CN113688525A - Power transmission system long air gap pilot channel thermodynamic characteristic space-time evolution simulation method - Google Patents
Power transmission system long air gap pilot channel thermodynamic characteristic space-time evolution simulation method Download PDFInfo
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Abstract
The invention discloses a long air gap pilot channel thermodynamic characteristic space-time evolution simulation method for a power transmission system, which comprises the following steps of: s110, carrying out a long air gap positive polarity operation impulse discharge test to obtain related physical parameters; s120, establishing a relation model between the refractive index of the pilot channel and the temperature; obtaining the lead initial critical temperature and the time-space evolution rule of the temperature field in the lead development stage under different test conditions; s130: the method comprises the steps that the space charge of a circulation region is equivalent to the space charge of the circulation region through annular simulation charge, and simulation charge is configured on an electric field line to establish a space charge calculation model, so that the calculation of the total space charge of the circulation region is realized; and analyzing the relationship between the temperature field of the discharge channel and the charge quantity of the charge injected into the pilot channel generated by the ionization of the flow injection area of the pilot head, thereby establishing a pilot discharge thermal expansion model. The method can improve the defects of the prior art, determine the lead initial critical temperature and the influence factors of the electrodes with different curvature radiuses, and has important significance for researching a discharge mechanism and optimizing an external insulation design.
Description
Technical Field
The invention relates to the technical field of long-gap discharge simulation, in particular to a time-space evolution simulation method for thermodynamic characteristics of a long-gap pilot channel of a power transmission system.
Background
The research on the long air gap discharge mechanism has important significance for optimizing the external insulation design and the lightning protection design of the power transmission system. The pilot discharge is an important link for maintaining the development of the long-gap discharge, is the leading process of the long-gap breakdown, and has important significance for researching the long-gap discharge mechanism by acquiring key parameters of the development process. Since the thermal characteristics are important characteristic quantities of the thermal ionization of the pilot discharge, researchers gradually focus the research of the pilot discharge on the thermal characteristics.
The traditional calibration schlieren technology is used as a common optical temperature measurement method for measuring the pilot temperature field at present, the traditional calibration schlieren technology has the limitation of smaller observation range, and the advantages of large observation range, high measurement accuracy and strong anti-interference on the theoretical level of the background schlieren technology provide wide prospects for field application, but the two factors of high spatial resolution and high temporal resolution need to be considered simultaneously when the pilot discharge channel temperature field is actually measured. The high-speed photography technology generally adopted at present ensures the high time resolution of the discharge image, but simultaneously limits the spatial resolution of the image, and cannot provide the detail information of the discharge channel. Therefore, a practical background schlieren observation platform needs to be designed and built, all system optical path systems such as a discharge gap distance, a high-speed camera focal length, a background particle density and size are optimally configured, a background schlieren imaging rule is explored, and full 'surface' display of the whole temperature field of the pilot channel is realized on the premise of meeting time and space factors.
Due to the non-axial symmetry development characteristics of the pilot channel, the conventional Abel inverse transformation algorithm cannot be applied to pilot discharge refractive index radial distribution calculation, the problem of boundary integral singularity is ignored, and the calculation precision of a temperature field cannot be guaranteed; the temperature rise and the temperature field development of the gas of the pilot discharge channel are complex time-varying processes comprising electricity and thermal fluid dynamics, the processes of instantaneous collision, migration, diffusion and the like of charged particles are neglected by a temperature inversion algorithm which is commonly adopted at present, and the contribution of the charged particles to the refractive index is neglected at the initial stage of the pilot stage, so that large errors exist in simulation and measurement data. Therefore, it is necessary to correct the temperature inversion model according to the movement and density change of the charged particles at different temperature stages of the pilot.
Disclosure of Invention
The invention aims to solve the technical problem of providing a power transmission system long air gap pilot channel thermodynamic characteristic space-time evolution simulation method, which can solve the defects of the prior art, determine the pilot initial critical temperatures and influence factors of electrodes with different curvature radii and has important significance for researching a discharge mechanism and optimizing an external insulation design.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows.
