Results of a Model for Premixed Combustion Oscillations

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Active Control Combustion oscillation control using low frequency, cyclic fuel injection was successfully tested on an experimental combustor. A complete description of the experiment and details of the combustor configuration can be found in Richards, et al. (1995). A time history of a pressure signal from the combustor with no cyclic fuel injection is shown in Figure 10. A 4.5 kPa RMS pressure signal is established at 300 Hz given premix fuel and air flow rates of 0.4 g/s and 9.9 gis, respectively. Figure 11 shows model results using a similar geometry and operating conditions. A 4.0 kPa RMS pressure signal is established at approximately 280 Hz. Active control of the oscillation using low frequency injection was attempted by adding 0.067 gls of fuel through the pulse injector. The fuel injector was operated with a frequency of 50 Hz and a pulse width of 7 ms. The resulting pressure oscillation of the combustor along with the injector status is shown in Figure 12. A step in the injector signal signifies the injector is open. The low frequency inj ection is shown to decrease the pressure oscillation by a factor of 0.3 from the uncontrolled case shown in Figure 10. Figure 13 shows model results using a low frequency injection through the bypass port. Pressure oscillations are shown to decrease by a comparable percentage. Figure 14 shows a time history of the pressure signal and the injector status from the time at which the inj ected fuel is first introduced into the nozzle until the oscillations are significantly suppressed. The model demonstrates that under these specified conditions, the limit cycle can be broken by periodically altering the equivalence ratio. The low frequency fuel inj ection is able to break the limit cycle by shifting combustion back and forth between stable and unstable conditions. This was demonstrated numerically by observing the oscillating behavior at (steady) equivalence ratios representing the instantaneous conditions corresponding to the injector open or closed states. Experiment and model both show that a relatively small amount of fuel injected appropriately can break a limit cycle and thus stabilize combustion oscillations. Inlet Air Temperature Initial difficulties in repeating experimental data from day to day led to the identification of inlet air temperature as a relevant parameter in combustion instability research. Although differences in inlet air temperature were severely affecting oscillation levels, the root cause was not immediately apparent. Figure 15 shows experimental and numerical data from a combustor similar to the one shown in Figure 2 operating at an equivalence ratio of 0.75 and an air flow rate of 19.3 g/s. The only parameter altered in Figure 15 is the temperature of the fuel and air mixture entering the nozzle. Experimental data shows that a significant increase in the pressure oscillation occurs at approximately 283 K. Numerical data sho\\'s a more severe, yet similar transition at approximately 293 K. It should be noted that the simplicity of the model in conjunction with the multiple, interdependent instability mechanisms previously alluded to make exact prediction of an experimental temperature transition very difficult. Nonetheless, a better understanding of the root cause of the transition is provided by the model. The model demonstrates that changes in 7