Stall margin improvement in a low-speed axial compressor rotor using a blockage-optimised single circumferential casing groove

The stall margin of tip-critical axial compressors can be improved by using circumferential casing grooves. From previous studies, in the literature, the stall margin improvement due to the casing grooves can be attributed to the reduction of the near casing blockage. The pressure rise across the compressor as the compressor is throttled intensifies the tip leakage flow. This results in a stronger tip leakage vortex that is thought to be the main source of the blockage. In this paper, the near casing blockage due to the tip region aerodynamics in a low-speed axial compressor rotor is numerically studied and quantified using a mass flow-based blockage parameter. The peak blockage location at the last stable operating point for a rotor with smooth casing is found to be at about 10% of the tip chord aft of the tip leading edge. Based on this information, an optimised single casing groove design that minimises the peak blockage is found using a surrogate-based optimisation approach. The implementation of the optimised groove is shown to produce a stall margin improvement of about 5%.

SINGLE AND MULTIPLE CIRCUMFERENTIAL CASING GROOVE FOR STALL MARGIN IMPROVEMENT IN A TRANSONIC AXIAL COMPRESSOR

The stability limit of a tip-stalling axial compressor is sensitive to the magnitude of the near casing blockage. In transonic compressors, the presence of the passage shock could be a major cause for the blockage. Identification and elimination of this blockage could be key to improving the stability limit of the compressor. In this paper, using numerical simulation, the near casing blockage within the transonic rotor, NASA Rotor 37, is quantified using a blockage parameter. For a smooth casing, the blockage at conditions near stall has been found to be maximum at about 20% of the tip axial chord downstream of the tip leading edge. This maximum blockage location is found to be consistent with the location of the passage shock-tip leakage vortex interaction. A datum single casing groove design that minimises the peak blockage is found through an optimisation approach. The stall margin improvement of the datum casing groove is about 0.6% with negligible efficiency penalty. Furthermore, the location of the casing groove is varied upstream and downstream of the datum location. It is shown that the stability limit of the compressor is best improved when the blockage is reduced upstream of the peak blockage location. The paper also discusses the prospects of a multi-groove casing configuration.

Axial Flow Compressor Stability Enhancement: Circumferential T-Shape Grooves Performance Investigation

This work focuses its attention on possibilities to enhance the stability of an axial compressor using a casing treatment technique. Circumferential grooves machined into the case are considered and their performances evaluated using three-dimensional steady state computational simulations. The effects of rectangular and new T-shape grooves on NASA Rotor 37 performances are investigated, resolving in detail the flow field near the blade tip in order to understand the stall inception delay mechanism produced by the casing treatment. First, a validation of the computational model was carried out analysing a smooth wall case without grooves. The comparisons of the total pressure ratio, total temperature ratio and adiabatic efficiency profiles with experimental data highlighted the accuracy and validity of the model. Then, the results for a rectangular groove chosen as the baseline case demonstrated that the groove interacts with the tip leakage flow, weakening the vortex breakdown and reducing the separation at the blade suction side. These effects delay stall inception, improving compressor stability. New T-shape grooves were designed keeping the volume as a constant parameter and their performances were evaluated in terms of stall margin improvement and efficiency variation. All the configurations showed a common efficiency loss near the peak condition and some of them revealed a stall margin improvement with respect to the baseline. Due to their reduced depth, these new configurations are interesting because they enable the use of a thinner light-weight compressor case as is desirable in aerospace applications.

Single and Multiple Circumferential Casing Groove for Stall Margin Improvement in a Transonic Axial Compressor

The stability limit of a tip-stalling axial compressor is sensitive to the magnitude of the near casing blockage. In transonic compressors, the presence of the passage shock could be a major cause for the blockage. Identification and elimination of this blockage could be key to improving the stability limit of the compressor. In this paper, using numerical simulation, the near casing blockage within the transonic rotor, NASA Rotor 37, is quantified using a blockage parameter. For a smooth casing, the blockage at conditions near stall has been found to be maximum at about 20% of the tip axial chord downstream of the tip leading edge. This maximum blockage location is found to be consistent with the location of the passage shock-tip leakage vortex interaction. A datum single casing groove design that minimises the peak blockage is found through an optimisation approach. The stall margin improvement of the datum casing groove is about 0.6% with negligible efficiency penalty. Furthermore, the location of the casing groove is varied upstream and downstream of the datum location. It is shown that the stability limit of the compressor is best improved when the blockage is reduced upstream of the peak blockage location. The paper also discusses the prospects of a multi-groove casing configuration.

Casing-groove optimisation for stall margin in a transonic compressor rotor

Purpose This paper aims to understand the aerodynamic blockage related to near casing flow in a transonic axial compressor using numerical simulations and to design an optimum casing groove for stall margin improvement using a surrogate optimisation technique. Design/methodology/approach A blockage parameter (Ψ) is introduced to quantify blockage across the blade domain. A surrogate optimisation technique is then used to find the optimum casing groove design that minimises blockage at an axial location where the blockage is maximum at near stall conditions. Findings An optimised casing groove that improves the stall margin by about 1% can be found through optimisation of the blockage parameter (Ψ). Originality/value Optimising for stall margin is rather lengthy and computationally expensive, as the stall margin of a compressor will only be known once a complete compressor map is constructed. This study shows that the cost of the optimisation can be reduced by using a suitably defined blockage parameter as the optimising parameter.

