Optimization of a Single-stage Air Starter Turbine

Grigorii Popov

, Oleg Baturin, Vasilii Zubanov

, Daria Kolmakova

, Anastasia Korneeva

and Andrei Volkov

Samara National Research University, Samara, Russia

Keywords: Auxiliary Power Unit, Air Turbine, Optimization, Joint Operation, Gas Turbine Engine, Start-up Time,

Torque.

Abstract: The paper describes the process of optimizing the blades of a starter air turbine for a gas turbine engine launch

system. This task was initiated by the necessity to use an existing turbine when starting a new engine. During

the study, it was found that the turbine, although it ensures the conditions for the joint operation with the

auxiliary power unit turbine and meet the strength constraints, it does not allow to start the engine within the

required time. As a result of studies using mathematical optimization methods involving commercial

programs, a variant was found to modernize the baseline turbine, which provides an acceptable value of torque

on the shaft with an adequate start-up time at all operating modes with minimal design changes.

NOMENCLATURE

GTE - gas turbine engine

APU - auxiliary power unit

NGV - nozzle guide vane

ATS - air turbo starter

IGV - inlet guide vane

𝐾



- mass flow parameter

𝐾



- power parameter

P - pressure

T - temperature

G - mass flow rate

𝛼



- IGV stagger angle

𝜋 - pressure ratio

Δ𝐺 - losses (bleeding) of air flow in the aircraft

Δ𝑝

∗

- total pressure losses in the aircraft ducts

Δ𝑇

∗

- total temperature losses in the aircraft ducts

N - power

𝑝



- ambient pressure

𝑡



- ambient temperature

n - rotational speed

M - torque

 - time

https://orcid.org/0000-0003-4491-1845

https://orcid.org/0000-0003-0737-3048

https://orcid.org/0000-0003-2806-3073

1 INTRODUCTION

The start-up of an aircraft gas turbine engine is an

important mode that largely determines the safety,

operational efficiency and reliability of the engine

and the entire aircraft. The gas turbine engine start-up

system includes a set of various devices and units: a

starter, auxiliary power unit, air and fuel

communications, automatic control system,

transmission, power supply system, ignition system,

etc. For reliable engine starting, the operation of all

these systems must be consistent with each other

(Inozemcev et al., 2008).

Currently, the civil aircraft engine starting system

is often based on an air turbine mechanically

connected to the GTE rotor, receiving compressed air

from the APU compressor (Figure 1).

The group of the paper’s authors is employed by

the Department of Aircraft Engine Theory of Samara

National Research University (Samara University,

2020) and has extensive experience in studying and

improving the working process of various

components of a gas turbine engine (Marchukov et

al., 2017; Matveev et al., 2018). Currently, the

scientific group takes part in joint work in the

interests of various enterprises that are the part of the

Popov, G., Baturin, O., Zubanov, V., Kolmakova, D., Korneeva, A. and Volkov, A.

Optimization of a Single-stage Air Starter Turbine.

DOI: 10.5220/0009769501550162

In Proceedings of the 10th International Conference on Simulation and Modeling Methodologies, Technologies and Applications (SIMULTECH 2020), pages 155-162

ISBN: 978-989-758-444-2

155

United Engine Corporation (Russia) (United engine

corporation, 2020). In particular, one of the

Customers set the task of assessing the feasibility of

using an air turbostarter manufactured at the

enterprise to launch a new turbofan engine of the

same class. The design of the considered ATS is

shown in Figure 2.

Figure 1: Concept scheme of the starting system with an air

turbine.

Figure 2: Design of the investigated starter air turbine.

The team was assigned the following tasks:

1) the ATS operation must be coordinated with

the operation of the APU at all operating modes

(under various atmospheric conditions, flight

speeds and altitudes);

2) start-up time must be minimized (the minimum

allowed time was set);

3) the torque on the output shaft must not exceed

the maximum value according to the strength

conditions of the reduction gearbox and gear

box of the engine drives.

In other words, the main task was to modify the

flow part of the existing ATS so that it would satisfy

the above conditions when working in a modified

launch system (another gas turbine engine and APU).

Moreover, the design of the baseline turbine must be

kept as much as possible to reduce costs. Thus, the

task is a typical optimization problem in which the

turbine geometry must be changed (considering

various constraints: structural and strength) to

improve the required criterion (start-up time).

In open scientific and technical sources, there are

many publications on the modernization and

optimization of the turbine workflow (Marchukov et

al., 2018; Châtel et al., 2019., Asgarshamsi et al.,

2014). However, all of them are aimed at improving

the efficiency of turbines or their reliability. In current

case, the unusual optimization criteria take place and

it is necessary to check the condition for the joint

operation of the ATS and APU and to calculate the

start-up time of the gas turbine engine during

optimization.

