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Show the ~aseline le~el .can usually be attributed to a mixture supply variation. Although the overall equlv~lence ~a~lo IS held constant, the oscillation magnitude varies around the baseline level as the axIal posloon of the bypass port is altered. This deviation from the baseline is due to air supply variation. Model results for a comparable configuration are shown in Figure 5. A similar pattern is exhibited. The variation in the RMS pressure as the bypass port location is altered is due to changes in the fuel transport time. The transport time is defined as the total amount of time it takes for a "pocket" of fuel to advect down the nozzle to the flame front, effectively mix, and subsequently combust. Transport time is the summation of advection time, mixing time, and reaction time. If the transport time is such that a fuel pocket generates heat at the peak of combustor pressure, the oscillation will grow. Note that the horizontal axes in Figures 4 and 5 are offset. The model predicts that the axial position of the bypass port at which oscillations are damped is further downstream than the experiment indicates. This is to be expected because the stirred reactor model neglects the mixing portion of the transport time. In order to equate the total transport times, the model essentially adds to the advection time by moving the bypass port location further downstream. Figures 6 and 7 respectively show experimental and numerical RMS pressure versus bypass port location for an air flow rate of 18.9 g/s and an equivalence ratio of 0.7. The figures indicate that the axial location that damps the oscillation is moved further upstream as the air flow rate is increased. Given that the oscillating frequency, or cycle time, is relatively constant from 17 g/s to 18.9 gis, this result is to be expected since the increase in nozzle velocity decreases the transport time. Nozzle Geometry Geometry has been shown to be a dominant factor in determining the stability of a fuel nozzle. Even slight machining tolerances have the potential to cause varying stability behavior (Scheuerman, 1996). PCOM is valuable to understanding the effect of some simple geometrical changes. Figure 8 shows how a slight decrease (1.5%) in nozzle diameter can affect the stability of a nozzle. Note that the data in Figure 5 serves as the baseline (air flow = 17 g/s and equivalence ratio = 0.7). Figure 8 indicates that the axial port which most efficiently damps the oscillation is moved upstream as the nozzle diameter is decreased. The decrease in nozzle area has increased the nozzle velocity for a given flow rate, and therefore decreased the transport time to the combustor. It should be noted that an instability associated with air supply variation, as depicted in Figures 5 and 8, is not the only mechanism which could be affected by a change in ge<?metry. For example, a modest change to the diameter or the length of a fuel nozzle will alter the nozzle impedance at the combustor interface and thereby alter the characteristic~ of an instabil~ty due to mixture supply variation. Figure 9 shows the effect of a 7:5% decr~ase In the no~le diameter. This significant geometrical change has moved . t?e nozzle In . to a fatrly sta~le . reg~me, as indicated by the "No Bypass" data. The instablhty due to mixture feed varIation IS no longer present. 6 |