Method for measuring liquid film thickness of horizontal gas-liquid slug flow based on ultrasonic doppler effect

Abstract

The present application relates to a kind of horizontal gas-liquid slug flow liquid film thickness measurement methods based on ultrasonic Doppler effect, comprising the following steps: build horizontal gas-liquid slug flow ultrasonic Doppler measurement system, obtain echo signal using double-crystal ultrasonic probe, and digital echo signal is collected;Digital echo signal is sequentially subjected to orthogonal demodulation, low-pass filtering processing, and signal only containing Doppler information related to the velocity of moving bubble is obtained;Doppler signal is reconstructed;Doppler frequency shift information is extracted, and the velocity sequence of bubble movement at the radial position of pipeline is obtained;The velocity-time distribution matrix of bubble is obtained;Liquid film area and liquid plug area are identified;The velocity of Taylor bubble position is zero, and the dispersed gas bubble at the bottom of Taylor bubble can cause Doppler frequency shift, so that non-zero velocity value is presented in velocity-time distribution, the position of the farthest non-zero velocity value of liquid film area from double-crystal ultrasonic probe is detected, and the measurement of slug flow liquid film thickness is realized;Liquid film thickness sequence is obtained.

Description

Technical Field

[0001] This invention relates to a method for measuring the thickness of a liquid film in a horizontal gas-liquid slug flow based on the ultrasonic Doppler effect. Background Technology

[0002] Gas-liquid slug flow is widely present in important industrial production fields such as oil and gas extraction and chemical production. In horizontal pipeline transportation, gas-liquid slug flow exhibits a quasi-periodic motion with alternating liquid film and liquid slug regions. In the liquid slug region, water is the continuous phase, filled with dispersed bubbles; the liquid film region consists of a Taylor bubble and the liquid film below it, containing a small number of moving bubbles. As the gas flow rate increases, the concentration of dispersed bubbles in both the liquid slug and liquid film regions continuously increases, and the motion gradually becomes more complex, exhibiting three typical flow structures: low-filling slug flow, high-filling slug flow, and pseudo-slug flow. Liquid film thickness is an indispensable and important parameter for measuring the gas holdup in slug flow and studying the interphase heat / mass transfer laws; its accurate measurement is of great significance for optimizing chemical production.

[0003] Ultrasonic methods offer the advantage of being non-invasive compared to conventional optical and electrical methods, making them suitable for fluid measurement in non-transparent pipes and adaptable to harsh industrial production environments. Traditional ultrasonic liquid film thickness measurement is based on the time-of-flight (TOF) of ultrasonic waves (Zhai et al., IEEE Transactions on Instrumentation and Measurement, 2021, 70:1-10). By detecting the delay time of ultrasonic waves reflected from the interface from emission to reception, combined with the velocity of ultrasonic waves in water, the gas-liquid interface position can be detected and the liquid film thickness measured. It is worth noting that when the gas flow rate in slug flow is high, the bubble concentration in the liquid film region increases significantly, and even a bubble layer phenomenon may occur. In this case, the echo signal of the bubble layer cannot be ignored, making it difficult to detect the gas-liquid interface position and resulting in errors between the measured liquid film thickness and the true value. Therefore, it is necessary to explore novel methods for measuring liquid film thickness in slug flow. Summary of the Invention

[0004] The purpose of this invention is to propose a method for measuring the liquid film thickness of a horizontal gas-liquid slug flow based on the ultrasonic Doppler effect. The ultrasonic Doppler method measures the spatiotemporal distribution of velocity in a horizontal gas-liquid slug flow, effectively distinguishing between the liquid film region and the slug region, detecting the velocity of the farthest bubble in the liquid film region from the ultrasonic probe, and measuring the liquid film thickness of the slug flow by identifying the location of the farthest bubble velocity. The technical solution is as follows.

