A method for measuring air velocity field in narrow flow channel using oblique air-coupled ultrasonic transducer

Abstract

The present invention discloses a method for measuring the airflow velocity field of a narrow flow channel by using an oblique air-coupled ultrasonic transducer, which has the following characteristics and includes the following steps: Step 1, first fix the piezoelectric ceramic in the transducer housing at an angle of α, and then bond a glass bead matching layer with a thickness of 1 / 4 wavelength to the front end of the piezoelectric ceramic to form an air-coupled ultrasonic transducer array. Step 2, the air-coupled ultrasonic transducer array obtains the downstream and upstream ultrasonic echo signals of different transceiver channels. Step 3, construct a transit time matrix based on the ultrasonic echo signal, and calculate the airflow velocity of different transceiver paths of the air-coupled ultrasonic transducer array based on the principle of time difference method. Step 4, solve the coordinates and velocity vector of the intersection of the two-dimensional plane sound path in the measurement area, and use the reconstruction algorithm based on the airflow velocity to obtain the airflow velocity field in the measurement area, and finally realize the accurate measurement of the airflow velocity field in the narrow flow channel.

Description

Technical Field

[0001] The invention relates to the technical field of flow field measurement, and in particular to a method for measuring an air flow velocity field in a narrow flow channel by using an oblique air-coupled ultrasonic transducer. Background Art

[0002] The measurement of air velocity fields in various narrow flow channels has received extensive attention in many fields such as power machinery and fluid machinery. The complex flow in the blade channel is critical to the efficiency, reliability and service life of the gas turbine. Only after fully understanding the flow laws can we actively control the relevant flows and achieve efficient, safe and reliable operation of the corresponding units. Therefore, a safe, reliable and non-interference measurement method and technology are needed to meet the measurement needs of the gas flow field in narrow flow channels.

[0003] Ultrasonic flow field measurement is a non-contact measurement method with the characteristics of no interference to the flow field, low cost, safety and wide adaptability to fluid characteristics. At the same time, ultrasonic waves also have the characteristics of high measurement accuracy, good directionality, and strong propagation ability in the medium. It can be combined with the time difference method to realize non-contact visual measurement of two-dimensional flow fields. However, the ultrasonic waves emitted by conventional ultrasonic transducers are greatly attenuated in the air; at the same time, for the measurement of velocity fields in narrow flow channels, when the transmitting and receiving transducers are far apart, it is difficult to obtain the echo signal after reflection due to the small distance from the front end of the transducer to the reflecting surface and the limited diffusion angle of the ultrasonic transducer. Based on this, the present invention proposes a method for measuring the airflow velocity field in a narrow flow channel using an oblique air-coupled ultrasonic transducer. Summary of the invention

[0004] The present invention is made to solve the above problems, and aims to provide a method for measuring the air flow velocity field in a narrow flow channel by using an oblique air-coupled ultrasonic transducer.

[0005] The present invention provides a method for measuring the airflow velocity field of a narrow flow channel by using an oblique air-coupled ultrasonic transducer, which has the following characteristics and includes the following steps: Step 1, first fix the piezoelectric ceramic in the transducer housing at an angle of α, and then bond a glass bead matching layer with a thickness of 1 / 4 wavelength to the front end of the piezoelectric ceramic to form an air-coupled ultrasonic transducer array. Step 2, the air-coupled ultrasonic transducer array obtains the downstream and upstream ultrasonic echo signals of different transceiver channels. Step 3, construct a transit time matrix based on the ultrasonic echo signal, and calculate the airflow velocity of different transceiver paths of the air-coupled ultrasonic transducer array based on the principle of time difference method. Step 4, solve the coordinates and velocity vector of the intersection of the two-dimensional plane sound path in the measurement area, and use the reconstruction algorithm based on the airflow velocity to obtain the airflow velocity field in the measurement area, and finally achieve accurate measurement of the airflow velocity field in the narrow flow channel.

