Evolution of flame surface in buoyant and nonbuoyant turbulent nonpremixed reactions

Said Elghobashi, University of California, Irvine

The evolution of the stoichiometric mixture fraction Fst isosurface is investigated using direct numerical simulations of turbulent nonpremixed reacting flow with infinite and finite reaction rates, with or without buoyancy. After the initial transient, Fst isosurface area increases monotonically with time; the rate of increase for the buoyant case is an order of magnitude higher than that of the nonbuoyant case. For the buoyant case, the Gaussian curvature of the isosurface indicates the presence of steep ridge-like topology and the mean reaction rate per unit surface area is twice that of the nonbuoyant case. The flame stretch equation:

(1/dA) d(dA)/dt = St + un grad.n

shows that, for the buoyancy case, the tangential strain rate St is dominant and positive at the troughs whereas the curvature term (un grad.n) is dominant and negative at the crests of the Fst isosurface. Thus, the surface area of the crests decreases while that of the troughs increases in time resulting in the steep-ridge topology. The strain field associated with the buoyancy-generated vortex-quadruple is responsible for this behavior.

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Last Modified: November 3, 1997