A flue gas flow rate detection system and method based on double-core optical fiber sensing

Abstract

This invention discloses a flue gas velocity detection system and method based on dual-core optical fiber sensing. The method includes using a dual-core optical fiber probe, which has a transmitting fiber core and a receiving fiber core arranged in parallel within a quartz cladding. Two optical signals of different wavelengths emitted by a light source are coupled to the transmitting fiber core via wavelength division multiplexing (WDM) to achieve dual-wavelength common-path transmission. The receiving fiber core picks up the echo signal scattered by flue gas particles. After signal processing and temperature compensation, transient thermo-optical correction is performed, along with environmental refractive index and geometric correction, to output the final flow velocity data. This invention can achieve real-time, high-sensitivity, and high-precision measurement of flue gas velocity in small-diameter flues under high-temperature and dusty environments. It can be widely applied to combustion experiments, emission measurement, industrial flue monitoring, and airflow characteristic research in carbon emission monitoring systems.

Description

Technical Field

[0001] This invention belongs to the field of gas sensing and fluid measurement technology, and relates to a flue gas velocity detection system and method based on dual-core optical fiber sensing. Background Technology

[0002] Existing flue gas flow detection methods mainly include traditional instruments such as Pitot tubes, thermal anemometers, and ultrasonic flow meters. At the same time, fiber optic sensing technology is also gradually being applied to this field. However, both types of technologies have obvious limitations in flow velocity detection under small-diameter and complex working conditions.

[0003] In traditional measuring instruments, the Pitot tube is bulky, and its insertion into the flue significantly disturbs the flow field, leading to errors in flow velocity measurement. Thermal anemometers are easily contaminated in high-temperature, dusty environments, and their probe sensitivity decays rapidly, requiring frequent cleaning and calibration. Ultrasonic flow meters are greatly affected by temperature gradients and changes in flue gas density, experiencing severe signal attenuation at temperatures above 100°C, and have limited measurement resolution, making them unsuitable for the wide-temperature conditions of laboratories. In laboratory flue gas ducts with small diameters of 20-50mm, space is limited and the flow field is prone to inhomogeneity. Traditional instruments struggle to balance high sensitivity and anti-interference performance, and since most are contact-based, probes are prone to wear and tear, maintenance costs are high, and long-term online monitoring is difficult.

[0004] Existing fiber optic flow detection mainly falls into three categories based on principle: light intensity attenuation type, which has a simple structure but low sensitivity and cannot meet the needs of detecting minute flow velocities; scattering Doppler type, which has high accuracy but complex devices, often employing a separate structure, resulting in signal transmission delay; and heat dissipation type (based on FBG), suitable for clean gases but susceptible to temperature fluctuations, making it unsuitable for dusty flue gas environments. While most fiber optic sensing solutions can detect flow velocities, an effective temperature compensation mechanism for the wide temperature range of 20-150℃ in laboratory flue gas has not yet been established. Furthermore, some solutions feature large probes that are difficult to embed in small-diameter flue gas ducts, making them unsuitable for the multi-point and miniaturized deployment requirements of laboratories.

[0005] Therefore, developing a miniaturized and highly stable fiber optic flow velocity detection device that can adapt to high temperatures, dust, and narrow flues has become an urgent need for current technological development. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a flue gas velocity detection system and method based on dual-core fiber optic sensing. By employing a dual-core fiber optic integrated structure and a dual-wavelength temperature compensation algorithm, it can effectively achieve high-precision real-time detection of flue gas velocity in small-diameter flues (typically 20-50mm) under wide temperature and dust conditions ranging from 20℃ to 150℃.

[0007] The technical solution adopted in this invention is as follows: A method for detecting flue gas velocity based on dual-core optical fiber sensing, comprising: A dual-core fiber optic probe is used, which includes a transmitting fiber core and a receiving fiber core arranged in parallel within a quartz cladding. The entire probe is encapsulated in a quartz sleeve, and the probe tip is used to extend into the flue gas field to be measured. The other end of the dual-core fiber optic probe is connected to the light source and the detection unit. The light source emits two optical signals of different wavelengths, which are coupled to the transmitting fiber core through wavelength division multiplexing to realize dual-wavelength common path transmission. The receiving fiber core picks up the echo signal scattered by the flue gas particles. The echo signal is processed for signal processing and temperature compensation, including: extracting and calculating frequency domain features to obtain a preliminary calculated value of flue gas velocity, and combining it with time domain autocorrelation for auxiliary verification. Based on the echo power of two different wavelengths, transient thermo-optical correction is performed on the verified preliminary calculated value. At the same time, environmental refractive index and geometric correction are performed to achieve temperature compensation processing of the signal from two aspects and output the final flow velocity data.

