Flow rate measuring device and method based on ultra-low field nuclear magnetic resonance

Abstract

The application discloses a flow velocity measuring device based on ultra-low field nuclear magnetic resonance, comprising a magnetic shielding device, a polarization magnet, a fluid pipeline, a precession field coil, an atomic magnetometer and a pulse coil, and discloses a flow velocity measuring method based on ultra-low field nuclear magnetic resonance. The device and the method utilize the principle of nuclear magnetic resonance, and use the atomic magnetometer to measure, thus having higher sensitivity compared with ultrasonic, turbine measurement and other measuring methods; the device can realize measurement in a low-frequency frequency range with low radio frequency coil efficiency; the measuring environment is an ultra-low field, the flow velocity of fluid in a metal pipeline can be measured, and the limitation of the pipeline material on traditional nuclear magnetic resonance measurement is effectively overcome; the device can realize measurement of fluid in the pipeline without being connected to the pipeline, and is less affected by environmental magnetic field changes in the ultra-low field environment; the measuring method is non-invasive measurement, and does not disturb the measured fluid; the flow velocity measurement is not affected by factors such as viscosity of the fluid, and the application range of the flow velocity measurement is wider.

Description

Technical Field

[0001] The present invention belongs to the technical field of ultra-low field nuclear magnetic resonance, and specifically relates to a flow velocity measurement device based on ultra-low field nuclear magnetic resonance, and also relates to a flow velocity measurement method based on ultra-low field nuclear magnetic resonance, which is suitable for measuring the flow velocity of a fluid. Background Art

[0002] Currently, flow meters have been widely used in industrial production and our daily lives, such as turbine flow meters, ultrasonic flow meters, and electromagnetic flow meters. While they have high measurement accuracy, they also have significant limitations. Among them, turbine flow meters infer the fluid flow volume based on the rotational speed of the turbine blades driven by the fluid, and then combine this with the pipe diameter to calculate the fluid flow rate. This measurement method has high measurement accuracy and a wide range, but it is only suitable for flow and velocity measurements of low-viscosity liquids. It requires access to both ends of the turbine flow meter pipe, which has a large restriction on the pipe diameter. At the same time, the contact measurement of the turbine blades will also cause contamination to the fluid. Ultrasonic flow meters use the reflection of ultrasonic signals by the fluid, measure the time difference between the ultrasonic emission and the reception of the ultrasonic signal, and combine this with the speed of sound to derive the fluid flow rate. This measurement method is non-contact and does not require direct contact with the fluid, which will not affect the flow state of the fluid. It has high measurement accuracy, but it will be affected by the physical properties of the liquid such as fluid density, temperature, and bubble content. The operation process is relatively complicated and calibration is required before each measurement. Electromagnetic flowmeters, based on the principle of electromagnetic induction, measure the induced potential generated by a fluid in a fixed magnetic field. Using the relationship between flow velocity and induced potential, they can infer the flow velocity of conductive fluids. While they offer high sensitivity and a wide measurement range, they are limited in their ability to measure the state and properties of the fluid being measured. The fluid must have a higher conductivity than distilled water, and magnetic field fluctuations can also affect measurement accuracy.

[0003] Existing flow velocity measurement methods based on the principle of nuclear magnetic resonance mainly adopt the coil measurement method, using Tesla-level magnetic fields for polarization and using radio frequency coils for measurement. This measurement method usually requires the use of a more complex pulse sequence to obtain high-intensity, high-frequency signals, and then infer the fluid flow velocity based on the signals. This places extremely high performance requirements on the pulse signal source, which in turn increases the cost of the device. Due to the limitations of the magnet and coil diameter, this measurement device requires the pipeline to be switched in actual application, which increases the complexity of the measurement and has strict requirements on the pipeline material. It cannot measure the fluid velocity in metal pipelines or pipelines containing magnetic or easily magnetized materials. Summary of the Invention

[0004] The purpose of the present invention is to address the above-mentioned problems existing in the prior art and to provide a flow velocity measurement device based on ultra-low field nuclear magnetic resonance, and also to provide a flow velocity measurement method based on ultra-low field nuclear magnetic resonance.

