Opposed-flow Diffusion Flames Research Articles

Theoretical analysis of polydisperse water spray extinction of opposed flow diffusion flames

We present a new mathematical analysis of the influence of a polydisperse spray of water droplets on the extinction of opposed flow diffusion flames. The current interest in the use of fine-water mists for fire extinguishment provides the motivation for our research. Previous experimental and numerical work in the literature used the opposed flow configuration as a convenient paradigm for extracting the relative importance of the various mechanisms at play in extinction of diffusion flames. In the current work we present an analytical approach to describe the problem in which a spray of water droplets of arbitrary polydispersity is present in the oxidant stream that opposes the fuel stream, the latter two being responsible for the formation of a diffusion flame. Transport effects are included through (possibly non-unity) Lewis numbers. The analysis permits arbitrary water spray polydispersity. Our level of modeling currently focuses on the effect of the water mist's endothermicity as the mechanism that primarily determines conditions for extinction. Here, we examine how the initial water droplet size distribution produced by the spray influences the spatial distribution of heat loss, and how the extinction of the diffusion flame is consequently effected. We consider six initial droplet size distributions, three having the same Sauter Mean Diameter and three having the same D 20. In both cases the distributions have rather different forms: (1) a quasi-monodisperse spray, (2) a bimodal spray and (3) a “normal”-spray. It is shown that the different sprays lead to quite different extinction characteristics despite having the same SMD or D 20. Under certain circumstances it is shown that conditions can be found that delineate between effective and ineffective use of the water droplets for suppression. The results strongly underscore the sensitivity of flame suppression to the water mist's actual droplet size distribution.

The chemical structures of opposed flow diffusion flames of C3 oxygenated hydrocarbons (isopropanol, dimethoxy methane, and dimethyl carbonate) and their mixtures

Species concentrations and temperatures were measured across opposed flow diffusion flames of three C3 oxygenated hydrocarbons: isopropyl alcohol, dimethoxy methane (DMM), and dimethyl carbonate (DMC). In addition, measurements were made with propane for comparison. Measurements were repeated for mixtures of propane and oxygenated hydrocarbons to investigate synergistic effects. The experiments were conducted at atmospheric pressure by withdrawing samples from the flame using a quartz microprobe coupled to an online gas chromatograph (GC). Gas analysis was also performed using HPLC techniques. Profiles were made of temperature and of more than 16 species, including oxygenated and aromatic hydrocarbons. The isopropyl alcohol results suggest that the presence of the OH group favors the formation of propylene, as well as acetaldehyde and acetone, relative to the propane case. For DMM and DMC, the lack of CC bonds greatly reduces the formation of ethylene, acetylene, and propylene. For example, relative to the propane flame, the ethylene levels of isopropanol, DMM, and DMC flames were reduced by 41, 77, and 93%, respectively. In the case of DMM, significant quantities of formaldehyde and methyl formate were measured. All intermediate hydrocarbon levels were lower for DMC relative to DMM. The principal intermediate of DMC was formaldehyde. Mixtures of propane and oxygenates generally had lower hydrocarbon levels than would be expected based on only additive effects. The information provided should be of considerable utility in providing better insight into oxygenated hydrocarbon combustion, as well as in developing and validating chemical kinetic mechanisms.

Modeling aerosol formation in opposed-flow diffusion flames

The microstructures of atmospheric pressure, counter-flow, sooting, flat, laminar ethylene diffusion flames have been studied numerically by using a new kinetic model developed for hydrocarbon oxidation and pyrolysis. Modeling results are in reasonable agreement with experimental data in terms of concentration profiles of stable species and gas-phase aromatic compounds. Modeling results are used to analyze the controlling steps of aromatic formation and soot growth in counter-flow configurations. The formation of high molecular mass aromatics in diffusion controlled conditions is restricted to a narrow area close to the flame front where these species reach a molecular weight of about 1000 u. Depending on the flame configuration, soot formation is controlled by the coagulation of nanoparticles or by the addition of PAH to soot nuclei.

Formation of carbon nanotubes in counter-flow, oxy-methane diffusion flames without catalysts

In oxygen enriched methane diffusion flames, carbon nanotubes were discovered to be formed in the region on the fuel-rich side of the flame front at an oxygen enrichment of 50%. No catalyst was employed. An opposed flow diffusion flames with varying strain rate and oxygen content in the oxidizer stream was used. Substantial quantities of nanotube material are produced at atmospheric pressure in this continuous (non-batch) process. Thermophoretic sampling of the flame and collecting the carbon material deposited near the exhaust was done. Both confirm the growth of carbon nanotubes and other carbon clusters.

