The effect of baffles on self-excited azimuthal modes in an annular combustor

Abstract

In this paper the effect of inserting baffles on the self-excited azimuthal modes and unsteady heat release rate in an annular combustor are investigated experimentally. Particular attention is given to their effect on the time-varying behaviour of azimuthal modes observed in recent experiments and Large Eddy Simulations (LES) in azimuthally symmetric annular chambers. This time-varying behaviour causes the azimuthal modes to switch back and forth between spinning and standing wave modes. With the addition of a single baffle, the azimuthal symmetry of the chamber was broken leading to a coupling between the clockwise (CW) and anticlockwise (ACW) azimuthal acoustic waves. This eliminated the time-varying behaviour and promoted standing wave modes. It was found that three or more baffles were required to achieve significant damping of the modes. Since almost perfect standing wave modes occurred with the addition of a single baffle, high-speed chemiluminescence measurements were obtained to characterise the unsteady heat release rate at the pressure node and anti-node. Previous studies have shown that maximum and minimum heat release rate is produced at the anti-nodes and nodes respectively. As found previously, peak fluctuations at the anti-nodes result from axisymmetric fluctuations in heat release rate caused by pressure fluctuations driving axial mass flow fluctuations at the burner inlet. However, at the pressure nodes, an anti-symmetric structure was observed producing negligible heat release rate via the mechanism of cancellation. This latter result suggests that transverse velocity fluctuations play an important mechanistic role in ensuring that negligible heat release rate is produced at the node.