Flame Standoff Ratio Research Articles

Single ammonia droplet combustion in a high-pressure environment in microgravity

Since ammonia is a carbon neutral fuel, it is expected to be widely utilized in the near future. Although there have been many researches on ammonia combustion, no research has been conducted on ammonia droplet combustion, which is a fundamental research on spray combustion, because ammonia is in a gaseous state at room temperature and atmospheric pressure. This study investigated the combustion characteristics of single ammonia droplets in microgravity for the first time. Single ammonia droplets were formed in high pressure air and were successfully ignited in microgravity. In an early stage of burning, the droplet vaporized with an almost constant vaporization rate of 0.87–1.0 mm2/s. After such a quasi-steady period, the vaporization rate decreased over time. In accordance with the decrease in the vaporization rate, the flame standoff ratio also decreased over time. After that, the droplet exhibited unique phenomena such as droplet disruption, puffing, or droplet re-expansion although the fuel was initially pure. These types of unique behavior are caused by the diffusion of water vapor, a combustion product, to the droplet surface, and dissolution and accumulation of water on the droplet surface. Since water has a lower volatility than ammonia, the concentration of dissolved water at the droplet surface increases rapidly as the droplet diameter decreases. As a result, the vaporization of ammonia is suppressed and the flame standoff ratio decreases. Additionally, as the water concentration at the droplet surface increases, the boiling point at the droplet surface also increases, resulting in superheating of the liquid ammonia and homogeneous bubble nucleation. The growth of bubbles caused droplet disruption, puffing, and droplet re-expansion.

Oxygen droplet combustion in hydrogen under microgravity conditions

The combustion of single liquid oxygen droplets in gaseous hydrogen is investigated experimentally under microgravity conditions to shed light on spray combustion processes in rocket engines. Using a drop tower apparatus, experiments are performed varying the ambient pressure between 0.1–5.7 MPa, which corresponds to a reduced pressure of oxygen pr between 0.02–1.12. The combustion is investigated using high-speed shadowgraph imaging to track the droplet shape and OH-chemiluminescence to identify the flame zone. At low pressures (pr<0.15), the droplet shape is found to change significantly during combustion likely due to the formation of a water ice layer around the droplet. Small jets of oxygen appear to break out of this ice layer, leading to an observed increase in linear and angular momentum of the droplet. At higher pressures, the visible effect of ice formation near the droplet surface decreases. The combustion process at different pressures in the subcritical and the supercritical regime is compared and discussed. The pressure has a limited influence on the flame standoff ratio, whereas it influences the burning rate constant substantially. Specifically, the experimental data suggest a maximum of the burning rate constant near the critical pressure, which is consistent with several experiments on hydrocarbon droplet combustion.

Combustion characteristics of aromatic-enriched oil droplets produced by pyrolyzing unrecyclable waste tire rubber

The purpose of this work is to study the combustion characteristics of aromatic-enriched tire pyrolysis oil (TPO) produced from typical small vehicles and truck tires. Pyrolysis oils (NRTO: natural rubber tire oil and SRTO: synthetic rubber tire oil) from two raw radial tire materials were produced and condensed at 25 °C and 130 °C in a three-stage auger pyrolysis reactor under negative pressure. Combustion particularities were acquired using a single-droplet suspension combustion device and evaluated by five different indicators: kinetics, flame shape, burning rates, flame stand-off ratio (FSR) and mass transfer parameters. It was found that TPO was a potential fuel with higher calorific value (40–44 MJ/kg) and lower combustion apparent activation energy (10–40 MJ/mol) than that of bio-oil and solid fuels. Oil produced from synthetic rubber and condensed at 25 °C exhibited higher stable burning rate (1.93 ± 0.03 mm2/s) than that of nature rubber pyrolysis oil. Comparing with traditional fuels, tire oil droplets displayed higher surface local heat transfer coefficient, but the mass transfer number (B ≈ 2.5) and FSR (<3.1) under stable combustion conditions were much lower. The combustion behavior of TPO has partial similarity with unsaturated hydrocarbons and ester biofuels. The reported results support the well-characterized TPO condensates for subsequent atomized combustion in realistic industrial burners.

