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

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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.

Introduction

The use of swirl-stabilised combustion is widespread, including power station burners, gas turbine combustors, internal combustion engines, refinery and process burners [1]. The mechanisms and benefits of swirl stabilised combustion are well documented and depend in most instances on the formation of a central toroidal recirculation zone which recirculates heat and active chemical species to the root of the flame, allows flame stabilisation and flame establishment to occur in regions of relative low velocity where flow and the turbulent flame velocity can be matched, aided by the recirculation of heat and active chemical species [1], [2]. These processes are illustrated in Fig. 1.1 and arise as follows:

  • Swirling flow generates a natural radial pressure gradient due to the term w2/r.

  • Expansion through a nozzle causes axial decay of tangential velocity and hence radial pressure gradient.

  • This in turn causes a negative axial pressure gradient to be set up in the vicinity of the axis, which in turn induces reverse flow and the formation of a CRZ.

  • Where the tangential velocity distribution is of Rankine form [1] (i.e. free/forced vortex combination), the central vortex core can become unstable, giving rise to the PVC phenomena.

  • The formation of the CRZ is thus dependent on the decay of swirl velocity as swirling flow expands.

A typical toroidal recirculation zone formed at the exhaust of a swirl burner is shown in Fig. 1.2 for a swirl number of 1.57 and shows the large bubble of time mean recirculated flow that is formed with here 12% of the flow being recirculated [3].

With confinement this process is modified, the rate of decay of swirl velocity is considerably reduced, hence the size and strength of the CRZ formed [1], [2]. This is illustrated by results from a swirl burner furnace system for the combustion of low calorific values gases from carbon black plants [4]. The combustion system is illustrated in Fig. 1.3 and consists of a variable swirl number burner with separate flow controls for axial and tangential premixed air and fuel. This is fired into a refractory lined chamber, the confinement ratio for the swirl burner, Do/De is 2, whilst the Lfurn/Do ratio for the furnace is 2.5. Isothermal velocity results are shown in Fig. 1.4, Fig. 1.5, Fig. 1.6. The tangential velocity distribution, Fig. 1.4, close to the burner exit at x/Do=0.11 shows a peak velocity of ∼17 m/s at r*=0.55; by x/Do=0.33 this peak velocity has been maintained whilst moving radially inwards to r*=0.35. These tangential velocity profiles are then conserved until the end of the furnace. This initial change in tangential velocity profiles induces complex axial velocity profiles and reverse flow zone patterns, Fig. 1.5, and also a PVC close to the burner exhaust. Throughout the furnace a region of forward axial exists on the axis, extending to r*∼0.3–0.4. An annular reverse flow zone, centred at r*=0.5, develops between x/Do=0.11 and 0.69, virtually disappearing by x/Do=1, although there is evidence of a weak intermittent zone to the end of the furnace. Associated velocity vectors are shown in Fig. 1.6 and show the development of the annular CRZ.

The conservation of swirl velocity and hence, angular momentum along the furnace length causes the PVC formed near the burner exit to be of higher frequency, but lower amplitude, than that formed by a free, unconfined, expansion. Moreover, this conservation of swirl velocity also means that there is potential for the formation of further PVCs in the furnace exit downstream. This is discussed later in Section 2.

Despite the advantages of swirl stabilised combustion there is a well known propensity for instability to develop and again there is an extensive literature in this area [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19].

Recent focus has been on lean premixed combustion (LPC) as used with many modern gas turbine systems to ensure low NOx emissions. Premixed flames are by nature more susceptible to static and dynamic instabilities due to the lack of inherent damping mechanisms. The resulting absence of diffusive mixing times leaves flames sensitive to acoustic excitation from sound waves with flame response dependent upon the amplitude, frequency and nature of acoustic wave impingement. If conditions are favourable, periodic fluctuations in the heat release will match the natural resonant frequency of one or more of the geometrical components of the combustor, or related natural fluid mechanic mechanisms, resulting in self-excited thermo-acoustic instabilities. The mechanism responsible for the maintenance of limit-cycle heat-driven oscillations was originally proposed by Rayleigh [20] and refers to the relationship between the pressure wave and rate of heat release. This paper discusses natural fluid dynamic and related instabilities, occurring in swirl combustors and related systems, which can excite or increase periodic heat release. A major focus here is the influence of vortex core precession and precessing vortex cores (PVC).

