Proceedings of TURBO EXPO 2006:
Proceedings of GT2006
ASME Turbo Expo 2006 – Power for Land, Sea & Air
ASME Turbo Expo 2006: Power
for Land, Sea and Air
May 8-11, 2006, Barcellona, Spain
May 8-11, 2006, Barcelona, Spain
GT2006-90905
GT2006-90905
HEAVY DUTY GAS TURBINE SIMULATION: GLOBAL PERFORMANCES
ESTIMATION AND SECONDARY AIR SYSTEM MODIFICATIONS
Carlo Carcasci, Bruno Facchini, Stefano Gori
DE: "S.Stecco" Dipartimento di Energetica
University of Florence
Via Santa Marta, 3
50139 Florence (I)
Luca Bozzi, Stefano Traverso
Ansaldo Energia
Via N. Lorenzi, 8
16152 Genova (I)
c.carcasci@ing.unifi.it
Stefano.Traverso@aen.ansaldo.it
Δ
ABSTRACT
This paper reviews a modular-structured program ESMS
(Energy System Modular Simulation) for the simulation of aircooled gas turbines cycles, including the calculation of the
secondary air system.
The program has been tested for the Ansaldo Energia gas
turbine V94.3A, which is one of the more advanced models in
the family Vx4.3A with a rated power of 270 MW. V94.3A
cooling system has been modeled with SASAC (Secondary Air
System Ansaldo Code), the Ansaldo code used to predict the
structure of the flow through the internal air system.
The objective of the work was to investigate the tuning of
the analytical program on the basis of the data from design and
performance codes in use at Ansaldo Energy Gas Turbine
Department.
The results, both at base load over different ambient
conditions and in critical off-design operating points (fullspeed-no-load and minimum-load), have been compared with
APC (Ansaldo Performance Code) and confirmed by field data.
The coupled analysis of cycle and cooling network shows
interesting evaluations for components life estimation and
reliability during off-design operating conditions.
= Deviation to Base Load ISO 15iC condition
Acronym
APC
CAC
FSNL
GT
VIGV
SAS
= Ansaldo Performance Code
= Cycle Analysis Code
= Full Speed No Load
= Gas Turbine
= Variable Inlet Guide Vane Compressor
= Secondary Air System
Subscript
Amb
exh
GT
tot
=
=
=
=
Ambient
Exhaust
Gas Turbine
total
INTRODUCTION
The energy market development in the last decade has been
influenced by several driving factors. Rising of power demand,
liberalization and emerging environmental regulations
increased the competition and the need for technology
innovation of gas turbines operating in combined cycle.
Modern heavy-duty gas turbines operate with high turbine
inlet temperature requiring complex secondary air systems to
ensure that turbine blades and vanes are supplied with the
necessary amount of cooling air.
Improvements of the secondary air system are basic
elements to increase reliability of such critical components.
Cooling air feeding allows controlling the metal temperature of
all critical parts along the hot gas path (turbine blading, rotor
disks, vane carrier, etc.). The material temperature modifies the
creep behavior and the number of allowable load cycles.
NOMENCLATURE
&
= mass flow rate (kg/s)
m
p
= Pressure
T
= Temperature (K)
W
= Power (MW)
Greek Symbols
β
= Compressor pressure ratio
1
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As a matter of fact a complete integration of all simulation
codes is, practically, impossible, but the coupling of CACs and
SAS codes meets several designers demands, in terms of
energy system control at each load condition, guarantees
reliability and allows a reliable performance prediction.
Generally, already cited CACs do not allow the coupled
analysis with SAS. On the other hand, various examples of
SAS simulators are reported in the literature (e.g. [1] and [3])
and some software house pushes the capabilities of own fluid
network solver to model the cooling system of gas turbines [4].
Everywhere, few works in literature deals with the proper
methodologies to solve the interactions between CAC and SAS
codes, considering the fact that boundary conditions of each
code (CAC/SAS) are the output results of the other one.
In this paper the coupling of energy system simulation
software, the ESMS code, with the SASAC (the Ansaldo code
for the SAS simulation) is presented. ESMS is an in-house
code, developed at the Dipartimento di Energetica “Sergio
Stecco” of University of Florence [5] and it is able to perform
thermo-dynamical and design/off-design simulations of any
energy system based on gas turbine engine. ESMS predicts
main gas turbine components behaviour and matching, on the
base of simplified models for multi-stage compressor and
turbine. A detailed comparison with standard APC is planned to
allow ESMS validation and tuning for the V94.3A gas turbine
model.
Both codes are easily modifiable by the authors, enhancing
the possibility of coupling. The cooling system has been
modelled by implementing in ESMS the SAS characteristic
curves obtained by SASAC.
The coupled ESMS/SASAC gas turbine simulation at
partial load conditions and/or extreme ambient conditions
enables the estimation of off-design performances of main gas
turbine components but, also, a detailed prediction of realistic
behaviour of secondary air system and blade cooling.
On the other side, because of the enormous thermal energy
passing through the turbine cascades, it can be removed only
by a correspondingly large cooling air mass flow. Such airflow
does not pass through the combustion chamber; as a result the
specific work decreases with the increasing of cooling air
requirement, as well as the overall thermal efficiency [1].
In addition to the problem of the cooling system design,
pollutant emission reduction in the last years becomes
fundamental for large energy systems. The Lean Premixed
Combustion proves to be the best way to reach this goal. On
the other hand, the increase of cooling mass flow rate
negatively influences lean combustion, reducing air/fuel ratio
and then the available coolant for combustion liner [2].
