Extended combustion model for single boron particles – Part I: Theory

Abstract

Ramjet engines have significant advantages when compared to conventional rocket motors concerning specific impulse, manoeuvrability, and range. Boron particle addition to the propellant of ducted rockets further increases this potential due to a very high heating value. However, the combustion of boron particles is a very complex process because of an inhibiting oxide layer covering the particles. This layer has to be removed before vigorous combustion can start. The boron particle combustion process runs in two distinct stages. In the literature review presented in this article two combustion models for single boron particles are outstanding. A very detailed model by the Princeton/Aerodyne group features hundreds of elementary reactions and considers all physical processes in the particle environment. It is very elaborate and, thus, not suitable for incorporation into three-dimensional CFD-calculations at present. The second model developed at Penn State University takes on a global approach with only a few reactions which makes it promising for CFD applications. A careful analysis of this model revealed some inconsistencies, errors and drawbacks which gave rise to the new model presented in this paper. The new model comprises a consistent formulation of the heat and mass transfer processes in the particle environment based on a quasi-steady approach, accounts for boron evaporation which is a relevant process despite the high boiling point of boron, and it considers the influence of forced convection on the particle conversion. The chemical reaction rates adopted from the original model were revised and are slightly changed, the differential equations to be solved are corrected and an iterative solution algorithm is introduced. A careful validation of the model is presented in Part II of this paper showing that the new model is suitable for boron particle sizes relevant for ramjet combustion chambers.