Effects of liquid properties on atomization and spray characteristics studied by planar two-photon fluorescence

Abstract

In this work, planar two-photon laser-induced fluorescence (2p-LIF) is applied for the first time to analyze the fluid dependent spray structure and atomization behavior of water and ethanol in a quantitative way. A commercial six-hole DISI (Direct-Injection Spark-Ignition) injector was studied at different injection pressures, operated with liquids containing the LIF dye fluorescein. Specifically for DISI-injectors, the fluid-dependent atomization is very complex and not fully understood due to the cavitating, turbulent nozzle flow that dominates the spray formation. Optical access and analysis of the near-nozzle spray are often challenging due to multiple light scattering in dense regions which is reduced by 2p-LIF measurements using a femtosecond laser. This allows high-contrast spray imaging close to the nozzle, resulting in an improved identification of single liquid structures of the spray. Thus, a higher accuracy of sizing is possible. Compared to water, the ethanol spray shape shows increased cone angles in the nozzle near-field of about 6%, which cannot be explained by classical atomization theory based on aerodynamic breakup. The larger cone angle of ethanol was attributed to its larger viscosity, which could decelerate the flow at the wall of the injection hole, affecting the velocity profile of the emerging jet. The atomization shows a main jet breakup distance of 7–10 mm in which the structure sizes decreased drastically, specifically for water. For the size of the liquid structures in the near-nozzle region, which show dimensions of about 80–130  μm, ethanol exhibited about 2% smaller Feret's diameters than water for the tested time steps at 20 MPa. This effect is even more distinct for other injection pressures and positions at a further distance to the injector. For all investigated conditions and measurement positions downstream of the nozzle, ethanol showed on average about 24% smaller structures compared to the water spray. Although this trend is in accordance with the classical atomization theory based on the aerodynamic breakup mechanism, other effects, such as cavitation and nozzle-flow induced breakup, contribute to this behavior.