Disruptive Burning Research Articles

The characterization of disruptive combustion of organic gellant-laden ethanol fuel droplets

The future of high-performance rocket propulsion systems depends on superior burn rates, shorter ignition delay times, and eco-friendly nature. Gelled fuel propellants are such candidates which are pivotal to achieving these performance parameters. Gel fuels are multi-component fuels, and their combustion behavior is different from conventional solid and liquid propellants. To develop a fundamental understanding of the vaporization and combustion of gel fuels, single droplet experiments in a pendant mode setup are performed for ethanol-based organic gel fuels by capturing high-speed video data. Primarily, two gel fuel combinations are taken for experiments. The gel fuels are non-metalized ethanol-based fuels containing organic gellants Hydroxypropyl methylcellulose (HPMC) and methylcellulose (MC). Both fuels have shown three distinct combustion stages namely transient heat up (stage I), disruptive burning (stage II), and carbonization (stage III). The demarcation of the stages is done by visual evidence backed by high-fidelity high-speed video data. In stage I, phase separation occurs which leads to the formation of gellant shells. Organic gellants tend to form viscoelastic shells which significantly influence the jetting behavior during the disruptive burning (stage II). The combustion of gel droplets in stage II exhibits characteristic differences in bursting mechanism, for instance, oscillatory and transient bursting in HPMC and transient bursting in MC. Moreover, droplet-bursting behaviors influence jetting attributes and burn rates. Besides, the bursting of gel droplets during combustion is also dependent on the thickness of the gellant shells, the thin and weak shells are prone to bulk droplet motion due to active jetting, whereas the thick and rigid shells act as kinetic energy dampeners and thereby inhibit the bulk motion of droplets. Apart from disruptive burning, carbonization is a significant part of the combustion during which the suppressed jetting behavior is seen due to the traces of fuel trapped in the gellant shells. A significant rise in burn rate is observed due to jetting during the disruptive burning stage in contrast to the other two stages of combustion for both the gel fuels under study. Furthermore, combustion residue analysis is carried out to assess the variation in shell morphology across the stages. Overall, the current study provides detailed insights into the combustion characterization of organic gel fuels, which can help in fabricating gel fuel compositions.

Single droplet combustion of iron nitrate-based precursor solutions: Investigation of time- and size scales of isolated burning FSP-droplets

Flame spray pyrolysis (FSP) is a powerful and versatile technique for the rapid and scalable synthesis of functional nanoparticles. The route of nanoparticle formation depends on the mass transfer of liquid precursor droplets into the gas phase, significantly deciding the quality of generated nanoparticles. Therefore, the study of time- and size scales of isolated burning FSP-droplets plays an important role in better understanding and promoting the use of this technique. Hereby, we have used a low-cost iron nitrate-based precursor-solvent system to investigate the disruptive burning process of single droplets with initial droplet diameters ranging from 48 µm to 125 µm. Different timescales at the level of steady droplet combustion and µ-explosions were correlated to droplet sizes. It has been observed that the total lifetime of a droplet converges towards the duration of the µ-explosion process for small droplet sizes causing the µ-explosion to occur immediately after ignition. In order to study the effect of the solvent composition of ethanol and 2-ethylhexanoic acid (2-EHA), the volumetric fraction of 2-EHA was varied from 20% up to 80%. An increasing fraction of 2-EHA causes the onset of the disruption to occur later and the droplet to become more concentrated with precursor. The results of this work provide fundamental insights into single droplet combustion and contribute to a better understanding of the use of low-cost precursor-solvent systems in FSP.

Time-Varying Oscillatory Response of Burning Gel Fuel Droplets.

