Oxidizer Side Research Articles

Vortex-induced extinction behavior in methanol gaseous flames: A comparison with quasi-steady extinction

Using a combination of HCHO planar laser-induced fluorescence and laser Doppler velocimetry measurements, the extinction behavior of methanol counterflow diffusion flames was examined experimentally under conditions in which the extinction was brought about by a vortex generated on the oxidizer side. Comparisons were made with quasi-steady extinction results for the same flames. It was found that the flames can withstand instantaneous strain rates as much as two-and-a-half times larger than the quasisteady ones. The finding was rationalized phenomenologically by comparing the characteristic times of the problem, that is, the mechanical time, the chemical time, and the vortex turnover time. Specifically, estimates of these times yielded the following ordering: sch svort sm. As a result, the vortex introduced an unsteady effect in the outer diffusive-convective layer of the flame, while the inner reactive-diffusive layer behaved in a quasi-steady manner. Consequently, the flame was subject to a damped strain rate through the outer layer. Results from a simple analytical model showed that the difference between vortexinduced extinction and quasi-steady extinction was much more modest in terms of instantaneous scalar

Response of counterflow diffusion flame stabilized on a methanol pool to suppressant doping

Response of a counterflow diffusion flame over a methanol pool to suppressant doping was investigated to understand inhibition processes. Experimentally, flame temperatures, locations, and fuel gasification rates were measured as a function of nitrogen concentration added to air, while flame-extinguishing concentrations of nitrogen, CHF 3 , and C 3 HF 7 were measured as a function of oxidizer velocity, with two different thermal conditions of the fuel pool. Computations were performed using a counterflow flame model with fuel gasification and heat gain/loss at the pool surface and with detailed chemistry and transport in the gas phase. Flame responses to nitrogen, CO 2 , CHF 3 , C 3 HF 7 , and CF 3 Br addition were computed at constant oxidizer velocities. The model well predicts the measured flame-extinguishing concentrations. The maximum flame temperature was reduced monotonically with the addition of nitrogen in both the experiment and computation, while no remarkable reduction was computed with CHF 3 , C 3 HF 7 , or CF 3 Br doping, resulting in significant difference in the flame temperature at extinction. Nevertheless, the condition of the vaporizing pool was insensitive to the suppressant variation. The computation further shows that the addition of CHF 3 and C 3 HF 7 promotes dominant chain branching reaction on the oxidizer side of the flame, while simultaneously inhibiting it in the fuel oxidation region. None of local flame parameters such as peak H, O, or OH concentration reflects universally the suppressant performance in the flame, while the total integrated amount of these flame radicals is reduced linearly over the entire range of the suppressant concentrations and shows excellent agreement at the extinction turning points. A concept of limit integrated amount of the reactive flame radicals is suggested. Difference between the suppressants in the sensitivity of the flame-extinguishing concentrations to the heat balance at the fuel pool is also demonstrated and interpreted based on the flame temperature effect on the suppressant performance.

98/02301 A study on final volatile yield of pulverized coal pyrolysis with thermobalance

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O2+(CO2, N2, and [Ar+CO2]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.

98/02295 Soot microstructure in steady and flickering laminar methane/air diffusion flames

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O2+(CO2, N2, and [Ar+CO2]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.

On micro-buoyancy spherical diffusion flames and a double luminous zone structure of the hydrogen/methane flame

Almost spherically symmetry diffusion flames of substantial dimension (diameter ∼ several centimeters) were established in earth gravity by ejecting an O2/N2 mixture from a porous sphere into a subatmospheric pressure environment consisting of low-molecular-weight fuels such as methane and hydrogen. In the present investigation, a double-luminous zone structure, consisting of a green luminous zone near the fuel side, a blue or violet luminous zone near the oxidizer side, and a dark spece between the two luminous zones, was observed for subatmospheric diffusion flames of either air or an O2/N2 mixture against a mixture of hydrogen with a small quantity of methane. The two luminous zones eventually merge with increasing methane addition, characterized by the green luminous zone shifting toward the blue luminous zone. Computational simulation using the experimental boundary conditions shows that the blue and green luminous zones respectively correspond to the main consumption layers of H2Ō2 and CH4: that the breakdown of CH4 is primarily due to its attack by H, leading to the formation of CO, additional H2 and eventually H2O and CO2 and that the electronically excited CO2 and C2 are respectively responsible for the blue and green luminescence. This double-luminous zone structure also exhibits a secondary peak in the heat release rate profile: important reactions accounting for the two heat release peaks were identified.

