THE RATE OF OXYGEN EVOLUTION FROM AVIATION

TURBINE FUEL WITHIN AIRCRAFT FUEL TANKS

by

Adam Paul Harris

A thesis submitted in partial fulfilment of the requirements of the University of the West

of England, Bristol for the degree of Doctor of Philosophy

Faculty of Health and Life Sciences, University of the West of England, Bristol

February 2012

ii

Abstract

Managing the effects of dissolved air evolution from aviation fuel has presented long-standing

issues for the design and operation of aircraft fuel systems. This phenomenon, known colloquially

as fuel outgassing, is responsible for a broad spectrum of fuel system issues, including; increased

fuel tank flammability, two-phase flow in pipes, fuel pump cavitation and fuel tank over-

pressurisation. The rate and effects of oxygen evolution from Jet A-1 aviation turbine fuel is

studied here using experimental techniques, dimensional modelling and aircraft flight testing. The

rate of fuel agitation present within a laboratory fuel tank was demonstrated to have the greatest

effect on the rate of oxygen evolution from the fuel. Oxygen evolution rate increased

hyperbolically with increasing fuel agitation rate under pressure and temperature conditions

consistent with an aircraft fuel tank during flight. Dimensional modelling was used to estimate

the rate of oxygen evolution in an Airbus A320-200 aircraft fuel tank from measurements made

on a dimensionally similar laboratory model. The extrapolated rate of oxygen evolution from

similarity laws was found to be over 200% greater in the A320 inner wing fuel tank than that

measured in the laboratory model. Further work is required to validate the similarity laws of fuel

outgassing with flight test data if dimensional modelling is to be adopted for estimating fuel

outgassing rates in aircraft fuel tank flammability studies. Flight testing on an Airbus A340-300

aircraft revealed the effect of fuel outgassing on a nitrogen inerted Centre Wing Fuel Tank

(CWT) ullage to be minimal. CWT ullage oxygen concentration increased primarily due to

atmospheric air inspired via the vent system, resulting from a reducing fuel quantity in the CWT.

This unexpected result is believed to have been influenced by a combination of the fuels

tendency to absorb nitrogen from the ullage during CWT refuel, a large ullage to fuel ratio and

near quiescent CWT fuel conditions.

iii

In memory of the 230 passengers and flight crew who lost their lives in the TWA 800 accident on

July 17th 1996 over the Atlantic Ocean

iv

Acknowledgements

I would like to thank Prof. Norman Ratcliffe for his unending support and motivation. His advice,

guidance and enthusiasm aided the writing of this thesis in innumerable ways.

I would like to express my gratitude to Dr. Joseph Lam, Dr. Kevin Golden and Dr. Paul White for

their assistance and guidance with mathematical and statistical analysis of test data. I would also

like to thank Dr. Fritjof Korber for constructive feedback and words of wisdom during my

progression examination.

I gratefully acknowledge the support of Moreton Sandford and James Henshaw at Airbus, Filton

for their participation in stimulating and thought provoking technical discussion on the physics of

fuel outgassing. I would like to thank David Oram for his suggestions and profound advice on

tackling the mathematical challenges encountered within this work. My thanks are also extended

to Mike Wilson and Philippe Foucault at Airbus, Toulouse who made flight testing on an A340

aircraft a reality. In addition, I gratefully acknowledge the financial support provided by Airbus.

I will be forever indebted to the eminent Dr. Thomas Szirtes for introducing me to the fine art of

dimensional modelling and for sharing his skills developed over an illustrious career. I would like

to thank Penny Szirtes for her first rate secretarial and administrative support, which eased trans-

Atlantic communications.

Finally, I would like to thank my wife, Katie, whose encouragement, support and enduring

patience has enabled me to focus the last five years of my life on the completion of this work.

v

Table of Contents

Abstract ........................................................................................................................................... ii

Acknowledgements ....................................................................................................................... iv

Chapter 1 Introduction.................................................................................................................. 1

1.1 Background ............................................................................................................................ 1

1.2 Scope of Research .................................................................................................................. 3

1.3 Thesis Layout ......................................................................................................................... 4

Chapter 2 The Behaviour of Dissolved Air in Aviation Turbine Fuel ...................................... 7

2.1 Introduction ............................................................................................................................ 7

2.2 Air Solubility in Aviation Fuel .............................................................................................. 7

2.2.1 Ostwald Coefficient ........................................................................................................ 8

2.2.2 Bunsen Coefficient .......................................................................................................... 8

2.2.3 Absorption Coefficient .................................................................................................... 8

2.3 Effect of Pressure on Air Solubility in Aviation Fuel .......................................................... 10

2.4 Effect of Temperature on Air Solubility in Aviation Fuel ................................................... 11

2.5 Composition of Air in Aviation Fuel ................................................................................... 19

2.5.1 Effect of Fuel Temperature on Dissolved Air Composition ......................................... 21

2.5.2 Composition of Air Released from Aviation Fuel ........................................................ 21

2.5.3 Effect of Fuel Properties on Air Solubility ................................................................... 23

2.6 Mechanisms of Dissolved Air Evolution ............................................................................. 25

2.6.1 Diffusive Mass Transfer to Gas Bubbles in Liquids ..................................................... 25

2.7 Conclusions .......................................................................................................................... 27

Chapter 3 The Effect of Dissolved Air Evolution on Aircraft Fuel Systems .......................... 29

3.1 Introduction .......................................................................................................................... 29

3.2 Fuel Tank Inerting Systems ................................................................................................. 30

3.2.1 Effect of Air Evolution on Inerting System Performance ............................................. 32

3.3 Engine Fuel Feed Systems ................................................................................................... 35

3.3.1 Functionality of the Engine Fuel Feed System ............................................................. 35

3.3.2 Effect of Air Evolution on Engine Fuel Feed ............................................................... 38

3.3.3 Vapour/Liquid Ratio ..................................................................................................... 38

3.3.4 Effect of Air Evolution on Engine Backing Pumps ...................................................... 41

vi

3.4 Fuel Quantity Indication Systems (FQIS) ............................................................................ 43

3.5 Vent Systems ....................................................................................................................... 44

3.5.1 Effect of Air Evolution on Vent System Performance .................................................. 45

3.6 Conclusions .......................................................................................................................... 49

Chapter 4 Measurement of Air, Oxygen and Nitrogen in Aviation Fuels .............................. 51

4.1 Introduction .......................................................................................................................... 51

4.2 Physical Methods ................................................................................................................. 52

4.3 Chemical Methods ............................................................................................................... 54

4.3.1 Gas Chromatographic Methods .................................................................................... 54

4.3.2 Electrochemical Methods .............................................................................................. 56

4.4 Optical Sensing Methods ..................................................................................................... 58

4.4.1 Optical Absorption Spectroscopy ................................................................................. 58

4.4.2 Measurement Principle ................................................................................................. 59

4.4.3 Luminescence Quenching ............................................................................................. 60

4.4.4 Measurement Principle ................................................................................................. 60

4.5 Conclusions .......................................................................................................................... 62

Chapter 5 Development of an Experimental Apparatus to Determine Oxygen Evolution

Rate from Aviation Turbine Fuel ............................................................................................... 64

5.1 Preliminary Considerations .................................................................................................. 64

5.1.1 Preliminary Tests .......................................................................................................... 65

5.2 Experimental Design ............................................................................................................ 66

5.2.1 Overall Assembly .......................................................................................................... 66

5.2.2 Thermal-Altitude Test Chamber ................................................................................... 68

5.2.3 Fuel Tank ...................................................................................................................... 69

5.2.4 Fuel Agitation ............................................................................................................... 71

5.2.5 Impeller Pumping Capacity .......................................................................................... 75

5.2.6 Impeller Pumping Capacity in Aviation Fuel ............................................................... 81

5.2.7 Fuel Sparging ................................................................................................................ 85

5.2.8 Oxygen Sensing ............................................................................................................ 87

5.2.9 Tunable Diode Laser Absorption Spectroscopy (TDLAS) Oxygen Sensor ................. 88

5.2.10 Polarographic Oxygen Sensor ..................................................................................... 92

5.3 Simulation of Aircraft Fuel Tank Pressures ......................................................................... 94

vii

5.3.1 Climb Profiles ............................................................................................................... 95

5.3.2 Pressure Profiles ............................................................................................................ 96

