HeterologouschargeOfelectrodeDue to the existence of potential difference, charge transfer occurs betweendischarge。After discharge, the charge decreases, the potential difference decreases, and the discharge stopsPower SupplyReturn the potential difference to the state before discharge, and then discharge again.This cycle formspulseDischarge.
Chinese name
Pulse discharge
Foreign name
impulsive discharge
Conditions
Pulse voltage is higher than DC discharge ignition voltage
Pulse discharge is a form of discharge produced by applying a pulse voltage between two electrodes of the discharge tube.
The principle circuit for generating pulse discharge is shown in Figure 1.
Figure 1 Electrical schematic diagram of pulse discharge
When working, firstDC power supplyThe capacitor C is charged by electricity R. When the charge reaches a certain amount, a high-voltage pulse is added to both ends of the trigger wire wound on the discharge tube.The gas in the tube is ignited, and then capacitor C discharges through the discharge tube, releasing a large amount of electric energy stored in the capacitor in the form of light energy in a very short time, forming a very strongflash of light。Discharge and then shut down.
From the beginning to the end of the pulse discharge, the whole process is gradually changed, so it is an unsteady discharge.The discharge process can be divided into two stages: the occurrence process and the development process.In the occurrence phase, the discharge hasSpark dischargeIn the development stage, it is similar to the high currentArc discharge。The basic physical process is described as follows:
Under the action of high pulse, the gas in the discharge tube breaks down, forming uneven and thin spark channels.ionizationUnder the action of the axial electric field, the generated electrons move towards the anode at a high speed, further ionizing the gas gap and forming an electron torrent.While electrons propagate, photons are generated due to the shock. Photoionization caused by these photons plays a great role. The secondary electron propagation formed by photoionization converges with the main electron propagation, connecting the entire discharge channel, so that the electric energy stored on the capacitor can be rapidly released and collected through the spark channel, while the gas tree in the discharge channel is strongly heated and ionized,The heated gas will cause the discharge channel to increase. After heating for several microseconds, the cross-section of the discharge channel will reach its maximum value.This is the process of spark growth and the stage of pulse discharge.
When the spark grows to a certain extentcapacitorWhen more energy is released, the pulse current is very large (thousands of amperes per square centimeter).Therefore, it will form in the discharge tubeHigh temperature plasma。At this time, the cathode surface has a strong electric field and high temperature, which will produce significantField emissionandThermionic emissionAnd to meet the high current required at the development stage.During the whole pulse discharge process, the duration of this phase is relatively long (about 50 microseconds).Most of the energy stored in the capacitor will be converted intoLight energy。This is the development stage of pulse discharge.
With the release of energy stored in the capacitor, the voltage applied at both ends of the discharge coil drops, the electric field in the tube weakens correspondingly, and the electric pre temperature drops quickly.When the energy supplied by the capacitor to the discharge tube is not enough to maintain the discharge, the discharge will be extinguished.
The basic process of pulse discharge discussed above is realized by discharging the energy storage capacitor.During discharge,electric currentThe direction remains the same, initiallyCurrent intensityIt increases sharply, and decreases rapidly after a maximum value.The condition for generating this pulse discharge is that the maximum pulse voltage should be higher than the ignition voltage of DC discharge.This type of discharge is called DC pulse discharge.[1]
Pulse discharge circuit
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In order to eliminate the sulfuration phenomenon of the battery and extend the service life of the battery, the charger has designed a pulse discharge circuit, which has a pulse repair effect on the sulfuration battery.The circuit is composed of IC3 (CD401), VT5 (negative pulse switch tube) and other components.
The non gates A and B of IC3 and C15, C16, R24, R25 form a multivibrator circuit. The oscillation pulse is input to pin 2 of IC3 to control inverter C. Pin 1 of IC3 is controlled by pin 1 of IC2. In the charging state, pin 1 of IC3 is high level, and inverter C outputs the discharge control pulse. After the non gate processing, it drives the Darlington tube composed of VT5 and VT6 to amplify and realize pulse discharge.[2]
Regularity of pulse discharge
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When the pulse voltage is applied to the air gap, due to the statistical characteristics of the discharge delay, the air gap discharge voltage is not only related to the peak value of the applied pulse voltage, but also related to the waveform of the pulse voltage. Therefore, during the pulse discharge test, the waveform must be unified before the discharge voltage can be compared.The standard waveform specified by China is 1.5/40 pulse wave (see Figure 2 below), which is mainly analogLightning overvoltageeffect.
