A method for identifying gas-liquid two-phase flow patterns in pipelines based on pulsed Doppler technology

Abstract

The present invention discloses a method for identifying the flow pattern of gas-liquid two-phase flow in a pipeline based on pulse Doppler technology. Based on the pulse Doppler principle, the difference in Doppler frequency shift and amplitude of echo signals between the gas and liquid phases is utilized, the Doppler signal in the echo signal is extracted through an orthogonal demodulation method, and a time-frequency spectrum is drawn using short-time Fourier transform. The size of bubbles in the measurement channel is then judged based on the time-frequency spectrum, thereby identifying the flow pattern of the fluid in the pipeline. The method has the advantages of fast detection rate, high accuracy, stable performance and wide application range.

Description

Technical Field

[0001] The present invention belongs to the field of fluid mechanics detection, and in particular relates to a method for identifying the flow pattern of gas-liquid two-phase flow in a pipeline based on pulse Doppler technology. Background Art

[0002] Gas-liquid two-phase flow is widely present in nature and in human production and life. With the development of science and technology, gas-liquid two-phase flow is also widely used in the fields of petrochemical industry, pipeline transportation, nuclear industry, and aerospace. For example, in the cooling circulation system of a nuclear power plant reactor, the gas-liquid two-phase flow in the core will affect the normal operation of the power plant; in the long-distance transportation of oil, a mixed oil and gas method is often used, and the gas-liquid two-phase flow in the pipeline will affect the transportation efficiency; in the metallurgical industry, the gas-liquid two-phase flow in the molten pool will affect the flow and cooling of molten steel. Gas-liquid two-phase flow often leads to accidents due to its complex and changeable flow characteristics. Accurate and real-time identification and detection of gas-liquid two-phase flow can effectively reduce the probability of accidents in industrial production. Therefore, exploring different gas-liquid two-phase flow identification methods is of great significance both in scientific research and production and life.

[0003] Methods for detecting gas-liquid two-phase flow are primarily categorized as invasive and non-invasive. Invasive methods primarily include hot-wire flowmeters and conductive probes, both of which can detect fluid disturbances and identify the flow pattern of the gas-liquid two-phase flow. However, invasive sensors are typically placed within the fluid, which can interfere with the flow. Therefore, the impact of sensors within the fluid on the flow pattern must be considered when processing experimental data. Non-invasive methods primarily include optical, ultrasonic, and electrical methods, which cause less disturbance to the pipeline. Summary of the Invention

[0004] The purpose of the present invention is to overcome the defects of the prior art and provide a method for identifying the flow pattern of gas-liquid two-phase flow in a pipeline based on pulse Doppler technology with fast detection rate, high accuracy, stable performance and wide application range.

[0005] To solve the above technical problems, the present invention proposes a method for identifying the flow pattern of gas-liquid two-phase flow in a pipeline based on pulse Doppler technology, which is characterized by comprising the following steps:

[0006] Step 1: First, connect the ultrasonic transducer to the ultrasonic detector, the ultrasonic detector to the digitizer, and the digitizer to the computer; then add an appropriate amount of tap water to the water tank, turn on the water pump to form circulating water in the pipeline; finally, turn on the air pump to inject compressed air into the pipeline, and control the gas flow through the regulating valve;

[0007] Step 2: Place the ultrasonic transducer close to the pipe to be tested, and make the incident angle of the ultrasonic wave emitted by it 45 degrees. The ultrasonic detector generates a pulse signal under the control of the computer. The ultrasonic transducer converts the signal into ultrasonic wave and transmits it to the pipe to be tested. At the same time, the ultrasonic wave enters the pipe and generates an echo signal due to the reflection of the sound intensity when it encounters a moving scatterer;

[0008] Step 3: The computer uses an orthogonal demodulation method to extract the required Doppler signal from the echo signal received by the ultrasonic transducer;

[0009] Step 4: Use short-time Fourier transform to extract the Doppler amplitude of various flow types in the echo signal, draw the probability density function, and set the Doppler amplitude threshold;

[0010] Step 5: Draw a time-frequency spectrum diagram based on the set threshold, and identify various flow types by observing the color changes of the Doppler amplitude in the time-frequency spectrum diagram.

