Flammability of combustible materials in reduced oxygen environment

Flammability of combustible materials in reduced oxygen environment is studied using theoretical analysis and laboratory-scale experiments. The experimental results show that the limiting oxygen concentration (LOC) at extinction and re-ignition can be measured using a variable oxygen concentration technique with uncertainties of ±0.5% (vol.). The imposed external radiant heat flux only affects the LOC measurements for the fuels tested in this work when the flux is less than 30 kW/m². Comparison of measured LOCs in parallel panel and horizontal geometries suggests that the latter appear to be more conservative given the same total incident heat flux to the fuel surface. A validation parallel panel test for fire propagation using PMMA at 15% (vol.) oxygen concentration confirmed fire propagation when the ambient oxygen concentration is higher than, but approaching the measured LOC. LOC calculations using critical flame temperature and Spalding's B-number theory provide qualitative agreement with the experimental data. However, quantitative comparison of calculations and measurements shows that the calculated LOCs are sensitive to the selection of flame temperature. Furthermore, calculations also show that LOCs do not change significantly if the ambient temperature varies no more than 40 K. Based on the experimental results, most of the combustible materials studied in this work such as polyethylene, oak and corrugated paper board will not be adequately protected if the ambient oxygen is maintained at 15% (vol.) or higher.     

Experimental measurements of upward flame spread on a vertical wall with external radiation

The overall objective of the project is to gain an understanding of the flame spread phenomenon under simulated surrounding fire conditions. In this phase of the project, emphasis is placed on obtaining experimental data for upward flame spread with applied external radiation on practical wall materials. A second phase (not yet reported) is the development of a numerical flame spread model and the experimental results presented here will be used for comparison with model predictions. Flame height, and in some cases pyrolysis height, were recorded as functions of time for 120 cm×30 cm samples; and these data were used to quantitatively investigate the effect of external radiation on several materials. Infrared heating panels were used to supply radiant fluxes of up to 15 kW/m² to the sample. Many wood-based materials do not exhibit flame spread to the top of the sample when ignited without applied external flux. With moderate levels of external radiation (5–10 kW/m²), many of these materials sustained flame spread to the top of the sample. With increasing external radiation levels, flame spread was also more rapid. A comprehensive series of tests was run on particle board to investigate the effect of igniter strength, preheat of the sample, and sample thickness. Igniter strength was not a significant factor and did not cause the flame spread to be sustained; the effect of preheat, even at moderate levels of radiant flux, was important; and sample thickness had a slight effect, with thicker samples burning slower. Total heat feedback to the sample was measured and the maximum values for various samples are reported. Experimental data obtained in this project will be used to aid in the development and validation of a numerical flame spread model.     

An estimate of the correction applied to radiant flame measurements due to attenuation by atmospheric CO2 and H2O

A narrow band statistical model has been used to estimate the uncertainty introduced into radiative heat flux measurements from fires which is attributable to attenuation by atmospheric H₂O and CO₂. The flames were assumed to be soot-dominated with blackbody emission characteristics. The ambient surroundings near the flames were assumed to be homogeneous with the total pressure being fixed at one atmosphere. Atmospheric CO₂ concentrations were held constant at 0.04 kPa and the water vapor concentrations varied between 0.55–5.63 kPa based on temperature and relative humidity. The remaining partial pressures were accounted for by O₂ and N₂. Correlations to estimate atmospheric attenuation are given over a range of conditions that include path length (10–200 m), ambient temperature (19–35°C), source temperature (1000–1600°C) and relative humidity (0.25–1.0) as parameters. The results of these calculations indicate that, over this range of conditions, the radiant flux can be attenuated by as much as 42%.     

Skin temperatures of a pre-cooled wet person exposed to engulfing flames

In a television show, a wetted bare-skinned person slid through engulfing kerosene pool fire flames. The 0.74 s flame exposure resulted in pain and light sun burns. The heat and mass transfer involved in this dangerous stunt have been analyzed in order to evaluate whether or not the thin water layer represented an important heat protection measure. It is estimated that the wetted person was exposed to heat fluxes in the range of 80–90 kW/m². Analytical solutions of the heat equation were used to evaluate water-spray pre-cooling, heating during flame exposure and post-flame relaxation of skin temperature gradients. It is shown that the water layer carried on the skin into the flames represented limited heat protection. The 30 s cold water-spray pre-cooling prior to the flame exposure was the most important heat protection mechanism. Larger flames of higher emissivity, longer period of flame exposure, warmer pre-cooling water or shorter pre-cooling period would most likely have resulted in severe skin burns.     

