Elsevier

Combustion and Flame

Volume 226, April 2021, Pages 182-189
Combustion and Flame

Effect on combustion of oxide coating formed on aluminum nanoparticles burned in steam

https://doi.org/10.1016/j.combustflame.2020.11.040Get rights and content

Abstract

A model of combustion of aluminum nanopowder in water vapor has been analyzed with allowance for the formation of a condensed phase of aluminum oxide on aluminum particles. Various processes affecting the growth rate of the oxide coating have been considered including adsorption and surface diffusion of Al2O3 molecules on aluminum particles, and condensation of the molecules on the oxide coating. Model calculations have yielded the time profiles of system temperature, phase and fractional composition, as well as the shape of oxide coated aluminum particles. The heterogeneous processes forming the condensed phase of aluminum oxide on Al particles have shown to increase heat generation in the system. On the other hand, shielding aluminum particles by oxide coating retards aluminum evaporation. The competition of these processes during the formation of oxide coating is one of the factors affecting the combustion of aluminum nanopowder in water vapor.

Introduction

Metal powders (boron, magnesium, aluminum, etc.) have been used to advantage as ingredients and main components of rocket engine fuels [1,2]. Recent years have seen significant growth of interest in the combustion of nanoscale aluminum particles [3,4], for this process is assumed to give rise to new promising technologies [2,[5], [6], [7]–8]. Unlike micron-sized particles, whose combustion is controlled by diffusion processes, the combustion rate of nanoscale particles is determined on the basis of the kinetic approach [9], [10], [11]–12]. Combustion of nanoscale particles proceeds at lower melting and ignition temperatures, which makes it possible to accelerate the combustion process in comparison with the combustion of micron-sized particles [13].

Storozhev and Yermakov [14] proposed a model of high-temperature combustion of an ensemble of aluminum nanoparticles in water vapor. We refer to the process as high-temperature combustion when the system temperature is higher or around the melting point of aluminum oxide. At these temperatures, the evaporation of aluminum plays a significant role in the combustion. Schematically, the model can be described as follows. At time t = 0, a preheated mixture of water vapor and monodispersed non-oxidized aluminum aerosol particles is introduced into the system. Evaporation of aluminum particles leads to gas-phase chemical reactions involving aluminum atoms and other components, including aluminum suboxides. This results in the formation of gaseous aluminum oxide and its transition to the condensed phase in the form of small aerosol particles (c-phase). The formation of the c-phase includes the processes of nucleation (formation of condensed phase nuclei), growth of aerosol particles Al2O3(c) due to condensation of Al2O3(g) and coagulation of particles. Here and hereafter, the subscript c denotes the condensed phase, and the subscript g denotes the gas phase. This model was further improved by the inclusion of heterogeneous reactions on the surface of c-phase particles proceeding with the participation of gaseous aluminum suboxides and atomic oxygen [15]. It should be noted that the formation of the c-phase plays an important role in the combustion process, since it is accompanied by a significant part of the total heat release (about 50%).

The following statements are assumed in the model:

  • a)

    The process is adiabatic and occurs under constant pressure (1 atm);

  • b)

    Evaporation from the surface of aluminum aerosol particles and condensation on the aluminum oxide particles occurs in the kinetic regime (Knudsen number Kn = Lav/R >> 1, where R is the radius of the particles and Lav is the mean path length of free motion of gas molecules);

  • c)

    The temperature distribution and concentration of components in the system is uniform at each time; no heat and mass transfer occurs between the elements of the system;

  • d)

    Heterogeneous chemical reactions on the aluminum particles surface are not taken into account in the model.

The combustion process of an ensemble of aluminum nanoparticles differs significantly from the combustion of a single micron particle. In the second case, the phenomena of heat and mass transfer are largely determined by high gradients of temperature and concentrations of components, and mass transfer is controlled by diffusion.

Numerical analysis carried out with this model showed that the formation of Al2O3 begins before the evaporation of Al particles has completed. This alumina can produce an oxide coating on Al particles during combustion and slow down the Al evaporation process. This effect plays a significant role in the combustion of micron-size particles [16]. The role of heterogeneous chemical reactions in the combustion of micron-sized aluminum particles was reported by Glorian at al. [17] and [18].

The formation of oxide coating on Al particles produces a multidirectional effect. On one hand, it can slow down the evaporation from the Al surface. On the other hand, the additional channel of c-phase formation increases heat release during combustion. In this paper we intend to evaluate the impact of this oxide coating process. For this purpose we augmented our model [14,15] with a block of equations describing the growth and shape of oxide coating on aluminum nanoparticles. Calculations based on the extended model have allowed us to estimate the influence of oxide coating formation on combustion process.

Section snippets

Shape of oxide coating on aluminum particles

Free surface energy plays a significant role in the energy balance of small particles, and this role increases in inverse proportion to particle size. It is known that aluminum oxide can form “caps” on the surface of aluminum particles even on micron-size particles (see, e.g., Beckstead [19]). The oxide cap formed on micron Al particles as a result of combustion was observed in various experimental images of scanning microscopes (see, e.g., Dreizin [20]). The probability of this phenomenon is

Growth of oxide cap

Growth of oxide caps on Al particles may proceed in two ways: (1) condensation of Al2O3 gas molecules on the cap surface and formation of aluminum oxide c-phase due to heterogeneous reactions involving suboxides of aluminum on cap surface; (2) adsorption of Al2O3 gas molecules on Al surface and surface diffusion of Al2O3 to the border of the oxide cap. The first way of aluminum oxide c-phase formation has been studied in detail in our recent paper [15]. In this paper, we also consider the

Results and discussion

The numerical calculations of this work were performed based on the earlier developed model of high-temperature combustion of aluminum nanoparticles in water vapor [14,15] supplemented by Eqs. (1)–(13) describing the process of oxide shell formation on aluminum particles. It should be noted that some parameters (for example, interfacial surface energy and diffusion coefficient) are not available in the literature. Therefore, we varied these parameters within certain limits with the purpose to

Conclusion

The paper describes a model of the dynamics of combustion of aluminum nanopowder in water vapor, which takes into account the formation of c-phase (oxide cap) on the aluminum particles. The dynamics of cap formation was modeled with allowance for the minimization of system's free energy. This approach took into account the processes of cap growth due to condensation of Al2O3(g) gas molecules directly on the cap, as well as their adsorption on the surface of aluminum particles with subsequent

Declaration of Competing Interest

None.

Acknowledgments

Financial support from the State jobs V.L. Talrose Institute for Energy Problems of Chemical Physics, N.N. Semenov Federal Research Center of Chemical Physics, Russian Academy of Sciences is gratefully acknowledged.

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