Experimental device and experimental method for explosion release of combustible gas in communicating container

Abstract

The invention discloses an experimental device and an experimental method for combustible gas explosion discharge of a communicating container, and belongs to the field of combustible gas explosion protection. The experimental device comprises a communicated discharge container, a gas distribution system, a high-voltage ignition system, a data acquisition system, a high-speed schlieren system and a synchronous control system. The experimental device and the experimental method can be used for experimental research on combustible gas explosion release of the communicating container; the influence rules of factors such as static action pressure (Pstat), discharge caliber (Dv), discharge coefficient (Kv), spherical explosion container volume ratio (V1 / V2), communication pipeline length (L), communication pipeline diameter (D), fuel gas equivalence ratio (phi), ignition position (IIn) and ignition energy (E) on flammable gas explosion discharge overpressure and discharge flame are analyzed; and the formation factors of internal and external discharge overpressure peak values (a peak value Pb formed by rupture of a discharge diaphragm, a peak value Pex formed by external secondary explosion, a peak value Ph formed by Helmholtz oscillation, a peak value Pac formed by sound wave coupling and a peak value Pred formed by a maximum flame area), a discharge gas cloud structure, a discharge flame form and an external pressure wave evolution rule are explored.

Description

Technical Field

[0001] The invention relates to an experimental device and an experimental method for studying explosion release of combustible gas in a connected container, belongs to the field of combustible gas explosion protection, and particularly relates to an experimental device and an experimental method for studying explosion release of combustible gas in a connected container. Background Art

[0002] As an important product of the petrochemical industry, flammable gas is widely used in many fields. It faces huge safety challenges in production, transportation and storage. Since flammable gas is usually stored in a closed container, once air is mixed in due to poor sealing performance, improper operation, increased ambient temperature and other factors, an explosive mixture may be formed. When the flammable concentration of the mixed gas reaches its explosion limit, if there is an ignition source such as sparks or high temperature, it will easily trigger an explosion. The explosion will not only generate a large amount of heat and strong shock waves in an instant, but also cause the explosion overpressure inside the container to rise sharply. The accumulation of explosion overpressure will cause devastating damage to the closed container and other equipment structures, and may trigger a chain reaction. For chemical equipment and surrounding personnel, this sudden explosion will not only cause equipment damage, but also cause serious casualties and economic losses. Therefore, in-depth research on the explosion characteristics of flammable gas in a closed container and the improvement of its release technology are of great practical significance, providing key technical guarantees for safe production in the chemical industry.

[0003] Explosion relief refers to the rapid release of high-pressure gas inside a closed container through a specific pressure relief structure when an explosion occurs in a closed container, avoiding equipment damage caused by overpressure accumulation. As an economical, direct and effective protective measure, explosion relief can play an important role in reducing internal explosion overpressure and protecting the structural integrity of equipment. Therefore, designing a reasonable explosion relief system in a flammable and explosive environment has become an important means to ensure equipment safety and reduce accident losses. However, the explosion relief process is not a single pressure relief behavior, but a complex physical process involving multiple factors such as turbulent flow, pressure oscillation, and external explosion. The explosion in the container will cause strong pressure fluctuations and turbulent effects, making the rise and fall of pressure during the explosion relief process full of dynamic uncertainty. The study of the explosion relief of combustible gas in connected containers not only helps to clarify the law of pressure parameter changes during the explosion process, but also provides guidance for the design and optimization of explosion relief. By analyzing the performance of the explosion relief effect under different working conditions, the pressure relief design can be further optimized to achieve more effective safety protection.

[0004] During chemical production, transportation and storage, combustible gases often need to be transferred and transported between multiple closed containers. Closed containers are connected by pipes to form a complex system. When one of the containers burns or explodes for some reason, the flame and pressure wave will quickly propagate along the connecting pipe, and then affect the adjacent containers. This explosion propagation phenomenon not only aggravates the scope of the explosion hazard, but also due to the different structural characteristics of the pipeline length, diameter, etc., it is easy to form a high-temperature and high-pressure flame front in the connecting pipe, which may produce more violent explosion reactions in the downstream container, making the explosion more destructive in the explosion-transmitting container. Such explosion shock and pressure wave propagation may cause serious damage to the container structure, and even trigger a chain rupture of the entire system, posing a major safety threat to surrounding facilities and personnel. In addition, this explosion propagation between multiple containers increases the difficulty of explosion venting design, requiring in-depth experimental and theoretical research on the influence of connecting pipes on explosion propagation characteristics. Therefore, conducting research on the explosion venting of combustible gases in connected containers is crucial to improving the safety of the chemical industry and optimizing the explosion venting design. Summary of the invention

[0005] In order to solve the problems existing in the prior art, the present invention aims to provide an experimental device and an experimental method for studying the explosion release of combustible gas in a connected container, through which the static action pressure (P stat ), discharge diameter (D v ), discharge coefficient (K v ), spherical explosion container volume ratio (V1 / V2), connecting pipe length (L), connecting pipe diameter (D), gas equivalence ratio (Φ), ignition position (Ign) and ignition energy (E) and other factors on the overpressure and flame of combustible gas explosion release, and explore the peak value of internal and external overpressure release (the peak value P formed by the rupture of the release diaphragm) b , the peak value P formed by the external secondary explosion ex , the peak value P formed by the Helmholtz oscillation h , the peak value P formed by acoustic wave coupling ac , the peak value P formed by the maximum flame area red ), the formation factors of the released gas cloud, the released flame morphology and the evolution law of the external pressure wave.

