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Central Recirculation Zone Visualization in ConfinedSwirl Combustors for Terrestrial Energy
Agustin Valera-Medina,∗ Nicholas Syred,† and Phil Bowen‡
Cardiff University, Cardiff CF24 3AA, Wales, United Kingdom
DOI: 10.2514/1.B34600
This study characterizes the central recirculation zones formed under combustion conditions, with natural gas as
fuel with different geometries and degrees of premixing using a swirl combustor firing into a confinement
representative of gas turbine combustors. Phase-locked particle image velocimetry is used as the main method of
characterization. The technique enables characterization of the time-dependent behavior of the central recirculation
zone and a three-dimensional reconstruction of its boundaries. The central recirculation zone typically had an
asymmetric lobed shape and precessed about the central axis. Partially premixed combustion at near-stoichiometric
equivalence ratios reduced the coherence of the central recirculation zone and often caused it to nearly disappear,
although a small remnant of recirculation could still be found in the three-dimensional space. Lower equivalence
ratios strengthened the central recirculation zone considerably, for both non-premixed and partially premixed
combustion. Although the central recirculation zone was asymmetric in shape and precessed about the central axis,
the precessing vortex core, commonly found in these flows, was found to be significantly suppressed especially when
central fuel injectors were used. Its occurrence then became intermittent and irregular. A quarl exit nozzle with a
divergent lip was found to reduce flame attachment to the fuel injector.
Nomenclature
D = exit diameter of burner, 78 mmDcrz = maximum diameter of central recirculation zone, mL = length of central recirculation zone beyond burner
exit, mNUcrz = Uaxrev∕Uannulus, %Re = Reynolds number defined using average exit velocityS = geometrical swirl numberS� = swirl number modified by combustionUannulus = average axial velocity in annulus surrounding fuel
injector, m∕sUaxrev = average axial velocity in central recirculation zone
derived from particle imaging velocimetry velocityvectors, m∕s
X = downstream distance from burner exit, mφp = premixed equivalence ratioφt = overall equivalence ratio
I. Introduction
SWIRL-STABILIZED combustion is probably one of the mostwidely used techniques for flame stabilization and NOx
reduction, as reviewed in [1–6].Swirl combustors generate the well-known central recirculation
zone (CRZ) by flow expansion from the burner exit. This is becausethe swirling flow generates significant radial pressure gradients; theexpansion causes axial decay of swirl velocity, which in turn reducesthe radial pressure gradients creating negative axial pressure gra-dients in and around the central axis. This in turn gives rise to theCRZ[5,6]. This zone is established in the center of the flowdue to the effectof the swirling flow. It is a form of vortex breakdown and serves as aflame stabilization regionwhere the hot products and active chemicalspecies are mixed with the incoming mixture of air and fuel. Thiszone plays an important role in the stabilization of the flame and
mixing process of the combustion products that are recirculatedupstream. The region is designed to occur in the primary combustionzone where lean combustion is usually used in gas turbines to lowerNOx emissions [7].When the swirl combustor fires into a confined space, as in gas
turbines, significant alterations occur in the axial decay of swirlvelocity, and hence the axial and radial pressure gradients and theCRZ characteristics [8–11]. Other effects include the sticking of theshear-flow layer to the confinement wall for confinement ratios of upto at least 5 in area [12]. Syred [1] and Gupta et al. [6] showed thatunder isothermal conditions the effect of a circular confinementwas to increase the recirculated mass flow. Syred and Dahmen [9]made extensive measurements of the flowfields under isothermalconditions across a range of swirl numbers and geometries in a swirlburner/furnace unit. They showed the generation of large CRZs in thefurnace as the expanding shear flow stuck to the walls, often creatingan external recirculation zone (ERZ) in the corner of the furnace. Thestriking feature of the flow was the formation of a central axial flowjet embedded inside of the CRZ, despite the absence of any centralfuel injector and associated flow.More recently, Valera-Medina et al.[2] demonstrated the appearance of two intertwined CRZs underisothermal conditions using particle image velocimetry (PIV) on aperspex version of the combustor used here. Quite often, the CRZwas composed of two highly three-dimensional (3-D), separate struc-tures, intertwined with a precessing vortex core (PVC). All ofthese structures were precessing in a regular manner about thecentral axis.Detailed data are more limited under combustion conditions.
Claypole and Syred [13] and Claypole [14] used a swirl burner ofsimilar design, characterizing the CRZs formed under combustionconditions for different swirl numbers. Recirculated mass flow wasreduced during combustion for all swirl numbers. The effect ofequivalence ratio as φ → 1 was to produce lower values of re-circulated mass flows due to higher combustion temperaturesreducing swirl number. Beltagui et al. [10] used hubless and annularvaned swirlers firing into different furnaces, showing that the internaldiameter of the confinement space was crucial in determining thevalue of recirculated mass flows. Larger values of the internaldiameter increased the recirculation considerably. The dimensions ofthe CRZ were governed by the furnace dimensions for a given swirlnumber.Many studies have focused on industrial swirl combustors
interacting with a representative confinement [15–17]. Roux et al.[15] predicted that a radial swirler firing into a square enclosure
Received 27 February 2012; revision received 15 May 2012; accepted forpublication 15 June 2012; published online 5 December 2012. Copyright ©2012 by the American Institute of Aeronautics and Astronautics, Inc. Allrights reserved. Copies of this paper may be made for personal or internal use,on condition that the copier pay the $10.00 per-copy fee to the CopyrightClearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; includethe code 1533-3876/12 and $10.00 in correspondence with the CCC.
