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Effect of electric fields on the burning velocity of various flames. [Mixture of CHâ and CâHâ]
... Experimental investigations have shown that electrostatic fields could affect the macroscopic behaviour of the flame, with an impact on the flame shape, stability and also emissions [5,6]. Studies on hydrogen flames with oxygen and air as oxidizers have also shown that, depending on the equivalence ratio, electric fields with various strengths have an effect on flame speed and flame shape [7,8]. ...
... In other studies, where hydrogen blends were investigated, it was shown that the addition of hydrogen to hydrocarbons could change the flame response to the action of external electrostatic fields [9]. The effects of electrostatic fields were justified as the result of the competing effect between ionic wind and flow velocity, while other studies on electrostatics effects on hydrocarbon combustion also pointed out the key role of electron dynamics on the reaction process [5]. Although some progress in the understanding of electrostatic actions on hydrogen and hydrocarbon combustion has been made, there are still open questions on whether the electrostatic fields could affect the chemistry itself. ...
... Electrons move and are shared or transferred between atoms to achieve equilibrium and more stable states. The study of redistribution and motion of sub-atomic particles (specifically electrons) is fundamental for a detailed understanding of chemical reactions under external electrostatic fields [5]. An effort to include the explicit treatment of electrons in reactive MD is represented by the development of eReaxFF force field [16]. ...
The effect of external electrostatic fields on the reaction kinetics of hydrogen is investigated using reactive molecular dynamics simulations to give more insight into the use of electromagnetic interactions to control the reactivity of hydrogen systems. Combustion of hydrogen in both pure oxygen and air is studied. Furthermore, two different methods to compute the charge distribution, namely the Charge Equilibration (QEq) and Charge Transfer with Polarization Current Equalization (QTPIE) methods, which model the medium as an ideal conductor and a dielectric, respectively, are used to investigate the role of atomic charges on the reaction kinetics. Results show that the distribution of atomic charges is the major factor to determine the response of the system to external electrostatic fields and that close-range charge transfers are not sufficient to result in any significant change of reactivity for the range of electrostatic fields investigated in this work. Results also show that above a given threshold of the strength of the external electrostatic field, the presence of nitrogen in the systems facilitates a drop in hydrogen’s half-life compared to pure hydrogen-oxygen systems when charge transfers are not limited to close interactions. This threshold depends on ambient conditions and it is a result of the availability of charges that hydrogen and oxygen molecules could acquire. Further computations with electron force field (eFF) show that the electric field could alter the total energy and electro-static potential between the electron-nuclei pair, but the energy variations are at least an order of magnitude lower than the kinetic energy of the system. This suggests that electron dynamics may have a secondary role in the change of reaction kinetics.
... As an example of this debate, consider the experiments measuring the flame speed under the effect of an electric field. Among the data published, we can find slight flame deceleration [7,8], slight flame acceleration [9,10,11,12,13] and strong flame acceleration of up to 200 % [2,4,14,15]. Three different mechanisms have been postulated to explain the effect of the electric field on flames: ionic wind, kinetic enhancement by non-thermal electrons and ohmic heating. ...
... Three different mechanisms have been postulated to explain the effect of the electric field on flames: ionic wind, kinetic enhancement by non-thermal electrons and ohmic heating. Chemical effects via activation of reactions has been identified by [2,9] as the main mechanism for flame acceleration in planar flames while body forces (ionic wind) are, apparently, solely responsible for the observed response in diffusion and premixed flames in complex geometries [16,17,5,18]. The effect on the flame speed of the heating of the background gas as a consequence of the higher temperature of the non-thermal electrons (ohmic heating) has been reported as important, among others, in the microwave induced flame speed enhancement experiments carried out by [19,20,13,15] and in the computations by Bradley & Ibrahim [21]. ...
... As a consequence of this, the heat added in this region, associated mainly with the ohmic heating, reduces, limiting the formation of Z and Z + through the temperature-dependent reactions I and III. This result is also consistent with the results reported by Jaggers and Von Engel [2] and by Murphy et al. [4] for pre-mixed methane air-flames over a range of equivalence ratios. ...
We examine in this work the effect of an external electric field on the propagation velocity of a laminar, one-dimensional and lean premixed flame, with the final goal of clarifying the relative importance of each of the three different mechanisms postulated in the literature to explain the effect of electric fields on flames: ionic wind, kinetic enhancement by non-thermal electrons and ohmic heating. The one-dimensional model proposed here expands the four-reactions scheme previously presented by Sánchez-Sanz, et al. (2015) to include the effect of non-thermal electrons and activated neutral molecules on flame acceleration. Two additional reactions are included in the model to complete a minimum set of six elementary reaction capable of qualitatively reproduce the results observed in classical (Jaggers, and Von Engel, (1971).) and recent (Volkov et al., 2013; Murphy, et al., 2014,) experiments. The limit of weakly ionized plasmas is used to integrate the Boltzmann equation and to derive an explicit expression for the electron temperature proportional to the square of the electric field. The numerical integration of the conservation equations gives the flame propagation velocity for a given set of parameters. The results reveal the importance of the electric field polarity on flame acceleration, finding faster flames for positive electric fields than for equally intense negative fields. At low-intensity fields, our results indicate that the ionic wind, and the associated redistribution of the charged particles, is the main mechanism inducing flame acceleration. In more intense fields, the combined effect of the ionic wind and the heat transfer from the high-temperature electrons to the background gas induces a significant increase in the temperature field upstream and downstream of the flame front. Associated with this temperature increase, relevant changes on the flame speed are computed for positive, intense electric fields, while only modest flame accelerations are observed for equally intense negative fields, behavior that reproduces qualitatively the measurements by Murphy et al. (2014). The reduced sensitivity to an external electric field when the mixture approaches stoichiometry, observed experimentally by Jaggers, and Von Engel (1971) and Fang et al. (2015), is also reproduced by the model proposed in this work.
