Opposed Flame Spread

There are three independent sources of heat that will lead to the spread of a flame and that will define the boundary condition  

The external heat transfer can be independently controlled, and hence, will not be addressed here. Heat transfer through the solid is given by the following expression  

Heat transfer through the gas phase has been observed to be mainly due to conduction therefore, radiation is generally neglected. The following expression gives an estimate of the heat flux from the flame to the surface and how that heat is transferred inward into the material  

Estimation of the relative importance of both these parameters was presented in [15], It can be seen that for thin fuels, the heat transferred through the gas is dominant, while for thick materials, it is the opposite. 

The abovementioned results correspond to Poly (methyl) Methacrylate (PMMA) under downward burning conditions, and are not general to all the materials or configurations. In most cases, heat transfer through the gas phase will be dominant for most conditions, mainly because the fire grows as the contribution from the flame radiation increases, enhancing the relative importance of the gas-phase heat transfer. A number of papers summarizing different results can be found in the work by Fernandez-Pello and Hirano [15], 

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Flame spread in gas flows moving in the opposite direction of the flame propagation (opposed flame spread)... 

It is important to note that the physical parameters controlling opposed flame spread are the same as those controlling ignition, with the added parameter associated with the flame heat flux. Depending on the approach to be followed, the flame input can be either defined by the flame temperature,... 

The analysis of co-current flame spread is very similar to that of opposed flame spread. However, it is further complicated because the flame covers the fuel thus, the flame length is a further parameter that needs to be analyzed. The flame length can be represented empirically as being proportional to the pyrolysis length or can be calculated using boundary layer theory and the assumption of infinitely fast gas-phase chemistry [22], Despite the added complexity, co-current flame spread is controlled by the same physical parameters as ignition or opposed flame spread. 

Solutions of these fire retardant formulations are impregnated into wood under fliU cell pressure treatment to obtain dry chemical retentions of 65 to 95 kg/m this type of treatment greatly reduces flame-spread and afterglow. These effects are the result of changed thermal decomposition reactions that favor production of carbon dioxide and water (vapor) as opposed to more flammable components (55). Char oxidation (glowing or smoldering) is also inhibited. 

I. Wichman, Theory of opposed-flow flame spread, Prog. Energy Combust. Sci. 18 553-593,1992. 

Harkelroad, M., Quintiere, J., Walton, W., "Radiative Ignition and Opposed Flow Flame Spread Measurements on Materials", Report No. DOT/FAA-CT-83/28, FAA Technical Center, Atlantic City Airport, N.J., 1983... 

Figure 8.2 Dynamics of opposed flow surface flame spread...

Since accidental fire spread mostly occurs under natural convection conditions within buildings and enclosures, some examples of configurations leading to opposed or wind-aided types of spread are illustrated in Figure 8.3. Flame spread calculations are difficult... 

This follows by a steady state energy balance of the surface heated by qe, outside the flame-heated region S. It appears that a critical temperature exists for flame spread in both wind-aided and opposed flow modes for thin and thick materials. Tstmn has not been shown to be a unique material property, but it appears to be constant for a given spread mode at least. Transient and chemical effects appear to be the cause of this flame spread limit exhibited by 7 smln. For example, at a slow enough speed, vp, the time for the pyrolysis may be slower than the effective burning time ... 

Thus, for opposed flow spread, the steady state thermal flame spread model appears valid. In wind-aided flame spread, it seems appropriate to modify our governing equation for the thermally thin case as... 

Table 8.1 Opposed flow properties for lateral flame spread on a vertical surface [16]...

Figure 8.12 (a) Flame front movement for a wood particle board under opposed flow spread in... 

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])...

Femandez-Pello, A.C., Ray, S.R. and Glassman, I., Flame spread in an opposed forced flow the effect of ambient oxygen concentration, Proc. Comb. Inst., 1980, 18, pp. 579-89. 

Opposed flow flame spread, representing spread on a horizontal surface, e.g. floor, large chair, mattress, etc. 

Extensive studies have been conducted both analytically and experimentally to study the opposed and concurrent flame spread [14,15], One frequently used approach is to isolate the effects... 

FIGURE 3.6 Schematic of opposed flow flame spread. 

There are many mechanisms involved in the propagation of a flame through a condensed fuel [13], The problem is extremely complicated therefore, the simplest case will be used here, namely, the opposed-flow flame spread. The tip of the flame controls the opposed-flow flame spread and all the transport mechanisms involved are presented in Figure 3.8. 

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] ... 

Flames can spread over a solid surface in two modes. In the wind-aided flame spread mode, flames spread in the same direction as the surrounding airflow. The second mode is referred to as opposed-flow flame spread, which occurs when flames spread in the opposite direction of the surrounding airflow. These two modes are illustrated for flame spread over a flat surface in Figure 14.2. 

The rate of opposed-flow flame spread can be determined from the following equation, derived theoretically by deRis.61... 

It is of interest to know under what conditions opposed-flow flame spread ceases. Quintiere and Harkleroad proposed the minimum surface temperature for spread, Ts min, as a convenient criterion.38... 

By far the most common and most practical approach to measure the rate of flame spread over a flat surface involves recording the location of the flame tip (wind-aided spread) or flame front (opposed-flow spread) as a function of time based on visual observations. However, in the case of wind-aided flame spread, it is very difficult to track propagation of the pyrolysis front (boundary between the pyrolyzing and nonpyrolyzing fuel) as it is hidden by the flame. This problem can be solved by attaching fine thermocouples to the surface at specified locations as ignition results in an abrupt rise of the surface temperature. This approach is very tedious and not suitable for routine use. An infrared video camera has been used to look through the flame and monitor the upward advancement of the pyrolysis front in a corner fire.62... 

The LIFT apparatus can be used for obtaining opposed-flow flame spread properties. The apparatus is briefly discussed in Section 14.3.2.3.2 and is described in detail in ASTM E 1321. A schematic is shown in Figure 14.7. 

The main objective of conducting LIFT flame spread experiments is to obtain material properties for predicting opposed-flow flame spread. On the basis of the analysis by deRis (see Equation 14.8), the flame heating parameter can be determined from the slope of a linear fit of l/yjvp(y) plotted as a function of qe (y), where y is the distance from the hot end of the specimen. The minimum surface temperature for spread, Tsmin, is calculated according to an expression akin to Equation 14.4 from the incident heat flux at the maximum distance from the hot end reached by the flame. 

The test method described in ASTM E 162 also evaluates opposed-flow flame spread characteristics of a product, and is referred to in regulations that pertain to various modes of transportation. 

Flame spread (opposed flow and wind-aided flame spread, fire growth rate, etc.)... 

Atreya, A. and Baum, H.R. Model of opposed-flow flame spread over charring materials. In Chen, J.H. and Colket, M.D. (eds.). Proceedings of the 29th Symposium (International) on Combustion, Volume 29, July 21-25, Sapporo, Japan. Pittsburgh, PA Combustion Institute, 2002, Part 1, pp. 227-236. 

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