An optical fiber system and method for online testing of free water content in aviation fuel

Abstract

The present invention discloses an optical fiber system and method for online testing of free water content in aviation fuel. The testing system includes a single-wavelength laser light source with multiple wavelengths, a first optical fiber flange, a hollow-core optical fiber liquid sample cell, a second optical fiber flange, a photoelectric probe with a pigtail, a microfluidic sampling pump, a liquid sample outlet, a constant temperature device, and a host computer. The present invention online measures the dependence of the Mie scattering loss of free water microspheres suspended in a hollow-core optical fiber liquid sample cell filled with aviation fuel on different laser wavelengths to calculate the radius of the free water microspheres in the aviation fuel and the number of free water microspheres per unit volume, thereby measuring the free water content in the aviation fuel. The optical fiber testing system has a compact structure, small size, light weight, and requires a small sampling volume. It can quickly and accurately test free water content and is suitable for real-time online monitoring of trace free water content in aviation fuel.

Description

Technical Field

[0001] The present invention relates to the field of optoelectronic information technology, and in particular to an optical fiber system and method for online testing of free water content in aviation fuel. Background Art

[0002] Aviation fuel is divided into two categories: aviation gasoline and kerosene. Aviation gasoline's primary components are hydrocarbons such as alkanes and aromatics, while aviation kerosene is primarily composed of hydrocarbons such as alkanes, aromatics, and olefins. During extended flight at high altitudes and low temperatures, trace amounts of water dissolved in the fuel can precipitate as free water and freeze into ice, potentially blocking valves or filters and causing engine fuel supply malfunction. This can lead to high-altitude engine shutdowns and flameouts, jeopardizing flight safety. For example, for safety reasons, international standards for aviation kerosene limit the water content to 0-15 ppm, while Chinese standards stipulate that the free water content in kerosene cannot exceed 30 ppm. Therefore, online monitoring of the water content in the fuel tank before and during flight is crucial.

[0003] The main methods currently used for testing the water content of petroleum products are: (1) Karl Fischer coulometric titration (for example: Chinese invention patent, application number: CN201811519465.2, "A method for analyzing the content of trace free water in aviation kerosene"), and (2) chemical reaction colorimetric method (for example: Chinese invention patent, application number CN 106990105A, "Test paper for detecting the content of trace water in aviation fuel and its preparation method"). Both of the above methods cannot achieve online real-time testing, and require specialized testers and test equipment, which are not friendly technical means for aircraft users and fuel processing workers. In addition, for example, the German optek-Danulat company has developed an optical scattering method, which tests the scattered light intensity of a single-wavelength laser or a narrow-band incoherent light source in the forward, reverse, and certain scattering angle directions, and calculates the radius of the trace free water microspheres in the fuel and the number of microspheres per unit volume based on the dependence of the scattering intensity on different scattering angles, thereby determining the content of trace free water. Among them, G. Mie deduced from Maxwell's equations in 1908 and found that the Mie scattering intensity produced by perfect spherical microspheres has a strict mathematical solution (G. Mie, Annalen der Physik 330 (3), 377 (1908)). However, this method is based on the principle of spatial geometric optics and requires a larger liquid sample pool and more block optical elements to achieve more accurate optical testing. However, in actual scenarios, most high-altitude aircraft using aviation fuel have very strict restrictions on the size and load of the equipment they carry. Therefore, there is an urgent need to develop an online test fiber optic system and test method for the free water content in aviation fuel that is compact, lightweight, and user-friendly for non-optical professionals. Summary of the Invention

[0004] The present invention aims to provide an optical fiber system and method for online testing of free water content in aviation fuel. By introducing multiple laser wavelengths and examining the wavelength dependence of Mie scattering loss in a hollow-core optical fiber liquid sampling cell over a sufficiently large wavelength band, the radius and content of free water microspheres in aviation fuel can be accurately determined.

