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Nonlinear Rocket-Engine Longitudinal Combustion Instability


With recent improvements in computer architecture, there are renewed interests in studying combustion instability in rocket engine via Computational Fluid Dynamics (CFD) simulations. Most existing numerical simulations are computationally expensive, with each simulation taking weeks to months to perform. The first objective of this dissertation is to develop a computationally inexpensive numerical solver. The solver, written in an axisymmetric formulation, is used to study longitudinal combustion instability in a single-injector rocket engine. For different stability domains, the results generated by the solver show good agreements with both experimental results and expensive 3D simulations. Compared to other axisymmetric solvers, the developed solver is at least an order of magnitude faster while using a more detailed chemical mechanism. Because of the canonical configuration of the rocket engine, fundamental understandings of the self-excited instability mechanisms under various stability domains are identified. Increasing the pressure oscillation leads to increase in mixing; making the flame inside the combustion chamber more compact. Pressure-Heat Release Rate coupling location plays a crucial role in driving the instability.

Subsequently, the flame dynamics under various oscillatory conditions are examined. Significant local extinctions and reignitions occur in the unstable cases compared to the stable cases. The complex chemical mechanism is proven to play an essential role in predicting the correct oscillation amplitude. Partially premixed flames are found in the combustion chamber.

In the last chapter, for an unstable oscillation, wall heat loss and chamber modifications was found to provide stabilizing effects. The stabilized chamber is then triggered using different types of disturbances with various durations. In some cases, a limit cycle behavior with much larger pressure oscillations than the initial conditions is observed. This triggered instability of the longitudinal mode in this dissertation is a novel contribution to the field.

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