Book Description

Vacuum-based investigations into the dynamics of gas-surface collisions for high vapor pressure liquids, including water and hydrocarbon fuels, are complicated by the dense vapor cloud that forms above the surface of the liquid. Earlier surface forming techniques, relying on a liquid-coated wheel, were limited to vapor pressures of 0.005 Torr. Narrow diameter liquid jets circumvent this limitation because their small surface area minimizes evaporation; the sparse vapor cloud surrounding the thin liquid jet, suppresses collisions between solvent vapor molecules and impinging or evaporating probe gas molecules. We use liquid microjets to investigate the interfacial behavior of surrogates of JP-8, a military grade jet fuel, under conditions that mimic the temperature of a gas turbine combustion chamber. Collisions between gases and liquids control the heating and vaporization of fuel droplets in jet engines. These dynamics can be investigated by gas-liquid scattering experiments. We explored collisions of oxygen and neon with dodecane (0.1 Torr vapor pressure) and compared them to collisions with a low vapor pressure liquid, squalane (10-8 Torr). The extent of energy transfer and thermalization are remarkably high and similar for the two hydrocarbon liquids. These studies suggest that hot gas molecules readily transfer their energy and heat and vaporize fuel droplets during the combustion process. We also used liquid microjets to explore the evaporation of inert gases from fuel and fuel surrogates. We find that most gas species evaporate with a Maxwell-Boltzmann distribution of kinetic energies (2RTliq) under collision-free conditions; they follow an evaporation pathway that involves momentary binding (trapping) at the surface before they desorb. However, two weakly binding atoms, helium and neon, evaporate with super-Maxwellian distributions. The measured average energies of evaporating helium atoms range from 14% more than 2RTliq for dodecane (a pure hydrocarbon liquid) to 70% more than 2RTliq for a 7.5 M LiBr/water solution. These experiments imply that He atoms evaporate ballistically, rapidly traversing the interfacial region at high energies whose magnitude are determined by the packing, bonding, and masses of interfacial solvent molecules. The evaporation of H2 was found to be sub-Maxwellian, as predicted from its light mass and moderate polarizability.