Mechanism Of Flame Spread Research Articles

Downward flame spread over PMMA sheets in quiescent air: Experimental and theoretical studies

To explore the flame spread mechanisms over noncharring material, downward flame spread over polymethyl methacrylate (PMMA) sheets from 1.5 to 10 mm thick is studied experimentally in the quiescent air in an open space. The experimental results show that the flame spread rate decreases with increasing thickness of sheet and it tends to a constant value for thick samples. Also the angle of the pyrolysis region is roughly constant for different sheet thicknesses. In this study, a heat transfer model is proposed to examine the rate of heat transfer for noncharring materials. Based on this model and the experimental results, a correlation between the flame spread rate and thickness is derived, in which the flame spread rate is inversely proportional to the sheet thickness. The proposed model estimated the convection heat flux and the gradient of temperature in the solid and gas phases. The experimental data are in good agreement with the results of the theoretical model.

Propagation and extinction mechanisms of opposed-flow flame spread over PMMA for different sample orientations

To understand the propagation and the extinction mechanisms of flame spreading along a combustible solid, opposed-flow flame spread along a thick slab of PMMA was experimentally investigated for four different sample orientation angles from 0° (horizontal) to −180° (ceiling flame spread) in several airflow rates. The detailed temperature distribution in the condensed phase during flame spread was measured using holographic interferometry (HI) and infrared thermography (IR). The particle-track laser sheet (PTLS) technique was employed to measure the local entrainment velocity to the flame front at the leading edge. This study found that a flame spread rate with a constant speed is proportional to the net total heat transfer rate. The heat transfer rate from the gas phase, Q y , is about 60% of the total heat transfer rate, Q T , under the no-imposed-flow horizontal flame spread condition ( u ¯ = 0 m / s ). However, the heat transfer rate through the condensed phase, Q x , increases with increasing airflow above 80% of Q T near the extinction limit. The radiative heat loss from the surface, Q R , increases with increasing opposed-flow rate and reaches a maximum of 13% of Q T at u ¯ = 0.65 m / s , while the heat transfer rate in ceiling flame spread is different than in horizontal or downward flame spread under nonopposed and slow-opposed flow conditions. The Q y in nonopposed flow ceiling flame spread is double that in horizontal flame spread, and is 85% of Q T . Despite adequate heat feedback to the condensed phase near the flame leading edge, the flame spread rate decreases rapidly as it approaches the extinction limit. In order to interpret the absence of flame spread or the extinction mechanism, we introduce the Damköhler number including the local air entrainment velocity and the burning rate at finite flame thickness. PTLS showed that the local entrainment velocity at the flame leading edge increased the opposed-flow rate. This leads to a decrease in the Damköhler number. When the Damköhler number reaches a critical value (at most <1), the flame cannot spread, retreats, and then is finally extinguished.

Scaling and instability analyses on flame spread over liquids

Stability and scaling analyses were applied to experimental data obtained by this group and other researchers on pulsating flame spread over liquids. Data to be analyzed include recent findings of cyclic appearance of a cold temperature valley at the liquid surface-created surface-wave ahead of the spreading flame, and main-pulsation of 0.5–2 Hz and sub-pulsation of 5–10 Hz. Our stability analysis is performed to understand the mechanism of instability on the liquid surface ahead of a flame’s leading edge, which is thought of as the major cause for pulsating flame spread. The scaling analysis is performed to explore the role of four independent (gravity, surface-tension, viscose, and inertia) forces on the mechanisms of flame spread. These four forces form three independent pi-numbers: Marangoni ( Ma) number, Weber ( We) number, and Froude ( Fr) number, all of which include the critical length scale ratio: (height of sub-surface circulation)/(horizontal length of preheated liquid surface). We combined the wave equation obtained from the stability analysis, the three pi-numbers, and the critical length scale ratio, and used them as a universal formula to describe flame spread over liquids. Using this formula, flame spread mechanism over four different types of alcohols was divided into two separate regimes: the thin liquid pool and the thick-liquid pool. For the thin liquid pool, the flame spread rate was correlated with ( Fr/Ma 0.5) −1.0, while for the thick-liquid pool it was correlated with ( Fr/Ma 0.5) −1.5. Change of flame spread pattern from the uniform to the pulsating can be described with temperature difference between the flash point and bulk liquid temperature. For the thin liquid pool this temperature difference is correlated with Ma −0.5, while for the thick-liquid pool it is correlated with Ma −1. The frequency of pulsation is correlated with We −1.0 for the thin liquid pool, while it is correlated with We −1.5 for the thick-liquid pool.

