Under-ventilated Conditions Research Articles

Bench Scale Generation of Smoke Particulates and Hydrocarbons from Burning Polymers

The soot and hydrocarbon generation behaviour from four common polymers has been investigated under different fire conditions using different bench scale apparatuses. While well-ventilated flaming typically has high CO2/CO ratios and high combustion efficiency, in under-ventilated flaming, higher yields of the many products of incomplete combustion are generally observed, including carbon monoxide, soot and hydrocarbons. Hydrocarbon and soot production from polyethylene, polystyrene, polyamide and polyvinyl chloride were investigated using the steady state tube furnace (ISO TS 19700) under well-ventilated, small under-ventilated and large under-ventilated fire conditions; and in the smoke density chamber (ISO 5659-2) under the three standard test conditions; and the results have been compared to published data from the fire propagation apparatus (ASTM E 2058). The steady state tube furnace and the fire propagation apparatus show good agreement, for hydrocarbon and soot yields. Conversely it proved impossible to identify correlations with these tests for the soot and hydrocarbon yields in the smoke density chamber. To investigate the soot generation further, the particle size distribution was determined using a cascade impactor. This showed the predominance of 1-5 ?m particulates for all polymers, except PVC, in under-ventilated conditions, and a smaller yield and a more even particle size distribution for well-ventilated burning. For PVC, no significant difference was observed for different fire conditions, with no clear trend of particle size distribution.

Fire Retardant Effects of Polymer Nanocomposites

Among the many and varied applications of nanotechnology, the dispersion of nanoscopic fillers to form polymer nanocomposites with improved fire behaviour illustrates the potential and diversity of nanoscience. Different polymers decompose in different ways and fire retardants act to inhibit the decomposition or flaming combustion processes. Polymer nanocomposites form barriers between the fuel and air, reducing the rate of burning, but beyond that there is little consistency in their effects. It is shown that the decomposition products of polypropylene are changed by the presence of nanoclay, although there is only a small influence on the mass loss rate. The rheological properties of molten polymer nanocomposites are radically different from those of virgin polymers, and these will profoundly affect the heat transfer through the material, resulting in a shorter time to ignition and lower peak in the heat release rate, typical of polymer nanocomposites. The dispersion of nanofillers within polymers is generally measured in the cold polymer, but since this does not reflect the condition at the time of ignition, it is proposed that temperature ramped rheological measurements are more appropriate indicators of dispersion. The influence of polymer nanocomposite formation on the yields of toxic products from fire is studied using the ISO 19700 steady state tube furnace, and it is found that under early stages of burning more carbon monoxide and organoirritants are formed, but under the more toxic under-ventilated conditions, less toxic products are formed.

Mixture fraction analysis of combustion products in the upper layer of reduced-scale compartment fires

A mixture fraction analysis is performed to investigate the characteristics of chemical species production in smoky compartment fires burning condensed-phase hydrocarbon fuels. A series of fire experiments were conducted in a 2/5 scale compartment based on the ISO-9705 room. Gas species and soot measurements were made at two locations in the upper layer of the compartment. The fuels considered, heptane, toluene, and polystyrene, generate highly smoky fires over a range of natural ventilation conditions. The mass fractions of measured chemical species, such as unburned hydrocarbons, carbon monoxide, carbon dioxide, oxygen, and soot are presented as a function of mixture fraction and compared with state relationships based on the idealized reactions of a hydrocarbon fuel. The results show that plotting the local composition as a function of the mixture fraction adequately collapses hundreds of species measurements from an assortment of compartment conditions with varying heat release rates and spatial locations into a few coherent lines or bands. It is also shown that about half (or more) of the fuel carbon may exist in the form of carbonaceous soot in the upper layer of smoky compartment fires. Inclusion of soot in the mixture fraction analysis allows identification of fuel-rich or under-ventilated conditions.