[0005] The above-mentioned purpose of the present invention is achieved by the following technical means:

[0006] The flow velocity measuring device based on ultra-low field nuclear magnetic resonance includes a polarizing magnet, a precession field coil, a pulse coil, a magnetic shielding device, an atomic magnetometer, and a fluid pipeline. The magnetic shielding device includes a plurality of magnetic shielding tubes which are fixed in sequence. All magnetic shielding tubes are placed horizontally and have a common central axis. A through hole is provided in the center of the two end faces of each magnetic shielding tube. A support plate parallel to the horizontal plane is provided in the magnetic shielding tube with the smallest circular cross-sectional diameter. The support plate is fixedly connected to the inner wall of the magnetic shielding tube with the smallest circular cross-sectional diameter. A flow The support plate is located below the fluid pipeline, and both ends of the fluid pipeline respectively pass through the through holes at both ends of each magnetic shielding tube and extend out of the magnetic shielding tube. The precession field coil is wound and fixed on the outer wall of the fluid pipeline, and both ends of the precession field coil pass through the magnetic shielding device and extend out of the magnetic shielding device; the pulse coil is arranged outside the fluid pipeline, and the magnetic field center of the pulse coil is located in the fluid pipeline. The polarization magnet is arranged outside the magnetic shielding tube, and the polarization magnet is arranged on the magnet bracket. The sensor of the atomic magnetometer is fixedly connected to the outer wall of the fluid pipeline, and one side of the polarization magnet is arranged adjacent to one end of the fluid pipeline.

[0007] The direction of the magnetic field generated by the precession field coil, the direction of the magnetic field of the polarizing magnet, and the flow direction of the fluid are all the same.

[0008] As described above, the pulse coil includes two groups of coils, which are respectively arranged on both sides of the fluid pipeline. The two groups of coils are respectively arranged on corresponding coil supports, and the coil supports are fixedly connected to the support plate. The magnetic field centers of the two groups of coils are located in the fluid pipeline.

[0009] As mentioned above, the polarized magnet includes two pairs of NS magnets, which are arranged opposite to each other. The direction of the magnetic field between the two pairs of NS magnets is the same as the flow direction of the fluid in the fluid pipeline and the direction of the magnetic field inside the precession field coil. Each magnetic shielding tube is provided with a signal line through-hole. The pulse coil, precession field coil, and control signal lines of the atomic magnetometer all pass through the signal line through-hole into the interior of the magnetic shielding tube, and are then respectively connected to the pulse coil, precession field coil, and atomic magnetometer. The control signal line of the pulse coil is also connected to the pulse circuit.

[0010] It also includes an adjustable speed centrifugal pump and a liquid storage container. The outlet pipe of the adjustable speed centrifugal pump passes through the polarized magnet and is then connected to one end of the fluid pipeline. The pipe section on the outlet pipe located in the polarized magnet is parallel to the magnetic field direction of the polarized magnet. The other end of the fluid pipeline is connected to the inlet of the liquid storage container, and the water inlet pipe of the adjustable speed centrifugal pump is connected to the outlet of the liquid storage container.

[0011] The flow velocity measurement method based on ultra-low field nuclear magnetic resonance, using the flow velocity measurement device based on ultra-low field nuclear magnetic resonance as described above, comprises the following steps:

[0012] Step 1: Initialize the atomic magnetometer, then pass a current that generates a μT-level magnetic field into the precession field coil, and make the direction of the magnetic field generated by the precession field coil, the direction of the magnetic field between the two pairs of NS magnets of the polarizing magnet, and the flow direction of the fluid all the same;

[0013] Step 2: Test the atomic magnetometer until it works normally.

[0014] Step 3: Calibrate the intensity and time of the 90-degree DC pulse signal, set the calibration value in the pulse circuit, and trigger the pulse circuit to output a 90-degree DC pulse signal to the pulse coil when measuring the FID signal;

[0015] Step 4: Regulate the flow rate of the fluid using an adjustable-speed centrifugal pump, and then measure the initial measurement value D(v) and the linear attenuation slope f of the FID signal of the fluid, respectively. This specifically includes the following process:

[0016] Multiple sets of data are continuously collected within a set time during the measurement process. Each set of data is fitted once to obtain a linear decay slope f. The measurement time t is controlled to be within 1 / 10 of the transverse relaxation time T2 of the fluid. Multiple initial measurement values ​​D(v) and multiple linear decay slopes f of the FID signal are obtained. The initial measurement values ​​D(v) and the linear decay slopes f are then averaged respectively.

