Burner-stabilized Stagnation Flame Research Articles

Kinetic modeling of carbonaceous particle morphology, polydispersity and nanostructure through the discrete sectional approach

Carbon nanoparticle (CNP) formation from hydrocarbons combustion is of high interest not only for the study of pollutant (soot) emissions, but, above all, in the area of advanced materials. CNP optical and electronical properties, relevant for practical applications, significantly change with their size, morphology, and nanostructure. This work extends a detailed soot kinetic model, based on the discrete sectional approach, to explicitly incorporate the description of CNP polydispersity, maintaining the CHEMKIN-like format. The model considers various nanosized primary particles, generated from liquid-like counterparts through the carbonization process, which successively grow or aggregate forming fractal structures. The model is validated against experimental measurements from the literature including CNP volume fraction, several morphological characteristics, number density and particle H/C ratio. Data are taken from 19 laminar flames, in different configurations (counterflow diffusion flames, premixed flat flames established on the McKenna-type burner and burner-stabilized stagnation flames) and over a wide range of operating conditions (P=1–10 atm, Tmax=1556-2264 K). The model captures the measured trends of all the analyzed CNP properties as a function of equivalence ratio, residence time and fuel type in premixed flames, and pressure and strain rate in counterflow flames. Model deviations from the experiments are discussed, also in comparison with other state-of-the-art soot models based on different approaches. Sensitivity analyses are performed on carbonization, coalescence, and aggregation rates, which have the largest impact on CNP morphology and are characterized by larger uncertainty compared to elementary chemical pathways.

Predictions of soot formation in burner-stabilized stagnation flames using various reaction mechanisms

Predictions of soot formation in burner-stabilized stagnation flames using various reaction mechanisms

Experimental investigation on the size-dependent maturity of soot particles in laminar premixed ethylene burner-stabilized stagnation flames

To understand soot maturation mechanism, we investigate the size-evolution of soot maturity in a series of laminar premixed burner-stabilized stagnation ethylene flames, with the maximum flame temperature Tf,max ranging from 1630 to 1869 K and the equivalence ratio φ ranging from 2.0 to 2.3. The soot particle size distribution is obtained by using the orifice sampling technique together with a scanning mobility particle sizer, while the maturity of soot is evaluated based on its oxidation characteristics and Raman spectra. The results show that at a fixed φ (say 2.2), soot particles grow bigger and become more mature concurrently in higher Tf,max flames (1809 and 1869 K). In contrast, although particle size increases obviously with time in lower Tf,max flames (1630 and 1724 K), soot maturity remains almost unchanged, indicating that particle growth is mainly controlled by particle coagulation or PAH condensation, instead of surface reactions in these flames. At a fixed Tf,max (say ∼ 1725 K), soot size growth is faster at higher φ as expected. However, when comparing soot particles of similar sizes, the particles from lower φ flames are more mature than those from higher φ flames, where the particle growth rate is too high to allow sufficient residence time for particle maturation.

A comparative study on soot particle size distributions in premixed flames of RP-3 jet fuel and its surrogates

Soot particle size distribution functions (PSDFs) of RP-3 jet fuel and its three surrogates, n-decane and two n-decane/1, 2, 4-trimethylbenzene blends were studied on the burner stabilized stagnation flame platform. The PSDFs of all the flames showed transitions from the unimodal (nucleation mode only) to the bimodal (nucleation and accumulation modes) distribution as the separation distance increased. However, the surrogates did not well capture soot behaviors of RP-3 jet fuel. At most heights, the flame of the surrogate blend of higher 1, 2, 4-trimethylbenzene fraction showed the strongest nucleation and accumulation strengths, whereas the flame of RP-3 jet fuel had the lowest nucleation strengths and n-decane had the lowest accumulation strengths, respectively. Besides, the PSDF of the surrogate blend of higher 1, 2, 4-trimethylbenzene fraction showed later shifts from the nucleation mode to the accumulation mode and higher particle growth rates than the other flames. The PSDF of the n-decane flame shifted, at an earlier time, from the nucleation mode to the accumulation mode and showed lower particle growth rates. RP-3 jet fuel showed intermediate mobility diameters of the trough and median mobility diameters at the accumulation mode. The computed acetylene and benzene concentration profiles of the blend of higher 1, 2, 4-trimethylbenzene fraction were higher, suggesting its stronger nucleation strengths and faster particle growth rates, agreeing well with the experimental observation. On the other hand, RP-3 jet fuel produced larger number density of particles firstly and then lower number density than the other fuels with increased heights.

