Numerical Simulation to Characterize Homogeneity of Air-Fuel Mixture for Premixed Combustion in Gas...

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NUMERICAL SIMULATION TO CHARACTERIZE HOMOGENEITY OF AIR-FUEL MIXTURE FOR PREMIXED COMBUSTION IN GAS TURBINE COMBUSTOR

Kamalika Chatterjee Department of Mechanical Engineering

Jadavpur University, Kolkata West Bengal, India

Email: [email protected]

Arkadeep Kumar Department of Mechanical Engineering

Jadavpur University, Kolkata West Bengal, India

Email: [email protected]

Souvick Chatterjee Department of Mechanical

Engineering Jadavpur University, Kolkata

West Bengal, India Email: [email protected]

Achintya Mukhopadhyay Department of Mechanical



Swarnendu Sen Department of Mechanical



ABSTRACT Homogeneity in mixing of air and fuel in premixed

combustion for a gas turbine combustor is a critical criterion to

ensure efficient combustion and less environmental hazards.

The current work deals with determining this homogenous

characteristic of air-fuel mixture through computational

simulation to specify homogeneity for a particular premixing

length and equivalence ratio required for gas turbine

combustion. A 3-D geometry of combustion chamber with

combustion zone of internal diameter 6 cm is constructed. A

premixing tube is augmented with the combustion chamber

which has one air inlet port at the bottom and 3 fuel inlet ports.

Air-fuel mixture is considered to enter the combustion zone with

inlet swirl. The homogeneity of the mixture is found out at the

dump plane and other important planes from simulation done

with ANSYS FLUENT®. for the meshed geometry. The results

show whether mixing of air and fuel is full or partial and the

extent of partial premixing. The parameters varied in the

ANSYS FLUENT®. based simulation are the premixing length

i.e. port of entry of fuel, the fuel flow rate i.e. the equivalence

ratio and the air flow rate.

Keywords: Homogeneity, Premixing, Equivalence ratio, Dump

plane.

INTRODUCTION Efficient combustion is the key integral part in power

generation related sectors like gas turbine, aircraft engines and

industrial burners wherein diffusion flame has long been in use.

But with production and emission of NOx due to high

temperature, the challenge for engine manufacturers for quite

some time has been to develop low NOx combustors. New

concepts introduced to the gas turbine industry include lean

premixed combustion (LPM), rich-burn quick-quench lean-burn

(RQL) combustion, catalytic combustion and selective catalytic

reduction (SCR) [1, 2]. Among these, the lean premixed

combustion appears the most effective mainly because of its

two-fold advantages [3]:

Reduction of flame temperature owing to the

presence of excess air which eliminates

production of thermal NOx.

Low exhaust temperature increases the lifetime of

turbine blades and other mechanical components.

The primary drawback of such LPM combustors is

reduction in the operability range between the flashback and

blowout regimes which decreases the overall stability of the

combustion process [4]. The popular stabilization mechanisms

used for sustaining the flame in the combustor are bluff body

flame holders [5, 6], dump plane swirler [7-9]and pilot flame

[10]. A failure in sustaining the flame in the combustors lead to

the phenomenon called lean blowout (LBO). To avoid such

unwanted phenomena, current combustors are typically

operated with a wide margin above the lean blowout limit which

Proceedings of the ASME 2012 Gas Turbine India Conference GTINDIA2012

December 1, 2012, Mumbai, Maharashtra, India

GTINDIA2012-9602

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tends to increase the NOx formation from the engine. Hence, a

significant optimized design balancing operation economy and

reduction of emission is crucial. The current work is aimed at

addressing a part of this issue where a CFD analysis is carried

out on dump plane swirler geometry to investigate the

homogeneity of air-fuel mixture.

Though many sensing techniques for predicting the

proximity of combustor to lean blowout have been reported in

the past, the general trend is to study the flame dynamics near

the blowout limit at a fixed degree of premixedness. The degree

of premixedness influences heat release fluctuations which

being the primary cause of dynamic instability should

subsequently affect the LBO limit [11, 12]. Several

experimental studies characterizing the blowout phenomenon

along with secondary measurements like strain rate at flame

base, flame base pulse rate before the onset of lift-off have

been done using modern optical diagnostic tools like PLIF and

PIV [13, 14]. Optical OH chemiluminescence has also been

used by Muruganandam et.al [15] to observe extinction and re-

ignition events near the blowout event which has been called as

precursor events. Similar method has also been used to detect

the location of the reaction zone and heat-release rate in a gas-

turbine combustor model [16]. LBO prediction is a very

important aspect in combustion scenario and various methods

have been established for the same over the years. The very

primitive of such methods was based on monitoring pressure in

the combustion chamber [17]. Nakae et al.[18] maintained the

steady state condition by varying the air flow rate depending on

the response of CO emission. Nair and Lieuwen [19] extended

the characteristics of blowout phenomenon in terms of acoustic

emission on different configuration of combustion setup.

