Structure Of Premixed Flames Research Articles

Chemical Structure of Lean and Stoichiometric Laminar Flames of Methylcyclohexane at Atmospheric Pressure

Methylcyclohexane (MCH, C7H14) is a typical component in hydrocarbon fuels and is frequently utilized in surrogate fuel mixtures as a typical representative of alkylated cycloalkanes. However, comprehensive experimental studies on speciation during its combustion remain limited. This research investigates for the first time the chemical structure of laminar premixed flames of lean and stoichiometric mixtures (φ = 0.8 and 1.0) of MCH/O2/Ar under atmospheric pressure. Using probe-sampling molecular-beam mass spectrometry (MBMS), the spatial distribution of 18 compounds, including reactants, products, and intermediates, in the flame front was measured. The obtained results were compared with numerical simulations based on three established chemical–kinetic models of MCH combustion. The comparative analysis demonstrated that while the models effectively describe the profiles of reactants, primary products and key intermediates, significant discrepancies were observed for various C2–C6 compounds. To indicate the roots of the discrepancies, a rate of production (ROP) analysis was performed in each simulation. ROP analyses revealed that the primary cause for the discrepancies could be attributed to the overprediction of the rates of initial stages during MCH decomposition. Particularly, the role of non-elementary reactions was emphasized, indicating the need for refinement of the mechanisms based on new experimental data.

Characteristics of Fuel-Borne Nitrogen Pollutants in Hydrogen-Ammonia Mixtures Using Argon-Oxygen Atmosphere Under Engine Conditions

Abstract Amid climate change, reducing reliance on fossil fuels and transitioning to renewable energy sources is crucial. Ammonia, a carbon-free and renewable fuel, shows significant potential as an alternative energy source. By incorporating hydrogen as an additive, its flammability can be enhanced to suit existing spark ignition engines. However, understanding the characteristics of nitrogen pollutant emissions (i.e., NOx, which includes NO and NO2, and N2O) from ammonia-hydrogen combustion is challenging due to contributions from both fuel-borne and air-borne nitrogen. However, understanding the characteristics of nitrogen pollutant emissions from ammonia-hydrogen combustion is challenging due to contributions from both fuel-borne and air-borne nitrogen. Therefore, a comprehensive understanding of fuel-borne nitrogen pollutants during ammonia-hydrogen combustion is essential. This study focuses on investigating fuel-borne nitrogen pollutants in an argon-oxygen atmosphere, thereby eliminating nitrogen from the oxidizer and its role in thermal NOx formation. The research examines the formation and evolution of fuel-borne nitrogen pollutants during ammonia-hydrogen combustion under engine-like conditions. Results indicate that fuel-borne nitrogen pollutants act as intermediates, potentially originating from chemical equilibrium. While fuel NO predominantly forms in the burning zone, it undergoes reduction in the burned zone. N2O, absent in thermal NOx mechanisms, shows significant concentrations in the burning zone and is mostly converted to N2, leading to limited N2O in the final fuel-borne nitrogen pollutant concentration. Lean burn conditions, hydrogen addition, and oxyfuel combustion promote fuel NOx formation. Additionally, the equivalence ratio affects the ammonia-hydrogen premixed flame structure due to the de-NOx effect of ammonia. Overall, these findings highlight that fuel-borne nitrogen pollutant mechanisms differ from thermal NOx mechanisms, necessitating specially designed reduction technologies for clean spark ignition engines.

Thermoacoustic parametric instability of premixed ammonia flames propagating downwards in an open-closed tube

