Temporal and spatiotemporal dynamics of the premixed bluff-body-stabilized flame under different combustor lengths

This paper presents the experimental investigation of the spontaneous temporal and spatiotemporal dynamics of a lean-premixed methane-air flame stabilized by a cylindrical bluff-body under different combustor lengths. The dynamic pressure and simultaneous heat release rate measurements, along with the flame visualization as well as the two-dimensional Particle Image Velocimetry (2D-PIV), were conducted to examine the temporal and spatiotemporal evolution of the flame dynamics within the combustion chamber. When continuously increasing the length of the combustor (Lc) at the constant inlet condition (both constant Reynolds number (Re) and equivalence ratio (∅)), we observe that the combustion mode switches from the steady-state combustion to the thermoacoustic combustion and finally to the intermittent combustion. By inspecting the simultaneously acquired time-resolved heat release rate and the dynamic acoustic pressure fluctuations, our study confirms that the steady combustion mode corresponds to a desynchronized aperiodic pattern, the thermoacoustic combustion mode corresponds to a perfect phase synchronized periodic pattern, and the intermittent combustion mode corresponds to an intermediate or transitional state between the previous two (the steady and the thermoacoustic) state. Finally, we investigate the hydrodynamic stability regime corresponding to the above-mentioned three combustion modes. Our study successfully provides comprehensive experimental evidence that even when the inlet conditions are kept constant, increasing the length of the combustor enhances the thermoacoustic positive feedback process, eventually resulting in different hydrodynamic states.