Secondary effects in combustion instabilities leading to flashback

FOLLOWING previously reported studies of turbulent combustion instabilities leading to flashback, an investigation was carried out of the secondary effects, namely those due to buoyance and contraction at the combustor outlet. As before, the experiments were performed in an oblong, rectangular cross-section combustion tunnel where the effects of a bluff-body flame holder were produced by a rear-facing step behind a streamlined inlet nozzle. Whereas in the previous studies the step was mounted at the bottom, this time it was either situated at the top in order to explore the effects of buoyancy or it was supplemented by a front-facing step to create a trough-shaped enclosure simulating the geometry of a gas turbine combustion chamber. Definite, albeit relatively small, effects of buoyancy were established, while the contraction at combustor outlet had a significant effect in curbing the tendency to flashback. Contents In order to study the essential features of fluid mechanical instabilities in a premixed charge combustor, we have been using a rectangular 5.1x18 cm cross-section combustion tunnel, providing the simplest geometrical configuration for a predominantly planar flow. Principal features of the flowfield created by a bluff-body flame holder have been modeled there by the wake of a rear-facing step, 2.54 cm high, extending over the full width of the test section. In all of the tests commercial grade propane was used as fuel; it was premixed with air in a duct immediately ahead of the test section provided with suitable packing to produce a homogeneous lean mixture at atmospheric pressure and room temperature. In our previous investigation, conducted by the use of this facility,1 three distinct types of unstable behavior were identified: humming, buzzing, and chucking, each distinguishable by a characteristic frequency and a particular flow pattern. Humming is associated with the highest frequency (-170 Hz), evidently being controlled by a standing longitudinal sound wave that accentuates the vortex-pairing pattern of the free shear layer behind the edge of the step. In fact, we have demonstrated that a similar effect can be induced externally by a loudspeaker operating at the same frequency band. Buzzing and chucking manifest the instabilities of the flow system. The former is associated with a flapping motion of the flame up and down across the combustion chamber at a much lower frequency ( — 70 Hz). The latter is characterized