To solve the problem, an algorithm was developed

for matching the working process of the APU and the

air turbine used to start the engine. This method was

described in detail in (Zubanov et al., 2019).

The essence of the method is that the characteristics

of the APU and ATS are converted to a general form

of the dependence of the reduced flow parameter K

on the expansion ratio of air in the turbine 𝐾

 



𝑓𝜋



. The intersection of the lines of the APU and

turbine operating modes suggests that the conditions

for joint work are fulfilled at these modes. At the

intersection points of the characteristics 𝐾

 



𝑓𝜋



 and 𝐾

 

𝑓



𝜋





, the parameters at

the APU outlet 𝑝



∗

, 𝑇



∗

, 𝐺



are

calculated and with their help the characteristics of

the turbine (in particular, the dependence of power on

rotational speed) are determined under the found

conditions. Then, using the ATS power

characteristics, the start time of the gas turbine engine

is determined. The algorithm for its calculation is

described in (Zubanov et al., 2019). The conformity

of the maximum torque to the strength constraints is

also checked.

Thus, based on the intersection points of the above

characteristics, the physical characteristics of the

ATS are found when working together with the APU

at all its modes.

The flow parameter K

is a physical quantity that

is calculated using the following expression:

𝐾





𝐺



∙



𝑇



∗

𝑝



∗

(1)

where 𝐺



- the value of air mass flow rate

through the ATS;

𝑇



∗

- the value of the total temperature at

the ATS inlet;

𝑝



∗

- the value of the total pressure at the

ATS inlet.

SIMULTECH 2020 - 10th International Conference on Simulation and Modeling Methodologies, Technologies and Applications

156

When calculating the flow parameter K

for the

APU, leakages and pressure and temperature losses in

the pipelines of the launch system on the way to the

turbine are considered.

𝐾

 





𝐺





∆G



∙



𝑇





∗

∆𝑇

∗



𝑝





∗

𝛥𝑝

.

∗

(2)

𝜋





𝑝



∗

𝛥𝑝

.

∗

𝑝



(3)

where 𝐾

 

– APU air mass flow parameter

determined at the inlet of the ATS considering losses

in the pipelines;

𝜋



– pressure ratio in the ATS determined by

the parameters of the air at the inlet to the ATS

considering losses in the pipelines;

𝐺



– the value of the mass flow rate of air

taken from the APU considering losses in the

pipelines, 





:

𝐺





𝐺



∆G

(4)

𝑇



∗

– the value of the temperature of air taken

from the APU considering losses in the pipelines,

К:

𝑇





∗

𝑇



∗

∆𝑇

∗

(5)

𝑝



∗

– the value of the total pressure of air

taken from the APU considering losses in the

pipelines, 







:

𝑝



∗

𝑝



∗

𝛥𝑝

.

∗

(6)

𝑝



– atmospheric pressure for altitude in standard

atmospheric conditions, 







.

Values of losses of mass flow rate, temperature

and pressure in the system are set by the Customer

according to the experience of operating an engine of

a similar class. The dependence 𝐾

 

𝑓𝜋





obtained for the used APU is shown in Figure 3.

Characteristics of the turbine of the form

𝐾

 

𝑓𝜋



 (Figure 4) were obtained by CFD

modelling of the working process in the investigated

turbine (Figure 2). Its mathematical model was

created using NUMECA software system. The model

included a structural grid of finite volumes of 92

thousand elements. The value of the parameter y+ did

not exceed 1. The Spalart-Allmaras turbulence model

was used. In general, the settings of the numerical

model of an air turbine corresponded to the typical

settings used in the calculation of the working process

in turbines (Popov et al., 2018).

Figure 3: Characteristics 𝐾

 

𝑓𝜋



 of used APU.

Figure 4: Mass flow characteristics 𝐾

 

𝑓𝜋



 of

investigated air turbine.

The calculation results obtained using the created

calculation model were compared with the

experimental data provided by the customer (Figure

5). The relative error in determining the ATS power

was less than 2.5%, and the air mass flow rate was

less than 2%.

The characteristics 𝐾



𝑓𝜋



 for the ATS and

APU were combined at one diagram (Figure 6).

Optimization of a Single-stage Air Starter Turbine

157

Power characteristics

Mass flow characteristics

Figure 5: Comparison of calculation results obtained with

the created computational model with experimental data.

Figure 6: Combined flow characteristic of the APU and

ATS.

As can be seen from Figure 6, all the lines of APU

and ATS operating modes intersect, which indicates

that the condition for their joint operation is fulfilled

at all operating modes. However, it was found during

the calculation, that the start-up time of the gas

turbine when using the ATS exceeds the value

specified in the technical requirements by 12.6%. At

the same time, the maximum torque on the turbine

shaft did not exceed the permissible value. Thus, it is

necessary to increase the turbine power (without

exceeding its limit value) and the air flow through it

in order to reduce the start-up time to use the turbine

for a new GTE start system.

