combustion chamber liners

Proceedings of ICFD 10: Tenth International Congress of Fluid Dynamics

December 16-19, 2010, Stella Di Mare Sea Club Hotel, Ain Soukhna, Red Sea, Egypt ICFD10-EG-30I3

Keynote Speakers: Dr. S. A. Channiwala & Dr. Digvijay B. Kulshreshtha Copyright © 2010 by ICFD 10

NUMERICAL SIMULATION APPROACH AS DESIGN OPTIMIZATION FOR MICRO COMBUSTION CHAMBERS

Dr. S. A. Channwala* & Dr. Digvijay Kulshreshtha#

*In-charge Director & Professor, Mechanical Engineering Department,

S.V. National Institute of Technology, Surat – 395007, India #Assistant Professor, Mechanical Engineering Department,

C. K. Pithawalla College of Engineering and Technology, Surat – 395007, India

We take this opportunity to thank the Organizers of 10th ASME International Congress on Fluid

Dynamics to present a keynote paper to this audience of International Experts in the field of

Fluid Dynamics and Combustion.

INTRODUCTION

The design of gas turbine combustion chamber is based on combined theoretical and empirical

approach and the design of combustion chamber is a less than the exact science. A technical

discussion on combustion technology status and needs will show that the classic impediments

that have hampered progress towards near stoichiometric combustion still exist. The process of

combustor design has taken a new meaning over the past several years as three dimensional

codes and other advanced design and validation tools have finally changed the approach from a

“cut and burn” technique to a much more analytical process.

Mixing processes are of paramount importance in the combustion and dilution zones. In the

primary zone, good mixing is essential for high burning rates and to minimize soot and nitric

oxide formation, whereas the attainment of a satisfactory temperature distribution (pattern factor)

in the exhaust gases is very dependent on the degree of mixing between air and combustion

products in the dilution zone. A primary objective of combustor design is to achieve satisfactory

mixing within the liner and a stable flow pattern throughout the entire combustor, with no




parasitic losses and with minimal length and pressure loss. Successful aerodynamic design

demands knowledge of flow recirculation, jet penetration and mixing, and discharge coefficients

for all types of air admission holes, including cooling slots.

Good amount of literature is available on modeling of the process of combustion for kerosene

and hydrocarbon fuels (Wooley et al. [1], Phillipe et al. [2], Z. Wen et al. [3], E. Reismeier et al.

[4], Sierra et al. [5] B. Zamuner [6], Grinstein et al. [7], Caraeni et al. [8], Gran et al. [9], Shyy et

al. [10, 11], Cooke et al. [12]). Charles K. Westbrook et al. [13] have reviewed the progress in

the field of computational combustion over last 50 years encompassing 3D DNS and LES

approaches. They have observed that many commercial CFD codes uses unstructured grid which

offer the advantage of being more suitable to massively parallel computing environment, as well

as an ability to deal with complex geometries.

The paper presents the design of tubular and annular combustion chamber followed by three

dimensional simulations in tubular and annular combustor with full film cooling to investigate

the velocity profiles, species concentration and temperature distribution within the liner. The fuel

under consideration is hydrogen and primary zone equivalence ratio variation from 0.5 to 1.6

were simulated. Reactive flow calculations were carried out with 19 reversible reactions and nine

species. The computational approach attempts to strike a reasonable balance to handle the

competing aspects of the complicated physical and chemical interactions of the flow and the

requirements in resolving the three-dimensional geometrical constraints of the combustor

contours, cooling slots, and circular dilution holes. The modeling employs non-orthogonal

curvilinear coordinates, second-order accurate discretisation, multi-grid iterative solution

procedure, the SST k -ω turbulence model, and a combustion model comprising of an assumed

probability density function flamelet concept. The complicated mixing process can be better

understood with more detailed information supplied by the numerical simulation. Accordingly,

in present study an attempt has been made through CFD approach using CFX 12 to analyze the

flow patterns within the combustion liner and through different air admission holes, namely,

primary zone, intermediate zone, dilution zone and wall cooling, and from these the temperature

distribution in the liner and at walls as well as the temperature quality at the exit of the




combustion chamber is obtained for tubular and annular combustion chambers designed for gas

turbine engine.

