1. Nick Syred Cardiff School of Engineering Wales, U.K.
Updating Swirl, Vortex, Cyclonic Flows, and Combustion with 40 more years experience
Nick Syred
Cardiff School of Engineering
Wales, U.K.

2. Rationale for this presentation
This 40 year old paper has stood the test of time, however there has been much filling in of gaps of knowledge in this time which this presentation tries to address
More details-see my review paper; Syred, N., A Review of Oscillation Mechanisms and the role of the Precessing Vortex Core (PVC) in Swirl Combustion Systems , Progress in Energy and Combustion Systems, vol 32, no 2, p , 2006-as well as subsequent publications

3. Objectives
Describe processes whereby swirling flow, combustion, fluid circuits or system acoustics interact to produce instability and large amplitude oscillations
Quantify and characterise the effect of instability on the structure of the flow, recirculation zones and coherent structures
Associate flow structures with driving/damping regions.
Study the possibility of enhancing flow stability by simple design changes as a methods of passive control.

4. Vortex breakdown- as swirl is increased from very low levels
Left-various vortex breakdowns,
followed by formation of a large
central recirculation zone and
quite often a precessing vortex
Core.
Critical Swirl No ~0.5 as
Vortex Breakdown starts,
Depends on flow geometry
Below spiralling PVC
Vortex breakdown- as swirl is
increased from very low levels
a free jet gradually expands until
It reaches the point of Vortex of
Breakdown and flow reversal starts
to form on axis. A little bubble of
recirculating flow forms, often
followed by a spiralling vortex, often
Called the precessing vortex core (PVC)

5. Sarpkaya’s curves of the
Vortex breakdown, showing
Position as a function of
Swirl No. and Re. No.
Clearly here the Vortex
Breakdown only reaches
the back wall for high Re
and Swirl Nos.
Thus flow very sensitive to
movement of Vortex
Breakdown bubble and
associated CRZ
Will show later problems
with operating close to vortex breakdown.
CRZ can vibrate longditudinally

6. Formation of CRZ, crucial to swirl burners, for flame stabilization.
Also occur in exhausts
of vortex amplifiers, vortex
diodes, helps to increase
flow resistance in vortex flow
state by reducing flow area.
CRZs cause problems, however
as can be excited by external
stimulation., combustion,
acoustics, control circuits etc.
CRZ vibrates axially coupling
with other resonances.
Size, shape and extent of CRZ
strongly affect by swirl no., system geometry, equivalence
ratio with combustion.
CRZs and the precessing vortex
core (PVC) can also interact

7. Combustion and Swirling Flow
Various Processes in Gas and Liquid fuelled Combustors and associated Characteristic times
Process
Time Scale milliseconds
Acoustic disturbance of 100 Hz 10
Acoustic disturbance of 500 Hz 2
Chemical kinetic ignition delay, φ=0.7 ~1
Chemical kinetic ignition delay, φ=1 ~3
Convection of disturbance 100 mm at 10 m/s
Convection of disturbance 100 mm at 50 m/s
PVC frequency say 100 Hz
Evaporation of 10 micron hydrocarbon droplet
0.3
Evaporation of 50 micron hydrocarbon droplet 8
Growth rate of an acoustic disturbance 10mm diameter liquid jet
125
Propagation of an acoustic disturbance 100 mm at 330 m/s (300K)
Propagation of an acoustic disturbance 100 mm at 600 m/s (1000K)
0.17

8. Note sensitivity of ignition time to φ perturbations for φ< 0.8

9. Φ perturbations less likely to cause reaction rate perturbations
for φ > 0.8, hence problems with LPP GT combustors as φ<0.8

11. Typical Siemens High Swirl DLE Combustor

12. Typical Flow Aerodynamics in a Natural gas Fired
Swirl Stabilised Gas Turbine Combustor (Turrell et al 2004)

14. Three dimensional time dependent coherent structures formed
in exhaust of swirl burners

15. Swirl Combustors and Instability
Despite the effect of φ fluctuations on instability, there are other important influences
It is quite well known that with large pressurised process plant involving long pipe work runs and cyclone dust separators resonances can occur between pipe work acoustics and the cyclone dust separators
Remedy centre body in cyclone exhaust- destroys PVC, centre body at base of cyclone useful as well-stabilises vortex core
Fluidics-vortex amplifiers well known to be unstable mid characteristic
Thus coupling between fluid dynamics/coherent structures and system acoustic possible
How?? – Interaction with combustion- acoustics coupling ?
Acoustic streaming effects?