Combustion characteristics of novel bishomocubane propellants in oxygen-enriched environments

The combustion characteristics of three novel energetic cage hydrocarbon compounds – bis(nitratomethyl)-1,3-bishomocubane (DNMBHC), diazido-dimethyl-bishomocubane (DADMBHC), and 1,3-bishomocubane dimer (BHCD), and an RP-1 surrogate fuel (RP-1-s) were investigated through droplet combustion experiments. As these novel compounds were envisioned as potential additives to RP-1 in semi-cryogenic engines, where liquid oxygen (LOX) would be used as the oxidizer, it was necessary to test these fuels in oxygen-enriched environments. Studies were conducted in two different environments – one containing 50% O2 and the other having 80% O2 with balance N2 in both cases. The influence of oxygen concentration on the variation of droplet and flame dimensions with time, time period of combustion, flame temperature, and sooting propensities were studied using videography and color-ratio pyrometry. With an increase in oxygen concentration, the flame temperatures were found to increase with a concurrent reduction in the flame stand-off ratios, resulting in faster combustion. As compared to trials in ambient air, the effect of microexplosions generated as a result of condensed phase pyrolysis reactions for the novel BHC fuels, was slightly diminished owing to the shorter combustion durations. Despite the reduced intensity of microexplosions, DNMBHC and DADMBHC droplets were found combust approximately 2.39 and 1.78 times faster than a similar-sized RP-1-s droplet with 80% ambient oxygen. Among the three novel BHC fuels, only DNMBHC fared better than RP-1-s in terms of maximum flame temperatures and had sooting propensities comparable to that of the benchmark fuel and thus was assessed as the most promising candidate.

Sub-millimeter sized multi-component jet fuel surrogate droplet combustion: Physicochemical preferential vaporization effects

Isolated droplet burning behaviors of fuel mixtures that all share the fully pre-vaporized combustion behaviors of the same “global” Jet-A real fuel are investigated numerically to elucidate the effects of preferential vaporization. Predictions are generated using three such multi-component hydrocarbon mixtures (Mixture-1: n-decane/iso-octane/toluene 42.7/33.0/24.3, Mixture-2: n-dodecane/iso-octane/1,3,5 trimethyl benzene 49.0/21.0/30.0 and Mixture-3: n-hexadecane/iso-octane/1,3,5 trimethyl benzene 36.5/31.0/32.5 molar ratios), each having very different distillation properties. Simulations are performed using a transient one-dimensional sphero-symmetric model, involving numerically reduced detailed chemical kinetics, and multi-component gas-phase diffusive transport. The interactions among the different liquid-phase components are modeled using UNIFAC activity coefficient methodology. The predictions using Mixture-1 are found to be in good agreement with previously published drop-tower experiments for 550 µm droplet combustion in air. Stagnant and internally mixed liquid-phase behaviors are both considered. Near fully mixed internal conditions and including sooting effects (observed in the experiments) result in predictions that are in good agreement with experimental drop diameter and flame-standoff ratio histories. By comparing predictions for the three mixtures, preferential vaporization effects exhibit strong non-linear dependences on initial droplet diameter and ambient pressure. At low pressures and for droplet sizes typical of those found in multi-phase gas turbine combustors, comparable diffusion and vaporization characteristic times favor frozen limit vaporization. However, at pressures typical of those found in applications, batch distillation dominates and preferential vaporization chemical property differences are found to become as significant as physical property effects in influencing combustion behaviors.

Effect of carbon-based nanoparticles on the ignition, combustion and flame characteristics of crude oil droplets

The use of in-situ burning (ISB) as a clean-up response in the event of an oil spill has generated controversy because of unburned hydrocarbons and products of incomplete combustion left behind on an ISB site. These substances threaten marine life, both in the ocean and on the ocean floor. Treating crude oil as a multicomponent liquid fuel, this manuscript investigates the effect of carbon-based nanomaterials, acetylene black (AB) and multi-walled carbon nanotube (MWNT), on the combustion and flame characteristics of crude sourced from the Bakken formation (ND, USA). Sub-millimeter droplets of colloidal suspensions of Bakken crude and nanomaterials at various particle loadings were burned, and the process was captured with CMOS and CCD cameras. The resulting images were post-processed to generate burning rate, ignition delay, total combustion time, and flame stand-off (FSR) ratio data for the various crude suspensions. A maximum combustion rate enhancement of 39.5% and 31.1% was observed at a particle loading of 0.5% w/w acetylene black nanoparticles and 0.5% w/w multi-walled carbon nanotubes, respectively. Generally, FSR for pure Bakken was noted as larger than for Bakken with nanoparticle additives. These results are expected to spur further investigations into the use of nanomaterials for ISB crude oil clean-ups.

