MICROSCALES OF TURBULENT COMBUSTION

A novel approach leading to the microscales of complex turbulent flows is reviewed. The approach is illustrated in terms of the classical mi croscales proposed by Taylor, Kolmogorov, Oboukhov-Corrsin and Batchel or. A thermal mesomicroscale between the Kolmogorov and Batchelor scal es, eta(theta) = (eta eta(B)(2))(1/3) is introduced. Here eta and eta( B), respectively, denote the Kolmogorov and Batchelor scales, nu and a lpha the kinematic and thermal diffusivities, and epsilon the rate of turbulent mechanical energy per unit mass. The foundations of the well -known correlation for forced convection over a flat plate, Nu(l) simi lar to Re-l(3/4) Pr-1/3, l being an integral scale, is interpreted in terms of l, eta and eta(theta). The approach is utilized to construct the microscales of forced and natural diffusion flames. In terms of a dame Batchelor scale eta B = (nu(beta)D(beta)(2)/epsilon)(1/4), the th ermal scale is extended to a flame mesomicroscale, eta(beta). Here nu( beta) = nu (1 + B) and D-beta = D (1 + B), respectively, denote the fl ame momentum diffusivity and the b-property diffusivity, nu and D bein g the usual diffusivities and B the transfer number. A model for the f uel consumption in forced flames is proposed in terms of eta(beta). Th e model correlates well with the existing experimental data. For buoya ncy driven flames, a Kolmogorov scale eta(beta) similar to (1 + sigma( beta))(1/4) (D-beta(3)/B)(1/4). is proposed. Here sigma(beta) = nu/D-b eta denotes a flame Schmidt number, B being the rate of buoyant energy production. The limit of this scale for sigma(beta) --> 0 turns out t o be a flame Oboukhov-Corrsin scale, eta(c) similar to (D-beta(3))(1/4 )/B). A model for the fuel consumption in buoyancy-driven flames is pr oposed in terms of eta(beta). The model correlates well with the exist ing experimental data.For oscillating (on the mean) flows, a Kolmogoro v scale, eta similar to (nu(3)/epsilon)(1/4)/[1 + omega(nu/epsilon)(1/ 2)](1/2) is proposed. Here omega denotes the frequency of externally i mposed-internally induced flow. The two limits of this scale correspon ding to omega --> 0 and omega --> infinity are the usual Kolmogorov an d Stokes scales. A model for heat transfer in pulse combustor tailpipe s is proposed in terms of eta. The model correlates well with the exis ting experimental data.