Flame Spread Phenomena Research Articles

Effects of thermal radiation on near-limit flame spread in a low convective microgravity environment

A numerical study is conducted to investigate the effects of thermal radiation on flame spread phenomenon close to limiting oxygen conditions in a low convective spacecraft environment. To understand the role of radiative heat loss from the flame, a set of simulations is performed where gas radiation is completely suppressed. The results show that radiation exerts a dual influence on flame spread rate depending on oxygen concentration. If there is more radiative heat loss from the flame, the limiting oxygen concentration increases, and the size of the flamelet decreases. It was observed that in the absence of gas radiation, oscillating near-limit flamelets stabilise and merge into a single flame. Further, the effects of surface radiation were studied by varying emissivity of the solid fuel and it was noted that a more emissive fuel is more susceptible to the formation of flamelets.

Study on the role of soot and heat fluxes in upward flame spread using a wall-resolved large eddy simulation approach

The present study aims to obtain further understandings of vertical flame spreading phenomena by analysing the influences of soot and individual heat flux components on PMMA walls using large eddy simulation. Total heat flux consists of convective and radiative components, but it is not clear which one has a significant role in fire spread. The computational code used is an in-house version of FireFOAM 2.2.x, which has recently undergone specific development and validation for flame spread studies by the authors. The present study has conducted numerical simulations for flame spread and full wall fire configurations. By scale-up of the PMMA size from 0.4 to 1.0 m, the convective heat flux decreased by 41.4% at the location of the pyrolysis front, radiative heat flux increased by 86.9%, and radiative heat flux due to soot grew by 215.2%. As the pyrolysis height increases from 0.3 to 1.0 m, the convective heat flux decreased by 26.8% at the location of the pyrolysis front. The radiative heat flux increased by 96.8%, and its components of combustion of the gaseous fuel and soot grew by 55.9% and 233.3%, respectively. Moreover, the ratio of radiative heat flux to total heat flux increased by 66.5%, and that of soot to radiative heat flux grew by 73.9%. The contribution of soot to radiative heat flux almost linearly increased against the pyrolysis height and that was higher at a higher pyrolysis height.

Effects of electric current and sample orientation on flame spread over electrical wires

Electrical wire is one of the primary causes of electrical fires. Experiments were conducted to study the effects of electric current and sample orientation on flame spread over polyethylene (PE) insulated copper wires. The flame spread velocity showed a non-monotonic trend with the inclination angle, i.e., the flame spread velocity firstly decreased and then increased with the increasing inclination angle. In the upward flame spread configuration, the molten polymer flew down along the wire if the wire was inclined, which resulted in a periodic flame spread phenomenon, and a sub-flame was observed behind the primary flame. The periodic time was shorter for the smaller inclination angle or the lower electric current. The downward spread seemed to be more susceptible to the electric current. A flame spread model considering the effects of inclination angle and electric current was developed. The heat transfer analyses in the preheating region were conducted to study the controlling mechanism of flame spread.

Experimental analysis of the pyrolysis of solids exposed to transient irradiation. Applications to ignition criteria

Ignition and flame spread theory is fundamental for evaluating the risk posed by a material during the early stages of a fire. This paper presents an experimental investigation aimed at understanding the parameters which govern the ignition of solids exposed to transient irradiation. Emphasis is placed on the conditions at ignition, including an energy balance to describe surface phenomena and the link between gas phase and solid phase processes. Experiments were performed in a fire calorimetry apparatus and incorporated a gas analysis system to study gas phase composition. Samples of Polyamide 6 (PA6) measuring 85 × 85 × 20 mm were used. Experiments were carried out to independently measure temperature in the solid phase and mass loss rate (MLR) over time. The MLR at ignition was calculated to be between 2.0 and 6.0 g/(m2 s) for all but 3 experiments, were outliers presented values of 7.9, 10.7 and 14.4 g/(m2 s). Temperature was recorded through the thickness of the solid, at depths of 4, 8, 12 and 16 mm from the surface. A regression analysis was used to calculate the surface temperature at ignition for all experiments, and it was found to vary between 270 and 325 °C for all but one experiment, were a temperature of 402 °C was recorded. The temperature distribution in the solid phase was used to estimate the net absorbed heat flux at the surface by applying Fourier’s law; with values ranging between 2.0 and 9.8 kW/m2. From the gas analysis, it was possible to assess the identity, mass flux and concentration of three dominant species produced before ignition: carbon monoxide, methane and hexane. These results are of value for the physical modelling of ignition and flame spread phenomena, allowing for more accurate criteria to describe the onset of ignition under a range of heating conditions.

