Proceedings of ASME Turbo Expo 2004
Power for Land, Sea, and Air
June 14–17, 2004, Vienna, Austria
GT2004-53112
CFD Simulation of the Flow Within and Downstream of a HighSwirl Lean Premixed Gas Turbine Combustor
Mark D Turrell and Philip J Stopford
ANSYS CFX
The Gemini Building, Harwell IBC, Fermi Ave.
Didcot, Oxfordshire OX11 0QR, UK
and
Khawar J Syed and Eoghan Buchanan
Demag Delaval Industrial Turbomachinery Ltd.,
PO Box 1,
Lincoln LN5 7FD UK
ABSTRACT
CFD analysis of the flow within a high-swirl lean
premixed gas turbine combustor and over the 1st row nozzle
guide vanes is presented. The focus of the investigation is the
fluid dynamics at the combustor/turbine interface and its impact
on the turbine.
For the configuration in question, temperature indicating
paint observations of the nozzle guide vanes, acquired during
engine development tests, show features consistent with the
presence of a highly rotating vortex core emerging from the
combustor.
The configuration was modelled by a fully compressible
reacting CFD analysis, whose domain stretched from the exit of
the combustor swirl generator to downstream of the 1st row
nozzle guide vanes. The CFD analysis, when using a Reynolds
stress turbulence model, predicted a highly rotating vortex core.
The predicted interaction between the core and the nozzle guide
vanes were consistent with the temperature indicating paint
observations. The interaction is dominated by the vortex core
being attracted to the locus of lowest static pressure.
INTRODUCTION
Turbine design processes have evolved over decades and
are capable of yielding very good preliminary designs, due to
the large amount of empirical input and experience that they
embody. As input, the design process requires key cycle
performance data, design constraints and combustor exit
conditions. In the case of the latter, typically the combustor exit
temperature pattern, in terms of the OTDF and RTDF, is
required. The empiricism, which underpins the turbine design
process, has been established largely upon experience gained
on conventional non-premixed combustor applications. Lean
premixed dry low emissions (DLE) combustors, however,
exhibit exit flows that can be substantially different from nonpremixed combustor designs, due to a significantly greater
degree of swirl acquired by the flow. High swirl is required
given that flow reversal through vortex breakdown is the
favoured method for flame stabilisation. Additionally, a large
proportion of the combustor air is swirled in order to ensure a
lean premixed reaction zone. An example of such a combustor
is the DLE combustor of Demag Delaval Industrial
Turbomachinery (the company) [1-3] which is the subject of
the present paper.
Swirling flows can exhibit several phenomena that are of
interest to combustor and turbine design, e.g. precessing vortex
cores, vortex breakdown and sub/super critical vortex cores and
their transition [4]. With respect to combustor exit flows, the
characteristics of the vortex core are of most significance.
There have been many experimental, numerical and theoretical
studies that have looked into the nature of the vortex core [4].
The numerical and analytical studies have employed
simplifications, e.g. axi-symmetry, inviscid flow, low Reynolds
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Copyright © 2004 by ASME
number and constant density. Though some studies have been
extended to include phenomena such as compressibility [5],
they still fall short of addressing all the complexities of
practical combustor flows. Practical systems exhibit flows of
high Reynolds number and therefore, for practical reasons,
require some modelling of the turbulence as well as the heat
release process. Issues which affect the vortex core state are the
inlet velocity and pressure distribution, turbulence, the heat
release process and the resultant density field. CFD offers a
way of predicting the vortex core state, however, processes
requiring modelling can have a substantial impact on the
results, as does the accuracy of the numerical solution.
THE DLE COMBUSTOR
The company’s dual fuel DLE can-type combustion
technology is applied across its range of small industrial gas
turbines from the Typhoon at ca. 5MW to the Cyclone at
13.4MW [1-3]. In order to cover this power range, rather than
having a single combustor design and applying different
numbers of combustors to each engine type, the combustors
have been scaled. The Typhoon, Tempest and Cyclone have 6
combustors per engine and the Tornado has 8.
