Width effects on downward flame spread over poly(methyl methacrylate) sheets
TL;DR: In this paper, the downward flame spread over poly(methyl methacrylate) sheets with different dimensions was conducted in comparison with the downward flames spread over samples under infinite dimensions.
Abstract: The experiments of downward flame spread over poly(methyl methacrylate) sheets with different dimensions were conducted in this study. In comparison with flame spread over samples under infinite wi...
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TL;DR: In this paper, the influence of vertical channel with various structure factors (α) on downward flame spread over extruded polystyrene (XPS) foam is investigated, and a model is established to predict the total, convective and radiative heat fluxes transferred to preheating zone.
Abstract: The influence of vertical channel with various structure factors (α) on downward flame spread over extruded polystyrene (XPS) foam is investigated. The flame shape, flame height (Hf), temperature in the channel (Tv) and on the curtain wall surface (Tc), flame spread rate (Vf), and induced air flow (Q) are obtained. Hf first drops and then increases as α rises, attributing to similar changing trends of induced airflow rate and Froude number. Concerning the temperature history, Tc > Tv at the initial stage while Tc T c ‾ (average value) and T v ‾ drop as α rises, and T v ‾ > T c ‾ under most conditions. A model is established to predict the total, convective and radiative heat fluxes transferred to preheating zone. The convective heat flux is more dominant than the radiative heat feedback from the curtain wall. Vf first rises and then drops as α increases. For α
27 citations
TL;DR: In this article, side-edge effects on downward flame spread over two parallel polymethyl methacrylate (PMMA) slabs under different pressure environments were investigated. But the results showed that the flame spread rate is controlled by ignition along the side-Edge, rather than at the center of the samples, for experiments with both single and two parallel slabs.
Abstract: This paper presents an experimental and theoretical study of side-edge effects on downward flame spread over two parallel polymethyl methacrylate (PMMA) slabs under different pressure environments. Identical experiments of downward flame spread over thin PMMA slabs with side-edges unrestrained were conducted at different altitudes in Hefei (102 kPa), Geermu (73.2 kPa) and Lhasa (66.3 kPa). Experimental results show that the flame spread rate is controlled by ignition along the side-edge, rather than at the center of the samples, for experiments with both single and two parallel slabs. Based on these results, a thermal model is developed which describes flame spread along the edge and quantitatively agrees with experimental results. In the parallel-slab case, convective heating appears to influence the spread rate only when the separation distance is very small, with radiative heating playing a more important role as separation distance increases. The angle of the pyrolysis front, formed between the faster side-edge spread and slower center-region spread, hardly changes with pressure, but changes significantly with separation distance, due to differing modes of heat transfer between the side-edge and center region. In addition, variations of flame height with pressure and separation distance are reasonably interpreted from diffusion flame theory.
26 citations
TL;DR: In this article, the effects of the nozzle inclined angle and diameter as well as the heat release rate on the characteristics of syngas jet flame were investigated. And the model can be used to estimate the horizontal extension length of the syngga jet flame for nozzles with different diameters and inclined angles.
Abstract: As an important industrial raw material, syngas can be used for hydrogen production, and its main components include hydrogen and carbon monoxide. Experiments were conducted on syngas (ratio of CO and H2 is 1:1) to study the effects of the nozzle inclined angle and diameter as well as the heat release rate on the characteristics of syngas jet flame. Three nozzles with different diameters, 5 mm, 10 mm, and 15 mm, were used in this study. Four different inclined angles of the nozzle corresponding to 0°, 30°, 60°, and 90° were employed. It was found that the normalized horizontal extension length of jet flame (L/D) varies with nozzle diameter and fuel flow rate, and increases exponentially as the dimensionless heat release rate ( Q ∗ ) increases for the nozzle with inclined angles of 0°–60°. By modifying the dimensionless heat release rate and fitting the experimental data, a unified empirical model, which is the first of its kind to date, is established. The model can be used to estimate the horizontal extension length of the syngas jet flame for nozzles with different diameters and inclined angles. For jet flame at an inclined angle of 90°, the normalized jet flame width was exponentially proportional to the modified Froud number with a power of 0.39.
22 citations
TL;DR: In this paper, a coupled model of heat and mass transfer describing the feedback between gas-phase flame and solid fuel has been defined by non-stationary two-dimensional elliptic equations applied both for gas phase and liquid fuel.