[0005] A method for measuring the thickness of a liquid film in a horizontal gas-liquid slug flow based on the ultrasonic Doppler effect includes the following steps:

[0006] (1) A horizontal gas-liquid slug flow ultrasonic Doppler measurement system was built, and echo signals were obtained by using a dual-crystal ultrasonic probe to acquire digital echo signals.

[0007] (2) The digital echo signal is sequentially subjected to quadrature demodulation and low-pass filtering to obtain a signal u containing only Doppler information related to the velocity of the moving bubble. dpl (t);

[0008] (3) Reconstruct the Doppler signal. The method is as follows: Let the Doppler angle be θ. For the delay time τ from the transmission to reception of the ultrasonic pulse, extract the demodulated and low-frequency filtered signal u of different ultrasonic pulses. dpl (t) amplitude information; the delay time τ corresponds to a certain radial position x of the pipe, that is, x=τc sinθ / 2, where c is the propagation speed of the ultrasonic wave; the extracted amplitude information is reconstructed into a signal according to the pulse sequence, and this reconstructed signal is the Doppler signal at the radial position x of the pipe;

[0009] (4) Let the repetition frequency of the ultrasonic pulses generated by the dual-crystal ultrasonic probe under the action of the excitation signal be f. prf The frequency of the Doppler signal is the Doppler frequency shift f caused by the moving bubble. D Select the pulse repetition number N pulse The Doppler signal at the radial position x of the pipe is divided into different time segments, each time segment being N. pulse / f prf Seconds; perform Fast Fourier Transform on signals within different time segments to extract Doppler frequency shift information f. D The Doppler frequency shift of each time segment is converted into the corresponding bubble velocity v using the following formula:

[0010]

[0011] The velocity sequence v(t) of the bubble at the radial position x in the pipe is obtained;

[0012] (5) Repeat the above steps for all radial positions in the pipe to obtain the spatiotemporal distribution matrix of bubble velocity v(x,t), where x represents the radial position in the pipe and t represents time; this spatiotemporal distribution matrix v(x,t) can describe the spatiotemporal distribution of bubble velocity in slug flow.

[0013] (6) Identify the liquid film region and liquid plug region from the velocity spatiotemporal distribution matrix v(x,t), as follows:

[0014] ① Input the spatiotemporal distribution matrix of velocity v(x,t), and search for the maximum velocity value of the matrix, denoted as V. max V max This represents the maximum velocity of the air bubble in the liquid plug region;

[0015] ② In the velocity spatiotemporal distribution matrix v(x,t), each column element v(:,t) corresponds to an instantaneous velocity profile; the maximum value of the search column element is set to v. max The maximum velocity value of the velocity profile at that moment; v max With V max By comparing and selecting an appropriate velocity threshold α, if v max <αV max If the velocity of the bubble at any position at time t does not conform to the characteristics of a moving bubble in the liquid plug region, then time t is identified as the liquid film region; if v max ≥αV max Count the number of non-zero elements in the upper half of the pipe in v(:,t), denoted as C. num Select an appropriate threshold β for the number of velocities, when C num When the time is less than the threshold β, time t is identified as the liquid film region; otherwise, it is identified as the liquid plug region.

[0016] ③ Construct a binary identifier signal S mark The signal consists of 0 and 1, where 0 and 1 represent the liquid film region and the liquid plug region, respectively;

[0017] ④ Based on the binary identifier signal S mark The system corrects for any erroneous signals that may occur, and the corrected binary identifier signal is still denoted as S. mark ; Using the corrected binary identifier signal S mark Identify the liquid film area and the liquid plug area;

[0018] (7) The velocity at the location of the Taylor bubble is zero. The dispersed bubbles at the bottom of the Taylor bubble can cause a Doppler frequency shift, thus presenting a non-zero velocity value in the velocity spatiotemporal distribution. By detecting the location of the non-zero velocity value farthest from the dual-crystal ultrasonic probe in the liquid film region, the thickness of the slug flow liquid film is measured. When the bubble closest to the bottom of the Taylor bubble passes through the measurement line, the starting point and ending point of the reflected ultrasonic signal generated by the bubble are set as P1 and P2, respectively. The radial positions of the pipe corresponding to P1 and P2 are x k and x j The radius r of the moving bubble is calculated using the following formula:

[0019]

[0020] Where r is the radius of the moving bubble; the bubble closest to the bottom of the Taylor bubble generates a local velocity v in the instantaneous velocity profile. f Local velocity v f The position of occurrence is represented by x k -x j express;

[0021] Due to the relationship between the liquid film thickness h and x k There exists a difference Δh:

[0022] Δh=rr cosθ

[0023] The liquid film thickness h is expressed as:

[0024]

[0025] (8) Obtain the liquid film thickness sequence H(t).

[0026] Furthermore, the horizontal gas-liquid slug flow ultrasonic Doppler measurement system includes a dual-crystal ultrasonic probe (T, R), an excitation module, a signal conditioning module, an FPGA module, and a signal acquisition and data transmission module. The dual-crystal ultrasonic probe is installed at an angle of Doppler angle θ on the lower pipe wall. The excitation module generates an excitation signal under the control signal of the FPGA. The piezoelectric material inside the dual-crystal ultrasonic probe generates pulsed ultrasonic waves under the action of the excitation signal, and converts the ultrasonic waves reflected by the moving bubbles in the gas-liquid two-phase flow in the pipe into the original echo signal. After amplitude amplification and bandpass filtering by the signal conditioning module, it is converted into a digital echo signal by the signal acquisition module.

[0027] Furthermore, the digital echo signal is sequentially subjected to quadrature demodulation and low-pass filtering to obtain a signal u containing only Doppler information related to the velocity of the moving bubble. dpl (t), the method is as follows: the amplified echo signal is divided into low-frequency and high-frequency components by orthogonal demodulation. The high-frequency component contains the second harmonic of the ultrasonic probe center frequency and Doppler frequency shift information, while the low-frequency component is only related to the Doppler frequency shift. A low-pass filter is introduced to filter out the high-frequency component in the orthogonal demodulated signal, obtaining a signal u that contains only Doppler information related to the velocity of the moving bubble. dpl (t).

[0028] Furthermore, a speed threshold α = 0.65 was selected.

[0029] Further, the method of step (7) is as follows: using the standard deviation of the liquid film thickness sequence in each liquid film region as the threshold, the liquid film thickness at each time moment is compared with the liquid film thickness at adjacent time moments. If the difference in liquid film thickness at a certain time moment is greater than the standard deviation, the liquid film thickness at this time moment is judged to be an outlier. The liquid film thickness at this time moment is corrected to the average value of the liquid film thickness at adjacent time moments, thereby removing the outlier liquid film thickness. The liquid film thickness sequence of each liquid film region is subjected to exponential fitting to obtain the final liquid film thickness sequence H(t).

[0030] The present invention has the following advantages due to the adoption of the above technical solutions:

[0031] (1) The present invention uses an ultrasonic measurement method to measure gas-liquid slug flow, which can achieve non-invasive detection and can measure opaque pipes.

[0032] (2) The present invention extracts the liquid film thickness based on the spatiotemporal distribution of ultrasonic Doppler velocity, which makes up for the fact that the traditional time-of-flight method results in a low liquid film thickness measurement value due to the echo signal of the bubble layer in high gas flow.

[0033] (3) Based on the ultrasonic Doppler motion bubble reflection model, the position and bubble size at the gas-liquid interface are extracted from the velocity profile, and a liquid film thickness compensation algorithm is proposed to avoid the error in liquid film thickness measurement when the bubble is large. Attached Figure Description

[0034] Figure 1 Schematic diagram of a horizontal gas-liquid slug flow ultrasonic Doppler measurement system

[0035] Figure 2 The process of extracting ultrasonic Doppler signals: (a) amplifying the echo signal; (b) quadrature demodulating the signal; (c) digital demodulating the signal; (d) Doppler signal.