... These ions can be moved and excited when subjected to an external electric field [2]. The effects of this can be used to extinguish flames [3][4][5][6], to increase the flammability limits [5][6][7][8][9][10][11][12][13][14][15], to reduce the pollutants emitted [8,9,[16][17][18][19][20][21][22][23][24][25][26][27][28], effect the temperature (by entraining air) [9,17,29], modify the burning velocity [7,13,30], or to increase/decrease the heating to surfaces surrounding a flame. The effects have been well documented; however, the results are sometimes contradictory. ...
... These ions can be moved and excited when subjected to an external electric field [2]. The effects of this can be used to extinguish flames [3][4][5][6], to increase the flammability limits [5][6][7][8][9][10][11][12][13][14][15], to reduce the pollutants emitted [8,9,[16][17][18][19][20][21][22][23][24][25][26][27][28], effect the temperature (by entraining air) [9,17,29], modify the burning velocity [7,13,30], or to increase/decrease the heating to surfaces surrounding a flame. The effects have been well documented; however, the results are sometimes contradictory. ...
... For example, it is unclear whether an electric field can be used to change the burning velocity. Jaggers and Von Engel found that this was possible [7] but this is contradictory to other reports by Bowser and Weinberg [31] and also Jaggers et al. [32]. One of the reasons for this could be that the experimental set-up was not the same. ...
There has been extensive work to show how electric fields can influence combustion. However, many different set ups are used. This work shows how different set ups produce different field strengths and that the field is not always uniformly distributed. The field strength is modelled using Ansys Maxwell. The type of material used is discussed and the set up of apparatus. It is recommended to use parallel plates for experimentation. Parallel plates produce the most uniform field this allow's it's influence to be directly investigated and related to the field strength. © 2011 Timothy J. C. Dolmansley et al.
... The use of externally applied electric fields to modify combustion has been a topic of interest for more than a century, with the earliest experimental investigation on modifying flame geometry published at the turn of the nineteenth century [1]. Over the following decades, numerous experimental studies applied electric fields to modify flames, including expanding blow-off and extinction limits, enhancing flame speed and stability, and reducing soot formation [2,3,4,5,6,7]. Marcum, et al. [8] detailed the principal results from experimental studies through 2005. ...
... The one-dimensional premixed flame code in Cantera software [36] was modified for the current model and is used to solve Equations (1), (5), and (6) , while the BOLOS solver [37] is used for solving Boltzmann's equation. The solution strategy is to gradually build up the elements of the model over three stages to ensure convergence is achieved at each step. ...
This study models the effect of microwave electric field on a premixed flame to achieve an accurate prediction of flame speed enhancement by considering non-thermal electron for both kinetics and mass transport. The results compare well against experimental data, and show that using electron energy distribution function (EEDF) to calculate recombination rates of electron is the key to improve the prediction of the electron number density and the flame speed. The resulting technique also agrees well with the flame speed theory to explain the mechanism of flame speed increase by ohmic heating, which is by increasing the flame temperature. The model can also predict the efficiency of flame speed enhancement for a microwave electric field. Finally, the model can be improved by incorporating recombination cross sections of major ions, if they are available.
... While van den Boom et al. (2009) presented different results which is an 8% increase in flame speed using the similar flat flame burner experimental setup. Jaggers and Engel (1971) conducted experiment using a tube flame with transverse electric fields, which showed a 100% increase in flame speed. Duan et al. (2015) and Meng et al. (2012) showed 10% increase in flame speed using spherical propagation flame with the DC/AC electric field. ...
... In addition, each experiment is repeated at least three times to quantify the experimental uncertainty. Mass burning rate is calculated from pressure history with Elbe's equation (Jaggers and Engel, 1971). Flame initiation time is defined as the time between the moment of ignition and 10% mass burning rate, and it is used to evaluate the effect of electric field on the initial flame propagation process. ...
The effects of DC electric field on laminar premixed CH4/air spherical propagation flames with excess air ratio λ = 1.0, 1.2, 1.4 were investigated in a constant volume combustion chamber at elevated pressures up to 0.5 MPa. Mesh electrodes were used to generate electric field inside the chamber. The flame front structure, flame displacement speed, and pressure-related combustion parameters were derived to evaluate the effects of electric field on flame propagation. The results show that the mean flame displacement speed increases in the electric field direction with the increase of applied voltages and it is more significant at lean conditions. The mean flame displacement speed decreases in the direction perpendicular to the electric field with the voltage at low pressures, while it increases with voltage due to flame instability induced by the electric field at higher pressures. The effect of electric field on flame displacement speed is more obvious with pressure rise. Electric-induced flame instability combined with the hydrodynamic instability both promoted at high pressure lead to much more cracked structure and enhance the flame displacement speed. An obvious acceleration stage during the flame propagation under electric field is also observed. The combustion peak pressure slightly increases and the timing of peak pressure is in advance with the increasing of applied voltage. The flame initiation time derived from pressure decreases with voltage and it is more obvious at higher pressures. In this study, a new ionic wind velocity calculation method was developed based on the ionic wind development degree and conservation of momentum. As pressure increases, the corrected ionic wind velocity decreases, and the tendency is consistent with the experimental results about increment of flame displacement speed at elevated pressures.
... [4,5]). Many previous studies gave strong indication that the ionic wind is mainly responsible for flame stabilization, minimization of pollutant emissions and changes in the flame front [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28], which are important aspects for technical applications [20][21][22][23][24]. ...
