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.     

Piloted ignition times,critical heat fluxes and mass loss rates at reduced oxygen atmospheres

Ignition, pyrolysis and burning of materials in reduced oxygen atmospheres occur when recirculating combustion gases are mixed with the air flowing into an enclosure. Still the incoming air can be sufficient for the complete combustion of the pyrolysis gases. Thus, for the prediction of fires in enclosures it is essential to understand the ignition and burning of materials in a reduced oxygen atmosphere even when plenty of oxidizer is available for complete combustion. Previous work employing gaseous fuels has shown that under these conditions, but before extinction, burning of gaseous fuels issuing from a nozzle is complete but radiation from the flames decreases owing to the reduction of their temperature. Complementary to that work, piloted ignition of solids is investigated here at reduced oxygen concentrations by measuring the ignition times and mass loss rates of the solid at ignition.These results were obtained in a cone calorimeter modified to supply air at reduced oxygen concentrations. Two types of plywood, normal and fire retardant 4 mm thick were examined at three imposed heat fluxes 25, 35 and 50 kW/m² and at oxygen concentrations of 21%, 18% and 15% by volume. Because heating at these heat fluxes and material thickness corresponds to intermediate thermal conditions (i.e. neither thin nor thick), novel analytical solutions are developed to analyze the data and extract the thermal and ignition properties of the material. The same novel solutions can be applied to modeling concurrent or countercurrent flame spread. Moreover, a theory for piloted ignition explains why the ignition times and mass pyrolysis rates are weakly dependent on reduced oxygen concentrations.     

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.     

Thermal analysis of wood–cement panels: Heat flux and indoor temperature measurements in test cells

The aim of the present paper is to characterize the physical and mechanical properties and to evaluate the thermal performance of wood–cement panels using wood flour originated from lumber industries rejects. The research comprised of several steps: (1) physical and mechanical analysis; (2) heat flux measurements using small-scale test cells of 1 m³ of internal volume and (3) indoor temperature measurements under summer and winter climatic conditions in Curitiba, Brazil (latitude 25.5°S, elevation 917 m above sea level). Reference material for indoor temperature comparisons was a prototype made with ordinary ceramic bricks, plastered on both sides. Air temperature measurements were carried out with data loggers, heat flux plates were attached to an exposed north facade of each test cell, while incoming solar radiation was measured with an experimental solarimeter.     

Deflagrations involving stratified heavier-than-air vapor/air mixtures

The distribution of explosion damage in a structure is a major indicator of the type of explosive material involved and its location. A solid-phase explosive material typically produces localized or “seated” damage, where a vapor/air explosive mixture typically produces generalized, omnidirectional damage. Investigators have been taught that the finding of more intense blast damage to lower portions of an enclosure indicates that the vapors were heavier than air, while explosion damage to upper portions indicates a lighter-than-air gaseous fuel. Most of the explosion pressure data in the literature deal with well-mixed mixtures that are uniform in concentration prior to ignition. This study explores the pressure distributions produced by the ignition of shallow (0.05–0.2 m deep) layers of hexane vapor created by the evaporation of liquid in a still, isothermal compartment. The floor-level vapor layers thus produced were ignited by an electric arc and the pressures at five different locations in the room were monitored. It was found that pressures increased in an exponential fashion over a period of 300–400 ms after ignition until the relief panel failed (at ∼5–6 kPa). The peak pressures observed at all five locations in the compartment coincided in time (to within ±5 ms) and intensity suggesting that the pressures produced within the 3.6 m×2.4 m×2.4 m chamber equilibrated very quickly. Any failure of the compartment, then, would be the result of failure of the weakest part of the confining structure, rather than the result of pre-ignition distribution of the fuel/air mixture. A small (∼−2 kPa), but reproducible negative pressure peak was observed some 60–70 ms after the maximum positive pressure. This finding shows that negative pressure peaks can be produced by deflagrating vapor/air mixtures that could exert physical effects on lightweight debris dislodged by the initial positive pulse.     

