Numerical Investigation of Diffuser Flow Field and Rotating Stall in a Centrifugal Compressor With Vaned Diffuser

2017 ◽  
Author(s):  
Yang Zhao ◽  
Jiayi Zhao ◽  
Zhiheng Wang ◽  
Guang Xi
Keyword(s):  
Centrifugal Compressor ◽  
Leading Edge ◽  
Suction Side ◽  
Rotating Stall ◽  
Tip Leakage Flow ◽  
Vaned Diffuser ◽  
Flow Angle ◽  
Diffuser Flow ◽  
Flow Phenomena ◽  
Reversal Flow

The diffuser rotating stall in a centrifugal compressor with vaned diffuser is one of important unsteady flow phenomena, which limits the operating range of the compressor. In this paper, the unsteady CFD analysis on a low-speed centrifugal compressor has been performed to investigate the flow characteristic in the diffuser and the propagation of the diffuser rotating stall. The flow behaviors at the outlet of the impeller at design and off-design conditions are firstly investigated. It is found that a reversal flow, induced by the tip leakage flow, exists near the shroud at the impeller outlet and becomes serious with the mass flow rate reduced. Due to the span-wise variation of the flow angle at the diffuser inlet and the inversed pressure gradient in the passage, the leading-edge vortex (LEV) generates on the diffuser leading edge. The LEV then induces the secondary flow in the diffuser passage and then causes the hub-corner separation. Furthermore, the propagation of the diffuser rotating stall is presented in details. The suction-side separation near the hub induces the blockage in the passage. And the shedding vortex from the suction side moves toward the leading edge of the adjacent blade. When the vortex reaches to the leading edge of the adjacent blade, the incidence increase and a new separation occurs on the suction side. With the development of the new separation, the passage becomes blocked gradually and the upstream stalled passage recovers to a normal condition. The rotating stall propagates along the direction of the impeller rotation at about 4.5% of the impeller rotational speed.

Structure of Diffuser Stall and Unsteady Vortices in a Centrifugal Compressor With Vaned Diffuser

2016 ◽  
Author(s):  
Nobumichi Fujisawa ◽  
Sota Ikezu ◽  
Yutaka Ohta
Keyword(s):  
Centrifugal Compressor ◽  
Leading Edge ◽  
Rotating Stall ◽  
Design Flow ◽  
Tip Leakage Flow ◽  
Vaned Diffuser ◽  
Tip Leakage ◽  
Close Proximity ◽  
Edge Vortex ◽  
Flow Blockage

The characteristics of a diffuser rotating stall and the evolution of a vortex generated on the diffuser leading edge (i.e., leading-edge vortex (LEV)) in a centrifugal compressor were investigated using experiments and numerical analyses. The experimental results showed that both impeller and diffuser rotating stalls occurred at 55 and 25 Hz during off-design flow operation. Both the stall cells existed only on the shroud side of the flow passages, which is in close proximity to the source location of the LEV. The numerical results showed that the LEV is a combination of a separated vortex near the leading edge and the extended tip-leakage flow from the impeller. In the partial flow operation, the LEV develops as the velocity decreases in the diffuser passages and forms a huge flow blockage within the diffuser passages. Therefore, the LEV may be considered to be one of the causes of diffuser stall in the centrifugal compressor.

Investigation of an Inversely Designed Centrifugal Compressor Stage: Part 1 — Design and Numerical Verification

2003 ◽  
Author(s):  
M. Zangeneh ◽  
M. Schleer ◽  
F. Plo̸ger ◽  
S. S. Hong ◽  
C. Roduner ◽  
...  
Keyword(s):  
Centrifugal Compressor ◽  
Design Method ◽  
Leading Edge ◽  
Inverse Design ◽  
Numerical Verification ◽  
Appreciable Effect ◽  
Tip Leakage Flow ◽  
Flow Angle ◽  
Compressor Impeller ◽  
Blade Geometry

In this paper the 3D inverse design code TURBOdesign-1 is applied to the design of the blade geometry of a centrifugal compressor impeller with splitter blades. In the design of conventional impellers the splitter blades normally have the same geometry as the full blades and are placed at mid-pitch location between the two full blades, which can usually result in a mis-match between the flow angle and blade angles at the splitter leading edge. In the inverse design method the splitter and full blade geometry is computed independently for a specified distribution of blade loading on the splitter and full blades. In this paper the basic design methodology is outlined and then the flow in the conventional and inverse designed impeller is compared in detail by using CFD code TASCflow. The CFD results confirm that the inverse design impeller has a more uniform exit flow, better control of tip leakage flow and higher efficiency than the conventional impeller. The results also show that the shape of the trailing edge geometry has a very appreciable effect on the impeller Euler head and this must be accurately modeled in all CFD computations to ensure closer match between CFD and experimental results. Detailed measurements are presented in part 2 of the paper.

