RANS and LES Modeling of the GE10 Burner

doi:10.4236/epe.2011.35076

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Energy and Power En gi neering, 2011, 3, 607-615

doi:10.4236/epe.2011.35076 Published Online November 2011 (http://www.SciRP.org/journal/epe)

RANS and LES Modeling of the GE10 Burner

Vladimir L. Zimont1, Vincent Moreu1, Valerio Battaglia1, Roberto Modi2

1CRS4, Science and Technology Park Polaris, Pula, Italy

2GE Oil & Gas Nuovo Pignone, Florence, Italy

E-mail: {zimont, moreau}@crs4.it, [email protected]

Recieved April 28, 2011; revised May 29, 2011; acce p ted June 10, 2011

Abstract

The paper presents 1) the numerical results of RANS (Reynolds Averaging Navier-Stokes) simulations for

two versions of the premixed combustion GE10 burners: the old one with non-premixed and modified one

with swirled premixed pilot flames; and 2) the numerical results of joint RANS/LES (Large Eddy Simula-

tion) modelling of the ONERA model burner and a simplified GE10 combustor. The original joint

RANS/LES approach is based on using the Kolmogorov theory for modelling sub-grid turbulence and com-

bustion intensity and using RANS numerical results for closure the LES model equations. The main conclu-

sion is that developed joint RANS/LES approch is the efficient timesaving tool for simulations both the av-

erage and instantaneous fields of parameters in gas turbine and boiler burners with premixed combustion.

Keywords: Turbulent Premixed Combustion, Gas Turbine Burner, Joint RANS/LES Simulation

1. Introduction

The proposed work is devoted to lean premixed combus-

tion technology, which is nowadays well established

within industrial gas turbine industry in order to reduce

nitric oxides (NOx) emission. The main numerical mod-

eling tool for industrial gas turbine combustion is RANS

codes, which yield averaged fields and integral charac-

teristics of the flow. The main part of this presentation is

devoted to RANS simulations of two variants of the

GE10 gas turbine combustor, which include 1) simula-

tions of the premixed combustion in the chamber; 2)

simulations of the preliminary partial mixing of gas fuel

and air; 3) simulations of the non-premixed pilot flame

(old version GE10); 4) premixed pilot flame (new ver-

sion GE10); and 5) air jets cooling system of the cham-

ber. In conditions of industrial gas turbines, instanttane-

ous combustion takes place in non-laminar (microturbu-

lent) and strongly wrinkled sheets with small-scale

structure, which fundamentally cannot be resolved by

model RANS and LES equations. In the presented simu-

lations, we used our Turbulent Flame Closure (TFC)

model [1-3], where this fundamental problem of model-

ing (“challenge of turbulent combustion”) is resolved in

the context of the Kolmogorov hypothesis of statistical

equilibrium of small-scale turbulent structures general-

ized for the case of turbulent combustion. This model

was already used for RANS simulations of the gas tur-

bine combustion and these results where presented in

IGTACE, Florida, 1997 (97-GT-395) published in [4]

and JPGC, 2001 [5]. (This mode l is now implemented in

the commercial codes Fluent and CFX.)

RANS results are important but not sufficient as non-

stationary characteristics of the flow are also important

in gas turbine applications. In academicals works, an

attempt to replace RANS tool by LES one is ongoing.

We think that “LES instead of RANS” in industrial ap-

plication is untimely and we proposed in [6] a joint

RANS/LES approach where the mean fields are simu-

lated by the RANS tool while the corresponding non-

stationary fields are simulated by the LES one using for

modeling some information from the preceding RANS

simulation. The paper [6 ] contains numerical illustrations

of this approach concerning mainly “academic” flames.

We achieved agreement between RANS and LES sub-

problems by using in fact the same combustion models in

both sub-problems. In this paper, we present numerical

results in the context of the joint RANS/LES approach

for gas turbine com b us tors.

We found support to our approach in the invited lec-

ture [7]. In the conclusions, the authors write: “The fu-

ture tools for gas turbine designs will be based on classi-

cal Reynolds Averages codes to predict main flows but

will also rely on Large Eddy Simulation tools coupled to

acoustic codes” (in our example both RANS and LES

sub-problems were stated in incompressible formulation).

V. L. ZIMONT ET AL.

608

We notice that proposed in this paper approach was dis-

cussed at the ASME ATI Conference in Italy “Energy:

production, distribution and conservation” in May, 2006

and was submitted to the proceeding of the conference,

but it was not published in a journal. We use this joint

RANS/LES approach in our work with the industries and

hope that this publication would be useful for engineers

working in the field modeling of industrial combustion.

