Showing papers on "Premixed flame published in 2009"
TL;DR: A comprehensive review of the advances made over the past two decades in this area is provided in this article, where various swirl injector configurations and related flow characteristics, including vortex breakdown, precessing vortex core, large-scale coherent structures, and liquid fuel atomization and spray formation are discussed.
1,048 citations
TL;DR: In this article, various factors affecting the determination of laminar flames speeds from outwardly propagating spherical flames in a constant-pressure combustion chamber were considered, with emphasis on the nonlinear variation of the stretched flame speed to the flame stretch rate, and the associated need to nonlinearly extrapolate the stretched flames speed to yield an accurate determination of the Laminar flame speed and Markstein length.
445 citations
TL;DR: In this article, an experimental and numerical study on laminar burning characteristics of premixed premixed methane-hydrogen-air flames was conducted at room temperature and atmospheric pressure.
443 citations
01 Jan 2009
TL;DR: In this paper, the trajectories of outwardly propagating spherical flames initiated by an external energy deposition are studied theoretically, numerically, and experimentally by using hydrogen/air mixtures.
Abstract: The trajectories of outwardly propagating spherical flames initiated by an external energy deposition are studied theoretically, numerically, and experimentally by using hydrogen/air mixtures. Emphasis is placed on how to accurately determine the laminar flame speeds experimentally from the time history of the flame fronts for mixtures with different Lewis numbers and ignition energies. The results show that there is a critical flame radius only above which is the linear and non-linear extrapolation for flame speeds valid. It is found that the critical radius depends strongly on the Lewis number. At large Lewis numbers, the critical radius is larger than the minimum flame radius used in the experimental measurements, leading to invalid flame speed extrapolation. The results also show that there is a maximum Karlovitz number beyond which propagating spherical flame does not exist. The maximum Karlovitz number decreases dramatically with the increase of Lewis number. Furthermore, the results show that the ignition energy has a significant impact on the flame trajectories. It is found that the unsteady flame transition causes a flame speed reverse phenomenon near the maximum Karlovitz number with different ignition energies. The occurrence of flame speed reverse greatly narrows the experimental data range for flame speed extrapolation. The strong dependence of flame trajectory on ignition energy and the existence of the flame speed reverse phenomenon are also confirmed by experimental results. Published by Elsevier Inc. on behalf of The Combustion Institute.
357 citations
TL;DR: A review of molecular-beam mass spectrometry of premixed, laminar, low-pressure flat flames has been provided in this paper, focusing on critical aspects of the experimental approach including probe sampling effects, different ionization processes, and mass separation procedures.
329 citations
01 Jan 2009
TL;DR: In this article, a mixture of n-decane 80% and 1,2,4-trimethylbenzene 20% by weight, called the Aachen surrogate, is selected for consideration as a possible surrogate of kerosene.
Abstract: Experimental and numerical studies are carried out to develop a surrogate that can reproduce selected aspects of combustion of kerosene. Jet fuels, in particular Jet-A1, Jet-A, and JP-8 are kerosene type fuels. Surrogate fuels are defined as mixtures of few hydrocarbon compounds with combustion characteristics similar to those of commercial fuels. A mixture of n-decane 80% and 1,2,4-trimethylbenzene 20% by weight, called the Aachen surrogate, is selected for consideration as a possible surrogate of kerosene. Experiments are carried out employing the counterflow configuration. The fuels tested are kerosene and the Aachen surrogate. Critical conditions of extinction, autoignition, and volume fraction of soot measured in laminar non premixed flows burning the Aachen surrogate are found to be similar to those in flames burning kerosene. A chemical-kinetic mechanism is developed to describe the combustion of the Aachen surrogate. This mechanism is assembled using previously developed chemical-kinetic mechanisms for the components: n-decane and 1,2,4-trimethylbenzene. Improvements are made to the previously developed chemical-kinetic mechanism for n-decane. The combined mechanisms are validated using experimental data obtained from shock tubes, rapid compression machines, jet stirred reactor, burner stabilized premixed flames, and a freely propagating premixed flame. Numerical calculations are performed using the chemical-kinetic mechanism for the Aachen surrogate. The calculated values of the critical conditions of autoignition and soot volume fraction agree well with experimental data. The present study shows that the chemical-kinetic mechanism for the Aachen surrogate can be employed to predict non premixed combustion of kerosene.
