flow characteristics of an annular gas turbine combustor...
TRANSCRIPT
Journal of Scientific & Industrial Research
Vol. 65, November 2006, pp. 921-934
Flow characteristics of an annular gas turbine combustor model for
reacting flows using CFD
S N Singh*, V Seshadri, R K Singh and T Mishra
Department of Applied Mechanics, IIT Delhi, New Delhi 110 016
Received 16 March 2005; revised 28 June 2006; accepted 18 July 2006
Computational Fluid Dynamics (CFD) approach can reduce the expenses as well as time to provide an insight into the
characteristics of flow and combustion process inside combustion chamber at design stage. Geometry of combustor
simulated for present investigation is a 45° sector of an annular combustor. Primary, secondary and dilution holes are
simulated on the inner and outer liner walls with swirler being placed at the center of the liner dome. Flow has been
analyzed in the annulus region. The results are fed as input for the flow analysis in the liner. Uniform velocity distribution is
obtained in the annulus passage around the liner. For the liner flow, it is observed that on moving axially from nozzle to
outlet, velocity and temperature contours become more uniform and symmetric in circumferential plane. Mass fraction of
CH4 and O2 decreases whereas concentration of CO2, NO and H2O increases in the axial direction (nozzle to outlet).
Keywords: Annular gas turbine combustor, Computational fluid dynamics, Liner holes, Species concentration, Temperature
contours
IPC Code: F15D1/00; G09B23/08
Introduction Annular combustors, which offer maximum
utilization of available volume, fewer requirements of
cooling air and high temperature application1 are one
class of combustors most commonly used. A well-
designed gas turbine combustor2 should have
complete combustion and minimal total pressure loss
over a wide range of operating conditions. Flow
characteristics3 in the annulus passage surrounding the
liner is equally important as the flow is fed into the
liner through the annulus passage. Bharani et al4 have
shown that the bulk of the flow remains close to the
outer liner wall between the rows of primary and
dilution holes while it shifts towards the liner mid
plane after the row of dilution holes. Bharani et al5,
using a prototype reverse flow combustor, have
shown that swirl has no significant effect on the flow
split through primary holes while the split through
dilution holes decreases for Swirl number up to 0.55.
Ahmed & Nejad6 have carried out experimental
investigation of turbulent swirling flow in a
combustor model for coaxial swirling jets with dump
diffusers. Green & Whitelaw7 have suggested that the
standard k-ε model gives better results than the other
turbulence models in turbulence combustion
prediction. Mongia8 has shown the difficulty in the
specification of boundary conditions, inferior
resolution of which hampers the ability of
computational models to predict combustor
characteristics. Mohan et al9 have numerically
investigated annuli flow and effect of inlet swirl on
the flow split through the liner holes of annular
reverse flow combustor model. Garg et al10
and
Singh et al10
have reported the effect of height of
inner and outer annuli for an elliptical dome shape
combustor for cold flow simulation using
computational fluid dynamics (CFD).
Cadiou & Grienche12
have conducted hot flow
studies on the liner with and without primary holes for
a reverse flow annular combustor. John & Torel13
have investigated the temperature profile and
concentration of CO, CO2, O2 and Nox for can type
combustor. Murthy14
has developed an algorithm for
one-dimensional analysis for flow and heat transfer in
straight tubular, tubo-annular and annular combustors.
Present analysis is an attempt to study reacting flow in
the annular combustor geometry suggested by
Garg et al10
and Singh et al11
using CFD.
Methodology
A commercial available CFD code ‘FLUENT’15
has been used for the analysis. The details of the
mathematical models are given in the manual of the
code. Brief discussion of the model is as follows:
__________
*Author for correspondence
E-mail: [email protected]
J SCI IND RES VOL 65 NOVEMBER 2006
922
The governing equations for mean flow in reduced
form for steady incompressible turbulent flows are,
( ) mi
i
Sux
=∂
∂ρ …(1)
Mass conservation equation is valid for both
incompressible and compressible flows. Sm is mass
added to continuous phase from dispersed phase.
