Showing papers on "Diffusion flame published in 2007"

Sooting behavior dynamics of a non-buoyant laminar diffusion flame

Abstract: Local soot concentrations in non-buoyant laminar diffusion flames have been demonstrated to be the outcome of two competitive processes, soot formation and soot oxidation. It was first believed that soot formation was the controlling mechanism and thus soot volume fractions could be scaled with a global residence time. Later studies showed that this is not necessarily the case and the local ratio of the soot formation and oxidation residence times is the prime variable controlling the ultimate local soot volume fractions. This ratio is a strong function of geometry and flow field, thus a very difficult variable to properly quantify. This study presents a series of microgravity, low oxidizer flow velocity, experiments where soot volume fraction measurements have been conducted on a laminar, flat plate boundary layer type diffusion flame. The objective of the study is to determine if the above observations apply to this type of diffusion flames. The fuel is ethylene and is injected through a flat plate poro...

Stabilization, propagation and instability of tribrachial triple flames

Abstract: A tribrachial (or triple) flame is one kind of edge flame that can be encountered in nonpremixed mixing layers, consisting of a lean and a rich premixed flame wing together with a trailing diffusion flame all extending from a single point. The flame could play an important role on the characteristics of various flame behaviors including lifted flames in jets, flame propagation in two-dimensional mixing layers, and autoignition fronts. The structure of tribrachial flame suggests that the edge is located along the stoichiometric contour in a mixing layer due to the coexistence of all three different types of flames. Since the edge has a premixed nature, it has unique propagation characteristics. In this review, the propagation speed of tribrachial flames will be discussed for flames propagating in mixing layers, including the effects of concentration gradient, velocity gradient, and burnt gas expansion. Based on the tribrachial edge structure observed experimentally in laminar lifted flames in jets, the flame stabilization characteristics including liftoff height, reattachment, and blowout behaviors and their buoyancy-induced instability will be explained. Various effects on liftoff heights in both free and coflow jets including jet velocity, the Schmidt number of fuel, nozzle diameter, partial premixing of air to fuel, and inert dilution to fuel are discussed. Implications of edge flames in the modeling of turbulent nonpremixed flames and the stabilization of turbulent lifted flames in jets are covered.

Intrinsic Flame Instabilities in Premixed and Nonpremixed Combustion

Abstract: The focus of this article is on intrinsic combustion instabilities in both premixed and nonpremixed systems, identifying, in particular, the roles of differential and preferential diffusion, thermal expansion, and heat losses. For premixed flames, the hydrodynamic instability resulting from thermal expansion plays a central role and is particularly dominant in large-scale flames. It is responsible for the formation of sharp folds and creases in the flame front and for the wrinkling observed over the surface of expanding flames. In contrast, instabilities in diffusion flames, which give rise to cellular and oscillating flames, are mainly driven by diffusive-thermal effects, with thermal expansion playing a secondary role. The discussion also includes instabilities of edge-flames in unmixed reactants, which possess stability characteristics of both premixed and diffusion flames, but with a distinct mode of instability.

A wide-ranging kinetic modeling study of methyl butanoate combustion

Abstract: A detailed chemical kinetic model has been used to study methyl butanoate (a model compound for biodiesel fuels) oxidation over a wide range of conditions. New experimental results obtained in a jet stirred reactor (JSR) at 0.101 MPa, Φ = 1.13 and 800 < T (K) < 1350 were obtained and used to test and modify an earlier model. In addition, new experimental data generated in an opposed-flow diffusion flame at 0.101 MPa and in the Princeton variable pressure flow reactor (VPFR) at 1.266 MPa, 0.35 < Φ < 1.5 and 500 < T (K) < 900 are presented and compared against the revised model. The numerical model consists of 295 chemical species and 1498 chemical reactions and gives a good description of the data. Experimentally, the oxidation of methyl butanoate shows very little low temperature and negative temperature coefficient behaviour, with hot ignition occurring at about 800 K. Modeling results show similar diminished low temperature oxidation character, but reasonably reproduce hot ignition behaviour found in the VPFR. At higher temperature conditions, the model well describes the intermediate species found in the jet stirred reactor and in opposed flow diffusion flame experiments.

