<div>Due to the depletion of fossil fuel reserves and emission challenges associated with its usage, there is a need for alternative aviation fuels for future propulsion. The alternative fuels with handling, storage and combustion characteristics similar to conventional fuels can be used as “drop-in” fuels without significant changes to the existing aviation infrastructure. Fire safety characteristics of alternative aviation fuels have not been studied intensively and therefore research is needed to understand these characteristics. In this study, fire safety characteristics namely hot surface ignition (HSI) and flame spread phenomena are investigated for alternative aviation fuels. </div><div><br></div><div>HSI is defined as the process of a flammable liquid coming in contact with a hot surface and evaporating, mixing and reacting with the surrounding oxidizer with self-supporting heat release (combustion). If all the conditions are adequate, the fuel may completely turn into combustion products following the ignition process. This work presents results from more than 5000 ignition tests using a newly developed reproducible test apparatus. A uniform surface temperature stainless steel plate simulating the wall of a typical exhaust manifold of an aircraft engine is used as the hot surface. Ignition tests confirmed that the ignition event is transient and initiates at randomly distributed locations on the hot surface. The results show many significant differences and some similarities in the ignition characteristics and temperatures of the different fuels. In this work, hot surface ignition temperatures (HSITs) are measured for nine hydrocarbon liquids. Five of these fuels are piston engine based, three fuels are turbine-engine based and one fuel is a pure liquid, heptane. The piston engine based fuels are given by FAA and are confidential and hence labeled as test fuels A, B, C, D for this study. The HSITs of these fuels are measured and compared against a baseline fuel 100 LL aviation gasoline (100LL Avgas). HSITs of conventional turbine engine based fuels namely Jet-A, JP-8, and JP-5 are also measured. </div><div><br></div><div>Flame spread along liquid fuel has been one of the important combustion phenomena that still requires more in-depth research and analysis for the deep understanding of the chemical processes involved. Flame spread rate determines how fast the flame spreads along the fuel surface and it is an important parameter to study for fire safety purposes. For the flame spread rates study, a novel experimental apparatus is designed and fabricated. The experimental apparatus consists of a rectangular pan, a fuel heating system, an autonomous lid actuation system, a CO2 fire extinguisher system, and a laser ignition system. The flame spread phenomenon is studied for a conventional aviation fuel namely, Jet-A and three alternative aviation fuels namely, hydro-processed ester fatty acids (HEFA-50), Fischer-Tropsch – IPK (FT-IPK) and synthetic iso-paraffin (SIP). The experiments are conducted for a wide range of initial fuel temperatures ranging from 25°-100°C for Jet-A, HEFA-50, FT-IPK and from 80-140°C for SIP as the flash-point of SIP is 110°C and is ~3 times higher than that of other three fuels. The flame spread rate of all fuels increases exponentially with increasing fuel’s initial temperature. Flame spread rate is as low as ~5 cm/sec for Jet-A, HEFA-50, FT-IPK for 25°C initial fuel temperature and goes to as high as 160 cm/sec for 80°C initial fuel temperature. For SIP based jet fuel, flame spread rate is ~160 cm/sec for initial fuel temperature of 140°C. Additionally, it was also found that the flame propagation consists of two types of flames: a precursor blue flame located ahead of the main yellow flame. These flames are more evident over the fuels’ surface with initial fuel temperatures higher than their respective flash-points. The precursor blue flame propagates like a premixed flame and the main yellow flame propagates like diffusion combustion.