Measurement and Statistical Simulation of Multiple Reignitions in Vacuum Switches

When the gap length of a vacuum switch at current zero is too small, successive reignitions and extinctions may occur under certain circuit conditions leading to overvoltages. This process of "virtual chopping" is more likely to happen when the contacts accidentally open shortly before current zero. In order to determine the values of the overvoltage and its probability when opening the contacts randomly, a computer program was developed which simulates the transient recovery voltage (TRV) in a single-phase circuit and determines the highest voltage for each switching operation. The moment of contact opening is simulated by pseudorandom numbers. Input data of the calculation are the circuit parameters, the reignition voltage of the gap versus time after current zero, and its capability of quenching the high-frequency current oscillation occurring after a reignition. These recovery data of the opening gap were determined experimentally for some contact materials using a special switching device in a synthetic test circuit with current injection.     

A numerical study of reignition induced by a diffusion flame

We present a numerical study of the reignition of a cold reactant mixture by the interaction with a nearby diffusion flame. This reignition mechanism may be an important process in turbulent non-premixed flames at high rates of strain where quenched sections of the stoichiometric surface are folded by the turbulent flow and come in close proximity with other burning flame sections. We consider an idealized one-dimensional setup containing the fundamental ingredients that are expected to contribute to this reignition mode. One- and two-step irreversible chemical mechanisms with heat release levels typical of practical hydrocarbon fuels are considered. It is observed that a slow moving reignition kernel originates on the high-temperature region of the burning flame in the one-step chemistry case owing to small leakage of oxidizer from the cold-mixture side. This kernel gradually moves, increasing the local temperature above that provided by diffusion and eventually leads to thermal runaway with the formation of a deflagration wave. The reignition time depends on the chemistry details, the Damköhler number, but in any case it cannot exceed the mixing time. This implies that the flame-induced reignition time is essentially bounded from above by mixing. Unless one of the free streams is hotter, in which case auto-ignition (as opposed to reignition) may proceed first, the reignition time is chemistry dependent. In the case of two-step chemistry, the reignition pathway is different initially owing to leakage of the radical species, but it approaches that of the one-step chemistry case shortly thereafter. It is observed that the only difference between the two cases is in the initial phase of the evolution of the reignition kernel. This phase appears to be very sensitive to the chemistry details, a general aspect of ignition. A parametric study is carried out to elucidate the effect of each non-dimensional quantity on the reignition time for the one-step chemistry case.     

An experimental investigation on high-frequency vacuum arcinterruption at small gap length

An experimental study of high-frequency (126, 230 kHz) vacuum arc interruption behavior and the voltage escalation processes at a small gap length (⩽1 mm) for three contact materials (Cu, CuCr, and CuTeSe) is discussed. Two experimental methods have been used: current injection in a low-voltage circuit and in a 10-kV AC circuit. Experimental results of the high-frequency current interruption ability and the dielectric breakdown voltage are presented. Three kinds of breakdown are distinguished: the reignition voltage, the breakdown voltage, and the cold breakdown voltage. It has been found that the interruption ability is directly related to the reignition voltage     

Interrupting capacity of vacuum interrupters depending on thefrequency of current

Results of arc reignition voltages during current interruption of frequencies from 5.9 to 60 kHz by a short vacuum gap are presented. Measured arc reignition voltages depend on current amplitude and frequency, the values of preliminary arc current at the moment of switching on the HF current, and the discharge mode in the preceding current semiwave. Threshold amplitudes of the first semiwave of currents l_m1l and I_m1h as a function of frequency are determined. I_m1l and I_m1h divide current into three ranges to which different kinds of arc reignition voltage distribution correspond. Particularly large dispersion of reignition voltages takes place in the current range from I_m1l to I_m1h. The threshold current I_m1h is inversely proportional to frequency in the range from about 10 to 60 kHz, which is in agreement with the elaborated mathematical model     

Reignition phenomena after high-frequency current interruption withshort vacuum gaps

Based on a large number of measurements of high-frequency (HF) current interruptions in vacuum at small contact gaps (⩽600 μm), the statistical reignition behavior of vacuum switching devices after a HF current zero is investigated. Three types of reignitions can be classified. Statistical evaluation of post-arc current measurements for different parameters at current zero and different HF current ignition processes gives information about the stress of the gap as a result of the transient recovery voltage after a HF current zero (HF-TRV) and the accumulated post-arc charge. Comparing post-arc current values at the beginning of the HF-TRV and at the moment of reignition reveals a production of charge carriers during the recovery interval. Possible reasons for the different types of reignitions are discussed     