5.4 Oxygen Sensor Measurement Uncertainty .......................................................................... 99

5.4.1 Measurement Uncertainty Calculation Methods ........................................................... 99

5.5 Summary ............................................................................................................................ 105

Chapter 6 Experimental Examination on the Rate of Oxygen Evolution from Aviation

Turbine Fuel ............................................................................................................................... 106

6.1 Introduction ........................................................................................................................ 106

6.2 Experimental Design .......................................................................................................... 106

6.3 Experimental Method ......................................................................................................... 107

6.3.1 Repeatability of Experimental Method ....................................................................... 109

6.3.2 Calculation of Oxygen Evolution Rates ...................................................................... 110

6.3.3 ASTM D2779-92 Oxygen Solubility Estimates ......................................................... 110

6.3.4 Non-Linear Regression Analysis ................................................................................ 112

6.4 Experimental Results ......................................................................................................... 115

6.4.1 Measurement of Oxygen Partial Pressures ................................................................. 115

6.4.2 Rate of Oxygen Evolution Results Fuel Agitation Rate .......................................... 119

6.4.3 Rate of Oxygen Evolution Results Fuel Temperature ............................................. 121

6.4.4 Rate of Oxygen Evolution Results Rate of Ullage Pressure Change ....................... 124

6.4.5 Time Constant of Oxygen Evolution Results ............................................................. 126

6.5 Statistical Analysis of Test Data ........................................................................................ 128

6.6 Discussion of Results ......................................................................................................... 130

6.6.1 Effect of Fuel Agitation Rate ...................................................................................... 130

6.6.2 Limitations of the Reciprocal Model .......................................................................... 139

6.6.3 Statistical Significance of Fuel Agitation Rate ........................................................... 140

6.6.4 Effect of Fuel Temperature ......................................................................................... 140

6.6.5 Effect of Rate of Change of Ullage Pressure .............................................................. 145

6.6.6 Time Constant of Oxygen Evolution, ...................................................................... 148

6.7 Conclusions ........................................................................................................................ 151

Chapter 7 Dimensional Modelling of Oxygen Evolution Rate from Aviation Turbine Fuel in

Aircraft Fuel Tanks ................................................................................................................... 153

7.1 Introduction ........................................................................................................................ 153

viii

7.2 Dimensional Similarity and Modelling .............................................................................. 154

7.3 Identifying Relevant Variables .......................................................................................... 155

7.3.1 Consideration of Other Physical Variables ................................................................. 158

7.4 The Dimensional Method .................................................................................................. 160

7.4.1 Sequence of Physical Variables in the Dimensional Set ............................................. 161

7.4.2 Scale Factors ............................................................................................................... 161

7.4.3 The Model Law ........................................................................................................... 162

7.5 Model Design ..................................................................................................................... 163

7.5.1 Flight Test Data ........................................................................................................... 163

7.5.2 A320 Engine Fuel Feed System Performance Analysis ............................................. 165

7.5.3 Model Fuel Agitation Rate (2) .................................................................................. 168

7.5.4 Model Rate of Ullage Pressure Change ( 2p ) ............................................................. 170

7.6 Model Tests ........................................................................................................................ 172

7.6.1 Surface Tension Measurements .................................................................................. 173

7.6.2 Measurement of Oxygen Partial Pressures in the Model ............................................ 174

7.7 Results ................................................................................................................................ 176

7.7.1 Rate of Oxygen Mass Release .................................................................................... 176

7.8 Discussion .......................................................................................................................... 179

7.8.1 Oxygen Evolution Rate Measurements from Dimensionally Dissimilar Models ....... 180

7.8.2 Reducing the Number of Dimensionless Variables Fusion of Physical Variables .. 183

7.8.3 Altering Variable Sequence in the Dimensional Set ................................................... 186

7.8.4 Relationship between Dimensionless Variables ......................................................... 187

7.8.5 Analytical Sensitivity Analysis ................................................................................... 190

7.9 Incorrect Determination of Impeller Rotational Speed ...................................................... 194

7.10 Conclusions ...................................................................................................................... 197

Chapter 8 Effect of Fuel Outgassing on Oxygen Concentration in an Aircraft Fuel Tank

Ullage........................................................................................................................................... 200

8.1 Introduction ........................................................................................................................ 200

8.2 Fuel Tank Flammability Assessment Method ................................................................... 201

8.2.1 FRM Performance Degradation due to Fuel Outgassing ............................................ 203

8.2.2 FTFAM Oxygen Evolution Time Constants ............................................................... 203

8.3 Flight Test Aircraft A340-300 MSN001 ......................................................................... 204

ix

8.3.1 A340-300 CWT Fuel System ...................................................................................... 205

8.4 Ullage Oxygen Concentration Measurement ..................................................................... 207

8.4.1 Flight Test F1437 ........................................................................................................ 212

8.5 Results from Flight Test F1437 ......................................................................................... 212

8.6 Discussion .......................................................................................................................... 217

8.6.1 Effect of Air Evolution on Inerted CWT Ullage ......................................................... 218

8.6.2 Validation of Fuel Outgassing Model Law ................................................................. 221

8.7 Conclusions ........................................................................................................................ 223

Chapter 9 Conclusions and Future Work................................................................................ 225

9.1 Conclusions ........................................................................................................................ 225

9.1.1 Measurement of Oxygen Evolution Rates .................................................................. 225

9.1.2 Effect of Aircraft Fuel Tank Environment on Oxygen Evolution Rate ...................... 226

9.1.3 Time Constant of Oxygen Evolution .......................................................................... 227

9.1.4 Dimensional Modelling of the Fuel Outgassing Phenomenon ................................... 228

9.1.5 Effect of Fuel Outgassing on an Inerted Aircraft Fuel Tank Ullage ........................... 228

9.2 Future Work ....................................................................................................................... 230

9.2.1 Experimental Set-up and Further Testing ................................................................... 230

9.2.2 Validation of Fuel Outgassing Model Law ................................................................. 231

9.2.3 Dissolved Oxygen Sensor Development ..................................................................... 232

Appendix A Rolls Royce Engine Fuel Feed Pipe Arrangements used in V/L Measurements

..................................................................................................................................................... 244

Appendix B Sizing Calculation for Fuel Tank Sparger Surface Area .................................. 245

Appendix C Performance Calculations for In-line Fuel Vapour Condenser ....................... 248

Appendix D Conversion of RAE Oxygen Release Rates ........................................................ 254

x

List of Figures

Figure 2-1 Ostwald solubility coefficients for air gases in aviation turbine fuels as a function of

temperature (Coordinating Research Council, 2004, Section 2 pg. 42) ......................................... 12

Figure 2-2 Gas chromatography measurements of air solubility in Avtur aviation fuel as a

function of fuel temperature (Ross, 1970, p.29) ............................................................................ 13

Figure 2-3 Clausius-Clapeyron equation predictions of Ostwald solubility coefficients compared

with CRC solubility data for oxygen dissolved in Jet A-1 aviation fuel as a function of

temperature .................................................................................................................................... 17

Figure 3-1 Generic On Board Inert Gas Generation System (OBIGGS) schematic (Langton,

Clark, Hewitt and Richards, 2009, p.228)...................................................................................... 31

Figure 3-2 Effect of dissolved air evolution from aviation kerosene on the oxygen concentration

of an on demand nitrogen purged fuel tank ullage as determined by (Gatward and Wyeth, 1951,

p.13) ............................................................................................................................................... 34

Figure 3-3 Schematic of a twin engine fuel feed system of a commercial transport aircraft

(Langton, Clark, Hewitt and Richards, 2009, p.67) ....................................................................... 36

Figure 3-4 Photograph of the Eaton Aerospace FRH280002 air release valve incorporated within

the engine fuel feed gallery of the Airbus A340-500/600 aircraft (Eaton Corporation, 2004) ...... 37

Figure 3-5 Simplified engine fuel feed system schematic (Langton, Clark, Hewitt and Richards,

2009, p.10) ..................................................................................................................................... 38

Figure 3-6 Vapour/liquid ratio in the horizontal feed pipe as a function of altitude (Wright, 1976,

p.16) ............................................................................................................................................... 40

Figure 3-7 Effect of engine fuel feed pipe geometry on vapour/liquid ratio as a function of

altitude (Wright, 1976, p.16) .......................................................................................................... 41

Figure 3-8 Wing vent arrangement of the Airbus A340-600 aircraft (Langton, Clark, Hewitt and