Soot zone structure and sooting limit in diffusion flames: Comparison of counterflow and co-flow flames

Soot zone structures of counterflow and co-flow diffusion flames have been studied experimentally using the soot extinction-scattering, polycyclic aromatic hydrocarbon fluorescence, and laser Doppler velocimetry measurements. The counterflow flame has been numerically modelled with detailed chemistry. Results show that two different categories of sooting flame structures can be classified depending on the relative transport of soot particles to flames. These are the soot formation-oxidation flame and the soot formation flame. The soot formation-oxidation flame characteristics are observed in counterflow flames when located on the fuel side and in normal co-flow flames. In this case, soot particles are transported toward the high temperature region or the flame and experience soot inception, coagulation-growth, and oxidation. The soot formation flame characteristics are observed in counterflow flames when located on the oxidizer side and in inverse co-flow flames. In this case, soot particles are transported away from the flame without experiencing oxidation and finally leak through the stagnation plane in counterflow flames or leave the flame in inverse co-flow flames. Sooting limit measurements in both flames also substantiate the two different sooting flame structures and their characteristics.

Relationships between bifurcation and numerical analyses for ignition of hydrogen—air diffusion flames

Linear bifurcation and numerical techniques are employed to determine critical conditions for ignition in steady, counterflow, nonpremixed hydrogen-air systems, with varying degrees of nitrogen dilution of the fuel, at temperatures larger than the crossover temperature associated with the second explosion limit for hydrogen. Analysis of profiles of the radical pool at ignition reveals that, irrespective of the degree of dilution of the fuel or oxidizer streams, the O-atom steady state fails on the oxidizer side of the mixing layer. Therefore, at least three overall steps, with O and H atoms as the chain-branching species, are necessary to describe the ignition process. A simplified model with variable density, specific heat and transport properties, and with Stefan-Maxwell approximations for the diffusion velocities, is proposed to describe the structure of the H 2O 2N 2 weakly reactive mixing layer. Results of bifurcation analysis with this flow-field model and a three-step reduced chemical-kinetic scheme show excellent agreement with results of numerical integration of the full conservation equations with detailed chemistry for all degrees of dilution of the fuel feed.

The effect of changes in the flame structure on the formation and destruction of soot and NOx in radiating diffusion flames

In this study, soot and NO x production in four counterflow diffusion flames with different flame structures is examined both experimentally and theoretically. The distance between the maximum temperature zone and the stagnation plane is progressively changed by changing the inlet fuel and oxidizer concentrations, thus shifting the flame location from the oxidizer side to the fuel side of the stagnation plane. One flame located at the stagnation plane is also examined. Detailed chemical, thermal, and optical measurements are made to experimentally quantify the flame structure, and supporting numerical calculations with detailed chemistry are also performed by specifying the boundary conditions used in the experiments. Results show that as the radical-rich, high-temperature reaction zone is forced into the sooting zone, several changes occur in the flame structure and appearance. These are the following: (1) The flames become very bright due to enhanced soot-zone temperature. This can cause significant reduction in NO formation due to increased flame radiation. (2) OH concentration is reduced from superequilibrium levels due to soot and soot-precursor oxidation in addition to CO and H 2 oxidation. (3) Soot-precursor oxidation significantly affects soot nucleation on the oxidizer side, while soot nucleation on the fuel side seems to be related to C 2 H 2 concentration. (4) Soot interacts with NO formation through the major radical species produced in the primary reaction zone. It also appears that the Fenimore NO initiation mechanism becomes more important for low-temperature flames and when N 2 is added to the fuel side, due to higher N 2 concentrations in the CH-rich zone.