Gel fuel droplets exhibit disruptive burning due to the rupture of their gellant shell, which causes the release of unreacted fuel vapors from the droplet interior to the flame in the form of jets. In addition to pure vaporization, this jetting allows convective transport for fuel vapors, which accelerates gas-phase mixing and is known to improve droplet burn rates. Using high-magnification and high-speed imaging, this study found that the viscoelastic gellant shell at the droplet surface evolves during the droplet's lifetime, which causes the droplet to burst at different frequencies, thereby triggering a time-varying oscillatory jetting. In particular, the continuous wavelet spectra of the droplet diameter fluctuations show that the droplet bursting exhibits a nonmonotonic (hump-shaped) trend, where the bursting frequency first increases and then decreases to a point where the droplet stops oscillating. The changes in the shell structure are captured by tracking the temporal variation of the area of rupture sites, spatial movement of their centroid, and the degree of overlap between the rupture areas of successive cycles. During the initial period (immediately following its formation) when the shell is newly formed, it is weak and flexible, which causes it to burst at increasingly high frequencies. This is because the area at and around the rupture site becomes progressively weaker with each rupture in an already weak shell. This is shown by a high degree of overlap between the areas of successive ruptures. On the other hand, the shell flexibility during the initial period is demonstrated by a reversal in the motion of rupture site centroids. However, at later stages when the droplet has undergone multiple ruptures, the depletion of the fuel vapor causes accumulation of gellant on the shell, thus causing the shell to become strong and rigid. This thick, strong, and rigid shell suppresses droplet oscillations. Overall, this study provides a mechanistic understanding of how the gellant shell evolves during the combustion of a gel fuel droplet and causes the droplet to burst at different frequencies. This understanding can be used to devise gel fuel compositions that result in gellant shells with tailored properties, and therefore, control the jetting frequencies to tune droplet burn rates.

Experimental study of droplet combustion and diesel engine characteristics for Azolla biodiesel

ABSTRACT This study pertains to studying the feasibility of the third-generation biodiesel obtained from one of the algae species known as Azolla microphylla by exploring their fundamental droplet combustion behavior and diesel engine characteristics. Firstly, the droplet evolution and burn rate are investigated for diesel, Azolla100, and Azolla50 (50% biodiesel +50% diesel) based on the experimental study of suspended droplet combustion. The diesel droplet showed steady combustion with linear regression for the decrease in droplet surface with time throughout its lifetime, while the Azolla100 and Azolla50 droplets showed a linear trend initially, and after a certain point, they resulted in a non-linear trend as a result of disruptive burning. The evaporation and burn rate was found to be higher for Azolla100 and Azolla50 than diesel during the steady burning period and thereafter it decreased with increasing Azolla concentration. The time evolution of droplet combustion images indicated that with increasing biodiesel concentration, the combustion duration was decreased due to secondary droplet ejections and microexplosion, and the residue burning duration was increased. The microexplosion increased the rate of combustion, however, droplet ejection resulted in incomplete combustion. Secondly, the engine experiments were performed for Azolla50 at different fuel injection pressures. The results showed that in-cylinder pressure and Brake thermal efficiency (BTE) for Azolla50 at 300 bar injection pressure were lower than diesel due to limitations with the physical properties of biodiesel. In order to improve the engine characteristics of Azolla50, this study increased the fuel injection pressure to 900 bar. As a result, the BTE for Azolla50 at 900 bar injection pressure is improved by 9.2% and 10.2% at low and full load conditions, respectively, compared to Azolla50 at 300 bar injection pressure. Overall, the spray-driven combustion for Azolla50 is limited by the physical properties of the biodiesel, which affects the mixture formation. On the other hand, the microexplosion and droplet ejection observed with the biodiesel during the combustion study would favor the combustion by improving the atomization and mixing process.

