CN117419777A - Liquid drop speed measuring method based on conical optical fiber probe - Google Patents
Liquid drop speed measuring method based on conical optical fiber probe Download PDFInfo
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- CN117419777A CN117419777A CN202311356594.5A CN202311356594A CN117419777A CN 117419777 A CN117419777 A CN 117419777A CN 202311356594 A CN202311356594 A CN 202311356594A CN 117419777 A CN117419777 A CN 117419777A
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- 239000000523 sample Substances 0.000 title claims abstract description 48
- 239000013307 optical fiber Substances 0.000 title claims abstract description 22
- 239000007788 liquid Substances 0.000 title claims abstract description 20
- 238000000034 method Methods 0.000 title claims abstract description 16
- 230000009466 transformation Effects 0.000 claims abstract description 15
- 239000000835 fiber Substances 0.000 claims description 18
- 238000005259 measurement Methods 0.000 claims description 15
- 230000010355 oscillation Effects 0.000 claims description 9
- 238000000691 measurement method Methods 0.000 claims description 4
- 230000001681 protective effect Effects 0.000 claims description 3
- 229910001220 stainless steel Inorganic materials 0.000 claims description 3
- 239000010935 stainless steel Substances 0.000 claims description 3
- 239000012071 phase Substances 0.000 description 17
- 238000005516 engineering process Methods 0.000 description 9
- 238000000605 extraction Methods 0.000 description 9
- 230000003287 optical effect Effects 0.000 description 4
- 238000003672 processing method Methods 0.000 description 4
- 230000008901 benefit Effects 0.000 description 3
- 238000011088 calibration curve Methods 0.000 description 2
- 238000007796 conventional method Methods 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 238000010291 electrical method Methods 0.000 description 2
- 239000012530 fluid Substances 0.000 description 2
- 239000007791 liquid phase Substances 0.000 description 2
- 238000005070 sampling Methods 0.000 description 2
- 238000012360 testing method Methods 0.000 description 2
- 238000013519 translation Methods 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 1
- 238000013459 approach Methods 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 238000005253 cladding Methods 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 238000011010 flushing procedure Methods 0.000 description 1
- 238000003703 image analysis method Methods 0.000 description 1
- 238000001000 micrograph Methods 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 238000000917 particle-image velocimetry Methods 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 230000001902 propagating effect Effects 0.000 description 1
- 238000002310 reflectometry Methods 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 238000012552 review Methods 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
- 238000004513 sizing Methods 0.000 description 1
- 239000007921 spray Substances 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
- 230000005514 two-phase flow Effects 0.000 description 1
- 239000011800 void material Substances 0.000 description 1
Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
- G01F1/66—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by measuring frequency, phase shift or propagation time of electromagnetic or other waves, e.g. using ultrasonic flowmeters
- G01F1/661—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by measuring frequency, phase shift or propagation time of electromagnetic or other waves, e.g. using ultrasonic flowmeters using light
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
- G01F1/66—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by measuring frequency, phase shift or propagation time of electromagnetic or other waves, e.g. using ultrasonic flowmeters
- G01F1/662—Constructional details
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- Electromagnetism (AREA)
- Fluid Mechanics (AREA)
- General Physics & Mathematics (AREA)
- Investigating Or Analysing Materials By Optical Means (AREA)
Abstract
The invention relates to a liquid drop speed measuring method based on a conical optical fiber probe, which comprises the following steps: setting up a measuring system, wherein the sensitive tip of the optical fiber probe is conical, and the sensitive tip positively rushes in the incoming flow direction; collecting output signals of a photoelectric detector; identifying drop signals dispersed in the photodetector output signal; obtaining a trending pre-signal; performing continuous wavelet transform CWT on the detrending pre-signal using Morlet wavelet as a wavelet basis function; extracting the instantaneous frequency of the wavelet coefficients from the continuous wavelet transform CWT output by phase transformation, sharpening the CWT output; summing the amplitudes of the wavelet coefficients at the same frequency, wherein the frequency value indicated by the peak value of the sum of the amplitudes is taken as the peak frequency of the liquid drop pre-signal; the peak frequency f of the droplet pre-signal is converted to the droplet velocity.
Description
Technical Field
The invention belongs to the technical field of flow measurement, and relates to a method for measuring liquid drop velocity.
