Soot Formation In Diffusion Flames Research Articles

Sooting characteristics of hydrocarbon compounds and their blends relevant to aviation fuel applications

The sooting characteristics of neat hydrocarbon compounds from different molecular classes and their blends that are relevant to aviation fuels were systematically investigated in the present work. Smoke points and maximum soot volume fractions of 10 neat hydrocarbon compounds and 33 binary fuel blends were measured in a standard wick-fed flame burner. The threshold sooting indices of test fuels were derived based on the fuel uptake flow rate with threshold imaging method. At the smoke point conditions, flames of all test fuels were noted to have relatively close maximum soot volume fractions, thereby suggesting that the maximum soot concentration contained by a diffusion flame without escaping smoke is relatively insensitive to the effects of fuel molecular structure. At the constant fuel carbon flow rate conditions, it was found that the soot formation generally follows: naphthalenes > monocyclic aromatics > cycloalkanes >iso-alkanes >n-alkanes. The relationships between soot formation and various sooting tendency indices were further examined. For given constituents, their binary blend result at varying blending ratio showed that the maximum soot volume fraction can be approximately linearly correlated with aromatics content, hydrogen content, hydrogen deficiency, and threshold sooting index; the slope varies with different blends. At a given constant fuel carbon flow rate, the results of all test fuels demonstrated that the reciprocal of smoke point is able to correlate linearly with the maximum soot volume fraction through a single curve. Therefore, the present findings indicated that smoke point is more effective even than threshold sooting index in predicting soot formation in diffusion flames.

Reconstruction and simulation of temperature and CO2 concentration in an axisymmetric flame based on TDLAS

Two-dimensional (2D) distributions of temperature and species concentration are crucial information to understand PAH inception and soot formation in diffusion flames. We report a tunable diode laser absorption spectroscopy (TDLAS) measurement to reconstruct the temperature and carbon dioxide concentration profiles in axisymmetric methane co-flow diffusion flame. The TDLAS sensor selected the v3 bandhead region of CO2 molecule at 4.2 um to ensure high temperature sensitivity and avoid ambient background absorption. A Fourier analysis Abel inversion transform algorithm was used to reconstruct 2D distribution of attenuation coefficient and showed higher inversion accuracy compared with other commonly used inversion methods. Evaluation of the reconstruction accuracy of the present Fourier analysis and traditional Onion Peeling algorithm was also performed using the simulated temperature and CO2 concentration profiles from CFD calculation. The experimental reconstructed temperature and CO2 volume fraction profiles were obtained and compared with thermocouple measurement and CFD simulation. The results show that the temperature from TDLAS measurement agrees well with CFD simulation. Flame temperature measured by thermocouple is lower compared with TDLAS and CFD because of the perturbation introduced by insertion of the thermocouple, especially at the edge of the flame.

The influence of gravity levels on soot formation for the combustion of ethylene-air mixture

The reduced mechanism coupled with 2D flame code using CHEMKIN II to investigate the effect of gravity on flame structure and soot formation in diffusion flames. The results show that the gravity has a rather significant effect on flame structure and soot formation. The visible flame height and peak soot volume fraction in general increases with the gravity from 1g decreased to 0g. The peak flame temperature decreases with decreasing gravity level. Comparing the calculated results from 1g to 0g, the flame shape becomes wider, the high temperature zone becomes shorter, the mixture velocity has a sharp decrease, the soot volume fraction has a sharp increase and CO and unprovided species distribution becomes wider along radial direction. At normal and half gravity, the flame is buoyancy controlled and the axial velocity is largely independent of the coflow air velocity. At microgravity (0g), the flame is momentum controlled.

Sooting behaviour of n-heptane laminar diffusion flames at high pressures

The effect of pressure on sooting behaviour of n-heptane is studied in co-flow n-heptane/air laminar diffusion flames at pressures above atmospheric in a high pressure combustion chamber. The fuel is diluted with either nitrogen or helium to keep a non-smoking flame at elevated pressures, and the selected fuel mass flow rate of n-heptane provided diffusion flames in which the soot was completely oxidized within the visible flame envelope. The flame stability proved to be a challenge and stable flames were possible only at certain pressures for a sufficiently long duration to permit measurements. The soot volume fractions and temperatures were measured by spectral soot emission as a function of pressure for nitrogen-diluted n-heptane flames at 2, 5 and 7atm. For helium-diluted n-heptane flames, line of sight soot emission data at 3, 4, and 5atm are presented at two heights above the burner exit. Comparison of limited nitrogen-diluted n-heptane data to previous measurements of soot yields indicate that soot formation in diffusion flames of n-heptane seems to be slightly more sensitive to pressure than that in aliphatic gaseous fuel diffusion flames within the pressure range considered in this work.

