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Journal ArticleDOI

Flame spread and interactions in an array of thin solids in low-speed concurrent flows

02 Jul 2009-Combustion Theory and Modelling (Taylor & Francis Group)-Vol. 13, Iss: 3, pp 443-459
TL;DR: In this paper, a steady, two-dimensional flame-spread model with flame radiation was employed and adapted in this work and some of the features were presented, including flow channelling effect and flame radiation interactions.
Abstract: Flame spread in an array of thin solids in low-speed concurrent flows was investigated and numerical solved. A previous steady, two-dimensional flame-spread model with flame radiation was employed and adapted in this work. The flame structures of spreading flames between parallel solids were demonstrated and some of the features were presented, including flow channelling effect and flame radiation interactions. The channelling effect is caused by flow confinement by the presence of the other solids; the flows through the hot combustion gases are accelerated downstream drastically. Radiation interactions between flames and solids contributed to a less heat-loss system, and radiation re-absorption by flames resulted in a larger flame with higher temperature, which increased the conductive heat fluxes to the solids and flame spread rate. Consequently, the extinction limit for the interacting flames is extended beyond the low-speed quenching limit for a single flame. The influence of the separation distance o...
Citations
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Journal ArticleDOI
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

Journal ArticleDOI
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...

13 citations


Cites background from "Flame spread and interactions in an..."

  • ...In a recent work, Shih [14] numerically studied the flame interactions in an array of thin solids for low speed concurrent flows in a microgravity environment....

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Journal ArticleDOI
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

Journal ArticleDOI
TL;DR: In this article, the authors analyzed the flame front speed in the downward combustion of multiple parallel samples of thermally thin fuels at normal gravity and far from extinction conditions and derived an analytical approximation for the burning rate that generalizes the classical de Ris formula for those cases where radiative effects cannot be neglected.
Abstract: We analyze the flame front speed in the downward combustion of multiple parallel samples of thermally thin fuels at normal gravity and far from extinction conditions. In contrast with the single sample case, where conduction through the gas-phase is the dominant heat transfer mechanism, in the multiple parallel samples case, radiative heat fluxes may become very relevant, which compromises the application of the well-known formula of de Ris for determining the burning rate. Here we study the downward combustion of multiple parallel sheets by (1) obtaining new experimental data at different oxygen atmospheric levels; (2) generalizing a previous comprehensive energy balance model now expected to be valid for a wide range of scenarios; and (3) deriving an analytical approximation for the burning rate that generalizes the classical de Ris formula for those cases where radiative effects cannot be neglected. The comparison with own as well as with external data reveals the strengths and weaknesses of these type...

8 citations

Journal ArticleDOI
TL;DR: In this article, the authors derived analytical expressions that correspond to upper bounds for the speed of the flame front propagating downwards, which are functions of several parameters that are not taken into consideration in the de Ris expressions, such as the activation energy for solid-and gas-phase reactions.
Abstract: De Ris formulae for the flame spread rate over solid fuels that propagate against an external fluid flow for both the thermally thin and thick regimes have become well-known. In some cases, it is expected that predictions from these formulae will fail since they are independent of parameters that may modify the burning velocity. Based on the technique developed by Benguria and Depassier, we derive in this article new analytical expressions that correspond to upper bounds for the speed of the flame front propagating downwards. These upper bounds are functions of several parameters that are not taken into consideration in the de Ris expressions, such as the activation energy for solid- and gas-phase reactions. Burning velocity values obtained for these upper bounds are compared with new data and with other experimental results found in the literature, showing a better agreement than the de Ris formulae for a wide variety of cases.

6 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


"Flame spread and interactions in an..." refers methods in this paper

  • ...The elliptic gas phase equations are solved numerically using SIMPLER algorithm [ 15 ] and the equation of radiative transfer is solved by S–N discrete ordinates method [16, 17 ], in which the solid angle is discretised by selecting an ordered set of directions spanning the total solid angle of 4π with predetermined weights....

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Journal ArticleDOI
W. A. Fiveland1
TL;DR: In this paper, the methode des ordonnees discretes discretees au calcul numerique du transfert radiatif de chaleur dans une enceinte rectangulaire bidimensionnelle remplie d'un milieu gris absorbant, emissif and diffusant isotrope.
Abstract: Utilisation de la methode des Sn ordonnees discretes au calcul numerique du transfert radiatif de chaleur dans une enceinte rectangulaire bidimensionnelle remplie d'un milieu gris absorbant, emissif et diffusant isotrope. Presentation des resultats pour les approximations S 2 , S 4 et S 6 et comparaison aux solutions exactes obtenues par la methode numerique des zones de Hottel

593 citations

Journal ArticleDOI
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

Book ChapterDOI
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


"Flame spread and interactions in an..." refers methods in this paper

  • ...where ξ and η are x and y direction cosines respectively and K is the mean absorption coefficient which is based on the Plank mean absorption coefficient [13] and has been calibrated against the narrow-band results through a quasi-one-dimensional flame to account for the spectral effects [14]....

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Journal ArticleDOI
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.

179 citations