This work investigates the self-excited thermoacoustic parametric instability of downward propagating laminar premixed ammonia flames in an open-closed combustion tube. NH3/O2/N2 flames were propagated at different laminar burning velocities at equivalence ratios of ɸ = 0.8 and ɸ = 1.2. Different unstable flame regimes were observed through pressure fluctuation measurements and synchronized high-speed imaging capturing the lateral and longitudinal perspectives of flame propagation. The observed propagation speed of quasi-planar flames showed a strong correlation with the calculated laminar burning velocities of NH3/O2/N2 mixtures. For the first time, the cellular structures of laminar premixed ammonia flames at the onset of parametric instability were clearly observed from the present experimental approach. The experimental cellular flame wavenumbers and acoustic velocity fluctuation amplitudes at the onset of parametric instability are in qualitative agreement with the theory of stability of laminar flames under acoustic excitation. Experiments have shown that NH3/O2/N2 flames are more unstable than CH4/O2/N2 flames at the same laminar burning velocity independent of Lewis number within the range of tested conditions. The influence of activation energy on parametric instability is investigated through a comparative thermoacoustic analysis between two different fuels at varied equivalence ratios. Ammonia flames are characterized by large overall activation energy and Zeldovich numbers compared to hydrocarbon flames. This work elucidates how this unique property characterizes the acoustic instability of downward propagating premixed ammonia flames in a tube based on classical theories on acoustic instability of laminar premixed flames.

The premixed flame structure and combustion mechanism of clean hydrogen mixed with multi-component natural gas

Hydrogen is widely taken for the most potential clean energy for the moment. Currently, pipeline transport of natural gas mixed with hydrogen is one of the primary approaches of hydrogen transportation. However, compared to pure natural gas (CH4), the explosion and combustion of natural gas mixed with hydrogen may lead to more severe consequences, and its reliability needs to be considered. Existing research of combustion and explosion using methane instead of natural gas have yet to be considered for application to real multi-component natural gas. To address this issue, real multi-component natural gas is configured, and the premixed flame and combustion characterization parameters of natural gas/H2/air are obtained through experiment. The chemical reaction kinetics of natural gas/H2/air mixtures are analyzed using Chemkin Pro software. These results prove that the laminar burning (LB) velocity varies between 0.33 m/s and 2.34 m/s by adding H2. The primitive reactions R1 (H2 + OH → H + H2O) and R3 (H + O2 → O + OH) predominantly contribute to enhancing the LB velocity. The Markstein lengths rang from −0.19 mm to −0.049 mm, most lewis numbers are less than 1, the flame thicknesses and critical destabilization radii are reduced. Additionally, hydrodynamic instability and thermal-diffusion instability are strengthened by adding the hydrogen. As for smaller hydrogen content, the average size of flame cell from about 8 mm program first rises and then falls cyclic fluctuations, fluctuation amplitude gradually reduces, fluctuation frequency accelerates. In larger hydrogen content, this fluctuation amplitude is reduced and the average size of flame cell decreases. In addition, H2 enhances the explosive strength of natural gas. Finally, methane instead of natural gas may exaggerate the velocity of laminar burning and exaggerate or weaken the ignition time delay of natural gas with different hydrogen contents. The current research might provide crucial information for the safe transport of hydrogen mixed with natural gas.

Numerical simulation of an industrial radiant tube burner using OpenFOAM

This study presents a numerical investigation into the effects of physical models on the prediction accuracy of the wall temperature distribution in an industrial radiant tube burner. Utilizing a reacting flow solver based on OpenFOAM, we explored the effects of various physical models, including those for chemistry, combustion, heat transfer, and radiation properties. The choice of combustion model significantly influences prediction accuracy, playing a more dominant role than the chemistry mechanism. Moreover, the simulations captured a distinctive triple flame structure inside the burner, representing the coexistence of rich premixed, non-premixed, and lean premixed flame structures. Conditional scatter plots displayed the development of both premixed and non-premixed flame structures, converging on the fuel-lean side. Notably, accurate prediction of wall temperature distribution depends on the incorporation of a precise heat transfer model, coupled with a detailed radiation property model. Regarding the distribution of tube surface temperature in the main radiation zone (a distance from the burner nozzle greater than 1 m), the most accurate prediction exhibits a maximum deviation of less than 56 K and an average deviation of 24 K compared to experimental results. The simulation closely matched experimental data for exhaust concentration of NO within an error margin of 20 ppm. However, discrepancy was observed in the CO concentration, which was attributed to the simplified representations of fuel chemistry and composition, as well as the difficulties in accurately capturing the unsteady flame dynamics near the wall.