Considering these aspects, two main facts arise:
• The design of high performances and low emissions gas
turbines requires minimum cooling and sealing air
consumption, which entails an extremely accurate definition
of vane and blade cooling and secondary air systems.
• To meet the strict customers requirements, related to low
emissions, reliability and high performances, the correct
estimation of power plant behaviour over a variety of
operating conditions has to be extremely detailed, taking
into account interactions between the main flow and the
secondary air system.
The latter instance implies for designers and commercial
personnel to be equipped with reliable calculation tools (inhouse developed or commercial) properly modified for specific
needs. In particular, Cycle Analysis Codes (CACs) allow the
designers to select proper energy system configurations.
In this field, several new codes have been created or
existing codes improved in the last years by research centres
and software houses (GateCycle by GE Enter Software, GSP
by USA National Aerospace Laboratory). To predict off-design
behaviour, these codes need to be not limited to thermodynamic
analysis, but also able to perform a simplified description of
each component and/or to introduce their characteristic curves.
On the other side, commercial offers to the customer are
defined making use of so-called “performance codes”, specific
for each machine, based on the matching of performance maps
and characteristic curves of the main gas turbine components.
Both types of approach require comparison and validation,
and particularly for the “performance codes”, calibration is
necessary by means of measurements carried out from field at
different loads and ambient conditions.
Moreover, the detailed design of specific components
and/or internal systems as SAS is based on specific programs
(3-D CFD or through-flow codes, 1-D flow codes, flow
network solver, etc.).
The coupling between the simulation cycle and the analysis
of singular components and internal systems becomes
fundamental to provide a real “picture” of the GT in all typical
operating conditions (base-load, partial load, star/stop cycle),
from different points of view (performance, overall mass
balances, temperature/pressure trends along the gas path, etc.).
GAS TURBINE DESCRIPTION
Vx4.3A family runs as one of the most advanced gas
turbine model series on the market, representing the latest
generation Ansaldo-Siemens technology gas turbine range.
Vx4.3A models are single-casing, single-shaft gas turbines
having a disc-type rotor held up with a pre-stressed central tierod. Rotor discs are splinted together by radial facial serrations
named as Hirth-couplings, which connect adjacent discs
permitting the transmission of turbine torque to the compressor.
This rotor configuration provides great stiffness with a
relatively low weight and permits the rotor parts to be bathed in
air from all sides, which prevent thermal stresses and rotor
distortion during load changes and rapid starts [2]. The rotor is
supported by two bearings, located outside the pressurized
region. This ensures excellent running qualities and constant
proper alignment. The front bearing casing is fixed to a ring
that rests on two supports by means of radial struts guiding the
airflow entering the compressor. A rigid one-piece cylinder,
comprised in the exhaust casing, supports the turbine bearing.
2
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Copyright © 2006 by ASME
VII
E1
E2
E3
E2
E1
E3
VE
KE
E2i
E3i
Figure 2. Flow path of cooling/sealing air in the Ansaldo
V94.3A gas turbine
ones provide flow paths into the rotor. The extractions are
located such that the cooling and sealing air is achieved with
the lowest energy losses and thus a high overall efficiency is
reached. From the operating point of view the SAS is
subdivided into the cooling system for turbine blades, the
cooling system for turbine vanes and the sealing system.
The cooling air for the turbine blades of stages 2, 3 and 4 is
extracted upstream of the 11th and 13th compressor stage and
led through two different passages in the central hallow shaft
marked by different grey scales in figure 2. The cooling air, by
centrifugal and pressure force, is driven towards the cavities
between the turbine rotor disks from which it passes through
radial bores to the rotor cascades.
The cooling air for the first stage vanes and blades is
directly extracted at the compressor exit. For the turbine vanes
either it is led to the tip of the vanes, passing radially outward
to supply the leading edge up to midspan, or led to the root of
the vanes, passing radially inward to supply the trailing edge up
to the midspan. The stationary blade rows of the remaining
stages are supplied by the outer extractions, located upstream
of the 6th, 10th and 14th compressor stage respectively.
The outer extraction lines feed three different chambers
(respectively E1, E2 and E3 in Fig. 2) contained between the
rear outer casing and the turbine stationary vane carrier. Metal
rings divide the first two chambers. A strong interaction
between the cooling lines occurs because of the cross-flow
passing through these separating rings.
The blade roots are cooled by passages in the erection part
(vane carrier) supplied by the outer extractions. The same
stationary secondary air path as the cooling air path for vanes
provides the sealing of clearances between rotor and stator.
Four blow-off lines carry the quantity of air sufficient to
ensure stable operation of the compressor at low speed. Two
lines start from the annular gaps downstream the 5th compressor
stage. The other two lines are located respectively in
correspondence with the 2nd and the 3rd outer extraction.
Uniform blow-off of air around the circumference, combined
with undisturbed discharge, prevents the excitation of
vibrations in neighbouring blade rows.
Figure 1. Ansaldo V94.3A gas turbine layout
Table 1. Performance data of Ansaldo gas turbines
As it can be seen in Fig. 1, a lever system permits to vary
the pitch of the first row of compressor vanes in order to adjust
the volume of inlet air to the needs of start-up, shutdown and
part-load operation.
Ansaldo supplies pre-designed combined cycle units
incorporating gas turbines, both of mentioned Vx4.3A model
series and of the older Vx4.2 family (Table 1), named as
COBRA (Combined Brayton Rankine) plants.
The Secondary Air System
The cooling/sealing systems of Vx4.3A models have similar
features. Figure 2 shows a section of the V94.3A gas turbine
and the complex flow paths of secondary air system are
highlighted. There are five extractions for compressed air
visible in the compressor section: three on five are led through
extraction pipes connected to the outer casing while the other
3
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Rotor components are warmed and cooled by circulation of
air through cooling air passages and hirth teeth gaps.
