ON THE INFLUENCE OF GRADIENT TRANSPORTS UPON THE BURNING VELOCITIES OF DUST FLAMES

Combining numerics and asymptotic methods in the limit of large Boltzm ann and Zel'dovich (Ze) numbers, we study how the burning speed (U) of rich, adiabatic dust-flames is affected by gradient (e.g. turbulent-) transport of heat and species. Depending on the gradient-to-radiative diffusivity ratio eta (a Planck number) evaluated in the high-tempera ture region, two asymptotic regimes are identified and analyzed: eta = O(Ze(-1)) leads to U(eta) < U(0), due to reactant spreading by diffus ion, whereas eta = O(1) and large enough ultimately yields the opposit e trend, due to enhanced overall conductive effects. An analytical com posite expression of U(eta,Ze) which encompasses both effects is provi ded and allows one to estimate the minimum burning speed; the latter d ecreases with Ze increasing.