Mechanism Of Flame Spread Research Articles

Mechanisms of flame spread over convex polymethyl methacrylate building material

Polymethyl methacrylate (PMMA) is a widely used combustible building material in convex structures, such as highway sound barriers and building façades. However, most flame spread studies focus on flat surfaces. This study investigates flame spread along convex surfaces through a series of burning experiments with varying curvatures and widths. The average path flame spread rate increases along with a bigger curvature and width. By introducing a dimensionless average path flame spread rate, an empirical correlation linking the average path flame spread rate with sample width and curvature is proposed. In reality, flame spread over convex surface exhibits a distinct time-varying process. For a large curvature, the transition of the flame shape from a wall-fire regime to a pool-fire regime is observed during the process. Moreover, as curvature and width increase, the flame spread transitions from a two-stage process (regime I) to a three-stage process (regime II). In regime II, the transient flame spread rate initially decreases, then increases, and finally decreases again. This behavior is due to the shift from convection dominance to radiation dominance, although radiant heat transfer significantly influences regime I as well. During this transition, a two-peak phenomenon in radiant heat flux emerges, indicating the onset of regime II. Based on this phenomenon, a power-law correlation between curvature and sample width as the critical criterion for triggering regime II is determined. The results of this study have implications concerning the fire safety design of buildings.

Study on the influence of restricted distance on fixed-width discrete of discontinuous flame spread

This study investigates the impact of varying distances between vertical walls on the propagation of discrete flames in PMMA (Polymethyl Methacrylate) slabs. The primary objective is to analyze the dynamics of flame spread in confined spaces, with a particular focus on how the spacing between vertical walls and the number of PMMA slabs influence flame characteristics. Experimentally, flame spread rate, mass loss rate, heat release rate, and pyrolysis front dynamics are examined under different confined space distances ranging from 1 to 5 cm using thermally thick PMMA slabs. The findings reveal a significant influence of confined space distance on flame propagation. In narrower spaces (1 to 2 cm), a stack effect leads to a rapid increase in flame height and mass loss rate. As the confined distance increases, this effect gradually diminishes, resulting in a decrease in flame height and mass loss rate. Additionally, the flame spread rate initially rises with increasing confined distance, reaching a peak at 2 cm, before decreasing. Through the application of the law of conservation of energy, the dominant mechanisms of flame spread are deeply analyzed under various confined space conditions. These insights are crucial for understanding and predicting fire spread in confined spaces.

Experimental study on the influence of restricted distance on the discrete flame spread of different widths

In this paper, the effects of varying material widths on the propagation of discrete fires within diverse confined space conditions are investigated experimentally. The width-induced mechanisms influencing the flame spread of discrete solid surfaces in distinct confined spaces are comprehensively discussed, encompassing flame characteristics, pyrolysis front dynamics, flame spread rate, and mass loss rate. The findings reveal that the dimensionless flame height and width conform to a negative power relationship at varying restricted distances. Furthermore, a positive correlation between the width of the PMMA in confined spaces and both the flame spread rate and the mass loss rate is established. Notably, when the width is fixed, the confined space plays a dominant role in the propagation of flames. As the restricted distance decreases, the flames become more slender. Additionally, flame height, flame spread rate, and mass loss rate increase with the widening of the material. However, as the restricted distance becomes smaller, the effect of width becomes less pronounced. Under conditions of same restricted distance, an increased width results in a higher radiation factor, thereby enhancing the thermal feedback of the wall to the material, which in turn accelerates the spread rate of discrete flames. Moreover, due to the dominant role of radiant heat feedback provided by the confined walls to the discrete PMMA panels, the proportion of heat transferred by convection in the flames is minimal. Consequently, the influence of the convective coefficient's width variation on flame spread is relatively minor within confined spaces. Ultimately, an energy conservation model under confined spaces was established, which facilitated the analysis of the comprehensive mechanism of flame spread influenced by varying confined spaces and material widths.

