|The RPMTurbo Advantage
RPMTurbo is a professional engineering consultancy specializing in linear
flow analysis for turbomachinery.
Linear flow analysis can be used to analyse turbomachinery blade flutter
(self-excited vibration due to the interaction of the natural blade
motion and unsteady aerodynamic forces).
Flutter can have a significant economic cost due to increased development costs,
losses due to development delays and increased maintenance costs.
RPMTurbo provides manufacturers of turbomachinery with high quality flutter
analysis based on 3D viscous unsteady linear flow simulations in a short time-frame
(weeks not months) that helps to identify potential flutter problems earlier
in the design process.
RPMTurbo has developed a linear flow solver that has many advanced features
Because RPMTurbo has developed its own steady and linearized flow solvers,
RPMTurbo can perform customized analysis.
RPMTurbo delivers high quality analysis to its clients
because it fully understands the capabilities of its flow solvers,
and the analysis is performed by an expert with more than 10
years of industrial experience.
The combination of advanced flow modeling, customized analysis and
extensive industry experience gives RPMTurbo's customers
a unique advantage.
Wet Steam Flow Modeling
The long blades at the last stage of modern steam turbines are prone to flutter.
Typically steam condensation has occurred by the last stage.
However, ideal gas is usually assumed when performing flutter analysis for
The accuracy of the flutter analysis can be improved by considering the
flow effects of the steam condensation.
RPMTurbo has developed an equilibrium wet steam flow model
that accounts for steam condensation.
Flow properties such as pressure, temperature, speed of sound and viscosity of
wet steam are calculated using formulae from
The International Association for the Properties of Water and Steam.
The plot below compares the steam quality (steam mass fraction) calculated by RPMTurbo
at the exit of an industrial steam turbine with data from the manufacturer
(published with permission).
The wet steam gas model can make a significant difference to the
predicted steady and unsteady flow compared with the ideal gas model.
Standard Configuration 11 are shown below.
The first plot shows the steam quality contours as predicted by the wet steam model.
The next two plots show the surface isentropic Mach number from the steady solution and
the unsteady surface pressure due to a torsion mode.
Complex Mode Shapes
The mode shapes of turbomachinery blades can be complex
if there is significant structural
coupling, due to blade connections such as tip shrouds or snubbers.
The plot below shows how the blade connections
cause the blade mode shape to have both bending and torsion components
at different phases.
These complex modes can be either forward running (same direction as turbine rotation)
or backward running (opposite direction).
RPMTurbo can perform flutter analysis on turbomachinery blades
with complex mode shapes.
Complex numbers are used to represent the mode shape, for example in the mode
shown above, the real component would be the axial bending and the imaginary
component would be the torsion.
The three-dimensional mode shapes (real and imaginary) calculated via
a finite element method are interpolated to a three-dimensional CFD mesh.
The unsteady flow response is calculated by RPMTurbo's linearised
flow solver and then the flutter risk is determined.
The plot below shows the aerodynamic damping calculated due to forward and backward
running complex modes for the Standard Configuration 11 turbine profile.
12th ISUAAAT London September 2009
The 12th ISUAAAT (International Symposium
on Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines)
will be held at Imperial College London from the 1st to 4th September 2009.
This conference is not to be missed as it is only held
every three years, and as the title
suggests, is the premier conference for aeroelasticity and
aeroacoustics in turbomachinery.
The last ISUAAAT held in Moscow in 2006 was a memorable experience, and
I am looking forward to seeing everyone again in London.