Flow rate measurement method and flow rate measurement device

Abstract

To provide a flow rate measuring method and a flow rate measuring device capable of accurately measuring the flow rate of a fluid to be measured with a simple device configuration and with little risk of breakdown. [Solution] An acoustic matching layer 3 is provided between a pipe 1 through which the fluid to be measured flows and a piezoelectric vibrator 2, and as the flow rate of the fluid flowing in the pipe 1 changes, the thickness of the boundary layer on the inner wall of the pipe changes, and this change causes a change in the mechanical load on the piezoelectric vibrator 2, and the admittance A of the piezoelectric vibrator 2 is calculated as a result.The flow rate of the fluid flowing in the pipe 1 is calculated from the admittance A of the piezoelectric vibrator 2 based on the correlation between the admittance of the piezoelectric vibrator 2 and the flow rate of the fluid flowing in the pipe 1, which has been calculated in advance.

Description

[Technical Field]

[0001] Ultrasonic flow measurement is used in a variety of applications, including various processing and control systems, by transmitting a pulsed or continuous ultrasonic signal through a fluid, detecting the transmitted signal, and processing the detected signal to determine the characteristics of the fluid and its flow rate (or velocity). [Background technology]

[0002] For example, as shown in FIGS. 4 and 5, Patent Document 1 describes an ultrasonic flow measurement device in which a holding body 22 is detachably attached to a pipe body 21 made of a flexible material through which a fluid to be measured flows, the holding body 22 having inner wall surfaces on both sides and a groove portion 23 for clamping the pipe body, and having latching arms 24a, 24b that are hung above the groove portion 23 and fastened and latched so as to narrow the width of the groove portion 23, thereby making the width of the groove portion 23 a predetermined size, and a pair of ultrasonic transmitter-receivers 25a, 25b are attached to the inner wall surface of the groove portion 23 on the upstream side and downstream side along the pipe body 21, and an ultrasonic beam from one of the ultrasonic transmitter-receivers is transmitted into the fluid in the pipe body 21, and the ultrasonic beam that has passed through the pipe body 21 is received by the other ultrasonic transmitter-receiver.

[0003] However, because the ultrasonic flow measurement device described in Patent Document 1 measures flow rate based on the propagation time of ultrasonic waves, good measurement results may not be obtained if the distance the ultrasonic waves travel through the fluid is short. In other words, it is difficult to perform measurements using a tubular member with a small inner diameter. While it is conceivable to increase the travel distance through the fluid by arranging a pair of ultrasonic transmitting and receiving elements at a distance along the longitudinal direction of the tubular member (reducing the inclination angle of the transmitting and receiving direction relative to the longitudinal direction), this may result in problems such as an increase in the size of the entire measurement device or total reflection of the ultrasonic waves at the interface between the members.

[0004] Furthermore, Patent Document 2 describes, as shown in Figures 6 and 7, "a transmitting transducer 32 that is installed on the wall of a fluid pipe 31, generates an ultrasonic pulse, and causes the ultrasonic pulse to enter the fluid to be measured flowing through the fluid pipe 31; a plurality of receiving transducers 33 that are installed two-dimensionally on the wall of the fluid pipe 31 opposite the transmitting transducer 32, and output detection signals of amplitudes corresponding to the intensity of received ultrasonic waves; and a plurality of receiving transducers 33 that detect the displacement amount of the ultrasonic pulse in the pipe axis direction from the detection signals of the plurality of receiving transducers 33 that are arranged in the pipe axis direction among the receiving transducers 33." the angles formed by a line segment passing through the exit end of the transmitting transducer 32 and the center of the pipe as viewed from the exit end of the transmitting transducer 32 are defined as opening angles α1 and α2, and the measurement region from the transmitting transducer 32 to a predetermined receiving transducer 33 is defined as measurement lines 34a and 34b, and a flow velocity distribution measuring means for determining the flow velocity at a predetermined position determined by a perpendicular line V from the center of the pipe to the two measurement lines 34a and 34b from the amount of displacement detected for each of the two measurement lines 34a and 34b, which have slightly different opening angles." In FIG. 6, 35 is a signal oscillator that outputs a transmission signal S1 to be supplied to the transmitting transducer 32, 36 is a detection circuit consisting of a signal amplifier that amplifies a detection signal output from the receiving transducer 33, the detection signal having a magnitude corresponding to the intensity of the incident ultrasonic wave, and a peak detection circuit that reads the peak value of the signal amplifier output, and each detection circuit 36 ​​has its pulse reception timing set by a timing signal S2 supplied from the signal oscillator 35, 37 is a data acquisition circuit consisting of a digital multiplexer that collects all the peak values ​​read by each detection circuit 36, and 38 is a data processing device that includes a flow velocity distribution measurement circuit 39 that measures the flow velocity distribution from the displacement detection signal output from the data acquisition circuit 37, a flow rate measurement circuit 40 that calculates the flow rate of the measured fluid flowing in the fluid pipe 31 from the flow velocity distribution data measured by the flow velocity distribution measurement circuit 39, and a display unit 41 that displays the measurement results. The flow velocity distribution measurement circuit 39 calculates the average flow velocity based on a predetermined formula from the amount of displacement of two measurement lines 34a and 34b having slightly different opening angles α1 and α2.