Near limit flame spread over thin solid fuels in a low convective microgravity environment

A few microgravity experiments have reported formation of flamelets at near extinction of freely propagating opposed flow flame over solid fuels. Inspired by these observations, about which little is known so far, an established and elaborate numerical model in 3D is used explore opposed flow flame spread phenomena near oxygen extinction limit in quiescent and low convective environment (with flow velocity, U∞ = 5 cm/s). Simulations were carried out for various fuel widths show that 1 cm wide fuel has the least oxygen extinction limit among fuel widths varying between 0.5 cm and the 2D limit even after accounting for the presence of flamelets over wider fuels (except in 2D limit). Unlike quiescent environment where flamelets were found to be inherently unsteady and eventually extinguished, steadily propagating flamelets were also obtained in low convective space environment. The flamelet oscillations which arise due to thermo-diffusive instability were found to have time period close to sum of times scales in gas phase and solid phase. A scaling analysis showed that fuel area density, flow velocity and fuel Lewis number are key factors that influence size of a steadily propagating flamelets. Several of these features have been observed in experiments and predicted in the present simulations.

Study experimental of flame-spread rate and spread limit distance ofbio-solardroplets in microgravity combustion

The most important issues in spray combustion science are how to understand the mechanism of the combustion of liquid fuel spray, especially in the flame-spread phenomenon. In spray combustion, combustion is the existence of the group is very important in order to obtain the stable combustion. Therefore, the flame spread among fuel droplets affects the occurrence of stable combustion in the spray combustion engine. This study was focused on phenomenon on diesel combustion engines. This research was conducted to study the behavior of flame spread rate of flame spread and limit the distance of bio-diesel liquid droplets. This research method was used experimental research. Microgravity condition was Obtained through the free fall tower with the height of the tower used is 6 m. Observed droplets suspended was placed on SiC fibers with different distance and droplet size 1 mm. This study observed Also the influence of the flame propagation direction of the burning droplet to the next burning droplets that lies in the direction and perpendicular to the direction of the flame propagation. The results showed that the bio-diesel fuel droplet Igniter (I) could burn next droplet (A)at a distance (S/d0) = 8.

Downward flame spread and extinction phenomena of externally heated electric wire:

Downward flame spread and extinction phenomena of externally heated electric wire:

Large eddy simulation of upward flame spread on PMMA walls with a fully coupled fluid–solid approach

A fully coupled fluid–solid approach has been developed within FireFOAM 2.2.x, a large eddy simulation (LES) based fire simulation solver within the OpenFOAM® toolbox. Due consideration has been given to couple the radiative heat transfer and soot treatment with pyrolysis calculations. Combustion is modeled using the newly extended eddy dissipation concept (EDC) for the LES published by the authors’ group. Soot formation and oxidation are handled by the published extension of the laminar smoke point concept to turbulent fires using the partially stirred reactor (PaSR) concept also from the authors’ group. The gases radiation properties are evaluated using the established weighted sum of grey gas model while soot absorption coefficient is calculated using a single Planck-mean absorption coefficient. The effect of in-depth radiation is treated with the relatively simple Beer's law and the solid surface regression length is calculated from the local pyrolysis rate. Systematic validation studies have been conducted with several published experiments including simple pyrolysis test without the gaseous region, small scale wall fires and large scale flame spread. The predictions are in very good agreement with the relevant experimental data, demonstrating that the present modeling approach can be used to predict upward flame spread over PMMA with reasonable accuracy. Further parametric studies have also been conducted to demonstrate the effectiveness of the present modifications to capture the underlying physics. The detailed field predictions for vortex structures and flame volume including laminar–turbulent transition have also been analysed to uncover further insight of the unsteady flame spread phenomena. Potentially, the model can be used to aid further fundamental studies of the flame spread phenomena such as investigating the effects of width, inclination angles and side walls on flame spread as well as the predictions of flame spread in practical applications.