Richards, 2009, p.46) ..................................................................................................................... 45

Figure 3-9 Maximum tank pressure rise and fuel volume carry-over as a function of vent pipe ID

from tests conducted by (Ross, 1972, p.21) ................................................................................... 48

Figure 3-10 Calculated rate of air evolution and maximum tank pressure rise from Avtur aviation

turbine fuel carryover measured by (Ross, 1972, p.21) ................................................................. 48

Figure 4-1 Air solubility measurement apparatus schematic developed by Cann of the

RAE(Cansdale, 1978, p.21) ........................................................................................................... 53

xi

Figure 4-2 Anatomy of an electrochemical oxygen sensor based on the polarographic design

(Hach Lange, 2009, p.15) .............................................................................................................. 56

Figure 4-3 Diagrammatic description of the TDLAS measurement principal (Frish, 2004, p.5) .. 60

Figure 5-1 Photograph of pilot apparatus set-up ............................................................................ 66

Figure 5-2 Schematic & photograph of the small-scale experimental fuel tank set-up and oxygen

evolution rate measurement apparatus ........................................................................................... 67

Figure 5-3 Weiss WK/1000/70-100/DS ATEX certified thermal-altitude test chamber with 1m3

internal volume .............................................................................................................................. 69

Figure 5-4 Range of fuel depth within the Airbus A380-800 wing fuel tanks at maximum fuel

capacity (LH wing-box shown) ..................................................................................................... 70

Figure 5-5 Experimental fuel tank dimensions (all dimensions in mm) ........................................ 71

Figure 5-6 Closed-loop PID controlled mixing impeller system architecture ............................... 74

Figure 5-7 IKA R1373 mixing impeller rotational speed in Jet A-1 fuel at 20C achieved by the

pneumatically driven impeller mixing system over the controllable airflow range ....................... 75

Figure 5-8 Dimensions of the IKA R1373 paddle type mixing impeller ....................................... 77

Figure 5-9 Radial flow pattern of a paddle type mixing impeller in a liquid tank ......................... 78

Figure 5-10 Photograph of the experimental set-up to measure impeller torque using a Brookfield

R/S Rheometer and water tank of identical internal dimensions to the experimental fuel tank .... 79

Figure 5-11 IKA R1373 impeller torque as a function of rotational speed measured using a

Brookfield R/S Rheometer in water at 17C with power-law regression fit and extrapolated

impeller torque values at 450 and 500 rpm .................................................................................... 80

Figure 5-12 Dimensionless plot of Np as a function of Re number for the IKA R1373 impeller

running in water at 17C over a 50 rpm to 400 rpm speed range .................................................. 83

Figure 5-13 RP4 paddle type impeller with Npstd = 3.4 and Nqstd = 0.62 (Post, 2011) .................. 83

Figure 5-14 IKA R1373 impeller pumping capacity as a function of impeller Re number running

in water at 17C ............................................................................................................................. 84

Figure 5-15 Calculated fuel agitation rate as a function of impeller Re number over a 0C to 40C

fuel temperature range (agitation rate of water shown for comparison) ........................................ 85

Figure 5-16 Photographs of the air metering system for in-tank sparging of aviation turbine fuel86

Figure 5-17 Re-saturation curve for dissolved air in Jet A-1 aviation fuel whilst agitating with the

mixing impeller at 300 rpm during sparging ................................................................................. 87

Figure 5-18 Schematic of Oxigraf O2G1 TDLAS oxygen sensing system architecture ............... 90

xii

Figure 5-19 Photograph of the Oxigraf O2G1 TDLAS oxygen sensor used to measure oxygen

concentration in the experimental fuel tank ullage ........................................................................ 91

Figure 5-20 Photograph of the in-line condenser designed to strip hydrocarbon fuel vapours from

the gas sample train of the Oxigraf O2G1 TDLAS oxygen sensor (exterior insulation removed) 92

Figure 5-21 Orbisphere polarographic oxygen sensor installation geometry within the

experimental fuel tank (all dimensions in mm) ............................................................................. 93

Figure 5-22 Climb rate performance envelope of the Boeing 777-200 aircraft at altitudes up to

40,000 ft (James and ODell, 2005, p.3) ........................................................................................ 95

Figure 5-23 Linear experimental climb profiles with rates ranging from 1000 ft/min to 5500

ft/min .............................................................................................................................................. 96

Figure 5-24 Pressure vs. time profiles developed to simulate the effect of aircraft altitude .......... 97

Figure 5-25 Fuel tank vent valve functionality at a climb rate of 2500 ft/min and 75% fuel load 98

Figure 5-26 Photograph of the Orbisphere polarographic oxygen sensor, compensation pressure

sensor and oxygen calibration gas supply hose inside the plastic bag ......................................... 101

Figure 5-27 Oxygen concentration measurement plot for the TDLAS oxygen sensor and oxygen-

in-nitrogen calibration gas certified to 5.04 % oxygen by volume .............................................. 102

Figure 5-28 Measured oxygen concentrations in certified oxygen-in-nitrogen calibration gases for

the polarographic and TDLAS oxygen sensors demonstrating a high level of linearity over a 5%

to 35% oxygen concentration range ............................................................................................. 103

Figure 6-1 Partial pressure of dissolved oxygen measured in Jet A-1 fuel as a function of time for

three separate tests conducted under identical environmental conditions to establish experimental

repeatability where =0.96707 kg/s, p =495.1 Pa/s and fuel temperature =20C ...................... 109

Figure 6-2 Mass of oxygen released from Jet A-1 fuel as a function of time calculated using

ASTM D2779-92 for three separate tests conducted under identical environmental conditions

where =0.96707 kg/s, p = 495.1 Pa/s and fuel temperature =20C ........................................... 112

Figure 6-3 Instantaneous mass release rates of oxygen from Jet A-1 fuel as a function of time for

three separate tests conducted under identical environmental conditions in the experimental fuel

tank where=0.96707 kg/s, p = 495.1 Pa/s and fuel temperature =20C ................................... 114

Figure 6-4 Oxygen evolution from Jet A-1 aviation fuel in the experimental fuel tank at a

temperature of 0C, fuel agitation rate of 0.355 kg/s and a 295.64 Pa/s rate of ullage pressure

change .......................................................................................................................................... 115

xiii

Figure 6-5 Dissolved and ullage oxygen partial pressures as a function of time in the experimental

fuel tank at a temperature of 0C, fuel agitation rate of 0.355 kg/s and a 295.64 Pa/s rate of ullage

pressure change ............................................................................................................................ 117

Figure 6-6 Oxygen mass released from Jet A-1 fuel and Janoschek regression model fit as a

function of time in the experimental fuel tank at a temperature of 0C, fuel agitation rate of 0.355

kg/s and a 295.64 Pa/s rate of ullage pressure change ................................................................. 118

Figure 6-7 Instantaneous oxygen mass release rate as a function of time for Jet A-1 fuel in the

experimental fuel tank at a temperature of 0C, fuel agitation rate of 0.355 kg/s and a 295.64 Pa/s

rate of ullage pressure change ...................................................................................................... 118

Figure 6-8 Oxygen release rate at t = as a function of fuel agitation rate at a fuel temperature of

20C and rate of change of ullage pressure ranging from 73.85 to 405.75 Pa/s .......................... 120

Figure 6-9 Oxygen release rate at t = as a function of fuel agitation rate at a fuel temperature of

40C and rate of change of ullage pressure ranging from 73.85 to 405.75 Pa/s .......................... 120

Figure 6-10 Oxygen release rate at t = as a function of fuel agitation rate at a fuel temperature

of 0C and rate of change of ullage pressure ranging from 73.85 to 405.75 Pa/s ........................ 121

Figure 6-11 Oxygen release rate at t = as a function of fuel temperature and a 73.85 Pa/s rate of

change of ullage pressure ............................................................................................................. 122

Figure 6-12 Oxygen release rate at t = as a function of fuel temperature and a 184.86 Pa/s rate

of change of ullage pressure ........................................................................................................ 122

Figure 6-13 Oxygen release rate at t = as a function of fuel temperature and a 295.64 Pa/s rate

of change of ullage pressure ........................................................................................................ 123

Figure 6-14 Oxygen release rate at t = as a function of fuel temperature and a 405.75 Pa/s rate

of change of ullage pressure ........................................................................................................ 123

Figure 6-15 Oxygen release rate at t = as a function of the rate of ullage pressure change at a

fuel temperature of 20C .............................................................................................................. 124