The relative position of pyrolysis onset and flame front location for downward flame spread

The interaction of solid and gas phase phenomena in downward flame spread is studied under buoyant conditions. Phosphor thermometry (PT) was used to determine the spatiotemporally resolved surface temperatures both ahead and beneath the flame while CH* chemiluminescence allowed for the quantification of the flame location relative to the fuel surface. The combination of these measurements allowed for the superposition of the flame front location and the surface temperature distribution. The standoff distance (∼1.1 mm) and preheated distance along the surface (∼3.5 mm) were quantified. The application of PT allowed for the continued measurement of surface temperatures in the pyrolyzing region beneath the flame, where the surface temperature was found to remain relatively constant between 300 and 350 °C. The combination of measurements allowed for the calculation of the gas-phase heat flux distribution ahead of the flame front using a point-source approximation, which compared well with previous studies. PT and CH* measurements were used to expand previous work to quantify the thermal gradients through the solid both ahead of and behind the flame front. Surface temperature measurements were coupled with thermogravimetric analysis (TGA) and a simple pyrolysis model to determine that the onset of pyrolysis occurs approximately 1 mm ahead of the flame front. This approach was also used to illustrate that pyrolysis gases were generated in sufficient volume to result in a flammable mixture at the leading edge of the flame. This study highlights the benefit of optical diagnostic techniques in exploring the controlling mechanisms of flame spread, and provides insight into the interaction of solid and gas-phase phenomena in downward flame spread.

Effect of silver inhibition on the ceramic foam as flame suppression

Aluminium dust explosions pose significant safety and economic challenges in various industrial processes. Due to this, the current research explores an innovative approach by inhibiting the silver nanoparticles (Ag NPs) to ceramic porous form substrate as a flame suppressant in order to mitigate the risks associated with these explosions. The antimicrobial and non-toxic qualities of silver are also attractive to be applied in medical and food technology. However, the interfacial adhesion between the metallic (nanosilver) and non-metallic (silica-based-ceramic) is still vaguely studied due to the mechanical and surface energy mismatch between the organic surface and the inorganic layers. From this study, the physicochemical and mechanical properties of the silver-coated ceramic foam were analyzed using X-ray diffraction, field emission scanning electron microscopy with energy dispersive X-ray, thermogravimetric analysis, and compression test. From the mechanical testing, it was found that the percentage increase of maximum load for silver-ceramic foam from the original ceramic foam was about 60%. The results indicate that silver-coated foam has a better compressive strength of 0.93 MPa as compared to 0.58 MPa by the original ceramic. The inhibition effect of Ag NPs powder on the explosion pressure evolution and flame spread mechanism of aluminium powder at different concentrations and particle sizes was tested using the Hartmann experimental system.

Enhancement of flame spread and droplet dynamics behavior of biodiesel constituents with ethanol and MWCNT-OH additions

The interactive droplet combustion between three adjacent droplets of biodiesel constituents was investigated to obtain flame spread and droplet dynamics behavior enhancement with ethanol and hydroxyl-functionalized multi-walled carbon nanotubes (MWCNT-OH) additives. The experiment was conducted at atmospheric pressure and under normal gravity using spatial and temporal tracking. This study found that ethanol effectively increases flame spread velocity and shortens droplet heating time at a close droplet distance. Meanwhile, MWCNT-OH shortens flame spread induction time at a large droplet distance when thermal conduction time mainly controls the flame spread. Both additives improve the volatility and thermal conductivity of fuel blends, extending the flame spread limit distance with strong reciprocal interaction between droplets and accelerating combustion with a higher burning rate. Different flame spread mechanisms were observed and determined by the fuel additives. Child droplet ejection and microburst flame encourage rapid ignition and flame spread to the next unburned droplet. The puffing and microexplosion mechanisms of the fuel blends were determined by the nucleation type preceding the child droplet ejection. Combining ethanol and MWCNT-OH as fuel additives is the most effective way to improve combustion and flame spread performance because ethanol induces nanoparticle burns at the earlier combustion stage.

