Effects of fuel droplet size on soot formation in spray flames formed in a laminar counterflow

The effects of fuel droplet size on soot formation in spray flames formed in a laminar counterflow are investigated experimentally and numerically. Sauter mean diameter (SMD) of quasi-monodispersed fuel spray (n-decane) is carefully controlled independently from the other spray characteristics using a frequency-tunable vibratory orifice atomizer, and the two-dimensional spatial distributions of soot volume fraction and soot particle size are measured by laser induced incandescence (LII) and time resolved LII (TIRE-LII), respectively. In addition, the soot formation processes are examined in detail by a two-dimensional direct numerical simulation (DNS) employing a kinetically based soot model with flamelet model. The results show that the soot formation area and location are strongly affected by the SMD of the fuel spray. As the SMD of the fuel spray increases, the average soot formation area expands, whereas local suppression of soot formation is instantaneously observed in the spray flames because of the appearance of groups of unburned droplets. The size of soot particles tend to be larger in the outer part of the soot formation area compared to soot in the inner part. This is because the surface growth of soot particles markedly proceeds compared to the coagulation and oxidation.     

NO formation in counterflow partially premixed flames

An experimental and computational study of NO formation in low-strain-rate partially premixed methane counterflow flames is reported. For progressive fuel-side partial premixing the peak NO concentration increased and the NO distribution along the stagnation streamline broadened. New temperature-dependent emissivity data for a SiO₂-coated Pt thermocouple was used to estimate the radiation correction for the thermocouple, thus improving the accuracy of the reported flame temperature. Flame structure computations with GRIMech 3.00 showed good agreement between measured and computed concentration distributions of NO and OH radical. With progressive partial premixing the contribution of the thermal NO pathway to NO formation increases. The emission index of NO (EINO) first increased and then decreased, reaching its peak value for the level of partial premixing that corresponds to location of the nonpremixed reaction zone at the stagnation plane. The observation of a maximum in EINO at a level of partial premixing corresponding to the nonpremixed reaction zone at the stagnation plane seems to be a consistent feature of low (<20 s⁻¹)-strain-rate counterflow flames.     

Analysis of the laminar flamelet concept for nonpremixed laminar flames

The goal of this paper is to investigate the application of the laminar flamelet concept to the multidimensional numerical simulation of nonpremixed laminar flames. The performance of steady and unsteady flamelets is analyzed. The deduction of the mathematical formulation of flamelet modeling is exposed and some commonly used simplifications are examined. Different models for the scalar dissipation rate dependence on the mixture fraction variable are analyzed. Moreover, different criteria to evaluate the Lagrangian-type flamelet lifetime for unsteady flamelets are investigated. Inclusion of phenomena such as differential diffusion with constant Lewis number for each species and radiation heat transfer are also studied. A confined co-flow axisymmetric nonpremixed methane/air laminar flame experimentally investigated by McEnally and Pfefferle (Combust. Sci. Technol. 116-117 (1996) 183-209) and numerically investigated by Bennett, McEnally, Pfefferle, and Smooke (Combust. Flame 123 (2000) 522-546), Cònsul, Pérez-Segarra, Claramunt, Cadafalch, and Oliva (Combust. Theory Modelling 7 (3) (2003) 525-544), and Claramunt, Cònsul, Pérez-Segarra, and Oliva (Combust. Flame 137 (2004) 444-457) has been used as a test case. Results obtained using the flamelet concept have been compared to data obtained from the full resolution of the complete transport equations using primitive variables. Finite-volume techniques over staggered grids are used to discretize the governing equations. A parallel multiblock algorithm based on domain decomposition techniques running with loosely coupled computers has been used. To assess the quality of the numerical solutions presented in this paper, a verification process based on the generalized Richardson extrapolation technique and on the grid convergence index (GCI) has been applied.     