Chemical effects of CF 3H in extinguishing counterflow CO/air/H 2 diffusion flames

The relative importance of introducing CF 3 H as a fire suppressant with the oxidizer or the fuel stream, and its chemical and thermal effects on the extinction condition of counterflow CO/air/H 2 diffusion flames, are investigated both experimentally and numerically. In experiments, the extinction strain rate is evaluated by measuring the jet exit velocities at extinction and the jet separation distance. In numerical calculations, the potential flow approximation is introduced to describe the outer flow field and the strain rate at extinction is determined from the axial velocity gradient on the oxidizer side. By employing mixture fraction concepts, the shift in flame location within the mixing layer caused by increasing the CF 3 H mole fraction or switching CF 3 H from air to fuel stream is shown to affect the measured and predicted extinction strain rates significantly. Flame structure and reaction pathway analyses have been used to identify the ratecontrolling reactions and the influence of introducing the supressing agent from the oxidizer or fuel side on the finite-rate chemistry. Subsequent sensitivity analysis has shown that two alternate CF 3 -consumption reaction pathways can either increase or decrease the extinction strain rate, similar to the sensitivities found in a recent premixed burning velocity study. Finally, based on the numerical calculations, the overall chemical and thermal effects of CF 3 H are demonstrated by freezing the CF 3 H chemistry and by replacing the heat capacity of CF 3 H with that of N 2 for the frozen case.

Acetylene and ethylene mole fractions in methane/air partially premixed flames

Mole fractions of acetylene in flames play a major role in soot formation. An experimental and computational study of the structure of non-premixed and partially premixed methane/air flames was condducted, using a counterflow flame, with emphasis on investigating C 2 H 2 and C 2 H 4 mole fractions. The operating conditions were selected to maintain a fixed stretch rate on the oxidizer side for different levels of partial premixing. Moderately high stretch rates were used to prevent the buoyancy effects. The high stretch rates and selection of methane as the fuel resulted in flames with negligible soot formation based on their blue color. However, measurable mole fractions of C 2 H 2 and C 2 H 4 existed at all levels of partial premixing. Local measurements of species mole fractions were performed using sampling and gas chromatography. Numerical simulations involved 30 species, variable diffusivities, and C 2 chemistry consisting of 278 steps. Analysis of formation and consumption of C 2 H 2 and C 2 H 4 was completed using quantitative reaction path diagrams (QRPD). The results show that the predictions of C 2 H 2 and C 2 H 4 mole fractions are significantly higher than measurements. Unavoidable errors associated with probe sampling are responsible for only a part of the discrepancies. A preliminary investigation of C 3 -C 6 chemistry shows that only a small portion of the error results from the neglect of conversion to higher hydrocarbons. Therefore, critical studies of the rate constants of the relevant reactions, which are identified using the QRPD, are warranted.

Structural response of counterflow diffusion flames to strain rate variations

The structural response of counterflowing methane/oxygen/nitrogen diffusion flames to aerodynamic straining was experimentally and computationally investigated. The temperature and major species concentration profiles were experimentally determined as functions of the applied strain rate by using spontaneous Raman scattering. The experimental situations were further computationally simulated with detailed reaction mechanisms and transport properties. The computed results were found to be in close quantitative agreement with the experimental data. Results demonstrate that, in contrast to counterflow premixed flames, a strained, counterflow diffusion flame has less flexibility to freely adjust its location in response to strain rate variations such that its structure in the direction normal to the flame surface is quite sensitive to variations in the strain rate. Specifically, the counterflow diffusion flame becomes thinner with increasing strain rate a, with its thickness varying inversely with √ a. This leads to increased amount of reactant leakage, progressive reduction in the flame temperature, and eventually extinction of the flame. Computational results further show that while the heat release rate of premixed flames is characterized by a single sharp maximum, for diffusion flames a secondary maximum described by a distinctively different reaction submechanism exists on the oxidizer side of the primary heat release zone. The nature and extent of the negative heat release rate on the fuel side of the flame, as well as its relation to the presence of a dent around the same location on the temperature profile as observed in previous experiments, were also discussed.

