Experimental study of the performance and endurance of carbon ﬁber brush seals for aero-engine bearing chambers

Abstract

Over the last decades, it has been progressively acknowledged that reducing the specific fuel consumption and the emission of pollutants as well as improving the thrust-to-weight ratio involves extensive research on advanced sealing technologies. Amongst these, brush seals are particularly well considered for their excellent leakage performance, their low friction properties, and their ability to cope with inevitable rotor excursions during flights.

This thesis presents the experimental work that has been carried on in order to characterize carbon brush seals performance in function of the bristle pack geometry and the operating conditions. The analyzed parameters are the bristle free length, the density, and the interplate distance.

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Acknowledgements

Carrying on this PhD thesis has been a life defining experience. This work could not have been performed without the support and help of many people.

First, I would like to thank my supervisor Prof. Patrick Hendrick, for giving me the chance to enter the world of academic research, for the fundings that allowed me to pursue my work and studies while living more than decently as a PhD student, for the technical and moral support, and for maintaining his trust in my abilities, especially throughout the first years where we struggled developing a reliable test bench.

It is gratefully acknowledged that the research leading to this PhD thesis has received fund-ing by the European Union Seventh Framework Program FP7/2007-2013 under grant agreement n 314366 (E-Break project led by Safran Helicopter Engines). I would like to thank Safran Air-craft Engines for their collaboration and help during the development of the test bench and the related test campaign.

I would like to thank Olivier Berten for his advice and for his everlasting good mood, and the laboratory technicians for their help and knowledge: Adrien Fita-Codina (thank you for all the evenings and week-ends you spent in the lab to help me), Pascal Beine, Dany Carlens, Lionel Lambert, Yves Simon, Abdelouahid Tarhach and Narcisse ”Charly” Yoppa Wanou. Special thanks go to Pierre Dupont, from Schaeffler, for his priceless help in designing the test bench.

I thank my fellow collegues and friends from the ATM Department, for making this place such a fun environment to work in. I do not thank my officemate Shayan for his bullying over some of the most pathetic performances of my favorite football team, PSG. However, I will truly miss our heated football discussions, and his constant jokes that lighten the atmosphere. Many thanks to Johan, Diako, Laurent, Guilherme, Jan, Mariano, Alessandro and J¨oelle for making our floor so lively, and to the notorious Italian mafia that invaded the ATM Department, for the great memories that were shared outside work.

I also would like to thank my childhood friends, and my university friends for their support and encouragements over the last few years. Every kind word counted, and every moment spent with them helped easing up the pressure.

Finally, the biggest ackowledgments go to my family, my brother Nabil and my parents, for their education, their advice, their constant moral support and their unconditional love. I would have certainly never made it that far without your sacrifices. I owe you this forever.

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List of Figures

1.1 A typical lubrication system with three bearing chambers . . . 6

1.2 Bearing chamber locations . . . 9

1.3 Bearing chamber schematics. . . 12

1.4 Corrugated heat exchangers ACOC. . . 13

1.5 SACOC principle. . . 14

1.6 Labyrinth seal. . . 15

1.7 Labyrinth seal principle . . . 16

1.8 Fanno curve . . . 16

1.9 Different types of labyrinth seals . . . 17

1.10 Ring seal. . . 19

1.11 Ring seal principle . . . 19

1.12 Circumferential carbon seal. . . 20

1.13 Feedback loops . . . 20

1.14 Carbon face seal . . . 21

1.15 Carbon face seal principle . . . 22

1.16 Finger seal close-up. . . 23

1.17 Finger seal . . . 24

1.18 Hydraulic seal. . . 25

1.19 Side view of a brush seal with clearance and interference configurations. . . 28

1.20 Face view of a brush seal. . . 28

1.21 Example of Metallic Brush Seal manufactured by Centurion . . . 29

1.22 Schematic illustration of a ceramic brush seal assembly. . . 31

1.23 Normalized wear rate of aramid fibers Vs. normalized wear rate of Haynes 25 fibers 32 1.24 Normalized leakage performance of aramid fibers Vs. normalized leakage perfor-mance of Haynes 25 fibers . . . 33

