Abstract
Nearly a century ago, the liquid self-balancing device was first introduced by M. LeBlanc for passive balancing of turbine rotors. Although of common use in many types or rotating machines nowadays, little information is available on the unbalance response and stability characteristics of this device. Experimental fluid flow visualization evidences that radial and traverse circulatory waves arise due to the interaction of the fluid backward rotation and the baffle boards within the self-balancer annular cavity. The otherwise destabilizing force induced by trapped fluids in hollow rotors, becomes a stabilizing mechanism when the cavity is equipped with adequate baffle boards. Further experiments using Particle Image Velocimetry (PIV) enable to assess the active fluid mass fraction to be one-third of the total fluid mass. An analytical model is introduced to study the effects of the active fluid mass fraction on a flexible rotor supported by flexible supports excited by bwo different destabilizing mechanisms; rotor internal friction damping and aerodynamic cross-coupling. It is found that the fluid radial and traverse forces contribute to the balancing action and to improve the rotor stability, respectively.
Highlights
There has been considerable work in the area of the dynamics of rotors with internal damping, with regard to internal friction arising from micro-slip at shrink-fit interconnections of built-up rotors
This paper shows experiments that indicate that the otherwise unstable mechanism consisting of trapped fluid in hollow rotors may become a stabilizing means if the cavity is equipped with radial baffles yielding; a) a fluid distribution within the cavity, and b) a raised fluid modes of vibration such that a fluid backward traveling wave occurs contributing to increase the threshold speed of instability of a flexible rotor
Eq (19) shows that for the rotor to be unstable due to internal friction damping, the rotational speed must exceed the undamped natural frequency plus a destabilizing term depending on the internal friction damping, plus a rising term depending on the traveling backward mass of the LeBlanc balancer and its geometrical properties
Summary
There has been considerable work in the area of the dynamics of rotors with internal damping, with regard to internal friction arising from micro-slip at shrink-fit interconnections of built-up rotors. Gunter [3] developed a linear rotordynamic model in which internal friction was modeled as a cross-coupled force He demonstrated that if external damping is added, the threshold speed could be greatly improved. Back in 1914, LeBlanc [7] first introduced a passive dynamic balancing device for turbine rotors consisting of an annular cavity partially filled with a liquid of high viscosity. This paper shows experiments that indicate that the otherwise unstable mechanism consisting of trapped fluid in hollow rotors may become a stabilizing means if the cavity is equipped with radial baffles yielding; a) a fluid distribution within the cavity, and b) a raised fluid modes of vibration such that a fluid backward traveling wave occurs contributing to increase the threshold speed of instability of a flexible rotor. Balance ring blades and fluid inertia are found to play an important role in the onset of instability
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