Effect of gravity on laminar premixed gas combustion I: Flammability limits and burning velocities

Ronney, Paul D.; Wachman, Harold Y.

doi:10.1016/0010-2180(85)90139-7

Cited by 120 publications

(54 citation statements)

References 16 publications

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“…The observation that microgravity smolder rates fall between normal-gravity upward and downward configurations is similar to observed lean premixed flame propagation rates in the standard flammability limit tube [19] and large combustion vessel [20,21]. The reaction front smolder temperature in microgravity is seen to also fall between those encountered in normal-gravity upward and downward tests.…”

Section: Forced Flow Testssupporting

confidence: 72%

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Space shuttle based microgravity smoldering combustion experiments

Walther

Fernandez‐Pello

Urban

1999

Combustion and Flame

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Results from four microgravity smoldering combustion experiments conducted aboard the NASA Space Shuttle are presented in this work. The experiments are part of the NASA funded Microgravity Smoldering Combustion (MSC) research program, aimed to study the smolder characteristics of porous combustible materials in a microgravity environment. The objective of the study is to provide a better understanding of the controlling mechanisms of smolder for the purpose of control and prevention, both in normal-and microgravity. The microgravity smolder experiments reported here have been conducted to investigate the propagation of smolder through a polyurethane foam sample under both diffusion driven and opposed forced flow driven smoldering. The present experiments, although limited, are unique in that they provide the only available information about smolder combustion in microgravity in sample sizes large enough to allow the self-propagation of the smolder reaction throughout the sample length. Two quiescent tests at ambient oxygen concentrations of 35% and 40% and two opposed forced flow tests with air as oxidizer, were conducted aboard the NASA Space Shuttle (STS-69 and STS-77 missions).The MSC data are compared with normal-gravity data to determine the effect of gravity on smolder, and are used to verify present theoretical models of smolder combustion. It is found that for the present test conditions, the microgravity opposed flow smolder reaction temperatures, propagation velocities, toxic compound production and reaction extent lie between those of normal-gravity upward and downward tests. Thermogravimetric analysis shows little effect of gravity on the kinetics of the smolder process in these cases. Neither of the two quiescent, microgravity cases resulted in self-sustained smolder propagation, while the normal-gravity downward cases propagated vigorously. The difference in these results shows that gravity has a significant effect on smolder combustion, at least for the sample size tested. Correlation of the forced flow smolder velocity data with a heat transfer based model, indicates that simplified heat transfer models of smolder propagation can effectively describe vigorous smolder, away from limiting conditions such as extinction and flaming.

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Section: Forced Flow Testssupporting

confidence: 72%

“…Post-flight gas and TGA analyses are conducted on the microgravity smolder products (gas and solid). [19][20][21][22][23][24][25][26][27][28][29]1996). The experiments investigate the propagation of smolder along a polyurethane foam sample under both diffusion-driven (quiescent) and forced-flow-driven opposed smoldering.…”

Section: Flight Hardwarementioning

confidence: 99%

Space shuttle based microgravity smoldering combustion experiments

Walther

Fernandez‐Pello

Urban

1999

Combustion and Flame

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“…2a), comparison between computation and microgravity experiments in spherically expanding flames [20,21] and tubes [22] and earth-gravity experiments specially designed for low S L [23] is more favorable without reabsorption because optically-thin conditions applied to these experiments. The discrepancy between computation and experiment is significant even though the chemistry and transport models employed predict S L away from the limits quite well [24].…”

Section: Comparison With Experimentsmentioning

confidence: 99%

Effects of radiative emission and absorption on the propagation and extinction of premixed gas flames