Figure 6-16 Oxygen release rate at t = as a function of the rate of ullage pressure change at a

fuel temperature of 0C ................................................................................................................ 125

Figure 6-17 Oxygen release rate at t = as a function of the rate of ullage pressure change at a

fuel temperature of 40C .............................................................................................................. 125

xiv

Figure 6-18 Semi-log transformation and linear regression fit of oxygen release rate as a function

of fuel agitation rate at a fuel temperature of 40C and a rate of change of ullage pressure of

295.64 Pa/s ................................................................................................................................... 131

Figure 6-19 Log-log transformation and linear regression fit of oxygen release rate as a function

of fuel agitation rate at a fuel temperature of 40C and a rate of change of ullage pressure of

295.64 Pa/s ................................................................................................................................... 132

Figure 6-20 Reciprocal transformation and linear regression fit of oxygen release rate as a

function of fuel agitation rate at a fuel temperature of 40C and a rate of change of ullage pressure

of 295.64 Pa/s .............................................................................................................................. 132

Figure 6-21 Reciprocal model fit to oxygen release rate multiplied by 106 as a function of fuel

agitation rate at a fuel temperature of 40C and a rate of change of ullage pressure of 295.64 Pa/s

..................................................................................................................................................... 133

Figure 6-22 Oxygen release rate 106 plotted as a function of impeller power at a fuel

temperature of 40C and a rate of change of ullage pressure of 295.64 Pa/s .............................. 135

Figure 6-23 Reciprocal of oxygen release rate 106 plotted as a function of impeller power at a

fuel temperature of 40C and a rate of change of ullage pressure of 295.64 Pa/s ....................... 136

Figure 6-24 Reciprocal model fit to oxygen release rate multiplied by 106 as a function of

impeller power at a fuel temperature of 40C and a rate of change of ullage pressure of 295.64

Pa/s ............................................................................................................................................... 137

Figure 6-25 Experimental data determined by the Shell Oil company for the volumetric release

rate of air from aviation kerosene at varying levels of fuel agitation (Ross, 1972, p.22) ............ 139

Figure 6-26 Oxygen diffusion coefficient in Jet A-1 over a 0C to 40C temperature range

estimated from the Wilke-Chang correlation for diffusion of dilute gases in liquids .................. 141

Figure 6-27 Rate constant of nitrogen gas evolution as a function of fuel temperature in RJ-1 fuel

subjected to horizontal shaking (Coordinating Research Council, 1958, Figure 39) .................. 142

Figure 6-28 Diffusivity of oxygen in Jet A-1 aviation fuel as a function of fuel viscosity over a

0C to 40C temperature range .................................................................................................... 145

Figure 6-29 % air by vol. at STP dissolved in aviation gasoline as a function of time in agitated

and unagitated conditions at 43C (110F) (Beal, Hilburger and Porter, 1945) .......................... 147

Figure 6-30 Comparison of the half-life of oxygen evolution from oxygen saturated RJ-1 fuel

with for oxygen evolution from Jet A-1. Both fuels were subjected to agitation by either shaking

or stirring at a temperature of ~20C ........................................................................................... 150

xv

Figure 7-1 The Dimensional Set for oxygen evolution rate from aviation turbine fuel ............... 160

Figure 7-2 Flight test data from flight No.0073 from the A320-200 MSN 659 aircraft (Bonjour,

2009, p.14) ................................................................................................................................... 164

Figure 7-3 Fuel tank configuration of the Airbus A320-200 aircraft (Walker, 2005, p.5) .......... 165

Figure 7-4 Schematic of the A320-200 engine fuel feed system architecture (left hand wing and

centre tank shown) (Walker, 2005, p.38) ..................................................................................... 167

Figure 7-5 Photograph of an A320-200 aircraft inner wing collector cell illustrating fuel boost

pumps and sequence valves from which fuel is ejected resulting in collector cell fuel agitation.

View is from the wing front spar looking aft towards RIB 1....................................................... 168

Figure 7-6 IKA R1373 mixing impeller pumping capacity as a function of impeller rotational

speed ............................................................................................................................................ 169

Figure 7-7 Pressure as a function of time curve generated from Equation 7-11 and exponential

regression equation fit for the A320-200 aircraft climb to 11582.4 m in flight No.0073 ............ 171

Figure 7-8 Ullage pressure and altitude as a function of time profiles for A320-200 aircraft (flight

No.0073) and model determined from flight test data and the Model Law ................................. 172

Figure 7-9 Physical model test set-up .......................................................................................... 174

Figure 7-10 Mass of oxygen released from Jet A-1 aviation fuel as a function of time in the model

tests .............................................................................................................................................. 176

Figure 7-11 Instantaneous mass release rates of oxygen from Jet A-1 aviation fuel as a function of

time in 3 separate model tests with identical test conditions ....................................................... 177

Figure 7-12 Oxygen partial pressures as a function of time due to outgassing of dissolved air from

aviation turbine fuel under reduced ullage pressure and agitated fuel conditions during Test 2. 184

Figure 7-13 Reconstruction of the Dimensional Set with variables pu and pf combined (pf pu) . 185

Figure 7-14 Re-arranged variables in the Dimensional Set ......................................................... 186

Figure 7-15 Dimensionless plot of 1 as a function of 3 using data from Test 1 in the ullage

depressurisation phase ................................................................................................................. 189

Figure 7-16 Relationship between (pf -pu) and p represented by a linear mathematical function 189

Figure 7-17 An empirical model describing the behaviour of 2Om in terms of p during ullage

depressurisation in Test 1 ............................................................................................................. 190

Figure 7-18 Absolute sensitivity of oxygen evolution rate resulting from a 1 to 10% change in the

rate of change of ullage pressures simulated in the dimensional modelling tests ........................ 192

xvi

Figure 7-19 Relative sensitivity of oxygen evolution rate resulting from a 1 to 10% change in the

rate of change of ullage pressure simulated in the dimensional modelling tests ......................... 193

Figure 7-20 Pumping capacity of IKA R1373 impeller running in Jet A-1 aviation fuel at 20C as

a function of impeller Re number ................................................................................................ 194

Figure 8-1 Flow chart illustrating the main computations used in the Fuel Tank Flammability

Assessment Method (FTFAM) and FRM performance (Summer, 2008, p.7) ............................. 202

Figure 8-2 Airbus A340-300 MSN001 flight test aircraft (Mervelet, 2009) ............................... 204

Figure 8-3 Schematic of A340-300 CWT fuel system architecture (Airbus Technical Description

Volume 3A System AI/ED-N 433.0023/91 Iss. 6) ...................................................................... 205

Figure 8-4 A340-300 architecture (Airbus Technical Description Volume 3A System AI/ED-N

433.0023/91 Iss. 6) ....................................................................................................................... 206

Figure 8-5 Oxigraf O2N2 flight test oxygen analyser and O2G8 gas sampling system architecture

(schematic courtesy of Oxigraf Inc.) (McCaul, 2007, p.29) ........................................................ 207

Figure 8-6 Photograph of the Oxigraf O2N2 flight test ullage oxygen analyser and O2G8 oxygen

sampling system installation within the Airbus A340-300 MSN001 flight test aircraft cabin .... 208

Figure 8-7 Cross-section of the A340-300 MSN001 centre wing fuel tank (Left-hand side)

showing ullage oxygen concentration and temperature sensing locations (all dimensions in mm)

(Boulet, 2009, p.4) ....................................................................................................................... 210

Figure 8-8 Illustration of ullage oxygen gas sampling points - float valve arrangements in the

centre wing fuel tank of the A340-300 MSN001 flight test aircraft (Boulet, 2009, p.5) ............. 211

Figure 8-9 Illustration of ullage oxygen gas sample return points - float valve arrangement at R10

within the centre wing fuel tank of the A340-300 MSN001 flight test aircraft(Boulet, 2009, p.6)

..................................................................................................................................................... 211

Figure 8-10 Flight test data from flight No. F1437 showing ullage oxygen concentration

measurements (left hand side) and corresponding aircraft altitude, CWT fuel quantity and fuel

temperature (right hand side) ....................................................................................................... 215

Figure 8-11 Effect of fuel transfer from CWT to inner feed tanks on CWT ullage oxygen

concentration during flight no. F1437. Fuel temperature and altitude data removed for clarity . 216

Figure 8-12 Comparison of theoretical CWT ullage oxygen concentration increase resulting from

air inspired via the vent system due to fuel transfer with measured ullage oxygen concentration.