[0005] To achieve the above objectives, on the one hand, the present invention discloses an optical fiber system for online testing of free water content in aviation fuel, comprising a laser light source, a hollow-core optical fiber liquid sample pool, a photoelectric probe with a pigtail, a microfluidic sampling pump, a liquid sample outlet, and a host computer, wherein the laser light source is located on one side of the hollow-core optical fiber liquid sample pool, and the pigtail of the laser light source is coupled to the hollow-core optical fiber input end of the hollow-core optical fiber liquid sample pool via a first optical fiber flange; the photoelectric probe is located on the other side of the hollow-core optical fiber liquid sample pool, and the hollow-core optical fiber output end of the hollow-core optical fiber liquid sample pool and the pigtail of the photoelectric probe are coupled to each other via a second optical fiber flange; the microfluidic sampling pump is connected to an aviation fuel tank and is used to take a sample from the aviation fuel tank and inject the fuel sample into the hollow-core optical fiber liquid sample pool;

[0006] The laser light source (1) is composed of n single-wavelength lasers with different wavelengths, the output line width of the single-wavelength laser is no more than 5nm; the wavelengths are λ1, λ2, ... λ i ,……,λ n , and λ1<λ2<……<λ i ...<λ n , coupling the output of the pigtail into the hollow-core optical fiber liquid sample pool filled with aviation fuel, and the photoelectric probe receives the multi-wavelength laser power output by the hollow-core optical fiber liquid sample pool filled with aviation fuel to perform online real-time testing;

[0007] The laser light source, the hollow-core optical fiber liquid sample cell, the photoelectric probe, and the microfluidic sampling pump are respectively connected to the host computer. The host computer is used to generate an accurate solution for Mie scattering at different wavelengths based on different free water microsphere radii, use a numerical approximation method for fitting, and calculate the free water microsphere radius and free water content in the aviation fuel liquid sample according to the principle of least squares.

[0008] Furthermore, it also includes a constant temperature device, which is closely attached to the bottom of the hollow-core optical fiber liquid sample pool; the constant temperature device is connected to the host computer, and the host computer controls the constant temperature device to keep the hollow-core optical fiber liquid sample pool at a constant temperature.

[0009] Furthermore, the shortest laser wavelength λ1 and the longest laser wavelength λ n Satisfy the relationship λ n ≥1.1λ1.

[0010] Furthermore, the refractive index of the material surrounding the hollow core of the hollow core optical fiber used in the hollow core optical fiber liquid sample pool is higher than the refractive index of aviation fuel in the working band, the diameter of the hollow core is 20-2000 microns, and the length of the hollow core optical fiber is 0.1-10 meters.

[0011] Furthermore, the operating wavelength band of the photoelectric probe covers the wavelengths of all single-wavelength lasers in the laser light source.

[0012] Furthermore, the first optical fiber flange is divided into a first input chamber and a second input chamber by a first optical window that is transparent to all wavelengths of the laser light source. The first input chamber is used to fix the output fiber pigtail of the laser light source and is exposed to the environment; the second input chamber is used to fix the input end of the hollow-core optical fiber liquid sample pool; the input end of the hollow-core optical fiber liquid sample pool and the insertion part of the first optical fiber flange are sealed; the side of the second input chamber is connected to the output end of the microfluidic sampling pump.

[0013] Furthermore, the second optical fiber flange is divided into a first output chamber and a second output chamber by a second optical window that is transparent to all wavelengths of the laser light source. The first output chamber is used to fix the output end of the hollow-core optical fiber liquid sample pool; the output end of the hollow-core optical fiber liquid sample pool and the insertion part of the second optical fiber flange are sealed; the second output chamber is used to fix the input pigtail of the photoelectric probe and is exposed to the environment; the side of the first output chamber is connected to the liquid sample discharge port.

[0014] Furthermore, when the microfluidic sampling pump takes samples from the aviation fuel tank, the aviation fuel liquid flows into the second chamber of the input end, and flows into the hollow-core optical fiber core through the gap between the first optical window and the input end of the hollow-core optical fiber liquid sample pool, and fills the hollow-core optical fiber core along the entire length of the hollow-core optical fiber; thereafter, the aviation fuel liquid flows into the first chamber of the output end through the gap between the second optical window and the output end of the hollow-core optical fiber liquid sample pool, and flows into the liquid sample discharge port.

