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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

Flammability Aspects of a Cotton-Fiberglass Fabric in Opposed and Concurrent Airflow in Microgravity

Microgravity combustion tests burning fabric samples were performed aboard the International Space Station. The cotton-fiberglass blend samples were mounted inside a small wind tunnel which could impose air flow speeds up to 40 cm/s. The wind tunnel was installed in the Microgravity Science Glovebox which supplied power, imaging, and a level of containment. The effects of air flow speed on flame appearance, flame growth, and spread rates were determined in both the opposed and concurrent flow configuration. For the opposed flow configuration, the flame quickly reached steady spread for each flow speed, and the spread rate was fastest at an intermediate value of flow speed. These tests show the enhanced flammability in microgravity for this geometry, since, in normal gravity air, a flame self-extinguishes in the opposed flow geometry (downward flame spread). In the concurrent flow configuration, flame size grew with time during the tests. A limiting length and steady spread rate were obtained only in low flow speeds ( 10 cm/s) for the short-length samples that fit in the small wind tunnel. For these conditions, flame spread rate increased linearly with increasing flow. This is the first time that detailed transient flame growth data was obtained in purely forced flows in microgravity. In addition, by decreasing flow speed to a very low value (around 1 cm/s), quenching extinction was observed. The valuable results from these long-duration experiments validate a number of theoretical predictions and also provide the data for a transient flame growth model under development.

Ferkul, Paul V.↗
NASA NTRS1984

Gravitational effects on flames spreading over thick solid surfaces

A theoretical model for the downward spread of a flame in the gas over a semi-infinitely thick fuel bed is presented. Gas-phase chemistry is assumed to proceed at an infinite rate while the fuel bed is taken to pyrolyze at the surface in an Arrhenius fashion. Spread rates and flame shapes are computed for two different ambient oxygen concentrations for gravitational accelerations both below and above that of the earth. Predicted spread rates decrease and actual flame size increases with decreasing gravity. Results agree favorably with available, experimental results and expected results from reduced gravity experimentation.

Vedha-Nayagam, M.↗
NASA NTRS2007

Experimental Measurements of Two-dimensional Planar Propagating Edge Flames

The study of edge flames has received increased attention in recent years. This work reports the results of a recent study into two-dimensional, planar, propagating edge flames that are remote from solid surfaces (called here, free-layer flames, as opposed to layered flames along floors or ceilings). They represent an ideal case of a flame propagating down a flammable plume, or through a flammable layer in microgravity. The results were generated using a new apparatus in which a thin stream of gaseous fuel is injected into a low-speed laminar wind tunnel thereby forming a flammable layer along the centerline. An airfoil-shaped fuel dispenser downstream of the duct inlet issues ethane from a slot in the trailing edge. The air and ethane mix due to mass diffusion while flowing up towards the duct exit, forming a flammable layer with a steep lateral fuel concentration gradient and smaller axial fuel concentration gradient. We characterized the flow and fuel concentration fields in the duct using hot wire anemometer scans, flow visualization using smoke traces, and non-reacting, numerical modeling using COSMOSFloWorks. In the experiment, a hot wire near the exit ignites the ethane air layer, with the flame propagating downwards towards the fuel source. Reported here are tests with the air inlet velocity of 25 cm/s and ethane flows of 967-1299 sccm, which gave conditions ranging from lean to rich along the centerline. In these conditions the flame spreads at a constant rate faster than the laminar burning rate for a premixed ethane air mixture. The flame spread rate increases with increasing transverse fuel gradient (obtained by increasing the fuel flow rate), but appears to reach a maximum. The flow field shows little effect due to the flame approach near the igniter, but shows significant effect, including flow reversal, well ahead of the flame as it approaches the airfoil fuel source.

Villa-Gonzalez, Marcos↗
NASA NTRS1997

Shuttle Fire Tests Are Radiant

Flame spreading is a phenomenon familiar to everyone who has witnessed an accidental fire. Yet, because of the complexity of the physical and chemical processes that are involved, the theoretical understanding of fires and flame spreading is a relatively new science. Flames spread along solid materials in a process where heat from the flames vaporizes the fuel just ahead of the moving flame. The vaporized fuel mixes with oxygen from the air and reacts chemically with it, producing the flame. On Earth, the spread rate of the flame is directly affected by the rate at which the fuel and oxygen are mixed with the help of buoyant convection.