Strained flamelets for turbulent premixed flames, I: Formulation and planar flame results

A strained flamelet model is proposed for turbulent premixed flames using scalar dissipation rate as a parameter. The scalar dissipation rate of reaction progress variable is a suitable quantity to describe the flamelet structure since it is governed by convection-diffusion-reaction balance and it is defined at every location in the flamelets, which are represented by laminar flames in reactant-to-product opposed flow configuration. The mean reaction rate is obtained by using the flamelets reaction rate and the joint pdf of the progress variable and its dissipation rate. The marginal pdf of the progress variable is presumed to be β-pdf and the pdf of the conditional dissipation rate is taken to be log-normal. The conditional mean dissipation rate is obtained from modelled mean dissipation rate. This reaction rate closure is assessed using RANS calculations of statistically planar flames in the corrugated flamelets and thin reaction zones regimes. The flame speeds calculated using this closure are close to the experimental data of Abdel-Gayed et al. (1987) [27] for flames in both the regimes. Comparisons with other reaction rate closures showed the benefits of the strained flamelets approach.     

Numerical and experimental study on silica generating counterflow diffusion flames

Temperatures and concentrations of OH radicals in silica generating counterflow oxy-hydrogen diffusion flames are measured using a broadband coherent anti-Stokes Raman spectroscopy (CARS) and a planar laser induced fluorescence (PLIF) techniques to study thermo-chemical effects of SiCl₄ addition to flames. Numerical analysis considering detailed chemical reactions including silica generating reactions is also conducted. The experimental results demonstrate that temperatures decrease in preheated zone due to the increase in specific heat of the gas mixture while the decrease is mitigated in particle formation zone due to the heat release through hydrolysis and oxidation reactions of SiCl₄. Also, OH concentrations significantly decrease in silica formation flame, which can be attributed to the consumption of oxidative radicals during the silica generating reactions of SiCl₄ and depletion of OH by HCl. The numerical simulation agrees well for flames having relatively low flame temperatures of 1750 K but underestimates the decrease in OH concentration for high temperature flame over 2700 K. The disagreement for the high temperature flames would imply possible OH consumption via direct reactions between OH radicals and silicon chlorides, which is expected to be highly sensitive to temperature.     

Stable negative edge flame formation in a counterflow burner

In nonpremixed combustion, edge flames can form as a region of flame propagation or flame recession. Forwardly propagating edge flames, as occur in lifted flames, have a local gas velocity at the flame edge that is from unburned partially premixed fuel and air into the flame. These flames represent an ignition process, and permit the flame itself to either stabilize against an incoming gas stream or propagate into unburned fuel and air. Negative edge flames represent the opposite case of a local gas velocity from burned products through the flame edge. The negative edge flame represents a local extinction process, and occurs, for example, during vortex-induced extinction of a nonpremixed flame sheet. A technique for generating steady negative edge flames in a standard counterflow burner is presented, which permits detailed examination of their properties. A coannular counterflow burner is used to create a strain gradient that quenches a central diffusion flame. Unlike previous research on strain-induced flame edges, the axisymmetric flow field ensures gas flow from products through the edge. Measurements of the edge flame's sensitivity to global strain rates and fuel mixtures are presented, along with measurements of the edge flame structure using OH fluorescence and CH emission imaging.     

Implementation of two-equation soot flamelet models for laminar diffusion flames

The two-equation soot model proposed by Leung et al. [K.M. Leung, R.P. Lindstedt, W.P. Jones, Combust. Flame 87 (1991) 289-305] has been derived in the mixture fraction space. The model has been implemented using both Interactive and Non-Interactive flamelet strategies. An Extended Enthalpy Defect Flamelet Model (E-EDFM) which uses a flamelet library obtained neglecting the soot formation is proposed as a Non-Interactive method. The Lagrangian Flamelet Model (LFM) is used to represent the Interactive models. This model uses direct values of soot mass fraction from flamelet calculations. An Extended version (E-LFM) of this model is also suggested in which soot mass fraction reaction rates are used from flamelet calculations. Results presented in this work show that the E-EDFM predict acceptable results. However, it overpredicts the soot volume fraction due to the inability of this model to couple the soot and gas-phase mechanisms. It has been demonstrated that the LFM is not able to predict accurately the soot volume fraction. On the other hand, the extended version proposed here has been shown to be very accurate. The different flamelet mathematical formulations have been tested and compared using well verified reference calculations obtained solving the set of the Full Transport Equations (FTE) in the physical space.     

