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Influence of precessing vortex core on flame flashback in swirling hydrogen flames
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 0 2 3 3 e2 0 2 4 1
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Influence of precessing vortex core on flame flashback in swirling hydrogen flames €nborn*, Parisa Sayad, Jens Klingmann Alessandro Scho Division of Thermal Power Engineering, Lund University, PO Box 118, SE-221 00, Lund, Sweden
article info
abstract
Article history:
This study examines the influence of vortex core precession on flame flashback of swirl-
Received 11 July 2014
stabilised hydrogen flames. Theoretical considerations suggest that the angular velocity
Received in revised form
of a swirling flow is reduced as vortex precession causes it to acquire an eccentric motion
26 August 2014
around the central axis of the burner. The eccentric motion of the vortex generates a
Accepted 1 October 2014
secondary flow, which is thought to reduce the angular velocity and tangential momentum
Available online 28 October 2014
available to the primary flow, and thereby reduce the flashback propensity at the centre of the vortex core. Experiments measuring the influence of the eccentric motion of the flame
Keywords:
tip on flame flashback behaviour were conducted using high-speed sequences of OH*-
Hydrogen
chemiluminescence images. Temporal analysis of a large sample of images revealed the
Flashback
existence of a systematic rotational frequency of the flame tip around the central axis of
Gas turbine
the burner. Analysis of the radial position of the flame tip in relation to its axial propa-
Precessing vortex core (PVC)
gation velocity showed that flashback velocity increased as the flame tip eccentricity
OH-chemiluminescence
approached zero and flashback velocity decreased as the eccentricity amplitude of the flame tip reached larger values. This suggested that flame eccentricity caused by vortex core precession may be detrimental to upstream flame propagation and may be effective in inhibiting flame flashback in swirl-stabilised flames. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is a potential future storage medium for intermittent renewable electricity, with production efficiencies currently between 65 and 82% [1,2]. Large-scale reconversion of hydrogen to electricity and heat can be achieved efficiently via combustion in gas turbine based power cycles. Gas turbines typically require lean-premixed combustion approaches to keep combustion temperatures low and limit nitrogen oxide (NOx) formation. Lean-premixed combustion is difficult to achieve for hydrogen in air, since the high flame
propagation velocity and the fast chemical reactions can readily cause flame flashback and autoignition in burners [3e10]. Accurate understanding of the operating limits and of the factors extending combustor operation for lean hydrogen and air mixtures is thus crucial to the development of hydrogen-powered gas turbines. Swirl-stabilised burners commonly comprise a central recirculation zone in which vortex breakdown is used to aerodynamically anchor the flame in position by recirculating burned gases in the flow [11]. Flame stabilisation and flashback often depend on the interaction of the flame with its
* Corresponding author. Division of Thermal Power Engineering, Lund University, P.O. Box 118, SE-221 00, Lund, Sweden. Tel.: þ46 737 2364 06; fax: þ46 46 222 47 17. € nborn). E-mail address: [email protected] (A. Scho http://dx.doi.org/10.1016/j.ijhydene.2014.10.005 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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flow-field. Ashurst [3] and Umemura et al. [4] showed that the baroclinic production of vorticity is conducive to upstream flame propagation and flashback through the central region of swirling flows. This effect was shown to increase with rotational velocity of the swirl and with the density gradient across the flame. This type flashback is commonly described as combustion-induced vortex breakdown (CIVB), and has been described extensively in recent studies [5e10]. Swirling flows often feature a precessing vortex core, which may cause the flame tip to rotate eccentrically about the central axis of the burner. This phenomenon has been delineated by a large number of experimental and numerical studies, and has been comprehensively summarised in a review by Syred [12]. The fundamental mechanism for vortex core precession is usually the interaction of swirl flow, acoustics and combustion heat release [12]. The aim of the present study is to examine the effect which precession of the vortex core exerts on the axial flame propagation in terms of its direction and velocity. The radial eccentricity of the flame tip is studied with respect to its axial propagation by visualising a swirl-stabilised hydrogen flame and its tip using OH*-chemiluminescence. The combustor flow-field is characterised using particle image velocimetry (PIV). The correlation between flame tip eccentricity and its axial propagation suggests that the presence of vortex precession may hinder flame flashback, and that its absence may promote flashback.
Theory A simplified planar model of the tangential velocity field in the premixing tube may be used to describe the flow conditions as the flame acquires a degree of eccentricity from the central axis of the premixing tube. Two representative conditions may be defined as: 1. Concentric flow 2. Eccentric flow The tangential velocity flow-field during concentric and eccentric flow conditions may be described using Fig. 1. At
concentric flow conditions, the gases rotate symmetrically around the centre C of the premixing tube. At eccentric flow conditions, the tangential velocity of the gases may be divided into primary and secondary flow components. The primary flow consists in a rotation of the gases around a point E, offset from C. The secondary flow consists in a rotation of point E around the centre of the premixing tube C. The secondary flow is a result of the flow imbalance around point E, which causes a rotation of E in the same direction as that of the primary flow at an angular velocity u. Preliminary Laser Doppler Anemometry (LDA) measurements carried out in this experimental apparatus by Sayad et al. [10], have shown that the tangential velocity at the centre of the burner inlet varies linearly with distance from its centre. If the velocity field in a two-dimensional horizontal plane across the premixing tube is approximated by that of a forced vortex, and the boundary layer is ignored, the tangential velocity is vc ¼ uc$r, where r is the distance from the centre of the premixing tube C. In an eccentrically offset swirl flow, the primary tangential velocity becomes ve ¼ ue$r a, where a is a velocity offset at the central axis. If a constant axial velocity u is assumed, then the axial flux of tangential momentum Uc for symmetric flow around C may be written according to Equation (1): Z0 Uc ¼ p
ZR ruvC r2 dr þ p
ruvC r2 dr
R
0
Z0
ZR ruuC r3 dr þ p
¼ p R
1 ruuC r3 dr ¼ pruuC R4 2
(1)
0
Similarly, if a constant axial velocity u is assumed, then the axial flux of tangential momentum Ue for eccentric flow around C may be described by Equation (2): Z0 Ue ¼ p
ZR ruve r2 dr þ p
R
ruve r2 dr 0
ZR
Z0 ruðue r aÞr dr þ p
¼ p
2
R
1 ruðue r aÞr2 dr ¼ pruue R4 2
0
(2) Thus, if the angular velocity of the tangential flow is the same for concentric and eccentric conditions, so that uc ¼ ue, then their tangential momentum would be equal, so that Uc ¼ Ue. If the momentum admitted into the premixing tube remains constant under symmetric and eccentric flow conditions, then the flux of tangential momentum around the central axis of the premixing tube U may be assumed to be constant. For symmetric flow around C, the total flux of tangential momentum may be written as U ¼ Uc. For eccentric flow conditions the total flux of tangential momentum is distributed between the primary and secondary flows, as shown by Equation (3), where Us is the axial flux of tangential momentum of the secondary flow. U ¼ Ue þ Us
Fig. 1 e Eccentric movement of swirl centre, tangential velocity profile around the eccentric centre of swirl, and secondary flow.
