Chapter 6

Secondary Air System

The basic thermodynamic cycle describes the process in terms of the primary air
flows in a gas turbine. Of course, in addition to the main stream there is a secondary
air system which provides the turbine cooling air, sealing air (prevents hot gases
entering the turbine disk cavities and bearing chambers) and controls the axial load
on the bearings (Fig. 6.1-1). By its nature the secondary air system (SAS) is very
complex involving many different sources for the air with flows returning at
numerous positions within or leaving the main gas path. This section shows how a
greatly simplified SAS can be used in the performance model whilst still providing
an accurate simulation of both overall performance and component parameters.
The SAS consists of a multitude of relatively small elements plus a few bigger
air flows dedicated to turbine cooling. The SAS consists of two subsystems, the
internal and the external system. The first subsystem deals with the air flows that
are required for the safe operation of the engine. These are the turbine cooling air,
bearing and rim sealing air, bearing thrust control, active tip clearance control,
handling bleeds. These bleeds are usually quantified as percentages of the relevant
compressor entry mass flow. The pressure of an individual flow is of no interest,
except for computing the thermodynamic efficiency of a cooled turbine.
The second subsystem, the external air system, provides air needed for aircraft
purposes (cabin ventilation, de-icing of nacelle and aircraft parts, cross starting of
engines). These secondary flows are mostly specified as an absolute value in kg/s.
Both air pressure and temperature at the engine interface are of interest to the
customer—the aircraft.

6.1 SAS in the Performance Model

It is common practice in a performance model to replace the numerous flow lines by

a manageable number of secondary streams. The model remains “fit for purpose” as
it maintains energy balances, with realistic flows, pressures and temperatures at the
© Springer International Publishing AG, part of Springer Nature 2018 687
J. Kurzke and I. Halliwell, Propulsion and Power,
https://doi.org/10.1007/978-3-319-75979-1_19
 688
6 Secondary Air System

Fig. 6.1-1 Simplified secondary air system

6.1 SAS in the Performance Model 689

recirculating HP leak to LPT exit

a b
handling bleed c
LPT NGV Coolg

LPT cooling
a HP leakage to bypass
leakage from bypass overboard bleed b NGV cooling
c HPT cooling

Fig. 6.1-2 Schematic of secondary air system for a 2-shaft turbofan (© Copyright CFMI)

main thermodynamic stations as well as representing the overall process

adequately.
Figure 6.1-2 shows the most important secondary air flow paths in a turbofan
performance model. Solid lines represent the invariant part of the internal air sys-
tem: the mass flow is a constant fraction of the HP compressor entry flow for any
off-design case. Dashed lines indicate the variable parts of the internal and external
air systems.
Performance programs in industry often model the secondary air system in great
detail. You might be tempted to consider more secondary flow paths than shown in
Fig. 6.1-2 with the aim of a more accurate simulation. What is the effect of sim-
plifying the secondary air system model on the overall performance simulation
quality?
Let us examine the influence of two different variants of the SAS model on the
performance characteristics of a turbofan. The first one employs a medium complex
description of the turbine cooling system while, in the second, all turbine cooling air
flows are zero. Figure 6.1-3 shows the enthalpy-entropy diagrams of the two cycle
design points.
Turbine temperatures and efficiencies of the cycle without secondary air system
have been adjusted in such a way that the HPT pressure ratio, thrust and specific fuel
consumption are the same. The differences between the two models are primarily in
the turbine: The model without turbine cooling air shows significantly lower turbine
690 6 Secondary Air System

Enthalpy

ηHPT=0.88

ηHPT=0.831

ηLPT=0.872 ηLPT=0.9

Entropy

Fig. 6.1-3 Mixed flow turbofan enthalpy—entropy diagram with and without turbine cooling air

inlet temperatures and the turbine efficiency numbers are also lower. Apart from that,
there are only very small differences between the two cycle design points.
Figure 6.1-4 shows how the two models behave in an off-design simulation: The
turbine temperature levels are different, but their tendencies from full to part load
are the same. It is noteworthy that there is no difference in the specific fuel con-
sumption, a measure of thermal efficiency.
The conclusion from this exercise: a simplified secondary air system model does
not significantly influence the quality of the overall performance simulation.
However, if you are interested in the true gas temperatures or turbine efficiencies the
implementation of the SAS must be well understood.

