Flame dynamics of a variable swirl number system and instability control

Swirling flame dynamics is investigated using a radial swirler equipped with movable blades. This device allows to continuously change the swirl number and to examine various flame configurations while keeping fixed values of mass flow rate, equivalence ratio, and other system parameters. This is used to reveal two types of thermo-acoustic instabilities, the first giving rise to a low frequency bulk oscillation while the second corresponds to a higher frequency mode. Because the swirl number plays a central role in this analysis, it is here deduced from measured axial and azimuthal velocity profiles under stable operating conditions. It is first found that values deduced from experimental profiles are lower than those estimated from standard expressions relying on geometrical parameters of the swirler. A modified expression derived for swirlers without central insert is proposed that improves swirl number estimates. Systematic variations of the swirl number indicate that the flow features a central recirculation zone when this number is sufficiently large and that a precessing vortex core (PVC) exists under cold flow and reactive conditions for measured swirl numbers exceeding 0.45. Velocity, chemiluminescence, and pressure signals are then used to identify frequencies corresponding to the PVC, self-sustained thermo-acoustic oscillations, and the difference frequency of the previous components. The flame transfer function when the burner is submitted to harmonic flow rate modulations is then determined by progressively increasing the swirler blade angle. As the swirl number is augmented, it is found that the phase lag decreases for a fixed forcing frequency while the transfer function gain covers a broader frequency range. These data are finally used in a linear stability analysis to identify the origin of combustion instabilities observed in the present experiment. It is shown that the time lag between velocity perturbations at the burner outlet and resulting heat release rate disturbances strongly depends on the swirler blade angle. It is also demonstrated that the adjustable blade angle swirler could be used in principle to control the flame dynamics and avoid regions of instability.