Broadened mechanism of the flammability limit of lean premixed mixture via increasing bluff-body temperature

Abstract

To provide the guideline for enlarging the flammability limit by the means of increasing the bluff-body temperature, the present study investigates the effect of the bluff-body temperature on the flammability limit of lean methane-air premixed mixture in a wide temperature range. It is interesting to experimentally observe that the flammability limit of lean methane-air premixed mixture can be significantly enlarged by the high-temperature bluff-body. At first, the flame behavior and structure features at various bluff-body temperatures are revealed. The bilateral flame fronts get away from each other and split into two parts at the flame base gradually with the increased bluff-body temperature. In the case of the high-temperature bluff-body, the heat release rate HRR value sharply increases first and then decreases to a specific value gradually. The diffusion and convection fluxes of the fresh methane which arrives at the flame front increase when the bluff-body temperature increases. Subsequently, the broadened mechanism of the flammability limit in the case of the high-temperature bluff-body is revealed quantitatively in terms of the effects of flow recirculation, stretch, preferential transport, and conjugate heat transfer. The analysis indicates that the effects of flow recirculation and preferential transport do not contribute to improving the flame anchoring performance in the case of the high-temperature bluff-body. The negative stretch rate near the flame base in the case of the high-temperature bluff-body is beneficial to anchoring the flame. In addition, when the bluff-body temperature increases, the preheating effect on the fresh mixture significantly increases and the heat-loss effect decreases, which greatly contributes to improving the flame anchoring performance. The stretch and conjugate heat transfer effects are the main factors that broaden the flammability limit. This study provides a new strategy for extending the operating range of lean premixed flame by controlling the bluff-body temperature and expands our understanding of the lean premixed flame dynamics stabilized by the bluff-body.

Novelty and Significance Statement

Lean premixed combustion is regarded as a promising technology to achieve cleaner and higher efficiency combustion of fossil fuels. To provide the guideline for enlarging its flammability limit via controlling the bluff-body temperature, the effect of the bluff-body temperature on the flammability limit of lean methane-air premixed mixture is studied. It is observed that the flammability limit can be significantly enlarged by the high-temperature bluff-body, and the corresponding broadened mechanism is revealed quantitatively in terms of the flow recirculation, stretch, preferential transport, and conjugate heat transfer effects. Such detailed visualization of the main factors that enlarge the flammability limit in the case of the high-temperature bluff-body is provided for the first time. This study expands our understanding of enlarging the flammability limit of lean premixed mixture via controlling the bluff-body temperature.