An experimental investigation of the phosphorescence properties of liquid and vapour
acetone is presented with the goal of introducing a novel technique for improved two-phase flow
visualization. Commonly applied laser induced fluorescence (LIF) investigations of two-phase
flows are challenging, in particular because of the large disparity in fluorescence intensity between
the two phases and the ensuing effect of halation around the liquid droplets. The phosphorescence
properties of liquid and gaseous acetone are utilized in order to decrease this disparity and acquire
more quantitative images of the vapour phase surrounding the droplets. For this purpose, the
phosphorescence lifetimes of acetone vapour in air and nitrogen bath gas, as well as those of liquid
acetone was investigated. A 308nm XeCl excimer laser was used as the excitation source.
Acetone vapour in air does not emit phosphorescence due to strong oxygen quenching;
however, in nitrogen, a slowly decaying signal with a lifetime of around 920ns is observed. Liquid
acetone phosphorescence is strongly affected by self-quenching, as well as the amount of oxygen
dissolved in it. The phosphorescence lifetime of liquid acetone that has been exposed to ambient air
for a prolonged period of time is around 73ns. When, instead, liquid acetone is degassed (purged
with nitrogen in order to remove any dissolved oxygen), its lifetime increases to 213ns. Introducing
liquid acetone that has not been previously purged in a nitrogen atmosphere allows for the on-going
diffusion of oxygen out of the liquid to be directly imaged. The phosphorescence intensity varies
depending on the degree of oxygen diffusion out of the liquid and thus, a well-defined condition
required for two-phase flow imaging is only achieved for the degassed liquid acetone in nitrogen
case.
Direct comparison of the phosphorescence intensities of liquid and vapour acetone along with
their respective decays has revealed the potential to optimize the disparity between the two signals
for two-phase flow imaging. In order to examine the effectiveness of visualizing the
phosphorescence rather the fluorescence, experiments with an acetone droplet stream were
conducted in different environments. The phosphorescence images allow for the liquid-vapour
interface to be accurately located and for the vapour phase to be clearly visible in contrast to
standard LIF images, which show strong halation around the droplets resulting in both an apparent
increase in the spatial extent of the droplets and an over-prediction of the vapour phase
concentration near the interface.