[0036] Figure 3 Instantaneous velocity calculation method based on ultrasonic Doppler signals: (a) Doppler signal; (b) velocity sequence

[0037] Figure 4 Spatiotemporal distribution of velocity in horizontal gas-liquid sluice flow

[0038] Figure 5 Flowchart for identifying the liquid film region and liquid slug region in a horizontal gas-liquid slug flow

[0039] Figure 6 The identification process of the liquid film region and liquid slug region in a horizontal gas-liquid slug flow: (a) spatiotemporal distribution of velocity; (b) number of non-zero velocity values ​​in the upper half of the pipe; (c) binary marker signal; (d) corrected binary marker signal.

[0040] Figure 7 Schematic diagram of the extraction principle of slug flow liquid film thickness based on ultrasonic Doppler effect: (a) Bubble distribution diagram in the liquid film region; (b) Model diagram of moving bubbles crossing the measurement line, where the measurement line is a straight line along the tilt direction of the ultrasonic probe; (c) Instantaneous velocity profile diagram of the liquid film region.

[0041] Figure 8 Extraction results of liquid film thickness sequence in horizontal gas-liquid slug flow: (a) spatiotemporal distribution of slug flow velocity; (b) liquid film thickness sequence; (c) corrected liquid film thickness sequence and fitted liquid film thickness curve.

[0042] Figure 9 Liquid film thickness obtained by ultrasonic Doppler method and TOF method: (a) Doppler method; (b) TOF method

[0043] Figure 10 Comparison of average liquid film thickness obtained by ultrasonic Doppler method and TOF method: (a) U sw=0.73m / s; (b)U sw =1.18m / s Detailed Implementation

[0044] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. The present invention includes:

[0045] (1) Design as follows Figure 1 The diagram shows a horizontal gas-liquid slug flow ultrasonic Doppler measurement system. The system includes a dual-crystal ultrasonic probe (T, R), an excitation module, a signal conditioning module, an FPGA module, and a signal acquisition and data transmission module. The dual-crystal ultrasonic probe is mounted at a Doppler angle θ on the lower pipe wall. The excitation module generates a sinusoidal pulse train as the excitation signal under the control of the FPGA. The piezoelectric material inside the dual-crystal ultrasonic probe generates pulsed ultrasonic waves under the excitation signal, with an ultrasonic pulse repetition frequency of f. prf Simultaneously, the ultrasonic waves reflected by moving bubbles in the gas-liquid two-phase flow within the pipeline are converted into raw echo signals. After amplitude amplification and bandpass filtering by the signal conditioning module, they are converted into digital signals by the signal acquisition module. After preprocessing within the FPGA, the signals are uploaded to the host computer for further extraction of liquid film thickness information.

[0046] (2) Press Figure 2 The process shown involves sequentially performing quadrature demodulation and low-pass filtering on the digital echo signal within the FPGA:

[0047] ① The amplified echo signal output by the conditioning module, such as Figure 2 As shown in (a), it can be expressed by the following formula:

[0048]

[0049] In the formula, A(t) is the amplitude of the amplified echo signal, f0 is the center frequency of the ultrasonic probe, and f D The Doppler frequency shift is caused by the motion of reflected bubbles in the fluid relative to the ultrasonic probe. This represents the phase difference between the received signal and the transmitted signal.

[0050] ② The sinusoidal reference signal u inside the FPGA ref (t)

[0051] u ref (t)=Bsin(2πf0t) (2)

[0052] Orthogonalizing the two yields Figure 2 The orthogonal signal shown in (b) can also be represented by the following formula:

[0053]

[0054] The amplified echo signal is divided into two components, low frequency and high frequency, by quadrature demodulation. The high frequency component contains twice the frequency of the center frequency of the ultrasound probe and Doppler frequency shift information; the low frequency component is only related to the Doppler frequency shift.

[0055] ③ A low-pass filter is introduced to remove high-frequency components from the quadrature demodulated signal, obtaining signal u. dpl (t), such as Figure 2 As shown in (c).

[0056]

[0057] At this point, the signal only contains Doppler information related to the velocity of the moving bubble.