[0008] In the above technical solution, the dual-core fiber optic probe further includes two parallel-arranged transmitting fiber cores and receiving fiber cores, both of which are provided with a quartz cladding on their outer sides. Outside the quartz cladding, from the inside out, there are also a polyimide coating layer, a filling layer, a quartz sleeve, and an anti-adhesion coating, and the probe front end is provided with a micro-angle chamfer.

[0009] Furthermore, the diameter of the transmitting fiber core and the receiving fiber core is 45~55μm, the center distance between the two fiber cores is 70~90μm, the diameter of the quartz cladding is 180~220μm, the thickness of the polyimide coating is 10~20μm, the diameter of the quartz sleeve is 1.5~2.0mm, and the quartz sleeve at the front end of the probe is machined with a 5~10° micro-angle chamfer.

[0010] Furthermore, the two optical signals of different wavelengths are 1550nm and 1310nm, respectively. The 1550nm laser is a probe laser used to capture the flow velocity of flue gas particles, and the 1310nm laser is used to provide reference data for back-end temperature drift compensation.

[0011] Furthermore, the signal processing includes: sampling and filtering the received signal to obtain an effective signal sequence, performing a fast Fourier transform to obtain a frequency domain power spectrum, searching for the main frequency point with the highest energy in the power spectrum, and calculating the Doppler frequency shift; The mapping relationship between flow velocity and frequency shift is established based on the Doppler physical model. Through dynamic calibration under standard flow velocity, the effective cosine value of the angle between the beam axis and the flow field vector direction is obtained and used as a structural constant to compensate for installation deviation. Based on the pre-stored effective cosine value of the angle, the preliminary solution value of flue gas velocity is obtained according to the real-time Doppler frequency shift solution.

[0012] Furthermore, the time-domain autocorrelation auxiliary verification specifically involves: performing time-domain autocorrelation operation on the effective signal sequence, extracting the characteristic delay time of flue gas particles passing through the sensing area by locking the first significant peak displacement excluding zero, calculating the flue gas reference velocity based on the characteristic delay time according to the equivalent optical center distance between the transmitting fiber core and the receiving fiber core, verifying the consistency between the flue gas reference velocity and the preliminary calculated value of the flue gas velocity, and eliminating false spectral peaks.

[0013] Furthermore, the temperature compensation process specifically includes: Based on the preliminary calculated value of flue gas velocity after verification, a transient optical compensation operator and an environmental refractive index and geometric correction operator are introduced to eliminate the interference of fiber refractive index changes with temperature and fluid environment fluctuations on the Doppler frequency shift signal. Finally, the flue gas velocity is the product of the preliminary calculated value of flue gas velocity and the transient optical compensation operator, environmental refractive index and geometric correction operator.

[0014] Furthermore, the transient optical compensation operator is obtained using the following method: The absolute temperature of the flue gas field is obtained, and the ratio of the echo power of two different wavelengths is calculated, based on the correction coefficient obtained in the pre-existing constant temperature bath experiment. The functional relationship between the power ratio and the echo power ratio is used to convert the power ratio into a correction coefficient, which is the transient optical compensation operator. The constant temperature bath experiment involves fixing the probe element inside a standard wind tunnel and placing it in a constant temperature bath, under standard flow rates. Take different temperature points Experiments were conducted to obtain the system output flow rate. and the power of the two echo optical paths , The correction factor is determined by the ratio of the standard flow rate to the system output flow rate. , The ratio is determined as the power ratio, thereby obtaining the corresponding dataset of power ratio and correction coefficient, and fitting the functional relationship between the two.

[0015] Furthermore, the environmental refractive index and geometric correction operator are obtained using the following method: Obtain the absolute temperature of the flue gas flow field According to the simplified form of the Lorentz-Lorentz equation, the refractive index of the flue gas With absolute temperature Inversely proportional, a correction term is introduced: , in, The reference refractive index at room temperature Reference absolute temperature; Simultaneously considering the disturbance of the angle between the beam axis and the flow field vector direction caused by the structural thermal expansion of the probe, an equivalent thermal expansion coefficient is introduced. By measuring the deviation of flow velocity measurement caused by temperature change in a known standard flow field, a linear fitting algorithm is used to extract the proportional constant between temperature change and flow velocity measurement gain as the equivalent thermal expansion coefficient to compensate for the measurement drift caused by thermal expansion and contraction of mechanical structure. The environmental compensation operator is obtained by synthesis. : .

