Acoustic Streaming and System Resonance Effects in High Intensity Swirl Burners Nicholas Syred, Jonathon Lewis, Agustin Valera-Medina, Phil Bowen Cardiff School of Engineering Wales, UK

Aims and Objectives We analyse and discuss the main sources of coupling which can occur in swirl burners via experimental results gained over many years and how this can interact and modify the combustion aerodynamics of the system. We start by considering the well known Geometric swirl Number which is then modified by combustion: Sg= Axial flux of Angular Momentum Re*Axial Flux of Axial Momentum Then with combustion S*=Sg*T isothermal T combustion s* is a function of Φ (ie flame temp.), and location/shape of flame, hence mode of fuel entry/addition, diffusion, partially premixed and fully premixed.

AB C 3 flames from same swirl burner S=2 (8 slit inlets no fuel gun) A-premixed B-partially premixed via tangential fuel entry C- diffusion fuel entry solely through 1 inch orifice in baseplate Fuel natural gas- mixture ratio~1.6 in all cases (equiv Ratio~0.67) Note differences in flame shape, heat release rate, hence altered Swirl Number due to combustion

Effect of Equivalence Ratio on Averaged reversed Axial Velocity in CRZ, S=1.08. Fuel natural gas. Nomenclature, refers to 25 l/min diffusive and to 80 l/min premixed. Clearly different modes of fuel entry give different CRZz

Diffusion flames- stable to pressesional influences, ie no PVC Top heavy fuel oil power station swirl burner- rotary cup atomiser Bottom Natural Gas fired 100KW Swirl Burner, Sg=1 Diffusion Fuel entry along axis Note similarities in flame shape and tulip shaped base region. Flames very stable to tangential or precessional disturbances, but can be excited by axial resonances PVC normally substantially damped. Note high centre line axial velocities can re-excite the PVC into a double system Many pf flames from front wall fired burners are similar to that from heavy fuel oil

Why is axial fuel entry along centre line so stable? Beer et al (C&F Vol 16, p 39, 1971)developed a modified Richardson No. to describe the re-laminarizataion or restabilisation of turbulent axial burning (or helium) central jets in a swirling environment R i *= (1/ρ)(dρ/dr)(W 2 /r) (dU/dr) 2 Ratio of centrifugal force fields and density gradients to the shear forces. Stabilization effects begin when R i *> 0 Worked well with Beer’s case.

In the case of central fuel injection & high fuel concentration in CRZ region gives high CRZ temps- surrounded by(initially) much lower temperature flow- hence strong +density gradients. Hence initial section of flow stable as R i * >1. Not so with premixed/partially premixed combustion where highest temps in outer annular shear flow, CRZ recycles cooler burnt products, hence density gradients -ve hence give s–ve R i * We shall now show how londitudinal acoustic coupling ¼ wave or helmholtz can completely modify the combustion aerodynamics

Resonance coupling rig Telescopic tube attached to small swirl burner S~2 and oscillation behaviour investigated as a function of Φ & tube length. Strong ¼ wave resonant acoustic coupling found especially around L~110cms and Φ~ 1.5 (equivalence ratio ~0.67 Although axial fuel entry/diffusion combustion stable to precessional perturbations not so for axial ones- tangential/partial premixing with the PVC in fact damps this oscillation

This is the tangential fuel entry case, frequencies virtually same as axial fuel entry. However Amplitudes reduced by ~ 60% Presence of PVC with tangential fuel entry damps longditudinal resonance. Thus PVC does not couple with ¼ wave resonances but attenuates or partially damps them

Oscillation Frequencies recorded in and around a Proprietory Gas Turbine Swirl Burner in the HPOC Note the almost constant frequency over a very wide range of Fuel flow and hence Φ. Clearly helmholtz resonance Near blowoff Flashback

Configurations of 100Kw Swirl Burner used to investigate Combustion Oscillations

a) Rms amplitude of oscillation as a function of Φ b) Corresponding Maximum Frequencies

Phase locked LDA Nat. Gas Partially premixed a) Axial velocities b) Axial directional Intermittency c) Tangential velocities d) Tang. Directional Intermittency Measurements taken just above burner exit