[0058] ④ Regarding the delay time τ from ultrasonic pulse transmission to reception, such as Figure 2 (c) shows the extracted amplitude information of the demodulated signals from different ultrasonic pulses, as indicated by the dashed line. This delay time τ corresponds to a radial position x in the pipe, i.e., x = τc sinθ / 2, where c is the propagation speed of the ultrasonic wave. The extracted amplitude information is used to reconstruct a signal based on the pulse sequence. This reconstructed signal is the Doppler signal at the radial position x in the pipe, as shown below. Figure 2 As shown in (d).

[0059] (3) The frequency of the Doppler signal is the Doppler frequency shift f caused by the moving bubble. D Select the pulse repetition number N. pulse The Doppler signal at the radial position x of the pipe is divided into different time segments, such as... Figure 3 As shown in (a), each time segment is N. pulse / f prf Seconds. Perform a Fast Fourier Transform (FFT) on the signal within different time segments to extract the Doppler frequency shift information f. D According to the Doppler velocity measurement principle, the Doppler frequency shift of each time segment is converted into the corresponding bubble velocity v according to formula (5):

[0060]

[0061] Thus, the velocity sequence v(t) of the bubble at the radial position x in the pipe can be obtained, such as Figure 3 As shown in (b).

[0062] (4) Repeat the above steps at all radial positions of the pipe to obtain the spatiotemporal velocity distribution of the moving bubbles in the slug flow, such as... Figure 4 As shown, the vertical axis represents the radial position x within the pipe, the horizontal axis represents time t, and the color intensity of the color scales represents the bubble velocity magnitude. The velocity spatiotemporal distribution diagram visually illustrates the velocity distribution in the liquid film and slug regions of the slug flow.

[0063] (5) Press Figure 5 The process shown is for Figure 4 The velocity spatiotemporal distribution v(x,t) shown is used to identify the liquid film region and the liquid plug region:

[0064] ① Input the spatiotemporal distribution matrix of velocity v(x,t), and search for the maximum velocity value of the matrix, denoted as V. max Because the velocity of bubbles in the slug region of a slug flow is significantly greater than that in the liquid film region, V max This represents the maximum velocity of the air bubble in the liquid plug region.

[0065] ② In the spatiotemporal distribution matrix v(x,t), each column element v(:,t) corresponds to an instantaneous velocity profile. The maximum value of the search column element is set to v. max This corresponds to the maximum velocity value of the velocity profile at that moment. Let v max With V max By comparing and selecting an appropriate velocity threshold α, if v max <αV max If the velocity of the bubble at any position at time t does not conform to the characteristics of a moving bubble in the liquid plug region, then time t is identified as the liquid film region. If v max ≥αV max Proceed to step ③.

[0066] ③Count the number of non-zero elements in v(:,t) located in the upper half of the pipe (i.e., x∈[D / 2,D], where D represents the inner diameter of the pipe), denoted as C. num ,like Figure 6 As shown in (b). Selecting a suitable threshold β for the number of velocities, when C... num When the time is less than the threshold β, time t is identified as the liquid film region; otherwise, it is identified as the liquid plug region.

[0067] ④ Construct a binary identifier signal S mark This signal consists of 0s and 1s, where 0 and 1 represent the liquid film region and the liquid plug region, respectively, as shown below. Figure 6 As shown in (c).

[0068] ⑤ When the velocity in the liquid film region or the non-zero velocity value in the liquid plug region in the upper part of the pipeline is relatively low at a certain instant, the binary identification signal S mark There are error signals, such as Figure 6 As shown in (c). If S mark If the value at a certain instant is not equal to the values ​​at the preceding and following instants, the identification result at that instant is considered incorrect, and it is corrected to be consistent with the preceding and following instants. The corrected binary identifier signal S mark like Figure 6 As shown in (d).

