THE EFFECT OF DUCT CROSS-SECTIONAL SHAPES ON METROLOGICAL PERFORMANCE OF ULTRASONIC FLOWMETERS

摘要

This paper describes prospects of measuring range extension on transient and laminar flow regimes for ultrasonic time-of-flight fiowmeters due to creation measuring ducts with special cross-sectional shapes. The appropriateness of this idea consists that the flow profile correction factor, which considers a difference between averaged fluid velocity along the path of ultrasound propagation and area-averaged fluid velocity is not identical for laminar, transitive and turbulent conditions. Even on the assumption of axially symmetric flows the correction factor changes more than on 25 % at transition from one condition to another. In this connection there was an object in view how to eliminate the influence of a velocity profile on accuracy of ultrasonic flow measurements. For that computer models of ultrasonic meters with several variants of cross-sections have been developed and due to CFD techniques the flow profile sensitivities of various meter configurations are investigated in different Reynolds number flows. The selection of optimum shape of the measuring duct for ultrasonic fiowmeters has been appraised based on getting of as much as possible stable correction factor in a wide flow range and the minimal pressure loss on a meter. Results show that ultrasonic fiowmeters with specially designed duct shapes could be used for increasing accuracy of non-liquid meter calibration and also for elimination of the error caused by variations of hydrodynamic flow characteristics.