Pyrolysis Front Research Articles

Pyrolysis front detection in carbon phenolic composites using x-ray computed tomography

Carbon phenolic composites are used as thermal protection systems (TPS) materials on space capsules to protect them from the hot aerothermal environment. The phenolic resin in the composite material decomposes (pyrolyzes) at low temperatures resulting in a pyrolysis front within the material. The detection of the pyrolysis front after exposure to heat has historically been achieved by physically sectioning cross-sections of the material. We combine the phase contrast retrieval method to reconstruct x-ray computed tomography scans along with image convolution to identify the pyrolysis front in carbon phenolic composites. Unlike the standard filtered back projection method that captures only the carbon phase, the phase contrast retrieval method uses both the attenuation coefficients and refractive indices to illuminate all three phases (carbon, resin, and voids) of carbon phenolic composites. Image convolution is applied on scans reconstructed using the phase contrast retrieval method to develop a density map of the composite to locate the pyrolysis front. The analysis is performed on a sample of phenolic impregnated carbon ablator that was tested in an arc-jet facility. For the sample analyzed, the depth of the pyrolysis front from the surface of the sample is calculated to be 2.150 ± 0.148 mm. Although the proposed approach is applied to detect the pyrolysis front, the tools can be used to illuminate the structure of any carbon phenolic composite, and we propose the use of the phase contrast retrieval method as a methodological standard to analyze carbon phenolic composites used on space capsules.

Effect of the Backboard on the Pyrolysis Front Morphology of Downward Flame Spread: A Simulation Study

In order to investigate the influence of the backboard on the morphology of the pyrolysis front, this study employed the fire dynamics simulator (FDS) to conduct simulation research, aiming to quantitatively analyze the reasons for the differences in the morphology of the pyrolysis front under two conditions: with and without the backboard. By comparing the surface temperature distribution, flow field, and air entrainment velocity at the bottom of the pyrolysis front made of polymethyl methacrylate (PMMA) under different conditions, it was found that the presence of the backboard not only reduced the surface temperature of PMMA at the bottom of the pyrolysis front and the temperature difference between the edge and middle, but also changed the flow field at the edge and suppressed the horizontal air entrainment velocity at that position, especially for thinner PMMA. These factors ultimately result in a larger angle of the pyrolysis front during the burning stable stage under the condition with the backboard compared to the condition without the backboard. In addition, the trend of horizontal air entrainment velocity at the edge position under different conditions also provide data support for the qualitative observation and analysis of previous experiments phenomena.

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.

An engineering model for creeping flame spread over idealized electrical wires in microgravity

Flame spread over an insulated electrical wire is a major source of fire scenario in a space vehicle. In this work, an engineering model that predicts the creeping flame spread over cylindrical wires in microgravity is developed. The model is applied to interpret experimental data obtained in parabolic flights for wires composed by a 0.25 mm radius nickel-chromium (NiCr) metallic core coated by low-density polyethylene (LDPE) of different thicknesses ranging from 0.15 mm to 0.4 mm. The model relies on the assumption that, in the pyrolysis region, the NiCr and the LDPE are in thermal equilibrium. This assumption is supported by more detailed numerical simulations and the model reduces then to solving the heat transfer equations for both NiCr and LDPE in the pyrolysis region and in the region ahead of the flame front along with a simple degradation model for LDPE, an Oseen approximation of opposed oxidizer flow and an infinitely fast gas-phase chemistry. The flame spread rate (FSR) is controlled by two model parameters, which are measurable from intrinsic material and ambient gas properties: the convective flame heat flux transferred to the solid ahead from the flame front and the gaseous thermal heat length near the flame front. These parameters are then calibrated from experimental data for a given wire geometry and the calibrated model is validated against experimental data for other wire geometries and ambient conditions. The heat transfer mechanisms ahead of the pyrolysis front are investigated with a special emphasis on the LDPE thickness and the conductivity of the metallic core. In addition to NiCr, metallic cores of lower and higher conductivities are considered. The polymer is shown to be thermally thick for all tested wire geometries and core conductivities. The flame heat flux is found to dominate the heat transfer in the preheat zone where it applies. The core has nevertheless a significant impact in the heating of the LDPE with its contribution increasing with the core conductivity and when decreasing the LDPE thickness.

