Tethered methanol droplet combustion in carbon-dioxide enriched environment under microgravity conditions

Tethered methanol droplet combustion in carbon dioxide enriched environment is simulated using a transient one-dimensional spherosymmetric droplet combustion model that includes the effects of tethering. A priori numerical predictions are compared against recent experimental data. The numerical predictions compare favorably with the experimental results and show significant effects of tethering on the experimental observations. The presence of a relatively large quartz fiber tether increases the burning rate significantly and hence decreases the extinction diameter. The simulations further show that the extinction diameter depends on both the initial droplet diameter and the ambient concentration of carbon dioxide. Increasing the droplet diameter and ambient carbon dioxide concentration both of them lead to a decrease in the burning rate and increase in the extinction diameter. The influence of ambient carbon dioxide concentration on extinction shows a sharp transition in extinction for larger size droplets (d_o > 1.5 mm) due to a change in the mode of extinction from diffusive to radiative control. In addition predictions from the numerical model is compared against a recently developed simplified theoretical model for predicting extinction diameter for methanol droplets, where the presence and heat transfer contribution of the tether is not taken into account implicitly. The numerical results suggest some limitation in the theoretical modeling assumptions for favorable comparisons with the experimental data.     

On the extinction characteristics of alcohol droplet combustion under microgravity conditions – A numerical study

Quasi-steady burning and extinction of droplets are of interest from both fundamental and application viewpoints. The latter is related to combustor performance and fire safety issues in reduced gravity environments. Influences of diluent in the atmosphere on isolated droplet combustion characteristics including extinction provide insights to fire extinguishment phenomena and the effectiveness of various diluents as fire suppressants. Extinction of pure methanol and methanol–water droplets ranging from 1.5 to 7 mm size, for varying levels of ambient carbon-dioxide, helium and oxygen concentration – burning in a quiescent microgravity environment were studied numerically to compare the effectiveness of fire suppressant diluent selection and determining the limiting oxygen index. The results show distinct regimes of diffusive and radiative extinction. The transition from diffusive to radiative extinction is strongly influenced by the ambient diluent selection, especially by carbon dioxide concentration. Results for helium as the diluent showed increased burning rate and extinction due to diffusive heat loss. An “extinction characteristic” correlation is proposed that depends on burning rate, ambient diffusivity and flame standoff ratio. Recent methanol droplet experiments conducted over a wide range of operating conditions onboard the International Space Station were found to yield results that agree well with the proposed “extinction characteristic” correlation.     

Activation energy asymptotics for methanol droplet extinction in microgravity

An activation energy asymptotic theory for methanol droplet combustion in microgravity is presented by extending earlier models to account for time-dependent water dissolution or evaporation from the liquid droplet. The model predictions for droplet extinction diameter as a function of its initial diameter are shown to compare favorably with experimental results for methanol burning in air.     

Autoignition and burning rates of fuel droplets under microgravity

This paper presents a mathematical model for the unsteady evaporation, ignition, and combustion of isolated fuel droplets under microgravity. The model consists of a large structured system of differential algebraic equations, where the numerical complexity is due both to the stiff nature of the kinetic mechanism and to the flame structure around the droplet. A very general, detailed kinetic scheme, consisting of ∼200 species and over 5000 reactions, is used to describe the gas-phase combustion of different fuels. Several comparisons with experimental measurements, carried out under various operating conditions, confirm that the proposed model is a useful tool for characterizing low-temperature and high-temperature ignition delay times. The predicted explosion diagrams of n-alkanes, as well as their ignition delay times, agree with the experimental measurements; the various oxidation regions are closely reproduced too. In addition to this, recent experimental results, relating to the influence of the initial diameter on droplet burning rates in cold and hot environments, are also presented and discussed. Lastly, an analysis of the extinction diameters for the combustion of n-heptane droplets allows a discussion of the role of radiative heat transfer, as well as further emphasizing the importance of the low-temperature oxidation mechanisms.     

