Central recirculation zone analysis in an unconfined tangential swirl burner with varying degrees of premixing |
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Authors: | A Valera-Medina N Syred P Kay A Griffiths |
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Institution: | (1) Turbomachinery Department, CIATEQ, Parque Industrial Bernardo Quintana, Manantiales no. 23A, Queretaro, CP 76246, Mexico;(2) School of Engineering, Cardiff University, Queen’s Building, The Parade, Cardiff, Wales, CF24 3AA, UK |
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Abstract: | Swirl-stabilised combustion is one of the most widely used techniques for flame stabilisation, uses ranging from gas turbine
combustors to pulverised coal-fired power stations. In gas turbines, lean premixed systems are of especial importance, giving
the ability to produce low NOx systems coupled with wide stability limits. The common element is the swirl burner, which depends
on the generation of an aerodynamically formed central recirculation zone (CRZ) and which serves to recycle heat and active
chemical species to the root of the flame as well as providing low-velocity regions where the flame speed can match the local
flow velocity. Enhanced mixing in and around the CRZ is another beneficial feature. The structure of the CRZ and hence that
of the associated flames, stabilisation and mixing processes have shown to be extremely complex, three-dimensional and time
dependent. The characteristics of the CRZ depend very strongly on the level of swirl (swirl number), burner configuration,
type of flow expansion, Reynolds number (i.e. flowrate) and equivalence ratio. Although numerical methods have had some success
when compared to experimental results, the models still have difficulties at medium to high swirl levels, with complex geometries
and varied equivalence ratios. This study thus focuses on experimental results obtained to characterise the CRZ formed under
varied combustion conditions with different geometries and some variation of swirl number in a generic swirl burner. CRZ behaviour
has similarities to the equivalent isothermal state, but is strongly dependent on equivalence ratio, with interesting effects
occurring with a high-velocity fuel injector. Partial premixing and combustion cause more substantive changes to the CRZ than
pure diffusive combustion. |
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