Momentum conservation equation for turbulent flow
is written as,
2
3
ji i i
j ij
j i j j i i
uu u upu
x x x x x xρ µ µδ
∂∂ ∂ ∂∂ ∂= − + + − + ∂ ∂ ∂ ∂ ∂ ∂
( )' '
i i j
j
F u ux
ρ∂
+ + −∂
…(2)
These equations are of the same general form as
the original equations except for some additional
terms. The additional terms are the Reynolds stresses
and these need to be modeled for closure solutions.
The Boussinesq hypothesis16
is used to relate the
Reynolds stresses to the mean velocity gradient as
ij
i
it
i
j
j
itji
x
uk
x
u
x
uuu δµρµρ
∂
∂+−
∂
∂+
∂
∂=−
3
2''
…(3)
where k is the turbulent kinetic energy and δij is the
Kronecker delta and µt is the eddy viscosity.
For reacting flow, energy equation also needs to be
solved, which is given as
( ) h
k
iik
ip
b
i
i
i
Sx
u
x
H
c
k
xHu
x+
∂
∂+
∂
∂
∂
∂=
∂
∂ 'τρ …(4)
Sh is the heat of chemical reaction, H is the total
enthalpy and is computed as sum of each mass
fraction from Eqs 5 and 6
∑='
''j
jj HmH …(5)
)( ,'',' jrep
T
T
ojjpj ThdTCH
j
′∫ += …(6)
Reynolds stresses in the time averaged momentum
equations need to be approximated and represented by
additional equations, which are required to be solved
for closure solution of fluid flow problem. In present
investigation, two-equation turbulence model
(standard k-ε)17
has been used.
Standard k-εεεε Model
Equations for k-ε model are
Mk
ik
t
ii
i YGx
k
xx
ku +−+
∂
∂
+
∂
∂=
∂
∂ρε
σ
µµρ
…(7)
KCG
kC
xxxu k
i
t
ii
i
2
21
ερ
εε
σ
µµ
ερ εε
ε
−+
∂
∂
+
∂
∂=
∂
∂
…(8)
Gk is generation term for turbulent kinetic energy
due to mean velocity gradient and is given by
i
j
jikx
uuuG
∂
∂−= ''ρ …(9)
Eddy or turbulent viscosity, µt is computed from
ερµ µ
2
t
kC= …(10)
where Cµ is constant.
Dilatation dissipation (YM) is modeled as per
Sarkar and Balakrishnan18
and is given as
2
t
2
M MY ρε= …(11)
where, Mt is turbulent Mach Number, defined as
RT
kM t
γ= …(12)
For incompressible flow YM is normally neglected.
Values of the empirical constants used are C1ε= 1.44,
C2ε = 1.92, Cµ = 0.09, σk = 1.0 and σε = 1.3. These
values have been found to work fairly well for a wide
range of wall bounded and free shear flows.
Reaction Modeling
Combustion is the major energy release mechanism
and it always gives off heat and gases as a product.
SINGH et al: FLOW CHARACTERISTICS OF ANNULAR COMBUSTOR USING CFD
923
Combustion reaction is assumed to be single step,
irreversible reaction following finite rate chemistry.
CH4 + 2O2 → CO2+2H2O …(13)
Heat formations (or standard state enthalpy) for the
fuel species can be calculated from the known heating
value, and are computed as
( )∑ ′−′′=∆=
N
ikiki
o
ihH
1',',', υυ …(14)
where, o
i,h is the standard state enthalpy (J/kg) for each
chemical species. ki ,'υ ′ is the stoichiometric coefficient
for reactant 'i in the reaction k and ki ,'υ ′′ is the
stoichiometric coefficient for product 'i .
Species Transport Equation
CFD approach is based on the transport
equation for each species. The local mass
fraction of each species, mi, through the solution
of convection diffusion for the ith species is
expressed as
( ) i,'ii,'ii,'i
i
'ii
i
SRjx
mux
++∂
∂−=
∂
∂ρ …(15)
where, iiR ,' is mass rate of creation or depletion of
species 'i by chemical reaction and iiS ,' is the mass
rate of creation or depletion by addition from the
dispersed phase plus any user defined source.
