1. Illustration on How to Study Response of a Ventilation Fan Subjected to High Frequency Loading.
In this work the question was, why
did the fan fail in fatigue during
operation? Did the initial design have
an 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 Analysis
CFX (Navier-Stokes solver) used for calculation of unsteady pressure around the fan, which is the load exciting the fan.
Flow Outlet
CFX Velocity contours
Rotor and stator details
Flow Inlet
LINFLOW 1.4

3. CFX Unsteady Pressure Up-stream the 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 Ventilation Fan
ANSYS Structural Model of the Fan
To perform an aeroelastic stability
and response analysis, a structural
dynamics model is needed. In this
case the ANSYS FE-program was used
to develop the needed model.
LINFLOW 1.4

5. The ANSYS Structure Dynamics Used by LINFLOW
ANSYS Modal Results for the Fan
LINFLOW 1.4

6. The LINFLOW Aeroelastic Model of the Fan
The LINFLOW linearized
fluid dynamics model of the
fan include both the fan blade
and a wake model. LINFLOW
uses boundary elements to
descretize the fluid dynamics,
which means that no flow domain
grid is needed.
Flow Conditions in this case:
Pressure = Pa Density = 1.2 kg/m3
Angular Velocity = 870 – 920 rpm
Cp/Cv = 1.4
Blade angle of attack 63o
LINFLOW 1.4

7. LINFLOW Steady Fluid Dynamics
In LINFLOW the aeroelastics is
studied around some mean steady
flow condition. In this case we check
that 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 that
the fan is driven by.
Picture show velocity contours on the surface of the blades..
LINFLOW 1.4

8. 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 problem
In the RPM range at which the fan is operated.
LINFLOW 1.4

9. LINFLOW Response/Spectrum analysis, 1
LINFLOW Aeroelastic Response Analysis of the Fan with Clean Blades.
Real part of the stress field on the model
Imaginary part of the stress field on the model
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
LINFLOW 1.4

10. LINFLOW Response/Spectrum analysis, 2
LINFLOW Response Analysis of the Fan with Dirt build-up on the blades.
Real part of the stress field on the model
Imaginary part of the stress field on the model
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 so
that vibration amplitude due to subjected load increased to levels above the fatigue limit for the material.
LINFLOW 1.4