Analyses of flame response to acoustic forcing in a rocket combustor

High frequency combustion instabilities in liquid propellant rocket engines are spontaneously occurring pressure fluctuations that are coupled with unsteady combustion processes. Under the right conditions the unsteady fluctuations can grow to a point where they affect the operation of the combustion chamber. The cause of combustion instabilities, including which processes are responsible and under what conditions they arise, are not yet fully understood. The ability to predict and prevent combustion instabilities during the design of new combustion chambers, through better understanding, would dramatically reduce the uncertainty and risk in the development of new engines. An experimental combustor, designated BKH, is used to conduct high frequency combustion instability experiments. BKH operates with liquid oxygen and gaseous hydrogen propellants at supercritical conditions analogous to real rocket engines. The chamber features an acoustic excitation system that imposes an acoustic disturbance representative of a high frequency instability upon a cluster of five coaxial injection elements in the center of the chamber. The response of the elements to the imposed acoustic disturbance is observed using high speed optical diagnostics. The main aim of this project is to develop methods for predicting the flame response to high frequency acoustic forcing representative of combustion instability phenomena. BKH is employed as an experimental and numerical test case for investigating the flame response. Modelling and complementary data analysis methods are developed and applied to model the chamber flow field, identify and predict the excited acoustic disturbance, identify the flame response using optical data, and to predict the flame response numerically. The BKH experiments are first characterised by modelling the chamber numerically and determining the local acoustic disturbance acting upon the flame. A steady state chamber model with supercritical oxygen-hydrogen combustion was computed using a specialised CFD code. The model results indicate the secondary injection in BKH has a strong influence on the resulting flame distribution. A method for reconstructing the acoustic field from dynamic pressure sensor data was developed to determine the local acoustic disturbance acting upon the combustion zone over a range of excitation frequencies. A low-order acoustic modelling approach is also shown to predict the resonant mode frequencies and the evolution of the acoustic field. The flame response to the imposed acoustic disturbance is identified by analysing optical data from BKH experiments and unsteady CFD modelling. Multi-variable dynamic mode decomposition (DMD) analysis is used to isolate the flame response to the imposed acoustic disturbance in shadowgraph and OH* imaging data. Wave-like structures propagating along the surface of the liquid oxygen (LOx) jet and a phase difference of 45° between acoustic pressure and observed intensity fluctuations were identified. An unsteady model of an injection element subjected to representative acoustic forcing is used to predict the flame response for a range of excitation amplitudes. Velocity ratio fluctuations caused by acoustic coupling with the oxidiser post in a pressure antinode are identified. The trend of exponential decay of the length of the LOx core with increasing transverse acoustic amplitude excitation is reproduced numerically and the flattening and flapping motion of the flame was further investigated using the numerical results.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2017