Semi-Analytical Modeling of Pyrolysis Front and Ignition Temperatures of Thermally Reactive Single Solid Particles

A semi-analytical approach is developed for predicting pyrolysis front temperature in a charring solid undergoing thermal decomposition. The pre-reaction heating stage is described using an analytical formulation and invoking the concept of thermal penetration depth. The solution for the solid conversion stage accounts for decomposition enthalpy, the convective flow of volatiles, and a reaction front characterized by a uniform temperature that progresses toward the inner layers. This method incorporates empirical relations into the analytical model. Two scenarios are considered. First, the solution of the pyrolysis model combined with the data of conversion time versus external heat flux leads to an algebraic expression that reveals the existence of a maximum pyrolysis-front temperature. Explicit relations are derived for both the extremum pyrolysis temperature and optimum applied heat flux. In the second case, an expression is derived for the ignition temperature of a solid fuel (e.g., wood) by incorporating ignition delay time measurements into the heating stage model. The newly derived expression allows the ignition temperature to be described as a function of the Biot number and external heat flux. The relation obtained for the ignition temperature explains the experimental trends reported in some previous studies where two local extremums were observed for the ignition temperature in the absence of volatile reactions.

An analysis of the sensitivity of the rate of buoyancy-driven flame spread on a solid material to uncertainties in the pyrolysis and combustion properties. Is accurate prediction possible?

In this study, upward flame spread over a combustible solid in a vertical corner configuration was simulated using two widely used computational tools – ThermaKin2Ds and the Fire Dynamics Simulator (FDS) – and predicted sensitivity of fire growth dynamics to variations in material properties was examined to determine which ones carry measurement uncertainties that translate into the largest errors in predicted behavior. The nominal value of each material property was calculated as the average of measured values of a balanced set of 26 materials that represent the combustible polymeric solids widely used in industrial, transportation, and construction applications. The upper and lower boundaries of each property were defined based on an uncertainty quantification of each specific measurement. These average material properties and related uncertainties were determined based on an extensive literature review presented in this work.A measure of average flame spread velocity was defined (based on the time derivative of pyrolysis front location) as the model output to characterize fire growth dynamics. Overall, ThermaKin2Ds and FDS generally demonstrate similar sensitivity to model inputs. For both simulation tools, uncertainties in reaction kinetics (Ea/ln(A)) and heat capacity of the virgin material, had the most significant effect on calculated average flame spread velocity, resulting in changes in this output (on average, for both simulation tools) of approximately 20%. FDS simulations also demonstrated a strong sensitivity to variations in heat of combustion when it was adjusted from its mean value to its lower or upper uncertainty bounds (17% change in average flame spread velocity; 2, 3 times as sensitive as ThermaKin2Ds results). In ThermaKin2Ds, an approximate 10% sensitivity in predicted average flame spread velocity was observed to variations in thermal conductivity of the virgin material, while in FDS this output demonstrated limited sensitivity to this parameter change. Uncertainties in the density of the virgin material and its char yield also showed a notable impact on predicted average flame spread velocity in both simulation tools. All other material properties considered in this work were found to demonstrate negligible influence on average flame spread velocity, which indicates that these parameters are measured with sufficient accuracy to deliver reasonably accurate predictions of flame spread dynamics.

Simplified modelling of transient upward flame spread on combustible element

This paper presents the development and validation of a simplified model to simulate upward fire growth and flame spread on vertical surface with combustible materials. The model consists of the following main sub-models: a one-dimensional combined heat and mass transfer model perpendicular to the vertical surface to compute gaseous fuel production due to decomposition and combustion of the combustible material, a simplified model to calculate surface heat flux, and a transient flame growth model based on an existing steady state flame model that takes into account transient heat feedback due to combustion. Validation of the model is checked by comparison of the predictions of this model against FDS (Fire Dynamic Simulator) simulation results and experimental results wherever available, including the following: flame growth with variable heat release rates of solid materials under various levels of external irradiation, temperature distribution through thickness of the wall, and propagation of the pyrolysis front. To demonstrate capability of the new model, vertical burning and upward flame spread on 2.4 m high panels of wood and 5.0 m high panels of PMMA are carried out and compared with FDS simulations. The new model calculation results, including propagation of the pyrolysis front, total heat flux and heat release rate are in good agreement with FDS simulation results. However, the new model requires a fraction (a few minutes of CPU time) of that for carrying out the corresponding FDS simulation (CPU time of 2 days).

