Opposed flow flame spread over an array of thin solid fuel sheets in a microgravity environment
TL;DR: In this paper, a numerical study has been carried out to gain physical insight into the phenomena of opposed flow flame spread over an array of thin solid fuel sheets in a microgravity environment.
Abstract: In this work a numerical study has been carried out to gain physical insight into the phenomena of opposed flow flame spread over an array of thin solid fuel sheets in a microgravity environment. The two-dimensional (2D) simulations show that the flame spread rates for the multiple-fuel configuration are higher than those for the flame spreading over a single fuel sheet. This is due to reduced radiation losses from the flame and increased heat feedback to the solid fuel. The flame spread rate exhibits a non-monotonic variation with decrease in the interspace distance between the fuel sheets. Higher radiation heat feedback primarily as gas/flame radiation was found to be responsible for the increase in the flame spread rate with the reduction of the interspace distance. It was noted that as the interspace distance between the fuel sheets was reduced below a certain value, no steady solution could be obtained. However, at very small interspace distances, steady state spread rates were obtained. Here, due to...
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TL;DR: In this paper, an experimental investigation of diffusion flames spreading along thin solid fuels in concurrent and opposed configurations in a gravity induced flow is presented, where the multiple fuel sheets (2 and 3 sheets) are kept parallel to each other with the separation distance between them varied from 0.5 to 3 cm.
19 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.
17 citations
01 Jan 2017
TL;DR: In this paper, an elaborate 3D numerical model is used to predict the near limit fame-spread behavior of a self-propagating flame over a thin solid in microgravity, and two kinds of unsteady flame-spread phenomena were noted.
Abstract: Microgravity experiments have shown that near limit opposed flow flame-spread show unsteady oscillatory behavior marked with formation of flamelets. Here in this work an elaborate 3D numerical model is used to predict this near limit fame-spread behavior of a self-propagating flame over thin solid in microgravity. As the oxygen level is reduced steady, below certain value, flame-spread over wide thin solid fuel becomes unsteady. Two kinds of unsteady flame-spread phenomena were noted. In one a single flame oscillated back and forth near the fuel side edge and in other the multiple flamelets were formed which oscillated laterally. The former occurs at relatively higher oxygen levels and leads to formation of the later at still lower oxygen levels just ahead of extinction. It is noted that as oxygen level is reduced the amplitude of longitudinal oscillation increases leading to the splitting of flame which then exhibits lateral motion. The lateral oscillatory behavior spans over a wider range of oxygen level than that of longitudinal oscillatory flame behavior. The range of oxygen level over which the lateral oscillatory behavior is observed, increases with increase in fuel thickness and fuel width. The number of flamelets formed increases with increase in fuel width with typical flamelet size of 2–4 cm. Some of the flame behavior noted here are remarkably similar to those seen in the short duration drop tower test where heat loss from the flame was artificially enhanced. However, the numerical computations show that such oscillatory flames can exist for considerably long durations.
11 citations
TL;DR: In this paper, a three-dimensional transient computational fluid dynamics (CFD) combustion model is used to simulate concurrent-flow flame spread over a thin solid sample in a narrow flow duct.
Abstract:
The objective of this work is to investigate the aerodynamics and thermal interactions between a spreading flame and the surrounding walls as well as their effects on fire behaviors. A three-dimensional transient computational fluid dynamics (CFD) combustion model is used to simulate concurrent-flow flame spread over a thin solid sample in a narrow flow duct. The height of the flow duct is the main parameter. The numerical results predict a quenching height for the flow duct below which the flame fails to spread. For duct heights sufficiently larger than the quenching height, the flame reaches a steady spreading state before the sample is fully consumed. The flame spread rate and the pyrolysis length at steady-state first increase and then decrease when the flow duct height decreases. The detailed gas and solid profiles show that flow confinement has multiple effects on the flame spread process. On one hand, it accelerates flow during thermal expansion from combustion, intensifying the flame. On the other hand, increasing flow confinement reduces the oxygen supply to the flame and increases conductive heat loss to the walls, both of which weaken the flame. These competing effects result in the aforementioned nonmonotonic trend of flame spread rate as duct height varies. Near the quenching duct height, the transient model reveals that the flame exhibits oscillation in length, flame temperature, and flame structure. This phenomenon is suspected to be due to thermodiffusive instability.
