Fuels In Microgravity Research Articles

Flammability maps for microgravity burning of a small flat material in a quiescent ambient

A gas fuel supplied burner (BRE) was used to emulate burning of real solid and liquid fuels in microgravity to address the fire safety concerns of spacecrafts. The analytical solution of these flames was utilized to predict the steady flame temperature, and a critical temperature range of 1100–1200 K, was chosen to distinguish between self-extinguished and steady flames. Steady flames are examined on a flammability diagram composed of the emulated heat of combustion and heat of gasification. It is shown that for a given heat of combustion, there is an upper and a lower limit value of heat of gasification that will allow steady burning. The theory and the boundary of extinction experimental data support the limits. The upper limit is classed as radiation extinction with the lower like the fire-point in Earth gravity and is called small flame extinction limit. A theoretical prediction for steady burning is demonstrated to within 10% for a given atmospheric condition of pressure and oxygen, material diameter, and external radiative heat flux. The material properties of Δhc, heat of combustion; L, heat of gasification; and Tb, burning temperature allow for the prediction of burning in microgravity, and can serve as a method to evaluate fire safety of spacecrafts.

Ignition and combustion of metal fuels under microgravity: a short review

Metal fuels have a high potential for applications in space propulsion owing to their superior energy density and distinct combustion characteristics. Therefore, research into the combustion properties of metal fuels in microgravity has become necessary. The principles, applications, benefits, and limitations of experimental methods for microgravity combustion of metal fuels were compared in this review. Drop towers, aircrafts, sounding rockets or microgravity balloons, acoustic or ultrasonic levitation, electrodynamic levitation, space stations, and numerical simulations are common methods used to provide microgravity for combustion research. Among them, the devices providing microgravity environment on the ground has become a good choice for simulating microgravity environment owing to its convenience, reliability and relatively low cost. Further, the physical and chemical properties of common metal fuels, such as aluminum, boron magnesium, and so on, were introduced, and the existing research achievements on the combustion of metal fuels in microgravity are succinctly summarized in this paper. The melting process, ignition temperature, combustion temperature, flame structure, and extinction of metal fuel combustion will be affected by microgravity. Moreover, the necessity and insufficiency of the research are also illustrated. Finally, the existing and potential applications of metal fuels in space propulsion and space weapons are introduced. Metal fuels’ superior energy density and combustion performance can benefit deep-space exploration, particularly Mars exploration, and bring new insights into the design of space weapons. This paper may throw some new light on the study of metal fuel combustion in microgravity.

Evaluation of burning rate in microgravity based on the fuel regression, flame area, and spread rate

The fuel burning rate and heat-release rate (HRR) play key roles in determining the fire intensity and hazard. On Earth, the burning rate of a condensed fuel is normally measured by the mass loss, but in microgravity, the impossibility of measuring the weight loss with a balance makes the measurement of burning rate challenging. This work proposes three methods to quantify the burning rate of condensed fuels in microgravity by measuring (i) the regression rate of the fuel surface, (ii) the spread rate of the flame leading edge, and (iii) the flame-sheet area, which all rely on video imaging of the flame or fuel surface geometry. The accuracies of these methods are quantified first in the ground-based tests with representative fuels, 1) solid candle and PMMA rods with diameters from 8 to 15 mm, 2) liquid fuels including propanol, hexane, and kerosene, and 3) the methane and propane gases. Results show that the burning rate obtained optically by tracking the flame leading edge and the fuel regression were less accurate due to strong sensitivity to camera resolution and background light. Comparatively, measuring the flame-sheet area is easier and gives more accurate results, and microgravity PMMA-rod flame (BASS-II project in the International Space Station) show that the fuel mass flux across the flame sheet is almost constant (0.15 mg/cm2-s) for a given fuel configuration and environment. This work offers a useful way to measure fuel burning rate and HRR in spacecraft and provides a path for the performance-based spacecraft fire safety design.