The effect of duct size, sample size, and fuel composition on concurrent flame spread over large cellulose samples in microgravity

Concurrent flame spread data for thermally-thin charring solid fuels are presented from Saffire and BASS experiments performed in habitable spacecraft for three duct sizes, five sample sizes, two materials, and two atmospheres. The flame spread rates and flame lengths were strongly affected by duct size even for the relatively large ducts (> 30 cm tall). A transient excess pyrolysis length (i.e., flame length overshoot) was observed for the cotton fabric that burned away, which indicates that the transient excess pyrolysis length phenomenon is caused by more than just the flame moving into the developing boundary layer thickness as was the case with the SIBAL sample. A burnout time, defined as the pyrolysis length divided by the flame spread rate, normalized the pyrolysis length histories into a single curve with a steady burnout time of 22 s for the SIBAL fabric. The transient excess pyrolysis length is hypothesized to be a post-ignition flame growth transient for the essentially two-dimensional flames where the burnout time becomes very long until the preheat and pyrolysis lengths develop. The three-dimensional flames over narrow samples have lateral thermal expansion and lateral oxygen diffusion which allows them to transition to a steady state length without the transient excess pyrolysis length. Surface temperature profiles, nondimensionalized by the pyrolysis length, indicate that the temperature profiles exhibit the same shape across the pyrolysis zone. A surface energy balance calculation in the preheat region revealed that the heat flux increased rapidly at the pyrolysis front to near the critical heat flux for ignition. An estimate of the acceleration of the inviscid core flow in the duct due to thermal expansion and developing boundary layers on the duct walls and the SIBAL sample surface seems to explain the observed spread rate trends across three duct sizes and multiple sample sizes.

Investigation on the double-sided concurrent flame spread behavior of log planks

This paper presents an experimental and theoretical research on the double-sided concurrent flame spread behavior of inclined planks. Pine log planks with a size of 100 cm long, 30 cm wide and 1 cm thick were used with six angles of inclination (θ). Results showed that the flame and the pyrolysis front on the upper surface present an inverted "V" shape at θ =15° to 45°, and the lower surface flame is always clinging to the plank surface, which is significantly different from the combustion of small-sized wooden samples. The flame spread rate is positively correlated with θ. Besides, it is found that the pyrolysis length (Lpy), preheating length (Lpr) of upper surface and the flame length of lower surface (Lf,lowersurface) have a relation of Lf,lowersurface≈Lpy+Lpr, indicating the significant influence of the lower surface flame on the double-sided flame spread. Heat received by the upper surface is mainly the radiative heat flux from the flame on the surface, and a physical model considering flame sizes and shapes is established, whose results agree well with the experimental data. Theoretical analysis was conducted on the heat transfer from lower surface, and found the conductive heat flux from the lower surface flame is of the same order of magnitude with the radiative heat flux from the upper surface flame. It indicates the heat from the lower surface flame has large contribution to the flame spread behavior. In addition, the influence of tung oil, which is widely used in timber buildings, on the flame spread behavior was also researched. It shows that brushing tung oil on the plank surface could increase the flame size, spread rate and heat release rate.

Mechanism of accelerated concurrent flame spread over glass fiber reinforced unsaturated polyester resin composites with ATH/MH retardants under external radiation

This paper reports an experimental investigation of concurrent flame spread over composites, for the first time, with combined flame retardants and external radiation factors. Glass fiber reinforced unsaturated polyester resins without flame retardants (GFUPR) as well as those containing aluminum hydroxide (ATH) and magnesium hydroxide (MH), which were utilized widely in modern high-speed train, were systematically explored. Infrared measurement was employed to track the displacement of pyrolysis front. Because of the dilution of H2O generated during the decomposition of ATH and MH, the flame temperature (Tf) of GFUPR-ATH/MH is thought to be lower than that of GFUPR. Nonetheless, the most noteworthy finding is that the flame spread rate, flame preheat length and flame height for GFUPR-ATH/MH could be larger than that of GFUPR if concurrent airflow velocity exceeds 0.5 m/s under high external radiation. To interpret this unexpected while practically important phenomena, theoretical analysis based on the heat/mass transfer theory is established to explore the general mechanism. It is concluded that in the transition from natural-convection to forced-convection, the different in the flame convection in burning zone between the sample with high Tf (i.e. GFUPR) and the sample with low Tf (i.e. GFUPR-ATH/MH), will decrease rapidly. It is primarily because the sample with low Tf is earlier to enter in the region where forced-convection is controlled as the airflow velocity increases. The critical parameter (Fr2Re)-1 = 1 could be used to characterize this transition. As a result, in forced-convection region, the role of thermal conductivity of the fuel-gas mixture (kmix) may emerge, that the sample with high kmix but low Tf (i.e. GFUPR-ATH/MH) may impart more heat to burning zone than the sample with low kmix but high Tf (i.e. GFUPR), generating more combustible gases in the burning zone hence larger flame heat feedback to preheat zone. Moreover, this effect will be reinforced by the larger flame preheat length of GFUPR-ATH/MH, due to the temperature dependency of molecular diffusivity. This mechanism analysis is well corroborated by the experimental results. This new finding may facilitate understanding of the effect of flame retardants on the heat/mass transfer mechanism in flame spread under concurrent airflow and external radiation.