319 citations
TL;DR: In this paper, the authors presented new experimental data for n-butanol in three experimental configurations, including an improved detailed chemical kinetic mechanism (878 reactions involving 118 species) derived from a previously proposed scheme in the literature.
297 citations
TL;DR: In this paper, a direct numerical simulation of a three-dimensional spatially developing turbulent lifted hydrogen jet flame in heated coflow is performed with a detailed mechanism to determine the stabilization mechanism and the flame structure.
Abstract: Direct numerical simulation (DNS) of the near field of a three-dimensional spatially developing turbulent lifted hydrogen jet flame in heated coflow is performed with a detailed mechanism to determine the stabilization mechanism and the flame structure. The DNS was performed at a jet Reynolds number of 11,000 with over 940 million grid points. The results show that auto-ignition in a fuel-lean mixture at the flame base is the main source of stabilization of the lifted jet flame. A chemical flux analysis shows the occurrence of near-isothermal chemical chain branching preceding thermal runaway upstream of the stabilization point, indicative of hydrogen auto-ignition in the second limit. The Damkoehler number and key intermediate-species behaviour near the leading edge of the lifted flame also verify that auto-ignition occurs at the flame base. At the lifted-flame base, it is found that heat release occurs predominantly through ignition in which the gradients of reactants are opposed. Downstream of the flame base, both rich-premixed and non-premixed flames develop and coexist with auto-ignition. In addition to auto-ignition, Lagrangian tracking of the flame base reveals the passage of large-scale flow structures and their correlation with the fluctuations of the flame base. In particular, the relative position of themore » flame base and the coherent flow structure induces a cyclic motion of the flame base in the transverse and axial directions about a mean lift-off height. This is confirmed by Lagrangian tracking of key scalars, heat release rate and velocity at the stabilization point.« less
238 citations
TL;DR: A simple algebraic model for the Favre averaged scalar dissipation rate, c, in high Damkohler number premixed flames is obtained from its transport equation by balancing the leading order terms as discussed by the authors.
Abstract: A simple algebraic model for the Favre averaged scalar dissipation rate, c, in high Damkohler number premixed flames is obtained from its transport equation by balancing the leading order terms Recently proposed models for the dominant terms in the transport equation are revisited and revised The algebraic model incorporates essential physics of turbulent premixed flames, namely, dilatation rate, its influence on turbulence-scalar interaction, chemical reactions, and dissipation processes A realizability analysis is carried out to show that the algebraic model is always unconditionally realizable The model predictions of dissipation rate are compared with the DNS results, and the agreement is good over a range of flame conditions Application of the Kolmogorov-Petrovski-Piskunov (KPP) theorem along with the above algebraic model gives an expression for the turbulent flame speed Its prediction compares well with a range of experimental data with no modifications to the model constants
194 citations
TL;DR: In this article, a low-pressure premixed toluene/O2/Ar flame with the equivalence ratio of 1.90 was investigated using tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry.
Abstract: A low-pressure premixed toluene/O2/Ar flame with the equivalence ratio of 1.90 was investigated using tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Combustion intermediates up to C19H12 were identified by the measurements of the photoionization mass spectrum and photoionization efficiency spectrum. Mole fraction profiles of flame species were evaluated from the scan of burner position at photon energies near ionization thresholds. Furthermore, flame temperature was recorded by a Pt/Pt-13%Rh thermocouple. The comprehensive experimental data concerning the flame structure facilitate the discussion about the flame chemistry of toluene and other monocyclic aromatic fuels. Benzyl and benzene were found to be major primary intermediates of toluene degradation; and benzene is suggested to originate mainly from fuel degradation instead of radical recombination channels in fuel-rich monocyclic aromatic hydrocarbon flames. On the basis of the intermediate identification, comparison...
