Characterization of supercooled droplets in an icing wind tunnel using laser-induced fluorescence

Abstract

An innovative application of the laser-induced fluorescence (LIF) technique is developed to characterize droplets in an icing wind tunnel (IWT). Two fluorescent dyes are dissolved in water, and fluorescence detection is made in three spectral bands allowing to calculate two fluorescence ratios. A first ratio is used for measuring the temperature of the supercooled droplets with a rather good temperature sensitivity of 2.56%/°C. The second ratio is used to characterize the droplet phase change, namely discriminating a supercooled droplet from a fully or a partially solidified droplet and in this latter case estimating the ice fraction in the droplet. Due to the dissolution of fluorescent molecules in water, it is essential to assess the influence of the dyes on both the physical properties of the liquid and the supercooled (degree) state. Thus, experiments allowing the determination of these different parameters were carried out on de-ionized and seeded water. It was thus observed that only the surface tension of seeded water is affected with a decrease of 25% compared to de-ionized water. Measurements conducted with droplets deposited on a subcooled surface have demonstrated that the droplet solidification occurs with the same probability regardless of whether they are seeded by the dyes or not. Experiments firstly conducted at laboratory scale, involving droplet velocities up to 10 m/s, demonstrated that the second fluorescence ratio makes it possible to estimate the ice fraction for a given droplet population. Finally, the technique was applied to supercooled droplets generated in an icing wind tunnel (IWT) for air flow up to 200 m/s, close to the conditions encountered in aircraft flights. Temperature measurements, derived from the first fluorescence ratio, demonstrate that droplets are not in thermal equilibrium with the cold air flow, even when they are supercooled. The second fluorescence ratio made it possible to detect the phase transition in the droplets and to estimate the ice fraction within the droplet population. Visualization of the phase change of supercooled droplets evolving in an icing wind tunnel and instantaneous evolution of their volume ice fraction.