</div><div><br></div><div>This dissertation includes eight chapters. Chapter 1 gives an overview of the work done until now in the field of hot surface ignition. Following this review, the experimental apparatus designed and fabricated for this study are discussed in Chapter 2. This chapter also talks about the test matrix, data acquisition tools and concludes with the data analysis method. In Chapter-3, HSITs of 3 turbine engine based fuels and 5 piston engine based fuels are reported. This chapter also discusses the effect of drop height and curvature (flat v/s cylindrical) for two fuels, Jet-A, and heptane. This concludes the work done in the field of HSI in this dissertation. Chapter 4 talks about the past work reported by various researchers in the field of flame spread phenomenon and key learnings from their work. Chapter 5 discusses the experimental apparatus designed and fabricated for flame spread phenomenon study. In chapter-6, flame spread rates of 4 alternative aviation fuels are reported. This chapter also discusses the flame spread mechanism associated with slower (liquid-phase controlled) and faster (gas-phase controlled) flame propagation. Chapter 7 discusses flame propagation which consists two types of flames: a precursor blue flame and a main yellow flame. Chapter 8 concludes the key findings of the hot surface ignition and flame spread phenomenon study in this research work </div><div><br></div>

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<p>The research described in this dissertation focuses on several areas: developing analytical methods to distinguish structural isomers, identifying the chemical compositions of aviation fuels and evaluating the effectiveness of organic dopants to swell aircraft o-ring seals. Chapter 2 discusses fundamental aspects of mass spectrometry, and ionization methods and the instrumentation used to complete this research. </p> <p>Chapter 3 discusses and compares two activation methods used to distinguish ionized structural isomers. Ionized naphthene-containing aromatic structural isomers were subjected to collision-activated dissociation (CAD) in an ion trap (ITCAD) and to medium-energy collision-activated dissociation (MCAD) in an octupole collision cell, both in the energy-resolved mass spectrometry mode (ERMS). MCAD was shown to be superior over ITCAD at the structural differentiation of the ionized isomers. </p> <p>Determination of the chemical compositions of petroleum-based jet and diesel fuels, potential alternative fuels and fuel blending components by using a GCxGC/(EI)TOF MS is discussed in chapter 4. The ability to determine the chemical compositions of fuels and to correlate the identified compounds and their concentrations to the physical and chemical properties and aircraft performance of the fuels is vital for the development of future resilient, alternative fuels. The chemical compositions of petroleum-based fuels were found to be different from potential alternative fuels.</p> <p>Chapter 5 discusses the effectiveness of aromatic and nonaromatic compounds in swelling air craft o-ring seals, which prevents leaks in the fuel circulation systems. The aim of this study was to identify aromatic and nonaromatic compounds that most effectively swell o-ring seals. Steric effects were shown to decrease the efficiency of the compounds to swell seals. Ethylbenzene and indane were found to swell o-ring seals more effectively than any other compounds studied, including a currently approved alternative fuel. </p>

<div>The aviation industry is continuously growing amid tight restrictions on global emission</div><div>reductions. Alternative aviation fuels have gained attention and developed to replace the</div><div>conventional petroleum-derived aviation fuels. The replacement of conventional fuels with</div><div>alternative fuels, which are composed solely of hydrocarbons (non-petroleum), can mitigate</div><div>impacts on the environment and diversify the energy supply, potentially reducing fuel costs.</div><div>To ensure the performance of alternative fuels, extensive laboratory and full-scale engine</div><div>testings are required, thereby a lengthy and expensive process. The National Jet Fuel Combustion</div><div>Program (NJFCP) proposed a plan to reduce this certification process time and</div><div>the cost dramatically by implementing a computational model in the process, which can be</div><div>replaced with some of the testings. This requires an understanding of the influence of chemical/</div><div>physical properties of alternative fuels on combustion performance. The main objective</div><div>of this work is to investigate the spray characteristics of alternative aviation fuels compared</div><div>to that of conventional aviation fuels, which have been characterized by different physical</div><div>liquid properties at different gas turbine-relevant conditions.