Analysis of the turbine working process showed

that the required increase in the turbine power and

mass flow rate can be achieved with minimal changes

in the initial design only by modernizing the shape of

the nozzle guide vanes. In the other words, the

modernization of the existing ATS to the new

requirements can be performed while substantially

preserving the design. In fact, only one part will be

changed that is the nozzle block.

2 PARAMETRIC STUDIES

To search for a possible solution, parametric studies

were conducted on the influence of these parameters

on the start-up time.

2.1 Changing the NGV Stagger Angle

During the study of the influence of the stagger angle

of NGV on the flow rate of the working fluid and the

power of the turbine, the angle varied in the range of

+2 relative to the initial value. In total, five points

were considered (including the initial geometry) with

a step of 0.5. A larger change in the angle will lead

to a change in the design of adjacent parts and a

significant amount of alteration of the original design.

The obtained dependences of the mass flow

parameter 𝐾



, start-up time and torque on changes in

NGV stagger angle are shown in Figure 7. It can be

seen that with a maximum increase in the NGV

stagger angle, the start-up time of the gas turbine

engine was reduced by only 7.6%. This value is 5.7%

higher than the limit value (i.e., such a turbine does

not satisfy the requirements). The maximum value of

torque was 27.5% less than the maximum allowable,

which meets the requirements of the technical

specifications. Thus, changing only the stagger angle

within the existing constraints on the axial length of

the NGV part is not enough to increase the turbine

power and to reduce the start-up time of the GTE.

2.2 Trimming the Trailing Edge of

NGV

Five variants for trimming the trailing edge of the

NGV were considered (Figure 8). The dependences

of the flow rate parameter 𝐾



, start-up time and

torque on the value of trimming the NGV trailing

edge are obtained. They are shown in Figure 8. It can

SIMULTECH 2020 - 10th International Conference on Simulation and Modeling Methodologies, Technologies and Applications

158

be seen from the presented results that, the start-up

time was reduced by 10.7% by trimming the NGV

trailing edge, which is 2.1% more than the maximum

allowable τ_max. The maximum torque value was

21.8% less than the maximum allowable M

torque_max

Figure 7: Dependences of the parameters of the starter

turbine and the start-up time of the gas turbine engine on

the stagger angle of the ATS NGV.

Figure 8: The hub and shroud sections of the NGV variants

with trimming trailing edge.

Thus, trimming the NGV trailing edge also does

not provide the GTE start-up time specified in the

technical task. During the study, it was found that the

proposed modernization options cause significant

deviations of the inlet flow angles for the rotor blade,

which are the reason of significant dynamic stresses

in it.

Thus, none of the considered variants of ATS

modernization satisfies the requirements of the

technical specifications with its design is preserved to

the maximum. For this reason, it was decided to use

mathematical optimization methods to find an

acceptable variant.

Figure 9: Dependences of the mass flow parameter 𝑲

𝑮

start-up time and torque on the value of trimming the NGV

trailing edge.

3 OPTIMIZATION ALGORITHM

FOR STARTER AIR TURBINE

A further search for a new shape of ATS NGV was

carried out using multicriteria mathematical

optimization methods. For this, an algorithm was

developed that is built around the IOSO optimizer

program (IOSO, 2020). The choice of the IOSO

program is due to the large number of its successful

application in the tasks of aircraft engine field,

including by the authors of the paper (12-17

Marchukov et al., 2019; Marchukov et al., 2019;

Marchukov et al., 2018; Marchukov et al., 2017;

Salnikov et al., 2019; Buyukli et al., 2017). The

specified program was used as a finished commercial

product. No upgrades were made to optimization

algorithms. A description of the algorithms used in

the program can be found on the website and

publications of the program developer.

Optimization of a Single-stage Air Starter Turbine

159

The schematic diagram of the developed ATS

optimization algorithm is shown in Figure 10. At each

optimization step, the IOSO PM optimizer generates

a vector of input parameters x

, x

, …, x

. The

values of the input parameters describe the profile

geometry of the nozzle guide vanes in a parametric

form. The vector of variable parameters is transferred

to the reprofiling unit, in which the Numeca

AutoBlade program (Numeca AutoBlade, 2020)

converts the blades and saves them as geometry files

in the *.GeomTurbo format. Then, Numeca

AutoBlade 5 creates a mesh model using new blades.

At the next step, CFD calculation is performed with

the new mesh model. Processing the results of CFD

modeling is performed by a special script using

Numeca FineTurbo (Numeca FineTurbo, 2020) and

small applications from the NET Framework library.

As a result, several output files are created containing

the ATS operation parameters of interest in the text

format. Then these parameters are transferred to the

IOSO optimizer, where the results are processed and

the current optimal variant of the ATS NGV is

selected, and a new set of input parameters x

, x

…, x

. is created. This process is iterative and runs

until the desired extremum is found, taking into

account the given constraints.