Effect of ambient pressure and radiation reabsorption of atmosphere on the flame spreading over thermally thin combustibles in microgravity

For the flame spread over thermally thin combustibles in an atmosphere, if the atmosphere cannot emit and absorb the thermal radiation (e.g. for atmosphere of O 2 -N 2 ), the conductive heat transfer from the flame to the fuel surface dominates the flame spread at lower ambient atmosphere. As the ambient pressure increases, the flame spread rate increases, and the radiant heat transfer from the flame to the fuel surface gradually becomes the dominant driving force for the flame spread. In contrast, if the atmosphere is able to emit and absorb the thermal radiation (e.g. for atmosphere of O 2 -CO 2 ), at lower pressure, the heat transfer from flame to the fuel surface is enhanced by the radiation reabsorption of the atmosphere at the leading edge of the flame, and both conduction and thermal radiation play important roles in the mechanism of flame spread. With the increase in ambient pressure, the oxygen diffuses more quickly from ambient atmosphere into the flame, the chemical reaction in the flame is enhanced, and the flame spread rate increases. When the ambient pressure is greater than a critical value, the thermal radiation from the flame to the solid surface is hampered by the radiation reabsorption of ambient atmosphere with the further increase in ambient pressure. As a result, with the increase in ambient pressure, the flame spread rate decreases and the heat conduction gradually dominates the flame spread over the fuel surface.

Opposed-flow ignition and flame spread over melting polymers with Navier-Stokes gas flow

A numerical model is constructed to predict transient opposed-flow flame spread behaviour in a channel flow over a melting polymer. The transient flame is established by initially applying a high external radiation heat flux to the surface. This is followed by ignition, transition and finally steady opposed-flow flame spread. The physical phenomena under consideration include the following: gas phase: channel flow, thermal expansion and injection flow from the pyrolyzed fuel; condensed phase: heat conduction, melting, and discontinuous thermal properties (heat capacity and thermal conductivity) across the phase boundary; gas-condensed phase interface: radiation loss. There is no in-depth gas radiation absorption in the gas phase. It is necessary to solve the momentum, species, energy and continuity equations in the gas along with the energy equation(s) in the liquid and solid. Agreement is obtained between the numerical spread rate and a flame spread formula. The influence of the gas flow is explored by comparing the Navier-Stokes (NS) and Oseen (OS) models. An energy balance analysis describes the flame-spread mechanism in terms of participating heat transfer mechanisms.

Propagation and extinction mechanisms of opposed-flow flame spread over PMMA

To understand the propagation and the extinction mechanisms of spreading flame along a solid fuel, opposed-flow flame spread along a thick polymethyl methacrylate slab was experimentally investigated for several airflow rates from 0 to 0.67 m/s. The detailed temperature structure during flame spread near the flame leading edge was measured using holographic interferometry and IR thermography. The particletract laser sheet (PTLS) technique was employed to measure the local entrainment velocity to the flame front at the leading edge. This study found that the flame spread rate with a constant speed is proportional to the net total heat transfer rate. The heat transfer rate from the gas phase, Qy, is about 60% of the total heat transfer rate, QT, in the no imposed flow condition (u=0m/s). However, the heat transfer rate through the condensed phase, Qx, is over 80% of QT in the opposed flow near the extinction limit (u¯≈0.65m/s). The radiative heat loss from the surface, QR, increases with increasing opposed-flow rate and reaches at most 13% of QT at u¯≈0.65m/s. In spite of enough heat feedback to the condensed phase near the flame leading edge, the flame spread rate rapidly decreases close to the extinction limit. In order to interpret the extinction mechanism, we introduce the burning rate at the finite flame sheet with finite chemical reaction rate. The PTLS result shows that the local entrainment velocity at the flame leading edge increases with increasing opposed-flow rate. When the local entrainment velocity exceeds the burning rate at the leading edge, the flame may not be sustained. The flame retreats and finally it is extinguished.