Study of the dynamic stability of the flame near LBO has

also been a key topic of investigation for many researchers.

Similar parametric study of the effect of different equivalence

ratios on the dynamic behavior of a lean premixed gas turbine

combustor has been reported by Fichera et al. [20]. Other

experimental work coupled with linear and nonlinear analysis of

the dynamic behavior of such a system have been addressed on

numerous occasions [21, 22].Though lean combustion is a

popular strategy for achieving very low NOx levels, it is

extremely susceptible to both dynamic stability and lean

blowout. Chaudhari in his PhD thesis [23] studied in great detail

the experimental characteristics of LPG-air flame along with the

blowout phenomenon in a swirl stabilized dump combustor. He

also supplemented this work along with a study of flame

dynamics from thermo-acoustic instability point of view. In this

work, we used a similar geometry for cold flow analysis to

study the extent of premixing prior to the combustion since

premixing of reactants play a major role in combustion

dynamics.

Computational Fluid Dynamics (CFD) is a key tool for the

study of combustion dynamics. Most of CFD studies have so far

addressed the dynamics of flames for the study of thermo-

acoustic stabilities that develop in a lean premixed combustion

process. Chatterjee [24] has elaborated a finite volume

approach to simulate reacting flows in laminar and turbulent

stabilized flames. Other sophisticated techniques like Large

Eddy Simulation (LES) and Lattice Boltzmann Method (LBM)

have also been used for simulating premixed combustions of

different scale [25, 26]. But realizing the fact that the key

element for an efficient lean premixed combustion is a

homogeneous mixture, this study is primarily addressed at this

goal. This simple yet effective phenomenon is crucial but not

many studies exist on monitoring this.

The variation of equivalence ratio and premixing extent

determines several different characteristics of flame as has been

found through experiments done earlier [23]. If homogeneity

could be specified with variation of equivalence ratio,

premixing length and air flow rate it will lead to

characterisation of flame with variation of homogeneity.

GEOMETRICAL MODEL DEVELOPMENT The combustion chamber 3D geometry is generated using

in the ANSYS platform. The combustion zone is cylindrical

with 20cm length and 6cm internal diameter. Before the

combustion zone a premixing tube is provided with maximum

premixing length of 35cm and diameter of 2.3 cm. Dump plane

is considered as the plane at which the combustion zone starts

with an abrupt increase of cross-section. At the bottom of the

premixing tube i.e. 37.5 cm from the dump plane the air inlet

port of 1.5cm diameter is provided. There are 3 fuel inlet ports

each of 6mm diameter at 10 cm distance from each other, the

uppermost being 15cm below the dump plane. The fuel inlet

ports at distance 35cm, 25cm and 15cm from the dump plane

are named as port 1, port 3 and port 5 respectively. Just below

the dump plane a swirler of length 1.5 cm and blade angle with

axial direction 60o is placed. The number of blades on the

swirler is 8 and blade thickness is 1mm. Diameter of the swirler

Port 1

Port 3

Port 5

Premixing Tube

Swirler section

Combustion

zone

Air Inlet

Port

Fig.1: Geometry of the Set-Up with the Mesh

X

Y



section is 2.9 cm. Mesh generated is unstructured and in

standard form as shown in Fig.1. The number of nodes and

elements are chosen as 3371 and 12346 respectively based on

grid sensitivity study.

COMBUSTOR NUMERICAL SIMULATION For numerical simulation of the generated meshed

geometry a 3D, pressure-based, steady, species transport and

viscous turbulent standard k-models are utilized. For species

transport model methane-air mixture is used for all cases. For

fuel entry through each port the air flow rate of 85 litres per

minute (LPM) and 70 LPM are used, the corresponding air inlet

velocity being 8.062 m/s and 6.605m/s. Port of entry determines

the premixing length. For entry of fuel through port 1, port 3

and port 5 the premixing length are 35 cm, 25 cm, and 15 cm

respectively. The fuel flow rate is varied to get 3 different

equivalence ratios (φ) as 1, 0.785 and 0.605 for each fuel entry

port. These equivalence ratios signify variation of fuel flow rate

from stoichiometric value to a value near the experimentally

determined LBO. For these 3 equivalence ratios the fuel inlet

velocities are respectively 1.062 m/s, 0.767 m/s, 0.531 m/s.