The importance of solid phase longitudinal conduction in flame spread over cylindrical and flat fuels: A comparative scale analysis

This work presents a scale analysis to explore the role played by forward (longitudinal) solid conduction in the mechanism of opposed-flow flame spread over flat and cylindrical fuels. Closed-form expressions in terms of known parameters and properties are derived for a non-dimensional conduction number, which compares solid phase forward conduction with the gas to solid conduction in the preheat zone. The corresponding expressions for cylindrical fuels are related to the flat fuel expressions using a curvature factor that depends on fuel radius and flow velocity. The results of the analysis are compared to existing experimental data for flame spread over thick flat and cylindrical fuels. For thin fuels, a comprehensive numerical model is used to evaluate the conduction number for comparison with the prediction of the analysis. An energy balance at the leading edge, taking into account the longitudinal conduction through solid, produces a spread rate formula that is related to the well-known thermal limit through the conduction number.

Effect of solid surface curvature and wall heat loss on the downward flame spread along the edge of thin PMMA sheets

Flames propagating along the sample edges (edge flame) will transform two-dimensional flame spread into three-dimensional flame spread, causing a significant interference with results of material flammability test. A potential method to minimize above effect is utilizing an inert wall to inhibit the side surface of sample with a designed air gap distance. In this work, the downward flame spread along the sample edge with various air gap distances is systematically studied. Cast PMMA with thicknesses from 0.4 to 2.5 mm were used as samples. The general trend of flame spread rate (FSR) at edge with respect to air gap distance can be summarized into three regimes: when the air gap distance is less than 2 mm, the edge flame is quenched by wall heat loss, and the overall flame spread behavior is two-dimensional. Then, the edge flame appears after the air gap distance exceeds 2 mm, resulting in a sharp increase in FSR. This critical air gap distance for the onset of edge flame is less sensitive to the sample thickness. Finally, as the air gap distance continues to increase, the FSR gradually approaches the asymptotic value of the uninhibited sample. With the increase in air gap distance, thermally-thin assumption loses validity as a result of the reduction in residence time of solid phase (larger FSR). To interpret the general trend of FSR with respect to air gap distance, a modified flame spread model based on conventional thermal theory is developed. The effect of air gap distance on flame spread along sample edge is characterized by two correction factors, one of which describes solid surface curvature at sample edge, the other of which is the effective flame temperature considering the heat loss from the spreading flame to the metal wall. The new model well reproduces the experimental results, and further supports the significance of heat loss in the trend of flame spread rate. This work helps to understand the flame spread mechanism under effects of solid surface curvature and heat loss, while also providing scientific support for optimization of material flammability testing standards.

Experimental study on coupled combustion of PMMA counter-directional flame spread in the horizontal direction

An experimental study was carried out to address the effects of coupled combustion, and sample widths on counter-directional flame spread over poly(methyl methacrylate) (PMMA) boards in the horizontal direction. Typical flame propagation parameters were observed and analyzed, including flame form, flame front, characteristic angles, rate of spread, and radiative heat flux density. Experimental results showed that the counter-directional fire propagation process could be divided into four stages based on the flame morphology variation, and the evolution of the flame front was presented as well. The characteristic angles, defined as half of the flame front angle, show the velocity variation between the side-edge and center flame at different spread phases. Compared with unilateral flame spread, the radiative heat flux density for the counter-directional flame spread grows faster, and the maximum value is about twice. Moreover, the maximal radiative heat flux increases for a wider sample, and a positive linear relationship exists between the dimensionless mass loss rate and characteristic angle against the breadth length ratio. The research outcomes of this study offer a better understanding of the counter-directional flame spread mechanism from aspects of scale change and heat transfer.

