Reignition Phenomena Research Articles

‘Hybrid’ multiple mapping conditioning on passive and reactive scalars

A two-dimensional, hybridized multiple mapping conditioning (MMC) model is used to model local extinction and reignition phenomena in homogeneous, isotropic decaying turbulence. The equations are solved in a prescribed, jointly Gaussian reference space with stochastic reference variables emulating the fluctuations of the mixture fraction and normalized sensible enthalpy conditioning variables. In “pure” MMC the joint PDF of the conditioning scalars is a solved quantity. Here we use a hybrid method where the time evolution of the marginal PDF for mixture fraction is solved and a presumed β-PDF is used for the conditional distribution of the normalized sensible enthalpy. Model results are compared with DNS in three flame cases with varying levels of local extinction, up to global extinction. Results for principal chemical species are in very good agreement with DNS and those for intermediate species are also satisfactory. The doubly conditioned MMC yields results which are considerably more accurate than those by modeling with conditioning on mixture fraction alone. A transformation of the Gaussian reference space casts the MMC model in the same form as conditional moment closure (CMC). The great advantage is that the MMC model contains the doubly conditioned scalar dissipation terms in closed form and these are generally found to be in good agreement with the DNS data.

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.

The influence of chemical mechanisms on PDF calculations of nonpremixed piloted jet flames

Seven different chemical mechanisms for methane are used in PDF model calculations of the Barlow and Frank flames D, E, and F in order to investigate the ability of these mechanisms to represent the local extinction, reignition, and other chemical phenomena observed in these nonpremixed piloted jet flames. The mechanisms studied range from a 5-step reduced mechanism to the GRI3.0 mechanism which involves 53 species. As in several other recent studies, we use the PDF method based on the joint probability density function of velocity, turbulence frequency, and composition. Extensive tests are performed to ensure the numerical accuracy of the calculations, to relate them to previous calculations based on the same model, and to reexamine the sensitivity of the calculations (especially of flame F) to uncertainties in the pilot temperature and the treatment of radiation. As has been observed in other studies of laminar and turbulent nonpremixed flames, we find that the GRI3.0 mechanism overpredicts the levels of NO, typically by a factor of 2. Apart from this, the GRI3.0 and GRI2.11 mechanisms yield comparably good agreement with the experimental data for all three flames, including the level of local extinction and the conditional means of major and other minor species. Two augmented reduce mechanism (ARM1 and ARM2) based on GRI2.11 and containing 16 and 19 species are slightly less accurate; while the 5-step reduced mechanism and two C 1 skeletal mechanisms containing 16 species display significant inaccuracies. An examination of the autoignition and laminar-flame behavior of the different mechanisms confirms (with some exceptions) expected trends: there is an association between long ignition delay times, small extinction strain rates, and high levels of local extinction. This study again demonstrates the ability of the joint PDF method to represent accurately the strong turbulence–chemistry interactions in these flames, and it clarifies the necessary level of description of the chemical kinetics.

Numerical Prediction of Diluted Hydrogen Turbulent Jet Diffusion Flame Using a Scalar Probability Density Function Method Considering Detailed Chemical Kinetics

A scalar probability density function (PDF) approach considering a detailed chemical kinetics is applied to a diluted hydrogen jet diffusion flame to evaluate the performance. The flame is formed on the nozzle of an inner diameter of 6 mm, with a fuel exit velocity of 30 m/s, surrounded by two annular pipes issuing airs of higher and lower velocities of 30 m/s and 3 m/s, respectively. The flow field has been solved on the basis of the k-ε two equation model. The modeled PDF transport equation has been solved by an Eulerian Monte-Carlo method. The results are compared with those of two flamelet calculations, which are based on the conventional laminar flamelet model method and a scalar PDF method based on the conserved scalar approach. The present PDF method predicts the maximum temperature on the center axis close to the experimental one than the other methods. The temperature downstream of the maximum temperature position has however been overestimated due to the underestimate of the mixing. The present PDF method has reproduced the extinction and reignition phenomena, which cannot be easily predicted by the conventional flamelet calculation. The variation in the diffusion flame structure due to the flame stretch, which was experimentally confirmed by Barlow et al., has been also reproduced. Moreover, the PDF calculation has revealed that intermediate radical species are created largely under the flame stretch effect.

