About: Premixed flame is a(n) research topic. Over the lifetime, 9890 publication(s) have been published within this topic receiving 255931 citation(s).
29 Dec 1998-
Abstract: Machine generated contents note: About the AuthorPreface to the Second EditionPreface to the Third EditionList of Symbols and Abbreviations1 Fire science and combustion 1.1 Fuels and the Combustion Process 1.2 The Physical Chemistry of Combustion in Fires Problems2 Heat transfer 2.1 Summary of the heat transfer equations 2.2 Conduction 2.3 Convection 2.4 Radiation Problems3 Limits of flammability and premixed flames 3.1 Limits of flammability 3.2 The structure of a premixed flame 3.3 Heat losses from premixed flames 3.4 Measurement of burning velocities 3.5 Variation of burning velocity with experimental parameters 3.6 The effect of turbulence Problems4 Diffusion flames and fire plumes 4.1 Laminar jet flames 4.2 Turbulent jet flames 4.3 Flames from natural fires 4.4 Some practical applications Problems5 Steady burning of liquids and solids 5.1 Burning of liquids 5.2 Burning of solids Problems6 Ignition: The initiation of flaming combustion 6.1 Ignition of^
01 Jan 1988-
Abstract: The laminar flamelet concept covers a regime in turbulent combustion where chemistry (as compared to transport processes) is fast such that it occurs in asymptotically thin layers—called flamelets—embedded within the turbulent flow field. This situation occurs in most practical combustion systems including reciprocating engines and gas turbine combustors. The inner structure of the flamelets is one-dimensional and time dependent. This is shown by an asymptotic expansion for the Damkohler number of the rate determining reaction which is assumed to be large. Other non-dimensional chemical parameters such as the nondimensional activation energy or Zeldovich number may also be large and may be related to the Damkohler number by a distinguished asymptoiic limit. Examples of the flamelet structure are presented using onestep model kinetics or a reduced four-step quasi-global mechanism for methane flames. For non-premixed combustion a formal coordinate transformation using the mixture fraction Z as independent variable leads to a universal description. The instantaneous scalar dissipation rate χ of the conserved scalar Z is identified to represent the diffusion time scale that is compared with the chemical time scale in the definition of the Damkohler number. Flame stretch increases the scalar dissipation rate in a turbulent flow field. If it exceeds a critical value χ q the diffusion flamelet will extinguish. Considering the probability density distribution of χ , it is shown how local extinction reduces the number of burnable flamelets and thereby the mean reaction rate. Furthermore, local extinction events may interrupt the connection to burnable flamelets which are not yet reached by an ignition source and will therefore not be ignited. This phenomenon, described by percolation theory, is used to derive criteria for the stability of lifted flames. It is shown how values of ∋ q obtained from laminar experiments scale with turbulent residence times to describe lift-off of turbulent jet diffusion flames. For non-premixed combustion it is concluded that the outer mixing field—by imposing the scalar dissipation rate—dominates the flamelet behaviour because the flamelet is attached to the surface of stoichiometric mixture. The flamelet response may be two-fold: burning or non-burning quasi-stationary states. This is the reason why classical turbulence models readily can be used in the flamelet regime of non-premixed combustion. The extent to which burnable yet non-burning flamelets and unsteady transition events contribute to the overall statistics in turbulent non-premixed flames needs still to be explored further. For premixed combustion the interaction between flamelets and the outer flow is much stronger because the flame front can propagate normal to itself. The chemical time scale and the thermal diffusivity determine the flame thickness and the flame velocity. The flamelet concept is valid if the flame thickness is smaller than the smallest length scale in the turbulent flow, the Kolmogorov scale. Also, if the turbulence intensity v′ is larger than the laminar flame velocity, there is a local interaction between the flame front and the turbulent flow which corrugates the front. A new length scale L G =v F 3 /∈ , the Gibson scale, is introduced which describes the smaller size of the burnt gas pockets of the front. Here v F is the laminar flame velocity and ∈ the dissipation of turbulent kinetic energy in the oncoming flow. Eddies smaller than L G cannot corrugate the flame front due to their smaller circumferential velocity while larger eddies up to the macro length scale will only convect the front within the flow field. Flame stretch effects are the most efficient at the smallest scale L G . If stretch combined with differential diffusion of temperature and the deficient reactant, represented by a Lewis number different from unity, is imposed on the flamelet, its inner structure will respond leading to a change in flame velocity and in some cases to extinction. Transient effects of this response are much more important than for diffusion flamelets. A new mechanism of premixed flamelet extinction, based on the diffusion of radicals out of the reaction zone, is described by Rogg. Recent progress in the Bray-Moss-Libby formulation and the pdf-transport equation approach by Pope are presented. Finally, different approaches to predict the turbulent flame velocity including an argument based on the fractal dimension of the flame front are discussed.
01 Apr 2000-Combustion and Flame
Abstract: The present study reports an updated detailed chemical kinetic model for soot formation The model combines recent developments in gas-phase reactions, aromatic chemistry, soot particle coagulation, soot particle aggregation, and develops a new submodel for soot surface growth The model was tested against experimental profiles of major and minor chemical species, aromatics, soot volume fractions, and soot particle diameters reported in the literature for nine laminar premixed flames of ethane, ethylene, and acetylene The numerical agreement between the model predictions and experimental data is generally within a factor of 3 This level of agreement is very encouraging, considering the current uncertainties in the thermodynamics and kinetics of aromatics and soot chemistry The principal accomplishment of the present study is that the demonstrated level of agreement all around—main flame environment, aromatics, and soot—can be attained with a single reaction model
01 Aug 2009-Progress in Energy and Combustion Science
Abstract: Combustion instability remains a critical issue limiting the development of low-emission, lean-premixed (LPM) gas turbine combustion systems. The present work provides a comprehensive review of the advances made over the past two decades in this area. Recent developments in industrial dry-low-emission (DLE) swirl-stabilized combustors are first summarized. 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. Nonlinear behaviors of combustion processes observed in combustors are described. The influence of fuel preparation, combustor geometry, and operating conditions on combustion characteristics in swirl-stabilized combustors is examined. The mechanisms driving combustion instabilities, including hydrodynamic instabilities, equivalence ratio fluctuations, flame surface variations, and oscillatory liquid fuel atomization and evaporation are investigated. Instability stabilization methods, including both passive and active control techniques, are also reviewed. Finally, recent progress in both analytical modeling and numerical simulation of swirl-stabilized combustion are surveyed.
01 Jan 1985-Progress in Energy and Combustion Science
Abstract: General discussion is given of the structure and dynamics of wrinkled fronts of premixed flames in turbulent and laminar flow fields. Recent theoretical analyses and experimental results leading to complete description of the coupling between diffusive transport processes, the hydrodynamical phenomena associated with gas expansion, complex chemical kinetics and acceleration of gravity are presented in detail. The following topics are considered: stability and flammability limits of planar fronts, cellular flames, flame stretch, turbulent and self-turbulizing flames, hydrodynamic interactions between weakly turbulent gas flows and wrinkled flame fronts, molecular diffusion effects of intermediate species involved in chain reactions. The main objective of this paper is to provide a review for non-specialists of recent developments in flame theory, in sufficient detail to give the reader a comprehensive introduction to the field.