Experiments on the interaction between a vortex and a strained diffusion flame

Flame-vortex interactions constitute a basic problem in the analysis of turbulent combustion. Vortex rollup is also found to be one of the major driving mechanisms of combustion instabilities. While there are many analytic and numerical studies of the process, the number of detailed experiments is relatively limited. In particular, the nonpremixed case has not been explored, apparently because the experimental configuration is less easily designed. It is shown here that this case may be examined by employing a counterflow burner. A steady diffusion flame is established in this geometry and a vortex ring is generated from a cylindrical tube installed in one of the combustor nozzles. The vortex impinges on the flame from the oxidizer side entraining the reactive layer and producing a hole in the initial flame. The apparatus is described and initial results of visualizations and velocity measurements by laser doppler anemometry are discussed. It is found that the interaction leads to different outcomes. Strong vortices produce flame extinction and a subsequent blowout. In contrast, the flame is recovered after an interaction with a weak vortex.

The structure of inhibited counterflowing nonpremixed flames

Numerical simulations of the structure of laminar counterflow methane-air nonpremixed flames with chemical (chloromethane) and inert (nitrogen) agents added to the oxidizer stream were performed using a previously developed computer code. The computations were substantiated by measurements of the temperature and velocity fields. The simulated flame structure demonstrated that the addition of chloromethane to the oxidizer side of a nonpremixed flame leads to a broadened reaction zone, increased flame temperatures, decreased concentrations of key flame radicals, and decreased rates of important exothermic chemical reactions. The addition of nitrogen to the oxidizer side of the flame had a very different impact on the flame structure, leading to decreased flame temperatures and decreased concentrations of key flame radicals. The reaction pathways associated with inhibition by chloromethane were identified through an analysis of the calculated flame structure. Simulated flames inhibited by molecular chlorine and hydrochloric acid are also discussed.

Nonluminous diffusion flame of diluted acetylene in oxygen-enriched air

A soot-reducing mechanism of fuel dilution and oxygen enrichment in laminar diffusion flames is suggested. Analysis using the Burke-Schumann theory for the shape of overventilated diffusion flames has shown that there is a critical ratio of stoichiometric coefficients of the fuel and the oxidizer under which the gas flows from the fuel side to the oxidizer side throughout the flame. When this condition is satisfied, the soot growth region vanishes. A similar result is also found in a numerical simulation for diffusion flames that do not satisfy the Burke-Schumann assumption of uniform flow field. KIVA code is used for that purpose. The theoretically predicted direction of gas-flow across the flame sheet is verified in an experiment in a coaxial-flow diffusion flame. Soot cloud and velocity fields are visualized through a laser sheet method in the experiment. The fuel is a mixture of acetylene and nitrogen. The oxidizer is a mixture of oxygen and nitrogen. The compositions of the reactants are controlled so that the adiabatic flame temperature is kept constant to avoid the effect of temperature change. Experimental results show substantial reduction of scattered light intensity by fuel dilution and oxygen enrichment. When a sufficient amount of nitrogen is added to the fuel, nonluminous blue flames are obtained. At higher oxygen concentrations, blue flames are obtained at higher flame temperature region. When oxygen concentration in the oxidizer is 70 vol. %, blue flames are obtained up to 2250 K. The critical condition of the reactants for nonluminous flames agrees with the theoretical prediction when the oxidizer is ordinary air. In oxygen-enriched conditions, the fuel must be diluted more than theoretically predicted.

