The assumptions and the results of applying three fire modeling approaches to study three accidental fires that occurred in single-family dwellings, are presented in this work. The modeling approaches used are: a simplified analytical model of fire growth, a zone model (CFAST) and a field model (FDS). The fires predicted are: a house fire of suspected initial location but of unknown ignition source, a small-apartment fire initiated by the ignition of a sofa which extinguished due to oxygen depletion, and a one-story house fire started by a malfunctioning gas heater. The input to each model has been kept as independent as possible from the other models while consistent with the forensic evidences. The predictions from the models of the fires’ characteristics are analyzed in the context of the forensic evidences for each accidental fire to compare the models’ predictive capabilities. It is found that in spite of the differences in the sophistication of these three modeling approaches, the results were in relatively good agreement, particularly in the early stages of the fire. Simpler models can be used as a first step towards less approximate modelling or to confirm the order of magnitude of the results from more complex models. The results of this work can be used to reach conclusions about the complexity of the model required to describe a particular fire scenario.

The transition from forward smoldering to flaming in polyurethane foam is observed using in-depth thermocouples and ultrasound probing. The experiments are conducted with small parallelepiped samples vertically placed in an upward wind tunnel. Three of the vertical sample sides are maintained at elevated temperature and the fourth is exposed to an upward oxidizer flow and a radiant heat flux. An ultrasound probing technique is used to measure the line-of-sight average permeability of the sample at the same heights as the thermocouples. The smolder front propagation is tracked by both the thermocouples and ultrasound data, which show an increase in temperature and permeability upon passage of the smolder front. The permeability data also show that the transition to flaming is preceded by rapid fluctuations in permeability in the char region below the smolder front, indicating the formation of pores by secondary char oxidation. The pores provide locations for the onset of gas-phase ignition (i.e. transition to flaming). The results from all the tests indicate that the formation of pores is a necessary but not sufficient condition for the transition to flaming. Two novel measures of the intensity of the secondary char oxidation are introduced: the time derivative of permeability, and the secondary char oxidation velocity. The time derivative of permeability, which provides a measure of the pore formation rate, is found to increase as the oxygen concentration and/or radiant heat flux increase, and to indicate the likelihood of the transition to flaming. The permeability data offers a means to track the propagation of the secondary char oxidation, and to calculate the secondary char oxidation velocity, which is found to be strongly correlated to the transition to flaming. A simplified energy balance model is able to predict the dependence of the secondary char oxidation velocity on oxygen concentration and radiant heat flux.

Experiments were conducted to measure the flame propagation rate of a plug-flow flame through a combustible matrix of randomly oriented cubes of polyurethane foam in microgravity and normal gravity as a function of the forced air flow. The experiments in microgravity were conducted at the Japan Microgravity Center (JAMIC) drop tower, which provides 10s of microgravity. The normal gravity experiments were simulations of the microgravity experiments, and by comparison, were used to determine the effect of gravity on the flame propagation process. The experiment was conducted in a cylindrical geometry. Ignition was accomplished by means of a hot-surface igniter brought into direct contact with the foam at one end of the sample holder. The other end of the sample was sealed to a fan drawing air through the sample, which was adjustable using a variable DC power supply. In this configuration the flame propagation is flow-assisted. The flame propagation rate was determined by means of the temperature histories provided by thermocouples placed along the centerline of the sample. It is found that, both in normal and microgravity, as the air flow rate is increased the flame propagation velocity increases. Comparison between the normal and microgravity experiments shows that the microgravity combustion is greatly influenced by the ignition period. In microgravity the time to initiation of flame propagation is significantly longer than the corresponding time in normal gravity. This is due to the contribution of the buoyant flow that assists the forced flow during the initiation period in normal gravity. A simplified analytical model is presented for correlation of the velocity data.

Experimental observations are presented of the effect of flow velocity, oxygen concentration, and a thermal radiant flux, on the transition from smoldering to flaming in forward smoldering of small samples of polyurethane foam with a gas/solid interface. The experiments are part of a project studying the transition from smoldering to flaming under conditions encountered in spacecraft facilities, i.e., microgravity, low velocity variable oxygen concentration flows. Because the microgravity experiments are planned for the International Space Station, the foam samples had to be limited in size for safety and launch mass reasons. The feasible sample size is too small for smolder to self propagate because of heat losses to the surroundings. Thus, the smolder propagation and the transition to flaming had to be assisted by reducing heat losses to the surroundings and increasing the oxygen concentration. The experiments are conducted with small parallelepiped samples vertically placed in a wind tunnel. Three of the sample lateral-sides are maintained at elevated temperature and the fourth side is exposed to an upward flow and a radiant flux. It is found that decreasing the flow velocity and increasing its oxygen concentration, and/or increasing the radiant flux enhances the transition to flaming, and reduces the time delay to transition. Limiting external conditions for the transition to flaming are reported for this experimental configuration. The results show that smolder propagation and transition to flaming can occur in relatively small fuel samples if the external conditions are appropriate. The results also indicate that transition to flaming occurs in the char region left behind by the smolder reaction, and it has the characteristics of a gas-phase ignition induced by the smolder reaction, which acts as the source of both gaseous fuel and heat. A simplified energy balance analysis is able to predict the boundaries between the transition/no transition regions.