Effects of Ambient Atmosphere on Flame Spread over Solid Fuel. Effects of Ambient Temperature and Limiting Oxygen Concentration for Downward Flame Spread.

The downward flame spread over solid fuel has been studied. Paper sheets are used as samples, and experiments are conduced in a combustion chamber with a vertical duct to obtain uniform ambient atmosphere, keeping the inflow velocity constant. The gas composition and temperature can be changed to examine the effects of ambient atmosphere. The downward flame spread rate has been measured at lower oxygen concentration and preheated temperature (∼150°C). Results show that, flame spread rate is increased with an increase of ambient temperature. As the oxygen concentration is decreased, the flame spread rate is decreased monotonically. Then, the flame can not propagate and the flame extinction occurs at some oxygen concentration. We define this condition as the limiting oxygen concentration for downward flame spread. It can be reduced as the ambient temperate is increased. To discuss these experimental facts, we estimate the net heat flow into the preheat region, which is an important factor to sustain the flame spread. The net heat flow into the preheat region is decreased as the oxygen concentration is smaller. It is interesting to note that this heat flow near the flame extinction is smaller with an increase of the ambient temperature. These experimental data are useful to make clear the controlling mechanism of flame spread.

Opposed-flow flame spread over polymeric materials: influence of phase change

A numerical model for flame spread over polymeric surfaces is constructed. The dependence of the flame spread rate on phase change and thermal properties is investigated by varying three non-dimensional parameters, St, k̄ l , and C̄ Pl . Quantitative comparisons indicate that the numerical model provides excellent agreement to an analytical formula in the cases of variable latent heat of the phase change, variable liquid thermal capacity, and variable thermal conductivity. However, the deRis formula yields a constant spread rate higher than the numerical result and is independent of phase change. Qualitatively, with the increase of St, or with the decrease of k̄ l or C̄ Pl , the flame spread rate increases. In addition, k̄ l is the strongest determinant of the influence of the thickness of the liquid region. The mechanisms of flame spread at the steady state are interpreted by applying an energy balance principle for the control volume upstream of the flame leading edge. It is found that a ratio between the total heat applied to the condensed material upstream of the flame leading edge and the spread rate reveals the physical mechanisms that control the preheating of the condensed material to the ignition temperature. The dependence of flame structure on St, k̄ l , and C̄ Pl is studied. It is found that with the increase of St or k̄ l , or with the decrease of C̄ Pl , the size of the flame increases. These results indicate that flame size dependence follows the magnitude of the spread rate when the properties of the condensed material are variable.

Flame spread over liquid fuel films on metallic substrates

New experimental studies of parametric dependences of the flame spread velocity and limits for liquid fuel films on metallic substrates confirmed the main features of the physical model proposed previously. For thermally thin layered systems “fuel-substrate,” a steady-state regime of flame spread is possible. It is shown that the flame velocity depends on the effective thermal diffusivity of the layer system, and its value is determined mainly by the volumetric heat capacities of the components of the system and, to a lesser degree, by their thermal conductivities. The mechanism of flame spread includes a series of interrelated elementary processes: heat conduction over the substrate from the combustion zone to the preflame zone, heating and evaporation of the fuel by the substrate, formation of a combustible mixture, and heating of the metallic substrate by the combustion products. The flame edge is located at the liquid surface, where the temperature corresponds to the formation of a stoichiometric mixture under equilibrium conditions. The liquid fuel is completely evaporated from the substrate at temperatures below the boiling point.

Mechanisms of concurrent-flow flame spread over solid fuel beds

It is proposed that concurrent-flow flame spread over solid fuel beds will be steady under conditions where heat and momentum losses to the sides of the fuel samples and/or surface radiative losses are significant. These losses are argued to be unavoidable because the flame length will grow until these losses balance the heat and momentum generation rates. Approximate relations (with no adjustable parameters or necessity for supplemental measured quantities such as heat fluxes or pyrolysis times) are derived for steady spread rates in the presence of these losses for laminar and turbulent flow, buoyant and forced convection, and thin and thick fuels. Experimental tests of these relations were conducted for upward flame spread over thermally thin fuels. Varying pressures, oxygen mole fractions, and diluents were employed to cover a seven-decade range of the Grashof number. These experiments generally support the validity of the proposed mechanisms.

