Soot-field structure in laminar soot-emitting microgravity nonpremixed flames

doi:10.1016/S0082-0784(96)80347-X

Symposium (International) on Combustion

Volume 26, Issue 1, 1996, Pages 1291-1299

https://doi.org/10.1016/S0082-0784(96)80347-X Get rights and content

Due to the intrinsic interest in laminar flame regimes that cannot be attained in normal gravity (1 g), the microgravity environment (0 g) offers an attractive and promising perspective to enhance our understanding of soot formation mechanisms in diffusion flames. An experimental investigation conducted at the 2.2-s drop tower of the NASA Lewis Research Center is presented to define the soot-field structure within 0-g laminar jet nonpremixed flames which operate above their smoke point. Parallel work on earthgravity flames is also presented to facilitate comparisons and define the effect of gravity on the soot fields. The experiment considers jet diffusion flames of nitrogen-diluted acetylene burning in quiescent air at atmospheric pressure. A full-field laser extinction technique is utilized to determine the transient soot spatial distributions in axisymmetric flames with fixed flow conditions corresponding to Rc=O(100). Results are presented for a nonsmoking normal-gravity flame and its nonbuoyant counterpart which releases soot from its blunt tip.

Quantitative measurements on the transient postignition character of the soot field in 0 g are presented for the first time and indicate that millimeter-diameter burners result in brief soot-field transients in microgravity. At a fuel flow rate which is nearly twice that at the smoke point in 0 g, the soot field after the initial transient period sustains its annular structure throughout the luminous nonbuoyant-flamezone. The maximum soot volume fraction measured at 0 g is nearly double that at 1 g for the same flow rate, thus attesting to the higher sooting tendency of nonbuoyant flames. The results indicate that the prolonged residence times under microgravity promote soot growth more than oxidation, thus producing more soot in 0 g. Side-by-side comparisons of the soot distributions in 0-g and 1-g flames demonstrate the increased spatial resolution afforded in microgravity flames.

