Aerodynamics of an Aeroengine Intermediate Compressor Duct: Effects from an Integrated Bleed System

Doctoral thesis, 2020

With the successful development of high-bypass-ratio turbofan engines, major aerodynamic components have been optimized and continuous efficiency improvements are getting harder to maintain. Therefore, to meet the requirements of lower emission of greenhouse gases, auxiliary modules such as intermediate ducts are receiving increasing interest. The Intermediate Compressor Duct (ICD) is an S-shaped duct connecting the engine’s low- and high-pressure compressor systems. Improving the ICD design has the potential to favorably affect the engine’s emission levels by reducing the engine’s length and, therefore, its weight.

In this thesis, a state-of-the-art ICD is simulated using Computation Fluid Dynamics (CFD). The geometry of the ICD represents a test section from an experimental rig. The upstream flow conditions are essential to achieve realistic behavior in the ICD. Therefore, integrated design is considered, including a representative of the last stage from the upstream low-pressure compressor and an upstream rotor off-take bleed system. The bleed

system is an auxiliary module and ensures a stable operation during off-design conditions. Through the bleed system, pressurized air is extracted from the main flow-path and used for different applications. The effect an upstream bleed system has on the ICD is analyzed, where the stability and the flow physics are compared for different bleed ratios. To take advantage of the integrated design and increasing computational resources, higher fidelity CFD simulations, using hybrid RANS/LES turbulence models, are compared to more common industrially applied CFD models and validated using experimental data.

The results show that the stability of the ICD is compromised with high bleed ratios. The flow at the low-pressure compressor’s outlet guide vanes (OGVs) is separated and the separation is more severe at the inner casing. The increased separation is caused by a thicker inner casing boundary layer and the conservation of tangential momentum when extracting axial velocity through the bleed system. As a result, the ICD experiences separated flow at the critical point of diffusion. The separation at the critical point of diffusion increases in magnitude with increased bleed rates.

Comparing the hybrid models to the steady-state RANS models, the hybrid models are capable of predicting the circumferentially averaged total pressure profiles downstream of the ICD. However, the RANS simulations result in over-predicted losses due to over- predicted separation on the OGV blades. The experimental data had a relatively low resolution, and therefore, the hybrid methods need further validations. Furthermore, the hybrid methods are significantly more expensive but represent the transient flow field, whereas the RANS simulations only provide the time-averaged results.