Showing papers on "Diffusion flame published in 2004"
TL;DR: In this paper, a 3-Zones Extended Coherent Flame Model (ECFM3Z) is proposed to account for the diffusion of fuel and air towards the reactive layer.
Abstract: The Extended Coherent Flame Model of Colin et al. (2003) developed to model combustion in perfectly or partially mixed mixtures is adapted to also account for unmixed combustion. The ECFM model is based on a flame surface density equation which takes into account the wrinkling of the flame front surface by turbulent eddies and a conditioning averaging technique which allows precise reconstruction of local properties in fresh and burned gases even in the case of high levels of local fuel stratification. This model has been used with success in gasoline engines (Duclos et al., 1996; Duclos and Zolver, 1998; Lafossas et al., 2002; Henriot et al., 2003; Kleemann et al., 2003). In order to adapt the model to unmixed combustion for Diesel application, a description of the mixing state has been added. It is represented by three mixing zones: a pure fuel zone, a pure air plus possible residual gases zone and a mixed zone in which the ECFM combustion model is applied. A mixing model is presented which allows progressive mixing of the initially unmixed fuel and air. This new combustion model, called ECFM3Z (3-Zones Extended Coherent Flame Model), can therefore be seen as a simplified CMC (Conditional Moment Closure) type model where the mixture fraction space would be discretized by only three points. The conditioning technique is extended to the three mixing zones and allows to reconstruct, like in the ECFM model, the gas properties in the unburned and burned gases of the mixed zone. Application of the model to internal combustion engine calculations implies the necessity of auto-ignition modelling coupled to premixed and diffusion flames description. Auto-ignition is modelled following (Colin et al., 2004), while the premixed turbulent flame description is given by the ECFM. The diffusion flame is now accounted for thanks to the three zones mixing structure which represents phenomenologically the diffusion of fuel and air towards the reactive layer, that is the mixed zone. The ECFM3Z combustion model has already been presented (Beard et al., 2003) in a comparative work between Diesel experiments and corresponding calculations covering different engine operating points. Here, the model is presented in all its details and its behavior is analysed when the relative duration of injection and auto-ignition delay are varied in a direct injection Diesel engine. It is shown that the model is able to reproduce the relative importance of auto-ignition and diffusion flame on the total heat release, depending on the engine operating point considered.
419 citations
TL;DR: In this paper, the effect of steady strain on the transient autoignition of n-heptane at high pressures is studied numerically with detailed chemistry and transport in a counterflow configuration.
277 citations
TL;DR: In this article, the effect of hydrogen blending on lean premixed methane-air flames is studied with the direct numerical simulation (DNS) approach coupled with a reduced chemical mechanism, and two flames are compared with respect to stability and pollutant formation characteristics: a pure methane flame close to the lean limit, and one enriched with hydrogen.
273 citations
TL;DR: In this article, the effect of the fractal structure of soot aggregates on the rate of heat loss to the surrounding gas was investigated in the presence of relatively low laser fluences under which soot particles are heated to temperatures below 3500 K.
268 citations
TL;DR: In this paper, the heat recirculation in a porous burner is analyzed using a one-dimensional time-dependent formulation with complete chemistry, where heat is recirculated through solid conduction and solid-to-solid radiation from the matrix downstream of a flame to the matrix upstream of the flame.
241 citations
TL;DR: In this article, the use of small injector tip orifices was shown to enable non-sooting and low flame temperature combustion in two different ways as summarized below, while the injector was fully open (i.e. during the mixing-controlled phase of heat release for diesel combustion).