Homogeneity in air-fuel mixture with varying premixing length

and equivalence ratio is investigated with respect to contours of

mass fraction of methane (CH4) at six different important planes

inside the combustor: i) 30cm; ii) 20cm; and iii) 10 cm below

dump plane; iv) dump plane; v)1cm and vi) 5cm above dump

plane. The CH4 mass fraction provides a direct quantitative

measure of the homogeneity in the air-fuel mixture. Graphs are

also constructed to show distribution of mass fraction of

methane along a predefined diameter of some of these

important planes. Results are presented for most important

comparisons and variations.

RESULTS AND DISCUSSIONS

Dump plane, being the most important plane of such a

combustor setup, the methane mass fraction contour is plotted

across the dump plane for air flow rate of 85 LPM. The contour

plots Figs. 2 – 4 indicate that as premixing length decreases

with change of port from port1 to port5 homogeneity of the

mixture decreases. One notable observation in Fig. 2 is the

presence of blade profile for φ=0.605. This can be attributed to

the fact that a very lean mixture is flowing through the chamber

and the fuel is injected through the furthest port from the dump

plane.

Fig. 2: Methane mass fraction contour on dump

plane for air flow rate of 85 LPM and entry of

fuel through port1

φ=1 φ=0.785

φ=0.605



fuel through port3

φ=1 φ=0.785

φ=0.605



fuel through port5

φ=1 φ=0.785

φ=0.605



Homogeneity in air-fuel mixture increases but to a very less

extent with decrease of equivalence ratio for port 1 as is evident

from the contour plot Fig.2. Figure 5 (a) shows that the methane

mass fraction decreases at dump plane with decrease of

equivalence ratio. It is notable from Fig.5 (b), (c) that increase

of homogeneity with decrease of equivalence ratio is more

prevalent for entry of fuel through port 3 and port 5. The

difference between maximum and minimum methane mass

fraction values for port 1 are 0.0054, 0.0039 and 0.0027 while

that for port 5 are 0.0265, 0.0191, 0.0134 for equivalence ratio

1, 0.785 and 0.605 respectively. This shows more rapid

decrease of the difference for port 5 than port 1.

The methane mass fraction is monitored at a plane just

before the swirler for φ=1, 0.605 keeping the air flow rate at 85

LPM. This can provide a nature of the mixture just prior to

entering the combustion chamber along with highlighting the

effect of the swirler blades. Comparison of Fig.6 with Fig.5

respectively reveals the fact that swirler plays important role in

Fig.5. (a) Methane mass fraction distribution along a

diameter on dump plane aligned with y axis of

geometry for air flow rate 85 LPM for entry of

fuel through port 1

Fig.5. (b) Methane mass fraction distribution along a


geometry for air flow rate 85 LPM for entry of

fuel through port 3

Fig.5. (c) Methane mass fraction distribution along a


geometry for air flow rate 85 LPM for entry of fuel

through port 5

φ=1

φ=0.605

Fig.6. Curves of methane mass fraction distribution

along a diameter on a plane just before the

swirler i.e. after port 5



distributing the fuel along a cross-sectional plane, thus making

the mixture more homogenous though significant

inhomogeneities exist at the dump plane in spite of the swirler.

One interesting difference between the two planes (before

swirler and after dump plane) is the CH4 mass fraction plot

corresponding to the Port 5 inlet data. The CH4 mass fraction

has a tendency to rise with increase in y in a plane before the

swirler. But, further downstream, after the dump plane the mass

fraction shows more like a “sinusoidal” nature. Also,

noteworthy, is the y-axis range in the plane before the swirler.

Owing to an expansion of the chamber diameter close to the

dump plane, the y axis range is smaller before the swirler (Fig.

6) as compared to the one 5 cm after the dump plane (Fig.

8).Also, as discussed earlier, one interesting parameter to

monitor is the difference between the maximum and minimum

mass fraction value. This variation is substantially higher in the

case of φ=1 as compared to the φ=0.605 scenario for both the

planes. The increase of homogeneity of the mixture with

distance from the dump plane is clearly indicated from the

methane mass fraction distribution plots. A typical mass fraction

contour plot at a plane after the dump plane showing the

homogeneity is shown in Fig. 7. The fact that the variation in

the values of CH4 mass fraction decreases after the dump plane

as compared to the plane before the swirler blades establishes

this interpretation. Thus, both the expansion of the chamber

diameter and the presence of swirler blades help in obtaining a

homogeneous fuel distribution in the combustion chamber.

