Gas-phase radiative effects on the burning and extinction of a solid fuel
TL;DR: In this paper, the effects of gas-phase radiative effects on the burning and extinction of a solid fuel in a stagnation-point flow geometry were investigated using a statistical narrowband model with carbon dioxide and water vapor as the radiative participating media.
Abstract: Gas-phase radiative effects on the burning and extinction of a solid fuel in a stagnation-point flow geometry are investigated using a statistical narrow-band model with carbon dioxide and water vapor as the radiative participating media. The model, coupled to other flame conservation equations with a one-step overall gas-phase chemical reaction and Arrhenius solid pyrolysis relation, is solved numerically. Flame temperature, solid burning rate, and heat fluxes are examined as functions of stretch rate. Using ambient oxygen percentage and stretch rate as coordinates, A U-shaped extinction boundary is identified. The extinction behavior at low stretch rates is qualitatively similar to that predicted by earlier theory with only surface radiation loss. However, gas radiation introduces additional heat loss from the system and shrinks the solid flammable domain. In addition, gas radiation can cause a substantial decrease of flame temperature and constitutes a significant portion of the heat feedback to the solid at low stretch rates. In the second part of the paper, a computationally less intensive gray gas radiation model is tested. As with a number of earlier investigations, the use of Planck mean absorption coefficient is found to overpredict net emission and flame radiative loss. By multiplying a correction factor (less than 1) in front of the Planck mean absorption coefficient, it is possible to compute many global flame properties with reasonable accuracy. An empirically determined formula is given to find the value of this correction factor for a given flame. This is offered as an engineering approach for the flame radiation treatment.
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TL;DR: There has been a considerable amount of progress in studying flamelets, their structures and their responses to various perturbations as mentioned in this paper, however, the focus is narrower, namely on quasisteady flamelets and therefore the considerations of flamelet extinction that are presented in this paper are not the dynamics of extinction.
Abstract: In the past 25 years there has been a considerable amount of progress in studying flamelets, their structures and their responses to various perturbations. The term “flamelet” as used here really would mean “laminar flame” to most readers and is employed only because a major motivation is for ultimate use in connection with more complex flows, mainly turbulent. There is, however, no consideration here of how the knowledge reviewed may be employed in flamelet modeling of turbulent combustion. Not even time-dependent flamelets are addressed, although a few related references are provided. The focus is narrower, namely on quasisteady flamelets, and therefore the considerations of flamelet extinction that are presented concern quasisteady extinction, that is, not the dynamics of extinction. Even in this narrow context, it will be seen that a great deal has been accomplished. When such a long-term view is taken, it is found remarkable how much progress has been made. The progress is addressed separately for premixed, nonpremixed and partially premixed systems. Suggested directions of future research also are indicated. Despite the limited scope of the topic and the extensive advancement that has occurred, much more research remains to be done.
203 citations
01 Jan 1998
TL;DR: A review of research on the effects of gravity on combustion processes is presented, with an emphasis on a discussion of the ways in which reduced-gravity experiments and modeling has led to new understanding as discussed by the authors.
Abstract: A review of research on the effects of gravity on combustion processes is presented, with an emphasis on a discussion of the ways in which reduced-gravity experiments and modeling has led to new understanding. Comparison of time scales shows that the removal of buoyancy-induced convection leads to manifestations of other transport mechanisms, notably radiative heat transfer and diffusional processes such as Lewis number effects. Examples from premixed-gas combustion, non-premixed gas-jet flames, droplet combustion, flame spread over solid and liquid fuels, and other fields are presented. Promising directions for new research are outlined, the most important of which is suggested to be radiative reabsorption effects in weakly burning flames.
86 citations
01 Jan 2015
TL;DR: In this paper, the authors introduce fire safety standards for flammability evaluation of solid material intended for use in a spacecraft habitat, and the difference between the limiting value in microgravity and the indices given by the standard test methods on the ground is discussed.
Abstract: This paper introduces fire safety standards for flammability evaluation of solid material intended for use in a spacecraft habitat. Two types of existing standards include material evaluation by pass/fail criteria corresponding to Test 1 of NASA STD 6001B and evaluation by a flammability index such as maximum oxygen concentration (MOC) corresponding to the improved Test 1. The advantage of the latter is the wide applicability of the MOC index to different atmospheres in spacecraft. Additionally, the limiting oxygen index (LOI) method is introduced as a potential alternative index for the evaluation using the improved Test 1 method. When criteria based on an index such as MOC or LOI are applied for material screening, the discrepancy of the index to the actual flammability limit in microgravity such as minimum limiting oxygen concentration (MLOC) is essential information for guaranteeing fire safety in space because material flammability can be higher in microgravity. In this paper, the existing research on the effects of significant parameters on material flammability in microgravity are introduced, and the difference between the limiting value in microgravity and the indices given by the standard test methods on the ground is discussed. Finally, on-going efforts to develop estimation methods of material flammability in microgravity according to normal gravity tests are summarized.
74 citations
TL;DR: In this paper, a two-dimensional, opposed-flow, flame-spread model, with flame radiation, has been formulated and solved numerically, and a comparison of flammability limits and flame-spreading rates between opposing and concurrent spreading flames are made; both models contain the same assumptions and properties.