Abstract: Flame spread over the horizontal surface of polymethyl methacrylate (PMMA) has been studied numerically by a coupled model of heat and mass transfer describing the feedback between gas-phase flame and solid fuel Mathematical formulation has been defined by non-stationary two-dimensional elliptic equations applied both for gas phase and solid fuel The computational procedure is based on modification of the OpenFOAM open-source code Results of predictions have been compared with the data of comprehensive experimental investigation of the thermal and chemical structure of PMMA flame Good agreement has been obtained for the detailed gas-phase and the solid fuel temperature and species concentrations profiles, as well as for the macroscopic parameters: the flame spread rate, the total mass regression rate and the length of the pyrolysis zone Based on the analysis of thermal degradation of methylmethacrylate in inert surrounding, the concept of reduced molar weight for gaseous products of PMMA pyrolysis has been proposed, which provided better agreement for fuel distribution in the gas phase
17 citations
TL;DR: Results show that flame spread rate increases exponentially with pressure, and the exponent of pressure further shows an increasing trend with the thickness of the sample, which indicates a steady-burning stage.
Abstract: The present study is aimed at predicting downward flame spread characteristics over poly(methyl methacrylate) (PMMA) with different sample dimensions in different pressure environments. Three-dimensional (3-D) downward flame spread experiments on free PMMA slabs were conducted at five locations with different altitudes, which provide different pressures. Pressure effects on the flame spread rate, profile of pyrolysis front and flame height were analyzed at all altitudes. The flame spread rate in the steady-state stage was calculated based on the balance on the fuel surface and fuel properties. Results show that flame spread rate increases exponentially with pressure, and the exponent of pressure further shows an increasing trend with the thickness of the sample. The angle of the pyrolysis front emerged on sample residue in the width direction, which indicates a steady-burning stage, varies clearly with sample thicknesses and ambient pressures. A global non-dimensional equation was proposed to predict the variation tendency of the angle of the pyrolysis front with pressure and was found to fit well with the measured results. In addition, the dependence of average flame height on mass burning rate, sample dimension and pressure was proposed based on laminar diffusion flame theory. The fitted exponent of experimental data is 1.11, which is close to the theoretical value.
14 citations
Cites background or methods or result from "Width effects on downward flame spr..."
...This method is verified to be able to predict downward flame spread rate [10,14]....
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...Our previous study [14] has built a predictive model of 3-D downward fire spread over PMMA in free conditions, as shown in Figure 3....
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...In our previous study [14], a calculating formula was proposed to predict the steady flame spread rate based on heat transfer theory and energy conservation equation,...
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...Heat transfer at the leading edge of the flame is enhanced since the stand-off distance is smaller and oxygen supply is rich there [2,7,12,14]....
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...α, β, γ are characteristic angles that emerged on sample residue in the steady-burning stage and they are unchanged with sample width according to the experimental results and related studies [2,12,14]....
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References
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TL;DR: In this paper, a review of the progress that has been made to the understanding of chemical and physical processes, which occur during combustion of solid fuels, is presented, and the effects of bubble formation on the transport of volatiles during thermal degradation of non-charring fuels, described through a one-step global reaction, have been modeled.
Abstract: Some of the progress that, owing to modeling and numerical simulation, has been made to the understanding of chemical and physical processes, which occur during combustion of solid fuels, is presented. The first part of the review deals with thermal degradation processes of charring (2ood and, in general, cellulosic materials) and non-charring (poly-methyl-methacrylate) materials. Gas-phase combustion processes (ignition, flame spread and extinction) are discussed in the second part of the review. Solid fuel degradation has been described by kinetic models of different complexity, varying from a simple one-step global reaction, to multi-step reaction mechanisms, accounting only for primary solid fuel degradation, and to semi-global reaction mechanisms, accounting for both primary solid degradation and secondary degradation of evolved primary pyrolysis products. Semi-global kinetic models have been coupled to models of transport phenomena to simulate thermal degradation of charring fuels under ablation regime conditions. The effects of bubble formation on the transport of volatiles during thermal degradation of non-charring fuels, described through a one-step global reaction, have also been modeled. On the contrary, very simplified treatments of solid phase processes have been used when gas phase combustion processes are also simulated. On the other hand, the latter have also always been described through one-step global reactions. Numerical modeling has allowed controlling mechanisms of ignition and flame spread to be determined and the understanding of the interaction between chemistry and physics during thermal degradation of solid fuels to be improved. However, the chemical processes are not well understood, the few kinetic data are in most cases empirical and variations of solid properties during degradation are very poorly known, so that even the most advanced models do not in general give quantitative predictions.
434 citations
"Width effects on downward flame spr..." refers background in this paper
...Apparently, mass loss rate is concerned with the flame spread rate in steady-state stage, and the relationship is presented in equation (2)....
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...By substituting equation (12) into equation (2), the computed mass loss rate as a function of width and thickness is shown as follows...
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01 Jan 1969
TL;DR: In this article, a theoretical description of a laminar diffusion flame spreading against an air stream over a solid- or liquid-fuel bed is presented, where both a thin sheet and a semi-infinite fuel bed are considered.
