Counterflow Diffusion Flames Research Articles

Detailed chemical effects of ammonia as fuel additive in ethylene counterflow diffusion flames

Ammonia is considered one of the most competitive fuels due to its carbon neutrality. The chemical effects of NH3 are distinguished by kinetic analysis via adding NH3 as reactive NH3 and fictitious inert NH3. The flame temperature and the mole fraction profiles affected by the chemical effects of NH3 addition for important species and soot are analyzed, with special emphasis on soot and its important precursor polycyclic aromatic hydrocarbons (PAHs). The results illustrate that NH3 addition inhibits the production of A1-A4. The chemical effects of ammonia decrease the hydrogen abstraction–C2H2–addition (HACA) surface growth rate and PAH condensation rate, which further reduces soot volume fraction and average particle diameter D63. The ammonia decomposition pathways interact with ethylene decomposition pathways via the four reactions: NH3 + C2H5 = C2H6 + NH2, HCN + C2H5 = C2H6 + CN, NH2 + C2H4 = C2H3 + NH3, and CH2CH2NH2 = C2H4 + NH2. The dilution and thermal effects of NH3 are dominant effects on soot reduction, while the chemical effects further inhibit soot formation.

Sooting transition chemistry for iso-octane/n-butanol counterflow diffusion flames

This work focused on effects of n-butanol addition on sooting transition process in iso-octane counterflow diffusion flames using optical and chemical diagnostics. Three iso-octane flames were studied with pure iso-octane, a 20% n-butanol addition mixture and a 40% n-butanol addition mixture as fuels. Optical diagnostics was carried out to determine effects of n-butanol addition on the sooting transition process using a high-resolution digital camera based on image processing and transition point extraction methods. Online gas chromatograph was used to investigate the flame chemistry in the sooting transition process. The results showed that the sooting transition point was delayed with the increase of n-butanol addition, which indicated that the addition of n-butanol could suppress the soot formation. To reveal the mechanism for effects of n-butanol on sooting transition process, kinetic simulations were further performed. The experimental results of flame chemistry were compared with the predictions from two mechanisms for iso-octane and n-butanol mixture. Both mechanisms were found to be capable of well capturing the main variations of most species in flames. After n-butanol addition, the changes of fuel-specific primary decomposition products were more obvious. It was noted that the decreased concentration of carbon which was available for benzene formation was the main reason for the delay of sooting transition point with n-butanol additions. Moreover, the inhibition of PAHs growth from n-butanol addition also could contribute to this kind of delayed effects.

Incipient sooting tendency of oxygenated fuels doped in ethylene counterflow diffusion flames

The incipient sooting tendencies of oxygenated fuels in counterflow diffusion flames were systematically assessed by doping selected oxygenate fuels into the baseline fuel of ethylene with various mixing ratios. Laser light scattering and planar laser-induced fluorescence (PLIF) techniques were adopted. The critical oxygen mole fractions at different mixing ratios were identified with the use of laser scattering. It is found that the critical oxygen mole fraction varies with oxygenates, suggesting that the effect of mixing ethylene with oxygenates is highly sensitive to the types of oxygenate. While the doping of acetone moderately promoted the incipient sooting tendency, the doping of methanol, acetic acid, formic acid, and diethyl carbonate suppressed the formation of incipient soot. On the other hand, the doping of ethanol, dimethyl carbonate, diethyl ether, and dimethoxymethane had weak influences on the incipient sooting tendency. The production of polycyclic aromatic hydrocarbons (PAHs) of different sizes were measured by PLIF. The results show there was a strong correlation between the incipient sooting tendency and the production of large PAHs. One important finding is that, at their sooting limits, the total oxygen-to-carbon ratio of a stoichiometric fuel/oxygen mixture is linearly correlated with the mixing ratio of the ethylene-oxygenate mixture. This slope of these two quantities can be defined as an incipient sooting index (ISI) of oxygenated fuels for ranking their tendency of incipient sooting formation. Having the slopes correlated well with the oxygenates’ hydrogen/oxygen ratio, a reasonably accurate prediction of critical oxygen mole fractions from the molecule information of oxygenated fuels can be achieved.

