Abstract
The present work investigates the formation process and early stage evolution of turbulent swirling vortex rings, by using planar Particle Image Velocimetry (PIV) and Large Eddy Simulation (LES). Vortex rings are produced in a piston-nozzle arrangement with swirl generated by 3D-printed axial swirlers in experiments. Idealised solid-body rotation is applied in LES to evaluate the effect of nozzle exit velocity profile in experiments. The Reynolds number (Re) based on the nozzle diameter D and the slug velocity U0 in the nozzle is 20,000. The swirl number S generated ranges from 0 (zero-swirl vortex ring) and 1.1, covering the two critical swirl numbers previously identified in a swirling jet. Both PIV and LES results show that the formation number F decreases linearly as S increases, with the maximum F ≈ 2.6 at S = 0 (produced by the swirler with straight vanes) and minimum F = 1.9 at S = 1.1. The corresponding maximum attainable circulation in the nozzle axis parallel plane also diminishes with increasing S. Evolution of compact rings produced by a stroke ratio L/D = 1.5 reveals that circulation decay rate is largely proportional to S. The trajectory of the vortex core in the axial direction, hence the ring axial propagation velocity, decreases as S, while that in the radial direction and the radial propagation velocity, increase with S. An empirical scaling function is proposed to scale these variables.
Highlights
Turbulent pulsatile injection [1] and continuous swirling injection [2] are two ways of flow momentum delivery used in combustion applications to enhance fueloxidiser mixing
We report the results in detail since they do not seem available in literature for similar designs
Axial swirlers were used to generate swirl and S produced covers all the dynamic regimes and the two critical swirl number Scr1 and Scr2 identified in a continuous swirling jet at a lower Reynolds number (Re) [24]
Summary
Turbulent pulsatile injection (puff) [1] and continuous swirling injection (swirling jet) [2] are two ways of flow momentum delivery used in combustion applications to enhance fueloxidiser mixing. For a swirling jet, when the swirling momentum exceeds a certain threshold, it is featured by a bluff-body wake-like recirculation zone at the jet exit and self-excited azimuthal vortex structures, which break down to processing helical vortex cores. These characteristics are determined by the relative swirling strength [5] and have been extensively applied in jet engine combustion processes. The combination of these two types of flow generates swirling puffs or swirling vortex rings. Fundamental understanding of the primary non-reacting flow physics of the swirling vortex rings is desirable, which, is limited to the best of the authors’ knowledge
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