[0006] The method for measuring the airflow velocity field of a narrow flow channel by using an oblique air-coupled ultrasonic transducer provided by the present invention may also have the following characteristics: wherein, in step 1, the calculation process of the inclination angle α of the oblique air-coupled ultrasonic transducer is:

[0007]

[0008] Wherein, d is the distance between two adjacent ultrasonic transducers, D is the diameter of the ultrasonic transducer, h is the distance between the transducer array and the reflecting surface, n is the number of air-coupled ultrasonic transducers, λ is the wavelength of the sound wave, and λ=c / f, c is the propagation speed of the ultrasonic wave, and f is the operating frequency of the ultrasonic transducer.

[0009] In the method for measuring the airflow velocity field in a narrow flow channel by using an oblique air-coupled ultrasonic transducer provided by the present invention, the method may also have the following features: wherein step 2 specifically includes the following steps:

[0010] Step 2-1, the computer controls the ultrasonic transmitting board to generate a trigger pulse, and the trigger pulse is amplified by the preamplifier to obtain an ultrasonic excitation pulse. Step 2-2, the switching switch controls a certain air-coupled ultrasonic transducer to be excited in the air-coupled ultrasonic transducer array to complete an ultrasonic emission. Step 2-3, excite air-coupled ultrasonic transducer 1 to emit ultrasonic waves, and air-coupled ultrasonic transducers 2 to air-coupled ultrasonic transducers n receive reflected ultrasonic signals in turn. Step 2-4, amplify the ultrasonic signal, send it to the computer through the data acquisition card, and obtain n-1 groups of ultrasonic echo signals emitted by probe 1.

[0011] In the method for measuring the airflow velocity field of a narrow flow channel by using an oblique air-coupled ultrasonic transducer provided by the present invention, the following features may also be provided: wherein, step 3 specifically includes the following steps: step 3-1, in a scanning manner, excite air-coupled ultrasonic transducers 2 to air-coupled ultrasonic transducers n to emit ultrasonic waves in sequence, and the remaining air-coupled ultrasonic transducers receive ultrasonic reflection signals, and after a set of cycles is completed, n×(n-1) groups of ultrasonic signal data are obtained. Step 3-2, n×(n-1) different path transit times are calculated from the ultrasonic signal data to form an n×n acoustic time matrix (the main diagonal elements are 0). Step 3-3, (n×(n-1)) / 2 downstream and upstream acoustic time differences are obtained from the acoustic time matrix. Step 3-4, the downstream and upstream acoustic time differences are calculated according to the ultrasonic time difference method velocity measurement principle. Step 3-5, the airflow velocity of different transceiver paths of air-coupled ultrasonic transducer i-air-coupled ultrasonic transducer j is calculated.

[0012] The method for measuring the air flow velocity field in a narrow flow channel by using an oblique air-coupled ultrasonic transducer provided by the present invention may also have the following characteristics: wherein, in step 3-4, the calculation process of calculating the downstream and upstream acoustic time differences is as follows:

[0013]

[0014] Where i and j are transducer numbers, t ji is the transit time from transducer j to transducer i (reverse flow), t ij Δt is the transit time from probe i transmitting to probe j receiving (downstream). ij is the acoustic time difference between transducer i and j paths (i<j), u ij is the absolute speed of the transducer i and j paths (i<j), l is the propagation path, and c is the propagation speed of ultrasonic waves in air.

[0015] The velocity calculation process of the transducer i-transducer j path is:

[0016]

[0017] In the formula, θ ij is the angle between the ultrasonic propagation direction of transducer i-transducer j and the flow velocity direction.

[0018]

[0019] In the formula, l×cosθ ij =d ij , d ij is the distance between probes i and j.

[0020] In the method for measuring the air flow velocity field of a narrow flow channel by using an oblique air-coupled ultrasonic transducer provided by the present invention, the following features may also be provided: wherein, in step 3-5, a straight line tangent to the outer boundaries of n / 2 transducers is defined as the y-axis, a straight line perpendicular to the y-axis and tangent to the outer boundaries of the peripheral transducers is defined as the x-axis, the intersection of the x-axis and the y-axis is defined as the origin of the coordinate system, and the velocity calculation process of the ultrasonic transducer i-transducer j path in different directions is as follows:

[0021]

[0022] Where, d ij is the distance between transducers i and j, dx ij is the horizontal (x-axis) distance between the locations of transducers i and j, dy ij is the vertical (y-axis) distance between the locations of transducers i and j, ux ij is the x-direction velocity of the transducer i and j paths, uy ij is the y-direction velocity of the transducer i and j paths.