Numerical and experimental studies of ethanol flames

Abstract: Ethanol combustion is investigated on the basis of a new chemical-kinetic mechanism consisting of 192 elementary steps among 36 species, augmented by 53 additional steps and 14 additional species to address the formation of oxides of nitrogen and 43 steps and 7 species to address formation of compounds involving three carbon atoms. The mechanism is tested against shock-tube autoignition-delay data, laminar burning velocities, counterflow diffusion-flame extinction and measurements of structures of counterflow partially premixed and diffusion flames, the last of these newly completed and reported here for the first time. These measurements, on ethanol–air flames at a strain rate of 100 s −1 , employing prevaporized ethanol with a mole fraction of 0.3 in a nitrogen carrier stream, were made for the pure diffusion flame and for a partially premixed flame with a fuel-side equivalence ratio of 2.3 and involved thermocouple measurements of temperature profiles and determination of concentration profiles of C 2 H 5 OH, CO, CO 2 , H 2 , H 2 O, O 2 , N 2 , CH 4 , C 2 H 6 and C 2 H 2 + C 2 H 4 by gas chromatographic analysis of samples withdrawn through fine quartz probes. Computational investigations also were made of profiles of oxides of nitrogen and other potential pollutants in similar partially premixed flames of ethanol and other fuels for comparison purposes. The computational results are in reasonable agreement with experiment and perform as well as or better than predictions of other, generally much larger, mechanisms available in the literature. Further research is, however, warranted for providing additional and more stringent tests of the mechanism and its predictions.

Theoretical analysis of the evolution from ignition kernel to flame ball and planar flame

Abstract: Dynamics of flame kernel evolution with and without external energy addition has been investigated analytically and numerically. Emphasis is placed on the effects of radiation heat loss, ignition power and Lewis number on the correlation and transition between the initial flame kernel, the self-extinguishing flame, the flame ball, the outwardly propagating spherical flame and the propagating planar flame. The present study extends previous results by bridging the theories of the non-adiabatic stationary flame balls and travelling flames and allowing rigorous consideration of radiation heat losses. The results show that the effects of radiation heat loss play an important role in flame regimes and flame transition and result in a new isolated self-extinguishing flame. Furthermore, it is found that radiation heat losses significantly increase the critical ignition radius and result in three different dependences of the minimum ignition power on the Lewis number. Comparisons between the results from the tran...

Evidence for the transition from the diffusion-limit in aluminum particle combustion

Abstract: This work presents experimental evidence that the transition from gas-phase diffusion-limited combustion for aluminum particles begins to occur at a particle size of 10 μm at a pressure of 8.5 atm. Measurements of the particle temperature by AlO spectroscopy and three-color pyrometry indicate that the peak temperature surrounding a burning particle approaches the aluminum boiling temperature as particle size is decreased to 10 μm when oxygen is the oxidizer. This reduction indicates that reactions are occurring at or near the particle surface rather than in a detached diffusion flame. When CO 2 is the oxidizer, the combustion temperatures remain near the aluminum boiling temperature for particles as large as 40 μm, indicating that the flame is consistently near the surface throughout this size range. Burn time measurements of 10 and 2.8 μm powders indicate that burn time is roughly proportional to particle diameter to the first power. The burn rates of micron- and nano-particles also show strong pressure dependence. These measurements all indicate that the combustion has deviated from the vapor-phase diffusion limit, and that surface or near-surface processes are beginning to affect the rate of burning. Such processes would have to be included in combustion models in order to accurately predict burning characteristics for aluminum with diameter less than 10 μm.

A comparison of saturated and unsaturated C4 fatty acid methyl esters in an opposed flow diffusion flame and a jet stirred reactor

Abstract: Biodiesel fuels, made up primarily of fatty acid methyl esters (FAME), are advantageous because they are renewable and generally have lower pollutant emissions. In order to study in detail the effect of the FAME molecular structure on the combustion chemistry, a saturated (i.e., methyl butanoate) and an unsaturated (i.e., methyl crotonate) C 4 FAME were oxidized in an opposed flow diffusion flame and a jet stirred reactor. Some consistent trends were seen in both experiments. Both fuels have similar reactivity. The experimental results show that methyl crotonate combustion produces much higher levels of C 2 H 2 , 1-C 3 H 4 , 1-C 4 H 8 , and 1,3-C 4 H 6 than methyl butanoate. The methyl butanoate combustion had higher levels of C 2 H 4 . In the opposed flow diffusion flames, the methyl crotonate also produced benzene while for methyl butanoate it was not detected. These species are relevant to soot formation. In addition, the experiments measured higher levels of 2-propenal, methanol, and acetaldehyde for methyl crotonate than for methyl butanoate. The reactions controlling these differences are discussed.