OpenFOAM solver for thermal and chemical conversion in porous media

We present the porousGasificationFoam solver and libraries, developed in the open-source C++ code OpenFOAM, for the comprehensive simulation of the thermochemical conversion in porous media.The code porousGasificationFoam integrates gas flow through a porous media with the models of heterogeneous (drying, gasification, pyrolysis, solid combustion, precipitation) and homogeneous (gas combustion) chemical reactions. Inside porous media transport equations are formulated applying the spatial averaging methodology. The mass and enthalpy transfer between solid and gas phases is suitable for systems out of the thermal equilibrium. The convection and radiation modes of the heat transfer are included for gas and solid phases, and the immersed boundary technique is applied for the porous media inside the computational domain.We validate the elements of the model against a set of experimental and theoretical results. Amongst them, Thermogravimetric Analysis experiments of thermal conversions of two wooden particles: one of millimeter size the other of centimeter size. Simulations feature reaction schemes and physical parameters established in the literature. We show the influence of the porous media size on the gasification process. The millimeter particle remains uniform, while for the centimeter setup, the pyrolysis front is reproduced. The spatial patterns in physical conditions modify the course of chemical reactions and influence media composition and structure evolution. Another important example is a gasifier where we obtain a self-sustaining front propagation because of an exothermic heterogeneous reaction. Program summaryProgram Title: porousGasificationFoamCPC Library link to program files:https://doi.org/10.17632/s6sj9kgp69.1Developer's repository link:https://github.com/pjzuk/porousGasificationFoam (foam-extend-4.1); https://github.com/btuznik/porousGasificationFoam (OpenFOAM 8)Licensing provisions: GNU General Public Licence version 3Programming language: C++External routines/libraries: OpenFOAM 8, foam-extend-4.1Nature of problem: The developed software is a solver and a set of libraries for simulating the thermal conversion in porous media, including drying, pyrolysis, gasification, and combustion. The model includes the flow of the reactive gas mixture and the transfer of mass and enthalpy between the gas and solid phases. The porous medium can change during the process. The model is transient and enables three-dimensional simulations.Solution method:: The finite-volume method is used in the code for equation discretization. A set of governing equations is solved for the gas and solid phases, including mass conservation of gas species and solid components, momentum conservation in the gas phase, and equations of enthalpy conservation in both phases. The porous media region inside the whole computational domain is defined using the immersed boundary approach.

Analysis of upward- and downward flame spread over vertical installed polyethylene-insulated electrical wires

Upward- (UFS) and downward (DFS) flame spread along a vertical installed electrical wire has been examined experimentally to gain better understandings of the heat transfer mechanism and burning behavior over electrical wires. UFS is considered as a much more hazardous fire scenario than DFS, however, UFS over electrical wires was very limited addressed. Fundamentally, the controlling heat transfer boundary over the preheating zone, hence the flame spread behaviors are quite different between DFS and UFS. This problem is more complicated for an electrical wire, for which the metal conduction is involved additionally, playing an important role in the heat transfer process. Thirteen types of polyethylene-insulated electrical wires with Cu- and NiCr-core were used to investigate the effect of metal core thermal conduction and wire configuration for both UFS and DFS. During the flame spread process, the pyrolysis length of UFS was enlarged, while only portion of virgin material of DFS were burnt as the result of distinct dripping. By further concerning the curvature effect, the dependence of the flame length on heat release rate per unit width (i.e. perimeter for cylinder) can be well explained by non-dimensional analysis. It turns out that the growth of flame length will be restrained as radius decreases significantly, and can be categorized in to two regions, one is controlled by laminar molecular diffusion followed the other controlled by turbulent entrainment. The variation of flame spread rate (FSR) with wire configuration and metal core thermal conduction are well interpreted by a theoretical model, which describes the heat recirculation through flame-insulation-core comprehensively based on thermodynamics theories, and attempts to consider the effects of temperature gap between core and insulation at pyrolysis front, burning fraction and melting heat. The validity of this new model is well confirmed by measured FSR. An important finding is that a highly conductive core will trigger an acceleration of FSR over the wire, primarily because heat flux in gas phase is more sensitive to the change of core thermal conductivity than heat conduction through core. Even though the heat diffusion length increases at the same time, however, the heat flux in gas phase grows faster than the corresponding heat loss through core, thereby compensate the corresponding heat loss. This work could facilitate understanding of heat transfer mechanism and burning behavior not only on the electrical wires, but also on the composite material with high thermal-conductive media.