Background
Annular flow is a common two-phase flow and is widely used in the fields of oil and gas wells, nuclear reactors, evaporators and condensers, food chemical industry and the like. The annular flow has a gas core flowing at a high speed and a liquid film flowing along the pipe wall, disturbance waves at the interface of the liquid film are sheared by a gas phase to form liquid drops flowing along with the gas, and the speed and the size of the liquid drops are important characteristics for describing the annular flow and directly influence the characteristics of the annular flow such as heat transfer, pressure drop and the like. Currently, the main measurement techniques of droplet velocity can be divided into optical methods and electrical methods. The optical method mainly comprises a phase Doppler wind speed method, a particle image velocimetry method and a particle/liquid drop image analysis method, and the electrical method mainly adopts a multi-conductivity probe to measure the speed and the size of liquid drops.
In recent years, fiber optic probe technology has attracted attention due to its excellent performance and small fluid turbulence, and it can identify fluids with different refractive indices, and is suitable for high-concentration spray droplet measurement. Compared with multi-optical fiber and multi-conductivity probes, the single optical fiber probe reduces interference to flow and has the advantages of high sensitivity, high response speed, electromagnetic interference resistance and the like. Depending on the principle of drop velocity measurement, the use of single fiber probes can be divided into conventional methods and fiber reflectometer (FOR) techniques. Conventional methods determine the velocity of the droplet based on the transition time of the high and low levels of the signal as it contacts the probe, which requires a corresponding calibration curve for each sensor, adding to the complexity of the measurement. In contrast, FOR technology has not been widely used. The principle is that an optical signal transmitted in an optical fiber is reflected and refracted when propagating to the probe tip-air interface, wherein the reflected light is folded back at the probe tip interface, and the refracted light irradiates into the air and illuminates a specific working area. Once a droplet near the tip of the probe enters the working area of the probe, the reflected light from the surface returns to the probe core and interferes with the reflected light returning at the interface of the probe tip, the frequency of the interference wave being a function of the interface velocity of the droplet. The non-contact speed measurement technology does not need to provide a calibration curve for the sensor, and has the advantages of convenient operation, high measurement precision and good repeatability. However, when the contact position of the probe with the droplet is severely deviated from the center of the droplet, reflected light of the surface of the droplet cannot effectively enter the probe core, and thus only the speed of a part of the droplet can be obtained. In addition, the processing method of the signal determines to a large extent the proportion and accuracy of the extracted droplet velocity. In the past, the velocity and size of the low velocity dispersed phase have been measured using a flat head probe [1] - [3]. In terms of signal processing, a stable peak frequency sequence is extracted by performing Fast Fourier Transform (FFT) after windowing an oscillating signal to determine the speed of a dispersed phase, which meets the measurement requirement of a low-speed stable dispersed phase. In the gas core of an annular flow, a high velocity and highly dispersed droplet velocity distribution can cause the frequency and duration of the oscillating signal to vary over a considerable range. Because the period of the oscillation waveform in each fixed window is limited, a stable frequency sequence is difficult to form, so that the speed ratio determined by the method is relatively low, the resolution is poor, and the accurate extraction of the annular flow liquid drop speed is difficult to ensure.
Literature of related arts
[1]Lim H J,Chang K A,Su C B,et al.Bubble velocity,diameter,and void fraction measurements in a multiphase flow using fiber optic reflectometer[J].Review of Scientific Instruments,2008,79(12):202.
[2]Do J,Chuang W L,Chang K A.Oil droplet sizing and velocity determination using a fiber-optic reflectometer[J].Measurement Science and Technology,2020,31(6):065301.
[3]Do J,Chang K A.Application of single-probe fiber optic reflectometry on phase discrimination and velocity and size determination in an oil–gas–water three-phase flow[J].Measurement Science and Technology,2021,32(10):105303.
