Flame spread velocity

Equation 3.32 provides an expression for the flame spread velocity based only on the parameters of the problem and on the experimental conditions, but still relies on the presence of an unknown constant that needs to be determined experimentally. 

In a similar manner, the flame spread velocity can be obtained for thermally thin materials ... 

Perrins, L.E. and Pettet, K., Measurements of flame spread velocities, Journal of Fire and Materials, 5, 85, 1974. 

The particular flame-spreading behaviour of Enerfoil has been investigated by Yeh et al. [10]. They found that for two aligned films, the maximum flame spread velocity would be obtained with a gap distance of 100 pm (Figure 16.5). Surprisingly, the flame-spread velocity drops with increasing pressure (Figure 16.6). 

Figure 16.5 Flame spreading velocity as a function of gap width in different atmospheres [10].

S has been approximated for flames stabili2ed by a steady uniform flow of unbumed gas from porous metal diaphragms or other flow straighteners. However, in practice, S is usually determined less directly from the speed and area of transient flames in tubes, closed vessels, soap bubbles blown with the mixture, and, most commonly, from the shape of steady Bunsen burner flames. The observed speed of a transient flame usually differs markedly from S. For example, it can be calculated that a flame spreads from a central ignition point in an unconfined explosive mixture such as a soap bubble at a speed of (p /in which the density ratio across the flame is typically 5—10. Usually, the expansion of the burning gas imparts a considerable velocity to the unbumed mixture, and the observed speed will be the sum of this velocity and S. ... 

In any gas burner some mechanism or device (flame holder or pilot) must be provided to stabilize the flame against the flow of the unbumed mixture. This device should fix the position of the flame at the burner port. Although gas burners vary greatly in form and complexity, the distribution mechanisms in most cases are fundamentally the same. By keeping the linear velocity of a small fraction of the mixture flow equal to or less than the burning velocity, a steady flame is formed. From this pilot flame, the main flame spreads to consume the main gas flow at a much higher velocity. The area of the steady flame is related to the volumetric flow rate of the mixture by equation 18 (81,82)... 

Increasing the surface area of a combustible solid enhances the ease of ignition. Hence dust burns more rapidly than the corresponding bulk solid combustion of dust layers can result in rapid flame spread by train firing . Solid particles less than about 10 pm in diameter settle slowly in air and comprise float dust (see p. 51 for settling velocities). Such particles behave, in some ways, similarly to gas and, if the solid is combustible, a flammable dust-air mixture can form within certain limits. Larger particles also take part, since there is a distribution of particle sizes, and ignition can result in a dust explosion. 

Determination of the flame spread parameter, (fc. The following well known expression has been given (( ), (6), etc) for the velocity of the flame front for a slab initially at the temperature T = Tg... 

Figure 8.9 Control volume energy conservation for a thermally thick solid with flame spread steady velocity, Up...

Figure 8.16 Flame spread rate over thick PMMA sheets as a function of the opposed forced flow velocity for several flow oxygen mass fractions (Femandez-Pello, Ray and Glassman [6])...

Calculate the upward spread velocity at 0.5 m from the floor. The flame height from the floor is 1.8 m and the heat flux from the flame is estimated at 3 W/cm2. 

Early theoretical treatments of bluff-body stabilized flame spreading have been based, in general, on the assumption that the flame is a discontinuous surface separating gas streams of different densities and temperatures [1, 15-17]. These theories neglect the finite thickness of turbulent flame zone and predict the increase of the spreading rate both with the density ratio across the flame, and with the increase in the laminar flame velocity of fuel-air mixture. This does not correspond to experimental observations (e.g., [8, 10]). 

Smolder propagation is generally treated as a flame spread problem, thus, a similar thermal analysis to the one presented in Section 3.5.5.1 is conducted for both opposed and concurrent smoldering. Many expressions for a smoldering propagation velocity can be found in the literature. Here, we will use only the one presented by Torero et al. for illustration [27] ... 

From Eqs. (2.11) and (2.12) it follows that the flame spread rate in the first case is inversely proportional to the material thickness and is independent of the incoming oxidant flow velocity. In the second case it is proportional to v, and does not depend on the material thickness. The functional relationship between v and the initial material temperature T is also different. Preheating of the material reduces AT = T — T, thereby promoting flame spread. 

The Steiner Tunnel test (ASTM E 84) is used to classify the fire-spread potential of products used in wall and ceiling linings [4], and is used to classify expanded polystyrene foam. In this method, specimens are placed on the ceiling of a 24 ft long tunnel. An 88 kW natural gas burner is placed at one end of the tunnel and a forced-air draft with a velocity of 1.22 m/s is introduced. The flame spread is recorded as a function of time and an arbitrary index is calculated from the measurements. 

Near the point where the two streams first meet the chemical reaction rate is small and a self-similar frozen-flow solution for Yp applies. This frozen solution has been used as the first term in a series expansion [62] or as the first approximation in an iterative approach [64]. An integral method also has been developed [62], in which ordinary differential equations are solved for the streamwise evolution of parameters that characterize profile shapes. The problem also is well suited for application of activation-energy asymptotics, as may be seen by analogy with [65]. The boundary-layer approximation fails in the downstream region of flame spreading unless the burning velocity is small compared with u it may also fail near the point where the temperature bulge develops because of the rapid onset of heat release there,... 

This equation may be employed to calculate a spread velocity if q, p, and Ah can be estimated. The concept of a critical temperature 7 for ignition of the fuel in the presence of a flame may be introduced to provide an approximate expression for Ah, namely. 

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