[0006] The technical solution adopted by the present invention is as follows:

[0007] An experimental device for explosive release of combustible gas in a connected container, which includes a connected release container, a gas distribution system, a data acquisition system, a high-speed Schlieren system and a high-voltage ignition system;

[0008] The communicating discharge container is connected to the first spherical explosion container and the second spherical explosion container by a communicating pipe; a spherical pressure sensor and a spherical gas distribution inlet are arranged on the side wall surface of the spherical explosion container, a spherical discharge flange is installed on the upper wall surface, and a spherical ignition electrode is arranged on the lower wall surface; fixing flanges for connecting the first spherical explosion container and the second spherical explosion container are arranged at both ends of the communicating pipe, a pipe discharge flange and a pipe pressure sensor are arranged on the top surface of the communicating pipe, a pipe ignition electrode, a pipe gas distribution inlet and a photodiode are arranged on the bottom surface, the photodiode is fixed in a hollow photodiode fixing bolt, and the bottom of the photodiode fixing bolt is filled with quartz glass for auxiliary pressure bearing; the discharge flange clamps the discharge diaphragm and the plastic gasket, and a sealing rubber ring is arranged between the discharge flange and the spherical explosion container and the communicating pipe;

[0009] The connected discharge container is provided with multiple groups of external pressure sensors through an external pressure sensor rectifier plate bracket, and the external pressure sensors are used to monitor the external pressure of the connected discharge container after discharge;

[0010] In the gas distribution system, the gas cylinder and the air cylinder are connected to the gas distribution pipeline through PU gas pipes respectively, and the gas distribution pipeline is connected to the gas distribution inlet of the sphere and the gas distribution inlet of the pipeline respectively; a pressure gauge and a vacuum pump are also arranged on the gas distribution pipeline;

[0011] The data acquisition system includes an external pressure sensor rectifier board bracket, a sensor power supply box and a data acquisition instrument;

[0012] The high-speed Schlieren system includes a high-speed camera, a background Schlieren plate and a computer. The background Schlieren plate is placed behind the measured flow field when the discharge is connected to the discharge container to obtain a high-speed time-series background Schlieren pattern of the discharge gas cloud structure, the external secondary explosion and the external area flow field of the jet flame;

[0013] The high-voltage ignition system comprises a high-voltage ignition package and a timing controller, and the timing controller is electrically connected with the high-speed camera, the high-voltage ignition package and the data acquisition instrument.

[0014] The sensor power supply box is electrically connected to the spherical pressure sensor, the pipeline pressure sensor and the external pressure sensor.

[0015] The gas distribution pipeline is also provided with a pressure gauge and a vacuum pump.

[0016] The timing controller is started by a low-voltage electrical push-button switch, which synchronously triggers the data acquisition instrument and the high-speed camera, and then controls the high-voltage electric ignition package to output high-voltage current to generate an arc at the tip of the spherical ignition electrode and the pipe ignition electrode.

[0017] The spherical relief flange is adjusted to adopt different diameters, and the pipeline relief flange is adjusted to adopt different diameters, and the relief pressure under different diameters is tested.

[0018] At least two photodiodes are symmetrically arranged on the communication pipe.

[0019] The diameter of the pipeline discharge flange is the radius of the connecting pipeline.

[0020] Specifically, the device includes two spherical explosion relief containers, a connecting pipeline, a gas distribution system, a data acquisition system, a high-speed schlieren system and a high-voltage ignition system.

[0021] The explosion relief container includes two spherical explosion containers of different volumes. A high-precision piezoelectric pressure sensor and an air inlet of a gas distribution pipeline are installed on the side wall of the spherical explosion container. The pressure sensor is fixed in a hollow pressure sensor fixing bolt. Discharge flanges of different calibers are installed in the center of the upper wall, and a flange of an ignition electrode is arranged in the center of the lower wall. Discharge diaphragms of different static action pressures are clamped on a circular groove of the discharge flange where a plastic gasket is placed by bolts, and a sealing rubber ring is provided on the top of the spherical explosion container to ensure the air tightness of the container after the discharge flange is fixed to the spherical explosion container. The connection The connecting pipe includes several pipes with different length-to-diameter ratios, and fixed flanges are arranged at both ends to facilitate the series connection of two spherical explosion containers. The extended part of the pipe outside the flange is cut according to the corresponding spherical surface to ensure that the interior of the two spherical explosion containers are in a complete spherical shape after the series installation. High-precision piezoelectric pressure sensors, photodiodes, ignition electrodes and air inlets of the gas distribution pipeline are installed on both sides of the connecting pipe, and a discharge flange is installed at the top center of the connecting pipe. The photodiode is fixed in the hollow photodiode fixing bolt, and a piece of quartz glass is filled at the bottom of the bolt to assist in pressure bearing.

[0022] The gas distribution system includes a gas cylinder, an air cylinder, a pressure reducing valve, a pipeline needle valve, a pressure gauge, a vacuum pump and a PU air pipe. The data acquisition system includes a piezoelectric pressure sensor, an external pressure sensor rectifier board bracket, a sensor power supply box, a photodiode and a data acquisition instrument. The piezoelectric pressure sensor converts the pressure signal into an electrical signal, and the photodiode converts the optical signal into an electrical signal, which is then recorded by the data acquisition instrument according to a certain sampling frequency and time length.

[0023] The high-speed Schlieren system includes a high-speed camera, a background Schlieren plate and a computer. The high-speed camera captures the flow field image of the external discharge area of ​​the explosion discharge. The background Schlieren plate is placed behind the measured flow field to obtain the high-speed time-series background Schlieren pattern of the external area flow field such as the discharge gas cloud structure, external secondary explosion and jet flame, and finally the captured image is transmitted to the computer.

[0024] The high-voltage ignition system includes an ignition electrode, a high-voltage ignition package and a timing controller. The high-voltage ignition system starts the timing controller through a low-voltage electrical push-button switch, synchronously triggers a data acquisition instrument and a high-speed camera, and then controls the high-voltage electric ignition package to output a high-voltage current, thereby generating an arc at the tip of the ignition electrode to ignite the combustible gas.

[0025] The two spherical explosion containers and the connecting pipe have a high degree of overall explosion resistance and pressure bearing. The discharge opening at the top of the spherical explosion container can be installed with discharge flanges of different calibers, and can clamp different discharge calibers (D v ) and static action pressure (P stat ) discharge diaphragm, can achieve different discharge diameters (D v ) and static action pressure (P stat ) on the explosive discharge of combustible gas in a connected container; two photodiodes are symmetrically placed at a certain distance in the center of the connected pipeline to measure the average flame speed of the pipeline; the opening diameter of the discharge flange at the top of the connected pipeline is the pipeline radius, which can realize the experimental study on the influence of different discharge port positions and numbers on the connected container on the explosive discharge of combustible gas in the connected container; after the discharge diaphragms of different thicknesses are installed on the discharge flange, air is slowly introduced into the container, and the internal pressure at the moment of the rupture of the discharge diaphragm is the static action pressure (P stat ); The threads of the flanges on both sides of the connecting pipe can be removed after disassembly, and pipes of different lengths and inner diameters can be replaced to realize the experimental study of the variation law of the explosion overpressure of the combustible gas in the connecting container and the impact of explosion discharge under different aspect ratios (L / D); Each flange connection on the two spherical explosion containers and the connecting pipe is equipped with a sealing rubber ring / gasket, and each thread is wrapped with raw tape before installation to fully ensure the overall airtightness of the device.

