Since the early work on axial compressors, the penalties due to radial clearances between blades and side walls are known and are an ongoing focus of research work. The periodic unsteadiness of the tip clearance vortex (TCV), due to its interaction with the stator wakes, has only rarely been addressed in research papers so far. The current work presents experimental and numerical results from a four-stage low-speed research compressor (LSRC) modeling a state-of-the-art compressor design. Time-resolved experimental measurements have been carried out at three different rotor tip clearances (gap to tip chord: 1.5%, 2.2%, 3.7%) to cover the third rotor's casing static pressure and exit flow field. These results are compared with either steady simulations using different turbulence models or harmonic Reynolds-averaged Navier–Stokes (RANS) calculations to discuss the periodical unsteady TCV development at different clearance heights. The prediction of the local tip leakage flow is clearly improved by the explicit algebraic Reynolds stress model (EARSM) turbulence model compared to the standard shear stress transport (SST) model. The harmonic RANS calculations (using the SST model) improve the prediction of time-averaged pressure rise and are used to analyze the rotor stator interaction in detail. The interaction of the rotor tip flow field with the passing stator wakes causes a segmentation of the TCV and results in a sinusoidal variation in blockage downstream the rotor row.
The demand of increasing pressure ratios for modern high pressure compressors leads to decreasing blade heights in the last stages. As tip clearances (TC) cannot be reduced to any amount and minimum values might be necessary for safety reasons, the TC ratios of the last stages can reach values notably higher than current norms. This can be intensified by a compressor running in transient operations where thermal differences can lead to further growing clearances. For decades, the detrimental effects of large clearances on an axial compressor's operating range and efficiency are known and investigated. The ability of circumferential casing grooves in the rotor casing to improve the compressor's operating range has also been in the focus of research for many years. Their simplicity and ease of installation are one reason for their continuing popularity nowadays, where advanced methods to increase the operating range of an axial compressor are known. In the authors' previous paper, three different circumferential groove casing treatments were investigated in a single-stage environment in the low-speed axial research compressor at TU Dresden. One of these grooves was able to notably improve the operating range and the efficiency of the single stage compressor at very large rotor TC (5% of chord length). In this paper, the results of tests with this particular groove type in a three stage environment in the low-speed axial research compressor are presented. Two different rotor TC sizes of 1.2% and 5% of tip chord length were investigated. At the small TC, the grooves are almost neutral. Only small reductions in total pressure ratio and efficiency compared to the solid wall can be observed. If the compressor runs with large TC, it notably benefits from the casing grooves. Both, total pressure and efficiency can be improved by the grooves in a similar extent as in single stage tests. Five-hole probe measurements and unsteady wall pressure measurements show the influence of the groove on the flow field. With the help of numerical investigations, the different behavior of the grooves at the two TC sizes will be discussed.
In high pressure compressors large tip clearances can occur in the rear stages especially during transient operation. This can lead to a reduced operating range and performance of the compressor. One simple means to reduce the negative effects of a rotor tip clearance increase can be circumferential groove casing treatments. Their ability to improve the operating range of a compressor is well-known for several decades. In this paper, an experimental study to investigate the influence of three different circumferential groove casing treatments on a low speed axial research compressor with very large rotor tip clearances (5% of annulus height) is presented. Two single-groove and one double-groove casing treatments have been tested. The grooves are positioned from 31% to 69% axial chord. The compressor characteristics of the smooth wall and the three groove configurations are discussed as well as selected five-hole probe measurements of the flow field downstream the rotor. It can be shown that all grooves are able to improve the operating range of the compressor. Furthermore, an improvement of efficiency due to the casing grooves can also be observed in certain operating points.
Experimental investigations in a single-stage low speed axial research compressor are presented. The influence of four different rotor tip clearances on the overall compressor performance and on the rotor tip flow field is investigated in configurations with and without circumferential groove casing treatments. Piezo-resistive pressure transducers are used to capture the unsteady flow field in the rotor tip region. The investigated casing groove is effectively working at the three largest investigated tip clearance sizes. The largest achieved operating range increase by the groove is 6.9%. The groove can delay the upstream movement of the flow interface between leakage flow and main flow and thus increase the stable operating range. Rotating instabilities are shown to exist at large tip clearance sizes in throttled operating conditions. Their amplitudes can be damped by the casing groove. No modal activities could be detected in the current single-stage compressor build.
Since the early work on axial compressors the penalties due to radial clearances between blades and side walls are known and an ongoing focus of research work. The periodic unsteadiness of the tip clearance vortex, due to its interaction with the stator wakes, has only rarely been addressed in research papers so far. The current work presents experimental and numerical results from a four stage low speed research compressor modeling a state of the art compressor design. Time-resolved experimental measurements have been carried out at three different rotor tip clearances (gap to tip chord: 1.5%, 2.2%, 3.7%) to cover the third rotor’s casing static pressure and exit flow field. These results are compared with either steady simulations using different turbulence models or harmonic RANS calculations to discuss the periodical unsteady tip clearance vortex development at different clearance heights. The prediction of the local tip leakage flow is clearly improved by the EARSM turbulence model compared to the standard SST model. The harmonic RANS calculations (using the SST model) improve the prediction of time-averaged pressure rise and are used to analyze the rotor stator interaction in detail. The interaction of the rotor tip flow field with the passing stator wakes cause a segmentation of the tip clearance vortex and result in a sinusoidal variation in blockage downstream the rotor row.
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