Counterflow Diffusion Flames Research Articles

Study of the reduced kinetic mechanism of methane/dimethyl ether combustion

The development of methane (CH4)/dimethyl ether (DME) combustion systems can reduce fossil energy consumption and pollutant emissions of gas turbines while enhancing fuel diversity. However, employing detailed kinetic mechanisms in the fluid dynamics calculations for the design and optimization of CH4/DME burner configurations is enormously costly for computational resources and time. Therefore, reduced kinetic mechanisms are essential for widespread implementation. In the current work, 1200 + iterations based on the genetic algorithm were performed with 30 target conditions, which extracted a reduced mechanism from the detailed CH4/DME chemical kinetic mechanism. It contained 35 species and 131 reaction steps that reasonably captured the detailed mechanism's main reaction path. Measurements from a shock tube verified that the reduced mechanism provides reasonable predictions of ignition delay times for pure methane, pure DME, and CH4/DME mixtures at T > 1300 K, 1050 K, and 1100 K, respectively. The addition of CH3O2 reaction schemes and modification CH3OCH2 related reactions extended the temperature range to T > 900 K, 700 K, and 760 K under stoichiometric and fuel-rich conditions. The accuracy of the reduced and optimized mechanism was also confirmed by other results obtained from laminar flames, counter-flow diffusion flames, and flow reactors. Accurate replication of the temperature and species profiles was achieved for the numerical simulation of the turbulent flames by the optimization mechanism, which also decreased nearly half the computational time compared to the detailed mechanism. Overall, there is a suitable balance between accuracy and computational efficiency, as shown by the reduced mechanism, promising potential computational cost savings for large-scale numerical combustion simulations.

The Effects of CO2 Additional on Flame Characteristics in the CH4/N2/O2 Counterflow Diffusion Flame.

The effects (chemical, thermal, transport, and radiative) of CO2 added to the fuel side and oxidizer side on the flame temperature and the position of the flame front in a one-dimensional laminar counterflow diffusion flame of methane/N2/O2 were studied. Overall CO2 resulted in a decrease in flame temperature whether on the fuel side or on the oxidizer side, with the negative effect being more obvious on the latter side. The prominent effects of CO2 on the flame temperature were derived from its thermal properties on the fuel side and its radiative properties on the oxidizer side. The results also highlighted the differences in the four effects of CO2 on the position of the flame front on different sides. In addition, an analysis of OH and H radicals and the heat release rate of the main reactions illustrated how CO2 affects the flame temperature.

Experimental and numerical investigations on extinction strain rates in non-premixed counterflow methane and propane flames in an oxygen reduced environment

The aim of this work is to contribute to the better understanding of the oxidation process of light hydrocarbons, low calorific value gas and components of natural gas in the presence of inert gases and the influence of oxygen reduction in exhaust gas recirculation situations. The extinction behavior of laminar counterflow diffusion flames of methane (CH4), ethene (C2H4) and propane (C3H8) under nitrogen diluted condition has been investigated experimentally and numerically. In the experimental set-up, a counterflow burner was employed to investigate the extinction behavior under atmospheric conditions (1 bar and 298 K). The present study highlights the non-linear influence on the decrease in the extinction strain rate (ESR) that were found with decreasing fuel content and decreasing oxygen content. The oxygen content on the oxide side was stepwise diluted with nitrogen in order to distinctly show the possible oxygen-depleted influence in the recirculation areas. Additionally, it has been shown that the distance between the nozzles has an impact on the ESR but not on the general rising trend with the increasing fuel content in the non-premixed flames. Moreover, to clarify the influence, the flame zones were visualized using a system of high-speed OH* chemiluminescence camera. It was possible to investigate the width, intensity and position of the flame front from the radical detection. This provided a comprehensive data base for the subsequent numerical validation. In order to complement the experimental investigations, extensive uncertainty analyses were conducted and a new laser-based approach for the detection of flame extinction were demonstrated for the low-visible flames. Besides the experimental study, ESRs of CH4 and C3H8 combustion obtained from the experimental measurements were compared with those predicted by the numerical simulations. The San Diego-2014 reaction mechanism was used to represent the detailed chemical reaction, in which the predicted ESRs agree well with the experimental data was shown. Additionally, the effects of radiative heat loss, molecular transport model and chemical reactions on the ESRs were studied numerically. The present results suggest that the radiative heat loss plays a minor role on the ESRs, the molecular transport model is more significant and the ESRs are quite sensitive to several key reactions.