The Use of CFD Calculations to Evaluate Fire-Fighting Tactics in a Possible Backdraft Situation

This paper is an attempt to integrate theoretical Computational Fluid Dynamics (CFD) calculations with practical fire-fighting tactics commonly used when arriving at the scene of an underventilated fire. The paper shows that CFD has a great potential in improving understanding and creating better effectiveness in the estimation of fire-fighting tactics. If burning has occurred in a lack of oxygen for a long time, excessive pyrolysis products may have accumulated in the fire compartment. If air is suddenly introduced in the compartment a backdraft may occur. The CFD code used for the simulations is fire dynamics simulator (FDS). In this paper, we focus on the conditions that can lead to backdraft, and not the deflagration or rapid combustion in itself. Therefore, the simulations focus on the gravity current and the mixing process between cold fresh air and hot smoke gases by considering a uniform temperature inside the building as initial condition. The different tactics studied include natural ventilation, positive pressure ventilation (PPV) and dilution by water mist. Their effectiveness is observed comparing them with a reference scenario, where no action is taken. The main objective of natural ventilation is to find the fire source, and the venting is more effective with several openings. Tactics involving PPV are very effective in evacuating the unburnt gases, but increases the mixing, and consequently the probability of backdraft during the early stage of operation. On the other hand, the addition of water mist can reduce the danger of backdraft by reducing the concentration of unreacted combustible gases below the critical fuel volume fraction (CFVF), where ignition cannot occur. If the dilution level is insufficient the danger of backdraft is increased, mainly because the process of gases evacuation is longer due to cooling, which reduces the density difference between hot and cold gases. During a fire-fighting operation, the choice of tactic depends mainly on whether there are people left in the building or not, but also on the fire-fighters’ knowledge of the building’s geometry and the fire conditions. If the situation shows signs of strongly underventilated conditions, the danger of backdraft has to be considered and the most appropriate mitigation tactics must be applied.

Carbon monoxide and smoke production downstream of a compartment for underventilated fires

The carbon monoxide, carbon dioxide and smoke yields measured downstream of a small scale compartment (volume 0.125m3 – 0.375m3) were studied for underventilated fires of propane. The flow rate of propane was increased gradually and species were collected under a hood. The heat release rate (HRR) of the combustion products was also measured (by oxygen depletion) and was found to initially increase as the flow rate of propane was increased (overventilated burning inside the compartment). Before external burning started, an intermediate plateau in the measured HRR was observed, corresponding to the Heat Release Rate =1500AH1/2 , where A and H are area and height of the opening respectively. The same behaviour was observed for all openings and remarkably all compartment geometries employed in this work. Further experiments indicated that the occurrence and extent of that plateau depends on the temperature inside the compartment and thus the growth rate of fire. Species production during this plateau period was investigated, as combustion was underventilated during this phase. It was observed that carbon monoxide (CO) and smoke yields increased during this period. The CO yield increased by a factor of 5, whereas the smoke yield by a factor of 3. Moreover, comparison and differences are discussed between the values of the ratio of carbon monoxide to smoke yield from our study and from the literature. The present results for the increased amounts of carbon monoxide and smoke are applicable if, during the fire growth, underventilated conditions develop without external burning. Current engineering calculations for smoke and carbon monoxide ca not predict the high concentrations of carbon monoxide and smoke measured in such a scenario. Whether these conditions can be developed will depend on whether the gas temperatures at the opening of the enclosure are able to ignite the unburned gases issuing from the enclosure. This in turn, depends on the fire growth rate, i.e. for a fast increase in the fuel supply rate the gas temperatures in the enclosure are lower than for a slower increase in the fuel supply rate due to (transient) heat losses to the walls of the enclosure and as result outside burning starts much later.

Fire tests to validate fire simulation codes: numerical results

Abstract Fire simulations have gained more and more significance, particularly in the context of deterministic as well as probabilistic fire safety analyses for operating nuclear power plants (NPP). An “International Collaborative Project to Evaluate Fire Models for Nuclear Power Plant Applications (ICFMP)” has been started in 1999, to make use of the experience and knowledge of a variety of expert institutions in this field worldwide to assess and improve, if necessary, the state-of-the-art with respect to modelling fires for application to nuclear installations/plants. In the frame of ICFMP two fuel fire experiments have been selected for the Benchmark Exercise No. 4 and four cable tray fire tests for the Benchmark Exercise No. 5, all of these performed in the fire compartment OSKAR of the iBMB (Institut für Baustoffe, Massivbau und Brandschutz) in Braunschweig. In this paper an overview of the calculated results will be given. The simulations of the two large fuel pool fire experiments still indicate difficulties to simulate under-ventilated conditions. The results provide a first indication on the uncertainties of such simulations, although it has to be considered, that the fire load was quite large compared to the compartment volume. The ignition of cables trays as well as the flame propagation on these trays could still not be simulated in an adequate way by the fire codes applied. Different boundary conditions such as pre-heating or the use of more complex cable insulation materials make the situation even more complex.