Numerical and experimental investigations on the influence of preheating and dilution on transition of laminar coflow diffusion flames to Mild combustion regime

A numerical and experimental study has been carried out to acquire knowledge about the structure and stabilization mechanism of coflow flames in their transition to the Mild combustion regime. In total, three CH₄/N₂/oxidizer coflow flames have been studied with a systematic dilution and preheating of the fuel and coflow streams. These flames comprise the non-preheated case (Case NP), preheated case (Case P) and Mild case (Case M), diluted and preheated from ambient temperature up to 1530 K. Radial profiles of temperature and species concentrations have been measured using spontaneous Raman scattering. Detailed computations have been performed by steady-state simulations of these cases using detailed chemistry with the GRI 3.0 mechanism, multi-component mixture-averaged transport and an optically thin approximation for radiative heat losses. An overall good agreement has been found between results of the detailed computations and experiments for Case NP, Case P and at lower axial distances for Case M. The importance of using multicomponent transport and radiative heat losses in the computations has been investigated by performing additional computations with more simplified models for Case NP. A comparison of computed temperature distributions indicates that the progressive preheating and dilution of the oxidizer and fuel leads to a reduction of the temperature rise in the reaction zone with respect to a non-reacting case; this rise in Case M is less than 200 K. Comparison of computed heat release and CH₂O distributions reveals that stabilization of Case NP and P occurs by an edge flame, while for Case M, it takes place by autoignition. Further investigations on the structure of Case M has been done by flamelet analyses in mixture fraction space. It is found that igniting flamelets, in contrast to steady flamelets, represent very well the structure of Case M at lower axial distances. This observation further emphasizes the stabilization of the Mild case by the autoignition phenomena.     

Synergistic effect on soot formation in counterflow diffusion flames of ethylene-propane mixtures with benzene addition

The characteristics of the formation of polycyclic aromatic hydrocarbon (PAH) and soot in counterflow diffusion flames of ethylene/propane mixtures have been investigated experimentally to identify the effect of fuel structure. The synergistic effect, that is, the enhancement of PAH and soot formation by the fuel mixing of ethylene and propane has been further analyzed to examine the suggested mechanisms based on the competition between PAH and soot growths through the H-abstraction-C₂H₂-addition (HACA) mechanism and the incipient ring formation through the propargyl recombination reaction. To mitigate the effect of incipient ring formation on the synergistic effect, a small amount of benzene was added to the fuel stream. Planar laser-induced incandescence and laser-induced fluorescence techniques were employed to measure relative soot volume fractions and PAH concentrations, respectively. Results showed that the synergistic effect on soot formation remained, even though the synergistic effects for relatively small aromatics mitigated with the benzene addition. Larger size PAHs have shown enhanced synergistic effects compared to smaller size PAHs regardless of benzene addition. These results implied that the role of propane mixing on the synergistic effect cannot be explained solely by the incipient ring formation via a propargyl recombination reaction; thus, it is suggested that the C₃ pathways could also contribute to the growth of PAH species.     

Effect of oxygen-rich combustion on soot formation in laminar co-flow propane diffusion flames

Oxygen-rich combustion is a new type of clean combustion technology with important application prospects. In this work, the effects of oxygen-rich combustion on soot formation in the propane/(O₂+N₂) laminar flow coaxial jets diffusion flame were numerically investigated by using the detailed gas-phase chemical reaction model with the mechanism of tetracyclic aromatic hydrocarbons and the complex thermodynamic properties and transport characteristics parameters. Soot surface growth follows the hydrogen-abstraction-carbon-addition (HACA) model. A hybrid gas-phase mechanism was adopted, which contains a DLR-based polycyclic aromatic hydrocarbons (PAHs) formation, growth model and a gas-phase model. Results show that the oxygen-rich combustion has a great influence on the flame temperature, especially the high temperature region. With the increase of oxygen concentration, the soot formation region of flame broadens and the maximum of soot volume fraction increase from 3.95 ppm to 10.87 ppm. The extra oxygen makes PAHs increased around the nozzle, leading to larger rate in early soot nucleation and surface growth, eventually more soot yield.     