</div><div>The experimental work focuses on the spray characteristics of standard and alternative</div><div>aviation fuels at three operating conditions such as near lean blowout (LBO), cold engine</div><div>start, and high ambient pressure conditions. The spray generated by a hybrid pressureswirl</div><div>airblast atomizer was investigated by measuring the drop size and drop velocity at</div><div>a different axial distance downstream of the injector using a phase Doppler anemometry</div><div>(PDA) measurement system. This provided an approximate trajectory of the largest droplet</div><div>as it traveled down from the injector. At LBO conditions, the trend of decreasing drop size</div><div>and increasing drop velocity with an increase in gas pressure drop was observed for both</div><div>conventional (A-2) and alternative aviation fuels (C-1, C-5, C-7, and C-8), while the effect of</div><div>fuel injection pressure on the mean drop size and drop velocity was observed to be limited.</div><div>Moreover, the high-speed shadowgraph images were also taken to investigate the effect of</div><div>the pressure drop and fuel injection pressures on the cone angles. Their effects were found</div><div>to be limited on the cone angle.</div><div><div>The spray characteristics of standard (A-2 and A-3) and alternative (C-3) fuels were</div><div>investigated at engine cold-start conditions. At such a crucial condition, sufficient atomization</div><div>needs to be maintained to operate the engine properly. The effect of fuel properties,</div><div>especially the viscosity, was investigated on spray drop size and drop velocity using both</div><div>conventional and alternative aviation fuels. The effect of fuel viscosity was found to be minimal</div><div>and dominated by the effect of the surface tension, even though it showed a weak trend</div><div>of increasing drop size with increasing surface tension. The higher swirler pressure drop</div><div>reduced the drop size and increased drop velocity due to greater inertial force of the gas for</div><div>both conventional and alternative aviation fuels at the cold start condition. However, the</div><div>effect of pressure drop was observed to be reduced at cold start condition compared to the</div><div>results from the LBO condition.</div><div>The final aspect of experimental work focuses on the effect of ambient pressures on the</div><div>spray characteristics for both conventional (A-2) and alternative (C-5) aviation fuels. Advanced</div><div>aviation technology, especially in turbomachinery, has resulted in a greater pressure</div><div>ratio in the compressor; therefore, greater pressure in combustors for better thermal efficiency.</div><div>The effect of ambient pressure on drop size, drop velocity, and spray cone angle was</div><div>investigated using the PDA system and simultaneous Planar Laser-Induced Fluorescence</div><div>(PLIF) and Mie scattering measurement. A significant reduction in mean drop size was</div><div>observed with increasing ambient pressure, up to 5 bar. However, the reduction in the mean</div><div>drop size was found to be limited with a further increase in the ambient pressure. The effect</div><div>of the pressure drop across the swirler was observed to be significant at ambient pressure of</div><div>5 bar. The spray cone angle estimation at near the swirler exit and at 25.4 mm downstream</div><div>from the swirler exit plane using instantaneous Mie images was found to be independent of</div><div>ambient pressure. However, the cone angle at measurement plane of 18 mm in the spray</div><div>was observed to increase with increasing ambient pressure due to entrainment of smaller</div><div>droplets at higher ambient pressure. Furthermore, the fuel droplet and vapor distribution in</div><div>the spray were imaged and identified by comparing instantaneous PLIF and Mie images.</div><div>Lastly, a semi-empirical model was also developed using a phenomenological three-step</div><div>approach for the atomization process of the hybrid pressure-swirl airblast atomizer. This</div><div>model includes three sub-models: pressure-swirl spray droplet formation, droplet impingement, and film formation, and aerodynamic breakup. The model predicted drop sizes as a</div><div>function of ALR, atomizing gas velocity, surface tension, density, and ligament length and</div><div>diameter and successfully demonstrated the drop size trend observed with fuel viscosity,</div><div>surface tension, pressure drop, and ambient pressure. The model provided insights into the</div><div>effect of fuel properties and engine operating parameters on the drop size. More experimental</div><div>work is required to validate the model over a wider range of operating conditions and</div><div>physical fuel properties.