... Altendorfner [24] found an increase in the NO-emission by up to 14% in premixed flames, which he correlated to a theoretically needed temperature increase of about 60 K (calculated with GRI-MECH 3.0 [29]). Besides the increase in NOx-emission, the temperature increase could also explain other observations such as increased chemiluminescence signal intensity or OH-LIF (laser-induced fluorescence) signal intensity [24,25,30] as well as slightly modified laminar burning velocity [14][15][16][17][18][19]. In principle, the chemiluminescence and OH-LIF signal increase could also be due to the pressure change induced by the ionic wind, however, this pressure change is typically very small (being about 0.0004 atm [4]) and it is expected to be negligible regarding variations in chemiluminescence or OH-PLIF signal intensity. ...
The general influence of electric fields on flames has been known for many years, the underlying mechanisms, however, are not finally identified yet. The changes observable in the flame structure and the pollutant emission when a flame is exposed to an electric field are mainly attributed to the ionic wind. Yet, especially the resulting flame temperature under the influence of electric fields is still an open research topic. Thus, in the present study the temperature distributions in a laminar premixed Bunsen type flame were measured for different air–fuel-mixtures using point-wise vibrational coherent-anti-Stokes Raman spectroscopy (vibrational-CARS) when a static electric field is activated. Additionally, CH2O- and OH-PLIF (planar laser-induced fluorescence) and particle image velocimetry (PIV) were applied to identify the best-suited locations for the temperature measurements and to visualize changes in the flame structure induced by the electric field for clarifying the underlying mechanisms.
... Recent investigations have reported on the effect of external electric fields on flame characteristics, for both premixed as well as diffusion flames. Transverse electric fields were used to modify the flame speed of various premixed air-fuel mixtures [14]. Droplet combustion studies showed the possibility of a change in flame shape and mass burning rate for sooting and non-sooting flames [15][16][17][18]. ...
... An external electric field can have two major effects on a flame, namely an effect on chemical kinetics as well as the generation of a body force, and different views have been expressed regarding the mechanism responsible for the observed modifications of flame behavior by the applied field. Some argue that these modifications result from kinetic effects caused by the collision of charged and neutral particles, which is a consequence of the redistribution of the charged particles resulting from their ionic mobility and the direction of the applied electric field [12,14]. Others, [15,16,18,19,22] argue that the body force due to the electric field is the primary mechanism responsible for the observed modifications of flame shape. ...
The effects of an externally applied electric field on the burning characteristics of a spherically symmetric fuel drop, including the flame structure, flame standoff distance, mass burning rate and flame extinction characteristics of the diffusion flame are studied. A reduced three-step chemical kinetic mechanism that reflects the chemi-ionization process for general hydrocarbon fuels has been proposed to capture the production and destruction of ions inside the flame zone. Due to the imposed symmetry, the effect of the ionic wind is simply to modify the pressure field. Our study thus focuses exclusively on the effects of Ohmic heating and kinetic effects on the burning process. Two distinguished limits of weak and strong field are identified, highlighting the relative strength of the internal charge barrier compared to the externally applied field. For both limits, significantly different charged species distributions are observed. An increase in the mass burning rate is noticed with increasing the strength of the electric field in both limits, with a small change in flame temperature. Increasing external voltages pushes the flame away from the droplet and causes a strengthening of the flame with a reduction in the extinction Damkhöler number.
... The effects of electric fields on flames have been extensively investigated and have shown positive abilities in flame stabilization and control (Ata et al. 2005;Belhi et al. 2010;Berman et al. 1992;Calcote and Berman 1989;Calcote and Pease 1951;Fialkov 1997;Hu et al. 2000;Kim et al. 2010;Lawton and Weinberg 1964;Maupin and Harris 1994;Starikovskii et al. 2008;Weinberg et al. 2006;Won et al. 2007;Yuan et al. 2001), burnt gas composition change and soot formation (Saito et al. 1999;Sepp and Ulybyshev 1997;Vega et al. 2007), and flame speed increase (Abrukov et al. 1981;Ata et al. 2005;Fowler and Corrigan 1966;Jaggers and von Engel 1971;Jaggers et al. 1972;Marcum and Ganguly 2005;van den Boom et al. 2009;Won et al. 2007) for both premixed and diffusion flames. These results show that significant modification of a flame is possible with relatively small electrical energy input compared to the thermochemical energy. ...
... This type of spreading of the flame is characteristic of flames that are subjected to increased ambient pressure. This spreading can cause a longer residence time for the preheat gas and a temperature redistribution near the burner head This increase in temperature and reduction in flame height suggest that 'effective' flame speed has increased that can contribute to more efficient and complete combustion (Jaggers and von Engel 1971;Marcum and Ganguly 2005). ...
The effects of millisecond-wide, pulsed current–voltage-induced behavior in premixed laminar flames have been investigated
through the simultaneous collection of particle image velocimetry (PIV) and chemiluminescence data with particular attention
paid to the onset mechanisms. Disturbances caused by applied voltages of 2kV over a 30-mm gap to a downward propagating, atmospheric pressure, premixed
propane/air flame with a flow speed near 2m/s and an equivalence ratio of 1.06 are investigated. The combined PIV and chemiluminescence-based
experimental data show the observed disturbance originates only in or near the cathode fall region very close to the burner base. The data also suggest that the coupling mechanism responsible
for the flame disturbance behavior is fluidic in nature, developing from the radial positive chemi-ion distribution and an
ion-drift current-induced net body force that acts along the annular space discharge distribution in the reaction zone in
or near the cathode fall. This net body force causes a reduction in flow speed above these near cathodic regions causing the
base of the flame to laterally spread. Also, this effect seems to produce a velocity gradient leading to the transition of
a laminar flame to turbulent combustion for higher applied current–voltage conditions as shown in previous work (Marcum and
Ganguly in Combust Flame 143:27–36, 2005; Schmidt and Ganguly in 48th AIAA aerospace sciences meeting. Orlando, 2010).