2. Principles of Modelling and the Equations

In this section we briefly describe a physical model of

premixed combustion at strong turbulence and fast

chemistry and present the equations used for both RANS

and LES simulations.

2.1. The Physical Model

In gas turbines, typical turbulent Reynolds numbers

RetuL





 are large and typical Damkohler numbers

tch



 are moderately large. Here, tLu





 and

ch L



 are the turbulent and chemical times,



the molecular heat conductivity and

S is the speed of

the normal laminar flame, these parameters being used in

the model as the physico-chemical characteristics of the

reactant. In this case, the speed

U and width



the thickened flamelets are controlled by the statistically

equilibrium small-scale turbulence and the chemical time



, and it results the following expressions [1-3]:

12 32

Lf L

UuDaSL Da







 (1)

The flamelet broadening takes place at43





 ,

i.e. when the size of the minimal eddies



is less than

the laminar flame width



. At the same time, the thic-

kened flamelet sheet is strongly wrinkled by turbulence

when



 and hence the condition for this combus-

tion mechanism is the following [1-3]:

3232 43

1Re

DaDa 

 (2)

So the mod el is val id at and ;

these conditions are common for large-scale gas turbine

combustors. For weaker turbulence, common for small-

scale laboratory burners, instantaneous turbulent com-

bustion takes place in a wrinkled laminar flamelet sheet.

Real combustion takes place in transient flames with

increasing width. The reason is that the large-scale wrin-

kles of the flamelet sheet remain statistically in non-equi-

librium at real residence time in the combustor. So the

width is controlled by the turbulent diffusion coefficient

t quite similarly to the nonreacting mixing layer. At

the same time, the small-scale winkles can be assumed

statistically in equilibriu m and as they give the main con-

tribution in the dimensionless flamelet sheet area

Re(10-10 )

t1

10Da 





Fthe turbulent flame velocity





0tf

UUFF is

controlled by large-scale turbulence parameters (in the

model, they are the r.m.s. velocity pulsation u



and the

integral scale ) and the chemical time

UuDa





[1-3].

2.2. RANS Equations of the TFC Premixed

Combustion Model

We analyze premixed flames adopting the flamelet for-

malism in terms of the progress variable c, using the

known bimodal approximation of the PDF p(c) where the

width of the thin instantaneous reaction zone is ignored.

In this case, we have the following expressions:









uub

iiu ib

TTc Tc

CC cCc

 



 







(3)

where ,

uu iu



and ,

bb ib



are the density,

temperature and species concentrations in unburned (re-

actants 0c



) and burned (equilibrium products 1c



)

gases.

The turbulent combustion front, moving with speed Ut

and having increasing brush width controlled by Dt, is

described by the following transport equations:



341212 14

(),

tut

tLu

ct c

DcU ca

UuS Lb









 









(4)

where



is an empirical constant equal to 0.5





valid for all fuels tested in [8,9] (methane 4, ethane

, propane 38

and even for hydrogen 2).

Equations (4) are combined with the average hydrody-

namic equations and the “

CH CH

-k



” model.

2.3. The LES Equations of the TFC Model

The main idea of our joint RANS/LES approach [6] is to

combine LES and RANS in a two-stage process. The

first step consists of the RANS simulation which yields

the averaged flow field; the second step entails LES us-

ing the mean turbulent dissipation rate (,)



 obtained

from RANS to estimate the subgrid turbulence u





and

subgrid flame speed



0tf. LES gives non-

stationary images of a RANS simulation.

UUFF



The model LES equation can be cast as follows:







ct uc

DcUc



 















(5)

V. L. ZIMONT ET AL.

609

lence model with wall functions.

where the subgrid flame speed and subgrid transport co-

efficients are the following:

3.1. Premixing Channel







*121414

13 131343

(),

(), ().

tLu t

UAuSU La

ubD c















 

(6) This part is comprehensive of all the mixing channel up

to the combustion chamber inlet and goes upstream quite

before the system which controls the mass flow rate, as

shown in left. Taking into account the symmetries of the

problem, we have restricted the simulation to an angular

sector of 30 degrees. Inside this sector, there are two

half-winglet (lying on the cyclic boundaries), two fuel

nozzles and one openin g of the flow controller. We used

about 2 millions cells which was the maximum reasona-

bly acceptable for stationary computations with the

computational power a vailable.