The effect of fuel area size on behavior of fires in a reduced-scale single-track railway tunnel

A set of experiments was carried out in a 1/9 reduced-scale single-track railway tunnel to investigate the effect of fuel area size on the temperature distribution and behavior of fires in a tunnel with natural ventilation. Methanol pool fires with four different fuel areas 0.6 × 0.3 m² (1 pan), 1.2 × 0.3 m² (2 pans), 2.4 × 0.3 m² (4 pans) and 3.6 × 0.3 m² (6 pans), were used in these experiments. Data were collected on temperatures, radiative heat flux and mass loss rates. The temperature distribution and smoke layer in the tunnel, along with overflow dimensions and radiant heat at the tunnel entrance were analyzed. The results show that as the fuel area enlarges, the fire gradually becomes ventilation-controlled and the ceiling temperature over the center of fire source declines. Burning at the central region of fire source is depressed due to lack of oxygen. This makes the temperature distribution along the tunnel ceiling change from a typical inverted V-shape to an M-shape. As observed in the experiments, a jet flame appeared at tunnel entrances and both the size and temperature of the flame increased with the enlargement of fuel area leading to a great threat to firefighters and evacuees in actual tunnel fires.     

Numerical study of the behaviour of a surface fire propagating through a firebreak built in a Mediterranean shrub layer

The efficiency of a firebreak, built in a shrubland has been studied numerically using a multiphase physical model. The physical mechanisms governing the propagation of the surface fire and the consequences upon the temperature signal and the radiative heat flux received by a target located at 1 m above the ground level, have been firstly studied before positioning the firebreak. The role played by the flame and the recirculation of hot gases to the ignition of unburned fuel (especially the dry grass) ahead of the fire front have been clearly identified. Four values of the firebreak width L_C (ranged between 5 and 20 m) and 3 values of wind velocities (ranged between 1 and 8 m/s) have been tested. The simulations show that above a threshold value of this parameter, even if a small amount of the fuel located on the opposite side of the firebreak was ignited, the released energy was not sufficient to sustain the propagation of the surface fire after crossing the firebreak.     

Experimental investigation on transverse ceiling flame length and temperature distribution of sidewall confined tunnel fire

This paper presents an experimental investigation on the transverse ceiling flame length and the temperature distribution of a sidewall confined tunnel fire. The experiments were conducted in a 1/6th scale model tunnel with the fire source placed against the sidewall, 0 m, 0.17 m and 0.35 m above the floor, respectively. Experiments of fire against a wall without a ceiling, 0.35 m above the floor in a large space, were also conducted as a control group. Results shows that for small heat release rate (HRR), the flame is lower than the ceiling and extends along the sidewall. With the increase of HRR and elevation of burner height, the flame gradually impinges on the ceiling and spreads out radially along it. The flame impingement condition and the flame shapes of the wall fire with and without ceiling are presented. From the viewpoint of the physical meaning of flame impinging on the ceiling, the horizontal flame length should be a function of the unburned part of the fuel at the impinging point. Based on the proportional relation between the flame volume and HRR, the effective HRR (Q_ef) at the ceiling is determined and the effective dimensionless HRR, Q^*_ef is defined to correlate the horizontal ceiling flame length. Additionally, predictive correlations of transverse ceiling temperature distribution are proposed for the continuous flame region, the intermittent flame region and the buoyant plume region under the ceiling, respectively.     