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A historical overview of experimental solid combustion research in microgravity
2022, Acta Astronautica
Studying solid combustion phenomena in microgravity environments can be complex, and this is furthered by many limitations and constraints in the available microgravity research platforms. Consequently, fire safety in spacecraft is also a complex subject. The main limitations found in the field are related to the microgravity quality, the duration of microgravity conditions, the rig capabilities in volume and size, time scales, length scales and the diagnostic systems, and these are therefore the focus in the current investigation. The laboratory capacity of ground-based platforms has remained somewhat stalled since 1990s, some drop towers have recently been upgraded to extend their performance. New space-based platforms have been or are being established and could extend the windows-of-opportunity to perform research. In addition, a discussion is provided on the implications of the fact that the phenomena studied in the experimental investigations and the type of material employed covers both programmatic and scientific needs. It is found that a handful of materials are most widely studied to quantify and characterise some of the phenomena, while some materials have been employed even in single experimental efforts. The current literature review provides a very comprehensive overview of previous experimental studies and the experimental methodologies utilised. Thus, this study can become an aid to planning for future studies.
Accessing the soot-related radiative heat feedback in a flame spreading in microgravity: Optical designs and associated limitations
2021, Proceedings of the Combustion Institute
Novel, high-fidelity results related to soot from microgravity flames were obtained by an international topical team on fire safety in space. More specifically, embedded optical techniques for evaluation of the soot-related radiative feedback to the base material from a spreading non-premixed flame in microgravity were developed. The configuration used a non-buoyant axisymmetric flame propagating in an opposed laminar stream over a Low Density PolyEthylene coating of an electrical wire. Within this context, both the standard Broadband Two Color Pyrometry (B2CP) and its recent extension Broadband Modulated Absorption/Emission (BMAE) technique can be deployed to measure the spatial distribution of soot temperature and volume fraction within the flame. Both fields are then processed to establish the field of local radiative balance attributed to soot within the flame, and ultimately the soot contribution to the radiative flux to the wire. The present study first assesses the consistency of the methodology contrasting an experimental frame and a synthetic one, the latter being produced by a signal modeling that processes fields delivered by a numerical simulation of the configuration as inputs. Using the synthetic signals obtained, the fields of local radiative balance within the flame are then computed and significant discrepancies were disclosed locally between the fields originating from the synthetic BMAE and B2CP inputs. Nevertheless, the subsequent evaluation of the soot-related radiative heat feedback to the wire shows that a weak deviation among the techniques implemented is expected. This finding is corroborated by similar evaluations conducted with experimental BMAE and B2CP measurements obtained in parabolic flights. As BMAE is implemented in an ISS configuration within the SCEM rig, BMAE and B2CP will soon provide long-duration soot observations in microgravity. In order to contrast the upcoming results, this current study quantifies discrepancies originating from the post-processing regarding soot temperature and volume fraction, and shows that the radiative feedback evaluation from both methods should be consistent.
Fire safety in space – Investigating flame spread interaction over wires
2016, Acta Astronautica
Citation Excerpt :
In a similar manner, the interaction among spreading flames has been the subject of several studies [13,14] but still merits further scientific investigation, in particular, for microgravity and low characteristic forced velocities conditions. The absence of natural convection allows a significant increase of the time scales associated with transport and combustion processes, increasing both soot concentration [15] and radiative emissions, especially from the soot continuum [16]. Transport thus changes the nature of the combustion processes and the interaction between the flame and the solid fuels in a manner that is still not fully understood.
A new rig for microgravity experiments was used for the study flame spread of parallel polyethylene-coated wires in concurrent and opposed airflow. The parabolic flight experiments were conducted at small length- and time scales, i.e. typically over 10 cm long samples for up to 20 s. For the first time, the influence of neighboring spread on the mass burning rate was assessed in microgravity. The observations are contrasted with the influence characterized in normal gravity. The experimental results are expected to deliver meaningful guidelines for future, planned experiments at a larger scale.
Arising from the current results, the issue of the potential interaction among spreading flames also needs to be carefully investigated as this interaction plays a major role in realistic fire scenarios, and therefore on the design of the strategies that would allow the control of such a fire. Once buoyancy has been removed, the characteristic length and time scales of the different modes of heat and mass transfer are modified. For this reason, interaction among spreading flames may be revealed in microgravity, while it would not at normal gravity, or vice versa. Furthermore, the interaction may lead to an enhanced spread rate when mutual preheating dominates or, conversely, a reduced spread rate when oxidizer flow vitiation is predominant.
In more general terms, the current study supports both the SAFFIRE and the FLARE projects, which are large projects with international scientific teams. First, material samples will be tested in a series of flight experiments (SAFFIRE 1-3) conducted in Cygnus vehicles after they have undocked from the ISS. These experiments will allow the study of ignition and possible flame spread in real spacecraft conditions, i.e. over real length scale samples within real time scales. Second, concomitant research conducted within the FLARE project is dedicated to the assessment of new standard tests for materials that a spacecraft can be composed of. Finally, these tests aim to define the ambient conditions that will mitigate and potentially prohibit the flame spread in microgravity over the material studied.
Phenomenological model of soot production inside a non-buoyant laminar diffusion flame
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Citation Excerpt :