Mass Diffusion in Turbulent Flow
For turbulent flow, mass diffusion of species 'i is
computed as
…(16)
iij ,' is diffusion flux of species 'i , which arises due to
concentration gradient. Di’m is the diffusion
coefficient for species 'i in the mixture. tSc is the
turbulent Schmidt number and is expressed as
t
tt
DSc
ρ
µ= …(17)
Reaction Rate Calculation (Finite Rate Chemistry)
Most traditional way to model the reaction rate is
the approach of finite rate chemistry where chemical
reaction is defined as
∑ →∑ oductPr
k'Ri
ttancaRe EE …(18)
The source of chemical species i´ due to reaction
rate 'iR is computed as the sum of the reaction
sources over the NR reactions that the species may
participate in
∑==
RN
1kk,'i'i'i R̂MR …(19)
'iM is the molecular weight of species 'i and
R kiˆ
,' is the molar rate of creation/destruction of
species 'i in reaction k computed as
( )', ',
', ', ', , ' , '' 1 ' 1
ˆ j k j kN N
i k i k i k f k j b k jj j
R k C k Cη η
υ υ′ ′′
= =
′′ ′= Γ − −
∏ ∏
…(20)
where, C j,r = molar concentration of each reactant and
product species j in reaction R (kgmol/m3), k,'jη′ =
forward rate exponent for each reactant and product
species j in reaction R, and k,'jη′ = backward rate
exponent for each reactant and product species j in
reaction R.
Γ represents the net effect of third bodies on the
reaction rate and is calculated as
∑= ′
N
'j'jk,j cγΓ …(21)
kj ,′γ is the third body efficiency of the j´th species in
the chemical reaction.
In the present reacting flow analysis, the reaction
rate is controlled by the mixing of the turbulent eddies
containing fluctuating species concentration namely
eddy dissipation model (EDM). Reaction is assumed
to be in continuous phase for the continuous species
only.
Eddy- Dissipation Model (EDM)
EDM is based on a detailed description of the
dissipation of turbulent eddies on the concept of
interaction between turbulence and chemistry of
flame. The total space is subdivided into reaction
i
'i
t
t
i
'im,'ij,'i
x
m
Scx
mDj
∂
∂
+
∂
∂−=
µρ
J SCI IND RES VOL 65 NOVEMBER 2006
924
space (fine structure) and surrounding fluid. All
reactions in the gas phase component are assumed to
take place within the reaction space, which represents
the smallest turbulent scale where all turbulent energy
is dissipated into heat. Influence of turbulence on the
reaction rate is taken into account by employing
Magnussem & Hjertager model19
, which gives the rate
of reaction ', ,i k
R which is given by the smaller of
the following two expressions
', ', ' '
,
' R
i k i k i
R k R
mR M A
k M
ευ ρ
υ= …(22)
', ', ' ''
,'
'pP
i k i k i N
R k Rj
mR M A B
k M
ευ ρ
υ=
∑∑
…(23)
where, pm and
Rm are mass fraction of product
species (P) and reactant (R) respectively. A and B are
empirical constants having values of 4.0 and 5.0
respectively.
EDM relates the rate of reaction to the rate of
dissipation of the reactant and product containing
eddies. (k/ε) represents the time scale of the turbulent
eddies. The model is useful for the prediction of
premix and diffusion problems as well as for partially
premixed reacting flows.
