CFD Analysis of Supercooled Large Droplets in Turbofan Engines 2011-38-0080
The study of Supercooled Large Droplets (SLD) has received greater attention in the Aviation industry since the ATR-72 accident in 1994, which was attributed to SLD. This type of icing cloud usually consists of droplets of up to a millimeter in diameter and mean volumetric diameter (MVD) greater than 40 microns1. The analyses of the ice accretion process with SLD have focused mainly on the wing and stabilizers, particularly on the leading edges where accretion can occur beyond the ice protected areas. There are several numerical and empirical models to predict the mass and shapes of ice accreted from SLD, but there are few published papers that focus on SLD accretion within aircraft turbofan engines2, 3, 4, 5, 6, 7, 8, 9.
SLD droplets have higher inertia than conventional icing droplets, which leads to their trajectories being less influenced by the aerodynamic forces. However, large droplets are more likely to breakup than smaller droplets when subjected to highly shear flows. In addition, SLD tends to splash on impact resulting in smaller droplets in the process.
CFD tools were considered as the most appropriate to simulate SLD dynamics (trajectories and droplet breakup). The CFD tools allowed modeling the dynamics of each particle and the global dynamic of the SLD flow, from the free stream up to the core inlet of turbofan aircraft engines. The simulations considered the drag2, wall impingement (splashing)10 and aerodynamic breakup12, 13. Empirical and analytical data2, 3, 4, 5, 6, 7, 9, 10, 11 have indicated that these first-order phenomena have a significant effect on the dynamics of the SLD size distribution. In addition, a three-dimensional Lagrangian droplet tracking method was employed to simulate these effects further into the engine; from the engine nozzle inlet to the core inlet.
The combined results of these two models showed that the high diameter SLD droplets break up at the nozzle inlet, where the relative velocity between the air and the droplets became more important. They also showed that the resulting droplet MVD at the core inlet were similar to those observed in conventional icing.