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dian Journal uf Engineering & vlaterial» SCiCIKCS[n '1)- 'Sl '91YoL [2, AU~l"t _Jt ), [JP, _. --

A numerical study of pre-diffuser optimization of an aero gasturbine combustion chamber

G Ananda Reddy & V Gunesan

Interual Combustion Engine Labur.uory. Department of Mechanical Engineering,lndi.u: Institute "fTechnull1gy "bdras, Chcnnai hO() ()3h, India

Received -J November :!(}(i3: <I"""I'Ie't! :COMal' :!()(}5

Tilis paper describes the .urcmpts made in the opiimiz.uion of pre-diffuser geometry of an "em gas turbine combustionchamber fur oblaining minimum total pressure losses and for good ,t~lIic pressure recovery, Optimization or pre-diffuser hasbeen "ttempted by changing the geometric parameters like length, divergence angle and wall contour. A 20" sector has beenconsidered tor the present study because nt' the periodicuy with respect to the air blast utumizer. The geometry is modeledusing (j ..\i'vlBIT and CATIA software, Meshing is done by (jAMBIT. The governing equations an: solved using theSIiVIPU:: algorulun with iterative multigrid nccelcr.nion. Turbulence effects arc simulated using tile rRNG) k-i: model. flowfield characteristics have been analyzed, Optimization is attempted from the point of view of achieving minimum pressure

loss, From the present study it is concluded that a 6' straight ,,'ailed pre-diffuser with area ratio of 1,6594 and a dump gapratio iD/H) (\1' 1.08 is found 10be most optimum, considering the (low development and minimum total pressure Ill",

(PC Cnd«: F23M

Gas turbine engines employ a diffuser between thecompressor discharge and cornbusrorts). The primaryfunction of a di ffus~r is to decelerate the compressordischarge flow so as to distribute the air evenly tovarious inlets on the combustor liner. The diffusionprocess must be accomplished with minimum totalpressure Illss because these parasitic losses have anadverse effect on the thermal efficiency, The flowuniformity around the liner is important to achieveefficient combustion. [0 prevent liner hot spots. and toprovide an acceptable combustor exit pattern factor,Basic confi;!,uration and desizn considerations for thecombustor :Iillusers are dis~usscd bv Lefebvre'. Arecent review bv Kleine elaborates on flowcharactcri"ti~'s Dr th~ combustor di ffusers.

A comllloniv used confizuration of combustor-diffuser in air-craft engines ~ is the dump diffuser.Wherein the compressor discharge-air is decelerated ina Short. C()I1\'entional pre-diffuser and then "dumped"lDto large chamber. which divides flow to the flametube, to the customer bleed and inner and outer annuliahroundthe combustor liner. The sudden expansion att e pre "t'" , I' , Id -c: 11..t.'1C'i" exit L'au~e~ tlo-v recircu anon In ruer,ump, whi':h helps to maintain a stable 110\v patternather in" , ' I"The ,SCilSilive to the engine operanng cone mons.t{ sUcick-n cxpunsion causes pressure losses,,however_ the required diffusion is accomplished In a, Ort length, \Inst of the losses occur in the dump

chamber, while nearly all of the static pressurerecovery takes place in the pre-diffuser".

The airflow path in the present diffuser system isdescribed as follows: The air exiting the compressoris decelerated in an annular pre-diffuser and it isdischarged into a dump chamber. Then, a portion ofdischarge-air flows through customer bleed holes forthe cooling, Remaining air will split and flow throughthe outer annulus. inner annulus and cowl. Finally, airwill reach flame tube through cowl, primary, dilution.and cooling ring holes on the outer and inner liners,The design requirements for the diffuser system are:Ii ) maximum static pressure recovery in the pre-diffuser and minimum total pressure losses in the pre-diffuser and dump chamber to improve turbineefficiency, (ii) uniform flow distribution in thecombustor. (iii) uniform and stable now conditions atoutlet. and (iv) required velocity reduction withinshortest possible distance,

Literature ReviewJeyachandran and Ganesan' investigated the turbulent

boundary layer uncler adverse pressure gradient andfound that tor mild pressure gradient the inlet velocitydistortion affects the boundary layer growth but forstrong pressure gradient tlows. there is little effect.

Jeyachandrun and Gunesan ' have investigateddiffuser now numerically and it was seen that the

[:-"1)1.\:-"1. I:Vi. \J.\TER. S(I. .. \LGLST ~llll:i

pcrtorrnuucc. etlecillelle.~~ .uid pressure ~raC!ienl llf ~lditlu..;er could ~I"O Ix predicted sllcce~,I'ully u,>in~p.uabo!ic cqu.uions. The prediction rnctbod 11~IS usedIor ::,5' :llld (,.' di ffusers Il'i thou I .md II·i rh ill let,·el(lcily dixtortiou :11 the II1ld, It is seen irurn therL''>ulh that rhc performance .md L'llL'L"lilelless or thL'dilTlher innease..; \li~llIly due t() the pre\ence (lrvclocirv distouion ~It inlel tor 25' diffuser and it isthe other \V~IV for the 6" diffuser It is further seen thatthe prexxurc gr:ldiL'lll increases with increase III thehalf COIlL' :lngle and alsll the inlet velocity distortioninLTe':tses the pressure gradienl. It \Vas concluded inboth lJf these investig.uions that the plausible resultsobtained hy the prediction need to be validated withexperiments.

