International Communications in Heat and Mass Transfer 36 (2009) 936–941

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International Communications in Heat and Mass Transfer

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CFD insight of the flow dynamics in a novel swirler for gas turbine combustors☆

Yehia A. Eldrainy a,⁎, Khalid M. Saqr b, Hossam S. Aly a, Mohammad Nazri Mohd Jaafar a

a Department of Aeronautical Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Malaysiab High-Speed Reacting Flow Laboratory, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Malaysia

☆ Communicated by W.J. Minkowycz.⁎ Corresponding author.

E-mail address: amaeyehia2@siswa.utm.my (Y.A. Eld

0735-1933/$ – see front matter © 2009 Elsevier Ltd. Aldoi:10.1016/j.icheatmasstransfer.2009.06.013

a b s t r a c t

a r t i c l e i n f o

Available online 19 July 2009

Keywords:Swirling flowVane swirlerGas turbineCFDSwirl numberRecirculation zone

We describe the flow dynamics inside a novel swirler conceptualized for gas turbine combustors. The supremeadvantage in this swirler is the ability to vary the swirl number for the same value of Reynolds number. Thesignificance of such advantage against contemporary configurations, which have constant swirl number, is quiteevident at low turbineoperating loads. Thenovel geometryandflowpattern aredescribed indetails in the presentwork. The results of four numerical simulations are presented and discussed to study the central recirculationzone, turbulence intensity, and pressure drop at different swirl numbers. The new concept is deemed to enhancethe combustion efficiencybecauseof its ability to adjust the swirl numberaccording to the turbineoperating load.The current study reports preliminary results which verify the concept behind the proposed swirler. However,intensive numerical and experimental studies are necessary to be carried out in order to characterize the flowdynamics produced by the novel swirler and its impact on the combustion process.

© 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Continuous combustion processes, such as those occurring in a gasturbine combustor, require the flame to be anchored at a zone withinthe flow field. In order to stabilize the flame, the incoming high-speedair streammust be decelerated to a velocity below the turbulent flamespeed. The flame stabilizes along the locus of points where the airvelocity is equal to the flame speed [1,2].

This flame stabilization can be achieved by creation of a toroidalreversal flow that entrains and recirculates a portion of the combus-tible gases to mix with the incoming air and fuel. One of the mosteffective ways of inducing recirculation in gas turbine combustorsis to fit a swirler in the dome around fuel injector. In this technique,swirl stabilization, a swirl velocity is imparted to the inlet air usingvane swirlers.

The degree of swirl in the flow is quantified by the dimensionlessparameter, Sn known as the swirl number which is defined as:

Sn =

RR0

ρUWr2dr

RRR0

ρU2rdrð1Þ

where U and W are the axial and tangential velocity componentsrespectively and R is the swirler radius. This number is a ratio of theaxial flux of angular momentum to the axial thrust.

rainy).

l rights reserved.

Swirling flows result from the application of a spiral motion, a swirlvelocity component being imparted to the flow by the use of somekinds of swirl generators. They can be of various designs; axial orradial; single or double (co-rotating or counter-rotating). The swirlervanes are often flat for ease of manufacturing; however, curved vanesmay sometimes be preferred due to their potentially better aerody-namics properties. Inside the combustor, the swirling flow spreads asit moves downstream, and the centrifugal force creates a low pressureregion in the centre of the flow. At a certain point downstream, thislow pressure region causes the vortex to collapse inwards on itself in aprocess known as vortex breakdown [3]. This creates a recirculationzone in the centre of the flow. This is essential to provide sufficienttime, temperature and turbulence for a complete combustion of thefuel [4,5]. Recirculation is generally not observed for swirl numbersbelow0.4, somost swirl-stabilized burners aredesigned for swirl num-bers greater than 0.6 [1,2].

Swirling flow, indeed, helps to stabilize the flame; in addition itproduces numerous effects which are beneficial to the combustionsystem. These effects primarily include promoting fuel and air mixingand assisting the control of combustion temperatures and emissions.This is because of the strong shear regions, high turbulence and rapidmixing rates produced by the swirling vortices and the resultingtoroidal recirculation zone. The various characteristics of swirl com-bustion are discussed extensively in Refs. [6] and [7].

