Unsteady Flow Field under Surge and Rotating Stall in a Three-stage Axial Flow Compressor

The unsteady flow structure between rotor blade-to-blade passages in a three-stage axial flow compressor is experimentally investigated by detailed measurements of unsteady performance characteristics,casing wall pressure fluctuations and their wavelet analyses.The main feature of the test compressor is a capacity tank facility connected in series to the compressor outlet in order to supply compression and/or expansion waves from downstream of the compressor.Research attention is focused on the post-stall characteristics of the surge and rotating stall which occur simultaneously.The influence of the compressor operating point on the unsteady performance curve shows that the surge cycle changes irregularly depending on the steady-state resistance characteristics,and the results of the wavelet analyses of the wall pressure fluctuations suggest that the surge cycle may selectively be determined by the rotating stall cell structure within the rotor cascade.

Unsteady Behavior of Surge and Rotating Stall in an Axial Flow Compressor

Unsteady behaviors as well as unsteady cascade flow fields of a single-stage axial flow compressor were experi- mentally investigated by detail measurements of unsteady performance characteristics and casing wall pressure and internal flow velocity fluctuations. The main feature of the test compressor is a capacity tank connected di- rectly to the compressor outlet in series through slits and a concentric duplex pipe, and also jet nozzles in order to inject compressed air toward the rotor tip region. Research attention is focused on the post-stall characteristics of surge and rotating stall which occur simultaneously. When the compressor was connected to the capacity tank, surge was generated with rotating stall in accordance with the capacitance increment of whole compressor system The surge behavior changed irregularly with throttling valve installed behind the compressor, and several types of surge cycles were observed. In addition, the surge cycle changed by jet injection to the rotor tip region. The re- suits suggested that the blockages of the cascade flow which were generated by a stall cell play an important role in deciding the surge behaviors.

Rotating Stall and Stall-Controlled Performance of a Single Stage Subsonic Axial Compressor

Activities by various authors on aerodynamics and control dynamics of rotating stall in axial compressor are first traced. Then, a process of stall cell evolution in a subsonic stage is discussed based on a 2-D CFD. A few numbers of vortices grow ahead of the rotor accumulating vorticity ejected from lightly stalled blades, and eventually organize a cell of circumferentially aligned huge vortices, which merge and recess repeatedly during the rotation. Such stall disturbance is intensified on trailing side of a circumferential inlet distortion and decays on the leading side. Considering these features, a new algorithm for stall warning is developed based on a correlation between pressure waveforms at each passing of a fixed blade. A remarkable change in the correlation level at near-stall provides a warning signal prior to the stall onset with sufficiently large time margin. This scheme is applied to achieve rotating stall prevention by actuating flaps installed on the hub. The last issue is on characteristics of forward swept blade which has much increased throttle margin with decreased tip loss. A 3-D computation shows that a secondary vortex generated in suction surface mid span interacts to reduce the tip leakage vortex that initiates the stall.

Natures of Rotating Stall Cell in a Diagonal Flow Fan

In order to clarify the natures of a rotating stall cell, the experimental investigation was carried out in a high specific-speed diagonal flow fan. The pressure field on the casing wall and the velocity fields at the rotor inlet and outlet were measured under rotating stall condition with a fast response pressure transducer and a single slant hot-wire probe, respectively. The data were processed using the “Double Phase-Locked Averaging (DPLA)”technique, which enabled to obtain the unsteady flow field with a rotating stall cell in the relative co-ordinate system fixed to the rotor. As a result, the structure and behavior of the rotating stall cell in a high specific-speed diagonal flow fan were shown.

Features of Rotating Stall Cell in a Diagonal Flow Fan (in Case of Mid-loading Rotor)

The structure and behavior of rotating stall cell were experimentally clarified in a diagonal flow fan. The specific-speed of the fan was 1140 （r/min, m^3/min, m）, and the total pressure-rise coefficient at design flow-rate was 0.345. The static pressure on outer casing wall and the total pressure at rotor inlet and outlet were measured using a high response pressure transducer. The measured data were processed by the use of DPLA technique, and the structure and behavior of rotating stall cell were obtained. As a result, the stall cell extent for circumferential and spanwise direction and the pressure distributions inside stall cell were clarified. The details of stall cell propagation were also shown.

Interactive Effects of Wind Tunnel Sidewalls on Flow Structures around 2D Airfoil Model

This paper presents the effect of wind tunnel sidewalls on the wind turbine airfoils with experimental and numerical methods.The test is carried out in a low-speed wind tunnel at Re=2.62×10^(5).Pressures acting on the airfoil surface are measured by a multiport pressure device.And,the oil flow visualization technique is used to investigate the flow field characteristics of the airfoil surface.Then,a numerical simulation was conducted with the measurement results.As a result,it is clarified the flow structures on the airfoil surface depend strongly on the angles of attack and the sidewalls.At small angles of attack,the three-dimensional separation caused by the interaction between the sidewall boundary layer and the airfoil boundary layer is very small,and only appears near the junction of the airfoil model and the sidewall.This corner separation becomes large with the increase of the angle of attack.At the middle part of the testing model,the boundary layer flow evolves into three-dimensional separation,i.e.,stall cell,when the separation develops to an appreciate extent.The stall phenomenon will further spread from the center line to sidewalls with the increase of the angle of attack;and then,its development will be limited by the sidewall boundary layer separation.Comparably,the simulation shows that the sidewall make the pressure coefficient Cpdecrease,and proper boundary condition can maintain two-dimensional flow at large angles of attack by eliminating the influence of corner vortices.