Variation of Swirl Number over an Oscillation Cycle in a rig very similar to that of figure 2a. The rig simulates at small scale a gas turbine combustor. Natural Gas partially premixed combustion

a)left Phase averaged Axial Velocities and right b) Tangential Velocities in the Furnace exhaust ab

Precession of coherent structures in vortex swirl flows well documented. Can take several forms including: Single precessing structures including the vortex core, CRZ, ERZ and the high momentum flow region (HMFR)embedded in the annular shear flow Double precessing structures-PVC2. Double CRZs also been seen Combustion induced vortex breakdown (CIVB) - CRZ can jump just past the burner exit to upstream in the burner. Different PVC states then present. Various analytical models have been developed to predict frequency of the PVC (ie. small perturbation analysis) initial excitation mechanism unclear In the authors experience longditudinal /helmholtz resonances always dominate and stream the flow preferentially. PVCs can damp longditudinal acoustic waves Illustrate precession via data showing how precessing wall flames can form - another form of vortex breakdown – Coanda vortex breakdown COVB

a)Generic 28 mm Exhaust swirl Burner b)b) Detail of the Exhaust Nozzle showing relationship c)to Base Plate. Note nozzle angle α and step size ΔY a b

Phase Locked Axial radial Plane Stereo PIV Images of Velocity Contours across exit Plane of Swirl Burner of Figure 9 a. A)OJF-MS shape. Re~15,500 ΔY/D=0.107 and α=45°. U vel. m/s B)COVB shape. Re~15,500, ΔY/D=0.000 and α=45°. Total UV velocity in [m/s]. A) Ordinary Swirl Jet Flow B) Coanda Jet Flow sticks to Plate

COVB Flat Flame attached to Baseplate. Re~25,000, ΔY/D=0.082 and α=45° Premixed Methane Air Flame T V

A)OJF-MS Superimposed Axial velocity contours on top of V_W vectors and B) V-W velocity vectors, respectively. Re~19,000, ΔY/D=0.082 and α=45°. PVC (red) and ERZ (black) encircled. Velocity in [m/s]. Direction of precession clockwise A)B)

COVB A) Superimposed Axial velocity contours on top of V-W vectors and B) V-W velocity vectors, respectively. Re~19,000, ΔY/D=0.082 and α=45°. PTV (black) encircled. Velocity in [m/s]. A) B)

St v Re for all the experiments. Note Shift in frequencies for ΔY/D=0.082.

Conclusions Swirl Burner Burner Flows are easily deformable and couple with external disturbances such as acoustic or system instabilities (ie compressor & turbine surges/instability/stall) One common form of londitudinal disturbance is that due to acoustic resonances, especially helmholtz and longitudinal acoustic waves. These swirl flows will readily couple, deform or stream with these disturbances often completely altering the combustion characteristics

Another common form of disturbance is precessional in nature generating the PVC, precessing CRZs and other coherent structures. The exact excitation mechanism is not clear. Some modes of combustion (ie axial fuel entry and diffusion combustion) can damp these precessional modes due to the right +ve density gradients. However they do not damp the longitudinal oscillations Strong evidence that presence of PVC type flows can damp longditudinal or helmholtz rsonances, not excite then as commonly thought

Vortex Amplifier

Pure tang. Flow- bottom of Characteristic. PVC/CRZ type flow Pattern but perfectly stable no resonant coupling. Note Sw~7, but PVC frequency much lower than found in burners S=1

Small Capacity Vortex Amplifier. Operating Characteristic shows Main Flow (Qs) and Control Flow (Qc) as a Function of Pressure Ratio from Tangential Inlet to Exhaust to that from the Radial Inlets to the Exhaust. System Swirl Numbers (Sw) along the characteristic also relate to static pressure curves later

Large Capacity Vortex Amplifier Operating Characteristic. The unstable and oscillatory sections shown

Sw=0 Sw=0.2 Sw=7.3 7 Sw=3.8 Sw=0.93 Static wall Pressure in Vortex Amplifier from Vortex Chamber into Outlet Diffuser. Note change from Negative to Positive Wall Pressure at Diffuser Throat as Swirl Number changes from 0.93 to 3.73