180 citations
TL;DR: In this paper, the principal burning characteristics of a laminar flame comprise the fuel vapour pressure, the burning velocity, ignition delay times, Markstein numbers for strain rate and curvature, the stretch rates for the onset of flame instabilities and of flame extinction for different mixtures.
TL;DR: In this paper, the laminar flame speed of hydrogen/carbon monoxide fuel mixtures is measured over a large range of fuel compositions (0-100% volume fraction for hydrogen in the mixture) by using a Bunsen burner.
TL;DR: In this paper, an experimental and numerical study on hydrogen-air flames at elevated pressures and temperatures was conducted, and the results showed good agreement between the computed results and experimental data.
TL;DR: In this paper, the effects of flow compression and flame stretch on the accurate determination of laminar flame speeds at normal and elevated pressures using propagating spherical flames at constant pressure or constant volume are studied theoretically and numerically.
Abstract: The effects of flow compression and flame stretch on the accurate determination of laminar flame speeds at normal and elevated pressures using propagating spherical flames at constant pressure or constant volume are studied theoretically and numerically. The results show that both the compression-induced flow motion and flame stretch have significant impacts on the accuracy of flame speed determination. For the constant pressure method, a new method to obtain a compression-corrected flame speed (CCFS) for nearly constant pressure spherical bomb experiments is presented. Likewise, for the constant volume method, a technique to obtain a stretch-corrected flame speed (SCFS) at elevated pressures and temperatures is developed. The validity of theoretical results for both constant pressure and constant volume methods is demonstrated by numerical simulations using detailed chemistry for hydrogen/air, methane/air, and propane/air mixtures. It is shown that the present CCFS and SCFS methods not only improve the a...
TL;DR: The effect of blockage ratio on the early phase of the flame acceleration process in an obstructed square cross-section channel was investigated in this paper, and it was determined that flame acceleration is more pronounced for higher blockage obstacles during the initial stage of flame acceleration up to a flame velocity below the speed of sound of the reactants.
TL;DR: A burner-stabilized, stagnation flame technique is introduced in this paper, where a previously developed sampling probe is combined with a water-cooled circular plate such that the combination simultaneously acts as a flow stagnation surface and soot sample probe for mobility particle sizing.
TL;DR: In this article, a general flamelet transformation that holds in the limits of both premixed and non-premixed combustion is developed, which makes use of two statistically independent variables: a mixture fraction and a reaction progress parameter.
TL;DR: In this paper, the microwave energy is deposited directly into the reaction zone through coupling to the weakly ionized gas in that region, resulting in a 20% increase in flame speed.
TL;DR: In this article, an atmospheric-pressure laboratory swirl burner has been developed wherein lean premixed methane/air is injected in an unconfined low-speed flow of air, and the flame is stabilized above the burner rim in a moderate swirl flow, triggering weak vortex breakdown in the downstream direction.
TL;DR: In this paper, a planar laser-induced incandescence (LII) method was used to measure soot volume fraction in a well-characterised, piloted, turbulent non-premixed flame known as the Delft Flame III.
TL;DR: In this article, the effects of combustor size and geometry, inlet velocity profile and slip-wall boundary condition on the flame temperature were investigated, and it was shown that a larger combustor gives higher flame temperature only when the flow velocity is below a certain level.
TL;DR: In this paper, the authors investigated the feasibility of the use of CMC/LES for ignition problems and examined which, if any, of the characteristics already observed in related experiments could be predicted.
TL;DR: A validated Large Eddy Simulation model of unsteady premixed flame propagation is used to study the phenomenology underlying vented gas explosions in the presence of obstacles, explaining both number and intensity of the overpressure peaks observed.
