Hot Kernel Research Articles

Dynamics of flame rings in a thermally-conductive narrow channel: a numerical experiment

A numerical experiment is conducted to examine the ignition of a near-flammability-limit low-Lewis-number mixture and the evolution processes of the resulting premixed flames in a circular thermally-conductive narrow channel. A diffusive-thermal model is adopted to describe the fuel mass conservation in the gas phase and energy conservation in both the gas and the channel wall. Spalding's ‘one-dimensional idealization’ approximation is introduced to simplify all these conservation equations to a two-dimensional form over the plane parallel to the wall surface of the channel. As a result, the half channel height h constitutes the primary parameter controlling the heat exchange rate between the gas and the wall, which serves as a conductive heat loss mechanism from the perspective of the gas phase. For relatively large h, following ignition at the centre of the channel by a hot ignition kernel, the resulting flame front suffers diffusive-thermal instability and quickly breaks into a number of discrete reaction cells, which propagate forward towards the boundary of the channel. When h becomes sufficiently small, the reaction cells close upon themselves and transform to ring-shaped flamelets, herein termed flame rings, a 2-D analogue of flame balls formed in 3-D unconfined space. The effects of the half channel height h on the formation and dynamics of the flame rings are examined, together with a demonstration of a possible means to manipulate the flame ring dynamics by exploiting the 2-D character of the narrow channel configuration. It is suggested that, as a simple ground-based buoyancy-suppression strategy, the presently considered narrow channel configuration can be conveniently employed to study the properties related to the flammability limit of combustible mixtures.

Vortex rings drive entrainment and cooling in flow induced by a spark discharge

Spark plasma discharges induce vortex rings and a hot gas kernel. We develop a model to describe the late stage of the spark induced flow and the role of the vortex rings in the entrainment of cold ambient gas and the cooling of the hot gas kernel. The model is tested in a plasma-induced flow, using density and velocity measurements obtained from simultaneous stereoscopic particle image velocimetry (S-PIV) and background oriented schlieren (BOS). We show that the spatial distribution of the hot kernel follows the motion of the vortex rings, whose radial expansion increases with the electrical energy deposited during the spark discharge. The vortex ring cooling model establishes that entrainment in the convective cooling regime is induced by the vortex rings and governs the cooling of the hot gas kernel, and the rate of cooling increases with the electrical energy deposited during the spark discharge.

Numerical and Experimental Investigation of the Channel Expansion of a Low-Energy Spark in the Air

ABSTRACT Numerical simulations were carried out in order to study the channel expansion of a low-energy spark in the air after the spark breakdown that resulted in the formation of initial arc channel. The spark-discharge model considers electric circuit parameters, gas-dynamic expansion processes, thermal gas ionization, non-equilibrium chemical reactions, electron heat transfer, and radiation. A specific feature of the model is its ability to simulate a spark channel expansion in the air when only electric circuit parameters (R, L, C) and the discharge gap length are given. The results of the numerical investigation were compared with experimental data based on schlieren images. In this article, the hot gas kernel evolution and plasma state inside the conducting channel are discussed. The efficiency and the rate of energy entering the spark are studied along with the distribution of thermodynamic gas parameters during the spark’s evolution. An extremely high efficiency of the low-energy electrical discharge and a high resistance of the discharge channel compared to other types of discharges were confirmed.

High-Speed Imaging of Forced Ignition Kernels in Nonuniform Jet Fuel/Air Mixtures