Validation of the Code
CFD code FLUENT 5.014
was validated against
experimental results of three-dimensional swirling
reacting turbulent flow inside the Can combustor20
,
which consists of fuel nozzle with swirler (Fig. 1). In
addition, there are six dilution holes equally spaced on
the circumference of the combustor wall. Hence
prediction was made in 60°-sector model by
considering 3D problem with symmetric boundary
conditions. The 60°-sector model consists of only one
dilution hole on the wall. Flow in the sector model
was solved for various degree of fineness of
computational mesh for checking the grid
independency and finally the number of meshing
element was found to be 78,000. The changes in
results were negligible for further increase in meshing
elements. Simulations were also carried out with
different turbulence models (RNG K-ε, K-ω model
and RSM model) to validate the results. Standard K-ε
model gave the best results. The two-equation
turbulence model (K-ε) has also been used for
economical reasons. For sake of brevity, results of
only standard K-ε model are presented. The
combustion has been simulated with the generalized
finite rate chemistry model and is modelled using
one-step reaction mechanism, assuming complete
conversion of fuel to CO2 and H2O. Reaction rate is
determined on the basis of assumption that turbulent
mixing is the rate limiting process with the turbulent
Fig. 1 — Geometry of the combustor used for validation20
SINGH et al: FLOW CHARACTERISTICS OF ANNULAR COMBUSTOR USING CFD
925
chemistry interaction modeled using EDM. Validation
of the code was further established by comparing the
predicted temperature contours at different sections
(Fig. 2). The penetration of primary jet in both the
figures is within 5% (CFD prediction 478 K, validated
results 500 K). Even away from the primary jet,
matching is reasonably good with deviation still being
within 5%. Temperature measured along the radius at
different sections down stream of the dilution hole in
form of contours is compared with the predicted
results at the same section (Fig. 3). Predicted contours
closely match the experimental trends; however,
predicted values are somewhat higher than
experimental values. Figure 3 show two recirculation
zones on both sides of the dilution hole. Deviations in
the results in this zone are of the order of 15%. In the
rest of the regions, deviation is of the order of 5%.
Penetration of jet at dilution holes is also almost same
and temperatures in this region also nearly match.
These deviations could be attributed to the
assumptions made in the combustion and turbulence
model. On the basis of reasonable matching of the
predicted and experimental results for the similar type
of combustor, commercial CFD code ‘FLUENT’ can
be considered to be validated for predicting reacting
flows in the annular combustor.
Fig. 2 — Comparison of static temperature contours in a can combustor17: a) Experimental results; b) CFD Predicted results
Fig. 3 — Comparison of temperature contours in the plane of dilution air in a can combustor: a) Experiment results7; b) Predicted results
J SCI IND RES VOL 65 NOVEMBER 2006
926
Geometry and Boundary Conditions
Combustor consists of 8 annular swirlers and 56
holes along the inner and outer circumference for
primary, secondary and dilution zones (Fig. 4). For
prediction, a 45o sector model is simulated with a
coaxial jet arrangement in the center of liner dome
and 7 holes each for primary, secondary and dilution
zones on the inner and outer circumference of the
liner. Combustor model consists of a pre-diffuser
followed by a dump diffuser, straight annular
confinement and liner. The liner dome is elliptical
with major axis perpendicular to the liner axis.
Coaxial jet arrangement has fuel in the central jet and
non-swirling or swirling annular jet for the oxidizer. It
also shows the plane selected for presentation of the
results inside the annulus. Mass fraction between the
two annuli is given as S = mo /mi, Dimensions of
various holes and coaxial jet are: Diam of primary and
secondary holes, 8; Diam of dilution hole, 12; Inner
diam of coaxial jet, 10; Outer diam of coaxial jet,
30 mm.
A 3-D 45o sector model (Fig. 5) was developed
from 2-D geometry using GAMBIT package of the
FLUENT code. The geometry was meshed with both
structured as well as unstructured mesh. Near the wall
region, boundary layer meshing scheme was opted
whereas in the rest of the region, tetrahedral meshing
scheme (hybrid grid) was employed. Boundary layer
meshing scheme is used, as it is useful for
computation of viscosity-dominated near wall regions
for turbulent viscous flows. Optimum numbers of
cells was arrived by checking the grid independence
with respect to the velocity vector and velocity
profile. Finally, total number of mesh was arrived to
be 78,000. In the 3-D geometry, one mesh element
contains 4 nodes, therefore the total number of nodes
are approx 300000.
Prediction has been carried out for air-fuel
mixture as working fluid. The flow in annulus and
liner have been analyzed separately. For solving the
annulus part, a flat velocity profile is fed upstream of
the pre diffuser having a velocity magnitude of
26 m/sec, which corresponds to an inlet Reynolds
number of 4.96×105 based on the inlet diam.