Temperature dependence of chemical effects of ethanol and dimethyl ether mixing on benzene and PAHs formation in ethylene counter-flow diffusion flames

The present work investigates the chemical, thermal, and dilution effects of ethanol and dimethyl ether mixing (10%/20%) on aromatics formation in ethylene counter-flow diffusion flames (CDFs) under different thermal conditions (Tmax≈1729/2000/2063 K). The fuel mixing effects were studied experimentally by gas chromatography measurements and computationally using two detailed chemical reaction mechanisms. The effects of ethanol and DME mixing on benzene and PAHs formation was found to be temperature-dependent in ethylene CDFs. Ethanol and dimethyl ether blending promoted benzene formation in high-temperature flames (2000 K and 2063 K) in comparison to low-temperature flames (1729 K). A slight reduction of benzene concentration was found after dimethyl ether addition under low-temperature conditions. The total effects of ethanol and dimethyl ether mixing on typical PAHs (C10H8, C14H10, C16H10) in low-temperature ethylene flames were also weaker than those in high-temperature flames. The temperature dependence of the total fuel mixing effects was dominated by chemical effects, while dilution and thermal effects were not significantly affected by the temperature. Pathway analysis showed that the temperature dependence of chemical mixing effects on benzene and PAHs formation originated from the C4+C2→C6→PAHs and C3+C3→C6→PAHs pathways.

Assessment on combustion chemistry of coal volatiles for various pyrolysis temperatures

Carrying out in-depth research on the formation characteristics and mechanisms of organic products in coal combustion will help to provide theoretical guidance for the emission reduction of coal-fired pollutants. Since the combustion of coal volatiles plays a vital role in the whole process of coal combustion, this work took coal pyrolysis gases as fuels and focused on the organic pollutants such as alkanes, olefins and aromatics in the combustion of coal pyrolysis gases. To explore the effects of three pyrolysis temperatures of 600 °C, 750 °C and 900 °C on the formation characteristics and mechanisms of hydrocarbon products during coal pyrolysis gases combustion, the online detection was carried out by gas chromatography (GC) and gas chromatography-mass spectrometer (GC-MS) on the counterflow diffusion flame platform, combined with chemical reaction kinetics simulations. The results showed that rising pyrolysis temperature had an inhibitory effect on the formation of light hydrocarbons and aromatic hydrocarbons, such as acetylene (C2H2), propyne (PC3H4), 1,3-butadiene (C4H6-13), vinylacetylene (C4H4), diacetylene (C4H2), benzene (C6H6), toluene (C6H5CH3) and naphthalene (C10H8), during the combustion of pyrolysis gases. Among three mechanisms used in this study, Creck mechanism was more accurate and applicable than KM2 mechanism and AramcoMech 3.0 mechanism. As the pyrolysis temperature rose, the rate of the reactions which dominated the formation of C2H2, propargyl (C3H3), C6H6 and C10H8 decreased continuously, because the decrease of CH3 concentration and the increase of H concentration inhibited the forward progress of these reactions. In addition, with rising pyrolysis temperature, less C3H3 was converted to butadiene (C4H6), C6H6 and Fulvene, and more inclined to its upstream reactants, allene (AC3H4) and PC3H4. The tendency of phenyl (C6H5) to form C6H6 was strengthened. Meanwhile, less C10H8 was generated from C4H4 and C6H5, but the dominant role of this reaction was still enhanced.

Numerical Investigation of Exergy Loss of Ammonia Addition in Hydrocarbon Diffusion Flames.

In this paper, a theoretical numerical analysis of the thermodynamics second law in ammonia/ethylene counter-flow diffusion flames is carried out. The combustion process, which includes heat and mass transfer, as well as a chemical reaction, is simulated based on a detailed chemical reaction model. Entropy generation and exergy loss due to various reasons in ammonia/ethylene and argon/ethylene flames are calculated. The effects of ammonia addition on the thermodynamics efficiency of combustion are investigated. Based on thermodynamics analysis, a parameter, the lowest emission of pollutant (LEP), is proposed to establish a relationship between the available work and pollutant emissions produced during the combustion process. Chemical reaction paths are also analyzed by combining the chemical entropy generation, and some important chemical reactions and substances are identified. The numerical results reveal that ammonia addition has a significant enhancement on heat transfer and chemical reaction in the flames, and the total exergy loss rate increases slightly at first and then decreases with an increase in ammonia concentration. Considering the factors of thermodynamic efficiency, the emissions of CO2 and NOx reach a maximum when ammonia concentration is near 10% and 30%, respectively. In terms of the chemical reaction path analysis, ammonia pyrolysis and nitrogen production increase significantly, while ethylene pyrolysis and carbon monoxide production decrease when ammonia is added to hydrocarbon diffusion flames.