Heat Transfer Pathways in Flame Spreading Over Thick Fuels as a Function of the Flame Spread Regime: Microgravity, Thermal, and Kinetic

The mechanism of flame spread over thermally thick solid fuels in an opposing flow is explored. The pathways of forward heat transfer are computed with mathematical models of increasing complexity in all regimes. New formulas are developed that reproduce numerical results for the dominant forward heat transfer pathways in the thermal and chemical kinetic regimes and compare well with experimental data. The heat transfer structure at the leading edge of the flame computed by the comprehensive model agrees quite well with available experimental and numerical results. In the thermal regime, forward heat transfer through the gas is dominant, while in the chemical kinetic and microgravity regimes. forward heat transfer through the solid may assume a significant role. In the latter regime the contribution of forward radiation heat transfer is found to be negligible, although gas-phase radiation has a profound effect on the flame structure. The sensitivity of the dimensionless contribution of forward heat transfer through the solid fuel to that through the gas. Rsfc, on the location of the flame leading edge reconciles seemingly contradictory results found in the literature. A semi-empirical predictive formula for Rsfc, is developed supported by numerical and experimental results. The result, expressed as: Rsfc,=Csfc, (Qsfc/Qgfc)=csfc(λs/λp) (1/F(Ls/Lg))0.5, requires the flame spread rate, Vf, to be available directly from expenment or correlation. Two limiting forms of this quantity are obtained for the thermal regime and the chemical kinetic blowoff limit; Rsfc, thermal=Csfc(λs/λg and Rsfc.blowoff=Csfc(λs/λg(1/F). Values of both limiting forms can be determined from known parameters of the problem.

A simplified theory for de ris flame over thick and thin fuel beds

The creeping flame spread problem in an opposed-flow environment over a flat fuel, called here de Ris flame, is revisited. Although the exact solution for the spread rates is well known (de Ris 1969; Delichatsios 1986), very little is definitively known about the flame structure, although a number of analytical and numerical studies have addressed this topic. None of the numerical studies have reproduced the exact solutions for the spread rates to date. The thick-fuel problem is reformulated in terms of five non-dimensional governing parameters retaining all the assumptions of the original theory; the thin fuel formulation is obtained as a limiting case. Direct numerical solution is presented first. The exact solution for the spread rate for thick fuel is reproduced for widely different sets of the controlling parameters. For thin fuel, however, the exact solution can be reproduced only when the flame hang-distance, the distance between the flame leading edge and the pyrolysis front, approaches zero. A finite hang-distance is found to cause an increase in spread rate by as high as 60%. This deviation from the exact formula is expressed in terms of a correction factor which is correlated with the hang-distance, which in turn is correlated with the known governing parameters. The thick-fuel flame is found to hug the surface in contrast to a lifted flame over a thin fuel. A simplified theory is proposed to predict the flame structure in the downstream region. The closed-form solutions for the conserved scalars reproduce the downstream flame structure for both the thick and thin fuel with remarkable accuracy and provide insight into the mechanism of flame spread.