[0026] After the two photodiodes distributed on the connecting pipe capture the light signals successively, the average speed of the pipe flame propagation can be calculated according to the "distance / time" to study the influence of different spherical explosion container volume ratios (V1 / V2), gas equivalence ratios (Φ) or connecting pipe length-to-diameter ratios (L / D) on the propagation speed of the combustible gas flame in the connecting container in the connecting pipe.

[0027] After the gas distribution system evacuates the connected container to a low vacuum degree through a vacuum pump, the gas distribution system realizes the configuration of the specified gas mixture concentration in the connected container by switching the pipeline needle valve and paying attention to the change of the pressure display number according to Dalton's law of partial pressure, so as to carry out experimental research on the impact of the explosion and release of combustible gas in the connected container under different gas equivalence ratios (Φ); the bottom surfaces of the two spherical explosion containers are equipped with one gas distribution inlet port, and the connecting pipeline is equipped with two gas distribution inlets to ensure that the overall gas distribution concentration of the connected container is uniform.

[0028] A piezoelectric pressure sensor is installed on the rectifier plate bracket of the external pressure sensor in the data acquisition system to reduce the influence of the explosion discharge flame on the pressure sensor and measure the pressure data in the specified direction.

[0029] In the high-speed schlieren system, the background schlieren pattern on the background schlieren plate is a number of randomly distributed square pixel points; the background schlieren plate is placed behind the measured flow field, and when the measured flow field has a density gradient caused by factors such as gas discharge, pressure wave compression or local heating, the non-uniform refractive index causes the light in the measured flow field to be deflected, and the pixel particles on the background schlieren plate captured by the high-speed camera are displaced, which indirectly reflects the density change of the measured flow field; the image data recorded by the high-speed camera is processed by a cross-correlation algorithm to present the gas cloud structure, flame shape and pressure wave evolution law of the measured flow field area, which can be used to compare with the synchronous pressure data to analyze the impact of the secondary explosion on the discharge pressure reduction rate during the combustible gas explosion discharge process.

[0030] The high-voltage ignition system is equipped with central ignition electrodes on both spherical explosion containers and the connecting pipe. Changing the output current and voltage of the high-voltage ignition package can produce different ignition energies, so as to realize experimental research on the influence of different ignition positions (Ign) and ignition energies (E) on the explosion discharge of combustible gas in the connecting container.

[0031] The above experimental device is used to carry out the explosion release experiment of combustible gas in the connected container, which specifically includes the following steps:

[0032] Step (1): According to the experimental plan, install a connecting pipe with a specified aspect ratio, fix the two spherical explosion containers (and the discharge flange) and the connecting pipe with bolts, install the discharge diaphragm, pressure sensor, and photodiode at the specified position, place a sealing rubber ring / gasket at the flange connection, wrap the raw tape at the threaded connection to ensure good airtightness, build a high-speed Schlieren system, connect the output end of the high-voltage ignition system to the specified ignition electrode pair, and connect the timing controller to the high-voltage ignition package, data acquisition instrument, and high-speed camera respectively.

[0033] Step (2): Place a rubber stopper at the discharge port, start the vacuum pump, evacuate the connected container to a low vacuum degree (<1.0 kPa), and after a short period of rest (about 2 minutes), observe the fluctuation amplitude of the pressure display to detect whether the air tightness is good; if the pressure value rises significantly in a short period of time, it means that the air tightness is poor, and it is necessary to check the position of the incompletely sealed parts and reseal them before testing the air tightness; if the pressure value does not rise significantly, it means that the air tightness is good and gas distribution can be carried out.

[0034] Step (3): According to Dalton's law of partial pressures, the premixed system combustible gas and air concentrations required for the specified gas equivalence ratio are configured, and after the pressure reading is stable, the rubber stopper is removed and allowed to stand for 5 minutes to allow the premixed gas inside the connected container to be evenly mixed.

[0035] Step (4): Check that the signal transmission and data acquisition lines of the pressure sensor, photodiode, ignition electrode and high-speed camera are correctly connected, adjust the high-speed camera to be able to fully capture the explosion release area and ensure that the edges of the pixel particles on the background pattern board are clear, adjust the data acquisition instrument and high-speed camera to the trigger state, press the low-voltage electrical push-button switch, and complete the trigger and ignition operations in sequence according to the timing controller preset.

[0036] Step (5): Save the pressure data on the data acquisition instrument and the image data captured by the high-speed camera, and use a blower to clean the discharge diaphragm debris and combustion products inside and near the container.

[0037] With the help of pressure sensors, photodiodes and high-speed Schlieren system, the internal and external explosion pressure data, flame signals and external image data of the explosion release of combustible gas in the connected container are obtained to study and analyze the static action pressure (P stat ), discharge diameter (D v ), discharge coefficient (K v ), spherical explosion container volume ratio (V1 / V2), connecting pipe length (L), connecting pipe diameter (D), gas equivalence ratio (Φ), ignition position (Ign) and ignition energy (E) and other factors on the overpressure and flame of combustible gas explosion release, and explore the peak value of internal and external overpressure release (the peak value P formed by the rupture of the release diaphragm) b , the peak value P formed by the external secondary explosion ex , the peak value P formed by the Helmholtz oscillation h , the peak value P formed by acoustic wave coupling ac , the peak value P formed by the maximum flame area red ), the formation factors of the released gas cloud, the released flame morphology and the evolution law of the external pressure wave, providing ideas and methods for the experimental research on the explosion release of flammable gas in connected containers.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows.

[0039] (1) The present invention provides an experimental device and an experimental method for studying the explosion and release of combustible gas in a connected container, which can obtain the influence rules of explosion pressure, pipeline flame speed and release flame in the process of combustible gas explosion and release. Moreover, the modular device design is conducive to realizing rich experimental variables, such as the discharge diameter, discharge port position, number of discharge ports, ignition position, volume ratio of spherical explosion container, length-to-diameter ratio of connected pipes, etc. On the other hand, the experimental device of the present invention can be transformed into other types of experimental devices according to experimental needs after adjustment and modification, such as spherical closed explosion experimental device, spherical explosion release experimental device, sphere-conduit release experimental device, etc.