(3)
If the total momentum U is constant for eccentric and symmetric flow conditions, then U ¼ Uc may be used to substitute U in Equation (3), yielding Equation (4). Thus, the axial
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the eccentric movement of the flame may be defined as per Equation (7), where fe is the frequency of the eccentric movement of the flame, Di is the diameter of the premixing tube, and ub is the bulk velocity of the unreacted gases in the premixing tube. St ¼
f e Di ub
(7)
Experimental apparatus and method Burner
Fig. 2 e Variable swirl burner with optically accessible premixing tube and combustion chamber.
flux of tangential momentum under eccentric conditions may be expressed as: Ue ¼ Uc Us
(4)
Since the secondary flow at eccentric conditions rotates in the same direction as the primary flow, the existence of a secondary flow must reduce the axial flux of tangential momentum in the primary flow by US, with respect to symmetric flow conditions. Thus if Uc > Ue, then Equation (1) and Equation (2) require that the angular velocity of the flow at concentric conditions must be higher than the angular velocity of the primary flow at eccentric conditions. This may be expressed by Equation (5). uc > ue
(5)
As mentioned in the introduction, Ashurst [3] and Umemura et al. [4] reported that upstream flame propagation may be enhanced by an increase with rotational velocity of the swirl flow. The swirl number S of a flow may be defined by Equation (6), where U is the axial flux of tangential momentum, R is the radius of the premixing tube and G is the axial flux of axial momentum [10]. S¼
U RG
The experiments were carried out in a variable swirl burner with optically accessible premixing tube and combustion chamber. An overview schematic of the burner can be seen in Fig. 2. The fuel and air premixing tube of the burner was supplied with premixed hydrogen and air mixtures, by admitting differing amounts of axial and tangential flow into a radial swirl-mixer. The swirl-mixer combined the axial flow of a central duct, with a tangential flow introduced through four tangential slots of 3 mm width and 10 mm height, as shown schematically in Fig. 3. The premixing tube consisted of a tube of 15 mm internal diameter and 93 mm length, of which the final 55 mm were made of quartz. The premixing tube led into a stepped expansion, which shall hereafter be referred to as the ‘dump plane’. The dump plane was manufactured from austenitic alloyed steel and was slotted on its outside, in the direction of camera access, in order to reduce its thickness to just 1 mm. The combustion chamber consisted of a cylinder of 60 mm internal diameter and 348 mm length, whose initial 120 mm were a quartz pipe of 3 mm wall thickness. This quartz combustion chamber liner was held in place by pneumatic pressure provided by four pneumatic actuators acting on the main combustion chamber. The pneumatic system allowed swift sealing and removal of the quartz liner during the experiments.
(6)
The reduction in angular velocity of the primary flow may thus cause a reduction in swirl number of the primary flow during eccentric conditions. A decrease in swirl number of the flow is well known to result in a decrease in centreline velocity and in a decrease in flashback propensity of swirl-stabilised flames [6e10]. A temporary increase in swirl eccentricity may consequently slow down or reverse the occurrence of flashback. The experiments presented herein, are aimed at investigating this hypothesis by visualising the influence of flame eccentricity on the propagation velocity of flashback. To characterise the frequency of the eccentric flame tip motion with respect to flow velocity, the Strouhal number for
Fig. 3 e Cross section of swirler, premixing tube and combustion chamber of variable swirl burner.
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The axial and tangential flows were metred, controlled and preheated individually using two laminar flow differential pressure mass flow controllers (Alicat MCR250), and feedback controlled air-heaters (Sylvania Sureheat Jet). The air was supplied by a compressed air system and filtered to 5 mm particle size. The hydrogen was supplied from bottles at a purity of 99.9%. All experiments were carried out at atmospheric pressure, an air inlet temperature of Ti ¼ 650 K and a constant air mass flow rate of 3.947 g/s. The fuel was controlled according to the desired fuel-air equivalence ratio 4, using a laminar flow differential pressure mass flow controller (Alicat MCR50), and divided into two streams by a second mass flow controller (Alicat MCR50), in order to keep the equivalence ratio in the axial and tangential flows equal. The fuel was injected more than ten diameters upstream of the swirl-mixer at a temperature of 298 K.
Simultaneous OH*-chemiluminescence and PIV A high-speed PIV system (LaVision Flowmaster) was used to measure the flow-field in the burner at a frequency of 2.5 kHz. A diode-pumped, dual cavity Nd:YF laser (Litron LDY 304-PIV) was used to illuminate the central plane of the burner at a sheet thickness of approximately 1 mm, using a diverging light sheet generated by an optical lens (f ¼ 20 mm). Two pulses of 527 nm wavelength, timed 27.5 ms apart from each other, were used to illuminate TiO2 particles seeded into the flow. The TiO2 particles consisted of primary particles of 20 nm diameter, forming particle clusters of 150e250 nm in size. They were seeded into the flow using a rotating-drum particle seeder (Scitek PS-10). Two images timed 27.5 ms apart were recorded at a repetition rate of 2.5 kHz, using a high-speed CMOS camera (Vision Research Phantom V 611). The images were recorded at 12 bit depth, with a resolution of 512 800 pixels2. Simultaneously, a second high-speed CMOS camera (Vision Research Phantom V 7.1) was used in conjunction with an image intensifier (Hamamatsu C4598), a band-pass filter (Acton Research 310.5 ± 5.75 nm) and a phosphate glass lens (UV-Nikkor 105 mm, f/4.5) to photograph OH*chemiluminescence of the flame around 306 nm. OH*chemiluminescence images were recorded at a resolution of 512 512 pixels2, at a frame rate of 2.5 kHz, each frame having an exposure time of 100 ms. The two cameras and the laser were synchronised using 5 V pulses from a programmable timing unit (LaVision). The two cameras were facing the burner from opposite directions, and oriented at right angles to the laser sheet.
background noise and calibrated spatially to the dimensions of the burner and premixing tube. In order to calculate the flame tip position, the images were individually thresholded to determine the flame boundaries, using the method described by Otsu [13]. The lowest vertical position of the flame was determined for the flame area. The equivalent radial direction of the flame tip was recorded and analysed using a fast Fourier transform subroutine available in the Matlab computer software. The vertical velocity of the flame tip was recorded as the gradient of the vertical position over three consecutive images.