Figure 2 Standard waveform of pulse
Under the action of pulse voltage with the same waveform, due to the load sharing of discharge delay.It will inevitably lead to the dispersion of the discharge voltage, that is, when the voltage with the same peak value is applied, the discharge may occur at the wave crest, or at the wave tail, or it may not occur (when the initial electron does not appear in time), and the voltage peak value will still be increased, but it is less likely to not occur breakdown.Because of the characteristic of pulse penetration, the volt one second characteristic of air gap discharge is generally used to describe the characteristics of air gap venous impulse discharge.
Figure 3 Method of determining volt one second characteristic
The volt one second characteristic is the relationship between the air gap pulse discharge voltage and the discharge time, as shown in Figure 3.The volt one second characteristic shows that the higher the voltage peak, the shorter the discharge time.The impulse voltage is the atmospheric overvoltage andSwitching overvoltageWhen it is necessary to specifically compare the pulse discharge characteristics of two air gaps in the high-voltage test, one method is to comprehensively compare their volt one second characteristics, and the other simple method is to specify that they should withstand a pulse voltage of a certain overvoltage multiple.[3]
application
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Pulse discharge sterilization technology
High voltage pulse discharge sterilization technology for liquid food is one of the most active topics in cold sterilization research.Pulse discharge sterilization is generally to place liquid food as a dielectric in the sterilization container, and two discharge electrodes insulated with the container are also placed in it;utilizeHigh voltage pulse generatorThe generated pulsed electric field carries out intermittent sterilization on food, or makes liquid food flow through the pulsed electric field for continuous sterilization.
1. Mechanism of pulse discharge sterilization
The sterilization mechanism and effect of electric pulse can be summarized into three aspects: strong current penetration sterilization effect, strong shock wave sterilization effect, and pulse discharge chemical sterilization effect.
High current penetration sterilization effect is also known as cell membrane perforation effect. The purpose of sterilization is to kill microorganisms by making use of the membrane perforation effect of microorganisms caused by high current.Microbial cell membrane is composed ofPhospholipid bilayerIt has certain charge, certain permeability and strength, and a certain potential difference between the inner and outer layers of the film.When an external electric field is applied to both ends of the cell, it will increase the internal and external potential difference of the membrane and cause a sharp increase in the permeability of the cell membrane;On the other hand, the opposite charges on the inner and outer surfaces of the membrane attract each other to produce squeezing force, as well as the impact of high-speed electrons and ions with great kinetic energy, resulting in holes on the cell membrane and irreversible damage. Moreover, because the applied external electric field is a pulse electric field, the voltage fluctuates violently in a very short time (microseconds, milliseconds),It will produce oscillation effect on the cell membrane, and the pores on the cell membrane will become larger. Under the combined effect of these factors, the cells will be irreversibly broken and tend to collapse eventually.
When a pulse discharge is generated in liquid food, the energy storage system (such as the energy storage capacitor bank) will release a large amount of stored energy in an instant, so that the liquid medium will be broken down to form a discharge channel, the gap resistance will rapidly drop from the insulation state to a fraction of an ohm, and a huge pulse impulse current will be generated in the discharge channel. Charged particles (electrons, positive ions, etc.) in the discharge channelHigh speed movement and collision also produce a lot of heat, which makes the liquid around the discharge channel vaporize instantaneously and form bubbles, generating a high-speed and violent expansion explosion and strong shock wave (4 × 10three～5×10fourm/s），Up to 10fiveThe pressure above Pa acts on the cell to break and crush the bacterial cell membrane.If the pulse discharge is constantly applied in the sterilization room, the shock wave will be generated continuously, and the internal of the bacterial cell will be in a continuous and intense forced vibration, thus accelerating the rate of bacterial death.