Experimental investigation of scalar-scalar-dissipation filtered joint density function and its transport equation

The filtered joint density function (FJDF) of a conserved scalar and its dissipation rate and the FJDF transport equation are studied experimentally in the fully developed region of a turbulent jet with a jet Reynolds number Rej=UjDj/ν of 40 000. Filtered variables are obtained by a two-dimensional box filter having a size of 90 scalar dissipation scales, realized by using three discrete sensors aligned in the cross-stream direction and by invoking Taylor’s hypothesis. One-dimensional filters are also used to examine the effects of filter scales. The results show that for small and large subgrid-scale (SGS) scalar variance the FJDF and its dynamics have qualitatively different characteristics. The conditional mean FJDF for small SGS variance is generally unimodal and the dependence of the dissipation on the SGS scalar is moderate. The dissipation rate is diffused toward its conditional mean and its production rate is generally smaller than its destruction rate. These characteristics are similar to those of scalar-scalar-dissipation joint probability density functions in a fully developed turbulent flow. On the other hand, the conditional FJDF for large SGS variance is bimodal and the dissipation shows strong dependence on the SGS scalar. Diffusion of the dissipation toward values much larger than its conditional mean is observed. The production of the dissipation generally exceeds its destruction, indicating nonequilibrium scalar dissipation. These characteristics are found to be associated with diffusion-layer-like structure in the SGS scalar for large SGS variance. In addition to the SGS scalar variance, the time scale of the SGS scalar also affects the FJDF. A current assumed FJDF model is also examined. The results provide new fundamental understanding of the structure of the small-scale scalar, which is important for improving large eddy simulation of nonpremixed turbulent combustion as well as for studying local extinction and reignition phenomena.

Reignition phenomenon of high-speed hydrogen jet diffusion flame

An experimental study deals with a high-speed hydrogen jet diffusion flame ejected vertically upward from a straight circular nozzle. Consideration is given to the reignition phenomenon that occurs after blow-off of the main flame. The Schlieren technique and image-processing method with the aid of the high-speed video camera are employed to visualize the flame shape, particularly the flame base near the nozzle tip and to investigate the time history of the flame morphology. It is found that: (i) the flame reignition phenomenon of hydrogen jet diffusion flames appears only in a certain region of mass flow rates; (ii) the small-sized flame-let remains in the vicinity of the nozzle rim at the mass flow rates that the reignition occurs; (iii) a further increase in mass flow rates makes the flame-let extinguish and no reignition occurs; (iv) the time interval of flame reignition extends with an increase in mass flow rates; and (v) the flow rates of the onset and end of the reignition and the existence of flame-let formed near the nozzle rim are affected by the rim thickness of the fuel nozzle. Copyright © 2002 John Wiley & Sons, Ltd.

Flame Reignition Phenomenon of Hydrogen Jet Diffusion Flames.

The flame reignition phenomenon that occurs after blowout of main flame is studied by using hydrogen fuel ejected vertically upward from a straight circular nozzle. Schlieren technique is employed to visualize the hydrogen flame, with focus on the region near the nozzle exit. It is found that : (1) The flame reignition phenomenon of hydrogen jet diffusion flames occurs only in certain nozzle diameters and flow rates ; (2) The small-sized flamelet (2∼3 mm) remains on the nozzle rim at the mass flow rates in which the reignition occurs. A further increase in mass flow rates causes the flamelet to extinguish and no reignition appear any more ; (3) The interval of name reignition increases with an increase in mass flow rates ; (4) Rim thickness does not affect the flow rates of reignition start and reignition finish. However, it influences the existence of flamelet formed above the nozzle rim.