Structures of Flow and Mixture-Fraction Fields for Counterflow Diffusion Flames with Small Stoichiometric Mixture Fractions

Asymptotic methods are used to evaluate a characteristic diffusion time (the recipro-cal of the so-called “scalar dissipation rate”) for counterflow diffusion flames. Attention is restricted to the practically important limit of small values of the stoichiometric mixture fraction, which results in large values of the stream function at the reaction sheet, a quantity that is employed here as a large expansion parameter. Three distinct layers are identified inside the viscous layer: (i) a convective-diffusive layer on the oxidizer side of the reaction sheet, (ii) a rotational inviscid layer on the fuel side of the reaction sheet, and (iii) a fuel-product mixing layer in the vicinity of the stagnation plane; all three of these layers are analyzed and matched. The matching produces analytic expressions for the stream function and mixture fraction at the reaction sheet and corrects formulas for the characteristic diffusion time previously derived on the basis of constant-density-flow or nonreactive-mixing-layer assumptions. Comparisons between analytical and numerical results demonstrate that the analytical results obtained in the present analysis provide accurate expressions for the characteristic diffusion times that are needed in analyses of flame structure and extinction.

Counterflow diffusion flames of quasi-monodisperse electrostatic sprays

The structure of counterflow diffusion flames of quasi-monodisperse electrostatic sprays of heptane has been studied by measuring droplet size, velocity and gas-phase temperature in two flames characterized by the same overall equivalence ratio. The first flame was found to behave much like a purely gaseous diffusion flame, since droplets evaporate completely before directly interacting with the flame. In the second spray flame, whose strain rate, initial mean droplet size and relative velocity were respectively 43%, 30% and 200% larger than in the first one, droplets have sufficient momentum to penetrate the flame and continue to burn in an oxygen rich environment. The flame appearance is also dramatically different the two cases. The flow strain rate flame appears as a thin blue sheet, whereas the other flame exhibits an additional thick orange region on the oxidizer side, whose luminosity is presumably due to the presence of small soot particles. A comparison between temperature profiles shows that the flame with the higher strain rate has a substantially broader temperature profile, in contrast with the typical behavior of purely gaseous diffusion flames. Furthermore, even though the two flames have identical overall equivalence ratios and nearly equal adiabatic flame temperatures, the high strain rate flame has a peak temperature 240 K higher than the other. This finding can be explained in terms of N 2 dilution effects: droplets cross the blue flame and continue to burn stoichiometrically in an oxygen rich environment while the inert gas concentration progressively decreases. In the course of droplet evaporation, the size distribution, initially monodisperse, broadens because of size-dependent evaporation rate and residence time.

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.

Preferential diffusion and concentration modification in sooting counterflow diffusion flames

An experimental investigation has been conducted on the influence of the mobility of inert additives on soot formation in propane and ethylene counterflow diffusion flames. Inerts used were helium, neon, argon, or krypton, and the results show that while the mobility of the inert has practically no effect when a small amount of inert is added to the oxidizer side, the influence is significant when added to the fuel side in that krypton, being the least mobile inert, yields the greatest soot loading while helium, being the most mobile, yields the least. By relating the spatially-resolved soot volume fractions to the corresponding profiles of temperature, velocity and species concentrations, it is demonstrated that this influence on soot loading is likely caused by concentration modifications of the fuel and the soot precursors due to the different mobilities of the inert additives.

Structure and behavior of diffusion flames in a pressure gradient

The structure of a diffusion flame embedded in a flow field parallel to the flame is studied under conditions where this external flow imposes an adverse pressure gradient. It is convenient to think of the physical problem as a flame lying along the flow direction of a divergent channel. The mathematical problem is reduced to a set of ordinary differential equations by (i) employing the Howarth transformation to eliminate the variable density and (ii) introducing a similarity solution somawhat in the manner of the Falkner-Skan treatment of boundary layer flows. Because the low-density gas near the flame respondes more readily to the pressure gradient than does the higher density gas, a reverse flow develops in the low density region which severely affects both the structure of the flame and the fuel consumption rate. For a flame with unit stoichiometry, the reverse flow eventually leads to extinction of the flame by separating the two shear layers that bound the fuel and oxidizer streams. For stoichiometry corresponding to methane-air, the flame situates itself near the oxidizer side of the reverse flow and has no tendency toward extinguishment.