SEARCH · Search NASA

Results for “FLAME”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22

All sources NASA NTRS DOE OSTI

NASA NTRS1994

The coupling between flame surface dynamics and species mass conservation in premixed turbulent combustion

Current flamelot models based on a description of the flame surface dynamics require the closure of two inter-related equations: a transport equation for the mean reaction progress variable, (tilde)c, and a transport equation for the flame surface density, Sigma. The coupling between these two equations is investigated using direct numerical simulations (DNS) with emphasis on the correlation between the turbulent fluxes of (tilde)c, bar(pu''c''), and Sigma, (u'')(sub S)Sigma. Two different DNS databases are used in the present work: a database developed at CTR by A. Trouve and a database developed by C. J. Rutland using a different code. Both databases correspond to statistically one-dimensional premixed flames in isotropic turbulent flow. The run parameters, however, are significantly different, and the two databases correspond to different combustion regimes. It is found that in all simulated flames, the correlation between bar(pu''c'') and (u'')(sub S)Sigma is always strong. The sign, however, of the turbulent flux of (tilde)c or Sigma with respect to the mean gradients, delta(tilde)c/delta(x) or delta(Sigma)/delta(x), is case-dependent. The CTR database is found to exhibit gradient turbulent transport of (tilde)c and Sigma, whereas the Rutland DNS features counter-gradient diffusion. The two databases are analyzed and compared using various tools (a local analysis of the flow field near the flame, a classical analysis of the conservation equation for (tilde)(u''c''), and a thin flame theoretical analysis). A mechanism is then proposed to explain the discrepancies between the two databases and a preliminary simple criterion is derived to predict the occurrence of gradient/counter-gradient turbulent diffusion.

Trouve, A.↗

NASA NTRS1995

Japan's research on gaseous flames

Although research studies on gaseous flames in microgravity in Japan have not been one-sided, they have been limited, for the most part, to comparatively fundamental studies. At present it is only possible to achieve a microgravity field by the use of drop towers, as far as gaseous flames are concerned. Compared with experiments on droplets, including droplet arrays, which have been vigorously performed in Japan, studies on gaseous flames have just begun. Experiments on ignition of gaseous fuel, flammability limits, flame stability, effect of magnetic field on flames, and carbon formation from gaseous flames are currently being carried out in microgravity. Seven subjects related to these topics are introduced and discussed herein.

Niioka, Takashi↗

NASA NTRS1998

Forced Flow Flame Spreading Test: Preliminary Findings From the USMP-3 Shuttle Mission

The Forced Flow Flame spreading Test (FFFT) is a study of flame spreading over solid fuels in very low-speed air flows. The FFFT experiment is part of research entitled Solid Inflammability Boundary at Low Speeds, (SIBAL) intended for operations on the Space Station. In the FFFT experiment, a series of 15 experiments conducted aboard the space shuttle during the United States Microgravity Payload (USMP-3) mission provided information about the structure and spreading characteristics of flames in low-speed, concurrent flows. The test samples included flat sheets of cellulose and cast cylinders of cellulose, burned in air at velocities of approximately 1 to 8 cm/sec. The test results have been successfully compared to theoretical predictions of the SIBAL program, a fundamentally based numerical simulation of concurrent flow flame spread. Additionally, some guidance for the design characteristics of the SIBAL flight experiment have been obtained including some verification of the theoretical predictions of flame size versus the required size of the SIBAL flow duct, and the effect of the presence of thermocouples in the vicinity of near-limit flames in microgravity.

Sacksteder, Kurt R.↗

NASA NTRS1999

Unsteady Diffusion Flames: Ignition, Travel, and Burnout (SUBCORE Project: Simplified Unsteady Burning of Contained Reactants)

An experimental apparatus for the examination of a planar, virtually strain-rate-free diffusion flame in microgravity has been designed and fabricated. Such a diffusion flame is characterized by relatively large spatial scale and high symmetry (to facilitate probing), and by relatively long fluid-residence time (to facilitate investigation of rates associated with sooting phenomena). Within the squat rectangular apparatus, with impervious, noncatalytic isothermal walls of stainless steel, a thin metallic splitter plate subdivides the contents into half-volumes. One half-volume initially contains fuel vapor diluted with an inert gas, and the other, oxidizer diluted with another inert gas-so that the two domains have equal pressure, density, and temperature. As the separator is removed, by translation in its own plane, through a tightly fitting slit in one side wall, a line ignitor in the opposite side wall initiates a triple-flame propagation across the narrow layer of combustible mixture formed near midheight in the chamber. The planar diffusion flame so emplaced is quickly disrupted in earth gravity. In microgravity, the planar flame persists, and travels ultimately into the half-volume containing the stoichiometrically deficient reactant; the flame eventually becomes extinguished owing to reactant depletion and heat loss to the walls.

