Experimental investigation on the influence of central airflow on swirl combustion stability and flame shape

Central airflow has been widely used to improve the performance of swirl burners in engineering applications. This paper reports an experimental investigation on the effects of such airflow on the combustion stability and shape of swirl flames. The results show that, for a low equivalence ratio, central airflow changes the flame shape from an "inverted cone" to a "rectangle" and significantly increases the flame height. Raising the speed of the central airflow increases the maximum temperature on the central axis of the swirl flame because the airflow enhances the upward momentum of the fuel. By contrast, for a high equivalence ratio, the swirl flame is prone to liftoff owing to the influence of the central airflow on the axial momentum of the fuel. In this case, increasing the fuel flow causes the swirl-flame blowout limit to increase and then decrease. This limit for different equivalence ratios is well described by dimensionless function. These findings will provide an important reference for the design of safe and high-performance swirl burners.