Numerical study of flame spread in a narrow flow duct in microgravity – effects of flow confinement and radiation reflection

A three-dimensional numerical study is performed to investigate concurrent-flow flame spread over thin solid fuels in microgravity. The model considers the burning scenarios of a recently concluded ISS microgravity experiment, Confined Combustion. Cellulose based thin samples are burned in a small flow duct. The height of the flow duct and the radiation reflectance of the duct wall are varied. Flame development and steady spread flame characteristics are compared with the experimental results at various duct heights. The numerical results demonstrate that the confinement imposed by the duct walls accelerates the flow during the combustion thermal expansion, enhancing the conductive heat transfer to the solid samples. When the duct height is below a critical height, the flow confinement limits oxygen supply to the flame, and the duct wall acts as a conductive heat sink. As a result of the interplay of these effects, the flame spread rate and pyrolysis length first increase and then decrease as the duct height decreases. Eventually, the flame fails to spread at a quenching duct height. In addition, side-leading concave (two-teeth fork shaped) flames are observed below the critical duct height. This flame shape increases the flame surface area and facilitates oxygen transport to the combustion zone. When the duct wall reflectance varies, a higher reflectance yields a longer pyrolysis length and a faster spread rate. This is due to enhanced heat input to both the solid sample surface and the gaseous flame. This effect is most significant for medium duct heights. At large duct heights, the duct wall is far from the flame and the sample. At small duct heights, while flame spread rate increases with the wall reflectance, the pyrolysis and flame length remain similar as combustion is limited by oxygen supply.

Effects of oxygen depletion on soot production, emission and radiative heat transfer in opposed-flow flame spreading over insulated wire in microgravity

The main objective of this article is to investigate experimentally and numerically the effects of reduced-oxygen contents on the soot production and emission from solid fuels in microgravity, which constitute an important issue in terms of fire safety for manned space missions. Due to its convenience for the implementation of soot-related optical diagnostics, the configuration of a flame spreading in an opposed flow over thin nickel chromium (NiCr) wires coated by low density PolyEthylene (LDPE) is considered. Experiments are conducted at a pressure of 101.3 kPa and an oxidizer velocity of 150 mm/s. The oxygen mole fraction in the oxidizer, XO2, is varied from 18% to 21% by nitrogen dilution of air. The modeling strategy lies on a surrogate fuel to mimic the combustion of LDPE by preserving its stoichiometry and the laminar smoke-point (LSP) flame height. The numerical model considers a detailed chemistry, a two-equation soot production model involving laminar smoke point (LSP)-based soot formation rates and oxidation by OH and O2, a radiation model coupling the Full-Spectrum correlated-k method with the finite volume method, and a simple degradation model for LDPE. Based on experimental evidence, the soot formation rate is scaled by the adiabatic flame temperature to account for thermal effects due to variation in XO2. The model reproduces quantitatively the increase in flame size, residence time, and soot volume fraction observed experimentally as XO2 is enhanced as well as the transition from a non-smoking to a smoking flame, which occurs for XO2 between 19% and 20%. The increase in soot volume fraction results of a combined enhancement in both residence time, owing to an increase in the fuel mass flow rate, and soot formation rate due to higher temperature in the soot formation region. The radiant fraction increases significantly with XO2 from about 17% for XO2=18% to about 36% for XO2= 21%. This increase in radiative losses is accompanied by a reduction of the temperature in the soot oxidation region. Therefore, for increasing XO2, the soot oxidation process is governed by a competition between oxygen-enhanced conditions that promote the formation of soot oxidizing species and the increase in radiative losses that dampens their formation. For the present flames, the first mechanism prevails as XO2 increases from 18% to 19% whereas the second dominates as XO2 is further increased, leading to smoking flames as actually observed for XO2=20 and 21%. The radiant fraction at the smoke-point transition and the soot oxidation freezing temperature are in line with those reported at normal gravity. Finally, model results show that, whatever XO2, the contribution of radiation to the heating process is negligible ahead of the pyrolysis front and is largely overcome by surface radiative losses along the pyrolysis region.