[0015] On the other hand, the present invention also discloses an online testing method for free water content in aviation fuel. Based on the above-mentioned optical fiber system for online testing of free water content in aviation fuel, the testing method comprises the following steps:

[0016] S1. The host computer calculates the exact solution for Mie scattering produced by free water scattering microspheres with a certain refractive index suspended in an aviation fuel liquid medium with a certain refractive index under certain temperature conditions based on the mathematical solution for Mie scattering produced by perfect spherical microspheres, and establishes a database of the dependencies between different free water microsphere radii and different laser wavelengths under certain temperature conditions;

[0017] S2. The host computer instructs the microfluidic pump to take a sample from the aviation fuel tank and fill the hollow-core optical fiber liquid sample pool;

[0018] S3. The host computer instructs the laser light source to turn on multiple single-wavelength lasers one by one. The laser outputs are coupled via pigtails into the input end of a hollow-core fiber optic liquid sample cell filled with an aviation fuel sample. The host computer receives the laser output power at each wavelength through a photoelectric probe at the output end of the hollow-core fiber optic liquid sample cell filled with an aviation fuel sample. The output power at each wavelength is compared with the laser output power at each wavelength of a hollow-core fiber optic liquid sample cell filled with an anhydrous aviation fuel standard sample under the same temperature and laser input power conditions. The scattering loss of the sampled aviation fuel sample at each wavelength is calculated.

[0019] S4. Based on the scattering loss values ​​of the hollow-core optical fiber liquid sample cell filled with real-time aviation fuel sampling at each laser wavelength obtained by the test in step S3, the host computer provides an initial value of the radius of the free water microsphere r0, calculates the sum of squares of the relative errors δ0 between the fitted scattering loss values ​​at different wavelengths when the water microsphere radius r0 is given in the database in step S1 and the measured scattering loss values ​​obtained in step S3, and gradually changes the radius value r of the free water microsphere in the direction of reducing the sum of squares of the relative errors according to the principle of least squares method. Finally, through the approximation method, the radius value r of the free water microsphere in the actual aviation fuel sample is obtained under the condition that the sum of squares of the relative errors δ between the fitted scattering loss values ​​at different wavelengths and the measured scattering loss values ​​is minimized;

[0020] S5. Calculate the number of free water microspheres per unit volume of the sample based on the radius of the free water microspheres suspended in the sample obtained in step S4 and the scattering loss values ​​of the hollow-core optical fiber liquid sample cell filled with the aviation fuel sample at each laser wavelength obtained in step S3, thereby obtaining the free water content in the aviation fuel sampled online in real time.

[0021] The beneficial effects of the present invention are:

[0022] (1) The present invention measures the dependence of the Mie scattering loss generated by free water microspheres suspended in a hollow-core optical fiber liquid sample pool filled with aviation fuel on different laser wavelengths online, and calculates the radius of the free water microspheres in the aviation fuel and the number of free water microspheres per unit volume, thereby measuring the free water content in the aviation fuel. Since free water is a perfect tiny sphere in high-purity aviation fuel, the Mie scattering generated by it as a scattering center has an exact solution. By examining the Mie scattering loss generated by the free water microspheres in the aviation fuel in the hollow-core optical fiber at more than four laser wavelengths, the radius of the free water microspheres in the aviation fuel and the free water content can be accurately fitted by nonlinear fitting based on the least squares principle;

[0023] (2) Under the conditions of the required test of free water content of the order of one part per million (ppm) and the free water microsphere radius of the submicron level, the Mie scattering loss generated is relatively weak; because the hollow-core fiber waveguide has a longer optical path than the traditional bulk scattering device, the use of a low-loss hollow-core fiber of meter-level length as the hollow-core fiber liquid sample pool can achieve effective coincidence of the optical signal and the scattering center of the free water microsphere that generates Mie scattering on the long optical path, so that the weak scattering signals at different wavelengths and their differences can be accurately measured, thereby achieving accurate testing of extremely low levels of free water in high-purity aviation fuel;

[0024] (3) This system can detect the free water content in aviation fuel online in real time. The overall system is small in size and light in weight, making it suitable for use on high-altitude aircraft. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a structural diagram of the online optical fiber testing system of the present invention;

[0026] Figure 2 This is a schematic structural diagram of a first optical fiber flange of an online optical fiber testing system of the present invention;

[0027] Figure 3 This is a schematic structural diagram of a second optical fiber flange of the online optical fiber testing system of the present invention;

[0028] Figure 4 is the dependence of the radius of free water microspheres on the laser wavelength;

