SQUEEZE FILM DAMPERS: OPERATION, MODELS AND TECHNICAL ISSUES

Appraisal of the art. Design considerations. Force Coefficients. Lubricant cavitation and air entrainment in SFDs. Response of a Rigid Rotor Supported on open-ended SFDs. (*) Digital video clips showing air entrainment in a SFD available at http://rotorlab.tamu.edu/TRIBGrou

Experimental identification of bearing force coefficients

Variable Filter method for bearing parameter identification

APPLICATION OF SQUEEZE FILM DAMPERS

The level of the vibrations and the presence of instability are the two most critical aspects regarding the operations of turbomachinery. To cope with this issues that may compromise the operation of the machines, squeeze film dampers (SFD) are often used in many industrial applications. Unfortunately, many complex phenomena characterize the dynamic behavior of these compo-nents and determine the high complexity of the modeling of these components. The most relevant phenomena involved in the characterization of SFDs are indi-viduated after a comprehensive investigation of the state of the art. Among them, the oil film cavitation, the air ingestion, and the effect of the inertia are intro-duced. A modeling strategy based on the Reynolds equation is then presented. The boundary conditions to be adopted for the feeding and discharging of oil are investigated and implemented. Eventually, the finite difference model is applied to a practical example to evaluate the possibility to minimize the vibration level and to reduce the effect of the instability if a SFD is added to a rotodynamic system. Meaningful information about the modeling of SFDs is provided in this work. The critical aspects of these components and their modeling are high-lighted and discussed

Analysis of Force Coefficients and Dynamic Pressures for Short-Length (L/D=0.2) Open-Ends Squeeze Film Dampers

Gas turbine engine manufacturers push for increasingly simpler squeeze film damper (SFD) designs that can still provide necessary damping to suppress rotor vibrations and offer stability to rotor-bearing systems. The work in this thesis addresses to industry needs by analyzing the experimental and predicted dynamic force performance of a very simply configured test SFD. The SFD incorporates three lubricant feedholes spaced 120° apart, a single short-length (L/D=0.2, L=2.54 cm) film land with no central feed groove, no end grooves for the provision sealing mechanisms (open-ends), and a nominal radial clearance c=0.267 mm (c/R=0.004). Analysis of the SFD performing whirl orbits with various amplitude (r) and departing from various static eccentricity (es) endeavors to reveal the dynamic performance of SFDs to events in gas turbine engine operation such as a blade loss, or a change in eccentricity. Circular (rX=rY) whirl orbits of the SFD with amplitude r/c=0.05 to 0.71, and departing from static eccentricity, es/c=0 to 0.86 lead to identification of the squeeze film force coefficients and measurements of the dynamic film pressures over the various dynamic operating conditions. Comparisons of experimentally identified force coefficients to those predicted by a finite element orbit-based model as well as those predicted by the classical short-length open-ends SFD theory strive to evaluate the accuracy of the state-of-the-art in SFD performance prediction. Comparisons of experimental results for the current SFD (termed damper 1) against that of two SFDs with similar configurations (dampers 2 and 3) examined in prior art, advance the simplicity of SFD design by highlighting the effects of a smaller radial clearance (in damper 2) and the effects of end grooves (in damper 3) on SFD dynamic force performance. Experimentally identified force coefficients for the current damper (#1) show moderate growth with orbit amplitude and strong nonlinear growth with static eccentricity, in particular at a largely off-centered position. Added mass coefficients for damper 1 increase with static eccentricity and, unexpectedly, with orbit amplitude. The experimentally identified SFD force coefficients for damper 1 exhibit excellent agreement with predicted force coefficients from the orbit-based model and the short-length open-ends model for whirl orbits departing from centered to slightly off-centered positions (e/c < 0.4) and with a small orbit amplitude (r/c < 0.4). The fluid film dynamic peak-to-peak (p-p) pressures exhibit a strong growth with orbit amplitude, and a moderate growth with static eccentricity. Inconsistent increases in p-p pressure with whirl frequency demonstrate the occurrence of air ingestion for motions with a large orbit amplitude or departing from a large static eccentricity. Comparisons of the current damper (damper 1) with damper 2 and 3 demonstrate that the damping and added mass force coefficients closely follow geometric ratios (L/c)^3 and (L^3/c), respectively, derived from the short-length open-ends SFD model. Damper 2, with ~half clearance (c2=0.122 mm), produces eight times more damping and 1.9 times more added mass than does damper 1. Damper 3 with similar clearance (c3=0.254 mm) and slightly longer film land length (Leff=2.97 cm), produces just 1.75 times more damping and 2.12 times more added mass than does damper 1