PIV calculation and image processing Flow-field calculations were conducted for image pairs recorded 27.5 ms apart from each other, at a repetition rate of 2.5 kHz. A cross-correlation on interrogation areas of 64 64 and subsequently 32 32 pixels2 with an overlap of 50% was computed using multiple passes and adaptive PIV subroutines available in the DaVis (v. 8.1.4) computer software. The images were calibrated spatially using a calibration plate with 2 mm diameter dots distributed in a rectangular grid of 4 4 mm2.
Results Flashback and blowout limits The operating limits in terms of their flashback and blowout limits of the combustor were determined experimentally for three different swirl numbers. The swirl numbers at the inlet to the combustor were determined in preliminary experiments reported by Sayad et al. [10] using LDA. Three swirl numbers were chosen that would illustrate the transition from high-swirl flow with recirculation zone caused by vortex breakdown (S ¼ 0.66), to low-swirl flow without recirculation zone (S ¼ 0.514). An overview of the three swirl numbers is given in Table 1. The blowout and flashback limits were determined by igniting the burner in the stable flame region and subsequently reducing or increasing the equivalence ratio in steps of D4 ¼ 0.01 respectively. The fuel-air equivalence ratio was held at each step for three minutes to stabilise the burner temperatures, after which the burner was operated three times for three minutes in order to assess whether blowout or flashback would take place. Flame disappearance during either phenomenon was verified using OH*-chemiluminescence images recorded by the high-speed camera. Fig. 4 shows that the stable operating limits for blowout and flashback became narrower at higher swirl numbers. The
High-speed OH*-chemiluminescence In a further series of experiments, the high-speed CMOS camera used in the PIV experiments (Vision Research Phantom V 611) was used in conjunction with the previously described optics, to record OH*-chemiluminescence of the flame at a higher frequency and resolution. OH*-chemiluminescence images were recorded at a resolution of 512 800 pixels, at a frame rate of 13.5 kHz, each frame having an exposure time of 73 ms. The OH*-chemiluminescence images were corrected for
Table 1 e Swirl numbers for different burner operating conditions. Swirl number
Tangential flow rate
Axial flow rate
S
ṁ
ṁ
[]
[g/s]
[g/s]
0.660 0.594 0.514
3.947 2.960 2.664
0 0.986776 1.283
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operating limit for flame flashback was reduced significantly at higher swirl numbers, while the blowout limit remained relatively unaffected by swirl number. This illustrates that stable flame operation at high equivalence ratios can be achieved by a reduction in swirl number, which may be implemented in gas turbine burners using variable swirl vanes.
Burner flow-field and flame position before flashback In order to characterise the flow-field and flame position, PIV and OH*-chemiluminescence images of the flame were recorded at the flashback limit, before flashback had occurred. Fig. 5 shows plots of the velocity vectors in the central plane of the burner, and the threshold limit of OH*chemiluminescence representing the flame boundary, for each swirl number. The velocity vectors indicate flow velocity by direction and magnitude scaled by length; where 1 mm length represents a speed of 10 m/s. Fig. 5 shows that for a swirl number of S ¼ 0.66, the flame was convex in shape and clearly detached from the burner inlet. It extended over a large volume downstream in the burner. At a swirl number of S ¼ 0.594, the flame was attached to the inlet of the burner and displayed a distinctive heart shape. For the more slowly rotating swirl flow of S ¼ 0.514, the flame was still attached to the burner inlet, but its borders are more compact and dome shaped. The PIV vector fields show that a recirculation zone exists for the higher swirl numbers of S ¼ {0.66; 0.594}, but that this recirculation zone was absent at the lowest swirl number of S ¼ 0.514. The differences visible in Fig. 5, in terms of flame detachment or attachment, and the presence or absence of recirculation zones, can lead to differences in the way flame flashback occurs, as shall be shown in high-speed chemiluminescence images in the results.
OH*-chemiluminescence sequences At the highest swirl number of S ¼ 0.66, the flame was initially detached from the burner inlet, as previously illustrated in
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Fig. 5. Fig. 6 shows a sequence of OH*-chemiluminescence images during flashback. The flame tip position calculated by the process described in method is marked in the images by a red asterisk. Flame flashback began by a small volume of intensive OH*chemiluminescence forming at the centre of the burner inlet, as shown at t ¼ 74 ms in Fig. 6. This volume of high OH*chemiluminescence intensity marking the flame, moved from a lifted position at t ¼ 0 ms to the inlet of the burner at t ¼ 74 ms, in an apparent step-change. At t ¼ 222 ms the flame propagated upstream into the premixing tube by forming a narrow flame filament located close to the central axis of the premixing tube. The ensuing image sequence from t ¼ 296 ms to t ¼ 519 ms shows that this filament grew in length and thickness, while allowing the flame tip to propagate further into the premixing tube. It is visible that the tip of the swirl performed small radial movements during the flashback event, which may have been the result temporary eccentricity of the swirl flow-field within the premixing tube. During flashback at the intermediate swirl number of S ¼ 0.594, the flame was initially attached to the burner inlet. Fig. 7 shows that flame flashback commenced as a protrusion of the flame into the premixing tube at t ¼ 148 ms. It should be noted here that chemiluminescence from the first millimetre of the premixing tube was attenuated by the dump plane of the burner, which caused the flame to appear separated into two pieces. Further propagation of the flame tip into the premixing tube of the burner could be observed between t ¼ 148 ms and t ¼ 519 ms, followed by a recession of the flame tip between t ¼ 593 ms to t ¼ 815 ms. The images suggest that the flame tip followed a rotary motion around the central axis of the inlet tube during some of the sequence. This motion is thought to represent an eccentric motion of the primary swirl flow, as earlier described in the theory. Detailed analysis regarding the frequency and amplitude of this motion will be provided in the results. At the lowest swirl number of S ¼ 0.514, the flame was initially straight and compact in size, due to the absence of a recirculation area, as was shown in Fig. 5. Fig. 8 shows that flashback was initiated at the centre of the burner inlet at t ¼ 74 ms by the flame forming a more conical shape of which the tip protruded into the premixing tube. The image sequence in Fig. 8 suggests that the flame tip did not follow an equally large eccentric movement as that visible for the higher swirl number of S ¼ 0.594, previously shown in Fig. 7.