TL;DR: In this article, numerical simulations of lean hydrogen flames interacting with turbulence are performed in an idealized setting using an adaptive low Mach number model with a numerical feedback control algorithm to stabilize the flame.
TL;DR: In this paper, an experimental study on laminar burning velocities and onset of cellular instabilities of the premixed methane-hydrogen-air flames was conducted in a constant volume combustion vessel at elevated pressures and temperatures.
TL;DR: In this paper, a combined experimental and modeling study is performed to clarify the isomer-specific combustion chemistry in flames fueled by the C 3 H 4 isomers allene and propyne.
TL;DR: In this paper, the combustion dynamics of propane-hydrogen mixtures are investigated in an atmospheric pressure, lean, premixed backward-facing step combustor, and the authors systematically vary the equivalence ratio, inlet temperature and fuel composition to determine the stability map of the combustor.
01 Jan 2009
TL;DR: In this article, the effects of pressure on soot formation and the structure of the temperature field were studied in co-flow methane-air laminar diffusion flames over a wide pressure range, from 10 to 60, in a high-pressure combustion chamber.
Abstract: The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow methane–air laminar diffusion flames over a wide pressure range, from 10 to 60 atm in a high-pressure combustion chamber. The selected fuel mass flow rate provided diffusion flames in which the soot was completely oxidized within the visible flame envelope and the flame was stable at all pressures considered. The spatially resolved soot volume fraction and soot temperature were measured by spectral soot emission as a function of pressure. The visible (luminous) flame height remained almost unchanged from 10 to 100 atm. Peak soot concentrations showed a strong dependence on pressure at relatively lower pressures; but this dependence got weaker as the pressure is increased. The maximum conversion of the fuel’s carbon to soot, 12.6%, was observed at 60 atm at approximately the mid-height of the flame. Radial temperature gradients within the flame increased with pressure and decreased with flame height above the burner rim. Higher radial temperature gradients near the burner exit at higher pressures mean that the thermal diffusion from the hot regions of the flame towards the flame centerline is enhanced. This leads to higher fuel pyrolysis rates causing accelerated soot nucleation and growth as the pressure increases.
01 Jan 2009
TL;DR: In this paper, the authors investigated the mechanism that causes periodic combustion oscillation in a Lean Premixed Prevaporized (LPP) Combustor, which was run with high flow rates of preheated air at elevated presures, with a realistic fuel injector.
Abstract: The mechanism that causes a periodic combustion oscillation in a Lean Premixed Prevaporized (LPP) Combustor was experimentally investigated. The combustor was run with high flowrates of preheated air at elevated presures, with Jet-A as the fuel, and with a realistic fuel injector. The injector provides LPP combustion and features a stable diffusion flame as a pilot surrounded by a premixed main flame. The combustor was stable over a wide range of conditions, but for off-design conditions oscillations associated with unsteady flashback and liftoff of the flame base were observed. These low-frequency combustor instabilities were a strong function of a global velocity gradient (U/d), which is analogous to the U/d gradient used to characterize stability limits of Bunsen flames. PIV data quantified the location and strength of shear-layer vortex shedding. The main and pilot flames were visualized using PLIF images of formaldehyde. These images showed that for design conditions the pilot flame is very stable and that the main premixed flame exists in the mixing layer. It was seen that as the global gradient U/d decreases, the local velocity gradient in the mixing layer also decreases, making the flame more susceptible to perturbations from the vortex-shedding. It is proposed that the unsteadiness in the combustion can be explained using classic theories of premixed flame flashback and blowout that was developed for Bunsen burners.
TL;DR: In this paper, a tri-variate model for the formation of soot particles is presented and further validated in high temperature premixed ethylene flames, premixed benzene flames, an acetylene counterflow diffusion flame and toluene pyrolysis in shock-tubes.