Jet, Wake and Intrinsic Motion in Impellers of Centrifugal Compressors

1996 ◽  
Author(s):  
Y. N. Chen ◽  
U. Seidel ◽  
U. Haupt ◽  
M. Rautenberg
Keyword(s):  
Reverse Flow ◽  
Vortex Pair ◽  
Leading Edge ◽  
Rotating Stall ◽  
Secondary Flows ◽  
Low Flow ◽  
Experimental Result ◽  
Vaned Diffuser ◽  
Measured Velocity ◽  
Intrinsic Motion

It was shown in a previous paper of the authors (1991) that jet and wake in the flow of the impeller of the centrifugal compressor are developed from the Dean’s type vortex pair formed in the curvature of the blade channel. The jet rotating against the sense of the impeller is weakened, and the wake rotating in the sense of the impeller is enhanced during travelling with the flow toward the outlet. This property is attributed to the conservation of the potential vorticity of the vortex. The experimental result obtained by Krain (1984) has confirmed this theory. The secondary flows found by Farge and Johnson (1990) enable the determination of the vorticity of the wake at the outlet of the impeller. It amounts to 6.9 Ω and 5.8 Ω for the radial-blading and the 60°-backswept blading impeller, respectively. The intensity of the vortex jet is weakened to undetectable value for both the impellers. The patterns of these secondary flow fields are also quite different between these two kinds of impellers. Whilst that of the former is controlled by the intrinsic motion, that of the latter is governed by the relative velocity along the blades. Furthermore, the experimental result obtained by the injection of colored dye at the impeller outlet and the measured velocity field around the impeller reveal an intense reverse flow in the radial blading impeller, travelling from the outlet toward the inlet, along the shroud. It can be shown that this reverse flow is caused by the intrinsic motion occuring in this impeller and impinging on the leading edge of the diffuser vane. As the rotating stall is introduced by the reverse flow, the low-solidity vaned diffuser, and still better the vaneless diffuser can therefore shift the stall line to a very low flow rate.

scholarly journals Investigation of Separation Phenomena in a Radial Pump at Reduced Flow Rate by Large-Eddy Simulation

2016 ◽  
Vol 138 (12) ◽  
Author(s):  
Antonio Posa ◽  
Antonio Lippolis ◽  
Elias Balaras
Keyword(s):  
Reverse Flow ◽  
Leading Edge ◽  
Suction Side ◽  
Secondary Flows ◽  
Vaned Diffuser ◽  
Flow Rates ◽  
Turbulent Statistics ◽  
Large Eddy ◽  
Radial Pump ◽  
Reduced Flow

Turbopumps operating at reduced flow rates experience significant separation and backflow phenomena. Although Reynolds-Averaged Navier–Stokes (RANS) approaches proved to be usually able to capture the main flow features at design working conditions, previous numerical studies in the literature verified that eddy-resolving techniques are required in order to simulate the strong secondary flows generated at reduced loads. Here, highly resolved large-eddy simulations (LES) of a radial pump with a vaned diffuser are reported. The results are compared to particle image velocimetry (PIV) experiments in the literature. The main focus of the present work is to investigate the separation and backflow phenomena occurring at reduced flow rates. Our results indicate that the effect of these phenomena extends up to the impeller inflow: they involve the outer radii of the impeller vanes, influencing significantly the turbulent statistics of the flow. Also in the diffuser vanes, a strong spanwise evolution of the flow has been observed at the reduced load, with reverse flow, located mainly on the shroud side and on the suction side (SS) of the stationary channels, especially near the leading edge of the diffuser blades.