Circulation of air makes rapid and more uniform the
temperature changes of discs and tie-rod. Compressor disc
assembly is split into three different blocks by three damping
elements in the form of clamps. Thus, leakage air enters the
teeth gap between the last two discs of the block and exits from
the upstream coupling gaps (see flow paths in Fig. 2).
Start
Compressor
Performance
Calculation
cooling air
mass flows
ptot, Ttot
(extractions)
INTERACTION BEETWEEN MAIN AIR-GAS PATH AND
COOLANT NETWORK
When the coolant paths and blade/vane cooling systems are
designed, the gas turbine geometries must be determined to
guarantee a sufficient coolant mass flow rate to the cooling
system. This mass flow rate is determined by pressure loss in
the path. When the gas turbine works in off-design condition
(part load or ambient temperature variation), the gas turbine
geometry is fixed and the coolant mass flow rate is determined
respectively by the pressure in the compressor extraction point
(inlet boundary conditions of SAS) and the pressure in the
turbine (outlet conditions of SAS). Thus, a strong interaction
between the coolant net and main air-gas path is present. So, to
determine the coolant mass flow rate in off-design condition, at
least, interaction between two codes are necessary: the CAC
and SAS codes. They can mainly interact in two ways, in this
paper the first method is used.
Secondary Air System
Calculation
cooling air
mass flows
ptot, Ttot
(blade & vane inlet)
thermal
boundary
conditions
SRBC Procedure
Blade
Cooling system
Calculation
CFD Solver
external heat
transfer
coefficient
FEM
Thermal
Analysis
cooling flows
and internal
heat transfer
coefficient
blade metal temperature
• Separated procedure: the code for coolant net simulation
allows determining the characteristic curves of SAS and
these can be introduced into CAC codes. But it is not easy
to determine a generic characteristic curve for a complex
geometry like an internal coolant passage.
Stop
Figure 3. Flowchart of interactions of codes
• Integrated Procedure: CAC code permits to determine the
boundary condition (inlet and outlet pressure, and inlet
temperature) for the second air system code, which can run
and determine the coolant mass flow rate and the outlet
temperature. These values can be imposed into CAC code
again, until the convergence is obtained (see scheme in Fig.
3). In this iterative procedure, can be added a procedure to
determine the blade metal temperature (Ansaldo utilizes
SBRC procedure [5]).
defined by a connecting a number of elementary components
representing different unit operations such as compressors,
combustion chambers, mixers and so on. Each component is
defined as a black box capable of simulating a given chemical
and thermodynamic transformation. The resulting set of nonlinear equations defining the power plant is then linearised (the
coefficients are, however, updated in the course of the
calculation). All equations are then solved simultaneously using
a classic matrix method; thus the procedure is essentially that of
the fully implicit linear approach.
Simulation of design and off-design conditions consists of a
two-step procedure. Off-design performance simulation
requires a geometric description of the different components
(e.g. the velocity triangle at mean radius and other cascade
parameters for the compressor or turbine, heat exchanger
surface areas, etc.). These data result from a design study.
When identifying the different parameters describing the
component geometry, knowledge of some plant data is
important to improve simulation results (e.g. the turbine
exhaust flow rate and the temperature). Off-design simulations
are based on fixed geometry (obtained in the course of the
design study), and this results in a reduction in the number of
input data.
The ESMS Cycle Analysis Code
Power plants based on GT engines are not very complex,
but, to simulate them, a flexible, very detailed and open-source
code is necessary. Gas Turbine designers use ad-hoc code to
simulate each component because a lot of details are necessary.
The code used was the ESMS code developed by some
authors of this paper. The reader is referred to references [6],
[7], [8], [9] and [10] for a complete presentation of the code,
related theory and some engineering applications.
