Investigation of Fire Safety Characteristics of Alternative Aviation Fuels
thesisposted on 05.12.2019 by Vikrant E Goyal
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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.
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.
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.
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