[0005] However, in Patent Document 2, after a plurality of receiving transducers 33 detect ultrasonic pulses, a flow velocity distribution in a direction perpendicular to the flow direction (flow velocity distribution in a cross section perpendicular to the axis of the fluid pipe 31) is calculated based on the displacement of the ultrasonic pulses in the pipe axis direction, and the flow rate of the gas fluid flowing in the fluid pipe 31 is calculated based on the flow velocity distribution. This leads to a complicated and large-capacity calculation, and there is a risk that the flow rate of the gas fluid cannot be measured accurately if calculation errors or disturbance factors are introduced in the process of calculating the flow velocity distribution from the displacement of the ultrasonic pulses and in the process of calculating the flow rate of the gas fluid from the flow velocity distribution.

[0006] Furthermore, as shown in FIG. 8, Patent Document 3 describes a flow rate measurement method, which comprises opening an inlet 43 at the lower end of a side wall 42 of a tank 41, injecting a fluid 44 at a constant flow rate, discharging the fluid 44 by overflowing it from the upper surface of the tank 41, attaching an ultrasonic vibrator 45 to the outer bottom surface of the tank 41, transmitting an electric signal from a signal generator 46 to vibrate the ultrasonic vibrator 45, measuring the voltage value and current value of the ultrasonic vibrator 45, and determining the resonant frequency or Q, or a change in impedance or admittance at the resonant frequency from the measurement results, thereby measuring the flow rate.

[0007] However, the ultrasonic vibrator 45 attached to the outer bottom surface of the tank 41 has the disadvantage that repeated mechanical stress gradually wears down the vibrating surface, causing it to stop functioning normally. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-219210 [Patent Document 2] International Publication No. 2008 / 004560 Brochure [Patent Document 3] Japanese Patent Application Publication No. 60-44825 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made in consideration of the problems associated with the prior art, and its purpose is to provide a flow rate measurement method and a flow rate measurement device that can accurately measure the flow rate of a fluid to be measured with a simple device configuration and that is less susceptible to failure. [Means for solving the problem]

[0010] As a result of extensive research by the present inventors to solve the above problems, the flow rate measuring method and flow rate measuring device of the present invention employ the following means. That is, the flow rate measurement method of the present invention is characterized in that a piezoelectric vibrator is attached to a pipe through which a fluid to be measured flows, and a change in the flow rate of the fluid flowing through the pipe causes a change in the thickness of the boundary layer on the inner wall of the pipe, and an admittance A of the piezoelectric vibrator resulting from this change in the mechanical load on the piezoelectric vibrator is calculated, and the flow rate of the fluid flowing through the pipe is calculated from the admittance A of the piezoelectric vibrator based on the correlation between the admittance of the piezoelectric vibrator and the flow rate of the fluid flowing through the pipe, which has been calculated in advance.

[0011] The flow measurement device of the present invention is characterized in that a piezoelectric vibrator is attached to a pipe through which a fluid to be measured flows, and the thickness of the boundary layer on the inner wall of the pipe changes as the flow rate of the fluid flowing through the pipe changes, and the admittance A of the piezoelectric vibrator is calculated as a result of this change in the mechanical load on the piezoelectric vibrator, and the flow rate of the fluid flowing through the pipe is calculated from the admittance A of the piezoelectric vibrator based on the previously calculated correlation between the admittance of the piezoelectric vibrator and the flow rate of the fluid flowing through the pipe. [Effects of the Invention]

