Sergio E. Diaz

A Model for Squeeze Film Dampers Operating With Air Entrainment and Validation With Experiments

Squeeze film dampers (SFDs) reduce vibrations and aid in suppressing instabilities in high performance rotor-bearing systems. However, air ingestion and entrapment, pervasive in open-ended dampers with low supply pressures, leads to a bubbly lubricant that severely reduces the dynamic film forces and the overall damping capability. Analyses based on conventional film rupture models, vapor or gaseous lubricant cavitation, fail to predict the actual performance of SFDs, and thus lack credibility in engineering practice. A modified Reynolds equation for prediction of the pressure in a homogeneous bubbly mixture flow is advanced along with an empirical formula for estimation of the amount of air entrained in an open-ended damper. Careful experimentation in a test SFD operating with controlled bubbly mixtures and freely entrained air evidenced similar physical behavior, guided the analytical developments, and provided the basis for validation of the model forwarded, Comparisons of predictions and test results show a fair correlation. A simple equation to predict the amount of air ingestion is also advanced in terms of the damper geometry, supplied flow and operating conditions. The criterion may lack practical implementation since the persistence of air entrainment increases with the frequency and amplitude of journal motions, unless enough lubricant is supplied at all operating conditions.

Analysis of squeeze film dampers operating with bubbly lubricants

Rotor-bearing systems supported on squeeze film dampers (SFDs) show large amplitude vibratory motions when traversing critical speeds. At these operating conditions air is drawn into the damper thin film clearance generating a bubbly mixture with the lubricant and producing SFD forces not readily predictable with currently available analysis. A continuum model is proposed for describing the motion of a bubbly fluid in an open ended SFD operating with circular centered journal orbits. Computed predictions for peak-peak dynamic pressures and fluid film forces agree reasonably well with experimental measurements conducted on a SFD test rig operating with a controlled air in oil mixture. The bubbly flow model provides an initial procedure towards the reliable design of SFDs in actual operating conditions.

A Method for Identification of Bearing Force Coefficients and Its Application to a Squeeze Film Damper with a Bubbly Lubricant

A general formulation of the instrumental variable filter (IVF) method for parameter identification of a n-DOF (Degrees Of Freedom) mechanical linear system is presented. The IVF is a frequency domain method and an iterative variation of the least-squares approximation to the system flexibilities. Weight functions constructed with the estimated flexibilities are introduced to reduce the effect of noise in the measurements, thus improving the estimation of dynamic force coefficients. The IVF method is applied in conjunction to impact force excitations to estimate the mass, stiffness, and damping coefficients of a test rotor supported on a squeeze film damper (SFD) operating with a bubbly lubricant. The amount of air in the lubricant is varied from nil to 100 percent to simulate increasing degrees of severity of air entrainment into the damper film lands. The experimental results and parameter estimation technique show that the SFD damping force coefficients increase as the air volume fraction in the mixtur...

Measurements of Pressure in a Squeeze Film Damper with an Air/Oil Bubbly Mixture

Squeeze film dampers (SFDs) provide damping to rotating machinery, thus reducing excessive vibration amplitudes, supressing instabilities, and isolating structural components. Large amplitude SFD journal motions while traversing critical speeds enhance the ingestion of air and lead to operation with a bubbly mixture of lubricant and air. This phenomena lacks firm analytical understanding and controlled tests presented here show its effect on SFD performance. The experiments report measurements of the dynamic pressure field in an SFD apparatus operating with a controlled bubbly mixture of oil and air. The journal describes circular-centered orbits at a fixed whirl frequency and the damper is fully submerged in an oil bath. The dynamic squeeze film pressures become less reproducible for consecutive cycles of journal orbital motion and the overall peak-to-peak film pressures decrease as the mixture void fraction (air/oil volume ratio) increases. An unstable zone of null squeeze pressure generation in the dyn...

Air Entrainment Versus Lubricant Vaporization in Squeeze Film Dampers: An Experimental Assessment of Their Fundamental Differences

Squeeze film dampers (SFDs) provide structural isolation and energy dissipation in air-breathing engines and process gas compressors. However, SFDs are prone to develop a flow regime where the ingestion of air leads to the formation of a bubbly lubricant. This pervasive phenomenon lacks proper physical understanding and sound analytical model-ing, although actual practice demonstrates that it greatly reduces the damper force response. Measurements of film pressures in a test SFD describing circular centered orbits at whirl frequencies varying from 0 to 100 Hz are presented for fully flooded and vented discharge operating conditions. The experiments demonstrate that operation with low levels of external pressurization, moderate to large whirl frequencies, and lubricant discharge to ambient leads to the entrapment of air within the damper film lands. The experiments also elucidate fundamental differences in the generation of film pressures and forces for operation in a flooded condition that evidences vapor cavitation. Damping forces for the vented end with air entrainment are just 15 percent of the forces measured for the flooded damper. Further measurements at constant whirl frequencies demonstrate that increasing the lubricant pressure supply retards the onset of air entrainment. Classical fluid film cavitation models predict well the pressures and forces for the lubricant vapor cavitation condition. However, prevailing models fail to reproduce the dynamic forced response of vented (open-ended) SFDs where air entrainment makes a foamy lubricant, which limits severely the damper film pressures and forces.

