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Journal ArticleDOI

Gravitational effects on flame spread through non-homogeneous gas layers

01 Jan 2002-Vol. 29, Iss: 2, pp 2561-2567
TL;DR: In this article, the authors examined a system in which a fuel concentration gradient exists normal to the direction of flame propagation and parallel with the gravitational vector, and showed that the absence of gravity results in a faster spreading flame, by as much as 80% depending on conditions.
Abstract: Flame propagation through non-uniformly premixed gases occurs in several common combustion situations. Compared with the more usual limiting cases of diffusion or uniformly premixed flames, the practical concern of non-uniform premixed gas flame spread has received scant attention, especially regarding the potential role of gravity. This research examines a system in which a fuel concentration gradient exists normal to the direction of flame propagation and parallel with the gravitational vector. This paper presents experimental and numerical results for flame spread through alcohol/air layers formed by diffusive evaporation of liquid fuel at temperatures between the flash-point temperature and the stoichiometric temperature. A gallery, which had either the top and/or one end open to maintain constant pressure, surrounded the test section. The numerical simulations and experiments conducted include normal and microgravity cases. An interferometer was used, in normal gravity only, to determine the initial fuel layer thickness and fuel concentration distribution before and during flame spread. Both the model and experimental results show that the absence of gravity results in a faster spreading flame, by as much as 80% depending on conditions. This is the opposite effect to that predicted by an independent model reported earlier in this symposium series. Determination of the flame height showed that the flame was taller in microgravity, an effect also seen in the results of the numerical model reported here. Having a gallery lid results in faster flame spread, an effect more pronounced at normal gravity, demonstrating the importance of enclosure geometry. The interferometry and numerical model both indicated a redistribution of fuel vapor ahead of the flame. Numerical simulations show that, despite the rapid flame spread in these systems, the presence of gravity strongly affects the overall flow field in the gallery.
Citations
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Journal ArticleDOI
01 Jan 2007
TL;DR: In this paper, the propagation speed of tribrachial flames is discussed for flames propagating in mixing layers, including the effects of concentration gradient, velocity gradient, and burnt gas expansion.
Abstract: A tribrachial (or triple) flame is one kind of edge flame that can be encountered in nonpremixed mixing layers, consisting of a lean and a rich premixed flame wing together with a trailing diffusion flame all extending from a single point. The flame could play an important role on the characteristics of various flame behaviors including lifted flames in jets, flame propagation in two-dimensional mixing layers, and autoignition fronts. The structure of tribrachial flame suggests that the edge is located along the stoichiometric contour in a mixing layer due to the coexistence of all three different types of flames. Since the edge has a premixed nature, it has unique propagation characteristics. In this review, the propagation speed of tribrachial flames will be discussed for flames propagating in mixing layers, including the effects of concentration gradient, velocity gradient, and burnt gas expansion. Based on the tribrachial edge structure observed experimentally in laminar lifted flames in jets, the flame stabilization characteristics including liftoff height, reattachment, and blowout behaviors and their buoyancy-induced instability will be explained. Various effects on liftoff heights in both free and coflow jets including jet velocity, the Schmidt number of fuel, nozzle diameter, partial premixing of air to fuel, and inert dilution to fuel are discussed. Implications of edge flames in the modeling of turbulent nonpremixed flames and the stabilization of turbulent lifted flames in jets are covered.

267 citations

Journal ArticleDOI
TL;DR: A review of studies dealing with the extinction of PPFs, which represent a broad family of flames, including double, triple (tribrachial), and edge flames, is provided in this paper.

63 citations

Journal ArticleDOI
TL;DR: A review of key studies in this subject area has been conducted and summarised, focussing mainly on the theory plus a notable experimental findings about combustion and the flame spread phenomena of fuel-soaked porous media as mentioned in this paper.

23 citations


Cites background from "Gravitational effects on flame spre..."

  • ...bed surface and the flame pillar grows up to the maximum height as the time elapses, it gradually decreases and flattened over the surface until the occurrence of spontaneous extinction [14,18,24,26,45,55]....