Formation of NO from combustion of volatiles from municipal solid wastes

An experimental and theoretical study has been performed on the formation of NO in the combustion of volatiles from municipal solid wastes. Single components and their mixtures were burned in a small-scale, fixed-bed reactor. Numerical simulations using the opposed flow diffusion flame program OPPDIF were performed to obtain a further understanding of the experimental results. Conversion factors for fuel-N to NO were determined for single components of newspaper, cardboard, glossy paper, low-density polyethylene (LDPE), and poly(vinylchloride) (PVC) and their mixtures, using gases with oxygen concentrations of 12, 21, and 40 vol %. For single components experiments at 100 vol % oxygen were also performed. The conversion factors for paper and cardboard varied from 0.26 to 0.99. The conversion factor for LDPE and PVC varied from 0.71 to 10.09 and 0.04 to 0.37, respectively. Conversion factors higher than 1.0 in the case of LDPE clearly show that NO is formed by thermal and/or prompt mechanisms. For mixtures, calculated conversion factors (based on a weighted sum of the conversion factors for single components) were compared with the experimentally determined conversion factors. Mixtures of paper and cardboard only gave different conversion factors with 40 vol % of oxygen. For mixtures of paper/cardboard and plastics, however, significant differences in the conversion factors were observed at all oxygen concentrations, when comparing experiments on a mixture of paper and plastics with the weighted sum of the single components. The explanation is found in the different combustion properties for paper/cardboard and plastic, which in this case make the formation of thermal NO from LDPE more favorable for the single component than in mixtures with other components. The simulations with OPPDIF confirmed the trends observed experimentally, and allowed an assessment of the contribution of the different mechanisms of NO formation.

Laser-induced Fluorescence Triple-integration Method Applied to Hydroxyl Concentration and Fluorescence Lifetime Measurements

We report quantitative hydroxyl concentrations obtained by using a new laser-induced fluorescence triple-integration method (LIFTIME), which is capable of rapid and continuous fluorescence lifetime measurements via a unique photon-counting technique. LIFTIME has been convolved with picosecond time-resolved laser-induced fluorescence to permit the rapid monitoring of instantaneous species concentrations in flames. Here, LIFTIME is used to measure hydroxyl concentrations and fluorescence lifetimes at a sampling rate of 1 Hz in eight premixed laminar flat flames and in one laminar opposed flow diffusion flame. Fluorescence lifetimes as a function of axial position are generally obtained with less than 5% uncertainty, while concentrations at the same locations are obtained with less than 10% uncertainty (95% confidence interval). The hydroxyl concentration measurements are shown to agree well with modeling predictions and with previous laser-saturated fluorescence measurements. The measurements are also compared with predictions based on existing quenching cross-section correlations.

Polycyclic Aromatic Hydrocarbon Formation in Opposed Flow Diffusion Flames of Ethane

The effect of fuel-side carbon density on the levels of polycyclic aromatic hydrocarbon (PAH) formation in atmospheric pressure, opposed flow, ethane diffusion flames has been studied using heated micro-probe sampling and gas chromatography/mass spectrometry (GC/MS). Mole fraction profiles of about 30 major and minor species, as well as trace aromatic and substituted aromatic hydrocarbons and PAHs (up to C 14H 10, e.g., cyclopenta[cd]pyrene), were quantified by direct gas analysis of samples withdrawn from within the flame without any pre-concentration. Results indicate that fuel-side carbon density and changes in flame temperature markedly influence the levels of benzene and PAH formed within opposed flow diffusion flames. Significant O 2 penetration was also noted on the fuel-rich side of the flame at a lower fuel dilutions, suggesting PAH formation may be also associated with O 2-catalyzed reactions.

Laser-Saturated Fluorescence Measurements of Nitric Oxide in Laminar Counterflow Diffusion Flames

This brief communication presents the first measurements of NO concentration in laminar counterflow diffusion flames using a nonintrusive technique, viz.laser-saturated fluorescence. The measurement results are compared with predictions from an existing opposed-flow diffusion flame code

Chemical kinetic modeling of a methane opposed-flow diffusion flame and comparison to experiments

The chemical structure of an opposed flow, methane diffusion flame is studied using a chemical kinetic model and the results are compared to experimental measurements. The chemical kinetic paths leading to aromatics and polycyclic aromatics hydrocarbons (PAHs) in the diffusion flame are identified. These paths all involve resonantly stabilized radicals which include propargyl, allyl, cyclopentadienyl, and benzyl radicals. The modeling results show reasonable agreement with the experimental measurements for the large hydrocarbon aliphatic compounds, aromatics, and PAHs. the benzene was predicted to be formed primarily by the reaction sequence of Allyl plus Propargyl equals Fulvene plus H plus H followed by fulvene isomerization to benzene. Naphthalene was modeled using the reaction of benzyl with propargyl, while the combination of cyclopentadienyl radicals were shown to be a minor contributor in the diffusion flame. The agreement between the model and experiment for the four-ring PAHs was poor.

