Flame surface area

The volume-source method is not only useful in a spherical approach, but can also be used in more arbitrary geometries, where it is possible to express the volume source strength in a product of burning velocity and flame surface area ... 

This concept can be generalized for more arbitrarily shaped clouds, provided that a reasonable estimate can be made of combustion process development in terms of burning velocity and flame surface area. According to Strehlow (1981), a conservative estimate of source strength is made by... 

The flame-surface area dependent on time is approximated by a plane cross-section moving at burning speed through the stationary cloud. 

Radiation heat flux is strongly time dependent, because both the flame surface area and the distance between the flame and intercepting surfaces vary during the eourse of a flash fire. The path of this curve ean be approximated by calculating the radiation heat flux at a sufficient number of discrete points in time. 

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... 

Considering the case of premixed flames, it is noted by Thomas and Williams [26] that sound radiation can be related to the rate of change of the flame surface area by assuming that the burning velocity is constant. Similarly, Ref. [35] and later Ref. [36] indicate that in the wrinkled flame regime, the rate of chemical conversion is directly linked to the flame surface area A(t). For a mixture of fresh reactants at a constant equivalence ratio, the pressure field is directly linked to the instantaneous flame surface ... 

Note <3>, equivalence ratio modulation frequency SPL, sound pressure level A, mean flame surface area fluctuation of flame area v, mean velocity at the burner outlet and v, imposed velocity... 

As mutual flame annihilation is believed to control and limit flame surface area in turbulent combustion, the previous results suggest that this mechanism could also be a source of intense noise radiation in turbulent combustors. 

Time traces OH light intensity I, flame surface area A, pressure fluctuahons p, and computed pressure fluctuations kdA/dt. Circles indicate extracted flame surface areas A in cm (S and A are used indifferently to designate the flame surface). Black circles marked a, b, c, d correspond to flame patterns presented in images from Figure 5.2.3. 

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 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. ... 

This emissive power is assumed to be over the whole flame surface area, and is significantly less than the emissive powers that can be calculated from point source measurements. Increasing the pool diameter reduces the emissive power due to the increasing black smoke outside the flame and the resulting obscuration effect on the luminous flame. 

Powders Precursors (ratio) Flame Surface area (m /g)... 

In line-symmetry, that is, a cylindrical flame between two plates, the overall flame surface area is proportional to the distance from the ignition point. Conse-quendy, deformation of the flame surface will have a stronger effect than in the point-symmetry case. 

In plane-symmetry, that is, a planar flame in a tube, the projected flame surface area is constant. There is hardly any flow field decay, and flame deformation has a very strong effect on flame acceleration. 

Divide radiated power by flame surface area. 

The Karlovitz stretch factor K denotes the rate of a flame front area change dAldt with respect to the whole flame surface area [5]... 

When the flame is subjected to small-scale curvatures, which is typical for lean H2 + air mixtures, an experimenter does not measure a normal velocity, but blindly measures some velocity depending on the level of the developing front. The front perturbation at an initial pressure of atmospheric or lower may be insufficient for a significant increase in the flame surface area. Therefore, even with the blind method of measurement in [26, 41], plausible results were obtained in the range between 0.05 and 0.1 MPa. 

Turbulence affects combustion by influencing a flame surface. This phenomenon has a dual effect. On the one hand, the turbulence raises the rate of combustion due to the intensive transfer of heat and active particles to the unburned gas and from the increase of the flame surface area resulting from its curvature and fragmentation. On the other hand, the turbulence causes reduction of the velocity by the local stretch-effect. Therefore, intensive turbulence may result in flame extinction. 

The instability of boundaries between gases of various densities is considerable in confined vessels, tubes and ducts. Flame acceleration due to Rayleigh -Taylor instability is explained by the fact that when the pressure wave crosses the flame front from the side of the combustion products (their density is less than that in the fresh mixture), the amplitude of all the flame front irregularities grows rapidly and the flame surface area increases. The extreme H2 + air mixture flame acceleration resulting from turbulence was mentioned earlier. In confined vessels, flame acceleration up to the detonation velocity is often caused by flame front instabilities when it interacts with compression waves. 

When the expansion level is large, the flame accelerates to high velocities approximately equal to the sound speed in the combustion products before DDT conditions are reached. This means that the flame propagates through obstacles, in obstructed channels or in a chain of connected volumes and gas flows are generated which are required to increase the flame surface area. 

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