Experimental Investigation Of Disruptive Burning Phenomena On Nanofuel Droplets

The use of sustainable and green fuels in the aeronautical industry has been implemented due to the environmental concerns and depletion of fossil fuels. The introduction of biofuels, a renewable energy source in the transportation sector, has shown advantages in terms of pollutant reduction. Recently, the addition of nanoparticles in the combustion of biofuel has been studied with the purpose of enhancing its combustion characteristics. Consequently, the present work evaluates nanofuel single droplet in a falling droplet method. In this way, the fiber suspension effect was neglected, and droplets with a diameter of 250 μm were evaluated. A comparison between pure biofuel and a nanofuel at two furnace temperatures (T = 800 ºC and T = 1000 ºC) was performed. Nanofuels can be defined as a novel class of fuel, where energetic or not energetic nanoparticles are stably suspended in liquid fuel. The preparation of nanofuels demands a methodic procedure to accomplish stable, long-term stability. Therefore, in the present work, the liquid fuel is a biofuel named NEXBTL (HVO) from NESTE. The nanofuel is composed of HVO and aluminum nanoparticles of 40 nm in a particle concentration of 1.0 wt.%. The combustion of pure biofuel and nanofuel was evaluated in a drop tube furnace. This droplet combustion facility consists of an electrically heated drop tube furnace (DTF), an illumination set, an image acquisition system, and an injector device. To follow the burning process from the injector tip until the micro-explosion, a CMOS high–speed camera (CR600×2, Optronics), coupled with a high magnification lens, was used. The image acquisition was pursued with 1000 fps with a resolution of 1200×500 pixels and an exposure time of 1/13000 s. Moreover, a Photron FASTCAM mini UX50, a high-speed camera with 1.3 Megapixel was used to acquire micro-explosions with more detail, and a high magnification lens was coupled to the camera. The results reveal that disruptive burning phenomena occur when aluminum nanoparticles are added to the biofuel. Consequently, a micro-explosion determines the end of the droplet lifetime, mainly affected by the furnace temperature. Disruptive burning phenomena identified in the experimental study are difficult to acquire since its duration is brief and occurs at the end of the droplet lifetime. Due to this, different camera operating ranges were considered. Several frame rates from 2000 to 5000 fps and exposure times of 1/16000s and 1/20000s were considered. The image data processing was performed in ImageJ and MATLAB software. First, the droplet size evolution was evaluated using ImageJ based on the droplet outline through the brightness gradient. Then, the bright spots provided from the micro-explosions were identified and counted by an algorithm developed in MATLAB. It was concluded that adding aluminum combustion in a proper particle concentration can potentially improve the biofuel combustion performance. Moreover, no puffing or micro-explosions were detected in the pure HVO. In contrast, nanofuels displayed puffing and micro-explosions at the end of the droplet lifetime. In addition, the furnace temperature affects the disruptive process, being more intense at T = 1000 ºC. It was observed that a higher number of fragments are ejected at the highest furnace temperature. In addition, the fragments after the explosion are mainly on the lower side, however more studies focusing on this topic should be performed since these disruptive events are unpredictable. The image data acquisition provided the visualization and description of micro-explosions in nanofuel droplets, as well as comprehension of its phenomenology.

The Addition of Particles to an Alternative Jet Fuel

The expansion of the research on nanoscale particles demonstrates several advantages in terms of stability and an increased surface area to volume ratio compared to micron-sized particles. Based on this, the present work explores the addition of aluminum particles in hydrotreated vegetable oil (HVO), an alternative jet fuel. To evaluate the influence of particle sizes, nano and micron particles (40 nm and 5 μm) in a particle concentration of 0.5 wt.% were stably suspended in HVO. This study evaluates droplet combustion with an initial diameter of 250 μm in a drop tube furnace under different furnace temperatures (600, 800, 1000 °C). A high magnification lens coupled with a high-speed camera provides qualitative and quantitative data regarding droplet size evolution and micro-explosions. Pure HVO and Jet A-1 were also tested for comparison purposes. The results reveal that the addition of aluminum particles enhances the alternative jet fuel combustion. Furthermore, decreasing the particle size and increasing the furnace temperature enhances the burning rate compared to the pure HVO. Pure HVO presents a burning rate nearly to 1.75 mm2/s until t/D02 = 0.35 s/mm2 at T = 1000 °C. When nanoparticles are added to HVO in a particle concentration of 0.5 wt.%, an improvement of 24% in burning rate is noticed. Conventional jet fuel and pure HVO do not present any disruptive burning phenomena. However, when aluminum particles were added to HVO, micro-explosions were detected at the end of droplet lifetime, regardless of the particle size.