Experimental study of concurrent-flow flame spread over thin solids in confined space in microgravity

Concurrent flow flame spread experiments are conducted over thermally thin solid fuels in microgravity aboard the International Space Station (ISS) under varying levels of confinement. Samples of cotton fiberglass blended textile fabric are burned in air flows in a small flow duct. Baffles are placed parallel to the sample sheet, one on each side symmetrically. The distance between the baffles is varied to change the confinement of the burning event. Three different materials of baffles are used to alter the radiative boundary conditions of the space that the flame resides: transparent polycarbonate, black anodized aluminum, and polished aluminum. In all tests, samples are ignited at the upstream leading edge and allowed to burn to completion. The results show that at low flow speeds (<17 cm/s), the flame reaches a steady state for all tested baffle types and baffle distances. The spread rates and flame lengths at the steady state increase first and then decrease when the baffle distance decreases, resulting in an optimal baffle distance for flame spread. Furthermore, there exists a limiting baffle distance below which the flame fails to spread. It is concluded that the confinement imposed by the baffles accelerates the flow during the combustion thermal expansion and the baffles reflect flame radiation back to the sample surface, both of which intensifying the burning. However, the confinement also limits the oxygen supply and introduces conductive heat loss away from the flame. At the same baffle distance and imposed flow speed, flame length and spread rate are largest 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, flame length and flame spread rate are similar for all baffle types at small baffle distances as the combustion is limited by the reduced oxygen supply.

Concurrent-flow flame spread over thin discrete fuels in microgravity

Microgravity experiments are performed to study concurrent-flow flame spread over an array of thin cellulose-based fuel samples, using NASA Glenn Research Center's 5.18 s drop tower. Sample segments are distributed uniformly, separated by air gaps, on a sample holder. The exposed width of each sample segment is 5 cm. Two segment lengths, 0.5 cm and 1 cm, are tested. The gap sizes are varied in different tests, ranging from 0.5 to 5 cm. In all tests, a low-speed air flow (30 cm/s) is imposed and the upstream-most fuel segment is ignited by an electrical ignition wire. Upon ignition, the flame spread is recorded by two video cameras from the front and side-view angles. Spread rates, flame lengths, and burning durations are extracted using a custom video processing code. Similar to continuous fuels, flame spread over discrete fuels is a continual process of ignition. A burning discrete fuel segment, before it is consumed, needs to ignite the subsequent segment in order to have flame propagation across the gap. During this process, larger gaps between samples reduce the effective fuel load, increasing the apparent flame spread rate. However, larger gaps also reduce the heat transfer between adjacent samples, decreasing the sample burning rate. As a result, as the gap size increases, the flame spread rate increases but the burning rate decreases. At the same gap size, the flame spread rate is higher for the shorter tested sample segments. When considering sample configurations of the same fuel ratio (fuel length over the summation of the fuel and gap lengths), the spread rates are similar. This trend remains until a critical gap size is reached and flame fails to propagate across the entire array of samples. The critical gap sizes are similar for the two tested sample segment lengths and are suspected to be determined by the flame length.

Effects of fuel Lewis number on self-propagating flame spread over thin solid fuels in microgravity

A numerical investigation is carried out to understand the role of fuel Lewis number (LeF=α/DF) on flame spread over thin solid fuel in a quiescent microgravity environment. A three-dimensional numerical model was used wherein fuel Lewis number, LeF was split into directional components along and perpendicular to the fuel surface to elucidate the directional effect of fuel vapor diffusion on flame, spread rate, and its extinction. It is shown that fuel vapor diffusion in the direction perpendicular to the fuel surface plays the most dominant role in the flame spread and extinction behavior. With this approach, the non-monotonic variation of flame spread rate with LeF, influence on limiting oxygen index are also explained. The spreading flame exhibited diffusive-thermal (D-T) instabilities near-extinction. The flame extinguished forming cellular type flame for LeF <1 and developed oscillatory behavior for LeF >1. It is shown that D-T instability induced in one direction triggers the instability in other direction as well. Sensitivity analysis revealed that fuel diffusion becomes increasingly important for narrow fuel strips.

