Laser induced phosphorescence imaging for the investigation of evaporating liquid flows

Abstract

The phosphorescence properties of liquid and gaseous acetone, following excitation at 308 nm, are studied and utilized in order to overcome two main challenges of two-phase flow laser induced fluorescence imaging: the large fluorescence intensity disparity between the two phases and the ensuing effect of halation. This is achieved on account of the different phosphorescence decay rates of the liquid and vapour phases, which allow for a more favourable signal ratio to be obtained. The benefits of visualizing the phosphorescence emission, instead of the fluorescence, are demonstrated by droplet stream experiments set up in different bath gases, at 1 atm and 297 K. The liquid–vapour interface can be accurately located, while the vapour surrounding the droplets is clearly visualized without any halation interference. The vapour phase phosphorescence signal was calibrated in order to quantify the vapour concentration around an evaporating droplet stream, and the results are compared to laser induced fluorescence images collected in the present study and results found in the literature. The effect of halation in the fluorescence images is shown to extend as far as 10 droplet diameters away from the interface for 161 μm droplets, resulting in a significant overprediction of acetone vapour mole fractions in that region. The vapour profile obtained by laser induced phosphorescence (LIP) imaging agrees with data found in the literature, for which a halation correction on fluorescence images was successfully performed. The demonstrated LIP technique for simultaneous vapour and liquid phase visualisation is only applicable to oxygen-free environments, as even trace quantities of oxygen completely quench the vapour phase phosphorescence emission.