The transonic NASA Rotor 37 is well known for the occurrence of rotating stall at operation points close to stall. Several experimental and numerical investigations have shown that the instabilities observed mainly origin near the blade tip. Due to this property this rotor is theoretically suitable for a successful application of casing treatments. The first issue of this paper is to numerically investigate the flow mechanisms leading to compressor stall at design and off-design conditions. The numerical simulations carried out demonstrate that at design speed the behaviour of tip leakage flow in combination with the angle of attack at the blade tips is the key factor which limits the flow stability. Whereas at off-design, a steep angle of attack near the blade tip causes vast flow separations at the blade’s suction side. The second issue of the paper is to design an adapted casing treatment for the NASA Rotor 37. This issue is very demanding since the design total pressure ratio is with 2.1 extremely high and therefore, high-loss blockage zones can be easily caused by a misdesigned casing treatment. Based on the observations made in the first part guidelines are suggested for designing a casing treatment which efficiently enhances the compressor’s flow stability at design and off-design conditions. Casing treatments geometries are presented which address these suggested guidelines. Three-dimensional time accurate CFD-simulations are carried out to verify these designed casing treatments.
This paper describes the impact of axial slots on the flow field in a transonic rotor blade row. The presented results are completely based on time-accurate three-dimensional numerical simulations of a high pressure compressor front stage with and without casing treatment. Two different axial positions of a casing treatment consisting of axial slots were tested for their impact on flow stability and efficiency. The first tested position (configuration 1) was chosen in a conventional way. The slots extend approximately from the leading up to the trailing edge of the rotor blades. As expected, the simulations of the compressor stage with this configuration showed a significant increase in flow stability near surge compared to the solid wall case. However, a non-negligible decrease in efficiency is also observed. Analyses of flow interactions between casing treatment and rotor blade rows under transonic conditions lead to the general conclusion that the stabilizing effect of circumferential grooves or axial slots mainly results from their impact on the tip leakage flow and its resulting vortex. A characteristic vortex inside the slots is observed in the simulations with the conventionally positioned casing treatment. This vortex removes fluid out of downstream parts of the blade passage and feeds it back into the main flow further upstream. The resulting impact on the tip leakage flow is responsible for the increased flow stability. However, the interaction between the configuration 1 casing treatment flow and the blade passage flow results in a significant relocation of the blade passage shock in the downstream direction. This fact is a main explanation for the observed decrease in compressor efficiency. A second slot position (configuration 2) was tested with the objective to improve compressor efficiency. The casing treatment was shifted upstream, so that only 25% of the blade chord remained under the slots. The simulations carried out demonstrate that this shift positively affects the resulting efficiency, but maintains the increased level of flow stability. A time-accurate analysis of the flow shows clearly that the modified casing treatment stabilizes the tip leakage vortex and reduces the influence on the flow inside the blade passage.
This paper describes the influence of casing treatments on the tip leakage flow and its resulting vortex. The presented results and conclusions are based on steady state numerical simulations of a high pressure compressor stage. Without casing treatments a significant change of behavior of the tip leakage flow can be observed near surge. This change is termed as vortex breakdown and occurs after passing the shock in the blade passage. The simulations indicate the losses in total pressure inside the vortex core as the main reason for the vortex breakdown. These losses mainly depend on the blade loading. Running the compressor stage at high pressure ratios these losses can reach such a high level that the total pressure inside the vortex measured in the rotating system of the rotor falls below the static pressure after the shock. This pressure difference works as a physical barrier for the low energy vortex core and prevents it from reaching the high pressure rotor outlet. Consequently, this blockage must lead to the onset of recirculation zones — the so called vortex breakdown. Different casing treatments have been tested on their ability to delay vortex breakdown and to move the surge line to lower mass flows. Numerical simulations show that configurations with axial slots as well as circumferential grooves weaken or even destroy the characteristic tip leakage vortex and reduce its resulting losses in total pressure. This reduction in losses delays or prevents the onset of vortex breakdown compared to the untreated case explaining the effectiveness of casing treatments. Observations indicate that casing treatments do not interfere with the vortex directly. The key mechanism seems to lay mainly in the interaction with the tip leakage alone. Taking advantage of existing pressure differences in the rotor blade row casing treatments remove tip leakage flow in zones of high pressure and interrupt temporarily the feeding of the vortex. The separated tip leakage reenters the main flow in zones of low pressure again. The way how this tip leakage bypass is realized defines the influence of casing treatments on efficiency and surge line.
This paper describes the impact of axial slots on the flow field in a transonic rotor blade row. The presented results are completely based on time-accurate 3-dimensional numerical simulations of a high pressure compressor front stage with and without casing treatment. Two different axial positions of a casing treatment consisting of axial slots were tested for their impact on flow stability and efficiency. The first tested position (configuration 1) was chosen in a conventional way. The slots extend approximately from the leading up to the trailing edge of the rotor blades. As expected, the simulations of the compressor stage with this configuration showed a significant increase in flow stability near surge compared to the solid wall case. However, a non negligible decrease in efficiency is also observed. Analyses of flow interactions between casing treatment and rotor blade rows under transonic conditions lead to the general conclusion that the stabilizing effect of circumferential grooves or axial slots mainly results from their impact on the tip leakage flow and its resulting vortex. A characteristic vortex inside the slots is observed in the simulations with the conventionally positioned casing treatment. This vortex removes fluid out of downstream parts of the blade passage and feeds it back into the main flow further upstream. The resulting impact on the tip leakage flow is responsible for the increased flow stability. However, the interaction between the configuration 1 casing treatment flow and the blade passage flow results in a significant relocation of the blade passage shock in the downstream direction. This explains the observed decrease in compressor efficiency. A second slot position (configuration 2) was tested with the objective to improve compressor efficiency. The casing treatment was shifted upstream, so that only 25% of the blade chord remained under the slots. The simulations carried out demonstrate that this shift positively effects the resulting efficiency, but maintains the increased level of flow stability. A time-accurate analysis of the flow shows clearly that the modified casing treatment stabilizes the tip leakage vortex and reduces the influence on the flow inside the blade passage.
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