Microexplosion of aluminum slurry droplets

Abstract

The microexplosion of a slurry droplet is experimentally and theoretically investigated. The microexplosion was considered to be caused by the shell formation and the following pressure build-up in the shell which would be promoted by the suppression of evaporation, subsequent superheating and heterogeneous nucleation of a liquid carrier. Experimentally, the microexplosion phenomena was examined for various surfactant concentrations and particle loading under dierent ambient temperature ranges (500±900 K). To closely investigate the pressure buildup and the heterogeneous nucleation, a numerical model was introduced by considering the three stages such as the shell formation, suppression of evaporation and pressure build-up inside. The microexplosion time was estimated by postulating the limit of superheat for heterogeneous nucleation. The simulation yielded a reasonably good agreement with experimental results for Al/n-heptane slurry droplets under various solid loadings (10±40 wt.%). #

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

“…(3) Finally, as the viscous shell inhibits vaporization, the flame moves closer to the surface (contraction), heating up the shell quickly beyond the heterogeneous nucleation temperature of xylene and 2-EH acid, initiating disruptions. Moreover, disruptions are only observed if Sn(II)2-EH is present, coinciding with the observations of Wong et al 73,74 and Byun et al 75 In addition, the disruption may also be initiated by soot or nanoparticles, which are formed in the flame and transported back to the droplet by thermophoresis as proposed by Shaw et al 31 Very small particles (d p 200 nm ) from the gas phase can get close to the droplet surface, as was shown by Ben-Dor et al 76 The probability of such an effect may increase as the flame contracts due to a decreasing vaporization rate ( Figure S1 of Supporting Information), thus reducing the force (i.e., drag force due to the Stefan flow) counteracting thermophoresis. Entrapped ambient gas ( Figure 9) may also draw particles into the liquid, where they can act as nuclei.…”

“…Moreover, Wong et al 73,74 showed that only the addition of low-volatile surfactants in slurry droplets suppressed evaporation, superheating, and heterogeneous nucleation of the carrier fuel. Likewise, Byun et al, 75 observed a decrease in the vaporization rate and droplet disruption only when surfactant was present in the slurry droplet. These findings are similar to the aforementioned observations of the Sn(II)2-EH and xylene/2-EH acid solution solutions, for example, (1) Sn(II)2-EH solutions burn initially without perturbation (following the d 2 -law), but the vaporization rate decreases as the concentration of Sn(II)2-EH increases.…”

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

“…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].…”

“…This stage is called ''surfactant flame" because we believe it is due to combustion of the surfactant or its pyrolysis products. None of the previous studies of slurry fuels involving micron-sized particles [6][7][8][9][10][11][12][13][14][15][16][17][18][19] has ever reported an observation of this flame. This stage occurred for all fuel mixtures with surfactant, regardless of the type of base fluid, particle size, or particle concentration.…”

“…Takahashi et al [8] proposed a three-step mechanism based on observations of burning JP-10/boron slurry droplets, which includes the d 2 -law combustion, shell formation, and disruption stages. Byun et al [13] proposed a stage in addition to those of Takiyuki's, called the pressure buildup stage. Two mechanisms to cause pressure build-up in droplet interiors were analyzed.…”