This paper describes experimental measurements of forced ignition of prevaporized liquid fuels in a well-controlled facility that incorporates nonuniform flow conditions similar to those of gas turbine engine combustors. The goal here is to elucidate the processes by which the initially unfueled kernel evolves into a self-sustained flame. Three fuels are examined: a conventional Jet-A and two synthesized fuels that are used to explore fuel composition effects. A commercial, high-energy recessed cavity discharge igniter located at the test section wall ejects kernels at 15 Hz into a preheated, striated crossflow. Next to the igniter wall is an unfueled air flow; above this is a premixed, prevaporized, fuel–air flow, with a matched velocity and an equivalence ratio near 0.75. The fuels are prevaporized in order to isolate chemical effects. Differences in early ignition kernel development are explored using three synchronized, high-speed imaging diagnostics: schlieren, emission/chemiluminescence, and OH planar laser-induced fluorescence (PLIF). The schlieren images reveal rapid entrainment of crossflow fluid into the kernel. The PLIF and emission images suggest chemical reactions between the hot kernel and the entrained fuel–air mixture start within tens of microseconds after the kernel begins entraining fuel, with some heat release possibly occurring. Initially, dilution cooling of the kernel appears to outweigh whatever heat release occurs; so whether the kernel leads to successful ignition or not, the reaction rate and the spatial extent of the reacting region decrease significantly with time. During a successful ignition event, small regions of the reacting kernel survive this dilution and are able to transition into a self-sustained flame after ∼1–2 ms. The low-aromatic/low-cetane-number fuel, which also has the lowest ignition probability, takes much longer for the reaction zone to grow after the initial decay. The high-aromatic, more easily ignited fuel, shows the largest reaction region at early times.

A thermodynamic model for the plasma kernel volume and temperature resulting from spark discharge at high pressures

In today’s internal combustion engines assisted ignition is predominantly achieved by conventional spark ignition. The spark ignition starts with the breakdown phase after which complex flow dynamics govern the energy deposition into the gas. The condition of the activated plasma kernel resulting from spark breakdown, among others, is important for the development of the flame kernel to a self-sustained flame. A simple thermodynamic model is formulated based on insight gained from available detailed plasma simulations and experiments. Under the assumption that the problem is cylinder symmetrical and that the breakdown energy is supplied quasi-instantaneously as a line source, the extent of the hot plasma kernel and the time for pressure equalization with the surrounding can be given by exploiting the properties of radially expanding blast waves. The so-defined control volume is treated as an open system, and a simple energy balance leads to a relation for the mean kernel temperature. The plasma volumes and mean temperatures predicted by this simple model are shown to correspond well with published results in the open literature based on full plasma simulations of the spark discharge available for pressures ranging from 1 to 8 bar. The model allows drawing some conclusions about the requirements for experimental methods, such as calorimetry, frequently used to characterize the efficacy of the spark discharge produced by ignition systems and the influence of pressure and breakdown energy on kernel volume and temperature.

Laminar syngas–air premixed flames in a closed rectangular domain: DNS of flame propagation and flame/wall interactions

The propagation of a lean laminar premixed syngas–air flame is investigated numerically in a confined rectangular domain with isothermal walls at a temperature that is lower than that of the unburned mixture. Initiated by a hot kernel, the flame propagates towards the cold walls, setting in motion the initially quiescent mixture and compressing the gases, so that propagation proceeds under varying flow and thermodynamic conditions. The complex flow field changes structure and its effect on the local propagation characteristics is quantified by following the local displacement speed together with the local flow velocity and stretch rate. The flame first quenches head-on at the horizontal walls and then propagates towards the lateral walls affected by side-wall quenching. During the final stage, thermodiffusive instabilities are triggered along the front whose thickness has become half of the initial mainly because of the increased pressure. The temporal evolution of the quenching distances, the wall heat flux distribution and the heat transfer to the cold walls are quantified during the whole process. Except for the stages of the initial flame kernel growth and the final consumption of the fuel in the near-wall region, the fuel is consumed at an almost constant rate.

Origin and reactivity of hot-spots in end-gas autoignition with effects of negative temperature coefficients: Relevance to pressure wave developments

