Showing papers on "Premixed flame published in 2012"
TL;DR: In this paper, a 3D direct numerical simulation (DNS) of a turbulent lifted ethylene jet flame in heated co-flowing air is used to identify the detailed structure and stabilization mechanism.
249 citations
TL;DR: In this paper, the effects of changes in intake temperature and pressure on the pre-ignition of turbo-charged engines were investigated. But the results were limited to the case of turbocharged engines, and they were concluded from the evidence of engine pressure records.
Abstract: Earlier studies of pre-ignitions at hot surfaces are first reviewed. The concept of a critical radius of a hot pocket of gas, closely related to the laminar flame thickness, that is necessary to initiate a propagating flame, has been used successfully to predict relative tendencies of different fuel–air mixtures to pre-ignite. As the mixture is compressed, the thickness of potential laminar flames decreases, and when this becomes of the order of the thermal sheath thickness at the hottest surface, pre-ignition can occur there, creating a propagating flame. Measured engine pre-ignition ratings are shown to correlate well with laminar flame thicknesses. Predictions are made concerning the effects of changes in intake temperature and pressure on the pre-ignition of different fuels.A growing current concern is occasional gas-phase, autoignitive, pre-ignitions that can occur in turbo-charged engines, giving rise to very severe autoignition and knock. It is concluded from the evidence of engine pressure records...
229 citations
TL;DR: In this article, the interaction of a helical mode with acoustic oscillations is studied experimentally in a turbulent swirl-stabilized premixed flame, and the authors show that the heat release rate perturbation associated with the nonlinear interaction of the helical model and the acoustic oscillation produces a "yin and yang" -type pattern rotating with the interaction frequency in the direction of the mean swirl.
197 citations
TL;DR: In this paper, a series of turbulent methane/air stratified flames stabilized on a swirl burner were investigated, and the corresponding three-dimensional surface density function and thermal scalar dissipation rate were investigated along with geometric characteristics of the flame such as curvature and flame thickness.
175 citations
TL;DR: In this article, a comparison of fundamental combustion properties for Jet-A and various alternative jet fuels provides insights into understanding the impact of fuel properties on combustion performance and developing a cost-effective combustion testing program that includes fundamental characterization.
168 citations
TL;DR: In this article, the authors used high-speed schlieren photography, pressure records and large eddy simulation (LES) model to study the shape changes, dynamics of premixed flame propagation and pressure build up in a closed duct.
166 citations
TL;DR: In this paper, a Rijke tube with three different temperature configurations resulting from a laminar premixed flame is considered to investigate its non-normality and the resulting transient growth of flow disturbances in triggering combustion instabilities.
157 citations
TL;DR: In this article, the authors used high speed photography and Schlieren system to capture the spark ignition process in the scramjet combustor fueled by hydrogen equipped with multi-cavities at Mach 4 flight condition.
156 citations
TL;DR: In this paper, the effects of hydrogen enrichment and steam addition on laminar burning velocity of methaneeair premixed flame were studied both experimentally and numerically using the slot burner method.
154 citations
TL;DR: In this article, the effect of external flame stretching on the laminar burning velocities of spherically outward expanding flames is investigated using high-speed laser tomography and a tomographic image correlation method.
145 citations
Book•
24 Oct 2012
TL;DR: In this paper, the authors present a simulation of an Incompressible Turbulent Flow Theory of Homogeneous Isotropic Turbulence Reynolds-Averaged Navier-Stokes Approach to Numerical Simulations of Turbent Flows Turbulant Diffusion Notes Phenomenology of Premixed Turbulents Combustion Mean Flame Brush Thickness and Burning Velocity Mean Structure of a Presumed PDFs for Modeling Both Premixed and Diffusion Burning Modes Summary References Index
Abstract: General Knowledge on Reacting Gas Mixtures Basic Characteristics of Gas Mixtures Chemical Reactions in Flames Balance Equations Unperturbed Laminar Premixed Flame Flow in a Laminar Premixed Flame AEA Theory of Unperturbed Laminar Premixed Flame Laminar Flame Speed and Thickness Summary A Brief Introduction to Turbulence Characteristics of an Incompressible Turbulent Flow Theory of Homogeneous Isotropic Turbulence Reynolds-Averaged Navier-Stokes Approach to Numerical Simulations of Turbulent Flows Turbulent Diffusion Notes Phenomenology of Premixed Turbulent Combustion Mean Flame Brush Thickness Turbulent Flame Speed and Burning Velocity Mean Structure of a Premixed Turbulent Flame Summary Physical Mechanisms and Regimes of Premixed Turbulent Combustion Flame Wrinkling and Flamelet Regime Variations in Flamelet Structure Due to Turbulent Stretching Flame Instabilities Regimes of Premixed Turbulent Combustion Flame Propagation along Vortex Tubes Concluding Remarks Influence of Premixed Combustion on Turbulence Countergradient Scalar Transport in Turbulent Premixed Flames Turbulence in Premixed Flames Summary Modeling of Premixed Burning in Turbulent Flows Fractal Approach to Evaluating Turbulent Burning Velocity Zimont Model of Burning Velocity in Developing Turbulent Premixed Flames Modeling of Effects of Turbulence on Premixed Combustion in RANS Simulations Models of Molecular Transport Effects in Premixed Turbulent Flames Chemistry in RANS Simulations of Premixed Turbulent Combustion: Flamelet Library Large Eddy Simulation of Premixed Turbulent Combustion Introduction to Nonpremixed Combustion Laminar Diffusion Flames Turbulent Diffusion Flames Summary Partially Premixed Turbulent Flames Effects Addressed in RANS or LES Studies of Partially Premixed Turbulent Flames Effects Ignored in RANS or LES Studies of Partially Premixed Turbulent Flames Use of Presumed PDFs for Modeling Both Premixed and Diffusion Burning Modes Summary References Index
TL;DR: In this paper, the structure of unconfined lean premixed methane-air flames stabilized on an axisymmetric bluff body has been examined for conditions increasingly closer to blowoff and during the blow-off event.