Effect of Curvilinear Element Blade for Open-Type Centrifugal Impeller on Stator Performance

2019 ◽  
Author(s):  
Kazuhiro Tsukamoto ◽  
Kiyohide Sakamoto ◽  
Kiyotaka Hiradate ◽  
Yasushi Shinkawa
Keyword(s):  
Rotating Stall ◽  
Centrifugal Impeller ◽  
Parameter Study ◽  
Tip Leakage Flow ◽  
Vaned Diffuser ◽  
Flow Angle ◽  
Open Type ◽  
Closed Type ◽  
Key Techniques ◽  
Stage Efficiency

Abstract The effect of a curvilinear element blade for an open-type centrifugal impeller on stator performance was investigated by experiment using an actual single stage compressor. This investigation focused on the stator part performance located at the downstream of the impeller for both a vane-less diffuser and a vaned diffuser. Centrifugal compressors are widely used in various industrial plants, and some customers require higher stage performance. The curvilinear element blade technique, which is one of the key techniques for increasing the efficiency of closed-type centrifugal impellers, was investigated, and it effected an increase in stator efficiency. For this reason, the effect of the curvilinear element blade for the open-type centrifugal impeller was investigated. Our previous study reported that the curvilinear element blade with the open-type impeller increased the impeller efficiency by decreasing the loss derived from the impeller tip leakage flow with the parameter study of the curvilinear element blade geometries using numerical simulations. This paper reports the results of the experimental verifications using the geometries from the previous report. Experimental results indicated that the compressor stage efficiency increased by 0.7% compared with that of the conventional impeller, which has a linear element blade by using the vane-less diffuser. However, a rotating stall occurred at a higher flow rate than that of the conventional case in the vane-less diffuser. This is due to the decrease of the impeller outlet flow angle derived from the effect of the curvilinear element blade, which makes the velocity distribution equal and reduces the blockage regions near the shroud side. On the other hand, the curvilinear element blade impeller could increase the stage efficiency by 1.2% over the conventional impeller by using the vaned diffuser. This is due to not only the impeller performance increase but also the diffuser performance increase derived from the equality of the flow distributions by the curvilinear element blade. In addition, there was no diffuser rotating stall.

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 Experimental Study of Stall Suppression and Associated Changes to the Flow Structures in the Tip Region of an Axial Low Speed Fan Rotor by Axial Casing Grooves

2017 ◽  
Author(s):  
Huang Chen ◽  
Yuanchao Li ◽  
Subhra Shankha Koley ◽  
Nick Doeller ◽  
Joseph Katz
Keyword(s):  
Flow Rate ◽  
Large Scale ◽  
Pressure Rise ◽  
Leading Edge ◽  
Suction Side ◽  
Rotating Stall ◽  
Flow Structures ◽  
Flow Angle ◽  
Low Speed ◽  
Flow Characterization

The effects of axial casing grooves on the performance and flow structures in the tip region of an axial low speed fan rotor have been studied experimentally in the JHU refractive index-matched liquid facility. The four-per-passage semicircular grooves are skewed by 45° in the positive circumferential direction, and have a diameter of 65% of the rotor blade axial chord length. A third of the groove overlaps with the blade front, and the rest extends upstream. These grooves have a dramatic effect on the machine performance, reducing the stall flow rate by 40% compared to the same machine with a smooth endwall. However, they reduce the pressure rise at high flow rates. The flow characterization consists of qualitative visualizations of vortical structures using cavitation, as well as stereo-PIV (SPIV) measurements in several meridional and (z,θ) planes covering the tip region and interior of the casing grooves. The experiments are performed at a flow rate corresponding to pre-stall conditions for the untreated machine. They show that the flow into the downstream sides of the grooves and the outflow from their upstream sides vary periodically. The inflow peaks when the downstream end is aligned with the pressure side (PS) of the blade, and decreases, but does not vanish, when this end is located near the suction side (SS). These periodic variations have three primary effects: First, substantial fractions of the leakage flow and the tip leakage vortex (TLV) are entrained periodically into the groove. Consequently, in contrast to the untreated flow, The TLV remnants remain confined to the vicinity of the entrance to the groove, and the TLV strength diminishes starting from the mid-chord. Second, the grooves prevent the formation of large scale backflow vortices (BFVs), which are associated with the TLV, propagate from one blade passage to the next, and play a key role in the onset of rotating stall in the untreated fan. Third, the flow exiting from the grooves causes periodic variations of about 10° in the relative flow angle around the blade leading edge, presumably affecting the blade loading. The distributions of turbulent kinetic energy provide statistical evidence that in contrast to the untreated casing, very little turbulence originating from a previous TLV, including the BFVs, propagates from the PS to the SS of the blade. Hence, the TLV-related turbulence remain confined to the entrance to groove. Elevated, but lower turbulence is also generated as the outflow from the groove jets into the passage.