Combustion characteristics of a single droplet of hydroprocessed vegetable oil blended with aluminum nanoparticles in a drop tube furnace

This study examines the burning characteristics and disruptive burning phenomena of single droplets of aluminum nanoparticles (n-Al) stably suspended in a biofuel (HVO). The biofuel used in the present work is a promising alternative fuel already tested in the aviation sector to reduce greenhouse gas and pollutant emissions. Experiments were conducted with two particle sizes (40nm and 70nm) and two particle concentrations (0.5 wt.% and 1.0 wt.%) to study its influence when added to the biofuel. The effect of size and concentration of the aluminum nanoparticles was studied at 1100 °C in a drop tube furnace. This experimental facility allows the study of combustion characteristics of falling droplets, ensuring there is no influence of the supporting fiber on the burning rate and disruptive burning phenomena occurrence. A CMOS high - speed camera coupled with a high magnification lens was used to evaluate the droplet size, burning rate, and micro-explosions. Based on this procedure, pure biofuel droplets were compared with those of biofuel blended with nanoparticles. The results suggest that the combustion characteristics of pure HVO can be enhanced with the addition of aluminum nanoparticles. Furthermore, by decreasing the particle size, a slight increase in the burning rate of nanofuels was noticed. Additionally, an increase in the particle concentration leads to a pronounced increase in the burning rate. The particle concentration also influences the delay and intensity of micro-explosions, disruptive burning phenomena detected at the end of the droplet lifetime.

Ignition, puffing and sooting characteristics of kerosene droplet combustion under sub-atmospheric pressure

The ignition, puffing and sooting characteristics of Chinese RP-3 kerosene droplet burning have been studied using high-speed, OH* chemiluminescence and soot thermal radiation imaging. The experiments were conducted in air at standard temperature and sub-atmospheric pressures ranging from 0.2 bar to 1 bar. The kerosene droplet was supported by a thermocouple tip and ignited by a retractable coiled heating wire. The results showed that the ignition delay time increased with a decrease of the ambient pressure, due to an increased distance between kerosene and oxygen molecules. Steady burning and disruptive burning were identified following the ignition. OH* chemiluminescence images showed a spherical flame and a longer flame standoff distance under a lower pressure. The puffing intensity was observed to be enhanced with a reduction of the ambient pressure, and a decreased pressure was found to lower the sooting emission.

Effect of crystallinity on droplet regression and disruptive burning characteristics of nanofuel droplets containing amorphous and crystalline boron nanoparticles

The present investigation deals with the droplet combustion characteristics of nanofuel droplets containing amorphous and crystalline boron nanoparticles at various particle loadings (0.25%, 1%, 2.5%, 5%, 7.5%, and 10% by weight). Characterization of pre-burnt particles in terms of particle size, morphology, and elemental boron content have been carried out using standard material characterization techniques such as SEM, TEM, XRD and TGA. The droplet burning process has been recorded using a high-speed imaging system. The diameter regression profiles show distinctly different characteristics for amorphous and crystalline particles loaded droplets. Amorphous particles loaded droplets show comparatively smooth regression with minor puffing coupled with shape oscillations at the early stage and micro-explosions at the later stage whereas the crystalline particles loaded droplets show sudden ejections and high-intensity micro-explosions. The morphology of the particle (crystallinity) is considered to be responsible for this difference in burning behaviour. A porous, permeable agglomerate shell forms in case of amorphous boron loaded droplet whereas a densely packed, impermeable agglomerate shell forms in case of crystalline boron loaded droplets during the early stage of burning. The micrographs of post-burning residues indeed reveal that blow holes are present in the agglomerate even at individual particle level for amorphous boron loaded case whereas there are no such blow holes present in crystalline boron loaded case. Thermograms, true colour images of flame and emission spectra show that the amorphous boron particles burn better than their crystalline counterpart.

Reaction Dynamics of Rocket Propellant with Magnesium Oxide Nanoparticles

The combustion behavior of rocket propellant grade 2 (RP-2) was investigated as a function of magnesium oxide (MgO) nanoparticles (i.e., 20 nm diameter) added at varied concentrations. The MgO nanoparticles were surface-treated with a long-chain carboxylic acid to aid their dispersion in RP-2. The fuel droplet regression rate, surface tension, and heat of combustion of RP-2 with MgO nanoparticle additives were measured to characterize combustion behavior. Heat of combustion and surface tension measurements varied negligibly among all samples indicating that calorific output and surface tension are not controlling parameters influencing fuel combustion behavior. However, fuel droplet regression rates were considerably increased by adding 0.5 wt % MgO from 0.225 to 66.16 mm/s, which is an improvement by 2 orders of magnitude. Further analysis showed that MgO particles enhance diffusive heat transfer, which promotes nucleation and disruptive burning throughout the three stages of regression, heating/evaporat...