[0040] (2) The present invention uses a piezoelectric pressure sensor to obtain the pressure change characteristics during the explosion and discharge of combustible gas, and uses a photodiode to obtain the average flame propagation speed in the conduit of the connecting container, so as to analyze the relationship between the internal overpressure and the average flame propagation speed of the combustible gas explosion and discharge of the connecting container, rather than obtaining the internal flame propagation speed by embedding a large-area glass window. This improves the pressure-bearing capacity of the overall device, ensures the feasibility of extreme high-pressure experimental conditions, and increases the application level of the experimental device.

[0041] (3) The present invention utilizes a high-speed schlieren system to obtain image data of the external area of ​​the explosion discharge, and obtains a dynamic visualization image of the external discharge flow field area, so as to study and analyze the evolution of the gas cloud structure, flame morphology and pressure wave from the dual dimensions of pressure and flow field, obtain the blocking mechanism of the secondary explosion and choked jet on the discharge and pressure reduction rate of the connected container, and develop a model and design method for the safe discharge of combustible gas explosions in connected containers.

[0042] (4) The present invention designs a connected container combustible gas explosion release experimental device, a connected pipe average flame speed calculation method, and a measured flow field schlieren image acquisition system. According to the operating steps, the static action pressure, release diameter, release coefficient, connected pipe length, connected pipe diameter, gas equivalence ratio, ignition position, ignition energy and other factors can be analyzed to determine the influence of the combustible gas explosion release overpressure and release flame. The formation factors of the internal and external release overpressure peaks, the release gas cloud structure, the release flame morphology and the external pressure wave evolution can be explored to provide ideas and methods for the experimental research on the combustible gas explosion release in connected containers. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a schematic diagram of the overall experimental device of the present invention.

[0044] Figure 2 It is a schematic diagram of the connection between the spherical explosion container and the connecting pipe.

[0045] Figure 3 It is a schematic diagram of the installation of the spherical explosion container discharge flange and ignition electrode flange components.

[0046] Figure 4 It is a schematic diagram of the installation of the connected pipeline pressure sensor and photodiode components.

[0047] Figure 5 It is a schematic diagram of the background schlieren of the measured flow field outside the explosion release.

[0048] Figure 6 This is the schematic diagram of the external pressure sensor rectifier board.

[0049] Figure 7 This is a schematic diagram of static action pressure measurement.

[0050] In the figure: 1, first spherical explosion container, 1a, second spherical explosion container, 2, spherical pressure sensor, 2a, pipeline pressure sensor, 2b, external pressure sensor, 3, spherical gas distribution inlet, 3a, pipeline gas distribution inlet, 4, spherical discharge flange, 4a, pipeline discharge flange, 5, spherical ignition electrode, 5a, pipeline ignition electrode, 6, discharge diaphragm, 7, plastic gasket, 8, sealing rubber ring, 9, connecting pipeline, 10, photodiode, 11, gas Gas cylinder, 12. Air cylinder, 13. Pressure reducing valve, 14. Pipeline needle valve, 15. Pressure gauge, 16. Vacuum pump, 17. PU air pipe, 18. External pressure sensor rectifier board bracket, 19. Sensor power supply box, 20. Data acquisition instrument, 21. High-speed camera, 22. Background pattern plate, 23. Computer, 24. High-voltage ignition package, 25. Timing controller, 26. Pressure sensor fixing bolt, 27. Photodiode fixing bolt, 28. Quartz glass. DETAILED DESCRIPTION

[0051] The specific implementation modes of the present invention will be described in detail below in conjunction with the accompanying drawings, but it should be understood that the protection scope of the present invention is not limited by the specific implementation modes.

[0052] Unless explicitly stated otherwise, throughout the specification and claims, the term “comprise” or variations such as “include” or “comprising”, etc., will be understood to include the stated elements or components but not to exclude other elements or components.

[0053] In this document, for the convenience of description, spatial relative terms such as "below", "below", "down", "above", "above", "upper", etc. may be used to describe the relationship between one element or feature and another element or feature in the accompanying drawings. It should be understood that the spatial relative terms are intended to include different orientations of the object in use or operation in addition to the orientation depicted in the figure. For example, if the object in the figure is turned over, the element described as being "below" or "below" other elements or features will be oriented "above" the element or feature. Therefore, the exemplary term "below" can include both the below and above directions. Objects may also have other orientations (rotated 90 degrees or other orientations) and the spatial relative terms used in this document should be interpreted accordingly.

[0054] An experimental device for explosion release of combustible gas in a connected container comprises a connected release container (composed of two spherical explosion release containers and a connected pipeline), a gas distribution system, a data acquisition system, a high-speed Schlieren system and a high-pressure ignition system; the connected explosion release container comprises two spherical explosion release containers 1 of different volumes, a high-precision piezoelectric pressure sensor 2 and a gas distribution pipeline air inlet 3 are installed on the side wall of the spherical explosion container 1, the pressure sensor is fixed in a hollow pressure sensor fixing bolt 26, a release flange 4 of different calibers is installed at the center of the upper wall, and a flange of an ignition electrode 5 is set at the center of the lower wall; a release diaphragm 6 of different static action pressures is clamped on a circular groove of the release flange 4 where a plastic gasket 7 is placed by bolts, and a sealing rubber ring 8 is provided on the top of the spherical explosion container 1 to ensure the air tightness of the container after the release flange 4 is fixed to the spherical explosion container 1.

[0055] The connecting pipe includes several pipes 9 with different aspect ratios, and fixed flanges are arranged at both ends to facilitate the connection of two spherical explosion containers 1 in series. The extended pipe part outside the flange is cut according to the corresponding spherical surface to ensure that the inside of the two spherical explosion containers 1 is in a complete spherical shape after the series installation. A high-precision piezoelectric pressure sensor 2, a photodiode 10, an ignition electrode 5 and an air inlet 3 of the gas distribution pipeline are respectively installed on both sides of the connecting pipe, and a discharge flange 4 is installed at the top center of the connecting pipe 9. The photodiode 10 is fixed in a hollow photodiode fixing bolt 27, and a piece of quartz glass 28 is filled at the bottom of the bolt to assist in bearing pressure;

[0056] The gas distribution system includes a gas cylinder 11, an air cylinder 12, a pressure reducing valve 13, a pipeline needle valve 14, a pressure gauge 15, a vacuum pump 16 and a PU air pipe 17; the data acquisition system includes a piezoelectric pressure sensor 2, an external pressure sensor rectifier plate bracket 18, a sensor power supply box 19, a photodiode 10 and a data acquisition instrument 20. The piezoelectric pressure sensor 2 converts the pressure signal into an electrical signal, and the photodiode 10 converts the optical signal into an electrical signal, which is then recorded by the data acquisition instrument 20 according to a certain sampling frequency and time length. The high-speed Schlieren system includes a high-speed camera 21, a background Schlieren plate 22 and a computer 23. The high-speed camera 21 captures the flow field image of the external discharge area of ​​the explosion discharge. The background Schlieren plate 22 is placed behind the measured flow field to obtain the high-speed time-series background Schlieren pattern of the external area flow field such as the discharge gas cloud structure, the external secondary explosion and the jet flame, and finally transmits the captured image to the computer 23.

