EFFECTS OF RECESS LENGTH OF BLUFF BODY ON LEAN-PREMIXED SWIRL-STABILIZED CNG-AIR FLAME STRUCTURE

The stability of lean premixed turbulent swirl flames has been investigated using a laboratory-scale swirl-stabilized gas turbine combustor by varying the position of the bluff body inside the premixing tube. The relative location of the bluff body with respect to the dump plane is characterized in terms of a recess length, which is varied during the experiment. The resultant flame structure and dynamics have been studied for different bluff body recess lengths and bluff body shapes based on high-speed OH*-chemiluminescent images. Herein, we employ two different tapered bluff body shapes of an area ratio of 40% (B40) and 60% (B60). For the bluff body B40, a stable flame is mainly attached to the burner at a higher equivalence ratio. For a lower equivalence ratio, a lifted and elongated flame is formed. The increase in the recess length brings the flame closer to the burner surface, and a stable and intense flame is observed. For the bluff body B60, a less stable elongated and columnar type of flame is formed, and the flame stability slightly improves with the increase in recess length. The change in recess length significantly improves combustor performance when B40 is used instead of B60. The increase in recess length is supposed to bring the recirculation created in the wake of the bluff body close to the exit of the premixing tube, resulting in better mixing of the inlet reactants and hot combustion products. This effect is less prominent for the B60 case, and high bulk velocity squeezing from the edges of B60 may not result in the effective mixing of inlet reactants and hot combustion products. The high bulk velocity for the B60 case may result in the delayed expansion of the inflow reactants, and all these combined effects may render ineffective for the B60 case.