Abstract
Abstract The mechanisms responsible for driving and damping of pressure oscillations in a laboratory combustor have been investigated. The chamber contains a turbulent methane/hydrogen/air premixed flame stabilized behind a rearward-facing step. Shadowgraph cinematography reveals the shedding of large vortices from the step at frequencies of the system acoustic modes. Variations in the fuel equivalence ratio and the mean flow speed result in a wide variety of nonlinear dynamical behavior. Typically, large cycle-to-cycle variations are observed such that energy may be added or subtracted over one cycle of oscillation but zero net energy change occurs many cycles of oscillation. A quantitative version of Rayleigh's Criterion is constructed by using the cross-spectral-density of the measured flame radiation and pressure. The results under one set of operating conditions show that large driving near the flameholder is balanced by equally large damping further downstream. A second set of conditions results in large energy addition to an acoustic mode balanced by attenuation at the mode subharmonic.
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