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Show Unsteady pressure is measured using a piezoelectric pressure transducer mounted on the nozzle exit flange, very near the flame zone. Thermocouples measure the temperature of fuel and air mixture just before it enters the nozzle, and the gas temperature at the exit of the combustor body. Time varying heat release is recorded with a PMT /line filter assembly viewing the 315 nm OR emission. The optical assembly was designed to view the entire flame zone. Results The simple model described is valuable to understanding the origin of combustion instabilities and the various means to control them. One can quickly examine instability trends associated with changes in equivalence ratio, mass flow rate, geometry, ambient conditions, and other relevant factors. PCOM obviously has somewhat limited use as a precise design tool due to its simplicity. Experimental evidence has shown that multiple, interdependent mechanisms affect the stability performance of a combustor, and not all of these complicated mechanisms are captured by the model. As an example, experimental results have shown that a pilot flame can have significant effects upon the stability of a combustor. The behavior of a pilot flame cannot be modeled in a stirred reactor. Given these limitations, PCOM has nonetheless proven valuable to the study of LPM combustion instability. Combustion Instability Mechanisms A combination of experimental testing and numeric simulation has helped identify five mechanisms that can drive combustion instability. These mechanisms are: • fuel supply variation • mixture supply variation • air supply variation • pilot instability • swirl instability A detailed explanation of the various instability mechanisms can be found in Richards (1996). One instability mechanism that PCOM has been especially helpful in clarifying is air supply variation. Instabilities via air supply variation occur when pressure fluctuations in the combustor produce corresponding changes in the nozzle air flow. Given a constant fuel flow, the result is a time varying fuel/air ratio. This fluctuation in the fuel/air ratio creates a fluctuation in the reaction rate, which results in an oscillatory heat release. If the resultant heat release fluctuation is in phase with the acoustic pressure, the feedback loop is closed and a limit cycle oscillation may be attained. For these experiments, all of the LPM air and a majority of the LPM fuel are premixed upstream and enter the nozzle through the LPM ports. A small percentage of the LPM fuel enters through a bypass port that is choked at the nozzle interface. Figure 4 shows the RMS pressure in the experimental combustor versus the bypass port position for an air flow rate of 17 g/s and an equivalence ratio of 0.7. The "No Bypass" data is a baseline measurement which indicates the level of instability when no fuel passes through the bypass port. An oscillation at 5 |
