Fully developed fires

One aspect of fully developed fires that will not be addressed here is their production of combustion products. When compartment fires become ventilation-limited, they bum incompletely, and can spread incomplete products such as CO, soot and other hydrocarbons throughout the building. It is well established that the yield of these incomplete products goes up as the equivalence ratio approaches and exceeds 1. More information on this issue can be found in the literature [1], 

Fully developed fire studies have been performed over a range of fuel loadings and ventilation conditions, but primarily at scales smaller than for normal rooms. Also the fuels have been idealized as wood cribs or liquid or plastic pool fires. The results have not been fully generalized. The strength of the dimensionless theoretical implication of Equation (11.38) suggests that, for a given fuel, the fully developed, ventilation-limited fire should have dependences as 

From Equation (11.21) and recognizing that the heat flux to the fuel depends on the flame and compartment temperatures, it follows that 

By rearrangement of these functional dependencies, it follows for a given fuel (L) that 

Investigators have developed correlations for experimental data in this form. 

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The FPL vertical wall furnace used in our study was described in some detail by Brenden and Chamberlain (6). This furnace is normally used to evaluate the fire endurance of wall assemblies. The basic guidelines for the furnace test method are given in the ASTM E-119 standard (5). The method was designed to evaluate the ability of a structure to withstand a standard fire exposure that simulates a fully developed fire. The furnace is gas fired, and its temperature is controlled to follow a standard time-temperature curve. A load may be applied to the assembly. The failure criterion can be taken as time at burnthrough, structural failure, or a specified temperature rise on the unexposed side of the wall—whichever comes first. The construction of the furnace is not specified in the ASTM E-119 standard. 

The fully developed fire is affected by (a) the size and shape of the enclosure, (b) the amount, distribution and type of fuel in the enclosure, (c) the amount, distribution and form of ventilation of the enclosure and (d) the form and type of construction materials comprising the roof (or ceiling), walls and floor of the enclosure. The significance of each phase of an enclosure fire depends on the fire safety system component under consideration. For components such as detectors or sprinklers, the fire development phase will have a great influence on the time at which they activate. The fully developed fire and its decay phase are significant for the integrity of the structural elements. 

Since the boundary surface will become soot-covered as the fire moves to a fully developed fire, it might be appropriate to set ew = 1. 

For a fully developed fire, conduction commonly overshadows convection and radiation therefore, a limiting approximation is that h hk, which implies Tw T. This result applies to structural and boundary elements that are insulated, or even to concrete structural elements. This boundary condition is conservative in that it gives the maximum possible compartment temperature. 

Since, for large fully developed fires, eg is near 1 or Tw Tg, then it follows that... 

The total heat losses in a fully developed fire can then be approximated as... 

Thomas, P.H. and Heselden, A.J.M., Fully-developed fires in single compartment, a co-operative research programme of the Conseil International du Batiment, Fire Research Note 923, Joint Fire Research Organization, Borehamwood, UK, 1972. 

For the fully developed fire, various correlations have sought to portray the temperature in these fires in order to predict the impact on structures. Chapter 11 highlights the CIB work on wood cribs and the corresponding correlation by Law [19]. It is instructive... 

When heat fluxes to the lower part of the compartment are high enough to ignite common combustible materials, a rapid transition occurs to a fully developed fire. This transition usually takes less than a minute and is referred to as flashover. When flashover occurs, it is no longer possible to survive in the fire compartment. All exposed combustible materials become involved in the fire (note burning rug and table top in Figure 14.1d). Commonly used criteria for the onset of flashover are a hot smoke layer temperature of 600°C and an incident heat flux at floor level of 20kW/m2. 

Apart from some specific areas such as fuel tanks or intumescent coatings, the performance in a fire test simulating a fully developed fire is often not of real interest with respect to the development of fire-retarded polymers, since common flame-retarded polymeric materials are used in applications... 

Testing codes within the scenario of a fully developed fire are based on intermediate, large, or full-scale testing. Specimens are typically in the dimension of several square meters and often, real components such as building columns are tested, or the whole product in the case of gas bottles. Tests like the small-scale test furnace based on specimens of 500 mm x 500 mm are exceptions. Intensive flame application or the use of furnaces realizing standard time-temperature curves are used to simulate the characteristics of fully developed fires. Thus, in particular the heat impact of convection and the surface temperature are clearly greater than in the tests discussed earlier. The fire properties investigated are often resistance to fire, or the fire or temperature penetration. 

A1 Fully developed fire in a room Single burning item in a room No flashover None... 

Figure 2-26 Fully-Developed Fire with the Savannah River in the Background. Courtesy of David Chung of the U.S. Environmental Protection Agency.

The Naval Ammunition Depot (NAD) at Crane, Indiana has developed improved fire-retardant phenolic foams containing blends of boric/oxalic acids as catalysts, as described above. The resultant foams were found to be extremely efficient fire barriers due to their high heat absorptivity, the amount of carbon and/or coke produced during pyrolysis, and the adhesion of the char to the burned materials. Other advantages of the foam during flaming and nonflaming pyrolysis are its low smoke emissions and lack of toxic fumes other than carbon monoxide. It takes one hour to reach 230°F (110°C) when a 13 Ib/tf (208 kg/m ) phenolic foam specimen 2.9 inches (7.4 cm) is exposed to a fully developed fire (41). 

Fully developed fire. All the combustible materials in the room are burning. In this stage, the amount of combustibles, the evolved heat, the smoke production, and the nature of the combustible gases are crucial. 

Fire propagation. Combustion is initiated also in adjacent systems, by virtue of heat evolved in the fully developed fire. For this reason, the most important factor in fire-spread, besides the flammability and amount of plastics in the system, is the fire endurance of the system boundaries. Evolved heat, produced smoke and combustible gases are also as important as in the other stages. 

There is therefore a need to use plastics parts with a higher level of fire safety meeting the requirements of UL94 Vertical tests so as to ensure adequate consumer protection. External fire sources, inadequate design, manufacturing faults or defects due to simple wear and tear, plus consumer misuse may lead to flashover and fully developed fires in a very short time if housings and backplates are not flame retarded to meet the vertical test criteria. 

Three stages characterize a fire growth from a small origin, full development, and decay. In a fully developed fire, the temperature in a confined space will reach 1,500-2,300 °F. Before or during full fire development, the contents of a room may burst into flames. This phenomenon is called flashover. 

The essential requirements for fire are heat, oxygen and fuel [120], as illustrated in Figure 7.2. Once combustion has occurred, the course of the fire often accelerates rapidly, passing from ignition initiation through fire propagation to fully developed fire and its decay (Figure 7.3) [121]. 

Figure 17.1 Temperature/time fire profile (a) Fire development phase (b) Fully developed fire (c) Receding fire. Source Author s own files)...

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