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.

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 93-161, 2006-as well as subsequent publications

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.

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)

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

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

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

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

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

Typical Siemens High Swirl DLE Combustor

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

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

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?

Visualisation of burning PVCs, one & two states

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

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

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

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.

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

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

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

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

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

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

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

Tangential Directional Intermittence Data Counter rotational instability substantially reduced by quarl

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

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

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

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

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

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

Pressure velocity relations in standing and travelling waves