Measurements of the Flow in Backshroud/CasingClearance of Precessing Centrifugal Impeller

Yoshida, Yoshiki; Tsujimoto, Yoshinobu; Ohashi, Hideo; Saitô, Akira; Ishizaki, Shin

doi:10.1155/s1023621x97000249

Cited by 5 publications

(3 citation statements)

References 10 publications

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“…Figure 18 shows the plot of Ω I /ω against by neglecting the fluid moment coefficients and using Eq. (5). The relation between the phase difference and the sign of Ω I was confirmed for all cases with the effect of fluid moment.…”

Section: For the Case With Fluid Force Momentssupporting

confidence: 53%

“…For the backshroud, the experimental results of fluid moment on a centrifugal impeller shroud in precessing motion were discussed based on the bulk flow model to elucidate the fundamental flow mechanism [4]. The flow in backshroud/casing clearance of precessing centrifugal impeller was measured in [5].…”

Section: Introductionmentioning

confidence: 99%

“…On the other hand, [4,5] discussed the instability in precession motion. Since the whirling and precessing modes occur simultaneously, Kanemori and Iwatsubo [8,9] studied the rotordynamic forces and moments on a long seal under whirling and precessing vibrations.…”

Section: Introductionmentioning

confidence: 99%

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Rotordynamic Instabilities Caused by the Fluid Force Moments on the Backshroud of a Francis Turbine Runner

Song¹,

Horiguchi²,

Ma³

et al. 2010

International Journal of Fluid Machinery and Systems

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Severe flexural vibration of the rotor shaft of a Francis turbine runner was experienced in the past. It was shown that the vibration was caused by the fluid forces and moments on the backshroud of the runner associated with the leakage flow through the back chamber. The aim of the present paper is to study the self-excited rotor vibration caused by the fluid force moments on the backshroud of a Francis turbine runner. The rotor vibration includes two fundamental motions, one is a whirling motion which only has a linear displacement and the other is a precession motion which only has an angular displacement. Accordingly, two types of fluid force moment are exerted on the rotor, the moment due to whirl and the moment due to precession. The main focus of the present paper is to clarify the contribution of each moment to the self-excited vibration of an overhung rotor. The runner was modeled by a disk and the whirl and the precession moments on the backshroud of the runner caused by the leakage flow were evaluated from the results of model tests conducted before. A lumped parameter model of a cantilevered rotor was used for the vibration analysis. By examining the frequency, the damping rate, the amplitude ratio of lateral and angular displacements for the cases with longer and shorter overhung rotor, it was found that the precession moment is more important for smaller overhung rotors and the whirl moment is more important for larger overhung rotors, although both types of moment due to the leakage flow can cause self-excited vibration of an overhung rotor.

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Section: For the Case With Fluid Force Momentssupporting

confidence: 53%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Rotordynamic Instabilities Caused by the Fluid Force Moments on the Backshroud of a Francis Turbine Runner

Song¹,

Horiguchi²,

Ma³

et al. 2010

International Journal of Fluid Machinery and Systems

Self Cite

View full text Add to dashboard Cite

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Rotordynamic Stability Predictions for Centrifugal Compressors Using a Bulk-Flow Model to Predict Impeller Shroud Force and Moment Coefficients

Gupta

Childs

2010

Journal of Engineering for Gas Turbines and Power

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An analysis is developed for a compressible bulk-flow model of the leakage path between a centrifugal-compressor impeller’s shroud and its housing along the impeller’s front and back sides. This development is an extension of analyses performed first by Childs (1989, ASME J. Vib. Acoust., Stress, Reliab. Des., 111, pp. 216–225) for pump impellers. The bulk-flow model is used to predict reaction force and moment coefficients for the impeller shroud. A labyrinth seal code developed by Childs and Scharrer ( 1986, ASME Trans. J. Eng. Gas Turbines Power, 108, pp. 325–331) is used to calculate the rotordynamic coefficients developed by the labyrinth seals in the compressor stage and also provides a boundary condition for the shroud calculations. Comparisons between the measured shroud moment coefficients by Yoshida et al. (1996, Proceedings of the 6th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, 2, pp. 151–160) and model predictions show reasonable agreements for the clearance flow and reaction moments. For the conditions considered, low Mach number flow existed in the shroud clearance areas and compressible-flow and incompressible-flow models produced similar predictions. Childs’ model predictions for the direct damping and cross-coupled stiffness coefficients of a pump impeller produced reasonable agreement; hence the present model was validated to the extent possible. A rotor model consisting of an overhung impeller stage supported by a nominally cantilevered rotor was analyzed for stability using the present bulk-flow model and an API standard Wachel–von Nimitz formula model (1981, J. Petrol. Technol., pp. 2252–2260). The bulk-flow model predicted significantly higher onset speeds of instability. Given that some compressors have been predicted to be comfortably stable using API standard Wachel–von Nimitz formula but have been unstable on the test stand, these results suggest that unidentified destabilizing forces and or moments are present in compressors. Seal rub conditions that arise from surge events and increase the seal clearances are simulated, showing that enlarged clearances increase the preswirl at the seals, thus increasing these seal’s destabilizing forces and reducing stability margins. These results are consistent with field experience. Predictions concerning the back shroud indicate that shunt-hole injection mainly acts to enhance stability by changing the flow field of the division wall or balance piston seals, not by influencing the back-shroud’s forces or moments. Effective swirl brakes at these seals also serve this purpose.

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Fluid Force Moment on the Backshroud of a Francis Turbine Runner in Precession Motion

Song

Horiguchi

Nishiyama

et al. 2010

Journal of Fluids Engineering

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The fundamental characteristics of rotordynamic fluid force moment on the backshroud of a Francis turbine runner in precession motion were studied using model tests and computations based on a bulk flow model. The runner is modeled by a disk positioned close to a casing with a small axial clearance. An inward leakage flow is produced by an external pump in the model test. The effects of the leakage flow rate, the preswirl velocity at the inlet of the clearance, and the axial clearance on the fluid force moment were examined. It was found that the fluid force moment encourages the precession motion at small forward precession angular velocity ratios and the region encouraging the precession motion is affected by the preswirl velocity. Through the comparisons of the fluid force moment with and without the rotation of the disk, it was found that the normal moment without the disk rotation did not have the effect to encourage the precession motion. Thus, the swirl flow due to disk rotation was found to be responsible for the encouragement of the precession motion.

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Measurements of the Flow in Backshroud/CasingClearance of Precessing Centrifugal Impeller

Cited by 5 publications

References 10 publications

Rotordynamic Instabilities Caused by the Fluid Force Moments on the Backshroud of a Francis Turbine Runner

Rotordynamic Instabilities Caused by the Fluid Force Moments on the Backshroud of a Francis Turbine Runner

Rotordynamic Stability Predictions for Centrifugal Compressors Using a Bulk-Flow Model to Predict Impeller Shroud Force and Moment Coefficients

Fluid Force Moment on the Backshroud of a Francis Turbine Runner in Precession Motion

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