96/00741 A study on the efficient conversion of Indonesia Pasir coal

The effects of centrifugal force on the combustion of propane-air and hydrogen-air mixtureswere evaluated in a combustion centrifuge. Tests using propane-air mixtures over a range of equivalence ratios, and hydrogen-air mixtures at an equivalence ratio of 0.6, showed that the flame-propagation rate was relatively independent of centrifugal force in acceleration fields of less than 200 times that of the earth (200 g), but increased approximately in proportion to the square root of the acceleration field at values above 200 g. Conversely, for a stoichiometric hydrogen-air mixture, the flame-propagation rates were independent of centrifugal force.A hypothesis, based on a “bubble transport” flamespreading mechanism, is proposed to explain the conflicting experimental results. Buoyant forces acting on a flame “bubble” move it in a direction opposite to that of the applied centrifugal force at a velocity corresponding to the condition at which the buoyant and drag forces on the bubble are equal. At large values of centrifugal force, the bubble velocity exceeds the turbulent flamespeed and controls the flamespreading rate; at lower values, the bubble velocity is less than the turbulent flamespeed and does not affect flame propagation. In this investigation, bubble velocities to 65 fps have been measured; these exceed conventional propane-air turbulent flamespeeds nearly fourfold.In many tests, an upper limit of the imposed acceleration field was reached, above which combustion could not be sustained. This limit, which was preceded by an abrupt decrease in flamespeed with increasing acceleration, appears to be a function of laminar flamespeed, and can be expressed approimately by g=4500Su, where Su is the laminar flamespeed for the mixture.

96/00753 Guide to CHP and on-site generation

The effects of centrifugal force on the combustion of propane-air and hydrogen-air mixtureswere evaluated in a combustion centrifuge. Tests using propane-air mixtures over a range of equivalence ratios, and hydrogen-air mixtures at an equivalence ratio of 0.6, showed that the flame-propagation rate was relatively independent of centrifugal force in acceleration fields of less than 200 times that of the earth (200 g), but increased approximately in proportion to the square root of the acceleration field at values above 200 g. Conversely, for a stoichiometric hydrogen-air mixture, the flame-propagation rates were independent of centrifugal force.A hypothesis, based on a “bubble transport” flamespreading mechanism, is proposed to explain the conflicting experimental results. Buoyant forces acting on a flame “bubble” move it in a direction opposite to that of the applied centrifugal force at a velocity corresponding to the condition at which the buoyant and drag forces on the bubble are equal. At large values of centrifugal force, the bubble velocity exceeds the turbulent flamespeed and controls the flamespreading rate; at lower values, the bubble velocity is less than the turbulent flamespeed and does not affect flame propagation. In this investigation, bubble velocities to 65 fps have been measured; these exceed conventional propane-air turbulent flamespeeds nearly fourfold.In many tests, an upper limit of the imposed acceleration field was reached, above which combustion could not be sustained. This limit, which was preceded by an abrupt decrease in flamespeed with increasing acceleration, appears to be a function of laminar flamespeed, and can be expressed approimately by g=4500Su, where Su is the laminar flamespeed for the mixture.

Effects of solid-phase properties on flames spreading over composite materials

The problem of downward flame spread over composite, thermally thick materials is theoretically investigated by means of a thermal, analytical model and a numerical model based on the reactive fully elliptic Navier-Stokes equations. The solid, made of a thermoplastic polymer and inert additives, undergoes in-depth endothermic pyrolysis, for the active part, with volatile monomer formation. The effective thermal conductivity of the composite material depends on the content of inert and the variable content of the active part, whose thermal conductivity varies between that of the polymer and that of the melted phase monomer. Even though surface regression is not taken into account, polymer consumption is modeled through a mass balance. The changes in the flame spread mechanisms with solid phase properties are investigated. The spread process is strongly affected by the solid perpendicular (to the spread direction) thermal conductivity which, when it decreases, causes a continuous increase in the spread rate. On the other hand, both numerical and analytical solutions give no dependence of the spread rate on the solid parallel (to the spread direction) thermal conductivity for a wide range of variation. In agreement with previous experimental results, at very large values of the latter, the finite-rate reaction model predicts a decrease in the spread rate. However, in contrast to certain experiments performed for non homogeneous composite samples, an increase of the spread rate with the effective solid parallel thermal conductivity or thermal capacity has not been found.

Pulsating Flame Spread Over Liquids

This study was undertaken as an effort to improve our understanding of the mechanisms of pulsating flame spread over liquids. A holographic interferometry technique was applied to obtain a time series of simultaneous temperature profiles in gas and liquid phases for pulsating spread over propanol. Liquid motion at the free liquid surface was visualized using a particle-track technique and recorded with a high speed, high resolution video-camera from which pulsation frequency and flame behavior were analyzed. Our experimental results support the hypothesis that the primary control mechanism of pulsating spread is subsurface-liquid flow, while the effect of gas-phase convection is secondary even though it governs flame pulsation. To evaluate this interpretation, two dimensional incompressible flow equations were numerically solved and fluid dynamic structure was simulated as a function of fuel-layer thickness. The calculation results explain the present and NASA's experimental results and support our proposed mechanism of pulsating spread.

