| Time-resolved simulations of combustion dynamics of homogeneous double-base propellants and the ensuing unsteady flowfield in a rocket motor are performed. The overall objective is to establish a unified theoretical/numerical framework accommodating propellant chemistry, turbulent combustion, and gas dynamics in order to predict self-sustained unstable motions in the motor known as combustion instability. The pathway to explore these intricate phenomena follows two fundamental steps.; Firstly, large-eddy-simulation (LES) of non-reacting, compressible flow in a nozzleless rocket motor with surface mass injection simulating propellant burning is performed. Three successive regimes of development: laminar, transitional, and fully turbulent flow, are observed. Transition to turbulence occurs away from the porous wall in the mid-section of the motor and the peak in turbulence intensity moves closer to the wall further downstream as the local Reynolds number increases. Coupling between the acoustic motions and turbulent fluctuations is then addressed by imposing periodic excitations, at different amplitudes and frequencies, at the head end of the motor simulating traveling acoustic waves in the stationary flow. The energy exchange mechanisms among the mean, periodic, and turbulent flowfields are studied to address the issue of acoustically induced turbulence. Excitations at certain frequencies within the broadband spectrum of turbulent motions may lead to resonance in the chamber causing early transition to turbulence.; Secondly, various physicochemical processes in double-base homogeneous solid propellant combustion are studied through time-resolved simulations of unsteady flowfield in the gas phase of the motor. Favre-averaged, spatially filtered conservation equations for a multi-component system are solved. Full account is taken for variable transport and thermodynamic properties and finite-rate chemical kinetics. The two-stage premixed flame, characterizing homogeneous propellant combustion, with primary, dark, and secondary flame zones, is modeled by two global reactions in the condensed phase and five reactions in the gas phase. The detailed flow structures and heat-release mechanisms in various parts of the chamber, with microscale motions above the propellant surface and macroscale motions in the bulk of the chamber are investigated. Emphasis is placed on the combustion wave structure in the laminar, transition, and fully turbulent regimes. Significant insight has been achieved in the unsteady flow evolution, propellant burning characteristics, and rocket motor internal ballistics. |