[0069] (6) Based on the spatiotemporal distribution of slug flow velocity, the velocity at the location of the Taylor bubble is zero. The dispersed bubbles at the bottom of the Taylor bubble can cause a Doppler frequency shift, thus presenting a non-zero velocity value in the spatiotemporal distribution. By detecting the location of the non-zero velocity value farthest from the dual-crystal ultrasonic probe in the liquid film region, such as... Figure 7 As shown in (a), the thickness of the liquid film in a slug flow can be measured. When the bubble closest to the bottom of the Taylor bubble passes through the measuring line, as... Figure 7 As shown in (b), the starting and ending points of the ultrasonic signal reflected by the bubble are set as P1 and P2, respectively. The radial positions of the pipe corresponding to P1 and P2 are x k and x j It can be derived that:

[0070]

[0071] Where θ is the Doppler angle and r is the radius of the moving bubble. The bubble generates a local velocity v in the instantaneous velocity profile. f ,like Figure 7 As shown in (c), the local velocity v f The location of occurrence can be represented by x. k -x j express.

[0072] Due to the relationship between the liquid film thickness h and x k There is a difference Δh:

[0073] Δh=rr cosθ (7)

[0074] Therefore, the liquid film thickness h can be expressed as:

[0075]

[0076] (7) Figure 8 (a) and (b) represent the spatiotemporal distribution of gas-liquid slug flow velocity and their corresponding liquid film thickness sequences h(t), respectively. Using the standard deviation of the liquid film thickness sequence in each liquid film region as a threshold, the liquid film thickness at each time step is compared with the liquid film thickness at adjacent time steps. If the difference in liquid film thickness is greater than the standard deviation, the liquid film thickness at that time is considered an outlier, and the liquid film thickness at that time is corrected to the average of the liquid film thicknesses at adjacent time steps. The liquid film thickness after removing outliers is as follows: Figure 8 (c) is shown by the dashed line. Finally, an exponential fit is performed on the liquid film thickness sequence for each liquid film region to obtain the final liquid film thickness sequence H(t), as shown in the figure. Figure 8 (c) is shown by the solid line.

[0077] Experimental verification and results:

[0078] To measure the liquid film thickness in the liquid film region of a horizontal gas-liquid slug flow, an ultrasonic Doppler measurement system was established, and horizontal gas-liquid two-phase flow experiments were conducted. The experimental media were tap water and air. The experiments were conducted with a fixed apparent velocity in the water phase and an adjusted apparent velocity in the gas phase. The liquid film thickness in the slug flow was measured using both ultrasonic Doppler and ultrasonic time-of-flight (TOF) methods under both low and high water phase flow rates (Zhai et al., IEEE Transitions on Instrumentation and Measurement, 2021, 70:9501410).

[0079] The spatiotemporal distribution of gas-liquid slug flow velocity was obtained using an ultrasonic Doppler measurement system. Figure 5 The process shown sets the velocity threshold α = 0.65 in the liquid plug region and the non-zero element count threshold β = 30 in the upper half of the pipe to identify the spatiotemporal distribution of velocity. Then, the farthest non-zero velocity value in the liquid film region is extracted to obtain the corresponding liquid film thickness sequence h(t). Figure 8 The method shown is used for further processing to obtain the final liquid film thickness sequence H(t).

[0080] The liquid film thickness sequence measured using the Doppler method and the time-of-flight method is as follows: Figure 9 As shown. The Doppler liquid film thickness sequence H of each liquid film region was extracted. UDi (t), where i represents the sequence number of the liquid film region, such as Figure 9 As shown in (a). Simultaneously, the time-of-flight film thickness sequence H under the same operating conditions was extracted. TOFi (t), such as Figure 9 As shown in (b), the corresponding average liquid film thickness δ is calculated according to formula (9). UD and δ TOF :

[0081]