*Gas Turbine Research Centre; [email protected].†Gas Turbine Research Centre; [email protected].‡Gas Turbine Research Centre; [email protected].
195
JOURNAL OF PROPULSION AND POWER
Vol. 29, No. 1, January–February 2013
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would produce a compact but irregular flame close to the burner exit,whereas Schlüter et al. [18] and Selle et al. [19,20] predicted theexistence of a spiral vortex in the outer part of the shear flow leavingthe swirler. Schildmacher et al. [21–24] extensively investigated thecombustion dynamics of vaned and diagonal swirlers firing intocombustion chambers. The effect of the equivalence ratio upon theoscillation pressure amplitude was demonstrated, and phase-lockedanalysis of the swirl number over an oscillation cycle showedvariations between 0.15 and 0.8, sufficient to cause the CRZ to onlyexist during part of the oscillation cycle. The major disturbances tothe combustion process included acoustic, equivalence ratios, entry,pressure, and swirl strength oscillations, as observed by Kim et al.[25]. Turrell et al. [26] and Bulat et al. [27] used time-dependentcomputational fluid dynamics (CFD) with simplified models ofcombustion to describe the combustion dynamics of a Siemensgas turbine combustor fired on natural gas. The presence of PVCswas shown under several circumstances and was used to explaincertain problems. The effect of geometry upon CRZs was alsoinvestigated [27], but no time-dependent experimental data werereported.Important parameters are the swirl number [1–6], the outlet
geometry, and the effect of combustion via the equivalence ratio. Inthis paper, a simple definition of swirl number based on geometry, S(extensively discussed elsewhere [1–6]), is used, which is suitable forcases where the density is constant, i.e., isothermal conditions.Combustion located downstream of the tangential inlets does notsignificantly affect the axial flux of angular momentum but cansignificantly alter the axial flux of axial momentum [1–6] dependingstrongly on the equivalence ratio, the rate of heat release, the locationof the flame front, and the mode of fuel injection. This can causesignificant lowering of swirl number such that at high equivalenceratios the CRZ can disappear [1–6].In this paper, a swirl combustor and associated confinement
[1,2,13,14,28–30] are characterized using phase-locked PIV toinvestigate the effects of various variables on the shape, strength, andtime-dependent characteristics of the CRZ. This is then analyzed togive normalized average CRZ velocities to enable determination ofthe effect of other parameters.
II. Experimental Approach
Experiments were performed in a 100 kW steel scaled version of a2 MW swirl combustor [1]. Two tangential inlets were used togetherwith blockages of 50, 25, or 0% of the width of the tangential inlet toprovide different swirl numbers. The nomenclature is fX-Yg, whereXreflects the blockage percent of one of its inlets and Y represents theblockage percent of the other inlet, i.e., f50–0g refers to the casewhere one inlet is blocked off by 50% of its width and the other isopen. The system was fed by a centrifugal fan providing airflow viaflexible hoses and two banks of rotameters for flow rate control and afurther bank for natural gas injection. The burner geometry isdepicted in Fig. 1. The burner was operated in the thermal input rangeof 16 to 98 kW to derive information more representative of high-power density units. The fuel was natural gas.
A. Geometries and Conditions Used
1. Injection Mode
Two different modes of natural gas injection were used: a non-premixed mode with fuel injected along the central axis via fuelinjectors and a premixed mode with entry in one or both tangentialinlets, with the injectors located before the inserts used for varyingthe swirl number. Overall equivalence ratio φ is reported as well asthe fuel proportion. The format �X-Y� here refers to X l∕min non-premixed natural gas injection and Y to that injected as premixed.Because of the combustion process, the Reynolds number (Re) isdefined from the burner throat diameter and under isothermalconditions. Different equivalence ratios were investigated, from verylean conditions at 0.11 to rich values at 1.66. Awide variation in theairflow and gas-flow rates was also made to visualize the progressivedevelopment of the CRZ.
2. Fuel Injectors
Three diffusive fuel injectors were used, extending from the burnerbaseplate to near the burner exit nozzle (Fig. 2). Firing positions ofthe various fuel injectors relative to the burner exit are as follows:1) The wide injector is 0.6D upstream of the exit.2) The perforated injector is 0.1D downstream of the exit.3) The narrow injector is 0.05D upstream of the exit.These designsweremindful of the requirement for dual/alternative
firing with liquid fuels. Injectors B and C were found at lean φ toproduce CRZs that extended over the fuel injector, causingoverheating. Thewide injector was thus developed, exhausting some0.6D upstream of the burner exit andwhere the CRZ and flame rarelyextended. This part of the study is an extension of previous work onthe same burner [29] but firing freely into the air.