A method for simulating the thermodynamic characteristics of a long air gap pilot channel of a power transmission system through space-time evolution comprises the following steps:
s110, carrying out a long air gap positive polarity operation impulse discharge test, shooting a background schlieren image of a discharge channel at a high speed, synchronously acquiring instantaneous optical power and a space electric field intensity signal, and acquiring parameters of a discharge development path, optical power and a field intensity jump amplitude;
s120, establishing an Abel inverse transformation optimization algorithm suitable for pilot temperature field inversion, and establishing a pilot channel refractive index and temperature relation model based on the contribution degree of particle density to the refractive index under different temperatures and ionization degrees; obtaining a pilot starting critical temperature and a temporal-spatial evolution rule of a temperature field in a pilot development stage under different test conditions by matching with optical power and a spatial field intensity signal;
s130: the method comprises the steps that the space charge of a circulation region is equivalent to the space charge of the circulation region through annular simulation charge, and simulation charge is configured on an electric field line to establish a space charge calculation model, so that the calculation of the total space charge of the circulation region is realized; and analyzing the relationship between the temperature field of the discharge channel and the charge quantity of the charge injected into the pilot channel generated by the ionization of the flow injection area of the pilot head, thereby establishing a pilot discharge thermal expansion model.
Preferably, in step S110, a background schlieren entity measuring system is designed and implemented, where the background schlieren entity measuring system includes a rear-projection LED array light source, a background curtain and a high-speed camera, the background curtain with distributed random dot matrix background particles is used as an observation object of the high-speed camera, when there is flow field disturbance between the camera and the curtain, an imaging position of the background particles on the camera is shifted to a certain extent before the disturbance, and the shift is directly related to refractive index distribution of the flow field; the refractive index of the non-uniform flow field can be reconstructed by back projection by analyzing the offset of each imaging point, and then a relation model of the refractive index and the channel temperature is established, and finally the temperature field of the pilot channel can be quantitatively reconstructed and visually displayed.
Preferably, the size of the background spot in the image is 3 pixels, the distance between the spot and the point is 2-4 pixels, and the background spot adopts a contrast form of a white dot with a black background or a black dot with a white background.
Preferably, in step S120, a model of the relationship between the refractive index and the temperature of the pilot channel is established based on the relationship between the density of each component particle and the refractive index in the pilot channel, the Saha equation and the gaseous state equation, and considering the contribution degree of the density of the particle to the refractive index under different temperatures and ionization degrees; in particular, the method comprises the following steps of,
electrons, neutral gas molecules, atoms and ions in the channel all affect the refractive index, and the total refractive index is equal to the sum of the refractive indexes of the components:
wherein n isiRepresents the refractive index of the ith particle,
when the wavelength of the incident light is constant, the refractive index of the channel and the density of each component are in a linear relationship as shown in the following formula, so in order to obtain the temperature of the discharge channel, the relationship between the density of each component and the temperature needs to be analyzed first,
the relationship between the temperature of the partially ionized plasma channel and the density of the constituents is described by the Saha ionization equation,
wherein N isjAnd Nj+1Respectively represent the density of the j and j +1 th ionized particles, m represents the mass of the particle, Ej+1Is the ionization energy of the (j + 1) th ionization of a single particle, k is the boltzmann constant, h is the planckian constant, Q is the partition function,
the air medium is simplified into a mixed gas containing only nitrogen and oxygen, the number of electronic charges in the air medium is equal to the number of ionic charges, as shown in the following formula,
the total component density N in the partially ionized plasma approximately satisfies the ideal gas state equation with the pressure p and the temperature T,
and (4) combining the equations to obtain a relation model between the density of each component and the temperature field.
Preferably, in step S120, an image interpolation method is introduced to perform digital image interpolation processing on the measured pilot discharge background schlieren image, so as to reduce the calculation error.