Frequency and amplitude of flame tip motion
Fig. 4 e Operational limits of the variable swirl burner in terms of blowout and flashback limits.
The influence of swirl eccentricity on flame flashback propagation was studied by analysing the radial eccentric and axial position of the flame tip in the time domain. Since eccentricity was strongest at S ¼ 0.594, images from flashback at this were used to perform this analysis. The other two conditions did not allow the reliable extraction of the flame tip frequency. A Fourier transform was applied to the radial position of the flame tip with respect to time during flashback, in order to determine characteristic frequencies of the flame tip. The frequency spectrum was calculated for instances in which the
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Fig. 5 e Flow-field and OH*-chemiluminescence at the flashback limit before flashback occurred. Vertical and horizontal coordinates in the images are given in millimetres and are measured from the centre of the burner inlet. The equivalence ratios were 4 ¼ 0.455 for S ¼ 0.66, 4 ¼ 0.263 for S ¼ 0.594, and 4 ¼ 0.186 for S ¼ 0.514.
flame tip was situated in the premixing tube, between 10 mm and 50 mm upstream the dump plane. Fig. 9 shows that at a swirl number of S ¼ 0.594, the flame time displayed a characteristic frequency of 2241 Hz. The image sample size was 4745 OH*-chemiluminescence images recorded in a single sequence of images, but other flashback image sequences revealed the same characteristic frequency. The Strouhal number of the eccentric motion of the flame under these condition has a value of 0.80, which is in reasonable agreement with values reported by Syred [12]. Fig. 9 suggests that a systematic radial motion of the flame tip existed in the burner. It is thought that this motion may be ascribed to an eccentric movement of the swirl field around the central axis of the premixing tube. According to the theory, an increase in the amplitude of the flow eccentricity may reduce the flashback propensity of the flow, while a reduction in flow eccentricity may restore the full flashback propensity of the flow. If temporary variations in the eccentricity of the flow-field exist, these may be
responsible for affecting the flashback velocity of the flow. In order to study this relation, the axial flame tip propagation velocity was compared to the radial eccentricity of the flame over a large sample of images. A negative axial flame tip velocity meant that the flame flashed back further upstream, while a positive axial flame tip velocity meant that the flame receded further downstream into the flow. Fig. 10 shows the influence of radial position of the flame tip on its axial velocity. The inverted conical shape of Fig. 10 suggested that at positive axial flame tip velocities, the flame tip showed a large degree of eccentricity around the central axis of the premixing tube, while at negative axial flame tip velocities, the flame tip was predominantly located in the vicinity of the central axis of the premixing tube, with low eccentricity. This implies that the upstream propagation of the flame was promoted as the flame tip eccentricity approached zero, and that it was inhibited as the eccentricity amplitude of the flame tip reached larger values.
Fig. 6 e OH*-chemiluminescence during flashback at a swirl number of S ¼ 0.66. *Flame-tip position.
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Fig. 7 e OH*-chemiluminescence during flashback at a swirl number of S ¼ 0.594. *Flame-tip position.
Fig. 8 e OH*-chemiluminescence during flashback at a swirl number of S ¼ 0.514. *Flame-tip position.
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It would be of interest to characterise re/rt in terms further burner geometry and operating conditions and clarify the causes for temporary eccentricity of the flow-field. Further insights into the relative importance of flame eccentricity with respect to turbulent fluctuations in velocity, equivalence ratio, temperature and pressure would be useful as future studies.
Summary and conclusions
Fig. 9 e Fast Fourier Transform (FFT) of the radial flame tip position with time during upstream and downstream movements of the flame during flame flashback.
This observation may be quantified by fitting a linear function to the maximum values of the modulus of radial position for the different values of axial flame tip velocity. A least squares approximation for the data shown in Fig. 10 yields Equation (8): VFt ¼ 2:03 þ 6:94re
(8)
Where VFt is the flame tip velocity, and re is the radius of eccentricity, i.e. the distance from point C to point E (Fig. 1). Equation (8) can be made non-dimensional by dividing VFt by the bulk velocity of the unburned gases (42 m/s), and by dividing re by the radius of the premixing tube rt (7.5 mm). This equation may be used to compare the observations made in the current geometry to other burner geometries. The results provide some evidence of vortex core precession having a detrimental effect on upstream flame propagation during flame flashback.
The influence of a precessing vortex core on flame flashback was investigated by analysing flame tip eccentricity of swirling hydrogen and air flames with respect to their axial propagation. Theoretical considerations suggested that the axial flux of tangential momentum in the swirl flow of a gas turbine burner may be divided between the primary swirl flow and a secondary flow created by eccentricity of the primary flow from the central axis of the premixing tube. The angular velocity and swirl number of the primary flow was thought to be reduced, as the flow was shifted into an eccentric flight path around the central axis of the burner. Radial and axial position of the flame, and flow-field were measured using OH*chemiluminescence and PIV in an optically accessible gas turbine burner. The experiments showed that a correlation between maximum radial distance from the central axis and axial propagation velocity of the flame existed, as was quantified by Equation (8). The results showed that the flame tip tended to propagate upstream during periods of low flow eccentricity, and receded downstream when the flame tip obtained a higher degree of eccentricity. The present results provide an indication of the influence of vortex core precession on flashback behaviour, showing an inhibiting effect on flame flashback through the premixing tube. Further insights into the relative importance of burner geometry and operating conditions on this effect, as well as systematic studies of turbulent fluctuations in velocity, equivalence ratio, temperature and pressure would be useful to characterise the relative importance of this effect. An unsteady numerical simulation of the flow-field would be useful future work that would allow further insights into the flashback behaviour during vortex core precession.