Size Distribution of Nascent Soot in Premixed n-Hexane, Cyclohexane, and Methylcyclohexane Flames

The evolution of particle size distribution function (PSDF) of nascent soot in premixed flames of n-hexane, cyclohexane, and methylcyclohexane was studied on the burner-stabilized stagnation flame platform. The cold gas velocities were changed to keep the maximum flame temperatures of different flames approximately constant. The PSDFs of all fuels tested transited from the unimodal (nucleation mode only) to the bimodal (both nucleation and accumulation modes) distribution with increased particle residence time. Soot nucleation and growth in the n-hexane flame were found to appear later than those in the cyclohexane and methylcyclohexane flames. In addition, the methylcyclohexane flame exhibited the highest nucleation and accumulation rate at the early stage of soot formation. However, the particle nucleation strength and the soot mass growth rate of the cyclohexane flame exceeded those of the methylcyclohexane flame at long particle residence times.

Soot formation and evolution characteristics in premixed methane/ethylene-oxygen-argon burner-stabilized stagnation flames

The soot formation and evolution characteristics in premixed methane/ethylene-oxygen-argon flames are studied experimentally and numerically. The soot particles sampled with an in-situ probe sampling method are measured for different light hydrocarbon fuels (i.e., methane and ethylene), heights above the exit surface of the burner (HAB), equivalence ratios, ϕ and Reynolds numbers of flame jet, Rej. A novel stochastically weighted operator splitting Monte Carlo (SWOSMC) method coupled with detailed soot model is developed to simulate the evolution of soot particle size distribution (PSD) in premixed methane/ethylene-oxygen-argon flames. The flame temperature decreases while geometric mean diameter of soot particles increases with increasing ϕ. With increasing ϕ, condensation and nucleation processes are enhanced in methane flame while coagulation and nucleation processes are enhanced in ethylene flame. Hence, these processes lead to an increase of total soot number concentration for methane flame but a decrease for ethylene flame. A distinctly different variation trend of the normalized total number density of soot particles for methane flame along with HAB can be found for low and high studied Rej ranges. Similarly, a distinctly different variation trend of geometric mean diameter of soot particles for ethylene flames along with HAB can also be found for low and high studied Rej ranges. The numerical simulation results obtained via the fully validated SWOSMC method show excellent agreement with experimental results in the present study. Consistent with the experimental results, the numerical simulation results show that both condensation and nucleation rates increase and become dominant processes in the methane flame while coagulation rate increases in ethylene flame with increasing ϕ. Meanwhile, the simulated nucleation and coagulation rates along the centerline of both methane and ethylene burner flames are reversed with increasing Rej. The present study not only verifies the competition phenomenon among different soot dynamic processes but also shows that this competition can be reversed for different Rej ranges. These results show that the soot formation and evolution characteristics in the studied premixed methane/ethylene-oxygen-argon flames are determined by competition among different soot dynamic processes.

Measurements of sooting limits in laminar premixed burner-stabilized stagnation ethylene, propane, and ethylene/toluene flames

The systematic investigation of the effect of two major experimental parameters (equivalence ratio and temperature) as well as fuel structure on the sooting limits of ethylene, propane, and ethylene/toluene flames was carried out in premixed burner-stabilized stagnation flames through qualitatively visual observations of flame yellow luminosity and quantitatively direct measurements of incipient soot particle size distributions by a scan mobility particle sizer. A strong temperature dependence of incipient soot formation was observed in the three flames. Detailed chemical kinetic modeling of the flame structure and the soot precursor chemistry was performed to explain the experimental observations. The results show that the incipient soot formation is prohibited in either low or high temperature flames. While the former is due to slow reactions induced by both low temperatures and small concentrations of soot precursors, the latter is attributed to the thermodynamic reversibility of polycyclic aromatic hydrocarbons (PAHs) at high temperatures. Sooting limits have been found to be not a function of concentrations of PAHs alone at a fixed flame temperature; sooting propensity generally increases with increasing fuel carbon/hydrogen atom ratio, presenting the following order: propane < ethylene < ethylene/toluene.