Squeeze Film Damper Modeling: A Comprehensive Approach

Squeeze film dampers (SFDs) are components used in many industrial applications, ranging from turbochargers to jet engines. SFDs are applied when the vibration levels or some instability threatens the safe operation of the machine. However, modeling these components is difficult and somewhat counterintuitive due to the multiple complex phenomena involved. After a thorough investigation of the state of the art, the most relevant phenomena for the characterization of the SFDs are highlighted. Among them, oil film cavitation, air ingestion, and inertia are investigated and modeled. The paper then introduces a numerical model based on the Reynolds equation, discretized with the finite difference method. Different boundary conditions for oil feeding and discharging are implemented and investigated. The model is validated by means of experimental results available in the literature, whereas different designs and configurations of the feeding and sealing system are considered. Eventually, an example of the application of a SFD to a compressor rotor for the reduction of vibration and correction of the instability is proposed. The paper provides an insight regarding the critical aspects of modeling SFDs, underscoring the limits of the numerical model, and suggesting where to further develop and improve the modeling

Measurements of the Dynamic Forced Response of an O-Rings Sealed Squeeze Film Damper Supplied with a Low Supply Pressure

Modern SFD designs are short in axial length to limit weight and part count and supplied with a low lubricant feed pressure to reduce operating costs related to lubricant storage and pumping power. O-rings (ORs) reduce lubricant side leakage, increasing the viscous damping within a constrained physical space and provide a modest centering support stiffness to the rotor. Continuing a long-term project characterizing SFDs for air breathing engines, the thesis details comprehensive measurements of the forced performance of an OR sealed damper (OR-SFD), with a film land length L=25.4 mm, 127 mm in diameter (D), and a radial clearance c=0.279 mm. The damper, with a slenderness ratio L/D = 0.2. undergoes centered whirl motions with amplitudes r=0.05c to 0.45c, over ω = 10 Hz to 130 Hz (max. squeeze film velocity vs=rω=102.5 mm/s). Lubricant ISO VG 2 supplied at 0.69 bar(g) fills an upstream oil plenum and flows through a single orifice with a check valve midway of the damper length (1/2L). Measurements of dynamic loads, along with the ensuing displacements and accelerations identify the parameters of the test structure, ORs and SFD. This research effort is the first to identify ORs force coefficients over a range of orbit amplitudes and assess its effects on the dynamic performance of the OR-SFD. The ORs force coefficients remain nearly invariant within the identification frequency range; however, they showcase significant orbit amplitude dependence. At r/c = 0.05 the OR centering stiffness (KOR) doubles the static stiffness (KOR,static), and as r/c→ 0.45, KOR approaches ½KOR,static, likely due to the extensive elastic deformation and slow recovery in the rings’ polymeric structure bonds. At r=0.05c and 0.10c, the ORs viscous damping coefficient (COR) contributes to ~10% of the total in the lubricated system (CL), while for r/c > 0.25, it contributes to just 3% of CL. For small orbit amplitudes (r ≤ 0.25c), the experimental SFD added mass (MSFD) and viscous damping (CSFD) coefficients are nearly equivalent to theoretical magnitudes for a fully sealed damper (no side leakage). However, as the orbit size grows to r → 0.45c, MSFD drops nearly 75% and COR decreases by ~40%. The reduction in force coefficients is due to the onset of both lubricant cavitation and air ingestion occurring for vs ≥ 24.5 mm/s. A prediction model delivers squeeze film added mass and viscous damping coefficients which are on average, 10% larger than those derived experimentally. Measured film dynamic pressures evidence both oil vapor cavitation and air ingestion, and video recordings depict a bubbly mixture in the lubricant return line and through the damper top end. Peak-peak film pressures for operation at vs ≥ 34 mm/s show the gas content prevents the generation of peak pressures proportional to vs. Moreover, pk-pk pressures inside the journal oil delivery plenum follow the same trend as those in the film land, showing the mechanical check valve installed in the journal allows for lubricant backflow. A novel approach enables the estimation of the gas volume fraction (GVF), which rapidly increases with vs. The simple procedure draws into a deflated balloon the material contents in the film, weighs the sample and identifies its volume to produce an estimation of the GVF. The research findings reveal more details on the effect of ORs to the forced performance of a damper and their limited ability to prevent air ingestion when operating at large squeeze velocities and a low lubricant feed pressure