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

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 2–4cm. 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.

Opposed-flow Flame Spread Over Solid Fuels in Microgravity: the Effect of Confined Spaces

Effects of confined spaces on flame spread over thin solid fuels in a low-speed opposing flow is investigated by combined use of microgravity experiments and computations. The flame behaviors are observed to depend strongly on the height of the flow tunnel. In particular, a non-monotonic trend of flame spread rate versus tunnel height is found, with the fastest flame occurring in the 3 cm high tunnel. The flame length and the total heat release rate from the flame also change with tunnel height, and a faster flame has a larger length and a higher heat release rate. The computation analyses indicate that a confined space modifies the flow around the spreading flame. The confinement restricts the thermal expansion and accelerates the flow in the streamwise direction. Above the flame, the flow deflects back from the tunnel wall. This inward flow pushes the flame towards the fuel surface, and increases oxygen transport into the flame. Such a flow modification explains the variations of flame spread rate and flame length with tunnel height. The present results suggest that the confinement effects on flame behavior in microgravity should be accounted to assess accurately the spacecraft fire hazard.

Enclosure effects on flame spread over solid fuels in microgravity

Enclosure effects on flame spread over solid fuels in microgravity

Theoretical prediction of piloted ignition of polymeric fuels in microgravity at low velocity flows

A numerical model developed for the prediction of the piloted ignition delay of solid polymeric materials exposed to an external radiant heat flux is used to predict the ignition delay and critical heat flux for ignition of solid fuels in microgravity at low velocity flows. The model considers the coupled thermochemical processes that take place in the condensed phase, including oxidative and thermal pyrolysis, phase change, radiation absorption, and heat and mass transfer in a multi-phase and multi-composition medium. Ignition is considered to occur when a critical pyrolysate mass flow rate is reached at the sample surface. Microgravity experimental surface temperature and ignition delay data previously obtained in a KC-135 aircraft are used to infer, in conjunction with the theoretical analysis, the critical mass flow rate for ignition. This value is then used to predict the ignition delay as a function of the external radiant heat flux, and the critical heat flux for ignition. Calculations are made for Polymethylmethacrylate (PMMA) and a Polypropylene/Fiberglass composite at airflows of 0.09 and 0.15 m/s under microgravity conditions and at 1.0, 1.75 and 2.5 m/s under normal gravity. The experiments and theoretical predictions show that the ignition delay and critical heat flux for ignition decrease as the forced airflow velocity decreases. It is predicted that at the tested lower velocities, the critical heat flux for ignition is close to half the value measured in normal gravity. The results have important implications since they indicate that materials could ignite easier under the conditions expected in spacecraft, and consequently stricter design specifications may be needed for fire safety.

Enclosure effects on flame spread over solid fuels in microgravity

Enclosure effects on the transition from a localized ignition to subsequent flame growth over a thermally thin solid fuel in microgravity are numerically investigated by solving the low Mach number time-dependent Navier-Stokes equations. The numerical model solves the two and three dimensional, time-dependent, convective/diffusive mass, and heat transport equations with a one-step global oxidation reaction in the gas phase coupled to a three-step global pyrolysis/oxidative reaction system in the solid phase. Cellulosic paper is used as the solid fuel and is placed in a slow imposed flow parallel to the surface. Ignition is initiated across the width of the sample or at a small circular area by an external thermal radiation source. Two cases are examined; an open configuration (i.e., without any enclosure) and the case with the test chamber used in our previous microgravity experiments. Numerical results show that the upstream centerline flame spread rate for the case with the enclosure is faster than that for the case without any enclosure. This is due to the confinement of the flow field and also thermal expansion initiated by heat and mass addition in the chamber. The confinement accelerates the flow in the chamber, which enhances oxygen transport into the flame. In the three-dimensional configuration with the spot ignition, the flame growth in the direction perpendicular to the flow is significantly enhanced by the confinement effects. The effect of the enclosure is most significant at the slowest flow condition investigated and the effect becomes less important with an increase in imposed flow velocity. The total heat release rate from the flame during a flame growth period increases significantly with the confinement and the enclosure effects should be accounted to avoid underestimating fire hazard in a spacecraft.

