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Show r- ----- --- ~~~--------------------~ i I Fig. 6. UM model results for mlx1ng and combustion In relatively complex flow, showing a quarl burner In an enclosed furnace. Air enters axtally through the annulus surrounding the fuel gun, and fuel Is Injected radially into the combustion air stream. The flue gas exit Is located In the center part of the furnace endwall. Shown are the instantaneous shape of the time-evolving computatlonal surface (top) on which the LIM equations for the local scalar energy dissipation rate profile moments are solved, together wtth the corresponding instantaneous velocity vector field (bottom). not generally involve pure fuel or pure air on either side of the layer, the correct local boundary values are Ci ~ C,± as n -+ ±too. Replacing the Ci's in (2) with their definitions in terms of the chemical species fields in (9) then leads to locally one-dimensional advection-diffusion-reaction equations for the chemical species fields Yj(x ,t). Thus the one-dimensionality of the conserved scalar field across each strained dissipation layer implies a locally one-dimensional structure for the underlying chemical species fields within the layer, though owing to the local boundary conditions this does not imply that the resulting Y,{x,t) fields must be layer-ltke. The resulting quasi-steady solutions Yin;o) can be equivalently given as mappings of the form Yp; •. VC·VC; C±). The scalar and scalar dissipation values thus determtne the local chemical composition at each point in the flow. It is shown in (5) that this mapping approach produces thin (flarnelet-like) mass fraction and reaction rate fields under conditions of relatively weak chemical nonequillbrium (I.e. fast chemical reactions!. and the natural emergence and dominance of broad (distributed) species concentratton and reaction rate fields for Increasing chemical eqUilibrium departures (Le . slow chemical reacttons). Thus. the model provides a physically-based framework that reconciles these two widely disparate views of the coupling between the fluid dynamics and reaction chemistry in turbulent combustion. The layer-like scalar dissipation structures at the core of the LIM model. together with this strained dissipation and reaction layer formulation. accounts for both the Mflarnelet" and Mdistributed" combustion regimes under differing equtlibrtum departures and locations in the flow . SAMPLE RESULTS FROM LIM MODEL CALCUlATIONS Figures 1-6 show sample results obtained by the LIM model deSCribed in the previous section. The first case presented. in Figs. 1 and 2. shows the differing effects of confinement on two-dimensional and axisymmetric bluff body flows . The model predicts a large effect of varying the confinement level in the two-dimensional case, with a smaller effect seen In axisymmetric cases. Moreover, owing to the differtng vortlcal structure of the recirculation zones, the entrainment and mixing properties are greatly affected by the level of confinement. These observations are in agreement with expertmental results for confinement effects in bluff body flows, and provide a partial validation of the results obtained with the LIM code. The second case. presented in Figs. 3- 5. shows mixing and combustion in a turbulent jet. Figure 3 shows the time-evolving |