[0016] This invention also provides a flue gas velocity detection system based on dual-core optical fiber sensing, comprising: A dual-core fiber optic probe, which includes a transmitting fiber core and a receiving fiber core arranged in parallel within a quartz cladding, is encapsulated entirely within a quartz sleeve. The probe tip is used to extend into the flue gas field to be measured. The light source and detection unit are connected to the other end of the dual-core fiber optic probe. The light source emits two optical signals of different wavelengths, which are coupled to the transmitting fiber core via wavelength division multiplexing to achieve dual-wavelength common-path transmission. The receiving fiber core picks up the scattered echo signal of the flue gas particles. The signal processing and temperature compensation unit receives the echo signals output by the light source and the detection unit, and performs signal processing and temperature compensation, including: extracting and calculating frequency domain features to obtain a preliminary calculated value of flue gas velocity, and combining it with time domain autocorrelation auxiliary verification, performing transient thermo-optical correction on the verified preliminary calculated value based on the echo light power of two different wavelengths, and performing environmental refractive index and geometric correction, thereby realizing temperature compensation processing of the signal from two aspects and outputting the final flow velocity data.

[0017] The beneficial effects of this invention are as follows: This invention is based on a miniaturized dual-core fiber optic probe design, utilizing optical scattering and the fiber Doppler effect, and employing a dual-wavelength, dual-closed-loop temperature compensation technique. It combines the ratio of the two wavelengths to perform transient thermo-optical correction on the fiber material, and updates the flue gas refractive index based on measured temperature while considering probe structural disturbances to achieve environmental state alignment calibration. This establishes a wide-temperature-range dynamic compensation model, effectively controlling flow velocity errors caused by temperature drift and solving the problem of cross-sensitivity in fiber optic sensing. It enables real-time, high-sensitivity, and high-precision measurement of flue gas velocity in small-diameter flues under high-temperature, dusty environments. The method and system of this invention can be widely applied to combustion experiments, emission measurement, industrial flue gas monitoring, and the study of airflow characteristics in carbon emission monitoring systems. Attached Figure Description

[0018] Figure 1 This is a schematic cross-sectional view of a dual-core optical fiber in one embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of an optical fiber probe in one embodiment of the present invention; Figure 3 This is a schematic diagram of the structure of a flue gas velocity detection system in one embodiment of the present invention; Figure 4 This is a flowchart illustrating the operation of the system in one embodiment of the present invention. Detailed Implementation

[0019] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] To address the core requirements of wide temperature fluctuation, high sensitivity, and long-term stable detection of flue gas velocity in laboratory small-channel environments, this invention provides a flue gas velocity detection system based on dual-core fiber optic sensing. According to a specific embodiment of the invention, the system mainly includes: a fiber optic probe module, a light source and detection unit module, a signal processing and temperature compensation unit module, and a mounting bracket, etc. These modules work together to achieve highly stable and highly sensitive flue gas velocity detection over a wide temperature range in small-diameter flues. The design of each module is described in detail below: As the core sensing unit of the device, the fiber optic probe in this invention employs a dual-core multimode quartz fiber, such as... Figure 1 As shown in the figure, this is a schematic diagram of the cross-sectional structure of a dual-core multimode optical fiber. From the inside out, it includes two symmetrically distributed single fiber cores, a common cladding, and a multi-layer protective structure. At the very center are the transmitting fiber core 1 and the receiving fiber core 2, responsible for transmitting and receiving optical signals respectively; their small spacing ensures signal correlation. Surrounding the fiber cores is a quartz cladding 3, used for total internal reflection light guiding. Outside the cladding is a polyimide coating layer 4 and a high-temperature inorganic adhesive filling layer 5, providing mechanical protection and stress buffering. Outside the filling layer is a quartz sleeve 6, providing overall structural strength. The outermost layer is an anti-adhesion coating 7, effectively preventing carbon soot and moisture from accumulating in the optical window. This structure, through its integrated dual-core design, improves system integration and anti-interference capabilities while ensuring optical performance.

[0021] The light source and detection unit is the coherent signal source and photoelectric conversion center of the device, responsible for generating high-quality laser and extracting weak Doppler frequency shift signals. This module can be integrated into an aluminum alloy cavity and mainly consists of a dual-wavelength drive unit (containing a 1550nm probe laser and a 1310nm reference laser), a wavelength division multiplexer (WDM), and a high-sensitivity PIN-TIA photoelectric detection component.

[0022] The core of the module employs a dual-wavelength common-path transmission and zero-difference coherent detection architecture. WDM technology combines two laser beams into a single-core transmission. The 1550nm light captures flow velocity due to its high scattering sensitivity to flue gas particles, while the 1310nm light serves as a physical reference for real-time monitoring of link loss, providing baseline data for backend temperature drift compensation. The module utilizes Fresnel reflection light from the probe end face as the "local oscillator," which is mixed with the returned particle-scattered light on the detector surface. This zero-difference detection method down-converts the high-frequency optical signal to a kHz-level intermediate frequency signal, and, combined with a microsecond-level TEC temperature control system, locks the wavelength, significantly enhancing the system's ability to capture the motion state of minute particles and its zero-point stability. To ensure signal purity under high temperature and strong radiation conditions, the module can also integrate a narrowband interference filter with a full width at half maximum (FWHM) of only 2nm at the front end of the detector. This design effectively blocks ambient stray light and flue gas thermal radiation interference, improving the system's signal-to-noise ratio by approximately 20dB. The weak current collected is processed by a high-gain bandpass amplifier circuit and converted into a high-quality voltage signal, which is then sent to the signal processing module.