Excitation Mechanism for Standing Stall of Centrifugal Compressors

1998 ◽  
Author(s):  
Y. N. Chen ◽  
D. Hagelstein ◽  
U. Haupt ◽  
M. Rautenberg
Keyword(s):  
Pressure Wave ◽  
Reverse Flow ◽  
Incidence Angle ◽  
Leading Edge ◽  
Forward Direction ◽  
Rotating Stall ◽  
Low Flow ◽  
Experimental Investigations ◽  
Centrifugal Compressors ◽  
Suction Surface

The standing stall of the centrifugal compressors appears with pulsating or switching pattern at operating points slightly away from the stall line. It is a weak form of the rotating stall and stands still in the absolute frame. The reverse flow of the compressed warm fluid travelling from the impeller’s outlet along the shroud surface towards the inlet is not yet powerful enough to generate rotating stall. The experimental investigations revealed that in the low-flow-rate off-design region, the inlet flow to the impeller has a large positive incidence angle. Nose bubbles are formed on the suction surface of the blade after the leading edge. Once the reverse flow as a pressure wave reaches the inlet of the blades, the nose bubble is stagnated to an enlarged size. The corresponding disturbance sends a rarefaction wave in the forward direction into the impeller. This wave of cool fluid meets the reverse pressure wave of the warm fluid at a circular front around the circumference of the impeller. Since this circular front has a weak baroclinicity, it cannot develop into Rossby waves which initiate the rotating stall. Instead it will either pulsate concentrically or switch linearly. We then experience a standing stall with the corresponding pattern.

EFFECT OF AXIAL CASING GROOVE GEOMETRY ON ROTOR-GROOVE INTERACTIONS IN THE TIP REGION OF A COMPRESSOR

2021 ◽  
pp. 1-54
Author(s):  
Subhra Shankha Koley ◽  
Huang Chen ◽  
Ayush Saraswat ◽  
Joseph Katz
Keyword(s):  
Rotor Blade ◽  
Leading Edge ◽  
Secondary Flows ◽  
Low Flow ◽  
Flow Angle ◽  
Flow Rates ◽  
Piv Measurements ◽  
Tip Leakage ◽  
Large Vortex ◽  
Blade Leading Edge

Abstract This experimental study characterizes the interactions of axial casing grooves with the flow in the tip region of an axial turbomachine. The tests involve grooves with the same inlet overlapping with the rotor blade leading edge, but with different exit directions located upstream. Among them, U grooves, whose circumferential outflow opposes the blade motion, achieve a 60% reduction in stall flowrate, but degrade the efficiency around the best efficiency point (BEP) by 2%. The S grooves, whose outlets are parallel to the blade rotation, improve the stall flowrate by only 36%, but do not degrade the BEP performance. To elucidate the mechanisms involved, stereo-PIV measurements covering the tip region and interior of grooves are performed in a refractive index matched facility. At low flow rates, the inflow into both grooves, which peaks when they are aligned with the blade pressure side, rolls up into a large vortex that lingers within the groove. By design, the outflow from S grooves is circumferentially positive. For the U grooves, fast circumferentially negative outflow peaks at the base of each groove, causing substantial periodic variations in the flow angle near the blade leading edge. At BEP, interactions with both grooves become milder, and most of the tip leakage vortex remains in the passage. Interactions with the S grooves are limited hence they do not degrade the efficiency. In contrast, the inflow into and outflow from the U grooves reverses direction, causing entrainment of secondary flows, which likely contribute to the reduced BEP efficiency.

Numerical investigation on vibration and pressure fluctuation characteristics in a centrifugal pump under low flow rate

2020 ◽  
pp. 095440622096972
Author(s):  
Like Wang ◽  
Jinling Lu ◽  
Weili Liao ◽  
Pengcheng Guo ◽  
Guojun Zhu ◽  
...  
Keyword(s):  
Centrifugal Pump ◽  
Pressure Fluctuation ◽  
Reverse Flow ◽  
Leading Edge ◽  
Low Flow ◽  
Main Shaft ◽  
Unstable Flow ◽  
Tip Leakage ◽  
Flow Vortex ◽  
Tip Separation Vortex