The company’s dual fuel DLE combustor is a device that
benefits from a high degree of swirl in terms of robust low
emissions combustion. In fact, the factors which limit the
degree of swirl are turbine rather than combustor
characteristics. For high levels of swirl, complex flow patterns
can exist at turbine entry rendering the established turbine
design process deficient.
Engine development tests on the 13.4MW Cyclone engine
[6], suggested the possible existence of a highly rotating vortex
core, generated within the combustor, which interacts with the
1st row NGVs. Whether or not such a vortex core exists, and the
understanding of the implications that arise from it, are
therefore of interest.
CFD analysis has an important role to play in both
understanding the full ramifications of combustor exit flow on
the turbine and in establishing combustor designs that are
acceptable to the turbine. This however can only be realised if
the CFD analysis is capable of predicting sufficiently
accurately the key fluid dynamic features.
The focus of the present paper is the validation of CFD
analysis applied to the combustor/turbine interface in the case
of a high-swirl lean premixed DLE combustor. The flow within
the combustor and over the 1st row NGVs of the Cyclone
machine is computed and the results are evaluated in terms of
their consistency with temperature indicating paint observations
obtained during the engine development tests.
Main Burner
Pilot Burner
PreChamber
Radial Swirler
Figure 1 : The Dry Low Emission combustor construction.
The combustor consists of three main sections (figure 1):
[i] the pilot body, which houses the pilot fuel galleries and
injectors for both gaseous and liquid fuel, [ii] the main burner,
which houses the main air swirler and main gas and liquid fuel
systems and [iii] the combustor, which includes a narrow inlet
duct, called the prechamber. The combustor is of a double skin
construction and is cooled through impingement cooling. The
cooling air is then either exhausted into the combustor through
dilution holes downstream of the main reaction zone, or enters
the combustor uniformly over the inner liner through effusion
cooling holes. The combustor considered in the present work is
the effusion-cooled variant. A transition duct, located
downstream of the combustor, conditions the flow from the
circular combustor exit to a sector of the turbine entry annulus.
In the next section, the company’s DLE system is
described. The temperature indicating paint results acquired
during engine development tests are then presented.
Subsequently the CFD model and procedure is described.
Finally the CFD results are presented and evaluated.
NOMENCLATURE
CFD
DLE
NGV
OTDF
RTDF
SSG
Computational fluid dynamics
Dry low emissions
Nozzle guide vane
Overall temperature distribution factor
Radial temperature distribution factor
Differential Reynolds stress model of
Speziale, Sarkar and Gatski
Double Skin Impingement
Cooled Combustor
Combustion air
Main fuel
Reaction supported by
internal and external
recirculation zones
Pilot fuel
Combustion air
Figure 2 : Schematic of the dry low emissions combustor
concept.
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Copyright © 2004 by ASME
Figure 2 shows a schematic of the combustion concept.
The main combustion air enters through a single radial swirler
at the head of the combustor. The flow then turns through a
right angle into the prechamber followed by a sudden
expansion into the combustion chamber. The swirl number is
sufficiently high to induce a vortex breakdown reverse flow
zone along the axis. This is termed the internal reverse flow
zone. In the concept, this reverse flow zone remains attached to
the back surface of the combustor, thereby establishing a firm
aerodynamic base for flame stabilisation. In the wake of the
sudden expansion, an external reverse flow zone is established.
The flame is stabilised in the shear layers around the internal
and external reverse flow zones.
seems plausible that it would be drawn towards the suction side
of the central NGV, given the low static pressure in this region.
Further, given that the combustor has a clockwise swirl
(looking downstream) the deflection of the leading edge and
hub platform cooling flows are consistent with this assumption.
Example of central nozzle
guide vane
The fuel, both gas and liquid, is introduced in two stages:
the main, which results in a high degree of premixedness and
hence low NOx emissions, and the pilot, which is steadily
increased as the load demand decreases in order to ensure flame
stability. The pilot is arranged, such that as the pilot fuel split
increases, the fuel is biased towards the axis of the combustor.
