Diffusion flames buoyancy effects

The physics and modeling of turbulent flows are affected by combustion through the production of density variations, buoyancy effects, dilation due to heat release, molecular transport, and instabiUty (1,2,3,5,8). Consequently, the conservation equations need to be modified to take these effects into account. This modification is achieved by the use of statistical quantities in the conservation equations. For example, because of the variations and fluctuations in the density that occur in turbulent combustion flows, density weighted mean values, or Favre mean values, are used for velocity components, mass fractions, enthalpy, and temperature. The turbulent diffusion flame can also be treated in terms of a probabiUty distribution function (pdf), the shape of which is assumed to be known a priori (1). 

Buoyancy is a major effect in most diffusion flames and even in premixed systems it can be responsible for much of the flame dynamics. The computational problem introduced here is one of... 

If due to buoyancy the fuel gases attain a velocity vb in the flame zone after leaving the port exit, continuity requires that the effective radial diffusion distance be some value rb. Obviously, continuity requires that p be the same for both cases, so that... 

Premixed flames may be influenced by the Darrieus-Landau hydrodynamic instability [1, 2] when the chemical heat release is sufficiently large. Hydrodynam-ically unstable premixed flames are not always observed, however, because of stabilizing influences of buoyancy and thermal diffusion. The long-wavelength flame-surface wrinkles are attenuated by buoyancy for downward propagating flames, and thermal-diffusive effects stabilize small-wavelength wrinkles when the... 

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