Investigation of Unsteady Flow Field in a Vaned Diffuser of a Transonic Centrifugal Compressor

Ibaraki, Seiichi; Matsuo, Tetsuya; Yokoyama, T.

doi:10.1115/1.2720505

Cited by 23 publications

(5 citation statements)

References 8 publications

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“…Whether the diffuser can be studied in isolation from the impeller has been the subject of much research. It is now firmly established that the unsteadiness resulting from the jet-wake impeller exit flow persists through the diffuser (see, for example, [19,20]). However, it has been suggested, using both numeric analysis and careful experimentation, that the effects of unsteadiness on the diffuser flow are small relative to the effects of the time-mean spanwise flow non-uniformity in the impeller exit flow [21][22][23].…”

Section: Approachmentioning

confidence: 99%

An Investigation of Stall Inception in Centrifugal Compressor Vaned Diffusers

Everitt¹,

Spakovszky²

2011

Volume 7: Turbomachinery, Parts A, B, and C

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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 non-uniform spanwise flow profile at the impeller exit.

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Section: Approachmentioning

confidence: 99%

An Investigation of Stall Inception in Centrifugal Compressor Vaned Diffusers

Everitt¹,

Spakovszky²

2011

Volume 7: Turbomachinery, Parts A, B, and C

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“…Ibaraki et al 3 measured the meridional velocity distributions at different crosssections along a transonic impeller chord to reveal the loss generation mechanism of the impeller flow using the Laser Doppler Velocimetry (LDV) technique. Ibaraki et al 4 applied particle image velocimetry (PIV) to measure the flow characteristics inside a vaned diffuser. This suggests that the impeller wake still exists downstream toward the diffuser throat and continues to mix in the spanwise direction.…”

Section: Introductionmentioning

confidence: 99%

Investigation of transient flow characteristics inside a centrifugal compressor for design and off-design conditions

Zhao

Wang

Zhao

et al. 2017

Proceedings of the Institution of Mechanical Engineers, Part A:

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As the flow rate decreases from the design to the rotating stall condition, the enhanced impeller-diffuser interaction considerably deteriorates the flow condition at the diffuser inlet, which may trigger stall and blade vibration problems. In order to reveal the underlying mechanism and estimate the impact of the interaction within the impeller and diffuser passages, measurements using the wireless acquisition technique and high-frequency response system have been conducted on a 1.5 stage centrifugal compressor with the vaned diffuser. The details of its transient flow characteristics suggest that the effects of the impeller sweep extensively propagate in the diffuser passage. Distinctions of the shroud reversed vortex effect exist between the main and splitter blades at the impeller outlet, which initiates the predominant passage passing frequency at the diffuser inlet under small flow rate condition. In addition, the present study explains why the dominant disturbance shifts back and force between the passage passing frequency and blade passing frequency for different positions of the diffuser and flow rates. Through flow throttling, the diffuser reaction toward the impeller passage considerably strengthens due to the growing pressure potential near the convex surface of the diffuser vane, which is associated with the reversed flow.

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“…The results obtained by him show that the flow in the impeller and volute casing is periodically unsteady and confirm the circumferential distortion of the pressure distribution at the impeller outlet and in the volute casing. Ibaraki et al [16] in their work have found that a complex three-dimensional flow with distortion between the shroud and the hub is observed.…”

Section: Introductionmentioning

confidence: 96%

“…A host of articles [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] is available which explicitly explore the impeller diffuser interactive phenomenon by both experimental and numerical methods.…”

Section: Introductionmentioning

confidence: 99%

CFD Analysis on the Effect of Radial Gap on Impeller‐Diffuser Flow Interaction as well as on the Flow Characteristics of a Centrifugal Fan

Karanth

Sharma

2009

International Journal of Rotating Machinery

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The flow between the impeller exit and the diffuser entry (i.e., in the radial gap is generally considered to be complex). With the development of PIV and CFD tools such as moving mesh techniques, it is now possible to arrive at a prudent solution compatible with the physical nature of flow. In this work, numerical methodology involving moving mesh technique is used in predicting the real flow behavior, as exhibited when a target blade of the impeller is made to move past corresponding vane on the diffuser. Many research works have been undertaken using experimental and numerical methods on the impeller-diffuser interactive phenomenon. It is found from the literature that the effect of radial gap between impeller and diffuser on the interaction and on the performance of the fan has not been the focus of attention. Hence numerical analysis is undertaken in this work to explore and predict the flow behavior due to the radial gap. This has revealed the presence of an optimum radial gap which could provide better design characteristics or lower loss coefficient. It is found that there is a better energy conversion by the impeller and enhanced energy transformation by the diffuser, corresponding to optimum radial gap. The overall efficiency also found to increase for relatively larger gap.

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Investigation of Unsteady Flow Field in a Vaned Diffuser of a Transonic Centrifugal Compressor

Cited by 23 publications

References 8 publications

An Investigation of Stall Inception in Centrifugal Compressor Vaned Diffusers

An Investigation of Stall Inception in Centrifugal Compressor Vaned Diffusers

Investigation of transient flow characteristics inside a centrifugal compressor for design and off-design conditions

CFD Analysis on the Effect of Radial Gap on Impeller‐Diffuser Flow Interaction as well as on the Flow Characteristics of a Centrifugal Fan

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