Experimental study of the channel effect on the flame spread over thin solid fuels
TL;DR: In this article, the speed of the flame front when it propagates within a narrow channel (closed cross section), within a channel with lateral walls only and through a free cross section (plain case) was investigated.
Abstract: We experimentally burn thin solid fuels and obtain the speed of the flame front when it propagates (1) within a narrow channel (closed cross section), (2) within a channel with lateral walls only and (3) through a free cross section (plain case). The latter configuration is the classical one and it has been extensively studied with analytical, numerical and experimental methods by other authors. Our experiments have been carried out at different geometrical configurations and angles of inclination of the sample and also at several values of oxygen molar fraction. All experiments are restricted to purely buoyant flow. Our main results are as follows: (1) sidewalls reduce the flame spread rate in a non-monotonous trend when varying its height; (2) in horizontal flame spread, two simultaneous flame fronts that propagate at different velocities may arise in the channel case at high oxygen levels. The fastest flame front speed may be higher than that obtained in the plain case; (3) in upward flame spread, the channel effect configuration produces the highest flame front speed. We finally analyze the correlation of the downward flame front speed data in terms of the Damkohler number.
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TL;DR: In this paper, an experimental study on combustion and fire safety of energy conservation materials (extruded polystyrene, i.e., XPS) in vertical channel with front openings of building facade is conducted, and effects of channel structure factor and curtain wall coverage rate (β) are revealed.
Abstract: Experimental study on combustion and fire safety of energy conservation materials (extruded polystyrene, i.e., XPS) in vertical channel with front openings of building facade is conducted, and effects of channel structure factor (α) and curtain wall coverage rate (β) are revealed. The XPS flame in the vertical channel is turbulent. There is a correlation between the flame height and pyrolysis length: x f = m x p n , where m and n vary with the change in structure factor and coverage rate. Upward flame spread rate decreases first and then increases as β rises. When 0 ≤ β 0.2 , the restraining effect of vertical channel on air entrainment dominates, while for 0.2 ≤ β ≤ 0.8 , the heat feedback from curtain wall dominates. When β = 0, the influence of α on flame spread rate is not significant. The flame spread rate increases with increasing α when 0.2 ≤ β ≤ 0.8 . A model is established to predict the flame spread rate under different coverage rates and structure factors. Compared with experimental results, the prediction error is smaller than 10%. Larger values of flame height and flame spread rate correspond to higher fire hazard. This work is beneficial for fire safety assessment of building thermal insulation materials and optimal design of energy-saving curtain wall.
15 citations
TL;DR: In this article, the authors used the MSU Narrow Channel Apparatus (MSU-NCA) to investigate opposed flow flame spread over samples of thermally thick polymethylmethacrylate (PMMA).
Abstract: The Michigan State University Narrow Channel Apparatus (MSU-NCA) was used to investigate opposed flow flame spread over samples of thermally thick Polymethylmethacrylate (PMMA). Three different fuel thicknesses were tested for mean airflow velocities 8-58 cm/s. The sample thicknesses were 6.6 mm, 12.1 mm and 24.5 mm, respectively. The measured flame position versus time determined the spread rate. Flame spread rates ranged between 0.02 - 0.07 mm/s depending on fuel thickness and mean opposed flow. Complete sample burnout occurred for the 6.6 mm and 12.1 mm samples at the critical flow velocity of 30 cm/s ± 5 cm/s and higher. The flame spread results appeared to be independent of flow velocities for this range (>30 cm/s): this plateau regime is identified as the regressive burning regime. The 24.5 mm thick samples never completely burned through, but they entered the regressive burning regime at 41.4 cm/s flow velocity. The nature of surface regression and its influence on the spread mechanism in this regime at high flow velocities was discussed for completely burned through samples (6.6 mm and 12.1 mm) and partially burned through samples (24.5 mm). For 12.1 mm thick samples, the flame spread results were compared with the same material (PMMA) and similar thickness (12.7 mm) results from the 1981 Fernandez-Pello et al. study. Their tests used a wind tunnel having a different length and cross-section than the MSU-NCA. The comparison was favorable when employing the stretch rate theory of flame spread incorporating the appropriate numerically computed stretch rate. Since buoyancy was an important factor in the 1981 study, when the buoyant stretch was also included, excellent agreement was obtained between the Fernandez-Pello et al. data and the current NCA data. The results demonstrated the effectiveness of the stretch rate theory for markedly different experimental configurations.
