Elsevier

Combustion and Flame

Volume 158, Issue 2, February 2011, Pages 354-368
Combustion and Flame

Combustion characteristics of fuel droplets with addition of nano and micron-sized aluminum particles

https://doi.org/10.1016/j.combustflame.2010.09.005Get rights and content

Abstract

The burning characteristics of fuel droplets containing nano and micron-sized aluminum particles were investigated. Particle size, surfactant concentration, and the type of base fluid were varied. In general, nanosuspensions can last much longer than micron suspensions, and ethanol-based fuels were found to achieve much better suspension than n-decane-based fuels. Five distinctive stages (preheating and ignition, classical combustion, microexplosion, surfactant flame, and aluminum droplet flame) were identified for an n-decane/nano-Al droplet, while only the first three stages occurred for an n-decane/micron-Al droplet. For the same solid loading rate and surfactant concentration, the disruption and microexplosion behavior of the micron suspension occurred later with much stronger intensity. The intense droplet fragmentation was accompanied by shell rupture, which caused a massive explosion of particles, and most of them were burned during this event. On the contrary, for the nanosuspension, combustion of the large agglomerate at the later stage requires a longer time and is less complete because of formation of an oxide shell on the surface. This difference is mainly due to the different structure and characteristics of particle agglomerates formed during the early stage, which is a spherical, porous, and more-uniformly distributed aggregate for the nanosuspension, but it is a densely packed and impermeable shell for the micron suspension. A theoretical analysis was then conducted to understand the effect of particle size on particle collision mechanism and aggregation rate. The results show that for nanosuspensions, particle collision and aggregation are dominated by the random Brownian motion. For micron suspensions, however, they are dominated by fluid motion such as droplet surface regression, droplet expansion resulting from bubble formation, and internal circulation. And the Brownian motion is the least important. This theoretical analysis explains the different characteristics of the particle agglomerates, which are responsible for the different microexplosion behaviors that were observed in the experiments.

Introduction

Metals such as aluminum have higher combustion energies and have been employed as energetic additives in propellants and explosives [1]. Recent advances in nanoscience and nanotechnology enable production, control, and characterization of nanoscale energetic materials, which have shown tremendous advantages over micron-sized materials. Because of the high specific surface area, metal nanoparticles offer shortened ignition delays, decreased burn times, and more complete combustion than micron-sized particles [1], [2], [3].

Using nanoscale energetic materials as fuel additives to enhance combustion of traditional liquid fuels is an interesting concept. The high energy density of metals, particularly aluminum, could significantly improve power output of engines and thus reduce consumption of liquid fuels and consequently result in less CO2, NOx, etc. In addition to higher energy density, fuel additives have potentials to shorten ignition delay time and enhance fuel oxidation by catalytic effect. Studies on ignition and combustion behavior of liquid fuels with nanoscale additives, however, are rare. Tyagi et al. [4] used a simple hot-plate experiment to study the effects on the ignition properties of diesel fuel when small quantities of aluminum and aluminum oxide nanoparticles were added. It was observed that the ignition probability for the fuel mixtures containing nanoparticles was significantly higher than that of pure diesel. Beloni et al. [5] recently studied combustion of decane-based slurries with metallic nano additives using a lifted laminar flame burner, considering pure aluminum, mechanically alloyed Al0.7Li0.3, and nanocomposite 2B + Ti as additives. Their effects on flame length, flame speed, flame emissions, and temperatures were measured. These studies, though limited, have shown promise of using nanoscale additives to enhance the combustion of liquid fuels.

Slurry fuels, which are mixtures of liquid and solid fuels, were under serious consideration as high-energy fuels a few decades ago. Aluminum, boron, and carbon particles (5–200 μm) were added to liquid fuels as a “liquid fuel extender” in the sense that less hydrocarbon and more plentiful solid fuel (e.g., coal) can be used [6]. The burning characteristics of slurry droplets involving micron-sized boron [7], [8], [9], [10], aluminum [11], [12], [13], carbon, and a blend of aluminum and carbon [14], [15], [16], [17] particles at relatively high solid loadings (40–80 wt.%) were studied experimentally. A few theories and postulates were proposed based on the experimental observations [10], [18], [19], and a review was provided by Choudhury [6]. In general, during the initial phase of oxidation of the droplet, semiporous hollow shells or densely packaged shells, consisting of the particle agglomerates, may form and thereby cause an increase in diameter as a result of swelling. Solid particles, heated by flame radiation and/or exothermic reaction, may initiate local boiling or liquid phase decomposition at the surface. Several possible events can take place, either individually or jointly, during slurry droplet combustion. A key event is the disruption/microexplosion behavior, which was first discovered by Takahashi et al. [8] for slurries of boron/JP-10 droplets. Two boron samples, amorphous (0.20–0.32 μm) and crystalline (3.57 μm), were used in the experiment. This study demonstrated that disruption of the primary droplet results in secondary atomization, which substantially enhances the overall burning rate of the primary droplet and provides a means for dispersal and ignition of the boron. This behavior was also evidenced by a few other studies involving aluminum and carbon slurries [13], [17].