[0023] According to a specific embodiment of the present invention, the single fiber core has a diameter of 50 μm, and two cores are arranged symmetrically in parallel within a quartz cladding with a diameter of 200 μm, with a center-to-center distance of only 80 μm. One core is connected to a 1550 nm / 1310 nm coupled light source as the transmitter, and the other core is connected to a PIN photodetector as the receiver. The outer layer of the optical fiber is coated with a 15 μm high-temperature resistant polyimide layer to ensure structural stability under high-temperature conditions. The entire probe is encapsulated within a high-purity quartz sleeve with an outer diameter of only 1.8 mm. This ultra-miniaturized design greatly reduces the flow field blockage effect within a narrow channel of 20–50 mm, maintaining the integrity of the original flow field.

[0024] To enhance environmental adaptability, the probe's front end, on the side where the flue gas flows in, can be machined with a 5-10° micro-angle chamfer, with a length of 1.0 mm. This structure induces smooth airflow and suppresses vortex shedding on the leeward side, thereby reducing low-frequency pulsation noise in the signal. Simultaneously, the outer surface of the quartz sleeve is coated with a 10 μm thick polytetrafluoroethylene-fluoride ceramic composite anti-adhesion coating 7. Utilizing the ultra-low surface energy characteristics of fluorides, this coating effectively prevents carbon smoke and water vapor from accumulating in the optical window, solving the problem of signal attenuation over time in dusty environments. Furthermore, in some embodiments, an alumina ceramic thermal insulation partition structure can be installed in the middle section of the probe, combined with a high-temperature resistant inorganic adhesive seal at the rear end, ensuring the thermomechanical stability of the optical precision components at 150°C. The probe's end is a connector 8 for connecting to the light source and detection unit, enabling the input and output of optical signals. A schematic diagram of the overall structure is shown below. Figure 2 As shown.

[0025] According to some embodiments of the present invention, such as Figure 3As shown, physical fixation can be achieved by creating a through hole in the flue gas duct wall for probe insertion and axially welding a threaded reinforcing base at the hole location. The fiber optic probe 10 is inserted into the flue gas duct via a flange structure (its position can be locked after alignment to ensure the geometric angle between the beam axis and the flow direction). Maintaining geometric stability and airtightness under pulsating pressure conditions of 20-150℃, the probe front end is located in the main flow area, used to emit laser and receive the scattered echo signal from particles in the flue gas, realizing in-situ acquisition of flow velocity information. The optical signal is transmitted to the light source and detection unit 20 via optical fiber. This module can integrate dual-wavelength laser driving, photoelectric detection, and signal conditioning. Specifically, the laser outputs light of a specific wavelength, which is coupled into the probe via optical fiber; the returned scattered light is converted into an electrical signal by the photodetector, and then amplified and filtered to improve the signal-to-noise ratio and suppress environmental interference. The processed analog signal enters the signal processing and temperature compensation unit 30 to complete high-speed sampling and digital processing, including frequency domain analysis (FFT to extract Doppler frequency shift) and time domain analysis (autocorrelation to estimate characteristic frequency). The reliability of the results is improved through a dual-mode consistency discrimination mechanism. At the same time, combined with temperature data, compensation and correction are performed using the dual-wavelength ratio and thermal effect model to obtain high-precision flow velocity results. The final calculation results are transmitted to the host computer data analysis software through the communication interface to realize real-time display, storage, and trend analysis of flow velocity data. It can also be output to the field display and control terminal 40. The system features a compact structure, strong anti-interference capabilities, and suitability for high-temperature and dusty environments, making it suitable for online monitoring of industrial flue gas.

[0026] The signal processing and temperature compensation unit receives the echo signals output from the light source and detection unit, and performs signal processing and temperature compensation, including: extracting and calculating frequency domain features to obtain a preliminary calculated value of the flue gas velocity, and combining this with time-domain autocorrelation for auxiliary verification; performing transient thermo-optical correction on the verified preliminary calculated value based on the echo power of two different wavelengths; and simultaneously performing environmental refractive index and geometric correction. This achieves temperature compensation processing of the signal from two aspects, outputting the final flow velocity data; specifically, it includes signal processing and temperature calibration. According to one embodiment of the present invention, the core task of signal processing is to convert the weak electrical signal converted by the photodetector into a high-precision flow velocity value through frequency domain analysis and time domain verification, including the following: 1) Raw signal acquisition and preprocessing Photocurrent captured by the detector First, it is converted into a voltage signal by a transimpedance amplifier. Because flue gas flow is random, continuous voltage signals... Includes Doppler frequency shift signal Additive white Gaussian noise : ; The system performs high-frequency discrete sampling via an ADC, with a sampling frequency of... To obtain discrete sequences To remove baseband DC components and high-frequency circuit noise, the sequence needs to be processed by a digital bandpass filter. The effective signal after filtering is... .