Experimental and numerical study of transient compartment fires

This paper reports results of experiments and numerical modeling for fires in small-and medium-scale enclosures in underventilated conditions resulting in flame exhaust to the atmosphere. Delays from ignition to the occurrence of a stable flame outside the enclosure were measured experimentally for three types of fuels (methane, propane, and propylene) with a specified fuel supply rate. Characteristic scales dependent on fuel supply rate, fuel source location, compartment geometry, and vent opening parameters are introduced and used to establish a unified nondimensional dependence of the flame exhaust time on fuel flow rate. For the smallest and largest enclosures used in the experiments, three-dimensional numerical calculations of the dynamics of fire development are performed by large eddy simulation. Calculated reaction rate isosurfaces are presented, which demonstrate the compartment fire evolution and its oscillations after exhaust into the atmosphere. It is shown that the calculation results agree qualitatively with experimental observations and that the flame exhaust times are in good agreement with the experimental correlation.

COMPUTATIONAL STUDY ON TOXIC GASES RELEASED FROM COMPARTMENT FIRES SUPPRESSED WITH HALOGENATED AGENTS

The evolution of toxic gases in the hot layer of an enclosure fire is simulated by a batch perfectly stirred reactor model using the SENKIN program from the CHEMKIN distribution package. The initial species composition was obtained from experimental data both for slightly underventilated and highly underventilated conditions. The formation rates of toxic gases were studied using a comprehensive kinetic mechanism, including the GRI 3.0 submechanism representing hydrocarbon oxidation, the National Institute of Standards and Technology (NIST) hydrofluorocarbon mechanism, and the NIST CBrF3 and CF3I inhibition mechanisms for the initial mole fractions of CBrF3 and CF3I ranging from 0.5 to 5.0%. An analysis of modeling results, including the prediction of the equilibrium concentration, provided important insight into the effects of adding CBrF3 and CF3I on the formation of toxic gases in the hot layer. Sensitivity analysis was performed under the highly underventilated conditions, leading to the identification of the chemical reactions that have the most significant influence on the formation of toxic products.

Characterization of the combustion products in large‐scale fire tests: comparison of three experimental configurations

AbstractThe storage of large amounts of polymers and other bulk chemicals is a potential hazard in the case of fire. There is at present a lack of knowledge about the implications of such fires. In particular the role of the ventilation conditions on fire chemistry has warranted investigation. A set of indoor, large‐scale combustion experiments, conducted on five different materials is described in this article. The main test series was conducted using the ISO 9705 room, where both well‐ventilated and under‐ventilated conditions were attained by restricting the opening of the room. The degree of ventilation was determined using a phi meter. Furthermore, in addition to measuring the traditional fire‐related parameters, extensive chemical characterization of the combustion products was made.Two additional series of experiments were also performed. In one series of tests the size of the enclosure was increased and the fuel was placed in a storage configuration to simulate a real storage situation. In the other test series, three of the materials were tested as large‐scale open pool fires.The results from the three configurations are compared regarding yields of combustion products as a function of the degree of ventilation. For a number of toxic combustion products a clear dependence of the production on the equivalence ratio was found. Further, placing the fuel in a storage configuration did not significantly change the outcome of the combustion. Thus, the ISO 9705 room is of a size and scale that can be taken as a model for representing real‐scale fires. Additionally it has been demonstrated that an advantage of the ISO 9705 room is the ability to alter the ventilation conditions. Copyright © 2001 John Wiley & Sons, Ltd.

Flame radiation from polymer fires

Abstract Enclosure fires can be divided into two ventilation regimes: well-ventilated and under-ventilated combustion. The influence of fire ventilation on flame radiation is very important in enclosure fires especially for under-ventilated conditions. An approximate model for predicting flame radiation for both well- and under-ventilated fires is proposed on the basis of the Γ-correlation, in which the role of flame sootiness and heat release is considered. This paper is an extension of work performed earlier and reported in Tewarson et al. (Combustion and Flame, 95 (1993) 151-169) [1] . The results are calculated for several typical polymers, and the relationship between the flame radiant fraction and the fuel’s smoke-point has been examined. Additionally, flame radiation is predicted by the slightly modified Global Flame Radiation (GFR) Model of de Ris and Orloff. The comparisons between experimental and predicted data are satisfactory. This study attempts to provide: (1) a deeper scientific understanding of the effect of ventilation conditions on flame radiation, and (2) available correlation, for the analysis of enclosure fires.