[0057] The high-voltage ignition system includes an ignition electrode 5, a high-voltage ignition package 24 and a timing controller 25. The high-voltage ignition system starts the timing controller 25 through a low-voltage electrical push-button switch, synchronously triggers the data acquisition instrument 20 and the high-speed camera 21, and then controls the high-voltage electric ignition package to output a high-voltage current, thereby generating an arc at the tip of the ignition electrode 5 to ignite the combustible gas.

[0058] The two spherical explosion containers 1 and the connecting pipe 9 have a high degree of explosion resistance and pressure resistance. The discharge opening on the top of the spherical explosion container 1 can be installed with discharge flanges of different diameters, and can clamp different discharge diameters D v and static action pressure P stat The discharge diaphragm 6 can realize different discharge diameters D v and static action pressure P stat Experimental study on the effect of the discharge diaphragm on the explosive discharge of combustible gas in the connected container. Two photodiodes 10 are symmetrically placed at a certain distance in the center of the connecting pipe 9, which can realize the measurement of the average flame speed of the pipeline; the opening diameter of the discharge flange at the top of the center of the connecting pipe 9 is the pipeline radius, which can realize the experimental study on the effect of different discharge port positions and numbers on the connected container on the explosive discharge of combustible gas in the connected container. After the discharge diaphragms of different thicknesses are installed on the discharge flange 4, air is slowly introduced into the container. The internal pressure at the moment the discharge diaphragm ruptures is the static action pressure P of the discharge diaphragm. stat The threads of the flanges on both sides of the connecting pipe 9 can be removed after disassembly, and pipes of different lengths and inner diameters can be replaced to realize the experimental study of the variation law of the explosion overpressure of the combustible gas in the connecting container and the impact of explosion discharge under different length-to-diameter ratios L / D. Each flange connection on the two spherical explosion containers 1 and the connecting pipe 9 is equipped with a sealing rubber ring / gasket, and each thread is wrapped with raw tape before installation to fully ensure the overall airtightness of the device.

[0059] After the two photodiodes 10 distributed on the connecting pipe 9 capture the light signals successively, the average speed of the pipeline flame propagation can be calculated according to the "distance / time" to study the influence of different gas equivalence ratios Φ or connecting pipe length-to-diameter ratios L / D on the propagation speed of the combustible gas flame in the connecting container in the connecting pipe.

[0060] After the gas distribution system evacuates the connected container to a low vacuum degree through the vacuum pump 16, according to Dalton's law of partial pressure, the specified gas mixture concentration configuration in the connected container is realized by switching the pipeline needle valve 14 and paying attention to the change in the pressure gauge 15, so as to carry out experimental research on the impact of the explosion and release of combustible gas in the connected container under different gas equivalence ratios Φ. The bottom surface of the two spherical explosion containers 1 and the connecting pipeline 9 are equipped with 1-2 pipeline gas distribution inlets 3a to ensure that the overall gas distribution concentration of the connected container is uniform.

[0061] A piezoelectric pressure sensor 2b is installed on the external pressure sensor rectifier plate bracket 18 in the data acquisition system to reduce the impact of the explosion release flame on the pressure sensor 2b and measure the pressure data in a specified direction.

[0062] In the high-speed Schlieren system, the background Schlieren pattern on the background Schlieren plate 22 is a number of randomly distributed square pixels; the background Schlieren plate 22 is placed behind the measured flow field. When the measured flow field has a density gradient caused by factors such as gas release, pressure wave compression or local temperature rise, the non-uniform refractive index causes the light in the measured flow field to be deflected, and the pixel particles on the background Schlieren plate captured by the high-speed camera are displaced, which indirectly reflects the density change of the measured flow field. The image data recorded by the high-speed camera is processed by the cross-correlation algorithm to present the gas cloud structure, flame morphology and pressure wave evolution law of the measured flow field area, which can be used to compare with the synchronous pressure data to analyze the impact of secondary explosion on the release pressure reduction rate during the combustible gas explosion release process.

[0063] The high-voltage ignition system is equipped with a central ignition electrode 5 on both the two spherical explosion containers 1 and the connecting pipe 9. Changing the output current and voltage of the high-voltage ignition package can produce different ignition energies, so as to realize experimental research on the influence of different ignition positions Ign and ignition energy E on the explosion discharge of combustible gas in the connecting container.

[0064] The experimental method of the experimental device for explosion release of combustible gas in a connected container includes the following steps:

[0065] Step (1): According to the experimental plan, install a spherical explosion container with a specified volume ratio and a connecting pipe with a specified length-to-diameter ratio, fix the two spherical explosion containers (and the discharge flange) and the connecting pipe with bolts, install the discharge diaphragm, pressure sensor, and photodiode at the specified position, place a sealing rubber ring / gasket at the flange connection, wrap the raw tape at the threaded connection to ensure good airtightness, build a high-speed Schlieren system, connect the output end of the high-voltage ignition system to the specified ignition electrode pair, and connect the timing controller to the high-voltage ignition package, the data acquisition instrument, and the high-speed camera respectively.

[0066] Step (2): Place a rubber stopper at the discharge port, start the vacuum pump, evacuate the connected container to a low vacuum degree (<1.0 kPa), and after a short period of rest (about 2 minutes), observe the fluctuation amplitude of the pressure display to detect whether the air tightness is good; if the pressure value rises significantly in a short period of time, it means that the air tightness is poor, and it is necessary to check the position of the incompletely sealed parts and reseal them before testing the air tightness; if the pressure value does not rise significantly, it means that the air tightness is good and gas distribution can be carried out.

