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To provide the theoretical basis to suppress the unstable flames under the coupling effect of flow and heat recirculation, the present work experimentally studies the ultra-lean dynamics of a holder stabilized 40%H2–60%CH4–air premixed flame in a preheated mesoscale combustor. The regime diagram of the flame behaviors at various operating conditions is obtained. It is observed that the blow-off limit first increases slightly and then decreases sharply (the anomalous blow-off limit) with the decreased Re value. Three types of the flame behaviors (i.e., the conventional stable flame, the stable residual flame, and the periodic oscillating residual flame) are found before the flame blow-off. In addition, with the decreased Reynolds number, the operating range for the stable residual flame broadens first and then narrows, but that of the periodic oscillating residual flame decreases monotonically, which are observed for the first time. The results show that, with the decreased Reynolds number, the flame root of the conventional stable flame anchors almost at the same location right behind the holder, while the flame tips obviously shift upstream. With the decreased equivalence ratio, the left and right flame tips in the downstream channel shift toward each other and finally merge into a single flame tip, which results in the formation of the stable residual flame. When the equivalence ratio decreases further, the periodic oscillating residual flame occurs. The flame tip periodically oscillates up and down over time. In the end, the blow-off dynamics of the stable residual flame and periodic oscillating residual flame are revealed.

This work experimentally investigates the ultra-lean dynamics of a 40% H2–60% CH4 flame near the laminar critical limit in a preheated mesoscale combustor with a flame holder. These experiments are conducted to verify a conjecture we proposed in a previous publication and reveal the ultra-lean flame dynamics under the synergistic effects of heat and flow recirculation. Notably, not only is our conjecture confirmed, but also some novel flame behaviors are found. As the equivalence ratio ϕ is decreased from 0.500 to 0.320, the conventional stable flame, stable residual flame, periodic residual flame with repetitive local extinction and re-ignition (periodic RFRER), and periodic oscillating residual flame are observed in sequence. For the stable residual flame (0.370 ≥ ϕ ≥ 0.355), the left and right flame roots reside directly behind the flame holder, and the flame tip stays near the combustor exit. For the periodic RFRER (0.350 ≥ ϕ ≥ 0.340), observed experimentally for the first time, the flame roots reside at almost the same location, but the flame tip oscillates up and down over time with pinch-off events. For the periodic oscillating residual flame (0.335 ≥ ϕ ≥ 0.320), found for the first time, the stable flame roots also reside at almost the same location, but the residual flame tip oscillates up and down over time without a pinch-off event. When ϕ decreases to 0.315, the oscillating residual flame extinguishes, and its blow-off dynamics are revealed in detail.