Solid Rocket Motor Internal Ballistics Simulation Using Eulerian Multiphase Models
Creators: name:  M. GROSSI 
name:   D. BIANCHI 
name:   G. COCIRLA 
name:   B. FAVINI 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
SOLID ROCKET MOTOR

description: Nowadays aft-finocyl solid propellant motors represent an essential asset for European launchers, since their employment in Vega and Ariane 6 launchers. Aft-finocyl solid rocket motors (SRMs) are inherently three-dimensional and geometrically complex. As the propellant burns, the geometry changes in a time-dependent manner, triggering an ongoing flow modification. As a a matter of fact, the internal flowfield of a modern solid motor is characterized by multiphase features. Modern propellants are laden with metal particles, mainly aluminum, that compose up to 20% of the total mass. The primary reason for the inclusion of such fuel particles into the solid propellant stems from the significant heat release obtained as the aluminum particles burn to form aluminum oxides. As the propellant burns, the propellant combustion interface regresses and the particles added to the propellant melt to form liquid droplets. A complex mechanism follows, resulting in the release of aluminum agglomerates and their injection into the accelerating core flow. The final size of the aluminum oxide particles is found to vary from a few micrometers to a few hundred micrometers, providing a polydisperse multiphase flow. Collisions between particles and breakup events near the motor nozzle generate further modification of particles distribution, exacerbating the already complex flowfield. Despite of its favourable effect on specific impulse, inclusion of Al particles in the propellant has some side effects. Indeed, it may lead to slag accumulation in submerged nozzles, thermoacoustic instability due to distributed combustion and, moreover, the alumina droplets are the main source of nozzle scouring from particulate impact, contributing to nozzle erosion. Two-phase flow losses in the nozzle expansion process caused by the velocity- and thermal-lag are also hindering the performance enhancement due to particle addition. Contrary to chemical erosion, mechanical one lacks of comprehensive theoretical/numerical models. Hence a step in this direction would be extremely valuable in achieving an improved understanding of the behavior of nozzle erosion in its entirety. However, such studies present serious challenges. Indeed, beyond the difficulty of defining a model able to cope with the interaction between particles flows and nozzle wall material, the first issue regards the correct assessment of particles trajectory, dimension, and distribution within the motor. In SRMs state of the art, flowfield analyses have been mainly treated using hybrid Eulerian/Lagrangian methods or pure Lagrangians one. Nevertheless, the subjects of such analyses have usually been solid motors with simple geometry or large segmented boosters, thus multiphase flowfield within aft-finocyl solid motors have still to be properly characterized. This work is consequently devoted to the investigation of the internal flowfield of aft-finocyl motors with particular focus on the nozzle region. Multidimensional CFD analyses based on fully Eulerian methods will be the main investigation tools. Since the interest in tracking large droplets characterised by high inertia, such methodology is not suited for accomplish this task due to the risk of generating the so called delta-shocked, thus, in this work, an advanced Eulerian modelling is taken into account. Moreover, thanks to a parametric analysis, focused on several key elements such as particles size and geometry configurations, the most important flow features affecting the motor behavior will be highlighted, contributing to provide a more clear scenario on multiphase flows within aft-finocyl motors class.