Numerical simulation of Lewis number effects on lean premixed turbulent flames

Abstract: A dominant factor in determining the burning rate of a premixed turbulent flame is the degree to which the flame front is wrinkled by turbulence. Higher turbulent intensities lead to greater wrinkling of the flame front and an increase in the turbulent burning rate. This picture of turbulent flame dynamics must be modified, however, to accommodate the affects of variations in the local propagation speed of the flame front. Classical flame analysis characterizes these local variations in propagation speed by the Markstein number which represents the response of the flame front to curvature and strain. In this paper, we consider lean premixed flames for three different fuels having widely varying fuel Lewis numbers corresponding to widely varying Markstein numbers. In particular, we present numerical simulations of premixed turbulent flames for lean hydrogen, propane and methane mixtures in two dimensions. Each simulation is performed at turbulence conditions similar to those found in laboratory-scale experiments and is performed using detailed chemical kinetics and transport properties. We discuss the effect of Lewis number on the overall flame morphology and explore the dependence of local flame propagation speed on flame curvature. We also explore the relationship between local flame speed and experimentally accessible variables such as OH concentration. Finally, we focus on the low Lewis number case, hydrogen, in which the flame front is broken indicating local extinction.

Emission spectroscopy of flame fronts in aluminum suspensions

Abstract: Spatially resolved emission spectra from Bunsen-type flames stabilized in aluminum suspensions in air and oxygen–argon/helium mixtures were obtained using a mechanical-optical scanning system. A low resolution (1.5 nm) spectrometer was used to acquire the broad spectra over the 350–1000 nm range, and a high-resolution (0.04 nm) instrument was used for observation of AlO molecular bands and non-ionized atomic aluminum. The temperature of condensed phase emitters in the flame was derived using polychromatic fitting of the continuum spectra to Planck’s law. AlO temperature was found by fitting of the theoretically calculated shape of the band to experimental data. Peak temperatures of the condensed emitters were found to be approximately 3250 K in aluminum-air flames and approximately 3350 K for oxygen–argon/helium flames. Temperatures derived from AlO spectra coincide with the temperature of the condensed emitters with measurement accuracy and are only 100–200 °C lower than the computed equilibrium flame temperatures. The radial distribution of the temperature profile of the continuous emitters was found via Abel deconvolution and recovered the double-front structure of the Bunsen flame cone, with the outer flame being attributed to a diffusion flame of the fuel-rich products with ambient air. The observation of atomic aluminum lines seen in emission from the outer flame edge and partial self-absorption from the inner flame confirms the structure associated with the double-front structure. The implications of these results for the regime of particle combustion in a dust flame are discussed.

Effects of CO2 dilution on turbulent premixed flames at high pressure and high temperature

Abstract: Turbulent premixed flames of mixtures of CH4 and air diluted with CO2 at high pressure and high temperature were experimentally studied to clarify the effects of exhaust gas recirculation (EGR), especially the effects of CO2 in EGR gases, on turbulent flame characteristics The maximum CO2 dilution ratio, defined as the ratio of the molar fraction of CO2 to those of air and CO2, was 01 The mixture was preheated up to 573 K and the maximum pressure was 10 MPa Bunsen-type turbulent premixed flames of the mixtures were stabilized in a high-pressure chamber OH-PLIF visualizations of the flames were performed By analyzing the OH-PLIF images, turbulent burning velocity, mean volume of turbulent flame region, and mean fuel consumption rate were calculated Results showed that the turbulent burning velocity, ST, normalized using laminar burning velocity, SL, became smaller when the mixture was diluted with CO2 When the turbulent flame region was defined as the region between 〈c〉 = 01 and 〈c〉 = 09, the mean volume of the flame region increased in the case of CO2 dilution Moreover, the mean fuel consumption rate in the flame region decreased with increasing CO2 dilution ratio This effect was stronger than the decrease in mass fraction of fuel due to CO2 dilution The decrease in the smallest wrinkling scale of the flame front with increasing turbulence Reynolds number in the case of CO2 dilution was more significant than that in the case of no CO2 dilution, corresponding well to the scale relation due to turbulence and intrinsic flame instability proposed previously These results, as well as the previously reported effects of the profiles of the heat-release region on combustion oscillation, imply that exhaust gas recirculation for high-pressure, high-temperature turbulent premixed flames is effective for restraining combustion oscillation of premixed-type gas-turbine combustors