Turbulence/flame/wall interactions in non-premixed inclined slot-jet flames impinging at a wall using direct numerical simulation

In the present work, three-dimensional turbulent non-premixed oblique slot-jet flames impinging at a wall were investigated using direct numerical simulation (DNS). Two cases are considered with the Damköhler number (Da) of case A being twice that of case B. A 17 species and 73-step mechanism for methane combustion was employed in the simulations. It was found that flame extinction in case B is more prominent compared to case A. Reignition in the lower branch of combustion for case A occurs when the scalar dissipation rate relaxes, while no reignition occurs in the lower branch for case B due to excessive scalar dissipation rate. A method was proposed to identify the flame quenching edges of turbulent non-premixed flames in wall-bounded flows based on the intersections of mixture fraction and OH mass fraction iso-surfaces. The flame/wall interactions were examined in terms of the quenching distance and the wall heat flux along the quenching edges. There is essentially no flame/wall interaction in case B due to the extinction caused by excessive turbulent mixing. In contrast, significant interactions between flames and the wall are observed in case A. The quenching distance is found to be negatively correlated with wall heat flux as previously reported in turbulent premixed flames. The influence of chemical reactions and wall on flow topologies was identified. The FS/U and FC/U topologies are found near flame edges, and the NNN/U topology appears when reignition occurs. The vortex-dominant topologies, FC/U and FS/S, play an increasingly important role as the jet turbulence develops.     

Kinetic effects of NO addition on n-dodecane cool and warm diffusion flames

The kinetic effects of NO addition on the flame dynamics and burning limits of n-dodecane cool and warm diffusion flames are investigated experimentally and computationally using a counterflow system. The results show that NO plays different roles in cool and warm flames due to their different reaction pathway sensitivities to the flame temperature and interactions with NO. We observe that NO addition decreases the cool flame extinction limit, delays the extinction transition from warm flame to cool flame, and promotes the ignition transition from warm flame to hot flame. In addition, jet-stirred reactor (JSR) experiments of n-dodecane oxidation with and without NO addition are also performed to develop and validate a n-dodecane/NO_x kinetic model. Reaction pathway and sensitivity analyses reveal that, for cool flames, NO addition inhibits the low-temperature oxidation of n-dodecane and reduces the flame temperature due to the consumption of RO₂ via NO+RO₂?NO₂+RO, which competes with the isomerization reaction that continues the peroxy radical branching sequence. The model prediction captures well the experimental trend of the inhibiting effect of NO on the cool flame extinction limit. For warm flames, two different kinds of warm flame transitions, the warm flame extinction transition to cool flame and the warm flame reignition transition to hot flame, were observed. The results suggest that warm extinction transition to cool flame is suppressed by NO addition while the warm flame reignition transition to hot flame is promoted. The kinetic model developed captures well the experimentally observed warm flame transitions to cool flame but fails to predict the warm flame reignition to hot flame at similar experimental conditions.     

Experimental and numerical studies of arc restrikes in low-voltagecircuit breakers

This paper is devoted to the study of the arc restrike phenomenon in low-voltage circuit breakers. We focused our interest on the type of arc restrike that can be described as a sudden reignition in the arcing contact region while the arc was situated in the quenching chamber a few tens of microseconds before it occurs. Our experimental investigations have established that the critical arcing contact region is still crossed by a so called residual current on the order of several amperes. A gas temperature around 4200 K was derived from electrical measurements in this region before the arc restrike occurrence, We also demonstrate that the restrike takes place through the growth of the remaining current of several amperes in the arcing contact region. A numerical approach was carried out with a two-dimensional hydrodynamic code in order to simulate the gas behavior in the arcing contact region before and during the arc restrike phenomenon. The same temperature as the measured one is calculated just before the restrike. It is demonstrated that the current density appears to be the most sensible quantity. A critical value of 5 A/cm² was calculated     

A DNS evaluation of mixing and evaporation models for TPDF modelling of nonpremixed spray flames