These previous studies have revealed some general burning characteristics of slurry fuels involving micron-sized particles. However, many puzzles remain. The events that can take place either individually or jointly during slurry droplet combustion have not been understood clearly [6] because of the complexity of the three-phase physics. Furthermore, other issues have not yet been fully studied, e.g., slurry preparation, slurry rheology, effects of surfactants on suspension quality and on combustion behavior, reaction kinetics and mechanisms of metal particles/agglomerates, and particle agglomeration mechanisms within a combusting droplet. Significantly, no studies have been made of slurry droplets involving nanoparticles, which could be different because of energy conversion and particle dynamics at different length scales, nano vs. micron.

The idea of the present paper is to suspend metallic nanoparticles in liquid fuels and to explore the differences between nanosuspensions and micron suspensions that have been studied previously. Nanoparticles have shown such advantages as higher reactivity and burning rate over micron-sized particles. But more important, nanoparticles are much easier to disperse and suspend in liquid fuels than microparticles are, the latter tending to settle quickly as a result of gravity. This is because nanoparticles have an extremely high ratio of surface area to volume; thus the interaction between particle surface and the surrounding liquid is strong enough to overcome difference in density. Moreover, the larger surface area of nanoparticles can be utilized for surface functionalization, making stabilized suspension possible to maintain for a very long time in practical applications. Also, ionic groups in liquid fuel can be absorbed onto particle surfaces to form a charged layer, which results in repulsive forces. These forces between nanoparticles increase as a result of the larger specific surface areas of nanoparticles, and this may reduce agglomeration to some extent [20]. The dispersion and suspension of metal nanoparticles in liquids have been critical issues in nanofluids research, which uses metal nanoparticles to enhance thermal conductivity of liquids for better cooling of micro- and nano-electromechanical systems (MEMS or NEMS), power electronics, light-emitting diodes, and semiconductor lasers [21].

The objectives of the present study are: (1) to investigate dispersion and suspension of nanoparticles in various liquid fuels and (2) to explore the difference between nanosuspension (<100 nm) and micron suspension (1–200 μm), especially the effect of particle size on droplet burning characteristics. The paper starts with fuel formulation methods, including particle dispersion, deagglomeration and fuel characterization. The droplet combustion experiment and diagnostic methods are then described. Several distinctive combustion stages are identified for the general burning behavior of a nanosuspension. The effects of surfactant and base fluid on suspension quality and droplet burning characteristics are discussed. In particular, the burning behavior of nanosuspensions was compared to that of micron suspensions. The results show that the characteristics and structures of particle agglomerates formed during droplet evaporation and combustion are responsible for the different burning behaviors. Lastly, a theoretical analysis is conducted to provide understanding of particle collision mechanisms and aggregate rates within a droplet. This analysis further reveals the effect of particle size on agglomeration formation and burning, which is consistent with the present experimental observations.

Section snippets

Fuel preparation and characterization

The preparation of fuel mixtures, a key step in developing knowledge of the nanofluid-type fuels, does not simply mean to disperse particles in liquids. Special handling is needed to achieve homogeneous, stable, long-term suspension and a low level of particle agglomeration. Many studies have shown that sonication and the adding of surfactants can reduce the coagulation of nanoparticles in nanofluids. The theory of ultrasonic-induced cavitation in liquids is well known [22]. When a liquid is

Distinctive combustion stages for nanosuspension

Figure 3 shows the burning sequence of a stabilized n-decane/nano-Al droplet. The particle concentration is 10 wt.%, and the surfactant concentration is 2.5 wt.%. The droplet size and temperature histories are shown in Fig. 4. Because of interference with the fiber, the droplet actually looks more elliptical than spherical. Thus we used a characteristic diameter D, which was defined as [7]D=(Dh2Dv)13where Dh and Dv are the horizontal and vertical diameters of the droplet, respectively. The

Conclusions

The burning characteristics of n-decane and ethanol droplets containing nano and micron-sized aluminum particles were investigated. The emphasis was to explore the difference between nanosuspensions and micron suspensions and to understand the effect of particle size on suspension quality and droplet burning characteristics. Other factors – such as the effect of particle concentration, the type of metal particles with or without surface functionalization, and the type of the

Acknowledgment

This work has been supported by the Army Research Office under contract W911NF-10-0133 with Dr. Ralph Anthenien as technical monitor.

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