Reignition phenomena after high-frequency current interruption with short vacuum gaps

Based on a large number of measurements of high-frequency (HF) current interruptions in vacuum at small contact gaps (/spl les/600 /spl mu/m), 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.

Simulations of turbulent methane jet diffusion flames with local extinction effects

Two-dimensional large eddy simulations of the development of piloted turbulent non-premixed jet flames of methane and air exhibiting significant finite-rate chemistry effects are presented. A partial equilibrium/two-scalar exponential (PDF) combustion submodel is applied at the subgrid scale level. Subgrid scale motions are modelled with a first-order closure employing a prognostic equation for the SGS energy. Statistical independence of the joint PDF scalars is relaxed and the necessary moments are obtained from their full transport equations under simplifying assumptions. Extinction is accounted for by comparing the local turbulent Damkohler number against the ratio of the Gibson scalar scale to the reaction zone thickness in mixture fraction space. The post-extinction regime is modelled via a Lagrangian formulation employing the solution of a transport equation for a mixing progress variable that follows a linear deterministic relaxation to its mean value (IEM). Comparisons between simulations and measurements suggested the ability of the method to calculate adequately the partial extinction and re-ignition phenomena observed in the experiments.

A lagrangian model for predicting turbulent diffusion flames with chemical kinetic effects

The present study is a new step in the development of a lagrangian turbulent combustion model devotedto the prediction of non-premixed flames in which kinetic effects occur. Taking into account, in a refined manner, the interaction between chemical reaction and turbulence, the model includes a spectrum of turbulent timescales instead of a single mean timescale. The more recent improvement of the model consists in including a library of chemical delay times derived from a six-step reduced reaction mechanism for methane-air combustion, which is valid over a wide range of equivalence ratio. The predictions of the combustion model in which the library was implemented were compared with experimental data on turbulent jet diffusion flames near extinction. These calculations were performed with a finite volume technique devoted to variable density Navier-Stokes equations. A standard two-equations turbulence model was used and provided reliable results for dynamic variables (mean velocity and kinetic energy of turbulence). Comparisons between computed and measured mean mixture fraction and mixture fraction root mean square (rms) are quite satisfactory, proving that scalar transport is correctly modeled. The more salient point is the prediction of partial extinction and reignition phenomena, in agreement with experiments. This result clearly demonstrates the ability of the turbulent combustion model to simulate the interaction between turbulence and chemical reactions, which occurs in flames displaying kinetic effects.

Investigations of multiple reignition phenomena and protection scheme of shunt reactor current interruption in GIS substations

Field measurement and computer analysis concerning 150 MVA shunt reactor current interruptions at a 275 kV gas insulated substation (GIS) were carried out to verify that high-frequency current interruptions can be caused by certain system structures on the circuit breaker power supply side. A shunt reactor current-interrupting test was conducted with phase angle control, which is considered to be the best method for preventing multiple reignitions, using short-circuit test facilities. The possibility of stable reignition-free interruption was confirmed, even when the longest arcing time is preset. >

Analysis of Re-Ignition of Multiple Break Switches Devices Application to Shunt Reactor Current Interruption

The interruption of shunt reactor current has been studied on a Hydro-Quebec substation. The study describes the test results based on particular characteristics of the circuit breaker: the influence of the auxiliary chamber synchronism and the influence of the opening resistor values of the breaker are first recorded. Their theoretical models are then developed to appraise the transient voltage across the breaker terminals and the overvoltages phenomena experienced. The reignition phenomena is then modeled according to the CIGRE Working Group 13-02 Recommendatatives and a good correlation between the test results and the models output are confirmed.

A study on discharge lamp current waveforms in constant wattage ballast systems.

This paper is concerned with the experimental investigation of the effects of the performance characteristics of the choke type constant wattage ballast on the lamp current waveforms or the values of the lamp current crest factor. The re-ignition phenomenon of the discharge lamp under AC operation is represented by an incomplete switch which consistes of a switch and an effective resistances in both the abnormal glow and the arc discharge regions. The high peaky lamp current is attributed to the transient phenomenon of the lamp operating circuit when the discharge lamp re-ignites. The effects of several ballast circuit elements, such as the magnitudes of both the capacitance and the series linear reactor, the magnetic characteristic and the geometry of the saturable auto-transformer, on the lamp current crest factor are experimentally investigated.

