Turbo Rotor Structure Interaction Simulation Centrifugal Chiller

© 2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

2008 International ANSYS Conference

Rotor-Structure Interaction Simulation of Centrifugal Chiller

Pavak MehtaTrane (Ingersoll Rand)Dave LoomanANSYS, Inc.


Outline

• Trane Centrifugal Chiller• Background

– Current Rotordynamics design• ANSYS Rotordynamics Model Validation

– Rotor Only Model • ANSYS Rotordynamics Model Application

– Centrifugal Chiller Model• Summary


Trane Centrifugal Chiller

• Water cooled chiller: 100 to 3000 tons capacity• Power frequency: 50 Hz or 60 Hz

Rotor housed inside compressor

Compressor supported on shells

Heat Exchange Shells sitting on the soft pads


Background

• Current Rotordynamics Design Approach– Tool: Specialized commercial code– Model: Rotor supported on bearings held rigidly– Output: Critical speed and shaft response– Limitation: Exclusion of structural resonances, Variable

bearing stiffness (impractical)

• New Rotordynamics Design Approach– Tool: ANSYS v 11 sp1– Model: Rotor supported on bearings housed in chiller– Output: Critical speed, shaft and chiller response,

dynamic bearing forces, etc– Limitation: Structural resonance correlation w/ test


Schematics of Rotordynamics Model

Current Model New ANSYS Model

xy

zShaft

Impeller

Motor rotor

Bearings

Shaft

Impeller

Motor rotor

Bearings

Chiller


Outline



– Rotor Only Model • ANSYS Rotordynamics Model Application



Rotor Only FEA model

• Linear brick elements (solid 185)• Impellers modeled as 3D point mass with rotary

inertia• Shaft components connected using contact elements• Rotor-shaft (w/ clearance) connected using

constrained equations• Stationary reference frame model


Rotor Support Bearing Model

• 2D spring/damper with cross-terms• Stiffness and damping varies with speed• In house code used to calculate stiffness matrix

Bearing element COMBI214


Rotor Only Campbell DiagramConstant Bearing Stiffness

1st Bending mode – Forward Whirl

1st Bending mode – Backward Whirl

1x - excitation

Operation Speed ANSYS: Solid line

In-house: Dotted line

System rigid body mode

Critical Speed

Speed (RPM)

Nat

ural

Fre

quen

cy (H

z)


Rotor Only Campbell DiagramVariable Bearing Stiffness

System rigid body mode1x - excitation

1st Bending mode – Forward Whirl

1st Bending mode – Backward Whirl

Critical Speed

ANSYS: Solid line

In-house: Dotted lineOperation Speed

Speed (RPM)

Nat

ural

Fre

quen

cy (H

z)

Variable bearing stiffness provides accurate critical speed prediction


Rotor Critical Mode Animation


Rotor OnlySynchronous Forced Response

In-house ANSYS (+2.5%)ANSYS: Solid line

In-house: Dotted line

Horizontal Shaft tip magnitude

Vertical Shaft tip magnitude

Speed (RPM)

Dis

plac

emen

t Mag

nitu

de (m

ils)

Operation Speed


Rotor Only Orbit at Critical Speed

Vert

ical

dis

plac

emen

t (m

ils)

Horizontal displacement

Minor axis

Major axis

Shaft tip orbit at resonance

In-house

ANSYS

ANSYS provides confidence in rotor synchronous response prediction


Outline



– Rotor Only Model• ANSYS Rotordynamics Model Application



Chiller FEA Model

• 3D representation of bearing support

• Mixed, solid-shell element model

• Stationary reference frame analysis

• 1 MDOF model

Compressor

Shells

Rotor


Rotor Only vs. Chiller Synchronous Forced Response

Speed (RPM)

Dis

plac

emen

t Mag

nitu

de (m

ils) Operation Speed



Rotor: Solid line

Chiller: Dotted line

High shaft response due to rotor-structural interaction is visible


Analysis vs. TestComparison of Shaft Response

Speed (RPM)

Dis

plac

emen

t Mag

nitu

de (m

ils) Operation Speed



Response shows good qualitative comparison

Analysis: Dotted line

Test: Solid line


Prediction of Dynamic Forces

2

1

3

1

4

3

2

4

Speed (RPM)

Dyn

amic

Bea

ring

Forc

e (lb

s) Operation Speed

Dynamic forces will help with reliability design of bearings

2


Prediction of Impeller-housing clearance

Speed (RPM)

Dis

plac

emen

t (m

ils)

Dynamic clearance prediction allows fine tuning of flow paths

Clearance

Shaft displacement

Housing (internal surface) displacement


Outline



– Rotor Only Model• ANSYS Rotordynamics Model Application



Summary

• Prediction of rotor system resonances with varying bearing property and whirl description is possible – within 2-5% accuracy of in-house code – Rotor Only– no classical splitting visible over speed – Chiller

• Prediction of synchronous harmonic forced response with bearing property as a function of speed– Results matched well w/ in-house code – Rotor Only– Magnitude and frequency trend matched with Test – Chiller

• Accurate structural FEA model is essential for rotor-structure interaction– For better test and analysis correlation


Benefits of ANSYS Rotordynamics on Chiller Design

• Allows 3D structure (chiller) representation for accurate rotor-structure interaction predictions

• Shaft synchronous and/or asynchronous forced response and chiller structural response can be predicted

• Structure and rotor design can be fine tuned in the early design stage and thus saves product launch time-to-market and development resources


Questions?

Thank You

Turbo Rotor Structure Interaction Simulation Centrifugal Chiller

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