13 citations
TL;DR: In this article, the downward flame spread over polymethyl methacrylate (PMMA) samples is studied experimentally under the spacing scenarios of 2.5mm, 7mm, 13mm, 19mm and 25mm.
Abstract: To explore the flame spread mechanism over non-charring materials, the downward flame spread over polymethyl methacrylate (PMMA) samples is studied experimentally under the spacing scenarios of 2 mm, 7 mm, 13 mm, 19 mm and 25 mm. The experimental results show that: (1) As the spacing increases, the flame height, the length of the preheating zone and mass loss rate all increase first and then decrease. When the spacing is 13 mm, each value reaches the maximum. (2) As the spacing increases, the flame spread speed increases first and then decreases, approaching the single burning PMMA slab finally. In this study, a heat transfer model is proposed to examine the spacing effect over PMMA slabs. According to experimental results, a correlation between the flame spread rate and spacing is derived. Besides, experimental data agree well with the theoretical model.
12 citations
TL;DR: In this paper, the same authors conducted concurrent flow flame spread experiments over thermally thin solid fuels in microgravity aboard the International Space Station (ISS) under varying levels of confinement.
Abstract: Concurrent flow flame spread experiments are conducted over thermally thin solid fuels in microgravity aboard the International Space Station (ISS) under varying levels of confinement. Samples of cotton fiberglass blended textile fabric are burned in air flows in a small flow duct. Baffles are placed parallel to the sample sheet, one on each side symmetrically. The distance between the baffles is varied to change the confinement of the burning event. Three different materials of baffles are used to alter the radiative boundary conditions of the space that the flame resides: transparent polycarbonate, black anodized aluminum, and polished aluminum. In all tests, samples are ignited at the upstream leading edge and allowed to burn to completion. The results show that at low flow speeds (
10 citations
TL;DR: In this paper, the authors characterized thin fuel opposed flow flame spread in simulated microgravity for a range of gap heights and airflow velocities in a Narrow Channel Apparatus (NCA).
Abstract: This study characterizes thin fuel opposed flow flame spread in simulated microgravity for a range of gap heights and airflow velocities in a Narrow Channel Apparatus (NCA). One objective was to estimate gap heights that suppress buoyancy without promoting excessive heat losses to the channel walls. A corollary of this objective was to assess the dependence of heat losses on the channel height. A second objective was to determine the influence of global combustion stoichiometry on simulated microgravity flame spread in the NCA. Whatman 44 filter paper was used for NCA gap heights ranging from 6–20 mm (half-gap below and above sample) and average opposed flow velocities 1–40 cm/s. Flames at low flows were fuel rich when the forced flows were of the same magnitude as the diffusive flow. For thin fuels, a full gap of 10 mm (5 mm half-gap) provided a compromise between buoyancy suppression and heat loss. Calculations were made of flame stoichiometry and of the influence of the velocity profile on flame spread rates (comparing it with previous theory). This part of the analysis provided support for the velocity gradient theory of flame spread. The information provided in this work on the theoretical nature of opposed flow flame spread over thin fuels is based on experimental measurements in simulated microgravity conditions.
10 citations
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TL;DR: This article illustrates some basic features of error bars and explains how they can help communicate data and assist correct interpretation and suggests eight simple rules to assist with effective use and interpretation.
Abstract: Error bars commonly appear in figures in publications, but experimental biologists are often unsure how they should be used and interpreted. In this article we illustrate some basic features of error bars and explain how they can help communicate data and assist correct interpretation. Error bars may show confidence intervals, standard errors, standard deviations, or other quantities. Different types of error bars give quite different information, and so figure legends must make clear what error bars represent. We suggest eight simple rules to assist with effective use and interpretation of error bars.
717 citations
TL;DR: In this article, the viscosity and thermal conductivity for nitrogen, oxygen, argon, and air were derived using an approach adopted from previous work on the equation of state for air.