Methanol droplet extinction in carbon-dioxide-enriched environments in microgravity

Diffusive extinction of methanol droplets with initial diameters between 1.25 mm and 1.72 mm, burning in a quiescent microgravity environment at one atmosphere pressure, was obtained experimentally for varying levels of ambient carbon-dioxide concentrations with a fixed oxygen concentration of 21% and a balance of nitrogen. These experiments serve as precursors to those which are beginning to be performed on the International Space Station and are motivated by the need to understand the effectiveness of carbon-dioxide as a fire suppressant in low-gravity environments. In these experiments, the flame standoff distance, droplet diameter, and flame radiation are measured as functions of time. The results show that the droplet extinction diameter depends on both the initial droplet diameter and the ambient concentration of carbon dioxide. Increasing the initial droplet diameter leads to an increased extinction diameter, while increasing the carbon-dioxide concentration leads to a slight decrease in the extinction diameter. These results are interpreted using a critical Damköhler number for extinction as predicted by an earlier theory, which is extended here to be applicable in the presence of effects of heat conduction along the droplet support fibers and of the volume occupied by the support beads.     

Isolated n-heptane droplet combustion in microgravity: “Cool Flames” – Two-stage combustion

Recent experimentally observed two stage combustion of n-heptane droplets in microgravity is numerically studied. The simulations are conducted with detailed chemistry and transport in order to obtain insight into the features controlling the low temperature second stage burn. Predictions show that the second stage combustion occurs as a result of chemical kinetics associated with classical premixed “Cool Flame” phenomena. In contrast to the kinetic interactions responsible for premixed cool flame properties, those important to cool flame droplet burning are characteristically associated with the temperature range between the turnover temperature and the hot ignition. Initiation of and continuing second stage combustion involves a dynamic balance of heat generation from diffusively controlled chemical reaction and heat loss from radiation and diffusion. Within the noted temperature range, increasing reaction temperature leads to decreased chemical reaction rate and vice versa. As a result, changes of heat loss rate are dynamically balanced by heat release from chemical reaction rate as the droplet continues to burn and regress in size. At reaction temperatures below the turnover, heat loss over takes the heat release rate and extinction occurs. Should heat release exceed heat loss as the temperature increases to that for hot ignition, initiation of a high temperature burning phase may be possible. Parametric study on factors leading to initiation of the second stage burning phenomena are studied. Results show that both carbon dioxide and helium diluents can promote initiation of low temperature burning at smaller initial drop diameters than found with nitrogen as diluent. Small amounts of carbon dioxide and helium in the ambient is sufficient to activate the phenomena. The chemical kinetics dictating the second stage combustion and extinction process is also discussed.     

Laminar mass burning and entrainment velocities and flame instabilities of i-octane,ethanol and hydrous ethanol/air aerosols

The paper reports experiments employing the cloud chamber technique for creating fuel aerosols, in studies of premixed laminar flames. Spherical explosion flames were initiated at different times after the start of expansion of the original gaseous mixture to lower pressure. Flame speeds were measured close to atmospheric pressure, over a range of equivalence ratios of iso-octane, ethanol and hydrous ethanol with air. A methodology was developed for deriving mass burning velocities and entrainment velocities, as well as mass burning fluxes, from the measurements of aerosol number densities, droplet sizes and flame speeds. It was vital to estimate whether droplet evaporation was completed in the flame preheat zone. This was done by calculating the spatial progress of droplet evaporation for the different aerosols from values of the evaporation rate constants of the different fuels.     

The effect of monodispersed water mists on the structure, burning velocity, and extinction behavior of freely propagating, stoichiometric, premixed, methane-air flames

We present a computational model to describe the two-phase thermal and chemical interactions between a freely propagating premixed flame and fine droplets of water. The objective is to develop a fundamental understanding of flame structure and extinction in the presence of a water mist. The model and the computational algorithm must accommodate strong coupling between the droplet dynamics and the gaseous flow. The gas-phase conservation equations, which include elementary chemistry, are discretized and solved on an adaptive Eulerian mesh, while the droplet dynamics are represented in a Lagrangian framework. A modified arclength-continuation method is used to follow the solutions through the extinction turning point and thus predict flame-extinction limits. The model predicts how burning velocity and extinction conditions depend on droplet size and number density. The results compare very favorably with previously published theoretical analyses.     

