ID 24

Temperature and ice fraction measurements of supercooled droplets using Laser Induced Fluorescence

 

Abstract:

A new field of application for the Laser Induced Fluorescence technique is presented: both temperature and ice fraction measurements of supercooled droplet. A couple of two fluorescent dyes have been identified allowing a temperature measurement with a sensitivity of 2.55 %⁄°C by the use of a first fluorescent ratio R12. The mixture (water and both dyes) is calibrated in temperature using a suspended droplet until a temperature of -17°C. An in-depth spectral analysis made it possible to set up a second fluorescence ratio R31 that is strongly sensitive to phase change. This second ratio was tested on a suspended droplet as well as an experimental setup allowing the simultaneous measurement of water volume variation and R31. It appears a proportional evolution between the ratio and the water volume. Raman spectroscopy is a proven method for monitoring the phase change of supercooled water. Here LIF and RAMAN measurement are compared to corroborrate that LIF measurement allows a follow-up of the phase change. The LIF3c2d measurement is then used to determine both the temperature and the state of droplets falling in a cold environment (-54°C). Two configurations were tested: individual droplets generated by a syringe pump and a mono-sized droplets chain. In parallel, numerical simulations were carried out for both configurations (taking into account the internal circulation within the drop and the droplet interaction) and show results in good agreement with those obtained experimentally.