Large eddy simulations of turbulent flames using the filtered density function model

Abstract: Large eddy simulations (LES) of the Sandia/Sydney swirl burners (SM1 and SMA1) and the Sandia/Darmstadt piloted jet diffusion flame (Flame D) are performed. These flames are part of the database of turbulent reacting flows widely considered as benchmark test cases for validating turbulent-combustion models. In the simulations presented in this paper, the subgrid scale (SGS) closure model adopted for turbulence-chemistry interactions is based on the transport filtered density function (FDF) model. In the FDF model, the transport equation for the joint probability density function (PDF) of scalars is solved. The main advantage of this model is that the filtered reaction rates can be exactly computed. However, the density field, computed directly from the FDF solver and needed in the hydrodynamic equations, is noisy and causes numerical instability. Two numerical approaches that yield a smooth density field are examined. The two methods are based on transport equations for specific sensible enthalpy (hs) and RT, where R is the gas constant and T is the temperature. Consistency of the two methods is assessed in a bluff-body configuration using Reynolds averaged Navier-Stokes (RANS) methodology in conjunction with the transported PDF method. It is observed that the hs method is superior to the RT method. Both methods are used in LES of the SM1 burner. In the near-field region, the hs method produces better predictions of temperature. However, in the far-field region, both methods show deviation from data. Simulations of the SMA1 burner and Flame D are also presented using the hs method. Some deficiencies are seen in the predictions of the SMA1 burner that may be related to the simple chemical kinetics model and mixing model used in the simulations. Simulations of Flame D show good agreement with data. These results indicate that, while further improvements to the methodology are needed, the LES/FDF method has the potential to accurately predict complex turbulent flames.

Hydrocarbon Nanoparticles Formed in Flames and Diesel Engines

Abstract: The formation of nanoparticles in laboratory hydrocarbon flames is reviewed in terms of particle morphology, chemical composition, and health hazards. The nascent nanoparticles in the nucleation mode have been widely reported in diverse laboratory flames, and are distinguished by their occurrence as singlet particles that form translucent images in transmission electron microscopy (TEM). Their sizes range from about 10 nm or more down to 2 to 3 nm, the limit of resolution of the TEM, and they possess a liquid-like quality. These particles are widely considered to be the precursor stage to the more readily observed carbonaceous aggregates consisting of chained primary particles that are opaque to the electron beam of the TEM. Nanoparticles sampled from the inverse diffusion flame and the particle effluent from diesel engines show a strong resemblance by GCMS analysis, and they contain many of the stabilomer PAHs and their isomers in the 200 to 302 atomic mass range. Many of these chemical species have high...

Experimental study of spinning combustion in a mesoscale divergent channel

Abstract: Quasi-steady and unsteady propagations of methane and propane–air premixed flames in a mesoscale divergent channel were investigated experimentally and theoretically. The emphasis was the impact of variable cross-section area and the flame-wall coupling on the flame transition between different regimes and the onset of flame instability. Experimentally, for the first time, spinning flames were observed in mesoscale combustion for both lean and rich methane and propane–air mixtures in a broad range of equivalence ratios. The spinning flames rotated in either clockwise or counterclockwise direction with equal probability. The results showed that for a fixed equivalence ratio, there was a critical flow rate, above which flame starts to spin. The spin frequency was approximately proportional to the flame speed. It was also found that the spinning flame only occurred after the transition from fast flame regime to slow flame regime. The flame propagation speed and the effective Lewis number were obtained analytically. Experimental observation and theoretical analysis suggested that regardless of the magnitude of mixture Lewis numbers, the flame-wall coupling will significantly increase the effective Lewis number and lead to a new mechanism to promote the thermal diffusion instability.