[0082] Fixed apparent velocity U of the water phase sw Gradually increase the apparent velocity U in the gas phase sg Average liquid film thickness δ UD and δ TOF The comparison results are as follows Figure 10 As shown in the figure. It can be seen that as the apparent velocity of the gas phase increases, δ UD With δ TOF They exhibit the same changing pattern. It is noteworthy that at low apparent velocities in the aqueous phase, as the apparent velocities in the gas phase gradually increase, the number and size of bubbles in the liquid film region show an increasing trend, δ... UD Significantly greater than δ TOF At high apparent velocities in the aqueous phase, δ UD With δTOF The difference is small, but δ is high at high apparent gas phase velocities. UD Still slightly greater than δ TOF This indicates that when a bubble layer appears in the liquid film region, the ultrasonic time-of-flight method is limited by the reflected echoes from the bubble layer, resulting in a smaller liquid film thickness. In contrast, the ultrasonic Doppler method can effectively avoid this limitation and has higher measurement accuracy.

[0083] In summary, this invention proposes a method for measuring the liquid film thickness of horizontal gas-liquid slug flows based on the ultrasonic Doppler effect. By measuring the spatiotemporal distribution of velocity in the horizontal gas-liquid slug flow using the ultrasonic Doppler method, the liquid film region and the slug region of the slug flow are effectively distinguished. The velocity of the bubble furthest from the ultrasonic probe in the liquid film region is detected, and the liquid film thickness is measured by the location of the furthest bubble velocity. Compared with traditional ultrasonic time-of-flight methods, the method proposed in this invention has higher measurement accuracy for the liquid film thickness of horizontal gas-liquid slug flows.

Claims

1. A method for measuring the thickness of a liquid film in a horizontal gas-liquid slug flow based on the ultrasonic Doppler effect, comprising the following steps: (1) A horizontal gas-liquid sluice ultrasonic Doppler measurement system was built, and echo signals were obtained by using a dual-crystal ultrasonic probe to acquire digital echo signals; (2) The digital echo signal is sequentially subjected to quadrature demodulation and low-pass filtering to obtain a signal containing only Doppler information related to the velocity of the moving bubble. ; (3) Reconstruct the Doppler signal, the method is as follows: Let the Doppler angle be... For the delay time from ultrasonic pulse transmission to reception The demodulated and low-frequency filtered signals from different ultrasonic pulses were extracted. Amplitude information; delay time A radial position relative to the pipeline Correspondence, that is ,in The speed of ultrasonic wave propagation is given; the extracted amplitude information is used to reconstruct a signal based on a pulse sequence, and this reconstructed signal represents the radial position of the pipe. Doppler signal at the location; (4) Let the repetition frequency of the ultrasonic pulses generated by the dual-crystal ultrasonic probe under the action of the excitation signal be . The frequency of the Doppler signal is the Doppler frequency shift caused by the moving bubble. Select the number of pulse repetitions. , the radial position of the pipe The Doppler signal at that location is divided into different time segments, each time segment being... Seconds; perform Fast Fourier Transform on signals within different time segments to extract Doppler frequency shift information. ; The Doppler frequency shift of each time segment is converted into the corresponding bubble velocity using the following formula. : Obtain the radial position of the pipe velocity sequence of bubble motion ; (5) Repeat the above steps for all radial positions of the pipe to obtain the spatiotemporal distribution matrix of bubble velocity. ,in Represents the radial position within the pipe. t Represents time; The velocity spatiotemporal distribution matrix It can describe the spatiotemporal distribution of bubble velocity in slug flow; (6) Regarding the velocity spatiotemporal distribution matrix The method for identifying the liquid film region and the liquid plug region is as follows: ① Input velocity spatiotemporal distribution matrix The maximum velocity value of the search matrix is ​​denoted as , This represents the maximum velocity of the air bubble in the liquid plug region; ② Velocity spatiotemporal distribution matrix Each column of elements Corresponding to an instantaneous velocity profile; the maximum value of the search column element is set to The maximum velocity value corresponding to the velocity profile at that moment; and Compare and select an appropriate speed threshold. ,if Then at time t The velocity of the bubbles at each location does not conform to the characteristics of moving bubbles in the liquid plug region, and the time will be... Identified as a liquid film region; if ,statistics The number of non-zero elements located in the upper half of the pipe is denoted as . Select an appropriate threshold for the number of speeds. ,when Less than the threshold At that time, If identified as a liquid film region, it is identified as a liquid plug region; otherwise, it is identified as a liquid plug region. ③ Construct a binary identifier signal The signal consists of 0 and 1, where 0 and 1 represent the liquid film region and the liquid plug region, respectively; ④ Based on the binary identifier signal The system will correct any erroneous signals that may occur, and the corrected binary identifier signal will still be recorded as [previous value]. ; Using the modified binary identifier signal Identify the liquid film area and the liquid plug area; (7) The velocity at the location of the Taylor bubble is zero. The dispersed bubbles at the bottom of the Taylor bubble can cause a Doppler frequency shift, thus presenting a non-zero velocity value in the velocity spatiotemporal distribution. By detecting the location of the non-zero velocity value farthest from the dual-crystal ultrasonic probe in the liquid film region, the thickness of the slug flow liquid film can be measured. When the bubble closest to the bottom of the Taylor bubble passes through the measurement line, the starting point and ending point of the ultrasonic signal reflected by the bubble are respectively set as and ; and The corresponding radial positions of the pipes are respectively and The radius of the moving bubble is calculated using the following formula. r : in, Let be the radius of the moving bubble; the bubble closest to the bottom of the Taylor bubble generates a local velocity in the instantaneous velocity profile. Local velocity The location of appearance express; Due to the thickness of the liquid film and There is a difference : liquid film thickness Represented as: ； (8) Obtain the liquid film thickness sequence .