The effects of steam and water spray on NO formation in a methane–air counterflow diffusion flame

This paper presents a numerical investigation of the effects of steam and water mist/spray addition on NO formation in a methane–air counterflow diffusion flame. A detailed chemical mechanism (Glarborg et al. Modeling nitrogen chemistry in combustion, Prog. Energy Combust. Sci. 67 (2018), pp. 31–68) which contains 1397 reactions and 151 species has been employed to investigate the NO formation in a counterflow diffusion flame for a wide range of strain rates (50–400 s) in OPPDIF. The water spray/mist evaporation is represented as a first-order dynamic process with Arrhenius type dependence on temperature that can be expressed in a form similar to a first-order chemical reaction. Instead the process is represented as a first-order dynamic process by invoking a hypothetical chemical species and a surrogate reaction . This approach allows handling detail chemistry and droplet evaporation at a reasonable computational cost. This model is validated with CFD simulation results where droplets are modelled using the Discrete Phase Modelling approach. The effects of steam and water mist on the flame temperature and NO formation have been examined. It is found that steam/water mist inhibits NO formation by affecting both thermal and prompt pathways. It is seen that the percentage contribution of prompt NO in overall NO production significantly decreases with increasing steam dilution. Water mist causes an additional evaporative heat loss and results in a drop in percentage contribution of thermal NO in overall NO formation.

Multi-physics modeling of the ignition of polymer matrix composites exposed to fire

Advanced polymer matrix composites are widely used in modern aircraft and as such have to withstand fire to comply with safety regulations. The thermal degradation of these lightweight materials is a complex process involving chemical reactions in the solid and gas phases, potentially leading to flaming combustion. Here, we numerically investigate the heat feedback from the ignition of outgassing at the surface of a structural polymer matrix composite exposed to a pilot flame. Our model couples the thermal, physical and chemical processes in the solid phase to compute the composite degradation, as well as the production and movements of pyrolysates. A counterflow diffusion flame is used to model the ignition of the combustible effluent close to the sample surface, where the hot jet of the pilot flame impinges. The predictive capabilities of the method are demonstrated considering a carbon/epoxy composite subjected to a small-scale fire test. As input data for our model, material properties have been measured for virgin and partially burned samples via Differential Scanning Calorimetry, Xenon Flash Analysis and Thermogravimetric Analysis. The model is able to predict the backface temperature and time-to-ignition of the composite sample exposed to a flame with a good degree of accuracy.

Comparisons of the Uncoupled Effects of CO2 on the CH4/O2 Counterflow Diffusion Flame under High Pressure

A comprehensive numerical investigation of the uncoupled chemical, thermal, and transport effects of CO2 on the temperature of CH4/O2 counterflow diffusion flame under high pressure up to 5 atm was conducted. Three pairs of artificial species were introduced to distinguish the chemical effect, thermal effect, and the transport effect of CO2 on the flame temperature. The numerical results showed that both the chemical effect and the thermal effect of the CO2 dilution in the oxidizer side can decrease the flame temperature significantly, while the transport effect of CO2 can only slightly increase the flame temperature and can even be ignored. The reduction value of the temperature caused by the chemical effect of CO2 grows linearly, while that caused by the thermal effect increases exponentially. The RPchem and RPthermal are defined to explain the temperature reduction percentage due to the chemical effect and the thermal effect of CO2 in the total temperature reduction caused by CO2 dilution, respectively. The RPchem decreases with the increase of the pressure, the strain rate, and the CO2 dilution ratio, while the RPthermal behaves in the opposite manner. In the above conditions, the chemical effect plays a dominant role on the flame temperature reduction.

A study of the sooting behavior of hydro-processed renewable jet and petroleum jet fuels in a laminar counter-flow flame

In this study, the soot formation behavior of hydro-processed renewable jet and petroleum jet fuels (Jet-A1, and JP-5) was investigated using a laminar counter-flow diffusion flame. Laser-induced incandescence was utilized to measure the soot volume fraction. The soot particle morphology of the fuels was characterized using transmission electron microscopy (TEM). It was found that the highest soot volume fraction was obtained in the following order: JP-5 > Jet-A1 > HRJ. Modifications to the reactant concentrations were observed to strongly influence the soot volume fraction by as much as 81%, 60%, and 64% for HRJ, JP-5, and Jet A1, respectively, and led to an increase in the soot particle diameter by as much as 26%, 15% and 10% for HRJ, JP-5, and Jet-A1, respectively. Both petroleum jet fuels reported soot particle diameters that were double those of the HRJ fuels. The fuel with higher H/C ratio (i.e. HRJ) had the lowest soot volume fraction. The smoke point results were found to have a positive correlation with the soot volume fraction for all of the tested fuels.

