Showing papers on "Premixed flame published in 2006"

Stabilization of a Turbulent Premixed Flame Using a Nanosecond Repetitively Pulsed Plasma

Abstract: A nanosecond repetitively pulsed plasma (NRPP) produced by electric pulses of 10 kV during 10 ns at a frequency of up to 30 kHz has been used to stabilize and improve the efficiency of a 25-kW lean turbulent premixed propane/air flame (ReD=30000) at atmospheric pressure. We show that, when placed in the recirculation zone of the flow, the plasma significantly increases the heat release and the combustion efficiency, thus allowing to stabilize the flame under lean conditions where it would not exist without plasma. Stabilization is obtained with a very low level of plasma power of about 75 W, or 0.3% of the maximum power of the flame. In addition, they find that at high flow rates, where the flame should normally blow out, the NRPP allows the existence of an intermittent V-shaped flame with significant heat release, and at even higher flow rates the existence of a small dome-shaped flame confined near the electrodes that can serve as a pilot flame to reignite the combustor. Optical emission spectroscopy measurements are presented to determine the temperature of the plasma-enhanced flame, the electron number density, and to identify the active species produced by the plasma, namely O, H, and OH

Combustion Science and Engineering

Abstract: Introduction and Review of Thermodynamics Introduction Combustion Terminology Matter and Its Properties Microscopic Overview of Thermodynamics Conservation of Mass and Energy and the First Law of Thermodynamics The Second Law of Thermodynamics Summary Stoichiometry and Thermochemistry of Reacting Systems Introduction Overall Reactions Gas Analyses Global Conservation Equations for Reacting Systems Thermochemistry Summary Appendix Reaction Direction and Equilibrium Introduction Reaction Direction and Chemical Equilibrium Chemical Equilibrium Relations Vant Hoff Equation Adiabatic Flame Temperature with Chemical Equilibrium Gibbs Minimization Method Summary Appendix Fuels Introduction Gaseous Fuels Liquid Fuels Solid Fuels Other Fuels Size Distributions of Liquid and Solid Fuels Summary Appendix Chemical Kinetics Introduction Reaction Rates: Closed and Open Systems Elementary Reactions and Molecularity Multiple Reaction Types Chain Reactions and Reaction Mechanisms Global Mechanisms for Reactions Reaction Rate Theory and the Arrhenius Law Second Law and Global and Backward Reactions The Partial Equilibrium and Reaction Rate Expression Timescales for Reaction Solid-Gas (Heterogeneous) Reactions and Pyrolysis of Solid Fuels Summary Appendix Mass Transfer Introduction Heat Transfer and the Fourier Law Mass Transfer and Fick's Law Molecular Theory Generalized Form of Fourier's and Fick's Laws for a Mixture, with Simplifications Summary Appendix: Rigorous Derivation for Multicomponent Diffusion First Law Applications Introduction Generalized Relations in Molar Form Closed-System Combustion Open Systems Solid Carbon Combustion Droplet Burning Summary Conservation Relations Introduction Simple Diffusive Transport Constitutive Relations Conservation Equations Generalized Transport Simplified Boundary-Layer-Type Problems Shvab-Zeldovich Formulation Turbulent Flows Summary Appendix Combustion of Solid Fuels, Carbon, and Char Introduction Carbon Reactions Conservation Equations for a Spherical Particle Nondimensional Conservation Equations and Boundary Conditions Interfacial Conservation Equations or BCs Solutions for Carbon Particle Combustion Thermal NOx from Burning Carbon Particles Non-Quasi-Steady Nature of Combustion of Particle Element Conservation and Carbon Combustion Porous Char Summary Appendix: d Law and Stefan Flow Approximation Diffusion Flames - Liquid Fuels Introduction Evaporation, Combustion, and d2 Law Model/Physical Processes Governing Equations Solutions Convection Effects Transient and Steady-Combustion Results Multicomponent-Isolated-Drop Evaporation and Combustion Summary Combustion in Boundary Layers Introduction Phenomenological Analyses Generalized Conservation Equations and Boundary Conditions Interface Boundary Conditions Generalized Numerical Solution Procedure for BL Equations in Partial Differential Form Normalized Variables and Conservation Equations Similarity Solutions-BL Equations Applications of Generalized Similarity Equations to Various Flow Systems Solutions for Boundary Layer Combustion of Totally Gasifying Fuels Combustion Results for Fuels Burning under Convection Excess Fuel and Excess Air under Convection Summary Combustion of Gas Jets Introduction Burke-Schumann (B-S) Flame Modification to B-S Analyses Laminar Jets Planar Laminar Jets Circular Jets Summary of Solutions for 2-D and Circular Jets Stoichiometric Contours for 2-D and Circular Jets, Liftoff, and Blow-Off Jets in Coflowing Air: Jet Flame Structure in Strongly Coflowing Air for 2-D and Circular Jets Turbulent Diffusion Flames Partially Premixed Flame Summary Ignition and Extinction Introduction Modes of Ignition Ignition of Gas Mixtures in Rigid Systems: Uniform System Constant-Pressure Systems Ignition of Solid Particle Ignition of Nonuniform Temperature Systems-Steady-State Solutions Summary Deflagration and Detonation Introduction Conservation Equations Solutions for Rayleigh and Hugoniot Curves Flame Propagation into Unburned Mixture Summary Appendices Flame Propagation and Flammability Limits Introduction Phemenological Analysis Rigorous Analysis Flame Stretching Determination of Flame Velocity Flammability Limits Quenching Diameter Minimum Ignition Energy for Spark Ignition Stability of Flame in a Premixed Gas Burner Turbulent Flame Propagation Summary Interactive Evaporation and Combustion Introduction Simplified Analyses Arrays and Point Source Method Combustion of Clouds of Drops and Carbon Particles Terminology Governing Equations for Spherical Cloud Results Relation between Group Combustion and Drop Array Studies Interactive Char/Carbon Combustion Multicomponent Array Evaporation Summary Pollutants Formation and Destruction Introduction Emission-Level Expressions and Reporting Effects of Pollutants on Environment and Biological Systems Pollution Regulations NOx Sources and Production Mechanisms NOx Formation Parameters Stationary Source NOx Control CO2 Sequestration Carbon Monoxide: CO SOx Formation and Destruction Soot Mercury Emissions Summary An Introduction to Turbulent Combustion Introduction Turbulence Characteristics Averaging Techniques Instantaneous and Average Governing Equations Governing Differential Equations: Axisymmetric Case and Mixture-Fraction PDF Combustion Model Turbulent Combustion Modeling (Diffusion Flames) Probability Density Function Premixed and Partially Premixed Turbulent Flames: Modeling Approaches Summary Appendix I: Cylindrical Coordinate System with Particle-Laden Flow Problems Formulae Appendix A Appendix B References Index