DESIGN OF COMBUSTION CHAMBER

Basic Terminology

Figure 1 Typical Combustor Cross Section [14].

There is a need to discuss the basic combustor chamber terminology to understand the different

components of combustion chamber. Figure 1 is a cross-section of a generic diffusion flame

combustion chamber. The main dimensions of the combustion chamber are the casing and liner

area. The other dimensions are dependent of these areas and accordingly the design methodology

is given in the following section.

Casing Area

(A) Aerodynamic Consideration

For straight-through combustors the optimal cross-sectional area of the casing refA is determined

from considerations of overall pressure loss and combustion loading. However, for most

industrial combustors and some aircraft combustors, the casing area needed to meet the

combustion requirements is so low as to give an unacceptably high pressure loss. Under these

conditions the overall pressure loss dictates the casing size and refA is obtained as [14]:




( )

0.5210.5

3 3 3 4 3 4

3 3

12ref

ref

m T P PRAP q P

−

− −

⎡ ⎤⎛ ⎞ ⎛ ⎞Δ Δ⎢ ⎥⎜ ⎟= ⎜ ⎟⎢ ⎥⎜ ⎟ ⎝ ⎠⎢ ⎥⎝ ⎠⎣ ⎦ (B) Chemical Consideration

This method is used to size the chamber with the chemical considerations in mind. It attempts to

ensure a high value for the combustion efficiency. This efficiency is represented by θ parameter,

which is given by [14]:

( )1.75 0.75 3

3 exp2

ref ref

A

TP A D b

mθ

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠=⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠

Liner Area

At first sight it might appear advantageous to make the liner cross-sectional area as large as

possible, since these results in lower velocities and longer residence times within the liner, both

of which are highly beneficial to ignition, stability, and combustion efficiency. Unfortunately, for

any given casing area, an increase in liner diameter can be obtained only at the expense of a

reduction in annulus area. This raises the annulus velocity and lowers the annulus static pressure,

thereby reducing the static pressure drop across the liner holes. This is undesirable, since a high

static pressure drop is needed to ensure that the air jets entering the liner have adequate

penetration and sufficient turbulence intensity to promote rapid mixing with the combustion

products. These considerations suggest that a satisfactory criterion for mixing performance

would be the ratio of the static pressure drop across the liner LPΔ to the dynamic pressure of the

flow in the combustion zone pzq . If the ratio of liner cross-sectional area to casing cross-

sectional area is denoted by k , then the optimal value of k is that which gives the highest value

of L pzP qΔ . It can be shown [14] that:

( ) ( )( )

( )2 222

3 3 422

1 1 11 3

1

snL

pz pz p ref

m r kT PP kq T m q k

λ−

⎧ ⎫⎡ ⎤− + − −ΔΔ ⎪ ⎪⎣ ⎦= + −⎨ ⎬−⎪ ⎪⎩ ⎭




Airflow Distribution within Liner

An important aspect of combustor design is to determine the number, size, shape and disposition

of the liner holes to establish airflow pattern within the liner that will ensure easy light-up,

efficient and stable combustion, adequate wall cooling and delivery of gases to the turbine with a

suitable temperature profile. If the liner wall contains a row of n holes, each of which has an

effective diameter jd , then the total mass flow rate jm of air through these holes is given by:

( )20.5

33

15.254j

jL

mnd

PP T

=Δ⎛ ⎞⎜ ⎟

⎝ ⎠ Length of Liner

The length of liner is given by:

( )1

1ln 51

LL L

ref

PL D Aq PF

−⎛ ⎞⎛ ⎞Δ

= ⎜ ⎟⎜ ⎟⎜ ⎟−⎝ ⎠⎝ ⎠

DESIGN DATA

Saravanamutto et al. [15] has given cycle analysis and performance characteristics of individual

components of gas turbine engine while Desai N. M [16] and Pandya M. P. [17] has carried out

extensive work on laboratory scale gas turbine engine. The data for designing tubular type

combustion chamber is extracted from the work of these researchers by performing the cycle

analysis and presented in Table 1.