Use of the Kolmogorov viscosity



, which in con-

trast to the commonly employed Smagorinsky model of

sub-grid tu rbulen ce does not dep end on time, makes LES

modeling more numerically favourable.

In our analysis, we paid special attention to the prob-

lem of consistency of the results of RANS and average

LES sub-problems, as it is a key point of the consistency

of joint RANS/LES approach. This correspondence is

based on using the same combustion model in both

sub-problems. A difference in the hydrodynamic p ictures

can be in the situation where “-k



” turbulence model is

not accurate. In the presented simulations it is not this

case.

When the flow controller is not fully open, it induces

some swirl component in the flow. For the open con-

figuration, the symmetry is higher and we could have

expected the flow to have a symmetrical pattern of 15

degrees opening. Nevertheless, as shown on Figures 1(b)-

(d), the numerical solution is very far away from the

symmetrical expectation, at least concerning the flow

investing the fuel nozzles. The winglet downstream has

the effect to strongly damp an eventual global swirl

component, but a departure of moderate strength from

the uniform axial velocity profile at the chamber en-

trance. Moreover, the fuel concentration also showed a

not so moderate dispersion, warning about an eventual

non-perfect mixing of the instantaneous fuel concentra-

tion. It is not yet known whether the discrepancy from a

symmetrical solution comes from numerical limitation or

is an indication that the symmetrical solution is really

unstable.

3. Burner Descriptions and Simulations

We present the result of RANS numerical simulations of

two versions on the NP GE10 gas turbine combustion

system, which use non-premixed and premixed pilot

flames. The examples presented not only refer to the

simulation of the combustor aerodynamics: the main and

pilot turbulent flames, cooling air jets and so on, but also

to the mixing process in the premixing channel and the

flow inside the premixed pilot burner. All the simulations

presented in this section have been performed with

StarCD (V.3.15) complemented with user subroutines.

Reynolds numbers are in the range 106 - 107 and the tur-

bulent flow has been simulated using the “-



” turbu-

(a) (b) (c) (d)

Figure 1. (a): the mixing channel; (b) and (c): the pressure colored wall boundaries; (d) mixing coloured by the fuel concentration.

V. L. ZIMONT ET AL.

610

3.2. GE10 Burner with Non-Premixed Pilot

Flame

One forth of the combustion chamber has been simulated,

Figure 2. It includes 6 pilot burners at the chamber en-

trance and a variety of small secondary cold air inlet

disposed in rows and aimed at cooling the chamber wall.

It also includes one dilution air inlet close to the chamber

exit. The smallest inlet hole rows have been collectively

simulated as inlet annuli with the same mass flow rate.

The main flow inlet condition has been taken from the

mixing channel simulation with a cyclic replication to

obtain a 90 degree sector. The flame development is very

close to axial-symmetry. The small departures caused by

the main inlet condition and by the pilot burners do not

have large scale consequences. Intensive combustion takes

place in the relatively small part of the chamber. Figure

3 shows the mesh and 3D field of the temperature.

Chamber outlet

Numerical outlet

Cooling A

Dilution Air

Pilot Fuel

Main Air Inlet

Fuel Inlet

Symetry Axis

Mixing HeadLiner

Lower part

Flame brush

(a) (b) (c)

Figure 2. (a): Sketch of the combustion chamber; (b): RANS simulations of the combustor GE10 colored by the temperature;

(c): the model source term with the flow streamlines and iso-contours of the progress variable.

(a) (b)

igure 3. (a): Mesh of the combustion chamber inlet and cap; (b): 3D isosurfaces of the temperature 1000-1400-1700-2000K.

V. L. ZIMONT ET AL.

611

3.3. GE10 Combustor with Premixed Pilot

Flame

To lower to NOx formation, GE decided to test a variant

of the combustor where the 24 non-premixed pilot burn-

ers were replaced by 4 bigger but essentially premixed

pilot burners. This procedure allowed having less fuel

burnt in stochiometric condition. The pilot burner has

been numerically tested in stand-alone condition. Figure

4, left, shows the fuel concentration that remains high

only on a very narrow central region in exit of the burner.

In Figure 4, centre, we see how the flame develops on an

axial plane. Figure 4, right, show the n ew pilot burn er as

been numerically tested inserted in the combustion

chamber cap. The global reacting flow field is somehow

disturbed by the presence of the new pilot, Figure 5, and,

obviously, is not any more close to axial-symmetrical.

Mainly, the recirculation region is heavily perturbed as it

is shown on Figure 6, left and centre. The interaction of

the hotter pilot flame with the colder main flame can be

appreciated in Figure 6, right.