[0017] Step 5: Repeat step 4, changing the flow rate of the fluid each time, and obtaining initial measurement values ​​D(v) and linear attenuation slopes f of the FID signals of the fluids at different flow rates;

[0018] Step 6: Obtain the relationship between the linear attenuation slope f of the FID signal multiplied by 1 / D(v) and the actual flow rate by fitting;

[0019] Step 7: Obtain the linear attenuation slope and initial measurement value of the FID signal of the fluid to be measured with unknown flow rate, and then calculate the flow rate of the fluid to be measured.

[0020] The intensity and time calibration of the 90-degree DC pulse signal in step 3 specifically includes the following steps:

[0021] Step 3.1, changing the pulse time at a fixed pulse intensity, and measuring the FID signal of the fluid at a fixed flow rate at the fixed pulse intensity, wherein the pulse time corresponding to the group of signals with the largest initial measured FID signal value is the 90-degree DC pulse at the fixed pulse intensity;

[0022] Step 3.2: Then change the pulse intensity and repeat step 3.1 to measure the pulse time required for a 90-degree DC pulse at each pulse intensity;

[0023] Step 3.3: Compare the initial measurement values ​​of the FID signal of the 90-degree DC pulse at different pulse intensities. The one with the largest initial measurement value is the optimal pulse intensity, and the 90-degree DC pulse time corresponding to the optimal pulse intensity is the optimal pulse time.

[0024] As mentioned above, the fitting is performed based on the following formula in step 6:

[0025]

[0026] Where D(v) is the initial measurement value of the FID signal of the fluid with a flow velocity of v and a volume of V detected by the atomic magnetometer, and L d is the length of the pipe section on the fluid pipeline located in the test area, and f is the linear attenuation slope.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) The device and method of the present invention utilize the principle of nuclear magnetic resonance and use an atomic magnetometer for measurement. Compared with measurement methods such as ultrasound and turbine, they have higher sensitivity and can achieve measurement in the low-frequency range where the efficiency of radio frequency coils is low.

[0029] (2) The measurement environment in the device and method of the present invention is an ultra-low field, which can realize the flow velocity measurement of the fluid in the metal pipe, effectively overcoming the limitations of traditional nuclear magnetic resonance measurement on pipe materials.

[0030] (3) The device of the present invention can measure the fluid in the pipeline without being connected to the pipeline, and is less affected by changes in the ambient magnetic field in an ultra-low field environment.

[0031] (4) The measurement method of the present invention is non-invasive and will not cause disturbance to the fluid to be measured; and is not affected by factors such as the viscosity of the fluid, and the scope of application of flow rate measurement is wider.

[0032] (5) The device and method of the present invention have high measurement accuracy. The signal is transmitted through electromagnetic waves, and the density and turbidity of the fluid have little effect on the measurement accuracy.

[0033] (6) In addition to being able to measure the flow rate of liquids, the device and method of the present invention can also be further extended to detect the flow rate of hyperpolarized gases. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic structural diagram of the device of the present invention after the other components, excluding the adjustable speed centrifugal pump and the liquid storage container, are connected;

[0035] Figure 2 The present invention Figure 1 An internal cross-sectional view of the magnetic shielding device;

[0036] Figure 3 is a flow chart of the method of the present invention;

[0037] Figure 4 It is a schematic diagram of the overall structure of the device of the present invention;

[0038] Figure 5 1 is a diagram of measurement results of Example 2 of the present invention;

[0039] Figure 6 1 is a linear attenuation slope fitting result diagram of Example 2 of the present invention;

[0040] Reference numerals and corresponding component names:

[0041] 1-polarized magnet; 2-magnetic shielding cylinder; 3-magnet bracket; 5-precession field coil; 6-pulse coil; 7-atomic magnetometer; 8-support plate; 9-liquid storage container; 10-adjustable speed centrifugal pump. DETAILED DESCRIPTION