Effectiveness of vertical barriers in preventing lateral flame spread over exposed EPS insulation wall

Insulation panels made of organic, combustible materials are frequently used in the exterior thermal insulation systems (ETIS) for buildings. Such combustible insulation panels have been involved in several catastrophic building fires in recent years in China. One potential strategy to mitigate this fire hazard is to limit fire spread over the ETIS. The present work evaluates the effectiveness of vertical fire barriers in inhibiting fire spread over exposed insulation walls made of expanded polystyrene (EPS) panels. Reduced-scale experiments were carried out indoors using EPS panels with or without two vertical barriers made of non-combustible mineral wool, the fire started at the bottom center of the middle panel. The interval and width of the barriers were varied systematically, while the temperature distribution on the wall, the radiation heat flux from the fire, and the infra-red (IR) images were recorded. To demonstrate the validity of the concept, an outdoor, full-scale experiment was carried out using a 7-floor building. Our reduced-scale experiments showed that the installation of two vertical fire barriers successfully stopped the lateral flame spread, decreasing the peak temperatures of the two side panels by about 300 °C for all barrier configurations tested. When barrier width was fixed at 5 cm, an increase of the barrier interval from 30 to 90 cm led to increases in the peak temperatures, radiation heat flux, and the maximum rate of upward flame spread. By contrast, when barrier interval was fixed at 90 cm, an increase of the barrier width from 2 to 5 cm had little influence on the combustion dynamics of the middle panel but the peak temperature on the side panels dropped, consistent with the smaller heat transferred with wider fire barriers. In the regions of the side panels next to the barriers, pyrolysis and deformation could be observed with barrier widths of 2 and 3 cm, but not 5 cm. Finally, our outdoor, full-scale experiment demonstrated that a 30 cm wide vertical barrier made of air-filled cement successfully stopped the lateral flame spread over exposed EPS wall. The study highlights the effectiveness of vertical fire barriers in preventing the lateral flame spread over the exposed EPS insulation wall and provides another option for enhancing the fire safety of the combustible insulation systems.     

Flame heat transfer in storage geometries

This paper presents measurements of the heat flux distribution to the surface of four square towers exposed to buoyant turbulent flames.The steel towers represent an idealisation of a rack storage configuration at reduced scale. Each tower was 1.8 m high and 0.3 m×0.3 m wide. The fuel was supplied from a circular gas burner at the floor. Three different gaseous fuels were used: carbon monoxide (CO), propane (C₃H₈), and propylene (C₃H₆). These fuels cover a wide range of flame sootiness resulting in distinctly different flame heat fluxes. At the same overall heat release rates the peak heat fluxes from C₃H₈ flames were twice those from CO flames, whereas the peak heat fluxes from C₃H₈ flames were 2.8 times those from CO flames. Heat fluxes were measured by thermocouples spot-welded to the back of the steel sheets. They were measured at 52 different locations. This measurement method turns out to be simple, accurate and robust in addition to being inexpensive. Formulas are provided for the flame heat flux distribution in terms of the overall fire heat release rate, fuel sootiness and separation distance between the towers. The formulas are suitable for direct use by engineering models of fire growth in storage geometries. The paper also provides additional data needed for the development of more general CFD models capable of predicting fire growth of other geometries.     

Simulation of fire scenarios due to different vehicle types with and without traffic in a bi-directional road tunnel

This paper presents findings obtained by CFD modelling for simulating the effects of fire due to different vehicle types in a bi-directional road tunnel. Four different burning vehicles placed in the centre of the driving lane at tunnel middle length were considered. Peaks of the heat release rate (HRR) of: 8, 30, 50, and 100 MW were simulated for the two cars, the bus, the heavy goods vehicle (HGV), and the petrol tanker, respectively. The fire effects on tunnel structure and on environmental conditions along people evacuation path were especially evaluated. The effects of the traffic jam, in contrast with the isolated vehicles, on temperatures, radiant heat flux, visibility distance, and toxic gases concentrations, were also investigated. The worst scenario was identified to be that pertaining to the petrol tanker and more critical conditions were also found when the tunnel was full of vehicles. The maximum gas temperatures reached in the presence of traffic at the side wall (and at the tunnel ceiling reported in brackets) were found to be: 360 °C (170 °C) for the two cars; 740 °C (465 °C) for the bus; 835 °C (735 °C) for the HGV and 1305 °C (1145 °C) for the petrol tanker, respectively. The presence of the traffic, in contrast with the isolated vehicle, involved an increase in the maximum temperatures equal to 16–17% for the two cars, and contained in the range 12–29% with percentages increasing starting from the tanker, to the HGV and to the bus. In other words when the maximum temperatures produced by the isolated vehicle are very high (e.g. for the tanker), the presence of the traffic had a minor effect. With reference to environmental conditions along the evacuation path, the results showed that in the case of petrol tanker fire the emergency ventilation ensures a tenable level of temperature, radiant heat flux, and toxic gases concentrations up to 5 min from the fire starting. This time increases up to 6.5 min for the HGV and 8 min for the bus. This means that the tunnel users in order to be safe in all scenarios should leave the tunnel within 5 min after the fire starting. Toxic gases concentrations, however, were found to be below the limit values in all cases and also in the presence of traffic. In the light of the aforementioned results, tunnel occupants should be promptly informed of the fire risk and guided to the exit portals. This might be done by equipping the tunnel with illuminated emergency signs located along the tunnel length and by installing traffic lights before the entrances so that the tunnel can be closed in case of emergency. By activating the traffic lights at the portals and the emergency signs (more especially those at the ceiling) at the same time as the emergency ventilation is activated, safer conditions for the people evacuation are expected.     