TL;DR: In this paper, it was shown that preferential species diffusion is known to have important effects on local flame structure in turbulent premixed flames and differential diffusion of heat and mass can have significant effects on both local flame structures and global flame parameters, such as turbulent flame speed.
TL;DR: In this article, the authors investigate the transient upstream propagation of premixed hydrogen-air flames in the boundary layer of a fully developed turbulent channel flow and show that the leading edges of the upstream-propagating premixed flame are always located in the near-wall region of the channel and assume the shape of several smooth, curved bulges propagating upstream side by side in the spanwise direction and convex towards the reactant side of the flame.
Abstract: Direct numerical simulations are performed to investigate the transient upstream propagation (flashback) of premixed hydrogen–air flames in the boundary layer of a fully developed turbulent channel flow. Results show that the well-known near-wall velocity fluctuations pattern found in turbulent boundary layers triggers wrinkling of the initially flat flame sheet as it starts propagating against the main flow direction, and that the structure of the characteristic streaks of the turbulent boundary layer ultimately has an important impact on the resulting flame shape and on its propagation mechanism. It is observed that the leading edges of the upstream-propagating premixed flame are always located in the near-wall region of the channel and assume the shape of several smooth, curved bulges propagating upstream side by side in the spanwise direction and convex towards the reactant side of the flame. These leading-edge flame bulges are separated by thin regions of spiky flame cusps pointing towards the product side at the trailing edges of the flame. Analysis of the instantaneous velocity fields clearly reveals the existence, on the reactant side of the flame sheet, of backflow pockets that extend well above the wall-quenching distance. There is a strong correspondence between each of the backflow pockets and a leading edge convex flame bulge. Likewise, high-speed streaks of fast flowing fluid are found to be always colocated with the spiky flame cusps pointing towards the product side of the flame. It is suggested that the origin of the formation of the backflow pockets, along with the subsequent mutual feedback mechanism, is due to the interaction of the approaching streaky turbulent flow pattern with the Darrieus–Landau hydrodynamic instability and pressure fluctuations triggered by the flame sheet. Moreover, the presence of the backflow pockets, coupled with the associated hydrodynamic instability and pressure–flow field interaction, greatly facilitate flame propagation in turbulent boundary layers and ultimately results in high flashback velocities that increase proportionately with pressure.
TL;DR: In this paper, a time-resolved Particle image velocimetry was used to study transient interactions between hydrogen-enriched methane/air premixed flames and toroidal vortex structures.
TL;DR: In this paper, the utility of a recently developed method of chemical explosive mode analysis (CEMA) for CFLD was demonstrated with a variety of flames for n -heptane including auto-ignition, ignition and extinction in steady state perfectly stirred reactors (PSRs) and laminar premixed flames.
TL;DR: The turbulent flame speeds from the present expanding flames and those from the Bunsen geometry in the literature can be unified by a turbulent Reynolds number based on flame length scales using recent theoretical results obtained by spectral closure of the transformed G equation.