[0042] In order to facilitate those skilled in the art to understand and implement the present invention, the present invention is further described in detail below with reference to the embodiments. The embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0043] Example 1:

[0044] A flow rate measuring device based on ultra-low field nuclear magnetic resonance includes a magnetic shielding device, a polarizing magnet 1, a fluid pipeline, a precession field coil 5, an atomic magnetometer 7, and a pulse coil 6. The magnetic shielding device includes a plurality of cylindrical magnetic shielding tubes 2 that are sequentially sleeved and fixed. All magnetic shielding tubes 2 are placed horizontally and have a common central axis. A through hole is provided at the center of the two end faces of each magnetic shielding tube 2. A support plate 8 parallel to the horizontal plane is provided in the magnetic shielding tube 2 with the smallest circular cross-sectional diameter. The support plate 8 is fixedly connected to the inner wall of the magnetic shielding tube 2 with the smallest circular cross-sectional diameter. A fluid pipeline is also provided in the magnetic shielding tube 2 with the smallest circular cross-sectional diameter. The support plate 8 is located in the fluid Below the pipeline, the two ends of the fluid pipeline respectively pass through the through holes at both ends of each magnetic shielding tube 2 and extend out of the magnetic shielding tube 2. The precession field coil 5 is wound and fixed on the outer wall of the fluid pipeline, and the two ends of the precession field coil 5 pass through the magnetic shielding device and extend out of the magnetic shielding device; the pulse coil 6 is arranged outside the fluid pipeline, and the magnetic field center of the pulse coil 6 is located in the fluid pipeline. The polarization magnet 1 is arranged outside the magnetic shielding tube 2, and the polarization magnet 1 is arranged on the magnet bracket 3. The sensor of the atomic magnetometer 7 is fixedly connected to the outer wall of the fluid pipeline, and one side of the polarization magnet 1 is arranged adjacent to one end of the fluid pipeline, so that the fluid directly enters the fluid pipeline after passing through the polarization magnet 1 for thermal polarization.

[0045] The magnetic shielding tube 2 is used to provide a near-zero magnetic field (<10 nT) environment, which can ensure high-sensitivity measurement of the atomic magnetometer 7. The magnetic shielding tube 2 can be made of Permalloy.

[0046] The precession field coil 5 is wound with copper wire. Both ends of the precession field coil 5 pass through the magnetic shield, allowing the external magnetic flux lines generated by the precession field coil 5 to pass through the outside of the magnetic shield, thereby preventing the precession field coil 5 from generating residual magnetism that affects the working state of the atomic magnetometer 7. Current is applied to the precession field coil 5 to generate an ultra-low static magnetic field with a magnetic field strength of the μT level.

[0047] The direction of the magnetic field generated by the precession field coil 5, the direction of the magnetic field of the polarizing magnet 1, and the flow direction of the fluid are all the same. This allows the polarization direction of the flowing fluid to remain consistent from the polarization position to the measurement position, and the fluid is always in an ultra-low static magnetic field of the μT level during the flow process, thereby expanding the flow velocity measurement range and increasing the intensity of the nuclear magnetic resonance free induction decay (FID) signal.

[0048] The atomic magnetometer 7 is used to detect the FID signal after the fluid is pulsed and excited.

[0049] The pulse coil 6 includes two groups of coils, which are respectively arranged on both sides of the fluid pipeline. The two groups of coils of the pulse coil 6 are respectively arranged on corresponding coil brackets, and the coil brackets are fixedly connected to the support plate 8. The magnetic field centers of the two groups of coils of the pulse coil 6 are located in the fluid pipeline; the central area of ​​the two groups of coils of the pulse coil 6 is the area to be measured, and pulses are applied to the fluid at the position to be measured. Since the static magnetic field strength provided by the precession field coil 5 is extremely low, FID measurement of the fluid can be achieved without a complex pulse sequence, and a 90-degree DC pulse is usually applied.

[0050] Each magnetic shielding tube 2 is provided with a signal line through-hole corresponding to the position. The control signal lines of the pulse coil 6, the precession field coil 5, and the atomic magnetometer 7 all pass through the signal line through-holes into the interior of the magnetic shielding tube 2, and are then connected to the pulse coil 6, the precession field coil 5, and the atomic magnetometer 7 respectively. The control signal line of the pulse coil 6 is also connected to the pulse circuit.