톨루엔 혼합에 따른 대향류 확산화염 내 매연 생성에 대한 수치적 연구

A numerical simulation has been performed to investigate effects of toluene mixing on soot formation in pure ethylene opposed-flow nonpremixed flame. Mixture ratios of toluene were 3%, 5%, 10%, and 20%. Senkin code for 0-D simulation and oppdif code for 1-D simulation based on CHEMKIN III were utilized. 0-D results by senkin showed that concentrations of methyl radicals and benzene were increased with increasing toluene mixture ratio. This implied that the mixing of toluene in pure ethylene diffusion flame produces more PAHs and soot than those of pure ethylene flame. 1-D result of 10 % toluene reaction by oppdif code showed that production rate for H radical was a crucial factor for benzene formation. These results imply that methyl radical, benzene and H radical play a important role on soot formation in diffusion flames.

Soot precursor measurements in benzene and hexane diffusion flames

To clarify the mechanism of soot formation in diffusion flames of liquid fuels, measurements of soot and its precursors were carried out. Sooting diffusion flames formed by a small pool combustion equipment system were used for this purpose. Benzene and hexane were used as typical aromatic and paraffin fuels. A laser-induced fluorescence (LIF) method was used to obtain spatial distributions of polycyclic aromatic hydrocarbons (PAHs), which are considered as soot particles. Spatial distributions of soot in test flames were measured by a laser-induced incandescence (LII) method. Soot diameter was estimated from the temporal change of LII intensity. A region of transition from PAHs to soot was defined from the results of LIF and LII. Flame temperatures, PAH species, and soot diameters in this transition region were investigated for both benzene and hexane flames. The results show that though the flame structures of benzene and hexane were different, the temperature in the PAHs–soot transition region of the benzene flame was similar to that of the hexane flame. Furthermore, the relationship between the PAH concentrations measured by gas chromatography in both flames and the PAH distributions obtained from LIF are discussed. It was found that PAHs with smaller molecular mass, such as benzene and toluene, remained in both the PAHs–soot transition and sooting regions, and it is thought that molecules heavier than pyrene are the leading candidates for soot precursor formation.

Effects of Oxygen Concentration on PAHs Formation in a Counterflow Diffusion Flame

The effects of the oxygen concentration on polycyclic aromatic hydrocarbons (PAHs) and soot formation in a methane air counterflow diffusion flame are investigated experimentally. The oxygen concentration in the oxidizer is varied in the range from 0.18 to 0.22 (mole). PAHs measurement is performed by a gas chromatograph mass spectrometer (GC/MS). It is found that PAHs concentrations in the flame increase along with the increase of the oxygen concentration. This result shows that the increase of the oxygen concentration will promote soot formation in diffusion flames. The responses of Xi/XC6H6, which is the mole fraction ratio of the species i to benzene, to the change of the oxygen concentration are also examined to elucidate PAHs formation mechanism. In the naphthalene (2 rings) formation process, H-abstraction-C2H2-addition (HACA) mechanism would be more dominant than Marinov's model, in which the combination of 5-membered ring radicals contributes to PAlls formation. On the other hand, in phenanthrene (3 rings) formation process, not only HACA mechanism, but also Marinov's model can be responsible for its formation.

Determination of Soot Particle Size in a Diffusion Flame: A Dynamic Light Scattering Study

This article presents dynamic light scattering measurements of soot particles formed in a laminar propane-air diffusion flame. After a short introduction to the problem of soot formation in diffusion flames we give a brief survey of dynamic light scattering and of the special problems of such measurements and evaluation of data from flowing gas phases, including a short description of our experimental setup. In the experimental part we describe our measurements of soot particles in dependence of the height above burner and the distance from the lateral border of the flame. The particle size increases with ascending height above burner and decreases at fixed height towards the boundary layers of the flame.

