PHASE AVERAGING OF THE PRECESSING VORTEX CORE IN A SWIRL BURNER UNDERPILOTED AND PREMIXED COMBUSTION CONDITIONS

The flow patterns produced in and past the exhaust of a 100-KW swirl b urner have been investigated experimentally under piloted premixed com bustion conditions. The well-known three-dimensional time-dependent in stability called the precessing vortex core (PVC) dominates the flow a nd 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 o f the rotating flow fields. Measurements were obtained over successive slices of the flow, extending to X/De = 2.5 past the burner exit. A d escription of the flow was thus obtained in terms of phase averaged ta ngential, axial and radial velocities in tangential/radial and axial/r adial planes. The results confirm previously reported work on the same burner operated isothermally and show that the center of the vortex f low is displaced from the central axis of the burner, creating the PVC phenomena as the center of the vortex precesses around the central ax is 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 degrees. The RFZ acts as a feedback mechanism for th e 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 t angential velocity and gives the characteristic PVC signal. The displa ced RFZ is both rotating through a region of forward flow whilst also being of an intermittent nature, giving rise to the excellent flame st abilisation and mixing characteristics of these types of burners. Simi lar results were obtained for isothermal and premixed combustion condi tions providing the flame was stabilised close to the burner exit nozz le.