Within the last decade, numerous studies were conducted to char-acterize the flow generated by different types of swirlers. In additionto enhancing themixing and combustion efficiency, amainmotivationof such studies was the desirable reduction of NOx emissions. Danget al. studied the influence of geometric parameters on combustionefficiency in a dual-stage swirler combustor [8]. Their experimental

Fig. 2. Pathlines passing through the axial passages.

937Y.A. Eldrainy et al. / International Communications in Heat and Mass Transfer 36 (2009) 936–941

results showed that, the geometric parameters of dual-stage swirlerand the arrangement of primaryholes (swirler passage area, swirl vaneangle of the primary and secondary swirler, the distance of the flareexit from throat, etc) had a great influence upon the combustion per-formance of the combustor. The work of Fu et al. remarkably outlinedthe characteristics of swirling flows generated from counter-rotatingand axial swirlers in the last fewyears [9–12]. Theyprovided importantdata about the swirling flow physics generated by different swirldegrees and geometries, which indicated that the degree of swirl hada clear impact on the mean and turbulent flow fields. The flow field,downstream different triple annular swirlers (TAS), has been studiedextensively by Li and Gutmark [13,14]. They indicated that the CRZ,produced by such swirler configuration, is axisymmetric but the jetcontains imprints of the internal flow and has some asymmetricfeatures. Lee andMoon have experimentally investigated the effects ofan axial swirler with turbulence generator on the mixing efficiency offuel and air [15]. They evidently showed that the mixing in the com-bustor in the radial direction was significantly improved and flamesand temperaturewerewell distributed throughout the cross section ofthe combustor as the area of turbulence generator increased.

The aerodynamic and thermodynamic performance of vane swirlerswas extensively studied by Wu and Chung [16,17] and Lilley [18–20].These works highlighted the main features of the flow field generatedby vane swirlers. In addition, they implicitly pointed out that the stateof the art vane swirlers share one disadvantage, which is the constantswirl number independent of the amount of inlet flow rate. This hasbeen also discussed by the authors in recent studies [21–23]. Atrelatively low loads, the inlet air flow rate is correspondingly low. Thismeans that the produced swirl and turbulence, likewise, decrease,which reduce the intensity of the recirculation zone. For this reason,the performance of gas turbine combustors is reduced at lowoperatingconditions (i.e. low Reynolds number).

The abovementioned disadvantage occurs because the swirl isproduced merely when the inlet air streams passes through the in-clined vanes, and gain tangential velocity. This tangential velocity com-ponent, then, intercepts with the main axial velocity componentcreating swirl. Thus, if another source of tangential velocity exists,which is independent of the main air stream, it is possible to controlthe swirl intensity at the same swirl geometry.

2. Features of the new swirler

The concept of the proposed swirler is introduced in Fig. 1. The inletair flow is divided into two streams. One stream passes axially through a

Fig. 1. The concept of the new swirler with a cutout in the outer shroud showing two ofthe tangential vanes.

cylindrical passage with conventional vanes. The second stream passestangentially to the axial flow passage through multiple tangentialpassages. The axial flow stream is interrupted by the tangentialflow creating a swirling turbulent region. The axial and tangential flowstreamdirections are illustrated inFigs. 2 and3 respectively. The intensityof swirling flow is mainly governed by the ratio of the tangential to axialair mass flow rates. When the swirl velocity component is feeble at lowflow velocities, it is now possible to amplify it through the tangential airstreamwhich will boost the value of the swirl velocity component.

3. CFD simulation

3.1. Physical domain and mesh

The physical domain represents the 3D flow field inside the novelswirler geometry, discharging to a combustor via one port. The mixingof the tangential and axial flows takes place upstream the combustorinlet port. The dimensions of the physical domain are illustrated inFig. 4. The total volume of the problemphysical domainwas 1.313729×10−2 m3. A total number of non-uniform hexahedral cells of 1,040,674were implemented to get a minimum cell size of 3.924182×10−10 m3

in the zone of high gradients as in Fig. 5. The discretization of the phys-ical domain was carried out using GAMBIT 2.3 software.

