linflow 1.41 illustration on how to study response of a ventilation fan subjected to high frequency...
TRANSCRIPT
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.
LINFLOW 1.4 5
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
LINFLOW 1.4 6
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
LINFLOW 1.4 7
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
LINFLOW 1.4 10
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