[0067] Step (3): According to Dalton's law of partial pressure, the concentration of combustible gas and air in the premixed system required for the specified gas equivalence ratio is configured, and after the pressure indication is stable, the rubber stopper is removed and allowed to stand for 5 minutes to allow the premixed gas in the connected container to be evenly mixed;

[0068] Step (4): Check that the signal transmission and data acquisition lines of the pressure sensor, photodiode, ignition electrode and high-speed camera are correctly connected, adjust the high-speed camera to be able to fully capture the explosion release area and ensure that the edges of the pixel particles on the background pattern board are clear, adjust the data acquisition instrument and high-speed camera to the trigger state, press the low-voltage electrical push-button switch, and complete the trigger and ignition operations in sequence according to the timing controller preset.

[0069] Step (5): Save the pressure data on the data acquisition instrument and the image data captured by the high-speed camera, and use a blower to clean the discharge diaphragm debris and combustion products inside and near the container.

[0070] With the help of pressure sensors, photodiodes and high-speed Schlieren system, the internal and external explosion pressure data, flame signals and external image data of the explosion release of combustible gas in the connected container are obtained to study and analyze the static action pressure (P stat ), discharge diameter (D v ), discharge coefficient (K v ), connecting pipe length (L), connecting pipe diameter (D), gas equivalence ratio (Φ), ignition position (Ign) and ignition energy (E) and other factors on the overpressure and flame of combustible gas explosion release, and explore the peak value of internal and external overpressure release (the peak value P formed by the rupture of the release diaphragm) b, the peak value P formed by the external secondary explosion ex , the peak value P formed by the Helmholtz oscillation h , the peak value P formed by acoustic wave coupling ac , the peak value P formed by the maximum flame area red ), the formation factors of the released gas cloud structure, the released flame morphology and the evolution law of the external pressure wave, provide ideas and methods for the experimental research on the explosion release of combustible gas in connected containers. The classic law of explosion release of connected containers is as follows:

[0071] (1) Compared with single sphere explosion containers, the maximum explosion overpressure of the connected container is higher than the maximum explosion overpressure of any single sphere explosion container regardless of the ignition position and the length and diameter of the connecting pipe. Therefore, under the same discharge coefficient, the maximum explosion overpressure of the connected container is higher than the maximum explosion overpressure of any single sphere explosion container.

[0072] (2) For explosion release of interconnected containers, when the small spherical explosion container is used as the detonation container, the pressure accumulation in the interconnected container is greater and the maximum explosion overpressure is higher.

[0073] (3) As the length of the connecting pipe increases, the average flame propagation speed in the pipe increases, the pressure accumulation effect and pressure oscillation become more obvious, and the maximum explosion overpressure and pressure rise rate in the explosion container of the explosive sphere also increase accordingly.

[0074] (4) The maximum explosion overpressure of the connected container is positively correlated with the static action overpressure and negatively correlated with the size of the discharge orifice, but has almost nothing to do with the ignition energy.

[0075] (5) When the release position is set downstream in the explosion propagation direction, the external secondary explosion caused by the explosion release is more likely to occur. As the volume of the spherical explosion container and the length of the connecting pipe increase, the external secondary explosion overpressure is higher and the flame jet length is longer.

[0076] Example

[0077] The present invention discloses an experimental device and an experimental method for studying the explosion and discharge of combustible gas in a connected container. The complete experimental device includes a connected discharge container (two spherical explosion-relief containers are connected by a connecting pipe), a gas distribution system, a data acquisition system, a high-speed Schlieren system and a high-voltage ignition system. The two spherical explosion-relief containers and the connecting pipe in the present invention are made of Q345R steel, and the overall design pressure is at least 10MPa, including spherical explosion-relief containers of different volumes, connecting pipes with different aspect ratios and discharge flanges of different calibers. The selectable inner diameters of the spherical explosion-relief containers are 49.2mm, 62.1mm and 71.0mm, respectively, and the volumes are 0.5L, 1.0L and 1.5L, respectively. The 0.5L spherical explosion container is equipped with a discharge flange with a discharge diameter of 24.6mm, 34.78mm and 49.2mm, the 1.0L spherical explosion container is equipped with a discharge flange with a discharge diameter of 20.7mm, 41.4mm and 62.1mm, and the 1.5L spherical explosion container is equipped with a discharge flange with a discharge diameter of 35.5mm, 50.2mm and 71.0mm. The outside of each spherical explosion container is a square outer shell, and 8 M20 screw holes are provided on the upper side of the square shell for fixing the top discharge flange. The top discharge flange is equipped with a discharge flange cover of other discharge diameter sizes, and is fixed with 8 M12 screw holes, and the discharge diaphragm is clamped and fixed with a plastic gasket. Three external pressure sensors are provided on the outside of each discharge flange, which are 15cm, 30cm and 45cm away from the end face of the discharge flange, and are fixed on the vertical external pressure sensor rectifier plate bracket. A pressure sensor fixing bolt is provided on the left side of the square housing. The bolt type is a non-standard countersunk bolt M14, with a hole in the middle and equipped with threads to fix the pressure sensor. There are 8 M12 screw holes on the front side of the square housing for fixing the ignition electrode flange. There are 8 M20 threaded holes on the right side of the square housing for fixing the side flange of the connecting pipe. There is an NPT1 / 4 threaded hole at the bottom of the square housing for connecting the 8mm caliber PU air pipe.

[0078] The optional connecting pipe lengths are 345mm, 443mm, and 542mm, respectively, and the inner diameters are 16.4mm, 32.8mm, and 49.2mm, respectively. There are 9 length-to-diameter ratios (L / D=7, 9, 11, 10.5, 13.5, 16.5, 21, 27, and 33). A discharge flange with the same diameter as the pipe is provided at the top center of each connecting pipe and fixed with 8 M10 bolts. A pressure sensor opening is provided on each side of the discharge flange. An ignition electrode flange position is provided at the bottom of the connecting pipe and fixed with 8 M10 bolts. A gas distribution hole position and a photodiode hole position are provided on both sides of the ignition electrode flange position. The front end of the photodiode fixing bolt is a 15mm thick quartz glass for pressure bearing. The photodiode is installed in the middle opening of the M16 fixing bolt.

