Abstract
This paper concerns a computational study of the process of removing water from an aircraft’s fuel tank by pumping nitrogen enriched air (NEA) from the bottom of the tank. This is an important procedure for the smooth, efficient, and safe operation of the aircraft’s engine. Due to the low partial pressure of water in the pumped NEA, it absorbs water from the fuel. The water-laden bubbles enter the ullage, the empty space above the fuel, and escape into the environment. The effects of the number of NEA inlets and the NEA mass flow rate on the timescale of the NEA pumping were investigated using Computational Fluid Dynamics. The results reveal that the absorption of water by the NEA bubbles is low and is not affected by the number of the inlets used. Yet, the water content in the fuel decreases fast during the procedure, which is the desired outcome. We show that this is due to the relatively dry NEA entering the ullage and displacing the moist air, thus reducing the partial pressure of water at the fuel/ullage interface. This shift from equilibrium conditions forces water to evaporate from the fuel’s entire surface. Furthermore, the amount of water migrating from the fuel directly into the ullage is significantly greater than that absorbed by the rising bubbles. In turn, the rate of decrease of the water content in the ullage is determined by the total NEA mass flow rate and this is the dominant contributor to the draining time, with the number of NEA nozzles playing a minor role. We confirmed this by pumping NEA directly into the ullage, where we observe a significant decrease of water even when the NEA is not pumped through the fuel. We also show that doubling the mass flow rate halves the draining time. When considering the capability of most modern aircraft to pump NEA through the fuel as part of their inerting system, the proposed method for removing water is particularly attractive, requiring very little (if at all) design modification.
Highlights
A major consideration in the design of aircraft fuel tanks is the purity of the fuel
During temperature and pressure fluctuations, the water can separate from the solution forming a water-in-fuel emulsion: small water droplets suspended in the fuel
The water vapour that is absorbed by the rising nitrogen enriched air (NEA) is shown in Figure 4 for the different inlet cases
Summary
A major consideration in the design of aircraft fuel tanks is the purity of the fuel. The first line of defence is the sumps, which are regions at the bottom of fuel tanks that collect debris and water that settles from the fuel. Sumps are drained during the on-ground routine maintenance before flights to relieve the fuel tank of any gathered contaminants. Fuel systems contain strainers and filters that prevent large pieces of debris and finer sediment, respectively, from entering the engine. Jet fuel can dissolve a small amount of water, which often enters the fuel tank from the ullage, which is the empty vapour space at the top of the tank. Dissolved water is not considered to be a contaminant [1], and it is vaporised during combustion [2]. During temperature and pressure fluctuations, the water can separate from the solution forming a water-in-fuel emulsion: small water droplets suspended in the fuel. If the temperature drops below a Energies 2018, 11, 908; doi:10.3390/en11040908 www.mdpi.com/journal/energies
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