Phase averaging of the precessing vortex core in a swirl burner under piloted and premixed combustion conditions

The flow patterns produced in and past the exhaust of a 100-KW swirl burner have been investigated experimentally under piloted premixed combustion conditions. The well-known three-dimensional time-dependent instability called the precessing vortex core (PVC) dominates the flow and mixing patterns. The PVC and its associated cycle time were used to trigger a three-component laser anemometry system. Successive cycles were overlaid and phase averaged to give a three-dimensional picture of the rotating flow fields. Measurements were obtained over successive slices of the flow, extending to X/De = 2.5 past the burner exit. A description of the flow was thus obtained in terms of phase averaged tangential, axial and radial velocities in tangential/radial and axial/radial planes. The results confirm previously reported work on the same burner operated isothermally and show that the center of the vortex flow is displaced from the central axis of the burner, creating the PVC phenomena as the center of the vortex precesses around the central axis of symmetry. As a consequence of this displacement the reverse flow zone (RFZ) is also displaced, while also partially lagging behind the PVC by up to 180°. The RFZ acts as a feedback mechanism for the PVC phenomena. As a consequence of the displaced vortex centre, flow between the PVC centre and the wall is squeezed. Thus, due to angular momentum flux consideration, it produces a considerable increase in tangential velocity and gives the characteristic PVC signal. The displaced RFZ is both rotating through a region of forward flow whilst also being of an intermittent nature, giving rise to the excellent flame stabilisation and mixing characteristics of these types of burners. Similar results were obtained for isothermal and premixed combustion conditions providing the flame was stabilised close to the burner exit nozzle.