The most important feature of this modular simulation code
is the ability to simulate a new power plant configuration
without creating a new source program. The code easily allows
addition of new components. The power plant configuration is
4
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Copyright © 2006 by ASME
Plot
Plot
Plot
Tubo
Curva
Plot
Plot
Plot
Blocco_212
Tubo
Blocco_211
Plot
Plot
Plot
Plot
Nodo
Mix
Blocco_213
Plot
Plot
Plot
Ingr. Tubo
Plot
Tubo
Cur va
Plot
Blocco_235
Plot
Nodo
Mix
Plot
Plot
Blocco_35
Plot
Blocco_215
Blocco_318
Nodo
m-n
Blocco_219
Blocco_131
Diaframm a
Blocco_117
Plot
Plot
Nodo
m -n
Blocco_137
Plot
Blocco_316
Blocco_223
Blocco_118
Blocco_125
Tubo
Blocco_221
Blocco_126
Plot
Plot
Tubo
Blocco_224
Tubo
Plot
Blocco_119
Plot
Plot
Plot
Blocco_324
Plot
Condizioni
Uscita
Nodo
Blocco_326
n-1
Blocco_203
Blocco_201
Blocco_202
Plot
Blocco_204
Blocco_9
Tubo
Nodo
Condizioni
Uscita
Blocco_129
Condizioni
Uscita
Condizioni
Uscita
Plot
Plot
Plot
Blocco_123
Blocco_228
Plot
Blocco_302
Plot
Nodo
Blocco_331
Condizioni
Uscita
Plot
Nodo
Mix
Blocco_330
Blocco_328
Plot
Blocco_304
Blocco_57
Condizioni
Uscita
Blocco_233
Plot
Plenum
Blocco_52
(portapalette)
Blocco_303
Blocco_301
Plot
Blocco_227
Plot
Plot
Plot
Plot
Condizioni
Ingresso
Plot
Condizioni
Uscita
Plot
Plot
Blocco_229
Plenum
Plot
Plot
Curva
Plot
Nodo
Mix
Plenum
Blocco_231
portapalette
Blocco_327
Plot
Nodo
Blocco_51
n-1
Plot
Blocco_128
Plot
Plot
Nodo
Blocco_323
n-1
Plot
Plot
Plot
Nodo
Blocco_230
Blocco_28
Spillamento
Plot
Nodo
Plenum
Plot
Plot
Tubo
Blocco_305
Tubo
Blocco_132
Plot
Plot
Plenum
Plot
Plenum
Blocco_121
portapalette
Plot
Condizioni
Ingresso
Plot
Blocco_325
Plot
Spillam ento
Plot
Nodo
Blocco_120
n-1
Nodo
Blocco_222
n-1
Plot
Plot
Tubo
Plot
Blocco_6
Spillamento
Nodo
Blocco_127
n-1
Plot
Blocco_49
Blocco_226
Plot
Condizioni
Ingresso
Blocco_101
Plot
Nodo
Blocco_225
n-1
Tubo
Ingr. Tubo
Blocco_322
Plot
Plot
Plot
Blocco_105
Blocco_321
Tubo
Blocco_19
Sostegno
Plenum
Ingr. Tubo
Ingr. Tubo
Plot
Blocco_306
Plot
Plot
Plot
Plot
Plot
Nodo
m-n
Blocco_320
Blocco_24
Plot
Ingr. Tubo
Blocco_220
Ingr. Tubo
Nodo
Ingr. Tubo
Plot
Blocco_319
Tubo
Plot
Plot
Condizioni
Blocco_56
Uscita
Plot
Plenum
Plot
Condizioni
Uscita
Blocco_138
Plot
Plot
Ingr. Tubo
Plot
Plot
Ingr. Tubo
Blocco_130
Plot
Plot
Plot
Diaframm a
Plenum
Plenum
Diffusore
(div.-div.)
Nodo
1-n
Nodo
Blocco_116
Plot
Plot
Blocco_803
Plot
Plot
Plot
Plot
Feritoia
Nodo
Mix
Blocco_317
Plot
Plot
Plot
Blocco_217
Blocco_218 Nodo
Plenum
Nodo
1-n
Plenum
Blocco_336
Plot
Blocco_802
Nodo
Blocco_20
Blocco_307
Blocco_115
Diaframma
Nodo
Blocco_315
n-1
Feritoia
Blocco_11
Blocco_12
Plot
Plot
Plot
Nodo
(vel.2)
Plenum
Blocco_13
Plot
Blocco_33
Plenum
Plot
Plot
1 = Choked
Tenute
ad una aletta
Nodo
(vel.2)
Ingr. Tubo
Blocco_308
Nodo
1-n
Condizioni
Uscita
Blocco_206
Nodo
n-1
Blocco_314
Blocco_216
Plot
Plot
Plot
Plot
Diafram ma
Blocco_214
Plot
Blocco_313
Tubo
Blocco_309
Plot
Blocco_207
Blocco_312
Tubo
Ingr. Tubo
Blocco_311
Plot
Curva
Valvola
Blocco_310
Blocco_208
Blocco_106
Blocco_114
Curva
Tubo
Blocco_209
Blocco_112
Tubo
Tubo
Blocco_210
Plot
Blocco_111
Plot
Curva
Blocco_107
Curva
Blocco_110
Tubo
Blocco_109
Blocco_108
Condizioni
Uscita
Plenum
Blocco_332
portapalette
Plot
Plot
Plot
Plot
Plot
Blocco_329
Plot
Plot
401
Plot
402
Condizioni
Ingresso
Tubo
Blocco_500
Blocco_501
Plot
403
Plot
Plot
401
Blocco_503
Blocco_502
Plot
402
Plot
403
Spillam ento
Condizioni
Ingr esso
Tubo
Blocco_400
Blocco_401
Condizioni
Uscita
Nodo
Blocco_403
Blocco_402
Plot
Plot
Plot
Plot
Condizioni
Ingresso
Diffusore
(conv.-div.)
Blocco_4
Nodo
(vel.1)
Plenum
Spillamento
Ingr. Tubo
Tubo
Blocco_21
Plot
Blocco_22
Plot
Blocco 405
Unione 1-3D1
Plot
Plot
Plot
Plot
Plot_23
Plot
425
Ingr. Tubo
Tubo
Blocco_29
Blocco_30
Nodo
n-1
Plenum
(portapalette)
Condizioni
Uscita
Blocco_32
Blocco_33 Tv1 78 pale
Plot
Blocco_48
Blocco_31
Plot
425
Condizioni
Uscita
Blocco_424
Plot
425
Condizioni
Uscita
Flow
Function
Blocco_526
Blocco_535
1 = Choked 1
Flow
Function
Condizioni
Ingresso
Plot
409
Plot
408
Nodo
Plot
Plot
3D
Plot
Tenute
Plenum
Blocco_59
Blocco_61
Blocco_62
Plot
Plot
Plot
Blocco_36
Plenum Rot.
Blocco_46
(piede pala)
Nodo
Mix
Plenum Rot.
Blocco_422
(piede pala)
Plot
3D
0.001*0.5
Plot
Blocco_408
Tubo Rot.
(inclinato)
Blocco_134
Unione 1-3D
blocco 37
Tubo Rot.
(inclinato)
Plot 3d
Plot
Plenum Rot.
