Vortex-driven subharmonic bifurcation in a multi-flame Rijke tube

The Rijke tube is widely used in the literature to study combustion dynamics, offering a simple, self-excited setup for laboratory-scale experiments. While many studies utilize laminar multi-flame burners, most analyses focus on changes in pressure and global heat release rate, often overlooking interactions between individual flames. This study experimentally investigates the role of individual flame dynamics and their influence on neighboring flames in shaping overall pressure fluctuations. By varying the hydrogen percentage in premixed hydrogen/propane/air flames, we demonstrate how the system transitions from periodic oscillations to quasi-periodic oscillations and, ultimately, to half-integer subharmonic oscillations. Through high-speed imaging of individual flames in a seven-flame burner, we further reveal the emergence of an alternating oscillation pattern from interactions between flames and vortex shedding. This effect intensifies with increasing hydrogen content. Additionally, we compare the fundamental modes of the air column in the Rijke tube and the harmonics of flame oscillations to illustrate how energy within the pressure dynamics is redistributed among different frequencies.

Novelty and Significance Statement

Previous studies with Rijke tube often used these multi-flame configurations to create complex combustion dynamics, yet the genesis of this complexity remained unexplored. This research explores such flames from different perspectives. The novelty in our approach is that we visualized and analyzed the dynamics of individual flames, while previous studies focused only on collective dynamics. This paved the way for investigating local interactions, which unveiled that the interactions between vortices shed by neighboring flames lead to subharmonic bifurcations, a critical transition process for combustion dynamics.