[0023] In the method for measuring the airflow velocity field of a narrow flow channel by using an oblique air-coupled ultrasonic transducer provided by the present invention, the following features may also be provided: wherein step 4 specifically includes the following sub-steps: step 4-1, calculating all the intersection points of the sound paths in the two-dimensional plane of the measurement area by the center coordinates of the n transducers. step 4-2, calculating the x- and y-direction velocities at the intersection points of the sound paths of the transducer array, the calculation process is:

[0024]

[0025] Where sx(i,j,m,n) is the x-direction velocity of the intersection of the paths of transducer i,j and transducer m,n, and sy(i,j,m,n) is the y-direction velocity of the intersection of the paths of transducer i,j and transducer m,n. Step 4-3: Eliminate invalid, boundary, and repeated intersections to obtain the position vector c of the valid intersection. x (k), c y (k) and the velocity vector v x (k), v y (k) Based on the velocity vector at the intersection of the sound paths and the reconstruction algorithm, the velocity field distribution in the measurement area is obtained.

[0026] Functions and Effects of the Invention

[0027] According to the method of measuring the airflow velocity field in a narrow flow channel by using an oblique air-coupled ultrasonic transducer involved in the present invention, the specific steps are as follows: Step 1, first fix the piezoelectric ceramic in the transducer housing at an angle of α, and then bond a glass bead matching layer with a thickness of 1 / 4 wavelength to the front end of the piezoelectric ceramic to form an air-coupled ultrasonic transducer array. Step 2, the air-coupled ultrasonic transducer array obtains the downstream and upstream ultrasonic echo signals of different transceiver channels. Step 3, construct a transit time matrix based on the ultrasonic echo signal, and calculate the airflow velocity of different transceiver paths of the air-coupled ultrasonic transducer array based on the principle of time difference method. Step 4, solve the coordinates and velocity vector of the intersection of the two-dimensional plane sound path in the measurement area, and use the reconstruction algorithm based on the airflow velocity to obtain the airflow velocity field in the measurement area, and finally achieve accurate measurement of the airflow velocity field in the narrow flow channel.

[0028] Therefore, compared with conventional ultrasonic transducers, the method of measuring the airflow velocity field in a narrow channel by using an oblique air-coupled ultrasonic transducer array in the present invention can effectively improve the problems of small amplitude and low signal-to-noise ratio of the ultrasonic array received signal, and realize accurate measurement of the airflow velocity field in a narrow channel. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a schematic diagram of an ultrasonic transducer array in an embodiment of the present invention;

[0030] Figure 2 is a schematic diagram of an oblique air-coupled ultrasonic transducer in an embodiment of the present invention;

[0031] Figure 3 is a schematic diagram of an ultrasonic measurement system for airflow velocity field in a narrow flow channel in an embodiment of the present invention;

[0032] Figure 4 This is a comparison diagram of 8 transmission-1 reception echo signals when the inlet velocity is 20 m / s in an embodiment of the present invention;

[0033] Figure 5 is a schematic diagram of the locations of intersections of sound paths of different paths in the measurement area in an embodiment of the present invention;

[0034] Figure 6 It is a diagram of the reconstruction result of the sound path intersection velocity and velocity field when the inlet velocity is 5 m / s in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the following embodiments and drawings specifically illustrate a method for measuring the airflow velocity field in a narrow flow channel using an oblique air-coupled ultrasonic transducer of the present invention.

[0036] In this embodiment, a method for measuring the air flow velocity field in a narrow flow channel using an oblique air-coupled ultrasonic transducer is provided.

[0037] Figure 1 is a schematic diagram of an ultrasonic transducer array in an embodiment of the present invention.

[0038] Figure 2 Schematic diagram of an oblique air-coupled ultrasonic transducer in an embodiment of the present invention.

[0039] like Figure 1 As shown, 1, 2, 7, and 8 are oblique-mouth air-coupled ultrasonic transducers 100, and 3, 4, 5, and 6 are flat-mouth air-coupled ultrasonic transducers 200.

[0040] like Figure 2 As shown, the oblique air-coupled ultrasonic transducer 100 in this embodiment includes a piezoelectric ceramic 10 , a housing 20 , a matching layer 30 , a core wire 40 and a shielding wire 50 .