3. Exit Nozzles
Experiments were also carried out with different burner exitnozzles to analyze their effects on the CRZ and the position of theflame relative to the fuel injector (Fig. 3). The base case, where themajority of experiments were carried out, had no nozzle fitted (cases1–23 in Table 1). The three exit nozzles consisted of two parallelsided units, 0.8D and 0.9D, respectively, and one quarl nozzle of0.8Dwith a final conical section. A complete system description canbe found in [30].
4. Burner Confinement
The burner exhaust confinements are shown in Figs. 4a and 4b.Both units are formed from the same quartz, open-ended cylinder
Fig. 1 Burner geometry and dimensions in millimeters; D �80.04 mm.
Fig. 2 Injectors used for non-premixed natural gas fuel injection:a) wide injector, b) perforated, and c) narrow.
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(0.130 m length × 0.006 m thickness × 0.152 m i:d:) and are at-tached to the burner exit by appropriate fixing lugs and flanges. Theexpansion ratio for this combustor is 3.61. In Fig. 4b, a conicalcontraction of 45 deg angle is then added, with an aperture of 1.0D.The quartz tube enabled the PIV measurements to be carried outprimarily in the axial radial plane but also, to a very limited extent, inthe tangential radial plane, thus facilitating the 3-D reconstruction ofthe time-dependent nature of the CRZ. Because of reflection causedby the cylindrical shape of the quartz, masks were placed in frontof the charge-coupled-device camera to avoid burning its sensors.This reflection only occurred near the walls, therefore restrictingmeasurements only in this area. The posterior wall was paintedwith ablack, high-temperature-resistant paint to reduce signal noise. Allof the analyzed cases are shown in Table 1, which shows all of thedifferent geometries and flow parameters for reference.Experiments were carried out to characterize stable recirculation
patterns at different equivalence ratios. Axial radial planar velocityanalyses were performed for all cases.
B. Measurement Methodology
The CRZ is displaced from the central axis, precesses about it,and generates an asymmetric, precessing, crescent moon shape in ahigh-momentum flow region (HMFR) located in the shear layer[1,27,29,30]. Quite often, the vortex core precesses as well; this is
called the PVC and adds to the signal level. However, the primaryeffect was caused by the precession of the CRZ. This precession,HMFR, and associated regular pressure signal allow the derivation ofthe CRZ boundary and velocities as a function of phase angle.Pressure fluctuation measurements were made with a Yoga EM-1electret condenser microphone, with a frequency response of 20–16 kHz and sensitivity of −64� 3 dB. It was positioned 30 mmupstream of the burner exit [28–30], as used by others [31].This microphone condenser signal was redirected to a BNCModel
500 pulse generator, for which the Transistor-Transistor Logic signalwas sent to a Dantec PIV system. The latter consists of a dual-cavityNd:YAG Litron laser of 532 nm capable of operating at 5 Hz; aDantec Dynamics laser sheet optics (9080X0651) was used toconvert the laser beam into a 1-mm-thick sheet. To record the images,a Hi Sense MkII camera, model C8484-52-05CP, was used, with1.3 megapixel resolution at 8 bits. A 60 mmNikon lens was used forresolution purposes, which allowed a field of view of approximately75 × 75 mm, with a resolution of 5.35 pixels per mm and a depth ofview of 1.5 mm. The inlet air was seeded with a water nebulizer forthe isothermal cases and with aluminum oxide Al2O3 by a venturisystem positioned 2.0m upstream of the burner inlets for combustion
Fig. 3 Different exit nozzle inserts: 0.9D, 0.8D, and quarl, respectively.