Preferably, in step S130,
by configuring ring charges on the ball electrode, calculating the gap electric field distribution by using an equivalent ball electrode of an analog charge method, setting check points, adjusting the positions of matching points and analog charges and the number of the analog charges, and determining the boundary of the streamer region according to the minimum maintenance electric field required by the stable development of the positive streamer, the shape of the streamer region can be determined by the distribution of the background electric field, and N is configured in the ball electrodeRSimulating a ring charge, applying a voltage of U to the ball electrode0Taking M rays with the end of the ball electrode as the top point, namely electric field lines, on the axial section of the flow, wherein the length of each ray is the distance from the boundary point of the region to the end of the ball electrode, and N rays are respectively arranged on each rayiA horizontally arranged ring-shaped analog charge, and a ball electrode surfaceAt any point k (x)k,yk) The requirements are met,
and any point t (x) on the mth ray in the streamer regiont,yt) The requirements are met,
n can be taken from the surface of the ball electrodeRA potential matching point, taking N in the flow regiontotalWriting an analog charge matrix equation in a column mode at each potential matching point and solving to obtain the total quantity Q of space charges in the streamer regionsc，
The flow stream-guide system is formed by the guide and the head flow stream area, when electrons generated in the flow stream ionization process are injected into the guide channel through the guide head flow stream stem, the electrons collide with gas molecules in the channel to heat the flow stream stem, the temperature of the discharge channel is continuously increased along with the migration of a large amount of electrons, so that the gas pressure of high-temperature gas in the center of the guide channel is continuously and outwards migrated due to the temperature increase, the temperature of the surrounding gas is gradually increased due to the influence of thermal convection, the boundary of the guide channel is increased, the time constant of conversion from vibration energy to translational energy is small in the guide development process, the vibration energy can be considered to be fully converted into the translational energy, and based on energy conservation and neglect radiation loss and conduction loss, the thermal diameter expression of the guide channel in the development process can be obtained as
Wherein q (t) is the amount of injected charge as a function of time; to convert the electric field, E/NkThe converted electric field in the pilot channel is constant 0.55 × 10-15V/cm2；Nk0The relation between the temperature field of the discharge channel and the electron quantity flowing through the pilot channel is explored based on a time-varying evolution rule of the temperature field in the pilot development stage obtained by a test by combining the time-varying relation between the thermal diameter of the pilot channel and the injected charges obtained by deduction for the initial value of the number of gas molecules;
when the electrons generated by ionization in the flow region at the head part of the guide head flow into the guide channel through the flow stem at the head part of the guide head, the electrons collide with gas molecules in the channel to generate heat, so that the temperature of the flow stem rises to form a new guide, the positive-polarity guide is continuously developed forwards, and the charge density q of the guide line is setLIn order to realize the purpose,
wherein, the delta Q is the charge quantity injected into the front flow injection region in the pilot development process; Δ L is the lead development length; therefore, the lead development length in unit time is the lead development speed vL(ii) a The amount of the injected electric charge in the front flow injection region of the pilot in unit time is the current I of the pilot channelLThen the lead development speed is vL，
In view of the relation between the development speed of the pilot and the injected charge amount of the pilot front streamer area, a pilot discharge thermal expansion model can be established based on the relation between the thermal diameter of the pilot channel and the injected charge.
Adopt the beneficial effect that above-mentioned technical scheme brought to lie in:
1. the built background schlieren measurement system realizes large-size measurement of the temperature characteristic of the gap flow field, and improves the schlieren measurement range of the temperature characteristic of the existing discharge channel from centimeter level to meter level;
2. the components in the pilot channel temperature field analysis model participate in more types, and the calculation result is more accurate;
3. and (3) defining characteristic parameters such as pilot initial critical temperature and the like under different electrode curvature radiuses, and establishing a pilot channel thermodynamic model.
