Modeling of the Transformation Kinetics of Small Metal Particles during Combustion inside the Chamber of Hybrid Rocket Engines

Abstract

Metal powders as fuel ingredients of hybrid rocket motors are an important factor which can improve the engine properties. Through the fuel density increase, a compact engine form can be achieved and the construction mass ratio can be reduced. Furthermore, the addition of metal powders in the fuel structure can increase the regression rate, the combustion temperature and the corresponding specific impulse. But to achieve these goals, the combustion efficiency of metal particles must be high and the condensation of metal droplets, high particle agglomeration, the combustor wall slagging as well as the nozzle structure abrasion must be reduced or completely avoided. The challenging experiments with hybrid rocket engines (HRE) deliver a limited amount of information necessary to understand transformation mechanisms, despite of the application of sophisticated measurement techniques. So, the experiments should be combined with CFD multiphase simulations to enable understanding of combustion processes related to the condensed flow phase. Subsequently, this enables the management of the combustion process within HRE as well as the avoidance of negative effects by the application of powder ingredients with metal content. Within the AHRES (Advanced Hybrid Rocket Engine Simulation) program, the DLR Institute of Aerodynamics and Flow Technology in Braunschweig has carried out experiments with different fuel formulations, which included metal powders of aluminum and aluminum-magnesium alloys. Additionally, the transformation loop of small particles with metal content is simulated using an Euler-Lagrange coupling of the reacting gas flow and the condensed metallic phase. The conducted simulation includes time dependent transformations of Al and AlH3 particles (decomposition, melting, vaporization, heterogeneous combustion, condensation and solidification). In this paper, the used transformation models and the coupling procedure between the gas and the condensed phase are classified and explained. For given start and boundary conditions one simulation of the characteristic small single metal particles with corresponding trajectories is realized. The results are graphically displayed and extensively commented. They enable helpful conclusions about the utilization of metal particles as fuel ingredients for HRE.