Disclosure of Invention
The invention provides a signal processing method FOR sharpening wavelet transformation to extract the droplet speed, which aims to solve the problem of accurately measuring the droplet speed with high speed and high dispersion in annular flow and is based on droplet signals measured by the FOR technology, so that the measuring precision of the droplet speed by the FOR technology is improved, the proportion and the accuracy of the obtained droplet speed are increased, and the speed measurement of high-speed droplets in the annular flow is realized. The technical scheme of the invention is as follows:
a droplet velocity measurement method based on a tapered fiber probe, comprising the steps of:
1) Setting up a measuring system, wherein the sensitive tip of the optical fiber probe is conical, and the sensitive tip positively rushes into the incoming flow direction and is used for detecting the speed of liquid drops in the annular flow; the laser emitted by the laser is transmitted to the sensitive tip of the optical fiber probe through the coupler, and the reflected light at the sensitive tip of the optical fiber probe is transmitted into the photoelectric detector through the other path of the coupler;
2) Collecting output signals of a photoelectric detector;
3) Identifying a droplet signal dispersed in the output signal of the photodetector, locating an oscillation signal generated before the droplet contacts the optical fiber probe, and defining the oscillation signal as a pre-signal g (t);
4) Performing trend removal operation on the pre-signal g (T), eliminating noise interference, simultaneously keeping oscillation characteristics, estimating the trend T of the pre-signal by using a moving average window, and then subtracting the trend T of the pre-signal from the pre-signal g (T) to obtain a trend-removed pre-signal f (T);
5) Continuous wavelet transform CWT, denoted W, of the detrending pre-signal (t) using Morlet wavelet as wavelet basis function ψ (t) ψ f(s,u)；
6) Based on phase transformation w f By phase transformation w, proportional to the principle of continuous wavelet transformation CWT on the first derivative of the shift quantity u f Extracting the instantaneous frequency of the wavelet coefficient from the continuous wavelet transformation CWT output, sharpening the CWT output, and reassigning the instantaneous frequency value of the output wavelet coefficient to the center of the time-frequency domain to obtain the amplitude of the CWT sharpened wavelet coefficient;
7) Summing the amplitudes of the wavelet coefficients at the same frequency, wherein the frequency value indicated by the peak value of the sum of the amplitudes is taken as the peak frequency f of the liquid drop pre-signal;
8) Converting the peak frequency f of the droplet pre-signal to a droplet velocity:
where λ is the laser wavelength, n is the refractive index of air, and the velocity value v is the final measure of the drop velocity.
Further, the tapered fiber probe is wrapped by an L-shaped stainless steel protective shell and embedded into the measurement pipe.
Further, the phase is shifted w f The formula of (2) is as follows:
further, the formula for converting the peak frequency f of the droplet pre-signal into the droplet velocity is:
where λ is the laser wavelength, n is the refractive index of air, and the velocity value v is the final measure of the drop velocity.
Drawings
Fig. 1: the fiber optic probe is schematic of a measurement system for annular flow droplets.
Fig. 2: schematic of droplet velocity extraction process. (a) a pre-signal and a detrending pre-signal; (b) a droplet signal and a pre-signal; (c) Performing a Continuous Wavelet Transform (CWT) on the detrending pre-signal; (d) wavelet coefficient magnitude sums of CWT; (e) a time-frequency diagram after CWT sharpening; (f) wavelet coefficient magnitude sum after CWT sharpening.
Fig. 3: the quantity and quality comparison graphs of the liquid drop speeds extracted by the three signal processing methods.
Detailed Description
The invention will now be further described with reference to the drawings and examples.
The system for measuring the annular flow drop by the optical fiber probe is shown in fig. 1, the optical fiber probe is processed by corning SMF-28e+ low-loss optical fiber, the cladding diameter is 125um, and the fiber core diameter is 8.2um. The sensitive tip of the fiber optic probe was polished to a 35 ° taper angle as shown in the fiber optic probe micrograph of fig. 1. Subsequently, the fiber optic probe was wrapped with an L-shaped stainless steel protective housing and embedded in the center of the tube with an inner diameter of 50 mm. The sensitive tip of the fiber optic probe is positively impacted in the direction of incoming flow for detecting the droplet. The laser emits infrared light with the power of 5mW and the wavelength of 1550mm, and the infrared light is transmitted to the sensitive tip of the optical fiber probe through a Y-shaped coupler. Meanwhile, the reflected light at the sensitive tip of the optical fiber probe is transmitted into the photoelectric detector through the other path of the Y-shaped coupler, and the optical signal is converted into an electric signal and amplified. In the experimental process, the apparent flow rate of the gas phase in the pipeline is controlled to be 15m/s, the apparent flow rate of the liquid phase is controlled to be 0.2m/s, the stable annular flow is ensured to be formed in the test section, and the liquid drop flushing probe is continuously arranged in the gas core.