Drawings
FIG. 1 is a general flow diagram of the present invention;
FIG. 2 is a schematic diagram of a background schlieren system according to the present invention
FIG. 3 is a schematic diagram of an analog charge configuration according to the present invention.
In the figure: 1. a high brightness spot light; 2. a background curtain; 3. a flow field; 4. a lens; 5. a high-speed camera; 6. a background particle; epsilon, deflection angle; zDThe distance from the curtain to the flow field; zBThe distance from the curtain to the lens; f. the focal length of the lens; 7. a rod electrode; 8. an annular analog charge distribution point; 9. and (4) potential matching points.
Detailed Description
The method for simulating the thermodynamic characteristics of the long air gap pilot channel of the power transmission system in the embodiment comprises the following steps:
s110, carrying out a bar-plate and ball-plate long air gap positive polarity operation impact discharge test, shooting a background schlieren image of a discharge channel at a high speed, synchronously acquiring instantaneous optical power and a space electric field intensity signal, and accurately acquiring key physical parameters such as a discharge development path, optical power, a field intensity jump amplitude and the like.
When a certain gas medium has a density gradient field, the refractive index of the gas at the position with higher density is larger, and when a beam of light enters the medium, the light is deflected towards the direction of increasing the refractive index. The background schlieren technology is an optical measurement system constructed on the basis of the corresponding relation between gas refractive index and density gradient fields, mainly comprises 3 parts of a rear projection type LED array light source, a background curtain and a high-speed camera, and a schematic structural diagram of a background schlieren observation system to be built is shown in figure 2.
The background schlieren system takes a background curtain distributed with random dot matrix background particles as an observation object of a high-speed camera, when a flow field disturbance exists between the camera and the curtain, the imaging position of the background particles on the camera deviates a certain amount compared with the imaging position before the disturbance, and the deviation is directly related to the refractive index distribution of the flow field. The refractive index of the non-uniform flow field can be reconstructed by back projection by analyzing the offset of each imaging point, and then a relation model of the refractive index and the channel temperature is established, and finally the temperature field of the pilot channel can be quantitatively reconstructed and visually displayed.
In background schlieren measurement, the selection and fabrication of background patterns greatly affect the measurement result. Therefore, it must be optimally designed. According to the principle of solving the spot displacement by the cross-correlation algorithm, the background spot is preferably about 3 pixels in size in the image, and the distance between the spot and the spot is preferably 2-4 pixels. The shape of the background spot has little influence on the density field, and a contrast form of a white dot with a black background or a black dot with a white background is generally adopted to improve the image contrast and ensure the high quality of image processing. In the experiment, the size and density of the printed spots were chosen appropriately, taking into account the size of the discharge channel.
In addition, the larger the distance between the center of the flow field and the background curtain is, the larger the spot displacement is, the higher the system sensitivity is, and the detection of the density gradient of the flow field is easier to realize. However, with the great improvement of the system sensitivity, due to the pneumatic optical effect, the background spots can be imaged on the photosensitive surface of the camera in a fuzzy manner, so that the system resolution is reduced, and the calculation accuracy of the spot displacement detected by the cross-correlation algorithm is reduced; on the other hand, an increase in the amount of displacement requires a corresponding increase in the query sub-window set by the cross-correlation algorithm, which results in a decrease in spatial resolution. Therefore, in practice, the equipment distance needs to be reasonably selected according to the discharge condition and the measurement environment, and the organic unification of sensitivity and high resolution is realized.
And (5) building a background schlieren observation entity system and carrying out a positive polarity operation impact discharge test. During actual test, the test is carried out under a closed windless condition, so that the interference caused by natural flow of air is reduced, and the accuracy of an analysis result is ensured. When a high-speed camera for background schlieren observation is configured with a fixed-focus lens and is aligned with a background spot plate to perform PIV analysis, the misjudgment level is controlled as much as possible under the condition of striving for high resolution, and the size of a diagnosis window needs to be comprehensively judged and selected according to factors such as picture quality, test background and the like.
