Methods to Reduce the Resonant Stresses Level of Gas Turbine

Engines Compressor Rotor Wheels

Grigorii M. Popov, Aleksandr O. Shklovets, Aleksandr I. Ermakov and Daria A. Kolmakova

Department of Theory of Engine for Flying Vehicles, 34 Samara State Aerospace University (SSAU),

Samara, Russian Federation

Keywords: Gas Turbine Engine, Compressor, Blade, Support, Ansys, CFX, Forced Oscillations, Optimization.

Abstract: The approaches to reducing the alternating stresses in the compressor blades, arising at a resonance, are

discussed in paper. Maximum alternating stresses in blades of the fifth stage of intermediate pressure

compressor (IPC, that operating under the gas flow circumferential variation conditions, are defined on the

basis of the forced blade oscillations calculation method. Parametric CFD-model which allows to introduce

different stagger angles and circumferentially alternating blade pitch at the guide vanes of IPC fifth stage

was created to reduce the stresses. The flow circumferential variation was reduced by changing these

parameters and as a consequence the resonant stresses were decreased by more than 2.5 times.

1 INTRODUCTION

Circumferential variation of the gas flow in the

channel of gas-turbine engine (GTE) is the major

factor, exciting blades oscillations, which lead to

blade fatigue destructions (Hynes, Greitzer, 1987).

The problem is compounded by the fact that

circumferential variation of the gas flow is unsteady

and caused by large numbers of both upstream and

downstream channel elements (Kuz'michev,

Morozov, 1991). Therefore the problem of reduction

of blades gas-dynamic excitation is extremely

complex and usually solved by using a large number

of experiments (Kaya, 2003).

The blades of the fifth rotor wheel (RW5) of

five-stage intermediate pressure compressor (IPC) of

gas turbine engine was object of research in this

paper (Figure 1).

The casing of the engine middle support is

located downstream the fifth compressor stage. High

pressure compressor is located after the support

casing. There are seven unevenly distributed racks

of different cross-sections in the channel of support

casing. (Figure 2). These racks are the cause of

circumferential variation of the gas flow in GTE

passage, which leads to increased dynamic stresses

in the fifth RW blades, as a consequence, to its

breakage.

The goal of this work was to reduce the dynamic

stresses in the rotor blades of the IPC fifth stage by

means of the blades reprofiling and circumferential

variation flow reduction.

Figure 1: The scheme of investigated compressor blading.

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M. Popov G., O. Shklovets A., I. Ermakov A. and A. Kolmakova D..

Methods to Reduce the Resonant Stresses Level of Gas Turbine Engines Compressor Rotor Wheels.

DOI: 10.5220/0005108706190624

In Proceedings of the 4th International Conference on Simulation and Modeling Methodologies, Technologies and Applications (SIMULTECH-2014),

pages 619-624

ISBN: 978-989-758-038-3

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

Figure 2: Casing of middle support.

2 METHOD FOR DYNAMIC

STRESSES CALCULATION

Method for dynamic stresses calculation in RW

blades of the IPC fifth stage was created at the first

stage (Shklovets, Popov, Kolmakova, 2012). This

method consisted of four stages.

1. Calculation at the required engine mode (take-

off mode and idle mode) is performed for the IPC

sector model in the software package NUMECA

Fine Turbo. The sector model also comprises inlet

guide vane (IGV) of high pressure compressor

(HPC), middle and intermediate engine supports.

Radial profiles of total pressure, total temperature

and flow direction in the section behind the rotor

wheel of IPC fourth stage were determined as the

result of this calculation.

2. The “full circle” compressor model calculation

with boundary conditions obtained in the IPC sector

model calculation is performed in the software

package Ansys CFX (Bochkarev, Dmitriev, Kulagin,

Makeenko, Mosoulin, Mossoulin, 1993). Full circle

model consists of the following blade rows: IPC

fourth guide vane (GV4), fifth rotor wheel, fifth

guide vane (GV5), middle support and HPC inlet

guide vane (IGV). Gas-dynamic load having an

effect at the IPC fifth RW blades and considering

gas flow circumferential variation caused by the

middle support racks was determined in this

calculation.

3. The calculation of the fifth RW blades natural

frequencies is performed in the software package

Ansys Mechanical. Based on this calculation, RW

frequency diagram construction and IPC rotor

speeds, at which the resonance may occur, are

carried out.

4. Gas load is represented as a combination of

backward traveling waves of load (harmonic waves)

using the object-oriented programming language

APDL, built in Ansys. Further dynamic calculation

at the resonance mode in resonance with the most

dangerous harmonic is performed. This method is

presented schematically in Figure 3.