They have made attempts to compare the predictedresults with experimental results. It was found that fill'the 60 diffuser. the velocity profil« showed,atisfact()I,)' agreemcnt near the wall and only fair Ileal'the axis, Further. it W:IS found that the agreementbetween the predicted values and the experimentalval uex of houndurv layer characteri-aics such asdisplacement thickness, shape factor and blockagefactor arc quite s.uisfuctory as in the case of velocityprofi lL's,

Klein' ill his work showed that the: radial velocity,which is more than the circurntcrenrial velocity is the:one which determine, the performance churucteri-uic-,of combustor diffuser.

Su and Zhou" carried out the numerical study 01combustor-diffuser flow interaction using the KIVA-3 V code, The: simulation was based on the solution of\;alier-Stnkes equations wirh phenomenologicalmodels ol turbulence. 'prays, and chemical reactionsThL' -wirler Il'as simplified III terms (If theconservation of Illass ami momentum. St.u ic presxurcrecovery coL't'ficielllS all11lg the inner and outer 1\ ailsof the pre-diffuser and combu-ror CISIIl~ I\CrL'obtained from the numerical solutions. which :lgrL'edI\L'II with publi-hed/mc.isurcd d.ua. FI()\\ licld-, in:1\i:t1 ;1Ilt! circumferential direction are analyzed,

Relation ('I .it' CIITic,:1 our numerical <imul.uion ofnon-reacting tlo,,\'S I'llI' industrial ~:IS turbinec.nnbusror gellmL'tril's, They evaluated the uppl icauonof the L"l)mpllt~itl'lil~t1 fluid dynamics (CFDI tocalculate tilL' Ill)I\· riek!'; ill illdu,t"i~t1 combustorsComparison wa-, made between Ihe -t.mdard k-Eturbulence model and .t modified version Ill' thestandard k-e turbulence model ill which a second timescale was added to the turbulent dissip.uion equation.Results obtained from theCFD calculation "ere

compared Ilith expcrimeruat data, For the elSe understudy, the standard k-E model was found to di Ifuse the\\\id .md axial momentum. which re,ullL'li III Ih

tinl'llllSistelll prediction of the rc-c ircul.u ion lone furihc test case.s, However. the modified k-r IlHlddshowed an improvcd prediction 1·L'gardi Ilg tilelocation. shape and SILt: llf the centLTlilW re.circulation zone. The large swirl and :I:xi;t! \L'locit\gradie'lllS, which are diffused hy the ..;taIHI:lrci k,,,mode], arc preserved by the modified model. Qndgood ~Igrel'ment was obtained between the cakulatrd.md the measured a:xi;t! and swirl I'elocities, The UIW,prediction of the. turbulent eddy ,isc()sity in regions orhigh shear. which is characteristic of the st~lIldare! k-fmodel could be taken care of by the Illoditiedturbulence model.

,-\gal'\\':ri 1'1 at.' carried (Jut an cxperirnemascornputationnl study of the cold Ilow in the combustordiffuser system of the. industrial gas turbinecombustors and impingement-cooled transitionpieces, The primary objectives were to determine!'low interucrion-; bet ween the prc-cli t'fusL'r .uid lbedump chamber. to evaluate. circumferential tlow non-uniformities around the transition pieces andcombustor«. :11lL1 to identity the pressure Ilhsmechanism. They conducted flow experiments in anapprox irnatel y one-th irei scale, Wall static pressure,and IL'locity prui'ilcs were measured at ,:ekcteulocations in the rest model. A th ree-d imensionulcomputational tluid dynamics analysis employing amulti-domain procedure was performed to suppiemelllthe flow measurements, The complex geometr«Icature-; of the test model wcre included ill theunulvsi«. The measured clara correlated well with thecomput.uiunx. ThL' results revealed strong Illler:letion,between the pre-diffuser and dump chamber flowS,The pre-diffuser exit film· was distorted ill.jicatin~thut the uniform exit conditions :ls,ulllL'd ill rheditlusel' design "LTc violated. The prcxsurc \;triedcircumrcrentiullv .uound the combustor casing andimpingement ,1~C\c, The circumferential (Il)\\' non-uniformiric-, increased toward the' inlet llf the [Urblilt'c xp.mdcr. .-\ venturi effect causing 11<111· t(1 ,:Iccck:':~t',llld decelerate ill the dump chamber \\-,:IS ltienlltlc 'The dump chamber contained xcverul re_cil'cldalIl'ilieC!ilJlb contributina III the IllS';eS, ,-\ re:di~tic tc~tm~del and thr.:e-dimc'llsional anulv "is usc,d in thiS

I ' I d ' . I - I' !l0\\stU( \I provu L' new insrgnt 111[(1 [It:

chumctcristics of practical combustor systelllS....., .... - I . Ll"in£!Karki I!I al . analvzcd the dump dltfu.,el ,') " -

