Study On Performance Of Vortex Flow Sensor At Low Flowrates

Vortex flow sensor is a kind of novel velocity-type flow instrument which has been developed fast in recent thirty years. It has many advantages such as low pressure loss, wide measurement range, stabilization and so on. Therefore, it is widely used in chemical industry, petroleum, metallurgy, food industry and other flow industry. To the piezoelectric vortex flow sensor that is commonly used in industry field, the measurement is easily disturbed by any external inferences because of its operation principle, which affects the measurement precision and range of vortex flow sensor. The problem is more important in low flowrate measurement. The digital signal processing method, structural optimization on vortex flow sensor—the location of piezoelectric probe and K-factor modified method with hardware implementation are investigated respectively in the thesis. The aim is to improve the measurement performance of vortex flow sensor at low flowrates.The mainly research works of this thesis are listed in the following.The improvement method of Hilbert-huang transform(HHT). Firstly, the characteristics of vortex signal at low flowrate are analyzed in time-domain and frequency-domain respectively. The source and representation of noise components are summarized. Secondly, according to these characteristics, the HHT method is improved in order to perform its better denoise function in weak vortex signal detection. Finally, by experimental comparison with the classic digital signal processing method FFT, it is indicated that the improved HHT method can not only extract the useful vortex signal exactly but also expand the low limit of measurement.Research on location of the piezoelectric probe in vortex flow sensor. Experiments are carried out in two-dimensional and three-dimensional vortex flow fields respectively with three trapezoidal cylinders. The optimum detection positions of the probe in the two flow fields are finally determined by analysis on signal intensity of pressure and velocity signals, signal-to-noise ratio, linearity and so on. Furthermore, a formula for the optimum position estimation is deduced, which discloses influencing factors on it. By experimental contrast to the original design of the vortex flow sensor, it is illustrated that the improved design has better measurement precision and lower limit of measurement. Moreover, three problems appeared in experiments are mainly discussed by numerical simulations and theoretical analysis: the differences of two-dimensional and three-dimensional vortex flow fields; study of vortex flow based on vorticity-transport equation; the variance of flow fields with trapezoidal cylinder and circular cylinder, those of which are useful for us to get more knowledge in vortex flow field, such as generation and shedding of vortices, characteristics of wake, distribution of velocity and pressure and so on.Nonlinearity modified method of K-factor based on CPLD, which is implemented by hardware. The spline interpolation method is adopted to approximate relation curve of flowrate and K-factor. The modified algorithm is realized by look-up-table(LUT) which is embedded in CPLD. Design and optimization on the LUT is paid more attention, becasure it is not only reducing the power of system but also saving the CPLD resources. Through experiments, it is indicated that the measurement range is expanded within accuracy level by vortex flowmeter with nonlinearity modified. This method is applicable in any other flowmeters which have the same nolinearity problem of K-factor.