Influence of self-circulating casing treatment with double-bleed ports structure on compressor performance

Casing treatment is an effective way to improve the stable working margin of the compressor and plays an important role in ensuring the stable and safe work of the compressor. On the premise that the numerical results are in agreement with the experimental values, the paper study on a new type of the self-circulating casing treatment with double-bleed ports structure was carried out on the NASA rotor 37 by numerical simulation. It is found that compared with the self-circulating casing treatment with single-bleed port structure, the self-circulating casing treatment with double-bleed ports structure is more favorable for improving the stall margin of the rotor, and the improvement of the stall margin obtained at the rotor design speed is as high as 9.57%. Compared with the other two self-circulating casing treatments with single-bleed port structure, the improvement of the stall margin is increased by 37.90 and 49.53%, respectively. The self-circulating casing treatment with double-bleed ports structure has a significant effect on the enhancing stability, but at the same time, it causes a greater efficiency loss, which reduces the design point efficiency of the rotor by 0.34%. Through detailed analysis of the flow field, it is found that the self-circulating casing treatment improves the flow capacity of the blade tip region, weakens the blockage degree of the blade tip region, and reduces the blade tip load, thereby improving the stall margin of rotor. Compared with the self-circulating casing treatment with single-bleed port structure, the self-circulating casing treatment with double-bleed ports structure is more advantageous for delaying the flow separation of the suction surface, delaying the occurrence of the blade tip stall to make the stability of rotor more favorable, and obtained stall margin improvement is the greatest.

Mechanism of affecting the axial rotor stability and performance with center offset degrees of axial skewed slots

A subsonic axial rotor with axial skewed slot casing treatment (ASSCT) was investigated with experimental and numerical methods to explore the effects of the center offset degree (Cod) on the rotor stability and performance. Cod is defined as the ratio of the central difference between the rotor tip section and ASSCT to the rotor tip axial chord length. The Cod values are selected as 1.16, 0.608, 0, and −0.36, respectively. When the ASSCT is located upstream, the value of the Cod is positive. The experimental and unsteady calculated results show that the stall margin improvement (SMI) and peak efficiency loss (PEL) are reduced when the slots move to the rotor upstream or downstream. The slots with 0 Cod (ASSCT1) achieve 50% SMI with 8.67% PEL. The slots with 0.608 Cod (ASSCT2) gain 48.5% SMI with 2.12% PEL. The slots with 1.16 Cod (ASSCT3) and −0.36 Cod (ASSCT4) gain −3.41%, 20.1% SMI, respectively. Considering the compromise between the SMI and PEL for the rotor, ASSCT5 was designed by changing the slot number of ASSCT2 from 180 to 90. The unsteady calculated result shows that ASSCT5 can gain 22.28% SMI and 0.485% PEL. The detailed analysis of the flow field in the compressor tip passage indicates that there are adverse effects made by the slots on the flow field near the rotor blade tip leading edge for ASSCT3. Moreover, the ability of reducing the range of low-velocity zones for ASSCT4 is much lower than that for ASSCT1 or ASSCT2, and ASSCT1 generates bigger flow losses in the rotor tip passage than ASSCT2. The positive effects and flow losses made by the slots with 0.608 Cod both become smaller with the slot number decreasing from 180 to 90.

Systematic Experimental Evaluations Aimed at Optimizing the Geometry of Axial Casing Groove in a Compressor

Performance and flow measurements are carried out to investigate the impact of varying the geometry of axial casing grooves on the stall margin and efficiency of an axial turbomachine. Prior studies have shown that skewed semi-circular grooves installed near the blade leading edge (LE) have multiple effects on the flow structure, including ingestion of the tip leakage vortex (TLV), suppression of backflow vortices, and periodic variations of flow angle. To determine which of these phenomena is a key contributor, the present study examines the impact of several grooves, all with the same inlet geometry, but with outlets aimed at different directions. The “U” grooves that have circumferential exits aimed against the direction of blade rotation achieve the highest stall margin improvement of well above 60% but cause a 2.0% efficiency loss near the best efficiency point (BEP). The “S” grooves, which have exits aimed with the blade rotation, achieve a relatively moderate stall margin improvement of 36%, but they do not reduce the BEP efficiency. Other grooves, which are aligned with and against the flow direction at the exit from upstream inlet guide vanes, achieve lower improvements. These trends suggest that causing high periodic variations in flow angle around the blade leading edge is particularly effective in extending the stall margin, but also reduces the peak efficiency. In contrast, maintaining low flow angles near the LE achieves more moderate improvement in stall margin, without the maximum efficiency loss. Hence, of the geometries tested, the S grooves appear to have the best overall impact on the machine performance. Velocity measurements and flow visualizations are performed in an axial plane located downstream of the grooves, near the trailing edge of the rotor. Reduced efficiency or performance co-occurs with elevated circumferential velocity in the tip region, but differences in the axial blockage are subtle. Yet, near the BEP, the regions with reduced axial velocity, or even negative velocity between the TLV and the endwall, are wider behind the U grooves compared to the S grooves. The vorticity profiles also show that at low flow rates the TLV is ingested entirely by the grooves, in contrast to the best efficiency point, where a considerable fraction of the TLV rollup occurs downstream of the grooves.