Premixed edge flame in a counterflow field with a stretch rate gradient

An edge flame was established in a counterflow field with a stretch rate gradient using twin rectangular burners which were misaligned by a few degrees. The stretch rate gradient was quantitatively defined as a function of the angle between the two burners and the distance from the edge of the burner, and thus the effect of stretch rate gradient on the behavior of the edge flame was investigated. The local chemical reaction rate at the edge of a CH₄/air flame was stronger than that at other parts of the flame. On the other hand, the reaction rate at the edge of a C₃H₈/air flame was weaker than that of other parts of the flame. The curvature of the flame edge of the CH₄/air flame was much larger than that of the C₃H₈/air flame. These results are thought to be due to the effect of the Lewis number. The ratios of the local stretch rate at the flame edge to the extinction stretch rate for planar twin flames with the same composition as the edge flame were 0.5 to 0.7 for the CH₄/air flame and 0.6 to 0.8 for the C₃H₈/air flame. These values were midway between those in the numerical simulation by Daou and Linan and those in the experiment by Liu and Ronney. Moreover, it was shown that an increase in the stretch rate gradient resulted in a lower local stretch rate at the flame edge. Behavior of the edge flames did not depend on the Lewis number of the mixture.     

Flame structure and global flame response to the equivalence ratios of interacting partially premixed methane and hydrogen flames

The interacting partially premixed methane and hydrogen flames established in a one-dimensional counterflow field were investigated numerically with the OPPDIF code and GRI-v3.0 was used to consider both fuels. The flame structure and response of the maximum flame temperature, heat-release rate, and flame speed to the equivalence ratios (Φ) and global strain rate (a_g) were investigated. The maximum temperature decreased with increasing a_g. The maximum temperature for cases with a stoichiometric hydrogen-side flame was higher than for other cases with the same a_g.The hydrogen-side flame played a key role in determining the maximum temperature. The maximum heat-release rates (MHRRs) for all cases show different trends. The MHRR of the methane-side flame was affected considerably by the interacting flame structure and hydrogen-side flame condition. However, the MHRRs of the hydrogen were independent of methane-side flame condition. For the cases where Φ of the methane-side flame was varied while the hydrogen-side flame was kept stoichiometric (Var-S), the MHRR and flame speed of the hydrogen-side flame were independent of the methane-side flame conditions. However, the methane-side flames had a negative flame speed except near-stoichiometric conditions. On the other hand, in the cases where Φ of the hydrogen-side flame was varied while the methane-side flame was kept stoichiometric (S-Var), the hydrogen-side flames had the MHRR and flame speed similar to those of an unstretched partially premixed hydrogen flame.     

The response of buoyant laminar diffusion flames to low-frequency forcing

Buoyant jet diffusion flames are frequently used to investigate phenomena associated with flares or fires, such as the formation and emission of soot, polycyclic aromatic hydrocarbons (PAH), and carbon monoxide (CO). To systematically investigate the influence of transient vortex-flame interactions on these processes, laminar jet flames may be periodically forced. Previous work has demonstrated that forcing the fuel stream at a (low) frequency close to the natural buoyant instability frequency will trigger the production of vortices on the air side of the high-temperature reaction zone, coupling the overall flame response to the forcing frequency. In the work reported here, measurements in methane/air and ethylene/air slot flames show that over a substantial range of forcing frequencies and amplitudes, the dominant, air-side vortex production is locked at precisely one-half the excitation frequency of the fuel stream. This phenomenon is examined in detail through the utilization of several laser diagnostic techniques, yielding measurements of both the frequency response of the flames and phase-locked images of the internal flame structure. Under some conditions the subharmonic response of the flame leads to transient separation of the PAH and soot layers from the surrounding high-temperature flame zone, potentially affecting the soot formation and radiation processes. This data should provide useful information for comparison with detailed modeling aimed to improve the understanding of the complex nature of the buoyant instability in jet flames.     