1.25 Comparison of the heat generations of carbon bristles and kevlar bristles at an interference of 0.1 mm . . . 34

1.26 Stiffness ratio Vs. pressure drop for a MTU metallic brush seal . . . 36

1.27 Pressure balance cavity in a brush seal . . . 36

1.28 Influence of the rotational speed on the bristles tip force . . . 37

1.29 Brush seal hysteresis curve. . . 38

1.30 Control volume of a bristle for blow-down explanation purpose. . . 39

1.31 Evidence of hydrodynamic lift of brush seals. . . 41

2.1 Principle schema of the test bench. . . 44

2.2 Cross section of the rotating unit.. . . 45

2.3 Close-up of the pressurization chamber. . . 46

2.4 Close-up of the torquemeter, timing belt, and motor. . . 46

2.5 Air circuit. . . 48

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List of Figures ix

2.7 Control panel of the test bench. . . 51

2.8 Carbon fiber brush seal plan. . . 53

2.9 Cross section of a cabon fiber brush seal.. . . 54

2.10 Zooms of brush seal carbon fibers. . . 54

2.11 Footprint of a brush seal without interference, and definition of the brush seal density. . . 55

3.1 Leakage flow of brush seal n◦7 with an interference of 0.75 mm at 4000 rpm.. . . 64

3.2 Friction torque of brush seal n◦7 with an interference of 0.75 mm at 4000 rpm. . 64

3.3 Leakage flow of a MTU metallic brush seal with in a line-to-line configuration at 4000 rpm. . . 64

3.4 MTU metallic brush seal torque with in a line-to-line configuration at 4000 rpm. 64 3.5 Proportionality between leakage flow and differential pressure.. . . 65

3.6 Typical brush seal leakage performance curve . . . 65

3.7 Leakage performance of brush seal n◦5 with an interference of 0.75 mm, as a function of the rotational speed. . . 66

3.8 Leakage performance of brush seal n◦1 in function of the interference at 4.000 RPM. . . 67

3.9 Leakage flow with identical fence heights, and δ = 1 mm. . . 68

3.10 Leakage flow with identical fence heights, and δ = 1.8 mm. . . 68

3.11 Leakage performance of brush seals n◦1, n◦3 and n◦5, with a line-to-line config-uraton, at 8000 RPM. . . 69

3.12 Axial displacement under the effect of shaft excursion. . . 70

3.13 Leakage performance of brush seals n◦1, n◦2, n◦3 and n◦4, with i = 0.465 mm, at 8000 RPM. . . 71

3.14 Evolution of the dynamic brush seal permeability in function of the relative in-terference. . . 72

3.15 Kp in function of i/L made independent from the fiber free length. . . 74

3.16 Kp in function of i/L made independent from the density. . . 75

3.17 Fiber pack strand modeled as a beam under differential pressure. . . 77

3.18 Fiber front displacement as a function of the fiber tip deflection. . . 77

3.19 Kp in function of i0 made independent from the inter-plate distance. . . 79

3.20 General equation of the brush seal Kp . . . 80

3.21 Kp increase of every seal from static to dynamic case in function of the interference. 81 3.22 Comparison of brush seals Kp in dry and wet conditions at 200 RPM, with oil at 50◦C. . . 82