Thermochemical and chemical kinetic data for fluorinated hydrocarbons

A comprehensive, detailed chemical kinetic mechanism was developed and is presented for C 1 and C 2 fluorinated hydrocarbon destruction and flame suppression. Existing fluorinated hydrocarbon thermochemistry and kinetics were compiled from the literature and evaluated. For species where no or incomplete thermochemistry was available, these data were calculated through application of ab initio molecular orbital theory. Group additivity values were determined consistent with experimental and ab initio data. For reactions where no or limited kinetics were available, these data were estimated by analogy to hydrocarbon reactions, by using empirical relationships from other fluorinated hydrocarbon reactions, by ab initio transition state calculations, and by application of RRKM and QRRK methods. The chemistry was modeled considering different transport conditions (plug flow, premixed flame, opposed flow diffusion flame) and using different fuels (methane, ethylene), equivalence ratios, agents (fluoromethanes, fluoroethanes) and agent concentrations. This report provides a compilation and analysis of the thermochemical and chemical kinetic data used in this work.

A Phenomenological Model for Smoke-Point and Soot Formation in Laminar Flames

Abstract Aglobal soot formation model is proposed, validated and applied based on the postulate that soot formation in laminar diffusion flames is controlled by gaseous reaction processes and not the available soot surface area. Although this model was not derived from detailed soot formation mechanisms, it is consistent with such mechanisms if the active sites for soot growth are so many that soot growth depends on the availability of gaseous species (e.g., C2H2 ). Global soot formation models in laminar diffusion models based on a surface growth process can not be reconciled with experiments. For the present model, a soot formation time in axisymmetric laminar diffusion flames below the smoke-point is shown to beproportional to the smoke-point height. Moreover the present model implies that smoke-point heights are proportional to pressure in agreement with experiments. Predictions of the model for flow. dilution and pressure effects on soot concentrations agree with experiments. both in laminar jet flow (2-D or axisymmetric) and opposed flow diffusion flames. The effects of radiant cooling on the flame and the flow structure are also briefly addressed in this work.

Relative importance of nitric oxide formation mechanisms in laminar opposed-flow diffusion flames

The relative importance of the Zeldovich, N 2O, and Fenimore mechanisms for nitric oxide formation in stretched laminar opposed-flow diffusion flames with CH 4 N 2 fuel is determined using model calculations that include detailed chemical kinetics (213 reactions and 45 molecular species) and realistic multi-component transport effects. For all values of stretch, the NO formation is dominated by the Fenimore mechanism, which contributes more than two thirds of the total NO formed. The remaining NO is formed approximately equally by the Zeldovich and N 2O mechanisms. The model calculations are compared with previous measurements in laminar opposed-flow CH 4 N 2 diffusion flames at three values of stretch (α = 42, 70, and 140 s −1). The model-experiment agreement (within 50 K for peak temperature and within a factor of 2 for peak NO x formed) supports the validity of the detailed chemical kinetic mechanism used. The Fenimore mechanism is expected to dominate NO formation in many laminar and turbulent hydrocarbon diffusion flames.

Thermal NO x in stretched laminar opposed-flow diffusion flames with CO/H 2/N 2 fuel

The effect of flame stretch (variations in velocity and concentration gradients) on thermal NO x formation has been studied in laminar opposed-flow diffusion flames. Detailed chemistry-transport model calculations show agreement (within 150 K for peak flame temperature and within 3 ppm for peak thermal nitric oxid concentrations) with previous experimental measurements in CO/H 2/N 2 laminar opposed-flow diffusion flames at three different velocity gradients (α = 70, 180, and 330 s −1). Major corrections were required to account for the finite spatial resolution of the probe sampling measurements. Additional model calculations were obtained over a wider range of stretch (α = 0.1–5000 s −1). Calculated NO x concentrations decreased dramatically as flame stretch was increased (with peak NO x values of 2300, 1100, 280, 20, and ≤1 ppm obtained for flames with α = 0.1, 1, 10, 100, and ≥500 s −1, respectively). This decrease was caused by declines in both the reaction time in high temperature flame zones (proportional to α −1) and in the net NO x formation rates. The net NO x formation rates are affected by flame stretch due to changes in peak flame temperature, superequilibrium O atom concentrations, NO destruction reactions, and N 2O formation reactions. Most of the NO x in flames at low stretch is formed by the Zeldovich mechanism, while the N 2O pathway dominates NO x formation in flames at very high stretch where the peak flame temperatures are lower. Reactions involving the formation and destruction of NO 2 occur in lean flame zones, but the amount of NO 2 formed is small (≤10 ppm). Both experiments and model calculations show that a very effective way to reduce thermal NO x formation in the forward stagnation regions of laminar opposed-flow diffusion flames (and possibly in turbulent diffusion flames as well) is to increase flame stretch.