Experimental study on the structure and propagation of the stretched hydrogen-rich turbulent premixed flames in the thin-reaction-zone regime

An experimental study on the structure and propagation of the stretched hydrogen-rich premixed turbulent flames in the thin-reaction-zone regime was conducted. A set of optical diagnostic techniques including the 2D/3D-particle image velocimetry, hot wire anemometer, intensified charge-coupled device (ICCD), and laser tomography visualization (LTV) methods were adopted to quantitatively measure the velocity field, turbulent intensity, and thickness of the stretched flame of the under various conditions. A new method to improve the measurements of the flame preheat zone thickness was developed by using the low-temperature available LTV method to identify the onset of the preheat zone and using the peak location of CH* to identify the reaction zone. The results showed that the flow field was in the inertial turbulent subrange and the turbulent flames obtained in the present counterflow configuration were categorized into the thin reaction zone regime in Borghi/Peters turbulent flame diagram. The experimental results showed that the flame structure of a turbulent H2-rich premixed flame was synergistically influenced by the hydrodynamic stretch, turbulence, and preferential diffusion. New evidence of the phenomenon accumulated that large-scale vortices wrinkled the surface of the flame while small-scale vortices penetrated into the flame preheat zone. Scaling laws for the turbulent flame propagation speed, apparent turbulent thermal conductivity, and were given with the consideration of the synergetic effect of hydrodynamic stretch, turbulence, and preferential diffusion. The scaling of the turbulent flame preheat zone thickness turned out to be sensitive, while those of the turbulent flame propagation speed and apparent turbulent thermal conductivity were insensitive, to the bulk flow strain rate.

A consistent MMC-LES approach for turbulent premixed flames

A multiple mapping conditioning (MMC) approach is coupled with large eddy simulations (LES) for the prediction of the flame propagation, the flame structure and finite rate chemistry effects including NO formation in turbulent premixed flames. The mixing term in MMC relies on a reference field that characterizes the chemical composition within the flame and it is typically taken to be the reaction progress variable. In LES, this reference field needs to be artificially thickened to be resolved on the LES grid. A novel MMC mixing time scale model that had been developed with the aid of direct numerical simulation (DNS), has been adapted to account for the thickening of the reference field. Equally, the particle selection mechanism for particle mixing has been modified to ensure localness of mixing in composition space and to preserve the prediction of a realistic (thin) flame thickness by the stochastic particles. A set of freely propagating turbulent premixed flames with varying equivalence ratios is used to test the performance of the MMC model. Results of MMC-LES are compared with reference DNS that include full chemistry. Additional simulations were carried out where MMC is coupled with quasi-DNS where the flow field is fully resolved but the reaction progress variable field is thickened and its source term is closed with a flamelet approach. This allows to isolate the different modelling assumptions invoked in MMC-LES. Deviations from the exact solution can then be associated with the MMC approach and with thickening of the reference field separately and they are small. The MMC-LES predictions of the turbulent premixed flame propagation speed and flame structure are very good and finite rate chemistry effects can be captured well which is demonstrated by good agreement of predicted NO with DNS data.

Inhibition of Laminar Premixed Flames of Dimethyl Ether by a Phosphorus-Containing Compound

ABSTRACT The present paper reports experimental and modeling results on the laminar burning velocity and structure of stoichiometric and fuel-rich premixed laminar flames of DME with and without TMP additives. The spatial variations of the mole fractions of H, OH, PO, PO2, HOPO, HOPO2 and some intermediate hydrocarbons in the one-dimensional burner-stabilized flames with various equivalence ratios are measured by the flame-sampling molecular beam mass spectrometry. The novel measurement data for the DME flames are used to validate a kinetic mechanism available in the literature for flame inhibition by organophosphorus compounds. The TMP inhibition effectiveness of DME flames has been determined and revealed to be lower than that of CH4/air flame. The speeds of CH4/air and DME/air flames were found to be sensitive to the same reactions involving phosphorus compounds, while the sensitivity of the methane flame speed to inhibition reactions is higher than that of DME/air flame. The observed differences were explained by the higher concentration of the chain carriers H, OH, and O in the DME flame. Based on the results obtained, we conclude that the phosphorus-containing inhibitors are promising candidates for reducing the flammability and ensuring the fire safety of DME mixtures with air.

