ХИМИЧЕСКАЯ ФИЗИКА, 2004, том 23, № 9, с. 22-27
ГОРЕНИЕ И ВЗРЫВ
УДК 541.126
CHARACTERISTICS OF SPREADING FLAME EDGES
© 2004 T. Hirano
National Research Institute of Fire and Disaster Received 01.10.2003
The characteristics of spreading flame edges have been investigated to a further extent from a novel point of view. This study deals mainly with the leading flame edges when the unburned material temperature is below its flash temperature, i.e. the flame is a diffusion flame. Behavior of a spreading diffusion flame strongly depends on the flow field near the leading flame edge, where main heat transfer to unburned part occurs. The reading flame edge is stable for stable flame spread. Although the leading flame edge cannot remain as a straight line, when the velocity of the ambient air-stream increases to that above a stable spread limit, it is stable in the changed shape if it continues spreading. It is pointed out that the characteristics of the leading edge of a spreading diffusion flame are far different from those of a premixed flame and a diffusion flame can move with the movement of pyrolysis region if its velocity is below 30 cm/s. Behavior of the leading flame edge in a concurrent gas-stream is not steady and our understanding about it is not enough, although it is practically important.
INTRODUCTION
Various studies on the mechanisms of flame spread across condensed-phase combustibles have been performed because in real fires flame spread is the main cause to make those serious [1-12]. When the temperature of a condensed combustible is below its flash temperature, there is no layer of flammable mixture ahead of the flame. In this case heat transfer from the leading flame edge to the unburned material ahead of it is necessary [1, 6-8, 12] and the flame is basically a diffusion flame. On the other hand, in the case when the temperature of a condensed combustible is above its flash temperature, a layer of flammable mixture is established and a premixed flame propagates through the layer [35, 8, 12]. The characteristics of the flames in these cases are quite different.
It can be easily confirmed in previous papers that the characteristics of a spreading flame depend not only on the gasification characteristics of the combustible but also on the ambient gas stream [6, 8, 12]. In an opposed air-stream, the flame-spread is suppressed, while in a concurrent flow, the flame-spread is enhanced.
For understanding the characteristics of the spreading flames, knowledge on behavior of those under various conditions is useful. The leading flame edge is a representative part of an individual spreading flame, so that for accumulating the knowledge on spreading flames, observation of leading flame edges should be a main purpose in the studies on the flame spread mechanisms. In only limited number of papers, however, observation of leading flame edges has been performed [13-21].
The objective of this study is to elucidate the characteristics of spreading flame edges to a further extent from a novel point of view. To avoid dispersion, this study deals mainly with characteristics of leading flame
edge spreading over a combustible of sub-flash temperature.
ASPECTS OF LEADING FLAME EDGES
In general, the mode of flame spread depends on the variables influencing heat transfer from reaction zone to unburned part of the combustible, across which the flame spreads. The ambient air-stream is a typical variable of those. Figure 1 shows variation of the mean py-rolysis spread rate over a thin combustible solid with downward ambient air-stream velocity [16]. In this
Free-stream velocity, cm/s
Fig. 1. Variation of mean pyrolysis spread rate with downward air-stream velocity [12, 16].
3 cm
_I
Scale
Fig. 2. Flow field change near the leading flame edge under conditions of Region II in Fig. 1 [12, 16]. Ambiert air-stream velocity: 120 cm/s; Particle track: 360 interruptions/s; Frame speed: 1 frame/s.
case, the flow field near the leading flame edge, where main heat transfer occurs, is under influence of ambient air-stream and gravity. Based on the observation, the types of flame spread are divided into three regions characterized by ranges of the air stream velocities. In Region I, representing the range of air-stream velocities from 0 to 85 cm/s, flame spread is accelerative, although the acceleration decreases with the increase of air-stream velocity. In this range, the flow field near the leading flame edge is basically similar to that under natural convection. In Region II, representing the range of air-stream velocities from 85 to 125 cm/s, a local flame spread rate fluctuates greatly, and flame spread is of intermediate characteristics between those of accelera-tive flame spread and steady flame spread. In Region III, representing the range of air-stream velocities from 125 to 190 cm/s, flame spread is almost steady and the behavior of the leading flame edge resembles that spreading stably in a downward air-stream.