Coolant tracks
do not remain
attached to
suction side
surface
Unlike concepts that involve a free-standing vortex
breakdown, such as the EV burner of ALSTOM Power [7], the
present combustion concept can benefit from very high swirl
numbers, which allows very firm aerodynamics for flame
stabilisation and is thus more robust against combustor
dynamics. The maximum swirl number that can be allowed is
governed by the characteristics of the vortex core. If a highly
rotating, axially-accelerating vortex core is produced, such as
identified in the experiments of Escudier and Keller [8], part
load emissions and turbine performance problems can arise.
High
temperature
Sweeping down
of coolant tracks
Example of non-central
nozzle guide vane
OBSERVATIONS OF THE NOZZLE GUIDE VANES
During the Cyclone development programme, temperature
indicating paint tests were performed to verify the temperatures
of the hot components. Observations of the 1st row NGVs
indicated high temperatures on the six vanes having their
leading edge closest to the axis of each of the six combustors.
Figure 3 shows an example of one of these vanes (termed a
central NGV) and, for comparison, an example of a non-central
NGV. The figure shows high temperatures on the suction side
of the central vane and at the hub platform immediately
downstream of this. A degree of flow visualisation is offered by
the leading edge film cooling, whose tracks can clearly be seen
on the temperature indicating paint. The tracks on the suction
side of the central vane can be seen to be deflected downwards
and seem to be rapidly detached from the surface of the vane.
Figure 4 shows the tracks of film cooling at the hub
platform in the region of the suction surface of the central vane
and its neighbours. The coolant associated with the neighbours
can be seen to follow the direction of the nozzle guide vanes,
whilst the coolant downstream of the suction side of the central
vane moves in a trajectory away from the suction surface.
The observations seem to be consistent with the presence
of a highly rotating vortex core, i.e. a Rankine vortex, such as
that observed in the experiments of Escudier and Keller [8], for
a sub-critical vortex core. If such a flow feature is present, it
Figure 3 : Temperature indicating paint results for the 1st Stage
NGVs. View of leading edge of central and non-central NGVs.
In view of these observations, the present CFD analysis
was commissioned to determine whether fluid dynamics
consistent with the NGV observations could be predicted. The
CFD analysis could then be applied to aid either the control of
the combustor fluid dynamics or to aid the better design of the
turbine to accept the combustor exit flow. The present paper is
concerned with the first issue of determining whether CFD
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Copyright © 2004 by ASME
analysis is capable of predicting features consistent with the
temperature indicating paint results.
for the reactions: CH4 + 3/2O2 → CO + 2H2O and CO + 1/2O2
→ CO2.
Central differencing has been applied to the diffusion terms
of all transport equations. Second order upwind differencing
has been applied to the convection terms. In the case of
bounded variables, however, first order upwinding has been
applied, when physical bounds would otherwise be violated.
Transient calculations were performed for the combustor,
for which the second order backward Euler scheme was used
with a time step of 0.4 msec.
The number of mesh nodes in the combustor model was
458,000 nodes (1,973,000 elements). The analysis was run for
26.0msec, with a time step of 0.4 msec and took a total of 330
hours CPU (130 hours wall clock time) on three Pentium IV 1.7
GHz processors with 3.6 Gb of RAM.
cen
tra
l
van
e
Figure 4 : Temperature indicating paint results for the 1st Stage
NGVs. View of trailing edge hub platform cooling.
FLUID DYNAMICS AND NUMERICAL MODELLING
The numerical model was based on the commercial
Computational Fluid Dynamics (CFD) software, CFX-5 [9].
Given that the domain of interest spanned both the combustor
and the NGVs, a reacting, fully compressible formulation was
adopted.
Grotjans et al. [10] have compared the predictions of
various turbulence models with laser Doppler anemometry
measurements for the tangential velocity profile in an industrial
hydrocyclone. The results show that the flow in the cyclone is a
strong Rankine-type vortex with a potential vortex at large
radius and solid body rotation near the axis. Only the Reynolds
stress model of Speziale, Sarkar and Gatski (SSG) [11]
reproduced this characteristic behaviour. In particular, the k-ε
model, even with curvature correction, failed to predict the
potential vortex and gives solid body rotation at all radii. While
demonstrating the importance of using a Reynolds stress model
for a highly swirled vortex flow, it is also apparent that the SSG
model is better able to capture the strong shear gradients near
the centre of the vortex. The SSG model has therefore been
adopted for the present work.