3.23 Kp,oil vs. Kp,dry for brush seals at 200 RPM with oil at 50◦C. . . 83

3.24 Comparison of brush seals Kp for different oil temperatures . . . 84

3.25 Kp,oil vs. Kp,dry for brush seals at 200 RPM with oil at 90◦C. . . 85

3.26 Evolution of Kp,oil decrease with the relative interference for brush seals n◦1, n◦2 and n◦5. . . 86

3.27 Evolution of Kp,oilincrease with the relative interference for brush seals n◦3, n◦4, n◦7 and n◦8. . . 86

3.28 Evolution of Kp,oil increase with the relative interference for brush seal n◦5 . . . 87

3.29 Diagram summarizing the effect of rotational speed on Kp in wet conditions . . . 87

3.30 Definition of the hysteresis proneness. . . 89

3.31 Normalized hysteresis H0 of brush seals in function of the fence height with N = 8000 RPM. . . 90

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List of Figures x

3.33 Hysteresis proneness of brush seals in function of the fence height with N = 8000

RPM, with and without oil. . . 92

3.34 Relative uncertainty of Kp in function of the maximum differential pressure reached during tests. . . 95

3.35 Leakage flow and seal torque with presence of fibers lift-off. . . 96

3.36 Leakage flow and seal torque with presence of fibers blow-down.. . . 96

3.37 Representative curve for e/e0. . . 97

3.38 Comparison of experimental data with flow models.. . . 98

4.1 Seal torque of brush seal n◦1 and brush seal n◦3. . . 105

4.2 Seal torque of a MTU metallic brush seal in quasi-static conditions. . . 105

4.3 Evolution of the brush seal n◦1 torque for ∆p = 0 in function of the interference and the rotational speed. . . 106

4.4 Evolution of the metallic brush seal torque with ∆p = 0 in function of the surface speed and differential pressure. . . 107

4.5 Residual torque of the test bench in function of the rotational speed at two different dates. . . 108

4.6 Evolution of brush seals torque with ∆p = 0 in quasi-static conditions. . . 109

4.7 Corrected seal torque with ∆p = 0 in quasi-static conditions in function of i/FH. 110 4.8 Evolution of the dimensionless friction torque of brush seals 1, 3, 5 and 7 with ∆p = 0 in quasi-static conditions in function of i/FH, made independent from the fiber free length. . . 111

4.9 Evolution of the fiber front displacement in function of the interference for dif-ferent fiber free lengths. . . 112

4.10 Evolution of the dimensionless seal torque with ∆p = 0, in quasi-static conditions in function of i/FH, made independent from the inter-plate distance effect. . . . 113

4.11 General equation describing carbon brush seals torque when ∆p = 0 in quasi-static conditions. . . 114

4.12 Evolution of Kω in function of the relative interference i0. . . 115

4.13 Influence of KLO on the seal torque decrease due to lift-off. . . 117

4.14 Evolution of the friction torque of brush seal 3 with rotor 6, and brush seal 1 with rotor 3, in function of the differential pressure. . . 118

4.15 Evolution of the coefficient KLO in function of the fence height for every seal. . . 119

4.16 Evolution of the τc0 - τinf ty in function of the fence height for every seal. . . 120

4.17 Evolution of the friction torque of brush seal 6 with rotor 4, in function of the differential pressure at 12000 RPM.. . . 121

4.18 Evolution of KBD in function of the interference for all seals. . . 122

4.19 Non-dimensional characterization of KBD. . . 123

4.20 Influence of 50◦C oil on the seal torque in quasi-static conditions. . . 124

4.21 Influence of oil temperature on the seal torque. . . 125

4.22 Influence of differential pressure under wet conditions for seal n◦5 and i = 0.6 mm.126 4.23 Characterization of the lift-off proneness coefficient under wetted conditions.. . . 127

4.24 Brush seal n◦ 1 with i = 0.6 mm torque decrease with ∆pc. . . 128

4.25 Brush seal n◦ 5 with i = 0.6 mm torque decrease with ∆pc. . . 128

4.26 Evolution of Kω,oil with the relative interference. . . 129

4.27 Comparison of Kω under dry and wet conditions. . . 129

4.28 Experimental setup featuring the IR Camera and the brush seal mounted on the motor. . . 132

4.29 Close-up of the brush seal mounted on the electric motor. . . 132

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List of Figures xi

4.31 Schematics of the heat transfer mechanisms of the system. . . 134

4.32 Meshing and boundary conditions. . . 136

4.33 Evolution of total heat dissipation in function of the sliding speed for i/L = 50 %.138 4.34 Evolution of heat dissipation in function of the interference for u = 17 m/s. . . . 139

