Multiphysical Effects on High-Speed Rotordynamics


Dikmen, Emre (2010) Multiphysical Effects on High-Speed Rotordynamics. thesis.

Abstract:A growing number of research projects focus on the development of rotating machinery
on a small scale. These machines generally operate at high speeds and as a result
multiphysical effects such as interaction with the surrounding air and thermal effects
become significant for the dynamics. Therefore, the multiphysical effects should be
modeled and coupled with the structural models in order to perform rotordynamic
analysis accurately. This thesis describes modeling approaches for flow-induced forces
in moderate flow confinements such as a casing and for temperature increase of fluid in
the confinement. Furthermore, a method to couple the flow force model and thermal
model with the structural model has been proposed. An experimental setup has been
designed and constructed in order to verify the simulations with experimental data.
The gap ratio (air gap/rotor radius) in the moderate flow confinement is two orders
of magnitude greater than the ones in small air gap geometries such as bearings and
seals (0.1 to 0.001). Due to high rotation speeds, the inertia effects become significant
as well as the viscous effects. A theoretical model for flow-induced forces in terms of
added mass, damping and stiffness was available in the literature for turbulent flow.
This model is extended for laminar flow and transition by using the suitable empirical
and analytical friction coefficients to model the shear stress.
Then a method to implement the flow forces to the rotor finite element model as a
spring damper and added mass at each node is proposed. Finite element modeling of
the rotor is based on Timoshenko beams (including the flexibility of the rotor shaft)
and each element has four degrees of freedom at each node.
As the rotation speed increases, the heat loss due to air friction and the
temperature increase in the air gap between rotor and stator become more significant.
Consequently, the change of air properties due to temperature change in the air gap
should be considered when calculating the flow-induced forces. Therefore, a thermal
model is established in order to calculate the heat dissipation and as a result, the
temperature increase of the air. In this model, the rotor, stator and the gap in between
is modeled as lumped thermal networks. The required convective heat transfer
coefficients and heat dissipation are calculated by empirical correlations. Afterwards,
the new air gap temperature is used to calculate the flow-induced forces with updated
air properties. In this way, thermal and fluid effects in medium gap confinements
are coupled with the rotordynamic models and their effects on critical speeds and stability are investigated. An experimental setup has been designed to verify the
developed models for multiphysical effects on a small scale. The design criteria have
been determined such that the rotor size is large enough to make multiphysical effects
measurable in the operation range. Flexible supports with changing stiffness are
designed to examine the effect of support stiffness. Experiments are performed as
spectrum measurements and modal analysis at different support stiffness with and
without casing. The developed modeling approach is verified by experimental results.
Finally, a simple way to overcome experimentally and theoretically observed
instability problems has been presented. Stationary damping is increased by mounting
viscoelastic inserts on the support disk. The spectrum and modal analysis experiments
are repeated for different viscoelastic materials and significant improvement has been
In summary, a method has been developed to couple flow induced forces with
structural FE model including the thermal effects. This method can be used
for the rotordynamic analysis of high-speed mini rotating machinery with medium
gap for both laminar and turbulent flow regimes. In addition, the usage of
viscoelastic materials to avoid theoretically and experimentally observed instability
is demonstrated.
Item Type:Thesis
Engineering Technology (CTW)
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