Abstract: Methods of producing non-sooting, low flame temperature diesel combustion were investigated in an optically-accessible, quiescent constant-volume combustion vessel under mixing-controlled diesel combustion conditions. Combustion and soot processes of single, isolated fuel jets were studied after auto-ignition and transient premixed combustion and while the injector was fully-open (i.e. during the mixing-controlled phase of heat release for diesel combustion). The investigation showed that small injector tip orifices could be used to produce non-sooting and low flame temperature combustion simultaneously. The use of small orifices was shown to enable non-sooting and low flame temperature combustion in two different ways as summarized below. A more detailed description of the experimental methods and results is provided in Ref. [1-3]. First, using an injector tip with a 50 micron orifice and ambient oxygen concentrations as low as 10% (simulating the use of extensive EGR), a fuel jet was non-sooting at typical diesel ambient temperatures (1000 K). Second, using the same injector tip at a reduced ambient gas temperature (850 K), but with 21% oxygen, it was shown that non-sooting, mixing-controlled combustion occurred at the lift-off length in a fuel-air mixture with a cross-sectional average equivalence ratio of approximately 0.6-suggesting that the quasi-steady combustion was fuel-lean andmore » thereby avoided the formation of a diffusion flame. The adiabatic flame temperature with reduced ambient oxygen concentration or fuel-lean combustion was approximately 2000 K, compared to typical diesel flame temperatures that exceed 2600 K. The 50 micron orifice results above were obtained using a No.2 diesel fuel. However, using an oxygenated fuel (20 wt% oxygen), the investigation showed that the same low temperature combustion, either with reduced ambient oxygen concentration or fuel-lean combustion, was realized with a 100 micron orifice. Although these single, isolated jets do not have jet-jet interactions that would occur in realistic engines, the results are useful for understanding limiting-case behavior of single-jet mixing and combustion during an injection event. The non-sooting and low flame temperature mixing-controlled combustion realized using small orifice tips suggests that the use of small orifices offers the potential for a simultaneous soot and NOx reduction in an engine, much like diesel HCCI combustion. However, further research is needed to determine whether these methods could be successfully implemented in real engines.« less
232 citations
TL;DR: In this paper, the transition of flame structure from a stable to an unstable state was studied in a lean-premixed swirl-stabilized combustor, and the inlet temperature and equivalence ratio were found to be two important variables determining the stability characteristics of the combustor.
165 citations
TL;DR: In this article, double-exposed flame images acquired through either a pentroof window or the piston crown were processed to calculate the intra-cycle flame growth and convection rates under 1500 RPM low-load conditions.
148 citations
TL;DR: In this article, it is shown that scalar dissipation is of great importance in the theory and modelling of combustion and other reacting turbulent flows and therefore, effort needs to focus on viable means of modelling it.
Abstract: Scalar dissipation is of great importance in the theory and modelling of combustion and other reacting turbulent flows. Measurements of scalar dissipation are found to lack the quality assurance of checks available from the conservation equations. Conditional averages of the scalar dissipation, so important in turbulent reacting flow theory and modelling, have qualitative and quantitative dependences that are very dependent on the details of the flow and mixing conditions. Accordingly, effort needs to focus on viable means of modelling it. Fluctuations of the scalar dissipation about the conditional mean are also important. Research results in this area need to be made more accessible to the combustion scientist. Heat release effects, so important in turbulent premixed combustion, are found to be much less important in non-premixed combustion.
134 citations
TL;DR: In this article, an axisymmetric coflow laminar methane/air diffusion flame at atmospheric pressure was modeled using complex thermal and transport properties and detailed gas-phase chemistry.
114 citations
TL;DR: In this article, the authors provide a window into the physics of extinction and reignition via three-dimensional simulations of non-premixed combustion in isotropic decaying turbulence using one-step global reaction and neglecting density variations.
Abstract: The goal of this study is to provide a window into the physics of extinction and reignition via three-dimensional simulations of non-premixed combustion in isotropic decaying turbulence using one-step global reaction and neglecting density variations. Initially non-premixed fields of fuel and oxidant are developing in a turbulent field. Due to straining, the scalar dissipation rate is initially increasing and its fluctuations create extinguished regions on the stoichiometric surface. Later in the process, the stoichiometric surface again becomes uniformly hot. Besides using Eulerian data, this research applies flame element tracking and investigates the time history of individual points (‘flame elements’) along the stoichiometric surface. The main focus of the study is the discussion of the different scenarios of reignition. This paper identifies three major scenarios: independent flamelet scenario, reignition via edge (triple) flame propagation, and reignition through engulfment by a hot neighbourhood. The results give insight into the role different scenarios play in the reignition process, reveal the physical processes associated with each scenario, and provide the relative frequency of reignition for each scenario.
TL;DR: In this paper, a co-flow diffusion flame at atmospheric pressure has been investigated experimentally using a combination of laser diagnostics and thermocouple-gas sampling probe measurements, and numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry and the dynamical equations for soot spheroid growth.