6.2 SAS Calculation

In the following, we limit ourselves to the SAS that corresponds to Fig. 6.1-2. It is
simple to derive energy balances, when the flow sources are at thermodynamic
stations as the temperatures are known. However, there are exceptions to this with
interstage ports used for overboard bleed and LPT flows. Also on multi-stage
turbines some SAS flows are returned mid-turbine. We now show how this can be
represented in the simplified SAS model.
6.2 SAS Calculation 691

No SAS Simulation

Thrust [kN]

Fig. 6.1-4 Models with and without secondary air system

6.2.1 Interstage Bleed

The specific compression work done on an interstage bleed (station 2x in Fig. 6.3.1)
is H2x
H2 . We relate this enthalpy difference to the overall specific work of the
HPC:

H2x
H25
DH2x;rel ¼ ð6:2-1Þ
H3
H25

The relative specific work DH2x; rel is easily estimated. If the bleed port is located
at the exit of the 7th stage of a ten-stage compressor, then the relative specific work
done on this bleed is DH2x; rel ¼ 0:7.
Generally, there is no need to calculate the bleed air pressure. The exception is
the overboard bleed needed for aircraft purposes—the customer bleed. For this
special case, we employ the polytropic HPC efficiency to calculate the main stream
total pressure at the bleed port. The pressure losses of the bleed port and in the pipe
to the aircraft-engine interface depend on many geometrical details and the amount
of bleed air. If an accurate bleed pressure value is needed then, typically, compute it
692 6 Secondary Air System

using the formula editor of your performance program, but note that these losses do
not affect the overall engine performance.

6.3 Turbine Cooling Air

Most secondary air flows recombine with the main stream. There the mass flow is
added and the temperature of the mixed stream follows from an energy balance. The
bleed air pressure is not considered in the mixing process.
Within the performance program, multi-stage turbines are simulated as equiva-
lent single stage turbines. The turbine cooling air joins the main stream upstream or
downstream of the turbine rotor.
Turbine cooling air must have sufficient pressure to allow it to join the main
stream. For a single stage HPT, this is simple to model as all HPT cooling flows in
the performance model (streams b and c in Fig. 6.1-2) are taken from compressor
delivery.

25 2x 3 4 45
4x

W cl

25 2x 3 4 45
2y 41 44

z*W cl

(1-z)*W cl

Fig. 6.3-1 HPT cooling with compressor interstage bleed

6.3 Turbine Cooling Air 693

In a two-stage turbine, the second stage can be cooled with bleed air from an
intermediate compressor stage. The cooling air joining the main stream in the
second stage vane contributes to the turbine power. How can this flow be modeled
using the SAS scheme from Fig. 6.1-2?
In Fig. 6.3-1 there is no route from the compressor interstage station 2x to an
imaginary turbine interstage stage 4x; such a station just does not exist in this
performance model. We replace the cooling bleed flow Wcl by two equivalent
flows: one which participates in the power generation ðz Wcl Þ and a second which
bypasses the turbine. ð1
z) Wcl . The value of z is the proportion of the turbine
power generated downstream of where the flow is introduced:

ðH4x
H44 Þ
z ¼ DHT4x;rel ¼ ð6:3-1Þ
ðH41
H44 Þ

We wish to maintain the compressor work in the cooling flow. The part of the
cooling flow taken from station 3 has more work done on it than before, so to
compensate we reduce the work on the non-working part by moving the offtake
position from station 2x to 2y.
The energy balance for the cooling flow is:


WCl ðH2x
H25 Þ ¼ zWcl ðH3
H25 Þ þ ð1
zÞWcl H2y
H25 ð6:3-2Þ

Which re-arranged into relative specific work gives

DH2x;rel
z
DH2y;rel ¼ ð6:3-3Þ
1
z

The working flow Wcl;3y is added to HPT NGV flow and the non-working flow
Wcl;2y to the LPT NGV flow in Fig. 6.3-1.
Next, let us look at the practical application of this approach. If the bleed port is
located right after the 7th stage of a ten-stage compressor then DH2x; rel ¼ 0:7. The
relative enthalpy drop from the second vane cooling air injection point to the
turbine exit is DHT; rel ¼ z ¼ 0:5. The relative work of the first imaginary stream is
1.0, that of the second imaginary stream—the non-working flow—is
DH2y; rel ¼ ð0:7
0:5Þ=ð1
0:5Þ ¼ 0:4. We account for this chargeable cooling air
by adding it to the input value, which intrinsically stands for the LPT NGV cooling
air WC LPT NGV/W25 in Fig. 6.3-1. The non-chargeable (i.e. the working) interstage
bleed air may be added to the HPT NGV cooling air in your performance program.
Alternatively, you can ignore it completely because this air goes through precisely
the same process as the main stream.
694 6 Secondary Air System

6.3.1 Multi-stage Turbines

Single spool gas turbines designed for power generation need many turbine stages
because the turbine pressure ratio equals (nearly) the compressor pressure ratio. The
Siemens SGT8000H heavy-duty gas turbine, for example, has four turbine stages
which are all air-cooled.
The first stage employs air from stage 13 (compressor exit) for cooling, the other
three turbine stages are cooled with bleed air taken after the compressor stages 11, 8
and 5. We must also include a corresponding turbine in the performance program
and this must be modeled as an equivalent single stage machine because we have
only one turbine map. So, in the simulation, the cooling air can join the main stream
either upstream or downstream of the sole turbine rotor.
Figure 6.3-2 shows the SAS of the single spool engine configuration of
GasTurb. There are two cooling air sources: one is an intermediate stage of the
compressor; the other is the exit of the compressor. The interstage bleed air can
re-unite with the main stream only downstream of the turbine because it has
insufficient pressure for use in cooling the first turbine stage. Air with compressor
exit pressure can join the main stream upstream of the first turbine rotor since the
sink pressure has been reduced at least by the burner pressure loss.