The dynamics of near limit self-propagating flame over thin solid fuels in microgravity

Microgravity experiments have shown that near limit opposed flow flame-spread show unsteady oscillatory behavior marked with formation of flamelets. Here in this work an elaborate 3D numerical model is used to predict this near limit fame-spread behavior of a self-propagating flame over thin solid in microgravity. As the oxygen level is reduced steady, below certain value, flame-spread over wide thin solid fuel becomes unsteady. Two kinds of unsteady flame-spread phenomena were noted. In one a single flame oscillated back and forth near the fuel side edge and in other the multiple flamelets were formed which oscillated laterally. The former occurs at relatively higher oxygen levels and leads to formation of the later at still lower oxygen levels just ahead of extinction. It is noted that as oxygen level is reduced the amplitude of longitudinal oscillation increases leading to the splitting of flame which then exhibits lateral motion. The lateral oscillatory behavior spans over a wider range of oxygen level than that of longitudinal oscillatory flame behavior. The range of oxygen level over which the lateral oscillatory behavior is observed, increases with increase in fuel thickness and fuel width. The number of flamelets formed increases with increase in fuel width with typical flamelet size of 2–4cm. Some of the flame behavior noted here are remarkably similar to those seen in the short duration drop tower test where heat loss from the flame was artificially enhanced. However, the numerical computations show that such oscillatory flames can exist for considerably long durations.

PMMA壁面の上方向への燃え拡がりのLarge eddy simulation

A fully coupled fluid-solid approach for flame spread simulations has been developed using FireFOAM. Radiative heat transfer and soot treatment are fully coupled with the pyrolysis calculations. For gaseous combustion, the newly extended eddy dissipation concept (EDC) for the large eddy simulation (LES) is used. Due to low Reynolds number, the laminar combustion model based on viscous diffusion is presented and coupled with the EDC model. The soot model is based on the laminar smoke point concept recently extended to turbulent fires using the partially stirred reactor (PaSR) concept. For radiation, the finite volume discrete ordinate method (FVDOM) is used with the gaseous absorption coefficients evaluated using the polynomial coefficient of temperature. In the solid region, one dimensional diffusion equation for sensible enthalpy is solved. The effect of in-depth radiation is taken into account using the relatively simple formation according to Beer's law. The Arrhenius type pyrolysis model developed by FM Global is used along with their measurements of the PMMA physical properties as input data. The validation study has been conducted with the published experiment. The predictions are in very good agreement with the relevant experimental data, demonstrating that the present modelling approach can be used to predict upward flame spread over PMMA with reasonable accuracy. Such a fully coupled predictive tool can be used to aid fundamental studies of the flame spread phenomena such as investigating the effects of width, inclination angles and side walls on flame spread.

Flame Spread and Extinction Over a Thick Solid Fuel in Low-Velocity Opposed and Concurrent Flows

Flame spread and extinction phenomena over a thick PMMA in purely opposed and concurrent flows are investigated by conducting systematical experiments in a narrow channel apparatus. The present tests focus on low-velocity flow regime and hence complement experimental data previously reported for high and moderate velocity regimes. In the flow velocity range tested, the opposed flame is found to spread much faster than the concurrent flame at a given flow velocity. The measured spread rates for opposed and concurrent flames can be correlated by corresponding theoretical models of flame spread, indicating that existing models capture the main mechanisms controlling the flame spread. In low-velocity gas flows, however, the experimental results are observed to deviate from theoretical predictions. This may be attributed to the neglect of radiative heat loss in the theoretical models, whereas radiation becomes important for low-intensity flame spread. Flammability limits using oxygen concentration and flow velocity as coordinates are presented for both opposed and concurrent flame spread configurations. It is found that concurrent spread has a wider flammable range than opposed case. Beyond the flammability boundary of opposed spread, there is an additional flammable area for concurrent spread, where the spreading flame is sustainable in concurrent mode only. The lowest oxygen concentration allowing concurrent flame spread in forced flow is estimated to be approximately 14 % O-2, substantially below that for opposed spread (18.5 % O-2).

Experimental Study on upward Flame Spread of Exterior Wall Thermal Insulation Materials

Abstract The objective of this paper was to gain an understanding of the upward flame spread phenomenon of thermal insulation materials as extruded polystyrene (XPS) and polyurethane (PU) foams. Plenty of these materials sustained flame spread under line n-heptane fire source regardless of edge effects. Different flame spread phenomena of XPS and PU were observed and the heat release rate was captured to analyze their burning performance. Flame height was recorded as a function of heat release. Relationship between flame height and total heat flux was revised to quantitatively investigate the effect of the line fire source on flame-retarded and non-retarded materials. This result was predicted in terms of measured fuel thermophysical properties, flame heights and heat feedback to the fuel surface. Total heat feedback to the sample was measured. The upward spread rate for vertical PU slabs during the early-stage burning was observed to increase exponentially with time.