Fuel temperature and altitude data removed for clarity ............................................................... 219

xvii

Figure 8-13 The effect of a nitrogen enriched ullage on the dissolved oxygen content of mildly

agitated Jet A-1 fuel at ambient temperature and barometric pressure ........................................ 221

xviii

List of Tables

Table 2-1 A summary of air, oxygen and nitrogen gas solubility measurements in aviation fuels

reported by various workers at an air partial pressure of 760 mmHg. Derry, Evans, Faulkner and

Jelfs (1952), Logvinyuk, Makarenkov, Malyshev and Panchenkov (1970), Astafev and Kozinova

(1988), Schweitzer and Szebehely (1950) ..................................................................................... 10

Table 2-2 Compositions of air dissolved in aviation fuels as determined at 25C using Gas

Chromatography (Ross, 1970, p.25) .............................................................................................. 20

Table 3-1 Effect of vent pipe ID on fuel tank pressurisation and fuel carryover due to air release

from supersaturated Avtur aviation turbine fuel subjected to agitation and a simulated altitude of

60,000 ft (Ross, 1972, p.21) ........................................................................................................... 47

Table 5-1 Outline specification of Weiss WK 1000/70-100/DS thermal-altitude test chamber .... 68

Table 5-2 IKA R1373 impeller pumping capacity from measured torque values in water at 17C.

Data values shown in red are calculated from extrapolated torque data ........................................ 82

Table 5-3 Fuel agitation rates calculated over a 200 rpm to 450 rpm impeller rotational speed

range............................................................................................................................................... 84

Table 5-4 Rate of change of ullage pressure for each climb profile .............................................. 97

Table 5-5 Certified oxygen-in-nitrogen calibration gas mixtures and associated standard

uncertainties assuming a normal distribution............................................................................... 103

Table 5-6 Uncertainty of measurement analysis of the TDLAS oxygen sensor over a 5% to 35%

oxygen by vol. concentration measurement range ....................................................................... 104

Table 5-7 Uncertainty of measurement analysis of the polarographic oxygen sensor over a ...... 105

Table 6-1 Experimental test conditions ....................................................................................... 107

Table 6-2 Time constants of oxygen evolution over the experimental ranges of fuel agitation, fuel

temperature and rate of change of ullage pressure ....................................................................... 127

Table 6-3 Two-way ANOVA results for oxygen release rate test data ....................................... 129

Table 6-4 Two-way ANOVA results for data ........................................................................... 129

Table 6-5 Calculated impeller power dissipated to the Jet A-1 aviation fuel at 40C over the range

of fuel agitation rates investigated ............................................................................................... 134

Table 7-1 Variables relevant to the rate of oxygen evolution from aviation turbine fuel using the

SI dimensional system ................................................................................................................. 156

xix

Table 7-2 Summary of flight test data taken from flight No.0073 on MSN 659 required for design

of the laboratory model (Bonjour, 2009, p.14) ............................................................................ 165

Table 7-3 Collector cell fuel agitation rate calculated from A320-200 MSN 659 flight test data

(flight No.0073) and analysis of engine fuel feed system performance parameters .................... 166

Table 7-4 Surface tension measurements of Jet A-1 aviation fuel used in the model tests

compared to published CRC Fuels Properties Handbook surface tension values for Jet A-

1(Intertek, 2011) .......................................................................................................................... 173

Table 7-5 Instantaneous oxygen mass release rates at t = and time constants of oxygen

evolution measured from Jet A-1 fuel in the dimensionally similar model tests ......................... 178

Table 7-6 Summary of dimensional modelling results on the rate of oxygen evolution from

aviation fuel in an A320 aircraft fuel tank ................................................................................... 179

Table 7-7 Oxygen release rates measured by the RAE under simulated flight conditions in

quiescent aviation kerosene using a dimensionally dissimilar laboratory model in comparison

with rates of oxygen release measured in the dimensionally similar model fuel tank (Bedwell,

1952, p.10) ................................................................................................................................... 181

xx

Nomenclature

Upper Case

A Constant, Condenser surface area 1, m2

B Bunsen solubility coefficient 1

D Bubble diameter, Impeller diameter m

DBA Gas-liquid diffusivity m2/s

E Activation energy J/mol

F Molar volume of solvent m3/mol

G Solubility of Oxygen 1

I Luminescence intensity in the presence of oxygen cd

I0 Luminescence intensity in the absence of oxygen cd

I Transmitted intensity W/m2

I,0 Initial laser intensity W/m2

K Rate constant of oxygen evolution from fuel s-1

L Ostwald solubility coefficient 1

Lo Ostwald solubility coefficient at 0C and 850 kg/m3 density 1

Lc Corrected Ostwald solubility coefficient 1

M Mass flow rate of condensate kg/s

Mg Molecular weight kg/mol

N Target gas number density, Impeller rotational speed molecule/m

3,

rev/min, s-1

(NA)avg Molar rate of gas diffusion from liquid to gas bubble

averaged over bubble surface area m

2/s

Np Impeller power number 1

Nq Impeller flow number 1

Npstd Ref. Impeller power number 1

Nqstd Ref. Impeller flow number 1

N Number of dimensionless variables 1

Nd Number of fundamental dimensions 1

O2,i % volume fraction of oxygen at time step, i 1

xxi

O2 % volume fraction of oxygen in ullage 1

P Gas Pressure, Power in stirred tank Pa, W

P(a) Pressure at altitude Pa

P1D Partial pressure of dissolved air in fuel upstream of fuel pipe Pa

P2D Partial pressure of dissolved air in fuel downstream of fuel

pipe

Pa

P2 Pressure in pipe Pa

Ps Fuel vapour pressure Pa

PTVP True vapour pressure of fuel Pa

Q Volumetric flow rate, Rate of heat transfer m3/s, m

2kg/s

3

Qimp Volumetric impeller flow m3/s

Qm Mass flow rate kg/s

R Gas constant J/kgK

Re Impeller Reynolds number 1

%S Percentage solubility 1

S(T) Temperature dependent line strength m-1

/moleculem-2

Sc Schmidt number 1

Sp Percentage air solubility 1

T Temperature K

Tfilm Condensate film temperature K

Tsat Vapour saturation temperature K

Twall Condenser wall temperature K

T0 Temperature difference between condenser wall and

saturated vapour

K

Tq Impeller torque Nm

V/L Vapour/liquid ratio 1

AV~

Molar volume of solvent m3/mol

xxii

Lower Case

a Constant 1

b Constant 1

c Constant 1

cA0 Solubility of gas A in solvent B 1

d Liquid density, Diameter kg/m3, m

g Acceleration due to gravity m/s2

g(-v0) Frequency dependence of the line strength cm

hcond Condensing heat transfer coefficient W/m2K

i Time step s

k Constant, Solubility coefficient, Boltzmann constant, Stern-Volmer

constant, Rate constant of oxygen evolution 1,1,J/K,

kf Thermal conductivity of condensate film W/m2K

kH Henrys Law constant Pa

m Constant 1

m Condensation rate kg/s

mf Mass of fuel in tank kg

mO2(t) Mass of oxygen dissolved in fuel at time, t kg

2Om Rate of oxygen evolution from fuel kg/s

mO2(t) Mass of oxygen released from fuel into ullage at time, t kg

n Number of measurements in set 1

p Gas partial pressure, Significance level, Shaping parameter Pa, mmHg,1

pO2 Partial pressure of oxygen Pa

pf Partial pressure of oxygen dissolved in fuel Pa

pt Total pressure in ullage Pa, mbar

pu Partial pressure of oxygen in ullage Pa

pv Vapour pressure of liquid, Vapour density Pa, kg/m3

p Rate of change of ullage pressure Pa/s

(pf pu) Delta oxygen partial pressure between fuel and ullage Pa

r Radius of solute molecule, Volume fraction m, 1

s Standard deviation 1

xxiii

t Fuel Temperature K,C,F

u Standard uncertainty of measurement 1

uc Combined standard uncertainty of measurement 1

z Altitude m

Greek Letters

Bunsen solubility coefficient, Fuel agitation rate 1, kg/s

Absorption coefficient 1

Condensate loading kg/sm-1

Surface tension N/m

Partial eta square (Effect size) 1

1/2 Half-life of oxygen evolution s

Heat of vapourisation J/kg

Fuel viscosity kg/(ms)

