Experimental investigation of combustion instability and NOx emissions control by adjustable swirl jet in lean premixed flames

This paper investigated the feasibility of using an adjustable swirl jet to control combustion instability and pollutant NOx emissions. The impact of three control variables of swirl jet media, swirl jet angles, and jet flow rates were discussed and analyzed. The results show that the adjustable swirl jet can realize the simultaneous control of thermoacoustic oscillation and NOx emissions. The suppression of dynamic pressure and flame heat release rate CH* presents three response states with the change of jet flow rate: high-amplitude oscillation, low -amplitude oscillation and stable combustion state. The experiment also indicates that the suppression effect when using Ar, co-swirl jet and high swirl jet intensity is better than that of He, counter-swirl jet and low swirl jet intensity, respectively. When using the Ar co-swirl jet with a swirl jet angle of 60 degrees, the pressure control reaches a maximum drop of 39.7 Pa. The reduction of pollutant NOx emissions under the He swirl jet is smaller than that of the Ar swirl jet. The NOx emissions reach the maximum decrease of 9.2 ppm under the action of the Ar counter -swirl jet. Meanwhile, the flame shape under the Ar swirl jet changes significantly, switching from a cap to a tablet shape. The Ar counter-swirl jet reduces the flame length considerably by 49 mm. These findings demonstrate that the adjustable swirl jet strategy can simultaneously mitigate combustion instability and NOx emissions, which may be conducive to the operation of premixed flame.