Gas dynamics and heat transfer inside a solid propellant crack during ignition transient

To study the gas dynamic and heat transfer phenomena inside a single isolated longitudinal solid propellant surface crack, two 3-D geometric models with different crack shapes were constructed. Concerning the influence of propagation of jet from the igniter on the flame spreading phenomena in the crack, flow region around the opening of the crack was also included in the above geometric models. A theoretical framework was then adopted to model the conjugate heat transfer in the combustion channel and the crack cavity. Numerical simulation results indicate that the ignition shock wave can spread into the crack cavity. Extremely high overpressure and pressurization rate were observed along the crack front. It is possible that the crack may propagate before the flame front reaches it. An ignited region located at the crack front near to the channel surface in downstream direction was generated long before the flame front reached the crack opening in both models.

On the role of radiation and dimensionality in predicting flow opposed flame spread over thin fuels

In this work a flame-spread model is formulated in three dimensions to simulate opposed flow flame spread over thin solid fuels. The flame-spread model is coupled to a three-dimensional gas radiation model. The experiments [1] on downward spread and zero gravity quiescent spread over finite width thin fuel are simulated by flame-spread models in both two and three dimensions to assess the role of radiation and effect of dimensionality on the prediction of the flame-spread phenomena. It is observed that while radiation plays only a minor role in normal gravity downward spread, in zero gravity quiescent spread surface radiation loss holds the key to correct prediction of low oxygen flame spread rate and quenching limit. The present three-dimensional simulations show that even in zero gravity gas radiation affects flame spread rate only moderately (as much as 20% at 100% oxygen) as the heat feedback effect exceeds the radiation loss effect only moderately. However, the two-dimensional model with the gas radiation model badly over-predicts the zero gravity flame spread rate due to under estimation of gas radiation loss to the ambient surrounding. The two-dimensional model was also found to be inadequate for predicting the zero gravity flame attributes, like the flame length and the flame width, correctly. The need for a three-dimensional model was found to be indispensable for consistently describing the zero gravity flame-spread experiments [1] (including flame spread rate and flame size) especially at high oxygen levels (>30%). On the other hand it was observed that for the normal gravity downward flame spread for oxygen levels up to 60%, the two-dimensional model was sufficient to predict flame spread rate and flame size reasonably well. Gas radiation is seen to increase the three-dimensional effect especially at elevated oxygen levels (>30% for zero gravity and >60% for normal gravity flames).

Experimental Study of Temperature Distribution and Flame Spread Over an Inert Porous Bed Wetted with Liquid Fuel

A combined experimental and modelling study was carried out to investigate the flame spread phenomenon over a porous bed wetted with finite quantity of super-flash liquid fuel. Measurements included flame spread rate, temperature distribution, and visual observation. Sand particles ranging from 0.5 to 5 mm and Propanol (flash point 12 °C) were used as porous bed and liquid fuel, respectively.Results corresponding to the rate of flame spread show that regardless of airflow speed and/or direction the rate of flame spread decreases as either the bed depth or particle size increases, however the flame spread deceleration rate is more distinct under opposed airflow.Temperature measurements and mathematical modelling results indicated the existences of three different temperature regions in the bed. The magnitude of temperature in upper region was significantly larger than those of the lower region. The modelling results were in good agreement with the experimental data.

Observation of Flame Spreading over Electric Wire under Reduced Gravity Condition Given by Parabolic Flight and Drop Tower Experiments

Ground-based, microgravity experiments attained by aircraft parabolic flight and drop tower on flame spread phenomenon over electric wire are performed. These are the preliminary tests for expected long-term microgravity experiments by sub-orbital or on orbit microgravity experiment. The main objectives of this study are (1) to confirm the apparatus can be work properly in microgravity and (2) to show the necessity of long-term microgravity experiments in order to observe the unsteady phenomenon. The flame spread rate and the total soot volume are important items as fundamental characteristics of the spreading flame. From the parabolic flight test, which can provide relatively long microgravity period, it is confirmed that the apparatus can work properly in microgravity. On the other hand, the quality of microgravity provided by aircraft is fair including G-jitters, and dependable data in the slow external flow velocity regime is hardly expected. Flame spread rate and the total soot volume are measured by drop tower, which can provide 10-4G microgravity environment. Although, in some conditions, the flame spread phenomenon seems to reach steady-state within the available microgravity time with the drop tower (∼ five seconds), the phenomenon includes periodical change in flame shape in reality. Consequently, to confirm the actual steadiness of the spread phenomenon, at least 10-4G microgravity environment and long-term microgravity environment is necessary.