Machine learning techniques to predict the flame state, temperature and species concentrations in counter-flow diffusion flames operated with CH4/CO/H2-air mixtures

The usage of artificial intelligence (AI) is increasing in many fields of research, since complex physical problems can be ‘learned’ and reproduced by AI methods. Thus, instead of numerically solving partial differential equations, describing the physical processes in detail, appropriate AI methods can be used to decrease the calculation time significantly. In the present study, artificial neural networks (ANNs) were used to predict temperature and species concentrations in a laminar counter-flow diffusion flame. To improve the accuracy of the ANNs, a support vector machine (SVM) was used to subdivide the wide range of operating conditions (air–fuel ratio, strain rate, fuel mixture) into ‘flame’ and ‘no flame’ cases. Due to classification with the SVM the prediction performance of the ANNs was optimized and an average error to the reference values (GRI3.0) below 10 K for all cases was detected, whereas the calculation time was decreased by a factor of about 4,800 (solving the transport equations with GRI3.0).

Numerical Study of PAHs and Soot Emissions from Gasoline–Methanol, Gasoline–Ethanol, and Gasoline–n-Butanol Blend Surrogates

Soot formation is an intricate phenomenon, and soot propensity of a fuel is interwoven with the fuel composition, physical and chemical properties, and combustion environment. The present study examines the hypothesis that in addition to the chemical composition of the fuel, the sooting nature of the fuel is closely coupled with its chemical property known as octane sensitivity (S). With this motivation, the present study numerically investigates the effects of gasoline surrogate composition and its property, octane sensitivity (S), on polycyclic aromatic hydrocarbons (PAHs) and soot emissions. Four-component toluene primary reference fuel (TPRF)–alcohol blends, comprising iso-octane, n-heptane, toluene, and one of the three different alcohols- methanol, ethanol, and n-butanol, are used as gasoline surrogates. A total of 320 TPRF–alcohol mixtures, with S in the range of 1–10, are examined under laminar counterflow diffusion flame conditions. A detailed chemical mechanism coupled with a comprehensive soot model, which includes reactions for soot inception, surface growth, PAH condensation, and oxidation, is adopted. The analysis indicates that the toluene content in the fuel mixture has a prominent effect, while the alcohol content and octane sensitivity of the fuel have a weak correlation with the PAHs and soot. Thus, it is not clear if any of these three variables, namely, toluene content in the fuel, alcohol content in the fuel, and S, are individually sufficient to characterize the PAHs and soot across various blends. For this reason, a new variable (XCHO) based on the elemental composition of the fuel mixture is identified and it is shown that XCHO along with S of the fuel characterize soot emissions satisfactorily. Further, a reaction path analysis indicates that the efficacy of alcohols in reducing soot emissions follows the order: methanol > ethanol > n-butanol.

Quantitative optical diagnostics on macroscopic soot onset for ethylene diffusion flames with ethyl ester addition.

A novel quantitative optical diagnostics method for determining the threshold of soot onset in counterflow diffusion flames was proposed and demonstrated. The method was based on the proportional discrimination of trichromatic luminescence and the nonparametric and unsupervised automatic threshold selection algorithm. The macroscopic soot onset threshold in ethylene diffusion flame with three ethyl esters additions could be precisely determined. It was found that the undesirable soot onset phenomenon for ethylene diffusion flames was significantly suppressed with ethyl ester addition. The method proposed here will be useful as a reference for soot diagnostics in other flames.

Experimental investigation of the effect of hydrogen addition on the sooting limit and structure of methane/air laminar counterflow diffusion flames

The influence of hydrogen (H2) addition on the sooting limit and structure of methane/air laminar counterflow diffusion flames has been studied. Soot limits have been obtained using the laser light scattering technique for CH4/H2 mixtures with H2 concentration varying from 0–20% by volume in the fuel mixture for different strain rates. The resulting soot limit map shows a significant increase in the fuel concentration at the soot limit with increasing amounts of H2. To understand this trend, temperature and species concentration profiles have been measured and compared under the influence of H2 addition in (a) an incipiently sooting and (b) in a sooting CH4/air flame. Flame temperatures have been measured using a thin-wire S-type thermocouple and species profiles have been measured using the gas chromatography technique (GC). The results indicate a significant decrease in the amounts of higher hydrocarbons such as benzene and naphthalene (which are considered important soot precursors) with an increase in H2 concentration for the sooting flame. To explain this decrease in the concentration of higher hydrocarbons, pathway analysis has been performed to elucidate the influence of H2 on the formation and consumption of soot forming species.