Abstract: New formulations for the viscosity and thermal conductivity for nitrogen, oxygen, argon, and air are given. Air is treated as a pseudo-pure fluid using an approach adopted from previous research on the equation of state for air. The equations are valid over all liquid and vapor states, and a simplified cross-over equation was used to model the behavior of the critical enhancement for thermal conductivity. The extrapolation behavior of the equations for nitrogen and argon well below their triple points was monitored so that both could be used as reference equations for extended corresponding states applications. The uncertainties of calculated values from the equations are generally within 2% for nitrogen and argon and within 5% for oxygen and air, except in the critical region where the uncertainties are higher. Comparisons with the available experimental data are given.
616 citations
TL;DR: In this paper, the steady-state flame spread over a thermally thin solid fuel is investigated, and qualitative agreement is obtained with experimental results in the near-extinction limit region.
Abstract: A theory for the steady-state flame spread over a thermally thin solid fuel is developed in this study. The model considers a laminar diffusion flame in a uniform opposed flow and includes the two-dimensional, elliptic, gas-phase energy, and species equations with one-step overall chemical reaction and second-order, finite-rate Arrhenius kinetics. The unsteady, solid-fuel equations neglect heat conduction ahead of the flame but include transient heating and Arrhenius pyrolysis kinetics and are coupled to the quasisteady gas phase. The equations are solved in the laboratory coordinate system. In this study the two-dimensional distributions of temperature and species are obtained, including the low reactivity zone in the flame region. The solid-fuel surface profiles indicate a region of almost uniform temperature (vaporization temperature) during pyrolysis for some parameter values; however, the value is not universally constant for the fuel and does depend on the ambient parameters (pressure, oxygen mass fraction, and magnitude of opposed velocity). The dependence of the flame-spread rate on the ambient parameters is investigated, and qualitative agreement is obtained with experimental results in the near-extinction-limit region. Quantitative agreement with experimental data depends on the choice of parameter values, especially the gas-phase kinetics model parameters, which are generally unknown. The flame-extinction limits due to increased opposed velocity, reduced pressure, and reduced ambient oxygen mass fraction are all obtained in the results calculated from this theory.
183 citations
TL;DR: A critical, historical review of the flame spread literature is given in this article, beginning with the first systematic studies of opposed-flow flame spread, including qualitative, simplified, and comprehensive numerical modeling.
Abstract: A critical, historical review of the flame spread literature is given, beginning with the first systematic studies of opposed-flow flame spread. Important modeling effects are described, including qualitative, simplified, μg and comprehensive numerical modeling. A brief discussion of subjects with the potential for further development is also given. Although this review focuses on flame-spread theory the emphasis is on the logical development, not the detailed mathematics.
179 citations
01 Jan 1981
TL;DR: In this article, the velocity of flame propagation over the surface of thick PMMA and thin paper sheets has been measured as a function of the velocity and oxygen concentration of a forced gas flow opposing the direction of the flame propagation.
Abstract: The velocity of flame propagation over the surface of thick PMMA and thin paper sheets has been measured as a function of the velocity and oxygen concentration of a forced gas flow opposing the direction of flame propagation. It is shown that although for thin fuels the flame spread rate always decreases as the opposed flow velocity increases, for thick fuels the dependence of the spread rate on the gas velocity is also a function of the ambient oxygen concentration. For low oxygen concentrations the flame spread rate decreases as the velocity of the gas flow increases. For high oxygen concentrations, however, the spread rate increases with the flow velocity, reaches a maximum and then decreases as the gas velocity increases. The velocity of the opposed flow at which the maximum occurs is a function of the oxygen concentration, decreasing as the concentration decreases. Following phenomenological considerations and simplified descriptions of the primary mechanisms occurring during the flame spread process, the experimental results are correlated by two non-dimensional parameters, one describing the gas phase kinetic effects and the other describing the process of heat transfer from the flame to the fuel. Such a correlation provides a powerful means of predicting the flame spread prcess as well as physical insight into the mechanisms controlling the propagation of the flame.
173 citations