Flame-Spreading Behavior in a Fin-Slot Solid Propellant Rocket Motor Grain (Part II)

To accurately predict the overall ignition transient for the reusable solid rocket motors of the space shuttle booster with head-end fin slots, it is necessary to have the knowledge of the flame-spreading rates in the fin-slot region. This paper is the second of a two part study and deals with the development of a flame-spreading correlation in the fin-slot region. A subscale (1:10) pie-shaped fin-slot motor was designed to perform diagnostic measurements for studying the flame-spreading behavior on the exposed propellant surface. Dynamic similarity was considered in the igniter design so the impinging jet had a similar exit angle onto the propellant surface in the fin-slot section. Flame-spreading measurements were gathered using a high-speed digital camera and nonintrusive optical measurement methods through an array of 36 near-infrared fast-response photodetectors installed perpendicular to representative regions of the propellant surface. Results showed that the flame-spreading phenomena was highly nonuniform, starting in the downstream portion of the fin-slot region before traveling back toward the igniter. A correlation was developed for the dimensionless flame-spreading time interval showing that it was inversely proportional to the pressurization rate to a power of 0.62, which depends strongly upon the flow parameters of the igniter induced flow and local propellant grain geometry.

Application of a Statistical Narrow Band Model to CFD Modelling of Upward Flame Spread Over PMMA Walls

A statistical narrow band (SNB) radiation gas model has been incorporated into a Reynolds-averaged Navier-Stoke (RANS) based computational fluid dynamics (CFD) code. The coupled CFD code is used in this work to simulate two upward flame spread scenarios: the first one concerns flame spread over a vertical PMMA wall, while the other represents flame spread along vertical corner walls. For comparison purpose, the weighted-sum-of-greygases (WSGG) model was also used. Simulation results indicate that the CFD model is capable of predicting reasonably the flame spread phenomenon over simple materials such as PMMA. Compared to the WSGG approach, the SNB model generally yields more accurate results thanks to a better predicted gas flow field which in turn improves the prediction of the radiative heat fluxes imposed on solid surfaces and consequently that of the flame spread rate and heat release rate.

Heat Transfer Mechanism of a Flash Flame Spread Along a Vertical or Inclined Napped Fabric

A napped fabric is known to catch fire easily, and the flame spreads along the napped surface very fast; i.e., a flash flame spread occurs. It may cause a person wearing the napped fabric to be burned alive. To avoid such a fire accident, flame spread phenomena along a napped fabric must be clarified for suppressing the flash flame spread along its surface. For this purpose, the flash flame spread along a napped fabric has been investigated experimentally. The absorbent cotton sheet napped mechanically was used as a sample of a napped fabric. Dependence of the flash flame spread rate on depth or density of a napped layer was studied experimentally. It was ascertained that the flame spread rate depended on both the depth and the density of the napped layer. Considering the experimentally obtained dependency, a heat transfer model of the flame spread along a napped layer was developed. From the model that considered the experimental results, it was revealed that a convective heat transfer governed the flame spread rate strongly in the case of upward flame spread along a napped fabric. Language: en

Effects of fuel Lewis number on flame spread over solids

Using a detailed two-dimensional numerical model, a systematic investigation has been made to study the effect of fuel Lewis number (Le F = α/D F) and mass transfer on flame spread over thin solids. The fuel Lewis number affects the flame spread rates for both concurrent and opposed flames over thin fuels. The dependence of the flame spread rate on Le F for these two spreading modes is, however, not the same. In opposed flame spreads (zero-gravity, self-propagation, and normal gravity downward propagation), the flame spread rate vs. Le F curve is non-monotonic with a maximum value occurring at an intermediate value of Le F = 0.5. In steady, concurrent spread in zero-gravity with low-speed flow and a constant flame length, the flame spread rate decreases with Le F in a monotonic manner. By using the computational model as a tool, the effects of fuel mass diffusion perpendicular to and parallel with the solid surface are isolated to obtain more physical insight on the two-dimensional aspect of fuel mass transfer on flame spread. In addition, the model has also been used to decouple the solid evaporation process so that the fuel diffusion effect in the gas-phase can be isolated. Both of these theoretical exercises contribute to the understanding of mass transfer effects on the flame spreading phenomena over solids.