Vibration characteristic is an important factor in evaluating operation stability of centrifugal pump. The vibration of main shaft was measured using a laser vibrometer, internal flow field was simulated via the shear stress transport turbulence model, and distribution law of vibration and pressure fluctuation in the impeller were analysed to explore the induction factor of vibration and the inherent relationship with pressure fluctuation in a semi-open centrifugal pump under low flow rate condition. Results of the numerical simulation are consistent with the experimental data. In addition to rotation frequency caused by impeller rotation, vibration frequency also includes characteristic frequency with high amplitude induced by unstable flow. The complex vortex in the impeller is composed of tip leakage vortex (TLV), reverse flow vortex, passage vortex and tip separation vortex. The primary tip leakage vortex (PTLV) formed by the streamline spills from 0 to 0.2λ where λ is the dimensionless distance from leading edge to trailing edge collides with tip leakage flow, the leading edge overflow and reverse flow vortex at the frequency of 1.6 fn ( fn is the rotating frequency) and 2.2 fn appear, respectively. The tip separation vortex formed in the tip clearance induced a frequency of 1.2 fn. The frequency of unstable flow phenomenon was consistent with the vibration frequency of main shaft, which induced the vibration of centrifugal pump.

scholarly journals Experimental Investigation of the Steady and Unsteady Relative Flow in a Model Centrifugal Impeller Passage

1993 ◽  
Author(s):  
M. Abramian ◽  
J. H. G. Howard
Keyword(s):  
Laser Doppler Anemometry ◽  
Quantitative Description ◽  
Rotating Stall ◽  
Secondary Flows ◽  
Operating Conditions ◽  
Low Flow ◽  
Centrifugal Impeller ◽  
Low Specific Speed ◽  
Specific Speed ◽  
Relative Flow

The behaviour of the relative flow in centrifugal turbomachines is extremely complex due to the existence of various fluid dynamic phenomena and their interaction. At design and off-design operating conditions, the relative flow is subject to stationary unsteadiness which includes flow separation and wakes associated with passage pressure gradients, secondary flows, and boundary layer stability. It may also be subject to periodic unsteadiness such as is the rotating stall and cyclic flow phenomena induced by the casing. This paper describes detailed measurements of the relative velocity field in a very low specific speed centrifugal pump impeller (Ns=515). Measurements were conducted by means of a recently developed rotating laser-Doppler anemometry system. Detailed quantitative description of the mean and fluctuating components of the primary and secondary velocity fields are presented for an impeller without volute at design, 50% design and shut-off conditions. The flow pattern in this low specific speed impeller with high blade loading is dominated by the relative eddy (a phenomenon also present in potential flow) which has suppressed suction side separation. When the impeller was fitted with a volute, the cyclic variation of the impeller exit flow, induced by the volute at low flow rates, is also presented.

An Investigation of Axial Pump Backflow and a Method for its Control

1988 ◽  
Author(s):  
M. Abramian ◽  
J. H. G. Howard ◽  
P. Hermann
Keyword(s):  
Flow Field ◽  
Laser Doppler Velocimetry ◽  
Recirculation Zone ◽  
Reverse Flow ◽  
Axial Flow ◽  
Leading Edge ◽  
Low Flow ◽  
Flow Conditions ◽  
Axial Pump ◽  
Swirl Velocity

The flow field within an axial flow inducer pump near the blade leading edge was explored by laser-Doppler velocimetry to extend the previous studies of the recirculation zone which is observed at low flow rates. Although a considerable region of upstream reverse flow and swirl was observed, the recirculation zone within the impeller was of limited axial extent and was confined to the pressure side of the passage. In an attempt to reduce the flow reversal, a series of perforated disks were placed in front of the inducer. The optimum disk geometry produced minor changes in the pump performance. LDV measurements of the flow field ahead and behind the disk showed considerable reduction of the swirl velocity under reverse flow conditions, with the observed upstream swirl opposite to the inducer rotation.

