Proceedings of the 5th European Combustion Meeting (2011)

1. Effect of Pilot Flame Instabilities on Pressure in Case of Assisted Combustion
D. Scarpa1
, C. Mongiello1
, N. Verdone2
, G. Calchetti1
, A. Di Nardo1
, M. Rufoloni1
1
Italian National Agency for New Technologies, Energy and Sustainable Economic Development – Italy
2
Sapienza Università di Roma - Dipartimento Ingegneria Chimica Materiali Ambiente – Italy
Abstract
Considering a particular gas turbine plant, were examined, after a thorough analysis of the characteristics of these
types of systems, problems relating to the phenomena of combustion burner fueled by natural gas. In particular,
attention was focused on a mixer body in order to identify the parameters of the premixed flow and its distribution
on the surface. This resulted in a series of parametric maps which have called the functional level of the plant. In
particular, it was received by the definition of an additional convergence criterion for the transport of passive scalar
in the context of the study of the mixing process. This information formed the basis for going to model the
phenomenon of interaction between the premixed flow and the pilot, the latter seen as a heat load to be delivered to
the ignition. With regard to operation with natural gas, is as interesting as what it has come from the characterization
of the pilot who, as a stabilizer of the process, results in a postponement of the place of full power, moving forward
the point of maximum temperature and ensuring, therefore, more safely. The findings can be found in the
comparison with experimental data from tests performed at ENEA. Such a simplified approach has allowed us to
come to determine the fluctuations of pressure, because it concerns orders of magnitude within the combustor.
Introduction
In modern power plants producing electricity is
obtained from the conversion of that provided by a
mechanical prime mover, usually consisting of a gas
turbine (TG), which is driven by the effluent of a
combustion chamber. In these machines are often
used gaseous fuel, generally natural gas (NG). As the
year goes on a wide range of load, the performance of
the plants are highly subject to operating conditions
and / or operation. Also comply with the laws on that
are becoming more restrictive, limiting further the
possibilities of use for this reason, choosing a certain
type of food for each fuel also requires very specific
performance requirements relating to that aspect of
'environmental impact. However, to date, the turbines
operate primarily with fossil fuels such because they
represent, for now, the only way technologically
feasible. This implies the need to try to optimize the
operation by implementing all the measures aimed at
making environmentally friendly performance, at
least until we will have other sources of energy can
be controlled accurately. Recently the company TG
manufacturers have started to operate with a view to
analyze carefully the phenomena that take place
inside the combustion chambers, preparing solutions
essentially little different from each other, in some
cases differing only in certain component parts, using
CFD design of the machine. In fact, compared to the
classic situation of a fuel spray, where it was in the
case of a liquid sprayed directly into the combustion
air flow, in turn divided into primary and secondary
air, is passed to the combustion chambers in which
the combustion air and fuel are completely mixed
together through the unique components of the
vortex, and then, by interaction with a properly sized
pilot in order to initiate the process chemical
reactions occur in as much as possible exothermic.
Ultimately, because of what was essential to
concentrate attention on the role taken by both the
diffusion flames, premixed that those who by their
mutual interaction in order to analyze the operation
and search for possible optimization of a plant with
similar characteristics, namely the TG Ansaldo-
Siemens V64.3A. The TG V64.3A is the smallest
turbine developed by Ansaldo and Siemens, the
technique of combining both companies, and
presented in January 1995. The characteristics of the
turbine are shown in Table 1.
Table 1. Characteristics of TG Ansaldo - Siemens
V64.3A.
Inlet Temperature [°C] 1190
Power [MW] 68
Operating Pressure [bar] 16.6
Exhaust Gas Flow [kg/s] 192
Exhaust Gas Temperature [°C] 590
Efficiency [%] 36.8
Emissions of NOx [ppm] 25
The innovative feature of V64.3A is the annular
combustion chamber, which consists of a single
flame tube contained between two circular enclosures
(Figure 1).