9 citations
01 Jan 2019
TL;DR: In this paper, an established and elaborate numerical model in 3D is used explore opposed flow flame spread phenomena near oxygen extinction limit in quiescent and low convective environment (with flow velocity, U∞ = 5 cm/s).
Abstract: A few microgravity experiments have reported formation of flamelets at near extinction of freely propagating opposed flow flame over solid fuels. Inspired by these observations, about which little is known so far, an established and elaborate numerical model in 3D is used explore opposed flow flame spread phenomena near oxygen extinction limit in quiescent and low convective environment (with flow velocity, U∞ = 5 cm/s). Simulations were carried out for various fuel widths show that 1 cm wide fuel has the least oxygen extinction limit among fuel widths varying between 0.5 cm and the 2D limit even after accounting for the presence of flamelets over wider fuels (except in 2D limit). Unlike quiescent environment where flamelets were found to be inherently unsteady and eventually extinguished, steadily propagating flamelets were also obtained in low convective space environment. The flamelet oscillations which arise due to thermo-diffusive instability were found to have time period close to sum of times scales in gas phase and solid phase. A scaling analysis showed that fuel area density, flow velocity and fuel Lewis number are key factors that influence size of a steadily propagating flamelets. Several of these features have been observed in experiments and predicted in the present simulations.
7 citations
References
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Book•
01 Jan 1980
TL;DR: In this article, the authors focus on heat and mass transfer, fluid flow, chemical reaction, and other related processes that occur in engineering equipment, the natural environment, and living organisms.
Abstract: This book focuses on heat and mass transfer, fluid flow, chemical reaction, and other related processes that occur in engineering equipment, the natural environment, and living organisms. Using simple algebra and elementary calculus, the author develops numerical methods for predicting these processes mainly based on physical considerations. Through this approach, readers will develop a deeper understanding of the underlying physical aspects of heat transfer and fluid flow as well as improve their ability to analyze and interpret computed results.
21,858 citations
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, a review of the literature in this area is presented, with a focus on gaseous radiation properties of gases and their applications in engineering applications, where the assumption is that the radiating gas under consideration is at the state of complete or local thermodynamic equilibrium and of negligible scattering effect.
Abstract: Publisher Summary This chapter aims to systematically develop the background information needed to formulate and evaluate thermal radiation properties of gases for engineering applications, and to review the literature of present works and approaches for future research in this area. The scope of the chapter is limited by the assumption that the radiating gas under consideration is at the state of complete or local thermodynamic equilibrium and of negligible scattering effect. The chapter introduces the general concepts concerning gaseous radiation and presents a review of the physics of atomic and molecular spectra. The radiation resulting from transitions of electronic, atomic, or molecular states has been discussed; they are line radiation, band radiation, and continuum radiation. The evaluation of total (engineering) emissivity and its applications to radiation from homogeneous gas bodies of complex geometry have been discussed. Consideration has been given to the appropriate absorption coefficients for use in the radiative transport calculations.
257 citations
01 Jan 1991
241 citations
"Opposed flow flame spread over an a..." refers background in this paper
...All thermal and transport properties are temperature dependent and are modelled following Smooke and Giovangigli [20]....
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...All thermal and transport properties are temperature dependent and are modelled following Smooke and Giovangigli [20]. μ μ∗ = κ/cp κ∗/c∗p = ρDi ρ∗D∗i = ( T T ∗ )0.7 , f or i = F, O2, CO2, H2O, N2 Here, T ∗(= 1250 K) is the mean of the adiabatic flame temperature in air and the ambient temperature....
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