Fig. 3. Pathlines passing through the tangential passages.

Fig. 4. Dimensions of the physical domain.

Fig. 5. Computational gird.

938 Y.A. Eldrainy et al. / International Communications in Heat and Mass Transfer 36 (2009) 936–941

3.2. Governing equations

The 3D flow was assumed to be incompressible and turbulent. Thegoverning equations are the conservation of mass and momentumas in Eqs. (2) and (3). The turbulence model used in the present studyis the standard k–ε model [24,25]. The equations of the turbulencekinetic energy (k) and its dissipation rate (ε) are Eqs. (4) and (5),respectively.

AρAt

+A

Axiρuið Þ = 0 ð2Þ

A

Atρuið Þ + A

Axiρuiuj

� �= − Ap

Axj+

A

Axjμ

Aui

Axj+

Auj

Axi− 2

3δij

Auj

Axi

!" #

+A

Axj−ρuVi uVj� �

ð3Þ

AkAt

+ uiAkAxi

= vtAui

Axj+

Auj

Axi−

" #Aui

Axj+

A

Axi

vtσk

AkAxi

� �− e ð4Þ

AeAt

+ uiAeAxi

= Ce1ekvt

Aui

Axj+

Auj

Axi

" #Aui

Axj+

A

Axi

vtσ e

AeAxi

� �− Ce2

e2

k: ð5Þ

3.3. Numerical solution details

The CFD code Fluent 6.3 [26] was used in order to solve the dis-cretized governing equations. Four numerical simulations were per-

formed at four different tangential to axial mass flow ratios in orderto examine the impact of the mass flow ratio on the length of the CRZ.In all the simulations, a steady state pressure based solver was used tosolve the governing equations. This solver uses an algorithm whichsolves the governing equations consecutively. The Semi ImplicitMethodfor Pressure Linked Equations (SIMPLE) algorithm [26] was used forpressure/velocity coupling. This algorithm satisfies the mass conser-vation equation by using a relationship between velocity and pressurecorrections.

The Quadratic Upstream Interpolation for Convective Kinetics(QUICK) scheme was implemented for the discretization of the con-vective terms in order to enhance the solution accuracy. Moreover, forthe case in hand, where the pressure variation between consequentmesh cells was expected to be significant, the pressure staggeringscheme was used. Such scheme computes the staggered pressurevalues at the face of each cell in order to capture the pressure non-uniformity [27].

4. Results and discussion

The vortex breakdown phenomenon in swirling flow is closelyrelated to the inlet flow Reynolds number ReD and swirl number Sn.Moreover, it is affected by the downstream boundary conditions [28].To explore the effects of the new swirler design on the swirling flowfield, the influence of tangential to axial inlet mass flow ratio on swirl-ing flow is investigated. This section presents a discussion of the CRZshape and the formation of additional recirculation zones at the cham-ber corners. In addition, the corresponding static pressure drop is studiedin order to comprehend theflowbehavior resulting from thenewswirler.

The swirl number is numerically computed for the four casesusing the integration shown in Eq. (1) on the swirler exit plane. The

Table 1Tangential and axial flow rates for the four cases.

Case a b c d

Tangential mass flow rate 0.05 0.075 0.1 0.125Axial mass flow rate 0.15 0.125 0.1 0.075Tangential to axial flow ratio 0.333 0.600 1.000 1.667Swirl number 0.586 0.801 1.220 1.463

Table 2Mach number and flame stagnation point for the four cases.

case a b c d

Tangential to axial flow ratio 0.333 0.600 1.000 1.667Flame stagnation point (axial distancefrom swirler inlet, mm)

303.322 398.344 527.350 748.461

939Y.A. Eldrainy et al. / International Communications in Heat and Mass Transfer 36 (2009) 936–941

different values of swirl number are listed in Table 1. It is clear that theswirl number increases with the increase of tangential air flow rate.In the present concept, the swirl number increased approximately 2.5times when the ratio of the tangential to axial flow rates increasedfrom 0.333 to 1.667.

