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, 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. © 2018 The Combustion Institute.

Amit Kumar

Department of Aerospace Engineering

Flame research

flow velocity

microgravity

MICROGRAVITY PROCESSING

Numerical models

Oxygen

EXTINCTION LIMITS

FLAME SPREAD

FLAMELETS

MICROGRAVITY ENVIRONMENTS

MICROGRAVITY EXPERIMENTS

OPPOSED FLOW FLAMES

QUIESCENT ENVIRONMENTS

Space environment

Fuels

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Proceedings of the Combustion Institute

Near limit flame spread over thin solid fuels in a low convective microgravity environment

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. © 2016 The Combustion Institute. Published by Elsevier Inc.

The dynamics of near limit self-propagating flame over thin solid fuels in microgravity

In this work a numerical investigation has been carried out to study the effect of g-jitter on zero-gravity (0ge) opposed flow spreading flame over thin solid fuels. For comparison simulations have also been carried out for normal gravity (1ge) downward spreading flames. G-jitter is emulated by gravity modulation of sinusoidal (Age sin(2πt/T ge)) gravity perturbation (g-perturbation) of a particular time-period (Tge) and amplitude (Age) over a selected base gravity level (0ge or 1ge). The response of flames at 0ge base gravity and at 1ge base gravity was different to the imposed g-perturbation. While at 0ge the mean and the amplitude of the oscillatory flame spread rate (FSR) magnified with increase in the time period of g-perturbation, interestingly for the 1ge flame a maximum mean FSR and oscillation amplitude occurs at certain perturbation time period. Further, at very small perturbation time-periods (Tge) the FSR at 1ge was lower than the steady state FSR. The amplitude of oscillatory FSR increased with increase in perturbation amplitude (Age). However, the 0ge flame which is little affected (compared to 1g e flame) at small perturbation amplitude (Age) is affected severely at large perturbation amplitude (Age). Both the gas phase and fuel pyrolysis (or fuel response) follow perturbation signal with a lag but fuel pyrolysis is more sluggish and lags behind gas phase. The phase lag between fuel pyrolysis and gas increases at smaller time-periods (Tge) and tends to enhance the effect of external perturbation whereas at larger time-periods (Tge) this lag inhibits the effect of external perturbation. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Gravity modulation study on opposed flame spread over thin solid fuels

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 oxygen starvation the flame spread rate decreased and eventually at some interspace distance the flame extinguished. With fuel emittance (equal to absorptance) reduced to '0' the flame spread rate was nearly independent of the interspace distance, except at very small distances where the flame spread rate dropped due to oxygen starvation. A flame extinction plot with the extinction oxygen level was constructed for the multiple-fuel configuration at various interspace distances. The default fuel with an emittance of 0.92 was found to be more flammable in the multiple-fuel configuration than in a single fuel sheet configuration. For a fuel emittance equal to zero, the extinction oxygen limit decreases for both the single and the multiple fuel sheet configurations. However, the two flammability curves cross over at a certain fuel separation distance. The multiple-fuel configurations become less flammable compared to the single fuel sheet configuration below a certain separation distance. © 2013 © 2013 Taylor & Francis.

Combustion Theory and Modelling

Opposed flow flame spread over an array of thin solid fuel sheets in a microgravity environment

In this work a flame-spread model is formulated in three dimensions to simulate opposed flow flame spread over thin solid fuels. The flame-spread model is coupled to a three-dimensional gas radiation model. The experiments [1] on downward spread and zero gravity quiescent spread over finite width thin fuel are simulated by flame-spread models in both two and three dimensions to assess the role of radiation and effect of dimensionality on the prediction of the flame-spread phenomena. It is observed that while radiation plays only a minor role in normal gravity downward spread, in zero gravity quiescent spread surface radiation loss holds the key to correct prediction of low oxygen flame spread rate and quenching limit. The present three-dimensional simulations show that even in zero gravity gas radiation affects flame spread rate only moderately (as much as 20% at 100% oxygen) as the heat feedback effect exceeds the radiation loss effect only moderately. However, the two-dimensional model with the gas radiation model badly over-predicts the zero gravity flame spread rate due to under estimation of gas radiation loss to the ambient surrounding. The two-dimensional model was also found to be inadequate for predicting the zero gravity flame attributes, like the flame length and the flame width, correctly. The need for a three-dimensional model was found to be indispensable for consistently describing the zero gravity flame-spread experiments [1] (including flame spread rate and flame size) especially at high oxygen levels (>30%). On the other hand it was observed that for the normal gravity downward flame spread for oxygen levels up to 60%, the two-dimensional model was sufficient to predict flame spread rate and flame size reasonably well. Gas radiation is seen to increase the three-dimensional effect especially at elevated oxygen levels (>30% for zero gravity and >60% for normal gravity flames). © 2012 Copyright Taylor and Francis Group, LLC.

On the role of radiation and dimensionality in predicting flow opposed flame spread over thin fuels

Journal	Data powered by TypesetProceedings of the Combustion Institute
Publisher	Data powered by TypesetElsevier Ltd
ISSN	15407489
Open Access	No