[0012] According to the flow rate measurement method and flow rate measurement device of the present invention, while having a simple device configuration in which a piezoelectric vibrator is simply attached to a pipe through which a fluid to be measured flows, the device utilizes the correlation between the admittance of the piezoelectric vibrator and the flow rate of the fluid flowing in the pipe to determine the admittance A of the piezoelectric vibrator, which is caused by a change in the thickness of the boundary layer on the inner wall of the pipe due to a change in the flow velocity of the fluid flowing in the pipe, and this change in the mechanical load on the piezoelectric vibrator, and then determines the flow rate of the fluid flowing in the pipe from the admittance A of the piezoelectric vibrator based on the previously determined correlation between the admittance of the piezoelectric vibrator and the flow rate of the fluid flowing in the pipe, thereby making it possible to accurately measure the flow rate of the fluid to be measured with little risk of malfunction.Furthermore, according to the present invention, since it is not necessary to calculate a cross-correlation function to estimate the propagation time difference of sound waves, high time responsiveness can be expected. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 shows a schematic configuration of a flow measurement device for carrying out the flow measurement method of the present invention, where FIG. 1(a) is a front view including a cross section of a pipe 1 made of PFA through which water flows, and FIG. 1(b) is a plan view of FIG. 1(a) with the pipe 1 removed. [Figure 2] FIG. 2 is a perspective view showing a state in which electrode films are provided on the entire upper and lower surfaces of the piezoelectric vibrator. [Figure 3] FIG. 3 shows the relationship between the water flow rate and the resonant frequency and admittance of the piezoelectric vibrator when the flow rate measurement method of the present invention is implemented, where FIG. 3(a) shows the case where the water temperature is 8°C, and FIG. 3(b) shows the case where the water temperature is 30°C. [Figure 4] FIG. 4 is a schematic cross-sectional view of the ultrasonic flow rate measuring device described in Patent Document 1. [Figure 5] FIG. 5 is a perspective view of the ultrasonic flow rate measuring device described in Patent Document 1. [Figure 6] FIG. 6 is a diagram showing the overall configuration of the ultrasonic flowmeter described in Patent Document 2. [Figure 7]FIG. 7 is a diagram showing two measurement lines with different opening angles of the ultrasonic flowmeter described in Patent Document 2. [Figure 8] FIG. 8 is a schematic cross-sectional view of an apparatus for carrying out the flow rate measuring method described in Patent Document 3. DETAILED DESCRIPTION OF THE INVENTION

[0014] Specific embodiments of the present invention will be described below, but the present invention is not limited to the following embodiments, and various modifications and alterations are possible within the scope of the technical scope of the present invention.

[0015] FIG. 1 shows a schematic configuration of a flow measurement device for implementing a flow measurement method of the present invention. FIG. 1(a) is a front view including a cross section of a pipe 1 made of perfluoroalkoxyalkane (PFA) through which water flows perpendicular to the paper surface. 2 is a piezoelectric transducer made of lead zirconate titanate (PZT), and 3 is a silicone acoustic matching layer that matches the acoustic impedance of the piezoelectric transducer 2 with the acoustic impedance of the fluid to be measured, water. An ultrasound diagnostic gel was interposed between the pipe 1 and the acoustic matching layer 3, and a similar gel was interposed between the acoustic matching layer 3 and the piezoelectric transducer 2 to secure these components. FIG. 1(b) is a plan view of FIG. 1(a) without the pipe 1. The pipe 1 has an outer diameter of 9.53 mm and an inner diameter of 6.35 mm. The piezoelectric transducer 2 has dimensions of 10 mm long, 10 mm wide, and 2 mm thick.

[0016] Acoustic impedance Z is defined by the sound velocity C in a material and density ρ as shown in the following equation (1). Z=ρ(kg / m 3 ) × C (m / s) (1)

[0017] The acoustic impedance of PZT, which is a piezoelectric vibrator used as a vibration means, is significantly different from that of water, which is the propagation medium of sound waves. For example, the acoustic impedance (Z0) of piezo ceramics such as PZT is 30×10 6 kg / m 2·s. The acoustic impedance of water (Z1) is 1.44×10 6 kg / m 2 ·s. As such, reflection occurs during sound wave propagation on a boundary surface with different acoustic impedances, weakening the strength of the transmitted sound waves. To solve this problem, a substance with an acoustic impedance that satisfies the relationship in equation (2) below is inserted between the piezoelectric vibrator, which is the vibration means, and water, which is the propagation medium of the sound waves, with their respective acoustic impedances Z0 and Z1. This reduces sound reflection and increases the strength of the transmitted sound waves. Z = (Z0 × Z1) 1 / 2 (2) The material having the acoustic impedance that satisfies the relationship of formula (2) is the acoustic matching layer. In this embodiment, a silicone sheet with a thickness of 0.5 mm was used.

[0018] When water flows through the pipe 1 of a flow measuring device such as that shown in Figure 1, the following two layers are formed inside the pipe 1. The first layer, or boundary layer, is formed in the area very close to the inner wall surface of the pipe 1, where the effects of viscosity are pronounced and the velocity gradient is very large, resulting in large shear friction stress. The second layer is formed in the entire inner area excluding this first layer, where the effects of viscosity are almost nonexistent and the flow is similar to that of an ideal fluid. The thickness of the boundary layer varies depending on the Reynolds number; generally, when the Reynolds number is small, the boundary layer thickness is relatively large, and when the Reynolds number is large, the boundary layer thickness is relatively small.

