Abstract of VISUALISATION OF VORTEX BREAKDOWN IN TURBULENT UNCONFINED JET FLOWS,
I.S. Carvalho and M.V. Heitor.
Laboratory of Combustion and Heat Transfer,
Instituto Superior Tecnico, Technical University of Lisbon, Department of Mechanical Engineering, 1096 Lisboa Portugal

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The spatial structure of the recirculating flow typical of coaxial swirling jets stabilised downstream of a model prefilming airblast atomiser with a conical shape is presented and discussed based on flow visualisation. The results include the analysis of the effect of swirling the outer jet on the mixing performance of the atomiser. For weak swirl levels, the instability and rolling-up of the shear layer established between the two jets leads to flow asymmetries associated with a spiral pattern and an abrupt change in the structure of the core of the flow. A comparison is established between this type of flow and the near-axisymmetric form of vortex breakdown characterised by the occurrence of a recirculation zone in the vicinity of the nozzle. A sequence of transitions between these two types of vortex flows is presented, and provides information on the topology of the flow after the onset of vortex breakdown.

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