And S120, establishing an Abel inverse transformation optimization algorithm suitable for pilot temperature field inversion, considering the contribution degree of particle density to the refractive index under different temperatures and ionization degrees, and establishing a pilot channel refractive index and temperature relation model. And obtaining the pilot initial critical temperature and the time-space evolution rule of the temperature field in the pilot development stage under different test conditions by matching with the optical power and the space field intensity signal.
The defects of the existing Abel inverse transformation algorithm in numerical calculation are considered, numerical calculation errors are reduced by adopting a method of constructing spline functions among sampling points, and the problem of integral singularities is solved through variable substitution. In the process of pilot development, the Abel inverse transformation is only suitable for the condition of axial symmetry when a rod pole is initially developed, and a channel is asymmetrically developed in a three-dimensional space at any inclination angle, so that an Abel inverse transformation algorithm is improved, and a filtering back projection reconstruction algorithm with higher universality is introduced.
And considering the contribution of each particle in the air medium to the refractive index during discharge, and selecting the dominant particle for calculation according to the density change condition of the particles at different stages.
Considering the main chemical reaction type possibly existing in the air medium and the temperature of partial ionized plasma generated by discharging in the process of flow-guide conversion is lower than 10 DEG5K, can be considered to be in a local thermodynamic equilibrium state. Electrons, neutral gas molecules, atoms and ions in the channel all affect the refractive index, and the total refractive index is equal to the sum of the refractive indexes of the components:
wherein n isiRepresenting the refractive index of the ith particle.
When the wavelength of the incident light is constant, the channel refractive index and the density of each component are in a linear relationship shown in formula (2), and therefore, in order to obtain the temperature of the discharge channel, the relationship between the density of each component and the temperature needs to be analyzed.
(a) Saha ionization equation
The relationship between the temperature of the partially ionized plasma channel and the density of the components can be described by the saha ionization equation (3).
Wherein N isjAnd Nj+1Respectively represent the density of the j and j +1 th ionized particles, m represents the mass of the particle, Ej+1Is the ionization energy of the (j + 1) th ionization of a single particle, k is the boltzmann constant, h is the planckian constant, and Q is the partition function.
(b) Quasi-neutral equation of state
The partially ionized plasma is macroscopically equal in total to the total of the negative charges of all the particles, and this state is called quasi-neutral. Since the air medium is generally simplified to a mixed gas containing only nitrogen and oxygen, the number of electronic charges in the air medium is equal to the number of ionic charges, as shown in equation (4):
(c) equation of state of gas
The total component density N in the partially ionized plasma and the pressure p and temperature T approximately satisfy the ideal gas state equation (5):
and (3) obtaining a relation model between the density of each component and the temperature field by combining equations (2) to (5). The pilot discharge thermodynamic model can analyze the time-varying rules of particle components, refractive index, air pressure and gas temperature, and combines electrical and optical signals to determine the corresponding relation between the injected charge of the long-gap flow and the critical temperature of flow-pilot conversion, thereby further disclosing the physical mechanism of pilot discharge.
And comparing and analyzing the images of the pilot initiation and the development background striae shadow under different gap distances and different electrode curvatures, analyzing the temperature rise speed of each point in the stem of the streamer and the change rules of the temperature rise speed of each point in a channel and the radius of the channel in the pilot initiation stage, and researching the influence rules of the gap distances and the electrode curvatures on the thermal characteristics of the pilot initiation. The relation between the thermal diameter of the pilot channel along with time and injected charges is researched by matching with a photoelectric observation system, and a foundation is laid for establishing a pilot mechanical model.
S130: the method comprises the steps that the space charge of a circulation region is equivalent to the space charge of the circulation region through annular simulation charge, and simulation charge is configured on an electric field line to establish a space charge calculation model, so that the calculation of the total space charge of the circulation region is realized; and analyzing the relationship between the temperature field of the discharge channel and the charge quantity of the charge injected into the pilot channel generated by the ionization of the flow injection area of the pilot head, thereby establishing a pilot discharge thermal expansion model.