Table 1 Design Data for Combustion Chamber [16, 17].

No Parameter Value

1 Inlet Pressure to Combustion Chamber 2.89 bar

2 Inlet Temperature to Combustion Chamber 870.266K

3 Mass Flow Rate of Air 0.10279 kg/s

4 Exit Temperature of Combustion Chamber 1200 K

5 Mass Flow Rate of Fuel (Hydrogen) 0.0008 kg/s

6 Designed Air Fuel Ratio (Hydrogen) 128.5




The design of the combustion chamber is carried out as outline in the literature [18, 19, 20] and

design procedure given in previous section. Table 2 gives the dimensional details of Hydrogen

fuelled tubular type micro gas turbine combustion chamber while figure 2 shows the dimensional

drawing of the combustion chamber.

Table 2 Dimensional Details of the Combustion Chamber [18, 19, 20].

Chamber

Casing

Area

(m2)

Liner

Area

(m2)

Air admission holes

Primary

Zone

Dilution

Zone Wall Cooling

Primary Zone Dilution Zone

n dh

(mm) n

dh

(mm) n

dh

(mm) N

dh

(mm)

Tubular 0.0019 0.0014 6 5.98 3 13.07 78 1.83 100 1.32

Figure 2 Detailed Dimensional Drawing of Micro Combustion Chamber.

Figure 3 shows the annular combustion chamber for hydrogen fuel designed as a part of

development of Micro Gas Turbine Engine under development jointly at S. V. National Institute

of Technology, Surat and C. K. Pithawalla College of Engineering and Technology, Surat. The

design of annular combustion chamber is carried out as per the design procedure for tubular




combustion chamber. The basic geometry is thereafter decided by selecting different design

parameters summarized in Table 3 (Thickness is not considered in defining the ratios).

Table 3 Details of Combustor Geometry.

1

1

i

o

RR

3

1

i

i

RR

3

3

o

i

RR

1

Flhh

3

1

iAA

3

1

oAA

3

3

o

i

AA

0.375 0.416 5.4 0.6 0.028 0.82 29.16

Figure 3 Annular Combustion Chamber (2D).

NUMERICAL SIMULATION

There are two ways to analyze the combustion chamber numerically. One way is to give input

conditions at inlet and all the air admission holes as per the design conditions. But in actual case,

the flow distribution in different zones cannot be controlled. This is the biggest drawback of

providing different inputs at different air admission holes. The second way of analyzing the

combustion chamber is to provide only one inlet at the inlet of the diffuser and let the flow divide

by itself into liner and casing, and from casing into different zones through air admission holes

and cooling slots. Such condition is the exact replica of the real case experimentation, in which

the air is supplied at the inlet diffuser with known conditions of pressure, temperature and

velocity, and then, allowed to divide between the casing and the liner with fuel injection at liner

entry.




Basic Assumptions and Boundary Condition

• The combustion chamber is analyzed by a single entry at diffuser

• The flow is allowed to divide by itself into liner and casing, and from casing into different

zones through air admission holes and cooling slots.

• Such condition is the exact replica of the real case experimentation, in which the air is

supplied at the inlet diffuser with known conditions of pressure, temperature and velocity,

and then, allowed to divide between the casing and the liner.

• Majority of the researchers working in the area of computational combustion have selected k

– ε model to capture physics of turbulence [21, 22]. The k – ε model under predicts

separation and is highly inaccurate with swirling flows and flows with strong streamline

curvature. In comparison to k – ε, SST k – ω accounts for the transport of the turbulent shear

stress and gives highly accurate predictions of the onset and the amount of flow separation.

Also, the model can be used with coarser near-wall mesh and produce valid results.

• A large variety of kinetic schemes and mechanisms varying from very complex schemes to

single step fast chemistry is available for hydrocarbon combustion [23, 24]. Few researchers

have observed that reduced kinetic and single step chemistry offers reasonable predictions

with limited computational power [25, 26]. But single step chemistry over predicts the

temperature levels along the combustor and hence gas-phase reaction model for the

combustion of hydrogen and air mixture consists of 19 reversible elementary reactions and

nine species was selected.

