Liquid-Vapor Imaging in Fuel Sprays Using Lifetime-Filtered Planar Laser-Induced Fluorescence

Many performance parameters in combustion systems are heavily dependent on the fuel injection process. Liquid-vapor imaging in spray applications has proven to be difficult because of the dominance of the liquid phase signal due to its higher number density. This work addresses this issue by employing temporal filtering in Planar Laser-Induced Fluorescence (PLIF) imaging. Temporal filtering takes advantage of the fact that although the vapor signal tends to be weaker, its fluorescence signal persists longer than the liquid signal under the same conditions. An experimental setup was designed with the capability to supply a two-phase flow in a controlled environment at atmospheric pressure. Fuels such as Jet-A, JP-10, and toluene were tested to determine their respective fluorescence lifetimes as well as their absorption and emission spectra. The effect of temporal filtering was then demonstrated for each material by characterizing the fluorescence decay profile for liquid and vapor under identical flow conditions and imaging system settings. A droplet subtraction method was also demonstrated using Jet-A in which a two-camera setup was utilized to subtract the liquid signal contribution from an image, leaving only a vapor measurement. Temporal filtering was also applied to exciplex tracers using various concentrations of N,N-Diethylmethylamine (DEMA) and fluorobenzene in hexane. The emission spectrum was determined for this combination in the liquid and vapor phases. The liquid, vapor, and cross-talk signals were quantified with increasing time delay after the laser pulse to determine the optimal timing for imaging. Finally, a demonstration of this technique was completed showing complete separation of the liquid and vapor signals