Fendell, Francis↗

NASA NTRS1999

Premixed Turbulent Flame Propagation in Microgravity

A combined numerical-experimental study has been carried out to investigate the structure and propagation characteristics of turbulent premixed flames with and without the influence of buoyancy. Experimentally, the premixed flame characteristics are studied in the wrinkled regime using a Couette flow facility and an isotropic flow facility in order to resolve the scale of flame wrinkling. Both facilities were chosen for their ability to achieve sustained turbulence at low Reynolds number. This implies that conventional diagnostics can be employed to resolve the smallest scales of wrinkling. The Couette facility was also built keeping in mind the constraints imposed by the drop tower requirements. Results showed that the flow in this Couette flow facility achieves full-developed turbulence at low Re and all turbulence statistics are in good agreement with past measurements on large-scale facilities. Premixed flame propagation studies were then carried out both using the isotropic box and the Couette facility. Flame imaging showed that fine scales of wrinkling occurs during flame propagation. Both cases in Ig showed significant buoyancy effect. To demonstrate that micro-g can remove this buoyancy effect, a small drop tower was built and drop experiments were conducted using the isotropic box. Results using the Couette facility confirmed the ability to carry out these unique reacting flow experiments at least in 1g. Drop experiments at NASA GRC were planned but were not completed due to termination of this project.

Menon, Suresh↗

NASA NTRS1999

Reflight of the Solid Surface Combustion Experiment: Flame Radiation Near Extinction

In flame spreading in quiescent and low-velocity opposing flows, effects of surface reradiation and flame radiation are important in establishing the spread rate and whether the flame, once ignited, survives to steady spread or extinguishes after a time long compared to the ignition event. A reflight of the Solid Surface Combustion Experiment (SSCE), supported by modelling, demonstrates that for thick, flat fuels, the ultimate fate of the flame is extinction rather than steady spread. A mismatch between the thermal scale in the gas, driven by radiation, and the species diffusion scale, driven by mass diffusion, develops such that the high temperature regions of the flame are ultimately located in a region to which oxygen cannot be supplied at a sufficient rate to sustain reaction, and extinction occurs. Results of the experiment conducted on Space Shuttle mission STS 85 on 9 August 1997 are reviewed. For the flat surface geometry, while the hydrodynamic phenomena associated with opposed-flow flame spread may be treated two dimensionally, the radiative effects are three dimensional, and so modelling the radiative processes, with the mismatch in dimensionality, is difficult. The cylindrical geometry at least one long compared to the radius, provides a configuration in which the radiative processes for spread in the axial direction are two dimensional, thus simplifying the modelling. The cylindrical geometry allows for the development of more sophisticated radiative models without the complication of dimensionality concerns, e.g., discrete transfer, which is discussed in detail by Bundy (1998). Additionally, the cylindrical geometry results in a "focussing" of the heat transfer to the surface and may allow for steady spread for radii that for thick fuels of the same half-thickness there is no steady spread.

Altenkirch, R. A.↗

NASA NTRS1999

Studies of Premixed Laminar and Turbulent Flames at Microgravity

Several topics relating to combustion limits in premixed flames at reduced gravity have been studied. These topics include: (1) flame balls; (2) numerical simulation of flame ball and planar flame structure and stability; (3) experimental simulation of buoyancy effects in premixed flames using aqueous autocatalytic reactions; and (4) premixed flame propagation in Hele-Shaw cells.