Squeeze Film Dampers: An Experimental Appraisal of Their Dynamic Performance

LectureLecture 15: Squeeze Film Dampers (SFDs) are effective means to ameliorate rotor vibration amplitudes and to suppress instabilities in rotor-bearing systems. A SFD is not an off-the-shelf mechanical element but tailored to a particular rotor-bearing system as its design must satisfy a desired damping ratio; if too low, the damper is ineffective, whereas if damping is too large, it locks the system aggravating the system response. In many cases, SFDs are also employed to control the placement of (rigid body) critical speeds displacing the machine operation into a speed range with effective structural isolation. Industry demands well-engineered SFDs with a low footprint to reduce cost, maintenance, weight, and space while pushing for higher operating shaft speeds to increase power output. Compact aero jet engines implement ultra-short length SFDs (L/D ? 0.2) to satisfy stringent weight and space demands with low parts count. A manufacturer, as part of a business plan to develop and commercialize energy efficient aircraft gas turbine engines, supported a multiple–year project to test novel SFD design spaces. In spite of the myriad of analyses and experimental result reported in the literature, there has not been to date a concerted effort to investigate the dynamic forced performance of a SFD through its many configurations: open ends vis-à-vis sealed ends conditions, and supply conditions with a fluid plenum or deep groove vis-à-vis feed holes directly impinging into the film land. This lecture presents experimental results obtained with a dedicated rig to evaluate short length SFDs operating under large dynamic loads (2.2 kN ? 500 lbf) that produced circular and elliptical whirl orbits of varying amplitude, centered and off-centered. The lecture first reviews how SFDs work, placing emphasis on certain effects largely overlooked by practitioners who often regard the SFD as a simple non-rotating journal bearing. These effects are namely fluid inertia amplification in the supply or discharge grooves, pervasive air ingestion at high whirl frequencies, and effective end sealing means to enhance damping. The bulk of the lecture presents for various SFD configurations comparisons of experimentally identified damping (C) and inertia or added mass (M) coefficients versus amplitude of motion (orbit size) and static eccentricity position, both ranging from small to large; as large as the film clearance! The experiments, conducted over six plus years of continued work give an answer to the following fundamental practitioners’ questions: (a) Dampers don’t have a stiffness (static centering capability), how come? (b) Why is there fluid inertia or added mass in a damper? Isn’t a damper a purely viscous element? (c) How much do the damping and added mass change when the film length is halved? What about increasing the clearance to twice its original magnitude? (d) How much more damping is available if the damper has end seals? (e) Is a damper with feed holes as effective as one containing a groove that ensures lubricant pools to fill the film? What if a hole plugs, is a damper still effective? (f) Do the amplitude and shape of whirl motion affect the damper force coefficients? (g) What happens if the damper operates largely off-centered; does its performance become nonlinear? (h) What do prevailing theoretical predictions correlate with the experimental record

Squeeze Film Dampers: A Further Experimental Appraisal Of Their Dynamic Forced Performance