In the present work, nonpremixed temporally evolving planar spray jet flames are simulated using both direct numerical simulation (DNS) and the composition transported probability density function (TPDF) method. The objective is to assess the performance of various mixing and evaporation source term distribution models which are required to close the PDF transport equation in spray flames. Quantities which would normally be provided to the TPDF solver by spray models and turbulence models are provided from the DNS: the mean flow velocity, turbulent diffusivity, mixing frequency, and cell-mean evaporation source term. Two cases with different Damköhler numbers (Da) are considered. The low Da case (Da-) features extinction followed by reignition while extinction in the high Da case (Da+) is insignificant. The TPDF modelling considers two mixing models: interaction by exchange with the mean (IEM) and Euclidean minimum spanning trees (EMST). Three models for distribution of the evaporation source terms are considered: EQUAL which distributes them in proportion to notional particles’ mass weight, NEW which creates new particles of pure fuel, and SAT which distributes the sources preferentially to notional particles close to saturation. It is found that the IEM model overpredicts the extinction when used with any evaporation model for both Da- and Da+ cases. The EMST model captures well the trend for extinction and reignition for the Da- case when it is coupled with the EQUAL evaporation model, but it overpredicts the extinction when coupled with the NEW or SAT evaporation model. For the Da+ case, all evaporation models reasonably capture the flame dynamics when coupled with EMST. The flame temperature in the mixture fraction space was examined to further assess the model performance. In general the EMST model results in narrow PDFs with little conditional fluctuation, while the IEM model produces bimodal PDFs with burning and partial extinction branches.     

Perturbed Arcs in Turbulent Axial Gas Flow. III. Numerical Simulation of Stochastically Disturbed Arc Discharges

Stochastically perturbed arcs are modelled by means of a Mayr type differential equation, the thermal constant τ being stochastically controlled by Gaussian fluctuations of given dispersion and correlation time: 1/τ = 1/τ₀ (1 + ζ). The ζ-signals are taken from an electronic noise source by an appropriately equipped intelligent CAMAC crate, which further processes the whole simulation program. Special cases treated are 1. stationary (in the mean) arc, 2. freely decaying arc, and additionally, 3. thermal reignition of a decaying arc with stiff recovery voltage.     

Experimental study on supersonic film cooling on the surface of a blunt body in hypersonic flow

The experimental study focuses on the heat flux on a double cone blunt body in the presence of tangential-slot super- sonic injection into hypersonic flow. The tests are conducted in a contoured axisymmetric nozzle with Mach numbers of 7.3 and 8.1, and the total temperature is about 900 K. The injection Mach number is 3.2, and total temperature is 300 K. A constant voltage circuit is developed to supply the temperature detectors instead of the normally used constant current circuit. The schlieren photographs are presented additionally to visualize the flow and help analyze the pressure relationship between the cooling flow and the main flow. The dependence of the film-cooling effectiveness on flow parameters, i.e. the blow ratio, the convective Mach number, and the attack angle, is determined. A semi-empirical formula is tested by the present data, and is improved for a better correlation.     

A Network of Analog Metal Oxide Semiconductor Circuits for Motion Detection with Local Adaptation to a Background Image

Inspired by a mechanism of biological vision systems, a model and a network of analog metal oxide semiconductor (MOS) circuits are proposed which display an optical flow with local adaptation to the relative velocity of a background image. A function of displaying an optical flow successfully worked as a result of simulations using the simulation program with integrated circuit emphasis (SPICE). A function of varying an optical flow at a certain instant by local adaptation was also demonstrated. The proposed network is suitable for the realization of a large-scale integrated circuit (LSI), which displays an optical flow with local adaptation to the local velocity of a background.     

A Model for DC Interruption in Diffuse Vacuum Arcs

A theoretical model for current interruption in a diffuse vacuum arc with dc commutation is described. Before current zero the interelectrode plasma is modeled as an ion-neutral fluid through which electrons are flowing. After current zero a positive ion sheath grows into the plasma from the former anode, driven by the transient recovery voltage. Using the basic laws of conservation, the decay of the plasma during commutation is evaluated numerically, enabling the post-arc current, the electric field at the former anode, and the power input to this electrode after current zero to be calculated. For copper electrodes, with a commutation time of 30 ?s, the ion density and velocity at current zero are 23 percent and 35 percent of their respective steady state values. The calculated post-arc currents of tens of amps are in good agreement with experimental data. The post-arc data generated with this model can be used to study reignition mechanisms and the interrupting capability of different contact materials.     

General Relativistic Thermoelectric Effects in Superconductors

We discuss the general-relativisticcontributions which occur in the electromagneticproperties of a superconductor with a heat flow. Theappearance of a general-relativistic contribution to themagnetic flux through a superconducting thermoelectricbimetallic circuit is shown. The response of theJosephson junctions to a heat flow is investigated inthe general-relativistic framework. Somegravitothermoelectric effects which are observable in thesuperconducting state in the Earth's gravitational fieldare considered.     