Structure of Laminar Opposed-flow Diffusion Flames With CO/H2/N2Fuel

Abstract The detailed structure of laminar opposed-flow diffusion flames with a fuel composition of 40% CO, 30% H2, and 30% N2 and an oxidizer composition of 79% N2 and 21% O2 has been calculated over a wide range of stretch conditions (α = 0.1-5000s-1) using 32 elcmenlal chemical reactions and realistic transport properties. Previously, when compared to experimental measurements at χ = 70, 180, and 330s-', this model was found to predict peak flame temperatures within 150K. widths of temperature profiles to 20%, and positions of temperature maxima to 0.1 cm. The present paper presents calculations over a wider range of stretch (χ = 0.1-5000s-1) and quantifies the effect of stretch on fundamental flame processes such as preferential diffusion. overlap of fuel and air. superequilibrium radical formation. deviations from partial equilibrium, and extinction. Preferential diffusion of H2 leads to superadiabatic temperatures in the α = 0.1 s-1 flame. Pronounced nonequilibrium effects occur even in the α = 0.1 ...

Stretched laminar flamelet analysis of turbulents H 2 and CO/H 2/N 2 diffusion flames

The stretched laminar flamelet approach to turbulent diffusion flames is tested by comparison of instantaneous and conditionally-averaged Raman measurements in turbulent H 2 and CO/H 2 /N 2 jet diffusion flames with stretched laminar opposed-flow diffusion flame model calculations and measurements. The H 2 turbulent jet diffusion flame measurements close to the fuel nozzle show instantaneous temperatures 300K less than equilibrium, peak OH concentrations three times larger than equilibrium, and substantial coexistence of H 2 and O 2 . Quantitatively similar effects are calculated by David, et al., for lightly stretched (α=100 s −1 ) opposed-flow laminar H 2 diffusion flames. The turbulent flame data lie in a narrow band between the highly stretched α=12,000 s −1 laminar flame calculation and the adiabatic equilibrium curve and approach adiabatic equilibrium downstream in the flame. Localized extinction is relatively unimportant in simple H 2 turbulent jet diffusion flames since only a few instantaneous Raman measurements are close to that calculated at extinction (α=12,000 s −1 ). With a fuel consisting of 40%CO/30%H 2 /30%N 2 , experimental data in both the turbulent jet flame and a laminar opposed flow diffusion flame show even larger nonequilibrium effects. The excellent correspondence between stretched laminar and turbulent flame data suggest that the laminar flamelet approach may provide better predictions than the two-scalar pdf approach of finite-rate chemistry in turbulent reaction zones because partial equilibrium assumptions are not required and localized extinction can be accounted for properly. However, differences between stretched laminar and turbulent flame data suggest that the laminar flamelet approach can overestimate the influence of preferential diffusion in flames containing H 2 and may not properly include the slow three body radical recombination reactions which are likely to occur outside of the relatively thin primary reaction zones.

Soot formation in an opposed flow polymer diffusion flame

This study describes an investigation of the mechanisms of soot formation in an opposed flow diffusion flame on a polyethylene rod. A specially designed burner which maintained the flame in a steady position in the laboratory frame of reference was built. Temperature distributions were measured using thin thermocouples while combined laser light scattering and absorption measurements were used to determine distributions of soot volume fractions, soot aggregate diameters and their number densities. The gaseous flow field as well as that of the soot particles were determined from a model developed for this flame. These results predicted the existence of two separate stagnation planes for the flame gases and the soot particles. Similar to what has been established for jet-like gaseous diffusion flames, nucleation in this flame was also found to take place in the pyrolysis region near the flame front. After nucleation, the particle grow by coalescence, agglomeration and surface growth as they move towards the burning polymer surface, with thermophoretic velocity first in concert with the convective velocity and then against the convective velocity of the gases. A secondary flame front surrounding the sides of the sooty zone cause a circumferential annulus of heavy soot loadings. The local rates of soot formation were determined from the soot mass conservation equation using measured axial distributions of the temperature and the soot volume fraction and calculated gas and soot particle velocities.