方形波パルスによる交流ＴＩＧ溶接アークの再点弧現象 ＩＩ

In our previous paper, we described some of reignition phenomena in which a rectangular current pulse having variable width 1-500 microsec was impressed to the gap for restriking at every reverse polarity, using alminum, mild steel and copper as a base metal.It is supposed that the reignition process with a rectangular current pulse depends only on the thermal energy due mainly to a pulsed arc discharge. Then we have defined those thermal energy as the Reignition Energy, and calculated these values in the critical conditions that the arc reignition was caused by minimum pulse voltage, pulse width and pulse current. The calculated results are compared with them of ordinary method in which the higher frequencies are superimposed.

Refractory electrode region recovery following arc interruption

Measurements are presented of the voltage fall across the electrode region of low-current carbon arcs when test current pulses are injected through the arc gap at some time (less than 100 µs) following rapid removal of the arc current. The results thus shed some light upon the thermal reignition phenomena of refractory electrodes. A model is proposed to describe the observed behavior at low injection current levels. This model may also be of assistance in describing the behavior of hot probes in high-pressure plasmas. Although the model is not extended to high current levels, a discussion is presented of the electrode region behavior for currents at which mechanisms apparently form, which are necessary for reignition and steady-state conduction through the electrode regions.

345-kV Switching Surge Tests Exploring Dynamic Voltage and Arrester Performance

This paper presents the results of switching surge tests on a 119-mile 345-kV circuit. For various methods of terminal switching, probability analysis is made of the magnitude and frequency of occurrence of transient and dynamic voltages when energizing the circuit through an autotransformer from systems with different soure impedances. Test results are compared with results of a model study on the same circuit. The good performance of current-limiting gap arresters is demonstrated in oscillographs showing arrester sparkover levels, discharge currents, reseal times, and reignition phenomena. The duration of dynamic voltages and the exposure of arresters to dynamic voltages above arrester rating are demonstrated. Data are presented on the transient performance and arcing period of 345-kV double-break disconnects when switching magnetizing currents of 200- and 400-MVA autotransformers.

ホットスタートに関する研究(第1報)

Basic experiments are carried out on the arc striking, arc reignition and flux melting phenomena of the low-hydrogen type electrodes. It has been confirmed that two defects of this kind electrodes, i. e., difficulty in arc striking end generation of blow holes at the start of weld, are eliminated by hot-start, an extra boost of welding current at the arc starting. A hot-start circuit without relays is achieved by using saturable reactors. Some experiments on blow holes are also given.

The recovery of electric strength of an arc-discharge column following rapid interruption of the current

A new technique for investigating the recovery period is described. A rectangular pulse of arc current is used and a probing voltage impulse is applied to the decaying arc channel at varied delay times after current interruption. The method has certain advantages over the normal a.c. method of testing.Reignition-voltage/time curves are presented for different conditions of electrode material, gas, gap length, arc current, arc duration and reignition polarity, with gas pressures at or near atmospheric. Observations are also made on the dynamic characteristics of the electrical discharge following reignition.Polished-copper and polished-tungsten electrodes require a reignition voltage of 355 volts within 3 microsec after current interruption for arc currents of 20 amp and less. Pipped-copper, pipped-tungsten, mild-steel, aluminium and graphite electrodes show continuing emission of electrons after current interruption to a varying degree, with corresponding reduction of the reignition voltage.The reignition phenomena are markedly affected by the polarity of the reignition. The arc anode normally attains a higher temperature than the cathode, so that continuing thermionic emission of electrons is more prevalent. Thus reignitions of reverse polarity from the original arc discharge tend to be lower than those of the same polarity.Different gases recover electric strength in the order: sulphur hexafluoride, carbon dioxide, oxygen, air, nitrogen and argon.