Laser frequency s-1

n Dimensionless variable 1

Fuel density, Gas density kg/m3

Surface tension N/m

f Fuel surface tension N/m

Time constant of oxygen evolution s

t Terminal bubble rise velocity m/s

B Association factor of solvent 1

1

Chapter 1

Introduction

1.1 Background

The tendency of dissolved air to emerge from aviation fuels during aircraft flight has presented

arduous fuel system design challenges since the advent of jet-propelled aircraft. During an

aircrafts ascent to cruising altitude, reducing atmospheric pressure above the fuels surface

promotes the release of dissolved air from the fuel. This phenomenon, known as fuel outgassing,

is closely related to the environmental conditions present within the aircraft fuel tank. The rate at

which air is evolved is governed by several key variables, namely; the degree of fuel agitation,

the rate of change of atmospheric pressure, degree of air supersaturation, air solubility and fuel

temperature. Under near quiescent fuel tank conditions the rate of air evolution is reported to be

slow where the fuel is highly supersaturated with air (Schweitzer and Szebehely, 1950). However,

operating fuel pumps and other fuel system equipment in air supersaturated fuel can produce

significant levels of fuel agitation where air release rates are high, causing a foam layer on the

fuels surface (Poulston and Thomas, 1959). Fuel outgassing can seriously impact the

performance and safe operation of an aircraft fuel system. Entrained air within the fuel flow to an

engine can lead to a condition known as vapour lock, which, if sustained, can result in fuel pump

cavitation and loss of engine operation (Beal, Hilburger and Porter, 1945; SAE International,

1997). Air evolution from supersaturated fuel has been responsible for over-pressurisation of fuel

tanks where tank venting systems have been designed with insufficient margin to accommodate

Supersaturation defines the condition when the amount of gas dissolved in a liquid is greater than the

amount which would correspond to the gas pressure above the liquids surface

2

air released at a high rate from the fuel (Beal, Hilburger and Porter, 1945; Lee 1974). The sudden

release of air, comprising up to ~35% oxygen by volume, can lead rapidly to the formation of

flammable fuel-air mixtures in the ullage of aircraft fuel tanks (Bragg, Kimmel and Jones, 1969).

Even the protection provided by nitrogen gas inerting systems, designed to mitigate the risk of

aircraft fuel tank explosion, can be compromised (Gatward and Wyeth, 1951; McConnell, Dalan

and Anderson, 1986; Cavage, 2005). Despite ongoing design efforts to manage the effects of fuel

outgassing, many problems still exist and arise during aircraft service. Previous studies on the

fuel outgassing problem have failed to examine the synergistic effects of key environmental

variables, such as pressure, temperature and fuel agitation on air release from aviation fuel.

Consequently, failure to simulate the true aircraft fuel tank environment within such studies has

lead to a gulf in understanding between fuel system designers and equipment manufacturers on

how to tackle the problem.

The problem of fuel outgassing has become more prevalent as greater demands are placed on fuel

system performance and operational safety. Aircraft fuel systems have become more complex as

a direct result of continually evolving requirements for larger, multi-engined aircraft flying longer

distances with greater fuel loads. Apart from supplying the propulsion system, fuel on-board

modern aircraft fulfils many other duties (Morris, Miller and Limaye, 2006). Fuel is used as a

heat sink for cooling hydraulic system oil, electronic equipment and for controlling aircraft flight

attitude and centre of gravity. Increasing fuel system complexity and the number of system

functions creates a greater susceptibility to the problems of fuel outgassing. In view of this it is

imperative that the aerospace industry fosters a need for comprehensive understanding of the

behaviour of air and its evolution characteristics within aviation fuel under all aircraft operating

Ullage describes the space occupied by air above the fuels surface in an aircraft fuel tank

3

envelopes. Such understanding will underpin the improved utilisation and development of multi-

phase computational fluid dynamics (CFD) models and mathematical modelling tools used in

fuel systems design. The design of fuel management systems for the next generation aircraft will

benefit as CFD modelling becomes more capable of accurately simulating the gas-liquid

environments encountered within aircraft fuel systems and tanks. Perhaps most importantly, it

will be possible to optimise fuel tank inerting strategies to counter the effects of oxygen-rich air

evolution on fuel tank flammability to further improve aircraft safety. Fuel tank inerting systems

in use on modern commercial aircraft are rather wasteful, generating far more nitrogen enriched

air than is necessary to replenish the gas lost to atmosphere. This increases airline operating costs

through increased fuel consumption and reduced engine performance. Characterising fuel

outgassing behaviour and its effect on flammability in aircraft fuel tanks will enable airlines to

adopt more economic inerting strategies, leading to cost savings and improved performance

without compromising safety. To achieve these goals this research will focus on quantifying the

rate of oxygen evolution from aviation turbine fuel over a range of fuel tank environments

encountered within the flight envelope of commercial transport aircraft.

1.2 Scope of Research

The work presented in this thesis covers the measurement of oxygen concentration in small-scale

laboratory fuel tanks and a flight test aircraft fuel tank from which oxygen evolution rates are

calculated. The effects of environmental variables, such as air pressure, temperature and the level

of fuel agitation on oxygen evolution rate are investigated to establish a clear understanding of

how these factors influence fuel outgassing behaviour. Dimensional modelling is used to project

oxygen evolution rate results from a small-scale fuel tank to an aircraft fuel tank to understand if

such an approach can provide a viable, cost effective alternative to flight testing.

4

The specific objectives are as follows;

To design and develop an environmentally conditioned experimental fuel tank capable of

simulating the environment within an aircraft fuel tank during flight. Oxygen sensing

apparatus and an experimental method for measuring the rate of oxygen evolution from

aviation turbine fuel in the fuel tank is to be developed.

To establish, from statistical analysis of empirical test data, the effect size of individual

environmental variables and associated interactions of these variables on the rate of

oxygen evolution from aviation turbine fuel.

To derive a Model Law of fuel outgassing rate using dimensional modelling so that

oxygen release rates from aviation turbine fuel measured in the laboratory can be

extrapolated to full scale aircraft fuel tanks.

To establish the effect of fuel outgassing on oxygen concentration within an aircraft fuel

tank ullage through aircraft flight testing.

1.3 Thesis Layout

This thesis is divided into three main sections. Chapters 2 to 4 form a literature review of

dissolved air-in-fuel behaviour, its effects on aircraft fuel system performance and methods for

measuring gas solubility and concentration in aviation fuels. The literature review creates a

foundation on which the experimental set-up, method and testing activities are developed in

Chapters 5 to 8. Finally, Chapter 9 presents a summary of the key conclusions from experimental

testing together with recommendations for further work. In detail, the thesis is organised as

follows:

Chapter 2 is a literature review of the work previously undertaken to study the behaviour of

dissolved air within aviation fuels and the effects of pressure and temperature on air, oxygen and

5

nitrogen gas solubility in aviation fuels. The influence of thermophysical fuel properties on gas

solubility is also reviewed.

Chapter 3 reviews literature on the effects of fuel outgassing on aircraft fuel system performance

and fuel tank flammability. Specifically, each fuel sub-system in modern commercial transport

aircraft is examined in terms of its susceptibility to the fuel outgassing phenomenon.

Chapter 4 examines the different approaches previously taken for measuring both dissolved and

gaseous phase oxygen in aircraft fuel tanks and systems. The suitability of each measurement

method is explored for determining the rate of oxygen evolution from aviation fuel in the

experimental phases of this research.

Chapter 5 covers the development of an experimental fuel tank, oxygen measuring apparatus and

an experimental method for determining oxygen evolution rates in a simulated aircraft fuel tank

environment.

Chapter 6 details the laboratory testing undertaken to determine oxygen evolution rates in a

simulated aircraft fuel tank environment and presents experimental results and a discussion of key

findings. A statistical analysis of the results is conducted to examine the effect size of each

environmental variable and associated interactions of each variable on the rate of oxygen

evolution from the fuel.

6

Chapter 7 explores the derivation of a Model Law of fuel outgassing from dimensional analysis,

which is used to extrapolate laboratory results of oxygen evolution rate from a dimensionally

similar experimental fuel tank to a full scale aircraft fuel tank. Oxygen evolution rate results are

contrasted with those generated by other workers from dimensionally dissimilar fuel tanks and

experiments.