[0027] 2) Frequency domain feature extraction For length of signal sequence Perform a Fast Fourier Transform (FFT) to obtain the frequency domain power spectrum. : , The system in the power spectrum The strongest peak frequency in the search energy The detected Doppler frequency shift was calculated. : , According to the Doppler physics model, particle velocity With frequency shift The mapping relationship is derived as follows: , in, For the refractive index of flue gas, The operating wavelength is 1550nm. This is the angle between the beam axis and the direction of the flow field vector. To eliminate the influence of installation deviations on measurement accuracy, this device employs a dynamic calibration scheme based on a known flow field to obtain the effective angle. At standard flow rate Under the experimental conditions, the Doppler frequency shift of the acquisition device output is the main frequency. The effective cosine value of the included angle under the current operating condition is obtained by reverse derivation using the above formula. This value is pre-stored as a structural constant for subsequent real-time calculation of the flow velocity.

[0028] This yields the preliminary flow velocity calculation value. : .

[0029] 3) Time-domain autocorrelation auxiliary verification mechanism To further enhance robustness, the system introduces a time-domain autocorrelation mechanism to verify the consistency of the frequency-domain results. Autocorrelation function Reflects the sequence Signal delay The similarity after sampling points is defined as follows: , The autocorrelation function is located using a peak search algorithm. The displacement corresponding to the first significant peak in the region The characteristic delay time of particles passing through the sensing area is obtained. ; Based on the physical structure of the probe module, the transmitting fiber core and the receiving fiber core have an equivalent optical center distance in space. Reusing time parameters The reference flow rate can be calculated independently. The system compares... and The consistency of the data effectively identifies and eliminates spurious spectral peaks caused by dust impact or electromagnetic transients. This time-frequency domain dual-mode verification logic enhances the measurement robustness of the device under low signal-to-noise ratio conditions, ensuring the continuity and authenticity of the flow rate data output.

[0030] According to a specific embodiment of the present invention, temperature calibration is the core guarantee for the system to achieve high-precision measurement over a wide temperature range of 20~150℃. Essentially, it eliminates the fiber optic thermo-optic effect (refractive index) through physical sensing and mathematical modeling. The interference of temperature changes and fluid environment fluctuations on the Doppler frequency shift signal.

[0031] 1) Physical structure and parameter definition of the compensation system The absolute temperature of the flow field is obtained by embedding a miniature K-type thermocouple in the probe base. .in, , These are the real-time optical powers of the two wavelengths sensed by the receiver; The refractive index of the gas to be measured inside the flue; The effective geometric angle between the beam axis and the direction of the flow field vector is given.

[0032] ① Transient optical compensation operator Acquisition Due to the refractive index of optical fibers It is temperature The coupling efficiency drift varies with different wavelengths at high temperatures, which is a function of the system. The system calculates the ratio of the two echo powers in real time. : , The ratio This directly reflects the minute shift in the optical constants inside the probe. According to one embodiment of the invention, calibration coefficients obtained beforehand in a constant temperature bath experiment are used... Convert the power ratio into a correction factor. : , The constant temperature bath calibration experiment is used to establish the functional relationship between the dual-wavelength echo power ratio and the flow rate correction coefficient. Before the experiment, the test system is set up, and the probes to be calibrated from the same batch and the high-precision temperature measuring elements are fixed in a standard wind tunnel. The whole system is placed in a constant temperature bath with a temperature control accuracy of ±0.1℃, equipped with an optical acquisition and synchronous data system, and fully preheated to eliminate zero-point drift of the equipment.

[0033] First, at a reference temperature of 20°C, a standard flow velocity was set using a standard wind tunnel. And record it. After the system stabilizes, continuously collect the flow velocity output signal and optical power signal over a period of time, and perform time averaging on the measurement results to obtain the system output flow velocity. and the corresponding optical power ratio The baseline correction factor is calculated using the following relationship:

[0034] Subsequently, temperature points were selected every 10°C within the range of 20°C to 150°C. After reaching thermal stability at each temperature point (temperature fluctuation less than ±0.1℃ and duration not less than 5 minutes), multiple sets of data are continuously collected, including the system output flow rate. and the power of the two echo optical paths , The collected data is averaged to reduce the impact of random noise.

[0035] Based on the processed data, the optical power ratio is calculated:

[0036] And the corresponding correction factor:

[0037] This yields the corresponding dataset for the power ratio and correction factor:

[0038] Establish functional relationships:

[0039] To improve data reliability, outlier removal is performed on the above dataset. When a data point deviates from the overall mean by more than a preset threshold, it is removed.