Figure 3: The scheme of method for calculation compressor rotor blades forced oscillations.

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3 THE MEASURES FOR BLADES

ALTERNATING STRESSES

MITIGATION

The following areas were selected as measures to

reduce the resonant stresses level:

 the usage of the blades with special Shvarov’s

profile at last rotor wheel;

 the usage of the guide vane in front of the

middle support, with the blades set having

different stagger angles and with

circumferentially alternating blade pitch.

Also, special attention was paid to the fact that

the number of blades with stagger angles different

from the standard should be as low as possible.

Parameterization of the full circle compressor

model was carried out for the introduction GV5

different stagger angles and circumferentially

alternating blade pitch.

At the first stage of creating a parametric model

the support racks and blades of GV5 were divided

into groups. For this purpose the development

drawing of the blade row in the circumferential

direction was carried out. The assumption that for

several blade groups will be given the same stagger

angle and blade pitch parameters was made for IPC

GV5 blades. When introducing the different stagger

angles the key factor was the minimum number of

changeable blades as the manufacturing a large

number of blades with different geometry greatly

increase the production costs. The quantity of the

variable blades in group was specified by the

number in brackets (Table 1).

Then, the maximum blade stagger angle within

the joint groups was determined. The stagger angle

was not changed for the GV5 blades located in the

racks plane of symmetry, the first and last blades in

the group. Changing of the stagger angles within the

groups were performed linearly. Blades arranged on

opposite sides of the racks plane of symmetry were

rotated in opposite directions relative to the initial

position (Figure 4). Moreover the blades located

closer to the front were rotated by a larger angle. If

the blade was rotated on closing (increase of the

stagger angle) before the angle value there is a sign

"+" if on opening (decrease of the stagger angle) -

sign "-".

The number of removable blades was not

restricted when blade pitch were changed. Parameter

of alternating blade pitch was set within 0.35...0.35

of the base pitch. The sign "-" means that in the

region between the blades the pitch is decreased,

while "+" - the pitch is increased. The number

indicates the maximum blade pitch increase

(decrease) in the group in relative values from the

nominal pitch with evenly spaced blades. Position of

extreme blades in groups was not changed when

introducing the circumferentially alternating blade

pitch. The law of pitch changing was also linear.

Thus, the universal parametric model which allows

to the introduction different stagger angles and

circumferentially alternating blade pitch at the IPC

fifth stage was created.

4 OPTIMIZATION

CALCULATIONS OF THE

DYNAMIC STRESSES IN THE

IPC FIFTH ROTOR BLADE

Calculations of dynamic stress for 11 variants of the

IPC fifth stage were performed using a parametric

model (Shklovets, Popov, Kolmakova, 2013). Blade

with a special Shvarov’s profile was used as the fifth

stage rotor blade in all the variants. The variants

differed from each in values of different stagger

angles and alternating blade pitch parameters of IPC

GV5. At the same time the first variant corresponded

to the base GV5.

The calculation results of the dynamic stresses

which arise at a resonance with the strongest 12th

harmonic, parameters of each variant as well as the

total number of GV variable blades of IPC fifth

stage are shown in Table 1.

Figure 4: Rotation scheme of guide vane blades.

MethodstoReducetheResonantStressesLevelofGasTurbineEnginesCompressorRotorWheels

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Table 1: The results of the parametric IPC model calculation.

Variant

number

Parameter of different stagger

angles, maximum stagger

angles (number of blades) for

the groups:

Parameter of alternating blade pitch for the

groups:

2, 5, 6 1, 3, 7 4 (3) 1, 7 2, 6 3 4 5

Number

variable

blades

Dynamic

stresses

MPa

1 0 0 0 0 0 0 0 0 0 86.7

2 0 0 0 0.3 0.3 0.3 0.3 0.3 0 115.98

3 3 (6) 3 (6) 3 (8) 0 0 0 0 0 42 43.791

4 3 (6) 3 (6) 3 (8) 0.3 0.3 0.3 0.3 0.3 42 31.877

5 3 (6) 6 (6) 9 (8) 0 0 0 0 0 42 37.536

6 0 0 0 0.35 0.35 0.35 0.35 0.35 0 86.95

7 0 0 0 -0.3 -0.3 -0.3 -0.3 -0.3 0 105.94

8 0 0 0 -0.15 -0.15 -0.15 -0.15 -0.15 0 137.741

9 6 (2) 6 (2) 6 (2) 0 0 0 0 0 14 46.737

10 0 6 (2) 6 (2) 0 0 0 0 0 8 57.214

11 6 (2) 6 (2) 6 (4) 0 0 0 0 0 16 44.426

From the data presented in Table 1, it is clear

that a significant reduction of the dynamic stresses

in the RW of IPC fifth stage at resonance with the

12th harmonic was achieved when applying the

Shvarov’s profile at rotor blades and introducing the

different stagger angles at GV of the IPC fifth stage.

