LINFLOW 1.4 1

Illustration on How to Study Response of a Ventilation Fan Subjected to High

Frequency Loading. In this work the question was, whydid the fan fail in fatigue during operation? Did the initial design havean aeroelastic problem or was the problem created by the dirt built up that was found on the failed fan blades?

Software tools used:+LINFLOW+ANSYS+CFX

LINFLOW 1.4 2

CFX and the Ventilation Fan AnalysisCFX and the Ventilation Fan AnalysisCFX (Navier-Stokes solver) used for calculation of unsteady pressure around the fan, which is the load exciting the fan.

Rotor and stator details

CFX Velocity contours

Flow Outlet

Flow Inlet

LINFLOW 1.4 3

CFX Unsteady Pressure Up-stream the CFX Unsteady Pressure Up-stream the Fan.Fan.

CFX used for calculation of unsteady pressure on the fan, which is the load exciting the fan.

Time history of unsteady Pressure that fan blades see.

An FFT analysis gave the frequency spectrum of the load that the fan blades are subjected to.

LINFLOW 1.4 4

The ANSYS Structural model of the The ANSYS Structural model of the Ventilation FanVentilation Fan

ANSYS Structural Model of the Fan

To perform an aeroelastic stabilityand response analysis, a structuraldynamics model is needed. In thiscase the ANSYS FE-program was usedto develop the needed model.

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The ANSYS Structure Dynamics Used by The ANSYS Structure Dynamics Used by LINFLOWLINFLOW

ANSYS Modal Results for the FanMode Number Frequency (Hz) Mode Type

0,1 37.75 Fan axel torsion0,2 57.42 Blades in bending motion0,3 80.93 Blades in bending motion0,4 109.7 Blades in cord bending0,5 136.8 Blades in torsion motion0,6 181.1 Blades in mix. torsion/bending0,7 184.2 Blades in mix. torsion/bending1,1 34.07 Fan hub bending1,2 34.07 Fan hub bending1,3 72.82 Blade bending1,4 72.82 Blade bending1,5 95.63 Blades in cord bending1,6 95.63 Blades in cord bending1,7 112.1 Blades in bending motion2,1 68.48 Blades in bending motion2,2 68.48 Blades in bending motion2,3 80.19 Blades in bending motion2,4 80.19 Blades in bending motion2,5 114.1 Blades in cord bending2,6 114.1 Blades in cord bending2,7 136.8 Blades in mix. torsion/bending3,1 71.38 Blades in bending motion3,2 71.38 Blades in bending motion3,3 87.99 Blades in bending motion3,4 87.99 Blades in bending motion3,5 136.8 Blades in mix. torsion/bending3,6 136.8 Blades in mix. torsion/bending3,7 181.12 Blades in mix. torsion/bending

Table 1, Structural eigenvalues for the fan blade model

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The LINFLOW Aeroelastic Model of the FanThe LINFLOW Aeroelastic Model of the Fan

The LINFLOW linearizedfluid dynamics model of thefan include both the fan bladeand a wake model. LINFLOWuses boundary elements to descretize the fluid dynamics, which means that no flow domain grid is needed.

Flow Conditions in this case:Pressure = 5530 PaDensity = 1.2 kg/m3

Angular Velocity = 870 – 920 rpmCp/Cv = 1.4Blade angle of attack 63o

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LINFLOW Steady Fluid Dynamics LINFLOW Steady Fluid Dynamics

Picture show velocity contours on the surface of the blades..

In LINFLOW the aeroelastics isstudied around some mean steadyflow condition. In this case we checkthat the lift on the blade corresponds to the force driving the flow through the fan and that the drag on the blade Corresponds to the moment thatthe fan is driven by.

LINFLOW 1.4 8

LINFLOW Aeroelastic Stability Anlysis of the Fan.LINFLOW Aeroelastic Stability Anlysis of the Fan.Aeroelastic Stability Check of the Fan

Aeroelastic Frequencies a.f.o. Flow Velocity. Damping requirement for neutral stability for the 5 first modes in table 1.

The calculation show no sign of aeroelastic stability problemIn the RPM range at which the fan is operated.

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LINFLOW Response/Spectrum analysis, 1LINFLOW Response/Spectrum analysis, 1LINFLOW Aeroelastic Response Analysis of the Fan with

Clean Blades.

Real part of the stress field on the model

Stess calc. location

Spectrum analysis with SRSS summation gave 19.4 Mpa stress level in the root of the blade shaft.Measurements on clean blade gave approx. 20 Mpa stress level at the same location

Imaginary part of the stress field on the model

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LINFLOW Response/Spectrum analysis, 2LINFLOW Response/Spectrum analysis, 2LINFLOW Response Analysis of the Fan with Dirt build-up

on the blades.

Spectrum analysis with SRSS summation gave 32.0 Mpa stress level in the root of the blade shaft.This is above the 27 Mpa stress level that is the limit above which fatigue problems start to appear.Conclusion, the reason for failure was that the dirt built-up had changed the aeroelastic charactiristics sothat vibration amplitude due to subjected load increased to levels above the fatigue limit for the material.

Real part of the stress field on the model Imaginary part of the stress field on the model