Numerical investigation of 3D two-phase turbulent flows in solid rocket motors

Abstract

A numerical investigation of the three-dimensional turbulent flow in a solid rocket motor with a submerged, vectored nozzle is presented in this paper. The simulations have been performed employing an Eulerian-Lagrangian approach: the full Navier-Stokes equations are solved numerically for the gas phase and a Lagrangian deterministic model is adopted for the discrete phase. Computations have been carried out on a realistic segmented solid rocket motor. Both an axisymmetric configuration and a 3-D geometry with a canted nozzle have been studied: a nozzle deflection of 6 degrees has been assumed. Three grid levels have been used in all computations to check grid independence. Very strong 3-D effects are predicted: an asymmetric pressure gradient is established at motor aft-end, and a vortical and circumferential flow pattern in the aft-dome is obtained. Alumina droplets trajectories have been computed for two diameters and different injection points; in most cases, three-dimensional, swirling paths have been predicted. Introduction Many solid rocket motors employ aluminized propellants to increase performance. During the combustion process liquid droplets of aluminum oxide are produced at the propellant grain surface, and are dragged into the combustion chamber by the internal flow. Some of these droplets, under certain circumstances, collect in the aft-end of the booster and give rise to alumina slag deposition which results in motor performance loss', damage of thermal protection, and possible sloshing and ejection of liquid agglomerates through the nozzle;' 3 this, in turn, is believed to cause pressure pulses and thrust imbalance. The possibilities that large lumps of alumina are exhausted through the nozzle is even greater during the thrust vector controlled phases of the vehicle ascent, in which the gimbaled nozzle operates at a certain deflection angle. This gimbaled position of the nozzle may induce strong threedimensional effects in the booster internal flow, especially in proximity of the motor aft-dome. These effects may lead to significant changes in the flow field patterns and in the alumina droplet trajectories with respect to an axisymmetric configuration, which may influence the deposition process significantly. Experimental evidence of 3-D motions in the booster aft-end region exists today. Slag movement and preferential accumulation in one sector of the motor were observed during the testing of a strategic-size solid rocket motor. Also, circumferential flows were observed by Waesche et al. in water tunnel tests when the submerged nozzle was gimbaled. Booster flow field characteristics in the motor aft-dome are very difficult to predict accurately due to the two-phase turbulent nature of the flow, mass injection from the grain surface, regions of strong recirculation and complex combustion chamber geometry, especially in three-dimensional configurations. Several numerical analyses of the internal flow field in a solid rocket motor directed to the investigation of the slag deposition phenomenon have been performed. A variety of models have been adopted: inviscid rotational or viscous flow models, including laminar and turbulent models, for the gas * Aerospace Engineer, Propulsion Section, Head; Member AIAA Aerospace Engineer, Computational Methods Section; currently with the Center for Turbulence Research, Stanford University, Stanford, CA Aerospace Engineer, Computational Methods Section, Head Copyright © 1998 by CIRA. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission American Institute of Aeronautics and Astronautics