Fire development in a 1/3 train carriage mock-up

To study what parameters that control the initial fire spread and the development to local flashover in a metro carriage, a total of six fire tests were conducted in a mock-up of a metro carriage that is about 1/3 of a full wagon length. They were carried out under a large scale calorimeter in a laboratory environment. The focus was on the initial fire development in a corner scenario using different types of ignition source that may lead to a fully developed fire. The ignition sources used were either a wood crib placed on a corner seat or one litre of petrol poured on the corner seat and the neighbouring floor together with a backpack. The amount of luggage and wood cribs in the neighbourhood of the ignition source was continuously increased in order to identify the limits for flashover in the test-setup. The tests showed that the combustible boards on parts of the walls had a significant effect on the fire spread. In the cases where the initial fire did not exceed a range of 400–600 kW no flashover was observed. If the initial fire grew up to 700–900 kW a flashover was observed. The maximum heat release rate during a short flashover period for this test set-up was about 3.5 MW. The time to reach flashover was highly dependent on the ignition type: wood cribs or backpack and petrol. A full developed carriage fire was achieved as a result of intense radiation from the flames and ceiling smoke layer. This was mostly dependent on the amount of fire load nearby the ignition source and how strong the vertical flame spread on the high pressure laminate boards mounted to walls and ceiling above the ignition source was, leading to a ceiling flame. In such cases, the seats alone did not contain sufficient fuel for the fire to spread within the train, and additional fuel (luggage) is required near the seats. For fully developed carriage fires, the fire starting on the seat in the corner spread to the opposite seat on the same side of the aisle, then horizontally spread to seats on the other side of the aisle, and finally a longitudinal flame spread along the carriage was observed. When and where the fire stopped or whether it reached a fully developed stage was mostly dependent on the amount of fire load nearby the ignition source and how strong the vertical flame spread on the high pressure laminate boards mounted to walls and ceiling above the ignition source was.     

Simultaneous improvements in flammability and mechanical toughening of epoxy resins through nano-silica addition

Polymers in transport, and many other engineering applications, are required to be mechanically tough as well as resistant to ignition and flame spread. These demands are often for many polymer types in competition, especially when adding flame retardants. With nano-silica addition, we show that improvements in both properties of a polymer can be achieved simultaneously. In this study, an epoxy resin is evaluated for its flammability and mechanical properties with step wise additions of nano-silica. The fracture toughness was significantly improved. In the single edge notch bending test, the addition of 36% nano-silica particles doubled the toughness and increased the flexure modulus by 50%. Flammability was studied via time to ignition at constant irradiation, and via a UL94 test coupled with mass loss and surface temperature measurements. Modelling for the heat transport and chemical kinetics in Gpyro was done and yielded good agreement with the temperatures measured. Adding up to 36% nano-silica, the time to ignition increased by 38% although a sharp decrease was observed around 24% SiO₂ addition. We show that the increased time to ignition is mostly due to a higher thermal diffusivity, increased inert content, as well as a strengthening of the residue outer skin, which acts as a mass barrier for pyrolysate. This outer skin was analysed using a scanning electron microscope coupled with an energy dispersive X-ray spectrometer. We found that in the skin the nano-silica particles agglomerate at the surface forming a strong continuous structure together with the char residue from the epoxy. Improvements in the flammability as seen in the UL94 test were measured with mass loss showing a 30% reduction after 20 s, and surface temperatures at the ignited end being up to 75 K lower compared to the pure epoxy. Modelling in Gpyro supported the temperature measurements taken. Despite the improvements seen, all samples ignited, failing the test with dripping and showing that the improvements recorded in time to ignition did not fully translate over to the UL94 test. Overall we show that the flammability and toughness of epoxy could be improved simultaneously with nano-silica. Using up to 36% nano-silica, the significant modification of thermal properties could be explored in relation to fire properties for epoxy. Increasing the thermal diffusivity as well as skin formation are the main parameters improving the flammability and show a path for potential improvements in other composites as well.     