Heat release characteristics of ammonia flames in MILD conditions

Ammonia (NH3) combustion is an attractive energy source with zero carbon dioxide (CO2) emissions and a potential hydrogen-combustion enabler. In addition, the availability of ammonia and its amenability to transportation make it appealing to future energy supply. To provide fundamental insights into the heat release characteristics of ammonia in moderate or intense low oxygen dilution (MILD) combustion, one-dimensional laminar and two-dimensional turbulent premixed flames are studied numerically. In particular, the flame structure and heat release distribution of ammonia and ammonia-hydrogen premixed flames are examined under conventional and MILD, and preheated conditions, respectively. The results show that stoichiometric ammonia MILD flames differ from normal ammonia flames producing NH2 earlier, which results in the formation of a radical pool that shifts the chemical pathway towards hydrogen-related chemistry. The NH2 radical serves as a precursor for the ammonia flame and characterizes the reaction zone, and plays a crucial role in bringing the explosive nature into the system. For fixed ratios of the root-mean-square turbulent velocity fluctuation to the laminar flame speed and integral length scale to the laminar flame thickness, the flame surface wrinkling is comparable between the stoichiometric ammonia flame at MILD and non-MILD conditions. Furthermore, the hydrogen enrichment of the lean ammonia flame results in a significant increase in heat release rate due to the preferential diffusion effect. The study also examines chemical markers to identify heat-releasing regions of the flames and proposes the use of NH2 × O (on a mass fraction basis) as a suitable indicator of regions with high chemical activity and correlates well with the heat release for the MILD and non-MILD ammonia flames as well as ammonia-hydrogen flames at both laminar and turbulent conditions.

Development of reduced and optimized mechanism for ammonia/ hydrogen mixture based on genetic algorithm

As carbon-free and renewable fuels, ammonia and hydrogen are promising fuels to be utilized in combustion devices. For this, the genetic algorithm was employed to reduce and optimize the mechanism for ammonia/hydrogen mixture, which was aimed to develop a compact and reliable skeletal mechanism for real engine simulation with ammonia as fuel. During the progress of GA reduction, the extracted sub-mechanisms was constrained high fidelity with original mechanism on predicting the combustion characteristics through the definition of penalty functions. Besides, the number of reactions and computer calculation time of the sub-mechanism were simultaneously tracked to reduce the chemical stiffness. The resulting mechanism contained 29 species and 63 reactions and was streamlined to be coupled into complex CFD calculation. The reduced mechanism was adjusted to better capture fundamental combustion data and the mechanism optimization was finished automatically through GA operation. To verify the reliability of the optimized mechanism, the experimental data including ignition delay times, laminar flame speed, JSR species concentration, premixed flame structure and HCCI engine in-cylinder pressure at various conditions was utilized to evaluate the performance of the models. As a result, the optimized mechanism was capable to reproduce those measurements and was credible for engine combustion simulation.

Turbulent flame speed based on the mass flow rate: Theory and DNS

Starting with an integral formulation of average mass flow rate through an ensemble of isotherms constituting a statistically planar, turbulent premixed flame, a scaling for the corresponding turbulent flame speed is derived without invoking Damköhler’s hypotheses. Major approximations and interim results are validated using a large Karlovitz number, unity Lewis number, Direct Numerical Simulation (DNS) dataset of n-heptane/air mixture, computed with reduced chemistry. A length scale quantifying the fluctuation distance of the isotherms within the premixed flame structure is analysed.