Autoignition and combustion characteristics of heptane droplets with the addition of aluminium nanoparticles at elevated temperatures

We investigate the effects of high ambient temperatures and various concentrations of nanoparticles (NPs) on the autoignition and combustion characteristics of heptane-based nanofluid droplets. A single, heptane (n-C7H16) droplet containing 0.5%, 2.5%, or 5.0% by mass of aluminium (Al) NPs mounted on a silicon carbide fibre was exposed to a rapid increase in temperature (from room temperature to temperatures in the range 600–850°C) at atmospheric pressure and under normal gravity, and the autoignition and combustion characteristics were observed. The ignition delay, burn rate, and combustion characteristics of pure and stabilised heptane droplets were also examined for comparison. The results show that, as with the pure heptane droplets, the ignition delay of the NP-laden heptane droplets (n-Al/heptane) followed an Arrhenius temperature dependence. The reduction in the ignition delay time with increasing temperature depended on the loading of NPs in the droplet. The overall activation energy obtained for dilute NP concentrations (0.5% by mass) was smaller, and that for dense (2.5% and 5.0%) concentrations of NPs was larger, than that of the pure heptane droplets. Consequently, the addition of 0.5% Al NPs to heptane resulted in ignition at 600°C, which is not observed with stabilised heptane or 2.5% and 5.0% n-Al/heptane droplets. At higher temperatures, the ignition delay of the n-Al/heptane droplets was comparable to that of pure heptane droplets. The combustion of the n-Al/heptane droplets did not follow the classical d2-law, in contrast to the combustion of the pure and stabilised heptane droplets. Regardless of the NP concentration, the n-Al/heptane droplets exhibited disruptive burning behaviour, which was characterised by multiple expansions and ruptures or ‘microexplosions’. During these microexplosions, the NPs were ejected from the droplets, and the intensity of the microexplosions increased with increasing temperature; consequently, the burning time and total combustion time of the droplet was reduced. Due to these intense and frequent microexplosions, almost no residue from the Al NPs remained on the fibre following combustion, and a separate Al flame was not observed. The average gasification (burn) rate of the n-Al/heptane droplets remained equal to that of pure heptane droplets at relatively low temperatures (600–700°C); however, at higher temperatures (750–850°C), it was significantly faster than that of the pure heptane droplets.

Disruptive burning of precursor/solvent droplets in flame‐spray synthesis of nanoparticles

Flame spray pyrolysis is an established technique for synthesizing nanoparticles in the gas phase through aerosol combustion of precursor/solvent droplets. The combustion characteristics of isolated micron‐sized precursor/solvent droplets are investigated experimentally. Pure solvent droplets burn uniformly and classically quasisteady, whereas precursor/solvent droplets manifest disruptive combustion behavior. The fast onset of droplet disruption, which occurs only for solutions with dissolved metal precursors, is not due to solid‐particle precipitation within the droplet. Instead, the mechanism of disruptive droplet burning is similar to that of slurry droplets, consisting of three main steps: (1) diffusion‐controlled burning of the high‐volatile solvent, (2) viscous‐shell formation due to decomposition of the low‐volatile metal precursor, and (3) subsequent disruption due to heterogeneous nucleation. The time sequence of the three steps depends on the concentration and decomposition characteristics of the metal precursor, shortening with increased concentration and higher incremental decomposition temperature. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4553–4566, 2013