Radiative extinction of gaseous spherical diffusion flames in microgravity

Radiative extinction of spherical diffusion flames was investigated experimentally and numerically. The experiments involved microgravity spherical diffusion flames burning ethylene and propane at 0.98 bar. Both normal (fuel flowing into oxidizer) and inverse (oxidizer flowing into fuel) flames were studied, with nitrogen supplied to either the fuel or the oxygen. Flame conditions were chosen to ensure that the flames extinguished within the 2.2 s of available test time; thus extinction occurred during unsteady flame conditions. Diagnostics included color video and thin-filament pyrometry. The computations, which simulated flow from a porous sphere into a quiescent environment, included detailed chemistry, transport, and radiation and yielded transient results. Radiative extinction was observed experimentally and simulated numerically. Extinction time, peak temperature, and radiative loss fraction were found to be independent of flow rate except at very low flow rates. Radiative heat loss was dominated by the combustion products downstream of the flame and was found to scale with flame surface area, not volume. For large transient flames the heat release rate also scaled with surface area and thus the radiative loss fraction was largely independent of flow rate. Peak temperatures at extinction onset were about 1100 K, which is significantly lower than for kinetic extinction. An important observation of this work is that while radiative heat losses can drive transient extinction, this is not only because radiative losses are increasing with time but also because the heat release rate is falling off as the flame expands away from the burner and the reactant supply to the flame decreases.     

Extinction of a premixed flame by a large variation in axial velocity

Unsteady flame propagation in a tube is examined by introducing a mean velocity variation larger than the burning velocity to a stabilized flame for a period longer than the reaction time scale. In our previous work, stabilized propane-air flames were classified as either one-dimensional or two-dimensional flames. The eventual extinction during the velocity increase was categorized as either acoustic extinction or boundary layer extinction. In this work, the effects of a nonunity Lewis number were estimated through experiments with a methane-air flame; the eventual extinction during the velocity decrease was investigated in more detail; and the growth of the extinction boundary layer was analyzed with a transient one-dimensional model of the flame stretch. In our experiments, the Lewis number did not affect the existence or characteristics of the critical velocity and the characteristic time for boundary layer extinction. An additional critical velocity was found, however, for acoustic extinction when the Lewis number was smaller than unity. In the transient one-dimensional model, the velocity transition along the flame was calculated with a continuity equation and an axial momentum equation. The spatial gradient of the burning velocity and the extinction criterion were simplified with the experimental results and some theoretical studies. The analysis shows that the unsteady flame stretch at the flame edge during a large axial velocity variation is the prevailing cause of the growth of the extinction boundary layer. These results provide some evidence that flame stretch affects the behavior of the flame edge; they also suggest the cause of the finger flame. The findings help explain the unsteady behavior of premixed flames near a flammability limit.     

Modeling of quasi-steady sodium droplet combustion in convective environment

In the event of accidental leakage of sodium from the systems of a Liquid Metal Fast Breeder Reactor (LMFBR), a spray of liquid sodium droplets may be formed which will burn by reacting with the surrounding atmospheric oxygen. In order to understand the burning characteristics of the complete spray, combustion of an individual sodium droplet forms the basis and this has been investigated in the present study. A comprehensive numerical model has been developed to analyze the isolated sodium droplet combustion in a mixed convective environment. The governing equations for mass, momentum, species and energy conservation have been solved in axisymmetric cylindrical coordinates using the Finite Volume Method (FVM). Finite rate kinetic mechanisms have been incorporated to simulate droplet burning, using available kinetics data for basic sodium oxidation reactions. Salient features of the numerical model include a global single-step reaction for sodium oxidation in air and the incorporation of property variations with temperature and concentration. An equilibrium mixture of sodium peroxide, sodium monoxide and sodium vapor is considered as the final reaction product, taking into account the effects of dissociation reactions. The numerical model has been validated with experimental results available in literature. Results for the fuel mass burning rates and flame shapes are presented for different sizes of droplets burning under different free-stream conditions. The model predicts the occurrence of envelope flame around a sodium droplet even at fairly high free-stream velocities.     

Radiation-enhanced evaporation of ethanol fuel containing suspended metal nanoparticles

Although the enhanced thermal conductivity of nanofluids has attracted much attention, their radiative properties have rarely been studied. The present paper quantitatively determined the radiative properties of various nanofluid fuels and their effects on droplet evaporation. The results show that radiation absorption can be significantly enhanced by adding a small amount of nanoparticles such as Al to the base fluid ethanol. The strong absorption of radiation energy by nanoparticles increases the nanofluid droplet temperatures and enhances droplet evaporation rate.     