Experimental study of upward flame spread behaviors over thermally thick vertical PMMA of various sample widths under normal wind

The effect of wind normal to the fuel surface on vertically upward flame spread is investigated over thermally thick PMMA slabs of various widths. The 1.2 cm-thick PMMA slabs of three sample width (10, 15, 20 cm) are vertically arranged subject to a horizontal wind (normal to fuel surface) ranging from 0 to 1.8 m/s. Flame size parameters, heat flux distribution and flame spread rate are measured and analyzed quantitatively. It is found that flame shape is much different under the strike of wind and flame spread rate decreases with wind velocity. To interpret the effect of normal wind, mechanism analysis is conducted based on flame spread theory. It is concluded that because of the transverse fuel diffusion, flame extending width increases with wind velocity, flame thickness and sample width in a relation of we∼δf2u∞ws. This leads to a lower flame height with increasing wind velocity, from which flame preheating length is shorter, being negative to flame spread. Flame thickness or flame volume increases with wind velocity which indicates a larger fuel burning rate due to the increasing flame heat flux in pyrolysis region. Two different modes are found for pyrolysis heat flux, that heat flux is controlled by buoyancy (flame) induced flow at low wind conditions while it could change to be mainly influenced by forced flow with the increasing wind velocity. Based on the analysis of the controlling mechanisms of flame spread under this configuration, a theoretical model is proposed to predict vertically upward flame spread process in normal wind, from which flame spread rate increases as a power function of time, and decreases with wind velocity. This work provides basic observation on upward flame spread progress in normal wind conditions and a detailed analysis of wind effect mechanisms on vertically upward flame spread behaviors.

Transition of heat transfer mechanism of concurrent flame spread over discrete fuels

Scenarios of flame spread over discrete fuel elements have been widely simulated by fuel arrays. While investigation on flame spread characteristics of discrete fuel arrays has been performed, the role of convective heat transfer is not yet addressed well. By designing 90 arrangements of fuel arrays, the effects of fuel spacing (denoted by S), inclination angle (θ), and column number (n) on convective heat transfer are studied. The distribution factor φsinθ is introduced to characterize the influences of porosity (φ) and θ. It is found that the Nusselt number (Nu) increases along with increasing φsinθ. Moreover, when φsinθ>0.511, the inflection points of the experimental Nu occur, corresponding to the transition of the heat transfer mechanism. By introducing the Richardson number (Ri), the transition is identified at Ricrit≈1, where the convection pattern is categorized into natural convection (Ri≥1) and forced convection (Ri<1). In addition, Nu correlations under Ri≥1 and Ri<1 are determined by combining the ratio of length and width of the burning zone, porosity, and inclination angle. Finally, based on the newly developed Nu correlations, the prediction models of mass loss rate are proposed, which match the experimental results well.

Determination of the dominant physical processes in downward-propagating flame spread over a solid fuel using machine learning

Flame spread over solid fuel (FSS) plays a key role in solid-fuel combustion and fire-related phenomenon. The mechanisms of flame spread over solid fuel are commonly described by means of dimensionless numbers or scaling analysis that describes the balanced relationship of several processes. However, these approaches rely on prior knowledge or explicit assumption of the relevant physical processes, and it is difficult to spatially distinguish among multiple physical processes. This work demonstrated a generalized way using an unsupervised machine learning method based on the Gaussian mixture models and sparse principal component analysis (GMM-SPCA) to automatically delineates the spatial domain of the FSS from a numerical simulation into several regions that are dominated by the balance between different physical processes. The idea of equation space is employed such that each coordinate in the equation space corresponds to a specific physical process as represented by the individual term in the corresponding governing equation. The dominant heat/mass transport processes for both gas and solid phases have been analyzed, and their spatial correspondence for the fields of temperature, flow, and species has been discussed. Some critical characteristics, such as the flame stand-off distance profile, the triple flame structure, and the pyrolysis zone of the solid fuel have been properly identified and quantified. It is demonstrated that the generalized GMM-SPCA method provides an intuitive insight into the heat and mass transfer processes of the FSS for further development of the flame spread model.