[0051] The polarizing magnet 1 includes two pairs of NS magnets, which are arranged opposite to each other. The direction of the magnetic field between the two pairs of NS magnets is the same as the flow direction of the fluid in the fluid pipeline and the direction of the magnetic field inside the precession field coil 5, so that the polarization direction of the fluid after thermal polarization is the same as the flow direction of the fluid.

[0052] The coil bracket, the support frame, and the magnet bracket 3 are all made of non-metallic materials.

[0053] Example 2:

[0054] The flow velocity measurement method based on ultra-low field nuclear magnetic resonance, using the flow velocity measurement device based on ultra-low field nuclear magnetic resonance described in the above embodiment 1, comprises the following steps:

[0055] In this embodiment, an adjustable-speed centrifugal pump 10 and a liquid storage container 9 are also included. The outlet pipe of the adjustable-speed centrifugal pump 10 passes through the polarized magnet 1 and is then connected to one end of the fluid pipeline. The pipe section of the outlet pipe located in the polarized magnet 1 is parallel to the magnetic field direction of the polarized magnet 1. The other end of the fluid pipeline is connected to the inlet of the liquid storage container 9, and the water inlet pipe of the adjustable-speed centrifugal pump 10 is connected to the outlet of the liquid storage container 9 to ensure the circulation of the fluid.

[0056] The measurement principle of the present invention is as follows: in nuclear magnetic resonance, the FID signal of the nucleus to be measured (such as H, Xe) in the fluid to be measured is measured, and the attenuation law of the FID signal is:

[0057]

[0058] Wherein, Q(t) is the FID signal intensity of the fluid with a volume of V detected by the atomic magnetometer 7 at time t, t is time, a is the proportional factor affecting the detection of the magnetic resonance signal, T2 is the transverse relaxation time, M is the magnetization value of the fluid per unit volume, and V is the volume of the fluid. In this embodiment, the fluid is located in the fluid pipeline, so the volume of the fluid in the test area is the volume of the pipe section of the fluid pipeline located in the test area. Assume that the cross-sectional area of ​​the fluid pipeline is S and the length of the pipe section of the fluid pipeline located in the test area is L. d , the volume of the fluid pipeline in the test area (that is, the volume of the fluid in the test area) is V, then V=SL d ;

[0059] Considering that the fluid flows along the pipeline during measurement, the magnetization value of the fluid at a certain flow rate will change during the flow, and after a 90-degree DC pulse is applied to the test area, the magnetization value of the fluid will also change over time when the fluid flows through the test area. In this embodiment, the flow rate of the fluid is set to v and the time is t. In this embodiment, the time when the fluid just flows into the test area is 0, and a 90-degree DC pulse is applied to the test area. Then, the relationship between the magnetization value of the fluid per unit volume flowing through the test area, time t, and flow rate v is:

[0060]

[0061] Where M(v,t) is the magnetization value of a unit volume of fluid with a flow rate of v at time t after entering the test area, and M(v) is the initial magnetization value of a unit volume of fluid with a flow rate of v when entering the test area (i.e., the magnetization value of a unit volume of fluid with a flow rate of v at time 0).

[0062] In addition, after the fluid is polarized by the polarizing magnet 1, the magnetization value of the fluid will also change during the process of flowing from the polarized position of the polarizing magnet 1 to the area to be measured. Assume that the length of the polarizing magnet 1 is L p , the distance between the polarized magnet 1 and the area to be measured is L flow , then the initial magnetization value M(v) of a unit volume of fluid with a flow rate of v when it enters the test area is based on the following formula:

[0063]

[0064] Where M0 is the initial magnetization value of the fluid per unit volume after being polarized by the polarizing magnet 1, and T1 is the longitudinal relaxation time;

[0065] Substituting formula (2) and formula (3) into formula (1), we can obtain the FID signal intensity Q(v,t) of the fluid with a flow velocity v and a volume V detected by the atomic magnetometer 7 at time t:

[0066]