The present study deals with the mechanisms for the hot-spot formation and pressure wave development associated with end-gas autoignition during knocking combustion of n-heptane/air mixture. The discussion is based on a one-dimensional (1-D) direct numerical simulation, where the compressible Navier–Stokes equations are solved with a detailed chemical kinetic mechanism of n-heptane, involving 373 species and 1071 reactions. The result demonstrates that the first trigger for a hot-spot formation is a compression wave generated by forced autoignition of a hot kernel and its reflection at a wall. The wall reflection of the propagating compression wave periodically produces an instantaneous temperature increase, which leads to the production of a larger amount of chemical species compared to that of other end-gas points. This non-uniform progress of chemical reaction process continues to exist at the wall, although the temperature increase is transient, resulting in faster autoignition and pressure wave generation at the wall. Thus, an important aspect of observing chemistry behaviors rather than temperature is demonstrated on the mechanism of hot-spot formation. The present study further addresses the reactivity of hot-spots on the relevance to pressure wave developments, wherein a zero-dimensional (0-D) ignition problem with pulsed compression waves is introduced. The higher reactivity of n-heptane/air mixture against pulse waves is observed with the faster ignition delay times in lower and higher initial temperature conditions. Conversely, the result at initial temperatures of 750—800 K indicates the lower reactivity with no significant effects of pulse waves on the ignition delay times. This is connected with the fuel characteristics of a negative temperature coefficient. Thus, in the 1-D simulations, a hot-spot with the high reactivity enhances spatial temperature difference in the end-gas region, leading to strong pressure wave generations. In contrast, a hot-spot with the low reactivity suppresses the pressure wave development with little spatial variation in temperature. The result demonstrates a significant aspect of hot-spot formation and reactivity on pressure wave development during knocking combustion.

A 3-D DNS and experimental study of the effect of the recirculating flow pattern inside a reactive kernel produced by nanosecond plasma discharges in a methane-air mixture

This paper presents 3-D DNS and experimental schlieren results used to study a non-equilibrium plasma discharge in a lean methane-air mixture. A detailed combustion mechanism and a plasma model, developed in our previous work, are used in the 3-D computations in order to study the impact of gas flow recirculation on the temporal evolution of species and gas temperature in the vicinity of the discharge zone. The results show that the formation of a fresh gas counterflow, with stagnation plane at the centre of the discharge and parallel to the electrodes, changes the topology of the hot kernel from an initial cylindrical shape to a toroidal one. This phenomenon leads to an increase of the area/volume ratio of the reactive kernel, that may result under certain conditions in kernel extinction. The results also show the importance of considering this 3-D gas flow recirculation to correctly predict the temporal evolution of the hot kernel. In particular, we show that the temperature and species concentrations in the central region of the discharge return to fresh gas conditions shortly after the end of the pulse. In the experimental case investigated here, this time is of the order of 150 µs. This result is particularly important for ignition by Nanosecond Repetitively Pulsed (NRP) discharges, because the gas conditions at the beginning of each successive pulse depend strongly on the time interval between pulses, thus on the pulse frequency

Least squares weighted twin support vector machines with local information

A least squares version of the recently proposed weighted twin support vector machine with local information (WLTSVM) for binary classification is formulated. This formulation leads to an extremely simple and fast algorithm, called least squares weighted twin support vector machine with local information (LSWLTSVM), for generating binary classifiers based on two non-parallel hyperplanes. Two modified primal problems of WLTSVM are attempted to solve, instead of two dual problems usually solved. The solution of the two modified problems reduces to solving just two systems of linear equations as opposed to solving two quadratic programming problems along with two systems of linear equations in WLTSVM. Moreover, two extra modifications were proposed in LSWLTSVM to improve the generalization capability. One is that a hot kernel function, not the simple-minded definition in WLTSVM, is used to define the weight matrix of adjacency graph, which ensures that the underlying similarity information between any pair of data points in the same class can be fully reflected. The other is that the weight for each point in the contrary class is considered in constructing equality constraints, which makes LSWLTSVM less sensitive to noise points than WLTSVM. Experimental results indicate that LSWLTSVM has comparable classification accuracy to that of WLTSVM but with remarkably less computational time.

Investigation of the effect of electrode geometry on spark ignition

High-speed schlieren visualization and numerical simulations are used to study the fluid mechanics following a spark discharge and the effect on the ignition process in a hydrogen–air mixture. A two-dimensional axisymmetric model of spark discharge in air and spark ignition was developed using the non-reactive and reactive Navier–Stokes equations including mass and heat diffusion. The numerical method employs structured adaptive mesh refinement software to produce highly-resolved simulations, which is critical for accurate resolution of all the physical scales of the complex fluid mechanics and chemistry. The simulations were performed with three different electrode geometries to investigate the effect of the geometry on the fluid mechanics of the evolving spark kernel and on flame formation. The computational results were compared with high-speed schlieren visualization of spark and ignition kernels. It was shown that the spark channel emits a blast wave that is spherical near the electrode surfaces and cylindrical near the center of the spark gap, and thus is highly influenced by the electrode geometry. The ensuing competition between spherical and cylindrical expansion in the spark gap and the boundary layer on the electrode surface both generate vorticity, resulting in the toroidal shape of the hot gas kernel and enhanced mixing. The temperature and rate of cooling of the hot kernel and mixing region are significantly effected by the electrode geometry and will have a critical impact on ignition. In the flanged electrode configuration the viscous effects generate a multidimensional flow field and lead to a curved flame front, a result not seen in previous work. Also, the high level of confinement by the flanges results in higher gas temperatures, suggesting that a lower ignition energy would be required. The results of this work provide new insights on the roles of the various physical phenomena in spark kernel formation and ignition, in particular the important effects of viscosity, pressure gradients, electrode geometry, and hot gas confinement.