Effects of DC Electric Fields on Combustion of Single Droplets under Microgravity.

This paper presents experimental results for effects of electric field on combustion of single droplets for sooting and non-sooting fuels in microgravity. The ambient gas was air at atmospheric temperature and pressure, and ethyl alcohol, n-octane, and toluene were used as fuels. The distance between electrodes is 50 mm and applied electric voltages ranged between 0 and 7 kV. The deformation of flame occurred and the burning rate constant increased with an increase in the electric field intensity for all of the fuels tested. It is supposed that the deformation of flame and the increase in the burning rate constant can be explained due to the movement of ions and soot particles which are charged positively and the flow induced by the movement. In consideration of not only the movement and induced flow but also behavior of soots, the experimental results of sooty flames can be explained.

Combustion behavior of single droplets for sooting and non-sooting fuels in direct current electric fields under microgravity

This paper presents experimental results for the effects of electric fields on the combustion of singledroplets for sooting and non-sooting fuels in microgravity. The ambient gas was air at atmospheric temperature and pressure, and ethyl alcochol, n -octane, and toluene were used as fuels. Applied electric field intensities ranged between 0 and 140 kV/m. The result showed clearly that the blue flame region, which corresponds to the reaction zone, is deformed toward the negative screen in electric fields for ethylalcohol. The deformation of luminous flames occurred for n -octane and toluence and is associated with the behavior of carbonaceous soot particles. The burning rate constant increased with an increase in the electric field intensity for all of the fuels tested. We suppose that induced flow to the negative screen exists by the movement of the positive ion molecules in electrc fields, and the induced flwo velocity was estimated using the analogy of flame displacement in forced convection. The burning rate constant in electric fields was predicted by the well-known empirical equation of burning rate constant in forced flows. For ethyl alchol and n -octane, experimental results are nearly consistent with predictions, while the measured burning rate constant is larger than the predicted results for toluence. The shadowgraph showed the reduction of the amount of soot that existed in the flame by electric fields for toluence, which results in the reduction of radiative heat loss of the flame. It is believed that this leads to an additional increase in burning rate constant in electric fields for toluene.

Interaction Effects on Combustion of Alcohol Droplet Pairs.

Experimental investigation was conducted on two droplet-array combustion of methanol and methanol/dodecanol mixture fuels in microgravity. For methanol, effects of ambient pressure and droplet spacing were examined. Results show that the droplet lifetime decreases with increasing spacing at relatively low pressure and the droplet lifetime becomes independent of spacing at higher-subcritical and supercritical pressures. For methanol/dodecanol mixture, effects of pressure, fuel composition were investigated in terms of occurrence of disruption. Disruption of droplet during combustion was demonstrated both for single droplet and droplet pairs.

Interaction Effects on Combustion of Alcohol Droplet Pairs.

Experimental investigation was conducted on two droplet-array combustion of methanol and methanol/dodecanol mixture fuels in microgravity. For methanol, effects of ambient pressure and droplet spacing were examined. Results show that the droplet lifetime decreases with increasing spacing at relatively low pressure and the droplet lifetime becomes independent of spacing at higher-subcritical and supercritical pressures. For methanol/dodecanol mixture, effects of pressure, fuel composition were investigated in terms of occurence of disruption. Disruption of droplet during combustion was demonstrated both for single droplet and droplet pairs.