Experimental and theoretical study on horizontal flame spread through arrays of wooden dowels

AbstractFlame spread along discrete fuel element is of great interest for both fundamental science and fire prevention, whose combustion properties may be different from those of homogenous materials. It has been a long time for researchers in fire science to find a simple and effective method to construct a discrete flame spread model. This work was motivated by the research of 2‐D horizontal discrete flame spread behaviors with different spacings and rows, establishing the prediction model of ignition time and global mass loss rate (MLR). In this work, burning characteristics of the 2‐D discrete flame spread were experimentally studied with six different row numbers (1–11 with an interval of 2) of cylindrical dowels (birch wood, 3 mm diameter × 50 mm height) under three selected spacings (6, 7, and 8 mm). Based on the heat transfer model developed, the theoretical models of ignition time and MLR were obtained and validated. Moreover, combined with previous studies, the numerical correlation between dimensionless flame height and dimensionless heat release rate for the 2‐D horizontal discrete flame spread scenario was established. This study provided an insight into the horizontal discrete flame spread mechanism in the aspect of heat and mass transfer, which may be applied in the prediction and assessment of discrete fuel fires in reality.

Opposite effects of typical solid inertants on flame propagation in Mg dust clouds versus dust layers

Adding solid inertants to combustible dust is an effective means of reducing dust explosion risk through both preventive and mitigative measures. However, since dust layers and dust clouds differ in their flame spread mechanisms, they may also differ in their response to solid inertants. In this paper, inerting efficacy of four commonly used solid inertants in Mg dust layers and dust clouds was investigated. As concentration increased, all four inertants effectively suppressed burning in Mg dust clouds. However, CaCO3, NaHCO3, and SiO2 greatly increased the fire hazard in Mg dust layers. CO2 produced by thermal decomposition of CaCO3 destroyed the MgO layer formed on the surface of Mg dust layers and caused more burning of Mg vapor in the gas phase. The water vapor produced by NaHCO3 reacted with Mg to generate hydrogen which intensified combustion. SiO2 emitted heat through a substitution reaction with Mg powder to thrust the mixed dust layer upwards, causing violent combustion similar to that in dust clouds. As inertant proportion increased, the fire hazard in dust layers was initially enhanced but was later decreased, although the fire hazard at 70% inertant remained much higher than that with pure Mg dust. Only NaCl, which is chemically stable and does not react with metal dust, can effectively inert both Mg dust layers and dust clouds. Research on the use of solid inertant technology to prevent or mitigate dust explosions should therefore consider the impact on both dust clouds and dust layers.

Generation and Propagation Characteristics of an Auto-Ignition Flame Kernel Caused by the Oblique Shock in a Supersonic Flow Regime

The auto-ignition caused by oblique shocks was investigated experimentally in a supersonic flow regime, with the incoming flow at a Mach number of 2.5. The transient characteristics of the auto-ignition caused by shock evolvements were recorded with a schlieren photography system, and the initial flame kernel generation and subsequent propagation were recorded using a high-speed camera. The fuel mixing characteristics were captured using NPLS (nanoparticle-based planar laser scattering method). This work aimed to reveal the flame spread mechanism in a supersonic flow regime. The effects of airflow total temperature, fuel injection pressure, and cavity length in the process of auto-ignition and on the auto-ignitable boundary were investigated and analyzed. From this work, it was found that the initial occurrence of auto-ignition is first induced by oblique shocks and then propagated upstream to the recirculation region, to establish a sustained flame. The auto-ignition performance can be improved by increasing the injection pressure and airflow total temperature. In addition, a cavity with a long length has benefits in controlling the flame spread from the induced state to a sustained state. The low-speed recirculating region created in the cavity is beneficial for the flame spread, which has the function of flame-holding and prevents the flame from being blown away.

