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Ph.D. Thesis Proposal
by
Yong Jea Kim
Simulation of Full-Scale Combustion Instabilities in Small-Scale Rigs using Actively Controlled Boundary Conditions
Advisor: Prof. Ben T. Zinn
10:30 am, Tuesday, September 6
Montgomery Knight Building, Room 317
ABSTRACT
The onset of combustion instabilities (CIs) has hindered the development and performance of combustion systems employed in industrial, power generation and propulsion systems for many decades. In an effort to solve this problem, many investigations to date sought to elucidate the feedback mechanism that drives these CI. Ideally, the experimental setup used in such studies should simulate the operating conditions (e.g., mean pressure and temperature, reactants and their supply system), geometry, and scale of the unstable system in order to properly reproduce the combustion process and acoustic oscillations that occur in the unstable full-scale engine to assure that all the parameters affecting the feedback mechanism are reproduced in the laboratory rig. Clearly, investigating the instability in an unstable “full-scale” engine would satisfy these requirements. However, investigating CI in the full-scale engine tests is not practical because of the exorbitant costs of such tests, and the inability to equip full-scale engines with diagnostic systems that could measure, e.g., the temporal and spatial dependence of the mean and acoustic pressures and velocities, temperature, compositions, and reaction rates. Because of these difficulties, most studies of CI to date were performed in “small-scale” setups that were geometrically similar to but smaller than the full-scale engine combustor. While testing with these small-scale setups reduced the cost of testing and produced important results, the acoustic modes excited in the small-scale setups had considerably higher frequencies and could not simulate the lower frequency oscillations that are excited in the unstable full-scale engines.
The above discussion indicates that in order to study the driving of CI in full-scale engines in small-scale rigs, the latter must simulate the acoustic environments, the combustion processes, and the interactions between these processes in the unstable full-scale engine. This study has been investigating the use of a small-scale rig with real time active boundary control (shown in above figure) to simulate the acoustic environment of the full-scale engine in the small-scale rig. To attain this goal, the active control system “generates” an acoustic impedance at location (II) of the small-scale rig that equals to the acoustic impedance at the corresponding location in the full-scale unstable engine. If accomplished successfully, the acoustic oscillations in the region between locations (I) and (II) in the small-scale rig and the full-scale engine would be identical. The developed active control system consists of the following three modules: (1) a “wave separation” module that determines the right and left going waves inside the small-scale rig from measured acoustic pressures; (2) a “simulation” module that determines the characteristics of the right and left going waves in the “missing part” of the full-scale engine (i.e., in the region to the right of locations (II) in the full-scale engine); and (3) an “actuator” module that determines the control signal for the actuator (e.g., speaker) at location (II) that “generates” the needed acoustic impedance at location (II) of the small-scale rig.
To date, a wave separation algorithm was formulated using the method of characteristics and successfully demonstrated numerically and experimentally. A real time active control system for one-dimensional acoustic oscillations was demonstrated using a tube equipped with speakers on its left and right hand sides. In this setup, the left speaker generated acoustic oscillations that simulate the driving by the combustion process, and the right speaker was actively controlled to simulate the acoustic field of the full-scale system. To date, this system was used to demonstrate the excitation of travelling acoustic wave CI by using the active control system to “generate” a non-reflecting boundary condition at the right boundary (II) of the tube. Current efforts are focusing on the simulation of a standing acoustic wave CI in a full-scale engine (i.e., the longer tube in the figure above) in the small-scale rig.
Additionally, a model describing the acoustic and combustion processes in an annular combustor experiencing tangential CIs was also developed to acquire capabilities for performing real time simulations of the “missing part” of the full-scale engine combustor experiencing such CI. The developed model was numerically solved to investigate the effect of the presence of mean tangential flow component in the engine. The results describe the basic driving/damping processes that control the investigated CIs. They also show that the presence and direction of the mean tangential flow component critically affect the characteristics of tangential (spinning) instabilities. The application of this model in the study of tangential CI in small-scale rigs will be studied in the future.
Committee Members
Prof. Ben T. Zinn (advisor),
Prof. Jechiel Jagoda, and
Prof. Krishan K. Ahuja.
