Counterflow Laminar Diffusion Flames Research Articles

Effects of syngas and methanol fuel substitution on ammonia counterflow diffusion flames

Ammonia (NH3) is increasingly recognized as a promising alternative fuel. Various blending options have been proposed that are aimed at improving its flammability and applicability across diverse energy systems. This study conducts a comprehensive investigation, employing both experimental and numerical approaches, to understand the impact of blending ammonia with CH3OH or syngas, at various H2:CO ratios, on flame extinction limits, structure, and NOx emissions in counterflow laminar diffusion flames. Concentration profiles of ammonia and intermediate species (O2, CO, CO2, H2, NO, and N2O) were measured using Fourier-transformed infrared (FTIR) and gas chromatographic (GC) techniques. The findings indicate that increasing the H2:CO ratio in syngas-blended flames increases the extinction limits and reduces NO and N2O emissions. Compared with ammonia/methanol flames the ammonia/syngas flames exhibit higher extinction limits, flame temperatures, and lower NO and N2O emissions. Multiple kinetic models were found to underestimate the extinction strain limits, with Wang et al.’s 2021 model offering the closest predictions. Notably, discrepancies in ammonia dissociation and oxygen leakage to the rich side of stoichiometry impact reaction zone characteristics, extinction limits, and NOx predictions. Further kinetic analyses reveal that reactivity in syngas-blended flames is dominated by heat and radicals-producing reactions, while methanol-blended flames are sensitive to reactions involving ammonia fragments, resulting in decreased reactivity compared to syngas-blended flames. The results and findings from this study yield valuable insights into optimizing ammonia blends for enhanced flame stability and reduced emissions across various energy applications. It also provides data for chemical kinetics model validation in non-premixed diffusion environment where both species chemistry and transport are encountered.

A Robust Reacting Flow Solver with Computational Diagnostics Based on OpenFOAM and Cantera

In this study, we developed a new reacting flow solver based on OpenFOAM (OF) and Cantera, with the capabilities of (i) dealing with detailed species transport and chemistry, (ii) integration using a well-balanced splitting scheme, and (iii) two advanced computational diagnostic methods. First of all, a flaw of the original OF chemistry model to deal with pressure-dependent reactions is fixed. This solver then couples Cantera with OF so that the robust chemistry reader, chemical reaction rate calculations, ordinary differential equations (ODEs) solver, and species transport properties handled by Cantera can be accessed by OF. In this way, two transport models (mixture-averaged and constant Lewis number models) are implemented in the coupled solver. Finally, both the Strang splitting scheme and a well-balanced splitting scheme are implemented in this solver. The newly added features are then assessed and validated via a series of auto-ignition tests, a perfectly stirred reactor, a 1D unstretched laminar premixed flame, a 2D counter-flow laminar diffusion flame, and a 3D turbulent partially premixed flame (Sandia Flame D). It is shown that the well-balanced property is crucial for splitting schemes to accurately capture the ignition and extinction events. To facilitate the understanding on combustion modes and complex chemistry in large scale simulations, two computational diagnostic methods (conservative chemical explosive mode analysis, CCEMA, and global pathway analysis, GPA) are subsequently implemented in the current framework and used to study Sandia Flame D for the first time. It is shown that these two diagnostic methods can extract the flame structure, combustion modes, and controlling global reaction pathways from the simulation data.

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.

Sooting tendency of n-heptane/anisole blends in laminar counter-flow flames with oxygen enrichment

Anisole has been recognized as a potential alternative fuel to lower soot emission in internal combustion engines. Thus, in the present study, soot formation by n-heptane/anisole blends in a counter-flow laminar diffusion flame have been measured by using the Line of Sight Attenuation (LOSA). Several main factors related to the sooting tendency were analyzed in detail, including the anisole proportion, oxygen content, and fuel and oxidant flow velocities. Increasing the anisole proportion (0–30% in mole fraction) and oxygen content (30–40% in volume fraction) in oxidant promoted soot formation in the flame, and the maximum soot volume fraction could be fitted as an exponential function of the anisole proportion. Higher fuel and oxidant flow velocities resulted in greater strain rates and a shorter residence time for the reaction mixture, which ultimately inhibited soot formation.

