A review of oscillation mechanisms and the role of the precessing vortex core (PVC) in swirl combustion systems

Abstract

This paper reviews the occurrence of the precessing vortex core (PVC) and other instabilities, which occur in, swirl combustion systems whilst identifying mechanisms, which allows coupling between the acoustics, combustion and swirling flow dynamics to occur. Initially, the occurrence of the PVC in free and confined isothermal flows is reviewed by describing its occurrence in terms of a Strouhal number and geometric swirl number. Phase locked particle image velocimetry and laser doppler anemometry is then used to describe the three-dimensional flow fields, which are generated when swirling flow is discharged into an open environment. This shows the presence of a rotating and precessing off centred vortex and associated central recirculation zone (CRZ), extending up to one burner exit diameter. The presence of axial radial eddies close to the burner mouth, in and around the CRZ, is clearly shown. Typically one large dominant PV is found, although many harmonics can be present of lower amplitude. The occurrence of these phenomena is very much a function of swirl number and burner geometry. Under combustion conditions the behaviour is more complex, the PVC occurrence and amplitude are also strong functions of mode of fuel entry, equivalence ratio and level of confinement. Axial fuel entry, except at exceptionally weak mixture ratios, often suppresses the vortex core precession. A strong double PVC structure is also found under certain circumstances. Premixed or partially premixed combustion can produce large PVC, similar in structure to that found isothermally: this is attributed to the radial location of the flame front at the swirl burner exit. Provided the flame is prevented from flashing back to the inlets values of Strouhal number for the PVC were excited by ∼2 compared to the isothermal condition at equivalence ratios around 0.7. Confinement caused this parameter to drop by a factor of three for very weak combustion. Separate work on unconfined swirling flames shows that even when the vortex core precession is suppressed the resulting swirling flames are unstable and tend to wobble in response to minor perturbations in the flow, most importantly close to the burner exit. Another form of instability is shown to be associated with jet precession, often starting at very low or zero swirl numbers. Jet precession is normally associated with special shapes of nozzles, large expansions or bluff bodies and is a different phenomenon to the PVC. Strouhal numbers are shown to be at least an order of magnitude less than those generated by the PVC generated after vortex breakdown. Oscillations and instabilities in swirl combustion systems are illustrated and analysed by consideration of several cases of stable oscillations produced in swirl burner/furnace systems and two where the PVC is suppressed by combustion. The first cases is a low frequency 24 Hz oscillation produced in a 2 MW system whereby the PVC frequency is excited to nearly six times that for the isothermal case due to interaction with system acoustics. Phase locked velocity and temperature measurements show that the flame is initiated close to the burner exit, surrounding the CRZ, but is located inside a ring of higher velocity flow. Downstream the flame has expanded radially past the high velocity region, but does not properly occupy the whole furnace. This allows the flame and swirling flow to wobble, exciting instability. The next family of oscillations reviewed occur in a 100 kW swirl burner/furnace systems whereby oscillations in the ∼40 Hz range are excited with flow fields akin to those found in pulsating combustors where the flow is periodically stopped in the limit cycle of oscillation. The phase locked velocity and temperature measurements show a number of mechanisms that can excite oscillation including substantial variations in shape and size of the CRZ during the limit cycle of oscillation, and wobble of the whole flame and flow as shown by negative tangential velocities close to the centre line. Analysis is then made of a high frequency ∼240 Hz oscillation in the same 100 kW swirl burner/furnace system, this oscillation being caused by minor geometry changes. The flame was shown to not fully occupy the furnace, allowing irregular wobble and precession of the flow and flame to develop, being especially noticeable close to the outer wall. The addition of an exit quarl to the swirl burner is shown to substantially reduce the amplitude of oscillation by eliminating the external recirculation zone (ERZ), reducing flow/flame wobble and variations in the size and shape of the CRZ. The quarl used was designed to largely occupy the space normally taken up by the ERZ. Two gas turbine combustor units firing into chambers are then considered, strong PVCs are developed under isothermal conditions, these are suppressed with premixing in the equivalence number range 0.5–0.75. PVC suppression is attributed to the equivalence ratios used, the burner configuration, location of the flame front and associated combustion aerodynamics. Other work on an industrial premixed gas turbine swirl burner and can showed the formation of strong helical coherent structures for equivalence ratios greater than 0.75. LES studies showed the PVC contributed to instability by triggering the formation of radial axial eddies, generating alternating patterns of rich and lean combustion sufficient to reinforce combustion oscillations via the Rayleigh criteria. Finally, it was concluded that coupling between the acoustics and flame/flow dynamics occurs through a number of mechanisms including wobble/precession of the flow and flame coupled with variations in the size and shape of the CRZ arising from changes in swirl number throughout the limit cycle. Remedial measures are proposed.