Inherently unsteady flame spread to extinction over thick fuels in microgravity

Results of an experiment for flame spread over thick PMMA in a quiescent, 50% O2 in N2, 1 atm, microgravity environment recently obtained aboard space shuttle mission STS 85 are described. Previous experimental results indicate that the spread process is unsteady with the spread rate decreasing with time. Although experiment time in the earlier experiments was insufficient to determine if steady spread is established or extinction occurs, computational modeling predicts extinction. The sample length was extended over that of the earlier experiments to determine the ultimate fate of the flame. Flame imaging shows that following ignition, the flame leading edge spreads at a continually decreasing rate for approximately 180 s, ceases to progress forward, and then retreats in the opposite direction for approximately an additional 360 s, at which time flame extinction occurs. Computational modeling, including gas and fuel surface radiation, captures the observed behavior, which is predicted for all oxygen concentrations up to pure oxygen at 1 atm. In the presence of a flow, a thin heated layer in the solid develops quickly with the heat transfer driving vaporization and steady spread, while in the quiescent environment, a heated layer of substantial thickness develops over time while the flame spreads, unsteadily, more slowly. As a result, radiation is important, and the length-scale characteristic of the temperature field in the gas is decreased in comparison to the mass diffusion scale, which grows with time. Ultimately, the mismatch in scales results in the flame being in a region to which oxygen is unable to diffuse at a sufficient rate, and the flame extinguishes. Such self-extinction at microgravity has implications for fire safety considerations in spacecraft.

Quiescent flame spread over thick fuels in microgravity

Experimental results for flame spread over thick PMMA in microgravity are reviewed. The results were obtained abouard three different space shuttle missions, STS-54, STS-63, and STS-64. For the three conditions, 50% O 2 in N 2 at 1 atm, 50% O 2 at 2 atm, and 70% O 2 at 1 atm, the flame-spread rate slowly decreases with time, which varied from about 50 s to over 300 s. Computational modeling that includes the effects of radiation captures the essential features of the flame position versus time trajectory. When computations are carried out past the experimental time, the flames eventually retreat and then extinguish after spread times of about 450–600 s. With respects to the flame, the flow velocity into the flame is the spread rate. Absent any additional flow to press the flame close to the surface to provide a heat flux that allows the heated layer in the solid to develop., the process remains unsteady. The thermal and mass diffusion scales each are approximately the thermal diffusivity of the gas divided by the spread rate. The computed temperature and oxygen fields show that the distances over which temperature changes take place are small compared to those over which oxygen diffuses. This effect is due to the radiation causing a reduction in the length scale characteristic of the temperature field compared to the mass diffusion scale. The mismatch in the actual thermal scale and the mass diffusion scale grows with time until the oxygen diffusion rate to the flame is unable to sustaint it. For fuels with thickness below some critical value, the fuel thickness is heated fast enough and the spread rate is high enough that the mismatch in the thermal and the mass diffusion scales is unimportant, and the spread rate is steady.

Unsteady flame spread over solid fuels in microgravity

For flame spreading over solid fuels at microgravity in a quiescent environment, experimental, and computational, results show that the spread rate following ignition is steady because the leading edge of the flame itself establishes the flow field into which it spreads. Spreading into an opposing forced flow is inherently unsteady because of the changing character of the boundary-layer flow with location. The development of an unsteady flame spread model is presented and applied to thermally thick polymethylmethacrylate. The unsteady model is constructed such that interfacial phenomena, e.g., fuel surface reradiation, are accounted for as volumetric source terms in differential, conservation equations, so that the solid and gas fields may be treated simultaneously without iteration between phases. Care must be taken such that communication between the solid and gas at the interface is computed accurately. For forced flows with velocity much larger than the spread rate, radiative processes are unimportant, but for the lower flow rates, comparable to the spread rate, the opposite is true. Solutions for a quiescent environment are difficult to obtain. Conduction scales become large, and conduction heat transfer from the flame to the solid is reduced. Because of this, quiescent environment solutions could not be obtained without at least an approximate treatment of gas-phase, flame radiation.