Acknowledgements This work was financially supported by the Swedish Research Council (Vetenskapsra˚det) under contract A0088001.
references
Fig. 10 e Influence of radial position of the flame tip on the axial velocity of the flame flashback.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Influence of precessing vortex core on flame flashback in swirling hydrogen flames €nborn*, Parisa Sayad, Jens Klingmann Alessandro Scho Division of Thermal Power Engineering, Lund University, PO Box 118, SE-221 00, Lund, Sweden
article info
abstract
Article history:
This study examines the influence of vortex core precession on flame flashback of swirl-
Received 11 July 2014
stabilised hydrogen flames. Theoretical considerations suggest that the angular velocity
Received in revised form
of a swirling flow is reduced as vortex precession causes it to acquire an eccentric motion
26 August 2014
around the central axis of the burner. The eccentric motion of the vortex generates a
Accepted 1 October 2014
secondary flow, which is thought to reduce the angular velocity and tangential momentum
Available online 28 October 2014
available to the primary flow, and thereby reduce the flashback propensity at the centre of the vortex core. Experiments measuring the influence of the eccentric motion of the flame
Keywords:
tip on flame flashback behaviour were conducted using high-speed sequences of OH*-
Hydrogen
chemiluminescence images. Temporal analysis of a large sample of images revealed the
Flashback
existence of a systematic rotational frequency of the flame tip around the central axis of
Gas turbine
the burner. Analysis of the radial position of the flame tip in relation to its axial propa-
Precessing vortex core (PVC)
gation velocity showed that flashback velocity increased as the flame tip eccentricity
OH-chemiluminescence
approached zero and flashback velocity decreased as the eccentricity amplitude of the flame tip reached larger values. This suggested that flame eccentricity caused by vortex core precession may be detrimental to upstream flame propagation and may be effective in inhibiting flame flashback in swirl-stabilised flames. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is a potential future storage medium for intermittent renewable electricity, with production efficiencies currently between 65 and 82% [1,2]. Large-scale reconversion of hydrogen to electricity and heat can be achieved efficiently via combustion in gas turbine based power cycles. Gas turbines typically require lean-premixed combustion approaches to keep combustion temperatures low and limit nitrogen oxide (NOx) formation. Lean-premixed combustion is difficult to achieve for hydrogen in air, since the high flame
propagation velocity and the fast chemical reactions can readily cause flame flashback and autoignition in burners [3e10]. Accurate understanding of the operating limits and of the factors extending combustor operation for lean hydrogen and air mixtures is thus crucial to the development of hydrogen-powered gas turbines. Swirl-stabilised burners commonly comprise a central recirculation zone in which vortex breakdown is used to aerodynamically anchor the flame in position by recirculating burned gases in the flow [11]. Flame stabilisation and flashback often depend on the interaction of the flame with its
* Corresponding author. Division of Thermal Power Engineering, Lund University, P.O. Box 118, SE-221 00, Lund, Sweden. Tel.: þ46 737 2364 06; fax: þ46 46 222 47 17. € nborn). E-mail address: [email protected] (A. Scho http://dx.doi.org/10.1016/j.ijhydene.2014.10.005 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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flow-field. Ashurst [3] and Umemura et al. [4] showed that the baroclinic production of vorticity is conducive to upstream flame propagation and flashback through the central region of swirling flows. This effect was shown to increase with rotational velocity of the swirl and with the density gradient across the flame. This type flashback is commonly described as combustion-induced vortex breakdown (CIVB), and has been described extensively in recent studies [5e10]. Swirling flows often feature a precessing vortex core, which may cause the flame tip to rotate eccentrically about the central axis of the burner. This phenomenon has been delineated by a large number of experimental and numerical studies, and has been comprehensively summarised in a review by Syred [12]. The fundamental mechanism for vortex core precession is usually the interaction of swirl flow, acoustics and combustion heat release [12]. The aim of the present study is to examine the effect which precession of the vortex core exerts on the axial flame propagation in terms of its direction and velocity. The radial eccentricity of the flame tip is studied with respect to its axial propagation by visualising a swirl-stabilised hydrogen flame and its tip using OH*-chemiluminescence. The combustor flow-field is characterised using particle image velocimetry (PIV). The correlation between flame tip eccentricity and its axial propagation suggests that the presence of vortex precession may hinder flame flashback, and that its absence may promote flashback.
Theory A simplified planar model of the tangential velocity field in the premixing tube may be used to describe the flow conditions as the flame acquires a degree of eccentricity from the central axis of the premixing tube. Two representative conditions may be defined as: 1. Concentric flow 2. Eccentric flow The tangential velocity flow-field during concentric and eccentric flow conditions may be described using Fig. 1. At
concentric flow conditions, the gases rotate symmetrically around the centre C of the premixing tube. At eccentric flow conditions, the tangential velocity of the gases may be divided into primary and secondary flow components. The primary flow consists in a rotation of the gases around a point E, offset from C. The secondary flow consists in a rotation of point E around the centre of the premixing tube C. The secondary flow is a result of the flow imbalance around point E, which causes a rotation of E in the same direction as that of the primary flow at an angular velocity u. Preliminary Laser Doppler Anemometry (LDA) measurements carried out in this experimental apparatus by Sayad et al. [10], have shown that the tangential velocity at the centre of the burner inlet varies linearly with distance from its centre. If the velocity field in a two-dimensional horizontal plane across the premixing tube is approximated by that of a forced vortex, and the boundary layer is ignored, the tangential velocity is vc ¼ uc$r, where r is the distance from the centre of the premixing tube C. In an eccentrically offset swirl flow, the primary tangential velocity becomes ve ¼ ue$r a, where a is a velocity offset at the central axis. If a constant axial velocity u is assumed, then the axial flux of tangential momentum Uc for symmetric flow around C may be written according to Equation (1): Z0 Uc ¼ p
ZR ruvC r2 dr þ p
ruvC r2 dr
R
0
Z0
ZR ruuC r3 dr þ p
¼ p R
1 ruuC r3 dr ¼ pruuC R4 2
(1)
0
Similarly, if a constant axial velocity u is assumed, then the axial flux of tangential momentum Ue for eccentric flow around C may be described by Equation (2): Z0 Ue ¼ p
ZR ruve r2 dr þ p
R
ruve r2 dr 0
ZR
Z0 ruðue r aÞr dr þ p
¼ p
2
R
1 ruðue r aÞr2 dr ¼ pruue R4 2
0
(2) Thus, if the angular velocity of the tangential flow is the same for concentric and eccentric conditions, so that uc ¼ ue, then their tangential momentum would be equal, so that Uc ¼ Ue. If the momentum admitted into the premixing tube remains constant under symmetric and eccentric flow conditions, then the flux of tangential momentum around the central axis of the premixing tube U may be assumed to be constant. For symmetric flow around C, the total flux of tangential momentum may be written as U ¼ Uc. For eccentric flow conditions the total flux of tangential momentum is distributed between the primary and secondary flows, as shown by Equation (3), where Us is the axial flux of tangential momentum of the secondary flow. U ¼ Ue þ Us
Fig. 1 e Eccentric movement of swirl centre, tangential velocity profile around the eccentric centre of swirl, and secondary flow.