Soot formation characteristics of ethylene premixed burner-stabilized stagnation flame with dimethyl ether addition

Effect of dimethyl ether (DME) addition on PAH and soot formation mechanisms as well as particle-size distribution function (PSDF) behavior of C2H4 premixed burner-stabilized stagnation (BSS) flame was studied. A wide range of DME addition from pure C2H4 to pure DME was considered. The sectional aerosol dynamics model with detailed gas- and particle-phase kinetic mechanism was employed for the simulations. The results indicate that soot growth rate by nucleation, and especially the H-abstractionC2H2addition (HACA) mechanism and polycyclic aromatic hydrocarbons (PAH) condensation increased significantly inside the stagnation boundary layer, due to strong flame-wall interaction. The PSDF curve in the post-flame region was unimodal, while that in the stagnation boundary layer was bimodal. The first peak of the PSDF curve in the stagnation boundary was resulted from the enhanced nucleation rate, and the second peak was due to the intensified soot surface growth by HACA reaction and PAH condensation. The rich premixed C2H4 BSS flame did not exhibit the synergistic effect of DME addition on PAH and soot formations. Soot formation rate decreased monotonously with DME addition for the C2H4 premixed BSS flame. Topology of soot PSDF behavior of the C2H4 premixed BSS flame changed significantly when DME addition exceeded above 20%.

Size Distribution of Soot Particles in Premixed n-Heptane and Methylcyclohexane Flames

The size distributions of soot particles from premixed n-heptane and methylcyclohexane flames were measured and compared at different particle residence times. The micro-orifice probe sampling technique and the scanning mobility particle sizer were used for the particle size measurement in the burner-stabilized stagnation flame configuration. For both flame series, the mixture C/O mole ratio was 0.6 and the maximum flame temperature was controlled around 1835 K to investigate the fuel structure effects on soot propensity. It was observed that, with the increased particle residence time, the particle size distribution function of both flames evolved from the unimodal (only the nucleation mode) to the bimodal (both the nucleation and coagulation modes) distribution. The particle nucleation strength and particle growth rate of the methylcyclohexane flame were quantitatively lower than those of the n-heptane flame. Considering the equivalence ratio difference between the two flames, the sooting propensity dis...

Size evolution of soot particles from gasoline and n-heptane/toluene blend in a burner stabilized stagnation flame

The evolution of soot particle size distribution function (PSDF) in premixed flames of gasoline (34% aromatics by volume) and a n-heptane/toluene blend (66% n-heptane/34% toluene by volume) was investigated in the burner stabilized stagnation (BSS) flame configuration, using the micro-orifice probe sampling and scanning mobility particle sizer (SMPS). The aim of this study is to illustrate the similarities and differences of sooting propensity between the real fuel and the simple hydrocarbon blend in premixed flame conditions. The mole ratio of carbon to oxygen (C/O) in the unburned gas was kept constant at 0.6 and similar maximum flame temperatures and temperature-time histories were kept between the two cases, so that we could focus on the fuel composition effects on sooting propensity. In addition, the size distribution, the total number density, and the volume fraction of soot were also compared to those previously measured for ethylene and propene flames under comparable conditions. It was observed that the particle size distributions of both gasoline and heptane/toluene flames evolve from the unimodal distribution (nucleation mode only) to the bimodal (both nucleation and coagulation mode) distribution. Compared to the heptane/toluene blend, the soot formation in gasoline flame features more persistent nucleation and much faster growth rate.

