Experiments in Fluids

, Volume 40, Issue 2, pp 177–187 | Cite as

An analysis of unsteady highly turbulent swirling flow in a model vortex combustor

Research Article

Abstract

This paper reports an experimental investigation of a non-reacting turbulent swirling flow in a practical vortex combustor. The flow was examined for the conditions characteristic of the presence of a breakdown zone and a strong flow instability appearing at swirl numbers S>0.5. Flow visualization techniques, LDA measurements and acoustic probes were employed to study the unsteady flow characteristics. Based on the experimental results a positive first helical mode of instability was identified with a wavelength and frequency depending on swirl. The wavelength was confirmed to grow monotonically with S, while the dominant frequency of the flow pulsations was found to have an unusual parabolic evolution with swirl, with a minimum at Smin=0.88. This finding was interpreted using a proposed kinematic model based on the contribution of two mechanisms: rotation and axial motion of the helical vortex. It was concluded that for S<Smin the instability frequency is essentially dominated by the axial translation of the spiral vortex being inversely proportional to S and therefore giving a decreasing trend. For S>Smin the frequency of the flow precession is more dependent on the angular transportation of the vortex core, which resulted in the expected growing dependence on S.

List of symbols

c

axial phase velocity, ω/k

cθ

angular phase velocity, ω/n

d

nozzle diameter

f

precession frequency

k

axial wavenumber, 2π/λ

n

azimuthal number

r, θ, z

cylindrical coordinates

rm

radius of maximum tangential velocity

Re

Reynolds number, u0d/ν (ν is the kinematic viscosity)

S

geometrical swirl number

Sr

Strouhal number, fd/u0

u0

mean axial velocity in the nozzle

uc

centerline axial velocity

um, wm

maximum axial and tangential velocities, respectively

ω

angular frequency, 2πf

Φ(z, θ, t)

phase function

λ

axial wavelength

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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  1. 1.Department of Mechanical Engineering, Instituto Superior TécnicoLaboratory of Thermofluids, Combustion and Energy Systems, Center for Innovation, Technology and Policy Research, IN+LisbonPortugal
  2. 2.Institute of ThermophysicsNovosibirskRussia

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