Abstract: Laminar, sooting, ethylene-fuelled, co-flow diffusion flames at atmospheric pressure have been studied experimentally and theoretically as a function of fuel dilution by inert nitrogen. The flames have been investigated experimentally using a combination of laser diagnostics and thermocouple-gas sampling probe measurements. Numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry and the dynamical equations for soot spheroid growth. Predicted flame heights, temperatures and the important soot growth species, acetylene, are in good agreement with experiment. Benzene simulations are less satisfactory and are significantly under-predicted at low dilution levels of ethylene. As ethylene dilution is decreased and soot levels increase, the experimental maximum in soot moves from the flame centreline toward the wings of the flame. Simulations of the soot field show similar trends with decreasing dilution ...
TL;DR: In this article, the extinction and suppression of diffusion flames is examined theoretically and the effects of oxygen reduction and external heat flux are examined compared to data in the literature, and an application of extinction in compartment fires is also examined.
Abstract: The extinction and suppression of diffusion flames is examined theoretically. The effects of oxygen reduction and external heat flux are examined compared to data in the literature. An application of extinction in compartment fires is also examined. The theory developed is based on a critical flame temperature, and that theory includes transient effects and the addition of water as well.
TL;DR: In this article, the authors investigated the effect of fuel structure on the formation of polycyclic aromatic hydrocarbon (PAH) and soot in counterflow diffusion flames of ethylene/propane mixtures and found that the role of propane mixing on the synergistic effect cannot be explained solely by the incipient ring formation via a propargyl recombination reaction.
TL;DR: In this article, the effect of heating height on the flame characteristics of a domestic gas stove was investigated. And the authors showed that flame structure, temperature distribution and thermal efficiency are greatly influenced by the heating height.
TL;DR: In this article, spatially resolved distributions of PAH absorbance and soot are obtained in sooting diffusion flames, and the absolute absorption coefficient of the PAH mixture is determined by comparing absorption measurements obtained by cavity ring-down spectroscopy at 1064 nm and 532 nm.
Abstract: Laser-induced incandescence is a technique which enables the measurement of soot volume fractions. However, the laser-induced soot emission might be affected by a fluorescence background generally ascribed to the polycyclic aromatic hydrocarbon compounds (PAHs) present at the soot location. In this paper, spatially resolved distributions of PAH absorbance and soot are obtained in sooting diffusion flames. The original method developed here consists in comparing the emission distributions induced by two different laser wavelengths: (1) at 1064 nm emission signals are exempt from PAH fluorescence and (2) at 532 nm both soot incandescence and PAH emission contribute to the total signal. In addition, the absolute absorption coefficient of the PAH mixture is determined by comparing absorption measurements obtained by cavity ring-down spectroscopy (CRDS) at 1064 nm and 532 nm. The proposed method can provide highly sensitive 2D imaging of PAHs and soot using the fundamental and the second-harmonic frequencies of a single YAG laser. Finally, 2D distributions of PAH absorbance and soot volume fraction calibrated by CRDS are obtained in two diffusion flames, particularly in a very low-sooting flame exhibiting a maximum PAH absorbance of 6×10-4 cm-1 and a maximum soot volume fraction of 3 ppb only. The respective spatial distributions of PAHs and soot are shown to vary with the initial C/O ratio.
TL;DR: In this article, the first tests of supereffective flame inhibitors blended with CO 2 have been performed in methane-air laminar co-flow diffusion flames stabilized on a cup burner.
TL;DR: In this paper, the radiative transfer in a turbulent jet diffusion flame has been calculated using the discrete ordinates and the ray tracing, using the correlated k-distribution method and the statistical narrow-band model.
TL;DR: In this paper, the soot microstructures for two flames established at ϕ = ∞ (i.e., non-premixed) and 5 were examined and the average primary soot particle diameter was ∼35 nm.
TL;DR: In this paper, the surface growth and oxidation properties of round laminar jet diffusion flames were studied experimentally at pressures of 0.1-1.0 atm, and the results were consistent with earlier measurements of soot surface growth rates in laminara premixed and diffusion flames involving a variety of hydrocarbons at atmospheric pressure.