TutorialSqueeze Film Dampers (SFDs) are effective means to ameliorate rotor vibration amplitudes and to suppress instabilities in rotor-bearing systems. A SFD is not an off-the-shelf mechanical element but tailored to a particular rotor-bearing system as its design must satisfy a desired damping ratio; if too low, the damper is ineffective, whereas if damping is too large, it locks the system aggravating the system response. In many cases, SFDs are also employed to control the placement of (rigid body) critical speeds displacing the machine operation into a speed range with effective structural isolation. Industry demands well-engineered SFDs with a low footprint to reduce cost, maintenance, weight, and space while pushing for higher operating shaft speeds to increase power output. Compact aero jet engines implement ultra-short length SFDs (L/D ≤ 0.2) to satisfy stringent weight and space demands with low parts count. A manufacturer, as part of a business plan to develop and commercialize energy efficient aircraft gas turbine engines, supported a multiple–year project to test novel SFD design spaces. In spite of the myriad of analyses and experimental results reported in the literature, there has not been to date a concerted effort to investigate the dynamic forced performance of a SFD through its many configurations: open ends vis-à-vis sealed ends conditions, and supply conditions with a fluid plenum or deep groove vis-à-vis feed holes directly impinging into the film land. This lecture presents experimental results obtained with a dedicated rig to evaluate short length SFDs operating under large dynamic loads (2.2 kN ≈ 500 lbf) that produced circular and elliptical whirl orbits of varying amplitude, centered and off-centered. The lecture first reviews how SFDs work, placing emphasis on certain effects largely overlooked by practitioners who often regard the SFD as a simple non-rotating journal bearing. These effects are namely fluid inertia amplification in the supply or discharge grooves, pervasive air ingestion at high whirl frequencies, and effective end sealing means to enhance damping. The bulk of the lecture presents for various SFD configurations comparisons of experimentally identified damping (C) and inertia or added mass (M) coefficients versus amplitude of motion (orbit size) and static eccentricity position, both ranging from small to large; as large as the film clearance! The experiments, conducted over six plus years of continued work give an answer to the following fundamental practitioners’ questions: (a) Dampers don’t have a stiffness (static centering capability), how come? (b) Why is there fluid inertia or added mass in a damper? Isn’t a damper a purely viscous element? (c) How much do the damping and added mass change when the film length is halved? What about increasing the clearance to twice its original magnitude? (d) How much more damping is available if the damper has end seals? (e) Is a damper with feed holes as effective as one containing a groove that ensures lubricant pools to fill the film? What if a hole plugs, is a damper still effective? (f) Does a flooded damper offer same force coefficients as one lubricated thru feed holes? (g) Do the amplitude and shape of whirl motion affect the damper force coefficients? (h) What happens if the damper operates largely off-centered; does its performance become nonlinear? (i) Is air ingestion a persistent issue with an open ends SFD? (j) How do predictions from accepted engineering practice SFD models correlate with the experimental record? Is an idealized SFD geometry representative of actual practice

Analysis of Force Coefficients and Dynamic Pressures for Short-Length (L/D=0.2) Open-Ends Squeeze Film Dampers

Gas turbine engine manufacturers push for increasingly simpler squeeze film damper (SFD) designs that can still provide necessary damping to suppress rotor vibrations and offer stability to rotor-bearing systems. The work in this thesis addresses to industry needs by analyzing the experimental and predicted dynamic force performance of a very simply configured test SFD. The SFD incorporates three lubricant feedholes spaced 120° apart, a single short-length (L/D=0.2, L=2.54 cm) film land with no central feed groove, no end grooves for the provision sealing mechanisms (open-ends), and a nominal radial clearance c=0.267 mm (c/R=0.004). Analysis of the SFD performing whirl orbits with various amplitude (r) and departing from various static eccentricity (es) endeavors to reveal the dynamic performance of SFDs to events in gas turbine engine operation such as a blade loss, or a change in eccentricity. Circular (rX=rY) whirl orbits of the SFD with amplitude r/c=0.05 to 0.71, and departing from static eccentricity, es/c=0 to 0.86 lead to identification of the squeeze film force coefficients and measurements of the dynamic film pressures over the various dynamic operating conditions. Comparisons of experimentally identified force coefficients to those predicted by a finite element orbit-based model as well as those predicted by the classical short-length open-ends SFD theory strive to evaluate the accuracy of the state-of-the-art in SFD performance prediction. Comparisons of experimental results for the current SFD (termed damper 1) against that of two SFDs with similar configurations (dampers 2 and 3) examined in prior art, advance the simplicity of SFD design by highlighting the effects of a smaller radial clearance (in damper 2) and the effects of end grooves (in damper 3) on SFD dynamic force performance. Experimentally identified force coefficients for the current damper (#1) show moderate growth with orbit amplitude and strong nonlinear growth with static eccentricity, in particular at a largely off-centered position. Added mass coefficients for damper 1 increase with static eccentricity and, unexpectedly, with orbit amplitude. The experimentally identified SFD force coefficients for damper 1 exhibit excellent agreement with predicted force coefficients from the orbit-based model and the short-length open-ends model for whirl orbits departing from centered to slightly off-centered positions (e/c < 0.4) and with a small orbit amplitude (r/c < 0.4). The fluid film dynamic peak-to-peak (p-p) pressures exhibit a strong growth with orbit amplitude, and a moderate growth with static eccentricity. Inconsistent increases in p-p pressure with whirl frequency demonstrate the occurrence of air ingestion for motions with a large orbit amplitude or departing from a large static eccentricity. Comparisons of the current damper (damper 1) with damper 2 and 3 demonstrate that the damping and added mass force coefficients closely follow geometric ratios (L/c)^3 and (L^3/c), respectively, derived from the short-length open-ends SFD model. Damper 2, with ~half clearance (c2=0.122 mm), produces eight times more damping and 1.9 times more added mass than does damper 1. Damper 3 with similar clearance (c3=0.254 mm) and slightly longer film land length (Leff=2.97 cm), produces just 1.75 times more damping and 2.12 times more added mass than does damper 1

