Experimental Study of High-Frequency Combustion Instability in a Continuously Variable Resonance Combustor (CVRC)

Abstract

An experimental investigation of high-frequency longitudinal combustion instability using a continuously variable resonance combustor (CVRC) is described. The continuously variable resonance condition is provided by varying the length of a tube carrying oxidizer into a combustor with a rearward-facing step. The a coustic length of the tube is varied between a quarter- and half-wave resonator. The data are analyzed and compared with the analytical results calculated using linearized Eule r equations. As the tube length changes the system transitions between stable and highly unstable operation, allowing the observation of several unique stability behaviors. When the system transitions into instability, higher harmonics appear simultaneously and instantaneously. The transition is accompanied by transitory, unorganized, high amplitude, sub-harmonic pressure fluctuations. When the system transitions into stability regime, the highe r harmonics disappear sequentially, led by the highest order. Small but measurable differences are measured in oscillation amplitude depending on the direction of the sweep (Ω wave to o wave or vice versa). For gaseous fuel experiments, the highest amplitudes of instability do not occur at the least damped acoustic configuration suggested by classical acoustic model s (Ω wave, perfectly coupled resonance condition). This indicates that the gas dynamic eff ects of the system on combustion is important. Comparisons are made between the measured frequency and mode shape data and results from a linearized Euler equation, indic ating generally good agreement, but highlight the need for an accurate combustion respo nse submodel.