Hub Clearance Research Articles

Hub Wall Rotation Influence on High-Speed Compressor Cascade Performance

The ine uence of the magnitude of the relative wall speed on the performance of an annular compressor cascade with hub clearance is examined. Five-hole probe measurements, conducted at the inlet and outlet of the cascade, are used to derive blade performance characteristics, in the form of loss and turning distributions. Characteristics are presented in the form of circumferentially mass-averaged proe les, whereas distributions on the exit plane provide information useful to interpret the performance of the blading. Four different rotational speeds of the hub have been examined, giving the possibility to observe the dependence of performance characteristics on hub rotational speed. Increasing the rotational speed is found to improvethe performance of the cascade by decreasing losses in the clearance region, while it affects the e ow in the entire passage.

Three-dimensional flow field downstream of an embedded stator in a multistage axial flow compressor: Part 1: Steady and unsteady flow fields

A comprehensive investigation of the three-dimensional unsteady flow and thermal field downstream of an embedded stator in a multistage compressor, acquired with a high-response hot-film probe and aspirating probe, is presented and analysed. Some of the earlier data (from a five-hole probe and a high-response Kulite probe) from the same compressor is used with the present data to provide an integrated interpretation of the flow and thermal fields. Part 1 includes a brief description of the facility, the development of the hot-film technique for the multistage flow field measurement and an interpretation of the various flow field features, such as the hub clearance flow and the suction surface-casing end-wall corner region. Part 2 covers velocity-velocity and velocity-temperature correlations and the assessment of their magnitudes in the average-passage equations. The results presented in this part of the paper (Part 1) indicate that major blockage in the stator hub end wall is caused by leakage flow and its possible subsequent roll-up into a vortex. Similar features exist in the suction surface casing end-wall corner due to secondary flow and its interaction with the upstream rotor end-wall flow. These are also the regions with the highest levels of unsteadiness. The transport of rotor wake towards the pressure side of the stator is confirmed. Both the revolution and the blade periodic velocity fluctuations are found to be significantly greater than the aperiodic fluctuations.

A Comparison Between the Design Point and Near-Stall Performance of an Axial Compressor

Detailed measurements have been made within an axial compressor operating both at design point and near stall. Rotor tip clearance was found to control the performance of the machine by influencing the flow within the rotor blade passages. This was not found to be the case in the stator blade row, where hub clearance was introduced beneath the blade tips. Although the passage flow was observed to be altered dramatically, no significant changes were apparent in the overall pressure rise or stall point. Small tip clearances in the rotor blade row resulted in the formation of corner separations at the hub, where the blade loading was highest. More representative clearances resulted in blockage at the tip due to the increased tip clearance flow. The effects that have been observed emphasize both the three-dimensional nature of the flow within compressor blade passages, and the importance of the flow in the endwall regions in determining the overall compressor performance.

Three-Dimensional Flows and Loss Reduction in Axial Compressors

Measurements have been performed in a low-speed high-reaction single-stage axial compressor. Data obtained within and downstream of the rotor, when correlated with the results of other investigations, provide a link between the existence of suction surface–hub corner separations, their associated loss mechanisms, and blade loading. Within the stator, it has been shown that introducing a small clearance between the stator blade and the stationary hub increases the efficiency of the stator compared to the case with no clearance. Oil flow visualizaton indicated that the leakage reduced the extensive suction surface–hub corner separation that would otherwise exist. A tracer gas experiment showed that the large radial shifts of the surface streamlines indicated by the oil flow technique were only present close to the blade. The investigation demonstrates the possible advantages of including hub clearance in axial flow compressor stator blade rows.

Stator Endwall Leading-Edge Sweep and Hub Shroud Influence on Compressor Performance

The use of stator endwall leading-edge sweep to improve axial-flow compressor stator row performance was examined experimentally. The aerodynamics of three stator hub (inner diameter) conditions, namely, a running clearance, a stationary clearance, and a shroud, were also investigated. Leading-edge sweep in the endwall regions of a stator blade can be beneficial in terms of loss reduction on the casing (outer diameter) end of a stator blade. It can also help at the hub end of a stator blade when either a stationary hub clearance or a hub shroud is used. A leading-edge sweep is detrimental (local loss increase) on the hub end of a stator blade when a running hub clearance is used. A running clearance is aerodynamically preferable to a stationary clearance.