(shaft cover)
Plot
424
Plot
424
Plot
423
Tubo Rot.
(inclinato)
Blocco_45
Blocco_410
Blocco_545
Plot
423
Plot
422
Blocco_39
Tubo Rot.
(radiale)
Plot
424
Plenum Rot.
Blocco_533
(piede pala)
Plenum Rot.
Blocco_524
(piede pala)
Blocco_421
Nodo
3-1D
Blocco_38
Plot
423
Tubo Rot.
(inclinato)
Blocco_523
Plenum Rot.
Tubo Rot.
Blocco_542 (inclinato)
Blocco_532
Plot
422
Blocco_420
Plot
3D
(assoradiale)
Plot
422
Plot
422
Plenum Rot.
Plot
415
Plot
416
Plot
417
Plenum Rot.
(piede pala)
Plot
423
Tubo Rot.
(inclinato)
Blocco_42
Plot
3D
Plot
414
Plot
Blocco_60
Plot
Plot
411
Display
Blocco_409
Blocco_135
Plot
424
Blocco_41
Blocco_407
Tenute
Condizioni
Uscita
Plot
Blocco_40
Tenute
Blocco_510
Plot
410
Plot
Plenum
Condizioni
Uscita
Blocco_136
Plot
Condizioni
Uscita
Blocco_35
1 = Choked
Plot
411
Tubo Rot.
(inclinato)
Tenute
Plot
Blocco_546
Plot
Plenum Rot.
Blocco_406
Radiale
Display1
1 = Choked
Plot
426
Flow
Function
Blocco_34
Blocco_507
Blocco_508
Blocco_509
Nodo
(vel.2)
Blocco_28
1 = Choked
Tubo
Condizioni
Uscita
Plot
Plot
Plenum
Blocco_27
39 rami
Plot
Tenute
Nodo
Tenute
Plot
410
Plot
Plot
Nodo
n-1
Blocco_23
Plot
Ingr. Tubo
Plot
408
Tubo
Blocco_25
Blocco_26
Blocco_7
Plot
407
Plot
409
Plot
Nodo
(vel.2)
Blocco_24
Plenum
Blocco_8
Blocco_5
Plot
406
Plenum Rot.
Blocco_506
Radiale
Plot
Plot
Plot
405
Blocco_404
Plot
407
Blocco_334
Plot
404
Plot
406
Blocco 505
Unione 1-3D1
Condizioni
Uscita
PLOT53
Plot
Plot
405
Blocco_504
Nodo
Blocco_133
n-1
Condizioni
Blocco_53
Uscita
Plot
404
Condizioni
Uscita
Nodo
Tubo Rot.
(rad.-ass.)
Blocco_43
Plot
418
Plenum Rot.
(assor adiale)
Blocco_541
Blocco_522
Plot
3D
(assoradiale)
Blocco_531
Plot
419
Plot
420
Plenum Rot.
Radiale
Plenum Rot.
Plot
422
(assor adiale)
Blocco_44
Plot
421
Blocco_540
Plenum Rot.
Passaggio
Rot.
Assiale
Blocco_412 [10.8 600 50]
Blocco_413
Brusco
Restr.
Rot.
Blocco_414
Tubo
Rot. Oriz.
Blocco_415
Brusco
Allarg.
Rot.
Tubo
Rot. Oriz.
Blocco_416
Blocco_417
Brusco
Restr.
Rot.
Tubo
Rot. Oriz.
Blocco_418
0
steps
Steps1
Plot
422
Blocco_419
conv
conv
Convergenza1
Plot
422
Plot
414
Passaggio
Rot.
Plot
415
Plenum Rot.
Assiale
Plot
416
Brusco
Restr.
Rot.
Plot
417
Tubo
Rot. Oriz.
Plot
419
Plot
418
Brusco
Allarg.
Rot.
Plot
419
Brusco
Restr.
Rot.
Tubo
Rot. Oriz.
Plot
420
Tubo
Rot. Oriz.
Plot
420
Brusco
Restr.
Rot.
Blocco_539
Plot
421
Tubo
Rot. Oriz.
Plot
421
Nodo
Blocco_521
Blocco_511
Blocco_512
Blocco_513
Blocco_514
Blocco_515
Blocco_516
Blocco_517
Blocco_518
Blocco_519
Plenum Rot.
Plot
421
Brusco
Restr.
Rot.
Plot
421
Tubo
Rot. Oriz.
Blocco_520
Plot
421
(assoradiale)
Plot
421
Brusco
Restr.
Rot.
Nodo
Blocco_530
Blocco_528
V94.3A
Tubo
Rot. Oriz.
Blocco_529
Plot
421
Blocco_537
Blocco_538
Plot
422
Figure 4. Simplified network modeling the secondary air system of the Ansaldo V94.3A gas turbine
The SASAC cooling network solver
The Secondary Air System (SASAC) code is developed in
Ansaldo and it runs in Matlab/Simulink® environment. A
network diagram (Fig.4) represents the secondary air system,
whose components are graphically connected together (within
the Simulink working sheet) through their input/output ports to
form the entire cooling/sealing system of the gas turbine.
Thanks to the graphical interface of the environment, the
arrangement of the air system and the effects of flow passage
size on pressure losses can be easily investigated.
The SASAC library makes available several modules (dark
grey marked blocks in the upper part of figure 4) for the
description of typical flow elements (such as pipes with blunt
or smoothed inlet-outlet pressure losses, bends, branches, etc.)
useful to describe the external system supplying sealing air and
coolant for turbine vanes. In addition to the aforementioned
elements, there are modules typical for the internal cooling
system of Vx4.3A gas turbines, where air is led through the
disc-type rotor (blocks in the lower part of Fig. 4). The most
important modules and some relating reference are listed.