- " I j' 'd LHIcomput.uional procedure, The procedure lS 1, ,C

the sCOnt'l

usiniI InoellII practI fuel!\ be :I)II inCOI

ti.1l1oI simll

Imod:tossedata,comlHowpredistreasirnudimeandsyrmthat'throrthroe

Fi:inve-diffustaticbut nregiapressJttenshap,minilsyrnrThesurraget d

Ccdumlcone!measratiodUmlhas Ipre-dWithstagOdiffu0.48:Was \,

, '

"!1"~!!!1II'111I'''!I'~ljl!''I'"'"!H!IIII!I''I!I!!I"''I!''!!!!I!lI!lllll/'I!!!II' 1111111"lllflllllllll" ,,! I I •.1, III I I 'I! H!llll:i U lilllilllllj' Hi. l'!l'!!:liu '

In:::DDY & GANESA:--J .".ERO G.".S TURBI7\iE CO\IBUSTION CHAt\lFlER

the soluri"n or the Navier-Stokex equations on a bodycontLlri1lin:.:~rid. The turbulence effects arc simulatedusing the high Reynolds number form of the k-:mode'l. The calculation method has been applied to apractil'al c'onriguratillil that includes support strut andfuel ll11uk, In the rirst case. the now is assumed iobe axicYll1ll1Clric. TIll' ctfccts of various hlod:age, ~lreiJlcorp()r~lll'd in the simplified ge()metry. This isfollo\\'c'd hv results of the three dimensionalsimuiall(ll1. l hc cstimate« provided by the simplifiedmodel for :;[alic pressure recovery and total pressurelosses ale wilhin 10<+ In 1.'i'.'0 Ill' the experimentaldata. The author says that the ax i-xymmctriccombust,)r analysis is adequate fill' most upplications.Howe\·er. lul] simulation is necessary for accuratepredicti')11 (If three-dimensional process including hotstrt:.!"s and pattern factor. The result of the fullsimulation provides an insight into the three-dimensional nature of the [low. The presence of strutsand fuel nuzzles results In significant non-symmetrical effects in the flow. The authors observedthat the model under investigation indicates the flowthrough :1I111ulus effectively. But flow distributionthrough the dome swirler is found to be non-uniform.

Fishenden and Stevuns" conducted ex peri mentalinvestigations on the performance of an annular dumpdiffuser for combustor and concluded that the majorstatic pressure rise occurs in the pre-diffuser portion.but most 1)1' the total pressure loss occurs in the dumpregion. The principal determinants of the totalpressure inss are the amount of diffusion beinaattempted downstream or the pre-diffuser and size andshape of the flame tube and also the dump gap. Theminimum overall pressure loss was obtained with asymmetncti velocity profile at pre-diffuser outlet.The geometry of the pre-di tfuser. flame tube andSurrounding annuli need to be carefully matched toget desired performance.

Corrottc et 01.1\1investi "ated the performance of thedump dd'i'u-;er by experimental method with the inletconditions heing generated by axial flow compressormeasun~l11ents. Studies \\ere made for the dump gap~atlo ot 1.0 (datum configuration) and three otherhump gaps (0.8, 0.65. 0.5). The following; conclusionas been drawn from the experiments: ~The overall

pre-dlffu cc, . . f d . . t" I\1/ ~cl per orrnance oes not vary SI!!111icant yStith dump gap. For the datum configu;ation. thedi~rnatlOn pressure loss coefficients between pre-O.4~;er 1:1!('tS,:lnd outlets an~, inner feed annuli wereW an(, U.··b respectively. the majority at this loss

as genera:ed around the flame tube head while most

,

ii

of the static pressure rise occurred wit h in the pre-diffuser. ,'.,Iso. the stagnation pressure Illsse,sincreased by J2c((llutel') and -+8(;((inner) ~h the dumpgap reduced from I.U to 0.5. This high value \\'as~lssociated with the relatively deep name tube und,hon system length. This increases the amount ()I'flo« curvature around tile I'I()W tube heud. whichbecomes more severe <IS the dump gap IS reduced,

Ohjecti veBased on the above literature survey it is ~el'll that

there IS considerable interest regarding flowde , eloprncnt in pre-diffuser for the combustionsystem. Hence. for the present study there arc threeruai n objecti vex: (i) Non-teneri ng tlow anal ys is tor thegl ven design configuration, (i i) optimization of pre-diffuser geometric parameters like length. divergenceangle ~1I1c!wall contours for minimum total pressureloss. and (i ii) to investigate the effect of increasing theinlet velocity on total pressure loss.

Computational Domain ConsideredThe combustor configuration under study IS

illustrated in Fig. I. The airflow leaving thecompressor enters the pre-diffuser section. wherelarge portion of the velocity head will be recovered.und then the flow enters into dump region of di ffuser.In this region, the flow will split into three streams.One stream enters the flame tube and the other twointo the surrounding feed annuli. The dump diffusercombustor design parameters are governed bygeometry and flow properties. These include thelength and area ratio (ratio between outlet to inlet areaof the [Ire-diffuser) of pre-diffuser, dump gap velocitydistribution and turbulence level at the compressorexit. Additional considerations are locations ofsupport struts and atomizer. These sub-componentsshould be designed and placed such that they have

Fi~, 1- Two-dimensional geometry of gas turbine combustor

284 INDIAN 1. ENG. MATER. SCL. AUGUST 2005

minimal effect on pressure loss and combustor exittemperature quality. The diffuser system should bedesigned such that there are no unwanted totalpressure losses. Further, it should deliver the desiredmass flow into various regions. In this study foroptimization of pre-diffuser, the length, divergenceangle and wall contour have been changed. Fig. 2shows the three-dimensional view of a 20° sector ofthe gas turbine combustor under investigation. A 20°sector IS taken for the prediction because of theperiodicity with respect to air blast atomizer.Therefore, by. applying periodic boundary conditions,predictions can be carried out without loss ofaccuracy. Further. it will save the computer time to agreat extent.