The actual mechanism of the coupling effect between the flow/flame dynamics appears to arise from flow instability feeding into unsteady heat release/combustion processes, which then feed instability via coupling with acoustic modes of oscillation and amplification via the Rayleigh criterion. Associated work has shown that in high pressure process plant containing large ductwork runs and cyclone separators low frequency high amplitude pressure oscillations can arise from coupling between natural modes of acoustic oscillation and the vortex core precession (PVC) generated in the cyclone separator, Yazabadi et al. [21], [22]. Similarly, Kurosaka [23] has shown that the cooling effect produced by the Ranque–Hilsch tube relies on the presence of strong PVCs, with up to six strong harmonics being readily detectable, typical fundamental frequencies were 2–7 kHz, being a near linear function of inlet velocity. Suppression of the PVC could be achieved by fitting 12 quarter wave damping tubes radially around the circumference of the tube and tuning their frequency to that of the PVC so that they worked in anti phase.

It is often difficult to analyse the role of the PVC, its influence on instability and indeed its presence in combustion systems. The occurrence of the PVC is a function of swirl number (S) [1], [2], the presence of a CRZ (normally S>0.6–0.7 for vortex breakdown, the PVC and a CRZ to occur [1], [2]), as well as the mode of fuel entry, combustor configuration and equivalence ratio. It has been shown that axial fuel entry normally suppresses the PVC amplitude substantially, whilst premixed fuel and air can restore its presence and indeed can considerably excite it [2]. This is of course extremely important with premixed and partially premixed combustors. Here again, the effect of confinement ratio on the swirling flow is important as discussed in Section 5.

This paper, thus commences with a review of relevant work and then uses recent and new data to analyse the role of the PVC and the associated CRZ, relative to other factors which influence instability in swirl combustion systems.

Section snippets

Vortex core and jet precession

The concepts to be discussed in this paper are initially best illustrated by reference to the swirl burner shown in Fig. 2.1. This is of simple configuration with two circular inlets firing into a circular chamber, which leads via a sudden contraction to the exhaust, normally 50% of the diameter of the main chamber. The area of the tangential inlets can be varied by removable inserts to give swirl numbers in the range 0.75 upwards. Fuel can be introduced by several methods, including axially

Combustion and the PVC

Combustion processes make the behaviour and occurrence of the PVC more complex. The form of the PVC and associated flows can be similar to that found in isothermal flows [1], [2], [26], [30], [37], [38], [50]. The use of axial or tangential fuel entry alone [1], [2], [26], [37], [38] can suppress the amplitude of the PVC by an order of magnitude or more and its frequency/occurrence becomes a complex function of flow rate, equivalence ration and mode of fuel entry. The PVC occurs more readily

Vortex breakdown, modelling of the PVC and related phenomena, comparison with experiment

The occurrence of the PVC is normally linked to the phenomena of vortex breakdown and the occurrence of CRZ. There is considerable evidence from analytical and experimental studies that precessional motion can exist at low swirl numbers when CRZs are not present, although there do appear to be significant differences to the PVC occurring after vortex breakdown; this is discussed later in this section. Sarpkaya [68] provided the first very detailed experimental study of the vortex breakdown

Oscillations in swirl burner furnace systems, related systems and associated driving mechanisms

In order to explain in part the driving mechanisms for instability in swirl stabilised combustion systems, it is useful to characterise the complex mechanisms occurring under oscillation conditions and flow conditions where the PVC is suppressed. There are few articles in the literature which quantify the processes occurring under regular, stable, oscillatory conditions whilst analysing the underlying processes, apart from Fick [30], Froud, [19], [85], Dawson, [83], [86], Syred et al. [18], [61]

Discussion

Most swirl combustion systems are designed with a Swirl number S>0.5 to generate a CRZ for flame stabilization purposes. When a PVC appears it is linked and possibly coupled with the CRZ. Typically, it is of helical form and is wrapped around a distorted asymmetrical CRZ. This flow combination also excites secondary flows especially radial axial eddies, and recent LES work indicates that these eddies, shed from the edge of an inlet shear flow can propagate downstream and help to initiate

Conclusions

This paper has reviewed recent work on instability and oscillations in swirl burner and combustion systems, using a range of existing and new data on open and confined swirl combustors, and related them to the occurrence of instability in such systems. Based on this, an analysis of the underlying mechanisms by which naturally occurring acoustic and other resonances can be reinforced is given. A number of remedial methods are discussed.

For the future, there is a need for many more fundamental

Acknowledgements

Professor N. Syred gratefully acknowledges the Royal Academy of Engineering award of a Global Research Award, also the facilities provided by the School of Mechanical Engineering, Adelaide University during his sabbatical leave. The financial support of the European Union via several programmes is acknowledged for much of the more recent work carried out at Cardiff University. The assistance of Dr Andy Crayford with the diagrams is gratefully acknowledged.

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