Modelling of swirling flow instabilities.
This research concentrates on the swirl motion, and in particular the flow structure
which develops under its action, in swirl burner/furnace systems. Although the
Reynolds numbers for such systems are usually large and well into the turbulent
regime, periodic oscillations and associated instabilities are still prevalent. The
predominant coherent structure is the so-called precessing vortex core (PVC) which is a
three-dimensional, time-dependent phenomenon. It is helical in shape, twisted against
the flow, and precesses around the geometric centre of the system, in the sense of the
The aim of this work was to numerically model this instability in a 2MW industrial-size
system, under isothermal conditions. A fully three-dimensional, time-dependent model
was developed using the CFD (Computational Fluid Dynamics) software FLUENT.
This study first presents an overview of publications on vortex breakdown, a similar
phenomenon observed initially on delta wings, in order to highlight its significant
features. A summary was also made of various recent studies, experimental and
theoretical, carried out at Cardiff University, in the same equipment as used in the
present work. This review allows a better understanding of the phenomenon and
constitutes a basis for further validation of the mathematical model.
Numerous flow pattern characteristics have- been predicted, which agree qualitatively
with different published studies, such as crescent shaped regions of maximum axial and
tangential velocities, off-centred reverse flow zone, and spiralling vortex core.
Quantitatively, the agreement is good, in terms of range of velocities and frequency.
However, the predicted flow pattern could. not be maintained in time and tended back to
axisymmetry, possibly due to numerical diffusion. Grid refinement could not, however,
be envisaged due to the practical limits of the available machines. Nevertheless, these
results are encouraging and prove that mathematical modelling of these complex flows
is a realistic objective.