Turbulent Combustion Modeling

Description

Lecture. - Belgique: Von Karman Instinute, 1999, 2001, 2003. 138 p.Foreword Numerical simulation of flames is a growing field bringing important improvements to our under-standing of combustion. The main issues and related closures of turbulent combustion modeling are reviewed. Combustion problems involve strong coupling between chemistry, transport and fluid dynamics. The basic properties of laminar flames are first presented along with the major tools developed for modeling turbulent combustion. The links between the available closures are enlighted from a generic description of modeling tools. Then, examples of numerical models for mean burning rates are discussed for premixed turbulent combustion. The use of direct numerical simulation (UN'S) as a research instrument is illustrated for turbulent transport occurring in premixed combustion, gradient and counter-gradient modeling of turbulent fluxes is addressed. Finally, a review of the models for nonpremixed turbulent flames is given.Contents Introduction Balance equations nstantaneous balance equations Reynolds and Favre averaging Favre averaged balance equations Filtering and Large Eddy Simulation Major properties of premixed, nonpremixed and partially premixed Flames Laminar premixed flames Laminar diffusion flames Partially premixed flames A direct analysis: Taylor’s expansion Scales and diagrams for turbulent combustion Introduction Turbulent premixed combustion diagram Introduction Combustion regimes Comments Nonpremixed turbulent combustion diagram Introduction Tools for turbulent combustion modeling Introduction Scalar dissipation rate Geometrical description G-field equation Flame surface density description Flame wrinkling description Statistical approaches: Probability density function Introduction Presumed probability density functions Pdf balance equation Joint velocity/concentrations pdf Conditional Moment Closure (CMC) Similarities and links between the tools Reynolds-averaged models for turbulent premixed combustion Turbulent flame speed Eddy-Break-Up model Bray-Moss-Libby (BML) model Introduction BML model analysis Recovering mean reaction rate from tools relations Reynolds and Favre averaging Conditional averaging - Counter-gradient turbulent transport Models based on the flame surface area estimation Introduction Algebraic expressions for the flame surface density SI Flame surface density- balance equation closures Analysis of the flame surface density balance equation Flame stabilization modeling A related approach: G-equation S Turbulent transport in premixed combustion Introduction Direct numerical simulation analysis of turbulent transport Introduction Results Physical analysis External pressure gradient effects Counter gradient transport - Experimental results To include counter-gradient turbulent transport in modeling Towards a conditional turbulence modeling ? Reynolds averaged models for nonpremixed turbulent combustion Introduction Fuel/Air mixing modeling Introduction Balance equation and simple relaxation model for x Models assuming infinitely fast chemistry Eddy Dissipation Model Presumed pdf: infinitely fast chemistry model (IFCM) . Flamelet modeling Introduction Flame structure in composition space Mixing modeling in SLFM Conclusion Flame surface density modeling, Coherent Flame Model (CFM) MIL model Conditional Moment Closure (CMC) Pdf modeling Turbulent micromixing Linear relaxation model, IEM/LMSE GIEM model Stochastic micromixing closures Interlinks PDF / Flame surface modeling Joint velocity/concentrations pdf modeling Large eddy simulation Introduction Unresolved turbulent fluxes modeling Smagorinsky model Scale similarity model Germano dynamic model Structure function models Unresolved scalar transport Simplest approaches for combustion modeling Arrhenius law based on filtered quantities Extension of algebraic Favre averaged approaches Simple extension of the Germano dynamic model LES models for non premixed combustion Linear Eddy Model Dynamic micro-mixing model Probability Density functions LES models for premixed combustion Introduction Artificially thickened flames G-equation Filtering the progress variable balance equation Numerical costs Conclusion