Impacts of Preferential Vaporization of Multi-Component Liquid Fuels on Near-Limit Combustion Behaviors

Combustion technologies used in energy conversion devices involve multi-phase and multi-component combustion behaviors associated with chemical kinetic characteristics coupled with spray dynamics. For reliable engine operation, the engine operation envelope is necessarily required to be determined by near-limit combustion behaviors such as flame flashback, lean blowout, ignition, and extinction, which are influenced by both fuel chemical and physical properties. Liquid fuels are composed of a wide range of molecular structures and weights, therefore exhibiting relatively large distillation temperature range. When fuel chemical properties change during fuel vaporization, preferential vaporization effects could play a considerable role in near-limit combustion behaviors. In this regard, the objective of this study is to evaluate the role of preferential vaporization on near-limit combustion behaviors. At first, the time scale analysis of fuel droplets for gasoline, Jet fuel, and diesel is discussed to highlight the potential importance of preferential vaporization. The potential of preferential vaporization of liquid fuels is evaluated to verify the change of global reactivity over distillation cuts. Derived Cetane Number (DCN) is introduced to represent the chemical reactivity potential of distilled fuels. The variation of DCN over the distillation cuts is discussed to elucidate the preferential vaporization impact on combustion behaviors. The impact of preferential vaporization on flame flashback is investigated by utilizing a spray burner that can control the extent of fuel evaporation. By formulating two binary mixtures to exhibit common combustion behaviors at the fully vaporized condition, but have considerably different vaporization characteristics, flame flashback behaviors are observed at fully and partially vaporized conditions with high-speed imaging and Planar Laser Induced Fluorescence (PLIF) technique. Using Phase Doppler Particle Analyzer (PDPA), the extent of fuel vaporization and flow fluctuation are evaluated to analyze the flame flashback behavior. The relative contribution of preferential vaporization on the flashback flow velocity is evaluated through feature sensitivity analysis. The impact of preferential vaporization on diffusion flame extinction is investigated by utilizing a counterflow burner combined with the spray burner to control the extent of fuel evaporation. The behavior of diffusion flame extinction is observed at fully and partially vaporized conditions. Binary mixtures are formulated to exhibit identical extinction behavior but have different vaporization characteristic. Transport weighted enthalpy (TWE) and radical index (Ri) are applied to analyze the behavior of flame extinction. Flow stretch induced by flow fluctuation is evaluated through PDPA measurement. The relative contribution of preferential vaporization on the diffusion flame extinction is evaluated through feature sensitivity analysis. The interaction of isolated single droplet with premixed/diffusion flames at near extinction conditions is discussed by utilizing a counterflow burner integrated with a piezo-electric assisted single droplet generator. The extinction strain rate of premixed/diffusion flames is measured at different droplet sizes with methane and air. Single and multi-component fuels are used to observe the impact of preferential vaporization near extinction condition. The extinction behaviors are analyzed based on TWE, Ri, and vapor pressure of liquid fuel droplets