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Summary: Fires in wood waste storages cause financial losses, are difficult to extinguish, and emit large amounts of fire effluents. The mechanisms related to fires in wood chip piles are not well elucidated. To find suitable preventive measures for handling such fires in wood waste, a better understanding of the physical properties of wood waste is needed. The present study investigates how granularity affects mechanisms of smoldering fire and transition to flaming in wood chip piles. Eighteen experiments with samples inside a top‐ventilated, vertical cylinder were conducted. Heating from underneath the cylinder induced auto‐ignition and smoldering fire, and temperatures and mass loss of the sample were measured. The results showed that granularity significantly affects the smoldering fire dynamics. Material containing larger wood chips (length 4‐100 mm) demonstrated more irregular temperature development, higher temperatures, faster combustion, and higher mass losses than material of smaller wood chips (length <4 mm). The larger wood chips also underwent transition to flaming fires. Flaming fires were not observed for small wood chips, which instead demonstrated prolonged and steady smoldering propagation. The differences are assumed to be partly due to the different bulk densities of the samples of large and small wood chips affecting the ventilation conditions. Increased knowledge about these combustion processes and transition to flaming is vital to develop risk‐reducing measures when storing wood chips made from wood waste in piles. [ABSTRACT FROM AUTHOR]

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.

Results are presented from an experimental study on the ignition of the combustion modified (fire retarded) polyurethane foam Pyrell® (35.3 kg/m3 and 64.0 kg/m3) in elevated oxygen concentrations, ranging from 30% to 60%. The samples are exposed to an external flow and variable radiant heat flux on one face, and insulated on the other faces. The experiments show that Pyrell undergoes a weak smoldering reaction that requires significant assistance in the form of external heat input in order to propagate. The results also show that given sufficient oxygen and radiant heat flux, the smoldering reaction can produce enough volatile fuel and heat to trigger a gas phase ignition, i.e. a transition from smoldering to flaming, in pores in the char region. The experiments also indicate that high-density Pyrell is more ignitable than low-density Pyrell, which could be explained by the greater solid surface area for smoldering reactions to take place.

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.

Abstract The transition from smoldering to flaming is basically a homogeneous gas-phase ignition reaction that has a high potential to engulf the entire fuel bed in flames. However, the transition mechanism, especially under windy environment, is poorly understood due to lack of theoretical study. This work proposes a two-dimensional computational model with multiphase formulations to simulate the forward horizontal smoldering and spontaneous transition to flaming of cellulosic insulation. A chemical scheme of three-step solid decomposition reactions combined with one global gas-phase reaction is used in the model. The multi-physical coupling of heat, mass and momentum fields is implemented to reveal the interactions between the packed insulation bed and the surrounding free flow. This model shows that the hotter surface char layer acts as an ignition source for heating the gas mixture, and both the surface char oxidation and sample pyrolysis reactions behave like a gaseous fuel supplier for gas-phase reaction. With favorable conditions of high-temperature combustible fuel gas and sufficient oxygen supply under windy environment, the transition ultimately occurs at 2-3 cm above the leading edge of smolder front on the top surface, which is totally different from the transition within the porous fuel bed under windless condition. The predicted effects of wind velocity and bulk density of fuel bed on the smoldering velocity agree well with literature data. The modeling results for the first time give an interpretation of the spontaneous smoldering to flaming transition over the fuel bed surface. [ABSTRACT FROM AUTHOR]