The measurement of the annular flow droplet velocity comprises the steps of:
1) And collecting output signals of the photoelectric detector. The Altai PCI8512 data acquisition card is controlled to continuously acquire the output signal of the photoelectric detector at the sampling frequency of 80MHz, and the duration time is 30 seconds.
2) The drop signal dispersed in the output signal of the photodetector is identified and the pre-signal is located. Due to the difference in refractive index between air and water, the output signal of the photodetector exhibits a high level when the fiber-optic probe-sensitive tip is in air, and a low level when a droplet touches the fiber-optic probe-sensitive tip. Fig. 2 (b) shows a droplet signal, where the reflected light at the droplet interface returns to the fiber core as the droplet approaches and eventually contacts the fiber probe, producing an oscillating pre-signal. An oscillation signal of 11 μs before the high-low level abrupt position is located as a pre-signal g (t) of the droplet, as shown in fig. 2 (b).
3) The pre-signal g (t) is trended to eliminate noise interference while preserving the oscillation characteristics. A trend T of the pre-signal is estimated using a moving average window of 20 data lengths, and subtracted from the pre-signal g (T), the reference voltage of the pre-signal is tiled at 0V, defined as a detrack pre-signal f (T), as shown in fig. 2 (a).
4) Continuous Wavelet Transform (CWT) of the detrending pre-signal f (t) is represented by formula (1)
Where s represents the scale, u represents the amount of translation, and Morlet wavelet is used as the mother wavelet ψ (t). The continuous wavelet transform converts the time domain signal into a time-frequency plane to obtain the magnitude of wavelet coefficient as shown in fig. 2 (c), which has the same time axis as the detrending pre-signal f (t) in fig. 2 (a), and converts the frequency value f of the vertical axis into the droplet velocity v by the formula (2)
Where λ=1.55 um is the laser wavelength and n=1 is the refractive index of air. As the drop gets closer to the sensitive tip of the fiber optic probe, a stripe of increasing intensity is found at a speed of about 10m/s, corresponding to a detrack pre-signal f (t) of increasing oscillating intensity.
5) The magnitudes of the wavelet coefficients at the same velocity are summed to obtain a velocity-magnitude sum curve, and the velocity indicated by its peak is located as a measure of the droplet velocity. The time-frequency plot of fig. 2 (c) presents a stripe of increasing brightness, the speed-amplitude and curve of fig. 2 (d) presents a significant peak at the corresponding bright stripe, this peak indicating a speed v=10.2 m/s as a measure of the drop speed.
In a more preferred embodiment, the Continuous Wavelet Transform (CWT) is preprocessed as follows to sharpen the CWT output:
using a phase transformation w f (s, u) extracting instantaneous frequency values of wavelet coefficients from the output of the CWT. The phase change w f (s, u) is proportional to the first derivative of CWT with respect to the translation u:
where the dimension s is defined as s=w 0 /w，w 0 Representing the small peak frequency, w refers to frequency. The phase transformation of formula (3) is applied, the instantaneous frequency value of the output wavelet coefficient is redistributed to the center of the time-frequency region, a clearer time-frequency diagram after CWT sharpening is obtained, as shown in fig. 2 (e), and the redistribution makes the output bright stripes more sharp than the CWT.
The velocity-amplitude and curve obtained after CWT sharpening is shown in fig. 2 (f), with the peak indicating a velocity v=10.11 m/s as the final measure of the drop velocity.
In order to verify the effectiveness of the signal processing method FOR measuring the droplet speed by using the FOR technology, a real-flow experiment FOR measuring the droplet speed by using the annular flow is designed, the apparent flow rate of a gas phase is set to be 15m/s, the apparent flow rate of a liquid phase is set to be 0.2m/s, and an annular flow which is stably developed is formed in a test tube section provided with an optical fiber probe. The optical fiber probe is inserted into the center of the pipeline, the control board continuously collects experimental data for 30 seconds at the sampling frequency of 80MHz, and the experimental data contains 1087 droplet signals in total through a droplet identification algorithm. All droplets are then velocity extracted by windowed FFT, continuous Wavelet Transform (CWT) and CWT sharpening methods, respectively, while three metrics are used to evaluate the droplet velocity recognition performance of the three methods, including droplet velocity extraction rate, velocity accuracy and velocity resolution, as shown in fig. 3.