(3)
If the total momentum U is constant for eccentric and symmetric flow conditions, then U ¼ Uc may be used to substitute U in Equation (3), yielding Equation (4). Thus, the axial
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the eccentric movement of the flame may be defined as per Equation (7), where fe is the frequency of the eccentric movement of the flame, Di is the diameter of the premixing tube, and ub is the bulk velocity of the unreacted gases in the premixing tube. St ¼
f e Di ub
(7)
Experimental apparatus and method Burner
Fig. 2 e Variable swirl burner with optically accessible premixing tube and combustion chamber.
flux of tangential momentum under eccentric conditions may be expressed as: Ue ¼ Uc Us
(4)
Since the secondary flow at eccentric conditions rotates in the same direction as the primary flow, the existence of a secondary flow must reduce the axial flux of tangential momentum in the primary flow by US, with respect to symmetric flow conditions. Thus if Uc > Ue, then Equation (1) and Equation (2) require that the angular velocity of the flow at concentric conditions must be higher than the angular velocity of the primary flow at eccentric conditions. This may be expressed by Equation (5). uc > ue
(5)
As mentioned in the introduction, Ashurst [3] and Umemura et al. [4] reported that upstream flame propagation may be enhanced by an increase with rotational velocity of the swirl flow. The swirl number S of a flow may be defined by Equation (6), where U is the axial flux of tangential momentum, R is the radius of the premixing tube and G is the axial flux of axial momentum [10]. S¼
U RG
The experiments were carried out in a variable swirl burner with optically accessible premixing tube and combustion chamber. An overview schematic of the burner can be seen in Fig. 2. The fuel and air premixing tube of the burner was supplied with premixed hydrogen and air mixtures, by admitting differing amounts of axial and tangential flow into a radial swirl-mixer. The swirl-mixer combined the axial flow of a central duct, with a tangential flow introduced through four tangential slots of 3 mm width and 10 mm height, as shown schematically in Fig. 3. The premixing tube consisted of a tube of 15 mm internal diameter and 93 mm length, of which the final 55 mm were made of quartz. The premixing tube led into a stepped expansion, which shall hereafter be referred to as the ‘dump plane’. The dump plane was manufactured from austenitic alloyed steel and was slotted on its outside, in the direction of camera access, in order to reduce its thickness to just 1 mm. The combustion chamber consisted of a cylinder of 60 mm internal diameter and 348 mm length, whose initial 120 mm were a quartz pipe of 3 mm wall thickness. This quartz combustion chamber liner was held in place by pneumatic pressure provided by four pneumatic actuators acting on the main combustion chamber. The pneumatic system allowed swift sealing and removal of the quartz liner during the experiments.
(6)
The reduction in angular velocity of the primary flow may thus cause a reduction in swirl number of the primary flow during eccentric conditions. A decrease in swirl number of the flow is well known to result in a decrease in centreline velocity and in a decrease in flashback propensity of swirl-stabilised flames [6e10]. A temporary increase in swirl eccentricity may consequently slow down or reverse the occurrence of flashback. The experiments presented herein, are aimed at investigating this hypothesis by visualising the influence of flame eccentricity on the propagation velocity of flashback. To characterise the frequency of the eccentric flame tip motion with respect to flow velocity, the Strouhal number for
Fig. 3 e Cross section of swirler, premixing tube and combustion chamber of variable swirl burner.
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The axial and tangential flows were metred, controlled and preheated individually using two laminar flow differential pressure mass flow controllers (Alicat MCR250), and feedback controlled air-heaters (Sylvania Sureheat Jet). The air was supplied by a compressed air system and filtered to 5 mm particle size. The hydrogen was supplied from bottles at a purity of 99.9%. All experiments were carried out at atmospheric pressure, an air inlet temperature of Ti ¼ 650 K and a constant air mass flow rate of 3.947 g/s. The fuel was controlled according to the desired fuel-air equivalence ratio 4, using a laminar flow differential pressure mass flow controller (Alicat MCR50), and divided into two streams by a second mass flow controller (Alicat MCR50), in order to keep the equivalence ratio in the axial and tangential flows equal. The fuel was injected more than ten diameters upstream of the swirl-mixer at a temperature of 298 K.
Simultaneous OH*-chemiluminescence and PIV A high-speed PIV system (LaVision Flowmaster) was used to measure the flow-field in the burner at a frequency of 2.5 kHz. A diode-pumped, dual cavity Nd:YF laser (Litron LDY 304-PIV) was used to illuminate the central plane of the burner at a sheet thickness of approximately 1 mm, using a diverging light sheet generated by an optical lens (f ¼ 20 mm). Two pulses of 527 nm wavelength, timed 27.5 ms apart from each other, were used to illuminate TiO2 particles seeded into the flow. The TiO2 particles consisted of primary particles of 20 nm diameter, forming particle clusters of 150e250 nm in size. They were seeded into the flow using a rotating-drum particle seeder (Scitek PS-10). Two images timed 27.5 ms apart were recorded at a repetition rate of 2.5 kHz, using a high-speed CMOS camera (Vision Research Phantom V 611). The images were recorded at 12 bit depth, with a resolution of 512 800 pixels2. Simultaneously, a second high-speed CMOS camera (Vision Research Phantom V 7.1) was used in conjunction with an image intensifier (Hamamatsu C4598), a band-pass filter (Acton Research 310.5 ± 5.75 nm) and a phosphate glass lens (UV-Nikkor 105 mm, f/4.5) to photograph OH*chemiluminescence of the flame around 306 nm. OH*chemiluminescence images were recorded at a resolution of 512 512 pixels2, at a frame rate of 2.5 kHz, each frame having an exposure time of 100 ms. The two cameras and the laser were synchronised using 5 V pulses from a programmable timing unit (LaVision). The two cameras were facing the burner from opposite directions, and oriented at right angles to the laser sheet.
background noise and calibrated spatially to the dimensions of the burner and premixing tube. In order to calculate the flame tip position, the images were individually thresholded to determine the flame boundaries, using the method described by Otsu [13]. The lowest vertical position of the flame was determined for the flame area. The equivalent radial direction of the flame tip was recorded and analysed using a fast Fourier transform subroutine available in the Matlab computer software. The vertical velocity of the flame tip was recorded as the gradient of the vertical position over three consecutive images.