• Wall boundary condition and heat loss does influence the flame structure and predictions of

temperature levels in combustion system [27, 28]. In present case, for the 3D calculations

with CFX, the adiabatic system model is used because of large mass flow rate of air through

annulus which keeps wall cooled casing nearly at ambient temperature.

GRID INDEPENDENCY

The three-dimensional grid independent study was carried out with number of nodes varying

from 145000 to 234000 nodes. The simulation results do not vary comprehensively between

nodes of 176000 to 234000 and hence a grid with 195000 nodes was selected for CFD

simulations. The grid spacing selected is similar to the Eulerian grid and is finest at the nozzle




exit and becomes gradually coarser away from the nozzle. Due to unstructured grid high

resolution advection scheme is selected for additional accuracy.

CFD MODELING OF HYDROGEN FUEL

The mixing process occurs within the combustion chamber which is also part of the simulated

region and so a non-premixed model was used. The gas-phase reaction model for the combustion

of hydrogen and air mixture consists of 19 reversible elementary reactions and nine species.

Third body efficiencies for all intermolecular reactions are 2.5 for H2, 16 for H2O, and 1.0 for all

the rest of species, as reported in Tien and Stalker [29]. The combustor modeling is carried out

using eddy dissipation model based on the concept that chemical reaction is fast relative to the

transport processes in the flow. When reactants mix at the molecular level, they instantaneously

form products. The CFD Models are summarized in table 4.

Table 4 CFD Models.

Fluid Model Thermal Energy

Turbulence Model SST k – ω

Combustion Model Eddy Dissipation

Radiation Model Discrete Transfer

Nitrogen Constraint

Combustion Reaction Hydrogen Air Multi step

CFX-RIF Generator was used to include 19 reversible reactions and 9 species for ANSYS CFX.

The solution convergence is judged according to the residuals of governing equations. The

results reported in this paper are based on the criteria that the residual of each equation should be

smaller than 1.0×10−6. Each simulation normally takes about 900 CPU hours.




TUBULAR MICRO COMBUSTION CHAMBER

Three dimensional non-reacting flows and reacting flow in micro combustion chamber have been

computed using ANSYS CFX. Figure 4 shows perspective view of the round combustor to

portray the three-dimensional (3D) non-reacting flow features. The flow re-circulation, mixing,

flow distribution through different air admission holes and the combustor is studied. The

boundary conditions are according to the design values specified in Table 1.

Figure 4 Flow Model for CFD Analysis.

EXPERIMENTAL INVESTIGATIONS

The numerically simulated micro-combustor was fabricated and its components are shown in

Figure 5. The tests were carried out under the following operating conditions:

Mass Flow Rate of air, Am•

= 0.1028 kg/s

Mass Flow Rate of fuel, Fm•

= 0.0008 kg/s




Equivalence Ratio, φpz = 0.9 (Primary Zone),

Overall Equivalence Ratio φ = 0.271

Inlet Pressure 3P = 2.89 bar

Inlet Air Temperature 3T = 870.26 K

Inlet Fuel Temperature, 2HT = 300 K

Figure 5 Main Components of Combustion Chamber.

MEASURED VALUES

The diagnostic techniques used in present experimentation are summarized with different

parameters of measurements.

Temperature: Type S Thermocouples

Pressure: Pressure Sensor

Velocity: Claw Type Yaw Meter

Flow Measurement: Orifice Meter with Differential Pressure Transducers




The uncertainty analysis for temperature, flow measurement, pressure and velocity is carried out

as per ASTM and ASME standards [30, 31]. The uncertainty in measurement is in the range of

±3% maximum with a confidence level of 95%.

ANNULAR COMBUSTION CHAMBER

Figure 6 Computational Domain of Annular Combustion Chamber.




Figure 7 Generated Grid.

Figure 7 shows the grid generated for annular combustion chamber isothermal swirling flow

analysis. The three-dimensional grid independent study was carried out with number of nodes

varying from 398712 to 640190 nodes. The simulation results do not vary comprehensively for

nodes of 436890 or higher and hence a grid with 545000 nodes was selected for CFD

simulations.