Grid size used in this study is 8.7 lakhs, which is oftetrahedral type. In order to test the adequacy of grid,the grid density was reduced by 50% and thenincreased by 50%. The variation of predicted velocityat the critical sections, namely the primary zone andsecondary zone with respect to 50% increased numberof grids were less than 4%. Hence for the presentstudy 8.7 lakhs cells were used. The minimum gridsize was 0.1 rnm and maximum was 2 mrn.

Governing EquationsThe time-averaged governing equations for fluid

flow in the Cartesian tensor form are given below:

Continuity equation

a .-(pu)-sax. I 11/,

The source term S", is the mass added to thecontinuous phase from the dispersed second phase(due to vaporization of liquid droplets). For the non-reacting flow S", takes a value of zero.

Fig. 2- Three-di mensional geometry of a 20° sector of aero gasturbine combustor 1

Momentum equation

t.p-

I

whe

a ap--(pu u ) =--ax I J ax.J I

+ 0;H;;',+ ::', - ~ 8" ;; IIa -,-, ,

+-(-pu.u)+F .. (2)ax 'J 'J

whe

TurbIn

usedequscallecareRNCforroodenerthe f

where p is the static pressure, ,Ll is the molecularviscosity, !5ij is Kronecker-delta function and F; is theexternal body force that arises from interaction withthe dispersed phase in the i direction. The second termin the right hand side represents the stress tensordenoted by Tij and the third term represents Reynolds'stresses. These are modeled using Boussinessq

,hypothesis. According to this hypothesis the Reynoldsstresses are related to the mean velocity gradients by:

=r:': ( au au'j 2 [ au 1s-pit. u . = u. __ i. + __ J. - -,- pk + /-l,~. U'j"I J t-', ax ax 3 uX

J I I

.(3).

where k is the turbulent kinetic energy and 111 isturbulent viscosity whose computational methoddepends on the type of turbulence model used.

Dp-D

Energy equation Here.(1)

_a_[u(pE+ P)l=-O-(kcrr_aT_- 'ZhJ +. U/Tij)crrloX; I ax; ax;) J J )

+S"

ener:as

.(4)

where «,« is the effective thermal conductivity (k+kl,

k, is the turbulent thermal conductivity), and IF is th~ .diffusion nux of the species j. S" includes the heat athe chemical reaction. In the above equation: wher

Ttdiffestane:rapid

/left

2

E=h-E+~p 2

. .(5)

where sensible enthalpy h is defined for ideal gases as

.(6)

I•

tiI

.. I~. fI

irH~leCLlI"iI.Ir;lslhcJ

. . i'IiOIl wliliI:•:.:ond termI'ss tensorRl'Yllolds'Lhsint'ssqReynoldslents by:

.(3)

nd ,LI, ismethod

\1

(T)" i

(-1)

I' (k+l«.I; is [hehea: of

ases as

[{EDDY & (jAi\'ESA, • AERO CAS n;RBIi'iE C();\lI3CSTION CHA\·IBER

where tn, is the mass fraction of the species j' and

= J C I' ,.d TF, ,

.. (7)

where T,,{ is 2l)~.lS K.

Turbulence model equationsIn [he present study, (RNG) k-». turbulence model is

used for turbulence modeling. Navier-Stokesequatiuns are sol vcd, using a mathematical techniquecalled "rellormalizatinn group" (RNG) method to takecare of physical modeling. These features make [heRNG k-i: model more accurate and reliable especial Iyfor swirl ing flows compared to the standard k-s:model. The transport equation for turbulent kineticenergy k and its dissipation rate E are obtained fromthe following transport equations

Dk a I dk 1 'P- == ._-,- a .u ..-, + C - PE-- rDt ax L ' r-f ax I .\fI ,

.(8)

and

Here, G, represents the generation of turbulent kineticenergy due to the mean velocity gradients calculatedas

... ( 10)

... (11)

Where C" is a constant.The R term in the epsilon equation marks the

difference in £ equation between the RNG andstandard k-£ model and it accounts for the effects ofrapid stain. It is given by:

C!,prr' (I - .!l) ,11.==_ 1)1) C

I+ {JIll k... (12)

where '7 =Sklc. 1)0 =-I-.JR. ~=().O 12. The values ofmodel constants used are given in Table I

AssumptionsThe following assumptions pertaining (0 the tlOIA' in

the combustor have been made in the present study:(i) now is considered as steady and incompressible.(i i) buoyancy effects are negligible. and iiil) radiationeffects are negligible.

Boundary ConditionsThe present problem has three types of boundaries.

They are inlet. outlet and wall. The W:'Ly theseboundary conditions arc prescribed is describedbelow .

