Flame-front propagation velocity

Detonation Propagation of a flame-driven shock wave at a velocity at or above the speed of sonnd in the nnreacted medinm as measnred at the flame front. The wave is snstained by chemical energy released by shock compression and ignition of the nnreacted medinm. The flame front is conpled in time and space with the shock front, and there is no pressnre increase significantly ahead of the shock-flame front. Propagation velocities in the range 1000-3500 m/s may be observed depending on the gas mixtnre, initial temperatnre and pressnre, and type of detonation. 

Figure 6.2 Dependence of flame-front propagation velocity of VN synthesis on (a) nitrogen pressure and sample density and (b) preheating temperature and diluent content.

When a vapor cloud bums, there is always a leading flame front propagating with uniform velocity into the unbumed cloud. The leading flame front is followed by a burning zone. 

The model is a straightforward extension of a pool-fire model developed by Steward (1964), and is, of course, a drastic simplification of reality. Figure 5.4 illustrates the model, consisting of a two-dimensional, turbulent-flame front propagating at a given, constant velocity S into a stagnant mixture of depth d. The flame base of width W is dependent on the combustion process in the buoyant plume above the flame base. This fire plume is fed by an unbumt mixture that flows in with velocity Mq. The model assumes that the combustion process is fully convection-controlled, and therefore, fully determined by entrainment of air into the buoyant fire plume. 

Consider a planar premixed flame front, such as that sketched in Figure 5.1.1. For the moment, we will be interested only in long length scales and we will treat the flame as an infinitely thin interface that transforms cold reactive gas, at temperature and density T p, into hot burnt gas at temperature and density T, A.-The flame front propagates at speed Sl into the xmbumt gas. We place ourselves in the reference frame of the front, so cold gas enters the front at speed = Su and because of thermal expansion, the hot gases leave the front at velocity 14 = Sl(Po/a)- The density ratio, Po/Pb, is roughly equal to the... 

Figure 8.8 Pyrolysis-front propagation velocity vp, and flame-tip propagation velocity Vf, as functions of the incident radiant energy flux, for a preheat time p of 2 min on a wood sample [13]...

Consider the condition, which determines the velocity of the curved flame front propagation in the channel. Inside the stagnation zone filled by combustion products the pressure is constant and is equal to the value at infinity (when x = oo). Because of Bernoulli s integral along the streamline restricting the stagnation zone, the gas motion velocity remains unchanged. Since at x = oo the flow is plane-parallel (ptJO = const, v — 0), distributions of velocity u and of the stream function are associated with the vorticity distribution ... 

This expression, together with the boundary conditions at the flame front [Eqs. (12)—(16)] and Eq. (6) for the flame surface shape, determines the combustion front propagation velocity U as a function of the normal flame... 

In premixed combustion, the velocity at which the flame front propagates (normal to itself) is called laminar burning velocity, 5l- This is a thermochemical transport property that depends on the equivalence ratio between fuel and air, the temperature of the unbumed mixture and the pressure. 

Hence, the flame front propagates during the time step dt by a larger absolute distance dSpp, and the fresh mixture is displaced by a distance dSpg = dSpp - dSg. The resulting absolute flame front velocity Vpp and the induced flow velocity in the fresh gas v g can be calculated by a simple mass balance supposing ideal gas behavior to be... 

The first experiments were performed in the empty explosion channel using different methane-air-mixtures. The dis-tance/time function of the flame front was measured by 13 ionization probes in the flame path. In the empty channel, this function was nearly linear, indicating an approximately constant propagation velocity of the flame front. This velocity is, however, much higher than that of a planar laminar flame front, the difference being due to a strongly... 

Most real flames are subjected to local surface hydrodynamic deformation including the one that causes changing of the local curvature. It affects the velocity of the flame front propagation (further referred to as flame velocity ) and it is called a stretch or a stretch effect. 

Deflagration Flame front propagation at relatively lower subsonic velocity than the sound speed in the reactants ahead of it (but not necessarily). 

Overdriven detonation is the condition that exists during a DDT before a state of stable detonation is reached. Transition occurs over the length of a few pipe diameters and propagation velocities np to 2000 m/s have been measnred for hydrocarbons in air. This is greater than the speed of sonnd as measnred at the flame front. Overdriven detonations are typically accompanied by side-on pressnre ratios (at the pipe wall) in the range of 50-100. A severe test for detonation flame arresters is to adjust the mn-np distance so that DDT occurs at the arrester, subjecting it to the overdriven detonation impulse. 

The cloud is consumed by a turbulent flame front which propagates at a velocity which is roughly proportional to ambient wind speed. 

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. 

Global velocity distribution behind flame front. Upward propagation in 5.15% methane/air mixture, (a) vector map, (b) and (c) scalar maps of axial and radial velocity components, respectively. Spots are caused by condensation of water vapor on the glass walls. 

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