RESULTS AND DISCUSSIONS

TUBULAR COMBUSTION CHAMBER

Non Reacting Flow Results

Figure 8 shows the velocity distributions at axial and radial locations of the combustion chamber

for both the CFD simulations and experimental investigations. The radial locations are

designated as radial 1, i.e. ratio of radial distance to radius of liner r/R = 0.35 and radial distance

2, i.e. ratio of radial distance to radius of liner r/R = 0.7. The low velocities are encountered in

the primary zone at axial as well as radial locations. These low velocities are beneficial for both




combustion stability and mixing. The fact is evident from figure 9, which shows the streamlines

from swirler outlet. Complete mixing is evident which is good for better mixing of fuel and air

inside the primary zone. Intense mixing and recirculation is observed at central core, which may

offer stable narrow flame [32, 33, 34].

Figure 8 Velocity Distributions for Non Reacting Flows.

Primary Zone = x/L < 0.4

Dilution Zone = x/L > 0.4




Figure 9 Streamlines through Swirler outlet.

Figure 10 Pressure Distributions for Non Reacting Flows.

Primary Zone = x/L < 0.4

Dilution Zone = x/L > 0.4




The velocity levels are slightly higher in the dilution zone compared to the primary zone. As

more air enters through the wall cooling and dilution zone, the velocity levels increases. This

may be due to the fact that pressure drop is manifested in the increased velocity levels for cold

flow studies. This pressure drop is graphically represented in Figure 10. Higher pressure drop is

witnessed in the dilution zone which leads to higher velocities near the exit of the combustion

chamber.

REACTING FLOW RESULTS

Figure 11 Fired Combustion Chamber.

The combustion chamber is first fired to ascertain the flame quality and thereafter pressure,

temperature and velocity measurement is carried out at designed overall equivalence ratio of




0.271. Velocity, pressure and temperature measurements are carried out at centerline of

combustion chamber and in the radial direction. Special attention is given to the fuel supply

system as hydrogen fuel has characteristics of backfire. To avoid the back fire, hydrogen is

supplied through a series of flame trappers and flame quenchers with non return valves to avoid

any accident in case of backfire. The combustion chamber fired at designed condition is shown

in Figure 13, while figure 14 shows the temperature distribution along the length of the

combustion chamber.

Figure 12 Temperature Distributions at Designed Equivalence Ratio.

The temperature levels in the entrance region near the fuel nozzle are lower and thereafter

increases and reaches maxima as shown in Figure 14. The temperature levels again decreases as

more and more air is available from dilution zone and wall cooling zone. The temperature levels

at radial locations, i.e., at r/R = 0.35 and 0.7 along the length of combustion chamber are found

to be decreasing due to gradual consumption of fuel.




The critical study of flame structure reveals that the present design offers narrow flame with

maximum temperature in primary zone being of the order of 1800 oC. H. J. Tomczak et al. [32]

has obtained these maxima as 2330 K, T. S. Chen et al. [28] has achieved it as 2100 K while

Jinsong Hua et al. [33, 34] has reported this peak temperature in the range of 2200 to 2400 K.

Cheng et al. [28] and Jinsong et. al. [33, 34] has observed significant effect of wall boundary

condition and heat losses on flame structure and prediction of temperature levels. The narrow

and short flame structure is reported by Cheng et al. [28] with constant wall boundary condition

while reduction of peak temperature level is observed by Jinsong et al. [33, 34] with heat losses.

The intense mixing and formation of re-circulation zone at the core of primary zone are

responsible for short and narrow flame structure as is observed through flow visualization

(Figures 9), in present study.

Figure 13 Velocity Distributions (Reacting Flows).

Similar trends for velocity and pressure distribution as found for cold for studies are found with

reacting flow studies (figures 13 and 14). However, the velocity and pressure levels are slightly




higher for reacting flows. It is important to note that the pressure drop across the combustion

chamber is around 10% of the inlet pressure which is quite tune with the published results for

tubular combustion chamber. CFD and experimental results qualitatively match while

quantitatively the results differ by around 20%. Similar results are obtained by Grinstein &

Fureby [7] for velocity profiles in LM6000 combustor.