Experimental and numerical study of upward flame spread and heat transfer over expanded polystyrene at different altitudes

Experimental and numerical study on upward, vertical flame spread was carried out on expanded polystyrene (EPS) at two different altitudes of 50 m and 3658 m to understand the effect that altitude has on flame spread of melted material. The experiments show that, with the altitude increase, the flame spread rate would decrease. At low altitude, the flame spread rate accelerated at later time, while at high altitude, the stagnant stage of pyrolysis front was formed. The numerical simulation could well give the trend of pyrolysis front evolution and the different flame spread behaviors with altitudes. It was found that, at low altitude, Raxincreased slowly in the turbulent period with relatively large entrainment air flow rate, mainly due to the increased and then decreased mass loss rate in the wall fire zone. While, at high altitude, the simulated increasing entrainment air flow rate, combined with the larger preheating length δf and the smaller flame heat flux qf", resulted in the stagnancy of pyrolysis front easily. It was hypothesized that extinction occurred eventually at high altitude due to competition between flame spread rate νf and preheating length δf.

Experimental investigations on two-sided upward flame spreading over thin flax fabric under the influence of restricted distance

Up to 2021, most previous work focused on upward flame spread over thin solid fuel completely attached to objects or with both sides freely exposed to the air, but did not take the restricted distance (distance between fuel and objects) effects into account. In this paper, the restricted distance effects on upward flame spread over thin solid fuels were investigated using 0.65 mm thick, 120 cm tall and 6.0 cm wide flax fabric sheets under various restricted distances of 1.0–3.5 cm. The essential parameters were monitored and analyzed simultaneously, including flame length, pyrolysis spread rate, surface temperature and ignition time. The main conclusions drawn are as follows: when the restricted distance is no more than 1.5 cm, the flame length on the unrestricted side is larger than that on the restricted side, whereas the variation exhibits the opposite trend when the restricted distance is beyond 1.5 cm. As the restricted distance increases from 1.0 to 3.5 cm, the flame length and flame spread rate first increase and then decrease, reaching a maximum value at 3.0 cm restricted distance, whereas the ignition time shows the opposite trend. The decrease rate of the surface temperature with the distance from the pyrolysis front first drops and then rises as the restricted distance increases, which qualitatively characterizes that the heat flux received by the virgin surface first increases and then decreases with restricted distance. The non-monotonic trends of heat flux received by the virgin surface and consequently the flame spread rate as a function of restricted distance are due to the combined restricted distance effects of the chimney effect, wall radiation and restricting oxygen supply. The results of this paper are not only helpful in better understanding the upward flame spread over a thin flax fabric under restricted distance, but also provide some basic data for fire prevention of thin solid fuels.

Structural analysis of biomass pyrolysis and oxidation using in-situ X-ray computed tomography

Detailed measurements are performed to examine the heterogeneous processes that occur within the solid fuel during the combustion of biomass. Specifically, the pyrolysis, char oxidation, and volume shrinkage are examined using X-ray computed tomography (XCT). Simultaneous 3D in-situ measurements of solid density and gas-phase temperature are acquired at a spatial resolution of 300 μm for 1.6-cm size samples of birch, douglas fir, oak and walnut. To simultaneously measure the heterogeneous combustion processes and the gas-phase temperature, the flow is doped with Kr, a high X-ray contrast agent. By varying the molar oxygen concentration from 0% to 21%, regimes of pyrolysis, smoldering, and flaming are identified for different types of biomass and grain orientations. Results from these multidimensional XCT measurements provide new insights about the fuel volume reduction, from which shrinkage parameters are experimentally determined. The simultaneous acquisition of gas-phase temperature measurements also highlights the direct dependence of flame anchoring upon grain orientation. In addition, the char oxidation is found to be limited to a narrow region with 2 mm thickness at the smoldering surface, while the propagation of the pyrolysis front is measured up to 1 cm within the sample. For some biomass materials, significant density fluctuations are measured at the millimeter scale, indicating non-homogeneous reaction rates.