[0011] The expression of the sound intensity reflectivity in step 2 is:

[0012]

[0013] Where, is the density of the scatterer, is the fluid density, c1 is the sound velocity in the scatterer, c2 is the sound velocity of the fluid, Z1 is the acoustic impedance of the scatterer, and Z2 is the acoustic impedance of the fluid.

[0014] When the orthogonal demodulation method is used to extract the Doppler echo signal in step 3, the ultrasonic detector first controls the ultrasonic transducer to continuously emit pulses at a frequency of The ultrasonic wave is then received by the ultrasonic transducer, where the echo signal n Pulses at the measurement position i Receiving time at t ni for:

[0015]

[0016] Where, d is the distance between the measured particle and the transducer; d It can be expressed as:

[0017]

[0018] in c is the propagation speed of ultrasound in liquid, t is the time from the transducer transmitting to receiving the signal. According to this formula, the position of the measured bubble in the pipeline can be determined;

[0019] Echo signal reflected by the measured particle f(t)It can be expressed as:

[0020]

[0021] Where, Pulse repetition time T prf The distance the measured particle moves.

[0022] Get echo signal f(t) After that, the echo signal is firstly orthogonally demodulated, that is, f(t) Multiply the sine component and cosine component respectively, and then eliminate the carrier component and fundamental frequency component through a low-pass filter. The continuous Doppler signal obtained is:

[0023]

[0024] exist t ni The continuous Doppler signal is sampled at all times to obtain a discrete Doppler signal D i ( n )for:

[0025]

[0026] Where, φ i For the same direction signal X I and orthogonal signals X Q The initial phase.

[0027] When using short-time Fourier transform to extract the Doppler echo signal in step 4, first perform short-time Fourier transform on the Doppler signal, then divide the signal into multiple adjacent windows, and use formula (7) to convert the signal of each window into a spectrum along several shorter time periods;

[0028]

[0029] The time spectrum diagram is T The Doppler amplitude and frequency shift at the moment can be expressed by formulas (8) and (9):

[0030]

[0031] The time-frequency spectrum obtained by short-time Fourier transform can intuitively show the time-frequency characteristics of the Doppler signal, and the bubble size on the measurement channel can be determined through the time-frequency spectrum.

[0032] Based on pulse Doppler technology, this paper proposes a method for identifying the flow pattern of gas-liquid two-phase flow by exploiting the differences in Doppler frequency shift and amplitude of echo signals between the gas and liquid phases. By extracting the Doppler signal from the echo signal through orthogonal demodulation, and using short-time Fourier transform to create a time-frequency spectrum, the time-frequency spectrum is used to determine the size of bubbles in the measurement channel, thereby identifying the flow pattern of the fluid in the pipeline. Compared with other fluid detection methods, this method has the following advantages:

[0033] 1. The transducer can be placed outside the closed pipe to avoid any interference with the fluid;

[0034] 2. A single transducer is used to transmit and receive ultrasonic waves, and the transducer is very small (usually about 8 mm in diameter), so the disturbance to the fluid is minimal;

[0035] 3. Pulse Doppler technology can be used to detect opaque liquids such as sewage, liquid metal, chemical reagents, etc., which cannot be detected by optical methods;

[0036] 4. Based on the Doppler frequency shift signal, pulse Doppler technology can detect the flow direction of the fluid at each location in the pipeline, thereby determining the velocity vector at that location;

[0037] 5. No complex ultrasonic beam forming system is required between the transducer and the pipe being tested. It only requires an appropriate amount of reflective particles in the pipe being tested to generate ultrasonic echoes. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Principle of pulse Doppler measurement method for pipeline flow pattern;

[0039] Figure 2 Ultrasonic echo signal processing method;

[0040] Figure 3 Schematic diagram of two-phase flow pattern in pipeline;

[0041] Figure 4 Pipeline two-phase flow pattern pulse Doppler identification and detection system;

[0042] Figure 5 Probability density function for different pipe ventilation rates;

[0043] Figure 6 Time-frequency spectra of different flow patterns. DETAILED DESCRIPTION