As mentioned in the previous section, knowledge on the aspects of leading flame edge is useful for understanding the flame-spread mechanisms. It would be easily supposed that behavior of the leading flame edge at an air-stream velocity in Region II would be complicated. Figure 2 shows the flow field change near the leading flame edge when the ambient air-stream velocity is 120 cm/s. It is seen that acceleration, deceleration, and vortexes appear and the behavior of leading flame edge depends on the flow field. This variation in the flow field causes the fluctuation of the distance from solid surface to leading flame edge, on which the amount of heat transfer would depend. Further, the gasification process is under influence of the heat transfer fluctuation, so that the combustion field, which depends on the combustible gas and oxygen supply, necessarily fluctuates. For understanding the flame-spread mechanisms in this range, we should make clear whole
processes mentioned above. The behavior of the leading flame edge is a representative aspect of the whole process and easy to observe.
Figure 3 shows the variation of flame propagation velocity with ambient air-stream velocity foe various values of initial methanol temperature [17]. It is seen that the flame propagation velocity depends both on the initial methanol temperature and the ambient air-stream velocity. It would be easily acceptable that the flame propagation velocity decreases with the increase of the opposed air-stream velocity. One would need a reasonable explanation, however, to accept that a flame is able to propagate through a flammable layer even when the air-stream velocity is larger than the flame propagation velocity in quiescent air. This behavior of the leading flame edge can be understood by considering the deceleration of gas in front of the leading flame edge [17].
In other cases of flame spread, the behavior of leading flame edge has been observed and close observation has led us to understand the mechanisms of flame spread [13-21].
STABILITY
In the case of flame spread in an opposed flow, heat transfers from the reaction zone to the unburned material ahead of the flame and increases the temperature to gasify the material. The gasified material and oxygen in the ambient air diffuse to the reaction zone and sustain the combustion. Major part of the heat needed for the flame spread comes from the leading flame edge, which is close to the material surface. For stable flame spread, the heat transfer sustaining the spread should be stable, so that the leading flame edge is stable for stable flame spread.
For downward flame spread along a thin combustible solid in an upward ambient air-stream under exter-
Vf, cm/s 600
-600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600
U, cm/s
Fig. 3. Variation of flame propagation velocity Vf with ambient air-stream velocity U for various values of initial methanol temperature Ti [8, 17].
Fig. 4. Record of burning paper configurations for every 6 s when the ambient air-stream velocity is 80 cm/s [15]. Thick and thin csolid lines indicate the configurations of the leading flame edges and blow off lines, respectively. Numbers above lines indicate seconds after the first photography of the burning paper was taken just after ignition.
nal radiation heating, the flame is stable for a certain range of the air-stream velocities [14, 15]. In this range of air-stream velocities, the flow field ahead of the leading flame edge changes only slightly. For a higher air-stream velocity, the approaching flow is decelerated. This approaching flow deceleration seems to make the leading flame edge stable [14].
When the upward air-stream velocity is beyond the limit of stable flame spread, the shape of the leading flame edge cannot remain to be horizontal. A typical record of burning paper configuration is shown in Fig. 4 [15]. In this case, the ambient air-stream velocity is 80 cm/s, which is just above the stable flame spread limit. A local blow off is observed just after ignition. The remaining burning-zones spread downward. A series of local blow offs are found to occur during the spread. Most leading flame edges are inclined or curved, and blow offs occur mainly at the locations where both inclined angle of the leading flame edge from the horizontal surface and curvature of the leading flame edge are small. This change in the shape influences the velocity field ahead of the leading flame edge. At the part ahead of the inclined leading flame edge, the profiles of the velocity component normal to the leading flame edge are similar to those in the case when a horizontal burning-zone spreads stably. Also, near the burning zone with a curved leading flame edge, the gas stream is decelerated due to stream tube expansion as it approaches the leading flame edge [15].
LIMIT OF DIFFUSION FLAME SPREAD
When the temperature of a condensed-phase combustible is below the flash temperature, the flammable compo
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