[0019] The Reynolds number Re is expressed by the following equation (3). Re=Um×D / ν (3) Um is the velocity of water flowing through pipe 1 (m / s), D is the diameter of pipe 1 (m), and ν is the dynamic viscosity of water (m 2 / s).

[0020] As shown in equation (3), Re changes with the velocity of the water flowing through pipe 1, and this change in Re also changes the thickness of the boundary layer. The change in boundary layer thickness changes the mechanical load on the piezoelectric element. As the mechanical load on piezoelectric element 2 changes, it is expected that the electrical characteristics of piezoelectric element 2 will change. The piezoelectric effect can be divided into two types: the longitudinal piezoelectric effect (which extracts power in the same direction as the applied force) and the transverse piezoelectric effect (which extracts power perpendicular to the applied force), depending on the relationship between the direction of the force applied to the piezoelectric material and the direction of the extracted power (electrode configuration). The longitudinal piezoelectric effect is the simplest power generation method, and a representative example is a shoe sole made of piezoelectric materials such as PZT or PVDF (polyvinylidene fluoride). However, it has the disadvantage of being prone to element damage due to stress concentration. On the other hand, transverse piezoelectric effect devices, which utilize the deflection of a cantilever-structured piezoelectric element to generate power perpendicular (in the thickness direction of the cantilever) to the strain direction (in the length direction of the cantilever), are widely used in practice.

[0021] Therefore, in this invention, we focused on the relationship between the flow rate of water flowing through pipe 1 in Fig. 1 and the resonant frequency and admittance due to the transverse piezoelectric effect of piezoelectric vibrator 2. As shown in Fig. 2, electrode films (silver) 11 and 12 were applied to the entire top and bottom surfaces of piezoelectric vibrator 2 in Fig. 1. An AC voltage signal generated by a function generator (not shown) was amplified by a power amplifier (not shown) and applied to piezoelectric vibrator 2 with electrode films 11 and 12. The temperature of the water flowing through pipe 1 was set to 8°C or 30°C, and the flow rate of the water was varied from 0 to 4.5 (liters / min). The frequency characteristics of the admittance of piezoelectric vibrator 2 were measured using an impedance analyzer (not shown). Fig. 3(a) shows the relationship between the water flow rate (horizontal axis) and the admittance (●) and resonant frequency (▲) when the water temperature was 8°C. Fig. 3(b) shows the relationship between the water flow rate (horizontal axis) and the admittance (●) and resonant frequency (▲) when the water temperature was 30°C.

[0022] As shown in Figures 3(a) and 3(b), it can be seen that there is a roughly inverse correlation between the water flow rate and the admittance of the piezoelectric vibrator. Therefore, in the flow rate measuring device configured as in Figure 1, if the relationship shown in Figure 3(a) or 3(b) is determined in advance, the flow rate of water flowing through pipe 1 can be estimated from the admittance of piezoelectric vibrator 2 obtained at that time. Naturally, the fluid flowing through pipe 1 is not limited to water. [Explanation of symbols]

[0023] 1 Piping 2 Piezoelectric vibrators 3 Acoustic matching layer 11, 12 Electrode membrane

Claims

1. A flow rate measurement method comprising the steps of: attaching a piezoelectric vibrator to a pipe through which a fluid to be measured flows; determining an admittance A of the piezoelectric vibrator resulting from a change in the thickness of the boundary layer on the inner wall of the pipe caused by a change in the flow rate of the fluid flowing through the pipe, and determining the flow rate of the fluid flowing through the pipe from the admittance A of the piezoelectric vibrator based on a previously determined correlation between the admittance of the piezoelectric vibrator and the flow rate of the fluid flowing through the pipe.

2. 2. The flow rate measuring method according to claim 1, further comprising providing an acoustic matching layer between the piezoelectric vibrator and a pipe through which the fluid to be measured flows.

3. A flow measuring device characterized by: attaching a piezoelectric vibrator to a pipe through which a fluid to be measured flows; determining an admittance A of the piezoelectric vibrator resulting from a change in the thickness of the boundary layer on the inner wall of the pipe caused by a change in the flow rate of the fluid flowing through the pipe, and determining the flow rate of the fluid flowing through the pipe from the admittance A of the piezoelectric vibrator based on a previously determined correlation between the admittance of the piezoelectric vibrator and the flow rate of the fluid flowing through the pipe.

4. 4. The flow rate measuring device according to claim 3, further comprising an acoustic matching layer provided between the piezoelectric vibrator and a pipe through which the fluid to be measured flows.

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