Influence of the n-dodecane chemical mechanism on the CFD modelling of the diesel-like ECN Spray A flame structure at different ambient conditions

Encouraged by the diversity of n-dodecane chemical mechanisms currently available, this investigation focuses on analysing the impact of using different fuel oxidation schemes on the diesel-like Engine Combustion Network (ECN) Spray A flame structure, simulated by means of an Unsteady Flamelet Progress Variable (UFPV) combustion model. The present research discusses systematically the characteristics of four n-dodecane chemical mechanisms in perfectly stirred reactors and counterflow laminar diffusion flames (flamelets) before the final evaluation in turbulent reacting sprays in order to describe the effects of adding different physical levels of complexity to the ignition of the mixtures. In addition, this analysis is complemented with the description of the effect of the boundary conditions on the flame structure.Results evidence the extreme importance of the low temperature chemistry including the period for which the cool flame extends. The different prediction of this stage between mechanisms leads to noticeable different laminar flame structures which in turn produce substantially distinct turbulent flames, especially in the vicinity of the lift-off length (LOL) in terms of reactivity and positioning in the Z-T map. Finally, simulations confirm the strong effect of the boundary conditions, especially for the ambient temperature, on the ignitable mixtures which directly impacts on the soot precursors formation.

Study of SiCl4/H2/O2 chemical kinetics and its application to fused silica glass synthesis

ABSTRACTA comprehensive computational model is first presented, which is capable of describing multispecies turbulent reacting flow and particle formation and deposition. Then a detailed mechanism for SiCl4/H2/O2 reaction system is proposed and applied to the prediction of a counterflow laminar diffusion flame. Good agreement in flame temperature is obtained between numerical results and experimental data, and the augmented 30-reaction SiCl4 submechanism in the detailed mechanism has a significant impact on the SiO2 formation rate. To employ the detailed mechanism in three-dimensional simulations, the reduction of reaction mechanism is performed to eliminate inconsequential species and reactions. The physiochemical process in a silica glass furnace is first simulated with the detailed mechanism to obtain realistic sample compositions. Then, the directed relation graph (DRG) method is employed to construct three comprehensive skeletal mechanisms by considering all accessed compositions in the fused silica glass synthesis, and further analyzethe governing chemical kinetics. The predicted total SiO2 deposition on glass ingot is found to depend on mechanism reduction, and the species SiCl and HOCl are important for the formation of SiO2 and OH. While only two species and seven reactions are removed from the detailed mechanism even though the reduction threshold error of 0.15 has been employed. It is also found that the number of important chemical species and reactions is significant only in a narrow region, which indicates a great potential in local adaptive mechanism reduction. Therefore, the dynamic adaptive chemistry (DAC) is used to generate a small locally valid skeletal mechanism. For the same accuracy in temperature and SiO2 mass fraction, DAC method obtains higher speedup factor than that of DRG method.

A 1D ionic transport model for nonlinear response analysis of a counterflow laminar diffusion flame in DC electric fields

The ionic wind effect is a well-known phenomenon in electric-field-assisted combustion in which the convective bulk flow can be modified by the collision of charged species with neutrals in the reaction region. This paper suggests an essential model based on 1D ionic transport equations to describe the ionic wind effect responsible for nonlinear responses of diffusion flame positions in a counterflow burner with DC voltage variations. The characteristics of ionized layers that substitute for counterflow diffusion flames were compared by applying the effects of one-way (unidirectional) and two-way (bidirectional) ionic wind models. The calculation results showed that the two-way ionic wind model was able to qualitatively predict the unique features of the flame positions in the present experiments and general trends in the electric current density reported in previous literature. The qualitative characteristics of the ionized layer can be classified into three regimes: (1) one-way ionic wind (OIW)-dominated regime, (2) transition regime, and (3) two-way ionic wind (TIW)-induced regime. The details are discussed in this article. The results of this study supported the importance of the two-way ionic wind effect for nonlinear responses of counterflow diffusion flames in DC electric fields.