Abstract: In this Letter we present turbulent flame speeds and their scaling from experimental measurements on constant-pressure, unity Lewis number expanding turbulent flames, propagating in nearly homogeneous isotropic turbulence in a dual-chamber, fan-stirred vessel. It is found that the normalized turbulent flame speed as a function of the average radius scales as a turbulent Reynolds number to the one-half power, where the average radius is the length scale and the thermal diffusivity is the transport property, thus showing self-similar propagation. Utilizing this dependence it is found that the turbulent flame speeds from the present expanding flames and those from the Bunsen geometry in the literature can be unified by a turbulent Reynolds number based on flame length scales using recent theoretical results obtained by spectral closure of the transformed G equation.
TL;DR: In this article, the major differences in the overall chemical pathways in these flames are discussed, especially with respect to the formation of undesired emissions, and from agreement and differences between experiment and model, suggestions for further work are given.
TL;DR: In this article, an experimental evaluation of such fundamental properties of hydrogen-air mixtures as flammability limits and laminar flame speed at sub-atmospheric pressures is presented.
TL;DR: In this article, a simple setup consisting of ducted laminar premixed conical flames was used to investigate the features of nonlinear thermoacoustic oscillations, and it was observed that the system undergoes a series of bifurcations leading to characteristically different nonlinear oscillations.
Abstract: Bifurcation analysis is performed on experimental data obtained from a simple setup comprising ducted laminar premixed conical flames in order to investigate the features of nonlinear thermoacoustic oscillations. It is observed that as the bifurcation parameter is varied, the system undergoes a series of bifurcations leading to characteristically different nonlinear oscillations. Through the application of nonlinear time series analysis to pressure and flame (CH * chemiluminescence) intensity time traces, these oscillations are characterized as periodic, aperiodic, or chaotic oscillations, and subsequently the nature of the obtained bifurcations is explained based on dynamical systems theory. Nonlinear interaction between the flames and the acoustic modes of the duct is clearly reflected in the high speed flame images acquired simultaneously with pressure time series.
TL;DR: In this paper, the propagation of a premixed flame inside of a confining vessel filled with combustible fluid is determined using large activation energy asymptotics, assuming that spatial and temporal variations in the transverse direction are weak compared to those in the direction normal to the flame surface.
Abstract: The propagation of a premixed flame inside of a confining vessel filled with combustible fluid is determined using large-activation-energy asymptotics. The flame structure is analysed assuming that spatial and temporal variations in the transverse direction are weak compared to those in the direction normal to the flame surface. The analysis considers weak pressure rise from confinement and also allows for mixtures that are both near and removed from stoichiometry, non-unity reaction orders, temperature-dependent transport coefficients, and general Lewis numbers. The resulting equations for flame propagation speed are expressed in a coordinate-free form and describe the evolution of an arbitrary shaped flame in a general confining flow. These expressions are specifically applied to the case of a spherical flame propagating inside a spherical chamber. The radius at which the confining vessel influences the flame propagation is determined and the various mechanisms influencing flame behaviour are discussed. The results give rise to a simplified asymptotic relationship that provides an improved equation that may be used to more accurately extrapolate unstretched laminar flame speeds from experimental measurements.
TL;DR: In this article, a study of non-premixed flames at supercritical-pressure conditions is presented, where flame stability in liquid rocket engines fueled with liquid oxygen and gaseous hydrogen is investigated.
TL;DR: In this article, the effect of large fraction of hydrogen and CO2 on flame radiation, chemical reaction, and intrinsic flame instability was investigated by using the outwardly spherical propagating flame method.
TL;DR: In this paper, the authors investigated the mechanisms contributing to flame stabilization and blowout in a nitrogen-diluted hydrogen transverse jet in a turbulent boundary layer cross-flow (JICF) with detailed chemistry.
TL;DR: In this article, the effects of diluents on laminar flame speeds of premixed DME/air flames were investigated in a constant volume bomb under initial temperature of 298 K and initial pressure of 0.1 MPa.
TL;DR: In this paper, a three-step mechanism for H2-air combustion has been implemented in a compressible explicit code and applied to the simulation of a supersonic lifted co-flowing hydrogen-air flame.
TL;DR: In this paper, an experimental investigation into the interactions that occur between two lean turbulent premixed flames stabilised on conical bluff-bodies when they are moved closer together is described.
TL;DR: In this paper, a model that synthesizes previous knowledge from experiments and simulations on spark ignition of gas and liquid-fuelled non-premixed recirculating flames has been developed.
TL;DR: In this paper, the authors investigate the flame stabilization mechanism and the conditions leading to the blowoff of a laminar premixed flame anchored downstream of a heatconducting perforated-plate/multi-hole burner, with overall nearly adiabatic conditions.
TL;DR: In this paper, the authors used measured flame describing functions (FDFs) to characterize the nonlinear response of swirling flames to incident perturbations and determine the regimes of instability of a generic system comprising an upstream manifold, an injector equipped with a swirler and a combustion chamber confining the flame.