Kinetic and radiative extinctions of spherical burner-stabilized diffusion flames

Extinction of steady, spherical diffusion flames stabilized by a spherical porous burner was investigated by activation energy asymptotics. An optically-thin radiation model was employed to study the effect of radiation on flame extinction. Four model flames with the same adiabatic flame temperature and fuel consumption rate but different stoichiometric mixture fraction and flow direction, namely the flames with fuel issuing into air, diluted fuel issuing into oxygen, air issuing into fuel, and oxygen issuing into diluted fuel, were adopted to understand the relative importance of residence time and radiation intensity. Results show that for a specified flow rate emerging from the burner, only the kinetic extinction limit at low Damköhler numbers (low residence times) exists. In the presence of radiative heat loss, extinction is promoted so that it occurs at a larger Damköhler number. By keeping the radiation intensity constant while varying the flow rate, both the kinetic and radiative extinction limits, representing the smallest and largest flow rates, between which steady burning is possible, are exhibited. For flames with low radiation intensity, extinction is primarily dominated by residence time such that the high-flow rate flames are easier to be extinguished. The opposite is found for flames suffering strong radiative heat loss. The kinetic extinction limit might occur at mass flow rates lower than what is needed to keep the flame outside of the burner and not observable. An extinction state on the radiative extinction branch can be either kinetic or radiative depending on the process.     

Dissolution kinetics of carbon in aluminum droplet combustion: Implications for aluminized solid propellants

An analytical model describing the kinetics of carbon dissolution in burning aluminum droplets has been developed in order to simulate its effects under solid rocket motor conditions. A carbon dissolution rate (k) was introduced in different droplet regression laws and depending on the heterogeneous kinetics between the Al surface and the surrounding gases. The model was validated using previous experiments performed by the authors on millimeter-sized Al droplets burning in several CO₂-containing atmospheres at atmospheric pressure (P=1 atm). It has been shown that the carbon dissolution is affected by the presence of hydrogen due to competition between CO and H₂ chemisorption. The model was then applied to aluminized propellants (AP/HTPB) at high pressures (P=60 atm) and high temperatures (T=3000 and 3500 K), as well as at various burning rates and adsorption conditions. Though the accuracy of the extrapolation results needs further improvement, it has been shown that the carbon dissolution process should not be neglected in order to achieve global understanding of the combustion of Al particles, particularly agglomerates.     

Influences of pressure on reduced-gravity combustion of HAN-methanol-water droplets in air

Reduced-gravity experiments were performed to investigate combustion characteristics of individual monopropellant droplets composed of hydroxylammonium nitrate (HAN), methanol, and water. The experiments were conducted using the 2.2 Second Drop Tower at the NASA John Glenn Research Center at Lewis Field in Cleveland, OH. Gravitational levels were about 10⁻⁴ times normal gravity and experiments were performed in air at about 25 °C and with pressures from 0.1 to 1 MPa. Droplets initially in the 1-mm size range were supported on quartz fibers and ignited using a hot wire that heated the gas phase. Initial droplet compositions varied from zero (initially pure methanol droplets) to a stoichiometric mixture of 69.4% HAN, 15.2% water, and 15.4% methanol by mass. Results indicated that increasing the pressure increased burning rates, delayed extinction, and promoted easier ignition of droplets. Decreasing the initial mass fraction of methanol reduced burning rates, increased the difficulty of ignition, and promoted gas-phase flame extinction. Internal bubbling was observed at certain initial droplet compositions. Aerosol formation was observed for higher HAN loadings at elevated pressures after the visible gas-phase flame had extinguished, which may be indicative of condensed-phase HAN reactions.     