Heat feedback to the fuel surface of a pool fire in an enclosure

As a part of an effort to determine the energy balance at the pool fire surface in compartments, a series of fire experiments were conducted to study heat flux of the flame in a vitiated environment formed with air and combustion products gases. This paper presents experimental results of the burning behaviour of a heptane pool fire in a reduced scale compartment equipped with a mechanical ventilation network. Measurements of heat fluxes, fuel mass loss rate, oxygen concentration and temperature are performed for heptane fires of 0.26 and 0.3 m diameter pans at different ventilation flow rates. An original method to separate effects of the radiant heat flux of the flame and of the external heat feedback to the fuel surface is developed. This was achieved by using an additional heat flux measurement located under the pool fire. A correlation was also developed to determine the temperature rise on the plume centerline in the compartment as a function of the heat release rate. The results indicate a decrease in the fuel mass loss rate, flame temperature and heat fluxes to the fuel surface as the oxygen concentration measured near the fuel decreases by varying the air refresh rate of the compartment. The flame radiation fraction shows a similar behaviour, whereas the convective fraction of the flame heat flux increases when oxygen concentration decreases. Based on these experimental findings, it was discussed that any classification of the burning regime of a pool fire should consider both the effects of pan diameter and the burning response to vitiated air.     

Effect of oxygen on flame heat flux in horizontal and vertical orientations

A systematic empirical and analytical study was conducted to directly quantify the effect of enhanced ambient oxygen concentration on flame heat flux at bench scale and its ability to represent large-scale flame heat flux of well-ventilated fires. The Advanced Flammability Measurements Apparatus was used to conduct bench scale horizontal and single wall vertical orientation experiments for black polymethylmethacrylate, propylene gas and black polyoxymethylene. The key aspect of this study was direct experimental measurements of flame heat flux back to the burning surface for 20.9–40% ambient oxygen concentrations over a range of applied heat flux. The total flame heat flux, as well as the radiative and convective components, was experimentally measured with various gages. To gain more insight into the effects of oxygen, the flame emissivity, flame height and flame temperature were measured and used to calculate the radiative and convective components of the flame heat flux. Gas burner experiments were conducted to decouple the solid and gas phase effects of the ambient oxygen. Large scale tests of black polymethylmethacrylate were conducted in a horizontal orientation and literature data was used for single wall vertical orientations for comparison to the bench scale, enhanced oxygen results. The main conclusion is that the flame heat flux in enhanced ambient oxygen bench scale does not simulate large-scale flame heat flux in horizontal orientations but simulates a more severe large-scale geometry (parallel wall) in vertical orientations and is useful for evaluation of materials’ vertical flame spread potential.     

An experimental study on the intermittent extension of flames in wind-driven fires

Experiments were conducted to study the intermittent extension of flames from wind-driven line fires using stationary burners. These fires are thought to share similar features with propagating wildland fires, where forward pulsations of flame have been observed to quickly ignite material far ahead of the mean flame front. However, stationary burners offer the ability to study the movement of the flame and its heating processes in greater detail than a spreading fire. In these stationary experiments, propane gas was used as a fuel with different burner sizes, 25–30 cm wide and 5–25 cm long in the direction of the flow. A specially-built wind tunnel was used to provide a well-characterized laminar flow for the experimental area. The free-stream flow velocity, measured by a hot-wire anemometer, ranged in the experiments from 0.2 to 2.7 m/s. The shape of the flame was measured using a high-speed video camera mounted perpendicular to the apparatus. A method was developed to track the extension of the flame close to the surface, simulating flame contact with unburnt fuel downstream of the fire. This extension length was then measured frame by frame and frequencies of flame presence/absence determined as a function of downstream distance. The location of maximum pulsation frequency, x_max, for each burner/wind configuration, was obtained using a level-crossing approach (essentially the variable-interval time-average (VITA) method). Further study indicates that x_max can be well estimated using mean flame properties. Probability distributions describing the location of the flame over time also showed that, the probability the flame extends far beyond the mean flame front is sensitive to increasing ambient winds and fire size.     