Thus the level of dynamic stresses at resonance with

the 12th harmonic was 86.7 MPa when using RW5

blades with the Shvarov’s profile and the base GV5.

And when the Shvarov’s profile at RW5 and also

different stagger angles and alternating blade pitch

parameters of IPC GV5 were used (variant No 4 in

Table 1, changeable GV5 blades value is 42) the

level of dynamic stresses at resonance with the 12th

harmonic was 31.9 MPa (reduced by 2.7 times).

However, the usage of RW5 blades with the

Shvarov’s profile and GV5 blades with different

stagger angles (variant No 9 in Table 1, changeable

GV5 blades value is only 14) the level of dynamic

stress is not much large than in variant No 4 – 46.7

MPa (reduced by 1.85 times).

4.1 Analysis of Circumferential

Variation Calculation for the IPC

Fifth Guide Vane Variants Giving

the Largest Dynamic Stresses

Reduction

The relative static pressure profiles in the cross

section following the IPC RW5 at mid-span for the

basic compressor case and compressor variant No 4

are shown in Figure 5 for the idle mode. The Figure

5 shows the basic version in blue and modernized

(variant No 4) – in red. Marked pressure peaks

correspond to the support racks and areas near them.

Figure 5: Relative static pressure variation in the section

following the IPC RW5 at mid-span.

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Base variant Variant No4

Figure 6: Static pressure field close to support rack in area 1.

Base variant Variant No4

Figure 7: Mach number field close to lower support rack.

Comparison of static pressure distribution fields

at mid-span the base compressor case and variant No

4 (Table 1) at area 1 are shown in Figure 6. A

similar comparison for the flow Mach number is

shown in Figure 7.

Apparently from the presented graphs and static

pressure distribution fields static pressure peaks

MethodstoReducetheResonantStressesLevelofGasTurbineEnginesCompressorRotorWheels

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decrease is observed for modernized variant (No 4)

compared to the base compressor case at the areas 1,

3 and 5. In addition, the more periodicity and more

uniform flow field is achieved for all specific areas.

5 CONCLUSIONS

Thus, the following results were achieved in the

course of this work:

1. Significant reduction of the dynamic stresses

in the IPC fifth rotor wheel at resonance with the

twelfth harmonic was achieved. More specifically,

the dynamic stresses were reduced by one half

compared with the RW5 with Shvarov’s profile and

base variant of GV5.

2. It was found that the different blades stagger

angles and alternating blade pitch introduction

allows to "flatten" the circumferential variation in

the cross section following the IPC RW5.

ACKNOWLEDGEMENTS

This work was financially supported by the Ministry

of education and science based on the Government

of the Russian Federation Decree of 09.04.2010 №

218 (code theme 2013-218-04-4777) and in the

framework of the implementation of the Program of

increasing the competitiveness of SSAU among the

world’s leading scientific and educational centers for

2013-2020 years.

REFERENCES

Hynes, T.P., Greitzer, E. M. 1987. A method for assessing

effects of circumferential flow distortion on

compressor stability. In Journal of Turbomachinery

vol. 109 (3), pp. 371-379.

Kuz'michev, V.S., Morozov, M.A. 1991. Conception of

method of pattern recognition of working process of

gas turbine engines in conditions of information

deficit. In Izvestiya VUZ: Aviatsionnaya Tekhnika

vol.(3), pp. 44 – 49.

Kaya, D. 2003. Experimental study on regaining the

tangential velocity energy of axial flow pump. In

Energy Conversion and Management vol. 44 (11), pp.

1817-1829.

Shklovets A.O., Popov G.M., Kolmakova D.A. 2012.

Issues of numerical investigations of forced axial

compressor blades oscillations. In Vestnik

dvigatelestroeniya vol. 2, pp. 223-227

Bochkarev, S.K., Dmitriev, A.Ya., Kulagin, V.V.,

Makeenko, S.V., Mosoulin, V.V., Mossoulin, A.A.

1993. Experience and problems of computer aided

thermogasdynamic analysis of testing results for gas-

turbine engines with complex schemes. In Izvestiya

Vysshikh Uchebnykh Zavedenij. Aviatsionnaya

Tekhnika, vol. 2, pp. 68-70.

Shklovets A.O., Popov G.M., Kolmakova D.A. 2013.

Optimization of the compressor stage blading of gas

turbine engine to ensure the dynamic strength in rotor

blade row. In Vestnik dvigatelestroeniya vol. 2, pp.

192-197.

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