Modeling temperature distribution and thermal property of asphalt concrete for laboratory testing applications

Material characterization from laboratory tests on asphalt concrete or predictions of pavement performance are meaningful only if temperature of the material is well taken into account. This paper discusses an analytical model to predict the transient temperature distribution within asphalt concrete and to determine its thermal properties. The paper also presents the laboratory test program designed to validate the model. Temperature measurements were carried out on a cylindrical specimen at different times after the specimen with a steady-state low temperature (3.5 °C) was placed inside an environmental chamber in a steady-state high temperature (36 °C). The temperature magnitude at different positions and its variation with time was recorded at a sampling rate of 1 min⁻¹. The analytical temperature models based on the classical planar wall and long cylinder were established to approximate the temperature distribution of asphalt concrete specimens with the geometry of a short cylinder or a beam. Thermal diffusivity as a function of thermal conductivity and heat convection is solved from the models, and then back-calculation was conducted to achieve the thermal properties using curve fitting. It was found that the analytical model could predict the measured temperatures reliably. For the materials used in this research, a thermal conductivity of 2.88 W/m °C and diffusivity of 1.42 × 10⁻⁶ m²/s were attained from the back-calculation. The time–temperature relationship, as determined from the prediction model, was found to be very sensitive to the geometric size and thermal properties of asphalt concrete.     

Heat release rates from heavy goods vehicle trailer fires in tunnels

Calculations of heat release rates (HRR) from four large-scale tests, with a mock-up of a Heavy Goods Vehicle (HGV) trailer, in a road tunnel are presented. Initial longitudinal ventilation rates within the tunnel were in the range of 2.8–3.2 m/s. Peak HRRs in the range of 66–202 MW were estimated. The peak HRRs were obtained between 7.1 and 18.4 min from ignition in the various tests. The HGV-trailer mock-up consisted of a steel rack system loaded with a mixed commodity of wood pallets and polyethylene pallets (Test T1), wood pallets and polyurethane mattresses (Test T2), furniture and fixtures with ten truck rubber tyres (Test T3), and paper cartons and polystyrene cups (Test T4). Each commodity was covered with a polyester tarpaulin in each test and ignited on the upstream, front end of the trailer. A comparison is made between the results presented here and other large-scale tests with HGV trailers in tunnels. The combined expanded relative standard uncertainty of the method used to determine the HRR was calculated to be 14.9%.     

Thermography as a technique for monitoring early age temperatures of hardening concrete

The exothermal character of cement hydration reactions causes concrete to endure temperature changes during the first days after casting, with associated volumetric deformations that may induce undesired cracking. The capability to predict temperature evolution in concrete since casting is thus important to back decisions that avoid detrimental thermal cracking in concrete structures. Even though several approaches exist to model the early age behavior of concrete, the laboratory or in situ verification of numerical predictions is scarce, and mostly done with embedded temperature sensors, with limited sampling points. The present research intends to evaluate the performance of the thermography technique in the continuous monitoring of surface temperatures of a hydrating 0.40 × 0.40 × 0.40 m³ concrete cube, in which embedded thermal sensors are also used. By using thermography, simultaneous monitoring of the visible surfaces of the specimen is possible, thus providing comprehensive information regarding the evolution of surface temperatures. The temperatures monitored with the thermography, as well as with the embedded temperature sensors, are finally used as a benchmark example for validation of a 3D finite element numerical code for thermal analysis developed by the authors. The use of thermography images for validation of finite element results is rather more advantageous than the use of standard single point temperature measurements, in view of the large facility and wide range of comparison provided by the simultaneous visualization of temperature surface color maps (measured and simulated).     

Self-ignition of natural fuels: Can wildfires of carbon-rich soil start by self-heating?