[0044] To more clearly explain the means and effects achieved by the present invention, the specific implementation methods, steps, and effects achieved by the method for identifying the flow pattern of gas-liquid two-phase flow in a pipeline based on pulse Doppler technology proposed by the present invention are described in detail with reference to the following examples:

[0045] The parameters in this example are: the center frequency of the ultrasonic transducer is 4 MHz; the pulse repetition frequency is f PRF 4kHz, pulse repetition time T PRF It is the reciprocal of the pulse repetition frequency, i.e. 0.25ms; the transmission frequency f 0 4MHz; the propagation speed of ultrasound in water c 1480m / s,

[0046] The measurement principle of pulse Doppler technology is as follows Figure 1 As shown, the transducer transmits ultrasonic waves to the pipe under test. When the ultrasonic waves are reflected by bubbles in the pipe under test, an echo signal is generated. At this time, the transducer switches to the receiving mode to receive the signal. The received signal is processed by the ultrasonic detector and digitizer and stored in the computer.

[0047] The acoustic intensity reflectivity in the pulse Doppler measurement principle is expressed by formula (1). In this example, the acoustic impedance of the gas phase is Z 1 is 40×10 5 , the acoustic impedance of the liquid phase Z 2 is 1.48×10 5 According to formula (1), the reflectivity of the gas-liquid interface is greater than that of pure water. Therefore, when bubbles appear in the measurement channel of the ultrasonic transducer, the echo signal intensity reflected by the bubbles is greater than the echo signal intensity reflected by pure water. Based on this, it can be determined whether the flow type in the pipeline is gas-liquid two-phase flow or pure water.

[0048] When the ultrasonic wave encounters a bubble, the position of the bubble can be determined according to formula (3). c is the propagation speed of ultrasound in liquid, t It is the time from the transducer transmitting to receiving the signal. According to this formula, the position of the bubble to be measured in the pipeline can be determined.

[0049] Various flow patterns in the tested pipeline such as Figure 2 Show, Figure 2 ( a ) indicates a small ventilation volume (below 2.0 L / min) and a bubbly flow. It can be seen that the bubbles in the tested pipe are small; Figure 2 ( b ) When the ventilation rate is increased to 2.2 L / min, it can be seen that due to the increase in ventilation rate, small bubbles gather above the pipe to form large bubbles. This flow pattern is called slug flow. Figure 2 ( c) is the case of further increasing the ventilation volume. When the ventilation volume is further increased to above 3.5L / min, there are both large and small bubbles in the pipeline, and they flow upward at a very high speed. The characteristic of this flow type is that the flow form is chaotic and difficult to measure.

[0050] Figure 3 Schematic diagram of the experimental setup for pulsed Doppler identification and detection of multiphase flow. The specific experimental steps are as follows:

[0051] 1. First, add tap water to the water tank and use a water pump to let the tap water flow into the pipe to be tested;

[0052] 2. Connect the ultrasonic transducer to the ultrasonic detector through a wire, connect the ultrasonic detector to the digitizer, and connect the digitizer to the computer;

[0053] 3. Then the computer controls the ultrasonic detector to send a pulse signal, which is converted into ultrasonic waves by the ultrasonic transducer and sent to the tested pipe at a transmission frequency of 4MHz;

[0054] 4. When ultrasonic waves enter the pipe being tested and encounter scatterers, they will generate reflected echoes. The echo signals are received by the ultrasonic transducer and finally stored in the computer through the digitizer.

[0055] The echo signal received by the transducer is processed as follows Figure 4 As shown, the ultrasonic echo signal received by the transducer contains Doppler frequency shift f d and transmission frequency f 0 The echo signal is converted into f(t) Multiply it with the sine component and cosine component, then filter out the fundamental frequency component and carrier through a low-pass filter to obtain the continuous Doppler signal, which is then handed over to the computer for processing.

[0056] First, the Doppler signal in complex discrete form is obtained from the echo signal according to formulas (2), (4), (5), and (6). The signal is expressed in array form as follows: Then, according to formula (7), the Doppler signal is short-time Fourier transformed to obtain the signal spectrum: .

[0057] According to formulas (8) and (9), the Doppler amplitude and frequency shift are extracted from the Doppler signal, and the probability density function and time-frequency spectrum can be plotted.

