Water drop runoff in aircraft fuel tank vent systems

Water condensation in aircraft fuel tank vent systems can run off to the fuel systems, where it can freeze to ice or support microbial growth in the fuel tanks. A laboratory scale test has been designed to investigate the ingress and runoff of water in the aircraft fuel tank vent pipes. The experiments are to determine the dual effects of air flow shear and hydrophobicity on water condensation in the vent pipes during descent from cruising altitudes. Results show only downslope runoff occurs and for large drop volumes where the height of the water drop is comparable with the height of the air flow boundary layer. Runoff is much more sensitive to drop volume and vent pipe inclination angle than air flow since the drops are within the air flow boundary layer. Downslope air flow has little effect on the runoff speeds. Downslope runoff speeds, where there is upslope air flow, exhibit large variations, when compared to those where there is downslope air flow. Upslope air flow can slow downslope runoff speeds of large volume drops by up to 400%. Runoff speeds may be up to 100 times greater with a hydrophobic coating than on the current inner vent pipe surface of anodised aluminium.