16. Visualisation of burning PVCs, one &
two states

18. Coherent Structures formed in exit of Swirl Burner, plus those in confined conditions
Results
Real flow Open Case.
Real flows Confined Conditions.

19. Linlet
Premixed
gas De
Dfurn = 2
Dnozz
Pilot
gas
Lfurn
Lnozz

20. Left hand slide shows low frequency oscillation,
70<1<40 Hz, based on 66 sets of experimental results.
Right hand slide shows high frequency oscillation discussed here
270<1<240 Hz, note how small geometric changes cause this change

21. Experiments Dynamic pressure Global OH chemiluminescence
2D triggered LDA
Post processed data for: spectral content, limit cycle resolved flowrates and residence times and Rayleigh Index.

22. Test Rig Configurations
Lf = 260, Ln = 120, De = 76 mm, Df = 150 mm

23. Pressure and Spectral Data
Top: expansion geometry 2a
Bottom: expansion geometry 2b

24. Flow Response and Stability Map
Pressure amplitude vs. Equiv. Ratio
Grey - expansion geometry 2a
White - expansion geometry 2b
Velocity derived PSD.
Black - Axial data
Red - Tangential data

25. Rayleigh Index and Spectra
LEFT: Simultaneous p’, q’, p’*q’ and K at x/De=1.56
RIGHT: p’ and q’ power spectra

26. Global Rayleigh Index Along Furnace Axis
Left: G(x) for expansion geometry 2a, limit plane for +ve values
at x/De = 1.4
Right: Expansion geometry 2b, limit plane for +ve values
at x/De = 1.3

27. Axial Phase Avg. Velocity Profiles
Note difference in height and structure of Recirculation Zones

28. Axial Directional Intermittence
Note enhanced stability of corner and central
recirculation for expansion geometry 2b

31. Tangential Directional Intermittence Data
Counter rotational instability substantially reduced by quarl

32. Distortions produced
in flame front by
acoustic coupling
of two transverse
modes of the
combustion chamber,
resulting in a helical
structure. The
iso-surface is of
the 1000K
temperature.
f~1200 Hz

33. Vortex Amplifier used for control of ventilation Flows for handling
Nuclear materials. Advantages no moving parts, fast response if leak
Develops. Works be modulating main high volume flow by vortex flow

34. Vortex Amplifier Characteristic, Case 1 low resistance pure supply
Flow: Case 5 high resistance low flow. Note jump in ‘characteristic’
Oscillations in this region as well as at point of vortex breakdown

35. Corresponding wall static pressure distributions: curve 1 low resistance
Diffuser gives very low throat pressure: curve 5 pure vortex flow
Thus wall pressure difference at throat varies enormously, easily
oscillates

36. Top no axial reverse flow Case 2
Bottom axial reverse flow-CRZ oscillates
Low frequency oscillations in flow

37. Conclusions
The subject of instabilities occurring in swirling flows has been reviewed in the context of swirl burner and vortex amplifiers with coupling between the fluidic dynamics, acoustic, combustion or circuits.
Considerable excitation commonly occurs and one of the main mechanisms appears to be axial oscillation of the central recirculation zone (CRZ) which amplifies small pressure fluctuations produced by other mechanisms
The precessing vortex core (PVC) can also acts as a stimulant to large scale oscillations. Here it often appears to interact with the CRZ increasing 3 dimensionality and propensity to oscillate
As many studies have shown swirling flow systems appear to be very sensitive to small scale perturbations which can be amplified by many mechanisms including CRZs and PVCs

38. Pressure velocity relations in standing and travelling waves