Masuya

Ronney

1998

Symposium (International) on Combustion

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113

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Premixed gas flames in mixtures of CH 4 , O 2 , N 2 and CO 2 were studied numerically using detailed chemical and radiative emission-absorption models to establish the conditions for which radiatively-induced extinction limits may exist independent of the system dimensions. It was found that reabsorption of emitted radiation led to substantially higher burning velocities and wider extinction limits than calculations using optically-thin radiation models, particularly when CO 2 , a strong absorber, is present in the unburned gas. Two heat loss mechanisms that lead to flammability limits even with reabsorption were identified. One is that for dry hydrocarbon-air mixtures, because of the differences in the absorption spectra of H 2 O and CO 2 , most of the radiation from product H 2 O that is emitted in the upstream direction cannot be absorbed by the reactants. The second is that the emission spectrum of CO 2 is broader at flame temperatures than ambient temperature, thus some radiation emitted near the flame front cannot be absorbed by the reactants even when they are seeded with CO 2 . Via both mechanisms some net upstream heat loss due to radiation will always occur, leading to extinction of sufficiently weak mixtures. Downstream loss has practically no influence. Comparison to experiment demonstrates the importance of reabsorption in CO 2 -diluted mixtures. It is concluded that fundamental flammability limits can exist due to radiative heat loss, but these limits are strongly dependent on the emissionabsorption spectra of the reactant and product gases and their temperature dependence, and cannot be predicted using gray-gas or optically-thin model parameters. Applications to practical flames at high pressure, in large combustion chambers and with exhaust-gas or flue-gas recirculation are discussed.

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“…Based on the aforementioned considerations, the main goal of this investigation was to conduct an experimental and computational study of FL's at variable pressures, and assess the validity of the previous theoretical findings [17]. The experiments were performed for lean CH 4 /air mixtures in a variable pressure, counterflow flame rig at 1-g as well as lg utilizing the drop tower at the National Microgravity Laboratory of China (NMLC) in Beijing. The experiments were modeled using detailed description of chemical kinetics, molecular transport, and thermal radiation and sensitivity analysis was implemented to provide insight into the kinetic mechanisms that control FL's.…”

Section: Introductionmentioning

confidence: 99%

“…Thus, experiments need to be conducted under micro-gravity or reduced-gravity (lg) conditions (e.g. [4][5][6][7]). …”

Section: Introductionmentioning

confidence: 99%

Extinction of lean near-limit methane/air flames at elevated pressures under normal- and reduced-gravity

Zhang

Ren

Wang

et al. 2011

Proceedings of the Combustion Institute

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The extinction limits of lean, near-limit, counterflowing, CH 4 /air twin premixed flames were studied experimentally at evaluated pressures and under normal-and micro-gravity conditions utilizing the 3.5 s drop tower of the National Microgravity Laboratory of China. The results showed that under micro-gravity conditions the natural convection is minimized and the flames become more planar and symmetric compared to normal gravity. In both normal-and micro-gravity experiments and for a given strain rate and fuel concentration, the flame luminosity was found to enhance as the pressure increases. On the other hand, at a given pressure, the flame luminosity was determined to weaken as the strain rate decreases. At a given strain rate, the fuel concentration at extinction was found to vary non-monotonically with pressure, namely it first increases and subsequently decreases with pressure. The limit fuel concentration peaks around 3 and 4 atm under normal-and micro-gravity, respectively. The extinction limits measured at micro-gravity were in good agreement with predictions obtained through detailed numerical simulations but they are notably lower compared to the data obtained under normal gravity. The simulations confirmed the non-monotonic variation of flammability limits with pressure, in agreement with previous studies. Sensitivity analysis showed that for pressures between one and 5 atm, the near-limit flame response is dominated by the competition between the main branching, H + O 2 ? OH + O, and the pressure sensitive termination, H + O 2 + M ? HO 2 + M, reaction. However, for pressures greater than 5 atm it was determined that the HO 2 kinetics result in further chain branching in a way that is analogous to the third explosion limit of H 2 /O 2 mixtures.

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Effect of gravity on laminar premixed gas combustion I: Flammability limits and burning velocities

Cited by 120 publications

References 16 publications

Space shuttle based microgravity smoldering combustion experiments

Space shuttle based microgravity smoldering combustion experiments

Effects of radiative emission and absorption on the propagation and extinction of premixed gas flames

Extinction of lean near-limit methane/air flames at elevated pressures under normal- and reduced-gravity

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