Chapter 8 describes the testing undertaken on a long-range flight test aircraft to establish the

effect of fuel outgassing on the concentration of oxygen within an aircraft fuel tank ullage during

flight.

Chapter 9 summarises the key conclusions made in the results Chapters, with recommendations

for future work.

7

Chapter 2

The Behaviour of Dissolved Air within Aviation Turbine Fuel

2.1 Introduction

As introduced briefly in Chapter 1, dissolved air is released from aviation fuel as the partial air

pressure above the fuels surface is reduced. Although problematic from a fuel handling and

management perspective, this behaviour is part of a more serious problem. The solubility

characteristics of air within aviation fuels results in an oxygen-rich dissolved air composition,

such that the released air is also oxygen-rich imposing serious flammability risks for aircraft fuel

tanks. The effect of temperature further compounds these issues as aviation fuel at higher

temperatures dissolves greater quantities of air. Within this Chapter a detailed review of the

factors which affect air solubility and the mechanisms of air evolution in aviation fuels will be

conducted. Experimental investigations carried out by previous workers to quantify air solubility

and its behaviour in hydrocarbon fuels and liquids under a variety of environmental conditions is

reviewed and the key findings relevant to this research presented to build an understanding prior

to experimental work.

2.2 Air Solubility in Aviation Fuel

The solubility of a gas in a liquid under equilibrium conditions can be expressed in a variety of

different ways (Markham and Kobe, 1941). Typically, the solubility of atmospheric gases in

aviation fuels are expressed using coefficients (Hetherington and Bellerby, 2004). These

coefficients differ depending on whether the gas pressure of interest, above the fuel, is expressed

In an aircraft fuel tank ullage air constitutes a partial pressure as the total ullage pressure is the sum of the

fuel vapour pressure and the air partial pressure

8

in terms of the partial pressure of that gas or the total pressure of all the gases present. The main

three coefficients used for expressing gas solubility in aviation fuels are the Ostwald, Bunsen and

absorption coefficients.

2.2.1 Ostwald Coefficient

The Ostwald coefficient, L is the ratio;

phasegaseousingasofionconcentrat

phaseliquidingasofionconcentratL

It is equivalent to the volume of gas dissolved by a unit volume of liquid where the gas volume is

measured at the temperature and partial pressure of the solution conditions. For aviation fuel the

Ostwald coefficient is expressed in mm3 gas/mm

3 fuel.

2.2.2 Bunsen Coefficient

The Bunsen coefficient, is defined as the volume of gas reduced to 273.15 K and 760 mmHg

that is dissolved by a unit volume of fuel under a gas partial pressure of 760 mmHg.

2.2.3 Absorption Coefficient

The absorption coefficient, is defined as the volume of gas reduced to 273.15 K and 760

mmHg which is dissolved by a unit volume of fuel when the total pressure is maintained at 760

mmHg. The total pressure is defined as a summation of the gas partial pressure and the fuels

vapour pressure.

Markham and Kobe (1941) established that the solubility coefficients, L, and were inter-

related via the expressions detailed in Equation 2-1 and Equation 2-2 as follows;

sp760

760 Equation 2-1

9

sp

TTL

760

760

273273 Equation 2-2

where T is temperature in Kelvin and Ps is the vapour pressure of the fuel in mmHg.

It can be seen clearly from Equation 2-2 that the Ostwald coefficient L, can be calculated from

through correction of the gas volume to the specific temperature at which gas absorption was

carried out.

Further research (Derry, Evans, Faulkner and Jelfs, 1952) found it convenient to express the

solubility of air, oxygen and nitrogen gases in aviation fuels from experimental measurements as

a percentage using the following expression;

pkS p Equation 2-3

where Sp is the number of ml of gas dissolving in 100 ml of liquid under a gas partial pressure, p

in mmHg. k is the experimentally determined solubility coefficient for each gas. Solubility

coefficients measured by Derry, Evans, Faulkner and Jelfs (1952) correlate well with those of

Logvinyuk, Makarenkov, Malyshev and Panchenkov (1970), Astafev and Kozinova (1988) and

Schweitzer and Szebehely (1950). Table 2-1 summarises the solubility data of air, oxygen and

nitrogen in various aviation fuels measured by those workers.

10

Table 2-1 A summary of air, oxygen and nitrogen gas solubility measurements in aviation

fuels reported by various workers at an air partial pressure of 760 mmHg. Derry, Evans,

Faulkner and Jelfs (1952), Logvinyuk, Makarenkov, Malyshev and Panchenkov (1970),

Astafev and Kozinova (1988), Schweitzer and Szebehely (1950)

Reference Fuel Type Temperature

(C)

Solubility Coefficient, k % Solubility

Air oxygen nitrogen Air oxygen nitrogen

Derry et al Kerosene 15.5 0.0184 0.0285 0.0157 13.98 4.54 9.43

Logvinyuk

et al

TS-1 20 0.0195 0.029 0.017 14.82 4.62 10.21

T-1 20 0.0183 0.027 0.016 13.9 4.3 9.61

T-6 20 0.0174 0.026 0.015 13.22 4.14 9.01

Astafev & Kozinova

RT 20 0.0215 0.0267 0.020 16.35 4.25 12.1

T-6 20 0.0177 0.0188 0.017 13.5 3.0 10.5

Schweitzer

&

Szebehely

Aircraft

Engine

Fuel

21.1 0.0226 - - 17.2 - -

2.3 Effect of Pressure on Air Solubility in Aviation Fuel

The solubility of air in aviation fuel is described by Henrys law, where the volume of air

dissolved is directly proportional to the partial air pressure above the fuels surface at a constant

temperature. Mathematically, Henrys law can be expressed as;

ckp H Equation 2-4

where p is the partial pressure of the solute in the gas above the solution, c the concentration of

the solute and kH is a constant (Henrys law constant), which is dependent on the solute, solvent

and the temperature. For a mixture of gases dissolved in a solvent, which is the case for air

dissolved in aviation fuel, Henrys law for each gas i.e. oxygen and nitrogen, applies

independently.

11

Derry, Evans, Faulkner and Jelfs (1952) have shown that the solubility of air gases in aviation

kerosene closely obeys Henrys law obtaining straight-line relationships between the volume of

gas evolved from the fuel and gas pressure above the fuels surface. This behaviour was

demonstrated to hold for air, oxygen and nitrogen separately. Ross (1970) also reported the

solubility behaviour of air in Avtur aviation turbine fuel at various fixed temperatures obeyed

Henrys law. Air solubility was found to decrease approximately linearly in Avtur aviation

turbine fuel with decreasing air partial pressure.

2.4 Effect of Temperature on Air Solubility in Aviation Fuel

In general the solubility of gases in non-aqueous solvents is reported to increase with increasing

temperature. Ostwald solubility data for air gases, published by the Coordinating Research

Council (2004), for a variety of military and civil aviation turbine fuels, exhibits this trend as

shown in Figure 2-1.

12

Figure 2-1 Ostwald solubility coefficients for air gases in aviation turbine fuels as a function

of temperature (Coordinating Research Council, 2004, Section 2 pg. 42)

Ross (1970) used gas chromatography to show the solubility of air in Avtur aviation turbine fuel,

at atmospheric pressure, increased with increasing temperature over a 26C to 120C range.

Solubility data was expressed as Ostwald, Bunsen and absorption coefficients as shown in Figure

2-2.

13

Figure 2-2 Gas chromatography measurements of air solubility in Avtur aviation fuel as a

function of fuel temperature (Ross, 1970, p.29)

Ostwald coefficients of air solubility increased linearly with increasing temperature over a 26C

to 120C temperature range. Bunsen coefficients increased almost linearly over this temperature

range, whilst the absorption coefficient reached a maximum at 60C and then decreased as

temperature increased beyond this value. The most likely explanation for a decreasing absorption

coefficient above a certain fuel temperature is attributable to the way in which the vapour

pressure of aviation turbine fuel increases exponentially with increasing temperature. In general,

the vapour pressure of aviation turbine fuel is low at temperatures below the fuels flash point. It

increases rapidly at temperatures above the flash point and becomes significant in the calculation

of in Equations 2-1 and 2-2. In the case of Avtur or Jet A-1, the flash point can vary between

14

38C and 66C. The fuel used by Ross (1970) may well have had a flash point of around 60C,

hence a rapid decrease in calculated absorption coefficient above this temperature.