[0067] From formula (4), it can be seen that when the fluid moves at a certain flow velocity v and the measurement time t<<T2, Let D(v)=aM(v)V, that is D(v) is the initial measurement value of the FID signal of the fluid with a flow velocity of v and a volume of V detected by the atomic magnetometer 7, which is given by the formula It can be seen that the FID signal of a fluid with a flow rate of v and a volume of V decays linearly, and the linear decay slope f is equal to Dividing the linear decay slope f by the initial measured value D(v) of the FID signal yields That is, the flow rate is proportional to the linear decay slope f divided by the initial measured value D(v) of the FID signal. Therefore, in actual measurements, we control the measurement time t to be within 1 / 10 of the measured fluid's T2, which is much shorter than the transverse relaxation time T2 of the fluid in the measurement environment. Then, by fitting the relationship between the linear decay slope f of the FID signal multiplied by 1 / D(v) and the actual flow rate, we can infer the fluid's flow rate based on the linear decay slope f of the FID signal multiplied by 1 / D(v).

[0068] Step 1: Initialize the atomic magnetometer 7, then pass a current that generates a μT-level magnetic field into the precession field coil 5, and make the direction of the magnetic field generated by the precession field coil 5, the direction of the magnetic field between the two pairs of NS magnets of the polarizing magnet 1, and the flow direction of the fluid all the same;

[0069] Step 2: testing the atomic magnetometer 7 until the atomic magnetometer 7 can work normally;

[0070] Step 3: calibrate the intensity and time of the 90-degree DC pulse signal, set the calibration value to the pulse circuit, and trigger the pulse circuit to output a 90-degree DC pulse signal to the pulse coil 6 when measuring the FID signal;

[0071] Step 3.1, changing the pulse time (e.g., gradually increasing from zero) at a fixed pulse intensity, and measuring the FID signal of the fluid at a fixed flow rate at the fixed pulse intensity, wherein the pulse time corresponding to the group of signals with the largest initial measured FID signal value is the 90-degree DC pulse at the fixed pulse intensity;

[0072] Step 3.2: Then change the pulse intensity and repeat step 3.1 to measure the pulse time required for a 90-degree DC pulse at each pulse intensity;

[0073] Step 3.3: Compare the initial measurement values ​​of the FID signal of the 90-degree DC pulse at different pulse intensities. The one with the largest initial measurement value is the optimal pulse intensity, and the 90-degree DC pulse time corresponding to the optimal pulse intensity is the optimal pulse time.

[0074] Step 4: Regulate the flow rate of the fluid by the adjustable speed centrifugal pump 10, and then measure the initial measurement value D(v) and the linear attenuation slope f of the FID signal of the fluid, which specifically includes the following process:

[0075] Multiple sets of data are continuously collected within a set time during the measurement process. Each set of data is fitted once to obtain a linear decay slope f. The measurement time t is controlled to be within 1 / 10 of the transverse relaxation time T2 of the fluid. Multiple initial measurement values ​​D(v) and multiple linear decay slopes f of the FID signal are obtained. The initial measurement values ​​D(v) and the linear decay slopes f are then averaged respectively.

[0076] Step 5: Repeat step 4, changing the flow rate of the fluid each time, and obtaining initial measurement values ​​D(v) and linear attenuation slopes f of the FID signals of the fluids at different flow rates;

[0077] Step 6. The relationship between the linear attenuation slope f of the FID signal multiplied by 1 / D(v) and the actual flow rate is obtained by fitting the following formula:

[0078]

[0079] Where D(v) is the initial measurement value of the FID signal of the fluid with a flow velocity of v and a volume of V detected by the atomic magnetometer 7, and L d is the length of the pipe section on the fluid pipeline located in the test area, and f is the linear attenuation slope.

[0080] Step 7: During actual measurement, the flow velocity of the fluid to be measured can be calculated by simply obtaining the linear attenuation slope and the initial measurement value of the FID signal of the fluid to be measured with unknown flow velocity.

[0081] As an implementable method, the winding coefficient of the precession field coil 5 is 4600 nT / mA, and a current of 0.2 mA is passed through the precession field coil 5. A magnetic field with a magnetic field strength B0 of 920 nT is generated inside the precession field coil 5. The proton gyromagnetic ratio γ is 42.58 MHz / T. The precession frequency ω is calculated to be 39.16 Hz using the formula ω=γB0. When the measurement time is t1=0.2 s, ωt1=8.716, and the FID signal can have at least 8 cycles for linear attenuation slope fitting.