Combustion of liquid fuels floating on water

The research presented consists of a study of the burning characteristics of a liquid fuel floating on water with emphasis in the phenomena known as boilover. The problem is of technical interest in the petro-chemical industry, particularly from the point of view of pollution and fires resulting from accidental liquid fuel spills in open water. Testing with multicomponent fuels gives information's about events that can occur in a practical situation, while testing with single component fuels permits obtaining fundamental information about the problem. It evidences the major effects caused by the transfer of heat from the fuel to the water underneath. One of these effects is the disruptive burning of the fuel known as boilover, that is caused by the water boiling and splashing, and results in a sharp increase in burning rate and often in the explosive burning of the fuel. It is shown that this event is caused by the onset of water boiling nucleation at the fuel/water interface and that it occurs at an approximate constant temperature that is above the saturation temperature of the water (water is superheated). These measurements conducted in two laboratories, address the major issues of the process by analyzing the effect of the variation of the parameters of the problem (initial fuel-layer thickness, pool diameter, and fuel type), on the burning rate, time to start of boilover, pre-boilover mass ratio, and boilover intensity. Finally, two types of modeling are proposed to describe the heat transfer in fuel and water phases: one simple for practical purposes, the other, more elaborated and transient, taking particularly into consideration the radiation in depth.

THIN-LAYER BOILOVER: PREDICTION OF ITS ONSET AND INTENSITY

Although in the burning of liquid fuel floating on water the fuel burning itself is similar to that of the simple fuel, the presence of the water underneath introduces a number of effects that are caused by the transfer of heat from the fuel to the water. One of the main effect is the disruptive burning of the fuel known as boilover that is caused by the water boiling and splashing, which results in the explosive burning of the fuel. From a practical point of view, it appears that there are two important aspects of the problem, one is the onset of boilover, the second its intensity. In the present work, the liquid heating is analyzed as a semi-infinite conduction problem with a surface suddenly increased to a constant temperature (boiling point), the penetration of the thermal wave being a function of time (of the order of , where α is the thermal diffusivity of the liquid). This analysis provides predictions for the times at which the nucleation of the water starts (assumed to occur when the water/fuel interface reaches a constant level of superheat ≃120°C). They are in satisfactory agreement with the measurements. These measurements were conducted in two laboratories and address the major issues of the thin layer boilover process by analyzing the effect of the variation of the key parameters of the problem (fuel layer thickness, pool diameter and fuel type). Knowing that the fuel layer remaining before nucleation is closely related to the thickness of the superheated water layer (water that gasifies), it is shown that the boilover intensity can be estimated on the basis of the pre-boilover fuel mass ratio. The thicker these layers, the more intense is the boilover. Fuels used as support of the proposed analysis include single-component fuels and multicomponent fuels.

Distinctive combustion stages of single heavy oil droplet under microgravity ☆

This report presents an investigation on the combustion of single droplets comprised of heavy oil and oil mixtures blending diesel light oil (LO) and a heavy oil residue (HOR). The tests were conducted in a microgravity facility that offered 10 s of free-fall time. Fine wire thermocouples supported the droplets, resulting in a measurement of droplet temperature history. Additional data were the droplet and flame size history. The results identified four distinctive burning stages between ignition and extinction for heavy oil (C class) and HOR–LO blends. They are, in succession, the start-up, inner evaporation, thermal decomposition (pyrolysis) and polymerization stages. The start-up stage denoted an initial transient period, where the LO components burned from the droplet surface and the droplet temperature increased rapidly. The latter three stages featured pronounced droplet swellings and contractions caused by fuel evaporation and decomposition inside the droplet. An evaporation temperature demarcated the start-up stage from the inner evaporation stage, and this temperature corresponded to a plateau in the temperature history of the droplet. Two additional temperatures, termed the decomposition and polymerization temperatures, indicated the ends of the evaporation and decomposition stages. These temperatures were similarly identified by plateaus or inflection points in the time–temperature diagram. The evaporation temperature gradually decreased with increasing the initial LO mass fraction in the droplet, whereas the other two temperatures were almost independent of the oil composition. All three temperatures increased with decreasing initial droplet diameter, but the dependence was very slight. Based on the results, the combustion of heavy oil droplets appears to be dominated by a distillation-like vaporization mechanism, because of the rapid mass transport within the droplets caused by the disruptive burning.