The comparison shows that the CWT significantly increases the minimum resolution of the extracted droplet velocity compared to the FFT, facilitating accurate extraction of the droplet velocity. In addition, CWT also increases to some extent the proportion of droplets that can be successfully extracted at a rate. In the extraction of the droplet velocity by CWT shown in fig. 2 (c) and 2 (d), it can be seen that the velocity band is thicker and the velocity resolution is worse. Based on CWT, the phase transformation used compensates the frequency diffusion effect of CWT while maintaining the time resolution of the signal by redistributing the signal energy, so that the time-frequency output after the phase transformation is sharpened on the basis of CWT, thereby improving the resolution of the droplet velocity and further remarkably improving the ratio of the droplet velocity successfully extracted by the FOR technique. Furthermore, the drop velocity accuracy of extraction after sharpening by CWT reached 97.8% in all drop samples, which can be attributed to its higher velocity resolution. FOR inherent reasons, FOR technology cannot capture velocity information FOR all droplets. However, with the benefit of the droplet velocity extraction algorithm proposed by the present invention, the number of droplets at extractable velocities increases from 384 (35.3%) to 592 (54.5%) far beyond the rate of velocity extraction (below 4%) by the past researchers using the FFT algorithm.
The invention solves the contradiction between time resolution and frequency resolution by utilizing the characteristic of self-adaptive window width brought by wavelet scaling transformation, is very suitable for local frequency analysis of liquid drop pre-signals, and realizes accurate frequency extraction of non-stationary liquid drop pre-signals with different speeds and durations in annular flow.
Claims (4)
1. A droplet velocity measurement method based on a tapered fiber probe, comprising the steps of:
1) Setting up a measuring system, wherein the sensitive tip of the optical fiber probe is conical, and the sensitive tip positively rushes into the incoming flow direction and is used for detecting the speed of liquid drops in the annular flow; the laser emitted by the laser is transmitted to the sensitive tip of the optical fiber probe through the coupler, and the reflected light at the sensitive tip of the optical fiber probe is transmitted into the photoelectric detector through the other path of the coupler;
2) Collecting output signals of a photoelectric detector;
3) Identifying a droplet signal dispersed in the output signal of the photodetector, locating an oscillation signal generated before the droplet contacts the optical fiber probe, and defining the oscillation signal as a pre-signal g (t);
4) Performing trend removal operation on the pre-signal g (T), eliminating noise interference, simultaneously keeping oscillation characteristics, estimating the trend T of the pre-signal by using a moving average window, and then subtracting the trend T of the pre-signal from the pre-signal g (T) to obtain a trend-removed pre-signal f (T);
5) Continuous wavelet transform CWT, denoted W, of the detrending pre-signal f (t) using Morlet wavelet as wavelet basis function ψ (t) ψ f(s,u)；
6) Based on phase transformation w f By phase transformation w, proportional to the principle of continuous wavelet transformation CWT on the first derivative of the shift quantity u f Extracting the instantaneous frequency of the wavelet coefficient from the continuous wavelet transformation CWT output, sharpening the CWT output, and reassigning the instantaneous frequency value of the output wavelet coefficient to the center of the time-frequency domain to obtain the amplitude of the CWT sharpened wavelet coefficient;
7) Summing the amplitudes of the wavelet coefficients at the same frequency, wherein the frequency value indicated by the peak value of the sum of the amplitudes is taken as the peak frequency f of the liquid drop pre-signal;
8) Converting the peak frequency f of the droplet pre-signal to a droplet velocity:
where λ is the laser wavelength, n is the refractive index of air, and the velocity value v is the final measure of the drop velocity.
2. The droplet velocity measurement method according to claim 1, wherein the tapered fiber probe is encased by an L-shaped stainless steel protective shell and embedded in the measurement pipe.
3. The method for measuring a droplet velocity according to claim 1, wherein the phase is shifted by w f The formula of (2) is as follows:
4. the method for measuring the velocity of a droplet according to claim 1, wherein the formula for converting the peak frequency f of the droplet pre-signal into the velocity of the droplet is:
where λ is the laser wavelength, n is the refractive index of air, and the velocity value v is the final measure of the drop velocity.
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