Numerical investigation on flow characteristics and emissions under varying swirler vane angle in a lean premixed combustor

The flow characteristics and NO and CO emissions were numerically investigated at a series of swirler vane angles in a lean premixed combustor. The superiority of different turbulence models was initially assessed and the realizable k-epsilon was recommendable. The results show that, central recirculation zone (CRZ) and outer recirculation zone (ORZ) appear at a critical swirler vane angle of 30 degrees. Increasing swirler vane angle decreases the areas of CRZ and ORZ, axial distance of CRZ center, flame length and area of high-temperature zone, but increases expansion angle and turbulent intensity. Both NO and CO concentrations at combustor exit sharply decline as vane angle increases from 25 degrees to 30 degrees, indicating that the onset of recirculation zones brings considerable reduction of NO and CO emissions. As vane angle increases from 30 degrees to 75 degrees, NO decreases while CO increases slightly. Analysis of NO formation sources shows that N2O pathway dominates NO formation under the investigation conditions, NNH pathway has the least contribution, thermal pathway and prompt pathway have comparably intermediate contributions. The high-pressure condition promotes N2O pathway due to its pressure-dependent feature, the very lean condition suppresses both thermal and prompt pathway due to relatively low flame temperature and strong oxidation circumstance, respectively.