Probing sooting limits in counterflow diffusion flames via multiple optical diagnostic techniques

Sooting limit representing the critical flame condition where soot starts to appear is a useful quantitative index to assess the sooting tendency of a particular fuel. Experimental determination of sooting limit is closely related to the establishment of a well-defined “nucleation” flame which facilitates further scrutinization of the soot formation process. The present study was devoted to probing the earliest soot appearance in typical counterflow diffusion flames. The sooting limit was defined as the critical fuel (or oxygen) mole fraction in the nitrogen-balanced fuel (or oxidizer) stream. Multiple optical diagnostic techniques including flame luminosity, light extinction and scattering, spectral soot emission (SSE) as well as laser-induced incandescence (LII) were used to determine the sooting limits. The results showed that the value of sooting limit can be quantitatively different among the used detection techniques. Flame luminosity and SSE were found to be most sensitive to the presence of nascent soot, followed by LII and laser light scattering, while light extinction is the least sensitive technique. The effects of the wavelength of probing light on sooting limits were also assessed for the laser extinction and scattering technique. The contribution of the present work can be summarized into the following two aspects: (1) a quantitative database of sooting limits as determined from different soot detection measurements is provided to facilitate the choice of nascent soot-probing technique; (2) the first appearance of soot particles in nucleation flames are fully characterized and interpreted through a cross-comparison among multiple diagnostics techniques. It is also the authors’ expectation that the present results may provide new insights into the understanding of soot nucleation process and the interaction between light and nascent soot.

Flame inhibition by aqueous solution of Alkali salts in methane and LPG laminar diffusion flames

Finely atomized water mist (diameter <100 μm) is a promising fire suppressant showing high effectiveness and no toxicity. The effectiveness can be improved further by mixing a small concentration of alkali compounds with pure water. The present work investigates the effectiveness of alkali compounds in counterflow diffusion flames of methane and LPG (38% propane, 62% n-butane by mass). In methane flames the inhibitors ranked by suppression effectiveness follows: KHCO3 ≈ CH3COOK > K2CO3 > K2C2O4.H2O ≈ Na2CO3 > NaHCO3 > CH3COONa > NaC3H5O3 > NaBr > C4H5NaO6.H2O. The effectiveness of the inhibitors increases linearly with solution concentration. Only the potassium salts were tested in LPG flames, and the ranking obtained is CH3COOK > KHCO3 ≈ K2CO3 > K2C2O4.H2O. The numerical models qualitatively capture the experimental trends; however, they underpredict the suppression effectiveness. Detailed sensitivity analysis and reaction pathway diagram analysis suggest that the rate constants of the recombination reactions, species entropy, and L-J collision diameters need careful consideration for improving the model predictions.

Quantitative measurement of mixture fraction in counterflow diffusion flames by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been widely employed in various flames to determine local element compositions. Compared with other laser-based measurement techniques, LIBS is a robust and fast-to-implement method that allows for real-time monitoring and remote analyses in various industrial applications. However, quantitative LIBS measurements of elemental ratios in the gas phase are challenging as the atomic emissions also depend on laser intensities, gas densities, and gas compositions. In this work, we propose a new spectral correction method to eliminate these influences. In detail, the plasma temperature is obtained by fitting spectral lines of one single element based on the local thermodynamic equilibrium (LTE) assumption of the laser-induced plasma. The plasma temperature is then utilized to correct the intensity ratios of spectral lines from different elements. The proposed method is first applied to pure air flow to correct the ratios of the O line at 716 nm and the N line at 819 nm. The corrected O/N ratios are independent of laser intensities and gate delays in the investigated range. Shot-to-shot fluctuations are also decreased after correction. This new method is then used to perform measurements in a counterflow diffusion flame. The elemental ratios of C/N, O/N, and H/N at different positions are obtained to derive the mass fractions of C, H, O, N as well as the mixture fraction. Agreement between experimental and simulation results indicates the feasibility of this method for quantitative measurement of mixture fractions in flames.