From the point of view of implementation, the flow system
elements are classified into distributed and lumped loss
modules. For distributed loss elements both energy and
momentum equations are written (in the relative system of
reference for rotating elements); whereas for lumped loss
modules (such as sudden contractions/expansions) only the
momentum equation is implemented and the energy balance is
reduced to the assumption Ttot=const (for stationary elements).
Depending on the characteristics of the secondary air
system elements, they have been modeled either by the basic
equation for flow through an orifice or expressing the pressure
losses as a multiple of the dynamic head by a loss coefficient
(incompressible flow-type approach).
Several CFD analyses (e.g. [18]) have been carried out by
the authors of the code to modify and/or calibrate the several
correlations, taken from literature, for the calculation of loss
and discharge coefficients. In a second stage, the entire code
has been tuned by means of gas turbine design data and field
measurements.
From the point of view of calculation, pressure is obtained
in correspondence of the nodes of the air system network (light
grey marked blocks in figure 4). They represent, in most of
cases, chambers of some volume in the turbine. In the modules
for stationary “chambers” the law of conservation of energy
and proper correlations to compute the quota of kinetic energy
converted into static pressure are implemented.
Specific algorithms are implemented for the flow elements
used to model chambers with rotating walls and stationary
casing. For instance, in the case of rim cavities, the core
temperature in the net wheel space is calculated by summing
the bulk gas temperature and the windage temperature rise [16].
• Elements for the conditions change from an absolute to a
relative system of reference.
• Rotating elements for the modelization of air flow through
rotating passages, rotating bores and between rotor disks
[11], [12], [13], [14] and [15].
• Static elements to describe the flow exchange between the
hot gas stream and the sealing air occurring in the statorrotor cavities [16].
• Elements for labyrinth seals [17].
5
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SIMULATION RESULTS
The bulk temperature computation assumes a flow-weighted
mixing of injected coolant flow and mainstream ingestion flow.
The windage temperature rise takes into account the rotor drag.
On the turbine side in Fig. 4 the boundary elements of the
network are, respectively, the modules for the leakage flows
through roots and the modules for the stationary and rotating
blades. They are described by black-box elements within which
the blade characteristic curves (pressure ratio -versus- reduced
mass flow) are implemented. Flow functions are calculated by a
procedure based on SRBC and ANSYS [5], which are the
programs in use at Ansaldo Energia Gas turbine Department to
model the internal cooling network of turbine blades and vanes.
It follows that the characteristic curves of cooling passages
calculated by SASAC take into account inside the matching
with the flow function of row cooling networks. This fact is
fundamental; otherwise it would not be easy to consider them
directly within the code for cycle analysis as well as to
distinguish the influence of single row coolant variation on
global performances of the gas turbine.
Steady-state computation of the network leads to the
solution of a highly non-linear system of equations made of
conservation equations and phenomenological correlations.
Once the boundary conditions (in terms of total/static
pressure/temperature, inlet/outlet flow rate, etc.) are applied to
specific modules of the network, the solution is performed in
four nested iterations:
Performance Calculation
For the simulation of the gas turbine cycle, ESMS code is
applied. The gas turbine is simulated splitting the compressor
in six parts so each cooling extraction can be simulated. Each
turbine stage is represented by a different module (for a better
modeling of cooling air injections); then splitters, mixtures and
ducts are used. As it can be seen in figure 5, ESMS code
permits to realise a network of modules having a considerable
relation with the actual layout of the GT described in figure 2.
Results of ESMS runs have been compared with data given
by Ansaldo Performance Code (APC Code). APC models the
gas turbine components by means of non-dimensional
performance maps. These curves are interpolated by the code to
match the characteristics of compressor and turbine in order to
define the working point of the gas turbine. Bleeds are taken
into account in the compressor curves and in the calculation of
the shaft power balance by proper correction factors, avoiding
the modelization of the secondary air system.
ESMS Code Validation
In order to validate the code, it is compared with results
from APC code in off-design conditions varying the ambient
temperature. For this simulation, the base load condition is
imposed for the ambient temperature of 15iC, and then the code
is run in off-design condition.
Mainly, the power output and the thermodynamic efficiency
are compared (Fig. 6 and 7, respectively). Increasing the
ambient temperature, the output power and thermodynamic
efficiency decreases because the air density decreases. A good
agreement is present for all range considered, particularly for
high ambient temperature.
Figure 8 shows the compressor pressure ratio trend. It
decreases with ambient temperature following the characteristic
curve. Even in this figure, there is a good agreement for high
temperature, whereas for low temperature the pressure ratio is
underestimated.
• Computation of the mass flow rate throughout determined
loss elements (rotating/stationary pipes/passages), for given
the pressure differential across the element.
• Computation of the pressure at the inlet of the remaining
stationary/rotating pipes/passages, for given outlet pressure
and mass flow (obtained at the previous iterative step).
• Computation of the node/chamber pressures to solve the
system of mass conservation equations applied to the
nodes/chambers, letting the nodal temperature to be fixed.
• Computation of the temperature in nodes/chambers and at
the outlet of the distributed loss elements, for given mass
flows.
Being the secondary air network strictly interconnected, at
the initial step the algorithm identifies the flow elements to be
considered for the computation of mass flows in order to
accelerate the convergence of the solution. In the remaining
modules the inlet pressure is calculated from outlet pressure
and mass flow.
In transient analysis the accumulations of mass, energy and
momentum are described by means of fluid-dynamic capacities,
thermal capacities and fluid-dynamic inductances, which
together govern the convergence of the solution.
