Simulation of full-scale combustion instabilities in small-scale rigs using actively controlled boundary conditions
Kim, Yong Jea
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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. Investigating CIs in actual “full-scale” engine tests are not practical because of the exorbitant cost of such tests, the large space required to house the full-sized engine, and the inability to equip full-scale engines with diagnostic systems. Because of these difficulties, most studies of CIs to date were performed in “small-scale” setups that were geometrically similar to but smaller than the full-scale engines combustors. 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 that did 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 CIs 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 developed a real time active acoustic boundary control approach to simulate the acoustic environment of the full-scale engine in the small-scale rig. For the study of the driving mechanism of longitudinal CIs, the small-scale rig consists of a shorter section of the full-scale engine and the “missing part” of the full-scale engine (consists of what has been “left over” after the small-scale rig has been removed from the full-scale combustor); see the first figure in Summary in this thesis. The goal of the actively controlled small-scale rig is to experimentally study the processes taking place in the corresponding region of small-scale rig section in an unstable full-scale engine. To attain this goal, the active control system (ACS) needs to generate an acoustic impedance at the actively controlled boundary of the small-scale rig that equals to the acoustic impedance at the corresponding location in the full-scale unstable engine. If this is accomplished, the acoustic oscillations in the small-scale rig and the corresponding region of small-scale rig section in full-scale engine would be identical. This study has developed a real time ACS, which enables the small-scale tube rig to simulate the longitudinal acoustic oscillations in the full-scale tubes (or engine), with the one-dimensional cold flow setup. In this setup, the speaker at the left end of the small-scale tube rig generated acoustic oscillations that simulate the driving by the combustion process, and the speaker at the right end was actively controlled to simulate the acoustic field of the full-scale system. It was demonstrated that the developed, actively controlled, small-scale, rig can simulate travelling and standing waves oscillations that are encountered in longer full-scale tubes. Additionally, this rig was used to demonstrate that standing acoustic waves CI in full-scale tubes having different lengths could be simulated in the developed, actively controlled, small-scale rig. This study also developed a theoretical model that determines in real time the acoustic boundary condition (BC) that must be generated by the ACS at the boundaries of a small-scale rig that simulates transverse (tangential) CI in an annular combustor similar to those used in gas turbines and jet engines. In this case, the small-scale rig consists of a small section of the annular combustor and the “missing part” of the full-scale engine (consists of what has been “left over” after the small-scale sector-rig has been removed from the annular combustor); see the second figure in Summary in this thesis. To determine the BCs that needed to be established at the boundaries of the actively controlled, small-scale rig, the developed model accounts for the effects of the combustion processes and flows through the reactants supply injectors and exhaust nozzles in the “missing part” of the engine, and for the presence of a tangential mean flow in the annular combustor. The developed model was numerically validated and used to investigate the effects of the exhaust nozzle, combustion process, and tangential mean flow component upon the characteristics of tangential CIs in an annular combustor. For example, its numerical solutions demonstrated that the presence and direction of the mean tangential flow component critically affect the characteristics of tangential (spinning) instabilities and an initially standing wave disturbance gradually transforms itself into a spinning wave that rotates around the annular combustor in the direction of the tangential mean flow; this finding is in agreement with previous experimental observations that have not been explained to date.