Behavior of fires spreading downward over thick paper

To explore the flame spread mechanisms over solid fuel sheets, downward flame spread over papersheets from 0.4 to 10-mm thick have been examined. The dependence of flame spread rate V on thickness δ indicates that the spreading behavior can bedivided into four types observed under conditions in four regions. The flame spread is found to take place for sample thickness δ <8.4 mm in the present experiments under the conditions of natural convection. Near the limiting thickness, unstable flame spread is observed, and its behavior is examined. The length L of pyrolysis region is measured as a characteristic length representing the pyrolysis area. In the range of δ <7.5 mm, the value of L increases as δ increases. A simple heat transfer model is proposed in this paper to examine the heat transfer for charring materials.It is assumed that the heat transferred through the solid surface ahead of the leading edge of the pyrolysis region is constant and that penetrating through the pyrolysis region is proportional to the area of the region. An equation as a function of V and L is derived from this model, which expresses the relation between L and ρδV . The heat flux penetrating through the pyrolysis region is estimated much smaller than that through the solid surface ahead of the leading edge of the pyrolysis region. It is also inferred that the flame stagnation is caused by the decrease of the fuel gas ejected from the unit area of the pyrolysis surface.

Mechanisms of Microgravity Flame Spread Over a Thin Solid Fuel: Oxygen and Opposed Flow Effects

Microgravity tests varying oxygen concentration and forced flow velocity have examined the importance of transport processes on flame spread over very thin solid fuels. Flame spread rates, solid phase temperature profiles and flame appearance for these tests are measured. A flame spread map is presented which indicates three distinct regions where different mechanisms control the flame spread process. In the near-quenching region (very low characteristic relative velocities) a new controlling mechanism for flame spread - oxidizer transport-limited chemical reaction - is proposed. In the near-limit, blowoff region, high opposed flow velocities impose residence time limitations on the flame spread process. A critical characteristic relative velocity line between the two near-limit regions defines conditions which result in maximum flammability both in terms of a peak flame spread rate and minimum oxygen concentration for steady burning. In the third region, away from both near-limit regions, the flame spread behavior, which can accurately be described by a thermal theory, is controlled by gas-phase conduction.

A study of flame spread over alcohols using holographic interferometry

This study was undertaken as an effort to improve our understanding of the flame spread mechanism over subflash-point liquids. Studies of flame spread over liquids are of interest due to our fundamental curiosity about its mechanism and practical concern about accidental occurrence of fires on liquid fuels. A holographic interferometry technique was applied to obtain detailed temperature profiles in the liquid phase for pulsating and uniform spread over liquids with subflash point. Subsurface liquid convection motion, which is composed of two different circulation zones, was found in the liquid phase for both pulsating and uniform spread regions. In the uniform region, energy balance calculations are made for the liquid phase from the obtained interferograms. The calculation confirms that the flame spread is governed by the convective heat carried by subsurface liquid convection. In addition, the flame spread behavior was recorded by a video camera. The spread rate and pulsation frequency were then deduced.

A Heterogeneous Model of Combustion of Porous Media

Abstract This paper is devoted to the theoretical investigation of combustion of porous media. For the first time the process of flame spread in a layer of a solid porous material with an external supply of a gaseous oxidizer has been analyzed in terms of a heterogeneous model. The multi-stage character of the process (filtration, sorption, chemical reaction) is taken into account. In many practically important cases (e.g., in the case of combustion of coarsely divided particles with the formation of compact films of a condensed product) the sorption stage plays the dominant role in the mechanism of thermal flame spread. This role is developed from a detailed analysis of heat and mass transfer in the model structure of a heterogeneous medium. Such an approach makes it possible to determine explicitly the dependence or the rate of combustion on both the filtration and sorption and structural characteristics of the solid phase (the solubility of gas in the solid phase, the diffusion activation energy, the sizes of pores and particles, etc.).