Figure 1. V64.3A the annular combustion chamber.
The combustion chamber of V64.3A has 24
burners arranged in a ring (one every 15°). Each
burner is equipped with an injector driver, operating a
diffusion flame, and most of the main fuel injectors,

2. 2
with a flue for the preparation of the air - fuel and
diagonal of a vortex (Figure 2).
Figure 2. Burner V64.3A.
In particular, the diagonal vortex, which has an
outer diameter of 35 cm and an axial length of 7.5
cm, consists of a row of 18 blades warped (one every
20 °) in which the incoming air stream is forced to
develop in a helical motion that helps to create a
mixing with fuel leaking from 10 equally spaced
holes, 1 mm in diameter, placed 5 on the intrados and
5 on the extrados of each pallet (Figure 3).
Figure 3. View of the nozzle vanes.
The disadvantages of this configuration are
many, in fact, although you can use more fuel jets,
get a uniform distribution of the air - fuel and
temperature is very difficult.
Specific Objectives
As mentioned, given a certain amount of mixing
ratio refers to the flow of incoming gas, there is no
guarantee that the resulting premixed flow conditions
occurring in a plug at each point output of the mixer.
Even less likely is that, in the area of interaction with
the pilot, where this is feasible, as it is also easy to
verify experimentally. Indeed it may happen that the
ratio of local mixing is much deviated from that
theory to mixtures is shown placing either poorer or
richer, all depending on the geometry of the mixer is
that the operating conditions and / or operation of the
TG . It is known that a pronounced non-uniformity of
the mixing ratio of the premixed flow profile is not
exactly an optimal situation for the operation of the
combustion chamber: while the points corresponding
to rich mixtures involving the possibility of formation
of hot spots (hot spots) with heat stress resulting in a
marked tendency to local corrosion (hot corrosion) to
the combustion chamber, the points correspond to
lean mixtures, however, are more difficult areas of
combustion, and congenital problems with the
ignition and complete combustion in order to reduce
the opportunities for training. It follows, therefore,
that a correct characterization of the geometry of the
equidistribution of blender influence the ratio of local
mixing as well as its ability to create turbulent fields
influence how spatial and thermo-fluid dynamics of
ignition of combustible gas mixture within the flow
combustion air. To this must be added the
consideration that the objective of seeking an optimal
equidistribution be achieved not only with respect to
the design conditions of the machine, but even at
partial loads. Therefore, starting from the fact that the
vortices form the first component on which action
can be taken to approach the problem assigned, it is
estimated that more appropriate to perform a
numerical simulation of transport phenomena of
diffusion and mixing of NG in air current in the
vortex present upstream of the combustion chamber
of V64.3A, assuming as an object of any geometric
changes. So if they have reconstructed the geometry
using GAMBIT (Figure 4).
Figure 4. Grid computing.
The computing grid consists of cells obtained
with tetrahedral volume sizes ranging from minimum
to maximum. The computer code used for numerical
modeling of the problem was that the FLUENT has
allowed us to represent the phenomenon in question
by setting the problem that is confirmed by
experimental results obtained from tests conducted at
the facility COMET-HP ENEA.

3. 3
Results and Discussion
They are then shown below the distributions of
temperature and concentration camps of the most
important species (air and methane) on the face of the
mixer output for a generic loading condition. As
shown in Figure 5, the higher temperatures (around
680 K) are spread outward to gradually get up to 640
K in the center of the face.
Figure 5. Temperature distribution on the face of the
mixer output.
The average temperature is 664 K. The
maximum temperature of 680 K, however, will be
taken into account to verify that there are phenomena
of ignition. The output section of the mixer has a
strong concentration difference between air and gas
from the fuel (Figure 6(a) and (b)). As for gas fuels,
there are areas of high concentration in
correspondence to each of the 18 blades through
know which flows into the vortex diagonal and
towards the center of the output section.
