On the Performance of Hot-Wire Water Content Probes in Supercooled Large Droplet (SLD) Conditions

Supercooled Large Droplet (SLD) icing conditions pose a significant risk to aviation, as the larger mass of impinging water can lead to complex ice accretions and runback into unprotected regions of an aircraft. A common method to measure the cloud’s liquid water content (LWC), a key parameter for icing, uses hot-wire probes. However, accurate in-flight measurement of LWC in SLD conditions is challenging due to physical effects like droplet splashing and deformation. This study provides a detailed performance evaluation of two hot-wire probes, the Science Engineering Associates (SEA) Multi-Element Probe and the SEA Ice Crystal Detector, by comparing their measurements against the Isokinetic Probe 2 as a reference standard.



Experiments were conducted in the NASA Icing Research Tunnel across a range of SLD conditions. Stokes number sweeps were performed at -10°C and -15°C, covering median volume diameters from 14.1 µm to 469 µm and airspeeds from 50 to 300 knots. Our analysis reveals a distinct drop-off in the collection efficiency of both hot-wire probes with increasing droplet size. High-speed video footage corroborates this finding, showing significant splashing off the hot-wire sensing elements, including the concave total water content element. Additionally, results show a persistent bias in the dry air power readings, likely caused by ice accumulation on the probe strut.



These findings provide crucial information on the effective operating range of modern hot-wire probes and suggest that new correction factors are needed for reliable measurements in high-droplet-size cases. While the observed collection efficiency trends can inform these corrections, the issue of dry air power bias warrants further investigation.