[0041] The method for measuring the air flow velocity field in a narrow flow channel by using the oblique air-coupled ultrasonic transducer 100 according to this embodiment comprises the following steps:

[0042] Step S1, first fix the piezoelectric ceramic 10 in the transducer housing 20 at an angle of α, and then bond a glass bead matching layer 30 with a thickness of 1 / 4 wavelength to the front end of the piezoelectric ceramic 10 for impedance matching between the piezoelectric ceramic and air. The oblique ultrasonic transducer has a larger sound beam diffusion angle than conventional transducers, which can effectively improve the echo signal amplitude and signal-to-noise ratio.

[0043] The calculation process of the diffusion angle θ of the air-coupled ultrasonic transducer when propagating in the air is:

[0044] θ=arcsin1.22λ / D≈70λ / D

[0045] Wherein, D is the diameter of the ultrasonic transducer, λ is the wavelength of the sound wave, and λ=c / f, c is the propagation speed of ultrasonic wave in air, and f is the operating frequency of the ultrasonic transducer.

[0046] Figure 3 Schematic diagram of an ultrasonic measurement system for airflow velocity field in a narrow flow channel in an embodiment of the present invention.

[0047] like Figure 3 As shown, in order to ensure that the ultrasonic transceiver-transmitter transducers (transducer 1-transducer 8, transducer 2-transducer 7) that are farthest apart can effectively receive the echo signal reflected by the reflector, the calculation process of the tilt angle α of the oblique air-coupled ultrasonic transducer is:

[0048]

[0049] Wherein, d is the distance between two adjacent ultrasonic transducers, D is the diameter of the ultrasonic transducer, h is the distance between the transducer array and the reflecting surface, n is the number of air-coupled ultrasonic transducers, λ is the wavelength of the sound wave, and λ=c / f, c is the propagation speed of the ultrasonic wave, and f is the operating frequency of the ultrasonic transducer.

[0050] Step S2, such as Figure 3 As shown, the air-coupled ultrasonic transducer array obtains the downstream and upstream ultrasonic echo signals of different transceiver channels, which specifically includes the following steps:

[0051] Step S2-1, the computer controls the ultrasonic transmitting board to generate a trigger pulse, and the trigger pulse is power-amplified by the preamplifier to obtain an ultrasonic excitation pulse;

[0052] Step S2-2, controlling a certain air-coupled ultrasonic transducer to be excited in the air-coupled ultrasonic transducer array by a switching switch to complete an ultrasonic wave emission;

[0053] Step S2-3, exciting the air-coupled ultrasonic transducer 1 to emit ultrasonic waves, and the air-coupled ultrasonic transducers 2 to n sequentially receive reflected ultrasonic signals;

[0054] Step S2-4, amplifying the ultrasonic signal, sending it to the computer via the data acquisition card, and obtaining n-1 groups of ultrasonic echo signals emitted by the probe 1.

[0055] Step S3, constructing a transit time matrix based on the ultrasonic echo signal, and calculating the airflow velocity of different transceiver paths of the air-coupled ultrasonic transducer array based on the time difference method principle, specifically includes the following steps:

[0056] Step S3-1, in a scanning manner, stimulate air-coupled ultrasonic transducers 2 to n to emit ultrasonic waves in sequence, and the remaining air-coupled ultrasonic transducers receive ultrasonic reflection signals. After one set of cycles is completed, n×(n-1) sets of ultrasonic signal data are obtained.

[0057] Figure 4 It is a comparison diagram of 8 transmission-1 reception echo signals when the inlet velocity is 20 m / s in an embodiment of the present invention.

[0058] like Figure 4 As shown in the figure, when the entrance velocity of the narrow flow channel is 20m / s, the echo signal amplitude obtained by using the oblique air-coupled ultrasonic transducer is significantly greater than that of the conventional transducer, and the signal-to-noise ratio is higher. Therefore, the use of the oblique air-coupled ultrasonic transducer can effectively solve the problem of weak transducer receiving signal and low signal-to-noise ratio.

[0059] Step S3-2, calculate n×(n-1) different path transit times from the ultrasonic signal data to form an n×n acoustic time matrix (the main diagonal elements are 0).

