Combustion modelling of solid propellants

Resumen

Resumen de la tesis: Among all energetic materials, the combustion of granulated, composite AP/HTPB and double-base solid propellants are studied in thesis. The improvement on the design and safety in industrial processes which use solid propellants are of major importance in industry of energetic materials. Unexpected detonation of granular solid propellants is a key safety issue. Moreover, ignition and subsequent combustion of granular solid propellants are also complex mechanisms of high importance in the manufacturing process of many energetic materials, not only because already mentioned safety issues, but also in due in the design of propulsion systems. Therefore, and in order to properly understand this problem, a model for the characterisation of the detonation process of granular solid propellants under shock tube conditions is developed. To be able to reach this objective, a two-phase model, which considers the conservation equations of mass, momentum and energy and constitutive relations of mass generation, gas-solid particle interaction, interface heat transfer and particle-particle stress is defined. The study performed in this thesis, improve the understanding and modelling of the deflagration-to-denotation (DDT) phenomenon in granular beds of solid propellants. Two different models have been used to calculate pressure, temperature and porosity distributions. The first model considered a modification of the particle momentum governing equation in order to prevent the porosity from reaching values below the minimum value of compaction defined for packed beds of spherical particles. The second model studied does not consider the porosity limiter in the momentum conservation equation but represents the limitation of the porosity directly in the code by preventing the porosity to reach values below a minimum. In addition, the results are obtained by employing several numerical schemes and are compared against those available in the bibliographical sources in order to assess their effectivity to predict the early stages of this transient combustion process. The results show that developed second model accurately represents the physical behaviour of the propellant combustion for all variables of interest becoming a predictive tool for the characterisation of granulated solid propellants. Modelling the combustion of composite propellants and double-base propellants is a key problem that has focused the interest of several researches in many industrial fields such as chemical engineering, aerospace engineering or safety in industrial processes. Regarding safety, not only gun tubes, but also rocket motors experiment pressure build-up which could increase the chamber pressure, causing the explosion of the rocket motor being of high importance to develop models which could represent accurately the combustion process of these materials. Modelling the combustion of these propellants has been also of high interest for researchers in terms of optimising chemical composition to fulfil industrial requirements, improve propellant design, development and testing activities. The analysis of the burning rate of these materials, which means their decomposition and their subsequent combustion, is one of the main objectives of combustion modelling. Therefore, in this work, a transient multidimensional numerical model to describe the combustion of composite and double-base solid propellants is presented. The kinetics of the model is described by considering firstly, a change of phase of the solid propellant from condensed to gas and secondly, a reduced chemistry scheme which defines simplified chemical reactions to represent the combustion itself. To couple both processes, mass and species conservation, as well as temperature continuity are imposed in the burning surface in which the burning rate will represent a key factor. Moreover, an energy balance is also applied at the burning surface which represents that heat that the gas transfers to the burning surface is invested firstly, in raising the surface temperature to produce the phase change and secondly, in warming the condensed phase by conduction. The results obtained in the combustion modelling of both, composite and double-base solid propellants, are compared against experimental test and results present in the existing literature. An absolute error between 0.4 mm/s and 1 mm/s is obtained being in the order of magnitude of the experimental error. In addition, the pressure oscillations produced in a rocket motor and the depressurisation suffered in a combustion chamber, as well as at the exit of a barrel, are very interesting technological problems which have not been deeply studied in the literature. Therefore, the results obtained in the numerical modelling of transient combustion of composite and double-base solid propellants are presented in this thesis too. To perform this analysis, a pressure ramp is applied during a short interval of time at the open side of the already defined geometries. The developed tool confirms its robustness for the prediction of composite and double-base propellants combustion behaviour in multidimensional scenarios with transient environmental conditions such as rocket engines and/or base bleed units in ballistics applications.