The user can try to input different values in the code for the
relaxation factors in order to accelerate the convergence.
IN
OUT
12
FUEL
1
2
3
E2
E1
5
4
I1
6
I2
E3
13
7
8
27
9
10
22
23
28
24
25
26
31
34
40
37
35
30
33
38
39
14
15
16
OUT
11
36
17
32
18
19
20
21
29
Figure 5. Flowchart of Ansaldo V94.3A gas turbine
enclosing the cooling/sealing air path
6
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Copyright © 2006 by ASME
300
280
260
240
ESMS code
APC code
220
200
ESMS code
APC code
180
260
WGT (MW)
WGT (MW)
280
240
160
140
120
100
80
220
60
40
20
200
-5
0
5
10
15
20
25
30
35
40
0
0.70
45
Tamb (°C)
0.80
0.85
0.90
0.95
1.00
min/min_nom
Figure 6. Comparison of ESMS and APC codes: GT power
vs. ambient temperature
Figure 9. Comparison of ESMS and APC codes: GT power
vs. inlet mass flow rate in FSNL condition
0.42
18
0.41
17
ESMS code
APC code
0.40
16
ESMS code
APC code
15
0.39
14
0.38
β
ηGT
0.75
13
0.37
12
0.36
11
0.35
10
0.34
-5
0
5
10
15
20
25
30
35
40
9
0.70
45
0.75
0.80
Tamb (°C)
Figure 7. Comparison of ESMS and APC codes: GT
efficiency vs. ambient temperature
ESMS code
APC code
β
Texh (K)
18
17
16
15
-5
0
5
10
15
20
25
0.90
0.95
1.00
Figure 10. Comparison of ESMS and APC codes:
Compressor pressure ratio vs. inlet mass flow in FSNL
20
19
0.85
min/min_nom
30
35
40
45
860
840
820
800
780
760
740
720
700
680
660
640
620
600
580
560
540
520
500
0.70
ESMS code
APC code
0.75
0.80
Tamb (°C)
0.85
0.90
0.95
1.00
m in/m in_nom
Figure 8. Comparison of ESMS and APC codes:
Compressor pressure ratio vs. ambient temperature
Figure 11. Comparison of ESMS and APC codes: Exhaust
temp. vs. inlet mass flow in FSNL condition
Presented results show that the ambient temperature
increasing makes decrease the pressure ratio and, fixing the
inlet temperature of turbine, the inlet temperature of hot gas in
each stage increases. From the point of view of cooling and
sealing flows, the compressor pressure ratio decreases, so the
air temperature growth decreases, but the inlet temperature of
first compressor stage is higher.
Figure 9, 10 and 11 show the comparison between ESMS
and APC codes for the main parameters of the simulation in
Full Speed No Load (FSNL) condition (with the ambient
temperature fixed to ISO condition). In this case, the VIGV is
used to decrease the power, so the inlet air mass flow rate is
decreased.
7
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20%
12%
Extraction Δ p
10%
0%
4%
0
-10%
45
FSNL
0%
-20%
-30%
-40%
E1
I1
E3
E2
I2
Comp. Outlet
45
-8%
E1
I1
E2
I2
-12%
E3
Comp. outlet
FSNL
-16%
Figure 12. Pressure deviation to Base Load ISO 15iC
condition for extraction points
Figure 14. Temperature deviation to Base Load ISO 15iC
condition for extraction points
GT stage inlet Δ p
10%
GT stage inlet Δ T
10%
0%
0%
0
45
FSNL
-10%
-10%
-20%
-20%
-40%
0
-4%
-50%
-30%
Extraction Δ T
8%
I Stage
II Stage
-30%
III Stage
IV Stage
-40%
0
45
I Stage
II Stage
III Stage
IV Stage
FSNL
GT outlet
-50%
-50%
Figure 15. Deviation to Base Load ISO 15iC condition of
temperature at turbine stage inlet
Figure 13. Deviation to Base Load ISO 15iC condition of
pressure at turbine stage inlet
20%
Results of FSNL simulation are compared with the two
extreme standard conditions for ambient temperature, named
respectively “cold day” and “hot day”, corresponding to 0iC
and 45iC of compressor inlet temperature. Simulation results
are related to the base load ISO 15iC operating condition.
In the cold day case, the pressure ratio increases both for
extraction points in the compressor (Fig. 12) and for gas
turbine stages (Fig. 13); these pressures decrease for 45 C. This
is mainly due mainly to the variation of pressure ratio (Fig. 8).
In FSNL condition, the pressure in the coolant extraction
points and in the turbine decreases as well as variations of
ambient temperature.
Fig. 14 and fig. 15 show the temperature respectively at the
extraction points and at the inlet of turbine stages. These
parameters are very important to determine the blade
temperature using SRBC procedure [5].
Some works in literature assume to scale the coolant mass
flow rates with respect to the inlet mass flow rate of the gas
turbine in any operating conditions. Simulation results notice
that this hypothesis is quite correct only for ambient
temperature variations, but it is not more valid for FSNL
condition (Fig. 16). This effect is mainly due to the no-linear
Coooling/sealing air Δ m
10%
0%
-10%
0
45
FSNL
-20%
-30%
-40%
I Stage
II Stage
III Stage
IV Stage
-50%
Figure 16. Deviation to Base Load ISO 15iC condition of
coolant mass flow for turbine stages
phenomena occurring in static and rotating cooling passages
and they are more relevant for strong variations. Globally, the
coolant mass flow rate is about 15i% (constant) for all ambient
temperatures investigated, but this value decreases to about
12i% of inlet air mass flow rate at FSNL.
