Full-scale fire modeling

The combustion reaction rate is controlled both by the availability of fuel and oxygen kinetic effects (temperature). In full-scale fire modeling, the resolvable length and time scales are usually much larger than those associated with the scales of the chemical combustion reaction, and it is common to assume that the reactions are infinitely fast. The local reaction rate depends on the rate at which oxygen and fuel are transported toward the surface of stoichiometric mixture fraction, shown in Figure 20.2 as a point where both oxygen and fuel mass fractions go to zero. For almost 20 years, the EBU or eddy dissipation models were the standard models used by the combustion CFD community. With the EBU, in its simplest form, the local rate of fuel consumption is calculated as [3] ... 

The Arrhenius form of the reaction results from the Maxwell speed distribution and the rate at which molecular bonds in gas-phase species are broken [44], In full-scale fire modeling, the finite reaction rates must be considered if one attempts to model things such as CO and soot production and oxidation, or ignition and extinction. However, then the simple mixture fraction formulation must be supplemented by additional variables keeping track of the reaction progress. 

For the small scale fire test methods it was possible to determine the mass of the sample burnt. In the full scale fire test this could not be done. To make gas emissions comparable between the fire models, the emissions of gases in the small scale fire tests have been reduced by the amount of material burnt in each case. 

A comparison of results for fire effluents from full scale and small scale fire tests has to be done in steps. A full scale fire is a developing event where temperature and major constitutions changes continously. A small scale fire test either take one instant of that developing stage and try model that or try to model the development in a smaller scale. On a priority one level rate of heat release, temperature, oxygen concentrations and the ratio of C02/C0 concentrations have to be similar for a comparison. The full scale fire experiments reaches a temperature of 900 C at the moment of flashover, while the small scale fire tests are reaching temperatures just above 400 °C for NT-FIRE 004 and the cone experiments. For the DIN 53436-method the temperature was set to 400 °C. 

This study demonstrate similar ranking of each material independent of test method used. At this stage it is premature to choose one test as a better small scale model of full scale fires. Each method needs further elaboration. 

Azone model calculatesthe fire environment by dividing each compartment in the model into two homogeneous zones. One zone is an upper hot smoke zone that contains the fire products. The other zone is a lower, relatively smoke-free zone that is cooler than the hot zone. The vertical relationship between the zones changes as the fire develops, usually via expansion of the upper zone. The zone approach evolved from observations of such layers in full-scale fire experiments. While these experiments show some variation in conditions within the zones, the variations are most often small compared to the difference between the zones themselves. 

In full-scale Are modeling, a diffusion flame structure is usually assumed. However, in many fire situations, such as underventilated fires, premixed or partially premixed flame theory may be more appropriate. The Burke-Schumann description of the diffusion flames can be used to conveniently represent the transport of gaseous species by a single scalar quantity called mixture fraction. For a simple one-step reaction ... 

Toxic potency values are most often assessed from the most suitable small-scale smoke toxicity test (NIST radiant test, using rats as the animal model, but only for confirmatory purposes, standardized in ASTM E 1678192 and NFPA 269193). The results from this test have been well validated with regard to toxicity in full-scale fires. However, such validation cannot be done to a better approximation than a factor of 3. This is illustrated by the fact that the range of the toxic potency of the smoke of almost all materials is so small that it pales in comparison with the ranges of toxic potencies of... 

This section provides some additional information about a few of the field scale experiments introduced in Section 8.5.1 under Modeling Methodologies Full Scale Fire Tests. 

Full-scale fire tests (model tests)... 

Laboratory tests often fail to give a true picture of the behaviour of plastics in a natural fire. This is the reason for conducting full-scale fire tests or so-called model tests for full-scale fires. 

Several modelled full-scale fire tests and their conclusions are now discussed, arranged according to the structures tested. 

The optical instrumentation is intended to correspond to the sensitivity of the natural human vision, and the correlations between the subjective and objective sensation are supported by model experiments in several countries. Detailed properties, indoor distribution and the flow conditions of the smoke are also evaluated in full-scale fire tests. 

Due to the data missing for the phenomenological models and the fact that these models are mostly not verified, the empirical and semi-empirical models are probably the best for the rapid practical applications required in risk assessments. Information in Tables 1 and 2 are data based on full scale fire tests carried out from approximately 1990 to present. Ref. 2 also presents data for other five fire types, but only the data summarised in Table 1 are used in this Paper as they show together with the data in Table 2 the widest range. 

Full scale tests are particularly valuable to obtain information on fire hazard. They can be used to validate small scale tests, and to validate mathematical fire models. The most important additional dimension full scale tests add are effects, e.g. radiation from the fire itself, which are difficult to simulate in a smaller scale. Full scale tests are very expensive and time consuming. It is essential, thus, to design them in such a way as to (a) make them most relevant (b) minimize their number and (c)... 

When it comes to the job itself, experts in the field are constantly at work developing new methods to prevent and control fires. They are coming up with chemical solutions to quench fires and computerized models that simulate and solve fire-related problems. They are also perfecting devices such as smoke detectors and indoor sprinkler systems, which are widely used and can help to avoid full-scale destruction by fire. 

There is an increased use of flammability tests, which measure fundamental properties as opposed to tests that simulate a specific fire scenario. The former can be used in conjunction with mathematical models to predict the performance of a material in a range of fire scenarios. This approach has become feasible due to the significant progress that has been made in the past few decades in our understanding of the physics and chemistry of fire, mathematical modeling of fire phenomena and measurement techniques. However, there will always be materials that exhibit a behavior that cannot be captured in bench-scale tests and computer models. The fire performance of those materials can only be determined in full-scale tests. 

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