TL;DR: In this article, a second-order CMC model for a detailed chemical mechanism is presented and applied to turbulent piloted jet diffusion flames, Sandia Flames D, E, and F. Second-order corrections are made for three rate-limiting steps of methane-air combustion, while first-order closure is employed for all other reaction steps.
TL;DR: In this paper, an experimental study has been performed on the effects of injection rate shaping on the combustion process and exhaust emissions of a direct-injection diesel engine, where boot-type injections were generated by means of a modified pump-line nozzle system, which is able to modulate the instantaneous fuel injection rate.
TL;DR: In this article, a series of experiments to study merged flame from multiple fire sources was carried out, where a porous 15 cm2 unit burner was used as a unit burner and propane was employed as a fuel.
Abstract: A series of experiments to study merged flame from multiple fire sources was carried out. The porous 15-cm2 burner was used as a unit burner and propane was employed as a fuel. Many burners with va...
TL;DR: In this paper, the authors investigated the combustion characteristics of porous heating burners under both steady-state and transient conditions and found that the flame speed in a porous heating burner decreases with an increase in the length of the porous bed.
TL;DR: In this article, the critical sooting C/O ratio was measured for a series of atmospheric-pressure, laminar premixed ethylene-oxygen-argon flames doped with a small amount of ferrocene.
TL;DR: In this article, a series of methane/air co-flowing non-premixed flames whose fuel was separately doped with 2000 ppm of cyclohexane, cycloenadiene, 1,3-cyclohexadiene and 1,4-cycloenadienes were measured.
TL;DR: In this article, the dynamics of an edge flame in a mixing layer is considered, which stands at a well defined distance from the plate separating the fuel and oxidizer streams, is stabilized by heat conducted back to the relatively cold plate.
TL;DR: In this paper, the extinguishment characteristics of CO2 as a fire-suppressing agent have been studied experimentally and numerically using a methane-air laminar co-flow diffusion flame stabilized on a cup burner.
01 Jan 2004
TL;DR: In this article, a method based on computational fluid dynamics (CFD) was proposed to reconstruct the transfer function of a turbulent premix swirl burner. But the method was only applied to a lean perfectly premixed swirl burner, and the results showed that the gain of the flame was larger than unity over a range of frequencies, even though fluctuations of heat release and velocity were normalized with their mean flow values.
Abstract: The introduction of lean premix combustion increases the susceptibility of the combustor to thermoacoustic instabilities. To control these instabilities, information about the dynamic behavior of the combustion process is necessary. The flame transfer function offers one possibility to describe the dynamic behavior of the combustion process. It relates velocity fluctuations through the burner to an overall heat release fluctuation caused by the flame. As the transfer function for turbulent premix swirl flames can not be derived accurately from first principles, an alternative approach is needed. This paper introduces and validates a method, based on computational fluid dynamics (CFD), to reconstruct flame transfer functions. A transient simulation of the turbulent reacting flow is performed with broad band excitation of the flow variables on the boundaries. On the basis of the resulting time series for velocity and heat release, the transfer function of the flame is reconstructed by application of a system identification procedure based on the Wiener-Hopf equation. This method is applied to a lean perfectly premixed swirl burner. The resulting transfer function is validated with experimental data up to frequencies of f = 400 Hz. Good qualitative agreement is observed between the two approaches. Remarkably, the absolute value of the flame transfer function (the ‘gain’ of the flame) is found to be larger than unity over a range of frequencies, even though fluctuations of heat release and velocity are normalized with their mean flow values. To gain insight into this phenomenon, the dynamic behavior of the flame is investigated in detail. This concerns in particular the interaction of velocity, heat release fluctuations, the swirl number, and fluctuations of flame position and shape. Instead of broad band excitation, single frequency excitation is applied on the boundary for these investigations. It is found that swirl number fluctuations are convected into the flame. At the frequency where the wavelength of those fluctuations agrees with the length scale of the flame, unburned gases accumulate in the combustor. The excess heat is released periodically, which causes the overshoot in the absolute value of the flame transfer function.Copyright © 2004 by ASME
TL;DR: Through variation of the burner conditions (stoichiometry, sampling height), it could be shown that nanoparticles and soot are entirely independent species.