The effects of incident electric fields on counterflow diffusion flames

Numerical simulations of counterflow laminar diffusion flames impinged by sub-breakdown DC electric fields are performed in this work using multi-component transport and a detailed chemical mechanism for methane–air combustion that includes elementary steps for the conversion of six electrically charged species. The electric field is induced by two electrodes located on the oxidizer and fuel sides and arranged parallel to the mixing layer in a configuration resembling the one recently studied experimentally by Park et al. (2016) [1], which unveiled significant electric-field effects on the aerodynamics. In these simulations, the electric drift of the charged species leads to a bi-directional ionic wind that is axially directed toward both injectors. The major components of the ionic winds are the H3O+ and O2− ions, which are steered by the electric field into the fuel and oxidizer streams, respectively. At sufficiently high electric fields of the order of a few kilovolts per centimeter, the ionic wind intricately couples with the aerodynamic field of neutral molecules flowing into the burner, in a manner that ultimately leads to non-negligible disturbances of the velocity field. The overall effect of these interactions consists of a decrease in the local strain rate and in the stoichiometric scalar dissipation rate, which increases the burning rate of the diffusion flame. The functional form of the scalar dissipation rate depends on the applied electric field, which may have consequences for the subgrid-scale modeling of these processes. In contrast to electrified one-dimensional premixed flames, here the current ceases to vary monotonically with the voltage as a result of the coupling with the aerodynamic field. Comparisons between the present numerical simulations and the experiments performed by Park et al. (2016) [1] are made that indicate qualitative agreement. Quantitative disagreements related to the saturation intensities and to the strength of the electric disturbances of the velocity field are discussed, and possible sources of these discrepancies are identified.

Evaluation of the approximated diffusion flamelet concept using fuels with different chemical complexity

• The approximated diffusion flamelet (ADF) model is assessed. • Three fuels of different chemical complexity are evaluated. • A method is proposed for reconstructing the autoignition of the homogeneous reactors. • A limitation in the chemical term is proposed for complex chemical fuels. • Excellent results are obtained in the autoignition range by the ADF model. The ability of flamelet models to reproduce turbulent combustion in devices such as diesel engines or gas turbines has enhanced the usage of these approaches in Computational Fluid Dynamics (CFD) simulations. The models based on turbulent look-up tables generated from counterflow laminar diffusion flames (DF model) permit drastic reduction of the computational cost of the CFD calculation. Nevertheless, for complex molecular fuels, such as n-heptane, the oxidation process involves hundreds of species and the calculation of the transport equations together with the ODE system that models the chemical kinetics for the DF solution becomes unaffordable for industrial devices where hundreds of flamelets are required. In this context, new hypotheses have to be introduced in order to reduce the computational cost maintaining the coherence of the combustion process. Recently, a new model known as Approximated Diffusion Flamelet (ADF) has been proposed with the aim of solving the turbulent combustion for complex fuels in a reduced time. However, the validity of this model is still an open question and has to be verified in order to justify subsequent CFD calculations. This work assesses the ADF model and its ability to reproduce accurately the combustion process and its main parameters for three fuels with different chemical complexity and boundary conditions by its comparison with the DF model. Results show that although some discrepancies arise, the ADF model has the ability to correctly describe the ignition delay and the combustion structure in the auto-ignition zone that is the most relevant one for industrial processes.

The characteristics and mechanism of the NO formation during oxy-steam combustion

Abstract Oxy-fuel combustion has been deemed a promising technology to reduce CO2 emissions from coal-fired power stations, and oxy-steam combustion technology is regarded to be the next-generation oxy-fuel combustion technology. The characteristics and mechanism of NO formation were numerically studied in the counter-flow laminar diffusion flame using methane as fuel. The comparisons of the N conversion rate in the oxy-steam combustion with that in air combustion under a similar temperature profile show that the emission of NO is enhanced due to high concentration of H2O during oxy-steam combustion compared with air combustion. The chaperone effect of H2O and a high mole fraction of H2O are important reasons for the enhancement of NO formation during oxy-steam combustion because H + NO + M ⇔ HNO + M and HNO + OH ⇔ NO + H2O are enhanced dramatically. The key elementary reaction for the enhancement of NO production during oxy-steam combustion is NH + H2O ⇔ HNO + H2 because it produces a large amount of HNO due to a high mole fraction of H2O. The nitrogen conversion rate through the pathway NH2 → NH → HNO → NO is dramatically enhanced and predominant during oxy-steam combustion. The nitrogen conversion rate increases with the increase in the mole fraction of H2O in O2, and equivalence ratio has a minimal impact on the NO formation during oxy-steam combustion.