Mixing and reaction of the oxidant and fuel (assumed to be
methane) are simulated by the eddy dissipation model of
Magnussen and Hjertager [12]. In this model, it is assumed that
the rate of turbulent combustion is the minimum of the
turbulent mixing rate of the reactants and products and the
chemical kinetic rate. The CH4 oxidation reaction is represented
by a two-step global mechanism of Westbrook and Dryer [13]
GEOMETRY AND BOUNDARY CONDITIONS
Figure 5 shows the combustor/NGV arrangement. At the
head of the combustor is the radial swirler, through which the
main combustion air enters. The flow within each of the swirler
channels is complex, exhibiting secondary flows which
influences the development of the swirl, as well as the fuel/air
mixing. In order to resolve the flow details, a very fine
computational grid is required for each of the swirler channels,
making a single domain calculation, extending from upstream
of the combustor to downstream of the NGVs unfeasible. The
domain was therefore represented by two computational
domains. The first extended from upstream of the swirler and
considered only a single swirler channel, utilising suitably
defined periodic boundaries. The domain included a sector of
the combustor with an axi-symmetric contraction at its exit.
This computation was used to determine the flow properties at
the exit of the swirler channel. These were then used as
upstream boundary conditions for the full combustor
calculation, which extends from the swirler exit to downstream
of the NGVs. The inlet boundary condition was mapped round
to represent the exit flow from each of the swirler channels.
Swirler
Combustor
Transition
duct
Nozzle
guide
vanes
Figure 5 : Geometry of the computational domain.
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Copyright © 2004 by ASME
The effusion cooling holes were not represented in the
computational grid, but account was taken of the effusion air by
introducing a uniform mass flux over the combustor surface.
Figure 6 shows the surface mesh for the combustor/NGV
domain, with a close-up view of the mesh in the region of the
NGVs.
Figure 8 shows the temperature at a plane through the
centre of the combustor and an iso-surface of relatively high
vorticity. The latter is high in boundary layers and shear layers,
but can also be high in the central part of a Rankine-type vortex
which exhibits a solid body rotation. Moving away from the
centre of the vortex, the vorticity drops, given that the outer
potential vortex is irrotational. A highly rotating vortex core is
indicated along the axis of the combustor.
Temperature
contours
Figure 6 : Surface mesh for CFD analysis.
RESULTS
Figure 7 shows a vector plot of the combustor front end
extracted at an arbitrary time step. The velocity vectors are
coloured with the component of velocity in the direction of the
combustor axis. The transient nature of the flow is clearly seen.
The most significant transient feature apparent within the
computations is a precessing vortex core.
Highly rotating
vortex core
indicated by
high vorticity
magnitude
Figure 8 : Temperature contours at a diametral plane through
the combustor and an iso-surface of relatively high vorticity.
The above vortex core characteristic is predicted using the
SSG Reynolds stress model, but is not present within a kepsilon model solution. This is illustrated in figure 9 where the
tangential velocity magnitude is shown. The SSG model results
in significantly greater values, especially immediately upstream
of the NGVs.
Normalised
velocity
1
k-ε model
0.8
0.6
0.4
Reynolds stress
model
0.2
0
Figure 7 : Vector plot at a diametral plane at the head of the
combustor. Vectors are coloured with the velocity component
in the direction of the combustor axis.
Figure 9 : Normalised tangential velocities at a diametral plane
through the combustor. Comparison between k-ε and Reynolds
stress turbulence models.
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Copyright © 2004 by ASME
Figure 10 shows the secondary flow velocity vectors just
upstream of the turbine. The vortex core can clearly be
identified. The vectors also show the impact of the global swirl
and the flaring out effect induced by the transition duct profile.
Figure 10 : Velocity vectors at the transition duct exit. Vectors
are coloured with the vorticity component in the direction of
the engine axis.
