Combustion Control by Vortex Breakdown Stabilization

Abstract

Flame anchoring in a swirl-stabilized combustor occurs in an aerodynamically generated recirculation region which is a result of vortex breakdown (VBD). The characteristics of the recirculating flow are dependent on the swirl number and on axial pressure gradients. Coupling with downstream pressure pulsations in the combustor affects the VBD process. The present paper describes combustion instability that is associated with vortex breakdown. The mechanism of the onset of this instability is discussed. Passive control of the instability was achieved by stabilizing the location of vortex breakdown using an extended lance. The reduction of pressure pulsations for different operating conditions and the effect on emissions in a laboratory scale model atmospheric combustor, in a high pressure combustor facility, and in a full scale land-based gas-turbine are described. The flashback safety, one of the most important features of a reliable gas turbine burner, was assessed by CFD, water tests, and combustion tests. In addition to the passive stabilization by the extended lance it enabled injection of secondary fuel directly into the recirculation zone where the flame is stabilized. Tests were conducted with and without secondary fuel injection. Measurements and computations optimized the location of the extended lance in the mixing chamber. The effect of variation of the amount of secondary fuel injection at different equivalence ratios and output powers was determined. Flow visualizations showed that stabilization of the recirculation zone was achieved. Following the present research, the VBD stabilization method has been successfully implemented in engines with sufficient stability margins and good operational flexibility. This paper shows the development process from lab scale tests to full scale engine tests until the implementation into field engines.