Experimental Investigation on the Effect of Hydrogen Addition on the Sooting Limit and Structure of Methane/Air Laminar Counterflow Diffusion Flames

Experimental Investigation on the Effect of Hydrogen Addition on the Sooting Limit and Structure of Methane/Air Laminar Counterflow Diffusion Flames

Numerical Study of Combustion Characteristics by Pressure and Oxygen Concentration in Counter-Flow Diffusion Flame Model

Numerical Study of Combustion Characteristics by Pressure and Oxygen Concentration in Counter-Flow Diffusion Flame Model

Effects of strain rate and Lewis number on forced ignition of laminar counterflow diffusion flames

Forced ignition is one of the most fundamental and important combustion problems. It is used in advanced engines and closely related to fire safety and accidental explosion. In this study, forced ignition of laminar counterflow diffusion flames with one-step chemistry is studied through two-dimensional simulations. The initial ignition kernel is simply modelled as a hot spot of unburned gas with a specified radius and uniform high temperature inside it. We focus on the subsequent ignition kernel development, during which an expanding axisymmetric triple flame appears under certain conditions. Non-monotonic change of the flame kernel radius with time is observed near the critical ignition conditions. The effects of strain rate and Lewis number on the ignition kernel development and critical ignition conditions are assessed. For the same initial hot spot, successful ignition can be achieved at relatively low strain rate and fuel Lewis number; while the ignition kernel quenches at relatively high strain rate or fuel Lewis number. The strain rate and Lewis number are shown to have great impact on the critical hot spot radius and on the minimum ignition energy. At relatively low strain rates, the minimum ignition energy increases linearly with the strain rate. This is consistent with previous theory on flame ball in a mixing layer. Besides, the optimum ignition position and the influence of the hot spot shape are investigated. It is demonstrated that the optimum ignition position is close to the stoichiometric surface and it depends on the Lewis number and global equivalence ratio.

An experimental and modeling study on sooting characteristics of laminar counterflow diffusion flames with partial premixing

The effects of partial premixing on soot formation were investigated both experimentally and numerically in counterflow diffusion flames (CDFs) of ethylene, propane and ethylene/propane binary mixtures. The partial premixing ratio was maintained at relatively low levels so as to minimize the hydrodynamic effects; while the chemical effects of partial premixing on soot formation can be highlighted. The experimental results and simulations consistently showed that partial premixing notably increased soot formation in ethylene flames; while it slightly reduced the soot formation in propane flames, suggesting that partial premixing played different chemical roles in the soot formation process of ethylene and propane flames. Kinetic analyses revealed that partial premixing showed opposing effects on propargyl formation of ethylene and propane flames; this in turn led to the opposing effects on the formation of benzene and polycyclic aromatic hydrocarbons (PAH), and finally on the soot inception process. For partial premixing in propane/ethylene mixtures, the observed synergistic effects with propane added to ethylene CDFs disappeared, indicating a chemical cross-linking effect between partial premixing and ethylene/propane mixture. In addition, the present soot model showed satisfactory performance in reproducing the mismatch between PAH and soot formation tendencies in ethylene and propane CDFs. This result implies that great care is needed when using PAH concentration as an indicator of sooting tendency of different fuels. The present modeling results provided new insights into the chemical role of partial premixing in the soot evolution process of ethylene, propane and their blends. Further investigations towards the partial premixing effects on soot formation of larger liquid hydrocarbons such as diesel surrogates are required.

Single-phase instability of intermediate flamelet states in high-pressure combustion

The single-phase instability of high-pressure, steady, laminar counterflow diffusion flame is studied using the Vapor–Liquid Equilibrium (VLE) theory. The a posteriori study focuses on the identification of the potentially unstable regions emerging throughout the full combustion states represented by the high-pressure counterflow diffusion flame solution; a specific emphasis is placed on the middle branch solutions which are characteristic of intermediary combustion states. The a posteriori analysis provides useful bounds on the multicomponent phase separation. We use a mixture-fraction space flamelet formulation with real-fluid thermodynamics; the results compare favorably to both a spatial-based flamelet and a two-dimensional direct numerical simulations (DNS) solution, at high-pressure conditions. The a posteriori, single-phase instability for a high-pressure LO2/GH2 flame is investigated by investigating the effects of operating pressure and inlet temperature on the flame structure; then the analysis is extended to a LO2/GCH4 flame to evaluate the fuel effect. It is found that a single-phase instability can appear at both the fuel and oxidizer inlets, and its location and extent is determined by the water vapor concentration and, more importantly, temperature. For the flamelets in the upper burning branch, the size and location of unstable region is nearly invariant to the scalar dissipation rate, while for the flamelets in the middle branch, the instability region near the fuel inlet expands significantly with the decrease of chemical reactivity.