The progress of the vortex core through the NGVs is
shown in figures 11 and 12, where the vortex core is visualised
by a surface of relatively high vorticity. Figure 12 shows that
the vortex core is directed towards the leading edge of the
central NGV. However, as expected, it is attracted by the low
pressure over the suction side of this vane. A second rotation is
set up near the hub over the pressure surface, due to a large
variation in incidence angle with span induced by the vortex
core. The vortex core passes the leading edge of the vanes at
approximately 40% span, but, as seen in figure 13, migrates
towards the hub through the NGV passage. This migration
appears to be due to the core being attracted by the locus of
lowest static pressure and an interaction between the vortex
core and the secondary flows set up within the NGV passage.
tral
Cen
N GV
Figure 12 : View of the vortex core over suction surface of the
central NGV. The vortex core is visualised by an iso-surface of
relatively high vorticity magnitude.
Peak gas
temperature
Central NGV
C en
tra
NGV l
Figure 13 : Upper picture: CFD results of the wall shear stress
on the suction surface of the central NGV and its neighbours.
The vortex core is visualised by an iso-surface of relatively
high gas temperature. Lower picture: Temperature indicating
paint observation of the suction surface of the central NGV.
Figure 11 : View of the vortex core approaching the leading
edge of the central NGV. The vortex core is visualised by an
iso-surface of relatively high vorticity magnitude.
Figure 13 shows a surface of high gas temperature in the
region of the NGVs. Given the nature of the temperature
distribution within the combustor, the highest temperatures are
located in the vortex core. The figure also shows the wall shear
stress over the NGVs. The wall shear stress, and therefore the
wall heat transfer coefficient, over the suction surface of the
central NGV is significantly greater than that over its
neighbours. The central NGV would therefore experience a
higher heat loading over the suction surface, because of higher
temperatures associated with the vortex core and enhanced heat
transfer coefficient induced by the core.
The above CFD results are consistent with the temperature
indicating paint observations depicted in figures 3, 4 and 13.
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Copyright © 2004 by ASME
The high temperatures over the suction surface of the central
NGV are due to the presence of a highly rotating vortex core.
High temperatures on the hub platform in the neighbourhood of
the trailing edge are due to the migration of the vortex core
towards the hub.
The CFD results presented in figures 7 to 13 are taken
from a single time step of the transient solution. However, with
respect to the interaction between the vortex core and the
NGVs, there is little variation in time as shown in figure 14.
The figure shows vorticity at a plane just downstream of the
leading edge of the NGVs. Results are presented at different
time steps, which represent the most extreme motion of the
vortex core. Though there is variation in the magnitude of the
vorticity and in its location, the vortex core is seen to be
confined to a region close to the suction surface of the central
NGV.
5.6 msec
7.6 msec
CONCLUSIONS
Development testing of the 13.4MW Cyclone gas turbine,
revealed high temperatures over the suction surface of the six
NGVs which have their leading edge closest to the axis of each
of the six combustors. High temperatures were also evident on
the hub platform immediately downstream of these vanes.
Observations of the trajectories of cool air emerging from
leading edge and hub platform film holes show the cooling air
to be deflected from the normal gas path. The observations
indicate the possible presence of a highly rotating vortex core
emerging from each of the combustors.
CFD analysis of the flow within the combustor and over the
NGVs, utilising the SSG Reynolds stress model, does indeed
predict a highly rotating vortex core. The core is seen to be
drawn over the suction surface, being attracted by the low static
pressure in this region. The core is at approximately 40% span
at the leading edge of the NGV, but migrates towards the hub
through the NGV passage. This is due to it following the locus
of lowest static pressure and due to the interaction between the
core and the secondary flows within the NGV passage. The
computed vortex core characteristics are in agreement with the
high metal temperatures and coolant paths observed during the
engine development tests.
REFERENCES
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Development of a Dry Low NOx Combustion System for the
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11.6 msec
14.4 msec
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Figure 14 : Vorticity at a plane just downstream of the leading
edge of the nozzle guide vanes. The middle of the three vanes is
the central NGV.
[7] Aigner M., Engelbrecht E.G., Eroglu A., Hellat, J. and Syed
K.J., 1999, “Development of an oil injection system optimised
to the ABB double cone burner”, ASME paper 99-GT-218.
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