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 Experimental Investigation of Unsteady Flow Phenomena in a Centrifugal Compressor Vaned Diffuser of Variable Geometry

1999 ◽  
Vol 121 (4) ◽  
pp. 763-771 ◽  
Author(s):  
F. Justen ◽  
K. U. Ziegler ◽  
H. E. Gallus
Keyword(s):  
Centrifugal Compressor ◽  
Pressure Fluctuations ◽  
Suction Side ◽  
Front Wall ◽  
Compressor Stage ◽  
Vaned Diffuser ◽  
Centrifugal Compressor Stage ◽  
Impeller Outlet ◽  
Flow Phenomena ◽  
Radial Gap

The behavior of vaned radial diffusers is generally considered to be due to the flow phenomena in the vaneless and the semi-vaned space in the diffuser inlet region. Even considering unsteady aspects, the adjacent diffuser channel is regarded as less important. The flat wedge vaned diffuser of the centrifugal compressor stage investigated allows an independent continuous adjustment of the diffuser vane angle and the radial gap between impeller outlet and diffuser vane inlet, so that information about the importance of these geometric parameters can be obtained. The time-dependent pressure distribution on the diffuser front wall and on the suction and pressure surfaces of the diffuser vanes reveal that in the semi-vaned space mainly the region near the vane suction side is influenced by the unsteady impeller-diffuser interaction. Downstream in the diffuser channel the unsteadiness does not decay. Here, pressure fluctuations are appearing that are distinctly higher than the pressure fluctuations in the vaneless space. An estimation of the influence of the unsteadiness on the operating performance of the centrifugal compressor stage is made by measurements at choke and surge limit for different diffuser geometries.

A Numerical Study of the Unsteady Interaction Effects on Diffuser Performance in a Centrifugal Compressor

2013 ◽  
Vol 136 (1) ◽  
Author(s):  
S. Anish ◽  
N. Sitaram ◽  
H. D. Kim
Keyword(s):  
Centrifugal Compressor ◽  
Numerical Study ◽  
Practical Importance ◽  
Leading Edge ◽  
Dominant Frequency ◽  
Vaned Diffuser ◽  
Flow Angle ◽  
Unsteady Characteristics ◽  
Cfd Method ◽  
Physical Phenomena

Interaction between rotating impeller and stationary diffuser in a centrifugal compressor is of practical importance in evaluating system performance. The present study aims at investigating how the interaction influences the unsteady diffuser performance and understanding the physical phenomena in the centrifugal compressor. A computational fluid dynamics (CFD) method has been applied to predict the flow field in the compressor, which has a conventional vaned diffuser (VD) and a low solidity vaned diffuser (LSVD). The radial gaps between impeller and diffuser and different flow coefficients are varied. The results obtained show that the major parameter that influences the unsteady variation of diffuser performance is due to the circumferential variation of the flow angle at the diffuser vane leading edge. The physical phenomena behind the pressure recovery variation are identified as the unsteady vortex shedding and the associated energy losses. The vortex core region as well as the shedding of vortices from the diffuser vane are triggered by the variation in the diffuser vane loading, which in turn is influenced by the circumferential variation of the impeller wake region. There is little unsteady variation of flow angle in the span-wise direction. This indicates that the steady state performance characteristics are related to the span-wise variation of flow angle, while the unsteady characteristics are contributed by the circumferential variation of flow angle. At design conditions, dominant frequency components of pressure fluctuation are all periodic and at near stall, these are aperiodic.

Evolution Process of Diffuser Stall in a Centrifugal Compressor With Vaned Diffuser

2018 ◽  
Author(s):  
Nobumichi Fujisawa ◽  
Tetsuya Inui ◽  
Yutaka Ohta
Keyword(s):  
Centrifugal Compressor ◽  
Leading Edge ◽  
Boundary Layer Separation ◽  
Rotating Stall ◽  
Evolution Process ◽  
Cfd Analysis ◽  
Pressure Measurements ◽  
Velocity Measurements ◽  
Vaned Diffuser ◽  
Computational Fluid Dynamics Cfd

The evolution process of a diffuser rotating stall in a centrifugal compressor with a vaned diffuser was investigated by experimental and numerical analyses. From velocity measurements, it was found that the diffuser stall propagated near the shroud side in the vaneless space. As the mass flow decreased, a stage stall rotated within both the impeller and diffuser passages, instead of a diffuser stall. The evolution process of the diffuser stall had three stall forms. First, the diffuser stall, which was rotating on the shroud side, shifted to the hub side. Then, the diffuser stall moved into the impeller passages and evolved to a stage stall. From computational fluid dynamics (CFD) analysis, a tornado-type vortex was generated first, near the hub side of the diffuser leading edge, when the diffuser stall was shifted to the hub side. Next, a throat area blockage was formed near the hub side because of the boundary layer separation in the vaneless space. Finally, the blockage within the diffuser passages expanded to the impeller passages and developed into a stage stall. From the pressure measurements along the impeller and diffuser passages, the magnitude of pressure fluctuation on the casing wall of the diffuser throat area also suddenly increased when the diffuser stall shifted to the hub side. Therefore, the evolution area of the diffuser stall was caused by the evolution of the blockage near the throat area of the diffuser passage.

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