Table 1 Analyzed casesa
Case Airflowrate, l∕min
Gas injection,l∕min b, c
Central injectionvelocity, m/s
Insert blockage S isothermal φt φp Central injector
1 600 25–0 0.97 25–25 0.98 0.40 0 Wide2 1000 25–0 0.97 25–25 0.98 0.24 0 Wide3 1600 25–0 0.97 25–25 0.98 0.15 0 Wide4 2200 25–0 0.97 25–25 0.98 0.11 0 Wide5 600 40–0 1.54 25–25 0.98 0.63 0 Wide6 1000 40–0 1.54 25–25 0.98 0.38 0 Wide7 1600 40–0 1.54 25–25 0.98 0.24 0 Wide8 2200 40–0 1.54 25–25 0.98 0.17 0 Wide9 600 25–40 0.97 25–25 0.98 1.03 0.63 Wide10 1000 25–40 0.97 25–25 0.98 0.62 0.38 Wide11 1600 25–40 0.97 25–25 0.98 0.39 0.24 Wide12 2200 25–40 0.97 25–25 0.98 0.28 0.17 Wide13d 600 25–80 0.97 25–25 0.98 1.66d 1.26 Wide14d 1000 25–80 0.97 25–25 0.98 0.99d 0.75 Wide15 1600 25–80 0.97 25–25 0.98 0.62 0.47 Wide16 2200 25–80 0.97 25–25 0.98 0.45 0.34 Wide17 2200 25–80 0.97 50–0 1.06 0.45 0.34 Perforated18 2200 25–80 0.97 50–0 1.06 0.45 0.34 Wide19 2200 25–80 0.97 50–0 1.06 0.45 0.34 Narrow20 2200 25–80 0.97 50–50 1.86 0.45 0.34 Perforated21 2200 25–80 0.97 50–50 1.86 0.45 0.34 Wide22 2200 25–80 0.97 50–50 1.86 0.45 0.34 Narrow23 2250 25–125 0.97 25–25 0.98 0.63 0.42 Wide24 2250 25–125 0.97 25–25 0.88 0.63 0.42 Wide25 2250 25–125 0.97 25–25 0.78 0.63 0.42 Wide26 2250 25–125 0.97 25–25 0.78 0.63 0.42 Wide
aThe Reynolds number (isothermal) varied from 11,000 to 51,000.bThe first entry is fuel injected through the central injector, whereas the second entry is premixed fuel injection.cThe last three cases were analyzed using the 0.9D, 0.8D, and quarl nozzles (Fig. 3), respectively.dThese locations are regions of very weak and intermittent CRZs.
Fig. 4 Swirl burner exhaust confinement: a) open-ended and b) conical.
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conditions. A flow of 250 l∕min of air was used to inject the seedingmaterial; this flowwas included in the determination of the final flowrate. The entire system was triggered at 90% of the highest peakobserved after 5 min of free running [8]. Velocity measurementsusing phase-locked PIV proved to be consistent under a wide varietyof conditions [2,29]. All CRZ boundary results are reported for the0–180 deg phase angle.The CRZ was characterized via the position of near-zero axial
velocities. After acquisition of the PIV data, a frame-to-framecorrelation technique was then carried out at 32 × 32 pixels, with anoverlap of 50%between frames to reduce noise. Seventy-five pairs offrames per plane were used to create an average phase-lockedvelocity map after tests showed 150 pairs of frames gave similarresults. This analysis was later extended to include the averagevelocity levels in the CRZ and directional intermittency. A vectorsubstitution of 2.8% was used. The velocity maps for the CRZs weredeveloped over the range −5.00 to 9.00 m∕s, where ∼97% of thevelocity vectors appeared, with the boundary taken at 0.27 m∕s: Thisvalue was chosen because it gave the most coherent CRZ boundary,as found for other experiments [29]. A velocity standard deviationanalysiswas also performed on the boundaries of eachCRZgiving anaverage value of 0.96 m∕s. This demonstrates a flapping movementof a small part of the CRZ, which was also observed in a previousprobability-density-function analysis [29]. The standard deviation inthe highly turbulent shear-flow region was often above 3.90 m∕s,thus confirming aCRZboundary that was quite stable. The frequencyof the CRZ was observed to be between 60 and 130 Hz, accordingto the analyzed configuration and the flow rate; the higher the flowrate, the faster the CRZ precessed. This is in accord with otherstudies [1,2,30].Experiments were initially conducted to create full 3-D recon-
structions of the CRZs at two particular conditions to facilitateunderstanding of the later work that merely used the 0–180 deg phasesection to characterize and compare each of the numerous CRZs.Investigated conditions were chosen from experiments that gave themost stable CRZ. Because of the precession of the CRZ and theplanar nature of the axial radial velocity planes, measurements wereobtained by moving the laser circumferentially around the outlet lipsequentially by 11.25 deg (32 planes in 360 deg) to allow a complete3-D reconstruction of the CRZ. Each phase plane will be referencedaccording to the position where the laser was placed during theiracquisition.
III. Results and Discussion
A. Preliminary Studies
Experiments were initially performed at low gas-flow rates andvalues of φt ≤ 0.4. First results showed some similarities to thoseobtained under isothermal conditions with the same exhaust con-finement [2], although the breaking up of the CRZ into two 3-Dhighly interwoven sections was not found.Under isothermal conditions the CRZs strongly interacted with a
PVC; somewhat different behavior was found under combustionconditions. Some evidence of the breakup of the main CRZ into twosmaller CRZs was found but could not be corroborated owing to thedifficulties of obtaining results under confined conditions. In contrastto the isothermal result, the existence of a regular PVC could not beconfirmed as in previous experiments [2] where the vortex could beclearly identified. However, erratic harmonics were observed in thetriggering signal, suggesting its suppressed presence. Combustionwith the tested fuel injectors and under the specified conditionsthus suppresses the strength and coherence of the PVC, withoutcompletely eradicating it, even at the lean values of φ used, while theCRZ still precesses. This appears to be a new finding.Figure 5 shows a high thermal input flame of 98 kW expanding
out of the burner nozzle (case 23 in Table 1) and hitting the walldownstream at about X∕D ∼ 1. There is recirculation of very hotgases by the ERZ shown by the glow of the plate surrounding theburner exit. Little reaction in the ERZ appears to occur, this beingconfined to the CRZ and boundary region within the shear flow. Theflame extended back into the combustor exhaust to the injector.
