An experimental study on a coaxial flow with inner swirl: Vortex evolution and flow field mixing attributes

A 2D particle image velocimetry study of a coaxial flow with inner swirl is presented. An inner swirling jet, produced by tangential injection, interacts with an annular flow generating a recirculating flow field with strong mixing attributes. The characteristics of the cross-plane velocity components of four different test cases are presented (two levels of tangential injection flow rate combined with two levels of annular flow rate) in order to study the mean and turbulent attributes of the swirling vortex. The main features of this complex flow field, which can be considered as the interaction of a typical swirling jet undergoing “vortex breakdown” with an outer annular flow with “backward facing step flow” characteristics, are investigated, focusing on the swirling jet's characteristics. The analysis of the mean and turbulent flow is based on a modified Rossby number, previously proposed by the authors, defined as the ratio of the streamwise velocity jump across the two streams over a typical tangential velocity, which is shown to represent the ratio of the pressure difference due to the streamwise velocity difference and the entrainment of the two flows to that due to the rotation of the swirling vortex. The angular momentum diffusion downstream is evaluated, to assess the mixing between the swirling vortex and the outer flow.