents the results of the large temperature difference tests with and without an upstream flow conditioner for one multi-path ultrasonic meter in the low-flow range of 0.15 m/s (0.5 ft/s) to 0.30 m/s (1 ft/s). Test conditions were selected to complement a computational fluid dynamics (CFD) study performed by Morrison and Brar [2004,2005] at Texas A&M University. The experimental results confirm that the gas flow in the ultrasonic meter was thermally stratified (as predicted by Morrison and Brar [2004]) and show the effects of thermal stratification on path velocities, meter diagnostic path velocity ratios, and on meter accuracy. The results show that the flow conditioner was relatively ineffective in smoothing the axial velocity profile distortion caused by thermal stratification in this low velocity range.

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