Carbon-rich soils, like histosols or gelisols, cover more than 3% of the Earth's land surface, and store roughly three times more carbon than the Earth's forests. Carbon-rich soils are reactive porous materials, prone to smouldering combustion if the inert and moisture contents are low enough. An example of soil combustion happens in peatlands, where smouldering wildfires are common in both boreal and tropical regions. This work focuses on understanding soil ignition by self-heating, which is due to spontaneous exothermic reactions in the presence of oxygen under certain thermal conditions. We investigate the effect of soil inorganic content by creating under controlled conditions soil samples with inorganic content (IC) ranging from 3% to 86% of dry weight: we use sand as a surrogate of inorganic matter and peat as a surrogate of organic matter. This range is very wide and covers all IC values of known carbon-rich soils on Earth. The experimental results show that self-heating ignition in different soil types is possible, even with the 86% inorganic content, but the tendency to ignite decreases quickly with increasing IC. We report a clear increase in ambient temperature required for ignition as the IC increases. Combining results from 39 thermostatically-controlled oven experiments, totalling 401 h of heating time, with the Frank-Kamenetskii theory of ignition, the lumped chemical kinetic and thermal parameters are determined. We then use these parameters to upscale the laboratory experiments to soil layers of different thicknesses for a range of ambient temperatures ranging from 0 °C to 40 °C. The analysis predicts the critical soil layer thicknesses in nature for self-ignition at various possible environmental temperatures. For example, at 40 °C a soil layer of 3% inorganic content can be ignited through self-heating if it is thicker than 8.8 m, but at 86% IC the layer has to be 1.8 km thick, which is impossible to find in nature. We estimate that the critical IC for a ambient temperature of 40 °C and soil thickness of 50 m is 68%. Because those are extreme values of temperature and thickness, no self-heating ignition of soil can be expected above the 68% threshold of inorganic content. This is the first in-depth experimental quantification of soil self-heating and shows that indeed it is possible that wildfires are initiated by self-heating in some soil types and conditions.     

Simulating longitudinal ventilation flows in long tunnels: Comparison of full CFD and multi-scale modelling approaches in FDS6

The accurate computational modelling of airflows in transport tunnels is needed for regulations compliance, pollution and fire safety studies but remains a challenge for long domains because the computational time increases dramatically. We simulate air flows using the open-source code FDS 6.1.1 developed by NIST, USA. This work contains two parts. First we validate FDS6’s capability for predicting the flow conditions in the tunnel by comparing the predictions against on-site measurements in the Dartford Tunnel, London, UK, which is 1200 m long and 8.5 m in diameter. The comparison includes the average velocity and the profile downstream of an active jet fan up to 120 m. Secondly, we study the performance of the multi-scale modelling approach by splitting the tunnel into CFD domain and a one-dimensional domain using the FDS HVAC (Heating, Ventilation and Air Conditioning) feature. The work shows the average velocity predicted by FDS6 using both the full CFD and multi-scale approaches is within the experimental uncertainty of the measurements. Although the results showed the prediction of the downstream velocity profile near the jet fan falls outside the on-site measurements, the predictions at 80 m and beyond are accurate. Our results also show multi-scale modelling in FDS6 is as accurate as full CFD but up to 2.2 times faster and that computational savings increase with the length of the tunnel. This work sets the foundation for the next step in complexity with fire dynamics introduced to the tunnel.     

Characterizing the ignition hazard from cigarette lighter flames

The abuse of cigarette lighters, especially by juveniles, poses a serious fire safety challenge. Little information is available on the cigarette ignition performance of these devices or the ignition hazard these devices present to other objects. The current investigation focuses on characterizing the ignition propensity of cigarette lighters through carefully designed and controlled experiments. Cigarette lighters with port-type and Bunsen-type burner designs were adjusted to produce 75 W flames for all experiments. Temperature, heat flux, and ignition propensity measurements were performed in the flame and plume regions above these lighters. Even with the small source sizes used in this investigation, the temperature measurements from both burner types followed the turbulent plume scaling laws and compared favorably with large-scale fire measurements. However, significant differences were observed in the measured heat fluxes from port- and Bunsen-type flames having the same energy release rate of 75 W.     

