Short Length Scale Pressure Pulse Interactions With Premixed Flames

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]

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1997 Best Paper Award—Turbomachinery Committee: A Study of Spike and Modal Stall Phenomena in a Low-Speed Axial Compressor

Journal of Turbomachinery ◽

10.1115/1.2841730 ◽

1998 ◽

Vol 120 (3) ◽

pp. 393-401 ◽

Cited By ~ 208

Author(s):

T. R. Camp ◽

I. J. Day

Keyword(s):

High Speed ◽

Three Dimensional ◽

Axial Compressor ◽

Short Length ◽

Operating Conditions ◽

Length Scale ◽

Stall Inception ◽

Low Speed ◽

The Stability ◽

Dimensional Instability

This paper presents a study of stall inception mechanisms in a low-speed axial compressor. Previous work has identified two common flow breakdown sequences, the first associated with a short length-scale disturbance known as a “spike,” and the second with a longer length-scale disturbance known as a “modal oscillation.” In this paper the physical differences between these two mechanisms are illustrated with detailed measurements. Experimental results are also presented that relate the occurrence of the two stalling mechanisms to the operating conditions of the compressor. It is shown that the stability criteria for the two disturbances are different: Long length-scale disturbances are related to a two-dimensional instability of the whole compression system, while short length-scale disturbances indicate a three-dimensional breakdown of the flow-field associated with high rotor incidence angles. Based on the experimental measurements, a simple model is proposed that explains the type of stall inception pattern observed in a particular compressor. Measurements from a single-stage low-speed compressor and from a multistage high-speed compressor are presented in support of the model.

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Giant Magnetoresistance Induced by Ultrathin Magnetic Layers

MRS Proceedings ◽

10.1557/proc-475-99 ◽

1997 ◽

Vol 475 ◽

Author(s):

G.J. Strijkers ◽

H.J.M. Swagten ◽

A.H.M. Mettler ◽

M.M.H. Willekens ◽

W.J.M. De Jonge

Keyword(s):

Giant Magnetoresistance ◽

Spin Valve ◽

Short Length ◽

Spin Valves ◽

Length Scale ◽

Magnetic Layers ◽

Ferromagnetic Layers ◽

Experimental Tool ◽

Spin Dependent Scattering

ABSTRACTWe introduce an interface selective structure, composed of a spin-valve on top of which a thick nonmagnetic back layer is deposited as a straightforward experimental tool to measure the GMR induced by ultrathin magnetic layers. The interface selectivity of spin-dependent scattering is evidenced by calculations and illustrated in both Co/Cu/Co and Ni80Fe20/Cu/Ni80Fe20 spin-valves by an almost discontinuous behavior in the GMR ratio. The temperture dependence of the extremely short length scale associated with this discontinuity is discussed in relation to the structure of ultrathin ferromagnetic layers.

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Effect of Tip Clearance on Stall Evolution Process in a Low-Speed Axial Compressor Stage

Volume 5: Turbo Expo 2004, Parts A and B ◽

10.1115/gt2004-53354 ◽

2004 ◽

Cited By ~ 20

Author(s):

Masahiro Inoue ◽

Motoo Kuroumaru ◽

Shinichi Yoshida ◽

Takahiro Minami ◽

Kazutoyo Yamada ◽

...

Keyword(s):

