Flow velocity measurement device and method based on ultralow field nuclear magnetic resonance

Abstract

The invention discloses a flow velocity measurement device based on ultralow field nuclear magnetic resonance, which comprises a magnetic shielding device, a polarized magnet, a fluid pipeline, a precession field coil, an atomic magnetometer and a pulse coil, and also discloses a flow velocity measurement method based on ultralow field nuclear magnetic resonance. According to the device and the method, the nuclear magnetic resonance principle is utilized, the atomic magnetometer is used for measurement, and compared with measurement methods such as ultrasonic measurement and turbine measurement, the device and the method have higher sensitivity; the measurement in a low-frequency frequency range with relatively low efficiency of the radio frequency coil can be realized; the measurement environment is an ultra-low field, the flow velocity of fluid in a metal pipeline can be measured, and the limitation of traditional nuclear magnetic resonance measurement on a pipeline material is effectively overcome; the device can measure fluid in a pipeline without being connected into the pipeline, and the influence of environmental magnetic field change in an ultra-low field environment is small; the measurement method is non-invasive measurement, so that the to-be-measured fluid cannot be disturbed; the method is not influenced by factors such as the viscosity of fluid, and the flow velocity measurement application range 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] At present, flowmeters have been widely used in industrial production and our lives, such as turbine flowmeters, ultrasonic flowmeters, electromagnetic flowmeters, etc., which have high measurement accuracy but also have great limitations. Among them, the turbine flowmeter reverses the fluid flow volume according to the rotation speed of the turbine blades driven by the fluid, and then calculates the fluid flow rate in combination with the pipe diameter. This measurement method has high measurement accuracy and a wide range, but it is only suitable for the flow rate measurement of low-viscosity liquids, and needs to be connected to both ends of the turbine flowmeter pipeline, which has a large restriction on the pipe diameter. At the same time, the contact measurement of the turbine blades will also pollute the fluid. The ultrasonic flowmeter uses the reflection of the ultrasonic signal by the fluid, measures the time difference between the ultrasonic emission and the reception of the ultrasonic signal, and combines the sound speed to derive the fluid flow rate. This measurement method is a non-contact measurement method, which does not require direct contact with the fluid, will not affect the flow state of the fluid, and has high measurement accuracy, but will be affected by the physical properties of the liquid such as fluid density, temperature, and bubble content, and the operation process is relatively complicated, and calibration is required before each measurement. Electromagnetic flowmeters are based on the principle of electromagnetic induction. They measure the induced potential generated by the fluid under a fixed magnetic field. The flow rate of the conductive fluid can be measured by inferring the flow rate based on the relationship between the flow rate and the induced potential. It has high sensitivity and a large measurement range, but it is limited to the state and properties of the fluid to be measured. The fluid to be measured must be a liquid with a higher conductivity than distilled water, and changes in the magnetic field will also affect the accuracy of the measurement.

[0003] The existing flow velocity measurement method based on the principle of nuclear magnetic resonance mainly adopts the coil measurement method, which uses a Tesla-level magnetic field for polarization and uses a radio frequency coil for measurement. This measurement method usually requires the use of a more complex pulse sequence to obtain a high-intensity, high-frequency signal, and then infers the fluid flow rate based on the signal. 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 applications, which increases the complexity of the measurement and has strict requirements on the material of the pipeline. It is impossible to measure the fluid velocity in a metal pipeline or a pipeline containing magnetic or easily magnetized materials. Summary of the invention

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

[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 polarization 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 sequentially sleeved and fixed, all of which 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. 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 fluid pipeline is provided with a support plate below the fluid pipeline, and two ends of the fluid pipeline respectively pass through the through holes at two 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 two 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, and the two groups of coils are respectively arranged on corresponding coil brackets, which are fixedly connected to the support plate, and 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, and the control signal lines of the pulse coil, the precession field coil, and the atomic magnetometer all pass through the signal line through-hole into the magnetic shielding tube, and are then respectively connected to the pulse coil, the precession field coil, and the 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, and 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, the atomic magnetometer is initialized, and then a current that generates a μT-level magnetic field is passed into the precession field coil, and 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 are all the same;

[0013] Step 2, testing the atomic magnetometer until the atomic magnetometer works normally;

[0014] 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 when measuring the FID signal;

[0015] Step 4: adjusting the flow rate of the fluid by means of an adjustable speed centrifugal pump, and then measuring the initial measurement value D(v) and the linear attenuation slope f of the FID signal of the fluid respectively, which specifically includes the following processes:

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

[0017] Step 5, repeating 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 multiple fluids with different flow rates;