4.35 Evolution of heat dissipation in function of the interference for u = 17 m/s, calculated with the FEM model and using seal torque data. . . 140

4.36 Evolution of the rotor outer temperature in function of the surface speed for i/L = 50 %. . . 141

5.1 Tribosystem typical wear and wear rate in function of the operating time. . . 143

5.2 Test bench approximation of the low pressure ∆p vs. time cycle. . . 145

5.3 Test bench approximation of the low pressure rotation speed vs. time cycle. . . . 145

5.4 Brush seal sample wear after endurance testing in dry conditions after 100 hours 146 5.5 fiber total length estimation . . . 147

5.6 Brush seal sample wear after endurance testing in wet conditions after 250 hours. 148 5.7 Brush seal sample wear after endurance testing in wet conditions after 250 hours. 148 5.8 Evolution of Kp with the operating time, with and without oil. . . 149

5.9 Evolution of τcwith the operating time, with and without oil. . . 149

5.10 Projection of Kp with the operating time, in dry conditions. . . 152

5.11 Projection of τcwith the operating time, in dry conditions. . . 152

5.12 Projection of Kp with the operating time, in wet conditions. . . 152

5.13 Projection of τcwith the operating time, in wet conditions. . . 152

6.1 Kp Vs. τ for wetted brush seals n◦2 and n◦5. . . 156

6.2 Hysteresis proneness of brush seals n◦2 and n◦5. . . 156

6.3 Brush seal Kp in function of the fiber free length and the interference, in wet conditions.. . . 157

6.4 Brush seal quasi-static torque in function of the fiber free length and the inter-ference, in dry conditions. . . 158