[0079] Static operating pressure of the discharge diaphragm (P stat ) is measured according to NFPA 68. After the discharge diaphragm to be tested is clamped and fixed on the discharge flange, air is introduced into the container at a slow and constant speed (≤0.1 bar / min) until the discharge diaphragm is ruptured under pressure. The rupture pressure at this time is the static operating pressure (P stat ). The static action pressure of the discharge diaphragm is related to the discharge diameter, thickness and material of the discharge diaphragm, and is one of the most important variable parameters in the explosion discharge experiment. Therefore, the static action pressure under different discharge conditions should be measured independently and repeated for more than three times, with an error of no more than 5% of the expected static action pressure value, in order to obtain a reliable measurement value.

[0080] The gas distribution system uses the partial pressure method to distribute gas. The gas is evenly and slowly introduced from a total of 4 gas distribution inlets on the spherical explosion container and the connecting pipe to ensure that the premixed gas inside the device is evenly distributed. Each gas distribution inlet is equipped with a needle valve (or ball valve) to prevent the explosion from entering the gas distribution pipeline. During the vacuuming process, in order to prevent the possible rupture of the discharge diaphragm due to internal and external negative pressure, a sealing plug is used to block the opening of the discharge flange to balance the internal and external air pressure, and then remove it after the gas distribution is completed. The premixed gas ratio is calculated according to the gas equivalence ratio. Dalton's partial pressure law is used. Based on the vacuum degree inside the connecting container and the concentration of combustible gas required for the experiment, the gas is distributed in the order of gas first and then air.

[0081] The high-voltage ignition system consists of two 1.5mm diameter copper rods for conducting electricity. The contact parts of the copper rods with the flange and fixing bolts are fixed after being covered with a polytetrafluoroethylene insulating tube to prevent leakage or wall ignition. There is a tungsten tip perpendicular to the copper rod at each end of the copper rod to achieve the tip discharge effect to create arc ignition. The two tips are collinear and opposite to each other, and the tip gap is controlled to be about 3mm. The length of the ignition electrode inside each container is different to control the ignition position to be in the center of the container. The ignition high-voltage current is output by the high-voltage ignition package, which converts the 220V, 0.25A AC input current into a 15kV, 40mA DC high-voltage current, and sets the continuous discharge time to 30ms in the internal program of the timing controller. The total ignition energy is roughly estimated to be about 120.0mJ by integrating the current, voltage and time. In addition, the internal program of the timing controller is set to trigger the high-speed camera and data acquisition instrument at the same time after the switch is pressed after a 4900ms delay, execute the ignition program at 5000ms, terminate the ignition at 5030ms, and the 100ms time interval ensures that the explosion pressure, photoelectric and image data after ignition can be fully collected to avoid time system errors and signal transmission delays. The ignition energy can be controlled by changing the length of the ignition delay time.

[0082] In order to accurately capture the pressure change trend during the rapid response of the gas explosion, the data acquisition system uses a high-precision piezoelectric pressure sensor, and sets the acquisition frequency and duration to 500kS / s and 200ms / div (10div in total), with a total of 1 million acquisition points. In order to avoid the unevenness of the end face of the pressure sensor and the adjacent inner wall after installation, the explosion pressure wave collides and interferes between the concave and convex surfaces, forming an abnormal peak of the pressure data, and a specific countersunk hollow thread is designed according to the thickness of the device wall, so that the end face of the pressure sensor is flush with the inner wall of the device. The external pressure sensor is installed on the rectifier plate, which also ensures the flushness of the end face of the pressure sensor. To avoid the influence of the release flame, a thin layer of high-performance thermal conductive silicone grease is applied to the end face of the sensor.

[0083] The high-speed Schlieren system sets the shooting frequency of the high-speed camera to 10,000 fps and the total shooting time to at least 5.0 seconds to ensure the continuity of the external discharge flow field image. This ensures that the film breaking moment, the gas cloud release process, and the external flame development process are fully captured in the discharge area. In order to visualize the evolution of the external discharge pressure wave, the background Schlieren technology is used. The refraction caused by the density difference of the external discharge flow field caused by the discharge pressure wave is used to calculate the relative displacement of the background Schlieren pattern particles through the cross-correlation algorithm, and the pressure fluctuation of the external flow field is presented by non-contact optical measurement. The particle pattern of the background Schlieren plate is composed of square blue blocks with a side length of 2 mm. The adjacent spacing is normally distributed, and the mean spacing is set to 3 mm. The deflection effect image group affected by the density difference field is processed using the PIVlab algorithm: first, the influence of light source stroboscopic and camera random noise is eliminated by averaging the background image; then, the differentiated image pairs are imported in batches using the combination of "average background image + time series image", and three iterative windows with different window points are set through the FFT algorithm; finally, the abnormal flow field vectors caused by factors such as image noise, discharge flames, and discharge diaphragm fragments are manually eliminated to obtain a visualized evolution image of the pressure wave in the flow field of the external discharge area.

[0084] When the above-mentioned connected container combustible gas explosion release experimental device is used for the experiment, the following steps are included:

[0085] Step (1): According to the experimental plan, install a spherical explosion container with a specified volume ratio and a connecting pipe with a specified length-to-diameter ratio, fix the two spherical explosion containers (and the discharge flange) and the connecting pipe with bolts, install the discharge diaphragm, pressure sensor, and photodiode at the specified position, place a sealing rubber ring / gasket at the flange connection, wrap the raw tape at the threaded connection to ensure good airtightness, build a high-speed Schlieren system, connect the output end of the high-voltage ignition system to the specified ignition electrode pair, and connect the timing controller to the high-voltage ignition package, the data acquisition instrument, and the high-speed camera respectively.

[0086] Step (2): Place a rubber stopper at the discharge port, start the vacuum pump, evacuate the connected container to a low vacuum degree (<1.0 kPa), and after a short period of rest (about 2 minutes), observe the fluctuation amplitude of the pressure display to detect whether the air tightness is good; if the pressure value rises significantly in a short period of time, it means that the air tightness is poor, and it is necessary to check the position of the incompletely sealed parts and reseal them before testing the air tightness; if the pressure value does not rise significantly, it means that the air tightness is good and gas distribution can be carried out.

[0087] Step (3): According to Dalton's law of partial pressures, the premixed system combustible gas and air concentrations required for the specified gas equivalence ratio are configured, and after the pressure reading is stable, the rubber stopper is removed and allowed to stand for 5 minutes to allow the premixed gas inside the connected container to be evenly mixed.