Of critical importance is the effect of oxygen concentration on the formation and oxidation, which connects soot history and local soot concentrations to the structure of the flow field in the vicinity of the flame. The global residence time, defined as the ratio of the characteristic flame length to the governing mass transport velocity, seems to be a key parameter controlling soot concentrations [2,14]. Konsur et al. [14] determined experimentally that the peak soot volume fraction decreased when the characteristic global residence time was reduced and Mortazavi et al. [17] extended similar observations to a wider range of conditions.
An original phenomenological model for soot production inside a laminar, flat plate boundary layer diffusion flame is presented. The model is compared with experimental measurements conducted in microgravity. For the experiments, the fuel, ethylene, is injected through a flat porous burner into an oxidizer stream flowing parallel to the burner surface. The oxidizer is a mixture of 35% oxygen and 65% nitrogen. The fuel and oxidizer velocities are systematically varied. The analysis of the data shows that the streamwise location of the maximum flame height can be considered an unambiguous characteristic length of the flame as opposed to the maximum visible flame length. Analysis of the streamwise location of the maximum flame height enables to establish the transition between “open-tip” and “closed-tip” behavior as well as scaling laws for the soot volume fraction. A scaled soot volume fraction is found to follow a linear relationship with the streamwise coordinate normalized by the burner length. This correlation appears to be valid for the whole range of conditions investigated, knowing that this range does not cover the blow-off regime.
Influence of thermal radiation on soot production in Laminar axisymmetric diffusion flames
2013, Journal of Quantitative Spectroscopy and Radiative Transfer
The aim of this paper is to study the effect of radiative heat transfer on soot production in laminar axisymmetric diffusion flames. Twenty-four C₁–C₃ hydrocarbon–air flames, consisting of normal (NDF) and inverse (IDF) diffusion flames at both normal gravity (1 g) and microgravity (0 g), and covering a wide range of conditions affecting radiative heat transfer, were simulated. The numerical model is based on the Steady Laminar Flamelet (SLF) model, a semi-empirical two-equation acetylene/benzene based soot model and the Statistical Narrow Band Correlated K (SNBCK) model coupled to the Finite Volume Method (FVM) to compute thermal radiation. Predictions relative to velocity, temperature, soot volume fraction and radiative losses are on the whole in good agreement with the available experimental data. Model results show that, for all the flames considered, thermal radiation is a crucial process with a view to providing accurate predictions for temperatures and soot concentrations. It becomes increasingly significant from IDFs to NDFs and its influence is much greater as gravity is reduced. The radiative contribution of gas prevails in the weakly-sooting IDFs and in the methane and ethane NDFs, whereas soot radiation dominates in the other flames. However, both contributions are significant in all cases, with the exception of the 1 g IDFs investigated where soot radiation can be ignored. The optically-thin approximation (OTA) was also tested and found to be applicable as long as the optical thickness, based on flame radius and Planck mean absorption coefficient, is less than 0.05. The OTA is reasonable for the IDFs and for most of the 1 g NDFs, but it fails to predict the radiative heat transfer for the 0 g NDFs. The accuracy of radiative-property models was then assessed in the latter cases. Simulations show that the gray approximation can be applied to soot but not to combustion gases. Both the non-gray and gray soot versions of the Full Spectrum Correlated k (FSCK) model can be then substituted for the SNBCK with a reduction in CPU time by a factor of about 20 in the latter case.
A numerical study on the effects of pressure and gravity in laminar ethylene diffusion flames
2011, Combustion and Flame
Citation Excerpt :
For example, the peak soot volume fraction in zero gravity is roughly 1.4, 1.6, 2.0, 2.7, and 2.2 times larger than the equivalent normal-gravity flame at 0.5, 0.7, 1, 2, and 5 atm, respectively. A similar factor-of-two enhancement of the peak soot volume fraction in micro-gravity was measured during drop-tower experiments [22,27,66] and predicted by Kong and Liu [36,37]. Kaplan et al. [18] predicted a much larger 11-fold increase in soot volume fraction for laminar ethylene–air jet diffusion flames in quiescent air.
The effects of pressure and gravity on sooting characteristics and flame structure were studied numerically in coflow ethylene–air laminar diffusion flames between 0.5 and 5 atm. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible, gaseous mixtures which include complex chemistry, detailed radiation heat transfer, and soot formation/oxidation. Soot formation/oxidation was modeled using an acetylene-based, semi-empirical model which has been verified with previously published experimental data to correctly capture many of the observed trends at normal-gravity. Calculations for each pressure considered were performed for both normal- and zero-gravity conditions to help separate the effects of pressure and buoyancy on soot formation. Based on the numerical predictions, pressure and gravity were observed to significantly influence the flames through their effects on buoyancy and reaction rates. The zero-gravity flames have higher soot concentrations, lower temperatures and broader soot-containing zones than normal-gravity flames at the same pressure. The zero-gravity flames were also found to be longer and wider. Differences were observed between the two levels of gravity when pressure was increased. The zero-gravity flames displayed a stronger dependence of the maximum soot yield on pressure from 0.5 to 2 atm and a weaker dependence from 2 to 5 atm as compared to the normal-gravity flames. In addition, flame diameter decreased with increasing pressure under normal-gravity while it increased with pressure in the zero-gravity cases. Changing the prescribed wall boundary condition from fixed-temperature to adiabatic significantly altered the numerical predictions at 5 atm. When the walls were assumed to be adiabatic, peak soot volume fractions and temperatures increased in both the zero- and normal-gravity flames, emphasizing the importance of heat conduction to the burner rim on flame structure.

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Soot-field structure in laminar soot-emitting microgravity nonpremixed flames

Prog. Energy Combust. Sci.

Acta Astronaut.

Combust. Flame

Combust. Flame

Combust. Flame

Prog. Energy Combust. Sci.

Int. J. Heat Mass Transfer

Prog. Energy Combust. Sci.

Combust. Flame

Combust. Flame

Combustion in Microgravity: Opportunities, Challenges, and Progress

AIAA Paper 90-0120