Soot inception temperature and the carbonization rate of precursor particles

The critical temperature for soot formation in diffusion flames has been the subject of prior measurements by various investigators. More recently, the presence of soot precursor particles in a variety of diffusion flames has been recognized, and the Arrhenius reaction rate constants for their conversion to carbonaceous soot have been reported. In this work the prediction of the soot inception temperature using the recent carbonization reaction rate data is investigated when temperature increases linearly with time. The temperature gradient and the detection sensitivity to the initial soot formation are two factors accommodated by the method of prediction. A good correlation is obtained between the predicted soot inception temperatures and the values previously measured by several investigators for eight hydrocarbon fuels of diverse molecular structures. Both the temperature gradient and the detection sensitivity of the experiments to the initial soot formation are significant differences among the several test protocols. The ability to predict the soot inception temperatures in diffusion flames for the wide range hydrocarbon fuels by means of the Arrhenius rate data are discussed.

Radiation fire modeling

The concept of radiation fire modeling is developed. It allows one to reproduce the behavior of largescale fires using model-scale fires at elevated pressure. It preserves large-scale fire heat fluxes and burn times by increasing the ambient pressure while maintaining the length-scale pressure-squared product l·,P 2 , invariant. The paper discusses the full range of modeled and non-modeled quantities. The radiation fire modeling hypothesis is based on two assumptions: (1) the flame heat flux comes primarily from soot radiation, and (2) soot formation in diffusion flames is second order in pressure and is characterized by the fuel smoke point. The radiation and second-order pressure dependence of soot formation in diffusion flames is carefully argued in the paper. The modeling concept was tested and confirmed by a series of model-scale pool-fire experiments performed in Factory Mutual Research's large pressure vessel at pressures up to 2.5 atm. These experiments are compared to pool-fire measurements performed at atmospheric pressure at diameters up to 1.2 m. The square exponent on pressure provides the best fit to the data. These pool-fire experiments, between the optically thin and optically thick limits, were specifically chosen to offer the most severe challenge to the modeling concept.

Influence of hydrogen addition to fuel on temperature field and soot formation in diffusion flames

Overventilated, coflowing axisymmetric laminar diffusion flames of ethylene, propane, and butane were used to study the influence on soot of hydrogen addition to the fuel. The flame temperatures were measured by CARS along the flame axis as well as at off-axis radial locations. CARS spectra taken in heavily sooting regions exhibited poor fits due to C2 absorbtion of part of the fundamental band of the nitrogen spectrum. It was found that C2 absorbtion was confined to frequencies greater than 2313 cm−1. We, therefore, implemented a strategy that fitted only the CARS spectra in the frequency range smaller than 2313 cm−1. Measured soot concentrations and the flame temperature data with and without hydrogen and helium dilution were evaluated and the relative influences of dilution and direct chemical interaction on soot formation, as a result of hydrogen addition, are presented. It is shown that when hydrogen or helium is added to the fuel as a diluent in moderate quantities, the changes in the temperature field of the coflow diffusion flames are negligible. When allowance is made for the influence of dilution, addition of hydrogen to the fuel side of an ethylene diffusion flame reduces the soot formation. For propane and butane flames, hydrogen addition does not show any influence on soot formation apart from the dilution effect.

Influence of nitrogen dilution and flame temperature on soot formation in diffusion flames

The line-of-sight soot surface temperatures and soot volume fractions were measured as a function of axial position in overventilated coflow laminar diffusion flames. A comparison of the influence of nitrogen dilution and the flame temperature on soot formation in diffusion flames of ethylene was made, and the relative importance of the two effects was quantified. To isolate the influence of the dilution and flame temperature, a reference condition was specified, such that the temperature of the reactants was 623 K without nitrogen dilution of the fuel gas. In dilution experiments, the temperature of reactants was at the reference temperature and the nitrogen dilution of the ethylene was varied from 0 to 0.78 mole fraction. In a second set of experiments, where dilution was zero, the flame temperature was varied by reducing the temperature of the reactants from 623 to 298 K in six steps. The reduction in soot formation is due to both lowered temperature and fuel concentration in dilution experiments, and due to only lowered temperature in undiluted flames since the temperature was varied by controlling the temperature of the reactants. The difference between the two sets of results, for the same flame temperature, gives the influence of dilution on soot formation. The observed soot concentrations in both diluted and undiluted flames, after allowance is made for differences in flame heights and diameters, are fitted to a simple Arrhenius rate expression where the preexponential factor is linear in fuel mole fraction and the measured activation energy is 200 kJ/mol. This rate expression correctly predicts the observation that the effect of dilution on the maximum soot volume fraction is greater than that of temperature up to a diluent fraction of about 0.7. At this point the contributions of both factors are equal; further increase in the diluent amount makes the contribution of the temperature larger.