An experimental and modeling study on stochastic aspects of microexplosion of binary-fuel droplets

Disruptive droplet burning is characterized by microexplosion occurrence during burning. The microexplosion phenomena of binary-fuel droplets are caused by internal boiling that is initiated by the homogeneous nucleation of bubbles in the liquid phase. Since homogeneous nucleation is a random process due to density fluctuation, some stochastic aspects appear in disruptive burning of the droplets. In order to investigate stochastic characteristics in disruptive burning, the microexplosion occurrence was studied experimentally and theoretically. In experiments, the burning droplet of an n -hexane/ n -hexadecanemixture was injected upward, and the induction time for microexplosion occurrence was obtained. Results show that microexplosion induction time is distributed over the quasi-steady vaporization period. The maximum frequency of microexplosion induction time and the occurrence probability of microexplosion increase with the initial droplet diameter. The microexplosion occurrence was modeled considering the homogeneous bubble nucleation rate. The present stochastic model well demonstrated the experimental results. The theory shows that these stochastic characteristics of microexplosion occurrence depend on the fifth power of the initial droplet diameter. The bubble nucleation leading to microexplosion depends on the timescale and the superheated liquid volume. The timescale in droplet combustion is proportional to the second power of the initial droplet diameter, and the volume of superheated liquid is proportional to the third power of the initial droplet diameter. For relatively large droplets, the microexplosion occurrence can be understood in a deterministic way. For smaller droplets, appearing in spray combustors, however, the microexplosion occurrence is more stochastic. The scatter in explosion behavior is also discussed considering the bubble nucleation location inside the droplet.

Spacelab and Drop-Tower Experiments on Combustion of Methanol/Dodecanol and Ethanol/Dodecanol Mixture Droplets in Reduced Gravity

This paper describes reduced-gravity experiments on the combustion of droplets composed of metha-nol/dodecanol and ethanol/dodecanol mixtures. The experiments used the NASA Glenn Research Center 2·2 s drop tower as well as facilities in Spacelab during the USML-2 flight. In the drop-tower experiments, initial droplet diameters ranged from 0·8 mm to 1.2 mm, initial dodecanol mass fractions, Y, (in both mixtures) were 0·1, 0·25 and 0·5, and the ambient gas mixture was either an 02/He mixture (with a molar O2 concentration of 0·5) or atmospheric air. Most drop-tower experiments were at 0·1 MPa ambient pressure, although two tests were at 0·04 MPa. In the Spacelab experiments, the methanol/dodecanol droplets burned in ambient shuttle air (0·21 oxygen mole fraction) at 0·1 MPa, the initial droplet sizes were in the range 4·2 mm to 5·4 mm, and the initial values of Y were 0·2 and 0·4. Burning rates varied with experimental conditions, and disruptive burning was very frequent. One of the largest methanol/dodecanol droplets extinguished in an oscillatory fashion. Data on flame contractions, in combination with existing theoretical analyses, were used to infer effective liquid species diffusivities, D, that apply to these experiments. The D values varied slowly with composition for 0·25 SYS 0·5. In this range, the theoretical values for liquid species diffusivities compare well with experimental values. The D values for Y < 0·25 varied rapidly with composition, and D values for Y = 0·1 were substantially smaller than for Y > 0·25. These variations in D are attributed to changes in liquid-liquid phase-equilibrium behaviors as liquid compositions change during combustion. Sooting was visually observed above threshold values of the initial dodecanol mass fraction (this threshold depended on the ambient conditions, and was different for methanol and ethanol), but only after flame contractions.