Atmospheric pressure conditions are specified at
different holes of the liner as outlet boundary
Fig. 4 — Plane representation of annulus geometry: A) 2-D Axi
symmetric geometry of annular combustor; B) 3-D geometry of a
45° sector of annular combustor
Fig. 5 — 3-Dimensional geometry of liner and orientation of
planes: a) 3-D geometry of liner; b) Orientation of vertical cross
sectional planes; c) Orientation of horizontal and vertical central
planes
SINGH et al: FLOW CHARACTERISTICS OF ANNULAR COMBUSTOR USING CFD
927
conditions at the holes and the coaxial jet
arrangement. Symmetry condition is imposed on both
the sidewalls. Outlet velocity profile obtained at
different holes was fed as input for the flow analysis
in the liner. For the liner flow, velocity profile
through the annular jet and uniform velocity for the
fuel jet was also specified as the input. At the outlet of
the liner, pressure outlet (atmospheric conditions)
boundary condition is specified. In the circumferential
direction for a 45°-sector model, symmetric boundary
conditions at the sides of the sector model were
specified.
Calculation for Air Fuel ratio
For complete combustion and better efficiency of
methane (CH4, density 0.668 kg/m3), fuel to air ratio
is given as21
F/A = 0.02929 …(24)
For reaction modeling in the combustion chamber,
mass flow rate of air (A) is taken as 0.2329 kg/sec.
Substituting the value of A in Eq. (24), mass flow rate
of the fuel (F) works out to 0.00668 kg/sec. Fuel
velocity has been calculated as
fuelfuelfuel VAF ××= ρ …(24)
Substituting the value of F, Afuel and ρfuel in above
expression, a value of Vfuel = 130 m/sec (Re = 2.4×105
based on the fuel jet diam) was obtained for fuel.
Flow analysis has been carried out for the above-
calculated air and fuel velocities with no swirl
condition. Velocity vectors as well as contours,
temperature contours and velocity profiles are plotted
for each case.
Orientations of planes for analysis (Fig. 5) are Mid
1 Plane (Mid Vertical Plane of the liner), Mid 2 Plane
(Mid Horizontal Plane of the Liner), Plane 1 (Vertical
cross-sectional Plane before 25 mm from Dilution
Holes) and Plane 2 (Vertical cross-sectional Plane at
the exit of the Liner).
Results and Discussion
Flow Analysis in the Annulus
Velocity (Fig. 6a) decreases gradually as the flow
progresses through the pre-diffuser (magnitude of
velocity at inlet of pre-diffuser is 26 m/sec and at
outlet it is 19.42 m/sec). After pre-diffuser, flow
enters into the dump diffuser where velocity further
reduces due to sudden enlargement of flow area
resulting in the formation of wall recirculation zone.
Velocity profile is almost same for both the annuli
except that the magnitude is slightly higher in the
inner annuli, perhaps due to reduced flow area as
compared to the outer annuli. Fig. 6b shows the
velocity vectors in the mid vertical plane for better
visualization of the flow. Flow apart from entering the
liner from the annuli, it also enters the liner through
the coaxial jet configuration where flow is nearly
axial and hence more flow enters the liner through
this jet. Maximum velocity of air in the annular jet is
61.52 m/sec.
Velocity Profiles at Primary, Secondary and Dilution Holes
Magnitude of velocity in inner annular holes is
slightly higher than outer annular holes (Figs 7 & 8),
whereas nature of profiles is nearly identical for
primary and secondary holes. Maximum velocity
magnitude for primary and secondary inner liner holes
is 58.28 m/sec, whereas it is 55.04 m/sec for outer
annuli. Magnitude of velocity at dilution holes is
higher than that in primary and secondary holes
(61.52 m/sec for inner, 58.28 m/sec outer). Velocity
entering the liner through these holes is nearly
uniform.
Flow Analysis in the Liner
Flow analysis for the liner is carried out by feeding
the air velocities (130 m/sec) obtained through the
Fig. 6 — Vector contours and vector plot for the annulus at
central mid Plane: a) Vector contours; b) Vector plot
J SCI IND RES VOL 65 NOVEMBER 2006
928
Fig. 7 — Velocity profiles at inner wall holes: a) Primary hole;
b) Secondary hole; c) Dilution hole
Fig. 8 — Velocity profiles at outer wall holes: a) Primary hole;
b) Secondary hole; c) Dilution hole
SINGH et al: FLOW CHARACTERISTICS OF ANNULAR COMBUSTOR USING CFD
929
annular jet and various liner holes along with fuel in
the central jet.