Abstract: Flame-spread phenomena over thin solids are investigated for purely forced-opposing and concurrent flows. A two-dimensional, opposed-flow, flame-spread model, with flame radiation, has been formulated and solved numerically. In the first part of the paper, flammability limits and spread rates in opposed flow are presented, using oxygen percentage, free-stream velocity, and flow-entrance length as parameters. The comparison of the flammability boundaries and spread-rate curves for two different entrance lengths exhibits a cross-over phenomenon. Shorter entrance length results in higher spread rates and a lower oxygen-extinction limit in low free-stream velocities, but lower spread rates and a higher oxygen-extinction limit in high free-stream velocities. The entrance length affects the effective flow rate that the flame sees at the base region. This affects the radiation loss and gas residence-time in an opposing way to cause the cross-over. Radiation also affects the energy balance on the solid surface and is in part responsible for the solid-fuel non-burn-out phenomenon. In the second part of the paper, a comparison of flammability limits and flame-spreading rates between opposing and concurrent spreading flames are made; both models contain the same assumptions and properties. While the spread rate in concurrent spread increases linearly with free-stream velocity, the spread rate in opposed flow varies with free-stream velocity in a non-monotonic manner, with a peak rate at an intermediate free-stream velocity. At a given free-stream velocity, the limiting oxygen limits are lower for concurrent spread, except in the very low free-stream-velocity regime, where the spreading flame may be sustainable in opposed mode and not in concurrent mode. The cross-over disappears if the two spread modes are compared using relative flow velocities with respect to the flames rather than using free-stream velocities with respect to the laboratory.
58 citations
TL;DR: In this paper, a narrowband radiation model is coupled to the OPPDIF program, which uses detailed chemical kinetics and thermal and transport properties to enable the study of one-dimensional counterflow H 2 /O 2 diffusion flames with CO 2 as dilution gas over the entire range of flammable strain rates.
Abstract: A narrowband radiation model is coupled to the OPPDIF program, which uses detailed chemical kinetics and thermal and transport properties to enable the study of one-dimensional counterflow H 2 /O 2 diffusion flames with CO 2 as dilution gas over the entire range of flammable strain rates. The effects of carbon dioxide dilution, ambient pressure and inlet temperature of opposed jets on the extinction limits and flame structures are compared and discussed. The extinction limits are presented using maximum flame temperature and strain rate as coordinates. Both high-stretch blowoff and the low-stretch quenching limits are computed. When the CO 2 dilution percentage is higher, the flame is thinner and flame temperature is lower. The combustible range of strain rates is decreased with increasing CO 2 percentage due to the effects of CO 2 dilution, which is categorized as dilute effect, chemical effect and radiation effect. In addition, the flame temperature of low-stretch diffusion flame with radiation loss is substantially lower than that computed with the non-radiation model. This large temperature drop results from the combined effect of flame radiation and chemical kinetics. The extinction limits and flame temperature are increasing with increasing atmospheric pressure and temperature, but the flame thickness is decreased with the pressure. At higher pressure and temperature, the extinction limits are extended more on the high-stretch blowoff limits, indicating the influence of the ambient pressure and temperature on the chemical reaction.
57 citations
References
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15 Aug 2003
TL;DR: Advances in Heat Transfer as mentioned in this paper provides in-depth review articles over a broader scope than in traditional journals or texts, which serve as a broad review for experts in the field and are also of great interest to non-specialists who need to keep up to date with the results of the latest research.
Abstract: Advances in Heat Transfer fills the information gap between regularly scheduled journals and university-level textbooks by providing in-depth review articles over a broader scope than in traditional journals or texts. The articles, which serve as a broad review for experts in the field are also of great interest to non-specialists who need to keep up-to-date with the results of the latest research. This serial is essential reading for all mechanical, chemical, and industrial engineers working in the field of heat transfer, or in graduate schools or industry. * Compiles the expert opinions of leaders in the industry* Fills the information gap between regularly scheduled journals and university-level textbooks by providing in-depth review articles over a broader scope than in traditional journals or texts* Essential reading for all mechanical, chemical, and industrial engineers working in the field of heat transfer, or in graduate schools or industry
1,591 citations
TL;DR: In this article, the growth curves calculated for Lorentz lines were used for molecular spectra representation using growth curves for the 1.1/s line intensity distribution function, and the growth curve was calculated for the 2.5/2.
Abstract: 1/s line intensity distribution function for molecular spectra representation using growth curves calculated for Lorentz lines
416 citations
TL;DR: In this paper, a theoretical analysis of diffusion flame extinction in the stagnation point region of a condensed fuel has been made including radiative heat loss from the fuel surface, and the extinction boundary consists of a blowoff and a radiative extinction branch.
Abstract: A theoretical analysis of diffusion flame extinction in the stagnation point region of a condensed fuel has been made including radiative heat loss from the fuel surface. In addition to the blowoff limit normally found when flame stretch rate is large, an extinction limit has been identified when the flame stretch rate becomes sufficiently small. This limit occurs as a result of flame temperature reduction when the rate of radiative loss becomes substantial compared with the rate of combustion heat release. A flammability map using oxygen mass fraction and stretch rate as coordinates shows that the extinction boundary consists of a blowoff and a radiative extinction branch. The merging point of the two branches defines a fundamental low oxygen flammability limit.
187 citations