[0029] In the figure, 1-laser light source, 2-first optical fiber flange, 3-hollow-core optical fiber liquid sample pool, 4-second optical fiber flange, 5-photoelectric probe, 6-microfluidic sampling pump, 7-liquid sample outlet, 8-host computer, 9-constant temperature device, 1a-output pigtail, 2a-first chamber at the input end, 2b-first optical window, 2c-second chamber at the input end, 4a-first chamber at the output end, 4b-second optical window, 4c-second chamber at the output end, 5a-input pigtail. DETAILED DESCRIPTION

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] like Figure 1As shown, an optical fiber system for online testing of free water content in aviation fuel includes a laser light source 1, a first optical fiber flange 2, a hollow-core optical fiber liquid sample cell 3, a second optical fiber flange 4, a photoelectric probe 5 with a pigtail, a microfluidic sampling pump 5, a liquid sample outlet 7, and a host computer 8. The laser light source 1 is located on one side of the hollow-core optical fiber liquid sample cell 3. The pigtail of the laser light source 1 is coupled to the hollow-core optical fiber input end of the hollow-core optical fiber liquid sample cell via the first optical fiber flange 2. The photoelectric probe 5 is located on the other side of the hollow-core optical fiber liquid sample cell 3. The hollow-core optical fiber output end of the hollow-core optical fiber liquid sample cell 3 is coupled to the pigtail of the photoelectric probe 5 via the second optical fiber flange 4. The microfluidic sampling pump 6 is connected to the aviation fuel tank and is used to take samples from the aviation fuel tank and fill the hollow-core optical fiber liquid sample cell 3 with fuel samples. The laser light source 1 is composed of n single-wavelength lasers of different wavelengths. The output linewidth of the single-wavelength lasers is no more than 5nm, and the wavelengths are λ1, λ2, ...λ respectively. i ,……,λ n Between, and λ1<λ2<……<λ i ...<λ n , coupled through the pigtail output into the hollow-core fiber optic liquid sample cell 3 filled with aviation fuel. The photoelectric probe 5 receives the multi-wavelength laser power output by the hollow-core fiber optic liquid sample cell 3 filled with aviation fuel and performs online real-time testing. The constant temperature device 9 is closely attached to the bottom of the hollow-core fiber optic liquid sample cell 3. The laser light source 1, hollow-core fiber optic liquid sample cell 3, photoelectric probe 5, microfluidic sampling pump 6, and constant temperature device 9 are respectively connected to the host computer 9.

[0032] like Figure 2 As shown, the first fiber optic flange 2 is divided into a first input chamber 2a and a second input chamber 2c via a first optical window 2b that is transparent to all wavelengths of the laser light source 1. The first input chamber 2a is used to secure the output fiber pigtail 1a of the laser light source 1 and is exposed to the environment. The second input chamber 2c is used to secure the input end of the hollow-core fiber liquid sample cell 3. The input end of the hollow-core fiber liquid sample cell 3 and the insertion portion of the first fiber optic flange 2 are sealed. The side of the second input chamber 2c is connected to the output end of the microfluidic sampling pump 6.

[0033] like Figure 3 As shown, the second optical fiber flange 4 is divided into a first output chamber 4a and a second output chamber 4c by a second optical window 4b that is transparent to all wavelengths of the laser light source 1. The first output chamber 4a is used to fix the output end of the hollow-core optical fiber liquid sample pool 3; the output end of the hollow-core optical fiber liquid sample pool 3 and the insertion part of the second optical fiber flange 4 are sealed; the second output chamber 4c is used to fix the input pigtail 5a of the photoelectric probe 5 and is exposed to the environment; the side of the first output chamber 4a is connected to the liquid sample discharge port 7.

