Flame sheets structure

Recall that we are assuming faem "C faff (°r fax, if turbulent flow). Anyone who has carefully observed a laminar diffusion flame - preferably one with little soot, e.g. burning a small amount of alcohol, say, in a whiskey glass of Sambucca - can perceive of a thin flame (sheet) of blue incandescence from CH radicals or some yellow from heated soot in the reaction zone. As in the premixed flame (laminar deflagration), this flame is of the order of 1 mm in thickness. A quenched candle flame produced by the insertion of a metal screen would also reveal this thin yellow (soot) luminous cup-shaped sheet of flame. Although wind or turbulence would distort and convolute this flame sheet, locally its structure would be preserved provided that faem fax. As a consequence of the fast chemical kinetics time, we can idealize the flame sheet as an infinitessimal sheet. The reaction then occurs at y = yf in our one dimensional model. 

Criteria for the validity of the flame-sheet approximation may be developed by analyzing the structure of the sheet (see Section 3.4). For calculation of flame shapes in the Burke-Schumann problem, the approximation usually is well justified, although uncertainties arise for strongly sooting flames. 

At large values of the ZePdovich number, the chemical reaction is confined to a thin sheet in the flow. For all purposes except the analysis of the sheet structure, the sheet may be treated as a surface—for example, G(x, t) 0—which in terms of the Cartesian coordinates (x, y, z) may be written locally as x = F(y, z, t) if the x coordinate is not parallel to the sheet in its local orientation. When analyses are pursued in outer-scale variables, it is convenient to work in a coordinate system that moves with the sheet. For the undisturbed flow, let the x coordinate be normal to the planar flame,... 

When disruptions of flame sheets become sufficiently extensive, there is appreciable nonreactive mixing of reactants and products at molecular scales. The extent of disruption increases as IJS decreases certainly if l/S becomes small compared with unity, then the turbulent flame no longer can be composed of wrinkled laminar flames. The true structures of turbulent flames in the limit of small values of l/S are unknown. 

Typical flame structures of HMX pellets are shown in Fig. 5-4 as a function of pressure. A thin luminous flame sheet stands some distance from the burning surface and a reddish flame is produced above this luminous flame sheet. The flame sheet approaches the burning surface as pressure increases 16. When the pressure is < 0.18 MPa, the luminous flame sheet is blown away from the burning surface, as shown in Fig. 5-4 (a). As pressure increases, the luminous flame sheet rapidly approaches the burning surface. However, it becomes very unstable above the burning surface and forms a wave-shaped flame sheet in the pressure range between 0.18 MPa and 0.3 MPa, as shown in Fig. 5-4(b). At further increased pressure, above 0.3 MPa, the luminous flame sheet becomes stable and one-dimensional just above the burning surface, as shown in Fig. 5-4(c). 

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