Challenges and artifacts of probing high-pressure counterflow laminar diffusion flames

An experimental study was conducted on two C2H4/O2/N2 laminar diffusion flames with identical compositions and strain rate, but operated at 0.29MPa and 2.5MPa, respectively, to assess pressure effects on the onset of soot formation. The low-pressure flame was permanently blue, whereas the high-pressure one was under conditions of incipient sooting. Both flames had nominally identical temperature–time history. Using gas sampling through quartz microprobes followed by GC/MS analysis and addressing potential artifacts associated with the diagnostic intrusiveness and probe-induced chemistry, we demonstrated that the flame structure was resolved even at the highest pressures, as shown by the good agreement with respect to major species between the experimental results and a one-dimensional computational model of the flame with detailed chemistry and transport. The increase in soot propensity correlates with an increase in the concentration of soot precursors like the aromatics and acetylene. Comparison with model predictions shows that the concentration of aromatics is systematically overpredicted. An ancillary computational study to estimate the sensitivity to changes in the velocity boundary conditions showed that minor species like the aromatics are sensitive to these details under certain conditions, because of changes in the temperature–time history that affects predominantly the slow chemistry.

Soot Formation in Co- and Counter-flow Laminar Diffusion Flames of Binary Mixtures of Ethylene and Butane Isomers and Synergistic Effects

Soot volume fractions of binary mixtures of butane isomers, ethylene-butane isomers, and propanebutane isomers were evaluated experimentally in diffusion flames on both co- and counter-flow burners. Soot volume fractions were measured by two-dimensional line of sight attenuation of a broadband arc lamp generated light in co-flow flames, whereas in counter-flow flames, attenuation of a small radius laser beam was used. Binary mixtures of iso-butane and n-butane did not show any synergistic effects on soot formation. On the other hand, either n-butane or iso-butane addition to ethylene caused a strong synergistic effect in both types of flames that the soot volume fractions were higher than those of the individual mixture components under the same flame conditions. Binary mixtures of propane and butane isomers, however, didnotdisplay any measurable synergisticeffect onsoot formation.These observations were discussed in the light of mechanisms proposed by previous investigators to explain the synergistic effects detected in the flames of binary mixtures. Current results cast doubt on the universality of the dominance of any of the mechanisms previously proposed to explain the synergistic effects observed with some binary hydrocarbon mixtures. Most of the soot formation studies focus on single-hydrocarbon fuels because of the complex nature of soot mechanismsinflames.Ontheotherhand,mixturesofhydrocarbons, especially binary mixtures, are studied, although to a limited extent, to identify the probable soot formation channels associated with potential interactions between pyrolysis products of the hydrocarbons in the mixture. Several of these studies identified a synergistic effect when two pure hydrocarbons are mixed as the fuel. The synergistic effect in soot formation is described as the measured (or computed) increasesofthesootyieldofabinaryhydrocarbonmixtureover the soot concentrations of any of the two components in the mixture individually under similar flame conditions. The synergistic effect is observed if the addition of one single component fuel to another intensifies some production channels for the formation of soot by the interaction of the pyrolysisproductsofthesefuels.Availablefuelmixtureresults in the literature provide some insight into the precursors that might be the culprit for the synergistic effects. However, there exist significant disagreement among researchers about the results of the fuel mixture experiments and the interpretation of these results. 1-3

Prediction and Measurement of Fuel Cell Flammability Safety Limits

Leakage of the fuel through a possible crack in the solid electrolyte or in the separator plate of a fuel cell, can cause uncontrolled combustion. An experimental investigation and theoretical prediction are made to simulate the above conditions. A cylindrical combustion chamber is supplied with an axial jet of methane from one side and air (or pure oxygen) jet from the other side to create a counter flow laminar diffusion flame. A disk with a small central hole is installed inside the cylinder perpendicular to its axis. It was found that the quenching distance, between oxidant nozzle and the disk, is of the order of 1.0 mm when oxygen is the oxidant while for air it is about 15 mm. These distances give an upper limit for the oxidant gap size of the fuel cell, based on safety considerations. Moreover, the measurements show that fuel cells are safer when using air as oxidant rather than pure oxygen. A new computer code is developed capable of simulating flow, heat transfer, and chemical reactions in 2D geometries, similar to the above flammability experiments. The governing laminar equations for momentum, mass, and mixture fraction are solved by a numerical iterative algorithm. The computed mixture fraction is used to calculate its dissipation rate. They are then used to access a flamelet library to obtain the local flame properties and/or its possible extinction. This flamelet library is generated separately, based on a 3-step reaction mechanism for CH4. The computed flammability distances for CH4–O2 and CH4–air flames are in reasonable agreement with, though slightly lower than, the corresponding measured quenching distances. This is a result of neglecting thermal radiation in the present combustion model.

Role of Elementary Reactions in Flame Structure and Unsteady Behavior of Two-Dimensional Fuel Jet Diffusion Flame.