Predictions of soot in laminar diffusion flames

A model for soot formation in diffusion flames is proposed in which a correlation between mixture fraction and root surface growth rate is used. The correlation is derived from measurements in a stagnation point diffusion flame. The caculation solves the boundary-layer form of the flow equations, which include the continuity equation, momentum, mixture fraction, and the enthalpy equations. The latter equation is included to account for radiation, which earlier studies found to be important in determining the flame temperatures in sooty flames and the rates of soot reactions

The influence of carbon dioxide and oxygen as additives on soot formation in diffusion flames

A study of carbon dioxide and oxygen addition on soot formation has been performed such that the effects of dilution, temperature and direct chemical participation have been isolated for the additives on both the fuel and oxidizer sides. By measuring soot inception limits in the counterflow flame and integrated soot volume fractions in the coflow flame, the influence of the additives on soot inception, growth and burnout has also been ascertained. Results demonstrate that carbon dioxide, whether added to the fuel or oxidizer side, can suppress soot formation chemically. The effect of oxygen addition is more complex. When added to the fuel side of an ethylene flame, the addition leads to an abrupt increase in the inception limit, indicating that the inception chemistry has been accelerated. The addition to propane, however, is initially suppressive and results in a significant reduction in the soot inception limit which is more than can be accounted for by dilution. The addition becomes promoting as the oxygen mole fraction approaches 40%. Finally, the effect of oxygen concentration on the oxidizer side, for both ethylene and propane flames, is almost totally thermal.

Studies of the effects of metallic and gaseous additives in the control of soot formation in diffusion flames

An examination of soot growth patterns on a charged probe in a fuel-rich ethylene diffusion flame, with both metallic salt and gaseous additives present, has been made. Alkali salts, which are effective soot inhibitors, cause neutralization of soot particles, leading to reduced particle size and subsequent enhanced oxidation. The addition of freon to the fuel led to greatly increased sooting, while SF 6 addition led to a reduction of soot emission. Both compounds are strongly electrophilic. The role of electrons in soot control was investigated but no correlation with electron density could be found.

Soot formation in diffusion flames: Flow rate, fuel species and temperature effects

A detailed study of soot particle formation in diffusion flames has been made for a series of laminar diffusion flames using a laser scattering/extinction technique. The effects of flow rate, fuel species and temperature on the evolution of the soot particle field have been investigated. Temperature effects have been examined through nitrogen dilution of the fuel. Fluorescence measurements from gas phase species in the initial particle formation region have been used to examine the relationship between these potential soot precursors and the observed particle formation. Flow rate, fuel species and temperature variations have been observed to significantly effect the annular region of the flame, where particles first appear. Increasing the fuel flow rate results in a larger soot volume fraction while the maximum particle size observed in the flame remains nearly constant. Thus, particle number concentration appears to be the quantity most sensitive to the flow rate variation. Temperature measurements made in the flames indicate that fuel flow rate increases result in higher temperatures in the particle formation region, leading to increased soot formation. At later stages, the flames with larger soot concentrations exhibit lower temperatures which facilitates soot particle survival in the oxidation region of the flame. Measurements made in flames with different fuels but similar adiabatic flame temperatures, indicate that the temperature exerts the strongest influence on soot formation. However, distinct effects due to fuel structure still need to be considered.

Kinetics of Soot Formation Investigations into the Mechanism of Soot Formation in Hydrocarbon Diffusion Flames

Abstract In most of the studies and theories about soot formation, the starting point of the discussion is the acetylene, disregarding the previous phase which covers the decomposition of the higher hydrocarbons to acetylene In our opinion the primary reaction step of soot formation in diffusion flames is this decomposition by a thermal cracking process in the gaseous phase. The decomposition rate depends mainly on the molecular structure of fuels We have shown that the dependence of soot formation on the fuel properties can be described by a simple linear relation between the concentration of soot (Rc) and a dimensionless characteristic fuel number BKZ. Instead of BKZ it is possible, to choose the free enthalpy of formation for characterising the decomposition properties of hydrocarbons also It is suggested that the clear distinction of the effect to be ascribed to the influence of the fuel on the one hand and the conditions of combustion on the other will give a better insight into the mechanism of soot...