... In modern studies about the interaction of a direct current electric field with a hydrocarbon flame, it is assumed that the field of low intensity ( < 3 kVꞏcm 1 ) does not interfere with the flame propagation speed and combustion kinetics [69], since the energy of the electric field applied to the flame is negligible comparing to the heat released at combustion reactions in the flame front [17]. Changes in the geometry of the flame are more dependent on the action of the volume force generated in the electric field, which manipulates the charged particles formed as a result of the natural ionization of the hydrocarbon flame. ...
The Doctoral Thesis examines the control of the swirling flame flow dynamics with an external static electric field by firing the gaseous products of thermal decomposition of pelletized straw, woody biomass, and peat with the aim of more efficient heat production with a decrease of flue gas emissions. The intensification of the downward vortex in the electric field has been determined, ensuring improved mixing of the air vortex with the biomass thermal decomposition gas flow, intensifying the convective mass transfer towards the heating surfaces, and increasing the amount of heat energy produced in the biomass thermochemical conversion process.
... Lately, as an alternative method in supersonic combustion, plasma-assisted combustion (PAC) has shown remarkable capabilities in improving fuel/air mixing, ignition and flame stabilization. The enhancement of supersonic combustion by PAC can be classified as kinetical [24][25][26][27][28][29][30], thermal [31][32][33][34][35], and plasma-induced aerodynamic effects [36][37][38]. Nonequilibrium plasmas, such as corona microwave, lowpressure glow, and nanosecond high-voltage discharges improve combustion by adding active radicals leading to the modification of chemical reaction pathways and therefore combustion times are shortened [27]. Experimental studies in these types of plasmas have shown that the introduction of radicals reduces the ignition delay time and kinetically enhances flame stability in supersonic environments [39]. ...
This work presents a simplified methodology to couple the physics of a nanosecond pulsed discharge to the process of supersonic combustion in a flat wall combustor configuration. Plasma and supersonic combustion are separately simulated and then coupled by seeding plasma-generated radicals on the combustion domain. The plasma model is built assuming spatial uniformity and considering only the kinetic effects of the nanosecond pulsed discharge. Therefore, a zero-dimensional kinetic scheme accounting for the generation of plasma species is utilized. For the combustion model, the complete set of Favre-averaged compressible Navier Stokes equations along with finite rate chemistry is solved through a control-volume based technique via the commercial software Ansys Fluent. The computational results are compared against experimental studies showing that the proposed methodology can capture the main kinetic effects of the nanosecond pulsed discharge on supersonic combustion. OH concentration contours reveal the presence of an enhanced flame when the plasma is applied following the trends from experimental OH PLIF images. In addition, time evolving temperature and OH concentration contours show that the ignition delay time is reduced with the application of the discharge.
... The mean electron energy governed by the reduced electric field may be a critical factor causing the difference in MAI performance under different ambient pressures [33]. As the ambient pressure increases, the mean free path of electrons is reduced. ...
The Microwave Assisted Spark Ignition system is a novel and potential ignition method to improve efficiency of spark ignition engines. To better understand the coupling mechanisms/characteristics between microwaves and spark plasma, experiments were conducted to visualize spark discharge and flame development with microwave emitted in a Constant Volume Combustion Chamber. The operation parameters varied included equivalence ratio, ambient pressure, microwave pulse repetition frequency, and peak power. To clearly illuminate the coupling mechanism between microwave and spark plasma, a small-diameter antenna device was specifically designed to minimize heat loss. The microwave assisted spark ignition and flame propagation characteristics of methane-air premixed spherical flames were studied first. Subsequently, non-reactive discharge was experimented to exclude the effects from combustion. Results showed that microwave increased the development speed of an early flame kernel by 60% with 1 kHz microwave pulse repetition frequency and 1000 W peak power under the condition of equivalence ratio 0.6 and ambient pressure 0.1 MPa. The enhancement was believed to be the result of enhanced chemical reaction and the induced flame deformation by collisions between high energy electrons and other particles. As the ambient pressure increased, the enhancement effect of microwaves on flame kernels was found to diminish. In this situation, non-reactive discharge and related MAI experiments showed that a higher pulse repetition frequency or peak power was beneficial to strengthen the effect of microwave as much higher energy absorption efficiency was achieved.
... The effects of electric fields on flames have been extensively studied, highlighting various phenomena, including reduced soot emission [1]- [5], enhanced flame propagation [6]- [10], and augmented flame stability [11]- [14]. Among various hypothetical effects of an electric field, ionic wind has attracted keen interest [15]- [17]. ...
Although ionic wind has been observed to play important roles in the effects of electric fields on flames, there is a lack of systematic quantification of ionic wind that allows interpretation of a flame's responses to electric fields. Here, we report on various responses of nonpremixed flames, such as the flame's dynamic responses and the generation of bidirectional ionic wind, in relation to the applied voltage and frequency of an alternating current (AC) in a counterflow burner. We find that although the Lorentz force acting on charged molecules initiates related effects, each effect is both complex and different. When the applied voltage is in the sub-saturated regime (small) as determined by the voltage-current behavior, flame movements and flow motion are minimally affected. However, when the applied voltage is in the saturated regime (large), flame oscillation occurs and a bidirectional ionic wind is generated that creates double-stagnation planes. The flame's oscillatory motion could be categorized in the transport-limited regime and in the oscillatory decaying regime, suggesting a strong dependence of the motion on the configuration of the burner. We also observed bidirectional ionic wind in visibly stable flames at higher AC frequencies. We present detailed explanations for flame behaviors, electric currents, and flow characteristics under various experimental conditions.