Radiant smoldering ignition of plywood

This paper investigates the role of self-heating in the smoldering ignition of 18 mm (three-quarter inch) thick maple plywood exposed to radiant heat fluxes between 6 and 15 kW/m² in the cone calorimeter for up to 8 h. The minimum heat flux for smoldering ignition was experimentally determined to be 7.5 kW/m². This compares favorably to predictions made using classical self-heating theory. The role of self-heating was explored via temperature measurements distributed within the specimens. Elevated subsurface temperature profiles indicated self-heating was an important ignition factor resulting in ignition at depth with smolder propagation to the surface and into the material. The ignition depth was shown to be a function of the heat flux with the depth moving towards the surface as the heat flux increased.     

Investigation of flame propagation over an inclined fuel wetted porous bed

This experimental study was conducted to investigate the rate of flame spread over an inclined porous solid (sand) wetted with finite quantities of fuel (iso-propanol). The study comprised experiments that were conducted over 15° and 30° inclined beds with depths ranging from 13.3 mm to 39.9 mm and consisting of average sand particle diameters ranging from 0.5 mm to 5 mm under quiescent, assisted and opposed airflow conditions.Analysis of the resulting data indicate that the rate of flame spread is significantly decreased by increasing the bed inclination angle or the airflow velocity and is applicable for both assisted and opposed directions. Furthermore, the rate of flame spread is decreased to the minimum value and actually ceased halfway along the bed with a 30° inclination angle. This behaviour was observed mainly for beds containing coarse sand particles. The rate of flame spread was higher for thinner beds rather coarse beds under any given airflow conditions. Finally, the rate of flame spread in upward direction was relatively quicker in comparison with downward direction counterpart.     

Study of ignition and extinction of small-scale fires in experiments with an emulating gas burner

The objective of this study is to explore mechanisms for ignition and extinction for condensed-phase fuels via the use of a gas-fueled burner. Flames were generated with a porous 25 mm circular burner using mixtures of methane and propane with nitrogen. The procedure was to specify a set of mass fluxes of nitrogen-fuel mixture that corresponded to the flash- fire- and extinction points and for the minimum mass flux where steady burning was achieved. The results show an increase in the critical mass flux with a decreased heat of combustion. The data fall into two regimes depending on the mixture flow rate; one buoyancy-driven (Fr<1) and one induced by momentum jet forces. The buoyancy-driven regime is geometrically consistent with the definitions of flash and fire points under natural convection conditions. The results for the momentum regime align reasonably with existing stagnant layer theory. Extinction theory is also suggested to give approximate results for the fire point. This argument is based on similar flame geometries for fire point and extinction and theoretical reasoning. An anchor point is proposed as the end point of ignition. Produced anchor point data result in a flammability diagram, below which quasi-steady burning occurs.     

Portable gasoline container headspace flammability

This work summarizes the findings of a multi-year study into the flammability hazards associated with portable gasoline containers (PGC's). In particular, this investigation focuses on identifying the limited conditions under which a flame can propagate through the pour spout and into the PGC, causing a deflagration. The first series of tests simulate quiescent gasoline storage in a 18.9 l (5 gallon) PGC with a child resistant spout and closure. The storage conditions are varied to include a range of liquid volumes (5 to 500 mL) and temperatures (−30 to 0 °C). The second series of tests simulates pouring of gasoline from the PGC and involves testing over a range of tilt angles from 61 to 73°. In both cases, vapor concentrations are obtained from a paramagnetic oxygen analyzer and from an infrared sensor calibrated for n-butane. It is found that the container tilt angle is a significant controlling parameter and that liquid volumes ranging from 5 to 30 mL in a 18.9 l PGC are capable of producing a flammable headspace region. Finally, a model is developed to predict the influence of these controlling parameters on the flammability hazard.