[0034] In this embodiment, a commercial semiconductor laser light source that has been developed in the three low-loss windows of traditional quartz communication optical fiber (i.e., 850, 1310, and 1550 nm bands) is directly used to construct a laser light source 1 with a pigtail output consisting of four wavelengths of single-wavelength laser light, with wavelengths of 850 nm, 1310 nm, 1550 nm, and 1625 nm, respectively. The hollow-core optical fiber in the hollow-core optical fiber liquid sample pool 3 is a hollow-core antiresonant optical fiber based on borosilicate glass (Schott DURAN glass), with a hollow-core core diameter of 100 microns and a hollow-core optical fiber length of 0.5 meters. The transmittance of the hollow-core optical fiber liquid sample pool filled with anhydrous No. 3 aviation kerosene in the range of 800-1700 nm is greater than 80%. Correspondingly, the liquid sampling volume in the hollow-core optical fiber core is only 4x10 -3 Milliliters. The indium gallium arsenide photoelectric probe 5 with a pigtail has an operating wavelength band covering 600-1700 nanometers. The constant temperature device 9 is used to control the temperature of the hollow-core optical fiber liquid sample pool to keep it in a constant temperature state of 25±0.1°C. The refractive index of the material surrounding the core of the hollow-core optical fiber used in the hollow-core optical fiber liquid sample pool 3 is 1.47 at a wavelength of 850 nanometers, which is at least 0.05 higher than the refractive index of aviation fuel in the operating wavelength band of 1.40±0.02. The hollow-core optical fiber core used in the hollow-core optical fiber liquid sample pool 2 has a diameter of 100 microns and a length of 0.5 meters.

[0035] Figure 4 The dependence of the free water microsphere radius on the laser wavelength is given by the exact solution of the Mie scattering of the free water microsphere radius, assuming that the free water microspheres in aviation fuel have the same radius. The sample temperature is 25±0.1℃. sca =ρπr 2 Q sca , μ sca The unit is m -1 , Q sca is the Mie scattering efficiency factor, ρ is the number of free water microspheres per unit volume (unit: m -3 ), r is the radius of the free water microsphere.

[0036] The present invention also discloses an online testing method for free water content in aviation fuel, comprising the following steps:

[0037] S1. Assuming that the free water microspheres have the same radius, the host computer 8 calculates and obtains an exact solution for Mie scattering under the condition of scattering centers of water microspheres of a certain refractive index suspended in a liquid medium of aviation fuel of a certain refractive index, and establishes a database of the dependency relationship between the radius of the free water microspheres and the wavelength of the multi-wavelength laser light source.

[0038] S2. The host computer 8 instructs the microfluidic pump 6 to take a sample and fill the hollow-core optical fiber liquid sample pool 3.

[0039] S3. The host computer 8 instructs the multi-wavelength laser light source 1 to turn on one by one and couple the laser light source 1 to the input end of the hollow-core optical fiber liquid sample cell 3 filled with the aviation fuel sample. The host computer uses the photoelectric probe 5 to measure the laser output power at each wavelength at the output end of the hollow-core optical fiber liquid sample cell 3 filled with the aviation fuel sample. The laser output power at each wavelength is compared with the laser output power at each wavelength of the hollow-core optical fiber liquid sample cell 3 filled with an anhydrous aviation fuel standard sample under the same temperature and laser input power conditions. The scattering loss of the sampled aviation fuel sample at each wavelength is calculated.

[0040] S4. Based on the scattering loss values ​​of the hollow-core optical fiber liquid sample pool 3 filled with real-time sampling aviation fuel at each laser wavelength obtained by the test in step S2, the host computer 6 attempts to provide an initial value r0 of the radius of the free water microspheres, calculates the sum of squares δ0 of the relative errors between the fitted scattering loss values ​​at different wavelengths when the water microsphere radius r0 is given in the database in step S3 and the measured scattering loss values ​​obtained in step S2, and then adjusts the free water microsphere radius r1:

[0041] Specifically, this embodiment adopts a dichotomy method.

[0042] First, select r1 = 0.5r0 and calculate the sum of squares of the relative errors δ1 between the fitted scattering loss values ​​and the measured scattering loss values ​​at different wavelengths when the radius of the water microsphere r1 is set.

[0043] Next, compare δ0 and δ1:

[0044] If δ0-δ1>b, where b is the acceptable relative error value at the end of the fitting process, for example, b=10 -3 , then continue to change the radius r of the water microsphere in the same direction, so that r2=0.5r1. On the contrary, if δ1-δ0>b, then change the radius r of the water microsphere in the opposite direction, so that r2=2r1.

[0045] ...,

[0046] Until the i-th step, δ i -δ i-1 ≤b, indicating that the radius r of the free water microsphere obtained by gradual approximation i The least squares principle is satisfied so that the sum of squares of the relative errors δ between the fitted scattering loss values ​​and the measured scattering loss values ​​at different wavelengths is minimized, and step S4 ends.