Modeling of fire suppression by fuel cooling

Fire suppression with water spray was investigated, focusing on cases where fuel cooling is the dominant suppression mechanism, with the aim to add a specific suppression model addressing this mechanism in Fire Dynamics Simulator (FDS), which already involves a suppression model addressing effects related to flame cooling. A series of experiments was selected, involving round pools of either 25 or 35 cm diameter and using both diesel and fuel oil, in a well-ventilated room. The fire suppression system is designed with four nozzles delivering a total flow rate of 25 l/min and injecting droplets with mean Sauter diameter 112 μm. Among the 74 tests conducted in various conditions, 12 cases with early spray activation were especially considered, as suppression was observed to require a longer time to cool the fuel surface below the ignition temperature. This was quantified with fuel surface temperature measurements and flame video recordings in particular. A model was introduced simulating the reduction of the pyrolysis rate during the water spray application, in relation to the decrease of the fuel local temperature. The numerical implementation uses the free-burn step of the fire to identify the relationship between pyrolysis rate and fuel surface temperature, assuming that the same relationship is kept during the fire suppression step. As expected, numerical simulations reproduced a sharp HRR decrease following the spray activation in all tests and the suppression was predicted in all cases where it was observed experimentally. One specific case involving a water flow rate reduced such that it is too weak to allow complete suppression was successfully simulated. Indeed, the simulation showed a reduced HRR but a fire not yet suppressed. However, most of the tests showed an under-estimated duration before fire suppression (discrepancy up to 26 s for a spray activation lasting 73 s), which demonstrates the need for model improvement. In particular the simulation of the surface temperature should require a dedicated attention. Finally, when spray activation occurred in hotter environments, probably requiring a combination of fuel cooling and flame cooling effects, fire suppression was predicted but with an over-estimated duration. These results show the need for further modeling efforts to combine in a satisfactory manner the flame cooling model of FDS and the present suggested model for fuel cooling.     

Abrasion resistances of cellulosic,synthetic, polyurethane,waterborne and acidhardening varnishes used woods

This study was performed to determine the abrasion resistances of some varnishes used on wood materials. For this purpose, test samples prepared from Scots pine, Oriental beech, European oak, Black poplar, Basswood and Black walnut woods, which met the requirements of ASTM D 358, were coated according to ASTM D 3023 standards with cellulosic (C), synthetic (Sn), polyurethane (Pu), waterborne (Wb) and acidhardening (Ah) varnishes. The abrasion resistance of samples after the varnishing process was determined in accordance with TS 4755. It was observed that, according to wood samples, the highest abrasion resistance was obtained in Black walnut (168.9 rpm), and the lowest abrasion resistance was obtained in Scots pine (50.63 rpm); according to varnish types, the highest abrasion resistance was obtained in acidhardening (213.4 rpm), and the lowest abrasion resistance was obtained in waterborne (45.44 rpm). In accordance with the interaction of the factors wood type, varnish type and layer type, the highest abrasion resistance was found at interaction of Black walnut + acidhardening + 3 layers (578.0 rpm), and the lowest abrasion resistance was found at interaction of Oriental beech + waterborne + 1 layer (11.50 rpm). Furthermore, it was found that interactions according to the varnish type and amount of layer thickness display differences; varnish types are efficient to the first degree and layer thickness to the second degree for abrasion resistance. In this respect, it can be stated that in wooden parquets and place floorings, in which the abrasion resistance is considerably important, the varnish application with three layers of acidhardening can provide an advantage.     

Heat of combustion in spreading wood crib fires with application to ceiling jets

Responding to a challenge raised with respect to a 1989 revision of a 1979 paper on the ceiling jet of t-squared fires, we have measured the heat of combustion in the growth phase of wood cribs made of sugar pine, the test fuel in the original work, needed to generalize the ceiling jet measurements to any combustible. The present determination of the chemical heat of combustion in the growth phase, 14.1 kJ/g, is a little higher than adopted in 1989 (12.5 kJ/g, from wood sample burning with diffusion flame) but still considerably lower than employed in 1979 (20.9 kJ/g, from oxygen bomb calorimetry). More importantly, the convective heat of combustion was measured as 11.5 kJ/g, which has been employed to update the ceiling jet equations for temperature and velocity in t-squared fires. An explanation is offered for the varying, and often higher than expected ceiling-level temperatures measured with thermocouples directly over the fire in the original experiments, suggesting that both plume lean and thermocouple insertion depth may have affected the indicated temperature.     

