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Experiments in Fluids

, Volume 50, Issue 6, pp 1611–1623 | Cite as

Central recirculation zone analysis in an unconfined tangential swirl burner with varying degrees of premixing

  • A. Valera-MedinaEmail author
  • N. Syred
  • P. Kay
  • A. Griffiths
Research Article

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.

Keywords

Equivalence Ratio Fuel Injector Premix Flame Premix Combustion Particle Image Velocimetry System 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

List of symbols

Ae

Tangential inlet area [m2]

CRZ

Central recirculation zone

D

Exit diameter of burner

Dcrz

Maximum diameter of CRZ

HMFR

High-momentum flow region

JMR

Jet momentum ratio, fuel jet axial momentum relative to that in the surrounding annulus [−]

L

Length of CRZ beyond burner exit, [m]

NUcrz

Uaxrev/Uannulus [−]

PVC

Precessing vortex core

Re

Reynolds number. Here defined using average exhaust velocity and gas kinematic viscosity under isothermal conditions [−]

re

Exit radius of the burner [m]

ri

Radius upon which the tangential inlet jets fire [m]

S*

Swirl number modified by combustion (Valera-Medina 2009a)

Sg

Geometrical swirl number (Syred 2006)

Uannulus

Average axial velocity in the annulus surrounding the fuel injector [m/s]

Uaxrev

Average axial velocity in CRZ derived from PIV measurements [m/s]

φp

Premixed equivalence ratio

φt

Overall equivalence ratio

Notes

Acknowledgments

Agustin Valera-Medina gratefully acknowledges the receipt of a scholarship from the Mexican Government (CONACYT) to carry out his PhD programme at Cardiff University and for the assistance of Malcom Seaborne during the set-up of the experiments.

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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  • A. Valera-Medina
    • 1
    Email author
  • N. Syred
    • 2
  • P. Kay
    • 2
  • A. Griffiths
    • 2
  1. 1.Turbomachinery DepartmentCIATEQ, Parque Industrial Bernardo QuintanaQueretaroMexico
  2. 2.School of EngineeringCardiff UniversityCardiff, WalesUK

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