Contrasting the results of Ross (1970) with Glendinning and Bedwell (1949) reveals some

contradiction in the behaviour of Bunsen solubility coefficients of oxygen and nitrogen in

aviation kerosenes with varying temperature. Both Ross (1970) and Glendinning and Bedwell

(1949) report a slight decrease in the Bunsen solubility of oxygen in aviation kerosene with

increasing temperature. However, for nitrogen solubility the results of these workers are not in

agreement. Ross (1970) reports a distinct increase in Bunsen coefficient with increasing

temperature and Glendinning and Bedwell (1949) a slight decrease in nitrogen solubility over a

similar temperature range. Barnett and Hibbard (1956) show Bunsen solubility coefficients for

air, oxygen and nitrogen in petroleum fractions of varying density increase with temperature over

a -30C to 60C range. Logically it would appear that nitrogen, being more abundant within the

dissolved air composition of aviation fuel, results in the Bunsen coefficient for air increasing with

fuel temperature as reported by Ross (1970) and Barnett and Hibbard (1956). Given the various

ways in which gas solubility can be expressed, it seems sensible that when considering the effect

of temperature on gas solubility, comparison between solubility results should be made, if

possible using the Ostwald solubility coefficient. The air solubility measurements of Ross (1970),

Derry, Evans, Faulkner and Jelfs (1952) and Barnett and Hibbard (1956) in kerosene based

aviation fuels when expressed as Ostwald coefficients were all in good agreement over a -40C to

120C temperature range. The gas volume is measured at the temperature and partial pressure

conditions of solution in each experimental method reducing the likelihood of possible errors

associated with conversion of solubility measurements to other temperatures and partial

pressures.

15

Markham and Kobe (1941) reported the Clapeyron equation had been used to relate the Ostwald

solubility of gases in liquids with temperature. Provided the heat of solution of the gas in the

liquid is constant over the temperature range of interest then it follows that;

212

1 11lnTTR

H

L

L Equation 2-5

where L1 and L2 are the Ostwald solubility coefficients at temperatures T1 and T2, H is the heat

of solution and R is the gas constant. Graphically, Ross (1970) showed ln L is a linear function of

1/T with the slope equal to H/R for air solubility in aviation fuels. Equation 2-5 is the

integrated form of the Clausius-Clapeyron equation. For air solubility data in aviation fuels

exhibiting variation with temperature it is perhaps more prudent to use an alternative form of the

Clausius-Clapeyron equation. Letting H =A then;

2RT

LA

dT

dL Equation 2-6

where T is equal to the fuel temperature. Integrating Equation 2-6 provides the solution for L for

constant A = a;

RT

akL exp Equation 2-7

where k is a constant. Now, if the heat of solution of air gases in aviation fuel is a linear function

of temperature e.g. bTaA then the solution is;

RT

aTkL R

b

exp.1 Equation 2-8

16

If A is considered to vary quadratically with temperature e.g. 2TcTbaA then the

solution is;

R

TcRT

a

TkL Rb

exp2

Equation 2-9

Equations 2-8 and 2-9 were fitted to Ostwald solubility vs. temperature data for Jet A-1 in Figure

2-1using SPSS17 non-linear regression analysis software. As can be seen from Figure 2-3

whether the heat of solution of air gases in aviation fuels is considered to vary linearly or

quadratically, the Clausius-Clapeyron equation closely describes the behaviour of dissolved air

gases in aviation fuel with varying temperature. Interestingly, the result obtained in Equation 2-8

is of the same form as that obtained by Valentiner in studies of inert gas solubility in water

reported by Markham and Kobe (1941);

cTbT

aL lnln

Equation 2-10

Senese (2009), described using a molecular model of gas solubility, how in organic solvents there

is usually net absorption of heat (endothermic reaction) when gases are dissolved. The energy

associated with forming a cavity between solvent molecules is greater than the energy released

during dissolution. Le Chateliers principle predicts that when heat is absorbed by the dissolution

process it will be favoured at higher temperature where gas solubility is expected to increase as

temperature rises (Holmes, 1996). By contrast, water exhibits a net release of heat (exothermic

reaction) when gases are dissolved because of much stronger intermolecular attractions between

dissolved gas molecules and surrounding solvent molecules where gas solubility is expected to

decrease with increasing temperature.

17

Figure 2-3 Clausius-Clapeyron equation predictions of Ostwald solubility coefficients

compared with CRC solubility data for oxygen dissolved in Jet A-1 aviation fuel as a

function of temperature

The solubility of air gases in aviation fuels can be estimated using the method given in American

Society for Testing Materials (ASTM), D2779-92 (2002). Utilising calculations based on the

Clausius-Clapeyron equation, Henrys law and the ideal gas law, the Ostwald solubility and mass

concentration of air gases in aviation fuels can be estimated over a -50F to 302F (-45.6C to

150C) temperature range. Universal Oil Products (UOP) Method 678-04 (2004) provides a

procedure for estimating the concentration of dissolved oxygen in liquid hydrocarbons from

measurements of dissolved oxygen partial pressure in a liquid sample, the Henrys law constant

and the Ostwald solubility coefficient estimated at the measurement temperature. As will be seen

18

in the experimental section of Chapter 6, these methods are used to determine the mass of

dissolved oxygen in the fuel, from which the rate of oxygen evolution is calculated. Considering

the numerous ways in which gas solubility in liquids can be expressed, as reviewed in this

Chapter, it can be concluded that the Ostwald solubility is perhaps the most relevant to this

research work.

Boeing and the US Air Force (2010) jointly examined the theoretical solubility of air gases

(oxygen and nitrogen) in bio-derived and Fischer-Tropsh-derived synthetic paraffinic kerosenes

using Ostwald solubility coefficients. Dissolved gas concentrations in mixtures of these bio-fuels

with petroleum aviation fuels were also examined. They contrasted the dissolved oxygen and

nitrogen mass concentrations with typical petroleum based fuels over a 0C to 120C temperature

range. Apart from the data showing a slight decreasing trend in dissolved oxygen concentration

with increasing temperature and a slight concentration increase of nitrogen with temperature, the

dissolved gas concentrations reported appear very high for all the fuels examined. Even the bio-

fuels lower density in comparison with petroleum derived fuel does not explain the magnitude of

the concentration values presented for each gas. The mass concentrations were calculated using

the ASTMD2779-92 method, which requires the fuel density, temperature and partial pressure of

the solute gas to be known. The error which appears to have occurred, resulting in higher than

expected concentrations, most probably stems from use of the air partial pressure instead of the

individual gas partial pressure used to compute the concentration of each dissolved gas. The

ASTMD2779-92 calculation method should report a mass concentration of oxygen (ppm) at 1

atmosphere air pressure, in Jet A-1 fuel with a density of 0.8 kg/l at 20C of 83.5ppm. Boeing and

the US Air Force report a dissolved oxygen concentration of ~400 ppm for the same fuel and

temperature. In close agreement with the ASTM method, UOP Method 678-04, using the authors

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measurements of dissolved oxygen partial pressure at 1 atmosphere in Jet A-1 fuel provides an

oxygen concentration estimate of 82 ppm at 20C. Correspondence with the Boeing Commercial

Airplane Company revealed that their theoretical solubility calculations using ASTMD2779-92

for both dissolved oxygen and nitrogen had indeed been based on the air partial pressure. This

leads to the assumption of either a pure oxygen or nitrogen atmosphere above the fuel surface,

which in an aircraft fuel tank ullage is unrealistic (Belieres, 2011).

2.5 Composition of Air in Aviation Fuel

When aviation fuel is exposed to the atmosphere oxygen and nitrogen are dissolved into the fuel

until the partial pressures of these dissolved gases are in equilibrium with the partial pressures of

the atmospheric gases. Oxygen is more readily dissolved than nitrogen where the oxygen

solubility coefficient can be 1.5 to 2 times that of nitrogen. As a result of differing solubility

coefficients, dissolved air in aviation fuel contains approximately 32% oxygen by volume with

the balance being nitrogen. Using a gas chromatographic method, Ross (1970) determined the

composition of air in terms of oxygen and nitrogen at 25C in a wide range of aviation fuels.

Table 2-2 presents this data.

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Table 2-2 Compositions of air dissolved in aviation fuels as determined at 25C using Gas

Chromatography (Ross, 1970, p.25)

Fuel Type Composition (% by volume)

Oxygen Nitrogen

Avgas 115/145 29.9 70.1