Temperature measurement and state determination of supercooled droplets using laser-induced fluorescence

An extension and a new application of the laser-induced fluorescence (LIF) is developed for supercooled droplet characterization: temperature measurements and phase droplet discrimination (liquid, solid or partially solid). In a first step, fluorescent dyes have to be selected to obtain a solution having a high temperature sensitivity. For that, fluorescent spectra at different temperatures of supercooled water within a cooled cell are analyzed and a couple of two fluorescent dyes have been identified. The water (with both dyes) was calibrated in temperature using a suspended droplet up to a minimum temperature of − 17 °C and a sensitivity of 2.56%/°C was obtained. A thorough study of the fluorescence spectrum allows also using LIF to determine the phase of the droplets. Furthermore, with the help of magnetic resonance imaging, it also possible to estimate the ice fraction in the case of supercooled water contained in a cell. Finally, the LIF technique is tested on supercooled droplets flying in a cold air environment for several injection conditions: droplets’ diameters up to 400 µm and a minimum temperature of − 45 °C. All detected droplets are classified according to their states and, for liquid droplets only, the temperature is obtained. Main results show a decrease of the supercooled droplets when the ambient air temperature decreases. Similarly, the number of liquid droplets increases with the ambient air temperature.