[0060] Step S3-3, obtain (n×(n-1)) / 2 downstream and upstream acoustic time differences from the acoustic time matrix.

[0061] Step S3-4, calculating the downstream and upstream acoustic time differences according to the ultrasonic time difference method velocity measurement principle.

[0062] The calculation process of downstream and upstream acoustic time difference is as follows:

[0063]

[0064] Where i and j are transducer numbers, t ji is the transit time from transducer j to transducer i (reverse flow), t ij Δt is the transit time from probe i transmitting to probe j receiving (downstream); ij is the acoustic time difference between transducer i and j paths (i<j), u ij is the absolute speed of the transducer i and j paths (i<j), l is the propagation path, and c is the propagation speed of ultrasonic waves in air.

[0065] The velocity calculation process of the transducer i-transducer j path is:

[0066]

[0067] In the formula, θ ij is the angle between the ultrasonic propagation direction of transducer i-transducer j and the flow velocity direction.

[0068]

[0069] In the formula, l×cosθij =d ij , d ij is the distance between probes i and j.

[0070] Step S3-5, calculating the airflow velocities of different transmitting and receiving paths of air-coupled ultrasonic transducer i-air-coupled ultrasonic transducer j.

[0071] Figure 5 It is a schematic diagram of the locations of intersections of sound paths of different paths in the measurement area in an embodiment of the present invention.

[0072] like Figure 5 As shown, the straight line tangent to the outer boundary of n / 2 transducers is defined as the y-axis, the straight line perpendicular to the y-axis and tangent to the outer boundary of the peripheral transducer is defined as the x-axis, the intersection of the x-axis and the y-axis is defined as the origin of the coordinate system, and the speed calculation process of the ultrasonic transducer i-transducer j path in different directions is as follows:

[0073]

[0074] Where, d ij is the distance between transducers i and j, dx ij is the horizontal (x-axis) distance between the locations of transducers i and j, dy ij is the vertical (y-axis) distance between the locations of transducers i and j, ux ij is the x-direction velocity of the transducer i and j paths, uy ij is the y-direction velocity of the transducer i and j paths.

[0075] Step S4, solving the coordinates and velocity vector of the intersection of the two-dimensional plane sound path in the measurement area, and obtaining the airflow velocity field in the measurement area using a reconstruction algorithm based on the airflow velocity, thereby finally achieving accurate measurement of the airflow velocity field in the narrow flow channel.

[0076] Wherein, step S4 specifically includes the following steps:

[0077] Step S4-1, calculating all the intersection points of the sound paths in the two-dimensional plane of the measurement area based on the center coordinates of the n transducers.

[0078] Step S4-2, calculate the x- and y-direction velocities at the intersection of the transducer array sound path, the calculation process is:

[0079]

[0080] Where sx(i,j,m,n) is the velocity in the x-direction of the intersection of the acoustic paths of transducers i,j and transducers m,n, and sy(i,j,m,n) is the velocity in the y-direction of the intersection of the acoustic paths of transducers i,j and transducers m,n.

[0081] Step S4-3, such as Figure 5As shown, invalid, boundary and repeated sound path intersections are eliminated to obtain the position vector c of the valid intersection point. x (k), c y (k) and the velocity vector v x (k), v y (k). Taking the 2×4 air-coupled ultrasonic transducer array as an example, a total of 27 effective sound path intersection points are obtained, that is, k = 1, 2, 3...27.

[0082] Figure 6 It is a diagram of the reconstruction result of the sound path intersection velocity and velocity field when the inlet velocity is 5 m / s in an embodiment of the present invention.

[0083] like Figure 6 As shown, according to the velocity vector at the intersection of the two-dimensional plane sound path and the reconstruction algorithm, the velocity field distribution of the measurement area is obtained, and the velocity field results at the intersection of the sound path are shown in Table 1.

[0084] Table 1 shows the velocity field results at the intersection of the sound paths when the inlet velocity is 5 m / s in an embodiment of the present invention.