The effect of acetylene black on droplet combustion and flame regime of petrodiesel and soy biodiesel

The addition of nanomaterials to base liquid fuels for the enhancement of combustion properties has long generated interest, since it is linked to improvement in combustion properties in petroleum-derived fuels. This work investigates the effect of acetylene black nanoparticles on soy-based biodiesel and petrodiesel. These nanoparticles were added in various proportions to base fuels to make suspensions. Sub-millimeter-sized spherical droplets of such suspensions were ignited using hot wire loops, and the combustion process was recorded using a high-speed camera. An infrared imaging camera was also used to record the droplet combustion process. Properties such as ignition delay, combustion rate, total combustion time, flame stand-off ratio, and droplet temperature were measured with post-processing of the resulting images. The results showed an increase in ignition delay of up to 10% in biodiesel and up to 8% in petrodiesel. An up to 13% decrease in biodiesel combustion rate and 1.5% decrease in diesel combustion rate was also observed. Moreover, a maximum of 14% decrease in the total combustion time of biodiesel was observed, compared to a 13% decrease in that of petrodiesel. The results are expected to spark further interest in the exploration of acetylene black as an additive for other liquid fuels.

Combustion of rocket-grade kerosene droplets loaded with graphene nanoplatelets—A search for reasons behind optimum mass loadings

Nanofluid-type fuels formulated by mixing minute quantities of graphene nanoplatelets (GNPs) with Isrosene, a refined form of kerosene utilized in semi-cryogenic engines by the Indian Space Research Organization (ISRO), were found to improve the convective heat transfer characteristics by 49% in a previous study. In this study, the combustion characteristics of such nanofluid-type fuels, such as initial mass burning rates, burning rate constants, and flame stand-off ratios, were determined for millimeter-sized droplets under ambient conditions. The impact of the specific surface area of the nanoplatelets and their relative mass loading in the nanofluid was determined. While the specific surface area did not have any impact on the combustion characteristics, the relative mass loading had a strong impact on the burning rate constants, with an increment of approximately 13.5% over Isrosene for a mass loading of 0.2%. There also existed an optimum value of mass loading, beyond which the burning rate constants. This anomalous behavior, which had earlier been conjectured to be due to various physico-chemical reasons, was attributed primarily to the gas phase conduction phenomenon, corroborated by the analysis of the temperature profiles in the gas phase, acquired using an infrared imaging camera.

Three stage cool flame droplet burning behavior of n-alkane droplets at elevated pressure conditions: Hot, warm and cool flame

Transient, isolated n-alkane droplet combustion is simulated at elevated pressure for helium-diluent substituted-air mixtures. We report the presence of unique quasi-steady, three-stage burning behavior of large sphero-symmetric n-alkane droplets at these elevated pressures and helium substituted ambient fractions. Upon initiation of reaction, hot-flame diffusive burning of large droplets is initiated that radiatively extinguishes to establish cool flame burning conditions in nitrogen/oxygen “air” at atmospheric and elevated pressures. However, at elevated pressure and moderate helium substitution for nitrogen (XHe > 20%), the initiated cool flame burning proceeds through two distinct, quasi-steady-state, cool flame burning conditions. The classical “Hot flame” (∼1500 K) radiatively extinguishes into a “Warm flame” burning mode at a moderate maximum reaction zone temperature (∼ 970 K), followed by a transition to a lower temperature (∼765 K), quasi-steady “Cool flame” burning condition. The reaction zone (“flame”) temperatures are associated with distinctly different yields in intermediate reaction products within the reaction zones and surrounding near-field, and the flame-standoff ratios characterizing each burning mode progressively decrease. The presence of all three stages first appears with helium substitution near 20%, and the duration of each stage is observed to be strongly dependent on helium substitutions level between 20–60%. For helium substitution greater than 60%, the hot flame extinction is followed by only the lower temperature cool flame burning mode. In addition to the strong coupling between the diffusive loss of both energy and species and the slowly evolving degenerate branching in the low and negative temperature coefficient (NTC) kinetic regimes, the competition between the low-temperature chain branching and intermediate-temperature chain termination reactions control the “Warm” and “Cool” flame quasi-steady conditions and transitioning dynamics. Experiments onboard the International Space Station with n-dodecane droplets confirm the existence of these combustion characteristics and predictions agree favorably with these observations.

