Infoscience

Thesis

Investigation on the Three-Dimensional Flow Mechanisms in Annular Axial Compressor Cascades for Aero Engines with Flow Control by Aspiration on the Hub and on the Blades

The axial compressor is an essential component of aero-engines and gas turbines. This work investigates how flow control by aspiration applied to the hub and to the blades of axial compressor wheels can improve the static pressure rise by preventing detrimental flow features that cause aerodynamic loss and flow non-uniformity. It also investigates if the improvement obtained by this technique can compensate the increased loss that occurs if the number of blades is reduced. This would allow reducing the mass of the compressor. The three-dimensional flow related to the technique of flow control by aspiration is analyzed, yielding a better understanding of the involved flow mechanisms and supporting the further development of this technique. To model the flow in the stator wheel of an axial compressor, different annular cascades are designed and manufactured. They are tested in the Non-Rotating Annular Test Facility of the EPFL. The flow on configurations without aspiration, with aspiration on the hub only and with aspiration both on the hub and on the blades is investigated for different aspiration rates. The last configuration is tested for two different blade numbers. This work analyzes the results obtained for same inlet flow Mach number in the high subsonic range and two different inlet flow incidence angles. Several measurement techniques are applied: aerodynamic five-hole probe measurements in the inlet and outlet planes, static pressure tap measurements on the blades and on the walls, Laser-Doppler-Anemometry measurements and skin friction line visualization. The analysis of the measurement results is supported by the analysis of numerical simulations based on the inlet flow conditions measured during the experimental investigations. The work is contextualized by a bibliographic research on the state of the art in the domain of flow control by aspiration in axial compressors as well as on the particular flow features occurring. A simple model based on the conservation of mass and total enthalpy is derived to clarify the influence of flow aspiration on the static pressure rise. An adequate set of parameters is defined to quantify the cascade performance. To identify particular three-dimensional flow features in the outlet plane measurements, the rotational flow field is analyzed in terms of vorticity and helicity. Particular attention is given to so-called secondary flow features that are induced by the streamwise vorticity occurring in the flow. An innovative method to extract the secondary velocity field from the outlet plane measurements is derived. It is based on the Helmholtz decomposition. The approach is implemented using a finite element method and is used to post-process the outlet plane measurements. The comparison with the results of a classical approach shows that the results of the new method are significantly improved in detail and plausibility. The investigation identifies a number of flow mechanisms occurring in the cascade that are compared for the different configurations. The tested aspiration system is shown to have a beneficial effect in several cases, especially for the lower inlet flow incidence angles, where an improved performance in terms of total pressure level, blade loading, static pressure rise and outlet flow uniformity is obtained. Also for some cases with reduced number of blades, thanks to the aspiration, a similar performance in terms of static pressure rise and total pressure loss is achieved as for the reference case with higher number of blades. Although more optimistic, especially on the influence of the aspiration on blade, the numerical results reproduce the main flow features and can thus be used to support the analysis. The detailed analysis of the flow mechanisms related to the aspiration shows and explains the occurrence of problematic recirculations in the aspiration ducts. This gives indications for the further development of the aspiration system.

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