Fig.7. Methane mass fraction contour on a plane 5 cm

after dump plane for air flow rate 85 LPM

Port 1 Port 3 Port 5

φ=1

φ=0.605

Fig.8. Methane mass fraction distribution along a

diameter on a plane 5 cm after dump plane

φ=1

φ=0.605



Fig.9 is important because it shows homogeneity of air-fuel

mixture and mixing patterns at different positions along

the length of the combustor. Comparing contours for the three

separate cases with inlet port variation in Fig. 9, it can be easily

detected that inside the combustion zone the completely

homogenous section is maximum in length for fuel entry

through port 1 and for port 5 inhomogeneity is maintained till

the end of combustion zone. Also, the effect of a lean mixture

can be clearly observed if we compare the contours in Fig. 9 for

φ=1 and φ=0.605. The red colored zone covers a smaller length

in the former case than the latter. Also from the plot

corresponding to φ=0.605, it is seen that for decrease of

equivalence ratio the length of homogenous section inside the

combustion zone has enhanced.

Fig.10 suggests that as air flow rate of 85 LPM allows the

equivalence ratio to drop to a lower value before LBO than that

for air flow rate of 70 LPM we get more homogenous mixture

just before LBO in case of 85 LPM. But for φ=1 we get more

homogeneity for air flow rate of 70 LPM. Again the results for

air flow rate of 85 LPM and 70 LPM are closer in case of fuel

entry through port 5 than entry through port 1.

From Fig.11 (a) it is observable that for equivalence ratio

1, any port of fuel entry provides more homogeneous mixture in

case of air flow rate of 70 LPM, but opposite happens near

LBO (Fig.11 (b)). This can be attributed to the fact that near

LBO equivalence ratio for air flow rate of 85 LPM is lower than

that for air flow rate of 70 LPM. The equivalence ratio value

near LBO was calculated to be 0.605 for air flow rate 85 LPM

and 0.7 for a 70 LPM air flow rate.

.

Fig.9. Methane mass fraction contour for a longitudinal

plane for different ports of fuel entry for air flow rate of

85 LPM (top) φ=1 and (bottom) φ=0.605

Fig.10. Methane mass fraction plotted on dump plane for

varying equivalence ratios and air flow rate corresponding

to fuel inlet at port 1 (Fig. a) and port 5 (Fig. b)

Entry through Port 1

Entry through Port 5

Fig.11(a). Methane mass fraction plotted on dump plane

for different ports

φ=1 a)

b)



CONCLUSIONS A 3D model of a pre-mixing chamber is built in ANSYS

platform to perform a comprehensive study on homogeneity of

air-fuel mixture. A set of swirler blades has also been

incorporated because of the immense practical utility of a

swirling flow in a gas turbine combustor. A comparative study

for varying mixing length is performed by changing the inlet

port of the fuel. The variation of mass fraction of fuel is

monitored at different sections of the premixing length for

different equivalence ratios. Also investigated is the

homogeneity of air-fuel mixture at the dump plane and a section

downstream of the same which serve as the gateway to the

combustion zone.

The numerical simulation and the results qualitatively

determine how the homogeneity of air fuel mixture in a gas

turbine combustor varies with variation of premixing length,

equivalence ratio and air flow rate. Dump plane results are

extremely important in this aspect as it signifies the first plane

after the mixture enters the main combustion zone. Figure 2 (c)

is significant as the blade profile could be seen vividly which

indicates a low mass fraction zone of methane i.e. the fuel. As

the premixing length increases homogeneity enhances in the

mixture. With variation of equivalence ratio the effect on

homogeneity depends on the premixing length. If the premixing

length is small we get pronounced increase in homogeneity with

decrease of equivalence ratio, but if the premixing length is

large this variation becomes less significant. Usually, for same

equivalence ratio less air flow rate provides more homogeneity

for the mixture. But this also depends on the premixing length.

Here we get more homogeneity for less air flow rate if

premixing length is more. If the premixing length is small we

get almost similar results with less air flow rate.

Thus, through simple CFD simulations, we get useful

insights on the premixing phenomenon of a combustor. The

dependence of a homogeneous mixture on various parameters

involved like premixing length, air flow rate and equivalence

ratio is studied. The data obtained is particularly useful for the

design of an actual efficient combustor as such a cost and time

effective CFD study can optimize the important parameters.

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Numerical Simulation to Characterize Homogeneity of Air-Fuel Mixture for Premixed Combustion in Gas...

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