Numerical study on CH4 laminar premixed flames for combustion characteristics in the oxidant atmospheres of N2/CO2/H2O/Ar-O2

The freely-propagating laminar premixed flames of CH₄–N₂/CO₂/H₂O/Ar-O₂ mixtures were conducted with the PREMIX code. The effects of the equivalence ratio and various oxidant atmospheres on the basic combustion characteristics were analyzed with the initial pressure and temperature of 1 atm and 398 K, respectively, O₂ content in the oxidant of 21%. The chemical reaction mechanism GRI-Mech 3.0 was chosen to determine the effects of the oxidant atmospheres of N₂/O₂, CO₂/O₂, H₂O/O₂, and Ar/O₂ on the adiabatic flame temperature, laminar burning velocity, flame structure, free radicals, intermediate species, net heat release rate and specific heat of the fuel/oxidant mixtures. The numerical results show that the maximum adiabatic flame temperatures and laminar burning velocities are at Ar/O₂ atmosphere. The mole fractions of CO and H₂ increased fastest at CO₂/O₂ atmosphere and H₂O/O₂, respectively. The mole fractions of CH₃ and H follow the order Ar/O₂> N₂/O₂>H₂O/O₂>CO₂/O₂. In addition, for 4 oxidant atmospheres, the peak mole fraction of C₂H₂ is following the order H₂O/O₂>Ar/O₂>N₂/O₂>CO₂/O₂ and the net heat release rate is following the order Ar/O₂>N₂/O₂>H₂O/O₂>CO₂/O₂ for all equivalence ratios.     

Experimental and computational study of methane counterflow diffusion flames perturbed by trace amounts of either jet fuel or a 6-component surrogate under non-sooting conditions

The chemical structure of a methane counterflow diffusion flame and of the same flame doped with 1000 ppm (molar) of either jet fuel or a 6-component jet fuel surrogate was analyzed experimentally, by gas sampling via quartz microprobes and subsequent GC/MS analysis, and computationally using a semi-detailed kinetic mechanism for the surrogate blend. Conditions were chosen to ensure that all three flames were non-sooting, with identical temperature profiles and stoichiometric mixture fraction, through a judicious selection of feed stream composition and strain rate. The experimental dataset provides a glimpse of the pyrolysis and oxidation behavior of jet fuel in a diffusion flame. The jet fuel initial oxidation is consistent with anticipated chemical kinetic behavior, based on thermal decomposition of large alkanes to smaller and smaller fragments and the survival of ring-stabilized aromatics at higher temperatures. The 6-component surrogate captures the same trend correctly, but the agreement is not quantitative with respect to some of the aromatics such as benzene and toluene. Various alkanes, alkenes and aromatics among the jet fuel components are either only qualitatively characterized or could not be identified, because of the presence of many isomers and overlapping spectra in the chromatogram, leaving 80% of the carbon from the jet fuel unaccounted for in the early pyrolysis history of the parent fuel. Computationally, the one-dimensional code adopted a semi-detailed kinetic mechanism for the surrogate blend that is based on an existing hierarchically constructed kinetic model for alkanes and simple aromatics, extended to account for the presence of tetralin and methylcyclohexane as reference fuels. The computational results are in reasonably good agreement with the experimental ones for the surrogate behavior, with the greatest discrepancy in the concentrations of aromatics and ethylene.     

Radiation-induced uncertainty in laminar flame speed measured from propagating spherical flames

Laminar flame speeds measured using the propagating spherical flame method are inherently affected by radiation. Under certain conditions, a substantial uncertainty in laminar flame speed measurement is caused by radiation, which results in a great concern for kinetic mechanism validation and development. In this study, numerical simulations with detailed chemistry and different radiation models are conducted to examine the effects of radiation on spherical flame propagation. The emphasis is placed on quantifying the uncertainty and corrections associated with radiation in laminar flame speed measurements using propagating spherical flames. The radiation effects on flame speeds at normal and elevated temperatures and pressures are examined for different fuel/air mixtures including methane, propane, iso-octane, syngas, hydrogen, dimethyl ether, and n-heptane. The radiative effects are conservatively evaluated without considering radation reflection on the wall. It is found that radiation-induced uncertainty in laminar flame speeds is affected in the opposite ways by the initial temperature and pressure. An empirical correlation quantifying the uncertainty associated with radiation is obtained. This correlation is shown to work for different fuels at normal and elevated temperatures and pressures. Therefore, it can be directly used in spherical flame experiments measuring the laminar flame speed. Furthermore, a method to obtain the radiation-corrected flame speed (RCFS) is presented and it can be used for laminar flame speed measurement using the propagating spherical flame method.     

