Stall Cell Research Articles

1017 高比速度斜流送風機の非定常内部流れ : 旋回失速セルを含む動翼下流流れ場(O.S.10-5 流体機械(3))(O.S.10 流体機械と流力振動)

1017 高比速度斜流送風機の非定常内部流れ : 旋回失速セルを含む動翼下流流れ場(O.S.10-5 流体機械(3))(O.S.10 流体機械と流力振動)

Comparative Studies on Short and Long Length-Scale Stall Cell Propagating in an Axial Compressor Rotor

In a low-speed compressor test rig at Kyushu University, multiple short length-scale stall cells appeared under a mild stall condition and turned into a long length-scale cell under a deep stall condition. Then, for the two types of stall cell, the pressure distribution on the casing wall and the velocity distributions upstream and downstream of the rotor have been measured by high-response pressure transducers and a slanted hot-wire, respectively. The time-dependent ensemble-averages of these distributions have been obtained phase-locked to both the rotor and the stall cell rotation using a “double phase-locked averaging technique” developed by the authors. The structures of the two stall cells are compared: The short length-scale stall cell is characterized by a concentrated vortex spanning from the casing wall ahead of the rotor to the blade suction surface. In the long length-scale stall cell, the separation vortices go upstream irregularly when blade separation develops in the front half of the cell, and re-enter the rotor on the hub side in the rear half of it. The unsteady aerodynamic force and torsional moment acting on the blade tip section have been evaluated from the time-dependent ensemble-averages of the casing wall pressure distribution. The force fluctuation due to the short length-scale cells is somewhat smaller than that for the long length-scale cell. The blade suffers two peaks of the force during a period of the short length-scale cells passing through it. The moment fluctuation for the short length-scale cells is considerably larger than that for the long length-scale cell.

Behavior of Multiple Short Length-Scale Stall Cells Propagating in an Axial Compressor Rotor.

Behavior of Multiple Short Length-Scale Stall Cells Propagating in an Axial Compressor Rotor.

Prediction and Measurement of Rotating Stall Cells in an Axial Compressor

This paper presents a parallel numerical and experimental study of rotating stall cells in an axial compressor. Based on previous theoretical and experimental studies stressing the importance of fluid inertia and momentum exchange mechanisms in rotating stall, a numerical simulation using the Euler equations is conducted. Unsteady two-dimensional solutions of rotating stall behavior are obtained in a one-stage low subsonic axial compressor. The structure and speed of propagation of one fully developed rotating stall cell together with its associated unsteady static pressure and throughflow field distributions are presented. The numerical capture of a stalled flow region starting from a stable high-flow operating point with an axisymmetric flow distribution and evolving at a reduced mass flow operating point into a rotating stall pattern is also discussed. The experimental data (flow visualization, time-averaged and unsteady row-by-row static pressure measurements) acquired in a four-stage water model of a subsonic axial compressor cover a complete characteristic line ranging from high mass flow in the stable regime to zero throughflow. Stall inception is presented together with clearly marked different operating zones within the unstable regime. For one operating point in the unstable regime, the speed of propagation of the cell as well as the static pressure spikes at the front and rear boundaries of the rotating stall cell are compared between computations, measurements, and an idealized theory based on momentum exchange between blade rows entering and leaving the stalled cell. In addition, the time evolution of the pressure trace at the rotor/stator interface is presented. This study seems to support the assumption that the cell structure and general mechanism of full-span rotating stall propagation are essentially governed by inertial effects and momentum exchange between the sound and stalled flow at the cell edges.

Propagation of Multiple Short-Length-Scale Stall Cells in an Axial Compressor Rotor

Evolution and structure of multiple stall cells with short-length-scale in an axial compressor rotor have been investigated experimentally. In a low-speed research compressor rotor tested, a short-length-scale stall cell appeared at first, but did not grow rapidly in size, unlike a so-called “spike-type stall inception” observed in many multistage compressors. Alternatively, the number of cells increased to a certain stable state (a mild stall state) under a fixed throttle condition. In the mild stall state the multiple stall cells, the size of which was on the same order of the inception cell (a few blade spacings), were rotating at 72 percent of rotor speed and at intervals of 4.8 blade spacings. With further throttling, a long-length-scale wave appeared overlapping the multiple short-length-scale waves, then developed to a deep stall state with a large cell. In order to capture the short-length-scale cells in the mild stall state, a so-called “double phase-locked averaging technique” has been developed, by which the flow field can be measured phase locked to both the rotor and the stall cell rotation. Then, time-dependent ensemble averages of the three-dimensional velocity components upstream and downstream of the rotor have been obtained with a slanted hot-wire, and the pressure distributions on the casing wall with high-response pressure transducers. By a physically plausible explanation for the experimental results, a model for the flow mechanism of the short-length-scale stall cell has been presented. The distinctive feature of the stall cell structure is on the separation vortex bubble with a leg traveling ahead of the rotor, with changing the blade in turn on which the vortex leg stands. [S0889-504X(00)00701-7]

A Two-dimensional Compressible Navier-Stokes Simulation of Transient Flows in Compressor Rotor/Stator Cascades. 3rd Report. Flow Structure of a Deep Rotating Stall Cell.

Detailed flow structure of a deep rotating stall cell in a single-stage axial compressor is studied based on a numerical simulation of simplified quasi-three-dimensional compressible Navier-Stokes equations a linear cascades system and unsteady flow measurements in a test compressor. The computational system is expanded to a maximum combination of 15 rotor blades/25 stator vanes, so that cell growth is not hampered by spatial constraint of the field. The simulation is performed at a blade Mach number of 0.3 to meet the rig test condition. It is found that the stall cell is composed of sub-cell vortices, each extending over 3-5 rotor pitches, which align circumferentially ahead of the rotor, and counter-rotating vortices occupying the stator passages. The computed cell speed agrees well with the measured speed. In the computed time trace of flow velocity at the rotor inlet, multiple peaks, corresponding to the sub-cell vortices, appear during a cell passage. These are also seen in the measured traces taken by split-film probes. The agreement indicates that multiple sub-cell vortices may constitute a deep stall cell in the real situation.

Stall Cell Development in an Axial Compressor

An experimental investigation of stall inception and stall cell development in a single-stage axial compressor is described. The stall inception was found to be naturally nonrandom: By artificially perturbing the flow the inception could be accurately fixed at a known location in the compressor. The stall cell was first detected behind the rotor at a small distance from the tip. The stall cell grew very rapidly in circumferential extent, but slowly in radial extent. After reaching the hub the cell decreased in size before reaching full development as a single full span rotating stall cell. Relationships between various parameters of the stall cell growth are presented. The growth is explained in terms of the cell blockage, and the mechanism for multiple stall inception is discussed.

The Measurement and Interpretation of Flow within Rotating Stall Cells in Axial Compressors

Detailed flow measurements obtained by a new measuring technique are presented for the flow in a stalled axial-flow compressor. Results were obtained from a wide range of compressor builds, including multi-stage and single-stage configurations of various design flow rates and degrees of reaction. Instantaneous recordings of absolute velocity, flow direction and total and static pressures have been included for both full-span and part-span stall. With the aid of these results, it has been shown that the conventional model of the flow in a stall cell is erroneous. An alternative model is proposed, based on the observation that the fluid must cross from one side of the cell to the other in order to preserve continuity in the tangential direction. An investigation of the experimental results also reveals the finer details of the flow in the cell and shows how these details are related to the design flow rate of the compressor. The influence of these cell details on the power absorbed by a stalled compressor are investigated, and consideration is given to the complex pressure patterns encountered in the compressor.