MMC-LES of spray combustion: Analysis of mixing timescales and flame structure

Large-eddy simulations (LES) of turbulent, pilot-stabilised, dilute acetone spray flames are performed using the sparse-Lagrangian multiple mapping conditioning approach (MMC-LES). The primary objective of this work is to assess the adaptability of the recently proposed dynamically evaluated molecular mixing timescale model for MMC-LES (Sharma et al., 2022, A fully dynamic mixing timescale model for the sparse Lagrangian multiple mapping conditioning approach, Combustion and Flame, 238, 111872), referred to as the dyn-aISO model, to the non-reacting and reacting two-phase flows. This work is the first application to perform a dynamic evaluation of the mixing timescale for turbulent spray combustion using the transported filtered density function (FDF) approach. A non-reacting case and four reacting flames with varying fuel mass loading are investigated, where the flame transitions from a diffusion flame to a premixed flame structure with a decrease in fuel loading. As is conventional for MMC-LES, a sparse resolution of one stochastic particle per six Eulerian finite-volume cells is used in this study, offering a much cheaper computing expense than the conventional transported FDF approach. The liquid evaporation is represented based on a non-equilibrium model at the droplet surface. The reaction chemistry is based on an augmented GRI 3.0 mechanism containing 39 species and 226 reactions to represent the oxidation of acetone. Predictions with the dyn-aISO model are compared with the constant coefficient anisotropic mixing time scale model and the isotropic C-K model. Thorough validation of liquid-phase statistics and gas-phase temperature is performed with available experimental data and popular studies from the literature. Analysis of the flame structure, with different timescale models, for both non-premixed and premixed type flames is also carried out. Quantification of the parameters affecting the timescale, such as model constants of the sub-Lagrangian-grid scale (slgs) mixture fraction variance (Cf) and its scalar dissipation rate (Scsgs−1Cν) is provided in comparison to their conventionally chosen static coefficient values. Predictions with the dyn-aISO model are quite good for the diffusion-type flames, while some discrepancies are noticed while predicting a premixed flame structure for lower liquid loading case having near stoichiometric jet-exit mixture composition.

Premixed combustion and emission characteristics of methane diluted with ammonia under F-class gas turbine relevant operating condition

Ammonia has been used on a small scale in other industrial equipment, such as gas turbines, as a carbon-free fuel. However, ammonia fuel suffers disadvantages such as high ignition temperature, low flame velocity and high NOx emissions. Doping with ammonia using a more reactive fuel, such as methane, can solve the above problems. Therefore, under the relevant operating conditions of the gas turbine (T = 723 K, p = 16.5 atm), the effect of ammonia content on the combustion and emission characteristics of laminar premixed methane flames was numerically investigated. This research uses the PREMIX code from ANSYS CHEMKIN-PRO 2020 and Okafor chemical kinetic mechanisms and provides a reference for our subsequent analysis of gas turbine operating conditions. Firstly, the emission data of major pollutants under different ammonia content (XNH3 = 0–1.0) and equivalent ratio (Φ = .6–1.4) were calculated. Then, the laminar premixed flame structure is analyzed under the lean fuel conditions associated with gas turbines (Φ = .6, .8). Finally, the effect of ammonia addition on the chemical reaction path of NO and CO emission was studied. The results show that ammonia/methane mixture fuel is more suitable for combustion at .6 < Φ < .8 under high temperature and pressure. High ammonia content (XNH3 > .6) and low equivalent ratio can reduce NO and CO emissions. The molar fractions of H, O, and OH radicals and flame temperature decreased with the increase in ammonia content. In addition, high temperature and high pressure conditions and ammonia content greatly influence the reaction path of NO and CO production. The increase in pressure resulted in a change in the primary reaction that produced NO. In conclusion, this study guides reducing the emission of NO and CO from lean side of gas turbine plants.

Two-dimensional simulation of cool and double flame formation induced by the laser ignition under shock-tube conditions

The laser ignition-induced spherical cool and double flame’s initiation, propagation, and transition are studied using a two-dimensional simulation with a detailed mechanism using n-heptane/O2/Ar/He mixture. The current study’s primary goal is to reveal how the radicals and flow field produced by the laser ignition lead to cool, hot, or double flame formation and to determine the flame propagation modes. It is found that after the laser spark, the over-driven shock wave results in vortex pairs, which significantly distorts the temperature field and the spatial distribution of the radicals. A torus-like hot flame is ignited at the ignition kernel center instantaneously. The results show that the laser-ignited hot flame can transit to a cool or double flame depending on the laser pulse energy. On the one hand, the hot flame extinguishes under a small laser ignition energy condition, at which the radicals at the hot flame front are transported by the vortex pair and lead to a cool flame formation. On the other hand, when the laser spark energy is large, a transient premixed double flame structure is observed. The double flame can be localized, and the appearance location is primarily affected by the spatial distribution of laser-induced radicals. The cool flame can coexist with a trailing hot flame for milliseconds, dramatically accelerating the hot flame propagation and fundamentally impacting the flame speed interpolation. Computational singularity perturbation (CSP) analysis identifies key reactions that belong to typical low-temperature chemistry in the double flame zone. A comparative simulation case with an initial temperature above the low-temperature chemistry regime confirms that no double flame structure can be formed under high-temperature conditions. The present study provides essential insight and guidance for the flame speed measurement using laser ignition under engine-relevant conditions.