4. Joint RANS/LES Modelling of Turbulent

Premixed Gas Turbine Combustion

To understand the non- statio nary features of the flow, we

performed a LES analysis of the burner. We used our

Joint RANS/LES approach [6] that strongly shortens the

LES analysis. The simulations were performed using the

Fluent package, a finite volume code which gives the

possibility to customize the models implemented. It has a

second order centered scheme fit to LES simulations

which require low dissipative numerical schemes.

In the next paragraphs, we present the validation of

this approach for a standard test case and then we show

results of the LES for a simplified geometry of the GE

burner previously described, not considering pilot system

and cooling jets.

4.1. The ONERA Standard Burner

To validate the Joint RANS/LES approach, we used a

standard test case known as the Moreau (ONERA)

burner [10]. It consists in a rectangular section burner

with the flame stabilized by a burned gas flow. The fuel

is a methane-air mixture with equivalence ratio equal to

0.84. The flow structures is mainly 2D due to the high

aspect ratio of the cross section, so it is possible to per-

form 2D simulations without losing accuracy in the re-

sults. In this test case, LES simulations are very sensitive

to inlet boundaries conditions for turbulence; in the pre-

sent work, we introduced a disturbance in the average

inlet velocity derived by the amplitude and length of the

upstream flow turbulent characteristics reported in [10].

The results of both RA NS and LES approac hes, Figure

7, show that the turbulent premixed flame has increasing

brush widt h and at the sam e time nearly co nstant speed, as

it can be seen by the practically constant angle with re-

spect to the main flow. In the figure measured in the ex-

periment mean temperature T and velocity U, and the

turbulent param eters (the rms fl uctuation of the velocity u'

and the integral scale L) have indexes “u” and “b” that

refer correspondingly to the flows of the unburned and

burned gases at the entrance of the burner. The upper

graph represents a result of the RANS simulation of the

progress variable : five isolines and two profiles in the

sections x = 0.1 and x = 0.5. The lower graph represent

the LES results: five isolines of the i nstantaneous progress

variable and following from averaging of the LES result

mean profiles for x = 0.1 and x = 0.5 and isolines of

the mean progress variables.



The isolines, which directly follow from LES model-

ing, clearly show the instantaneous stru cture of the flame

with the effects of the large scale vortices that convoluted

Figure 4. Left: premixed pilot burner with 2 coaxial swirling flows, coloured by the fuel concentration, the external flow ar-

rives already mixed in the burning region and only the central part keeps a high fuel concentration; center: pilot burner

flame with streamlines coloured by temperature; right: the computational mesh of the combustion chamber cap with the

ilot burner inserted. p

V. L. ZIMONT ET AL.

612

perature are not available to verify the accuracy of the

fluctuations resulting from the simulations, but observed

agreement between R ANS and a verage LE S data is a clue

of the reliability of the instantaneous data.

Figure 5. Field of equivalence ratio Φ with stream lines in a

section of the burner.

The lower graph in Figure 7 represent also a linear

distribution of a passive concentration z in the entrance

section x = 0, which was used in the simulations as a

boundary condition, and following from averaging of the

LES results the mean profilesin the section x = 0.1 and

x = 0.5. Performed in [6] comparative analysis of the

LES data for the progress variableand the passive con-

centrations z showed that in the flame the mean flux of

the passive concentration z is gradient, while the mean

flux of the progress variable c is predominantly counter-

gradient: it is gradient in the beginning of the flame and

then becomes counter-gradient in the main part of the

flame. The reason is that the flux of the passive concen-



and stretched the isosurfaces inside the flame. Compari-

son of the mean isolines and the profiles of the progress

variables simulated directly by the RANS simulation and

by averaging of the LES data (the uppe r a nd lower gra phs

in Figure 7) are similar. Figure 8 shows that the mean

profiles of the progress variable and the velocity, which

follow from the RANS and LES approaches, are close and

they are in reasonable agreement with the experimental

data from [10]. Measurements of r.m.s. velocity or tem-

Figure 6. Left: combustion chamber surface temperature. Centre: global flow temperature in the combustion chamber, trace

effects of the pilot burner, the cooling inlet rows and the dilution hole can be appreciated. Right: flow temperature and

streamlines on the plane containing the chamber and the pilot burner axes.