[0058] Plot the probability density function:

[0059] like Figure 1As shown, first fill the water tank with enough tap water. When there is enough tap water in the water tank, turn on the water pump to allow the tap water to flow into the experimental device. The computer controls the ultrasonic detector to generate a pulse signal with a frequency of 4MHz, which is converted into 4MHz ultrasonic waves through the ultrasonic transducer. The ultrasonic waves are injected into the pipe under test at an incident angle of 45°. When the ultrasonic wave encounters a scatterer in the pipe under test, an echo signal will be generated. The echo signal is digitized by the digitizer and stored on the computer. When the flow rate of the liquid in the pipe under test remains unchanged, the bubble volume increases with the increase of the air pump ventilation volume. When the bubble volume changes with the ventilation volume, the Doppler amplitude can be extracted from the echo signal to draw a probability density function graph. About 10,000 sets of data are counted at different ventilation volumes to draw a probability density function graph. Figure 5 The probability density function diagrams are shown when the ventilation volumes are 0.7 L / min, 1.2 L / min, 1.7 L / min and 2.2 L / min respectively; Figure 5 ( a ) is the probability density function at a ventilation volume of 0.7 L / min, and the Doppler amplitude of this ventilation volume is 14.6 mV; Figure 5 ( b ) is the probability density function diagram when the ventilation volume is 1.2 L / min, and the Doppler amplitude of the ventilation volume is 16.9 mV; Figure 5 ( c ) is the probability density function diagram when the ventilation volume is 1.7 L / min, and the Doppler amplitude of the ventilation volume is 20.2 mV; Figure 5 ( d ) is the probability density function diagram when the ventilation volume is 2.2L / min, and the Doppler amplitude of this ventilation volume is 105.6mV.

[0060] Depend on Figure 5 It can be seen that, given the same interface acoustic intensity reflectivity, the Doppler amplitude of bubbles tends to shift to the right as ventilation increases. When ventilation is sufficiently high, small bubbles transform into large ones, and the Doppler amplitude peak shifts significantly to the right. Therefore, the range of Doppler amplitude can be used to determine the presence and size of bubbles.

[0061] according to Figure 5 The legend of the time-frequency spectrum is divided according to the range of Doppler amplitudes for different bubble sizes: the probability of small bubbles appearing in the range of Doppler amplitudes less than 10mV is less than 7%, and the probability of small bubbles appearing in the range of greater than 100mV is less than 16.7%. Therefore, it can be considered that bubbles with a Doppler amplitude range of 10-100mV are small-sized bubbles, and bubbles with a Doppler amplitude range of more than 100mV are large-sized bubbles. Therefore, the legend of the time-frequency spectrum is set to 10-100mV.

[0062] Plot a time-frequency spectrum:

[0063] After setting the legend of the time-frequency spectrum through the probability density function diagram, the short-time Fourier transform is used to perform time-frequency-amplitude analysis on different echo signals, extract the Doppler frequency shift from the echo signal, and draw the time-frequency spectrum diagram. The time-frequency spectrum diagrams of pure water, bubbly flow, and slug flow can be obtained, such as Figure 6 As shown in the figure, the horizontal axis represents the number of channels (i.e., distance, with one channel equal to 0.74 mm), the vertical axis represents the Doppler shift, the legend represents the Doppler amplitude, and the background is a distinctive gray. As the Doppler shift increases, the color of the time-frequency spectrum gradually changes from white to black. It can be seen that when the amplitude is between 20mV and 100mV, the spectrum displays white, gray, dark gray, and black; when the amplitude is above 100mV, the spectrum displays black. Figure 6 ( a ) is the time-frequency spectrum of pure water without bubbles. It can be seen that the time-frequency spectrum at this time is all background color, and no bubbles are generated in the measured pipe. Figure 6 ( b ) is the time-frequency spectrum diagram for bubbly flow. It can be seen that most of the area of ​​the time-frequency spectrum diagram is the background color, but there is also a small white area, indicating that small bubbles pass through the measurement channel at this time. The smaller white area is because the size of the bubbles is small and the velocity range they occupy is small. Figure 6 ( c ) is a time-frequency spectrum for slug flow. The light gray area in the figure shows large white areas and small black areas. The small black areas represent the energy peaks of large bubbles in the echo signal, while the large white areas indicate large bubbles occupying a wide velocity range. This indicates that the pulse Doppler technology can identify various flow patterns in the measured pipeline.

