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Numerical simulation has been used in an attempt to explain the complex interactions between large swirling flow coherent structures and combustion. Despite some successes, the results obtained for more intricate cases with high flow rates, high swirl involving combustion, leave much to be desired .Therefore, systems that use high swirl numbers require extensive and expensive experimentation for optimisation. A relatively simple swirl burner design is thus used to characterise a whole family of coherent structures which arise from complex combusting flows.
For swirling flows some of these complexities were first recognised in the 1960’s. The swirl number (S), Reynolds number (Re) and Strouhal number (Sr) are commonly used for characterisation purposes. Detailed derivations of swirl number are extensively discussed elsewhere . A direct approach has been used deriving a geometrical swirl number (Sg) for each burner highlighted earlier .
The phenomenon of interest occurs for swirl numbers >0.5–0.6 after vortex breakdown and the formation of a central recirculation zone . There has been considerable steady state characterisation of swirl burners and related systems, but very little time dependent studies.
Vortex breakdown, typically at a swirl number of 0.5, is characterised by a transition from a supercritical to a subcritical state, the formation of a central recirculation zone (CRZ), and normally a precessing vortex core (PVC) which this work shows is one of a whole family of coherent structures. The CRZ is the phenomenon which is used extensively in combustion systems for extending flame holding and stability (whilst keeping the hot flame away from the walls) as well as minimising emissions via staged combustion techniques. In the combustion state the associated PVC can often be damped with diffusion controlled combustion, but premixing is well known to excite the amplitude of the PVC to the detriment of the system [13], whilst acoustic coupling can make the PVC reappear even in diffusion controlled combustion.
The PVC appears at moderate Re, normally after vortex breakdown and the formation of a CRZ. This structure appears to be a mechanism for the rapid transport of fluid, caused by the sudden precession of an internal vortex in the vicinity of the CRZ. Although some systems have been classified as precessing or stable swirling flows , the mechanism of this phenomenon is unclear. Some authors have studied the phenomenon under isothermal conditions , whilst others have theorised its nature as a series of small eddies generated by the CRZ .
This paper highlights an experimental approach to study the above to provide benchmarking for numerical simulation, whilst providing new descriptions for the time dependant nature of the CRZ which is so important for flame stabilisation.
2. Experimental details
Experimental studies were performed using a 100 kW Perspex model of a 2 MW swirl burner, which has been extensively analysed [13]. Two tangential inlets are used together with variable width inserts to give a range of swirl numbers (Sg). The system was fed by a centrifugal fan providing air flow via flexible hoses and two banks of rotameters, each feeding one inlet, allowing a Re range from 5700 to 61,000 (based on burner exit diameter). Fig. 1 shows details of the geometry of the system.
Two different inserts were used to change the swirl number. A third configuration used no insert. Nomenclature was developed to take account of the blocked area during the testing regime These are reported as IC-50_50, IC-50_0, IC-25_25, etc., where for instance IC-50_0 refers to an inlet condition with one tangential inlet half blocked off, the other being 100% open (0% obstruction). A total of nine configurations were analysed including three with different inserts in the two inlets and three with only one inlet. Fig. 2 shows a schematic representation of the burner with tangential inlet inserts. Both axi-symmetric or asymmetric flow conditions could be thus produced.