Spectral Power ◽

Axial Compressor ◽

Pressure Rise ◽

Rotating Stall ◽

Short Length ◽

Tip Clearance ◽

Evolution Process ◽

Length Scale ◽

Compressor Stage ◽

Low Speed

Effect of the tip clearance on the transient process of rotating stall evolution has been studied experimentally in a low-speed axial compressor stage with various stator-rotor gaps. In the previous authors’ experiments for the small tip clearance, the stall evolution process of the rotor was sensitive to the gaps between the blade rows. For the large tip clearance, however, little difference is observed in the evolution processes independently of the blade row gap. In the first half process, it is characterized by gradual reduction of overall pressure-rise with flow rate decreasing, and the number of short length-scale disturbances is increasing with their amplitude increasing. In the latter half a long length-scale disturbance develops rapidly to result in deep stall. Just before the stall inception the spectral power density of the casing wall pressure reveals the existence of rotating disturbances with broadband high frequency near a quarter of the blade passing frequency. This is caused by the short length-scale disturbances occurring intermittently. A flow model is presented to explain mechanisms of the rotating short length-scale disturbance, which includes a tornado-like separation vortex and tip-leakage vortex breakdown. The model is supported by a result of a numerical unsteady flow simulation.

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Experimental Investigation of Stall Inception Mechanisms of Low Speed Contra Rotating Axial Flow Fan Stage

International Journal of Rotating Machinery ◽

10.1155/2015/641601 ◽

2015 ◽

Vol 2015 ◽

pp. 1-14 ◽

Cited By ~ 6

Author(s):

Tegegn Dejene Toge ◽

A. M. Pradeep

Keyword(s):

Mass Flow ◽

Wavelet Transforms ◽

Axial Flow ◽

Short Length ◽

Length Scale ◽

Pressure Data ◽

Flow Conditions ◽

Axial Flow Fan ◽

Stall Inception ◽

Low Speed

The present paper is an attempt in understanding the stall inception mechanism in a low speed, contra rotating axial flow fan stage, using wavelet transforms. The rotors used in this study have relatively large tip gap (about 3% of the blade span) and aspect ratio of 3. The study was carried out near stall and at stall mass flow conditions for different speed ratios of rotor-2 to rotor-1. Unsteady pressure data from the casing wall mounted sensors are used to understand the stall inception mechanism. The wavelet transform clearly indicates that stall inception occurs mainly through long length scale disturbances for both rotors. It also reveals that short length disturbances occur simultaneously or intermittently in the case of rotor-1. The analysis shows the presence of a strong modal disturbance with 25–80% of the rotor frequency in the case of rotor-1 at the stall mass flow for all the speed combinations studied. The most interesting thing observed in the present study is that the frequency amplitude of the disturbance level is very small for both rotors.

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Long-to-Short Length Scale Transition: A Stall Inception Phenomenon in an Axial Compressor With Inlet Distortion

Volume 6: Turbo Expo 2005, Parts A and B ◽

10.1115/gt2005-68656 ◽

2005 ◽

Cited By ~ 1

Author(s):

Feng Lin ◽

Meilin Li ◽

Jingyi Chen

Keyword(s):

Axial Compressor ◽

Axial Flow ◽

Short Length ◽

Scale Model ◽

Future Research ◽

Length Scale ◽

Stall Inception ◽

Inlet Distortion ◽

Scale Transition ◽

Asymmetric Flows

A theoretical and experimental study of stall inception processes in a three-stage low-speed axial flow compressor with inlet distortion is presented in this paper. Since inlet distortion provides asymmetric flows imposing onto the compressor, the main goal of this research is to unveil the mechanism of how such flows initiate long and/or short length-scale disturbances and how the compression system reacts to those disturbances. It is found that the initial disturbances are always triggered by the distorted flows, yet the growth of such disturbances depends on system dynamics. While in many cases the stall precursors were the short length scale spikes, there were some cases where the compressor instability was triggered after the disturbances going through a long-to-short length scale transition. A Moore-Greitzer based (system scale) model was proposed to qualitatively explain this phenomenon. It was found that when the compressor operated in a region where the nonlinearity of the characteristics dominated, long length-scale disturbances induced by the inlet distortion would evolve into short length-scale disturbances before they disappeared or triggered stall. However, the model was not able to predict the fact that many disturbances that were triggered by the distorted sector(s) were completely damped out in the undistorted sector(s). It is thus suggested that in future research of compressor instability, one should consider the flows in blade passage scale, the dynamics in system scale and their interaction simultaneously.