Firstly, calculating the potential of a pilot head based on a spatial-temporal evolution rule of a temperature field in a pilot development stage obtained by a test, determining the shape of a pilot head streamer area, establishing a space charge calculation model, and calculating the amount of space charge in a discharge development process. The method comprises the steps of configuring ring charges on a ball electrode, calculating gap electric field distribution by using an analog charge method equivalent ball electrode, setting a check point, and adjusting positions of a matching point and analog charges and the number of the analog charges to reduce calculation errors. The stream region boundary is determined according to the minimum sustaining electric field required for the steady development of the positive polarity stream, and thus the stream region shape can be determined by the background electric field distribution. As shown in FIG. 3, assume that N is arranged in the ball electrodeRSimulating a ring charge, applying a voltage of U to the ball electrode0Taking M rays with the end of the ball electrode as the top point, namely electric field lines, on the axial section of the flow, wherein the length of each ray is the distance from the boundary point of the region to the end of the ball electrode, and N rays are respectively arranged on each rayiA horizontally arranged ring-shaped analog charge, and any point k (x) on the surface of the ball electrodek,yk) Satisfies the following conditions:
and any point t (x) on the mth ray in the streamer regiont,yt) Satisfies the following conditions:
n can be taken from the surface of the ball electrodeRA potential matching point, taking N in the flow regiontotalWriting an analog charge matrix equation in a column mode at each potential matching point and solving to obtain the total quantity Q of space charges in the streamer regionsc：
The guide and the head streamer region form a streamer-guide system, and electrons generated in the streamer ionization process collide with gas molecules in the channel when being injected into the guide channel through the guide head streamer stem to heat the streamer stem. Along with the migration of a large amount of electrons, the temperature of the discharge channel is continuously increased, so that the gas pressure of high-temperature gas in the center of the pilot channel is continuously and outwards migrated due to the temperature increase, the temperature of surrounding gas is gradually increased due to the influence of thermal convection, and the boundary of the pilot channel is increased. In the process of pilot development, because the time constant for converting vibration energy into translational energy is small, the vibration energy can be considered to be fully converted into translational energy, and based on energy conservation and neglecting radiation loss and conduction loss, the thermal diameter expression of a pilot channel in the development process can be obtained as follows:
wherein q (t) is the amount of injected charge as a function of time; to convert the electric field, E/NkThe converted electric field in the pilot channel is approximately constant 0.55 multiplied by 10-15V/cm2；Nk0The initial value of the number of gas molecules. Thus, derived pilot channel thermal diameter and injected charge time variation can be combinedAnd the relation between the temperature field of the discharge channel and the electron quantity flowing through the pilot channel is explored based on the spatial and temporal evolution rule of the temperature field in the pilot development stage obtained by the test.
According to the initial mechanism of the guide, when electrons generated by ionization of the flow region at the head part of the guide flow into the guide passage through the flow stem at the head part of the guide flow collide with gas molecules in the passage to generate heat, so that the temperature of the flow stem rises to form a new guide, the positive guide is continuously developed, and the charge density q of the guide line is assumed to beLComprises the following steps:
wherein, the delta Q is the charge quantity injected into the front flow injection region in the pilot development process; Δ L is the lead development length; therefore, the lead development length in unit time is the lead development speed vL(ii) a The amount of the injected electric charge in the front flow injection region of the pilot in unit time is the current I of the pilot channelLThen the lead development speed is vL：
In view of the relation between the pilot development speed and the injected charge amount of the pilot front streamer area, a pilot channel thermal expansion characteristic model can be established based on the relation between the pilot channel thermal diameter and the injected charge.
In the description of the present invention, it is to be understood that the terms "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on those shown in the drawings, are merely for convenience of description of the present invention, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be construed as limiting the present invention.
The foregoing shows and describes the general principles and broad features of the present invention and advantages thereof. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.
