Effect of Particle Size on Boron Combustion in Air
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Effect of Particle Size on Boron Combustion in Air A. P. Shparaa , D. A. Yagodnikova , and A. V. Sukhova
UDC 536.46
Published in Fizika Goreniya i Vzryva, Vol. 56, No. 4, pp. 112–120, July–August, 2020. Original article submitted December 3, 2018; revision submitted November 11, 2019; accepted for publication February 19, 2020.
Abstract: Mathematical modeling of combustion of micron-sized and nanometer boron particles in air with allowance for changes in the heat and mass transfer mechanism and a decreasing particle size is carried out. The Knudsen number is taken as an indicator of a transition from one state to another: a continuous medium assumption is valid for describing the heat and mass transfer mechanisms in the case where Kn < 0.01, the free molecular regime takes place for Kn > 10, and a transient state occurs for 0.01 < Kn < 10. Particle sizes at which different heat and mass transfer regimes occur with respect to boron combustion in air at pressures of 0.1–4 MPa are estimated. The time it takes for boron particles to combust within the framework of the assumption of a continuous medium and in a free molecular mode is determined. It is shown that calculation models for determining the burning time of boron particles with initial sizes close to micron-sized and nanodispersed ones should take into account a change in the heat and mass transfer mechanism with variation in the current particle radius during burnout. Keywords: boron, particle, combustion, modeling, micron size and nanosize. DOI: 10.1134/S0010508220040115 INTRODUCTION Boron has been considered as a highly efficient fuel for various types of jet and rocket engines for a long time. However, the main problem with its use in this role is that boron ignites as poorly as it burns. To date, a large number of theoretical and experimental works devoted to improving the flammability and combustion efficiency of boron have been carried out, and the results of these studies and the estimation of the possibility of their practical use are presented in [1]. An important conclusion follows from the analysis of the literature: the problem of boron combustion with a required combustion efficiency can be solved with neither mathematical modeling using more and more complex physicochemical models (see, e.g., [2, 3]) nor constructive and technological methods, such as organizing various schemes for boron combustion, adding particles of various flammable additives into the composition, particle surface modification, etc. a
Currently, the most promising direction for solving the problem of efficient combustion of boron is to reduce its particle size to a minimum possible size from the standpoint of manufacturing technology, down to a nanoscale. In this regard, it is important to develop computational models for particle combustion with sizes lying in the nanorange and near the lower bound of the microrange. The question is that most of the known physical models and the calculation methods created on their basis are rooted in the assumption of a continuous medium within which
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