Combustion characteristics and liquid-phase visualisation of single isolated diesel droplet with surface contaminated by soot particles

The combustion generated soot contamination effect on a single diesel droplet ignition and burning was investigated experimentally for the first time. Diesel droplet flame was used to contaminate the droplet to be investigated prior to ignition. Distinct differences in lifetime and stability of the burning of the neat and contaminated droplet samples were observed in their heating, boiling and disruptive phases. For a soot-contaminated droplet surface, the evaporation rate became weaker as a result of slower mass transfer thus contracted the flame formation. Contrary to the burning rate enhancement of droplet with stable and uniform suspension of particles observed by other researchers, the slightest contamination of soot particles in a fuel droplet surface can significantly reduce the burning rate. Denser agglomeration of soot can form a shell on the droplet surface which blocks the flow of gas escaping through the surface thus distort the droplet even further. At late combustion stage, bubbles are observed to rapture on the surface of the soot-contaminated droplet. Strong ejections of volatile liquid and vapour that would explode shortly after parting from the droplet are observed. It seems that the explosion and burning of ejected mixture have little interactions with the enveloped flame surrounding the primary droplet. Enhanced visualisation of droplet liquid-phase has clearly indicated the cause of declining trend in the burning rate and flame stand-off ratio of soot-contaminated diesel droplet. These insights are of significance for understanding the effect of fuel droplet contamination by combustion generated soot particles.

A front tracking method for particle-resolved simulation of evaporation and combustion of a fuel droplet

A front-tracking method is developed for the particle-resolved simulations of droplet evaporation and combustion in a liquid-gas multiphase system. One field formulation of the governing equations is solved in the whole computational domain by incorporating suitable jump conditions at the interface. Both phases are assumed to be incompressible but the divergence-free velocity condition is modified to account for the phase change at the interface. A temperature gradient based evaporation model is used. An operator-splitting approach is employed to advance temperature and species mass fractions in time. The CHEMKIN package is incorporated into the solver to handle the chemical kinetics. The multiphase flow solver and the evaporation model are first validated using the benchmark problems. The method is then applied to study combustion of a n-heptane droplet using a single-step chemistry model and a reduced chemical kinetics mechanism involving 25-species and 26-reactions. The results are found to be in good agreement with the experimental data and the previous numerical simulations for the time history of the normalized droplet size, the gasification rate, the peak temperature and the ignition delay times. The initial flame diameter and the profile of the flame standoff ratio are also found to be compatible with the results in the literature. The method is finally applied to simulate a burning droplet moving due to gravity at various ambient temperatures and interesting results are observed about the flame blow-off.

Extended lattice Boltzmann scheme for droplet combustion

The available lattice Boltzmann (LB) models for combustion or phase change are focused on either single-phase flow combustion or two-phase flow with evaporation assuming a constant density for both liquid and gas phases. To pave the way towards simulation of spray combustion, we propose a two-phase LB method for modeling combustion of liquid fuel droplets. We develop an LB scheme to model phase change and combustion by taking into account the density variation in the gas phase and accounting for the chemical reaction based on the Cahn-Hilliard free-energy approach. Evaporation of liquid fuel is modeled by adding a source term, which is due to the divergence of the velocity field being nontrivial, in the continuity equation. The low-Mach-number approximation in the governing Navier-Stokes and energy equations is used to incorporate source terms due to heat release from chemical reactions, density variation, and nonluminous radiative heat loss. Additionally, the conservation equation for chemical species is formulated by including a source term due to chemical reaction. To validate the model, we consider the combustion of n-heptane and n-butanol droplets in stagnant air using overall single-step reactions. The diameter history and flame standoff ratio obtained from the proposed LB method are found to be in good agreement with available numerical and experimental data. The present LB scheme is believed to be a promising approach for modeling spray combustion.