Experimental analysis of laser-induced spark ignition of lean turbulent premixed flames: New insight into ignition transition

Recently, Shy and his co-workers reported a turbulent ignition transition based on measurements of minimum ignition energies (MIE) of lean premixed turbulent methane combustion for a wide range of equivalence ratios. Our study used a similar approach to present an additional and complete MIE data set for laser-induced spark ignitions in a flow configuration and under turbulent conditions (i.e., for different ranges of length and time scales) that differed from those chosen by Shy and his colleagues. To extend the previous analyses to very lean premixed methane/air flames, the study examined a characteristic chemical time (τCB), which was defined as the start of the chain-branching reactions. Physically, this chemical time corresponded to the time when the initial chemical reactions of the ignition process released enough heat to compensate for heat losses and to enable a self-sustained reaction in successful ignition. Values for τCB, which were experimentally obtained from the temporal evolution of the mean hot kernel emissions (in laminar flow), strongly decreased with the equivalence ratio Φ for very lean flames and remained nearly constant and equal to 150μs for Φ>0.7. We obtained a clear turbulent ignition transition on the MIE as a function of the rms velocity (u′) and reconfirmed the two modes of ignition. Temporal kernel emission recordings served to describe the transition through the study of the interaction between the turbulence and the hot kernel at the time t=τCB. For low turbulence, when the smallest time scales of the turbulence τK were larger than the τCB values, the hot kernel/turbulence interaction occurred after the initiation of the chemical reactions. The amount of deposited energy required to initiate the reactions and to attain a self-sustained flame kernel was consequently similar to that found under laminar conditions. After the transition, the smallest time scales of the turbulence were smaller than the τCB values. Turbulence may have affected and interacted with the hot kernel before the initiation of chain-branching reactions occurred. Larger amounts of deposited energy were therefore required to compensate for this turbulent dissipation and to attain a self-sustained flame. A Peclet number (PeCB) was introduced, which is equal to the ratio of the turbulent diffusivity at the time of the initiation of the chemical reactions and the thermal diffusivity. These very scattering MIE data (dependent on Φ and u′) are plotted as a function of PeCB and collapsed into a single curve with two drastically different increasing slopes showing the ignition transition.

Spatiotemporal temperature and density characterization of high-power atmospheric flashover discharges over inert poly(methyl methacrylate) and energetic pentaerythritol tetranitrate dielectric surfaces

A flashover arc source that delivered up to 200 mJ on the 100s-of-ns time-scale to the arc and a user-selected dielectric surface was characterized for studying high-explosive kinetics under plasma conditions. The flashover was driven over thin pentaerythritol tetranitrate (PETN) and poly(methyl methacrylate) (PMMA) dielectric films and the resultant plasma was characterized in detail. Time- and space-resolved temperatures and electron densities of the plasma were obtained using atomic emission spectroscopy. The hydrodynamics of the plasma was captured through fast, visible imaging. Fourier transform infrared spectroscopy (FTIR) was used to characterize the films pre- and post-shot for any chemical alterations. Time-resolved infrared spectroscopy (TRIR) provided PETN depletion data during the plasma discharge. For both types of films, temperatures of 1.6–1.7 eV and electron densities of ∼7–8 × 1017/cm3 ∼570 ns after the start of the discharge were observed with temperatures of 0.6–0.7 eV persisting out to 15 μs. At 1.2 μs, spatial characterization showed flat temperature and density profiles of 1.1–1.3 eV and 2–2.8 × 1017/cm3 for PETN and PMMA films, respectively. Images of the plasma showed an expanding hot kernel starting from radii of ∼0.2 mm at ∼50 ns and reaching ∼1.1 mm at ∼600 ns. The thin films ablated or reacted several hundred nm of material in response to the discharge. First TRIR data showing the in situ reaction or depletion of PETN in response to the flashover arc were successfully obtained, and a 2-μs, 1/e decay constant was measured. Preliminary 1 D simulations compared reasonably well with the experimentally determined plasma radii and temperatures. These results complete the first steps to resolving arc-driven PETN reaction pathways and their associated kinetic rates using in situ spectroscopy techniques.

