Liquid rockets are prone to large amplitude oscillations, commonly referred to as thermoacoustic instability. This phenomenon causes unavoidable developmental setbacks and poses a stern challenge to accomplish the mission objectives. Thermoacoustic instability arises due to the nonlinear interaction between the acoustic and the reactive flow subsystems in the combustion chamber. In this paper, we adopt tools from dynamical systems and complex systems theory to understand the dynamical transitions from a state of stable operation to thermoacoustic instability in a self-excited model multielement liquid rocket combustor based on an oxidizer rich staged combustion cycle. We observe that this transition to thermoacoustic instability occurs through a sequence of bursts of large amplitude periodic oscillations. Furthermore, we show that the acoustic pressure oscillations in the combustor pertain to different dynamical states. In contrast to a simple limit cycle oscillation, we show that the system dynamics switches between period-3 and period-4 oscillations during the state of thermoacoustic instability. We show several measures based on recurrence quantification analysis and multifractal theory, which can diagnose the dynamical transitions occurring in the system. We find that these measures are more robust than the existing measures in distinguishing the dynamical state of a rocket engine. Furthermore, these measures can be used to validate models and computational fluid dynamics simulations, aiming to characterize the performance and stability of rockets.

A concurrent computational and experimental study of self-excited combustion dynamics in a model configuration of a lean direct injection (LDI) gas turbine combustor are described. Incoming air temperature and equivalence ratio were varied. Simulation at low equivalence ratio compared better with measurement and thus this condition was selected for a more detailed study of the underlying combustion dynamics mechanisms. First, hydrodynamic modes are investigated by conducting the simulation with an acousticallyopen combustor so that acoustic effects on the flow field are minimized. The Vortex Breakdown Bubble (VBB) proves to be an important flow structure that can easily interact with the acoustic field to sustain instability. Second, detailed cycle studies of the acoustically closed combustor simulation reveals enhanced mixing and vaporization of the JP-8 fuel spray due to acoustic compression wave. Dynamic Mode Decomposition (DMD) analysis is used to identify the coupling between axial acoustics and the vortex breakdown bubble in the lower frequency region. Presence of another important hydrodynamic mode, the Precessing Vortex Core (PVC) is also identified from the DMD analysis. The possibility of nonlinear coupling between the acoustics and PVC modes is indicated.

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Simulations of self-excited combustion instabilities in a model configuration of a lean direct injection (LDI) gas turbine combustor were performed and investigated with different operating conditions (air temperature and equivalence ratio). Concurrently, experimental data were obtained at the same conditions in a well-instrumented test combustor with the same configuration to validate the simulation results. The simulations are used to investigate the coupling between the acoustic and heat release modes and the important flow dynamics to understand the physics that lead to combustion instabilities in the LDI combustor. A Precessing Vortex Core (PVC) hydrodynamic instability was found to be significant in driving spray and flame responses. Detailed and systematic studies of the PVC instability are also performed using non-reacting simulations of an acoustically-open combustor to minimize the acoustic and combustion effects on the flow field.