Flow Visualization and Forces From a Squeeze Film Damper Operating With Natural Air Entrainment

Measurements of dynamic film pressures and high-speed photographs of the flow field in an open-ended Squeeze Film Damper (SFD) operating with natural free air entrainment are presented for increasing whirl frequencies (8.33-50 Hz), and a range of feed pressures to 250 kPa (37 psig). The flow conditions range from lubricant starvation (air ingestion) to a fully flooded discharge operation. The test dynamic pressures and video recordings show that air entrainment leads to large and irregular gas fingering and striation patterns. This is a natural phenomenon in SFDs operating with low levels of external pressurization (reduced lubricant through flow rates), Air ingestion and entrapment becomes more prevalent as the whirl frequency raises, and increasing the feed pressure aids little to ameliorate the loss in dynamic fbrced performance. As a result of the severity of air entrainment, experimentally estimated damping forces decrease steadily as the whirl frequency (operating speed) increases.

Sine Sweep Load vs. Impact Excitations and Their Influence on the Damping Coefficients of a Bubbly Oil Squeeze Film Damper

Squeeze Film Dampers (SFD) suppress excessive vibrations and rotordynamic instabilities in turbomachinery. However, air ingestion into the oil film is a pervasive phenomenon that affects their performance, complicating their analysis and design, and demanding careful experimentation. The type of force excitation affects the damping coefficients since the ensuing dynamic journal motions may lead to a rapid expulsion or to coalescence and entrapment of the air within the lubricant matrix. Experimental force coefficients from a small rotor-SFD apparatus operating with controlled mixtures of air and oil, i.e. emulating degrees of air entrainment, are obtatined from the dynamic response to sine sweep forces and impact loads. The parameter identification procedure renders damping coefficients that are sensitive to the type of force excitation. For impact tests, damping coefficients steadily increase for lubricant mixtures up to 50 % in air volume content. For unidirectional sine sweep load excitations, the damping coefficients are nearly constant even for mixtures with larger air volume fractions. The larger and sustained amplitudes of periodic journal motion induced in the sweep sine tests expel rapidly the air from the mixture, thus leaving a lubricant film that generates invariant dumping coefficients. Conversely, in the impact tests, the journal motions are of small amplitude and short duration thus providing larger damping values; the mixture behaves as a nearly incompressible fluid of larger viscosity. Presented at the 56th Annual Meeting Orlando, Florida May 20–24, 2001

Non-Linear Behaviors in the Motion of a Magnetically Supported Rotor on the Catcher Bearing During Levitation Loss:An Experimental Description

Recent developments improving load capacity foretell the practical implementation of Active Magnetic Bearings (AMB) on industrial level, pushed by the advantages of reduced wear and higher speeds that they make available. However, the possibility of an eventual power failure forces the use of back-up (catcher) bearings, which usually are of the ball bearing type. The back-up bearings present a clearance between the shaft and the inner race, such that there is not contact during normal operation. On a power failure or an emergency stop, the rotor is only supported by the catcher bearings. Thus, the rotor motion within the clearance results on a succession of impacts, contact and non-contact intervals producing a non-linear behavior of the system. The complexity of this non-linear behavior prevents the use of traditional methods for the design of the catcher bearings, calling for the need of extensive experimentation and previous experience in their dimensioning process. Here, the response of a rigid rotor, supported by a pivot on the drive side and a magnetic bearing on the other side, is measured during the emergency stop from several operating speeds. Non-linear analysis tools, such as Poincare Maps and Bifurcation Diagrams, are employed to demonstrate the non-linear characteristics of the motion, which in some conditions is shown to become chaotic with a vibration limit cycle. The rotor motion on the catcher bearings (with the magnetic bearing deactivated) is measured at constant running speeds. The limit cycles and chaotic attractors are described, showing the relation of the non-linear effects to the rotational speed.Copyright

Air Entrainment vs. Lubricant Vaporization in Squeeze Film Dampers: An Experimental Assessment of Their Fundamental Differences

Squeeze film dampers (SFDs) provide structural isolation and energy dissipation in air breathing engines and process gas compressors. However, SFDs are prone to develop a flow regime where the ingestion of air leads to the formation of a bubbly lubricant. This pervasive phenomenon lacks proper physical understanding and sound analytical modeling, although actual practice demonstrates that it greatly reduces the damper force response. Measurements of film pressures in a test SFD describing circular centered orbits at whirl frequencies varying from 0 to 100 Hz are presented for fully flooded and vented discharge operating conditions. The experiments demonstrate that operation with low levels of external pressurization, moderate to large whirl frequencies, and lubricant discharge to ambient leads to the entrapment of air within the damper film lands. The experiments also elucidate fundamental differences in the generation of film pressures and forces for operation in a flooded condition that evidences vapor cavitation. Damping forces for the vented end with air entrainment are just 15% percent of the forces measured for the flooded damper. Further measurements at constant whirl frequencies demonstrate that increasing the lubricant pressure supply retards the onset of air entainment. Classical fluid film cavitation models predict well the pressures and forces for the lubricant vapor cavitation condition. However, prevailing models fail to reproduce the dynamic forced response of vented (open ended) SFDs where air entrainment makes a foamy lubricant, which limits severely the damper film pressures and forces.Copyright

Simple Model for a Magnetic Bearing System Operating on the Auxiliary Bearing

The present work studies the behavior of a magnetic bearing supported rotor when the flow of electric current to the magnetic actuator is suppressed In this condition the rotor is supported by the auxiliary bearing, which has looseness with the rotor, generating a series of impacts between these components. For the study of this state, a model of a flexible rotor is proposed, and the impacts are simulated using kinematical restitution coefficient theory. The results obtained from the theoretical model are compared with experimental data taken on a test rig using tools for non linear systems analysis such bifurcation diagrams. The comparison shows that, besides the simplification of the contact, the model predicts ranges chaotic, quasi-periodic, and periodic motions in the test rig.Copyright