PIV calculation and image processing Flow-field calculations were conducted for image pairs recorded 27.5 ms apart from each other, at a repetition rate of 2.5 kHz. A cross-correlation on interrogation areas of 64 64 and subsequently 32 32 pixels2 with an overlap of 50% was computed using multiple passes and adaptive PIV subroutines available in the DaVis (v. 8.1.4) computer software. The images were calibrated spatially using a calibration plate with 2 mm diameter dots distributed in a rectangular grid of 4 4 mm2.
Results Flashback and blowout limits The operating limits in terms of their flashback and blowout limits of the combustor were determined experimentally for three different swirl numbers. The swirl numbers at the inlet to the combustor were determined in preliminary experiments reported by Sayad et al. [10] using LDA. Three swirl numbers were chosen that would illustrate the transition from high-swirl flow with recirculation zone caused by vortex breakdown (S ¼ 0.66), to low-swirl flow without recirculation zone (S ¼ 0.514). An overview of the three swirl numbers is given in Table 1. The blowout and flashback limits were determined by igniting the burner in the stable flame region and subsequently reducing or increasing the equivalence ratio in steps of D4 ¼ 0.01 respectively. The fuel-air equivalence ratio was held at each step for three minutes to stabilise the burner temperatures, after which the burner was operated three times for three minutes in order to assess whether blowout or flashback would take place. Flame disappearance during either phenomenon was verified using OH*-chemiluminescence images recorded by the high-speed camera. Fig. 4 shows that the stable operating limits for blowout and flashback became narrower at higher swirl numbers. The
High-speed OH*-chemiluminescence In a further series of experiments, the high-speed CMOS camera used in the PIV experiments (Vision Research Phantom V 611) was used in conjunction with the previously described optics, to record OH*-chemiluminescence of the flame at a higher frequency and resolution. OH*-chemiluminescence images were recorded at a resolution of 512 800 pixels, at a frame rate of 13.5 kHz, each frame having an exposure time of 73 ms. The OH*-chemiluminescence images were corrected for
Table 1 e Swirl numbers for different burner operating conditions. Swirl number
Tangential flow rate
Axial flow rate
S
ṁ
ṁ
[]
[g/s]
[g/s]
0.660 0.594 0.514
3.947 2.960 2.664
0 0.986776 1.283
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operating limit for flame flashback was reduced significantly at higher swirl numbers, while the blowout limit remained relatively unaffected by swirl number. This illustrates that stable flame operation at high equivalence ratios can be achieved by a reduction in swirl number, which may be implemented in gas turbine burners using variable swirl vanes.
Burner flow-field and flame position before flashback In order to characterise the flow-field and flame position, PIV and OH*-chemiluminescence images of the flame were recorded at the flashback limit, before flashback had occurred. Fig. 5 shows plots of the velocity vectors in the central plane of the burner, and the threshold limit of OH*chemiluminescence representing the flame boundary, for each swirl number. The velocity vectors indicate flow velocity by direction and magnitude scaled by length; where 1 mm length represents a speed of 10 m/s. Fig. 5 shows that for a swirl number of S ¼ 0.66, the flame was convex in shape and clearly detached from the burner inlet. It extended over a large volume downstream in the burner. At a swirl number of S ¼ 0.594, the flame was attached to the inlet of the burner and displayed a distinctive heart shape. For the more slowly rotating swirl flow of S ¼ 0.514, the flame was still attached to the burner inlet, but its borders are more compact and dome shaped. The PIV vector fields show that a recirculation zone exists for the higher swirl numbers of S ¼ {0.66; 0.594}, but that this recirculation zone was absent at the lowest swirl number of S ¼ 0.514. The differences visible in Fig. 5, in terms of flame detachment or attachment, and the presence or absence of recirculation zones, can lead to differences in the way flame flashback occurs, as shall be shown in high-speed chemiluminescence images in the results.
OH*-chemiluminescence sequences At the highest swirl number of S ¼ 0.66, the flame was initially detached from the burner inlet, as previously illustrated in
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Fig. 5. Fig. 6 shows a sequence of OH*-chemiluminescence images during flashback. The flame tip position calculated by the process described in method is marked in the images by a red asterisk. Flame flashback began by a small volume of intensive OH*chemiluminescence forming at the centre of the burner inlet, as shown at t ¼ 74 ms in Fig. 6. This volume of high OH*chemiluminescence intensity marking the flame, moved from a lifted position at t ¼ 0 ms to the inlet of the burner at t ¼ 74 ms, in an apparent step-change. At t ¼ 222 ms the flame propagated upstream into the premixing tube by forming a narrow flame filament located close to the central axis of the premixing tube. The ensuing image sequence from t ¼ 296 ms to t ¼ 519 ms shows that this filament grew in length and thickness, while allowing the flame tip to propagate further into the premixing tube. It is visible that the tip of the swirl performed small radial movements during the flashback event, which may have been the result temporary eccentricity of the swirl flow-field within the premixing tube. During flashback at the intermediate swirl number of S ¼ 0.594, the flame was initially attached to the burner inlet. Fig. 7 shows that flame flashback commenced as a protrusion of the flame into the premixing tube at t ¼ 148 ms. It should be noted here that chemiluminescence from the first millimetre of the premixing tube was attenuated by the dump plane of the burner, which caused the flame to appear separated into two pieces. Further propagation of the flame tip into the premixing tube of the burner could be observed between t ¼ 148 ms and t ¼ 519 ms, followed by a recession of the flame tip between t ¼ 593 ms to t ¼ 815 ms. The images suggest that the flame tip followed a rotary motion around the central axis of the inlet tube during some of the sequence. This motion is thought to represent an eccentric motion of the primary swirl flow, as earlier described in the theory. Detailed analysis regarding the frequency and amplitude of this motion will be provided in the results. At the lowest swirl number of S ¼ 0.514, the flame was initially straight and compact in size, due to the absence of a recirculation area, as was shown in Fig. 5. Fig. 8 shows that flashback was initiated at the centre of the burner inlet at t ¼ 74 ms by the flame forming a more conical shape of which the tip protruded into the premixing tube. The image sequence in Fig. 8 suggests that the flame tip did not follow an equally large eccentric movement as that visible for the higher swirl number of S ¼ 0.594, previously shown in Fig. 7.