Aluminum droplet combustion in fluorine and mixed oxygen/fluorine containing environments

Experimental and numerical results are reported on the combustion of single aluminum (Al) droplets, 187 μm diameter, that were laser ignited in quiescent environments of SF 6 and SF 6 /O 2 or Ar mixtures. Combustion measurements consisted of luminosity versus time traces, photographic observations, and electron probe microanalysis (EPMA) and electron microscope inspection of condensed-phase products of combustion surrounding burn-intercepted droplets. Planar laser-induced fluorescence (PLIF) was employed to image the gaseous intermediated reaction species ALO during droplet combustion in SF 6 /O 2 mixtures. Aluminum droplets burning in pure SF 6 burned without condensed-phase products in the surrounding envelope flames. Accordingly, no condensed-phase products were found on surfaces of droplets captured by quenching into an inert environment. In SF 6 , classical d 2 law burning behavior was observed. Droplets burned in SF 6 /Ar mixtures exhibited similar behavior, with diminished burning rates. Disruptive burning was not observed for pure SF 6 or any of the SF 6 /Ar mixtures. Condensed-phase deposits on quench plates intercepting burning droplets showed the surrouunding gaseous flame structure to consist of two zones for burning in SF 6 and three zones in SF 6 /O 2 mixtures. For each system, near the surface where the gas phase was aluminum rich, significant amounts of sulfur were found in the condensed products. PLIF showed the species profile of AlO to shift closer to the surface with increasing fluorine content in SF 6 /O 2 mixtures. Experimental results were compared with a local equilibrium combustion model. The model predicted burn rates within 20% and qualitatively predicted radial profile speciation. As the O/F ratio was increased, the final equilibrium products were predicted to shift from AlF 3 to AlF 2 O to Al 2 O 3 . The formation of AlF 2 O reduced condensed-phase products at the reaction temperature for a wide range of O/F ratios.

Combustion of unsupported water-in- n-heptane emulsion droplets in a convection-free environment

Results of an experimental study are reported for combustion of unsupported n-heptane droplets emulsified with water burning while levitated in microgravity where the effects of external convection are negligible. The results are compared to a pure heptane droplet and a mixture of two miscible liquids (methanol/dodecanol) burning under similar low convection conditions. The effect of water concentration on flame luminosity and sooting tendency, preferential vaporization of the emulsion components, and the role of the emulsifying agent in promoting disruptive burning were examined. The photographs showed less sooting for the emulsions relative to pure heptane, and a preferential vaporization process was revealed from the image analysis by an abrupt change in the vaporization rate. A period exists near the end of burning during which the droplet diameter is nearly constant, followed by a disruptive burning event. A quasi-steady complex chemistry analysis with variable properties shows that a frozen evaporation mode occurs after an initial delay period during which heptane is evaporating from the droplet. Predicted and measured burning rates are in good agreement during this later period.

Pyrolysis product absorption by burning benzene droplets

By burning droplets of benzene in a single-droplet combustor and performing phase-discriminating sampling of the liquid and gas phases of the droplet system, we have found that gas-phase pyrolysis products arise in the liquid phase of the droplet. The experiments are conducted at 1000 K and 21 mol % O 2 in the postcombustion gas from an oxygen-rich premixed methane flame. Disruptive burning, which has not previously been reported for a pure hydrocarbon in normal gravity conditions, is observed at the end of the droplet residence time (∼92 ms). Samples of the liquid phase have been taken at various times throughout the combustion lifetime and analyzed by high-pressure liquid chromatography. Compositional analysis using ultraviolet-visible absorbance spectra of the separated components of the samples reveals a wide variety of pure polycyclic aromatic hydrocarbons (PAH), substituted PAH, and cyclopenta-fused PAH. In addition, recent synthesis of new reference standards has enabled identification of cyclopenta-fused PAH—cyclopent[ hi ]acephenanthrylene, cyclopenta[ cd ]fluoranthene, and dicyclopenta[ cd, jk ]pyrene—which have never before been identified as benzene products. Because the droplet remains relatively cold (∼350 K) with respect to the gas phase in the oxygen-deficient zone between the droplet and the flame (∼2000 K), we conclude that these compounds are gas-phase pyrolysis products that are obsorbed into the droplet, rather than products of reactions within the droplet. These heavier species may play a role in observed terminal disruptive burning events by acting as additional droplet components that promote multicomponent effects. Analysis of species concentrations over time reveals the dominance of both ring rupture pyrolysis products such as phenylacetylene, triacetylene, and acenaphthylene, and biaryl pyrolysis products such as biphenyl. These four products in particular represent 70% of the identified mass of absorbed pyrolysis products, which accounts for up to 5% of the droplet mass at the end of its lifetime.