Experimental study on influence of air gap on upward flame spread over discrete fuel

In a real fire scenario, the distribution of fire loads may not be continuous. In this paper, PMMA blocks (8 cm × 10 cm × 1 cm) are used as experimental materials to carry out upward flame spread experiments with different air gaps (0–18 cm) to analyze changes in burning rate, flame shape, flame height and pyrolysis front position. It is found that as the air gap increases, the burning rate and flame height both increase first and then decrease, and the turning point of this trend is predicted to be between 6 and 8 cm. In addition, it is founded that the pyrolysis reaction is accelerated due to the additional air entertainment provided by the air gap when the pyrolysis front passes through the air gap. However, this effect becomes insignificant for a larger air gap.

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 oxygen depletion on soot production, emission and radiative heat transfer in opposed-flow flame spreading over insulated wire in microgravity

The main objective of this article is to investigate experimentally and numerically the effects of reduced-oxygen contents on the soot production and emission from solid fuels in microgravity, which constitute an important issue in terms of fire safety for manned space missions. Due to its convenience for the implementation of soot-related optical diagnostics, the configuration of a flame spreading in an opposed flow over thin nickel chromium (NiCr) wires coated by low density PolyEthylene (LDPE) is considered. Experiments are conducted at a pressure of 101.3 kPa and an oxidizer velocity of 150 mm/s. The oxygen mole fraction in the oxidizer, XO2, is varied from 18% to 21% by nitrogen dilution of air. The modeling strategy lies on a surrogate fuel to mimic the combustion of LDPE by preserving its stoichiometry and the laminar smoke-point (LSP) flame height. The numerical model considers a detailed chemistry, a two-equation soot production model involving laminar smoke point (LSP)-based soot formation rates and oxidation by OH and O2, a radiation model coupling the Full-Spectrum correlated-k method with the finite volume method, and a simple degradation model for LDPE. Based on experimental evidence, the soot formation rate is scaled by the adiabatic flame temperature to account for thermal effects due to variation in XO2. The model reproduces quantitatively the increase in flame size, residence time, and soot volume fraction observed experimentally as XO2 is enhanced as well as the transition from a non-smoking to a smoking flame, which occurs for XO2 between 19% and 20%. The increase in soot volume fraction results of a combined enhancement in both residence time, owing to an increase in the fuel mass flow rate, and soot formation rate due to higher temperature in the soot formation region. The radiant fraction increases significantly with XO2 from about 17% for XO2=18% to about 36% for XO2= 21%. This increase in radiative losses is accompanied by a reduction of the temperature in the soot oxidation region. Therefore, for increasing XO2, the soot oxidation process is governed by a competition between oxygen-enhanced conditions that promote the formation of soot oxidizing species and the increase in radiative losses that dampens their formation. For the present flames, the first mechanism prevails as XO2 increases from 18% to 19% whereas the second dominates as XO2 is further increased, leading to smoking flames as actually observed for XO2=20 and 21%. The radiant fraction at the smoke-point transition and the soot oxidation freezing temperature are in line with those reported at normal gravity. Finally, model results show that, whatever XO2, the contribution of radiation to the heating process is negligible ahead of the pyrolysis front and is largely overcome by surface radiative losses along the pyrolysis region.

Numerical analysis of the effect of fuel load on downward flame spread

This paper performs parameter studies varying sample thickness and volume density to investigate the effect of surface density on downward flame spread using Direct Numerical Simulation (DNS). Surface density is a significant property that affects fuel loads on the fuel surface greatly. The results reveal that all the thin fuels of 70 – 180 g/m2 attain a steady state, which means that the flame base and pyrolysis front move downstream with the same rate. Moreover, the propagating rates of the flame base and pyrolysis front decrease with the increasing surface density. Subsequently, the flame contour and flame heat feedback in preheated region are studied. Along the flame base downstream, dimensionless flame standoff distance grows gradually till a peak reaches at appropriately (Z-Zb )/Zf of 0.23, and then is reduced to a minimum near the flame front. Radiative heat flux occupies the dominant position upstream the flame base, and its distribution with normalized space (Z-Zb )/Zf is established. The present paper together with previous works can provide further understanding on the effect of fuel loads on downward flame spread.