InletVelocity boundary condition is used to define the

flow velocity along with all relevant scalar propertiesof the flow at the flow inlet. Velocity magnitude anddirection. the velocity components or the velocitymagnitude normal to the boundary specifies inletvelocity boundary conditions. Inlet velocity used inthe present study is 198.5 rn/s,

OutletOutflow boundary condition is used to model the

now at exit where the details of the flow velocity andpressure are not known prior to solution of theproblem. As these variables are not known for thecase under study, we have adopted this boundarycondition for the combustor exit. When this conditionis specified, the code extrapolates the requireciinformation from the interior. in the present studythere are five outlets. the flow rate weighting has beenset to indicate what fraction of total now rate takesplace through the given boundary. Table 2 gives themean mass now rate through bleed at di fferentlocations. These inputs are predetermined and cannotbe changed. The different outlets are shown in Fig. 3.

Table l-'\10dcl constants

C"

1.-12

CDO.l507 0.719-1

C 2£ cr, cr,0.719-1

Table 2-Mass flow split

S. No. Location '7cof bleed of total mass flow

OULer 6.97% (of tara I mass flow)

2 Inner front (rotor) 3.9-1'7<: (ofrota) mass flow)

3 Inner rear -I.OI(7c. (of total mass flow)-I Customer bleed 0.73% (of total mass flow)5 Flame Lube 84.35%( of totai rnass flow)

286 [NDIAN J. ENG. MATER. set. AUGUST 2005

Wall

In any flow. Reynolds number of the flow becomesquite low and turbulent fluctuations are dampedconsiderably near the walls and the laminar viscositystarts to playa significant role. In the present case,walls are assumed to be adiabatic with no slipcondition. Standard wall functions are used tocalculate the variables at the near wall-cells and thecorresponding quantities on the wall.

Results and DiscussionThe velocity magnitude contours at 0, 10 and 20°

planes are shown in Fig. 4. As expected, velocitymagnitude contours at 0 and 20° are almost same dueto the periodic boundary condition used. Due to thevelocity· reduction there will be static pressurerecovery due to energy transformation in the pre-diffuser. This static pressure recovery is the onewhich going to determine the downstream flowcondition and therefore the details of the downstreamflow field also presented. This can be clearly seen inFig. 4. Figure 5 shows the details of the velocity

Fig. 3-Location of different outlets

20311 •.02

I sae .•.0218361.02173e •. 02

I53E •. 02

I 52e .•.0214.2I!.02132 ••.• :)2

t zae .•.0)2

112e .•.02

1~e-0291':e .•Ol8138 •.01

7119 .•.01

6008 .•.015 CEle.OI405e •.Ol30:5e .• Ol203e •. Ol

102e .•.OI

OOOhOO

Fig. 4-Velocity magnitude contours at 0.10 and 20° plane

vectors inside the flame tube, just behind the swirler.As is evident there is clear re-circulation zone (FiE,. 5)that has developed just downstream of the swirler.This recirculation zone is formed due to the effect ofthe radial pressure gradient created by the swirler andthe interaction of opposing primary jets, which havebeen modeled carefully. This re-circulation in theprimary zone is very important for flame stabilizationso that the flame remains anchored to the atomizerand effects complete burning of the fuel in theprimary zone to provide a good exit pattern factor.

For the development of proper recirculation ZOnethe primary jets should penetrate fully into the flametube. As already mentioned the penetration of primaryjets into the flame tube can be seen clearly in Fig. 6.The interactions of primary opposing jets will createrecirculation that leads better mixing of air and fuel.These jets act as flame stabilizers and helps tostabilize the flame. Figure 7 shows the flow throughthe dilution holes. The function of the dilution air jetis to mix. with the products of combustion withincreasing air from dilution holes in order to coolthem to a value acceptable by the turbine and also toreduce the pattern factor at the exit of combustor.

The total pressure drop across the combustor is dueto the various blockages like the atomizer mount,swirler and swirl cone. The pressure drop is due to theflow across the primary, dilution and the coolingholes and also due to the skin friction along the wallsof the combustor casing, the flame tube, swirler andswirl cone. The total pressure drop across thecombustor for the case under consideration is 6.85%(Fig.8).

Figure 9 shows the static pressure contours at themid plane of combustor. In pre-diffuser the velocityreduces so that the dynamic head is converted into thestatic head. It could be seen in Fig. 4 that as the flowpasses through the pre-diffuser there is a veloClt~reduction and the corresponding static pressurincreases (Fi g. 9).

~ . tThe actual static pressure recovery coefficlen

(c 1=6P 1(0.S*pu2)) . is 0.6l2 for the case underP actua s . t

consideration. The ideal pressure recovery coefficleo. 2· ual tofor the di ffuser (Cp ideal = l-(A II A2)) IS eq . h

. ~C0.63809. Pressure recovery effectiveness, w alrepresents the ability of the diffuser to achieve iderecovery characteristics, is the ratio of the Cp actualto

id aUonthe Cp ideal. Thus, for the case under consr er hi hPressure recovery effectiveness is about 96% w C

sid howcan be considered as a very good value an s fthat the diffuser design as well as the performance athe pre-diffuser are quite reasonable.

2.

2.1.1.1.1.1.1.1.