Spatially and temporally resolved temperature measurements in counterflow flames using a single interband cascade laser.

Spatially and temporally resolved temperatures are measured in counterflow diffusion flames with a tunable diode laser absorption spectroscopy (TDLAS) technique based on direct absorption of CO2 near 4.2 µm. An important aspect of the present work is the reduction of the beam diameter to around 150 µm, thus providing high spatial resolution that is necessary to resolve the high axial temperature gradient in counterflow flames. The temperature non-uniformity was taken into account through both hyperspectral tomography and the multiline technique with profile fitting, with the latter one being capable of providing temporally resolved data. The proposed methods were used to measure four counterflow flames with peak temperature ranging from 1654 to 2720 K, including both non-sooting and sooting ones.

Automatic Construction of REDIM Reduced Chemistry with a Detailed Transport and Its Application to CH4 Counterflow Flames

Extinction limits are important quantities of counterflow diffusion flames. An accurate prediction of extinction limits is necessary for the design of engineering combustion devices involving flame...

A comparative study of the sooting tendencies of various C5–C8 alkanes, alkenes and cycloalkanes in counterflow diffusion flames

In this work, sooting tendencies of various vaporized C5-C8 alkanes, alkenes, cycloalkanes were investigated in a counterflow diffusion flame (CDF) configuration. Mie scattering and laser induced incandescence (LII) were respectively employed to measure the sooting limits and soot volume fraction, both of which were used as quantitative sooting tendency indices of the flames. Research foci were paid to analyze the impacts of fuel molecular structure on sooting tendencies, ranging from the length of the carbon chain, fuel unsaturation, presence and position of the branched chain as well as the cyclic ring structure. It was found that the present CDF-based sooting tendency data did not always agree with existing literature results that were obtained from either coflow or premixed flame experiments, indicating the importance of considering flame conditions when assessing fuel sooting tendencies. Several interesting observations were made; in particular, our results showed that the sooting limits of C5-C8n-alkane is comparable, regardless of the carbon-chain length, indicating an interesting fuel similarity pertinent to sooting tendency. In addition, we found that cyclopentane, with a five-membered ring, has even stronger sooting propensity than the six-membered cyclohexane; this trend occurs because cyclopentane prefers to decompose to more odd-carbon radicals of cyclopentadienyl and allyl, which are efficient aromatic precursors. Our results are expected to serve as necessary complements to existing sooting index data for a deeper understanding of the correlation between fuel molecular structures and their sooting tendencies.

Kinetic Monte Carlo statistics of curvature integration by HACA growth and bay closure reactions for PAH growth in a counterflow diffusion flame

This paper uses a Kinetic Monte Carlo model that includes processes to integrate curvature due to the formation of five- and seven-member rings to simulate polycyclic aromatic hydrocarbons (PAHs) growing in lightly sooting ethylene and acetylene counterflow diffusion flames. The model includes new processes to form seven-member rings via hydrogen-abstraction-acetylene-addition and bay closure reactions on sites containing partially embedded five-member rings. The model additionally includes bay closure and HACA bay capping reactions for the integration of five-member rings. The mass spectra of PAHs predicted by the model are assessed against experimental data, and the distribution of embedded five-member rings and seven-member rings is studied as a function of spatial location, molecule size and frequency of events sampled in the simulation. The simulations show that the formation of seven-member rings and the embedding of five-member rings is a competitive process. Both types of rings are observed more frequently as the simulation proceeds from the fuel outlet towards the stagnation plane. Approximately 15% of the events that integrate curvature resulted in the formation of a seven-member ring coupled to an embedded five-member ring, and the remaining 85% of events embedded five-member rings via the formation of six-member rings. The proportion of PAHs containing embedded five-member rings and/or seven-member rings is observed to be a function of PAH size, passing through a maximum for PAHs containing 15–20 six-member rings. However, the proportion of PAHs containing both types of ring increases with PAH size, where upwards of 10% of PAHs containing at least one five-member ring and 15 or more six-member rings also contain a seven-member ring.