[0018] Step 6, obtaining the relationship between the value obtained by multiplying the linear attenuation slope f of the FID signal 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, measuring the FID signal of the fluid at a fixed flow rate at a fixed pulse intensity, wherein the pulse time corresponding to a group of signals with the largest initial measurement value of the FID signal is the 90-degree DC pulse at the fixed pulse intensity;

[0022] Step 3.2, then change the pulse intensity, repeat step 3.1, and 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 under different pulse intensities, wherein 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 described 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 rate 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 area to be tested, 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 ultrasonic and turbine measurement methods, the device and method have higher sensitivity and can achieve measurement in the low-frequency range where the efficiency of the radio frequency coil 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 pipeline, effectively overcoming the limitations of traditional nuclear magnetic resonance measurement on pipeline 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 environmental 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. It is also not affected by factors such as the viscosity of the fluid, and the flow rate measurement has a wider application range.

[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 the flow rate detection of hyperpolarized gases. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic diagram of the structure of the device of the present invention after the other components except 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 a 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 is a measurement result diagram of Example 2 of the present invention;

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

[0040] Reference numerals and corresponding component names:

[0041] 1-polarized magnet; 2-magnetic shielding tube; 3-magnet support; 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 in conjunction with 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] Embodiment 1:

[0044] A flow rate measuring device based on ultra-low field nuclear magnetic resonance includes a magnetic shielding device, a polarized 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 which are sequentially sleeved and fixed, all of which are placed horizontally, all of which share a common central axis, and the centers of the two end faces of each magnetic shielding tube 2 are provided with through holes, 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, and 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 the two 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, and both ends of the precession field coil 5 pass through the magnetic shield, so that the external magnetic flux lines generated by the precession field coil 5 can 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. A current is applied to the precession field coil 5 to generate an ultra-low static magnetic field with a magnetic field strength of 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, so that the polarization direction of the flowing fluid from the polarization position to the measurement position can be kept consistent, and it is always in an ultra-low static magnetic field of 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.

[0049] The pulse coil 6 includes two groups of coils, which are respectively arranged on both sides of the fluid pipeline, and 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 respectively connected to the pulse coil 6, the precession field coil 5, and the atomic magnetometer 7. The control signal line of the pulse coil 6 is also connected to the pulse circuit.

[0051] The polarization 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] Embodiment 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, it also includes an adjustable speed centrifugal pump 10 and a liquid storage container 9. 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 measuring 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 magnetic resonance signals, T2 is the transverse relaxation time, M is the magnetization value of the fluid per unit volume, 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, assuming 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 when the fluid flows, and after a 90-degree DC pulse is applied to the test area, the magnetization value of the fluid will also change with 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 and the time t and the flow rate v when the fluid flows through the test area is:

[0060]

[0061] Wherein, 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 area to be tested, and M(v) is the initial magnetization value of a unit volume of fluid with a flow rate of v when entering the area to be tested (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 in the process of flowing from the position after the polarization of the polarizing magnet 1 to the area to be tested. Assume that the length of the polarizing magnet 1 is L p , the distance from the polarized magnet 1 to 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 area to be tested is based on the following formula:

[0063]

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

[0065] Substituting formula (2) and formula (3) into formula (1), we can obtain that 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 is:

[0066]

[0067] It can be seen from formula (4) 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 rate 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 velocity is proportional to the linear attenuation slope f divided by the initial measurement value D(v) of the FID signal. Therefore, in actual measurement, we control the measurement time t to be within 1 / 10 of the measured fluid T2, which is much smaller than the transverse relaxation time T2 of the fluid in the measurement environment. Then, by fitting the relationship between the linear attenuation slope f of the FID signal multiplied by 1 / D(v) and the actual flow velocity, we can infer the flow velocity of the fluid based on the linear attenuation slope f of the FID signal of the measured fluid 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, measuring the FID signal of the fluid at a fixed flow rate at a fixed pulse intensity, wherein the pulse time corresponding to a group of signals with the largest initial measurement value of the FID signal is the 90-degree DC pulse at the fixed pulse intensity;

[0072] Step 3.2, then change the pulse intensity, repeat step 3.1, and 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 under different pulse intensities, wherein 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: adjusting the flow rate of the fluid by the adjustable speed centrifugal pump 10, and then measuring the initial measurement value D(v) and the linear attenuation slope f of the FID signal of the fluid respectively, which specifically includes the following processes:

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

[0076] Step 5, repeating 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 multiple fluids with 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 rate 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 area to be tested, and f is the linear attenuation slope.

[0080] Step 7: During actual measurement, the flow rate 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 an unknown flow rate.

