Comparative analysis on the normal vector features of the conventional inverted flame and ultra-lean residual flame

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Residual behavior often appears near the flammability limit for lean premixed flames. Insight into it is beneficial to improve the lean premixed combustion performance further. Identifying the normal vector of the residual flame front plays a key role in revealing its structure and evaluating the weights of some main effects quantitatively. For the present residual flame with a high curvature, as the polynomial curve fitting method causes a noticeable error due to the Runge phenomenon, a non-equidistant central difference method is proposed. Subsequently, the normal vectors of the bluff-body stabilized residual flame and conventional inverted flame of 40%H2–60%CH4–air are obtained accurately and their differences are compared quantitatively. It is found that there are significant differences in the normal vector and flow structure features between the conventional inverted flame and residual flame behaviors. The normal vector of the conventional inverted flame always points at the upstream direction. However, the normal vector at the middle section of the residual flame front is almost parallel to the horizontal direction, and it points at the downstream direction next to the residual flame tip. In addition, the flow structure around the residual flame front is revealed. This is the first time the normal vector of the residual flame has been accurately visualized and the difference in normal vector feature between the residual flame and conventional inverted flame revealed. This work provides a universal and accurate method to identify the normal vector of the flame front and new insight into residual flame dynamics.