Unimolecular reactions of the resonance-stabilized cyclopentadienyl radicals and their role in the polycyclic aromatic hydrocarbon formation

Resonance-stabilized cyclopentadienyl radicals are important intermediate species in the combustion of transportation fuels. It not only serves as precursors for polycyclic aromatic hydrocarbon (PAH) formation, but also involves in the formation of fundamental PAH precursors such as propargyl and acetylene. In this work, the unimolecular reactions of the cyclopentadienyl radicals are theoretically studied based on high-level quantum chemistry and RRKM/master equation calculations. Stationary points on the potential energy surface (PES) are calculated at the CCSD(T)/CBS//M06–2X/6–311++(d,p) level of theory. The branching ratios of unimolecular reactions of the cyclopentadienyl radicals are analyzed for a broad temperature range from 500 to 2500 K and pressures from 0.01 to 100 atm. It is found that the isomerization reaction of the cyclopentadienyl radical via 1,2-hydrogen transfer dominates at low temperatures and high pressures, while the well-skipping decomposition reaction which forms propargyl and acetylene is important at high temperatures and low pressures. Both the decomposition reaction of the cyclopentadienyl radicals and its reverse reaction show pronounced pressure dependence, and their reaction rate constants are compared against available low-pressure experimental measurements and theoretical studies. The temperature- and pressure-dependent rate coefficients for important reactions involved on the C5H5 PES are calculated and updated in a chemical kinetic model. Impacts of the unimolecular reactions of the cyclopentadienyl radicals on the PAH formation are explored by the numerical modeling of a low-pressure cyclopentene counterflow diffusion flame.

The role of resonance-stabilized radical chain reactions in polycyclic aromatic hydrocarbon growth: Theoretical calculation and kinetic modeling

Resonance-stabilized radical chain (RSRC) reactions were recently proposed as alternative formation and growth pathways of polycyclic aromatic hydrocarbons (PAHs). In the present study, the growth of indene from the resonance-stabilized cyclopentadienyl radical following a series of RSRC sequences is investigated. Temperature- and pressure-dependent rate coefficients for acetylene addition reactions to the cyclopentadienyl, vinylcyclopentadienyl, cycloheptatrienyl, and benzyl radicals are determined based on high-level quantum chemistry and RRKM/master equation calculations. The acetylene addition to the resonance-stabilized cyclopentadienyl radical can generate resonance-stabilized vinylcyclopentadienyl, cycloheptatrienyl, and benzyl radicals. The formation reaction of cycloheptatrienyl has the highest rate constant compared to those of vinylcyclopentadienyl and benzyl. The acetylene addition to the above three resonance-stabilized C7H7 isomers forms indene. It is found that the indene formation reaction from C2H2 addition to benzyl has the highest rate constant, while that from C2H2 addition to cycloheptatrienyl is the lowest. The impacts of the investigated reactions on indene formation are then numerically explored in both counterflow diffusion and laminar premixed flame configurations. Results indicate that the RSRC pathways via C2H2 addition to vinylcyclopentadienyl and cycloheptatrienyl radicals have marginal contributions to indene formation.

Transient analysis of the nonmonotonic response of counterflow laminar diffusion flames in alternating current and step electric fields using a one-dimensional ionic transport model.

The electrohydrodynamic response of a counterflow laminar diffusion flame to applied alternating current (ac) electric fields is investigated experimentally and numerically. The flame positions are observed to show typical response to applied ac electric fields with high and moderate frequencies. The flame position does not respond above a threshold frequency corresponding to a certain collision response time, below which it oscillates in phase with the applied electric field. At a very low frequency (less than approximately 1 Hz), however, the flame position is observed to vary nonmonotonically as a function of time. To elucidate the nonmonotonic behaviors, a one-dimensional ionic transport model was employed by applying time-dependent electric fields. The responses of flame positions for ionized layers substituting for counterflow diffusion flames were systematically investigated with respect to one-way ionic wind (OIW) and two-way ionic wind (TIW) models. Consequently, it is demonstrated that the ionic models can produce not only harmonic flame oscillations for relatively low ac frequencies, but also free flame oscillations for stepwise voltages, which originated from the interaction between electrostatic force and ionic wind-induced force in the ionic system for both the OIW and TIW models.

A quasi-direct numerical simulation solver for compressible reacting flows

A new quasi-direct numerical simulation (q-DNS) solver named combustionFoam, capable of simulating reacting flows at arbitrary Mach number with a detailed chemical reaction mechanism, has been developed. This solver is implemented based on reactingFoam-solver from OpenFOAM-v7. The main improvements are a mixture-averaged formula transport model and ability to simulate reacting flows at arbitrary Mach number. The transport model is implemented by developing an data exchange interface to couple OpenFOAM® with Cantera. To damped nonphysical oscillations near shocks, a hybrid KT/KNP approach derived by Kraposhin et al. [18] is adopted. This solver was validated by two test cases: (1) a low speed steady two dimensional non-premixed counterflow diffusion flame; (2) a supersonic transient one dimensional detonation waves. Comparisons of the results with data from other solvers and literatures are carried out and good agreements are obtained.