[0040] Based on this, using optical power ratio Independent variable, correction coefficient Using the least squares method as the dependent variable, the fifth-order polynomial model is fitted:

[0041] The calibration coefficients of each order are obtained by minimizing the sum of squared fitting errors. After fitting is complete, the fitting results are substituted into the original data for back-substitution calculation, and the fitting error is evaluated.

[0042] ② Environmental refractive index and geometric correction operator Acquisition Because changes in the external ambient temperature directly alter the refractive index of the gas. This affects the mapping ratio between Doppler frequency and flow velocity. The system obtains the absolute temperature of the flue gas through thermocouples. According to the simplified form of the Lorentz-Lorentz equation, the refractive index of a gas is related to its absolute temperature. Inversely proportional. Introducing a correction term: , in, This is the reference refractive index at room temperature (e.g., 20°C). The reference absolute temperature is 293.15 K.

[0043] Meanwhile, taking into account the effect of thermal expansion of the quartz sleeve on the beam angle... The tiny perturbation introduces the coefficient of thermal expansion .

[0044] coefficient of thermal expansion It is determined based on the material properties of the probe's mechanical structure (such as quartz) combined with full-temperature-range calibration experiments. The deviation in flow velocity measurement caused by temperature changes (i.e., representing the geometric angle of the beam) is measured in a known standard flow field. The resulting deviation is compensated by using a linear fitting algorithm to extract the proportionality constant between the temperature change and the flow rate measurement gain, thereby compensating for the measurement drift caused by the thermal expansion and contraction of the mechanical structure.

[0045] The environmental compensation operator is obtained by synthesis. : , ③ Receive the original flow velocity solution value Based on this, the compensation operators of the two dimensions mentioned above are multiplied and synthesized. The final target flow velocity data is then obtained. for: , In summary, according to an embodiment of the present invention, the workflow of the flue gas velocity detection method based on dual-core optical fiber sensing of the present invention includes the following: 1) Light Source Emission Stage: After system startup, two narrow-linewidth lasers are synchronously driven by the semiconductor laser component to emit continuous optical waves with center wavelengths of 1550nm (probe light) and 1310nm (reference light), respectively. The two optical signals (continuous optical waves) of different wavelengths are coupled to the transmitting fiber core via a wavelength division multiplexer to achieve dual-wavelength common-path transmission. The optical signals are transmitted to the probe end located inside the flue, pass through the quartz window and enter the flow field to form a sensing optical field.

[0046] 2) Signal Scattering and Reception Stage: When flue gas particles pass through the measurement area, the frequency of the scattered light undergoes a Doppler shift due to the particle's velocity. The echo signal carrying the frequency characteristics is picked up by the receiving fiber core and converted into a weak electrical signal by the photodetector. The signal is amplified by the preamplifier circuit and then input to the signal processing module.

[0047] 3) Signal acquisition and filtering stage: The signal processing module uses a high-speed ADC to obtain signals at a frequency... (like )right Perform equal-interval sampling to generate discrete sequences. Meanwhile, the module extracts data in real time. and Real-time echo power of optical signal and .

[0048] 4) Data demodulation and autocorrelation analysis stage: for Perform a Fast Fourier Transform to search for the dominant frequency point with the highest energy in the power spectrum. Calculate Doppler frequency shift This allows for the calculation of the preliminary flow velocity based on frequency characteristics. ; Perform autocorrelation calculations synchronously By locking the first significant peak displacement other than zero point Extract the characteristic delay time of particles passing through the sensing area. The system utilizes physical center distance. Convert to reference flow rate .

[0049] 5) Temperature drift and compensation stage: A. Transient thermo-optical correction: The system calculates the instantaneous power ratio. Substitute the fifth-order polynomial fitting coefficients stored in memory Calculate the transient compensation operator This step offsets the coupling efficiency fluctuations caused by heating of the fiber optic material within microseconds.

[0050] B. Environmental Condition Alignment: Reading the absolute temperature of the fluid via embedded thermocouples. Real-time updates of gas refractive index Combined with the preset system comprehensive thermal expansion coefficient Generate environment correction operator .

[0051] 6) Flow velocity output and monitoring stage: The signal processing module performs the final product operation to synthesize the target flow velocity. The data processing unit outputs the real-time calculation results to the host computer monitoring system through the communication interface; the host computer software realizes the functions of data recording, chart display, historical backtracking and abnormal alarm. Figure 4 This is a flowchart illustrating a specific embodiment of the present invention.

[0052] Example: Laboratory small-channel flue gas velocity detection and verification 1. Experimental Apparatus Structure Selection and Physical Parameters In this embodiment, a stainless steel flue gas duct with an inner diameter of D=80mm is selected as the test object. The specific hardware configuration is as follows: Optical probe: The sleeve is made of quartz, with an outer diameter d=8mm and an insertion depth L=40mm (blocking ratio of 6.4%). The probe tip features a 10° wedge chamfer and internally encapsulates a single-mode fiber collimator with an equivalent center distance of... .