... For example, atomic oxygen has been exper imentally identified as a product of nanosecond pulsed discharges that can accelerate hydrocarbon combustion reactions [14]. In addition, more effective local heating [15], the associated hydrodynamic effects, and ionic wind [16,17] can also lead to ignition improvement in diluted fuel-air mixtures. Moreover, LTP does not induce significant heating at the cathode surface, which helps to increase spark plug service lifetime and reduce the ignition energy lost through heat transfer to the electrodes. ...
This paper focuses on the multi-dimensional simulation of non-equilibrium plasma generated by nanosecond pulsed discharge in air, at pressure values higher than atmospheric. Voltage profiles and electrode geometry closely match those from a complementary experimental study. Simulations highlight the transition between different post-discharge plasma regimes at increasing pressure and tie the characteristics of the streamers to the electric field distribution in the gap between the electrodes. Results from simulations match experimental observations and qualitatively capture the experimental trend in terms of regime transition pressure and structure of the streamers. As a result, this paper validates a numerical tool that captures the physical and chemical properties of the low-temperature plasma and contributes to expand the understanding of low-temperature plasma ignition processes.
... Electric field was found to support flames with greater flame strength than without a field. The experimentation was followed by Jaggers and Engel [7] estimation using experimental and numerical approach with DC, AC and hf electric field to get persuasive clarification. Important declaration of work detailed a DC field of 0.5 kV/cm increases the burning velocity by a factor of 2. Additionally, the results were expounded with modeling and estimation of changes in flame temperature. ...
In flame spread research, an important area of concern is establishing operating criteria in presence of an external energy source. The work is motivated by the need to understand the flame behavior in presence of an external electric influence to produce unique combustion results that are not simple interpolations. Through systematic experimentation, the effect of an external electric field on a downward spreading flame is investigated. An experimental setup was upraised and related energy interactions between the flame and the electric source are explored under diverse conditions to respond to the unique aspect of combustion. The role of controlling parameters viz., separation distance, electrodes symmetry, number of electrodes and arc impingement on flame and the pilot fuel were evaluated in terms of flame spread rate variation. Results shows that the presence of an external electric field significantly affects spreading of flame and the governing interaction mechanics between an electric and heat energy source strongly depends on the separation distance. As a potential energy source, the external electric field stimulates the role of a heat source and heat sink under varying conditions. Amalgamated configurations with increased number of electrodes were found to exhibit the counter-balancing features limiting the spreading flame behavior. The resultant flame behavior and extent of change in the spreading rate substantiates with the altered thermochemistry and related losses. The results direct in developing technological application for better fire safety provisions and efficient combustion.
... One of the most plausible explanations is the electro-hydrodynamic effect, also called the ionic wind effect, which refers to the generation of a body force due to the collision of charged particles with the surrounding neutral molecules, altering the convective transport in the bulk gases. Many studies reported the effects of the electrohydrodynamic forces on increasing the burning velocity [13,14], on modifying the characteristics of the laminar bunsen flames [9,15], and on extending the stabilization limits of the nonpremixed jet flames subjected to an electric field oriented in the streamwise direction [5,16,17]. The subject has been extensively discussed by Lawton and Weinberg [2] and Fialkov [3], providing qualitative physical understanding of the process. ...
Two-dimensional axisymmetric simulations for counterflow non-premixed methane-air flames were undertaken as an attempt to reproduce the experimentally observed electro-hydrodynamic effect, also known as the ionic wind effect, on flames. Incompressible fluid dynamic solver was implemented with a skeletal chemical kinetic mechanism and transport property evaluations. The simulation successfully reproduced the key characteristics of the flames subjected to DC bias voltages at different intensity and polarity. Most notably, the simulation predicted the flame positions and showed good qualitative agreement with experimental data for the current–voltage curve. The flame response to the electric field with positive and negative polarity exhibited qualitatively different characteristics. In the negative polarity of the configuration considered, a non-monotonic variation of the current with the voltage was observed, along with the existence of an unstable regime at an intermediate voltage level. With positive polarity, a typical monotonic current–voltage curve was obtained. This behavior was attributed to the asymmetry in the distribution of the positive and negative ions resulting from ionization processes. The present study demonstrated that the mathematical and computational models for the ion chemistry, transport, and fluid dynamics were able to describe the key processes responsible for the flame-electric field interaction.
... The effects of electric fields on flames have been observed and reported in the literature for a long time [441][442][443][444]. Past investigations of electric field application in traditional hydrocarbon combustion systems have demonstrated control of various flame characteristics (e.g. burning velocity [445][446][447], stability [448,449], shape [450][451][452], luminosity [453,454], extinction limit [455][456][457]) and pollutant formation (e.g. soot [450,452,454,458], NOx [455,459]). ...
... physics have shown a high diversity in the types of the MW discharge, and, as a consequence, a similar diversity in the discharge structure, which depends on gas pressure and temperature as well as on the electric field parameters. Jaggers et al. (1971) first studied the effect of electric fields on the burning velocities of different fuels. In that study measurements were made of the flame in methane-air, ethylene-air and town gas (coal gas)-air flames exposed to DC, 50 Hz and 5 MHz, below-breakdown electric fields. ...