Figure 14 Pressure Distributions (Reacting Flows).

ANNULAR COMBUSTION CHAMBER

An appropriate fluid flow texture is critical important for stable combustion in the liner, which

ensures proper fuel concentration field in the combustor and air distribution among the dilution

holes and tiny film-cooling holes [35]. The numerical simulation is carried out using SST k – ω

turbulence model with inlet boundary conditions of mass flow rates of air and fuel. The outlet is

taken as opening type with relative pressure at outlet of 0 Pa.




The flow from the pre-diffuser inlet to the combustor exit can be mainly divided into three zones,

namely back flow zone, mixing zone and trailing flow zone. In the first zone or back flow zone

the air leaves the swirl cone to the dome at high swirling velocity. After that, the swirling

mixture of the air and fuel flows forward and entraps the air in the center area of the liner, and at

the same time, the downstream air refills the region. As a result, a counter-rotating vortex pair

(CRVP) forms in the central region of combustor liner. Corresponding to a pair of large eddies in

this zone, there should be two symmetrical small eddies in the corners of the liner. In the

downstream area of the back flow zone, the outside mixture further mixes with the fresh air

injected from the dilution holes distributed on the liner wall. The injection flows retard the

revolution of the mixed gas and restrain the anterior reverse flow zone. As a result, the back flow

zone is cut off by the first pair of the dilution holes and the combustion should mainly occur in

the backflow zone under very hot condition, i.e., primary zone. Although a small amount of the

mixed gas is inhaled into the primay zone, most gas flows downstream and mixes with the cold

air from the second pair of the dilution holes. This makes the flow in the combustor more

uniform. (Figure 15)

Figure 15 Velocity Distributions in Annular Combustion Chamber.




Actually, the back flow zones can be regarded the areas where the fuel are ignited and the

combustion flame is stably held, or the flow back zones serve as igniters and flame-holder while

the CRVP downstream the second pair of the dilution holes may be a cut-valve to the flame. The

flow behavior in the combustor, including the flow back zones, eddies, and their size and

intensity, will apparently control the combustion behavior and then the performance of the

engine. The simulation and experimental results indicate the main inlet flow plays a critically

important role in forming a very well organized combustion, while the flows from the dilution

holes further assist and complete fuel combustion in later stage [18].

Figure 16 Normalized Distributions of Velocity and Pressure in Pre-diffuser Section.

Figure 16 shows the velocity and pressure distributions in the pre-diffuser section, normalized

with the combustor reference velocity and inlet pressure, respectively. The inlet velocities are

higher compared to the reference velocity and thereafter the values decreases gradually. This is

mainly due to flow diffusion leading to drop in velocity gradient and rise in pressure gradient.




But there is a drop in pressure rise at the pre-diffuser exit, probably due to diffuser losses mainly

due to skin friction losses.

Figure 17 Normalized Velocity Distributions in Dump Diffuser Region.

The flow is decelerated without separation in a short pre-diffuser and then discharged into dump

region. In this zone, flow is divided into three streams. The center stream enters the liner, while

the outer and inner streams proceeds as free jets around the head of liner, thereby undergoing

free surface diffusion, until they enter the outer and inner annuli that surrounds the liner. Because

of the sudden expansion at the pre-diffuser exit, the losses are high. Generally vortices are

formed in the dump region and hence the losses are high in dump diffuser section. Therefore, the

velocity distributions in the dump diffuser regime, normalized with respect to combustor

reference velocity are not uniform (figure 17).

In the pre-diffuser, the flow decelerates with comparatively small losses (figure 16), because its

boundaries are rigid. In the dump region, losses are high due to the shear action between the jets




and the stagnant air and due to large flow turning imposed by the head of liner. These high losses

are manifested in sudden drop of velocity at x/L locations of 0.06 and 0.09 (figure 17).

Particularly high losses will occur in the regions where the jets bend towards the outer and inner

annuli, because here the turbulent structure changes because of the centrifugal force which

strongly influences the turbulent stresses (figure 18 and figure 19).

