Burning velocity laminar

Before the size of the flammable portion of a vapor cloud can be calculated, the flammability limits of the fuel must be known. Flanunability limits of flammable gases and vapors in air have been published elsewhere, for example, Nabert and Schon (1963), Coward and Jones (1952), Zabetakis (1965), and Kuchta (1985). A summary of results is presented in Table 3.1, which also presents autoignition temperatures and laminar burning velocities referred to during the discussion of the basic concepts of ignition and deflagration. 

Gas or Vapor Flammability Limits (vol. %) Flash Point rc) Autoignition Temperature rc) Laminar Burning Velocity (mis)... 

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. 

In-cloud overpressure is dependent on outflow velocity, orifice diameter, and the fuel s laminar burning velocity. 

Burning velocity The velocity of propagation of a flame burning through a flammable gas-air mixture. This velocity is measured relative to the unbumed gases immediately ahead of the flame front. Laminar burning velocity is a fundamental property of a gas-air mixture. 

It can be seen in Figure 3.1.1 that the total surface area of the propane lean limit flame is much less than that of the methane one. This is because the laminar burning velocity for the limit mixture is much higher for propane than for methane. 

In contrast to the lean propane flame, the burning intensity of the lean limit methane flame increases for the leading point. Preferential diffusion supplies the tip of this flame with an additional amoxmt of the deficient methane. Combustion of leaner mixture leads to some extension of the flammability limits. This is accompanied by reduced laminar burning velocity, increased flame surface area (compare surface of limit methane... 

In the case of flame propagation in the lean limit methane/air mixture, the local laminar burning velocity at... 

Law, C.K., A compilation of experimental data on laminar burning velocities. Reduced Kinetic Mechanisms for Application in Combustion, Eds. N. Peters and B. Rogg, Springer-Verlag, Heidelberg, Germany, pp. 15-26, 1993. 

It is not possible to obtain exactly identical flow conditions for the configurations explored. The level of velocity fluctuation at the burner outlet also differs in the various cases. This level was adjusted to get an acceptable signal-to-noise ratio. In the results presented here, the specific heat ratio was taken as equal to y= 1.4, the sound speed Cq = 343 m/s corresponds to a room temperature T = 293 K. The air density is taken equal to = 1.205 kg/m. Laminar burning velocities are... 

The steady states of such systems result from nonlinear hydrodynamic interactions with the gas flow field. For the convex flame, the flame surface area F can be determined from the relation fSl = b zv, where Sl is the laminar burning velocity, the cross-section area of the channel, and w is the propagation velocity at the leading point. 

The cooling effect of the channel walls on flame parameters is effective for narrow channels. This influence is illustrated in Figure 6.1.3, in the form of the dead-space curve. When the walls are <4 mm apart, the dead space becomes rapidly wider. This is accompanied by falling laminar burning velocity and probably lowering of the local reaction temperature. For wider charmels, the propagation velocity w is proportional to the effective flame-front area, which can be readily calculated. On analysis of Figures 6.1.2b and 6.1.3, it is evident that the curvature of the flame is a function of... 

Gutkowski, A., Laminar burning velocity under quenching conditions for propane-air and ethylene-air flames, Archivum Combustionis, 26 163, 2006. 

Variations of the maximum Karlovitz number and laminar burning velocities with the equivalence ratio, showing the accessible domain when the maximum/= 170Hz is operated, (a) CH4/air mixtures (b) CH4 diluted with 20-60% N2 (c) CH4 diluted with 20-60% CO2 and (d) combined plots of these maximum-Ka = 170 Hz) lines from (a-c) for comparison. (From Yang, S.I. and Shy, S.S., Proc. Combust. Inst, 29,1841, 2002. 

Stone, R., Clarke, A., and Beckwith, P, Correlations for the laminar-burning velocity of methane/diluent/air mixtures obtained in free-fall experiments. Combust. Flame, 114, 546, 1998. 

The fact that the fuel/air ratio is spatially constant in HCSI engines, at least within a reasonably close approximation, allows substantial simplifications in combustion models. The burn rate or fuel consumption rate dm /dt is expressed as a function of flame surface area the density of the unburnt fuel/air mixture Pu, the laminar burning velocity Sl, and the fluctuations of velocities, i.e., E as a measure of turbulence, u. ... 

Embedded in such models, in which variations were developed [12] are further detailed. The laminar burning velocity is expressed as a function of fuel type, fuel/ air ratio, level of exhaust gas recirculation, pressure, temperature, etc. Furthermore, submodels have been developed to describe the impact of engine speed, port-flow control systems, in-cylinder gross-flow motion (i.e., swirl, tumble, squish), and turbulent fluctuations u. Thus, with a wider knowledge base of the parametric impact of external variables, successful modeling of... 

C"alculatc the laminar burning velocity as a function of pressure at 0.25,... 

Burning, Laminar, of Gases. Influence of pressure and temperature on the laminar burning velocity of stoichiometric acetylene-air mixtures using a constant-volume bomb method is described by M.L. Agrawal S.P. Sharma in UnivRoorkeeResJ(India), 8(3-4), Part 11, 81-102(1965) (in Engl)... 

Discuss how an increased pressure may affect the laminar burning velocity of methane. 

At the relatively low inlet velocity of U =30 cm/s, the flame is stabilized by heat transfer to the inlet manifold. This is essentially the situation in the typical flat-flame burner that is found in many combustion laboratories (e.g., Fig. 16.8). The laminar burning velocity (flame speed) of a freely propagating atmospheric-pressure, stoichiometric, methane-air flame is approximately 38 cm/s. Therefore, since inlet velocity is less than the flame speed, the flame tends to work its way back upstream toward the burner. As it does, however, a... 

The opposed-flow geometry has some important differences, as well as benefits, compared with the burner-stabilized flat flame (e.g., Fig. 1.1). One is that the strain field can be varied by controlling the flow rate, ranging from an essentially strain-free situation to a flame extinction. As discussed subsequently, this flow configuration can be used experimentally for the accurate measurement of laminar burning velocities [238,438,448]. 

For a given set of flow parameters, the strained flame speed is taken as the fluid velocity at the minimum in the profile just upstream of the flame. Law and collaborators developed an analysis that uses a series of variously strained flames to predict strain-free laminar burning velocities [238,438,448]. As the strain rate is decreased, the strained flame speed decreases and the flame itself moves farther from the symmetry plane. There is an approximately linear relationship between the strained flame speed and the strain rate. Thus, after measuring the velocity profiles (e.g., by laser-dopler velocimetry) for a number of different strain rates, the strain-free burning velocity can be determined by extrapolating the burning velocity to zero strain. 

In addition to the low-strain limit, which can be used to determine laminar burning velocities, the opposed-flow configuration can also be used to determine high-strain-rate extinction limits. As the inlet velocities increase, the flame is pushed closer to the symmetry plane and the maximum flame temperature decreases. There is a flow rate beyond which a flame can no longer be sustained (i.e., it is extinguished). Figure 17.11 illustrates extinction behavior for premixed methane-air flames of varying stoichiometries. 

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