Flames laminar

A number of chemiluminescent reactions have been studied by producing key reactants through pulsed electric discharge, by microwave dissociation, or by observing the reactions of atoms and free radicals produced in the inner cone of a laminar flame as they diffuse into the flame s cool outer cone (182,183). These are either combination reactions or atom-transfer reactions involving transfer of chlorine (184) or oxygen atoms (181,185—187), the latter giving excited oxides. 

Diffuse-thermal instabilities involve the relative diffusion reactants and heat within a laminar flame. These are the smaHest-scale instabilities (11). 

Laminar flame instabilities are dominated by diffusional effects that can only be of importance in flows with a low turbulence intensity, where molecular transport is of the same order of magnitude as turbulent transport (28). Flame instabilities do not appear to be capable of generating turbulence. They result in the growth of certain disturbances, leading to orderly three-dimensional stmctures which, though complex, are steady (1,2,8,9). 

Turbulent flame speed, unlike laminar flame speed, is dependent on the flow field and on both the mean and turbulence characteristics of the flow, which can in turn depend on the experimental configuration. Nonstationary spherical turbulent flames, generated through a grid, have flame speeds of the order of or less than the laminar flame speed. This turbulent flame speed tends to increase proportionally to the intensity of the turbulence. 

To analy2e premixed turbulent flames theoretically, two processes should be considered (/) the effects of combustion on the turbulence, and (2) the effects of turbulence on the average chemical reaction rates. In a turbulent flame, the peak time-averaged reaction rate can be orders of magnitude smaller than the corresponding rates in a laminar flame. The reason for this is the existence of turbulence-induced fluctuations in composition, temperature, density, and heat release rate within the flame, which are caused by large eddy stmctures and wrinkled laminar flame fronts. 

The mechanism of flame propagation into a stagnant fuel-air mixture is determined largely by conduction and molecular diffusion of heat and species. Figure 3.1 shows the change in temperature across a laminar flame, whose thickness is on the order of one millimeter. 

Heat is produced by chemical reaction in a reaction zone. The heat is transported, mainly by conduction and molecular diffusion, ahead of the reaction zone into a preheating zone in which the mixture is heated, that is, preconditioned for reaction. Since molecular diffusion is a relatively slow process, laminar flame propagation is slow. Table 3.1 gives an overview of laminar burning velocities of some of the most common hydrocarbons and hydrogen. 

What are the mechanisms by which slow, laminar combustion can be transformed into an intense, blast-generating process This transformation is most strongly influenced by turbulence, and secondarily by combustion instabilities. A laminar-flame front propagating into a turbulent mixture is strongly affected by the turbulence. Low-intensity turbulence will only wrinkle the flame front and enlarge its surface area. With increasing turbulence intensity, the flame front loses its more-or-less smooth, laminar character and breaks up into a combustion zone. In an intensely turbulent mixture, combustion takes place in an extended zone in which... 

Fuel-pair mixtures, in soap bubbles ranging from 4 to 40 cm diameter and with no internal obstacles, produced flame speeds very close to laminar flame speeds. Cylindrical bubbles of various aspect ratios produced even lower flame speeds. For example, maximum flame speeds for ethylene of 4.2 m/s and 5.5 m/s were found in cylindrical and hemispherical bubbles, respectively (Table 4.1a). This phenomenon is attributed to reduced driving forces due to the top relief of combustion products. 

Van Wingerden and Zeeuwen (1983) demonstrated increases in flame speeds of methane, propane, ethylene, and acetylene by deploying an array of cylindrical obstacles between two plates (Table 4.3). They showed that laminar flame speed can be used as a scaling parameter for reactivity. Van Wingerden (1984) further investigated the effect of pipe-rack obstacle arrays between two plates. Ignition of an ethylene-air mixture at one edge of the apparatus resulted in a flame speed of 420 m/s and a maximum pressure of 0.7 bar. 

Over the years, this concept was refined in several ways. A scale dependency was modeled by the introduction of scale-dependent quenching of combustion. The first stage of the process was simulated by quasi-laminar flame propagation. In addition, three-dimensional versions of the code were developed (Hjertager 1985 Bakke 1986 Bakke and Hjertager 1987). Satisfactory agreement with experimental data was obtained. 

Overpressure within a vapor cloud is dependent upon outflow velocity, orifice diameter, and laminar flame speed expressed in the following semi-empirical relation ... 

Sivashinsky, G. I. 1979. On self-turbulization of a laminar flame. Acta Astronautica. 6 569-591. 

Laminar Versus Turbulent Flames. Premixed and diffusion flames can be either laminar or turbulent gaseous flames. Laminar flames are those in which the gas flow is well behaved in the sense that the flow is unchanging in time at a given point (steady) and smooth without sudden disturbances. Laminar flow is often associated with slow flow from small diameter tubular burners. Turbulent flames are associated with highly time dependent flow patterns, often random, and are often associated with high velocity flows from large diameter tubular burners. Either type of flow—laminar or turbulent—can occur with both premixed and diffusion flames. 

For some processes, a burner in which there is no primary aeration may produce a flame. These laminar flames have a very low intensity of combustion and a luminous appearance. 

More recently, Rosen (R3), Spalding (S5), and Johnson and Nachbar (J4) have considered a simplified approach using the analysis of laminar-flame propagation velocities. According to these investigators, the principal exothermic reactions occur in the gas phase. Some of the heat liberated by these reactions is then transferred back to the solid surface to sustain the endothermic surface-gasification processes. Thus, the temperature profile within the reactive zone is quite similar to that of Rice and Crawford. However, gasification of the solid surface is assumed to be endothermic, while exothermic reactions were considered in the studies discussed previously. 

Dixon-Lewis G., Structure of laminar flames, Proc. Combust. Inst., 23 305-324,1990. 

Gorecki G., Analysis of laminar flames propagating in a vertical tube based on PIV measurements, PhD dissertation, Technical University of Lodz, Lodz, Poland, 2007. 

Rimai L., Marko K.A., and Klick D., Optical study of a 2-dimensional laminar flame Relation between temperature and flow-velocity fields, Proc. Combust. Inst., 19 259-265,1982. 

Reference stretch-affected flame speeds as a function of Karlovitz number for various (a) n-heptane/air and (b) iso-octane/air flames, showing how the reference stretch-affected flame speed is extrapolated to zero stretch to obtain the laminar flame speed. The unburned mixture temperature T is 360 K. Solid lines represent linear extrapolation, while dotted lines denote nonlinear extrapolation... 

Figure 4.1.7 summarizes the measured laminar flame speeds of efhylene/air, n-heptane/air, fso-octane/air. 

Measured laminar flame speeds of (a) ethylene/air, (b) n-heptane/air, (c) iso-octane/air, and (d) n-decane/air mixtures as a function of the equivalence ratio for various unburned mixture temperatures. 

When representing the dependence of laminar flame speed (S°) on mixture preheat temperature (TJ in the form of S°(T, (Z>)/S°(To,(Z>) = (T /Tq)", where Tq is the lowest unburned mixture temperature investigated for a given fuel/air composition, the current experimental data can be correlated well with n in the... 

The effect of nitrogen concentration variation on laminar flame speed was also experimentally studied at... 

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