Figure 5 also shows how the quartz tube terminated some 10 mm(0.13D) above the burner exit, and thus the shape and extent of theflame (and CRZ) here relied on visual/photographic observation asclearly PIV measurements could not penetrate any lower. Figure 6shows one of the vector maps obtained during the experimentalprocedure. The following three main features can be clearlyidentified: 1) the CRZ indicated by the direction of the vectors, 2) theshear flow surrounding the CRZ (note the asymmetric nature due tothe formation of an HMFR; Fig. 7), and 3) the formation of an ERZbetween the shear flow and the outer wall.
B. Three-Dimensional Reconstruction of the PrecessingCentral Recirculation Zone
The complete 3-D, precessing, shape of two particular CRZs werereconstructed in a 3-D analysis using the two exhaust confinementsshown in Figs. 4a and 4b for case 11 in Table 1. The CRZ boundarieswere reconstructed from the phase-locked PIVmeasurement data in a3-D space using measurements from every plane (32 planes every11.25 deg) for partially premixed combustion, φt � 0.386. This isthe same method as used in [2] to characterize various CRZs in thesame rig under isothermal conditions. Because the recirculation zoneextended downstream of the PIV data acquisition area, the 3-Dreconstruction appears flat at the top (Figs. 7a and 7b). The location ofthe HMFR that triggers the PIV system is also denoted. The CRZstructures precess in an anticlockwise direction. The lobed shape tothe right, toward the HMFR, of both CRZs is evident. The lobes forboth cases extended to considerable height fromX∕D ∼ 0.42 to 1.25.The confinement of Fig. 4b produced a more flattened CRZ than thatof Fig. 4a. These results differ considerably from those found under
Fig. 5 Flame image from case 23 in Table 1; φt � 0.63, and the highthermal input is 98 kW.
Fig. 6 Vector map of the experimental procedure. The CRZ can beobserved in the middle.
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isothermal conditions [2], where the single CRZ breaks up into twoseparate but intertwined CRZs and there was a strong presence of thePVC. There was some evidence in certain phase-angle planes thatwith the conical exhaust the CRZ could intermittently split into twointertwined structures, but no distinct PVC could be regularlydiscerned; the signal for the phase locking was generated bythe HMFR that appears to have developed from the precession of theCRZ, not the vortex core. These two figures illustrate that theprecessing CRZs are of somewhat irregular deformed balloon shape.For characterization of the numerous CRZs investigated later in thepaper, the 0–180 deg phase-angle axial radial velocities were used asthis gave a representation of thewidest section of the CRZ, as is clearfrom Fig. 7. Some results were obtained for tangential radialvelocities, as illustrated by the inset of the HMFR in Fig. 7.
C. Comparison of Central Recirculation Zone Boundaries
The reconstruction of the 3-D, precessing, boundaries of the twoCRZs illustrates the complex nature of the CRZs formed. CRZboundaries, axial velocities, and the average CRZ axial velocities inthe radial–axial 0–180 deg phase planes have been used to comparethe various CRZs formed. Reference to Fig. 7 and the point where thetriggering signal was obtained indicates that this phase angle willmeasure the maximum diameter of the CRZ and thus forms a goodbasis of comparison for all of the CRZs characterized.Figure 8 shows PIV velocity contours for a wide variety of
conditions (cases 1 to 16 in Table 1), S � 0.98, to illustrate how thechanges in equivalence ratio and mode of fuel injection distort theCRZ. The wide injector was chosen as it gave more stable phase-locking triggering signals.Figures 8a to 8d show PIV velocity maps of non-premixed flames
at lean equivalence ratios ≤0.4. Strong coherent stable recirculationzones are formed even at low airflow rates. The strength of the CRZincreases with airflow rate and reduction of φt. Moreover, the shapeof the CRZ is maintained as a slightly irregular, lobed structure,similar to the observations of Syred [1] and Dawson et al. [28]. In the
outer shear layer, the velocity is biased to the right (in accord with thephase locking) and matches the position of the HMFR discussedpreviously.Higher gas-flow rates (Figs. 8e to 8h) show strong CRZs devel-
oping with higher natural gas-flow rates. Shapes of the CRZs arevery similar for similar equivalence ratios; compare Figs. 8a and 8f.A coherent stable recirculation zone is formed for airflow rates≥1000 l∕min,φt � 0.38, whereas aweakCRZ formed at lower flowrates (Fig. 8e); here, the swirl number is reduced by combustion andthe flow is close to vortex breakdown. Irregular harmonics wereobserved in the triggering signal at moderate and high flow rates inthe range of 0–100Hz, similar to those found in isothermal flows [2].The generation of a region of high momentum (HMFR) in the shearflow on the right-hand side of most figures is again noticeable.With partial premixing [25–40], more intense flames developed