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Journal ArticleDOI
TL;DR: In this article, the effects of the ground conditions; surface inclination, the ground temperature, and the surrounding airflow velocities, on the propagation velocity of the flame tip and on the thickness of the flammable gas layer are investigated.
Abstract: The propagation of a precursor flame tip through a layered flammable gas mixture along a fuel-soaked ground is studied experimentally under the effect of the surrounding airflow. The behavior of the precursor flame tip is investigated by the simultaneous measurement of the images of color Schlieren photograph and a visualized chemiluminescence zone due to the OH radical in the flame tip. The effects of the ground conditions; the surface inclination, the ground temperature, and the surrounding airflow velocities, on the propagation velocity of the flame tip and on the thickness of the flammable gas layer are investigated. The thickness and the length of the OH luminescence zone in the flame tip are of the order of 1 and 2-3 cm, respectively. The tip of the OH luminescence zone propagates in the velocity boundary layer of the airflow. When the ground temperature is not so high, even above the stoichiometric temperature of combustion of the liquid fuel, no propagation of flame tip occurs in the counter airfl...

19 citations


Cites background from "Gravitational effects on flame spre..."

  • ...Consequently, the behavior of the precursor flame tip is affected strongly by the conditions of formation of a flammable gas layer: the air stream along the ground, the inclination angle of the ground surface, and the temperature of the ground [14,20,21]....

    [...]

Journal ArticleDOI
TL;DR: In this paper, a numerical model developed with the CFD code Fire Dynamics Simulator (FDS) was used to analyze the effect of reduced gravity and ambient pressure on the transport processes taking place in the piloted ignition of an externally irradiated solid fuel.

18 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

Journal ArticleDOI
TL;DR: In this article, a comprehensive survey of the refractive indices and dispersions of gases that are found in practical or laboratory combustion experiments is reported; a critical evaluation was used to obtain recommended values where experimental data are available; where they are not, sums of atomic and bond refractivities were used.

230 citations

Journal ArticleDOI
TL;DR: In this paper, it was shown that a layered, unconfined combustible mixture supports a flame propagation velocity of 4 to 5 times the laminar flame speed of the stoichiometric mixture of fuel and air.
Abstract: It is observed that a layered, unconfined combustible mixture supports a flame propagation velocity of 4 to 5 times the laminar flame speed of the stoichiometric mixture of fuel and air. This effect which has been observed by some and more recently in this laboratory, has never been adequately explained. It is in contrast to the classical soap bubble experiment, in which the flame propagation velocity is of the order of 7 to 8 times the laminar flame speed, that is, the order of the unburned to burned gas density ratio To explain this new effect two physical models with different degrees of complexity are explored here. The first is a step-wise premixed fuel-air and pure air case in a gallery of infinite length. The second deals with one of finite length. Both have been mathematically defined to give quantitative results so that the effects of environmental factors can be explicitly determined Solutions of the first model show that the maximum flame propagation speed occurs when the gallery is of...

64 citations

Journal ArticleDOI
TL;DR: In this article, a two-dimensional, transient, numerical model was proposed to simulate the pulsating and uniform spread of a flame in n-propanot and ethanol liquid fuel pools.
Abstract: Pulsating and uniform spread across n-propanot and ethanol liquid fuel pools is simulated via a two-dimensional, transient, numerical model which incorporates finite-rate chemical kinetics, variable properties, and a partially adaptive finite-difference gridding scheme. The model is compared to detailed, independent experimental data. The explanation and characterization of the pulsating and uniform flame spread phenomena are developed Pulsating flame spread requires a gas-phase recirculation cell just forward of the flame. δ flow This cell entrains evaporating fuel vapor. The size and existence of the recirculation cell is determined by the extent of liquid motion ahead of the flame (δ flow) and by opposed flow in the gas phase, naturally induced by buoyancy. The amplitude and period of the pulsations each increase with δ flow Over the range of pool depths that were investigated (2 to 10 mm) the liquid-phase flow is primarily affected by large surface-tension variations along the liquid surface and not b...

54 citations

Journal ArticleDOI
TL;DR: In this paper, a study was made of flame propagation along methane-air roof layers using a laboratory scale gallery, 8 feet long by 3 inches wide by 6 to 10 inches high.
Abstract: A study was made of flame propagation along methane-air roof layers using a laboratory scale gallery, 8 feet long by 3 inches wide by 6 to 10 inches high. The effects of various environmental and gas layer factors on the speed of the propagating flame were determined and dimensional analysis techniques were used to develop a relationship between these factors and flame speed. The model was designed to study the early stages of flame propagation along a methane-air layer ignited at the closed end of a tunnel.

46 citations