Effects of ammonia borane on the combustion of an ethanol droplet at atmospheric pressure

Adding compounds rich in hydrogen to liquid fuels has the potential to change combustion behavior and enhance performance. One potential additive is ammonia borane (AB), which contains 19.6 wt.% hydrogen and can be dissolved in anhydrous ethanol (up to 6.5 wt.%). The particular system studied here would have limited use due to energy density and stability but is studied as a model system. Single droplet combustion experiments were performed with AB concentrations in ethanol varying from 0 to 6 wt.%. Measurements performed using high speed (5 kHz) planar laser-induced fluorescence (PLIF) indicate that hydrogen gas addition from the decomposition of AB continues throughout the droplet burning process. The hydrogen addition leads to an increase in the D² law rate constant, k₀, of up to 16%. While AB (and residual material) participates throughout the combustion process, it dramatically impacts the combustion behavior at the end of the droplet lifetime as the concentration of AB residual grows within the droplet. This results in droplet shattering, causing fine atomization and rapid combustion of the remaining fuel. Boron is also oxidized in this short period of time, increasing the energy released. In combustors, droplet shattering could enhance mixing and increase combustion efficiency. Thus, the approach of adding compounds rich in hydrogen is a promising method to introduce H₂ gas to practical combustion systems, while enhancing performance.     

A theoretical analysis of the extinction limits of a methane-air opposed-jet diffusion flame

A theoretical analysis is described for a methane-air diffusion flame stabilized in the forward stagnation region of a porous metal cylinder in a forced convective flow. The analysis includes effects of radiative heat loss from the porous metal surface and finite rate kinetics but neglects the effects of gravity. The theoretically predicted extinction limits compare well with experimentally observed extinction limits from the literature.After the predicted limits compared well with the experimental limits, a parametric study of the effect of fuel surface emissivity and Lewis number was conducted with the numerical model. It was found that the computed blowoff limit is independent of radiative heat loss for high fuel blowing velocities but is a strong function of Lewis number. At low fuel blowing velocities, the extinction limit varies with both radiative heat loss and Lewis number. It is discovered, however, that even if thermal losses from the fuel surface are absent, the flame can extinguish at the fuel surface independently of Lewis number due to excessive reaction zone thinning.     

A Burke-Schumann analysis of dual-flame structure supported by a burning droplet

Droplet combustion experiments carried out onboard the International Space Station, using pure fuels and fuel mixtures, have shown that quasi-steady burning can be sustained by a non-traditional flame configuration, namely a “cool flame” burning in the “partial-burning” regime where both fuel and oxygen leak through the low-temperature-controlled flame-sheet. Recent experiments involving large, bi-component fuel (n-decane and hexanol, 50/50 by volume) droplets at elevated pressures show that the visible, hot flame becomes extremely weak while the burning rate remains relatively high, suggesting the possible simultaneous presence of “cool” and “hot” flames of roughly equal importance. The radiant output from these bi-component droplets is relatively high and cannot be accounted for only by the presence of a visible hot flame. In this analysis we explore the theoretical possibility of a dual-flame structure, where one flame lies close to the droplet surface, called the “cool flame”, and the other farther away from the droplet surface, termed the “hot flame”. A Burke-Schumann analysis of this dual structure seems to indicate that such flame structures are possible over a limited range of initial conditions. These theoretical results can be compared against available experimental data for pure and bi-component fuel droplet combustion to test how realistic the model may be.     

Surface tension effects during low-Reynolds-number methanol droplet combustion

A numerical investigation of methanol droplet combustion in a zero-gravity, low-pressure, and low-temperature environment is presented. Simulations have been carried out using a predictive, transient, and axisymmetric model, which includes droplet heating, liquid-phase circulation, and water absorption. A low initial Reynolds number (Re₀=0.01) is used to impose a weak gas-phase convective flow, introducing a deviation from spherical symmetry. The resulting weak liquid-phase circulation is greatly enhanced due to surface tension effects, which create a complex, time-varying, multicellular flow pattern within the liquid droplet. The complex flow pattern, which results in nearly perfect mixing, causes increased water absorption within the droplet, leading to larger extinction diameters. It is shown that, for combustion of a 0.43-mm droplet in a nearly quiescent environment (Re₀=0.01) composed of dry air, the extinction diameter is 0.11 mm when surface tension effects are included, and 0.054 mm when surface tension effects are neglected. Experimental work available in the literature for a 0.43-mm droplet reported extinction diameters in the range of 0.16 to 0.19 mm. Results for combustion in a nearly quiescent environment (Re₀=0.01) with varying initial droplet diameters (0.16 to 1.72 mm) show that including the effect of surface tension results in approximately linear variation of the extinction diameter with the initial droplet diameter, which is in agreement with theoretical predictions and experimental measurements. In addition, surface tension effects are shown to be important even at initial Reynolds numbers as high as 5.