Frequency and amplitude of flame tip motion
Fig. 4 e Operational limits of the variable swirl burner in terms of blowout and flashback limits.
The influence of swirl eccentricity on flame flashback propagation was studied by analysing the radial eccentric and axial position of the flame tip in the time domain. Since eccentricity was strongest at S ¼ 0.594, images from flashback at this were used to perform this analysis. The other two conditions did not allow the reliable extraction of the flame tip frequency. A Fourier transform was applied to the radial position of the flame tip with respect to time during flashback, in order to determine characteristic frequencies of the flame tip. The frequency spectrum was calculated for instances in which the
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Fig. 5 e Flow-field and OH*-chemiluminescence at the flashback limit before flashback occurred. Vertical and horizontal coordinates in the images are given in millimetres and are measured from the centre of the burner inlet. The equivalence ratios were 4 ¼ 0.455 for S ¼ 0.66, 4 ¼ 0.263 for S ¼ 0.594, and 4 ¼ 0.186 for S ¼ 0.514.
flame tip was situated in the premixing tube, between 10 mm and 50 mm upstream the dump plane. Fig. 9 shows that at a swirl number of S ¼ 0.594, the flame time displayed a characteristic frequency of 2241 Hz. The image sample size was 4745 OH*-chemiluminescence images recorded in a single sequence of images, but other flashback image sequences revealed the same characteristic frequency. The Strouhal number of the eccentric motion of the flame under these condition has a value of 0.80, which is in reasonable agreement with values reported by Syred [12]. Fig. 9 suggests that a systematic radial motion of the flame tip existed in the burner. It is thought that this motion may be ascribed to an eccentric movement of the swirl field around the central axis of the premixing tube. According to the theory, an increase in the amplitude of the flow eccentricity may reduce the flashback propensity of the flow, while a reduction in flow eccentricity may restore the full flashback propensity of the flow. If temporary variations in the eccentricity of the flow-field exist, these may be
responsible for affecting the flashback velocity of the flow. In order to study this relation, the axial flame tip propagation velocity was compared to the radial eccentricity of the flame over a large sample of images. A negative axial flame tip velocity meant that the flame flashed back further upstream, while a positive axial flame tip velocity meant that the flame receded further downstream into the flow. Fig. 10 shows the influence of radial position of the flame tip on its axial velocity. The inverted conical shape of Fig. 10 suggested that at positive axial flame tip velocities, the flame tip showed a large degree of eccentricity around the central axis of the premixing tube, while at negative axial flame tip velocities, the flame tip was predominantly located in the vicinity of the central axis of the premixing tube, with low eccentricity. This implies that the upstream propagation of the flame was promoted as the flame tip eccentricity approached zero, and that it was inhibited as the eccentricity amplitude of the flame tip reached larger values.
Fig. 6 e OH*-chemiluminescence during flashback at a swirl number of S ¼ 0.66. *Flame-tip position.
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Fig. 7 e OH*-chemiluminescence during flashback at a swirl number of S ¼ 0.594. *Flame-tip position.
Fig. 8 e OH*-chemiluminescence during flashback at a swirl number of S ¼ 0.514. *Flame-tip position.
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It would be of interest to characterise re/rt in terms further burner geometry and operating conditions and clarify the causes for temporary eccentricity of the flow-field. Further insights into the relative importance of flame eccentricity with respect to turbulent fluctuations in velocity, equivalence ratio, temperature and pressure would be useful as future studies.
Summary and conclusions
Fig. 9 e Fast Fourier Transform (FFT) of the radial flame tip position with time during upstream and downstream movements of the flame during flame flashback.
This observation may be quantified by fitting a linear function to the maximum values of the modulus of radial position for the different values of axial flame tip velocity. A least squares approximation for the data shown in Fig. 10 yields Equation (8): VFt ¼ 2:03 þ 6:94re
(8)
Where VFt is the flame tip velocity, and re is the radius of eccentricity, i.e. the distance from point C to point E (Fig. 1). Equation (8) can be made non-dimensional by dividing VFt by the bulk velocity of the unburned gases (42 m/s), and by dividing re by the radius of the premixing tube rt (7.5 mm). This equation may be used to compare the observations made in the current geometry to other burner geometries. The results provide some evidence of vortex core precession having a detrimental effect on upstream flame propagation during flame flashback.
The influence of a precessing vortex core on flame flashback was investigated by analysing flame tip eccentricity of swirling hydrogen and air flames with respect to their axial propagation. Theoretical considerations suggested that the axial flux of tangential momentum in the swirl flow of a gas turbine burner may be divided between the primary swirl flow and a secondary flow created by eccentricity of the primary flow from the central axis of the premixing tube. The angular velocity and swirl number of the primary flow was thought to be reduced, as the flow was shifted into an eccentric flight path around the central axis of the burner. Radial and axial position of the flame, and flow-field were measured using OH*chemiluminescence and PIV in an optically accessible gas turbine burner. The experiments showed that a correlation between maximum radial distance from the central axis and axial propagation velocity of the flame existed, as was quantified by Equation (8). The results showed that the flame tip tended to propagate upstream during periods of low flow eccentricity, and receded downstream when the flame tip obtained a higher degree of eccentricity. The present results provide an indication of the influence of vortex core precession on flashback behaviour, showing an inhibiting effect on flame flashback through the premixing tube. Further insights into the relative importance of burner geometry and operating conditions on this effect, as well as systematic studies of turbulent fluctuations in velocity, equivalence ratio, temperature and pressure would be useful to characterise the relative importance of this effect. An unsteady numerical simulation of the flow-field would be useful future work that would allow further insights into the flashback behaviour during vortex core precession.
Acknowledgements This work was financially supported by the Swedish Research Council (Vetenskapsra˚det) under contract A0088001.
references
Fig. 10 e Influence of radial position of the flame tip on the axial velocity of the flame flashback.
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