Transient Analysis of Rotating Stall Development in a Centrifugal Compressor With Vaned Diffuser

2019 ◽  
Author(s):  
Nobumichi Fujisawa ◽  
Masaki Takahashi ◽  
Yutaka Ohta
Keyword(s):  
Transient Process ◽  
Centrifugal Compressor ◽  
Transient Analysis ◽  
Leading Edge ◽  
Adverse Pressure Gradient ◽  
Rotating Stall ◽  
Vaned Diffuser ◽  
Surface Separation ◽  
Pressure Surface ◽  
Key Factor

Abstract The transient process of the rotating stall development in a centrifugal compressor with a vaned diffuser was investigated by experimental and numerical analyses. Previous studies show that a diffuser stall triggers a stage stall, which rotates through rotor and stator passages. The vortex evolution at the diffuser throat represents the key factor in diffuser stall development. The developed diffuser stall cell blocked the impeller exit, causing an impeller passage stall. This paper focused on two aspects regarding the transient process of the diffuser stall development. The first aspect is the process by which the vortex at the diffuser throat near the hub side, develops in the circumferential direction. Secondly, we investigated the mechanism of the diffuser stall expansion into impeller passages. The transient analysis of the diffuser stall development was conducted experimentally and numerically by closing the throttle valve abruptly. The hub side blockage was initiated near the cutoff by the strong adverse pressure gradient in the diffuser throat area. Therefore, the key factor in the diffuser stall evolution was the development of a throat blockage near the cutoff, obtained from both experimental and computational fluid dynamics results. Furthermore, the transient stall cell blocked the impeller passages and induced a hub side blockage at the throat of the impeller passages and the impeller leading edge separation. The pressure surface separation of the impeller at the trailing edge had a great impact on the development of the stall cell within impeller passages.

Unsteady Behavior of Diffuser Stall in a Centrifugal Compressor With Vaned Diffuser

2017 ◽  
Author(s):  
Nobumichi Fujisawa ◽  
Daiki Ema ◽  
Yutaka Ohta
Keyword(s):  
Centrifugal Compressor ◽  
Vortical Structure ◽  
Leading Edge ◽  
Rotating Stall ◽  
Longitudinal Vortex ◽  
Suction Surface ◽  
Vaned Diffuser ◽  
Edge Vortex ◽  
Flow Blockage ◽  
Unsteady Behavior

In this study, the unsteady behavior of a diffuser rotating stall in a centrifugal compressor with a vaned diffuser was investigated through experiments and numerical analyses. From the casing static pressure measurements, it was determined that the diffuser stall propagated at 25% of impeller rotational speed in the vaneless space. The numerical results revealed the presence of a typical vortical structure on the diffuser’s leading edge. Under partial flow condition, a tornado-type vortex was generated on the diffuser’s leading edge. Furthermore, a longitudinal vortex at the shroud/suction surface corner (i.e., leading edge vortex (LEV)) was induced by the rolling-up flow on the diffuser suction surface. As the velocity was decreased, the development of the tornado-type vortex and LEV forms a substantial flow blockage within the diffuser passages. Furthermore, the diffuser stall cell was caused by the systematic vortical structure which consisted of the tornado-type vortex, LEV, and vortex in the throat area of diffuser passages. In addition to this, the developed LEV interacted with the next diffuser leading edge and formed the throat area blockage with the passage of time. Then, the tornado-type vortex and LEV developed by the throat area blockage and diffuser stall cell, which was caused by the systematic vortical structure, propagated to the succeeding diffuser vane. Therefore, the diffuser stall in the centrifugal compressor was caused by the evolution of the tornado-type vortex and LEV.