Preparation and properties of a single molecule intumescent flame retardant

Melamine salt of pentaerythriol phosphate (MPP), as a new single molecule intumescent flame-retardant, was prepared from pentaerythritol, phosphoric acid, and melamine, and then incorporated into polypropylene (PP) with organic montmorillonite (OMT) to obtain flame retardant PP/MPP/OMT composites. The flammability and combustion behavior of flame retardant PP composites were characterized by using LOI, UL-94 test, and cone calorimeter, respectively. The results showed that the flame retardant properties of the composite containing 29.0 wt% MPP and 1.0 wt% OMT are the best among all the composites. The digital photographs after cone calorimeter test demonstrated that moderate OMT could promote to form the homogenous and compact intumescent char layer.     

Measuring fuel transport through fluorocarbon and fluorine-free firefighting foams

A flux chamber was designed to measure the transient fuel transport through a foam layer before significant degradation of foam occurred. The fuel transport rate through AFFF (fluorinated foam) was much slower than through RF6 (fluorine-free foam) with break-through times being 820 s and 276 s respectively over n-heptane. The fuel flux through AFFF covering three fuel pools (n-heptane, iso-octane, and methyl-cyclohexane) was also measured. AFFF had the smallest flux over iso-octane with a break-through time over 1900 s and the highest flux over methyl-cyclohexane with a break-through time under 80 s even though the fuels have similar vapor pressures at room temperature. Despite the lack of aqueous film formation on an iso-octane fuel pool, the fuel vapor flux through AFFF was much smaller relative to the methyl-cyclohexane pool, which enables film formation due to its higher surface tension than iso-octane. Our measurements of transient fuel flux show that the foam layer is a significant barrier to fuel vapor transport. The data suggest a transient mechanism based on the suppression of fuel adsorption onto bubble lamellae surfaces due to the oleophobicity of fluorocarbon surfactants, which is consistent with fuel solubility data. This suggests that surfactants that suppress fuel adsorption and solubility into bubble lamellae surfaces may reduce fuel transport through foams.     

Large-scale boilover experiments using crude oil

In order to know the mechanisms of boilover in a large tank, large-scale experiments using a 5 m diameter pan filled with crude oil were conducted. The initial fuel layer thickness was 0.45 m. At about 70 min after ignition, boilover occurred. The maximum irradiance was observed at the boilover and was about 22 times greater than that at steady burning. The increasing rate of the isothermal layer (hot zone) thickness was evaluated on the basis of measured temperature profile changes in the fuel. The measured periods from ignition to boilover coincide fairly well with those measured in the previous studies.     

Evaluation of energy-related and economic aspects of heating a family house with dendromass in the north-east of Poland

This paper presents an evaluation of energy-related and economic aspects of production of thermal energy to heat a family house with wood briquette. The object of the study was a detached house with an area of 247 m², situated in Olsztyn, in the north-east of Poland. The study lasted three years, from October 2006 to September 2009. The highest monthly consumption of wood briquette for thermal energy production: heating water for the central heating system and hot utility water production were recorded in January (1052–1333 kg/month). The average annual briquette consumption ranged from 6.36 to 6.72 t/year. With the mean lower heating value of briquette of 17.99 GJ/t, the mean consumption of energy in the fuel ranged from 114 to 121 GJ/year. The annual cost of heat production for a family house with briquette as fuel ranged from €572 to €651, during the 2006/2007 and 2008/2009 seasons, respectively. It would have been cheaper by €187–228 year^?1 to heat the house with seasoned willow chips, whereas using alternative fuels, such as hard coal (fraction 0.5–2.5 cm) oak pellets, natural gas and heating oil would have increased the cost of heat production. If the last of those fuels had been used, it would have increased the cost 3.5-fold as compared with wood briquette.