Analysis of Velocity Field
Recirculation zone is formed just at the down
stream of the primary and secondary holes for both
inner and outer walls (Fig. 9). There is no formation
of reverse flow down stream of dilution holes on the
outer wall but a reverse flow is found near the dilution
holes on the inner wall, which suggests the need for
modification of liner geometry.
Fuel velocity coming out from the nozzle at the
mid 1 plane (Fig. 10a) has a magnitude of 130 m/sec.
Air coming through the annular jet to the liner has a
maximum velocity of 65.01 m/sec and this flow
spreads in the radial direction occupying whole space
without formation of recirculation zone. This is due to
the blockage effect created by the primary jets forcing
the annular jets and fuel jet to spread in radial
direction. Velocity of air fuel mixture suddenly
reduces as it enters the liner to a value of 71.51/m/sec
at the center of the liner and again decreases to a low
value of 6.50 m/sec in the radial direction (close to the
wall). Air entering the liner through primary,
secondary and dilution holes helps to achieve a better
air-fuel mixture with uniform velocity profile at the
exit of the liner. Velocity contours at Mid 2 plane
(Fig. 10b) clearly shows a faster spread rate for the
flow that may be results in combustion process
completeness. The value of velocity (78.02 m/sec) at
the exit of liner is nearly uniform with slightly higher
value in the center. In plane 1, effects of primary and
secondary holes are seen and flow becomes more
uniform in circumferential direction due to better
mixing of air fuel mixture (Fig. 10c). The velocity in
the central zone decreases gradually from 65.01 m/sec
to 6.50 m/sec. In plane 2, velocities are high at the
center, which reduce gradually away from the center
(Fig. 10d). The velocity contours are symmetrical and
uniform a desirable feature for improved performance
of gas turbine.
Analysis of Temperature Contours
At nozzle and annular jet, temperature of inlet air
and fuel was taken as 300 K. Temperature contours at
mid 1 plane (Fig. 11a) of the liner shows the reaction
rates to be quite slow in this region resulting in low
temperature rise. After the initial region, temperature
increases gradually from 300 K to a maximum value
of 1948 K in the central region due to efficient
combustion of air fuel mixture. High temperature
zone is around 1948 - 2039 K and forms as a circular
band around the central region as a result of complete
combustion (Fig. 11b). Air fuel mixture flows axially
after the reaction and it diffuses away from the center
to ensure entrainment of more air along the centerline
of the combustor thereby reducing the temperature in
this region. Temperature contours depict a spread in
the circumferential direction with the shape change
from circular to elliptical for Plane 1 (Fig. 11c). The
reaction also intensifies at the down stream of the
secondary jet, forcing high temperatures (2039 K) due
to better mixing of air and fuel. Outlet temperature
contours are more uniform and flatter in the central
region, which may result in better performance of
turbine stage. In this plane (Fig. 11d), contours are
wider and completely elliptical in shape.
Analysis of Species Concentration
Mass concentration of different species (CH4, O2,
CO2, NO and H2O) is analyzed for mid-1-Plane.
Mass fraction of CH4
Mass fraction of CH4 (Fig. 12a) decreases in axial
direction (0.99 at the inlet and 0.05 at the outlet of
liner) due to efficient mixing of fuel with air.
Inspection of contour levels also indicates that large
fraction of the fuel is consumed in the initial region of
the liner.
Mass fraction of O2
Mass fraction of O2 (Fig. 12b) also reduces in the
axial direction because it is also involved in
combustion process. At the inlet, O2 coming through
holes and swirler was around 23% whereas at the
outlet of the liner it was found to be only 1.1%.
Mass fraction of CO2
Mid 1 plane (Fig. 12c) shows that concentration of
CO2 increases in the axial direction (0 at the inlet and
around 13% at the exit). CO2 is generated as a by-
product of the chemical reaction. Mass fraction
contours are symmetric due to proper combustion and
mixing of jets.
Mass fraction of H2O
Mass fraction of H2O is almost (Fig. 12d) same as
CO2 as it is also generated as a by-product of chemical
reaction. Differences are only in magnitude (0% at
inlet and maximum value of 10.9% at exit).