6.5 Brush seal heat generation during maximum take-off for the HP stage. . . 159

6.6 Brush seal heat generation during cruise for the HP stage. . . 159

6.7 Brush seal heat generation during ground idle for the HP stage. . . 159

6.8 Brush seal heat generation during flight idle for the HP stage. . . 159

6.9 Brush seal heat generation during maximum take-off for the LP stage. . . 160

6.10 Brush seal heat generation during cruise for the LP stage. . . 160

6.11 Brush seal heat generation during ground idle for the LP stage. . . 160

6.12 Brush seal heat generation during flight idle for the LP stage. . . 160

6.13 Shaft temperature in function of the brush seal design during maximum take-off for the HP stage. . . 161

6.14 Shaft temperature in function of the brush seal design during cruise for the HP stage. . . 161

6.15 Shaft temperature in function of the brush seal design during ground idle for the HP stage. . . 161

6.16 Shaft temperature in function of the brush seal design during flight idle for the HP stage. . . 161

6.17 Shaft temperature in function of the brush seal design during maximum take-off for the low pressure stage. . . 162

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List of Figures xii

6.19 Shaft temperature in function of the brush seal design during ground idle for the

low pressure stage. . . 162

6.20 Shaft temperature in function of the brush seal design during flight idle for the low pressure stage. . . 162

6.21 Total air consumption during the cycle for the HP stage.. . . 163

6.22 Total air consumption during the cycle for the LP stage. . . 163

A.1 Cross section of the bearing HCB7206-C-2RSD-T-P4S-UL . . . 170

C.1 Bristle pack strand modeled as a beam under differential pressure. . . 172

C.2 Beam modelization of the carbon fiber.. . . 173

C.3 Diagram forces on the fiber. . . 173

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List of Tables

1.1 Operating limits for different sealing technologies.. . . 25

1.2 Operating limits for brush seals. . . 26

1.3 List of main brush seals manufacturers. . . 27

1.4 Mechanical characteristics of brush seal bristles/fibers. . . 35

1.5 Oil temperature rise in function of speed and differential pressure. . . 42

2.1 List of the test bench variables and sensors. . . 50

2.2 Range of parameters of the test bench. . . 52

2.3 Brush seal samples. . . 56

2.4 Diameters of rotor discs. . . 56

3.1 Coefficients for the linear function defining Kp in function of the relative inter-ference. . . 73

3.2 Boundary conditions for f2 . . . 75

3.3 Coefficients f3 et f6 for brush seals 1 to 8 . . . 78

6.1 Operating conditions of a typical aero-engine . . . 156

6.2 Optimized brush seals geometrical parameters and performance. . . 164

6.3 Air consumption of different newly mounted seals per phase. . . 164

A.1 Technical data of the bearing HCB7206-C-2RSD-T-P4S-UL . . . 170

D.1 Brush seal Kp for N = 0 RPM . . . 176

E.1 Brush seal Kp for N = 0 RPM . . . 178

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Symbols

Symbol Definitions Units

a acceleration m s−2

A hysteresis induced leakage factor −

A0 normalized hysteresis induced leakage factor −

Cp specific heat J(kg K−1)

d bristle diameter mm

D seal diameter mm

e thickness mm

E Modulus of Young GPa

f1,2,...,7 leakage flow correlation coefficients

-F seal friction force N

F H fence height mm

h heat transfer coefficient by convection W (m−2K)

H hysteresis area

-i interference mm

i0 relative interference mm

I moment of intertia m−4

Kb bristle stiffness

-KBD blow-down proneness coefficient

-KL,δ seal torque correlation coefficients

-KLO lift-off proneness coefficient

-Kp brush seal permeability

-KT oil temperature coefficient (K−1)

Kω rotational speed coefficient

-˙

m mass flow rate g s−1)

L bristle free length mm

L0 normalized bristle length mm

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Symbols xv

Ltot bristle total length mm

N rotational speed RPM

p pressure Pa

Re Reynolds number

-Q heat generation rate W m2)

˙

Q volumetric flow rate L h−1)

S brush seal cross section m2

T temperature K

T S tensile strength MPa

u surface speed m s−1

w wear rate m3s−1

W seal normal load N

α thermal expansion coefficient K−1

β cant angle ◦

δ inter-plate distance mm

porosity

-θ number of strands in the axial direction

-λ thermal conductivity W(mK)−1

µ friction coefficient

-ν kinematic viscosity Pa s

ξ thermal effusivity WK−1 m−2 s−1/2

ρ density kg m−3

σ axial density bristles mm−2

τ friction torque Nm

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Subscripts

Subscripts Definitions

0 refers to the quasi-static conditions atm refers to the atmospheric conditions

ax refers to the axial direction

b refers to the bristle

bleed refers to the compressor bleed air

BM refers to the bain-marie

c refers to corrected values

dry refers to dry conditions

in refers to the inlet

inj refers to the oil injector

max refers to maximum values

pump refers to the pump

out refers to the outlet

rad refers to the radial direction

tank refers to the oil tank

t refers to the bristle tip

tot refers to total values

wet refers to the wet (lubricated) conditions

∞ refers to steady-state values

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Acronyms

Acronyms Definitions

ADU Analog (to) Digital Units

AGB Accesory GearBox

BTP Bristle Tip Pressure

CFD Computational Fluid Dynamics

FBC Front Bearing Chamber

FEM Finite Element Model

HP High Pressure

LP Low Pressure

MTO Maximum Take Off

RBC Rear Bearing Chamber

SFC Specific Fuel Consumption

ULB Universit´e Libre (de) Bruxelles

Experimental study of the performance and endurance of carbon ﬁber brush seals for aero-engine bearing chambers