The contours of reversed flow region (negative axial velocity)on stream-wise plane are shown in Fig. 6a–d. These contours revealthe dependency of the CRZ size and shape on the tangential to axialmass flow rate ratio. It is evident that the CRZ is controlled in this newswirler concept by varying the inlet mass flow ratio. Controlling theCRZ implies that the flame stability can also be controlled. Fig. 6 alsopoints out another major effect for varying the inlet mass flow ratio,namely, the diminishing of the corner recirculation zones (i.e. deadzones) as the tangential to axial flow rate ratio increases. These deadzones have a negative effect on the combustion process and the homo-geneity of the wall temperature as well [29]. Therefore, eliminating,or at least reducing, these unfavorable recirculation zones are vital forenhancing the combustion process. It is noteworthy here to state thatthese positive effects of increasing the tangential to axialflow rate ratiocome with the expense of increasing the pressure drop through thecombustor, and hence reducing its efficiency. Consequently, a carefuloptimization study must be performed in order to decide the range ofinlet mass flow ratios operating conditions.

The Mach number for the four different cases is 0.032 which is atypical value for real gas turbine combustors [30]. It is also importantto highlight the location of the flame stagnation point and its variationwith different tangential to axial mass flow ratios. It can be seen fromthe data in Table 2 that such pointmoves further downstreamwith theincrease of tangential to axial mass flow ratio (i.e. swirl number).

It is quite crucial to study the turbulence behavior as well. Fig. 7illustrates the turbulence intensity just after the swirler exit for thedifferent cases.With the occurrence of recirculation zones due to swirl,additional turbulence is produced in the shear layer between the re-verse flow and the surrounding forward flow and serves to enhance

Fig. 6. CRZ for different axial to tange

mixing in the combustor. The highest turbulence intensity is achievedat the maximum tangential to axial flow ratio, which reveals the con-junction of turbulence with the CRZ formation and quality. The locusof the maximum turbulence intensity can also be of significance inexplaining how the tangential and axial flow streams intercept. At thelowest tangential to axial mass flow ratio, the two stream interceptrelatively away from the swirler outlet.While in comparison,when thetangential mass flow is at maximum value, it intercept with the axialflow stream just at the swirler outlet. This comparison is evident bycomparing the turbulence intensity contours in Fig. 7a–d. Hence, it isperceived that the location of the maximum turbulence intensitydepends on the shape and size of the CRZ.

The static pressure is plotted in Fig. 8 along the centerline of thecombustor for the four cases. It is clear that the pressure drop increaseswith the increase of tangential to axial velocity ratio. This is physicallycomprehensible because the CRZ induces high shear stress regions,which dissipate the flow energy, pressure in this sense. This resultshapes the limitations of the presented swirler concept in terms ofvarying the swirl number. Such limitation, however, is positivelysuperior to the geometry andfixed swirl number limitations existing inthe state of the art swirler.

5. Conclusion

A new swirler for gas turbine combustors was presented. The newconcept enables the variation of swirl number at the same air inletmass flow rate. The performance and main characteristics of the newswirler were examined through four numerical simulations. Thesesimulations evidently proved that the swirl number significantlychanges with the variation of tangential to axial flow rate ratio, en-abling the tuning of swirl number according to turbine load. The newswirler is deemed to enhance the combustion efficiency because of itsability to produce high swirl number especially at low turbine loads.In addition, the new swirler eliminates the dead zones proved to have

ntial air velocity ratios (m s−1).

Fig. 7. Turbulence intensity at different tangential to axial mass flow ratios.

940 Y.A. Eldrainy et al. / International Communications in Heat and Mass Transfer 36 (2009) 936–941

negative effect on the combustion process. Immediate future work isdirected to comprehensively investigate the swirler isothermal perfor-mance. This is to be done through intensive numerical studies as wellas experiments. It is also vital to study the effect of tangential to axialmass flow ratio on the combustion efficiency, emissions and flamestability.

Fig. 8. Static pressure profiles along the combustor centerline.

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