This paper presents a brief selective overview of current trends in plasma assisted ignition for internal combustion engines. Short duration pulsed nonequilbrium plasmas show promise for improved engine performance, including extension of the lean limit, reduction of NOX and consistent cycle-to-cycle ignition timing. This paper presents methods for achieving these improvements, including the use of lasers, microwaves, and high voltage nanosecond pulse driven discharges. These approaches can provide multiple simultaneous ignition points, optimized localization of ignition within the combustion chamber, and precise timing. A classification of nonequlibrium pulse driven ignition systems from the physical point of view and a discussion of different breakdown mechanisms are included, along with a discussion of recent laboratory and road test results.
... Charged particles of positive and negative ions and electrons are generated in a flame zone through chemi-ionization and subsequent ion chemistry. External electric fields can influence the movement of charged particles, causing adjustments to various aspects of combustion including flame stability [1][2][3][4][5][6], flame propagation speed [7][8][9][10][11] and emissions [12][13][14][15][16]. ...
We develop a simplified model to better explain electric current response when direct current (DC) is applied to a flame. In particular, different current responses have been observed by changing the polarity of the DC in a sub-saturated current regime that results from the presence of ions and electrons in the flame zone. A flame zone was modeled as a thin, ionized layer located in one-dimensional DC electric fields. We derived simplified model-governing equations from species equations by implementing mobility differences dependent on the type of charged particle, particularly between ions and electrons; we performed experiments to substantiate the model. Results showed that the sub-saturated current and local field intensity were significantly influenced by the polarity of the DC because of the combined effect of unequal mobility of charged particles and the position of the ionized layer in the gap relative to two electrodes. When an energized electrode is close to the ionized layer, applying a negative DC causes a more rapid increase in current than by applying a positive DC to the same electrode. Results from our experimental measurement of current using counterflow diffusion flames agreed qualitatively well with the model predictions. A sensitivity analysis using dimensional and non-dimensional parameters also supported the importance of the mobility difference and the relative location of the ionized layer on the electric current response.
... It was found that a strong electric field increased flame speeds and improved stabilization due to collisional energy transfer (heating) between electrons and neutral molecules. At the same time, a strong electric field also induced flow motion due to the collisions between positive ions and neutral molecules [125]. The former effect was thermal and the latter effect was termed "ionic wind." ...
Plasma assisted combustion is a promising technology to improve engine performance, increase lean burn flame stability, reduce emissions, and enhance low temperature fuel oxidation and processing. Over the last decade, significant progress has been made towards the applications of plasma in engines and the understanding of the fundamental chemistry and dynamic processes in plasma assisted combustion via the synergetic efforts in advanced diagnostics, combustion chemistry, flame theory, and kinetic modeling. New observations of plasma assisted ignition enhancement, ultra-lean combustion, cool flames, flameless combustion, and controllability of plasma discharge have been reported. Advances are made in the understanding of non-thermal and thermal enhancement effects, kinetic pathways of atomic O production, diagnostics of electronically and vibrationally excited species, plasma assisted combustion kinetics of sub-explosion limit ignition, plasma assisted low temperature combustion, flame regime transition of the classical ignition S-curve, dynamics of the minimum ignition energy, and the transport effect by non-equilibrium plasma discharge. These findings and advances have provided new opportunities in the development of efficient plasma discharges for practical applications and predictive, validated kinetic models and modeling tools for plasma assisted combustion at low temperature and high pressure conditions. This article is to provide a comprehensive overview of the progress and the gap in the knowledge of plasma assisted combustion in applications, chemistry, ignition and flame dynamics, experimental methods, diagnostics, kinetic modeling, and discharge control.
... CFsBr + 'H^HBr + 'CFs (28) HBr + 'H ^ H2 + -Br ...
... The effects of electric fields on flames have been extensively investigated and shown to improve flame stabilization [1][2][3][4][5][6], change of burnt gas composition [7][8][9], and enhance flame speed [5,6,[10][11][12][13][14] for both premixed and diffusion flames. There are, however, still questions like which effects are mainly responsible for the observed changes; (1) an ionic wind responsible for changing flame shape, (2) a change in the chemical reactions due to ion/electron collisions, and or (3) a conversion of electrical energy into thermal energy. ...
The effect of millisecond wide sub-breakdown pulsed voltage-current induced flow perturbation has been measured in premixed laminar atmospheric pressure propane/air flame. The flame equivalence ratios were varied from 0.8 to 1.2 with the flow speeds near 1.1 meter/second. Spatio-temporal flame structure changes were observed through collection of CH (A-X) and OH (A-X) chemiluminescence and simultaneous spontaneous Raman scattering from N2. This optical collection scheme allows us to obtain a strong correlation between the measured gas temperature and the chemiluminescence intensity, verifying that chemiluminescence images provide accurate measurements of flame reaction zone structure modifications. The experimental results suggest that the flame perturbation is caused by ionic wind originating only from the radial positive space-charge distribution in/near the cathode fall. A net momentum transfer acts along the annular space discharge distribution in the reaction zone at or near the cathode fall which modifies the flow field near the cathodic burner head. This radially inward directed body force appears to enhance mixing similar to a swirl induced modification of the flame structure. The flame fluidic response exhibit a strong dependence on the voltage pulse width ≤10 millisecond.
... Although controversies abound in the literature on the influence of electric field on burning velocity, this is one aspect of combustion that has received much attention. Jaggers and Von Engel [6] reported that flame propagation speed can rise well above its value without the field because of the increase in reaction rate brought about by electron collisions. Interestingly, a study by Bowser and Weinberg [2] reported that there is very minimal, almost undetectable effect on burning velocity under saturation conditions and the reported minimal effect is due to positive ions rather than electrons. ...
