No abstract

A novel technique for controlling oscillating combustion is proposed and demonstrated. For oscillations that exist over a limited range of equivalence ratios, we suggest that periodic modulation around the unstable condition can effectively avoid the oscillating condition, but otherwise produces the desired timeaverage equivalence ratio. Tests of this concept were carried out in an atmospheric-pressure, swirl-stabilized combustor with a nominal heat input of 30 kW. The fuel was natural gas. We show that it was possible to control a 300-Hz oscillation by modulating the fuel flow at frequencies less than 20 Hz, reducing the observed rms pressure from 2.7 to 0.8 kPa. Limitations on when this technique may produce successful control are also discussed.

A number of recent articles have demonstrated the use of active control to mitigate the effects of combustion instability in afterburner and dump combustor applications. In these applications, cyclic injection of small quantities of control fuel has been proposed to counteract the periodic heat release that contributes to undesired pressure oscillations. This same technique may also be useful to mitigate oscillations in gas turbine combustors, especially in test rig combustors characterized by acoustic modes that do not exist in the final engine configuration. To address this issue, the present paper reports on active control of a subscale, atmospheric pressure nozzle/combustor arrangement. The fuel is natural gas. Cyclic injection of 14 percent control fuel in a premix fuel nozzle is shown to reduce oscillating pressure amplitude by a factor of 0.30 (i.e., −10 dB) at 300 Hz. Measurement of the oscillating heat release is also reported.

Recent regulations on NO emissions are promoting the use of x lean premix (LPM) combustion for industrial gas turbines. LPM combustors avoid locally stoichiometric combustion by premixing fuel and air upstream of the reaction region, thereby eliminating the high temperatures that produce thermal NO . Unfortunately, this style of comx bustor is prone to combustion oscillation. Significant pressure fluctuations can occur when variations in heat release periodically couple to acoustic modes in the combustion chamber. These oscillations must be controlled because resulting vibration can shorten the life of engine hardware. Laboratory and engine field testing have shown that instability regimes can vary with environmental conditions. These observations prompted this study of the effects of ambient conditions and fuel composition on combustion stability. Tests are conducted on a subscale combustor burning natural gas, propane, and some hydrogen/ hydrocarbon mixtures. A premix, swirl-stabilized fuel nozzle typical hardware (Cutrone et al. 1985). of industrial gas turbines is used. Experimental and numerical results Changes in instability behavior are typically attributed to combusdescribe how stability regions may shift as inlet air temperature, tor modifications or changes in operating conditions. In addition, humidity, and fuel composition are altered. Results appear to indicate laboratory and engine field testing have shown that instability regimes that shifting instability regimes are primarily caused by changes in can also change with environmental conditions and fuel composition. reaction rate.These field observations have seldom been reported in the literature.

Recent regulations on NO ' emissions are promoting the use of lean premix (L2M) combustion for industrial gas turbines. LPM combustors avoid locally stoichiometric combustion by premixing fuel and air upstream of the reaction region, thereby eliminating the high temperatures that produce thermal NO,. Unfortunately, this style of combustor is prone to combustion oscillation. Significant pressure fluctuations can occur when variations in heat release periodically couple to acoustic modes in the combustion. chamber. These oscillations must be controlled because resulting vibration can shorten the life of engine hardware.Laboratory and engine field testing have shown that instability regimes can vary with environmental conditions. These observedons prompted this study of the effects of ambient conditions and fuel composition on combustion stability. Tests are conducted on a subscale combustor burning natural gas, propane, and some hydrogen/ hydrocarbon mixtures. A premix, swirl-stabilized fuel nozzle typical of industrial gas turbines is used. Experimental and numerical results describe how stability legions may shift as inlet air temperature, humidity, and fuel composition are altered. Results appear to indicate that shifting instability regimes are primarily caused by changes in reaction rate.