Ignition of polyoxymethylene

An experimental opposed flow diffusion flame system has been developed with which polymer ignition may be carefully investigated. The effects of ignition source intensity, oxidizer flow rate, and oxidizer composition on the ignition of polyoxymethylene have been studied. Comparisons with available theoretical models have been made where possible. It is concluded that the polymer surface temperature at ignition is independent of ignition source intensity, oxidizer composition, and oxidizer flow rate; that the ignition delay time is independent of oxidizer velocity over the range of velocities for which a flat flame can be established in the opposed flow diffusion flame system; that gas phase absorption of radiation may affect the ignition process even for relatively small optical depths and low source intensities; that the ignition delay time decreases linearly with increasing oxygen concentration in oxygen nitrogen mixtures for low flame stretch rates; that ignition occurs during the transition stage when the free stream oxygen mass fraction is less than the oxygen mass fraction in the solid, and for higher free stream oxygen mass fractions it appears that oxygen diffusion to the surface is enhancing the gasification rate during the transition stage and/or decreasing the period of the transition stage; and that the various processes occurring in the gas phase during the ignition event appear to result in a net liberation of energy.

The structure of a poly(ethylene) opposed flow diffusion flame

An opposed flow diffusion flame was used to study the burning characteristics and flame structure of high purity poly(ethylene) near extinction conditions. Special attention was paid in the experimental design to insure a uniform oxidizer velocity profile which is essential for burning conditions near extinction. The influence of the sampling probe on the location of the flame was studied through the use of shear interferometry. Structural measurements were obtained and compared with other investigations of diffusion flames. Departures from the commonly assumed collapsed flame model of laminar diffusion flames were observed in terms of excessive CO concentrations and oxygen penetration into the fuel side of the flame. An upper bound on the importance of oxygen diffusion to the fuel surface and subsequent surface oxidation was placed at twenty percent of the energy required for fuel pyrolysis, with the remainder of the energy being delivered to the surface from the flame through heat transfer processes. As the oxygen concentration in the oxidizer flow was decreased and extinction conditions approached, the CO/CO2 ratio at the flame increased slightly, the oxygen concentration at the liminous flame zone decreased, the flame stand-off distance decreased, and the flame temperature decreased. Radial similarity in the composition and temperature profiles was established experimentally which confirms predictions and greatly simplifies the modeling of the opposed flow diffusion flame.

Opposed flow diffusion flame extensions

The Opposed Flow Diffusion Flame (OFDF) apparatus has been used for estimating the thermochemical properties of materials related to fire safety and for diffusion flame structure studies. Here the extension of the flame beyond the pyrolyzing fuel surface and the excess pyrolyzate are predicted. Since flame and stagnant plane locations may be sensitive to the usual Le=1 assumption, these additional predictions which are not so sensitive can provide useful estimates of fuel properties. Shvab-Zeldovich variables are used in analysis of steady, laminar, non-radiative, axisymmetric and Cartesian two-dimensional boundary layers. The thin diffusion flame lies within the boundary layer on the oxidant side of the stagnation plane. Much of the behavior of the velocity, enthalpy and species profiles is similar to the analogous free convection flame profiles. The results depend on five parameters; r , the mass consumption number; B, the mass transfer number; D c , a dimensionless heat of combustion: ϑ w , the fuel surface temperature; and Pr, the Prandtl number. The fuel pyrolysis rate is controlled primarily by B, while the flame extension is dominated by r and the flame temperature is influenced by D c . In comparison with polymer experiments, care is required to insure that the apparatus satisfies the model assumptions and that radiative emission from the fuel surface is taken into account. Good aggreement is obtained between predicted and experimental flame extensions for polymethylmethacrylate.

Combustion of polyvinyl chloride studied by a low-pressure moving-wire technique

The low-pressure moving-wire technique (MWT) provides a means of achieving steady-state polymer combustion. For charring polymers, the steady state is not controlled by the rate of char combustion, as is the case for the opposed-flow diffusion flame technique. At low pressure, the polymer flame is expanded to a size that is convenient for microprobe sampling. It is then possible to identify the flammable gases evolving under actual combustion conditions, thereby avoiding the common extrapolation from nonflame pyrolysis studies. As a demonstration of the technique, we made measurements of the structure of a polyvinyl chloride (PVC) flame at reduced pressure (0.10 bar). The material evolving from the burning polymer was estimated to be ≅0.4% benzene and ≅14% pentene isomers (and/or butenal). Butene isomers (and/or propenal) and lower-molecular-weight species accounted for the remainder. The total mass flux from the burning surface was 0.17 kg m −2 sec −1. The surface temperature at ignition was ≅660°K.