Claims (6)
1. A method for simulating the thermodynamic characteristics of a long air gap pilot channel of a power transmission system through space-time evolution is characterized by comprising the following steps:
s110, carrying out a long air gap positive polarity operation impulse discharge test, shooting a background schlieren image of a discharge channel at a high speed, synchronously acquiring instantaneous optical power and a space electric field intensity signal, and acquiring parameters of a discharge development path, optical power and a field intensity jump amplitude;
s120, establishing an Abel inverse transformation optimization algorithm suitable for pilot temperature field inversion, and establishing a pilot channel refractive index and temperature relation model based on the contribution degree of particle density to the refractive index under different temperatures and ionization degrees; obtaining a pilot starting critical temperature and a temporal-spatial evolution rule of a temperature field in a pilot development stage under different test conditions by matching with optical power and a spatial field intensity signal;
s130: the method comprises the steps that the space charge of a circulation region is equivalent to the space charge of the circulation region through annular simulation charge, and simulation charge is configured on an electric field line to establish a space charge calculation model, so that the calculation of the total space charge of the circulation region is realized; and analyzing the relationship between the temperature field of the discharge channel and the charge quantity of the charge injected into the pilot channel generated by the ionization of the flow injection area of the pilot head, thereby establishing a pilot discharge thermal expansion model.
2. The method for simulating the thermodynamic characteristics of the long air gap pilot channel in the power transmission system according to claim 1, wherein the method comprises the following steps: in step S110, designing a background schlieren entity measuring system, where the background schlieren entity measuring system includes a rear-projection LED array light source, a background curtain and a high-speed camera, and the background curtain with distributed random dot matrix background particles is used as an observation object of the high-speed camera, and when there is flow field disturbance between the camera and the curtain, the imaging position of the background particles on the camera is shifted to a certain extent compared with that before the disturbance, and the shift is directly related to the refractive index distribution of the flow field; the refractive index of the non-uniform flow field can be reconstructed by back projection by analyzing the offset of each imaging point, and then a relation model of the refractive index and the channel temperature is established, and finally the temperature field of the pilot channel can be quantitatively reconstructed and visually displayed.
3. The method for simulating the thermodynamic characteristics of the long air gap pilot channel in the power transmission system according to claim 2, wherein the method comprises the following steps: the size of the background spots in the image is 3 pixels, the distance between each dot and each dot is 2-4 pixels, and the background spots are in a black bottom white dot or white bottom black dot contrast mode.
4. The method for simulating the thermodynamic characteristics of the long air gap pilot channel in the power transmission system according to claim 1, wherein the method comprises the following steps: in the step S120, a model of the relation between the refractive index and the temperature of the pilot channel is established by considering the contribution degree of the particle density to the refractive index under different temperatures and ionization degrees based on the relation between the density of each component particle and the refractive index in the pilot channel, a Saha equation and a gas state equation; in particular, the method comprises the following steps of,
electrons, neutral gas molecules, atoms and ions in the channel all affect the refractive index, and the total refractive index is equal to the sum of the refractive indexes of the components:
wherein n isiRepresents the refractive index of the ith particle,
when the wavelength of the incident light is constant, the refractive index of the channel and the density of each component are in a linear relationship as shown in the following formula, so in order to obtain the temperature of the discharge channel, the relationship between the density of each component and the temperature needs to be analyzed first,
the relationship between the temperature of the partially ionized plasma channel and the density of the constituents is described by the Saha ionization equation,
wherein N isjAnd Nj+1Respectively represent the density of the j and j +1 th ionized particles, m represents the mass of the particle, Ej+1Is the ionization energy of the (j + 1) th ionization of a single particle, k is the boltzmann constant, h is the planckian constant, Q is the partition function,
the air medium is simplified into a mixed gas containing only nitrogen and oxygen, the number of electronic charges in the air medium is equal to the number of ionic charges, as shown in the following formula,
the total component density N in the partially ionized plasma approximately satisfies the ideal gas state equation with the pressure p and the temperature T,
and (4) combining the equations to obtain a relation model between the density of each component and the temperature field.
