| Ultrasonic flowmeter has the characteristics of wide measuring range,high temperature adaptability,no moving parts inside,easy to install and maintain,etc.It is widely used in trade settlement,energy metering,process control,environmental protection and other fields.In recent years,with the development of science and technology and the improvement of industrial production efficiency,ultrasonic flowmeters are facing with increasingly high measurement accuracy requirements.The design and calibration of the ultrasonic flow meter is based on the steady flow field assumption.In the actual measurement process,the ultrasonic flowmeter only collects part of the flow field information(the line velocity on the ultrasonic propagation path),and obtains the transient flow rate by conversion based on the empirical coefficient(the ratio of line velocity and the surface velocity in the steady flow field).When the velocity distribution of the actual measured medium is different from the calibration process,the ultrasonic flowmeter will have flow field adaptability problems,resulting in an increase in measurement error.In the flow field adaptability problem,the unsteady flow field pulsation caused by rotating or reciprocating components,vortex,turbulence,etc.will increase the measurement uncertainty.Solving this problem is critical for improving the accuracy and stability of ultrasonic flowmeter measurements.In order to reduce the influence of unsteady flow field pulsation on the measurement accuracy of ultrasonic flowmeters,the following three aspects were studied:Through the abstract vortex field model,the physical mechanism of measure-ment uncertainty caused by unsteady flow field pulsation is analysed;through the LDV experimental measurement and CFD numerical simulation,the characteristic scale and spatial distribution law of unsteady flow field pulsation on the ultrasonic propagation path are found,and the influence law of unsteady flow field pulsation on the line velocity measurement of ultrasonic flowmeter is analysed;through theoretical analysis and CFD numerical simulation,the optimization route of flow channel structure for reducing flow field pulsation error is summarized,and a new scheme of flow channel structure is designed.The specific work of the study is as follows:(1)Through theoretical analysis and two-dimensional flow field discussion,different kinds of unsteady fluid pulsation sources in ultrasonic flowmeters are studied.The results show that when there are no moving parts in the pipe,the unsteady flow field pulsation is mainly composed of the vortex caused by the separation of the boundary layer and turbulent eddies.The abstract vortex field model was constructed using MATLAB,and the influence of vortex on the uncertainty of line velocity measurement was analysed.The results show that the absolute value and the growth rate of the line velocity measurement uncertainty increase with the increase of the vortex scale in the direction of the ultrasonic propagation path.The study clarifies the physical mechanism of the uncertainty of the ultrasonic flowmeter measurement caused by the unsteady flow field pulsation.It is pointed out that the measurement accuracy of the ultrasonic flowmeter is mainly affected by the large-scale vortex on the acoustic propagation path.(2)A transparent ultrasonic flowmeter measurement test bench is established,and the transient velocity on the effective propagation path of the ultrasonic flowmeter was measured using LDV.Aiming at the defect that the LDV pointmeasurement data cannot reflect the vortex scale feature,the concept of pseudo-line average velocity is proposed.The distribution law of large-scale vortices on the ultrasonic propagation path is revealed by filtering out the small-scale vortex pulsation through the line averaging process.The one-dimensional turbulent energy spectrum is calculated and the axial characteristic length of the large-scale vortex is obtained.The results show that for a 40 mm diameter mono U-shaped ultrasonic flowmeter,the area where the pseudo-line average velocity standard deviation is large is the main introduction area of the ultrasonic flowmeter measurement error.When Re= 30000,the main introduction area of measurement error is located in the middle and rear part of the ultrasonic effective propagation path,50 mm<x<126 mm area,and the axial characteristic length of the large-scale vortex in the area is the scale as the pipe diameter(40 mm).This study obtained the characteristic scale and spatial distribution law of the unsteady flow field pulsation in the ultrasonic flowmeter,which indicated the direction for the optimal design of the flow channel structure.(3)The transient,flow field in the ultrasonic flowmeter was numerically simulated based on the DES model.The pulsation characteristics of the fluid line velocity on the ultrasonic propagation path are studied,and the variation law of the flow field pulsation error with the Reynolds number of the line velocity mea-surement is obtained.In order to discuss the relationship between the flow field pulsation error and the distribution law of large-scale vortices,the probability density of the axial helicity and one-dimensional turbulent energy spectrum on the ultrasonic effective propagation path are calculated.The results show that the distribution range of large-scale eddies significantly affects the measurement accuracy of ultrasonic flowmeters.The flow field pulsation error increases sharply in the range of 8000<Re<20000 due to the extension of the distribution of the pipe-scale vortices from the middle portion(around x = 28 mm)to the middle and rear portions of the ultrasonic effective propagation path.This study obtained the influence of unsteady flow field pulsation on the line velocity measurement of ultrasonic flowmeter,explained the main reason for the increase of flow field pulsation error,and laid a theoretical foundation for the optimal design of flow channel structure.(4)Based on above research,the target parameters of flow channel structure optimization are summarized,and two categories of flow channel structure optimization are discussed.Through the DES numerical simulation,the effects of two flow channel structure optimization categories are evaluated.The results show that the two categories of flow channel structure optimizations,reducing the cross-sectional area of the flow and introducing small-scale structures,can effectively reduce the measurement uncertainty of the ultrasonic flowmeter.The optimization that reducing the cross-sectional area of the flow is more suitable for large flow rate measurement,but the pressure loss is relatively large;the optimization that introducing small-scale structures is more suitable for small flow rate measurement,but the structure is relatively complicated and the processing is difficult.Based on the research results of the flow channel structure optimization method,a new flow channel structure design scheme is proposed.The experimental measurement results show that by introducing a small-scale structure,the new flow channel structure design scheme significantly improves the measurement accuracy of the commercial ultrasonic flowmeter with constriction at low flow rate and low cumulative flow rate.When the measured flow rate is 0.05 m3 h-1 and the cumulative flow rate is 0.675 L,the pulsation standard deviation ?E of the measurement error of the new ultrasonic flowmeter is reduced by more than 0.69%compared with that of the commercial ultrasonic flowmeter with constriction.The statistical research methods in this study enrich the error analysis theory of ultrasonic flowmeters.The research results on the vortex feature scale and spatial distribution law lay a theoretical foundation for effectively suppressing the flow field pulsation error.The research on the optimization method of flow channel structure provides technical support for the design of ultrasonic flowmeter. |
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