[0053] Laser source: A dual-wavelength semiconductor laser is used, with output wavelengths of respectively and The output power is constant at 20mW.

[0054] Signal processing unit: Equipped with a Xilinx Artix-7 series FPGA chip, configured with a 16-bit dual-channel high-speed ADC, sampling frequency... The frequency was set to 250kHz, and the number of FFT calculation points N=4096.

[0055] Preset parameters: Fifth-order polynomial coefficients calibrated offline and stored in memory. The overall thermal expansion coefficient of the system .

[0056] 2. Experimental testing conditions The experiment was conducted in a standard wind tunnel laboratory, simulating a flue gas environment ranging from 20°C to 150°C using a heater. A standard reference flow rate was provided by a high-precision Pitot tube (0.5% accuracy). The average particle size added to the flue gas was... Silica particles are used as the scattering medium.

[0057] 3. Experimental Results and Performance Analysis (1) Validation of time-domain auxiliary verification Under conditions of large dust concentration fluctuations (signal-to-noise ratio SNR < 5dB), the performance of single FFT demodulation and the "time-frequency dual-mode verification" of this device were compared: Single FFT method: Due to background noise interference, the main frequency capture frequently jumps, and the flow velocity curve shows a large number of abnormal spikes.

[0058] The device's verification method: based on autocorrelation delay time. Extracted reference flow rate More than 98% of spurious peaks were successfully identified and eliminated, and the smoothness of the output curve was improved by 85%.

[0059] (2) Temperature compensation accuracy comparison test Recorded at standard flow rate The measurement errors at different temperatures during continuous operation are shown in the table below:

[0060] Results Analysis: Experimental data show that without temperature compensation, the drift error at 150°C is as high as 18.8% due to the thermo-optical effect and changes in refractive index. After introducing the dual closed-loop compensation model of this scheme, the measurement error over the entire range is successfully compressed to within ±0.5%.

[0061] (3) Installation stability test Ten consecutive disassembly and reassembly tests were conducted on the probe. The standard deviation of flow rate measurement repeatability was determined after each reassembly, relying on the hard limiting washer and circumferential locking nut. This demonstrates that the side-entry threaded base connection structure has excellent geometric reset accuracy.

[0062] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0063] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0064] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0065] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0066] The embodiments described above are merely some preferred embodiments of the present invention, and are not intended to limit the invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A flue gas flow rate detection method based on dual-core optical fiber sensing, characterized in that, include: A dual-core fiber optic probe is used, which includes a transmitting fiber core and a receiving fiber core arranged in parallel within a quartz cladding. The entire probe is encapsulated in a quartz sleeve, and the probe tip is used to extend into the flue gas field to be measured. The other end of the dual-core fiber optic probe is connected to the light source and the detection unit. The light source emits two optical signals of different wavelengths, which are coupled to the transmitting fiber core through wavelength division multiplexing to realize dual-wavelength common path transmission. The receiving fiber core picks up the echo signal scattered by the flue gas particles. The echo signal is processed for signal processing and temperature compensation, including: extracting and calculating frequency domain features to obtain a preliminary calculated value of flue gas velocity, and combining it with time domain autocorrelation for auxiliary verification. Based on the echo power of two different wavelengths, transient thermo-optical correction is performed on the verified preliminary calculated value. At the same time, environmental refractive index and geometric correction are performed to achieve temperature compensation processing of the signal from two aspects and output the final flow velocity data.

2. The dual-core fiber sensing based flue gas flow rate detection method according to claim 1, wherein, The dual-core fiber optic probe includes two parallel transmitting and receiving fiber cores, both of which are covered by a quartz cladding. Outside the quartz cladding, from the inside out, there are also a polyimide coating, a filling layer, a quartz sleeve, and an anti-adhesion coating. The probe tip is provided with a micro-bevel.

3. The dual-core fiber sensor based flue gas flow rate detection method according to claim 2, wherein, The diameter of the transmitting and receiving fiber cores is 45~55μm, the center-to-center distance between the two fiber cores is 70~90μm, the diameter of the quartz cladding is 180~220μm, the thickness of the polyimide coating is 10~20μm, the diameter of the quartz sleeve is 1.5~2.0mm, and the quartz sleeve at the front end of the probe is machined with a 5~10° micro-angle chamfer.

4. The dual-core optical fiber sensing based flue gas flow rate detection method according to claim 1, characterized in that, The two optical signals of different wavelengths are 1550nm and 1310nm, respectively. The 1550nm laser is a probe laser used to capture the flow velocity of flue gas particles, and the 1310nm laser is used to provide reference data for back-end temperature drift compensation.