High Temporally Resolved Optical Measurement for Laser Ignition Process of Laminar Premixed Mixtures

This paper describes laser ignition process in a laminar premixed mixture of propane and air. The plasma was generated by focusing a second harmonic and several-nanosecond pulse emitted from a Q-switched Nd : YAG laser. Minimum ignition energy (MIE) was measured in different equivalence ratio. The growth process in the plasma and the hot kernel (flame kernel) around the MIE were observed using high-speed schlieren photography and a multi-fiber coupled to an ICCD spectrometer. The kernel size, the kernel expansion velocity and the emission spectrum history form the plasma and the kernel were determined under the ignition case and misfire case. Further, temperature of the plasma and the kernel were determined from the CN molecular spectra by comparing experimental and simulated (LIFBASE) spectra. The main results of this study are as follows : (1) In laser ignition method, combustion reaction begins through the cooling process in which the ion recombines to the atom, furthermore the atom recombines to the molecular after breakdown. (2) By analyzing the CH emission intensity history, it was considered that combustion reaction occurs confirmed about 100 μs after laser pulse. (3) The temperature of the plasma and the kernel decrease with time and there were almost no difference between fire and misfire. (4) The minimum ignition energy is the smallest near the stoichiometric mixture.

Solar spectral fine structure in 18-23 GHz band

On 30th June 1989 high sensitivity-spectral resolution observations of solar radio bursts were carried out in the frequency range of 18 - 23 GHz. The burst observed at 17:46 UT was different from the 60 bursts observed so far in the sense that it exhibited a frequency fine structure superimposed on the ongoing burst in its rising phase, i.e. an additional enhancement of the flux density of the order of 10 SFU, observed only in the 21 and 22 GHz frequency channels, lasting for about 4 s. Interaction of an emerging loop with an adjacent loop accelerated particles in that loop from which the broadband burst was emitted due to the gyrosynchrotron emission. The observed fine structure is interpreted as due to thermal gyro-emission at 6th harmonic of the gyrofrequency originated from a hot kernel with short lifetime located at the top of emerging loop. We derived the hot kernel source parameters, th e temperature as 8 ´107 K, the magnetic field as 1250 G and the density as 5 ´1012 cm-3.

Cyclotron lines in the spectra of solar flares and solar active regions

It was shown by Zheleznyakov and Zlotnik (1980a, b) that in complex configurations of solar magnetic fields (in hot loops above the active centres, in neutral current sheets in the preflare phase, in hot X-ray kernels in the initial flare phase) a system of cyclotron lines in the spectrum of microwave radiation is likely to be formed. Such a line was obtained by Willson (1985) in the VLA observations at harmonics of the electron gyrofrequency. This communication interprets these observations on the basis of an active region model in which thermal cyclotron radiation is produced by hot plasma filling the magnetic tube in the corona above a group of spots. In this model the frequency of the recorded 1658 MHz line corresponds to the third harmonic of electron gyrofrequency, which yields the magnetic field (196 ± 4) G along the magnetic tube axis. The linewidth Δf/f ∼ 0.1 is determined by the 10% inhomogeneity of the magnetic field over the cross-section of the tube; the line profile indicates the kinetic temperature distribution of electrons over the tube cross-section with the maximum value 4 × 106 K. Analysis shows that study of cyclotron lines can serve as an efficient tool for diagnostics of magnetic fields and plasma in the solar active regions and flares.