with generally lower axial velocities in the CRZs (Figs. 8i to 8l). Thisis to be expected as the overall equivalence ratios are higher withmore heat release, increased axial flux of axial momentum, andmorereduction of swirl number for a given value of φt. A coherent, stableCRZ developed for airflow rates ≥1000 l∕min, with a wobblingunattached CRZ at lower airflow rates and φt � 1.03. Clearharmonics in the high-momentum shear-flow region were observedatmoderate and highRe in the range of 0–100Hz for a first harmonic,followed by another one at 200–250 Hz, quite common in com-busting flows [1] but not observed under isothermal conditions. At600 l∕min airflow (Fig. 8i), φt � 1.030, the higher heat releaseincreases the axial flux of axialmomentum such that the swirl numberhas been reduced and the CRZ has been substantially reduced in size.Atφt � 0.620 (Fig. 8j), the effect of combustion is not so high and
a stronger CRZ has reestablished itself, the effect continuing as theairflow is increased and equivalence ratio decreases to φt � 0.281(Fig. 8l). When premixed fuel was increased to 80 1∕min (Figs. 8mto 8p), the result was the production of highly emissive flames at lowflow rates without a CRZ. As the airflow is increased, φt reduces to0.623 and 0.453 (Figs. 8o and 8p) and strong CRZs develop. Again,
Fig. 7 CRZ for Case 11, Table 1 for a) open and b) conical confinements. Isosurfaces created at 0.27 m∕s. HMFR as observed in [2]. Dimensionless scale[X∕D].
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the high-momentum region in the shear flow on the right-hand side isevident, even when there is no coherent CRZ at low flow rates. TheCRZs steadily strengthen as φt reduces from 0.623 to 0.453, and forairflow rates ≥1600 l∕min. Clear harmonics were observed in thetriggering signal in the range of 0–100 Hz and 200–250 Hz without aCRZ, possibly due to some form of the PVC or natural frequency ofthe vortex breakdown.Comparison of the CRZ boundaries for all of the above cases is
shown in Fig. 9. Higher deformation of the CRZ occurs in the sideopposite to the triggering where the HMFR is located. Initial CRZboundaries are fairly similar for airflow rates ≥1600 l∕min, whilethe confinement allows the CRZ length to increase substantially. It isencouraging that all of the CRZs appear to terminate close to theburner exit,minimizing the effect of flame damage to the fuel injector.Many of the differences between the CRZ boundaries can beattributed to the wide differences in φt (from 0.11 to 1.03), and thusthe different heat release patterns. For φt ≤ 0.63, the CRZ is mostregular at higher airflow rates.Phase-locked velocity vectors inside the CRZs in the above cases
were normalized with their corresponding averaged axial velocity in
the annulus surrounding the fuel injector (Fig. 2a) and presented as anormalized velocity (NUcrz in Fig. 10). The highest values of NUcrz
were produced by partially premixed combustion up to −13% atφt � 0.45 and φp � 0.34. Curves have been fitted to the partiallypremixed data to illustrate the differences produced by differentlevels of partial premixing; higher levels of premixed gas in the�25–80� l∕min cases produce higher values ofNUcrz compared to the�25–40� l∕min cases. General trends for non-premixed combustionare similar to the �25–40� l∕min partially premixed cases, wherepremixed combustion was not so dominant. The results illustratethe intermittent nature of some of the CRZs because the velocities(Fig. 10) always become positive as equivalence ratio increases.Diffusive combustion had the most effect. Regular precession of thecentral region of flow was always detected even where the averagevelocity vectors were positive; for non-premixed diffusive flames,this occurred up to equivalence ratios of ∼0.5, and for partiallypremixed ones up to equivalence ratios of 1.Figure 10 shows how the non-premixed combustion only creates a
strong CRZ (defined as having average velocities always in thenegative region) at low equivalence ratios. This appears to be because
Fig. 8 Confined PIV velocity analysis. Cases 1–16. Configuration as in Fig. 4a. Plane x-y.
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the fuel flows through the injector at low velocity around the CRZ,creating a region richer in fuel than the surroundings. Thus, foroverall lean combustion, the CRZ burns at a higher temperature thanthe rest of the flame, reducing the CRZ strength.When partially premixed conditions are compared to non-
premixed conditions, similar equivalence ratios generate strongerCRZs over a wider range of equivalence ratios. When only thepartially premixed cases are compared, higher reverse-flowvelocitieswere generated in the CRZ when the premixed gas flow wasaugmented from 40 to 80 l∕min. This appears to arise from changesin the heat release patterns causing differences in the distribution ofaxial flux of axial momentum, and hence swirl number, in the system.