1.1.

9.8.

. 7.6.5:4.3.2.1.

9.

1I"lm~IItIIlI""II!I!!I!'!""I!IIIIII'''''IIIIII!II'''''I!''''!'''"1I1I1I!1II1IIII'''"IIIIIIIIII"",IIIIIIIIIIIIIIIIII!I'

,wirier.Fi!o.5)

,wirier.leet orler andh havein the'zation)Illizern the)f.

ZoneflameIlllary:ig.6.~reate

fuel.)$ to'oughur jetwithcool'

so to

. due)LInt.i theding/ailsandthe

:5%

thecitytheow.ityure

~ntler.nt(0

:haltoIII

:h

REDDY & GANESAN: AERO GAS TURBINE COMBUSTION CHAMBER 287

Outer Primary Jets

Inner Primary Jets

Fig. 5-Exploded view of velocity vectors at just downstream of swirler (100 plane)

2.11e+02

2.00e+021.90e+021. ;ge.;.021.tge+021..588+02

1.47e+02 oj,

i~~~f~1::.k~f1j~9 . .188.;.018.43e+Ol

7.3ge.~OI6 339+01

5.27e+Ol4.22e+Ol3.17e+Ol2.12e4.01 Lc1.06e+Ol9.57e-02 :X

Fig. 7-Flow through dilution holes (100 plane)

mainly influenced by turbulence rather than heataddition. In the present study cold losses due tofriction and turbulence come into play. As expectedthat increase in inlet velocity increases the totalpressure losses. which can be clearly seeni n Fig. 10.The variation of static pressure recovery coefficientwith inlet velocity is shown in Fig. II. As the velocityincreases the static pressure recovery coefficientincreases marginally. To put this quantitatively a 50%increase in the inlet velocity (from 160-240 rn/s)produces an increase of about 1% (0.609 to 0616) instatic pressure recovery coefficient.

Optimization of pre-diffuser geometry

The pre-diffuser geometry has been optimized bychanging the geometric parameters such asdivergence angle, length and wall contour. The

Fig. 6-Flow through primary holes (l0° plane)

Parametric study for cold flow

In order to .study the effect of the inlet velocity onthe present design a parametric study has been carriedOur. It is a known fact that the total pressure losses areinfluenced by the inlet velocity. The inlet velocity ofthe deSign-combustor is 198.5 m/s. A parametricstUdy has been carried out with ± to and 20% of thedeSign velocity to establish the pressure recoverycoefficient as well as total pressure losses. The inletvelocity from 158.8 to 238.2 m/s in steps of 19.85rn!s. Table 3 shows how the total pressure losses,velocity outlet of flame tube and the static pressurereCOverycoefficient vary with inlet velocity.

It should be recognized that the pressure loss iscaUsed by two factors; one due to friction and theOther due to acceleration caused by heat addition. Itshould also be noted that the total pressure losses is

I

288

3.12et06

INDIAN 1. ENG. MATER. SCI., AUGUST 2005

3.0ge~06 Inlet Total Pressure=3111600

3.05e·OB

3.D2et06

2.9get06

2.9Se·QS

2.92et06

2.Bge·QS

2.86e •.06

2.82e"06

••2.7ge·06Outlet Total Pressure=2898455.4

Fig. 8- Total pressure COntours at the mid plane of combustor(10° plane)

3.129 .•.063.10e .•.063.08e .•.063.06e .•.06.1.04e .•.063.03e .•.063.01e .•.062.93e .•.06

~ 2.97e .•.062.96e+062.94e .•.06

2.92e+062.90e..·062.OOe .•.062.87e .•.062.SSe....062.83e .•.06

2.819+062.80e+062.lBe+062.70e+06

Fig. 9-Static pressure contours at the plane of combustor(10° plane)

Table 3-Results of parametric study

Velocity Velocity at Total St. Prs. Rec.inlet flame tube pressure Coeff.

(m/s) outlet loss (%) Cp=L'.P,I(0.5*pv2)(rn/s) (L'.PjPQ3)

158.80 52.83 4.41 0.609178.65 59.50 5.54 0.611198.50 67.07 6.85 0.612218.35 73.71 8.31 0.613238.20 80.55 9.87 0.616

following steps have been considered for optimizationof pre-di ffuser.

• Divergence angle has been optimized by changingthe angle from 6.5° to 50 in. steps of 0.50 withsame length and wall contour as originalconfiguration. It has concluded from thenumerical studies that the geometry with 6°divergence angle found to be the best possibleconfiguration.

Keeping the same divergence angle (6°) thelength of the diffuser has been varied from 96 to

•

165 180 195 210Inlet Velocity (m/sec)

225 240

WallCootou

F

divergenctdivergenctc0l11parati1effectivent

'fhe. varespect todivergenc~

Fig. 10- Variation of total pressure losses with inlet velocity

•

72 mrn in steps of 8mm. The pre-diffuser with 88rnrn length was found to be good.Keeping the same divergence angle of 6° andlength 88 rnrn, the wall contours have beenoptimized by changing the wall shape, which can :see from Fig. 12.