The controlling mechanisms of horizontal flame spread over thick rods in upward cross flow

This work studies the flame spread over horizontal thick PMMA (polymethyl methacrylate) rods with three radii under different oxygen concentrations and upward cross flow velocities. The flame spread rate and the limit oxygen concentration are measured. Far away from the extinction limit, the flame spread rate increases with the flow velocity but decreases with radius. Near the extinction limit, the flame spread rate is insensitive to the flow velocity and radius. The flame spread rate can be correlated by the stretch rate, and it is found that the flame spread rates for different radii are close at the same stretch rate. A scaling analysis shows that the flame spread rates are approximately square-root dependent on the stretch rate. Prior to the extinction, the flame has entered the regressive burning regime where the flame leading edge will not spread forwardly but continuously retreat. The flame extinction is dependent on the local stretch rate and in-depth heat conduction. For a given radius, the limit oxygen concentration increases with the stretch rate. For a given stretch rate, the smaller cylinder can sustain at lower oxygen concentration due to the less solid-phase heat loss.

Computational analysis of the mechanisms and characteristics for pulsating and uniform flame spread over liquid fuel at subflash temperatures

The present study aims to gain insight of the mechanisms and characteristics for pulsating and uniform flame spread over liquid fuel at subflash temperatures. A specific goal is to use the validated three-dimensional (3-D) numerical model to reveal fine details of the gas and liquid phase flows as well as the resulting flame characteristics, which are challenging to obtain experimentally. To facilitate the study, 3-D formulations have been developed to explicitly solve the transport equations in both phases. A compressible solver was formulated for flame propagation in the gas phase using a one-step chemical reaction expression and mixture-averaged diffusion coefficients for the gaseous species. An incompressible solver with temperature dependent thermo-physical properties was employed to describe the convective motions and heat transfer in the liquid fuel region. Validation has been conducted for both uniform and pulsating spreads over a narrow 1-propanol tray with varying fuel depths through comparing the predicted flame front evolution with published measurements. Further qualitative comparison has also been conducted for some predicted fine features of the gas and liquid phase flows and flame spreading characteristics with published experimental observations. For both the uniform and pulsating spread, the detailed flame structure including the main diffusion flame and a small stratified premixed flame at the front have been captured. Wherever relevant, the detailed predictions were also used to shed light on some discrepancies in previously reported features in different laboratory studies and numerical simulations. Finally, the detailed 3-D predictions were used to illustrate fine features of the subsurface convective flow and its relative position to the flame front, the relative magnitudes of the subsurface flow velocity and that of the spread rate as well as the role of the thermocapillary-driven subsurface flow in the flame spread mechanism.

Experimental study of downward flame spread and extinction over inclined electrical wire under horizontal wind

Flame spread and extinction behavior are significantly influenced by both fuel inclination and wind velocity, however, the coupling effect of two has not been well quantified yet. In this paper, downward flame spread and extinction over inclined electrical wire under horizontal wind is investigated. Polyethylene(PE)-insulated wires (0.5 mm/0.15 mm in core diameter/insulation thickness) of two representative core materials (Copper/Nickel-Chrome: high/low conductivity) are used as samples. Flame geometrical characteristics (flame length Lf, pyrolysis length Lp, gas-phase thermal length Lg), as well as the flame spread rate (FSR) are quantified. Results show that as the horizontal wind velocity increases, FSR demonstrates a non-monotonic trend before extinction, where four regimes are identified based on different heat transfer controlling mechanisms. On the other hand, FSR of Cu-core wire and NiCr-core wire show different dependence on inclination angle. FSR reaches the maximum when the flame is pushed by the horizontal wind to be parallel to wire, which can be explained by a balance of horizontal wind and buoyancy-induced flow (i.e. inclination). A quantified model based on the three characteristic lengths (Lf,Lp,Lg) and a proposed mixed-convective coefficient hmix combining the effect of inclination angle and wind velocity is established. The proposed model well represents the experimental FSR and interprets the controlling heat transfer mechanism. Moreover, the extinction limit is represented by a heat loss factor Rloss as a function of strain rate a, showing an enhanced quenching effect and a weakened blow-off effect with increased inclination angle. The flame with Cu-core is more difficult to extinguish at small inclination but becomes easier at large inclination than that with NiCr-core, indicating a transition of inner core role from “heat source” to “heat sink”. This work provides essential knowledge on flame spread and extinction mechanism over inclined fuel under wind.