Effects of curvature on soot formation in steady and unsteady counterflow diffusion flames

A numerical study has been conducted in order to understand the effects of flame curvature on soot formation in laminar non-premixed flames. For the fundamental understanding, the canonical configuration of a counterflow diffusion flame is employed as it exhibits the essential combustion physics associated with non-premixed flames. Numerical results for the steady counterflow ethylene flames confirmed that the response of soot in curved flames is governed by the intricate coupling between flow convection, soot kinetics, and differential transport of soot impacting the rates of soot formation sub-processes. The sensitivity of the soot formation in curved flames to the strain rate variation is also analyzed to understand their competing effects. Furthermore, the dynamic response of soot formation to the unsteady curvature is investigated by imposing harmonic oscillations to the curvature. Numerical results revealed that for an increase in the frequency of curvature oscillations, the soot formation gets attenuated and amplitudes of the induced oscillations exhibit large phase-shift with respect to imposed fluctuations. The higher characteristic time scales of larger-sized particles proved to control the overall dynamic response of soot under unsteady fluctuations of curvature. To examine the dynamic response of soot formation under a more complex case of flow-flame-soot interaction, the unsteady analysis of flames is extended by subjecting them to the simultaneous strain rate and curvature oscillations. The correlation between imposed frequencies and variability of soot formation response under fluctuating strain rate and curvature is elaborated.

Sensitivity of soot formation to strain rate in steady counterflow flames determines its response under unsteady conditions

This work reports an original experimental investigation on the sooting characteristics of counterflow diffusion flames (CDFs) subject to sinusoidal oscillations of strain rates. The study was motivated by the necessity to consider transient responses of soot formation when using the laminar flamelet model to simulate soot formation in turbulent flames. Acoustic forcing was applied through loudspeakers installed at the inlet end of both the oxidizer and fuel nozzles of the counterflow burner to generate strain rate oscillations at the desired frequency and amplitudes. Particle image velocity and planar laser induced incandescence were used respectively to measure flow field and the soot volume fraction in a time-resolved manner. Counterflow flames of C2H4 and C3H8 fuels were tested with oxidizer streams of various oxygen mole fractions (XO). The experimental data revealed notable differences in unsteady soot responses between CDFs of C2H4 and C3H8 at the same XO. However, after matching the soot loadings of the two CDFs to have the same sensitivity to strain rate at steady states, their responses to unsteady strain oscillation became almost identical. We experimentally demonstrated that the unsteady response of soot formation to oscillating strain rates, as characterized by phase lag and amplitude attenuations, are strongly correlated with the sensitivity of the soot loading to strain rate under steady conditions. The results may have important implications for the development of transient flamelet models that incorporate unsteady effects on soot formation.

Large Activation Energy Analysis of Nonadiabatic Strained Premixed Laminar Flames with Nonunity Lewis Numbers

ABSTRACT We present an asymptotic analysis of a strained premixed flame in the mixing layer between two counterflowing streams: one with fresh reactants at a temperature and other with the burned gases at temperature , which may be different from the adiabatic combustion temperature of the fresh gases. A one-step irreversible Arrhenius reaction model, of high activation energy, is used for the asymptotic analysis, together with the thermal-diffusive approximation of constant density and transport properties – easily generalized to variable density and transport properties with the use of a heat-conduction-weighted coordinate. The analysis for near unity Lewis numbers of the fuel by Libby, Liñán and Williams (1983) is extended here to arbitrary nonunity Lewis numbers, of relevance to a wide variety of applications, ranging from hydrogen-fueled combustors to heavy fuel systems. In analogy with Liñán’s analysis of counterflow diffusion flames, three asymptotic distinguished regimes are identified for premixed flames for large activation energies and the appropriate Damköhler numbers – the ratio of the characteristic diffusion and reaction times. These regimes are the premixed flame regime, the partial burning regime and the nearly frozen ignition regime. The analytical expressions obtained for these regimes, of the dimensionless reaction rate as a function of the Damköhler number, are seen to describe with good accuracy the results obtained from the numerical integration of the full problem.

Modeling soot particles as stable radicals: a chemical kinetic study on formation and oxidation. Part I. Soot formation in ethylene laminar premixed and counterflow diffusion flames

