Efficiency Gains in a Water-Enhanced Turbofan Compressor – A CFD Study

Abstract In pursuing energy-efficient, low-emission propulsion concepts, civil aviation takes a renewed interest in compressor water injection (CWI). The unprecedented heat capacity of water can be utilised for effective compressor cooling through evaporation, thus substantially reducing the highly temperature-sensitive nitrogen oxide (NOx) emissions formed in the combustion process. It is also well known that evaporative cooling increases thermodynamic cycle efficiency and thus improves specific fuel consumption. However, relatively little consensus exists on the exact effects of CWI on compressor efficiency and operating limits, raising concerns about the deployment of water injection in aero-engines. A generic turbofan compressor was studied in detail using Computational Fluid Dynamics (CFD) analysis to shed light on how finely atomised water affects compressor efficiency. The compressor layout comprised an integrated fan stage and a two-stage booster, and water was introduced at the inlet to the core. Take-off operation was simulated under dry (no water) and wet conditions with 1% and 2% injection rates (water to air mass) and varying droplet diameters of 10 and 20 microns. A universal, second thermodynamic law approach was used to quantify the overall entropy generation in the compressor. The study demonstrated that compressor inter-cooling by evaporating water reduces entropy generation of compression at a rate that can supersede the entropy yield due to phase change. For very small droplet sizes and a moderate injection rate, a staggering 4% efficiency gain was projected when compared to the dry case with no water injection, thus reaffirming the performance benefit of a wet-operated compressor.