Analysis of Pollutant
Pollution emission level from a combustor depends
upon the interaction between the physical and
chemical process and is strongly temperature
dependent. Dominant component of the pollutant is
J SCI IND RES VOL 65 NOVEMBER 2006
930
Fig. 9 — Vector plot at the Mid 1 plane of the liner
Fig. 10 — Vector contours at the different selected planes of the liner: a) Mid 1 plane; b) Mid 2 plane; c) Plane 1; d) Plane 2
SINGH et al: FLOW CHARACTERISTICS OF ANNULAR COMBUSTOR USING CFD
931
Fig. 11 — Temperature contours at the different selected planes in the liner: a) Mid 1 plane; b) Mid 2 plane; c) Plane 1; d) Plane 2
Fig. 12 — Mass fraction contours of different species in the liner at Mid 1 plane: a) Mass fraction of CH4; b) Mass fraction of O2
J SCI IND RES VOL 65 NOVEMBER 2006
932
Fig. 12 — Mass fraction contours of different species in the liner at Mid 1 plane: c) Mass fraction of CO2; d) Mass fraction of H2O;
e) Mass fraction of NO
SINGH et al: FLOW CHARACTERISTICS OF ANNULAR COMBUSTOR USING CFD
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nitrogen monoxide (NO) and evaluation of NO as
pollutant is based on thermal model. In thermal
model, temperature controlled oxidation of N2 leads to
formation of NO (Fig. 12e) whose emission level
changes with axial distance. As temperature increases
in the axial direction, oxidation of N2 increases
leading to increase in NO concentration (1e-06 at
inlet, 6e-06 at outlet).
Mass Split through Liner Hole
Mass split through dilution holes is found to be
maximum from both annulus spaces (Table 1). The
higher velocity through annulus passage deflects the
fluid core towards casing wall. Due to higher flow
momentum, fluid jumps the initial liner holes and
more fluid enters the liner through dilution holes.
It is also observed that the flow splits through primary
and secondary holes are nearly same for both inner
and outer annulus, whereas flow splits through
dilution holes is higher from the inner annulus
passage.
Conclusions An attempt has been made to simulate the
phenomenon of reacting three-dimensional turbulent
flow in the combustion chamber using CFD.
Methodology allows parameteric investigation for
optimizing the design of combustion chamber. Mass
splits of the total flow through the outer annuli and
inner annuli respectively have been found to be:
primary, 23.06, 22.51; secondary, 23.17, 22.93; and
dilution hole, 53.77, 54.56%. Recirculation zone
forms just downstream of the primary and secondary
holes at both the inner and outer wall. There is no
flow reversal at downstream of the dilution holes at
the outer wall; however, a large reverse flow is seen at
the inner wall. This phenomenon suggests the
necessity for modification of the liner shape. The flow
spreads uniformly in the axial direction and velocity
contours change from circular to elliptical shape in
the circumferential plane quantifying the spread rate.
The temperature contours are circumferentially more
uniform and symmetric. Temperature was found to be
maximum at the outlet of the liner. The mass fractions
of CH4 and O2 decrease whereas concentration of CO2
and H2O increases as combustion products move from
the inlet to the outlet.
References
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Table 1 — Mass split through different liners holes
Liner holes Inner holes
kg/sec
Outer holes
kg/sec
Primary 0.0212 0.0217
Secondary 0.0216 0.0218
Dilution 0.0514 0.0506
J SCI IND RES VOL 65 NOVEMBER 2006
934
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Nomenclature
C1ε, C2ε, Cµ,σk, σε Constants of turbulence model
Gk Generation term (kinetic energy)
K Turbulent kinetic energy
M Number of dependent variable
P Static pressure
Ri Sum residual for a dependent variable
Sm Mass added to the continuous phase
SNφ Normalizing factor
Tij Stress Tensor
Uavi Mass average inlet velocity
u Mean velocity
u’ Velocity perturbation
ub Bulk velocity
V Cell volume
Vf Mass flux (velocity) through the face
X Longitudinal coordinate
α Under relaxation factor
ε Turbulence dessipation rate
ρ Density of fluid
µ Dynamic viscosity
µt Turbulence viscosity (Eddy viscosity)
ν Kinematic viscosity
Subscript
i,j Indices of tensorial notation as 1,2,3