Although smouldering ignition of upholstery items remains a leading cause of residential fire deaths, relatively little research is conducted on the topic. An experimental investigation of the effect of sample size on the ignition and spread of smouldering and flaming in polyurethane foam under natural flow conditions is reported here. Polyurethane foam samples are used because this is a common material in modern, residential environments and one for which there exists significant quantities of previous experimental data in the literature. Samples of different square cross-section size and a fixed height of 150 mm are insulated on all sides except the top which is exposed to a radiant heat flux and is open to the air. Samples with side lengths of 50 mm, 100 mm, and 140 mm are studied. Ignition and spread dynamics are diagnosed using thirteen thermocouples located along the vertical centre line. The onset of smouldering ignition (13 $$\hbox{kW}\, \hbox{m}^{-2}$$, 8 $$\hbox{kW}\, \hbox{m}^{-2}$$ and $$7\,\hbox{kW}\, \hbox{m}^{-2}$$ for 50 mm, 100 mm and 140 mm sample sizes respectively) is observed at significantly lower heat fluxes that flaming (45 $$\hbox{kW}\,\hbox{m}^{-2}$$, 32 $$\hbox{kW}\,\hbox{m}^{-2}$$ and $$30\,\hbox{kW}\,\hbox{m}^{-2}$$ respectively). Critical heat fluxes for smouldering and flaming ignition increase with decreasing sample size, with smouldering ignition being significantly more sensitive to sample size than flaming ignition under the size range studied. Smouldering spread rates are measured in the range from 3 $$\hbox{mm}\, \hbox{min}^{-1}$$ to $$25\,\hbox{mm}\, \hbox{min}^{-1}$$ and found to be a strong function of the heat flux and depth of the smoulder front. The effect of sample size on smouldering has been theoretically proposed before but this is the first time that this effect has been demonstrated experimentally for ignition. The fact that large samples result in the lowest critical heat flux could have implications for testing procedures and translation of results from small-scale testing to real-scale in the built environment. [ABSTRACT FROM AUTHOR]

Abstract: A numerical transport model was developed to study the spontaneous transition from smoldering to flaming combustion in polyurethane foam. The numerical transport model is two-dimensional with an eight-step reduced reaction mechanism. The reaction mechanism includes seven heterogeneous and a global homogeneous gas phase reaction and is capable of simulating both forward and opposed smoldering combustion. The current study examines the transition to flaming in normal gravity for flow assisted forward smoldering combustion as a function of an externally applied heat flux and the velocity and oxygen concentration of a forced gas flow. Reaction rates, species profiles, gas phase temperatures, and condensed phase temperatures are examined. Three reactions were found to play a major role in leading to the prediction of transition to flaming. Favorable agreement of temperature response, time to spontaneously transition from smolder to flaming, and location of the transition event between simulation results and experimental data is demonstrated. [Copyright &y& Elsevier]

• Smouldering experiments of polyurethane foam under different inclination angles, for the first time, were conducted. • Three kinds of self-sustained smouldering modes are identified by experimental observation. • The transition to flaming is more likely to occur with the increase of the inclination angle. • Energy balance analysis indicates that the average smouldering spread rate increases linearly with the sine of the inclination angle. Polyurethane foam smouldering continues to attract the attention of the fire community and has been intensively investigated. However, scarce studies have compared the differences of smouldering spread in the vertical and horizontal configurations. In this work, smouldering experiments of polyurethane foam under different inclination angles, sample lengths and densities were conducted. The smouldering temperature and spread rate were measured. It was observed that in horizontal smouldering, only with a proper sample length, sufficient oxygen supply can achieve a self-sustained smouldering spread and yield enough char for the subsequent second char oxidation, and eventually trigger a transition to flaming. Results also show that the transition to flaming is more likely to occur with the increase of the inclination angle. The effective average spread rate increases with the inclination angle and sample length. A higher-density foam resulted in a longer smouldering-to-flaming transition time and a lower spread rate, possibly attributable to a decreased oxygen supply to the reaction zone due to decreased foam porosity and permeability. However, the higher-density foam generates more residual char for further oxidation and accumulates more heat and flammable pyrolysate gases to trigger the transition to flaming. The effect of buoyancy was theoretically interpreted based on the energy balance. An expression for the smouldering spread rate was proposed by considering the buoyancy-induced flow in the energy balance equation. A good agreement between the predictions and the experimental results confirms the critical effect of inclination angle on the spread rate of smouldering combustion. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]