[0088] Step (4): Check that the signal transmission and data acquisition lines of the pressure sensor, photodiode, ignition electrode and high-speed camera are correctly connected, adjust the high-speed camera to be able to fully capture the explosion release area and ensure that the edges of the pixel particles on the background pattern board are clear, adjust the data acquisition instrument and high-speed camera to the trigger state, press the low-voltage electrical push-button switch, and complete the trigger and ignition operations in sequence according to the timing controller preset.

[0089] Step (5): Save the pressure data on the data acquisition instrument and the image data captured by the high-speed camera, and use a blower to clean the discharge diaphragm debris and combustion products inside and near the container.

[0090] The above describes the specific details, basic principles, implementation methods and main features of the present invention. The implementation methods of the present invention described are part of the implementation cases of the present invention, not all of the implementation cases. Other technical solutions and implementation cases implemented by means of improvement and equivalent replacement are within the scope of protection of the present invention.

Claims

1. Experimental device for explosion release of combustible gas in connected containers, characterized by: The device includes a communication discharge container, a gas distribution system, a data acquisition system, a high-speed schlieren system and a high-voltage ignition system; The communicating discharge container is connected to the first spherical explosion container (1) and the second spherical explosion container (1a) by a communicating pipe (9); a spherical pressure sensor (2) and a spherical gas distribution inlet (3) are arranged on the side wall surface of the spherical explosion container, a spherical discharge flange (4) is installed on the upper wall surface, and a spherical ignition electrode (5) is arranged on the lower wall surface; fixing flanges for connecting the first spherical explosion container (1) and the second spherical explosion container (1a) are arranged at both ends of the communicating pipe (9); a pipe discharge flange (4a) and a pipe pressure sensor (2a) are arranged on the top surface of the communicating pipe (9); a pipe ignition electrode (5a), a pipe gas distribution inlet (3a) and a photodiode (10) are arranged on the bottom surface; the photodiode (10) is fixed in a hollow photodiode fixing bolt (27); the bottom of the photodiode fixing bolt (27) is filled with quartz glass (28) for auxiliary pressure bearing; the discharge flange clamps the discharge diaphragm (6) and the plastic gasket (7), and a sealing rubber ring (8) is arranged between the discharge flange and the spherical explosion container and the communicating pipe; The connected discharge container is provided with a plurality of groups of external pressure sensors (2b) via an external pressure sensor rectifier plate bracket (18), and the external pressure sensors (2b) are used to monitor the external pressure of the connected discharge container after discharge; The gas cylinder (11) and the air cylinder (12) in the gas distribution system are respectively connected to the gas distribution pipeline via a PU gas pipe (17), and the gas distribution pipeline is respectively connected to the sphere gas distribution inlet (3) and the pipeline gas distribution inlet (3a); a pressure gauge and a vacuum pump are also provided on the gas distribution pipeline; The data acquisition system comprises an external pressure sensor rectifier plate bracket (18), a sensor power supply box (19) and a data acquisition instrument (20); The high-speed Schlieren system comprises a high-speed camera (21), a background Schlieren plate (22) and a computer (23); the background Schlieren plate (22) is arranged behind the flow field to be measured when the discharge is connected to the discharge container, so as to obtain a high-speed time-series background Schlieren pattern of the discharge gas cloud structure, the external secondary explosion and the external area flow field of the jet flame; The high-voltage ignition system comprises a high-voltage ignition package (24) and a timing controller (25), wherein the timing controller (25) is electrically connected to a high-speed camera (21), the high-voltage ignition package (24), and a data acquisition device (20).

2. The experimental device for explosion release of combustible gas in a connected container according to claim 1 is characterized in that: The sensor power supply box (19) is electrically connected to the spherical pressure sensor (2), the pipeline pressure sensor (2a) and the external pressure sensor (2b).

3. The experimental device for explosion release of combustible gas in a connected container according to claim 1 is characterized in that: A pressure gauge (15) and a vacuum pump (16) are also provided on the gas distribution pipeline.

4. The experimental device for explosion release of combustible gas in a connected container according to claim 1 is characterized in that: The timing controller (25) is started by a low-voltage electrical push-button switch to synchronously trigger the data acquisition instrument (20) and the high-speed camera (21), and then the high-voltage electric ignition package (24) is controlled to output a high-voltage current to generate an arc at the tips of the spherical ignition electrode (5) and the pipe ignition electrode (5a).

5. The experimental device for explosion release of combustible gas in a connected container according to claim 1 is characterized in that: Adjust the discharge diameters of the ball discharge flange (4) and the pipe discharge flange (4a) and test the discharge pressures under different discharge diameters.

6. The experimental device for explosion release of combustible gas in a connected container according to claim 1 is characterized in that: At least two photodiodes (10) are symmetrically arranged on the communication pipe (9).

7. The experimental device for explosion release of combustible gas in a connected container according to claim 1 is characterized in that: The diameter of the pipeline discharge flange (4a) is the radius of the connecting pipeline (9).

8. The experimental method of the experimental device for explosion release of combustible gas in a connected container according to any one of claims 1 to 7, characterized in that: The following steps are involved: Step (1): According to the experimental plan, install a connecting pipe with a specified aspect ratio, fix two spherical explosion containers with bolts, build a high-speed Schlieren system, connect the output end of the high-voltage ignition system to the specified ignition electrode pair, and connect the timing controller to the high-voltage ignition package, the data acquisition instrument and the high-speed camera respectively; Step (2): Place a rubber stopper at the discharge port, start the vacuum pump, evacuate the connected container to a low vacuum degree of less than 1.0 kPa, and observe the fluctuation amplitude of the pressure indication after standing to detect whether the air tightness is good; Step (3): According to Dalton's law of partial pressure, the concentration of combustible gas and air in the premixed system required for the specified gas equivalence ratio is configured, and after the pressure indication is stable, the rubber stopper is removed and allowed to stand to allow the premixed gas in the connected container to be evenly mixed; Step (4): Adjust the high-speed camera to completely capture the explosion release area and ensure that the edges of the pixel particles on the background schlieren board are clear. After adjusting the data acquisition instrument and the high-speed camera to the trigger state, press the low-voltage electrical push-button switch to complete the trigger and ignition operations in sequence according to the timing controller preset; Step (5): Save the pressure data on the data acquisition instrument and the image data captured by the high-speed camera.

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