Modeling of Reaction Processes in Turbulent Flames with Special Emphasis on Soot Formation and Combustion
The present paper reviews features of the eddy-dissipation concept developed by the author for treating chemical reactions in turbulent flow.
An essential feature of this concept is that it takes into account the fact that the molecular mixing between reactants, which is associated with the dissipation of turbulence, takes place in concentrated, isolated regions that occupy only a small fraction of the total volume of the fluid.
The mass fraction occupied by the dissipative regions, as well as the mass transfer rate between these regions and the surrounding fluid, are determined from turbulence theory thus providing new general fluid mechanical information for the solution of reaction problems. This enables fast and accurate calculations of turbulent combustion phenomena.
The treatment of fast and slow chemical reactions in turbulent flow is discussed in relation to this concept. Comparison is made with experimental data.
Special attention is given to the modeling of soot formation and combustion in turbulent flames. A two-step model for the soot formation is applied, i.e., one rate equation for the formation of nuclei and one for particles. The interaction between the chemistry and the turbulence is modeled according to the eddy-dissipation concept. Comparison is made between experimental data and calculations for acetylene.
It is interesting to notice that when the same rate equations with the same constants are applied also for methane and propane, results are obtained which seem to be closely related to physical reality.
KeywordsFine Structure Mass Transfer Rate Soot Particle Diffusion Flame Soot Formation
constant or flux absorption coefficient
- C1, C2, CD
activation energy or blackbody emissive power
flatness factor or radiation flux sum
mixture fraction or linear branching coefficient
linear termination coefficient or gravitation constant
coefficient of linear termination on soot particle
reaction enthalpy difference
intensity of scattered light
turbulence kinetic energy
characteristic length of fine structures
exchange rate of mass with fine structures
mass concentration (kg/kg)
mass of soot particle (kg/part)
concentration of soot particles (part/m3)
nucleus concentration (part/m3)
rate of spontaneous formation of nucleus (part/m3/s)
rate of fuel combustion (kg/m3/s)
- Rn, c
rate of nucleus combustion (part/m3/s)
- Rn, f
rate of nucleus formation
- Rs, c
rate of soot combustion
- Rs, f
rate of soot formation
stoichiometric oxygen requirement to burn 1 kg fuel or soot
excess temperature of reacting fine structures
characteristic velocity of fine structures
rate of dissipation of turbulence kinetic energy
effective turbulent viscosity
turbulent Prandtl/Schmidt number
mass fraction occupied by fine structures
fraction of fine structures reacting
integral scale of turbulence
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