A thermal formulation for single-wall quenching of transient laminar flames

Improving our knowledge of flame-wall interaction is of relevance to performing near-wall combustion calculations. Quenching distance is to be determined accordingly, as a major parameter of flame quenching. For this purpose, an equation describing the behavior of single-wall flame quenching has been derived from a simplified model of laminar flame-wall interaction. It allows evaluating quenching distance from wall heat flux and mixture properties; a significant advantage of this formula is the absence of any empirical coefficient. To assess its reliability, the results computed with this equation have been compared to experimental data concerning laminar flame-wall interaction. For this purpose, single-wall quenching parameters have been recorded in both head-on and sidewall configurations. Quenching distance and wall heat flux have been measured simultaneously, during the combustion of quiescent methane-air mixtures in a constant-volume vessel. Quenching distance is determined through direct visualization, whereas wall heat flux is processed from the time evolution of wall surface temperature. The equation has been verified over the pressure range 0.05-0.35 MPa in stoichiometric and lean mixtures. It shows good agreement with experimental data at first order, with less than 20% variation.     

Effects of fuel inlet boundary condition on aromatic species formation in coflow diffusion flames

In a number of previous numerical studies, the fuel inlet velocity boundary conditions (BC) of coflow diffusion flames were specified at the exit of the fuel nozzle with a parabolic velocity profile. Such choices were based on the assumption that the flow inside the vertical fuel tube is fully developed and the buoyancy has negligible impact on the fuel flow at the nozzle exit. These assumptions, however, might not hold in practical experiments. This study demonstrates it is necessary to account for the effect of inlet BC location to accurately predict the nozzle exit velocity profile as well as the velocity, temperature profiles downstream, which are prerequisites for meaningful polycyclic aromatic hydrocarbon (PAH) and soot prediction in coflow diffusion flames. In particular, laboratory-scale laminar coflow diffusion flames at atmospheric pressure have been studied computationally with a focus on the effects of the fuel inlet velocity profile on PAH formation. Two sets of simulations were conducted which differ in the location specified for the fuel inlet boundary. In the first case, the fuel inlet boundary was specified at the nozzle exit while in the second case it was specified at a distance of 7 cm upstream of the nozzle exit. Parabolic velocity profiles were specified for both cases. In each set of simulations, flames with three different fuels (methane, ethylene and propane) were tested. Detailed high-temperature reaction mechanisms accounting for the formation of aromatic species were employed. The results showed that the fuel inlet BC location notably influence the predicted flow/temperature field and the resultant PAH concentration. Moreover, the effects become more notable with lower fuel stream velocities. It was also found that for propane with a density larger than air, recirculation zones were formed near the nozzle exit which exerted an additional influence on the flow development and temperature field as well as PAH formation. In addition, the effects of nozzle heating on flow development and PAH formation were also investigated.     

Structure of laminar sooting inverse diffusion flames

The flame structure of laminar inverse diffusion flames (IDFs) was studied to gain insight into soot formation and growth in underventilated combustion. Both ethylene-air and methane-air IDFs were examined, fuel flow rates were kept constant for all flames of each fuel type, and airflow rates were varied to observe the effect on flame structure and soot formation. Planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and polycyclic aromatic hydrocarbons (PAH PLIF), planar laser-induced incandescence of soot (soot PLII), and thermocouple-determined gas temperatures were used to draw conclusions about flame structure and soot formation. Flickering, caused by buoyancy-induced vortices, was evident above and outside the flames. The distances between the OH, PAH, and soot zones were similar in IDFs and normal diffusion flames (NDFs), but the locations of those zones were inverted in IDFs relative to NDFs. Peak OH PLIF coincided with peak temperature and marked the flame front. Soot appeared outside the flame front, corresponding to temperatures around the minimum soot formation temperature of 1300 K. PAHs appeared outside the soot layer, with characteristic temperature depending on the wavelength detection band. PAHs and soot began to appear at a constant axial position for each fuel, independent of the rate of air flow. PAH formation either preceded or coincided with soot formation, indicating that PAHs are important components in soot formation. Soot growth continued for some time downstream of the flame, at temperatures below the inception temperature, probably through reaction with PAHs.