2. The method for measuring the thickness of a liquid film in a horizontal gas-liquid sluice flow according to claim 1, characterized in that, The horizontal gas-liquid slug flow ultrasonic Doppler measurement system includes a dual-crystal ultrasonic probe, an excitation module, a signal conditioning module, an FPGA module, and a signal acquisition and data transmission module. The dual-crystal ultrasonic probe is positioned at a Doppler angle. The device is installed at an angle on the lower pipe wall. The excitation module generates an excitation signal under the control of the FPGA. The piezoelectric material inside the dual-crystal ultrasonic probe generates pulsed ultrasonic waves under the action of the excitation signal. The ultrasonic waves reflected by the moving bubbles in the gas-liquid two-phase flow in the pipe are converted into original echo signals. After amplitude amplification and bandpass filtering by the signal conditioning module, the signal acquisition module converts them into digital echo signals.

3. The method for measuring the thickness of a liquid film in a horizontal gas-liquid sluice flow according to claim 1, characterized in that, The digital echo signal is sequentially subjected to quadrature demodulation and low-pass filtering to obtain a signal containing only Doppler information related to the velocity of the moving bubble. The method is as follows: The amplified echo signal is divided into low-frequency and high-frequency components through orthogonal demodulation. The high-frequency component contains twice the harmonic of the ultrasonic probe's center frequency and Doppler frequency shift information, while the low-frequency component is only related to the Doppler frequency shift. A low-pass filter is introduced to remove the high-frequency component from the orthogonal demodulated signal, obtaining a signal containing only Doppler information related to the velocity of the moving bubble. .

4. The method for measuring the thickness of a liquid film in a horizontal gas-liquid sluice flow according to claim 1, characterized in that, Select speed threshold .

5. The method for measuring the thickness of a liquid film in a horizontal gas-liquid slug flow according to claim 1, characterized in that, The method for step (8) is as follows: using the standard deviation of the liquid film thickness sequence in each liquid film region as a threshold, the liquid film thickness at each time moment is compared with the liquid film thickness at adjacent time moments. If the difference in liquid film thickness at a certain time moment is greater than the standard deviation, the liquid film thickness at this time moment is judged to be an outlier. The liquid film thickness at this time moment is corrected to the average value of the liquid film thickness at adjacent time moments, thereby removing the outlier liquid film thickness. The liquid film thickness sequence of each liquid film region is subjected to exponential fitting to obtain the final liquid film thickness sequence. .

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