Investigation of laser induced phosphorescence properties of acetone

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.