5. The method for simulating the thermodynamic characteristics of the long air gap pilot channel in the power transmission system according to claim 4, wherein the method comprises the following steps: in step S120, an image interpolation method is introduced to perform digital image interpolation processing on the measured pilot discharge background schlieren image, so as to reduce the calculation error.
6. The method for simulating the thermodynamic characteristics of the long air gap pilot channel in the power transmission system according to claim 1, wherein the method comprises the following steps: in the step S130, the process is performed,
by configuring ring charges on the ball electrode, calculating the gap electric field distribution by using an equivalent ball electrode of an analog charge method, setting check points, adjusting the positions of matching points and analog charges and the number of the analog charges, and determining the boundary of the streamer region according to the minimum maintenance electric field required by the stable development of the positive streamer, the shape of the streamer region can be determined by the distribution of the background electric field, and N is configured in the ball electrodeRSimulating a ring charge, applying a voltage of U to the ball electrode0Taking M rays with the end of the ball electrode as the top point, namely electric field lines, on the axial section of the flow, wherein the length of each ray is the distance from the boundary point of the region to the end of the ball electrode, and N rays are respectively arranged on each rayiA horizontally arranged ring-shaped analog charge, and any point k (x) on the surface of the ball electrodek,yk) The requirements are met,
and any point t (x) on the mth ray in the streamer regiont,yt) The requirements are met,
n can be taken from the surface of the ball electrodeRA potential matching point, taking N in the flow regiontotalWriting an analog charge matrix equation in a column mode at each potential matching point and solving to obtain the total quantity Q of space charges in the streamer regionsc，
The flow stream-guide system is formed by the guide and the head flow stream area, when electrons generated in the flow stream ionization process are injected into the guide channel through the guide head flow stream stem, the electrons collide with gas molecules in the channel to heat the flow stream stem, the temperature of the discharge channel is continuously increased along with the migration of a large amount of electrons, so that the gas pressure of high-temperature gas in the center of the guide channel is continuously and outwards migrated due to the temperature increase, the temperature of the surrounding gas is gradually increased due to the influence of thermal convection, the boundary of the guide channel is increased, the time constant of conversion from vibration energy to translational energy is small in the guide development process, the vibration energy can be considered to be fully converted into the translational energy, and based on energy conservation and neglect radiation loss and conduction loss, the thermal diameter expression of the guide channel in the development process can be obtained as
Wherein q (t) is the amount of injected charge as a function of time; to convert the electric field, E/NkThe converted electric field in the pilot channel is constant 0.55 × 10-15V/cm2；Nk0The relation between the temperature field of the discharge channel and the electron quantity flowing through the pilot channel is explored based on a time-varying evolution rule of the temperature field in the pilot development stage obtained by a test by combining the time-varying relation between the thermal diameter of the pilot channel and the injected charges obtained by deduction for the initial value of the number of gas molecules;
when the electrons generated by ionization in the flow region at the head part of the guide head flow into the guide channel through the flow stem at the head part of the guide head, the electrons collide with gas molecules in the channel to generate heat, so that the temperature of the flow stem rises to form a new guide, the positive-polarity guide is continuously developed forwards, and the charge density q of the guide line is setLIn order to realize the purpose,
wherein, the delta Q is the charge quantity injected into the front flow injection region in the pilot development process; Δ L is the lead development length; therefore, the lead development length in unit time is the lead development speed vL(ii) a The amount of the injected electric charge in the front flow injection region of the pilot in unit time is the current I of the pilot channelLThen lead to the development ofVelocity vL，
In view of the relation between the development speed of the pilot and the injected charge amount of the pilot front streamer area, a pilot discharge thermal expansion model can be established based on the relation between the thermal diameter of the pilot channel and the injected charge.
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