This work investigates the electric field effect on gas temperature, radiative heat flux and flame speed of premixed CH4/O2/N2 flames in order to gain a better insight into the mechanism of controlling the combustion process by electrophysical means.
Experiments were performed on laminar Bunsen flames (Re<2200) of lean to rich mixture composition (φ =0.8–1.2) with slight
oxygen enrichment (Ω=0.21-0.30). The Schlieren flame angle technique was used to determine the flame speed, and thermocouple
measurements at the post flame gas were conducted. The radiative heat flux was measured by using a heat flux meter. At high
field strengths, coincident with the appearance and enhancement of flame surface curvatures, an apparent change in flame speed
and gas temperature was observed. However, the application of an electric field had no significant effect on flame speed and
temperature when the flame geometry was unaltered. This was supported by radiative heat flux showing negligible electric field
effects. The modification in flame temperature and flame speed under electric field was attributed to the field-induced flame
stretch due to the body forces produced by the ionic winds. This additional flame stretch, coupled with the influence of non-unity
Lewis number, accounts for such changes. This reinforces the idea that the action of an electric field on flames with a geometry
that remains practically undeformed produces very minimal effect on flame speed, temperature and radiative heat flux. A possible
mechanism of combustion control by the application of flame stretch using electric field was introduced.
... Electric fields are known to enhance stabilization of turbulent flame (Lee et al. 2005;Sakhrieh et al. 2005;Won et al. 2007;Won et al. 2008). The physical processes involved are not well identified yet, and several interpretations of the phenomenon are proposed: change in flame chemistry (Jaggers and von Engel 1971;Marcum and Ganguly 2005), ionic wind (Belhi et al. 2009;Hu et al. 2000). Then further investigations are required. ...
Simultaneous stereoscopic PIV, OH and acetone planar laser-induced fluorescence measurements are performed to analyze the
processes involved in the enhancement of flame stabilization by electric field. Instantaneous velocity and mixture fraction
fields are measured simultaneously at the base of a lifted flame to analyze whether the flow properties in front of the flame
when electric field is applied are compatible with a mechanism involving ionic wind. The measurements conditioned on the instantaneous
flame bases with and without the electric field are compared. The velocity in front of the flame decreases with electric field
what is in agreement with the assumption involving ionic wind. To analyze the mixture in front of the flame, a joined analysis
of velocity and mixture fraction is required to show the mixture stays near stoichiometry when the electric field is applied.
The need of a joined analysis illustrates the interest of performing the three laser diagnostics simultaneously.
Experimental investigations of the DC electric field effect on thermal decomposition of biomass, formation of the axial flow of volatiles (CO, H2, CxHy), mixing of volatiles with swirling airflow at low swirl intensity (S ≈ 0.2-0.35), their ignition and on formation of combustion dynamics are carried out with the aim to understand the mechanism of electric field influence on biomass gasification, combustion of volatiles and heat energy production. The DC electric field effect on combustion dynamics was studied by varying the positive bias voltage of the central electrode from 0.6 kV to 3 kV, whereas the ion current was limited to 2 mA. The results of experimental investigations confirm the field-enhanced biomass gasification with enhanced release of volatiles and the development of endothermic processes at the primary stage of thermochemical conversion of biomass determining the field-enhanced heat energy consumption with the correlating decrease of the flame temperature and heat energy production at this stage of flame formation. Further, the field-enhanced radial expansion of the flame reaction zone correlates with a more complete combustion of volatiles increasing the combustion efficiency by 3% and decreasing the mass fraction of CO, H2 and CxHy in the products, whereas by 10% increases the average volume fraction of CO2 and the heat energy production downstream the combustor increases by 5-10%
The effect of electric fields on the response of diffusion flames in a counterflow has been investigated experimentally by varying the AC voltage and frequency. The result showed that the flame was stationary with high AC frequency above the threshold frequency, and it increased with the applied voltage and then leveled off at 35 Hz. Below the threshold frequency, however, the flame oscillated with a frequency that was synchronized with the applied AC frequency. This oscillation can be attributed to the ionic wind effect due to the generation of bulk flow, which arises from the momentum transfer by molecular collisions between neutral molecules and ions, where the ions in the reaction zone were accelerated by the Lorentz force.
In this work, effects of direct-current (DC) electric fields on the flame propagation and combustion characteristics of premixed CH4/O2/N2 mixtures were experimentally investigated at excess air ratios of 0.8, 1.0, and 1.2, room temperature, and atmospheric pressure. Results show that the existence of the DC electric fields significantly affects the flame propagation and combustion properties. Specifically, the flame shape becomes a prolate spheroid, with the major axis in the electric field direction as a result of the movement of positive ions by the electric body force, and a further increase in the applied voltage distorts the flame front more significantly. Additionally, the flame propagation speed in the electric field direction (Sn) and corresponding unstretched laminar burning velocity (ul) are increased as the electric field becomes more intense, and this behavior is more pronounced for lean mixtures. Finally, the initial and main combustion durations defined by the pressure evolution profiles are shortened. The peak pressure and peak rate of pressure rise are increased with the increase of the electric field intensity just for lean mixtures. The observation of the laminar burning velocity and pressure evolution behavior substantiates the potential of the electric field in enhancing lean combustion.
An experimental study is performed of the effect of dc and ac electric fields on the concentration limits for propagation of a propane flame in air within a vertical tube with closed lower and open upper ends. It is established that both limits for upward propagation and the lower limit for downward propagation remain unchanged upon field application. A qualitative interpretation of the results obtained is offered.