Optimized two-step (OTS) chemistry model for the description of partially premixed combustion

Once properly optimized, single-step chemistry models are able to recover essential characteristics of flames such as burned gases temperature, flame propagation velocity and thickness. Ignition delay values can be reproduced as well. However, an improved description of either the premixed flame inner structure or the ignition process requires the consideration of the multi-step nature of chemical kinetics. Indeed, single-step chemistry models can neither describe properly the internal structure of laminar premixed flames nor the development of ignition processes. These limitations arise because single-step chemistry does not take into account intermediate species, the importance of which is essential. Thus, the present work is aimed at extending the recently-proposed optimized single-step (OSS) framework to multi-step chemistry. To this purpose an unbranched chain reaction with only two consecutive steps relevant to chain initiation and chain termination is considered. The corresponding kinetics model does involve two fictive species, the characteristics of which are optimized together with the two-step chemical scheme parameters, e.g., pre-exponential factors and activation energies. One of this fictive species is relevant to combustion products while the other represents the whole pool of radicals. The chemistry optimization procedure makes use of a well-defined in-house genetic algorithm and the resulting optimized two-step (OTS) model is assessed through comparisons made with data obtained from detailed chemistry computations used as reference. For similar (and even lower) CPU costs, the computations performed with the OTS model put into evidence significant improvements compared to the results obtained with the OSS description.

Local effects in vortex-flame interactions: Implications for turbulent premixed flame scaling and observables

A computational investigation was carried out to assess the effects of highly energetic vortices on the local structure of laminar premixed flames. The study involved a lean atmospheric methane-air flame interacting with vortex sizes ranging from five times to half of the laminar flame thickness. The characteristic velocities of the vortices were chosen to correspond to extreme turbulence intensities. Vortices smaller than the flame thickness were found to have a minimal effect on the flame structure due to the high rate of dissipation. Vortices with sizes greater than or equal to the flame thickness were found to induce flame siphoning and/or product-side local flame annihilation and as a result transport considerable amounts of formaldehyde and thermal energy to the unburned side thereby modifying the state of the reactants. The results of the present study for high activation energy reacting flows raise questions regarding the general validity of: (1) conjectured mechanisms of the so-called flame broadening under intense turbulence conditions; and (2) qualitative arguments regarding sub-flame-thickness vortex effects on the preheat zone in the context of the Borghi–Peters regime diagram, as originally postulated by Damköhler.

The influence of cooling air jets on the premixed flame structure and stability of air-cooled bluff-body flameholder

An air jet cooling method is adopted to reduce the temperature of the trailing edge wall of a bluff-body flameholder. The added cooling air jets would rebuild the flow field and reduce the temperature and fuel–air ratio in the near-field wake region of the bluff-body, thereby affecting the flame structure and stability. The influence of cooling air jets on the flame structure and lean blowout limits is investigated in a rectangular premixed combustor using a high-speed image capturing system and a fuel supply system. Experimental results demonstrate that the cooling air jets injected into the wake region could form cooling air vortexes, reduce the fuel–air ratio leading to partial flame extinction, create a flame hollow-out zone at the near-field wake, finally achieve excellent cooling effectiveness. The increasing flow rate of cooling air would shrink the recirculation zone that disappeared until the blowing ratio of 2 and reduce the fuel–air ratio in the near-field wake region. As a result, the flame hollow-out zone gradually disappeared, and the flame base and flame anchoring location moved upstream to the near-wall region; meanwhile, the flame expansion boundary in the near-field wake region was narrowed, the flame intensity was weakened slowly, and the flame stability was reduced considerably. Relatively, the cooling air temperature has little impact on the flame structure and stability. Notably, when the blowing ratio was 1, the increased mainstream velocity and decreased mainstream temperature could enlarge the flame hollowed-out zone, migrate the flame anchoring location in the central symmetric axis downstream, and reduce the flame stability. In addition, as the equivalence ratio increased at blowing ratio of 1, more fuel was entrained into cooling air vortexes, which caused the mixed reactants ignition; therefore, the flame hollowed-out zone was shortened, and the flame base and anchoring location moved upstream.