A Burke–Schumann analysis of diffusion-flame structures supported by a burning droplet

A Burke–Schumann description of three different regimes of combustion of a fuel droplet in an oxidising atmosphere, namely the premixed-flame regime, the partial-burning regime and the diffusion-flame regime, is presented by treating the fuel and oxygen leakage fractions through the flame as known parameters. The analysis shows that the burning-rate constant, the flame-standoff ratio, and the flame temperature in these regimes can be obtained from the classical droplet-burning results by suitable definitions of an effective ambient oxygen mass fraction and an effective fuel concentration in the droplet interior. The results show that increasing oxygen leakage alone through the flame lowers both the droplet burning rate and the flame temperature, whereas leakage of fuel alone leaves the burning rate unaffected while reducing the flame temperature and moving the flame closer to the droplet surface. Solutions for the partial-burning regime are shown to exist only for a limited range of fuel and oxygen leakage fractions.

Ignition and combustion characteristics of single droplets of a crude glycerol in comparison with pure glycerol, petroleum diesel, biodiesel and ethanol

The ignition and combustion characteristics of single droplets of a crude glycerol were experimentally studied and compared with those of pure glycerol, a petroleum diesel, a biodiesel, and ethanol. A single fuel droplet was suspended at the tip of a silicon carbide fibre undergoing heating, ignition and combustion in an electrically heated horizontal tube furnace at an air temperature ranging from 948 K to 1048 K. The ignition and combustion behaviour of the droplet were recorded using a CCD camera. Ignition delay time, total combustion time, burning rate, and flame standoff ratio were estimated. At a same temperature, the ignition delay time and total combustion time followed the order of pure glycerol > crude glycerol > ethanol > biodiesel > diesel, while the burning rate followed the order of crude glycerol > diesel > biodiesel > ethanol > pure glycerol, suggesting that the impurities, mainly water and methanol, had a profound influence on the combustion characteristics of crude glycerol. The flame standoff ratio slightly decreased after ignition but continuously increased with time afterwards for the crude glycerol and remained almost unchanged for the pure glycerol, indicating the significant influence of impurities on the quasi-steadiness of the flame.

Combustion characteristics of butanol isomers in multiphase droplet configurations

This study reports results of experiments on the isolated droplet burning characteristics of butanol isomers (n-, iso-, sec-, and tert-) under standard atmosphere conditions in an environment that promotes spherical combustion. The data are compared with predictions from a detailed numerical model (DNM) that incorporates complex combustion chemistry, radiative heat transfer, temperature dependent variable fluid properties, and unsteady gas and liquid transport. Computational predictions are generated using the high temperature kinetic models of Sarathy et al. (2012) and Merchant et al. (2013).The experiments were performed in a free-fall facility to reduce the effects of buoyancy and produce spherical droplet flames. Motion of single droplets with diameters ranged from 0.52mm to 0.56mm was eliminated by tethering them to two small-diameter SiC filaments (∼14µm diameter). In all the experiments, minimal sooting was observed, offering the opportunity for direct comparison of the experimental measurements with DNM predictions that neglect soot kinetics.The experimental data showed that the burning rates of iso- and sec-butanol are very close to that of n-butanol, differing only in flame structure. The flame stand-off ratios (FSR) for n-butanol flames are smaller than those for the isomers, while tert-butanol flames exhibited the largest FSR. DNM predictions based upon the kinetic model of Sarathy et al. over-predict the droplet burning rates and FSRs of all the isomers except n-butanol. Predictions using a kinetic model based upon the work of Merchant et al. agree much better with the experimental data, though relatively higher discrepancies are evident for tert-butanol simulation results. Further analyses of the predictions using the two kinetic models and their differences are discussed. It is found that the disparity in transport coefficients for isomer specific species for Sarathy et al. model fosters deviation in computational predictions against these newly acquired droplet combustion data presented in this study.

Partial-Burning Regime for Quasi-Steady Droplet Combustion Supported by Cool Flames

A simplified model for droplet combustion in the partial-burning regime is applied to the cool-flame regime observed in droplet-burning experiments performed in the International Space Station with normal-alkanes fuels resulting in expressions for the quasi-steady droplet burning rate and for the flame standoff ratio. The simplified predictions are found to produce reasonable agreement with the experimentally measured values of burning-rate constants but not with their apparent dependencies on pressure or on the initial droplet diameter. Good agreement is found, however, with newly measured and numerically calculated flame standoff ratios in this droplet combustion supported by cool flames.