An experimental technique for using microwave power to increase the flame speed of laminar premixed flames is discussed. The microwave energy is applied by means of a rectangular resonant cavity. A laminar flame burner is used to produce a free floating flame sheet within the confines of the rectangular cavity. Upon application of microwave power, the flame sheet is observed to move toward the burner indicating an increase in laminar flame speed. The resulting data has been analyzed and estimates of the flame speed increase have been calculated. A possible physical mechanism of microwave-induced flame speed enhancement is also presented, and the estimates give the flame speed increase consistent with the observations. I. Overview his paper presents research conducted jointly by Research Support Instruments (RSI) and the Applied Physics Group in the Department of Mechanical and Aerospace Engineering at Princeton University that has yielded results that could lead to a paradigm shift in combustion research being conducted on engines for both supersonic and hypersonic flight. Research to date has yielded laminar flame speed increases of up to 65%, and the physical models developed indicate that it may be possible to double or even triple the flame speed using a refinement of our techniques. In the experiments conducted, microwave energy is absorbed into the thin flame region and does not lead to breakdown elsewhere. The logical progression of this research would be to extend this technique such that a microwave resonator is incorporated into the design of a high-speed combustor. Some of the potential applications of a microwave-enhanced combustor are the following: • Increased scramjet combustor efficiency resulting from rapid spreading of the flame fronts in the turbulent combustion regime. • High gain control of the heat release process through the deposition of low levels of power into localized regions within the flame zones. This deposition process could be controlled as a function of time and thus be used to influence the acoustic mechanisms and mixing in the combustor. • Suppression of flameout within the combustor. The presence of the flame depresses the Q-value of the microwave cavity and thus the amplitude of the local electric field. A reduction in flame size would result in a corresponding increase in the Q-value, rapidly increasing the field strength and providing a mechanism for stabilization and/or rapid reignition of the flame through microwave breakdown. • A pulsed power mode could possibly be used for enhanced mixing through the creation of micro-explosions (high power pulses) and the local generation of flow vorticity.
Long autoignition delay time and low lateral flame propagation speed are among the key problems in developing high-speed combustors for ram/scramjet engines. Plasma-assisted combustion can help to solve these problems. Estimates indicate that uniform volumetric nonequilibrium cold plasma ignition of fuel-air mixtures in ram/scramjet combustors can require large amounts of power to be deposited into the flow. In this paper, we explore a possibility of using microwaves for increase of flame propagation speed, which would be complementary to plasma ignition, allowing the latter to be applied to a smaller volume. The results suggest that the flame propagation speed strongly depends on the Q of the microwave cavity. For high values of Q (~1000), the input microwave power requirement decreases sharply from kilowatts to hundreds of watts. A 20% increase in the premixed methane-air flame propagation speed was observed for 400 Watt of input microwave power which was operating at 2.45 GHz. It was found that the power absorbed by the flame increases with the increase in the input subcritical microwave power.Theoretical estimations suggest that a small amount of power (on the order 10 W) was absorbed in the flame. These theoretical estimates support the observed experimental data.
In this work we demonstrate that a small amount of microwave power below its breakdown threshold can be locally absorbed into a flame combustion zone. The absorbed microwave power can significantly change the flame speed of both laminar and turbulent flames. PIV technique was employed to measure the laminar flame speed. It was found that microwave assisted flame speed enhancement was greatly dependent on Q of the microwave cavity. Due to the unsteady nature of interaction, microwave assisted flame speed measurements were difficult to make, however, preliminary observations of the flame luminosity indicated that there was energy addition occurring without microwave breakdown and the flame speed was increased.
This report describes a Phase II effort that demonstrated the technological feasibility to extinguish fires using an electromagnetic (EM) pulse. Using this technology only electrical energy is used for the fire extinguishment process; no water or chemical are required. The experimental device employed 10 energy storage capacitors of 40 micro F total and operated at 15 kV. With this device, heptane, diesel and kerosene pool fires, as well as forced-flow flames of propane and butane were extinguished. The facility can be used to study extinguishment of already ongoing fires aw well as to study explosion mitigation. In addition, a specialized version for the letter application was designed, built and tested (0.5 micro F at 35 kV). There are some fire extinguishment methods, which employ electrostatic fields. In contrast to these methods, the present device employs an electromagnetic (EM) pulse. The duration of this EM pulse is only several microseconds. Pulse rise times of less than 100 nanoseconds have been achieved. The present process is the only practical process known to date for extinguishment of fires without a chemical agent. It is also the only process known at present that is fast enough to be considered for use in explosion mitigation applications.
It is well known that electric fields can influence combustion processes. When the magnitude of an external applied electric field exceeds the breakdown field of the fuel gas or fuel/oxidizer mixture, plasma effects dominate. The earlier work in the field of plasma-assisted combustion has demonstrated that dielectric-barrier-discharge (DBD)-driven nonthermal plasmas (NTPs) can increase flame speed and extend the combustion of hydrocarbon fuel gases into very lean-burn regimes. In this paper, results on the decomposition of ethane (C2H6) by DBDs at atmospheric pressure will be presented. The authors have chosen ethane for this paper because its gaseous electronics properties (electron-impact dissociation cross sections, drift velocity) are available in the literature. A subsequent paper will present results on the calculated yield of DBD-driven plasma decomposition products of ethane, as predicted by plasma-chemistry modeling. In this paper, results on experiments carried out to determine the decomposition products of ethane, as measured by gas chromatography are presented. An atmospheric-pressure DBD reactor processed a flowing gas stream of chemically pure ethane in the regime of plasma specific energy ranging from 1200 to 2400 J/std lit. The major stable decomposition products were H2, CH4, C2H2, and C2H4. These results are important in assessing the possibility of using NTPs to enhance the combustion of hydrocarbons
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