[0047] S5. Calculate the number of free water microspheres per unit volume of the sample based on the radius of the suspended free water microspheres in the sample obtained in step S4 and the scattering loss values ​​of the hollow-core optical fiber liquid sample cell 3 filled with the aviation fuel sample at each laser wavelength obtained in step S2, thereby obtaining the free water content in the aviation fuel sampled online in real time.

[0048] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the present invention is not limited thereto. Various changes that can be made within the scope of knowledge possessed by technicians in the relevant technical field without departing from the spirit of the present invention are all within the scope of protection of the claims of the present invention.

Claims

1. An optical fiber system for online testing of free water content in aviation fuel, characterized in that: The system comprises a laser light source (1), a hollow-core optical fiber liquid sample pool (3), a photoelectric probe with a pigtail (5), a microfluidic sampling pump (6), a liquid sample outlet (7) and a host computer (8), wherein: The laser light source (1) is located at one end of the hollow-core optical fiber liquid sample pool (3), and the photoelectric probe (5) is located at the other end of the hollow-core optical fiber liquid sample pool (3); The pigtail of the laser light source (1) is coupled to the hollow-core optical fiber input end of the hollow-core optical fiber liquid sample pool (3) via a first optical fiber flange (2); the hollow-core optical fiber output end of the hollow-core optical fiber liquid sample pool (3) and the pigtail of the photoelectric probe (5) are coupled to each other via a second optical fiber flange (4); The microfluidic sampling pump (6) is connected to the first optical fiber flange (2) and is used to sample from the aviation fuel tank and inject the fuel sample into the hollow-core optical fiber liquid sample pool (3); the liquid sample outlet (7) is connected to the second optical fiber flange (4) and is used to discharge the fuel sample flowing through the hollow-core optical fiber liquid sample pool (3); The laser light source (1) is composed of n single-wavelength lasers of different wavelengths, which are coupled into the hollow-core optical fiber liquid sample pool (3) filled with aviation fuel through pigtail output. The photoelectric probe (5) receives the multi-wavelength laser power outputted through the hollow-core optical fiber liquid sample pool (3) filled with aviation fuel and performs online real-time testing. The laser light source (1), the hollow-core optical fiber liquid sample pool (3), the photoelectric probe (5), and the microfluidic sampling pump (6) are respectively connected to the host computer (8). The host computer (8) is used to calculate the free water microsphere radius and the free water content in the aviation fuel liquid sample according to the least squares principle based on the measured scattering losses at different wavelengths and the exact solution of Mie scattering generated by perfect spherical microspheres, using a numerical approximation method for fitting.

2. The optical fiber system for online testing of free water content in aviation fuel according to claim 1, characterized in that: The invention also includes a constant temperature device (9), which is closely attached to the bottom of the hollow-core optical fiber liquid sample pool (3); the constant temperature device (9) is connected to the host computer (8), and the host computer (8) controls the constant temperature device (9) to keep the hollow-core optical fiber liquid sample pool (3) at a constant temperature.

3. The optical fiber system for online testing of free water content in aviation fuel according to claim 1, characterized in that: The output line width of the single-wavelength laser of the laser light source (1) is not greater than 5 nm, and the wavelengths are λ1, λ2, ..., λ i , ..., λ n , and λ1<λ2<...<λ i <...<λ n , its shortest laser wavelength λ1 and longest laser wavelength λ n Satisfy the relationship λ n ≥1.1λ1.

4. The optical fiber system for online testing of free water content in aviation fuel according to claim 1, characterized in that: The hollow core optical fiber used in the hollow core optical fiber liquid sample pool (3) has a refractive index of a material surrounding the hollow core higher than the refractive index of aviation fuel in the working band, a hollow core diameter of 20-2000 microns, and a hollow core optical fiber length of 0.1-10 meters.

5. The optical fiber system for online testing of free water content in aviation fuel according to claim 1, characterized in that: The operating wavelength band of the photoelectric probe (5) covers the wavelengths of all single-wavelength lasers in the laser light source (1).

6. The optical fiber system for online testing of free water content in aviation fuel according to claim 1, characterized in that: The first optical fiber flange (2) is divided into a first input chamber (2a) and a second input chamber (2c) by a first optical window (2b) that is transparent to all wavelengths of the laser light source (1). The first input chamber (2a) is used to fix the output fiber pigtail (1a) of the laser light source (1) and is exposed to the environment; the second input chamber (2c) is used to fix the input end of the hollow-core optical fiber liquid sample pool (3); the input end of the hollow-core optical fiber liquid sample pool (3) and the insertion portion of the first optical fiber flange (2) are sealed; and the side of the second input chamber (2c) is connected to the output end of the microfluidic sampling pump (6).