Abstract: A theoretical description is presented for a laminar diffusion flame spreading against an air stream over a solid- or liquid-fuel bed. Both a thin sheet and a semi-infinite fuel bed are considered. The burning process is described as follows: The hot flame heats the unburned fuel bed, which subsequently vaporizes. The resulting fuel vapor reacts with the oxygen supplied by the incoming air, thereby producing the heat that maintains the flame-spread process. The formulated model treats the combustion as a diffusion flame, for which the details of the reaction kinetics can be ignored by assuming infinite reaction rates. The model includes the chemical stoichiometry, heat of combustion, gas-phase conductive heat transfer, radiation, mass transfer, fuel vaporization, and fuel-bed thermal properties. The radiation is mathematically treated as a heat loss at the flame sheet and a heat gain at the fuel-bed surface. The calculated flame-spread formulas are not inconsistent with available experimental data. These results reveal much of the physics involved in a spreading, flame. For instance, the flame-spread rate is strongly influenced by (1) the adiabatic stoichiometric flame temperature, and (2) the fuel-bed thermal properties, except for the fuel-bed conductivity parallel to the propagation direction.
356 citations
TL;DR: In this paper, heat transfer and gas phase chemical kinetic aspects of the flame spread process are addressed separately for the spread of flames in oxidizing flows that oppose or concur with the direction of propagation.
Abstract: Recent advances in the experimental study of the mechanisms controlling the spread of flames over the surface of combustible solids are summarized in this work. The heat transfer and gas phase chemical kinetic aspects of the flame spread process are addressed separately for the spread of flames in oxidizing flows that oppose or concur with the direction of propagation. The realization that, in most practical situations, the spread of fire in opposed gas flows occurs at near extinction or non-propagating conditions is particularly significant. Under these circumstances, gas phase chemical kinetics plays a critical role and it must be considered if realistic descriptions of the flame spread process are attempted. In the concurrent mode of flame spread, heat transfer from the flame to the unburnt fuel appears to be the primary controlling mechanism. Although gas phase chemcial kinetics is unimportant in the flame spreading process, it is important in the establishment and extension of the diffusion ...
266 citations
TL;DR: In this article, heat transfer and flame height results are presented for wall samples burned at varying levels of external irradiance, and an approximate theoretical analysis is included to serve as a guide to identifying the important variables and their relationship for correlation purposes.
Abstract: New concepts are addressed for predicting the flame spread on materials from laboratory measurements. It focuses on heat transfer which precipitates and precedes upward flame spread on a vertical surface. Six materials have been featured in this study as well as in past related studies. Their flame spread properties are presented. In this particular study heat transfer and flame height results are presented for wall samples burned at varying levels of external irradiance. Also complementary results are presented for methane line burner wall fires. An approximate theoretical analysis is included to serve as a guide to identifying the important variables and their relationship for correlation purposes. Experimental results yield flame height proportional to energy release rate to the 2/3 power, and wall heat flux distributions are roughly correlated in terms of distance divided by flame height. These correlations appear to at least hold for the scale of these experiments: flame heights of 0.3 to 1....
135 citations
TL;DR: In this article, the authors measured temperature distributions within the PMMA sample in the vicinity of the leading edge of the flame front using holographic interferometry and found that the heat flow vector patterns within the sample showed the dominant heat transfer path from the gas phase into the unburnt fuel ahead of the vaporization point.
Abstract: Flame spread over a thermally thick slab of PMMA at several angles of sample orientation from theta = -90/sup 0/ (vertically downward flame spread) to theta = +90/sup 0/ (vertically upward flame spread) in air was investigated by measuring temperature distributions within the PMMA sample in the vicinity of the leading edge of the flame front using holographic interferometry. Samples with widths of 0.32, 0.47, 1.0, and 2.5 cm, a thickness of 2.5 cm, and a length of 30 cm were used. The measured net heat flux from the gas phase to the sample surface at the vaporization front of the sample is about 7 W/cm/sup 2/ for downward flame spread (theta < 0/sup 0/), 6.5 W/cm/sup 2/ at theta = +10/sup 0/, and 2.8 W/cm/sup 2/ at theta = +90/sup 0/. However, the total net heat transfer rate increases with an increase in the angle of sample orientation, because the characteristic heating length, defined as the distance from the adiabatic point on the sample surface to the vaporization point, increases with an increase in the orientation angle of the sample. The total net heat transfer rate into the sample from the gas phase is about 56% of the totalmore » net heat transfer input to the sample at theta = -90/sup 0/, 78% at theta = 0/sup 0/, 87% at theta = +10/sup 0/, and 99% at theta = +90/sup 0/. Therefore heat transfer from the gas phase into the unburnt fuel ahead of the vaporization point is the dominant heat transfer path for all angles of orientation. This was clearly demonstrated by the net heat flow vector patterns within the sample. The streamwise conductive heat transfer rate through the sample decreases with an increase in flame spread rate (increase in the sample orientation angle) due to insufficient time being available for the slow thermal wave to travel through the sample.« less
117 citations