Olson, Sandra L.↗
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 NTRS1985

Backward boundary layers in downward flame spread

The gas-phase combustion of a vertical fuel slab pyrolyzing at a finite rate under downward flame spread conditions is investigated theoretically. The backward boundary layer character of the flow is exploited, and a transformation that shifts the singularity associated with boundary-layer equations to a location, upstream of flame lift-off, where the surface temperature is nearly ambient, is employed. The boundary-layer flow is matched asymptotically for large Grashof number to a potential flow that allows the flow induced by the flame and along the upstream surface of the fuel bed to be calculated. Gas-phase temperature and velocity predictions are compared to experiment, and although analysis of the gas phase is emphasized, an approximate spread rate, based on a solid-phase energy balance, is calculated.

Vedha-Nayagam, M.↗
NASA NTRS1934

A Preliminary Motion-picture Study of Combustion in a Compression-ignition Engine

Motion pictures were taken at 1,850 frames per second of the spray penetration and combustion occurring in the N.A.C.A. combustion apparatus arranged to operate as a compression-ignition engine. Indicator cards were taken simultaneously with the motion pictures by means of the N.A.C.A. optical indicator. The motion pictures showed that when ignition occurred during injection it started in the spray envelope. If ignition occurred after injection cut-off, however, and after considerable mixing had taken place, it was impossible to predict where the ignition would start. The pictures also showed that ignition usually started at several points in the combustion chamber. With this apparatus, as the injection advance angle increased from 0 degrees to 40 degrees before top center, the rate of flame spread increased and the duration of the burning decreased.

Buckley, E C↗
NASA NTRSDate not supplied

Flammability of Materials on the Moon

Some materials have recently been found to exhibit increased flammability in Lunar gravity. The reduced buoyancy driven flow on the Moon may present a goldilocks region of decreased convective cooling, while still supplying adequate oxidizer to the flame zone. Combined with the desired elevated oxygen habitat of exploration architectures and the difficulty of rescue, the combination could present increased risk to crew. Flammability of Materials on the Moon will be delivered to the Lunar surface on an HLS lander, and will burn four samples; SIBAL fabric will be exposed to normal air while the PMMA rods will be exposed to the normoxic Space Exploration Atmospheres (SEA) with elevated oxygen in a small atmospheric chamber. Each fuel will have two tests each, one burning upward and one burning downward. The results of flame size, spread rate, atmosphere changes, and extinction oxygen concentration will then be analyzed and used to help quantify the risk and mitigation for future exploration missions.

flammability↗
NASA NTRS1991

Heat Transfer to a Thin Solid Combustible in Flame Spreading at Microgravity

The heat transfer rate to a thin solid combustible from an attached diffusion flame, spreading across the surface of the combustible in a quiescent, microgravity environment, was determined from measurements made in the drop tower facility at NASA-Lewis Research Center. With first-order Arrhenius pyrolysis kinetics, the solid-phase mass and energy equations along with the measured spread rate and surface temperature profiles were used to calculate the net heat flux to the surface. Results of the measurements are compared to the numerical solution of the complete set of coupled differential equations that describes the temperature, species, and velocity fields in the gas and solid phases. The theory and experiment agree on the major qualitative features of the heat transfer. Some fundamental differences are attributed to the neglect of radiation in the theoretical model.

Bhattacharjee, S.↗
NASA NTRSDate not supplied

Effects of Confinement on Flame Spread in Microgravity

Solid fuel combustion experiments aboard the ISS examine the effects of confinement on a concurrent, purely-forced-flow flame spread in microgravity environment. The results for a thin, cotton-fiberglass-blended textile fabric fuel are presented. Flat baffles of differing materials are used to alter the radiative boundary conditions with transparent polycarbonate, black anodized aluminum (reflectance ~ 0), and highly polished aluminum (reflectance ~ 1). The baffles are parallel to the fuel sheet and placed symmetrically on each side. The inter-baffle distance is varied to change the boundary conditions for the flow. In all tests, samples are ignited at the upstream leading edge and allowed to burn to completion. Results show that the flame reaches a steady length and spread rate at low flow speeds (< 15 cm/s) for all tested inter-baffle distances. As the distance decreases, the flame length and spread rate first increase then decrease showing an optimal inter-baffle distance. For all baffle types, the flame either fails to ignite or extinguishes before reaching the end of the sample when the inter-baffle distance is too small (~ 1 cm). This is attributed to the reduction of oxygen supply to the flame zone and heat loss to the baffles. The results also show at the same inter-baffle distance, flame length and spread rate are highest for polished aluminum baffles, and lowest for transparent polycarbonate baffles. The differences are most prominent at intermediate tested baffle distances. While the radiative heat feedback from the baffles is expected to increase when the baffle distance decreases, the combustion is limited by the reduced oxygen supply. Near this limit, flame lengths and spread rates are similar for all baffle types.