The goal of optimizing the flow part of the ATS is

to reduce the start-up time of a gas turbine engine

while reducing the torque on the turbine output shaft

(in order to increase the reliability). The presence of

two optimization criteria ensures more stable

operation of the IOSO optimizer (Kuzmenko et al.,

2007).

Based on the foregoing, the following criteria

were selected for optimization:

1) GTE start time (it must be reduced);

2) the torque on the turbine shaft (it must be

reduced).

The following parameters were set as constraints:

1) the torque at all modes must be less than the

maximum allowable value;

2) the flow parameter K

must be within the range

min

… K

max

, where the value K

min

corresponds to the minimum value of K

from

all operating modes of the APU, and K

max

to the maximum value of Kg from all operating

modes of the APU. This requirement is

necessary to fulfill the conditions for the joint

work of the APU and the ATS at all operating

modes.

The parametrization scheme for the sections of the

ATS nozzle guide vanes is shown in Figure 11.

Changing the geometry of the first nozzle guide vanes

was carried out in two control sections (hub and

shroud). Each section was described by 14

independent variables (chord, inlet and outlet design

angles, profile stagger angle, edge radii, position of

control points of the spline of the pressure side and

suction side, etc.). In total, 28 variables were used to

describe the geometry of the ATS nozzle guide vanes.

Such a statement of the optimization problem

makes it possible to find the Pareto front of optimal

solutions in which each value of the gas turbine

engine start-up time corresponds to the minimum

torque at which it can be achieved (Figure 12). Then,

in accordance with the task requirements, a variant is

selected that provides the minimum start-up time, that

can be achieved with a torque value acceptable under

the conditions of transmission strength. Obviously,

the requirement to reduce the start-up time will shift

the selected point on the Pareto front to the left as far

as possible, as much as the torque limitations allow.

Figure 10: Schematic diagram of the applied methodology for multicriteria optimization of ATS.

SIMULTECH 2020 - 10th International Conference on Simulation and Modeling Methodologies, Technologies and Applications

160

Figure 11: Parametrization scheme for the sections of the ATS nozzle guide vanes.

Figure 12: Pareto Front “Torque - Engine Start-Up Time” of the ATS variant at the most loaded operating mode.

4 OPTIMIZATION RESULTS

The optimization process was stopped after 200

calculation cycles. As a result, the Pareto front was

obtained between the values of the start-up time of the

gas turbine engine and the torque on the output shaft

of the ATS (Figure 12).

For the current problem, the case with maximum

torque and minimum start-up time is of the greatest

practical interest. The start-up time of the gas turbine

engine for the selected variant decreased by 13.7%

relative to the variant with the baseline ATS geometry

and was 1.3% less than the maximum permissible

according to the technical requirements. The

maximum torque among all the matched operating

modes of the APU did not exceed the maximum

allowable M

torque_max

by 18.5%. The value of the mass

flow parameter K

remained within the specified

limits K

min

...K

max

and was less than the value

max

by 3.8%. The latter circumstance ensured the

preservation of the coordinated work of the APU and

ATS at all operating modes. Thus, the main practical

goal of the study was achieved.

Figure 13 shows a comparison of the NGV cross-

sections of the optimized and baseline ATS variants.

The increase in the cross-sectional area and,

accordingly, the flow parameter K

, occurred due to

a decrease in the blade thickness, and due to a

decrease in the chord of the blade, especially in the

shroud section. Since the shape of only the stator

blades was adjusted, the strength of the most loaded

turbine elements did not change significantly.

Shroud section

Hub section

Figure 13: Blade-to-blade sections of ATS NGV: solid line

- the baseline variant; dashed line - optimized variant.

Optimization of a Single-stage Air Starter Turbine

161

5 CONCLUSIONS

This paper presents the results of the modernization

of the flow part of an existing air turbine for starting

system for application at another engine of the same

class. It was shown that the initial turbine satisfies the

requirements of working together with the APU at all

operating conditions, provides satisfaction of strength

requirements, but the start-up time when using it

exceeds by 12% the maximum time required in the

technical specifications. During the analysis, it was

found that it is necessary to increase the power of the

turbine by increasing air flow rate in order to fulfil the

requirements. Changing the parameters of the turbine

was carried out by changing the shape of the NGV.

Conducted parametric research did not allow to find

a solution. Therefore, the problem was solved using

the original algorithm using a commercial optimizer

program. As a result, a solution was found that made

it possible to find such a configuration of the turbine

flow path only by changing one element (NGV unit)

that ensures that the technical requirements in terms

of starting time are met with a margin of 2%. In this

case, the maximum torque on the turbine shaft is 18%

less than the permissible value and the conditions for

the joint operation of the turbine and the APU are

fulfilled at all operating modes.

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