Characterization and Control of Lean Blowout Using Periodically Generated Flame Balls

Abstract

Combustion near LBO involves the complex physical processes including turbulence, air/fuel mixing, and chemical kinetics. The goal of this paper was to identify the typical combustion behaviour near LBO of the burner and to develop an effective actuator that will have the necessary control authority without having adverse effects such as increased emissions. Early detection and effective extension of lean blowout (LBO) are the keys to ensure flight safety and low emissions for aero engines, and are of importance to industrial gas turbines for operation below regulated NOx limits. In addition, efficient actuation are crucial for effective active LBO control. An experimental investigation of LBO was carried out using a swirl-stabilized atmospheric combustor with separate pilot and premix gaseous fuel (natural gas) injection systems. Systematic tests were performed including measurements of pressure, OH chemiluminescence and emissions for different combustor lengths, fuel split ratios, preheat temperatures and air flow rates. Operation near LBO may involve excitation of undesired thermoacoustic instabilities that have to be mitigated. LBO was approached by reducing the fuel flow rate while keeping the air flow rate, the preheat temperature and the other parameters constant. Control of the LBO and thermoacoustic instabilities was achieved by generating periodic flame balls. The LBO could be extended by 13 % relative to the natural lean blowout limit at nearly 50% reduced NO emission in comparison to common pilot fuel modulation. A spark discharge system was installed at the pilot fuel injection location. The periodic spark discharge was synchronized with the pulsed fuel injection at a phase shift of 165° and an operating frequency of 22 Hz to produce flame balls that affected the main combustion region. The flame balls excitation provided an effective tool for controlling the premix combustion characteristics at the LBO.