EXPERIMENTS AND MODELS FOR OPERATION OF A SEALED ENDS SQUEEZE FILM DAMPERS: A STEP TOWARDS QUANTIFYING AIR INGESTION IN SQUEEZE FILMS

Squeeze film dampers (SFDs) in high-performance turbomachinery reduce rotor motion amplitudes as it traverses a critical speed or when the system has a dynamic instability, thus ensuring system reliability. To improve the damping capacity in aircraft engines within a limited space, piston ring (PR) seals are installed at the axial ends of a film land. Even though PRs effectively seal a SFD, a significant amount of a lubricant exits through the gap at the abutted ends of the PR (PR slit). However, when the squeeze-film pressure is lower than ambient pressure, air ingests into the film and mixes with the lubricant. The advanced turbomachines have a larger operating speed with a smaller lubricant supply than traditional turbomachines; hence, air entrainment in a sealed ends SFD becomes significant. This dissertation presents a computational physics model for a sealed ends SFD and open to ambient, hence prone to air entrainment; and delivers predictions benchmarked against experimental test results. The first embodiment is a SFD with a PR and an O-ring (OR) sealing the film land. In the tests, a known gas (air) volume fraction (GVF or β) in a mixture of air and ISO VG2. The PR and the O-ring (OR) that seal the film land are located in the grooves at the top and bottom of the journal, respectively. The supplied mixture discharges through the PR slit, located at the top axial end, into a vessel submerged within a large volume of lubricant. Another damper, which has same journal geometric parameters, is supplied with a pure lubricant of a supply pressure. Both the top and bottom axial ends are sealed with PRs and opened to ambient. Hence, the supplied lubricant exits through the PR slits into ambient, and the air ingests through the PR slits when the film pressure is below the ambient pressure. There are two distinctive models evaluating the evolution of gas volume fraction in a squeeze film land: (a) a volume of fluid (VOF) model and (b) a bubbly mixture. The models predicting the pressure field in the squeeze film implement the Reynolds equation, modified to include temporal fluid inertia effects, and uses physics-based inlet and outlet lubricant conditions through a feed hole and PR slits, respectively A parametric study produces the dynamic forced performance of the PR sealed ends SFD. The predictions show the time-space average GVF increases as the squeeze velocity (vs) increases. On the other hand, the GVF decreases as the supply pressure increases. The damper physical geometry also affects its dynamic forced performance. The GVF increases as the journal diameter increases; whereas the SFD axial length does not significantly change the GVF. The GVF reduces as the damper clearance increases. The GVF does not significantly change as the PR slit cross-sectional area varies. An oil supply pressure large enough to prevent air ingestion varies with damper geometry, lubricant inlet/outlet conditions, and the kinematics of the journal. The PR slits allows air ingestion even as the squeeze velocity is small. As the damper diameter to clearance ratio (D/c) increases, the GVF in the film increases. Most importantly, the location of the PR slit relative to the feedhole significantly affects the amount of air content in the film. When the PR slit faces to the feedhole, the film land is mostly filled with a pure lubricant. The GVF increases as the arc distance from the PR slit to the feedhole increases