In this work, a Rijke-Zhao tube with a unique structure and geometry is designed and experimentally tested. It has a mother tube (bottom stem), which splits into two bifurcating daughter tubes (i.e. upper branches) with different lengths. As a premixed laminar flame is placed inside the mother tube, it provides a mechanism to produce self-excited combustion oscillations, known as combustion instabilities. It is surprisingly found that the resulting flow fields (temperature and velocity) in the bifurcating branches are dramatically different. Theoretical analysis is conducted first to predict the air flow paths in the bifurcating branches. 2D numerical simulations of Rijke-Zhao combustion instabilities are then conducted to simulate our experiments. In order to gain insight on the mode selection and triggering in Rijke-Zhao tube, experiments are then conducted on the tube with one branch removed. It is found that the measured pressure spectrum agrees very well with that when there are two bifurcating branches. This indicates the critical role of the combustor length in mode selection and triggering. Thus, further analysis on the transient growth of flow disturbances and the 'self-aspirating' behavior of combustion instability is performed. It is found that longer combustor length, more pronounced non-normality. In addition, bifurcation diagram shows that the periodic limit cycle undergos subharmonic bifurcations, and finally transition to chaos, as the bifurcation parameter K is varied. Finally, a feedback control scheme is developed to real-time monitor and mitigate Rijke-Zhao combustion instabilities. Implementing the control scheme can quickly stabilize the Rijke-Zhao system and adapt to prevent the onset of a new limit cycle resulting from the changes of fuel flow rate. The sound pressure level is reduced by approximately 50 dB.
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