Probe effects in soot sampling from a burner-stabilized stagnation flame

Probe sampling of soot particles in laminar premixed flames is a common method for characterizing nascent soot formation. Probe intrusiveness into the flame can introduce significant uncertainty in interpretation of experimental data and comparison with numerical results. The aim of the present work is to study the probe-induced effects on soot sampling in a burner-stabilized stagnation (BSS) flame by numerical simulations. The thermophoretic effect is investigated first under non-reactive conditions. The relevant model formulation was tested against experimental data from the literature. Soot size distributions and global properties in the burner stabilized-stagnation flame configuration are studied using both a one-dimensional stagnation flow model and two-dimensional axisymmetric simulations using detailed kinetics and transport. A benchmark burner-stabilized stagnation flame fed with ethylene (Camacho et al., 2015 ) was employed as the target for detailed investigation, focusing on the quantification of the orifice flow effect on the soot size distribution. The results show that the orifice flow can introduce a notable impact on the local flow field, temperature, and particle residence time. Soot measurements have to be shifted some millimeters upstream from the stagnation surface because of the impact of the orifice on the local flow and temperature field. The extent of the spatial shift was quantified by comparing one-dimensional stagnation flow and two-dimensional axisymmetric simulations. The results showed that the spatial shift is weakly dependent on fuel chemistry, but it exhibits stronger dependencies on burner to stagnation separation, pressure drop across the orifice, unburned gas velocity, and the orifice diameter. The extent of spatial shift is parameterized with respect to these experimental parameters.

Particle size distribution of nascent soot in lightly and heavily sooting premixed ethylene flames

The evolution of the nascent soot particle size distribution function (PSDF) was determined by mobility sizing for two series of atmospheric pressure premixed ethylene flames in the burner stabilized stagnation flame configuration. The first series of flames has an equivalence ratio of 1.8, corresponding to conditions just above the sooting limit. The second series has an equivalence ratio of 2.5 and is quite heavy in soot production. In each series, six flames were tested in which the cold gas velocity is varied to obtain flame temperatures ranging from 1559 to 1941 K. The temperature profiles were carefully determined and the comparison to pseudo-one dimensional simulations was satisfactory. It was found that the evolution of the PSDFs with respect to flame stoichiometry, temperature and growth time is consistent with the understanding of kinetic competition during soot formation. Finite rate kinetic limitations are observed at lower temperatures and thermodynamic reversibility occurs at higher temperatures. The observed PSDF features are highly sensitive to competition among the various processes of soot formation, from nucleation to coagulation and gas–surface reactions. The PSDFs are mostly bimodal with both nucleation and coagulation mode particles present. The evolution of the PSDF indicates a strong contribution to the mass of coagulation-mode by the nucleation-mode particles. The measured PSDFs offer comprehensive, canonical data sets useful for testing models of soot formation.

Mobility size and mass of nascent soot particles in a benchmark premixed ethylene flame

The burner stabilized stagnation flame technique coupled with micro-orifice probe sampling and mobility sizing has evolved into a useful tool for examining the evolution of the particle size distribution of nascent soot in laminar premixed flames. Several key aspects of this technique are examined through a multi-university collaborative study that involves both experimental measurement and computational modeling. Key issues examined include (a) data reproducibility and facility effects using four burners of different sizes and makers over three different facilities, (b) the mobility diameter and particle mass relationship, and (c) the degree to which the finite orifice flow rate affects the validity of the boundary condition in a pseudo one dimensional stagnation flow flame formulation. The results indicate that different burners across facilities yield nearly identical results after special attention is paid to a range of experimental details, including a proper selection of the sample dilution ratio and quantification of the experimental flame boundary conditions. The mobility size and mass relationship probed by tandem mass and mobility measurement shows that nascent soot with mobility diameter as small as 15 nm can deviate drastically from the spherical shape. Various non-spherical morphology models using a mass density value of 1.5 g/cm3 can reconcile this discrepancy in nascent soot mass. Lastly, two-dimensional axisymmetric simulations of the experimental flame with and without the sample orifice flow reveal several problems of the pseudo one-dimensional stagnation flow flame approximation. The impact of the orifice flow on the flame and soot sampled, although small, is not negligible. Specific suggestions are provided as to how to treat the non-ideality of the experimental setup in experiment and model comparisons.