[0085] Table 1

[0086]

[0087] Functions and Effects of the Embodiments

[0088] According to the method of measuring the airflow velocity field in a narrow flow channel by using an oblique air-coupled ultrasonic transducer involved in this embodiment, the specific steps are as follows: Step 1, first fix the piezoelectric ceramic in the transducer housing at an angle of α, and then bond a glass bead matching layer with a thickness of 1 / 4 wavelength to the front end of the piezoelectric ceramic to form an air-coupled ultrasonic transducer array. Step 2, the air-coupled ultrasonic transducer array obtains the downstream and upstream ultrasonic echo signals of different transceiver channels. Step 3, construct a transit time matrix based on the ultrasonic echo signal, and calculate the airflow velocity of different transceiver paths of the air-coupled ultrasonic transducer array based on the principle of time difference method. Step 4, solve the coordinates and velocity vector of the intersection of the two-dimensional plane sound path in the measurement area, and use the reconstruction algorithm based on the airflow velocity to obtain the airflow velocity field in the measurement area, and finally achieve accurate measurement of the airflow velocity field in the narrow flow channel.

[0089] Therefore, compared with conventional ultrasonic transducers, the method of measuring the airflow velocity field in a narrow channel by using an oblique air-coupled ultrasonic transducer array in this embodiment can effectively improve the problems of small amplitude and low signal-to-noise ratio of the ultrasonic array received signal, and realize accurate measurement of the airflow velocity field in a narrow channel.

[0090] The above-mentioned embodiments are preferred examples of the present invention and are not intended to limit the protection scope of the present invention.

Claims

1. A method for measuring the air velocity field in a narrow flow channel using an oblique air-coupled ultrasonic transducer, characterized in that: The following steps are involved: Step 1, first fix the piezoelectric ceramic at an angle of α in the transducer housing, and then bond a glass bead matching layer with a thickness of 1 / 4 wavelength to the front end of the piezoelectric ceramic to form a 2*4 air-coupled ultrasonic transducer array in a plane. The two transducers at both ends of the air-coupled ultrasonic transducer array are oblique air-coupled ultrasonic transducers, and the four transducers in the middle of the air-coupled ultrasonic transducer array are flat air-coupled ultrasonic transducers. The calculation process of the inclination angle α of the oblique air-coupled ultrasonic transducer is: Where α>0, Wherein, d is the distance between two adjacent ultrasonic transducers, D is the diameter of the ultrasonic transducer, h is the distance between the transducer array and the reflecting surface, n is the number of air-coupled ultrasonic transducers, λ is the wavelength of the sound wave, and λ = c / f, c is the propagation speed of the ultrasonic wave, and f is the operating frequency of the ultrasonic transducer; Step 2, the air-coupled ultrasonic transducer array obtains the downstream and upstream ultrasonic echo signals of different transceiver channels, and a certain air-coupled ultrasonic transducer to be excited in the air-coupled ultrasonic transducer array is controlled by a switching switch to complete an ultrasonic emission; the air-coupled ultrasonic transducer 1 is excited to emit an ultrasonic wave, and the air-coupled ultrasonic transducers 2 to the air-coupled ultrasonic transducers n receive the reflected ultrasonic signals in turn; in a scanning manner, the air-coupled ultrasonic transducers 2 to the air-coupled ultrasonic transducers n are excited to emit an ultrasonic wave in turn, and the remaining air-coupled ultrasonic transducers receive the ultrasonic reflected signals. After a set of cycles is completed, n×(n-1) groups of ultrasonic signal data are obtained; Step 3, constructing a transit time matrix based on the ultrasonic echo signal, and calculating the airflow velocity of different transceiver paths of the air-coupled ultrasonic transducer array based on the time difference method principle; Step 4, solving the coordinates and velocity vector of the intersection of the two-dimensional plane sound path in the measurement area, and obtaining the airflow velocity field in the measurement area using a reconstruction algorithm based on the airflow velocity, thereby finally achieving accurate measurement of the airflow velocity field in the narrow flow channel.

2. The method for measuring the airflow velocity field in a narrow channel using an oblique air-coupled ultrasonic transducer according to claim 1, characterized in that: in, Step 2 specifically includes the following steps: Step 2-1, a computer controls an ultrasonic transmitting board to generate a trigger pulse, and a preamplifier amplifies the power of the trigger pulse to obtain an ultrasonic excitation pulse; Step 2-2, controlling a certain air-coupled ultrasonic transducer to be excited in the air-coupled ultrasonic transducer array by a switching switch to complete an ultrasonic emission; Step 2-3, exciting the air-coupled ultrasonic transducer 1 to emit ultrasonic waves, and the air-coupled ultrasonic transducers 2 to n sequentially receive reflected ultrasonic signals; Step 2-4, amplifying the ultrasonic signal, and sending it to the computer via a data acquisition card to obtain n-1 groups of ultrasonic echo signals emitted by the transducer 1.