NGV a
Recirculating Cooling
b Turbine
Overboard
Cooling
Bleed
c
Handling
Bleed

Fig. 6.3-2 GasTurb SAS for the single spool turboshaft

6.3 Turbine Cooling Air 695

5
5
8
40 41 11
S4 R4
S3 R3
S1 R1 S2 R2

from compressor stage 13

W Cl /W2 [%] 6.3 5.8 4.4 3.4 1.8 1.3 0.7 0.3 Σ = 24%
Work potential ΔHT,rel 1 .75 .75 0.5 0.5 .25 .25 0

Working W Cl/W2 [%] 6.3 4.35 3.3 1.7 0.9 0.325 0.175 0
Not Working W Cl/W2 [%] 0 1.45 1.1 1.7 0.9 0.975 0.525 0.3

Fig. 6.3-3 Secondary air system for a single spool gas turbine for power generation

Table 6.3-1 Energy balance for compressor interstage bleed

1 Source stage no 13 11 8 5
2 Work fraction 1 0.846 0.615 0.385
D Hcl 2x
4x
3 Sink S1 R1 S2 R2 S3 R3 S4 R4 Total
S2-R4
4 Rel. mass flow 6.3 5.8 4.4 3.4 1.8 1.3 0.7 0.3 11.9
WCl2x-4x/W2
5 Turbine work 1 0.75 0.75 0.5 0.5 0.25 0.25 0
fraction z
6 Working mass 6.3 4.35 3.3 1.7 0.9 0.325 0.175 0 6.4
fraction of W2
7 Turbine bypass 0 0.25 0.25 0.5 0.5 0.75 0.75 1
1−z
8 Non-working 0 1.45 1.1 1.7 0.9 0.975 0.525 0.3 5.5
mass fraction of
W2
9 Non-working 1 1 0.384 0.692 0.23 0.487 0.18 0.385 Mass
flow: Equivalent weighted
compr work mean
fraction D Hcl y; rel 0.453

The true cooling air scheme is much more complex than that in the performance
model. Let us consider as a numerical example the SAS schematic shown in
Fig. 6.3-3. It is somewhat similar to the real SGT-8000H cooling configuration; the
numbers we are using here have been published for a similar engine in Ref. [2].
696 6 Secondary Air System

1.45% non-working compressor exit bleed

49 5

6.3+4.35+6.4=17.05% working compressor exit bleed

4 41
Rotor
Stator

5.5% non working interstage bleed (45.3% compressor work)

Fig. 6.3-4 SAS of the equivalent single stage turbine

We can transform this detailed SAS to the simple model shown in Fig. 6.3-2.
Stator 1 cooling flow is simple to model as it is taken from compressor exit and
allocated to the NGV flow a in Fig. 6.3-2. Rotor 1 flow is split into a working and
non-working flows according to lines 5 and 7 in Table 6.3-1 and allocated to the
flows a and b respectively in Fig. 6.3-2.
Stators and Rotors 2–4 are cooled with interstage air which are split into working
and non-working flows. The working flow is taken from compressor exit and
included in the flow a in Fig. 6.3-2, with the non-working flows added to flow c in
Fig. 6.3-2. For each of the interstage flows the equivalent HPC specific work
fraction is calculated according to Eq. (6.3.3) as described in Sect. 6.3. The mean
specific work in the total flow c is derived from the mass-weighted mean of the lines
8 and 9 in Table 6.3-1.
The equivalent single stage turbine SAS is shown in Fig. 6.3-4.
It should be noted that the Stator 1 may also be regarded as part of the com-
bustion chamber. This does not change the overall cycle as the flows and tem-
peratures through the turbine rotor are not affected. However, the value for T4
reduces when the cooling flow is added upstream of the stator in the model.

6.4 References

1. Kurzke, J.: About simplifications in gas turbine performance calculations. Paper presented at
Turbo Expo 2007, Montreal, Canada ASME GT2007-27620, 2007
2. H2IGCC Low Emission Gas Turbine Technology for Hydrogen Rich Syngas Project under the
European Union’s Seventh Framework Programme for Research and Technological
Development SP4—Description of the models adapted or developed ad hoc for the
IGCC-CCS plants D4.2.2—final—long version http://www.h2-igcc.eu/Pdf