Results from two forward forced-flow smolder tests on polyurethane foam using air as oxidizer conducted aboard the NASA Space Shuttle (STS-105 and STS-108 missions) are presented in this work. The two tests provide the only presently available forward smolder data in microgravity. A complimentary series of ground-based tests were also conducted to determine, by comparison with the microgravity data, the effect of gravity on the forward smolder propagation. The objective of the study is to provide a better understanding of the controlling mechanisms of smolder for the purpose of control and prevention, both in normal- and microgravity. The data consists of temperature histories from thermocouples placed at various axial locations along the fuel sample centerline, and of permeability histories obtained from ultrasonic transducer pairs also located at various axial positions in the fuel sample. A comparison of the tests conducted in normal- and microgravity indicates that smolder propagation velocities are higher in microgravity than in normal gravity, and that there is a greater tendency for a transition to flame in microgravity than in normal gravity. This is due primarily to the reduced heat losses in the microgravity environment, leading to increased char oxidation. This observation is confirmed through a simplified one-dimensional model of the forward smolder propagation. This finding has important implications from the point of view of fire safety in a space-based environment, since smolder can often occur in the forward mode and potentially lead to a smolder-initiated fire. (C) 2004 Elsevier Inc. All rights reserved.

Results are presented from a model of forward smoldering combustion of polyurethane foam in microgravity. The transient one-dimensional numerical-model is based on that developed at the University of Texas at Austin. The conservation equations of energy, species and mass in the porous solid and in the gas phases are numerically solved. The solid and the gas phase are not assumed to be in thermal or in chemical equilibrium. The chemical reactions modeled consist of foam oxidation and pyrolysis reactions, as well as char oxidation. The model has been modified to account for new polyurethane kinetics parameters and radial heat losses to the surrounding environment. The kinetics parameters are extracted from thermogravimetric analyses published in the literature and using Genetic Algorithms as the optimization technique. The model results are compared with previous tests of forward smoldering combustion in microgravity conducted aboard the NASA Space Shuttle. The model calculates well the propagation velocities and the overall smoldering characteristics. Direct comparison of the solution with the experimental temperature profiles shows that the model predicts well these profiles at high temperature, but not as well at lower temperatures. The effect of inlet gas velocity is examined and the minimum airflow for ignition identified. It is remarkable that this one-dimensional model with simplified kinetics is capable of predicting cases of smolder ignition but with no self-propagation away from the igniter region. The model is used for better understanding of the controlling mechanisms of smolder combustion for the purpose of fire safety, both in microgravity and normal gravity, and to extend the unique microgravity data to wider conditions avoiding the high cost of space-based experiments.

An experimental investigation on the effects of buoyancy on opposed-flow smolder is presented. Tests were conducted on cylindrical samples of open-cell, unretarded polyurethane foams at a range of ambient pressures using the Microgravity Smoldering Combustion (MSC) experimental apparatus. The samples were tested in the opposed configuration, in which the flow of oxidizer is induced in the opposite direction of the propagation of the Smolder front. These data were compared with opposed-forced-flow tests conducted aboard STS-69, STS-77, and STS-105 and their ground-based simulations. Thermal measurements were made of the smolder reaction to obtain peak reaction temperatures and smolder velocities as a function of the ambient pressure in the MSC chamber. The smolder reaction was also observed using high-frequency ultrasound pulses as part of the ultrasound imaging system (UIS). The UIS measurements were used Lis a second means of providing smolder propagation velocities Lis well as to obtain permeabilities of the reacting samples. Results of forced-flow testing in normal gravity were compared to results in microgravity at a range of ambient pressures and forced flows. Results indicate that a critical oxidizer mass flux of roughly 0.5 to 0.8 g/m(2)s is required in normal gravity for a self-sustaining propagation in this configuration. In microgravity tests, self-sustained smolder propagation Was observed at a significantly lower oxidizer mass flux of 0.30g/m(2)s. Analysis Suggests that the removal of buoyancy-induced heat losses in microgravity allows for self-sustained propagation at an oxidizer mass flux below file critical value observed in normal-gravity testing. Normal-gravity tests also show that the smolder propagation velocity is linearly dependent oil the total oxidizer mass flux in an oxidizer-limited regime. Pressure effects on the chemical kinetics of a smolder reaction are interred by comparison of normal-gravity and microgravity tests and believed to be only weakly dependent oil Pressure (similar top(1/3)).