Difference in the Working Principle of Axial Slot and Tip Blowing Casing Treatments

Abstract

Tip blowing and axial slot casing treatments have shown their ability to enhance the stability of a transonic axial compressor with different effects on efficiency. For an effective application of these casing treatments, a good knowledge of the influence of the casing treatment on the rotor flow field is important. There is still a need for more detailed investigations, in order to understand the interaction between the treatment and the near casing 3D flow field. For transonic compressor rotors this interaction is more complex, as super- and subsonic flow regions alternate while interacting with the casing treatment. In the present study, an axial slot and a tip blowing casing treatment, which have been developed and optimized for the same tip critical transonic axial compressor rotor (reference rotor) by Streit et al. [1] and Guinet et al. [2], are subject of the investigation. Both casing treatment types showed their capabilities to enhance the compressor stability without losing by means of CFD simulations. Since the higher compressor stability allows a higher blade loading, Streit et al. reduced the blade number of the rotor. Thus, the efficiency was increased due to the reduction of friction losses. However, applying the tip blowing casing treatment to the reduced rotor shows a negative effect on the efficiency. Both casing treatment types recirculate flow from a downstream to an upstream location of the rotor and reinject it to enhance the near casing flow field. Although the working principle of the two casing treatment types are similar, the transfer of the casing treatments from the reference to the reduced rotor show different trends in efficiency. Therefore, the effect of recirculation cannot explain the difference in efficiency. Hence, applying axial slots must include additional flow features, compared to recirculation channels. Compensating effects as in circumferential groove casing treatments and other flow interactions between the near casing flow field and the slot flow are considered. These additional mechanisms of the axial slot casing treatment will be identified and isolated by comparing the two different casing treatment types. The numerical simulations are carried out on a 1.5 stage transonic axial compressor using URANS simulations.

Effect of axial slot casing treatment on a counter-rotating axial compressor performance and stability

“…The results show that the curved inclined casing treatment with 20 to 40% axial position and 33% porosity can improve the stall margin and improve the compressor efficiency. Inzenhofer et al(Inzenhofer et al, 2016) compared the difference between the ASCT and the self-recirculating casing treatment. They demonstrated that the ASCT improved the stall margin better and restrained the TLF more effectively.…”

Effect of axial slot casing treatment on a counter-rotating axial-flow compressor aerodynamic performance and stability

“…A parametric study of bend skewed casing treatment was carried out by Alone et al 15,16 The results showed that the curved inclined casing treatment with 20%-40% axial position and 33% porosity can improve the stall margin and improve the compressor efficiency. Inzenhofer et al 17 compared the difference between the axial slot casing treatment (ASCT) and the selfrecirculating casing treatment. The results demonstrated that ASCT has a greater potential for increasing stall margin by injecting momentum over a wider range of blade spans.…”

Stability Enhancement of a Single-Stage Transonic Axial Compressor Using Inclined Oblique Slots

“…From an extensive study with 21 designs for a low-speed large scale compressor and a transonic compressor, two slots geometries with isoscelestrapezoid shape were devised, which increased the peak efficiency by about 0.5%. An axial-slot design by Inzenhofer et al [12] nearly doubled the compressor's flow range with an improved pressure ratio. They concluded that the axial-slot enhanced the compressor's stability by alleviating the pressure difference at the blade tip and ingesting a part of the low-momentum leakage air stream.…”