5. The dual-core optical fiber sensing based flue gas flow rate detection method according to claim 1, wherein, The signal processing includes: sampling and filtering the received signal to obtain an effective signal sequence, performing a fast Fourier transform to obtain a frequency domain power spectrum, searching for the main frequency point with the highest energy in the power spectrum, and calculating the Doppler frequency shift; The mapping relationship between flow velocity and frequency shift is established based on the Doppler physical model. Through dynamic calibration under standard flow velocity, the effective cosine value of the angle between the beam axis and the flow field vector direction is obtained and used as a structural constant to compensate for installation deviation. Based on the pre-stored effective cosine value of the angle, the preliminary solution value of flue gas velocity is obtained according to the real-time Doppler frequency shift solution.

6. The dual-core optical fiber sensing based flue gas flow rate detection method according to claim 5, characterized in that, The time-domain autocorrelation auxiliary verification specifically involves: performing time-domain autocorrelation operation on the effective signal sequence, extracting the characteristic delay time of flue gas particles passing through the sensing area by locking the first significant peak displacement excluding zero, calculating the flue gas reference velocity based on the characteristic delay time according to the equivalent optical center distance between the transmitting fiber core and the receiving fiber core, verifying the consistency between the flue gas reference velocity and the preliminary calculated value of the flue gas velocity, and eliminating false spectral peaks.

7. The dual-core optical fiber sensing based flue gas flow rate detection method according to claim 1, wherein, The temperature compensation process specifically involves: Based on the preliminary calculated value of flue gas velocity after verification, a transient optical compensation operator and an environmental refractive index and geometric correction operator are introduced to eliminate the interference of fiber refractive index changes with temperature and fluid environment fluctuations on the Doppler frequency shift signal. Finally, the flue gas velocity is the product of the preliminary calculated value of flue gas velocity and the transient optical compensation operator, environmental refractive index and geometric correction operator.

8. The flue gas velocity detection method based on dual-core optical fiber sensing according to claim 7, characterized in that, The transient optical compensation operator is obtained using the following method: The absolute temperature of the flue gas field is obtained, and the ratio of the echo power of two different wavelengths is calculated, based on the correction coefficient obtained in the pre-existing constant temperature bath experiment. The functional relationship between the power ratio and the echo power ratio is used to convert the power ratio into a correction coefficient, which is the transient optical compensation operator. The constant temperature bath experiment involves fixing the probe element inside a standard wind tunnel and placing it in a constant temperature bath, under standard flow rates. Take different temperature points Experiments were conducted to obtain the system output flow rate. and the power of the two echo optical channels , The correction factor is determined by the ratio of the standard flow rate to the system output flow rate. , The ratio is determined as the power ratio, thereby obtaining the corresponding dataset of power ratio and correction coefficient, and fitting the functional relationship between the two.

9. The flue gas velocity detection method based on dual-core optical fiber sensing according to claim 7, characterized in that, The environmental refractive index and geometric correction operator are obtained using the following method: Obtain the absolute temperature of the flue gas flow field According to the simplified form of the Lorentz-Lorentz equation, the refractive index of the flue gas With absolute temperature Inversely proportional, a correction term is introduced: ， in, The reference refractive index at room temperature Reference absolute temperature; Simultaneously considering the disturbance of the angle between the beam axis and the flow field vector direction caused by the structural thermal expansion of the probe, an equivalent thermal expansion coefficient is introduced. By measuring the deviation of flow velocity measurement caused by temperature change in a known standard flow field, a linear fitting algorithm is used to extract the proportional constant between temperature change and flow velocity measurement gain as the equivalent thermal expansion coefficient to compensate for the measurement drift caused by thermal expansion and contraction of mechanical structure. The environmental compensation operator is obtained by synthesis. : 。 10. A flue gas velocity detection system based on dual-core optical fiber sensing, characterized in that, include: A dual-core fiber optic probe, which includes a transmitting fiber core and a receiving fiber core arranged in parallel within a quartz cladding, is encapsulated within a quartz sleeve. The probe tip is used to extend into the flue gas field to be measured. The light source and detection unit are connected to the other end of the dual-core fiber optic probe. The light source emits two optical signals of different wavelengths, which are coupled to the transmitting fiber core via wavelength division multiplexing to achieve dual-wavelength common-path transmission. The receiving fiber core picks up the scattered echo signal of the flue gas particles. The signal processing and temperature compensation unit receives the echo signals output by the light source and the detection unit, and performs signal processing and temperature compensation, including: extracting and calculating frequency domain features to obtain a preliminary calculated value of flue gas velocity, and combining it with time domain autocorrelation auxiliary verification, performing transient thermo-optical correction on the verified preliminary calculated value based on the echo light power of two different wavelengths, and performing environmental refractive index and geometric correction, thereby realizing temperature compensation processing of the signal from two aspects and outputting the final flow velocity data.

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