D. Effect of Swirl Number and Type of Fuel Injector
For cases with different fuel injectors, numbers 17 to 22 in Table 1,the CRZ boundaries were somewhat different; compare Fig. 11 toFig. 9. All of the CRZs formed extended back into the burner and, forthe perforated and narrow injectors, over the injectors. Similarly, theflame boundaries, located close to theCRZboundaries, also extendedover the fuel injectors toward the burner baseplate for mostequivalence ratios. The effect was most accentuated at lean values ofφt ≤ 0.4. Downstream of the CRZ, long weak CRZ tails formedextending into the confinement exhaust, effects also observed byothers [4,9,10,12,14]. Asymmetric airflow entry through one inletf50–0g, S � 1.06 (Fig. 11a), appears to be most affected by theperforated injector (Fig. 2b); here, the fuel is injected radially into theshear layer, producing a slightly more asymmetric CRZ. The fueljet from the narrow injector (Fig. 2c) can be clearly seen at the baseof the CRZ and appears to be precessing, an effect found by others[1]. However, it does not appear to affect the CRZ boundariessignificantly compared to results with the wide injector (Fig. 2a).With symmetric flow entry via equal-sized inserts f50–50g, and ahigher swirl number, S � 1.86 (Fig. 11b), CRZ boundaries are verysimilar while the effect of the high-momentum central fuel jet withthe narrow fuel injector has been suppressed. Comparing Fig. 11bwith Fig. 11a at a higher swirl number, the CRZs are slightly widerandmore compact close to the burner forX∕D ≤ 1; beyond this pointthe CRZ tail was much stronger at higher values of S. NormalizedCRZ axial velocities, NUcrz, are also shown in tabular form inFigs. 11a and 11b. If compared to Fig. 10 for S ∼ 1, the same value ofφt (overall equivalence ratio) and the wide fuel injection (Fig. 11a),NUcrz is some 20% lower than the result shown in Fig. 10 forsymmetrical inlets. The NUcrz results for the perforated and narrowfuel injectors are some20%higher than those for thewide injector. Asexpected, higher swirl numbers, S � 1.86 (Fig. 11b), producedvalues of NUcrz some 20 to 30% higher than at S � 1.06, this beingindependent of the type of fuel injector.Comparing the CRZ boundaries of Fig. 11a with those of Fig. 9, it
is seen that for similar swirl numbers, 1.06 compared to 0.98, verysignificant changes in CRZ boundaries have occurred via the use ofasymmetric flow entries with one air inlet 100% open and the other50% closed. TheCRZboundaries of Fig. 11a are very similar to thoseof Fig. 11b at a much higher swirl number. This appears to arisebecause in operation the flow at each air inlet was equal, beingmonitored via separate flow meters, and thus inlet velocities weredifferent and the associated swirl with the 50% closed inletdominated, producing CRZ boundaries closer to those at S � 1.86.
E. Effect of Burner Exit Nozzles
Three different exit nozzles (Fig. 3) were studied to investigatetheir effects on the CRZs. Swirl numbers varied from 0.98 to 0.78(cases 23 to 26 in Table 1). Other work indicated that beneficialimprovements in flame holding could be achieved under thesecircunstances [3,6]. The quarl nozzle produced the lowest levels offlame impingement on the fuel injector at lean equivalence ratios<0.5. CRZboundaries are shown in Fig. 12,with PIV velocity imagesshown in Fig. 13. Comparison is made with partially premixedcombustion with the wide fuel injector (Fig. 2a) and the openconfinement of Fig. 4a, at a higher thermal loading of 98 kW toproduce a comparison more repesentative of industrial practices.Otherwork [30] has shownbenefits from the use of the quarl nozzle interms of improved flashback limits and avoidance of flameimpingement on the wide injector.CRZ boundaries (Fig. 12) are similar as they leave the burner exit,
but reduce in length in accordwith reduction of nozzle diameter, from1.76D (D) to 1.14D (0.8D). The CRZs now develop closer to theburner exit and are not lifted as occurred in Fig. 9. Maximum CRZdiameter reduces proportionately with the reduction of nozzlediameter apart from the quarl nozzle. The quarl nozzle has the same0.8D as the parallel nozzle (Fig. 3), but the CRZ boundaries aresignificantly reduced in size. Precession of the CRZ is most evidentwith the quarl nozzle, but all CRZ boundaries are biased to the righttoward the HMFR. All of the CRZs are more compact than those
Fig. 9 CRZ boundary analysis, wide injector, as in Fig. 2a; S � 0.98.
Fig. 10 NUcrz as a function of φt; wide injector (Fig. 2), S � 0.98.