Experiments and theoretical approaches on the burning behaviors of single n-heptane droplet

This study was conducted to improve the theoretical prediction of the burning characteristics of an n-heptane droplet by comparing them with experimental results. To achieve this, numerical approaches were conducted by assuming that the droplet combustion can be described by both quasi-steady behavior for the region between the droplet surface and the flame interface, and transient behavior for the region between the flame interface and ambient surrounding. Comparisons were considered for droplet diameter (dt), flame diameter (df), flame standoff ratio (FSR), and viscous drag induced fluxes which are Stefan flux and thermophoretic flux for various initial droplet diameter (d0) and oxygen (O2) concentration conditions. It was revealed that the flame diameter (df) and flame standoff ratio (FSR) initially increase dramatically and approach quasi-steady behavior within the observation period, and the flame standoff ratio (FSR) increases a little with the initial droplet diameter (d0) both experimentally and theoretically. The value of flame diameter (df) decreases from its maximum value when oxygen (O2) concentration is increased from a value of 18% to 40%. The burning rate (K) constant becomes higher as the oxygen (O2) concentration increases since the increase of oxygen (O2) concentration produces a higher maximum flame temperature (df) which enhances the effective thermo-physical properties of the gas-phase bounded by droplet and flame front.

Computational Modeling of Unsupported and Fiber-Supported n-Heptane Droplet Combustion in Reduced Gravity: A Study of Fiber Effects

A detailed numerical investigation of combustion of unsupported and fiber supported n-heptane droplets in reduced gravity is presented. The primary focus is on the effects of support fibers on the droplet burning rates and flame structure. A 21-step n-heptane reaction mechanism consisting of 20 species is employed to model the combustion chemistry. The volume-of-fluid (VOF) method is employed to capture the liquid-gas interface while allowing for time-dependent two-phase multidimensional flows. Computed burning rates and flame stand-off ratios are compared with the experimental results of Jackson. Predicted flame structures are also validated with the experimental results of Mikami. The present computational results agree well with the experimental results. The results indicate that the support fibers can have significant impact on droplet burning rates and flame structure.

Methanol 연료 액적의 연소 특성에 관한 연구

The main purpose of this study is to provide basic information of droplet burning, extinction process and flame behavior of methanol fuel and improve the ability of theoretical prediction of these phenomena. For the improved understanding of these phenomena, this paper presents the experimental results on the methanol droplet combustion conducted under various initial droplet diameters (), ambient pressure (), and oxygen concentration () conditions. To achieve this, the experimental study was conducted in terms of burning rate (K) with normalized droplet diameter (), flame diameter () and flame standoff ratio (FSR) under the assumptions that the droplet combustion can be described by both the quasi-steady behavior for the region between the droplet surface and the flame interface and the transient behavior for the region between the flame interface and ambient surrounding.

N-Butanol droplet combustion: Numerical modeling and reduced gravity experiments

Recent interest in alternative and bio-derived fuels has emphasized butanol over ethanol as a result of its higher energy density, lower vapor pressure and more favorable gasoline blending properties. Numerous efforts have examined the combustion of butanol from the perspective of low dimensional gas-phase transport configurations that facilitate modeling and validation of combustion kinetics. However, fewer studies have focused on multiphase butanol combustion, and none have appeared on isolated droplet combustion that couples experiments with robust modeling of the droplet burning process. This paper presents such an experimental/numerical modeling study of isolated droplet burning characteristics of n-butanol. The experiments are conducted in an environment that simplifies the transport process to one that is nearly one-dimensional as promoted by burning in a reduced gravity environment. Measurements of the evolution of droplet diameter (Do=0.56–0.57mm), flame standoff ratio (FSR≡Df/D) and burning rate (K) are made in the standard atmosphere under reduced gravity and the data are compared against numerical simulation. The detailed model is based on a comprehensive time-dependent, sphero-symmetric droplet combustion simulation that includes spectrally resolved radiative heat transfer, multi-component diffusive transport, full thermal property variations and detailed chemical kinetic. The simulations are carried out using both a large order kinetic mechanism (284 species, 1892 reactions) and a reduced order mechanism (44 species, 177 reactions). The results show that the predicted burning history and flame standoff ratios are in good agreement with the measurements for both the large and reduced order mechanisms. Additional simulations are conducted for varying oxygen concentration to determine the limiting oxygen index and to elucidate the kinetic processes that dictate the extinction of the flame at these low oxygen concentrations.