Analysis for nine different geometries

Basically nine geometries were created foroptimization of pre-diffuser by varying the divergenceangle, length and wall contours. The performance ofthe nine different pre-diffusers is shown in Tables 4 to6. It can be seen from the tables that total pressurelosses, static pressure recovery coefficient and overalleffectiveness vary with the change of parameters likedivergence angle, length and wall contours of pre-di ffuser geometry.

Table 4 shows the variation of the diffuserperformance with the divergence angle for the fourdifferent cases studied. As the divergence angle IS

increased from 6° to 6.5°, the total pressure'loSsdecreases marginally and the effectiveness of thediffuser is almost same. Actual static pressurerecovery coefficient increases with increase indivergence angle. And also the ideal recoverycoefficient increases with increase in divergen.ceangle because of the increased outlet area of the pre

al-

. the totdiffuser. However, the overall decrement mPressure losses is almost nezliaible. As the divergence

o 0 ~~

anzle decreases from 6° to 5° the total pressure. 0 B hi'· re recover)'increases. ut t e actua static pressu alLcoefficient showed reasonable decrement. The overforeffecti veness of the diffuser also decreases

REDDY & GANESA : AERO GAS TURBINE COMBUSTION CHAMBER

0.620

0.618

c••'uE••oo~ 0.616

••,.ourl••~ O.6U -••..Il:

~0.612

38

Egio 6· U)

co ~88.0 <.eiN ~

t- 1

289

165 180

0.610 L-~~_~--, -,- ~_~ ..-. --,,--_~ __ -,

150 240195Inlet Velocity (mi •• c)

210 225

ld Fig. II-Variation of static pressure recovery coefficient with inlet velocity

.111

1-----Il_ -----

or:eof

Original geometryWall Contours - --I

r~----~·i [ ,,------__ I ------~

toret11.e

Model we;p.

I

II--_--.J

errrisisie'en Fig. 12-Different Pre-Diffuser Geometries

divergence angles from 6° to 5°. Hence, the originaldivergence angle of 6° can be considered to beComparatively zood with respect to overalleff 0llectiveness.

_ The variation of performance parameters with~~spect to the length of the pre-diffuser for fixedIVergence angle (6°) is shown in Table 5. The total

Model we]

•we stands for wall contour

y-e

Table 4-Pcrforlllance of pre-diffusers for different angles

Angle Actual static Ideal static Total Overallpressure pressure pressure effecti venessrecovery recovery loss

coefficient coefficient

6.5 0.6352 0.6620 6.81 96.2836.0 0.6119 0.6381 6.85 96.3355.5 05890 0.6l35 6.96 96.1615.0 0.5621 0.5854 7.07 95.921

Table 5-Perforlllance of pre-diffusers for different Length

Length Actual static [deal static Total Overallpressure pressure pressure e ffecti venessrecovery recovery loss

coefficient coefficient

72 0.5534 0.580 7.13 95.78880 0.5856 0.611 6.91 96.04288 0.6119 0.638 6.85 96.33596 0.6373 0.664 6.80 96.385

pressure losses decrease marginally with increasinglength of the diffuser. If the diffuser length increasesthe flame tube length decreases. For good combustionthere should be sufficient length of flame tube formixing, flame stabilizing and complete combustion offuel. Because of the above reasons the length of the'diffuser is not possible to increase and also the lengthbetween the compressor exit to the turbine inlet is

290 It D[AN J. E 'G. ~lATER. SCI.. AUGUST 2005

Table n- Performance of pre-diffusers for different wallcontours

Shape Actual static Ideal static Total Overallpressure pressure pressure effectivenessr~covC'ry recovery loss

coefficient coefficient

Straight (J.1i!!9 0.638 6.85 96.335Cuse ! 1l.5949 0.648 7.14 92.0li7

Case2 O.(]OX6 ll.tA8 7.00 94041i

Casd 0.6145 II.M8 6.90 95.1)22

fixed. Hence the original length of the diffuser tS

considered to be good for better performance andminimum total pressure loss.The effect of wall contour is studied for four differentcases and is shown in Table 6. From the results it isseen that straight wall diffuser ewes betterperformance and less total pressure losses than others.From the present investigation it may be concludedthat the original geometry of the pre-diffuser givesoptimum performance and therefore no change isrequired. The mass tlow split and the pressure lossesfor various geometries are given below.

ValidationValidation of total pressure losses for the present

combustor configuration could not be carried out inthe view of proposed combustor geometry is underdevelopment and no experimental data is available.However, Balasubramanium II has satisfactorilyvalidated the FLUENT code for the baselinecombustor configuration, which is similar and hasclose resemblance to the present configuration. It maybe noted that the main aim of the combustor design isto obtain minimum pressure loss, which depends onthe aerodynamics design. Further, to the best ofauthors' knowledge there are no satisfactory velocitymeasurements inside the combustion chamber eitherfor the cold or hot tlow. Hence, at present one has tobe satisfied with such validation of pressure loss only.A typical comparison between experimental andpredicted results of total pressure is shown in Fig. 13.

From the graph it is clear that the experimentalvalues are slightly higher than the predicted values.However the trend of predictions is quite satisfactory.This IS mainly attributed to the presence ofappendages and struts and other obstructions in theflow path in the experimental model and accuracy ofexperiments. It is to be noted that the presentpredicted results are quite reasonable and thereforethe results can be used with confidence. It should alsobe remembered that experimental data for a similar

"rC

8~.•

.~3oJ

!!"..!