Effect of sample thickness on concurrent steady spread behavior of floor- and ceiling flames

This paper presents an experimental investigation of the sample thickness effect on the concurrent flame steady spread behavior. Two configurations, ceiling flame and floor flame, as the fundamental diffusion combustion problems and representative scenarios of horizontal flame spread, are concerned herein. PMMA samples with thickness (δ) of 2, 3, 4, 5, 7, 9 mm were employed including both thermally-thin and thermally-thick conditions. A wind tunnel was used to provide the uniform concurrent airflow ranging from 0.3 m/s to 1.2 m/s in 0.3 m/s intervals. Flame characteristics and basic parameters to quantify the flame steady spread process have been studied, including flame spread rate (FSR), equivalent pyrolysis length as well as heat flux feedback in both pyrolysis and preheating zones. Results show that FSR decreases with sample thickness, being more sensitive to sample thickness for ceiling flame configuration than that for floor flame configuration. Equivalent pyrolysis length increases with sample thickness and ceiling flame's equivalent pyrolysis length is larger than floor flame's. Convection feedback is dominant under concurrent airflow regardless of the flame configuration, airflow velocity or sample thickness. In pyrolysis zone, the total heat flux to fuel surface of ceiling flame configuration is larger than that of floor flame configuration for thin samples due to buoyancy, but it decreases rapidly as sample thickness increases. In preheating zone, the heat flux decreases in power law for ceiling flame configuration while a piece-wise fitting is found for floor flame configuration, which is interpreted based on their inherent flame characteristic difference hence the thermal boundary layer conditions between these two configurations. To analyze the sample thickness effect on concurrent flame spread, a characteristic solid temperature (T*) is proposed by theoretical analysis, as an essential parameter to be used in the concurrent flame spread model to represent the coupling effect of heat input in gas phase and in-depth heat loss into the solid phase (or namely thermal inertia controlled by a characteristic thermal penetration depth). The calculated non-dimensional characteristic solid temperature (θ) shows to be less sensitive to flame configuration and concurrent airflow velocity for relatively thicker samples because the thermal penetration depth will tend to approach a limit. A general trend of its variation with sample thickness (δ) can be employed to predict the heat loss into solid phase and its predictive capability is verified by the experimental results. This work advances the fundamental understanding of the sample thickness effect on concurrent flame spread mechanism.

Concurrent flame spread over vertically oriented paper sheet: The effect of sample thickness

SummaryPacking or printing paper made out of cellulose is widely produced and used in daily life. The thickness of thin material affects the burning process significantly. Concurrent flame spread is an effective approach when analyzing and evaluating the fire hazard for combustible materials. Conducting this work will provide further understanding of the flame spread mechanism and reduce fire hazard. It will also enable investigation of the effect of sample thickness on concurrent flame spreading over the thin paper sheets experimentally. Numerically examining concurrent flame spread over thin paper sheet with varied thicknesses is also performed using Direct Numerical Simulation, which can provide more details on flame spread process. Qualitatively, the model predictions agree reasonably with the experiment in terms of the flame contour, burn duration, and flame spread rate. The moving rates of the flame base and pyrolysis front decrease with sample thickness both experimentally and numerically. The relationship between net heat flux, flame standoff distance, and solid burning rate is analyzed, with the burning rate exhibiting a similar trend to that of the net heat flux. A correlation of the flame net heat flux with the normalized distance in the pyrolysis region is also established where the flame heat flux displays a decay trend with the normalized space.