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Precursor Chemistry and the Structure of Silica Aerogels

MRS Proceedings ◽

10.1557/proc-121-691 ◽

1988 ◽

Vol 121 ◽

Cited By ~ 7

Author(s):

D. W. Schaefer ◽

C. J. Brinker ◽

J. P. Wilcoxon ◽

D.-Q. Wu ◽

J. C. Phillips ◽

...

Keyword(s):

Polymer Network ◽

Precursor Chemistry ◽

Silica Aerogels ◽

Short Length ◽

Base Catalysis ◽

Length Scale ◽

X Ray ◽

Second Stage ◽

X Ray Scattering ◽

Surface Tension Forces

ABSTRACTSmall-angle X-ray scattering is used to characterize the structure of aerogels prepared by two-stage polymerization processes. Second-stage catalysis controls the resistance to collapse during drying with the base-catalyzed system being most resistant. Base catalysis in the second stage leads to compaction of the polymer network on short length scales. This short-scale rigidity makes the networks sufficiently robust to withstand the surface tension forces present during solvent extraction and re-exposure to the atmosphere. Aging in solution also improves aerogel quality. In this case, a dissolution-repolymerization process leads to short length scale circuits and improved rigidity.

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Short and Long Length-Scale Disturbances Leading to Rotating Stall in an Axial Compressor Stage With Different Stator/Rotor Gaps

Volume 1: Aircraft Engine; Marine; Turbomachinery; Microturbines and Small Turbomachinery ◽

10.1115/2001-gt-0341 ◽

2001 ◽

Cited By ~ 2

Author(s):

M. Inoue ◽

M. Kuroumaru ◽

S. Yoshida ◽

M. Furukawa

Keyword(s):

High Frequency ◽

Wavelet Transforms ◽

Wave Length ◽

Axial Compressor ◽

Leading Edge ◽

Rotating Stall ◽

Short Length ◽

Length Scale ◽

Compressor Stage ◽

Stall Inception

The transient processes of rotating stall evolution have been investigated experimentally in a low-speed axial compressor stage with three stator-rotor gaps. The pressure traces at 8 circumferential locations on the casing wall near the rotor leading edge have been analyzed by the wavelet transforms. With the appropriate mother wavelets, the evolution of short and long length-scale disturbances leading to the stall can be captured clearly. Behavior of these disturbances is different depending on the stator-rotor gap. For the large and middle gap, the stall inception is detected by a spiky short length-scale disturbance, and the number of spiky waves increases to generate the high frequency waves. They becomes the short length-scale part-span stall cells at the mild stall for the large gap, while they turn into a big stall cell with growth of a long length-scale disturbance for the middle gap. In the latter case, therefore, the stalling process was identified with ‘high frequency stall inception’. For the small stator-rotor gap, the stalling process is identified with ‘long wave-length stall inception’, and supported the recent computational model for the short wave-length stall inception by showing that closing the rotor-stator gaps suppressed the growth of short length-scale disturbances. From the measurement of the pressure field traces on the casing wall, a hypothesis has been built up that the short length-scale disturbance should result from a separation vortex from a blade surface to reduce circulation. The processes of the stall evolution are discussed on this hypothesis.

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

Journal of Turbomachinery ◽

10.1115/1.1326085 ◽

2000 ◽

Vol 123 (1) ◽

pp. 24-30 ◽

Cited By ~ 55

Author(s):

M. Inoue ◽

M. Kuroumaru ◽

T. Tanino ◽

S. Yoshida ◽

M. Furukawa

Keyword(s):

Pressure Distribution ◽

Aerodynamic Force ◽

Axial Compressor ◽

Short Length ◽

Time Dependent ◽

Length Scale ◽

Suction Surface ◽

Pressure Transducers ◽

Double Phase ◽

Stall Cells

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.

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