8
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1.25
Desing point
1.20
1.15
2nd stage
blade
1.00
0.80
2nd stage circuit
0.60
0.40
1
2
0.20
1.10
1.05
1.00
0.95
old design point
0.90
0.00
0.50
0.60
0.70
0.80
0.90
1.00
ps2/p01
0.85
Figure 17. Characteristic curve of secondary air passage
nd
and 2 stage blade of V94.3A gas turbine
10.0%
new design
point
+10%
compressor
pressure
ratio
1.20
Feeding pressure
Non-dimensional reduced mass flow
1.40
0.80
0.60
0.80
1.00
1.20
1.40
1.60
Coolant mass flow
2
nd
stage blade cooling air Δ m (E3i extraction)
nd
Figure 19. Characteristic curves of 2
system in different design conditions
7.5%
horizontally in the gaps between the blade roots and enters the
downstream rim cavity (contained between the turbine disk and
the shroud ring of the 2nd stage vanes). The flow of sealing air
gives a contribution to cool the blade roots.
The flow characteristic of the compressor-turbine rotor
assembly has been calculated running the fluid network in
SASAC for different values of outlet mass flow rate. As shown
in Fig. 17, the matching between the characteristic curve of the
secondary air circuit and the flow function of the blade cooling
network defines the working point of the system, given the
rotational speed as a parameter (data are normalized with
respect to Base Load reference values).
Note that the flow function of the blade moves (rising or
dropping) on the “pressure ratio-reduced mass flow” plane
(circuit pressure ratio is defined as the extraction total pressure
divided by the static pressure in the turbine) in function of two
main parameters:
5.0%
2.5%
0.0%
+2.5%
+5%
+7.5%
+10%
+12.5%
stage blade cooling
+15%
Compressor pressure ratio
nd
Figure 18. Deviation of 2 stage blade coolant mass flow
for different compressor pressure ratios
Secondary Air Flow Calculation
The method to calculate the set of flow functions
implemented in the ESMS program for the cooling air system
of V94.3A gas turbine is outlined. As an example, the cooling
air passage for the 2nd stage turbine blade of the V94.3A model
is considered.
In this case, air extracted from the compressor is led
through the rotor assembly, where a series of rotating bores,
passages and restrictors are present. As indicated in the detail in
Fig. 17, the airflow path starts at the station 1 (corresponding to
the trailing edge of the 12th compressor stage) and it ends at the
blade root (station 2 where the static pressure, ps2, is indicated).
Note the analogy with the network model in Fig. 4.
The system provides cooling air for blades, roots and part of
the sealing air for the downstream rotor-stator cavity. The 2nd
stage blades are cooled by film cooling (at the leading edge)
and convection cooling. Air enters the blade through three
separate channels in the root and exits both leading edge film
cooling holes and trailing edge slots. Sealing air passes
• Temperature of hot gas over blade surface.
• Static pressure at the outlet of film cooling holes and
trailing edge slots.
The static pressure profile around the blade is calculated by
a CFD analysis performed in Base Load ISO 15iC operating
condition; then it is averaged over the airfoil in order to use a
unique value for the blade outlet static pressure. Total pressure
and temperature at the extraction points (named respectively p01
and T01 in Fig. 17) are calculated by averaging, over the radial
direction, the outputs of the through-flow compressor code.
Variations of extraction pressure and temperature make the
characteristic of air passage in the rotor translate up and down.
Matching the flow functions in Fig. 17 for different values
of the blade outlet pressure allows defining a unique curve of
9
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the secondary air passage. It is in order to calculate the bleed
reduced mass flow as a function of circuit pressure ratio. This
method allows taking into account, stage by stage, both the
cooling passage characteristic and the row cooling network
curve in the same function.
Integration of SAS solver with cycle analysis code also
permits to investigate the effects of new design solutions on
each circuit of the air system.
As an example, consider the case of improving the
compressor design in order to increase pressure ratio and
efficiency. It is clear that a reliable evaluation of intermediate
extractions plays a fundamental rule to provide the right
boundary conditions to the analysis of compressor
performances. ESMS/SASAC integrated codes give the
necessary information. They also make easier any type of
sensitivity analysis involving the parameters of the design
problem, such as illustrated in figure 18 (comparison is
performed in Base Load ISO 15iC condition). In this case it is
convenient plotting the relative total pressure at blade entrance
as function of the mass flow rate, in order to represent the
matching between flow functions, as illustrated in Fig. 19.
4.
5.
6.
7.
8.
9.
10.
CONCLUSION
A modular-structured program, the ESMS, has been set up
that can model Brayton cycles performed by air-cooled gas
turbines, including the calculation of the secondary air system.
The paper describes the application of the program to the
heavy-duty Ansaldo Energia gas turbine V94.3A. Selected
thermodynamic parameters from the performance code APC, in
use at Ansaldo Energy Gas turbine Department, permitted the
tuning of ESMS in design conditions.
Good agreement was found in critical off-design operating
points between the results of ESMS and data obtained from
field and by different codes, proving that ESMS has turned out
to be a valuable tool for the thermodynamic analysis of heavyduty gas turbines.
Results also show that the ESMS code may be used to
investigate the complex interactions between secondary air
system, compressor and turbine. For this application, ESMS
makes easier the introduction of the characteristic curves of
cooling/sealing air network with respect of most of commercial
CAC codes.
Considering that decreasing of emissions and increasing of
thermal efficiency are driving the competition in the today’s
market of heavy-duty gas turbines, the research efforts in codes
valuable for the optimization of the secondary air system
design are fundamental.
11.
12.
13.
14.
15.
16.
17.
18.
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