On the other hand, the chemical effect produced by pulse discharge in liquid materials can also accelerate the death of bacteria.Due to the large current of pulse discharge, the resulting strong magnetic field and electrolytic ionization, many plasma and basic particles will be generated in the liquid material, such as H+OH, H2O ionic group, O, H atom, Otwo、Htwo、Ozone molecule, photons, etc. They are extremely active under the action of strong current, and some basic particles can also pass through the cell membrane with increased permeabilityRibonucleic acidAnd other life substances to denature and die.On the other hand, the generated ozone molecule itself has strong bactericidal ability.
Generally speaking, the sterilization mechanism of pulse discharge when the discharge electrodes are parallel plates is mainly the cell membrane perforation effect, while the sterilization mechanism of streamer discharge when the electrodes are sharp is mainly the impact effect.
2. Characteristics of pulse discharge sterilization
Pulse discharge sterilization has the following three main characteristics:
1) The sterilization time is short and the energy consumption is low. The sterilization time is generally μ s~ms, and 99% of the bacteria can be killed with very low energy consumption. The sterilization power consumption per ton of liquid food is about 0.5~2.0 kW · h, which is 1% of the energy consumption for high-temperature sterilization.
2) The temperature rise during temperature rise small pulse sterilization is generally less than 5 ℃, so it can effectively preserve the nutrition and natural flavor of food.There is little difference between the processed food and fresh food in physical properties, chemical properties, nutritional components, etc.
3) The sterilization effect is obvious. The electric field intensity of 25kV/cm is used. When sterilization is conducted at 60 ℃, the survival rate of bacteria is logN/No=10-9The sterilization rate can meet the requirements of commercial sterility.[4]
Related terms
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Pulse signal: The voltage or current that changes suddenly in a short time is called pulse signal.It can be periodically repeated, non periodic or single.
Pulse reactor(pulse reactor): a reactor that can reach a supercritical state in a short time interval, thus generating very high pulse power and strong neutron flux, and can safely and reliably operate repeatedly.It can be divided into thermal neutron pulse reactor and fast neutron pulse reactor.China has built a uranium zirconium hydrogen pulse reactor, which uses uranium zirconium hydrogen as fuelreactor。It mainly uses hydrogen asModeratorWhen the power increases, the temperature will increase, the hydrogen slowing effect will weaken, and the reactivity will immediately decrease. The reactor has a large instantaneous negative temperature coefficient, so it operates in a pulse mode.Pulse reactor is used to train personnel, engage in research work and produce short liferadio isotopeIn addition, it can also be used for cancer treatment, neutron radiography, activation analysis and irradiation of fuels and materials.
Pulse power supply: The user's load needs to be powered on intermittently, that is, according to a certain time rule, power on the load for a certain period of time, and then power off for a certain period of time to form a cycle.Such repeated execution constitutes a pulse power supply.For example, in the sophisticated process of non-polar electrolytic capacitors, it is necessary to charge the capacitors forward for a period of time, then discharge them, then charge the capacitors backward for a period of time, and then discharge them, thus forming a forward → discharge (power off) → reverse → discharge → forward......, and so on repeatedly.
Pulse width: the duration of high level.It is often used as sampling signal or trigger signal of thyristor and other components.
Pulse circuit: the circuit for generating, shaping and transforming the pulse waveform.The pulse circuit is composed of two parts: inert circuit and switch.The function of the switch is to destroy the steady state and make the circuit transient.
Pulse dialing: a time-domain processing method, which uses the number of pulses to represent the number of numbers.Pulse dialing mode has strict requirements on pulse width, size, spacing and shape. If these parameters change due to line interference or other reasons, it may cause errors in number reception.On the other hand, because each pulse occupies a certain time (generally 100 ms for each pulse), this dialing mode is slow.
Pulse heating: It uses various waveform pulse currents to heat in an intermittent manner to meet some special process requirements.
Pulse wave: the signal waveform generated in the form of shock.