Interaction of turbulence and scalar fields in premixed flames

Abstract: The interaction of turbulence with progress variable, c, field in a premixed flame is studied. This interaction is characterized by the correlation c,ieijc,j¯, where the overbar denotes an appropriate averaging process, c,i is the gradient of c in spatial direction i and eij is the turbulence strain rate. The importance of this term is recognized via a transport equation for the square of the magnitude of the scalar gradient in turbulent premixed flames. It is also shown that the above correlation forms a natural part of the tangential strain rate which appears in the flame surface density, Σ, equation. It is well known that this correlation is negative signifying the scalar gradient production by incompressible turbulence via the preferential alignment of the scalar gradient with the most compressive principal strain rate. This physical picture is commonly adopted for turbulent premixed flames also. Contrary to this, analyses of direct numerical simulation data for a turbulent premixed flame having flame...

Plasma-Discharge Stabilization of Jet Diffusion Flames

Abstract: The authors examine three different types of plasma discharges in their ability to stabilize a lifted jet diffusion flame in coflow. The three discharges include a single-electrode corona discharge, an asymmetric dielectric-barrier discharge (DBD), and a repetitive ultrashort-pulsed discharge. The degree of nonequilibrium of this pulsed discharge is found to be higher than that for the DBD. Furthermore, this pulsed discharge causes the most significant improvement in the flame stability. The optimal placement of the discharge electrodes is investigated, and it is found that there is a close relation between this placement and the emission spectra, suggesting use of the emission spectra as a possible indicator of fuel/air mixture fraction. The optimal placement is mapped into mixture-fraction space by use of a fully premixed flame experiment of known mixture fraction. The result shows that the mixture fraction, which corresponds to the optimal placement, is much leaner than that of a conventional lifted jet flame

Flame propagation and combustion in some dust-air mixtures

Abstract: Some results of measurements of laminar burning velocities and of maximum flame temperatures for combustible dust-air mixtures (starch dust-air mixtures, lycopodium-air mixtures and sulphur flour-air mixtures) are presented. Thin (25 and 50 μm) thermocouples have been used to measure maximum flame temperatures. The results are compared with those obtained with other devices such as resistors, pyrometers and are compared to the theoretical values. It appears that the observed discrepancies seem principally to come from the relatively poor efficiency of the burning processes inside the flame front than to heat losses by radiation as suggested before. Two methods for determining laminar burning velocities have been used: the classical ‘tube method’ and a ‘direct method’ based on the simultaneous determination of the flame speed and of the mixture velocity ahead of the flame front using a tomographic technique. Two different tube diameters are considered as well as additional results obtained with a small burner. The validity of these techniques is firstly assessed by comparing the results obtained with CH4-air mixtures and secondly by considering their relevancy for combustible dust-air mixtures (influence of the size of the apparatus). In particular, the influences of heat flame by radiation and of flame stretching are considered.