Microgravity↗
NASA NTRS1999

Flame Spread Across Liquids: Experimental Results

The goal of our research on flame spread across a pool of liquid fuel is the quantitative identification of the mechanisms that control the rate and nature of flame spread when the initial temperature of the liquid pool is below the fuel's flash point temperature. Besides numerous experiments in drop towers and 1 g laboratories, we have flown five microgravity (mu-g) experiments on sounding rockets. As described in earlier papers, the first three flights examined the effect of forced opposed airflow over a 2.5 cm deep x 2 cm wide x 30 cm long pool of 1-butanol in mu-g. It was found that the flame spread is much slower and steadier than in 1 g where flame spread has a pulsating character. It was speculated that the flame spread in mu-g resembled the character of pseudo-uniform spread in 1 g; Ito et al later confirmed this conclusively in 1 g experiments. Much of the mu-g flame is also farther from the surface, dimmer, and with less soot, when compared to the 1 g flame. Three-dimensional liquid-phase flow patterns that control the liquid preheating were discovered in both 1 g and mu-g. Our numerical model, restricted to two dimensions, had predicted faster, pulsating flame spread in mu-g for opposed airflow. In examining the differences in the dimensionality of the model and experiment, it was noted that the experiment allowed gas expansion in the lateral direction (across the width of the pool), for which the model could not account. Such lateral expansion could reduce the expansion in the forward and upward directions. Because only these latter directions could be modeled, it was decided to artificially reduce the gas thermal expansion in the predictions. When this was done, satisfactory agreement could be obtained between the predicted and observed spread rates and the steadiness of the spread in microgravity. In 1 g, however, the predicted flame spread character also changed to pseudo-uniform, which disagreed with our 1 g experiments where the spread is pulsating. It was then speculated that gas-phase buoyant flow might oppose the lateral gas expansion, so that the 1 g experiments retained their pulsating flame spread character. If this speculation was valid, a difference in lateral gas expansion should be observable when comparing 1 g and mu-g experiments. Specifically, it was anticipated that greater flow divergence caused by lateral expansion would be measured in mu-g in the absence of a buoyant flow directed towards the flame.

Ross, H. D.↗
NASA NTRS1991

Radiation-Controlled, Opposed-Flow Flame Spread in a Microgravity Environment

The effects of surface and gas-phase radiation on the rate and the structure of laminar flame spread over thin fuels are investigated using a flame-spread model which consists of the continuity, momentum, species, and energy equations in the gas and the continuity and energy equations in the solid. Numerical calculations, complemented by scaling arguments, show that, at high velocities of the oxidizer flow, radiation effects are unimportant; the spread rate decreases with increasing opposing velocity due to finite-rate gas-phase kinetics. However, radiation becomes progressively important when the opposing velocity is below a certain value: the flame cools, shrinks in size, and its spread rate falls sharply with decreasing opposing velocity.

Bhattacharjee, Subrata↗
NASA NTRS1995

Brazil Fire Characterization and Burn Area Estimation Using the Airborne Infrared Disaster Assessment (AIRDAS) System

Remotely sensed estimations of regional and global emissions from biomass combustion have been used to characterize fire behavior, determine fire intensity, and estimate burn area. Highly temporal, low resolution satellite data have been used to calculate estimates of fire numbers and area burned. These estimates of fire activity and burned area have differed dramatically, resulting in a wide range of predictions on the ecological and environmental impacts of fires. As part of the Brazil/United States Fire Initiative, an aircraft campaign was initiated in 1992 and continued in 1994. This multi-aircraft campaign was designed to assist in the characterization of fire activity, document fire intensity and determine area burned over prescribed, agricultural and wildland fires in the savanna and forests of central Brazil. Using a unique, multispectral scanner (AIRDAS), designed specifically for fire characterization, a variety of fires and burned areas were flown with a high spatial and high thermal resolution scanner. The system was used to measure flame front size, rate of spread, ratio of smoldering to flaming fronts and fire intensity. In addition, long transects were flown to determine the size of burned areas within the cerrado and transitional ecosystems. The authors anticipate that the fire activity and burned area estimates reported here will lead to enhanced information for precise regional trace gas prediction.

Brass, J. A.↗
NASA NTRS2014

Combustion of Solids in Microgravity: Results from the BASS-II Experiment

The Burning and Suppression of Solids-II (BASS-II) experiment was performed on the International Space Station. Microgravity combustion tests burned thin and thick flat samples, acrylic slabs, spheres, and cylinders. The samples were mounted inside a small wind tunnel which could impose air flow speeds up to 53 cms. The wind tunnel was installed in the Microgravity Science Glovebox which supplied power, imaging, and a level of containment. The effects of air flow speed, fuel thickness, fuel preheating, and oxygen concentration on flame appearance, growth, spread rate, and extinction were examined in both the opposed and concurrent flow configuration. The flames are quite sensitive to air flow speed in the range 0 to 5 cms. They can be sustained at very low flow speeds of less than 1 cms, when they become dim blue and stable. In this state they are not particularly dangerous from a fire safety perspective, but they can flare up quickly with a sudden increase in air flow speed. Including earlier BASS-I results, well over one hundred tests have been conducted of the various samples in the different geometries, flow speeds, and oxygen concentrations. There are several important implications related to fundamental combustion research as well as spacecraft fire safety. This work was supported by the NASA Space Life and Physical Sciences Research and Applications Division (SLPSRA).