Figure 18 Normalized Velocity Distributions in Outer Annulus Casing.




Figure 19 Normalized Velocity Distributions in Inner Annulus Casing.

Figure 20 Velocity Distributions at Radial Locations at Swirler Exit.




Figure 21 Normalized Velocity Distributions in Annulus Liner.

The velocity distribution at the swirler exit is depicted in Figure 20 at different radial locations at

the distance of x/L of 0.29. The velocity levels are lowest at the radial location of 0.5 and

increases at one move near the wall of the annular liner. As expected, the introduction of swirl

resulted in a very rapid development of the velocity field (figure 21). Furthermore, a region of

reverse flow was created that extended to roughly the first rows of primary holes (figure 15). It is

interesting to note that whereas the swirling flow recovers in the axial direction, it did not

completely recover in the radial or tangential directions. High levels of radial and tangential

turbulence were observed in the central core region throughout the combustor. This high level of

turbulence is attributed to the existence of the precessing central vortex throughout the

combustor.




CONCLUSIONS

The design of micro gas turbine combustion chamber is carried out using hydrogen as fuel and

the design is then validated using Numerical and Experimental Approach. The qualitative and

quantitative agreement of CFD results with the Experimental Results suggests that the basic

assumptions and boundary conditions as well as the problem definition for CFD analysis can be

applied to understand the flow phenomena, temperature contours and air flow distribution for

combustion chamber. The consistent centerline temperatures achieved along the centerline of

combustion chamber validates the design methodology proposed and presented in this paper.

The maximum centerline temperature recorded by CFD simulation is in the vicinity of 1876 oC

while for Experimental Investigations is around 1700 oC. The pressure loss along the combustion

chamber is 10% of the inlet pressure. The velocity profiles show an increasing trend along the

length of combustion chamber, but low velocities are encountered in primary zone which is

beneficial for combustion stability.

A numerical CFD simulation was made to investigate the isothermal swirling flow characteristics

in an annular combustor for micro gas turbine engine. Three dimensional model is investigated

to study the flow behavior in pre-diffuser, dump region, liner, inner and outer annuli and swirler

exit. High Grid densities were specified to obtain best resolution for the various components of

the combustor, such as fuel and air inlet fluid boundary. The SST k – ω model were employed to

describe the fluid flow and cooling behavior of the combustor. The fluid flow in the tube can be

mainly divided into back flow zone, mixing zone and trailing fluid zone. This fluid flow texture

ensures the proper flow field in the combustor and affects the air distribution through different

air admission holes. High levels of radial and tangential turbulence were observed in the central

core region throughout the combustor. High losses are encountered in the dump region and

where the jets bend towards the outer and inner annuli. The design of these zones needs to be

taken utmost care to reduce the losses in the combustor. The flow behavior in the combustor,

including the flow back zones, eddies, and their size and intensity, will apparently control the

combustion behavior and then the performance of the engine. High velocity from primary and

dilution air admission holes of the order of 110 m/s is witnessed. Such high velocity from the air

admission holes ensures high static pressure drop, which is advantageous in mixing through air




admission holes. The formation of counter rotating vortices from the second row of primary zone

ensures cutoff of flame.

NOMENCLATURE

A Geometrical area

,h effA Total effective liner hole area

,h goemA Hole area, geometric

b Temperature dependence of reaction rates

DC Discharge Co-efficient

D Diameter

d Diameter

k Ratio of liner to casing area

L Length

m Mass flow rate

pm Ratio of primary-zone air flow to total chamber air flow

snm Ratio of air entering snout to total chamber air flow

n Number of holes

P Pressure

PF Pattern factor or Temperature traverse quality

q Dynamic pressure

r Radius

R Gas constant

T Temperature

LPΔ Liner pressure differential

λ Diffuser pressure-loss co-efficient

φ Equivalence ratio




SUBSCRIPT

A Air

a Air

f Fuel

F Fuel

j Jet value

L Liner

PZ Primary-zone

ref Reference value

3 Combustor inlet plane

4 Combustor exit plane

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