7. The optical fiber system for online testing of free water content in aviation fuel according to claim 6, characterized in that: The second optical fiber flange (4) is divided into a first output chamber (4a) and a second output chamber (4c) by a second optical window (4b) that is transparent to all wavelengths of the laser light source (1). The first output chamber (4a) is used to fix the output end of the hollow-core optical fiber liquid sample pool (3); the output end of the hollow-core optical fiber liquid sample pool (3) and the insertion part of the second optical fiber flange (4) are sealed; the second output chamber (4c) is used to fix the input pigtail (5a) of the photoelectric probe (5) and is exposed to the environment; the side of the first output chamber (4a) is connected to the liquid sample outlet (7).

8. The optical fiber system for online testing of free water content in aviation fuel according to claim 7, characterized in that: When the microfluidic sampling pump (6) takes a sample from the aviation fuel tank, the aviation fuel liquid flows into the second chamber (2c) at the input end, and flows into the hollow-core optical fiber core through the gap between the first optical window (2b) and the input end of the hollow-core optical fiber liquid sample pool (3), and fills the hollow-core optical fiber core along the entire length of the hollow-core optical fiber; thereafter, the aviation fuel liquid flows into the first chamber (4a) at the output end through the gap between the second optical window (4b) and the output end of the hollow-core optical fiber liquid sample pool (3), and flows into the liquid sample outlet (7).

9. A method for online testing of free water content in aviation fuel, based on the optical fiber system for online testing of free water content in aviation fuel according to any one of claims 1 to 8, characterized in that: The testing method comprises the following steps: S1, the host computer (8) calculates the exact solution of Mie scattering produced by free water scattering microspheres with a certain refractive index suspended in an aviation fuel liquid medium with a certain refractive index under certain temperature conditions based on the mathematical solution of Mie scattering produced by perfect spherical microspheres, and establishes a database of the dependence between different free water microsphere radii and different laser wavelengths under certain temperature conditions; S2. The host computer (8) instructs the microfluidic sampling pump (6) to take a sample from the aviation fuel tank to fill the hollow-core optical fiber liquid sample pool (3); S3, the host computer (8) instructs the laser light source (1) to turn on multiple single-wavelength lasers one by one, and the laser outputs thereof are coupled into the input end of the hollow-core optical fiber liquid sample pool (3) filled with the aviation fuel sample through the pigtail. The host computer (8) receives and tests the laser output power of each wavelength at the output end of the hollow-core optical fiber liquid sample pool (3) filled with the aviation fuel sample through the photoelectric probe (5). The laser output power of each wavelength is compared with the laser output power of the hollow-core optical fiber liquid sample pool (3) filled with the anhydrous aviation fuel standard sample under the same temperature conditions and the same laser input power conditions, and the scattering loss of the sampled aviation fuel sample at each wavelength is calculated; S4. According to the scattering loss value of the hollow-core optical fiber liquid sample pool (3) filled with real-time sampling aviation fuel at each laser wavelength obtained by the test in step S3, the host computer (8) gives the initial value r0 of the radius of the free water microsphere, calculates the sum of squares δ of the relative errors between the fitted scattering loss values ​​at different wavelengths and the measured scattering loss values ​​obtained in step S3 when the water microsphere radius r is given in the database in step S1, and gradually changes the radius value r of the free water microsphere in the direction of reducing the sum of squares δ of the relative error according to the principle of least squares method. Finally, through the approximation method, the radius value r of the free water microsphere in the actual aviation fuel sample is obtained under the condition that the sum of squares δ of the relative errors between the fitted scattering loss values ​​at different wavelengths and the measured scattering loss values ​​is minimized; S5. Calculate the number of free water microspheres per unit volume of the sample based on the radius of the free water microspheres suspended in the sample obtained in step S4 and the scattering loss value of the hollow-core optical fiber liquid sample cell (3) filled with the aviation fuel sample at each laser wavelength obtained in step S3, thereby obtaining the free water content in the aviation fuel sampled online in real time.

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