Combustion↗
NASA NTRS2013

Thickness and Fuel Preheating Effects on Material Flammability in Microgravity from the BASS Experiment

The Burning and Suppression of Solids (BASS) experiment was performed on the International Space Station. Microgravity combustion tests burning thin and thick flat samples, acrylic spheres, and candles were conducted. The samples were mounted inside a small wind tunnel which could impose air flow speeds up to 40 cms. The wind tunnel was installed in the Microgravity Science Glovebox which supplied power, imaging, and a level of containment. The effects of air flow speed, fuel thickness, fuel preheating, and nitrogen dilution on flame appearance, flame growth, and spread rates were determined in both the opposed and concurrent flow configuration. In some cases, a jet of nitrogen was introduced to attempt to extinguish the flame. Microgravity flames were found to be especially sensitive to air flow speed in the range 0 to 5 cms. The gas phase response is much faster compared to the solid and so as the flow speed is changed, the flame responds with almost no delay. At the lowest speeds examined (less than 1 cms) all the flames tended to become dim blue and very stable. However, heat loss at these very low convective rates is small so the flames can burn for a long time. At moderate flow speeds (between about 1 and 5 cms) the flame continually heats the solid fuel resulting in an increasing fuel temperature, higher rate of fuel vaporization, and a stronger, more luminous flame as time progresses. Only the smallest flames burning acrylic slabs appeared to be adversely influenced by solid conductive heat loss, but even these burned for over 5 minutes before self-extinguishing. This has implications for spacecraft fire safety since a tiny flame might be undetected for a long time. While the small flame is not particularly hazardous if it remains small, the danger is that it might flare up if the air convection is suddenly increased or if the flame spreads into another fuel source.

Combustion↗
NASA NTRS1994

Flame spreading over a thin solid in low-speed concurrent flow- Drop tower experimental results and comparison with theory

Flame spread over thin paper samples in low-speed concurrent flow is experimentally investigated in a 5.18 s drop tower. In the experiment, the oxygen molar percentage is varied from 30% down to the flame extinction limits and the forced flow velocity from 5.29 cm/s down to the quenching limits. Motion pictures are taken to observe flame shape, color, size, and spread rates. These quantities are compared with a theoretical model describing concurrent flame spread over thin solids in low-speed flows. The paper also discusses the similarity and difference between concurrent-flow and opposed-flow flame spread in microgravity and between low-speed and high-speed concurrent-flow flame spread. Finally the limitations of using a drop tower for flame spread research is assessed.

Grayson, G. D.↗
NASA NTRS1997

Flame Spread Across Liquids

The principal goal of our recent research on flame spread across liquid pools is the detailed identification of the mechanisms that control the rate and nature of flame spread when the liquid pool is initially at an isothermal bulk temperature that is below the fuel's flash point temperature. In our project, we specialize the subject to highlight the roles of buoyancy-related processes regarding the mechanisms of flame spread, an area of research cited recently by Linan and Williams as one that needs further attention and which microgravity (micro-g) experiments could help to resolve. Toward resolving the effects of buoyancy on this flame spread problem, comparisons - between 1-g and micro-g experimental observations, and between model predictions and experimental data at each of these gravitational levels - are extensively utilized. The present experimental and computational foundation is presented to support identification of the mechanisms that control flame spread in the pulsating flame spread regime for which long-duration, micro-g flame spread experiments have been conducted aboard a sounding rocket.

Ross, Howard D.↗
NASA NTRS1993

Analysis of Lewis Number Effects in Flame Spread

De Ris's formulation of the problem of flame spread over thin and thick solid fuel surfaces is extended to include the influence of nonunity Lewis numbers of oxidant and fuel vapors. A zeroth order formula for the flame spread velocity is deduced. To this order of approximation, the Lewis number of the oxidant affects the spread rate only through its effect on the flame temperature and the fuel Lewis number plays no role at all. Thin fuel predictions are compared with experiments on flame spread over thin paper samples in a variety of O2-diluent atmospheres. Good agreement between theory and experiment supports the theory and indicates that unity Lewis number assumptions can lead to rather inaccurate spread rate and flame temperature predictions.

Greenberg, J. B.↗