Inlet:

= 600K

= 60 m/s

= 2200 K

= 120 m/s

RANS

X(m)

0.50

0.1

y(m)

c = 0.1

c = 0.3

c = 0.5

c = 0.7

c = 0.95

0c



0.1



0.5



y(m)

(a)

+air



= 0.8

U

= 8 m/s

= 5.4 mm

U

= 23 m/s

= 1.6 mm

LES

X(m)

0.5

0.1



0.5



y(m)

0.1



(b)

Figure 7. Average and instantaneous field of progress variable (a): RANS simulation, (b) corresponding LES.

V. L. ZIMONT ET AL.613

Figure 8. Comparison of axial velocity (left) and temperature (right) with experimental data.

tration is controlled only by turbulent diffusion, while the

flux of the progress variable is controlled by turbulence

and the gasdynamic mechanism: different pressure-dri-

ven acceleration in the flame of relatively heavy reac-

tants and light products. The balance between turbulent

and gasdynamic mechanisms controls observed in the

premixed flame transition from gradient to counter-gra-

dient flux of the reacting c. It is to remark that LES de-

scribe this transition without any additional model [6]. In

the RANS version of the TFC model we overcame this

problem by including of the gasdynamic contribution in

the model chemical source [2]. So the transport term in

Equation (4) is controlled only by gradient turbulent dif-

fusion and hence there is no need to model in practical

simulation the counter-gradient transport phenomenon.

4.2. Simplified Version of the GE10 Combustor

To test the Joint RANS/LES approach, we performed

LES of a simplified geometry of the first version of the

GE burner previously described (the cooling jets and the

pilot system were not meshed). We used Fluent code and

simulated a 60˚ degree section of the burner applying

periodic boundary condition at the lateral sections. LES

average shows good agreement with RANS data. The

non-stationary images of the flow field give an idea of

the effects of large scale vortices on the flame displace-

ments.

In Figure 9(a) RANS and instantaneous LES progress

variable are overlapping on a section of the burner to

show the convolu tion of the instantaneous reactin g zone,

which travels and is stretched by vortex shedding from

the geometrical step. Figure 9(b) clearly shows the tur-

bulent structure originated at the inlet step and how they

evolve along the flow. LES gives the possibility to un-

derstand the trend of variation of the flame shape chang-

ing operative conditions and boundary conditions. It is

possible to create 3D visualizations to see the effects of

the vortex structures in the circumferential direction as

shown in Figure 10. Our joint RANS/LES approach,

obviously, does not exclude the possibility to use LES

modeling, that is independent on the RANS one and

based on using common now Smagorinsky model of the

sub-grid viscosity. In this connection, we would like to

mention the paper [11], which is devoted to the com-

parison of predictions of the premixed flame anchoring

in the double cone gas turbine burner using independent

RANS and LES approaches based on our TFC combus-

tion model. We notice only that our joint RANS/LES

approach permits to reduce significantly computation

time of LES.

5. Conclusions

The presented results consist of two parts:

1) Comprehensive (wherever possible) RANS nume-

rical simulations of aerodynamic systems of two versions

of the GE10 gas turbine combustor (with nonpremixed

and swirled premixed pilot flames): cold mixing cham-

bers of the combustor and the premixed pilot burner,

aerodynamic of the main and pilot flames together with

an actual system of air cooling jets.

2) Numerical illustrations of our original joint RANS/

LES approach applied to the gas turbine combustion

(using the standard model situation and a simplified ver-

sion of the GE10 combustor), which can be an effective

and economical tool for the analysis of both stationary

mean and nonstationary fields of parameters.

All simulations where performed in the context of the

TFC combustion model, which, in particular yielded

reasonable agreement between RANS and LES sub-

problems.

Our general conclusion is that RANS simulations re-

V. L. ZIMONT ET AL.

614

(a)

(b)

Figure 9. 2D visualization of middle-section. (a): Contour lines of progress variable (from 0.1 to 0.9 step 0.2) for RANS

(brown) and instantaneous LES (blue); (b); (b) iso-surfaces of LES and the field of the vorticity.

Figure 10. 3D visualization. Two isosurfaces of the progress variable (left) and the field of the vorticity (right).

main a necessary (but not sufficient) tool for practical

numerical analysis of the gas turbine premixed combus- tion and the joint RANS/LES approach can be a useful

tool for investigation of non-stationary characteristics of

V. L. ZIMONT ET AL.615

the flow, including unsteady combustion regimes. It

seems that complete replacement of the RANS tool by

the LES one is untimely at least for gas turbine applica-

tions.

6. References

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doi:10.1007/BF00785062

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Experimental Thermal and Fluent Science, Vol. 21, No.

1-3, 2000, pp. 179-186.

doi:10.1016/S0894-1777(99)00069-2

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