Four-dimensional laser-induced fluorescence and tomography of liquids

The bulk liquid deformation and breakup of droplets are inherently three-dimensional and important in various spray and multiphase industrial processes. Time-resolved volumetric laser-induced fluorescence (VLIF) is investigated to characterize four-dimensional (x,y,z,t) liquid breakup dynamics encountered in complex propulsion sprays. Experiments and ray-tracing simulations are used to characterize the liquid fluorescence signal and the connection of surface morphologies to imaging artifacts. High-speed VLIF of acetone is performed at rates up to 20 kHz using two high-speed cameras coupled to four-way image splitters to enable tomographic reconstruction with eight perspective views. Comparisons of reconstructed droplets to reference shadowgraphs are used to quantify the expected reconstruction error at different spatial scales. The liquid reconstructions for the current imaging arrangement are more accurate for larger structures and result in ∼5% error for features greater than 0.6 mm (∼10 voxels). Various fluorescence signal artifacts limit the reconstruction accuracy of liquid structures. Results from experiments and simulations indicate that the VLIF signal includes intensity hot spots caused by surface ripples from aerodynamic forces. Based on detailed liquid fluorescence ray-tracing simulations, it is found that internal reflections and signal trapping are the primary cause of these artifacts. These measurement artifacts depend on the liquid's index of refraction and surface topology and are linked to liquid laser-induced fluorescence. Despite these limitations, VLIF adequately resolved submillimeter amplitude oscillations and primary breakup temporal dynamics such as ligament and droplet formation. Potential means of improving accuracy and quantitative liquid fluorescence measurements are discussed.