[0082] A DC pulse with pulse intensity B1 = 22.6μT and pulse time t2 = 260us is selected, and the pulse angle θ = γB1t2 is calculated (γ = 42.58MHz / T is converted to 42.58×2πrad·T -1 ·s -1 After calculation, it can be seen that the pulse angle can reach 90 degrees. Execute steps 1 to 5 in sequence, and the measurement results are as follows Figure 5 shown.

[0083] Figure 6 The FID signal measured by the device when the ultrasonic flowmeter displays a flow rate of 0.397 m / s has a linear attenuation slope of 7.4595 and a fitting value of D(v) of 1.0, that is, the linear attenuation slope multiplied by 1 / D(v) is 7.4595. Figure 5 The flow velocity measured by the curve inversion device is 0.401m / s, which has a measurement deviation of 1% compared with the ultrasonic flow meter.

[0084] It should be noted that the embodiments described herein are merely illustrative of the spirit of the present invention. Persons skilled in the art may make various modifications, additions, or substitutions to the described embodiments without departing from the spirit of the present invention or exceeding the scope of the appended claims.

Claims

1. A flow velocity measurement method based on ultra-low field nuclear magnetic resonance, using a flow velocity measurement device based on ultra-low field nuclear magnetic resonance, the device comprising an atomic magnetometer (7), a precession field coil (5), a polarizing magnet (1), a pulse coil (6), and an adjustable speed centrifugal pump (10); It is characterized by: The above method comprises the following steps: Step 1: Initialize the atomic magnetometer (7), then pass a current that generates a μT-level magnetic field into the precession field coil (5), and make the direction of the magnetic field generated by the precession field coil (5), the direction of the magnetic field between the two pairs of NS magnets of the polarizing magnet (1), and the flow direction of the fluid all the same; Step 2, testing the atomic magnetometer (7) until the atomic magnetometer (7) works normally; Step 3: calibrate the intensity and time of the 90-degree DC pulse signal, set the calibrated current intensity and pulse time in the pulse circuit, and trigger the pulse circuit to output the 90-degree DC pulse signal to the pulse coil (6) when measuring the FID signal; Step 4: Adjust the flow rate of the fluid by the adjustable speed centrifugal pump (10), and then measure the initial measurement value of the FID signal of the fluid respectively. and linear decay slope , specifically including the following processes: During the measurement process, multiple sets of data are continuously collected within the set time, and each set of data is fitted once to obtain the linear attenuation slope. , and each measurement time Set to the transverse relaxation time of the fluid Obtain initial measurement values ​​of multiple FID signals within 1 / 10 of and multiple linear decay slopes , and then the initial measurement values and linear decay slope Averaged separately; Step 5: Repeat step 4, changing the flow rate of the fluid each time, and obtain the initial measurement values ​​of the FID signal of the fluid with different flow rates. and linear decay slope ; Step 6: Obtain the linear decay slope of the FID signal by fitting Multiply The relationship between the value obtained and the actual flow rate; Step 7: Obtain the linear attenuation slope and initial measurement value of the FID signal of the fluid to be measured with unknown flow rate, and then calculate the flow rate of the fluid to be measured.

2. The flow velocity measurement method based on ultra-low field nuclear magnetic resonance according to claim 1, characterized in that: The intensity and time calibration of the 90-degree DC pulse signal in step 3 specifically includes the following steps: Step 3.1, changing the pulse time at a fixed pulse intensity, and measuring the FID signal of the fluid at a fixed flow rate at the fixed pulse intensity, wherein the pulse time corresponding to the group of signals with the largest initial measured FID signal value is the 90-degree DC pulse at the fixed pulse intensity; Step 3.2: Then change the pulse intensity and repeat step 3.1 to measure the pulse time required for a 90-degree DC pulse at each pulse intensity; Step 3.3: Compare the initial measurement values ​​of the FID signal of the 90-degree DC pulse at different pulse intensities. The one with the largest initial measurement value is the optimal pulse intensity, and the 90-degree DC pulse time corresponding to the optimal pulse intensity is the optimal pulse time.