Abid, M.↗

NASA NTRS1999

Effects of Buoyancy in Hydrogen Jet Diffusion Flames

This project was carried out to understand the effects of heat release and buoyancy on the flame structure of diffusion flames. Experiments were conducted at atmospheric pressure in both normal gravity and microgravity conditions in the NASA LeRC 2.2 s drop tower. Experiments were also conducted in a variable pressure combustion facility in normal gravity to scale buoyancy and thus, to supplement the drop tower experiments. Pure H2 or H2 mixed with He was used as the jet fluid to avoid the complexities associated with soot formation. Fuel jet burning in quiescent air was visualized and quantified by the Rainbow Schlieren Deflectometry (RSD) to obtain scalar profiles (temperature, oxygen concentration) within the flame. Burner tube diameter (d) was varied from 0.3 to 1.19 mm producing jet exit Reynolds numbers ranging from 40 to 1900, and generating flames encompassing laminar and transitional (laminar to turbulent) flow structure. Some experiments were also complemented with the CFD analysis. In a previous paper, we have presented details of the RSD technique, comparison of computed and measured scalar distributions, and effects of buoyancy on laminar and transitional H2 gas-jet diffusion flames. Results obtained from the RSD technique, variable pressure combustion chamber, and theoretical models have been published. Subsequently, we have developed a new drop rig with improved optical and image acquisition. In this set up, the schlieren images are acquired in real time and stored digitally in RAM of an onboard computer. This paper deals with laminar diffusion flames of pure H2 in normal and microgravity.

Agrawal, A. K.↗

NASA NTRS1999

Interferometer Development for Study of Interactions between Flames on Parallel Solid Surfaces

The interactions between flames spreading over parallel solid sheets of paper are being studied in normal gravity and in microgravity. This geometry provides interesting opportunities to study the interaction of radiative and diffusive transport mechanisms on the spread process. These transport mechanisms are changed when the flame interacts with other flames. Most practical heterogeneous combustion processes involve interacting discrete burning fuel elements, consequently, the study of these interactions is of practical significance. Owing largely to this practical importance, flame interactions have been an area of active research, however microgravity research has been largely limited to droplets. Consideration of flame spread over parallel solid surfaces has been limited to 1-g studies. To study the conductive transport in these flames, an interferometer system has been developed for use in the drop tower. The system takes advantage of a single beam interferometer: Point Diffraction Interferometry (PDI) which uses a portion of the light through the test section to provide the reference beam. Like other interferometric and Schlieren systems, it is a line of sight measurement and is subject to the usual edge and concentration effects. The advantage over Schlieren and shearing interferometry systems is that the fringes are lines of constant index of refraction rather than of its gradient so the images are more readily interpreted. The disadvantage is that it is less able to accommodate a range of temperature gradients.

Goldmeer, J. S.↗

NASA NTRS1999

Low Stretch Diffusion Flames Over a Solid Fuel

A unique new way to study low gravity flames in normal gravity has been developed. To study flame structure and extinction characteristics in low stretch environments, a normal gravity low-stretch diffusion flame is generated using a cylindrical PMMA sample of varying large radii. Foutch and T'ien used the radiative loss as well as a densimetric Froude number to characterize the blowoff (small Da) and quenching extinction (large Da) boundaries in stagnation-point diffusion flames under various convective conditions. An important conclusion of this study was that the shape and location of the extinction boundary, as well as a number of important flame characteristics, were almost identical for the buoyant, forced, and mixed convective environments they modeled. This theory indicates it should be possible to understand a material's burning characteristics in the low stretch environment of spacecraft (induced by fans and crew movements) by understanding its burning characteristics in an equivalent Earth-based stretch environment (induced by normal gravity buoyancy). Similarly, the material's burning characteristics in Lunar or Martian stretch environments (induced by partial gravity buoyancy) can be assessed. Equivalent stretch rates can be determined as a function of gravity, imposed flow, and geometry. A generalized expression for stretch rate which captures mixed convection includes both buoyant and forced stretch is defined as a = a(sub f) ((1 + (a(sub b))exp 2/(a(sub b))exp 2))exp 1/2. For purely buoyant flow, the equivalent stretch rate is a(sub b) = [(rho(exp e)-rho(exp *)/rho(sub e)][g/R](exp 1/2). For purely forced flow, the equivalent stretch rate is characterized by either a(sub f)= 2U(sub infinity)/R for a cylinder, or a(sub f)=U(sub jet)/d(sub jet) for a jet impinging on a planar surface. In these experiments, the buoyant stretch is varied through R, the radius of curvature, but the buoyant stretch could also be varied through g, the gravity level. In this way the effect of partial gravity, such as those found on the Moon (1/6 g) or Mars (1/3 g) can be captured in the definition of flame stretch.