Effect of DC Electric Field on Turbulent Flame Structure and Turbulent Burning Velocity

ABSTRACT Effect of electric field on turbulent premixed flame structure and turbulent burning velocity is investigated through OH-PLIF technique. The ring-plate electrode configuration is used, which the ring electrode is high potential. Flame front structure and turbulent burning velocity are derived to estimate the effect of electric field on turbulent flame. Results show that electric field has similar effects compared with turbulence to some extent in this experiment, which could increase flame volume, turbulent burning velocity and decrease the flame surface density. But there are still substantial distinctions in detail. The perturbation induced by turbulence is the vortex, while that induced by electric field is the directional flow, and it can be verified through the PDF distribution of flame curvature with/without electric field. Effect of electric field is a local effect because it is dependent on the distribution of charged species. The research indicates electric field-assisted combustion is mitigated by the turbulence to some extent, and it may hinder the utilization of electric field at practical application for combustion enhancement, but other aspects need further researches. Two reasons are proposed to explain the mitigation of turbulence. Firstly, the response of turbulent burning velocity to the same perturbation decreases with turbulence intensity. Secondly, the wrinkled structure of turbulent flame mitigates the effect of electric field.

Effect of Ionic Wind Induced by DC Electric Field on Biogas/Air Turbulent Premixed Flame Structure

ABSTRACT Effect of DC electric field on lean premixed biogas turbulent flame structure at low and medium turbulence intensity was studied experimentally using OH-PLIF. OH signal distribution, turbulent burning velocity and flame structure geometric parameters were derived. Results show that downward electric field could broaden the downstream burned gas and push the flame tip down. The flame base is closer to the burner rim in downward electric field for the modification of thermal diffusion and flow modification caused by ionic wind. The effect on flame tip height reduction is more obvious for biogas flame with a higher CO2 ratio. According to the flame structure analysis, the turbulent burning velocity of biogas is increased slightly due to the modification of flow field by ionic wind generated by downward electric field, and the increment decreases with the increase of turbulence intensity. In addition, there’s a decrease in mean flame surface density and an increase in centerline flame brush thickness. The flame surface curvature PDF distribution is nearly not affected by the application of DC electric field. For turbulent flames, the effect of electric field is non-uniform and inconstant, which induces extra flow perturbation. This perturbation is different from turbulence, and the combined effects lead to the variation of flame structure.

Effect of non-ambient pressure conditions and Lewis number variation on direct numerical simulation of turbulent Bunsen flames at low turbulence intensity

The instantaneous flame front structure of high pressure turbulent premixed Bunsen flames has been analyzed for a wide range of characteristic Lewis numbers using a new Direct Numerical Simulation (DNS) database. High pressure turbulent premixed flames of lean-light fuels are likely to feature both thermo-diffusive and Darrieus–Landau instabilities. As the effects of these instabilities are eclipsed by intense turbulence, the present analysis focuses on flames located at the border of the wrinkled and corrugated flamelets regimes. The flame morphology has been characterized by the skewness and kurtosis of the probability density function (PDF) profiles of the mean and Gaussian curvatures. While skewness alone is not sufficient as a marker to distinguish between thermo-diffusive and Darrieus–Landau instabilities, it has been found that excess kurtosis of Gaussian curvature possibly can be used to distinguish between both instabilities. Further, the inner cut-off length and the fractal dimension have been computed for characterization of flame scales and for parameterization of wrinkling factor models. It has been observed that increasing pressure and decreasing Lewis number give rise to flame instabilities which results in an increased fractal dimension and a decreased inner cut-off scale. For high pressure flames with Lewis numbers around unity, the inner cut-off scale scales very well with the critical wavelength for flame instabilities determined from the theoretical analysis by Matalon and Matkowsky (1982). For small sub-unity Lewis numbers, the flames become unconditionally unstable and the critical wavelength loses its meaning, while the inner cut-off scales continues to decrease with Lewis number and the fractal dimension continues to increase up to a limit of about 7/3. The present findings are in excellent agreement with experimental observations from literature and theoretical analysis of flame instabilities.