3. The flow velocity measurement method based on ultra-low field nuclear magnetic resonance according to claim 2, characterized in that: In step 6, fitting is performed based on the following formula: ； Where, The flow rate detected by the atomic magnetometer (7) is , the volume is The initial measurement value of the FID signal of the fluid, is the length of the pipe section on the fluid pipeline located in the area to be measured, is the linear attenuation slope.

4. A device for implementing the flow velocity measurement method of ultra-low field nuclear magnetic resonance according to any one of claims 1 to 3, comprising a polarizing magnet (1), a precession field coil (5), and a pulse coil (6), characterized in that: It also includes a magnetic shielding device, an atomic magnetometer (7), and a fluid pipeline. The magnetic shielding device includes a plurality of magnetic shielding cylinders (2) that are fixed in sequence. All magnetic shielding cylinders (2) are placed horizontally. All magnetic shielding cylinders (2) share a common central axis. A through hole is opened in the center of the two end faces of each magnetic shielding cylinder (2). A support plate (8) parallel to the horizontal plane is provided in the magnetic shielding cylinder (2) with the smallest circular cross-sectional diameter. The support plate (8) is fixedly connected to the inner wall of the magnetic shielding cylinder (2) with the smallest circular cross-sectional diameter. A fluid pipeline is also provided in the magnetic shielding cylinder (2) with the smallest circular cross-sectional diameter. The support plate (8) is located below the fluid pipeline. The two ends of the fluid pipeline respectively pass through the through holes at the two ends of each magnetic shielding tube (2) and extend outside the magnetic shielding tube (2); the precession field coil (5) is wound and fixed on the outer wall of the fluid pipeline, and the two ends of the precession field coil (5) pass through the magnetic shielding device and extend outside the magnetic shielding device; the pulse coil (6) is arranged outside the fluid pipeline, and the magnetic field center of the pulse coil (6) is located in the fluid pipeline; the polarized magnet (1) is arranged outside the magnetic shielding tube (2), and the polarized magnet (1) is arranged on the magnet bracket (3); the sensor of the atomic magnetometer (7) is fixedly connected to the outer wall of the fluid pipeline, and one side of the polarized magnet (1) is arranged adjacent to one end of the fluid pipeline.

5. The device according to claim 4, characterized in that The direction of the magnetic field generated by the precession field coil (5), the direction of the magnetic field of the polarized magnet (1), and the flow direction of the fluid are all the same.

6. The device according to claim 5, characterized in that The pulse coil (6) comprises two groups of coils, the two groups of coils being respectively arranged on both sides of the fluid pipeline, the two groups of coils being respectively arranged on corresponding coil supports, the coil supports being fixedly connected to the support plate (8), and the magnetic field centers of the two groups of coils being located in the fluid pipeline.

7. The device according to claim 6, characterized in that The polarized magnet (1) includes two pairs of NS magnets, which are arranged opposite to each other. The direction of the magnetic field between the two pairs of NS magnets is the same as the flow direction of the fluid in the fluid pipeline and the direction of the magnetic field inside the precession field coil (5). Each magnetic shielding tube (2) is provided with a signal line through-hole. The control signal lines of the pulse coil (6), the precession field coil (5), and the atomic magnetometer (7) all pass through the signal line through-hole and enter the interior of the magnetic shielding tube (2). They are then connected to the pulse coil (6), the precession field coil (5), and the atomic magnetometer (7) respectively. The control signal line of the pulse coil (6) is also connected to the pulse circuit.

8. The device according to claim 7, characterized in that The invention also includes an adjustable speed centrifugal pump (10) and a liquid storage container (9), wherein the outlet pipe of the adjustable speed centrifugal pump (10) passes through the polarized magnet (1) and is then connected to one end of the fluid pipeline, and the pipe section of the outlet pipe located in the polarized magnet (1) is parallel to the magnetic field direction of the polarized magnet (1), the other end of the fluid pipeline is connected to the inlet of the liquid storage container (9), and the water inlet pipe of the adjustable speed centrifugal pump (10) is connected to the outlet of the liquid storage container (9).

Images