Flame spreading speed

Figure 8.4 displays a thermally thin solid of thickness d, insulated at its back face and undergoing surface flame spread. Any of the modes shown in Figure 8.1 are applicable. We consider the case of steady flame spread, but this constraint can be relaxed. The surface flame spread speed is defined as... 

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. ... 

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 ... 

From Figure 8.8, Vf > vp, so cbcf/dxp > 1. Hence, transient effects will cause a lower speed for wind-aided spread. Again, since the gas phase response time is much faster than the solid, we can use steady gas phase results for qf and <5f in these formulas for flame spread on surfaces. 

Flame Spread The speed at which a flame will cross the surface of a material, influenced by the physical form of the fuel, air supply, the moisture content of the fuel, specific gravity, size and form, the rate and period of heating, and die characteristics/nature of the heat source. A higher flame-spread critically affects the severity of the fire in a given period of time. 

When a flame traverses a pre-formed combustible mixture which is at rest in a horizontal tube, a zone of intense reaction moves along the tube with a definite velocity. If the gas mixture is streamed through the tube at an appropriate rate, the flame front itself may be maintained stationary at the mouth. In either case the relative velocity of flame front and gas mixture is the same. If the speed of the stream is less than the rate of travel of the combustion zone, the flame performs the operation known as striking back. If, on the other hand, the gas stream emerges from the tube faster than the combustion is propagated, the flame spreads forth from the mouth... 

Jo is the initial temperature of the gaseous mixture T/ is the temperature at the beginning of oxidation T/is the temperature at the beginning of the high-speed oxidation Tt is the temperature of combustion dp is the thermal preparatory zone dj is the thermal width of the flame front d is the chemical width of the flame front is the normal flame spread rate... 

Does the flame spread lead to fire propagation and, if so, at what speed ... 

These tests along with experience from fatal fires demonstrate that it is absolutely essential to restrict the speed of fire along internal surfaces. The degree to which flame spreads across the surface of a lining material is classified by a physical test which is detailed in British Standard BS 476 Part 1. 

A massive amount of propane is instantaneously released in an open field. The cloud assumes a flat, circular shape as it spreads. When the internal fuel concentration in the cloud is about 10% by volume, the cloud s dimensions are approximately 1 m deep and 100 m in diameter. Then the cloud reaches an ignition source at its edge. Because turbulence-inducing effects are absent in this situation, blast effects are not anticipated. Therefore, thermal radiation and direct flame contact are the only hazardous effects encountered. Wind speed is 2 m/s. Relative humidity is 50%. Compute the incident heat flux as a function of time through a vertical surface at 100 m distance from the center of the cloud. 

In Chapter 4 we examined the spread rate of a premixed flame and found that its speed, Su, depended on the rate of chemical energy release, m Ahc. Indeed, for a laminar flame, the idealized flame speed... 

Equation (8.6) demonstrates that as the face weight, pd, decreases the spread rate increases. Moreover, if a material undergoing spread is heated far away from the flame, such as would happen from smoke radiation in a room fire, Ts will increase over time. As Ts - Tig, an asymptotic infinite speed is suggested. This cannot physically happen. Instead, the surface temperature will reach a pyrolysis temperature sufficient to cause fuel vapor at the lower flammable concentration. Then the speed of the visible flame along the surface will equal the premixed speed. This speed in the gas phase starts at about 0.5 m/s... 

After ignition of the edge, the external heat flux is removed and it is no longer felt by the wood, in any way. A researcher measures the heat flux of the flame, ahead of the burning wood, as it spreads steadily and horizontally along the edge. That measurement is depicted in the plot below. Compute the flame speed on the wood, based on this measurement. 

A part of the subsample was spread by hand from the plastic bag into the No. 50 mesh basket, 29.5 cm wide by 10 cm high by 90.8 cm long. The basket was weighed and placed on the trolley. The fuel bed was lit by first dosing the leading edge of the bed with 40 cm of the methylated alcohol. The trolley was allowed to remain stationary for thirty seconds and the flame front usually took three minutes to reach steady state. The speed of the trolley was then controlled to keep the flame in a stationary position over the duct. A constant burning rate was usually maintained for 60 cm of travel. 

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