</div><div>Overall, this work provides valuable information to increase understanding of the spray</div><div>characteristics of conventional and alternative aviation fuels at various engine operating</div><div>conditions. This work can provide valuable data for developing an advanced computational</div><div>combustor model, ultimately expediting the certification of new alternative aviation fuels.</div></div>

In order for the aviation industry to meet the targets of the Paris agreement and reach net-zero by 2050, significant amounts of greenhouse-gas emissions are to be reduced. However, as the industry is essentially dependent on conventional jet fuel, it becomes necessary for alternative technologies to develop and phase out fossil-based fuels. The thesis aims to provide valuable insights into the challenges and potentials of alternative technologies, which include sustainable aviation fuel (SAF), hydrogen, and electric-powered aviation. Additionally, the thesis investigated the Science-Based Targets initiative, and challenged the interim 1.5oC aviation pathway. The findings emphasize the crucial role of stakeholder cooperation in achieving net-zero emissions by 2050. Moreover, the thesis underscores the need for significant investments in alternative technologies, in order to enable growth and make the solutions increasingly attractive compared to conventional jet fuel. Collaboration and innovation are essential for attaining environmental targets while balancing economic growth. The thesis also highlights the urgency of policies and regulations to promote additional SAF production investments in order to vastly increase the supply. Furthermore, while the Science-Based Targets initiative (SBTi) is an effective means of securing airlines' commitment to the Paris Agreement, the thesis concludes that the SBTi 1.5oC interim pathway for airlines is overly optimistic. While the SBTi 1.5oC interim pathway’s SAF estimates for 2050 could be achieved, the thesis suggests around 2 to 4 times lower SAF supply for 2030 compared to the SBTi’s estimates. This further emphasizes the airlines' difficulties in following the 1.5oC pathway and the need for the industry to accelerate its transformation and make space for alternative solutions in order to meet the environmental targets. / För att flygindustrin ska nå målen i Parisavtalet och uppnå netto-nollutsläpp år 2050 måste betydande mängder växthusgasutsläpp minskas. Eftersom branschen är i grunden beroende av konventionellt flygbränsle blir det nödvändigt att utveckla alternativa teknologier för att fasas ut fossilbaserade bränslen. Avhandlingens syfte är att ge värdefulla insikter i utmaningarna samt möjligheterna med alternativa teknologier, vilket inkluderar hållbart flygbränsle (SAF), väte och elektriskt driven flygning. Dessutom undersökte avhandlingen Science-Based Targets-initiativet och utmanade det interimistiska 1.5°C-målet för flygindustrin. Resultaten betonar den avgörande rollen som samarbetande intressenter spelar för att uppnå netto-nollutsläpp år 2050. Dessutom understryker avhandlingen behovet av betydande investeringar i alternativa teknologier för att möjliggöra tillväxt och göra lösningarna allt mer attraktiva jämfört med konventionellt flygbränsle. Samarbetet och innovationen är nödvändiga för att uppnå miljömålen samtidigt som ekonomisk tillväxt möjliggörs. Avhandlingen betonar också brådskan med att införa policys och regleringar för att främja ytterligare produktion av hållbart flygbränsle (SAF) för att drastiskt öka tillgången. Medan Science-Based Targets-initiativet (SBTi) är ett effektivt sätt att säkerställa flygbolagens åtagande att uppfylla Parisavtalet, drar avhandlingen slutsatsen att SBTi:s interimistiska 1.5°C-mål för flygindustrin är alltför optimistiskt. Medan SBTi:s SAF-estimat för 2050 skulle kunna uppnås, föreslår avhandlingen en SAF-tillgång som är ungefär 2-4 gånger lägre för 2030 jämfört med SBTi:suppskattningar. Detta understryker ytterligare svårigheterna för flygbolag att hålla sig till 1.5oC målet samt nödvändigheten för flygindustrin att accelerera omställningen och skapa utrymme för alternativa lösningar för att uppnå miljömålen.