Effect of the length of a plate flame holder on flame blowout limit in a micro-combustor with preheating channels

It is challenging to achieve a large blowout limit for micro-combustors due to the increased heat loss ratio and reduced residence time. For this, we recently developed a micro-combustor with a plate flame holder and two preheating channels. In this paper, the effect of the plate length (Lb = 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 mm) on the blowout limit of CH4/air flames was numerically investigated. The results show that the flame blowout limit increases firstly and then decreases with an increasing plate length. The largest blowout limit is obtained at Lb = 1.0 mm. Three neighboring cases, i.e., Lb = 0.5, 1.0 and 2.0 mm, are taken to analyze the underlying mechanisms responsible for this non-monotonic trend. The flame blowout process demonstrates that, the flame is extinguished due to "pinch-off phenomenon" at high inlet velocity, and the shorter the distance between the upper and lower flame fronts, the smaller the blowout limit will be. Numerical analysis reveals that the differences in flame blowout limits are a result of the combined effects of heat recirculation and local flow field at the entrance of the combustion chamber. The heat recirculation effect grows stronger with a decreasing length of flame holder, which results in a more obvious volumetric expansion at the entrance of the combustion chamber. Meanwhile, the plate flame holder has a redirection effect on the local flow field. As a result, the gaseous mixture enters the combustion chamber with a smallest acute angle in the case of Lb = 1.0 mm, which leads to a largest distance between the upper and lower flame fronts and a longest recirculation zone. Consequently, the flame blowout limit reaches a peak at Lb = 1.0 mm. The present work provides an important guideline to design such kind of micro combustors.