The application of water injection in jet engine compression systems is of renewed interest in the civil aviation industry due to its potential to cool the intake air and thus reduce the NOx emissions formed in the combustion process. A relatively unexplored area is the employment of this technique to low-pressure compressors in civil turbofan engines. In this study, a numerical experiment of water injection on an axial high aspect ratio transonic rotor is carried out. 

The analysis of the continuous gas phase is performed using a steady-state *Reynolds-Averaged Navier-Stokes* solver in conjunction with the *Shear Stress Transport Re-attachment Modification* turbulence model. The dispersed phase is analysed using a Lagrangian particle transport approach with two-way coupling between the discrete and continuous flow component. The particles are injected at uniform particle mass flux. The number of droplets that would be typically present in a spray injection is characterised by so-called parcels which are a statistical representation of droplet groups of the same characteristics. To quantify the compressor performance in the presence of heat exchange between the phases, an entropy-based compression efficiency evaluation method is implemented and tested for both the dry and wet case. 

The dry and wet rotor performance is tested at Sea-Level Static, *Internation Standard Atmosphere* day conditions alongside the rotor design speed line. The simulated points include the near-choke, near-peak-efficiency, design, and near-stall conditions. The water injection is analysed at a constant temperature but varying water-to-air ratio, droplet diameter and size distribution. The change in mass flow, velocity, flow angle and the stagnation values at the rotor exit are evaluated, together with the evaporation rate, enthalpy, entropy generation and thus the compression efficiency for both the dry and wet cases. Furthermore, the effect of the grid and statistical droplet models on the water injection predictions is analysed.

The results indicate that for a given water-to-air ratio, there is a minimum parcel size for which the results are statistically significant. The particle transport model exhibits a weak dependence on the mesh size due to the introduced changes in the main gas path. The study confirms that the entropy-based compression efficiency is an adequate measure for compressor performance and gives realistic results at both dry and wet operation. It is observed that water evaporation contributes to entropy generation across the rotor, and so deteriorates the compressor efficiency. The circumferentially averaged flow parameters are strongly sensitive to droplet diameter and water-to-air ratio. The changes are more pronounced at higher liquid fractions and smaller droplets as more water evaporates and the mass transfer is faster than for larger particles. Bigger droplets contribute less to the variation of the radial flow profiles but increase the axial velocity at the rotor exit due to their larger momentum. Furthermore, there is a more significant drop in the exit stagnation temperature along the upper blade span as the droplet evaporation increases in the shock area. The influence of water is shown to be relatively insensitive to the type of droplet size distribution.

Keywords: water injection, wet compression, axial compressor, transonic, isolated rotor, design speed, aerodynamic performance


Entropy Generation and Efficiency of a Transonic Rotor With Water Injection: A Numerical Study

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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