Based on recent theoretical and experimental evidences, this work proposes a detailed discrete sectional soot model considering particles and aggregates behaving as stable radicals, paving the way for the implementation of a game-changing assumption in soot kinetic models. The concentration of large closed-shell and open-shell polycyclic aromatic hydrocarbons (PAHs) levels out with the increase of their aromatic structure and, contrary to conventional assumptions in existing soot models, their reactivity approaches that of persistent radical species. These findings were implemented in the model here proposed to describe soot formation (Part I) and oxidation (Part II). While the number of lumped pseudo-species involved is significantly reduced compared to previous discrete sectional soot models since no distinction between the kinetics of closed-shell and open-shell particles is taken into account, the physical meaning and consistency with recent evidences improves together with model performances. Rate parameters of gas-phase reactions involving resonantly stabilized radical PAHs were used as a reference for soot particle kinetics. The model was validated against 65 target ethylene laminar premixed flames and counterflow diffusion flames, with a general good agreement between experimental data and numerical simulations at different pressures (P = 1–8 atm), maximum flame temperatures (Tmax∼1500–2400 K) and soot yields (fv∼0.01 to ∼170 ppm). Parity diagrams allow to identify systematic deviations of the present model from the experiments, while the comparison between all the flames included in the database highlights the relative contribution to soot formation and growth of the main reaction classes constituting the soot model, spotting analogies and peculiarities of the two flame configurations considered. Similar kinetic patterns characterize most of the flames within the database, independently of flame configuration and sooting tendency of the specific case. Large PAH condensation dominates the formation of the first soot nuclei, while the condensation of smaller PAHs up to four aromatic rings is primarily responsible for the growth of larger soot particles and aggregates. Finally, the contribution of the different fluid dynamics in premixed and counterflow flames to particle evolution, resulting in a higher hydrogenation level of soot particles in the latter, is discussed.

Simultaneous measurements of temperature, CO2 concentration and soot volume fraction in counterflow diffusion flames using a single mid-infrared laser

Simultaneous measurements of temperature, CO2 concentration and soot volume fraction in counterflow diffusion flames using a single mid-infrared laser

A study on the extinction condition in counterflow diffusion flames of methane and LPG under the influence of polydisperse water mist

The extinction condition in methane and LPG (mixture of propane and n-butane) flames under the influence of polydisperse water mist is investigated in a counterflow configuration. LPG is a standard cooking fuel used worldwide. Despite widespread use, there is no existing literature on the behaviour of LPG flames under the influence of water mist. The goal of the present work is to carry out extinction experiments with LPG and compare the behaviour with another fuel studied in greater detail, namely methane. In the present work, we investigate the effect of three different droplet size distributions with Sauter mean diameters (SMD) of 24 µm, 29 µm, and 43 µm, respectively. LPG flames show significantly higher resistance to extinction than methane flames in all the conditions studied. However, the addition of water mist shows similar suppression effectiveness in both flames. Mist with 43 µm SMD shows lesser effectiveness as compared to the other two cases. The experimental investigation is supplemented by numerical modelling in a one-dimensional framework. The numerical modelling predictions are in good agreement with the experimental measurements. The effect of water mist addition on the reaction kinetics is further investigated by Damköhler number analysis. A definition of Damköhler number is proposed to uniquely characterise strain-induced extinction in counterflow diffusion flames under the influence of water mist.

Non-premixed counterflow methane flames in DC/AC/NS electric fields

The response of counterflow diffusion flames to sub-breakdown DC and AC electric fields, as well as their superposition with ns pulse discharge waveforms, is studied in the plane-to-plane electrode geometry. Sub-breakdown DC and low-frequency AC electric fields cause the flame displacement toward the grounded electrode, in the direction of the applied field, indicating that the body force on the positive ions exceeds that on the electrons and negative ions. As the AC frequency increases, the flame response becomes less pronounced, due to the reduction of the electrohydrodynamic (EHD) body force impulse over the AC half-period. The electric field in the electrode gap is determined by ps Electric Field Induced Second Harmonic (E-FISH) generation, with absolute calibration using sub-breakdown ns pulses overlapped with the measured electric field waveform. The results show that the electric field distribution across the flame in the current saturation regime follows the Laplacian field. This indicates that the space charge density in the gap is too low to distort the applied DC or AC field, consistent with the kinetic modeling predictions. Combining a nanosecond pulse discharge with a sub-breakdown DC field generates a diffuse plasma across the entire gap. Time-resolved and spatially resolved measurements of the electric field in the discharge indicate the ionization wave propagation between the electrodes. The present results do not exhibit a detectable flame displacement enhancement by ns discharge pulses combined with a sub-breakdown field, observed previously. Kinetic modeling calculations show that the absence of this effect in the plane-to-plane geometry is due to the rapid plasma self-shielding. This indicates that alternative electrode geometries limiting the self-shielding would be more effective for the plasma / electric field enhanced flameholding and flame stabilization applications.