Olson, S. L.↗

NASA NTRS1999

Flame Spread and Extinction in Partial-Gravity Environments

Considerable progress has been made in understanding the mechanisms of spreading flames under certain conditions, nearly all under the influence of normal Earth gravity. Recently, several investigators have studied some aspects of flame spread in purely forced flows in microgravity. However, very few have considered (especially experimentally) purely-buoyant flow influences, using gravity as a variable. In addition to the scientific interest in understanding how variable gravity affects flame spread in purely-buoyant flow, prospective human exploration of the Moon and Mars provides an incentive to obtain practical knowledge for use in fire-safety related engineering and mission operations in those partial-gravity environments. The purpose of this research effort is to conduct a focused experimental effort to observe the behavior of flames spreading both upward (concurrent flow) and downward (opposed flow) over thin fuels in partial-gravity environments, and to extend an existing numerical model of flame spread to predict flammability and flame spread behavior in these two regimes. A significant aspect of the experimental effort is to use a special device to improve the simulated partial-gravity environment achievable aboard reduced-gravity aircraft facilities.

K R Sacksteder↗

NASA NTRS1999

Influence of Buoyant Convection on the Stability of Enclosed Laminar Flames

An investigation of the stability limits of Enclosed Laminar Flames (ELF) was conducted in the Middeck Glovebox (MGBX) facility on the STS-87 Space Shuttle mission (November to December 1997). The primary objective of the ELF glovebox investigation is to determine the effect of buoyancy on the stability of round, laminar, gas-jet diffusion flames in a co-flow air duct. Comparison tests were conducted in normal gravity to allow isolation and identification of the influence of buoyancy. The results were used to map the lift-off and blow-out stability limits as a function of the fuel and air velocities for the two buoyancy conditions. Approximately 50 tests were conducted during the Space Shuttle mission, using a 50/50 mixture (volume basis) of methane and nitrogen as the fuel. The experimental results verified the hypothesis that substantially greater velocities are required to destabilize the flame in microgravity. The increase in air velocity required to induce lift off in microgravity (compared to normal gravity) was nearly equal to the increase required to induce blow out. Furthermore, the air velocity increase was relatively independent of the fuel flow, except at low fuel flows. At high fuel flows, it was found that the microgravity flames tend to immediately blow out after lift off. This is in agreement with the free-jet theory which suggests that stable lifted flames are not possible for fuels with a Schmidt number of 0.5<Sc<l, such as the diluted methane used in this study. However, stable lifted flames were observed in microgravity at low fuel flows, and in normal gravity, despite the fuel's low Schmidt number. The discrepancy between the experimental data and the theory of Chung and Lee is presumably due to the differences in the flow condition; namely, the ducted flow versus a free jet, and a non-similar velocity profile versus a self-similar profile.

Brooker, John E.↗

NASA NTRS1999

Fuel Preheat Effects on Soot-Field Structure in Laminar Gas Jet Diffusion Flames Burning in 0-g and 1-g

An experimental investigation conducted at the 2.2-s drop tower of the NASA Lewis Research Center is presented to quantify the influence of moderate fuel preheat on soot-field structure within 0-g laminar gas jet diffusion flames. Parallel work in 1-g is also presented to delineate the effect of elevated fuel temperatures on soot-field structure in buoyant flames. The experimental methodology implements jet diffusion flames of nitrogen-diluted acetylene fuel burning in quiescent air at atmospheric pressure. Fuel preheat of approximately 100 K in the 0-g laminar jet diffusion flames is found to reduce soot loadings in the annular region, but causes an increase in soot volume fractions at the centerline. In addition, fuel preheat reduces the radial extent of the soot field in 0-g. In 1-g, the same fuel preheat levels have a more moderated influence on soot loadings in the annular region, but are also seen to enhance soot concentrations near the axis low in the flame. The increased soot loadings near the flame centerline, as caused by fuel preheat, are consistent with the hypothesis that preheat levels of approximately 100 K enhance fuel pyrolysis rates. The results show that the growth stage of particles transported along the soot annulus is shortened both in 1-g and 0-g when elevated fuel temperatures are used.

Konsur, Bogdan↗

NASA NTRS1999

Studies of Premixed Laminar and Turbulent Flames at Microgravity

Abid, M.↗

NASA NTRS2000

Transitional Gas Jet Diffusion Flames in Microgravity

Drop tower experiments were performed to identify buoyancy effects in transitional hydrogen gas jet diffusion flames. Quantitative rainbow schlieren deflectometry was utilized to optically visualize the flame and to measure oxygen concentration in the laminar portion of the flame. Test conditions consisted of atmospheric pressure flames burning in quiescent air. Fuel from a 0.3mm inside diameter tube injector was issued at jet exit Reynolds numbers (Re) of 1300 to 1700. Helium mole percentage in the fuel was varied from 0 to 40%. Significant effects of buoyancy were observed in near field of the flame even-though the fuel jets were momentum-dominated. Results show an increase of breakpoint length in microgravity. Data suggest that transitional flames in earth-gravity at Re<1300 might become laminar in microgravity.