3. The method for measuring the airflow velocity field in a narrow channel using an oblique air-coupled ultrasonic transducer according to claim 1, characterized in that: in, Step 3 specifically includes the following steps: Step 3-1, in a scanning manner, exciting air-coupled ultrasonic transducers 2 to n to emit ultrasonic waves in sequence, and the remaining air-coupled ultrasonic transducers receive ultrasonic reflection signals, and after one set of cycles is completed, n×(n-1) sets of ultrasonic signal data are obtained; Step 3-2, calculating n×(n-1) different path transit times from the ultrasonic signal data to form an n×n acoustic time matrix, with the main diagonal elements being 0; Step 3-3, obtaining (n×(n-1)) / 2 downstream and upstream acoustic time differences from the acoustic time matrix; Step 3-4, calculating the downstream and upstream acoustic time differences according to the ultrasonic time difference velocity measurement principle; Step 3-5, calculating the air flow velocity of different transmitting and receiving paths of air-coupled ultrasonic transducer i-air-coupled ultrasonic transducer j.

4. The method for measuring the air flow velocity field in a narrow flow channel by using an oblique air-coupled ultrasonic transducer according to claim 3, Features: Among them, in step 3-4, the calculation process of calculating the downstream and upstream acoustic time differences is: Where i and j are transducer numbers, t ji is the transit time of the reverse flow from transducer j to transducer i, t ij Δt is the transit time from transducer i transmitting to transducer j receiving downstream; ij is the acoustic time difference between the paths of transducers i and j, where i<j, u ij is the absolute velocity of the transducer i and j paths, where i < j, l is the propagation path, and c is the propagation velocity of ultrasonic waves in air; The velocity calculation process of the transducer i-transducer j path is: In the formula, θ ij is the angle between the ultrasonic propagation direction of transducer i-transducer j and the flow velocity direction, In the formula, l×cosθ ij =d ij， d ij is the distance between transducers i and j.

5. The method for measuring the airflow velocity field in a narrow channel using an oblique air-coupled ultrasonic transducer according to claim 3 is characterized in that: in, In step 3-5, the straight line tangent to the outer boundary of n / 2 transducers is defined as the y-axis, the straight line perpendicular to the y-axis and tangent to the outer boundary of the peripheral transducer is defined as the x-axis, the intersection of the x-axis and the y-axis is defined as the origin of the coordinate system, and the velocity calculation process of the ultrasonic transducer i-transducer j path in different directions is as follows: Where, d ij is the distance between transducers i and j, dx ij is the horizontal distance between the locations of transducers i and j, i.e., the x-axis distance, dy ij is the vertical distance between the locations of transducers i and j, i.e. the y-axis distance, ux ij is the x-direction velocity of the transducer i and j paths, uy ij is the y-direction velocity of the transducer i and j paths.

6. The method for measuring the airflow velocity field in a narrow channel using an oblique air-coupled ultrasonic transducer according to claim 1, characterized in that: in, Step 4 specifically includes the following steps: Step 4-1, calculating all sound path intersections of the two-dimensional plane of the measurement area based on the center coordinates of n transducers; Step 4-2, calculate the x- and y-direction velocities at the intersection of the acoustic paths of the transducer array, the calculation process is: Where, sx(i,j,m,n) is the velocity in the x direction of the intersection of the acoustic paths of transducers i,j and transducers m,n, and sy(i,j,m,n) is the velocity in the y direction of the intersection of the acoustic paths of transducers i,j and transducers m,n; Step 4-3, remove invalid, boundary and repeated sound path intersection points to obtain the position vector c of the valid intersection point x (k), c y (k) and the velocity vector v x (k), v y (k) Based on the velocity vector at the intersection of the sound paths and the reconstruction algorithm, the velocity field distribution in the measurement area is obtained.

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