Claims

1. A method for identifying the flow pattern of gas-liquid two-phase flow in a pipeline based on pulse Doppler technology, characterized in that The steps include: Step 1: First, connect the ultrasonic transducer to the ultrasonic detector, the ultrasonic detector to the digitizer, and the digitizer to the computer; then add an appropriate amount of tap water to the water tank, turn on the water pump to form circulating water in the pipeline; finally, turn on the air pump to inject compressed air into the pipeline, and control the gas flow through the regulating valve; Step 2: Place the ultrasonic transducer close to the pipe to be tested, and make the incident angle of the ultrasonic wave emitted by it 45 degrees. The ultrasonic detector generates a pulse signal under the control of the computer. The ultrasonic transducer converts the signal into ultrasonic wave and transmits it to the pipe to be tested. At the same time, the ultrasonic wave enters the pipe and generates an echo signal due to the reflection of the sound intensity when it encounters a moving scatterer; Step 3: The computer uses the orthogonal demodulation method to extract the required Doppler signal from the echo signal received by the ultrasonic transducer; specifically, after obtaining the echo signal, the echo signal is first orthogonally demodulated, that is, the echo signal is multiplied by the sine component and the cosine component respectively, and then the carrier component and the fundamental frequency component are eliminated through a low-pass filter to obtain a continuous Doppler signal. ni The continuous Doppler signal is sampled at every moment to obtain a discrete Doppler signal, where t ni is the reception time of the nth pulse at the measurement position i; Step 4: Use short-time Fourier transform to extract the Doppler amplitude of various flow types in the echo signal, draw the probability density function, and set the Doppler amplitude threshold; When using short-time Fourier transform to extract the Doppler echo signal, first perform short-time Fourier transform on the Doppler signal, then divide the signal into multiple adjacent windows, and use formula (1) to convert the signal of each window into a spectrum along several shorter time periods; The Doppler amplitude and frequency shift of the time-spectrum diagram at time T can be expressed by formulas (2) and (3): A(f,f D )=|U(f,f D )| 2 (2) The time-frequency spectrum obtained by short-time Fourier transform can intuitively show the time-frequency characteristics of the Doppler signal, and the bubble size on the measurement channel can be determined by the time-frequency spectrum. Step 5: Draw a time-frequency spectrum diagram based on the set threshold, and identify various flow types by observing the color changes of the Doppler amplitude in the time-frequency spectrum diagram.

2. The method according to claim 1, characterized in that The expression of the sound intensity reflectivity in step 2 is: Where ρ1 is the density of the scatterer, ρ2 is the density of the fluid, c1 is the speed of sound in the scatterer, c2 is the speed of sound in the fluid, Z1 is the acoustic impedance of the scatterer, and Z2 is the acoustic impedance of the fluid.

3. The method according to claim 1, characterized in that When the orthogonal demodulation method is used to extract the Doppler echo signal in step 3, the ultrasonic detector first controls the ultrasonic transducer to continuously emit pulses with a frequency of f prf The ultrasonic wave is then received by the ultrasonic transducer, where the nth pulse is received at the measurement position i at time t ni for: d can be expressed as: Where c is the propagation speed of ultrasound in the liquid, and t is the time from the transducer transmitting to receiving the signal. The position of the bubble to be measured in the pipe is determined according to this formula; The echo signal f(t) reflected by the measured particle can be expressed as: Where △x is the pulse repetition time T prf The distance the measured particle moves; After obtaining the echo signal f(t), the echo signal is first orthogonally demodulated, that is, f(t) is multiplied by the sine component and cosine component respectively, and then the carrier component and fundamental frequency component are eliminated through a low-pass filter. The continuous Doppler signal obtained is: In t ni The continuous Doppler signal is sampled at every moment to obtain the discrete Doppler signal D i (n) is: Where, Is the same direction signal X I and the orthogonal signal X Q The initial phase.

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