An Investigation of Stall Inception in Centrifugal Compressor Vaned Diffuser1

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
Jonathan N. Everitt ◽  
Zoltán S. Spakovszky
Keyword(s):  
Centrifugal Compressor ◽  
Short Wavelength ◽  
Leading Edge ◽  
Rotating Stall ◽  
Full Scale ◽  
Vaned Diffuser ◽  
Stall Inception ◽  
Tip Leakage ◽  
Leakage Flows ◽  
Blade Tip

In compression systems, the stable operating range is limited by rotating stall and/or surge. Two distinct types of stall precursors can be observed prior to full scale instability: the development of long-wavelength modal waves or a short-wavelength, three-dimensional flow breakdown (so-called “spike” stall inception). The cause of the latter is not well understood; in axial machines it has been suggested that rotor blade-tip leakage flow plays an important role, but spikes have recently been observed in shrouded vaned diffusers of centrifugal compressors where these leakage flows are not present, suggesting an alternative mechanism may be at play. This paper investigates the onset of instability in a shrouded vaned diffuser from a highly loaded turbocharger centrifugal compressor and discusses the mechanisms thought to be responsible for the development of short-wavelength stall precursors. The approach combines unsteady 3D RANS simulations of an isolated vaned diffuser with previously obtained experimental results. The unsteady flow field simulation begins at the impeller exit radius, where flow is specified by a spanwise profile of flow angle and stagnation properties, derived from single-passage stage calculations but with flow pitchwise mixed. Through comparison with performance data from previous experiments and unsteady full-wheel simulations, it is shown that the diffuser is accurately matched to the impeller and the relevant flow features are well captured. Numerical forced response experiments are carried out to determine the diffuser dynamic behavior and point of instability onset. The unsteady simulations demonstrate the growth of short-wavelength precursors; the flow coefficient at which these occur, the rotation rate and circumferential extent agree with experimental measurements. Although the computational setup and domain limitations do not allow simulation of the fully developed spike nor full-scale instability, the model is sufficient to capture the onset of instability and allows the postulation of the following necessary conditions: (i) flow separation at the diffuser vane leading edge near the shroud endwall; (ii) radially reversed flow allowing vorticity shed from the leading edge to convect back into the vaneless space; and (iii) recirculation and accumulation of low stagnation pressure fluid in the vaneless space, increasing diffuser inlet blockage and leading to instability. Similarity exists with axial machines, where blade-tip leakage sets up endwall flow in the circumferential direction leading to flow breakdown and the inception of rotating stall. Rather than the tip leakage flows, the cause for circumferential endwall flow in the vaned diffuser is the combination of high swirl and the highly nonuniform spanwise flow profile at the impeller exit.

scholarly journals A Rotating Laser-Doppler Anemometry System for Unsteady Relative Flow Measurements in Model Centrifugal Impellers

1993 ◽  
Author(s):  
M. Abramian ◽  
J. H. G. Howard
Keyword(s):  
Laser Doppler Anemometry ◽  
Fluid Dynamic ◽  
Rotating Stall ◽  
Secondary Flows ◽  
Operating Conditions ◽  
Laser Doppler ◽  
Low Flow ◽  
Centrifugal Impeller ◽  
Flow Measurements ◽  
Relative Flow

The behaviour of the relative flow in centrifugal turbomachines is extremely complex due to the existence of various fluid dynamic phenomena and their interaction. At design and off-design operating conditions, the relative flow is subject to stationary unsteadiness which includes the flow separation and wakes associated with passage pressure gradients, secondary flows, and boundary layer stability. It is also subject to periodic unsteadiness from the rotating stall and the cyclic flow phenomena induced by the casing. This paper describes the mechanical and optical design of a rotating laser-Doppler anemometry system which allows direct measurement of the relative flow by means of an optical de-rotator. By isolating the impeller rotational frequency from the sampling frequency, it allows direct time-average measurements of the stationary behaviour of the relative flow along with the ensemble (phase)-average measurements of its periodic behaviour. Its success is demonstrated with measurements conducted in a low specific speed centrifugal impeller fitted with a single volute. Sample results of the time-averaged blade-to-blade variation of total relative velocities along with their associated turbulence intensities are reported. The (periodic) cyclic variations of the impeller exit flow, induced by the volute at low flow rates, are also presented for the suction and pressure sides.

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