Agrawal, Ajay K.↗

NASA NTRS2001

Transport and Chemical Effects on Concurrent and Opposed-flow Flame Spread at Microgravity

Flame spread over flat solid fuel beds is a useful means of understanding more complex two-phase non-premixed spreading flames, such as those that may occur due to accidents in inhabited buildings and orbiting spacecraft. The role of buoyant convection on flame spread is substantial, especially for thermally-thick fuels. The conventional view, as supported by computations and space experiments, is that for quiescent mu-g conditions, the spread rate must be unsteady and decreasing until extinction occurs due to radiative losses. However, this view does not consider that radiative transfer to the fuel surface can enhance flame spread. In this work we suggest that radiative transfer from the flame itself, not just from an external source, can lead to steady flame spread at mu-g over thick fuel beds.

Son, Y.↗

NASA NTRS2001

Characterization of a Laminate Flat Plate Diffusion Flame in Microgravity using PIV, Visible and CH Emissions

Motivated by fire safety concerns and the advent of long-term micro-gravity facilities, a cooperative program has been developed to study the mechanisms and material properties that control flow assisted (co-current) flame spread. This program has used as a common fire scenario a reacting steady-state boundary layer. Preliminary studies explored the aerodynamics of a reacting boundary layer by simulating a condensed fuel by means of a gas burner. Stability curves for ethane air flames were obtained and different burning regimes were identified. An important feature of this study was the independent identification of the different mechanisms leading to the instability of the flow. It was observed that fuel injection velocity and thermal expansion independently contributed to the separation of the flow at the leading edge of the burner. The occurrence of separation resulted in complex three-dimensional flow patterns that have a dominant effect on critical fire safety parameters such as the stand-off distance and flame length. This work was extended to a solid fuel (PMMA) leading to a Sounding Rocket experiment (Mini-Texus-6). The solid phase showed similar flow patterns, mostly present at low flow velocities (<100 mm/s) but the results clearly demonstrated that the thermal balance at the pyrolyzing fuel surface is the dominant mechanism that controls both stand-off distance and flame length. This thermal balance could be described in a global manner by means of a total mass transfer or "B" number. This "B" number incorporates surface re-radiation, radiative feedback and in-depth heat conduction as first prescribed by Emmons. The mass transfer number becomes the single parameter that determines the evolution of these fire safety variables (flame length, stand-off distance) and therefore can be used as a ranking criterion to assess the flammability of materials. The particular configuration is representative of the NASA upward flame spread test (Test 1) therefore this approach can be used in the interpretation of the results obtained from this test. Nevertheless, complete validation of this approach has not been fully achieved due, mainly because all the measurements necessary to compare with the theoretical predictions have not been obtained. Following these studies two different directions have been taken. The first attempts to elucidate the details of the gas phase combustion reaction and the associated flow field by means of quantitative and qualitative measurements. The second approach, a more practical one, is to apply this methodology to the assessment of material flammability. The former is currently being conducted with a gas burner because it allows for easier control and longer experimentation time. The results obtained so far will be presented in more detail. The latter is a new program therefore only a brief summary of the objectives will be presented.

Joulain, P.↗

NASA NTRS2001

Quantitative Species Measurements in Microgravity Combustion Flames

Flame-vortex interactions are canonical configurations that can be used to study the underlying processes occurring in complicated turbulent reacting flows. The elegant simplicity of the flame-vortex interaction permits the study of these complex interactions under relatively controllable experimental configurations, in contrast to direct measurements in turbulent flames. The ability to measure and model the fundamental phenomena that occur in a turbulent flame, but with time and spatial scales which are amenable to our diagnostics, permits significant improvements in the understanding of turbulent combustion under both normal and reduced gravity conditions. In this paper, we report absolute mole fraction measurements of methane in a reacting vortex ring. These microgravity experiments are performed in the 2.2-sec drop tower at NASA Glenn Research Center. In collaboration with Drs. Chen and Dahm at the University of Michigan, measured methane absorbances are incorporated into a new model from which the temperature and concentrations of all major gases in the flame can be determined at all positions and times in the development of the vortex ring. This is the first demonstration of the ITAC (Iterative Temperature with Assumed Chemistry) approach, and the results of these computations and analyses are presented in a companion paper by Dahm and Chen at this Workshop. We believe that the ITAC approach will become a powerful tool in understanding a wide variety of combustion flames under both equilibrium and non-equilibrium conditions.

Silver, Joel A.↗