A direct numerical simulation of two-dimensional fuel jet flames developed in a co-flowing air stream was made considering rather complex finite chemistry in order to clarify the role of elementary reactions in flame structure and its unsteady behavior. The governing equations were discretized and numerically integrated using the finite volume method. The temperature dependence of thermodynamical properties was taken into account and the transport properties were calculated according to the simplified transport model proposed by Smooke [Reduced Kinetic Mechanisms and Asymptotic Approximations for Methane-Air Flames, (1991), p.1-28, Springer-Verlag]. Chemical reactions were described by Smooke's skeletal methane-air reaction mechanism. It was found that large-scale fluctuations were produced in the downstream of the jet flame where a decrease in temperature occurred at some instant, leading to local extinction. Upon comparison with the results of a counterflow laminar diffusion flame, it was also found that the hypothesis of the laminar flamelet model could be accepted even for the case of the unsteady jet flame with local extinction. Furthermore, better understanding of the extinction mechanism was obtained : In the turbulent region of the flame, fuel is converted by large vortices into the reaction zone at high supply rates, inducing CH3, CH3O, CH2O and HCO production. At some instant, the increase in mixing rates will result in a decrease in temperature, causing the consumption of available active radicals due to the activity of some exothermic reactions, and the slowdown of the reactions producing new OH, O and H radicals. The radical pool is no longer available for chain reactions to proceed, resulting in an extinction.

Role of Elementary Reactions on Flame Structure and Unsteady Behavior of Two-Dimensional Fuel Jet Diffusion Flame.

A direct numerical simulation of two-dimensional fuel jet flames developed in a co-flowing air stream was made considering rather complex finite chemistry in order to clarify the role of elementary reactions on flame structure and its unsteady behavior. The governing equations were discretized and numerically integrated by using a finite volume method. The temperature dependence of thermodynamical properties was taken into account and the transport properties were calculated according to the simplified transport model proposed by Smooke et al. Chemical reactions were described by the Smooke's skeletal methane-air reaction mechanism. It was found that large-scale fluctuations were produced in the downstream where a decrease in temperature occurred at some instants leading to local extinction. Comparing with the results of counterflow laminar diffusion flame, it was also found that the hypothesis of laminar flamelet model can be accepted even for the case of the unsteady jet flame with the local extinction. Furthermore, a better understanding of extinction mechanism was obtained : In the turbulent region of the flame, fuel is converted by the large-scale vortices into the reaction zone at high supply rates inducing CH3, CH3O, CH2O and HCO production. At some instant, the increase of mixing rates will results in a decrease of temperature causing the consumption of available active radicals due to the activity of some exothermic reactions, and the slow-down of the reactions producing new OH, O, H radicals. The radical pool is no longer available for chain reactions to proceed, resulting in an extinction.

Planar Laser Induced Fluorescence Measurement of [ OH] in the Stabilization Stage of a Spray Jet Flame

Planar laser induced fluorescence of OH is quantitatively applied in the lift-off region of a spray jet flame. The spray is produced by a coaxial air-blast injector fed with liquid methanol. OH is laser excited through the (0,1) vibrational band so that the shifted fluorescence emitted via the ( 0,0) and ( 1,1) bands can be spectrally discriminated against elastic scattering by the methanol drops. The lift-off location is determined at each laser shot making it possible to investigate the reactive structures in their own coordinate system. Statistical analysis of the fluorescence images shows that the lifted flame may develop in the remarkable structure of two diverging diffusion-like fronts as was predicted by modeling of spray combustion in a counterflow laminar diffusion flame. Calibration of the fluorescence imaging system into absolute OH concentration is performed in a laminar diffusion flame of methanol vapor where [ OH] is determined in situ by absorption measurement with the same optical device. The OH concentration conditioned on the elevation above the lift-off is found to hold a constant superequilibrium value about four times the equilibrium level.

Temperature and concentration measurements by CARS in counterflow laminar diffusion flames

The combination of broadband CARS thermometry on nitrogen with the narrowband technique, for detection of a second major species, is applied to counterflow laminar propane-air diffusion flames. Temperature profiles are measured for various conditions of strain and equivalence ratio. Comparisons with 1D calculations of the flow with a detailed kinetic mechanism are satisfactory. Some measurements of CO concentrations are performed. Feasability of temperature measurements when the propane is replaced by liquid fuel (heptane) is demonstrated.