(a)
(b)
Figure 6. Distribution of the concentration of CH4 (a)
and air (b) on the face of the mixer output.
By contrast, the combustion air is distributed
toward the outer surface. This can be explained since
the combustion, having a higher molecular weight
undergoes a greater effect of swirl imparted by the
impeller. In order to better assess the efficiency of I-
scelazione implemented has investigated the
distribution of the mixing ratio of surface area on the
local output of the mixer, ie the value taken by this
parameter in each grid cell. This survey was
conducted for three different loading conditions, with
100% pre-mixed operation, reported in the Technical
Annex supplied by Ansaldo Energia. The operating
conditions of such loading conditions are listed in
Table 2.
Table 2. Loading conditions tested for the burner
V64.3A and related conditions.
Load 1 2 3
Qa [kg/s] 0.323 0.452 0.645
Qf [kg/s] 0.010 0.014 0.020
αeff [-] 32.30 32.29 32.25
Ta [K] 681 681 681
ρa [kg/m3
] 8.4 8.4 8.4
pa [Pa] 101325 101325 101325
Tf [K] 290 290 290
ρf [kg/m3
] 12.28 12.28 12.28
Vf [m/s] 5.76 8.07 11.53
They are crucial because they represent the
initial values of thermo-fluid dynamics variables of
interest. The diagrams on the distribution of the air -
fuel for the three loading conditions are shown in
Figure 7. The evidence shows that there is no single
report were designed to achieve excellent mixing and
combustion in terms of thermodynamic point of view
of pollutant emissions. They also indicate the overall
value of α (αg), the actual one working of the
machine (αeff), the stoichiometric one (αs), the
statistical mean one (αms), the highest percentage one
(αmp) and the interval of significant variation of α (9
≤ α ≤ 51) in which focused attention.

4. 4
Figure 7. Percentage distribution diagrams of the
local value of the air - fuel.
A reworking of these diagrams was made only to
a more "general" of the data obtained for all three
conditions ac-rico, thus giving rise to the diagram
shown in Figure 8. The analysis of this diagram, in
particular, shows how mixing worse with increasing
load. In general, however, the behavior of the mixer
can not be defined nor fully functional compared to
the theoretical conditions of the project, nor
acceptable in the operating conditions of the unit of
electricity production. The optimization of the
distribution curve should be sought through detailed
analysis of the combustion chamber-ta, meaning they
interact vo-lume the outgoing flows from the I-
preselecting mixing with the pilot set up in the
middle of the latter.
Figure 8. Overlapping stability curves for the three
loading conditions analyzed.
It then becomes central to consider both the
influence of the parameters characterizing the flow
that the size of the premixed flame pilot operated in
the pressure-stoichiometric because, as it is that it
ensures stable operation of the combustor, such as the
minimum to maximum load.
Conclusions
The work showed a worsening in the functioning
of the increasing load. Of course, with a view to
resetting the geometry in order to optimize the
reaction process in the case of conventional fuel, it
would seem appropriate to take action by making
changes to the apparatus of injection as a lower
vorticity at the entrance to allow the flame front to
stretch without determine instability caused by
temperature gradients or flows too intense viscous
dissipation that Deplete the enthalpic content of the
current. Also a possible and very sustainable change
in input parameters (speed and swirling) could
provide a profile of the heat transfer as much oriented
toward the adjacent stream or with reduced thermal
characteristics of major importance. The study has
achieved its objective which was to analyze the vo-
operation and identification of possible guidelines ot-
victimization of TG Ansaldo - Siemens V64.3A.
Beyond the known problem of a purely fluid dynamic
of the "double infinity", and limitations related to the
characterization of the pilot, the lack of empirical
data has limited the ability to discriminate in the
choice of the kinetic scheme that is more efficient,
given the instability numerical re-encountered for
complex kinetic schemes. However, this is absolutely
neces-sary discriminating because of the different
behavior of premixed flows under varying kinetic
scheme.
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5. 5
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