一个新的三维混沌系统的分析、电路实现及同步

中心流形理论提供了一个将高维系统降维研究的方法，应用该理论研究了一个新的混沌系统的基本特性，给出中心流形上流方程，分析这个新的混沌系统的叉式分岔.通过构建电路实现了该混沌系统,从而验证了系统的混沌行为，证实了混沌吸引子的存在.同时说明了由于电路信号频率与数值信号频率的不同所带来的数值仿真与物理实现之间在应用上有着重要区别.最后利用单变量反馈控制方法实现了新系统的同步控制，并给出了完整的同步实现电路. 关键词： 三维混沌系统 中心流形 电路实现 同步     

Time-Resolved Laser Doppler Measurements in the Underexpanded Jet of a Model Gas-Blast Circuit Breaker under Arcing Conditions

A differential laser Doppler system has been used in conjunction with a self-triggered oscillographic raster display system for measuring particle concentrations and flow velocities in a model gas-blast circuit breaker. The flow velocities have been correlated with particle size to enable extrapolation to be made to determine the true plasma flow velocity. The results have been deconvoluted to take account of lateral movements of the arc column. The measurements have been taken with air as the host arcing medium and a flow through a 25-mm-diameter orifice sustained by an upstream pressure of about 7 bar (downstream pressure 1 bar). The arc was sustained by sinusoidal cuffents of peak values 3 and 8 kA and a frequency of 85 Hz. These two currents corresponded to conditions when the model circuit breaker successfully and unsuccessfully interrupted the arc-sustaining current, respectively. The results show that in the case of the 8-kA arc, the flow pattern is severely distorted from that under nonarcing conditions, suggesting that this is a contributory reason for the poorer perfonnance of the model circuit breaker at this higher current level.     

Conditional dissipation of scalars in homogeneous turbulence: Closure for MMC modelling

While the mean and unconditional variance are to be predicted well by any reasonable turbulent combustion model, these are generally not sufficient for the accurate modelling of complex phenomena such as extinction/reignition. An additional criterion has been recently introduced: accurate modelling of the dissipation timescales associated with fluctuations of scalars about their conditional mean (conditional dissipation timescales). Analysis of Direct Numerical Simulation (DNS) results for a passive scalar shows that the conditional dissipation timescale is of the order of the integral timescale and smaller than the unconditional dissipation timescale. A model is proposed: the conditional dissipation timescale is proportional to the integral timescale. This model is used in Multiple Mapping Conditioning (MMC) modelling for a passive scalar case and a reactive scalar case, comparing to DNS results for both. The results show that this model improves the accuracy of MMC predictions so as to match the DNS results more closely using a relatively-coarse spatial resolution compared to other turbulent combustion models.     

Flame propagation in gaseous nitrous oxide

The characteristics of the propagation of a nitrous oxide decomposition flame in a tube with an internal diameter of 70 mm were measured. It was demonstrated that the pattern of flame propagation and the extent of burnout are determined by the convective motion of the flame kernel because of a very slow burning of nitrous oxide. The laminar flame speed estimated from pressure oscillograms and calculated using thermal theory of flame propagation was found to be ～1 cm/s. The critical diameter of flame quenching in channels were measured to decrease from 10 to 4 mm as the pressure was increased from 15 to 20 atm. Because of the possibility of reignition of the fresh mixture behind the flame arrester by the outflowing combustion products, the channel should be significantly longer than 200 mm.     

Modeling of filtered heat release for large eddy simulation of compressible infinitely fast reacting flows

A priori and a posteriori studies for large eddy simulation of the compressible turbulent infinitely fast reacting shear layer are presented. The filtered heat release appearing in the energy equation is unclosed and the accuracy of different models for the filtered scalar dissipation rate and the conditional filtered scalar dissipation rate of the mixture fraction in closing this term is analyzed. The effect of different closures of the subgrid transport of momentum, energy and scalars on the modeling of the filtered heat release via the resolved fields is also considered. Three explicit models of these subgrid fluxes are explored, each with an increasing level of reconstruction and all of them regularized by a Smagorinsky-type term. It is observed that a major part of the error in the prediction of the conditional filtered scalar dissipation comes from the unsatisfactory modeling of the filtered dissipation itself. The error can be substantial in the turbulent fluctuation (rms) of the dissipation fields. It is encouraging that all models give good predictions of the mean and rms density in a posteriori LES of this flow with realistic heat release corresponding to large density change. Although a posteriori results show a small sensitivity to subgrid modeling errors in the current problem, extinction–reignition phenomena involving finite-rate chemistry would demand more accurate modeling of the dissipation rates. A posteriori results also show that the resolved fields obtained with the approximate reconstruction using moments (ARM) agree better with the filtered direct numerical simulation since the level of reconstruction in the modeled subfilter fluxes is increased.     

RLC交流稳态与谐振的微机处理

针对RC电路和RLC电路的交流稳态给出了具体的微机测试电路、实验方法及程序流程图