[0081] As an implementable method, the coefficient of the wound precession field coil 5 is 4600nT / mA, and a current of 0.2mA is passed into the precession field coil 5. A magnetic field with a magnetic field strength B0 of 920nT is generated inside the precession field coil 5, and the proton gyromagnetic ratio γ=42.58MHz / T. The precession frequency ω=39.16Hz can be calculated by the formula ω=γB0. When the measurement time is t1=0.2s, 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 known that the pulse angle can reach 90 degrees. Steps 1 to 5 are performed 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, the linear attenuation slope is 7.4595, and the value of the fitting D(v) is 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, and the measurement deviation from the ultrasonic flow meter is 1%.

[0084] It should be noted that the embodiments described in the present invention are merely examples of the spirit of the present invention. Those skilled in the art may make various modifications or additions to the described embodiments or replace them in similar ways, but they will not deviate from the spirit of the present invention or exceed the scope defined by the attached claims.

Claims

1. A flow velocity measurement device based on ultra-low field nuclear magnetic resonance, 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) which are sleeved and fixed in sequence. All the magnetic shielding cylinders (2) are placed horizontally. All the magnetic shielding cylinders (2) share a common central axis. A through hole is provided at the center of 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 cylinder (2) and extend out of the magnetic shielding cylinder (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 polarized magnet (1) is arranged outside the magnetic shielding cylinder (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 close to one end of the fluid pipeline.

2. The flow velocity measurement device based on ultra-low field nuclear magnetic resonance according to claim 1, characterized in that: 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.

3. The flow velocity measurement device based on ultra-low field nuclear magnetic resonance according to claim 2, characterized in that: The pulse coil (6) comprises two groups of coils, the two groups of coils are respectively arranged on both sides of the fluid pipeline, the two groups of coils are respectively arranged on corresponding coil brackets, the coil brackets are fixedly connected to the support plate (8), and the magnetic field centers of the two groups of coils are located in the fluid pipeline.

4. The flow velocity measurement device based on ultra-low field nuclear magnetic resonance according to claim 3, characterized in that: The polarized magnet (1) comprises two pairs of NS magnets, the two pairs of NS magnets 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 pulse coil (6), the precession field coil (5), and the control signal line of the atomic magnetometer (7) all pass through the signal line through-hole into the magnetic shielding tube (2), and then are respectively connected to the pulse coil (6), the precession field coil (5), and the atomic magnetometer (7), and the control signal line of the pulse coil (6) is also connected to the pulse circuit.

5. The flow velocity measurement device based on ultra-low field nuclear magnetic resonance according to claim 4, characterized in that: It 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 a 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 inlet pipe of the adjustable speed centrifugal pump (10) is connected to the outlet of the liquid storage container (9).

6. A flow velocity measurement method based on ultra-low field nuclear magnetic resonance, using any flow velocity measurement device based on ultra-low field nuclear magnetic resonance as claimed in claim 4 or claim 5, characterized in that: The following steps are involved: Step 1, the atomic magnetometer (7) is initialized, and then a current that generates a μT-level magnetic field is passed into the precession field coil (5), and 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 are 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 a 90-degree DC pulse signal to the pulse coil (6) when measuring the FID signal; Step 4: adjusting the flow rate of the fluid by means of an adjustable speed centrifugal pump (10), and then measuring the initial measurement value D(v) and the linear attenuation slope f of the FID signal of the fluid respectively, which specifically includes the following process: Multiple sets of data are continuously collected within a set time during the measurement process, and each set of data is fitted once to obtain a linear decay slope f, and each measurement time t is controlled to be within 1 / 10 of the transverse relaxation time T2 of the fluid, to obtain multiple initial measurement values ​​D(v) and multiple linear decay slopes f of the FID signal, and then the initial measurement values ​​D(v) and the linear decay slopes f are respectively averaged; Step 5, repeating 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 multiple fluids with different flow rates; Step 6, obtaining the relationship between the value obtained by multiplying the linear attenuation slope f of the FID signal by 1 / D(v) and the actual flow rate by fitting; 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.

7. The flow velocity measurement method based on ultra-low field nuclear magnetic resonance according to claim 6, 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, measuring the FID signal of the fluid at a fixed flow rate at a fixed pulse intensity, wherein the pulse time corresponding to a group of signals with the largest initial measurement value of the FID signal is the 90-degree DC pulse at the fixed pulse intensity; Step 3.2, then change the pulse intensity, repeat step 3.1, and 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 under different pulse intensities, wherein 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.

8. The flow velocity measurement method based on ultra-low field nuclear magnetic resonance according to claim 7, characterized in that: In step 6, fitting is performed based on the following formula: Where D(v) is the initial measurement value of the FID signal of the fluid with a flow rate 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 area to be tested, and f is the linear attenuation slope.

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