~g0I-

'0~ 't

I

~

bl'00

(--- EXpen"-EJ--- Predlclad !

•

••

•

, I I I I

iJ'"J'"

I K'"kc z:

k'ff'"

Ip",Pr '"Re '"T'"U'"X'"

Greek sQ=a=15=E=Jl=/it =p=pp=r=SubscripEjJ=i.i. k =K=Ref =T=

,~ lW 'wInlet Velocity (mi.)

,w

Fig. 13- Total pressure loss validation

combustor system for any other parameter shown inFig. I is almost non-existent because of thecomplexity of geometry and the now.

ConclusionsA numerical study has been performed to calculate

the nonreacting tlows in a typical aero gas turbinecombustor geometry. which contains a combination ofswirling and recirculating t1ows. The majorconclusions are:

I. Based on the static pressure recovery coefficientand diffuser effectiveness it can be concluded thatthe original diffuser design is quite satisfactoryfor wide rage of inlet velocities. Increasing theinlet velocity causes only marginal changes instatic pressure recovery.

2. With the increase in inlet velocity the totalpressure loss also increases.

3. Based on the optimisation studies on divergen~anele lenzth and wall contours, the onglfl=-, e» ·dconfiguration of 6° with 88 mrn length anstraight wall pre-diffuser is found to be the bestconfiguration.

Nomenclature.-\ = area of cross section. m2

A = sonic velocity. m/sCD = drag coefficientC; = specific heat. J/kg-KE = energy. JF = external body force. N

Greek svmhols

Q =' mc.ui r.uc Ill' nl{atiulla! rCIlSUf

illvcr:-.t.:' effective PrandrlllllillbL'[

KI\)J1L'cl-.cr~d('J(afunction

turbulent dissipation rate. Ill'/S'

molecular viscosity. kg/Ill-Sturbulent viscosity. kg/msdensity. kg/mdensity of the partickshear stress. N/Ill'

iz=J=K=k c =k'lr=p=Pr =Re =T=U=X=

a=8=E=f1=Ji.=p=pp=T=;

f

If

II,It

SubscriptsElf =ti. k =K=Ref =T=

,.

iI

iIrI

REDDY & GANESA.N: AERO GAS TURBI:-';E COiVIBt;STION CHAMBER291

sensible enthalpy. Jlkgdiffusion flux. kg/m'-sturbulent kinetic energy. n1'/s-mass transfer co-efficient. IlL'Scfti:ctil'e thermal conducuvirv. W/Ill-Kstatic prc"SSU[L', PaPrandr! numberReynolds numbertemperature. Kmc.m vclucity l,.'UmpllllC'!H. Ill!S

lungitudinal distance. III

ReferencesI Arthur H Lefebvre, Gus turbine combustion (Hemisphere

Pub. Cnrporation), 1993.

Klein A. r-«. .·\S;\/£J Fit"" Eng. J()'i (1'.183) ~5().~ kyachandran K & Gancsar, V. Turbulent boundary laver

development under adverse pressure gradiclH-r\ numericalprcdicuon. PmI'. 11/" NmiOlllli Coni Fluid Mechanics (lilt!FiIl,,1 PIlII'er. Reginnal Institute ot Technology. Jamshedpur.1';'lJtT No.E-X. I'.IXI.

.j Jevach:lndrJn K & Gancsan V. Numcncal prediction Ill'

pcrf()nnJfH.:e and cflcctivetlcss of conical ditlu.\L'r \vith inlet""Iocity distortion. Pmc I:!'" Nutionu! COliI' Flllid Mcchanir-.,(lId Fluid POII'('r. liT. Delhi. Paper Nu. CFM· 3. 19X.l.

Su K & Zhou C <), Numcricul mudding of gas turhinecombuxtor iJltc-gratt.:d with diffuser. Prm 3·/"; NUliul/ui Heatrr(IJI.,/~'r Con}, Pillsburg. Pcnllsyl\,;1/1ia.

(, Relation J L & Ballaglioli W F Ng, ..\SMr: J EI/g (;us'/'lIr"illn PIII\'('/'. 120 11l)'.I~) .jo(J-to7,

7 ,.\garwal J S. Kapar T T & Yang. ASME J Eng Gus Turbines('{llI'Cr, 120 (1l)l)X) ~-1-:n.

X Kark i K C. Oechsle V L & Mungia H C. ASidE J £I/g Gasliilbille.fP01!·er.II-1(I)(1992) 1-7.

9 Fishenden & Stevens S J. The Performance of AnnularCll11hustion-Dulllp ditfusers, AIAAISAE j(j" PropulsionCOII(California.Oct21_23, 197-1, 1-1~.

J() Brouwer Carroue J, Wood C P & Samuelsen (; S. ASMF .Ii:'lIg GllS Turhille Power 1II ( I Y89) 31-14.

II Batasuhranium V. Numerical predictioll or jlOlI' andcombustion ill a gas turbilll! combustion SV.I'/('II/, MS Thesis.Ie Engines Lab. lIT. Madras. 2001.

effective value (laminar + turbulent)tcnsorial notationturbulcm kinetic L'nergyreference valueturbulent