The prediction of flame heights and flame shapes of small fire whirls

Abstract: This paper adds vortex flows around Burke–Schumann diffusion flames to predict the flame heights and the flame shapes of small fire whirls. The resulting model matches the measurements of methanol flames in a previous laboratory experiment and the results of numerical calculations in this paper. Burgers Vortex is assumed inside the vortex core radius, while ideal flow is assumed outside the vortex core radius. The ideal flow is corrected for the viscosity changes inside and outside the flame. If the two vortices are combined, they can be approximated as a Sullivan Vortex. Both the experiments and the numerical calculations show that vortex flows stabilize the flame shape, allowing the flame height as defined in a regular diffusion flame to increase. In fact, regular diffusion flames chop off unburned fuel to form separate plumes. With vortex flow, the flame stretches as if the diffusion rate had been reduced. We adjust Roper’s flame height equation to account for the vortex flow and find that the flame height depends on the volume fuel rate and the vortex core radius. If more flows than that required to stabilize the flame were supplied, the radial flows start reducing the flame diameter near the pan, which in turn is balanced by an increase in the volume fuel rate. In the experiment, a balance between the flame temperature, the volume fuel rate, and the flame shape explains why the flame height stops increasing with vortex flows after a fire whirl is generated. In the numerical calculations, we find that the temperature gradient above the port, which controls the fuel evaporation rate, increases with the vortex flows.

Flame characteristics of turbulent lean premixed methane/air flames at high pressure: Turbulent flame speed and flame brush thickness

Abstract: The effects of operating conditions and turbulence on flame front position, turbulent flame speed and flame brush thickness of lean premixed methane/air flames at high pressure are investigated experimentally. A comparison of the measured turbulent flame speeds with existing correlations is presented and discussed. The measurements were performed in an axis-symmetric, generic combustor at pressures up to p = 1.44 MPa, equivalence ratios in the range Φ = 0.43–0.56, preheating temperatures T = 673–773 K and for an inlet bulk velocity range of u BULK = 30–60 m/s. The turbulence intensity and the integral length scale at the combustor inlet were varied by means of turbulence grids with different geometry and by changing the grid position in the inlet channel. The isothermal flow field was characterized by two-dimensional particle image velocimetry (PIV). The characterization of the flame front was based on the statistical analysis of two-dimensional instantaneous images of the laser induced fluorescence of the OH radical (OH-PLIF). The analyses revealed that pressure has no effect on the flame front position, the turbulent flame speed, or on the flame front fluctuation (flame brush thickness). Decreasing the fuel concentration from Φ = 0.43 to Φ = 0.56 results in a flame elongation by a factor of 2 and a corresponding decrease of the turbulent flame speed by a factor of 3. For the same variation of the fuel concentration the flame brush thickness increases by a factor of 2. Additionally, the flame brush thickness was consistently observed to vary proportionally to the square root of the flame front position. The experimental values of the turbulent flame speed are in satisfactory agreement with the existing correlations.

Calculating the soot particle size distribution function in turbulent diffusion flames using a sectional method

Abstract: Soot formation in a turbulent jet diffusion flame is modeled using an unsteady flamelet approach in post-process. In the present work, we apply a detailed kinetic soot model with a sectional method, and study the evolution of the particle size distribution. Detailed information on the evolution of the soot particle size distribution function is acquired. It is found that the particle size distribution function is bimodal throughout the flame. The transition from the small to large particle size distributions is strongly influenced by surface growth and oxidation reactions. We find that large particles are most likely to be emitted from the flame. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

NO Emission Behavior in Oxy-fuel Combustion Recirculated with Carbon Dioxide

Abstract: A numerical study is conducted to grasp the flame structure and NO emissions for a wide range of oxy-fuel combustion (covering from air-blown combustion to pure oxygen combustion) and various mole fractions of recirculated CO2 in a CH4−O2/N2/CO2 counterflow diffusion flame. Special concern is given to the difference of the flame structure and NO emissions between air-blown combustion and oxy-fuel combustion w/o recirculated CO2 and is also focused on chemical effects of recirculated CO2. Air-blown combustion and oxy-fuel combustion without recirculated CO2 are shown to be considerably different in the flame structure and NO emissions. Modified fuel oxidation reaction pathways in oxy-fuel combustion are provided in detail compared to those in air-blown combustion without recirculated CO2. The formation and destruction of NO through Fenimore and thermal mechanisms are also compared for air-blown combustion and oxy-fuel combustion without recirculated CO2, and the role of the recirculated CO2 and its chemica...