Cyclotron Lines in the Spectra of Solar Flares and Solar Active Regions

AbstractIt was shown by Zheleznyakov and Zlotnik (1980a, b) that in complex configurations of solar magnetic fields (in hot loops above the active centres, in neutral current sheets in the preflare phase, in hot X-ray kernels in the initial flare phase) a system of cyclotron lines in the spectrum of microwave radiation is likely to be formed. Such a line was obtained by Willson (1985) in the VLA observations at harmonics of the electron gyrofrequency. This communication interprets these observations on the basis of an active region model in which thermal cyclotron radiation is produced by hot plasma filling the magnetic tube in the corona above a group of spots. In this model the frequency of the recorded 1658 MHz line corresponds to the third harmonic of electron gyrofrequency, which yields the magnetic field (196 + 4) G along the magnetic tube axis. The linewidth Af/f ∼ 0.1 is determined by the 10% inhomogeneity of the magnetic field over the cross-section of the tube; the line profile indicates the kinetic temperature distribution of electrons over the tube cross-section with the maximum value 4 x 106 K. Analysis shows that study of cyclotron lines can serve as an efficient tool for diagnostics of magnetic fields and plasma in the solar active regions and flares.

Mechanism of flame kernel growth and effect of spark electrode shapes in ignition process by short duration sparks.

The formation and growth process of flame kernels in the ignition process by short-duration electric sparks has been investigated by using a numerical simulation and schlieren photographic observations. In the simulation, unsteady two-dimensional cylindrical coordinate system was employed and the governing partial differential equations, such as mass, momentum and energy equations were solved by the finite differential method. The calculated results show that the ellipsoidal hot gas kernel produced by a spark discharge develops into a torus and that there are two high-temperature regions, namely within the rings of torus and just between the spark electrodes. It is also found that a narrow groove is formed inside the torus. These calculated features of the hot kernel structures agree with those in schlieren photographs taken by the high speed camera. Effects of diameter of spark electrodes and shaped of electrode tips on the formation process of flame kernels are also studied.

Mechanism of flame kernel formation produced by short duration sparks

With reference to flame kernel developments in spark ignition of combustible mixtures, the formation process of hot kernels produced by short duration sparks (less than 0.3 μs) in an inert gas is modeled by using a set of partial differential equations with unsteady and two-dimensional cyclindrical coordinates. These equations are solved numerically. The results show that the gas flow pattern governs the kernel configuration and that the kernel shape has some characterized features such as a torus, a torus with a groove and a nontoroidal shape. The simulated results are successfully verified by optical measurements.

Numerical simulation on formation process of flame kernels.

The formation process of flame kernels in the ignition process by electric sparks has been investigated by using a numerical simulation, which employs unsteady two-dimensional cylindrical co-ordinate systems. In the simulation, effects of the value of the diameter of spark electrodes and the width of the spark gap on the calculated results are mainly examined. The calculated results show that the hot gas kernel produced by a spark discharge takes an ellipsoidal form at first and then develops into a torus. Under some conditions, there exist two high-temperature regions, namely within the rings of torus and just between the spark electrodes. It is also found that a narrow groove is formed inside the torus. These configurations of the flame kernels agree well with those found by schlieren photographs of the practical flame kernels which are taken by a flash-schlieren photographic technic.

Flame initiation in lean, quiescent and turbulent mixtures with various igniters

High speed flame photographs have been taken, following ignition, in an explosion bomb, equipped with four high speed fans to create turbulence. The different igniters studied, under quiescent and turbulent conditions of the the lean methane-air mixture, comprised o (i) a conventional two electrode spark system (ii) a surface discarge spark (iii) a plasma jet (iv) a plasma jet with wetted cavity (v) a torch from a pre-chamber Hot kernel volume, flame front wrinkling and flame speed were measured during the early stages of flame development. Systems (ii) to (v) were potentially superior to (i) in their ability to create a larger igniting volume of hot, reactive gas. The flame speeds for systems (iii) to (v) were enhanced by jet-generated turbulence and, additionally, by active species in the case of the plasma jet. In turbulent mixtures, after a few milliseconds the flame speed is predominantly a function of the turbulence. Ignition limits have been measured over a range of r.m.s. turbulent velocities and different igniters compared. With limit mixtures, the ratio of chemical to eddy lifetime is a useful correlating parameter. Ignition limits for the different igniters fall within an upper bound for flame propagation defined by a fixed value of this ratio.