Droplet temperature is one of the critical parameters which determine ice accretion on aircraft components. Recreating natural icing conditions in a wind tunnel still pose a challenge. A major difficulty is the determination of the water droplet temperature. Unlike naturally formed liquid droplets in the troposphere, where the droplet temperature can be assumed to be equal to the surrounding air temperature, the water injection in icing wind tunnels demands positive temperatures, limiting the residence time and thus the heat exchange with ambient air in the free flow Therefore, in-situ measurements of droplets temperature and size distribution is crucial for characterization of conditions in the tunnel.

The aim of task 4.3 in WP4 is to evaluate droplet temperature measurements by using the Global Rainbow Technique (GRT) in Icing Wind Tunnel (IWT) as well as to simulate the droplet temperature behavior during its short resident time. GRT is a non-intrusive technique based on the analysis of the light scattered by droplets around the rainbow angle. By using a rigorous theory, the average droplet temperature and size distribution can be extracted simultaneously. According to the IWT dimension, GRT-XL is developed to measure from outside of small IWT while GRT-Mini is developed to measure directly inside large IWT. A simulation code has been developed to quantify the effect of few large droplets in a cloud of smallest one on the temperature and size measurement, especially when the large and small droplet are at different temperature.

This report describes the first droplet temperature measurements carried out a small IWT at the Technical University of Braunschweig (TUBS) for small and large droplets, under Appendix C conditions. Additionally, the preliminary computational work carried out at TU Braunschweig to estimate the droplet temperature in the icing wind tunnel will be summarized in this report. The influence of the injection temperature, injection velocity and spray bar axial position are studied, to estimate droplet velocities and temperatures for different size distributions for supercooled droplets in the IWT test section. Furthermore, the preliminary temperature measurements of large droplets under FZDZ conditions inside RTA’s large IWT are evaluated.
Over the course of one week, the droplet temperatures have been successfully measured in the icing wind tunnel at TUBS. Real time measurements have been realized under adjustment of IWT settings in order to vary spray parameters such as: MVD, LWC, air temperature and velocity. The results show that the droplet temperature is a complex function of all these parameters. The measurements have been essentially focused on droplets with a MVD between 20 μm and 60 μm, permitting comparisons with numerical simulations. Moreover, some measurements have been carried out for SLD with MVD larger than 100 μm.
FZDZ studies, which have been performed during a two-day measurement period at RTA, IWT (Vienna, Austria) have been strongly perturbated by water condensation on the GRT recording lens. Nevertheless, measurements have been successfully carried out for one case.

These results demonstrate that droplet temperature measurements under application of GRT in IWTs are possible, showing that the main limitations are mainly related to technical issues due to water/ice deposition on the optical surface. To remove these difficulties, dedicated accessories have to be used to locally ventilate and warm the optical surfaces. An upgrade to the instrument might involve a heated optical inlet and the application of a covering sheath-flow above the optics, in order to prevent and remove any deposition. Adjustments to the proposed instrument accessories have to be tested for the employed test environments in each wind tunnel, specifically.

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