Kinetic modeling of particle size distribution of soot in a premixed burner-stabilized stagnation ethylene flame

A detailed model of soot formation is proposed, which consists of a gas-phase kinetic model for the pyrolysis and oxidation of selected hydrocarbon fuels and a kinetic mechanism of soot nucleation and mass/size growth through coagulation and surface reactions. The gas-phase model (Ranzi et al., 2012) was expanded to include the chemistry of Polycyclic Aromatic Hydrocarbons (PAHs) up to four-to-five ring PAHs, with a modular and hierarchical approach. The discrete sectional method was employed to solve the size evolution of the particle size distribution function (PSDF). Analogy and similarity rules were employed to describe heterogeneous reaction kinetics of soot surface reactions. A variable collision efficiency was assumed for the coalescence of small soot particles. Larger particles were assumed to undergo aggregation. The predicted PSDFs are found to be in reasonably good agreement with the experimental data for nascent soot measured in an atmospheric-pressure premixed ethylene–oxygen–argon flame in the burner-stabilized stagnation flame configuration. Sensitivity analyses of the PSDF, number density, and volume fraction were carried out with respect to the rate parameters of addition reactions of acetylene, PAHs, resonantly stabilized radical reactions, and coalescence and aggregation. The results show that the reaction of PAHs and acetylene with soot surfaces and the kinetics of coalescence and aggregation exhibit dominant effects on the detailed and global soot properties for the flame studied, in agreement with conclusions of a large range of previous modeling studies.

Kinetics of nascent soot oxidation by molecular oxygen in a flow reactor

Oxidation kinetics of soot is typically measured with well aged soot as substrates. Recent studies show that nascent soot can have structures and surface composition drastically different from mature, graphitized soot. In the present study, the kinetics of nascent soot oxidation by molecular oxygen was observed at temperatures of 950, 1000 and 1050K for molecular oxygen concentrations ranging from 1000 to 7800ppm at ambient pressure in a coupled burner-stabilized stagnation flame burner and laminar aerosol flow reactor. The reactant particles, 10–20nm in diameter, were freshly synthesized from premixed flames of ethylene, n-heptane, and toluene and introduced into the flow reactor continuously. The kinetics of particle oxidation was measured through electric mobility measurement of particle size distributions before and after oxidation. It was found that the specific oxidation rate is has a first-order dependency on gas-phase O2 concentration over the range of O2 concentration studied. The rate measured is an order of magnitude larger than that predicted by the classical Nagle Strickland-Constable (NSC) correlation. The result suggests that the surface of nascent soot is considerably more reactive towards oxidation than graphite or graphitized soot.

Simulation of Particle Synthesis by Premixed Laminar Stagnation Flames

ABSTRACTA sectional method for determining particle size distributions has been implemented within the particle tracking module included with CHEMKIN-PRO. The module is available for use with many types of reactor models, ranging from 0-D batch reactors to laminar flame simulations. Coupled with the Burner-stabilized Stagnation Flame (BSSF) Model, the sectional model offers a high-fidelity, robust, and efficient computational framework for simulating flame synthesis of particles in a laminar, premixed stagnation flame environment. The CHEMKIN-PRO coupling allows inclusion of detailed gas-phase chemistry that determines key particle-formation precursors, along with physical processes such as nucleation and coagulation of particles. These capabilities are demonstrated for two flame-particle systems of practical importance, viz. nanocrystalline titania synthesis and soot formation. The results are compared with experimental data obtained at the University of Southern California (USC) flame facility. Computed particle size distributions show good agreement with experimental data. Simulations have led to exploration of the parameter space for particle production and particle-size influences.

Quantitative measurement of soot particle size distribution in premixed flames – The burner-stabilized stagnation flame approach

A burner-stabilized, stagnation flame technique is introduced. In this technique, a previously developed sampling probe is combined with a water-cooled circular plate such that the combination simultaneously acts as a flow stagnation surface and soot sample probe for mobility particle sizing. The technique allows for a rigorous definition of the boundary conditions of the flame with probe intrusion and enables less ambiguous comparison between experiment and model. Tests on a 16.3% ethylene–23.7% oxygen–argon flame at atmospheric pressure show that with the boundary temperatures of the burner and stagnation surfaces accurately determined, the entire temperature field may be reproduced by pseudo one-dimensional stagnation reacting flow simulation using these temperature values as the input boundary conditions. Soot particle size distribution functions were determined for the burner-stabilized, stagnation flame at several burner-to-stagnation surface separations. It was found that the tubular probe developed earlier perturbs the flow and flame temperature in a way which is better described by a one-dimensional stagnation reacting flow than by a burner-stabilized flame free of probe intrusion.