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shown in Fig. 9 for the same overall value of φt. The NUcrz valuesare also extremely interesting (Fig. 12), with values increasingsequentially from −1.8% (D), −3.4% (0.9D), −12.3% (0.8D), and−22.1% (quarl). Thus, despite the lower swirl numbers with thesmaller diameter nozzles, the trends show reduced CRZ dimensions,but increased values ofNUcrz. This illustrates the benefits of the quarlnozzle in producingmore compact flames and reducing wall contact.The greatest contribution to the phenomenon is caused by the
divergent nature of the quarl nozzle, which increases the strength ofthe CRZ arising from changes in velocity decay and hence pressurefields. Figure 14 shows data from all tests summarizing the variationof maximum diameter of the CRZ with partially premixedcombustion; there is a near-linear variation inDcrz∕D as a function ofφp, although there is some small effect of the level of premixing, i.e.,φp∕φt. The data with the various exit nozzles fit into this curvereasonably well. Unfortunately, comparison of overall CRZ L∕Dratios was not possible as the CRZs in many tests extended beyondthe end of the measuring region.The highest levels of reversed axial velocity in the CRZ occur
with the quarl nozzle (Fig. 13d, and in accordance with Fig. 12) withthe CRZ being much more compact than with other cases; for thesame 0.8D diameter nozzle, there is a significant difference betweenthe parallel-sided nozzle (Fig. 13c) and the quarl nozzle, showing theeffect of the small conical divergence at the end of the quarl nozzle.The shear flows are always biased to the right, especially just past theburner exit, in accord with the HMFR, which triggers the phaselocking. With no exit nozzle (Fig. 13a), the shear flow sticks readilyto the outerwall, leaving a low-velocity central void.Moving throughthe various nozzles, this void slowly closes and fills with highervelocity flow, the effect being most noticeable with the quarl nozzle.All of the CRZs can be seen bending to the right and are precessingwith the HMFR.
F. Effect of Confinement
After comparing results between those obtained with combustion,but without confinement [29], it is clear that the structures, particu-larly the CRZ, are very significantly affected by the confine-ment of the flow and flame. Of special note is the preponderance of
Fig. 11 Effect of fuel injector and air inlet configuration for cases a) 17–19 and b) 20–22 in Table 1.
Fig. 12 CRZboundary analysis using different burner exit nozzles thanin Fig. 3.
Fig. 13 PIV velocity maps with and without various exit nozzles, Fig. 3. Cases 23–26. Plane x-y.
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positive velocity vectors, and hence values ofNUcrz, in CRZ regionsfor values ofφt > 0.5 for non-premixed combustion andφt > 0.65 to1 for partially premixed combustion. This was not observed withunconfined flames.Moreover, the confinement, togetherwithφp, hasa greater effect onDcrz, the nondimensionalized diameter of the CRZ,than that observed with the unconfined flames. This infers thatdifferent CRZs develop with confinement, with reshaping possiblysimilar to the equivalent confined isothermal cases [2,32]. This iscaused by changes in the tangential velocity distribution and down-stream pressure decay [1,5], which change the pressure gradientsgiving rise to the CRZs.
IV. Conclusions
A three-dimensional reconstruction of the central recirculationzone (CRZ) for the two exhaust confinements produced similar lobedprecessing CRZs at lean equivalence ratios. There was evidence ofmore complex structures developing, similar to those found underisothermal conditions. To the authors’ knowledge, the full 3-D time-dependent structure of such CRZs has never been previouslycharacterized experimentally under combustion conditions. Ofspecial note is that regular precession was found for all CRZsinvestigated, even when the normalized average velocity vectorswere positive at equivalence ratios higher than 0.5 to 0.7 (dependenton configuration). Under these conditions, the outer annular shearflows spread somewhat radially and reduced in magnitude, butnevertheless retained their coherence and surrounded a low-velocity,intermittent central region.When a swirl burner combustor is fired into a confined space,
variation in equivalence ratio and flow rate produced very significantchanges in the CRZ structure. For φ > 0.65, the CRZ becameintermittent and often disappeared. With this type of unit, there isnormally an external recirculation zone present in the corner formedwhere the confinement joins the burner exit. Disappearance of theCRZ can be undesirable as it can move the primary region of flamestabilization closer to the walls, potentially causing overheating.Values ofNUcrz decrease in accord with the equivalence ratio, the
mode of injection, and the level of premixing; maximum values werearound 15% using an open nozzle. This is partially caused by hotreaction products that lead to increases in the axial flux of axialmomentum, and hence decreases in the swirl number and thetendency for vortex breakdown. Leaner equivalence ratios strengthenthe CRZ, which becomes closer in structural terms to that foundunder isothermal conditions.Combustor exit nozzles were of benefit in restricting the CRZ size
and reducing the possibility of flame impingement on the fuelinjectors. The quarl nozzle was of particular benefit, producingcompact CRZswith higher values ofNUcrz due to associated changesin pressure distribution and pressure decay.Asymmetric flow entry at a value of S � 1.06 produced
boundariesmost similar to those produced by symmetrical flow entryat S � 1.86, with an enlarged CRZ in the combustor exhaust with along tail.
Acknowledgments
Agustin Valera-Medina gratefully acknowledges the receipt of ascholarship from the Mexican Government National Council ofScience and Technology (CONACYT) and the assistance ofMalcomSeaborne during the setup of the experiments.
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A. GuptaAssociate Editor
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